Laser Applications (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Advanced Materials and Technologies)

Landolt-Börnstein GROUP VIII: Advanced Materials and Technologies VOLUME 1 Laser Physics and Applications SUBVOLUME C L...

Author: D. Bäuerle | H.W. Bergmann | F. Dausinger | K. Dörschel | A. Gebhardt | M. Geiger | M. Grupp | H. Haferkamp | C. Hertzler | H. Hügel | O. Minet | M. Möhrle | G. Müller | W. O'Neill | W. Schulz | G. Sepold | H. Tiziani

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Landolt-Börnstein GROUP VIII: Advanced Materials and Technologies VOLUME 1 Laser Physics and Applications SUBVOLUME C Laser Applications Title Pages Contributors, Preface, Contents Contributors Preface Contents 1

Fundamentals

1

1.1 1.1.1

Fundamentals of laser-induced processes (H. HÜGEL, F. DAUSINGER) Introduction

3 3

1.1.2

Energy coupling

4

1.1.2.1

Fundamentals

4

1.1.2.2

Optical properties of metals

6

1.1.2.2.1

Temperature effects

9

1.1.2.2.2

Chemical effects

14

1.1.2.2.3

Roughness effects

14

1.1.2.3

Optical properties of ceramics

16

1.1.2.4

Scattering and absorption by particles

18

1.1.2.5

Non-linear absorption

24

1.1.3

Thermophysical and dynamical “response”

25

1.1.3.1

Condensed matter

25

1.1.3.1.1

Heat conduction

25

1.1.3.1.1.1

Fourier heat conduction

25

1.1.3.1.1.2

Two-temperature model

28

1.1.3.1.2

Phase transitions

30

1.1.3.1.2.1

Melting

30

1.1.3.1.2.2

Evaporation

32

1.1.3.1.3

Melt dynamics

34

1.1.3.1.3.1

Origin of driving forces

34

1.1.3.1.3.2

Resulting effects in laser machining

35

1.1.3.2

Interaction mechanisms in the gas and plasma phase

37

1.1.3.2.1

Basic ionization and absorption mechanisms

38

1.1.3.2.1.1

Bound electrons

38

1.1.3.2.1.2

Free electrons

40

1.1.3.2.2

Absorption and refraction effects in laser-induced plasmas

41

1.1.3.2.2.1

Plasma composition and temperature

41

1.1.3.2.2.2

Absorption

47

1.1.3.2.2.3

Refraction

50

1.1.3.2.3

Dynamical effects

52

1.1.4

Simplified dependences in laser processes

56

1.1.4.1

Energy coupling in laser processes

56

1.1.4.1.1

Coupling rate in laser cutting

56

1.1.4.1.2

Coupling rate in laser welding

57

1.1.4.2

Process windows

58

1.1.4.2.1

Power threshold

59

1.1.4.2.2

Factors determining process velocity

60

1.1.4.2.3

Factors determining efficiency

61

References for 1.1

62

2

Production engineering

73

2.1 2.1.1

Surface treatment (H.W. BERGMANN) Laser macro processing

75 75

2.1.1.1

Solid-state hardening

76

2.1.1.1.1

Physical basics

76

2.1.1.1.2

Material science basics

77

2.1.1.1.3

Production-related aspects

78

2.1.1.1.4

Time scheme of the irradiation

79

2.1.1.1.5

Observed degradations and their reasons

79

2.1.1.2

Remelting

81

2.1.1.2.1

Remelting of cast iron

83

2.1.1.2.2

Remelting of aluminum alloys

84

2.1.1.2.3

Remelting of titanium alloys

86

2.1.1.2.4

Remelting of magnesium alloys

88

2.1.1.2.5

Observed degradations of surface-remelted components and their reasons

89

2.1.1.3

Laser cladding

92

2.1.2

Thin-layer technologies

95

2.1.2.1

Laser cleaning

96

2.1.2.2

Laser cleaning and smoothing of cast iron

97

2.1.2.3

Surface alloying

97

2.1.3

Laser shock hardening

98

References for 2.1

101

2.2 2.2.1

Rapid prototyping (A. GEBHARDT) Layer manufacturing

105 106

2.2.1.1

Rapid-prototyping process chain

106

2.2.1.2

Prototypers

107

2.2.1.3

Characteristics of rapid-prototyping processes

108

2.2.1.4

Materials

108

2.2.1.5

Post-processing

113

2.2.1.6

Finishing

113

2.2.1.7

Functional metal parts

113

2.2.1.8

Rapid tooling

113

2.2.1.8.1

Indirect rapid tooling

115

2.2.1.8.2

Direct rapid tooling

116

2.2.2

Application of rapid-prototyping models

117

2.2.2.1

Model characteristics and model properties

117

2.2.2.2

Criteria for the use of rapid-prototyping models

117

2.2.2.3

Examples

119

2.2.2.4

Rapid manufacturing

121

2.2.3

Recent developments and future trends

121

References for 2.2

123

2.3 2.3.1

Thermal bending (M. GEIGER, F. VOLLERTSEN) Principle of laser forming

125 125

2.3.2

Mechanisms

125

2.3.3

Influence parameters

126

2.3.3.1

Threshold energy

127

2.3.3.2

Processing parameters

127

2.3.3.3

Material parameters

128

2.3.3.4

Geometric parameters

130

2.3.4

Bend radii

131

2.3.5

State of the art and trends

132

References for 2.3

133

2.4 2.4.1

Joining (H. HAFERKAMP) Introduction

137 137

2.4.2

Conduction welding

139

2.4.3

Deep-penetration welding

140

2.4.3.1

Capillary formation

142

2.4.3.2

Plasma formation

143

2.4.3.3

Humping effect

144

2.4.4

Material weldability

145

2.4.5

Thermal distortion

146

2.4.6

Tailored blanks

147

2.4.7

Soldering and brazing

149

2.4.8

Diode-laser applications

150

2.4.9

How to avoid quality degradation

151

References for 2.4

156

2.5 2.5.1

Laser separating (W. O'NEILL) Introduction

159 159

2.5.2

Cutting

160

2.5.2.1

Fusion cutting

160

2.5.2.2

Reactive-gas cutting

162

2.5.2.3

Sublimation cutting

163

2.5.3

Cleaning

163

2.5.4

Machining

167

2.5.4.1

Oxidation processes

168

2.5.4.2

Liquid-phase machining

168

2.5.4.3

Vapor-phase machining

168

2.5.5

Drilling

170

2.5.5.1

Piercing

172

2.5.5.2

Multiple-pulse drilling

172

2.5.5.3

Trepanning

173

2.5.5.4

High-speed drilling

175

2.5.6

Non-thermal ablation

176

2.5.7

Marking

177

2.5.8

Comparison with conventional processes

178

2.5.9

How to avoid quality degradation

180

References for 2.5

184

2.6 2.6.1

Cutting: Modeling and data (W. SCHULZ, C. HERTZLER) Diagnostics, monitoring and modeling

187 188

2.6.2

Experiments and diagnostics

189

2.6.3

Mathematical formulation

191

2.6.4

Inertial manifolds

192

2.6.5

Dimension in phase space

193

2.6.6

Spatial one-dimensional model

194

2.6.7

Spatial two-dimensional model and diffusive eikonal

196

2.6.8

Iterative refinement

199

2.6.9

Cutting data

202

2.6.9.1

Laser power

202

2.6.9.2

Modulation of the laser output (gating frequency)

203

2.6.9.3

Beam quality and power density distribution

203

2.6.9.4

Spatial and temporal beam stability

205

2.6.9.5

Polarization

205

2.6.9.6

Output mirrors of the laser unit

205

2.6.9.7

Beam alignment

205

2.6.9.8

Astigmatism

206

2.6.9.9

Deflection mirror

206

2.6.9.10

Focusing lens

206

2.6.9.11

Beam to nozzle alignment

206

2.6.9.12

Shape of nozzle exit

207

2.6.9.13

Cutting speed

207

2.6.9.14

Type of assist gas

207

2.6.9.15

Gas pressure

207

2.6.9.16

Focal position

208

2.6.9.17

Material type and composition

208

2.6.9.18

Thickness

208

2.6.9.19

Surface condition of the sheet metal

208

2.6.9.20

Cut shape

209

2.6.9.21

Kerf width

209

2.6.9.22

Dross formation

210

2.6.9.23

Mean roughness

210

2.6.9.24

Perpendicularity and slant tolerance

211

2.6.9.25

Drag lines

211

2.6.9.26

Pitting

211

2.6.9.27

Heat-affected zone

211

2.6.10

Machining data tables for cutting

212

References for 2.6

215

2.7 2.7.1

Laser systems for materials processing (G. SEPOLD, M. GRUPP) Laser macro systems

219 220

2.7.1.1

Laser sources

221

2.7.1.1.1

CO2-laser

221

2.7.1.1.2

Nd:YAG laser

222

2.7.1.1.3

High-power diode lasers

223

2.7.1.1.4

Fiber lasers and thin disc lasers

224

2.7.1.2

Laser beam guiding

224

2.7.1.2.1

Beam-guiding systems for CO2-lasers

224

2.7.1.2.2

Beam guiding for Nd:YAG-lasers

225

2.7.1.3

Beam-forming elements

226

2.7.1.3.1

Focusing optics

227

2.7.1.3.1.1

Laser cutting heads

228

2.7.1.3.1.2

Welding heads

228

2.7.1.3.1.3

Working heads for surface treatment

229

2.7.1.4

Handling devices

230

2.7.1.4.1

System concepts

230

2.7.1.4.1.1

1-dimensional systems

230

2.7.1.4.1.2


230

2.7.1.4.1.3


232

2.7.1.4.2

New developments

234

2.7.1.4.3

Special systems

235

2.7.1.4.4

Actuation and control of laser systems

236

2.7.1.4.5

Clamping devices

236

2.7.2

Laser microtechnology

237

2.7.2.1

Beam sources

238

2.7.2.2

Beam-guiding and -forming techniques

238

2.7.3

Conclusions and outlook

239

References for 2.7

241

2.8 2.8.1

Process monitoring and closed-loop control (W. WIESEMANN) Introduction

243 243

2.8.2

Basics of process monitoring and closed-loop control

244

2.8.2.1

General

244

2.8.2.2

Process-surveillance objectives and strategies

245

2.8.2.2.1

Support for scientific research by monitoring of process output parameters

245

2.8.2.2.2

On-line treatment fault probability assessment and documentation during serial production

246

2.8.2.2.3

Closed-loop control during serial production

246

2.8.2.3

Treatment quality indicators, span of surveillance

247

2.8.2.4

Process output parameter detection

249

2.8.2.4.1

Theoretical introduction

249

2.8.2.4.2

Radiation emission from the interaction zone

250

2.8.2.4.3

Radiation reflection and transmission at the interaction zone

250

2.8.2.4.4

Radiation detection and sensor arrangement

251

2.8.2.4.5

Two-dimensionally resolved radiation emission

252

2.8.2.4.6

Sound detection

253

2.8.2.4.7

Electrical-charge detection

253

2.8.2.4.8

Multiple-sensor fusion

253

2.8.2.5

Signal assessment methods

254

2.8.2.6

Control actions

256

2.8.3

State of the art of process monitoring and control technology

258

2.8.3.1

Cutting and drilling

259

2.8.3.1.1

General

259

2.8.3.1.2

Scientific research

259

2.8.3.1.2.1

Cutting

259

2.8.3.1.2.2

Drilling, piercing

260

2.8.3.1.3

Industrial applications

261

2.8.3.2

Welding

261

2.8.3.2.1

General and historical

261

2.8.3.2.2

Recent scientific research

262

2.8.3.2.2.1

Optical emissions, photo-diode detection

263

2.8.3.2.2.2

Optical emissions, camera detection

264

2.8.3.2.2.3

Reflected laser radiation

264

2.8.3.2.2.4

Electrical charge collection

265

2.8.3.2.2.5

Multiple sensor schemes

265

2.8.3.2.2.6

X-ray and visible-light shadowgraphy, holography, laser beam probe, ultrasonic inspection

266

2.8.3.2.3

Industrial applications

267

2.8.3.2.3.1

Monitoring systems

267

2.8.3.2.3.2

Closed-loop control systems

268

2.8.3.3

Transformation hardening

268

2.8.3.4

Cladding, alloying

269

2.8.3.5

Cleaning, caving

269

2.8.4

Outlook

270

References for 2.8

272

3

Life science, biological and chemical processing

277

3.1 3.1.1

Lasers in biology and medicine (O. MINET, K. DÖRSCHEL, G. MÜLLER) Light and heat transport in tissue

279 279

3.1.1.1

Light transport and optical parameters

279

3.1.1.2

Heat transport and thermal parameters

282

3.1.2

Laser-tissue interactions

285

3.1.2.1

Laser diagnostics by transillumination and induced fluorescences

285

3.1.2.2

Laser-induced photochemistry

287

3.1.2.3

Photothermal effects

287

3.1.2.3.1

Coagulation

288

3.1.2.3.2

Evaporation of tissue

288

3.1.2.4

Effects of short-pulsed laser radiation

288

3.1.2.4.1

Photoablation

289

3.1.2.4.2

Optical breakdown and plasma formation

290

3.1.2.4.3

Laser-induced shock waves and cavitations

291

3.1.2.5

Laser biostimulation

291

3.1.3

Medical laser systems

292

3.1.3.1

Typical medical lasers

292

3.1.3.2

Laser light delivery

292

3.1.3.2.1

Articulated arm

292

3.1.3.2.2

Light guide

292

3.1.3.2.3

Optical glass fiber

295

3.1.3.2.4

Hollow wave guide

296

3.1.3.3

Applicator systems

296

3.1.4

Laser applications in medicine

296

3.1.5

Medical laser safety

298

3.1.5.1

Medical laser pyrolysis products

298

3.1.5.2

Regulatory requirements for medical laser systems

301

3.1.5.3

Specific aspects of medical laser safety

302

References for 3.1

303

3.2 3.2.1

Laser chemical processing (D. BÄUERLE) Introduction

311 311

3.2.2

Pulsed-laser ablation

313

3.2.2.1

Surface patterning

314

3.2.2.2

The threshold fluence

317

3.2.2.3

Ablation rates

317

3.2.2.4

Material damage

319

3.2.2.5

Influence of an ambient atmosphere

320

3.2.2.6

Instabilities, structure formation

320

3.2.3

Materials etching

320

3.2.3.1

Etching of metals

321

3.2.3.2

Etching of semiconductors and insulators

321

3.2.4

Laser-induced chemical vapor deposition (Laser-CVD)

325

3.2.4.1

Microstructures

325

3.2.4.2

Thin-film formation

327

3.2.4.3

Adsorbed layers, hybrid techniques

329

3.2.5

Deposition from liquids

331

3.2.5.1

Electroless plating

332

3.2.5.2

Electrochemical plating

333

3.2.6

Pulsed-laser deposition (PLD)

333

3.2.6.1

Overview of materials and film properties

334

3.2.6.1.1

Inorganic materials

334

3.2.6.1.2

Organic materials

336

3.2.6.2

Nanocrystalline films

338

3.2.6.2.1

Nanocomposite materials

338

3.2.6.2.2

Size-selective ablation

339

3.2.6.3

Hybrid techniques

339

3.2.6.4

Laser-induced forward transfer

339

3.2.7

Chemical surface transformations

339

3.2.7.1

Doping

340

3.2.7.2

Alloying and synthesis

341

3.2.7.3

Oxidation, nitridation, reduction

341

3.2.7.4

Transformation of organic materials, laser lithography

342

References for 3.2

345

4

Optical data processing

353

4.1 4.1.1

Communication (M. MÖHRLE, H. VENGHAUS) Introduction

355 355

4.1.2

Heterostructures

356

4.1.3

Material systems

359

4.1.4

Diode laser structures

359

4.1.4.1

Ridge-waveguide (RW) lasers

360

4.1.4.2

Buried-heterostructure (BH) lasers

360

4.1.4.2.1

Conventional BH-lasers

361

4.1.4.2.2

Semi-insulating BH-lasers

362

4.1.4.2.3

Buried-Ridge-Stripe (BRS) lasers

362

4.1.5

Laser types

362

4.1.5.1

Multimode devices (Fabry-Perot (FP) lasers)

362

4.1.5.2

Single-mode devices

364

4.1.5.2.1

Distributed-feedback (DFB) lasers

364

4.1.5.2.2

Distributed-Bragg-reflector (DBR) lasers

367

4.1.6

Characteristics of 1.3 µm and 1.5 µm lasers

367

4.1.6.1

Fabry-Perot (FP) lasers

368

4.1.6.2

Distributed-feedback (DFB) lasers

371

4.1.6.2.1

General

371

4.1.6.2.2

Linewidth

373

4.1.6.2.3

HF characteristics

373

4.1.6.3

Lasers for uncooled operation

374

4.1.6.4

Lasers for supervisory channels

375

4.1.7

Fiber-based lasers

375

4.1.7.1

Fiber lasers

375

4.1.7.2

Hybrid Fabry-Perot fiber Bragg grating lasers

376

4.1.8

Integrated laser-modulator

376

4.1.9

Tunable lasers

378

4.1.9.1

Standard devices

378

4.1.9.2

Semiconductor lasers with enhanced tuning range

379

4.1.9.3

Commercial tunable single-chip (SC) lasers

380

4.1.9.4

Linewidth of widely tunable lasers

380

4.1.9.5

Wavelength tunable/selectable fiber laser

380

4.1.9.6

External-cavity laser

381

4.1.10

Monolithic integrations

381

4.1.10.1

Integrated spot size converter

381

4.1.10.2

Integrated multi-wavelength sources

382

4.1.10.3

Integrated mm-wave source

383

4.1.10.4

Transceiver

384

4.1.11

Lasers for advanced optical systems

384

4.1.11.1

Short-pulse sources

384

4.1.11.2

Self-pulsating lasers

384

4.1.12

Pump lasers for optical amplification

385

4.1.12.1

Pump lasers for erbium-doped fiber amplifiers (EDFAs)

385

4.1.12.2

Pump lasers for Raman amplification

386

4.1.13

Vertical-cavity surface-emitting lasers (VCSELs)

386

4.1.13.1

Short-wavelength VCSELs

387

4.1.13.2

Long-wavelength (1.3 µm, 1.55 µm) VCSELs

388

4.1.13.2.1

1.3 µm VCSELs

389

4.1.13.2.2

1.55 µm VCSELs

389

4.1.13.3

Tunable VCSELs

392

4.1.14

Reliability

392

References for 4.1

395

5

Metrology

403

5.1 5.1.1

High-precision optical metrology for surfaces (H.J. TIZIANI, M. TOTZECK) Introduction

405 405

5.1.2

Microstructure metrology

407

5.1.2.1

Resolution in optical imaging

407

5.1.2.2

Improving the optical resolution

410

5.1.2.2.1

Decreasing the wavelength

410

5.1.2.2.2

Increasing the numerical aperture (NA)

411

5.1.2.2.3

Decreasing the prefactor κ

411

5.1.2.3

Usage of a-priori information: From model-based imaging to threshold criteria

411

5.1.3

Methods and instrumentation

412

5.1.3.1

High-NA lenses

413

5.1.3.2

Field-measuring microscopy

414

5.1.3.2.1

Intensity microscopy

414

5.1.3.2.2

Microscopy with pupil filters

417

5.1.3.2.3

Interference microscopy

417

5.1.3.2.4

Polarization interferometry

420

5.1.3.3

Confocal microscopy

421

5.1.3.4

Near-field microscopy

425

5.1.4

Large-field metrology

428

5.1.4.1

Interferometry for spherical surfaces

428

5.1.4.2

Aspherics and their testing methods

429

5.1.4.3

Interferometry for aspherical surfaces

429

5.1.4.3.1

The computer-generated hologram (CGH) null

429

5.1.4.3.2

Computation and fabrication of CGH’s

431

5.1.4.3.3

CGH application and error reduction

432

5.1.4.4

Heterodyne interferometry

433

5.1.4.4.1

Principle of external reference

433

5.1.4.4.2

Scanning differential heterodyne interferometry

435

5.1.4.5

Shack-Hartmann sensors

437

5.1.5

A look into the future

438

References for 5.1

439

5.2 5.2.1

Environmental control (M. ULBRICHT) Introduction

443 443

5.2.2

Tunable diode laser spectroscopy (TDLAS)

443

5.2.3

Cavity ring-down spectroscopy (CRDS)

444

5.2.4

Photoacoustic spectroscopy (PAS)

445

5.2.5

Lidar

446

5.2.5.1

Backscatter lidar

446

5.2.5.2

Differential absorption lidar (DIAL)

447

5.2.5.3

Raman lidar

450

5.2.5.4

Fluorescence lidar

451

5.2.5.5

Doppler lidar

451

5.2.5.6

Lidar using intense femtosecond laser pulses

452

References for 5.2

453

6

Laser safety and ecology

457

6.1 6.1.1

Laser safety (H. WELLING) Hazard potentials

459 459

6.1.2

Norms and standards for laser safety

460

6.1.3

Effects of laser radiation and safety measures

461

6.1.3.1

Effects of laser radiation on biological tissue

461

6.1.3.2

Threshold limit values and laser classification

464

6.1.3.3

Safety measures

465

6.1.3.4

Hazard distances

465

6.1.3.4.1

Specular-reflected beam

466

6.1.3.4.2

Diffuse-reflected beam

466

6.1.4

Secondary hazard potentials and safety measures

466

6.1.4.1

Laser system and components

466

6.1.4.1.1

Electrical safety

466

6.1.4.1.2

Optical components

467

6.1.4.1.3

Laser gases

467

6.1.4.1.4

Handling devices

468

6.1.4.2

Secondary radiation

468

6.1.4.3

Explosive atmospheres and fire hazards

469

6.1.4.4

Emission of gases and fumes

469

6.1.4.4.1

Characteristics of laser-generated air contaminants

469

6.1.4.4.2

Extraction systems

473

6.1.4.4.3

Filtration

474

6.1.5

Risk assessment

475

6.1.6

Training and education

476

References for 6.1

477

Index

481

http://www.springer.com/978-3-540-00105-8

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

3

1.1 Fundamentals of laser-induced processes H. HÜGEL, F. DAUSINGER

1.1.1 Introduction Laser applications in industrial production are based on definite interactions of the beam with the workpiece. Since the electro-magnetic energy transported by the laser beam is providing the heat source for all the thermal processes, the mechanisms governing the conversion of these energy forms play a decisive role and represent the fundamentals of any laser-induced process. To make material processing an efficient and widely accepted technology, it is of great importance to utilize as much as possible of the energy radiated onto the workpiece. A sound knowledge of the basic phenomena involved and their dependences on laser beam and workpiece properties is decisive for a proper choice of the laser source to be used for a particular application and for the way the process itself should be conducted. The thermal processes of heating, melting or evaporation that will occur in the interaction zone – the area at, in, or above the workpiece where the laser beam directly interacts with material – basically depend on a balance of the heat fluxes released there and those being transported away from there by conduction, convection, or radiation. As a result, the geometry and the thermo-mechanical as well as optical properties of the interaction zone will adjust themselves accordingly. In other words, the phenomena of energy coupling and “the response” of the workpiece are closely interrelated. Consequently, in treating interaction mechanisms the effects of laser beam and material properties as well as geometrical conditions of the interaction zone have to be considered at the same time. Against this background, the fundamentals of energy conversion in laser-induced processes will be treated by answering the central questions: • • •

How much of the energy irradiated onto the workpiece will be released as heat? How does the workpiece “respond” to this energy supply? Are there – on the basis of answers to the first two questions – some generalized conclusions that directly predict the energy coupling in real processes?

Along these lines, this contribution is organized following the order of events generally occurring when a laser beam is focused onto the surface of a workpiece: In the very first instant, the electro-magnetic wave interacts with the electron system of the material. Some fraction of the radiation will be reflected, the rest will propagate into the workpiece experiencing an attenuation. It is the corresponding loss of the wave’s energy that is gained as heat by the material; this aspect is briefly reviewed in Sect. 1.1.2.1 together with methods to calculate reflectivity resp. absorptivity for condensed matter (solid and liquid). Pertinent theoretical and experimental data and figures for metals and ceramics are presented in Sects. 1.1.2.2 and 1.1.2.3. In processes such as cladding or alloying, the interaction of particles with the laser beam will modify the intensity reaching the workpiece surface and the absorption there; this aspect is addressed in Sect. 1.1.2.4. A short reference to non-linear effects concludes this section on energy coupling (Sect. 1.1.2.5). Section 1.1.3 deals with the “response” of the workpiece to heating. The phenomena are addressed according to the state of the material the radiation interacts with. In the first place, therefore, mechanisms governing the energy flux and balance in condensed matter will be treated in Sect. 1.1.3.1. The discussion includes heat conduction (Sect. 1.1.3.1.1), phase transitions (Sect. 1.1.3.1.2), and melt dynamics (Sect. 1.1.3.1.3). Whenever the electro-magnetic energy flux (intensity) is very high, e.g. in drilling or welding processes, the evaporated material will become ionized. In this extended interaction zone along the beam axis absorption and refraction occur (Sect. 1.1.3.2) leading to modifications of the energy cou-

Lando lt -Börnst ein New Ser ies VIII/1C

4

1.1.2 Energy coupling

[Ref. p. 62

pling. Basic ionization mechanisms are reviewed in Sect. 1.1.3.2.1 and typical plasma properties, absorption and refraction data are presented in Sect. 1.1.3.2.2. Dynamical effects of laser-induced plasmas such as the formation of shock waves are briefly dealt with in Sect. 1.1.3.2.3. In Sect. 1.1.4 an attempt is made to summarize the basic implications of the discussed mechanisms in some generalized formulations.

1.1.2 Energy coupling A description of the propagation of electromagnetic radiation and its interaction with matter can be given by the physical models of particles (photons) and electromagnetic waves. The latter is used in this chapter to discuss reflection, absorption and refraction mechanisms. The influence of various parameters that are of particular interest especially in laser materials processing will be shown.

1.1.2.1 Fundamentals The propagation of electromagnetic waves can be described theoretically on the basis of Maxwell’s equations, see e.g. [86Bor]. A general solution of the wave equation for the time-dependent electric field strength E in the case of a plane wave propagating in z direction is i

ω~ nz

E ( z, t ) = E0 eiω t e c0

where n~ = n + i k .

(1.1.1)

Herein, E0 denotes the amplitude of the electric field, ω the angular frequency, c0 the speed of light in vacuum, and ñ the complex refractive index with n and k representing the index of refraction and extinction, respectively. For k = 0 the amplitude of the electric field and hence the energy contained within the wave remain constant over time which corresponds to the free propagation in vacuum. If, however, k is larger than zero, the wave will be damped and will lose energy upon its propagation within an absorbing medium. This absorbed energy eventually will be transformed into heat and thereby will be available for the laser treatment process. The energy flux of the wave is determined by the Poynting vector whose time-averaged value corresponds to the intensity and can be expressed by the averaged square of the field strength

I = c0 ε 0 E 2 ,

(1.1.2)

with ε0 denoting the electrical permittivity. In absorbing materials, the light intensity decreases along a distance from z = 0 to z according to Beer’s law:

I ( z ) = I 0 e −α z .

(1.1.3)

The absorption coefficient α, which is a function of the vacuum wavelength λ0, can be derived by comparing the exponents resulting from (1.1.1) in conjunction with (1.1.2) with that of Beer’s law (1.1.3):

α=

4 k (λ0 ) . λ0

(1.1.4)

The distance after which the intensity is reduced by a factor of 1/e is called the absorption length or optical penetration depth lα , which is the reciprocal value of the absorption coefficient α :

lα = 1 / α .

(1.1.5)

Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]


5

The question whether a medium can be regarded as transparent or absorbing is answered by comparing lα with its geometrical dimension in direction of the laser beam propagation. A survey of values for the absorption length and complex refractive index for metals and ceramics at room temperature is given in the following sections. For metals k is larger than unity in the infrared and visible spectral range. Consequently, the absorption length for laser light with such wavelengths is smaller than the wavelength λ0. With lα being many orders of magnitude lower than the typical dimensions of the workpiece (and even that of the interaction zone), the absorbed energy acts as a surface heat source. On the other hand, in cases where lα is comparable to the thickness of the workpiece, e.g. for thin metal films or ceramic plates, volume heating has to be considered. In laser treatment processes the incident intensity I will not be coupled completely into the workpiece. The incident beam intensity is split up into a part reflected at the surface, a part absorbed within the bulk, another one, which is scattered by the material, and a part, which is transmitted through the workpiece. Those fractions can be described by the reflectivity R, the absorptivity A, the scattered fraction S and the transmissivity T, respectively. Due to conservation of energy these contributions have to fulfill the following relationship:

R + A+ S +T =1.

(1.1.6)

The starting value I0 for Beer’s law (1.1.3) is given by the intensity, which actually enters the workpiece, i.e. (1−R) I. The absorbed intensity A⋅I will contribute to the heating of the material and will thereby have a direct impact on the process. If scattering can be neglected this fraction can be evaluated by integrating (1.1.3) over the thickness of the workpiece while the transmissivity T corresponds to the remaining fraction of intensity at the bottom of the workpiece. Otherwise the attenuation and deflection of the beam due to scattering has to be taken into account which will be discussed in Sect. 1.1.2.4. If, additionally, the thickness of the material is much larger than the optical penetration depth lα transmission can be neglected such that A = 1–R. The reflectivity and beam deflection at the interface between two media can be computed as a function of incident angle and polarization by the so-called Fresnel equations. Their general form is given in various optics books (see e.g. [93Ped] or [75Jac]). For the special case of perpendicular incidence on the surface of a beam propagating from an optically thin medium (e.g. air with n ≅ 1) into matter with a complex index of refraction, the reflectivity R can be computed by the following equation: R=

2

2

2

2

( n − 1) + k (n + 1) + k

.

(1.1.7)

For oblique incidence on metals (n2+k2 >>1) and laser light with wavelengths λ longer than 0.5 µm the Fresnel equations can be simplified according to [90Pro] R|| =

(n cos ϕ − 1)2 + k 2 cos 2 ϕ (n cos ϕ + 1)2 + k 2 cos 2 ϕ

R⊥ =

(n − cos ϕ)2 + k 2 (n + cos ϕ)2 + k 2

,

,

(1.1.8)

(1.1.9)

where ϕ is the angle between the surface normal and the incident beam. R|| describes the reflectivity for the electric field vector parallel (p-polarization) to the plane of incidence (defined by the incident beam and the surface normal) whereas R⊥ denotes the reflectivity for its orientation perpendicular (s-polarization) to it. It is obvious that in the limit of perpendicular incidence (ϕ = 0) these expressions reduce to (1.1.7). In the following sections these theoretical findings will be further examined and compared to experimental data. Due to the different optical behavior for opaque and transparent materials this survey is split into separate sections for metals and ceramics.


6


[Ref. p. 62

1.1.2.2 Optical properties of metals Various methods have been developed to determine n and k describing the optical behavior of materials and their interfaces [91Pal]. Pure materials at room temperature with clean and smooth surface conditions were preferentially used in most investigations. A rather complete data base, [91Pal] and [81Wea], is at hand for such samples. A survey of data for these optical constants at different laser wavelengths is given in Table 1.1.1. It can be observed that the extinction index k usually is much larger than unity which leads to absorption lengths smaller than the wavelength, see Table 1.1.2.

Table 1.1.1. Optical constants n and k at room temperature for various metals. Ref.

Laser / Wavelength [µm] CO2

CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

n

k

n

k

n

k

n

k

n

k

Aluminum 25.3

89.8

8.67

48.6

1.21

10.6

2.8

8.45

0.19

2.94

91Pal

25.5

60.9

7.84

35.7

0.98

7.65

1.12

6.0

0.183 2.88

62Lan

31.2

104

8.59

48.2

1.99

7.05

30.4

4.49

15.4

83Ord

Iron 5.81

3.23

4.35

6.26

26.8

4.96

14.58

3.24

4.26

5.81

30.4

4.59

15.4

3.23

4.35

3.19

4.43

0.36

7.22

0.54

6.53

3.05

3.77

1.31

1.95

91Pal

1.01

0.88

62Lan

1.14

1.87

81Wea 85Ord

Steel 1.38

1.87

62Lan

Copper 10.8

47.5

3.81

27.5

9.89

62.4

3.07

32.8

11.6

49

3.6

26.6

8.31

63

3.26

33

0.26

5.26

1.47

1.78

91Pal

0.29

5.18

1.37

1.783

62Lan

0.12…0.26

5.07…5.26

83Ord 85Ord


Ref. p. 62]


7

Table 1.1.2. Absorption length lα for metals at different wavelengths and room temperature. lα [nm]


CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

8.9

8.2

8.1

7.5

6.7

91Pal

91Pal

91Pal

91Pal

91Pal

26

28.2

19.3

17.4

10.6

85Ord

91Pal

81Wea

81Wea

81Wea

13

15

12

12.7

11.1

85Ord

91Pal

62Lan

62Lan

91Pal

Aluminum

Iron

Copper

Table 1.1.3 summarizes absorptivity values of metals and some alloys at different wavelengths and various surface conditions found in literature. Table 1.1.3. Experimental absorptivity values of metals and some alloys at room temperature for various surface conditions. They are either obtained from measured n and k-data (I) or from direct absorptivity (II), emissivity1) (III), or reflectivity (IV) measurements. A [%]

Ref.


CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

1.41

4.13

13.05

7.6

1.65

3.33…4.03

13.5…15.3

7.55…8.0

Aluminum and Alloys Pure Al

1.15 1.05

1.42 1.7

13.23…13.57 6

12

91Pal (I) 62Lan (IV) 83Ord (I) 80Dec (IV)

1.2

89Bru (I)

2

91Yil (IV)

oxidized, 3 µm rough

5

6

72Tou (III)

oxidized, 115 µm rough

25

25

72Tou (III)

anodized

20…97

polished

2

6.25

93Ste (II)

milled

2.45

7.8…11.25

93Ste (II)

sandblasted

15.5

22.4

93Ste (II)

9…49 2

26…60

72Tou (IV)

(continued) 1

) According to Kirchhoff’s law [93Mod] absorptivity is equal to the spectral emissivity.


8


[Ref. p. 62

Table 1.1.3 continued. A [%]

CO2

CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

7.2

13.4

99Sei (II)

7.8

14.0

99Sei (II)

AlMg5Mn AlMgSi1 AlCu33

Ref.

Laser / Wavelength [µm]

3

91Fre (II)

Al alloy 6061-T6 chem. milled

1.0

1.0

5.0

9.0

72Tou (IV)

sandblasted

22.5

35.0

58.5

46.0

72Tou (IV)

2.4

6.7

35.3

8.5

35

38.5

83.8

62Lan (ns)

3.2

8.0

35.9

39.8

56.5

81Wea (I)

2.4

6.8

35.1

Iron and Alloys Pure Fe

3.5 5

91Pal (I)

85Ord (I)

33.0

47Pri (III)

33.2

95Ste (II)

35 polished

57.3

38

99Sei (II)

9

72Tou (IV)

2 µm rough

40

46

72Tou (IV)

15 µm rough

49

54

72Tou (IV)

oxidized

90.5

90

72Tou (IV)

35CD4 35NCD16

4.4

29.9

5.2

8.5

11

7

St1403/FeP04

95Ste (II)

29…31

95Ste (II) 00Sch1 (II)

38

45

99Sei (II)

Copper and Alloys Pure Copper

1.8

2.0

3.6

63.4 62.3

1.8

1.98

2.67

1.78…4.0

1.32

0.87…1.98

2.4

1.78

0.82

1.18

4.78

91Pal (I) 62Lan (IV) 83Ord (I) 85Ord (I)

0.8

89Bru (I) 4.5

5.5

11.0

17.0

72Tou (IV)

47.0

77.5

72Tou (IV)

CuZn37

5.2

5.8

99Sei (II)

CuSn6

5.3

6.2

99Sei (II)

2.52 polished oxidized

2.5

99Sei (II) 91Yil (IV)

3.0


Ref. p. 62]


9

In Fig. 1.1.1 the calculated absorptivity of iron, aluminum, and copper at room temperature for perpendicular incidence following (1.1.7) is shown. With increasing wavelength a nearly steady decrease is observed, interrupted only by some local maxima. According to (1.1.8) and (1.1.9), for light which is not normally incident the effect of the orientation of polarization becomes important. In Fig. 1.1.2 this dependence is shown for iron. For higher angles p-polarization leads to an increasing absorptivity with a maximum at the so-called Brewster angle, whereas s-polarization shows a steady decrease. The Brewster maximum is shifted towards larger angles for longer wavelengths. For circular polarization the absorptivity values correspond to the arithmetic mean value of p- and s-polarization, i.e. Ac = (A||+A⊥)/2.

II

ϕ Fig. 1.1.1. Dependence of absorptivity A on wavelength for Fe (I), Al (II) and Cu (III) at normal incidence; n, k-data from [91Pal, 81Wea].

Fig. 1.1.2. Angular dependence of absorptivity A for linearly polarized laser light for Fe at different laser wavelengths; n, k-data from [91Pal].

1.1.2.2.1 Temperature effects An early attempt to calculate absorptivity at elevated temperatures on the basis of the close relation of electronic and optical properties of the materials was made in [1903Hag]: A= 2

4 ⋅ f ⋅ ε0 , σ 0 (T )

(1.1.10)

where σ0(T) denotes the temperature-dependent dc electrical conductivity and f the optical frequency. This so-called Hagen-Rubens approximation is restricted to the long wavelength range. The limit of validity is shifted towards shorter wavelengths with decreasing conductivity. In the case of iron, (1.1.10) yields acceptable values at the wavelength of CO- and CO2-lasers, see e.g. [95Dau]. Since in metals the conductivity decreases with increasing temperature an even better coincidence of (1.1.10) with experimental data can be expected at higher temperatures such as the melting temperature. Calculations of optical constants or absorptivity as functions of temperature from elementary electron theory of metals were made by [1900Dru, 55Rob, 65Seb, 59Rob, 93Dau, 94Hue]. A short historical survey of this development is given in [93Dau]. The various approaches differ in the way by which the two mechanisms relevant for the absorption of laser radiation in metals are taken into account. First, the electrons in the conduction band are accelerated by the electric field of the wave and their movement is damped by collisions with other electrons or lattice vibrations and imperfections. Here, the interdependence of electrical conductivity and optical properties becomes obvious. This absorption mechanism is Lando lt -Börnst ein New Ser ies VIII/1C

10


[Ref. p. 62

often referred to Drude [1900Dru]. The acceleration can also be described as an increase of the energy of the electron respectively a lift to a higher energy level within an electronic band and, therefore, this absorption mechanism is called intraband absorption. Second, at higher excitation energies, the electron can overcome the bandgap and can be lifted from the valence band into the conduction band. This mechanism shows a resonance character [71Ash] with local maxima at definite wavelengths and is called interband absorption (see e.g. absorption peak at 800 nm for Fe in Fig. 1.1.1). Upon irradiation by a laser beam the surface temperature of a workpiece will undergo a considerable increase. Depending on the process, the surface may reach values just below melting temperature (martensitic hardening) to melting temperature (cutting, remelting) or up to vaporization temperature (welding, drilling). As a consequence, the thermophysical properties of the material, including absorptivity, will vary. With respect to a reliable description of energy coupling the temperature dependence of absorptivity has to be known. Temperature influences the intraband as well as the interband absorption. Generally speaking, the first mechanism increases with temperature because of an increase of the collision frequency of the free electrons. On the other hand, the interband absorption leads via an increase of damping due to the rise in temperature to a decrease of the maximum and to a broadening of the local absorptivity peak. At the melting point, band structures can change which can result in a vanishing of the interband absorption. Detailed investigations on the wavelength dependence of the optical constants for the liquid phase of several metals have been described in [69Mil] and [72Com]. In [69Mil] a vanishing of the 840 nm absorption peak for Al upon melting was observed, whereas in [94Hue] an intraband behavior was calculated even for temperatures far above the melting point. The temperature dependence of absorptivity for Fe and steel at 1.06 µm (Nd:YAG laser), 5 µm (CO laser) and 10 µm (CO2 laser) is shown in Fig. 1.1.3. At Nd:YAG-wavelength a negative gradient with temperature was obtained which is due to the influence of an interband transition. At higher wavelengths, absorptivity can be described by a pure intraband characteristic, which causes the positive temperature dependence. In case of Al a predominant influence of the interband absorption at 1.06 µm is not observed, which yields a positive temperature dependence for Nd:YAG and CO2-wavelength up to the melting point, as shown in Fig. 1.1.4. 10

35 30

8

25 6

20

4

15 10

2

5 0

200

600 1000 1400 Temperature T [°C]

1800

Fig. 1.1.3. Measured temperature dependence of absorptivity A at Nd:YAG- (squares), CO- (triangles) and CO2-wavelength (circles) of polished iron (I, V) and polished steel 35NCD16 (II, III, IV); exp. data [95Ste].

0 0

100

200 300 400 500 Temperature T [°C]

600

700

Fig. 1.1.4. Measured temperature dependence of absorptivity A at Nd:YAG- (squares) and CO2wavelength (circles) for polished Al 99.5; exp. data [95Ste].

The temperature dependence of absorptivity for alloys is influenced by the fact that, even at low alloying contents, the collision frequency of the “free” electrons is changed. It is expected that the interband absorption is less influenced than the intraband part. At higher alloying grade the band structure is changed distinctively and, hence, a simplified theoretical calculation of the absorptivity cannot describe the experimental findings very well [93Dau] (see Fig. 1.1.5). Calculated temperature dependences for different steels and Al-alloys reveal that, in case of a low alloying grade, absorptivity is not remarkably


Ref. p. 62]


11

influenced. For higher alloying grade (see Fig. 1.1.5, stainless steel 304) a coincidence of experimental and calculated data could not be obtained. An extension of Drude’s theory was performed in [76Wie] being in good coincidence with experimental values of stainless steel at wavelengths longer than 10 µm. Towards shorter wavelengths an increasing deviation was observed. In Fig. 1.1.6 the calculated temperature dependence for pure Al and some alloys at 1.06 µm and 10.6 µm wavelength is shown. For the solid as well as for the liquid phases the absorptivity of alloys is increased compared to the pure material. 15 c b a

13 Absorptivity A [%]

Absorptivity A [%]

15

10

5

11 9 7 5

c b a

II

3 1

0 0

600 200 400 800 Temperature T [°C]

1000

Fig. 1.1.5. Temperature dependence of absorptivity A at 10.6 µm; experimental (symbols) [90Ste] and calculated (lines) [93Dau] values for Fe (I), steel Ck45 (II), steel 35NCD16 (III) and stainless steel 304 (IV).

I 0

200 400 600 Temperature T [°C]

800

Fig. 1.1.6. Temperature dependence of absorptivity A at 10.6 µm (I) and 1.06 µm (II) for Al (a), AlMgSi1 (b) and AlMg5 (c), calculated data [95Dau].

Experimental and theoretical data for the temperature dependence of absorptivity of Al, Fe, Cu and steel of various authors are shown in Tables 1.1.4 and 1.1.5. The figures for the temperature dependence (∆A/∆T) represent a linear fit of the measured or calculated dependences and merely give a rough indication of the change in absorptivity for the indicated temperature range. The experimental results agree reasonably with each other for all materials shown. In [91Yil] values for Al and Cu have been obtained, which deviate – due to the influence of oxidation during the experiment – about one order from that of other authors. Theoretical models yielded values, which are in agreement with each other and with experimental values in case of CO2 wavelength. For Nd:YAG wavelength the values spread slightly. The only exception is the approach in [72Uji] considering a Drude term with a temperature dependence of the electron-phonon collision; for Al it yields a strong deviation from other theoretical and experimental data (except for Nd:YAG values in [91Yil]).


12


[Ref. p. 62

Table 1.1.4. Experimental results for the temperature dependence of absorptivity. Material

T-range

∆A/∆T

[°C]

[% / 100°C]

CO2

25…630

CO2

Laser

Surface

Method

Ref.

0.11

polished

calorimetric

93Ste

25…544

1.55

polished to 2.5 µm

reflectometric

91Yil

CO2

25…660

0.28

polished

ellipsometric

89Bru

Nd:YAG

25…630

0.16

polished

calorimetric

93Ste

CO2

25…1200

0.875

polished

calorimetric

95Ste

CO2

600…950

1.036

not specified

not specified

62Lan

Nd:YAG

25…700

–0.457

polished

calorimetric

95Ste

Nd:YAG

1200…1580

–0.526

2 µm RMS

emissivity

72Tou

CO2

25…550

0.27

polished

reflectometric

76Wie

CO2

25…1000

0.3

polished

calorimetric

95Ste

CO2

25…1000

0.55

polished

calorimetric

95Ste

CO

25…1000

0.65

polished

calorimetric

95Ste

Aluminum

Iron

Steel SS304 35NCD16

Ck45

Nd:YAG

25…1000

–0.35

polished

calorimetric

95Ste

CO2

25…1100

0.36

polished

calorimetric

93Ste

Nd:YAG

25…1100

–1.05

polished

calorimetric

93Ste

CO2

25…790

3.27

polished to 2.5 µm

reflectometric

91Yil

CO2

25…1100

0.16

polished

ellipsometric

89Bru

Nd:YAG

1160…1456

0.0005

liquid

ellipsometric

90Kri

Copper


Ref. p. 62]


13

Table 1.1.5. Theoretical results of the temperature dependence of absorptivity. Material

Laser

T-range

∆A/∆T

[°C]

[% / 100°C]

25…660

1.27

Model

Remarks

Ref.

Aluminum CO2

Nd:YAG

25…660

3.6

CO2

25…660

0.3

CO2

Tm…1600

0.11

Drude + temp.dep. el.-phonon coll. frequency

72Uji

72Uji good agreement with [95Dau, 89Bru] and [84Arn]

94Hue

94Hue

Nd:YAG

25…660

0.38

94Hue

Nd:YAG

Tm…1600

0.235

94Hue

CO2

25…Tm

0.3

CO2

Tm…1900

0.13

Nd:YAG

25…Tm

0.455

95Dau

Tm…1900

0.23

95Dau

CO2

25…660

0.326

CO2

660…1600

0.246

CO2

0…700

0.638

see above

95Dau

CO2

700…Tm

0.439

see above

95Dau

temp.-dep. Drude + temp.-dep. interband absorption

95Dau

95Dau

Drude + temp.dep. conductivity

84Arn 84Arn

Iron

CO2

Tm…3000

0.1

see above

95Dau

Nd:YAG

0…700

–0.98

see above

95Dau

Nd:YAG

700…Tm

0.082

see above

95Dau

Nd:YAG

Tm…3000

0.18

see above

95Dau

35NCD16

CO2

25…1000

0.687

see above

95Dau

35NCD16

CO

25…1000

0.547

see above

95Dau

35NCD16

Nd:YAG

25…1000

–0.57

see above

95Dau

Ck45

CO2

25…1000

0.698

see above

95Dau

Ck45

CO

25…1000

0.579

see above

95Dau

Ck45

Nd:YAG

25…600

–0.586

see above

95Dau

CO2

25…1000

0.39

see above

84Arn

CO2

25…1100

0.9

see above

72Uji

Nd:YAG

25…1100

2.45

Steel

Copper


72Uji

14


[Ref. p. 62

1.1.2.2.2 Chemical effects In most practical applications of laser processing, materials are not pure elements with smooth and clean surfaces but alloys with oxidized and, in general, rough surfaces. The more the technical surface conditions deviate from the physically clean ones, the less information about optical constants is available. In literature only few investigations are concerned with the influence of oxidation as the most important chemical process modifying surface properties and absorptivity. Oxidation processes can be divided into two groups, the natural oxidation and the laser or temperature induced oxidation. Metals kept in air are always covered with a so-called native oxide layer with a thickness of 10…50 nm [97Nan]. On the other hand, as oxidation is a time as well as a temperature-controlled process, with elevated temperatures caused by e.g. direct laser irradiation or heat conduction from the interaction zone, the oxide layer grows faster. As reported in [90Pro] for copper foils and steel, oxidation starts when the temperature is high enough for activation. Absorption then increases strongly and, moreover, shows an “oscillatory” character (see Fig. 1.1.7). This effect can be explained as follows. According to [90Pro, 97Nan] there is a direct effect of oxide layer thickness ξ on absorptivity due to interference of the incident and reflected beams at the metal-oxide interface. The change in absorptivity with oxide layer thickness dA/dξ can have a negative as well as a positive sign, which results in “oscillations”. Oxide layers in most cases do not consist of just one single oxide type. The oxide layer on copper is made up of two components, CuO/Cu2O, whereas on iron a three-component layer of FeO/Fe3O4/Fe2O3 is present. Data for optical constants of these oxides are given in [91Pal, 79Tan, 82Kar] and for emissivity values in [80Ise, 79Neu]. In [95Ste] the change in absorptivity during the growth of oxide layers on a ground steel (35NCD16) by heating the sample with a CO2-laser in atmospheric ambience is described. Applying simultaneously an Nd:YAG- and a CO2-probe laser, oscillations have been observed. In [94Jun] a detailed description of the influence of oxidation in the laser hardening process of C60 steel is given. A review of the interdependence of oxide formation on metal surfaces and roughness is given in [62Eub]. 1200

Cu

Absorptivity A [%]

0.6 0.5

1000 T

0.4

800 A

600

0.3

400

0.2

200

0.1 68

70 72 74 Irradiation time t [s]

Temperature T [°C]

0.7

Fig. 1.1.7. Influence of oxidation on absorptivity of Cu at CO2-wavelength [90Pro].

1.1.2.2.3 Roughness effects Although roughness of surfaces plays an important role in laser materials processing, not much information is found concerning this effect. A theoretical investigation on the influence of random and regular roughness for cases when the Root Mean Square (rms) of the roughness is much smaller than the wavelength of the laser radiation is given in [82Els]. The contribution of the so-called Roughness-Induced Absorption (RIA) is compared to the intrinsic absorptivity of the material. For p-polarization the RIA for


Ref. p. 62]


15

randomly rough surfaces contributes to 1…2 % of the intrinsic value whereas for periodically rough surfaces it can reach the same order as the intrinsic value. The absorptivity of p-polarization is characterized by peaks following sin ϕ = 1 – N λ/∆, where ∆ is the peak-to-peak periodicity of the roughness. This indicates the stimulation of surface plasmon2) resonances which was described by various authors [82Els, 82Bar]. In case the wavelength is of the order of or below the dimensions of characteristic surface structures, multiple reflection respectively multiple absorption within such a groove or depression occurs which increases energy coupling. A model calculating the absorptivity of a depression considering multiple reflection of radiation is presented in [68Aga] and yields

A=

RS 1 1 + ( − 1) Ai

(1.1.11)

with Ai being the intrinsic absorptivity of the material (the value for a smooth surface), RS the surface roughness factor, which is defined by the ratio of the surface of a depression and its projected area. For a single cavity at given width the absorptivity increases with RS, i.e. with increasing depth of the depression. A survey of absorptivities (which are according to Kirchhoff’s law equal to emissivities) in dependence of roughness calculated using (1.1.11) for various forms of cavities (cylindrical, conical, hemispherical) is given in [68Aga, 79Neu]. Some experimental results of absorptivity of randomly rough samples of steel and aluminum alloys are shown in Figs. 1.1.8 and 1.1.9. A tendency of increasing absorptivity with increasing roughness can be recognized with peaks occurring around particular values of roughness. Comparing the absorptivity of steel and Al-alloys in Fig. 1.1.9 the latter is characterized by a stronger influence of roughness, which is in accordance to [80Loe] showing a stronger influence of roughness for metals with low absorptivity. A varying influence of roughness depending on temperature is reported in [95Ste] and shown in Fig. 1.1.10.

Absorptivity A [%]

Absorptivity A [%]

60 55 50 45 40 35 II 30 I 25 20 15 10 5 0 0.5

III III III

1.0 1.5 2.0 2.5 3.0 Roughness Ra [µm]

3.5

4.0

Fig. 1.1.8. Dependence of absorptivity A on roughness Ra (average peak to valley height) for 35NCD16 at Nd:YAG- (squares), CO- (triangles) and CO2wavelength (circles), polished (I), ground (II) and milled (III) samples; exp. data [95Ste].

2

50 40 30 20 10 0

2

4

10 12 6 8 Roughness Rz [µm]

16

Fig. 1.1.9. Dependence of absorptivity A on roughness Rz (peak to valley height) at 808 nm for steel St1403 (squares), AlMg0.6Si1.2 (full circles) and AlMg5Mn (open circles); exp. data [99Sei]; all samples ground.

) Plasmons are collective longitudinal vibration states of an electron gas.


14

16


Absorptivity A [%]

50 40

[Ref. p. 62

VI V

30 IV 20 10

III

Fig. 1.1.10. Temperature dependence of absorptivity A for Ck45 at Nd:YAG- (squares) and CO2wavelength (circles); polished (I, IV), ground (II, V) and sandblasted surfaces (III, VI); exp. data [93Ste].

II I 0

200

400 600 800 1000 1200 Temperature T [°C]

The influence of roughness on the dependence of absorptivity at λ = 808 nm on the angle of incidence was investigated in [98Sei] for steel and aluminum samples. Even polished samples with only a few remaining scratches showed a reduced or less pronounced angular dependence compared to the one calculated with given n, k data. This can be explained by the fact that on the wall of a depression the angle of incidence and the polarization state (s- or p-polarization) differ from the macroscopically adjusted one. A manifold of angles and polarizations occur which lead to an averaging effect of the polarization influence. Theoretical considerations on the influence of roughness on polarization effects are given for example in [83Pop], showing a decrease of the angular dependence with increasing roughness.

1.1.2.3 Optical properties of ceramics Some data of optical properties of pure ceramics can e.g. be found in [85Pal] and are summarized in Tables 1.1.6 and 1.1.7. The reflectivities obtained from these data are typically much lower than those of metals. As can be seen from Fig. 1.1.11 its value for parallel polarization reaches, in contrast to metals, almost zero at the Brewster angle. Another important difference compared to metallic materials is found concerning the absorption lengths which are usually much larger for ceramics. Figure 1.1.12 shows the dependence of this optical penetration depth on wavelength for two ceramic materials. It is remarkable that in the spectral range between 0.5 and 10 µm the penetration depth can be up to three orders of magnitude larger than the wavelength. Table 1.1.6. Optical constants n, k at room temperature. Laser

Ref.

CO2 n

CO k

n

Nd:YAG k

n

Diode k

n

Excimer k

n

k

2.598

3.16

0.259

91Pal

2.08

2.278

0.0048

91Pal

SiC-crystal 0.0593

1.21

2.467 8.75×10–4

non-crystalline Si3N4


Ref. p. 62]


17

Table 1.1.7. Absorption length for ceramics at different wavelengths and room temperature. lα [nm]

Laser CO2

Ref. CO

AlN (Aluminum nitride)

Nd:YAG

Diode

208 000

182 000

Excimer

99Rai

SiC crystal

31

455

76

91Pal

4028

91Pal 99Rai

Si3N4 non crystalline technical

40 000…160 000

37 000…130 000

I

I

II

Fig. 1.1.11. Angular dependence of the Fresnel reflectivity of Si3N4 at different wavelengths. For 0.5 µm wavelength both parallel (||) and perpendicular (⊥) polarizations are shown, for all other wavelengths only parallel polarization. Calculation based on pure material data [85Pal].

Fig. 1.1.12. Optical penetration depth of SiC and Si3N4 (pure materials) in dependence on the wavelength, derived from experimental data in [85Pal].

For a detailed description of the energy coupling of ceramics, besides the absorptivity of pure crystalline material, the structure of the ceramic has to be taken into account. Technical ceramics consist of grains embedded in a glass phase both having different optical properties. Radiation thus is multiply scattered at the grain boundaries as well as at impurities. This results in smaller absorption lengths compared to those in pure materials. Hence, the scattered fraction in (1.1.6) can no longer be neglected. Scattering therefore decreases transparency and penetration depth but does not necessarily increase absorption. The reflectivity and the penetration depth of some technical ceramics are shown in Fig. 1.1.13 [99Rai]. The values are measured at low intensity and ambient temperature with a Fourier spectrograph. The various species of Si3N4 differ in grain size and sinter-additives. Compared to the values of reflection and penetration depth shown in Fig. 1.1.12, a marked reduction in penetration depth is observed.



[Ref. p. 62

α

18

a

b

Fig. 1.1.13. (a) Reflectivity and (b) penetration depth of some technical ceramic materials measured at ambient temperature [99Rai]. Sinter-additives (MgO for 1225 whereas Al2O3+Y2O3 for 1253 and 1254) and mean grain size (0.59 µm, 0.82 µm and 0.36 µm for 1225, 1253 and 1254, respectively) vary for the three different Si3N4 ceramics.

These values, measured at room temperature, are only valid and relevant at the beginning of the heating process. At process temperature the optical properties can be totally different. Optical properties such as transmission and reflection at high temperature are reported in [97Gar]: In the case of AIN reflectivity and optical penetration depth decrease with increasing temperature, whereas the values for Si 3N4 are nearly constant over a broad range of temperatures (see Fig. 1.1.14). At a certain temperature, a steep increase in reflection is observed. It has been shown that this temperature corresponds to the value where decomposition of the ceramic starts [99Rai].

Fig. 1.1.14. High-temperature reflectivity and penetration depth of technical AlN and Si3N4 ceramic samples in air atmosphere [97Gar].

1.1.2.4 Scattering and absorption by particles In most practical cases of material processing the laser beam propagates through a more or less well defined “atmosphere” above the workpiece before it reaches the solid or liquid surface. Depending on the process itself as well as on the way it is being conducted, this atmosphere consists of e.g. pure gas, gas mixtures or gases containing particles. These particles can be produced by condensation of clusters within a plasma plume [94Kar, 96Sch1, 98Cal, 98Luk], by melt expulsion [97May], phase explosion [96Kel] or can be provided to the interaction zone by an external flow of powder, e.g. in the case of laser cladding. Here, the influence of particles will be reviewed.


Ref. p. 62]


19

If multiple scattering is negligible the intensity of a beam is exponentially attenuated according to a relation similar to (1.1.3) with a coefficient resulting from the absorption and scattering in all directions by the particles. Although both processes occur simultaneously, one or the other may dominate, depending on the density of particles and their properties like chemical composition, size, shape and orientation, on the ambient atmosphere and on the properties of the incident beam. The extinction is the combined effect of absorption and scattering, the extinction coefficient αext is expressed as the sum of absorption coefficient αabs and scattering coefficient αsca :

αext = α abs + αsca = N (Cabs + Csca ) = NCext

(1.1.12)

with N denoting the number of particles per unit volume. Equation (1.1.12) also defines Cabs and Csca , the absorption and scattering cross sections, and Cext the resulting “effective” extinction cross section. The cross sections indicate the amount of energy absorbed or scattered by a particle per unit irradiation intensity and area. These cross sections are normalized to the geometrical cross section G of the particle, generally, which leads to dimensionless values:

Qext =

Cext C C , Qabs = abs , Qsca = sca , G G G

(1.1.13)

which are called the efficiency factors for extinction, absorption and scattering and indicate the absorption and scattering capability of an irradiated particle. The efficiency factors of spherical particles of arbitrary size and refractive index can be calculated by the Mie theory [57Hul, 83Boh]. They depend on two parameters: the complex refractive index n~ and the size parameter x x=

⋅d , λ

(1.1.14)

where d is the diameter of the particles and λ is the wavelength of the beam. The intensity of the scattered light I at a large distance R from the particle must be proportional to the intensity of the incident light I0 and R–2: I = I0

λ2 F (θ , ϕ ) , 4 2R2

(1.1.15)

where θ is the scattering angle measured from the forward direction of the light and ϕ is an azimuth angle. The dimensionless function F(θ,ϕ) defines the angular distribution of the scattered light and can be written for the special case of incident linearly polarized light and spherical particles:

F (θ ,ϕ ) = S 2 (θ ) cos 2 ϕ + S1 (θ ) sin 2 ϕ , 2

2

(1.1.16)

where Si(θ ) are the amplitude functions for the two lateral electric field components. For spherical particles the two complex amplitude functions depend only on the scattering angle θ and are functions of the size parameters x and the complex refractive index n~ [57Hul]. The efficiency factors can be calculated from:

Qsca =

π

1 dθ x 2 ∫0

{ S (θ ) 1

4 Qext = 2 Re{S (0)} . x

2

+ S 2 (θ )

2

} sin θ

,

(1.1.17)

(1.1.18)

For forward scattering the amplitude functions have the same value, i.e. S1(0) = S2(0) = S(0). Using the amplitude function F(θ ) = 1/2{⎜S1(θ )⎜2 + ⎜S2(θ )⎜2} the angular distribution of unpolarized light scattered by a particle can be calculated as well. In Fig. 1.1.15 examples of F(θ) for different size parameters according to [94Han] are shown; the incident light is at 180°. It is seen that the angular de-


20


[Ref. p. 62

pendence of the scattering is a strong function of the particle size parameter x. The angular dependence of the scattering becomes very strongly forward-directed for the range of x >> 1. This suggests that strong forward scattering of laser radiation may occur during laser cladding with blown particles, where powders with typically d > 40 µm are applied. Small particles, which appear during penetration welding, will scatter laser radiation away from the beam axis and in some cases, back towards the laser [94Han]. 400 300 200

120

90

60

150

x = 5.928

30

100 0 100 200

180

0

330

210

300 400

240

120

270

90

300

60

x = 1.186

60

30

0

180

330

210 240

120

270

90

300

x = 0.593 60

0.075 150

30

0.050

150

30

0.025 180

0

0

180

0

0.025

1 2

90

150

0.100

1 0

0.25 0.50 0.75 1.00 1.25

120

x = 1.778

3 2

1.25 1.00 0.75 0.50 0.25 0

330

210

3 240

270

300

0.050 0.075 0.100

330

210 240

270

300

Fig. 1.1.15. Angular distribution F(θ) of the light scattered by Fe-particles at λ = 10.6 µm for four different size parameters according to [94Han].

For particular values of the size parameter x and the refractive index n~ , the Mie theory can be simplified. In Fig. 1.1.16 the two main limiting cases are shown. For very small particles (x > 1) the power of the scattered light is proportional to the geometrical cross section of the particle and the geometrical-optics approach can be applied. In between these two limiting cases the more complex Mie theory has to be applied [87Ruc].


Ref. p. 62]


~d

21

2

log ( PS )

Mie theory Rayleigh approximation

~d

geometrical optics approximation

6

small

medium

Fig. 1.1.16. Power of the scattered light PS =

very large

∫ I dA versus A

particle diameter [87Ruc].

log ( d )

For welding and cutting with Nd:YAG- and CO2-lasers the particles fulfill the conditions underlying the Rayleigh approximation. In that case the absorption and scattering cross sections are given by [83Boh]

⎧⎪ m 2 − 1 ⎡ 4 x 3 n~ 3 ⎛ m 2 − 1 ⎞⎤ ⎫⎪ 1 ⎟⎟⎥ ⎬ ⇒ Cabs ∝ , Im ⎜⎜ 2 Cabs = d 2 x n~ Im⎨ 2 ⎢1 + λ 3 ⎪⎩ m + 2 ⎣ ⎝ m + 2 ⎠⎦ ⎪⎭ C sca

2 d 2 4~4 m2 −1 = x n 3 m2 + 2

2

⇒ C sca ∝

1 λ4

(1.1.19)

(1.1.20)

with the relative refractive index m

n~ , m= ~ nM

(1.1.21)

where n~ and n~M are the refractive indices of the particle and the medium, respectively. The scattering cross section in (1.1.20) is proportional to d 6 as shown in Fig. 1.1.16. If (m2–1)/(m2+2) is only weakly dependent on wavelength (which is not true e.g. for metallic particles) then the radiation scattered by a sphere under the condition of small x is proportional to 1/λ4. This approximation can be used for sufficiently small spheres with a refractive index not strongly dependent on wavelength over the region of interest [83Boh]. As can be seen from (1.1.19) and (1.1.20), scattering is increasing stronger than absorption for decreasing wavelengths. Depending on the properties of the particles the dominance of one mechanism over the other can change. For particle sizes in a region in between the two limiting cases, Rayleigh and geometrical optics, rather complex formulas have to be applied. Near the transition to the geometrical-optics approach the Mie theory yields the efficiency factor for extinction (the limiting value)

Qext = 2 ,

(1.1.22)

which corresponds to a cross section twice as large as its geometrical area. This paradox that, according to the Mie theory, a large particle removes twice the energy that is incident on it, is a consequence of some simplifying assumptions [83Boh]. Geometrical optics leads to value of exactly Qext = 1. In the process of laser cladding and alloying the laser beam may interact with a stream of powder in some region above the workpiece surface. This interaction leads to an attenuation of the energy flux


22


[Ref. p. 62

within the beam, a change in absorptivity as particles come to lie on the surface and to a heating of the particles due to the absorbed part of energy before they hit the workpiece surface. For the process of laser cladding and alloying depending on the particle diameter, refractive index and wavelength the Mie theory or geometrical-optics approach can be applied. The efficiency factors for Fe and Cu particles are calculated e.g. in [89Lag] according to the Mie theory. The considered wavelength is 10.6 µm and the powder diameters are between 5 and 115 µm. With this data one can estimate the extinction during the laser cladding process where usually powder diameters are between 40 and 150 µm. The assumed refractive index for Fe is n~ = 5.95 – 32i and for Cu n~ = 9 – 65.9i, respectively. As shown in Table 1.1.8 the scattering at Fe and Cu particles is dominating over absorption, in this case. Table 1.1.8. Efficiency factors for absorption and scattering for Fe and Cu particles at λ = 10.6 µm [89Lag]. Iron (Fe) d [µm] 5

15

25

35

45

55

65

75

85

95

105

115

Qabs

0.0468 0.0377 0.0350 0.0330 0.0330 0.0326 0.0322 0.0320 0.0318 0.0317 0.0316 0.0316

Qsca

2.2311 2.1534 2.1049 2.0779 2.0617 2.0505 2.0435 2.0377 2.0328 2.0290 2.0270 2.0270

Copper (Cu) d [µm] 5

15

25

35

45

55

65

75

85

95

105

115

Qabs

0.0169 0.0135 0.0126 0.0121 0.0119 0.0326 0.0116 0.0115 0.0114 0.0114 0.0113 0.0113

Qsca

2.1810 2.1393 2.0953 2.0718 2.0568 2.0505 2.0409 2.0358 2.0319 2.0287 2.0271 2.0271

1.0

1.0

0.9

0.9

0.8

0.7

0.6 0.5

a

Transmission T [%]

Transmission T [%]

Using the extinction factors for Fe following from Table 1.1.8 the transmission through a powder stream applied using a lateral nozzle was calculated in [93Gas], the results are presented in Fig. 1.1.17a. In Fig. 1.1.17b, the transmission through a stellite powder stream is seen which has been calculated according to [92Mar] by a geometrical approach. The attenuated power is calculated by assuming that the laser beam and powder stream are two intersecting cylinders. The results show that the attenuation is proportional to the powder feed rate and inversely proportional to the powder size. One can see in Fig. 1.1.17 that both methods yield similar results of the transmission.

40

5 g/min 10 g/min 15 g/min 20 g/min 25 g/min

100 60 80 Particle diameter d [µm]

0.8

0.7

0.6 0.5 40

120

b

5 g/min 10 g/min 15 g/min 20 g/min 25 g/min

100 60 80 Particle diameter d [µm]

120

Fig. 1.1.17. Dependence of the transmission through a powder gas stream on the particle size and powder feed rate at λ = 10.6 µm by using (a) Fe powder [93Gas] and (b) stellite powder [92Mar].


Ref. p. 62]


23

In [03Kin] and [96Haa] approaches were made for studying the absorption of particles, primarily. Starting from the described dependence of the Fresnel absorption on the angle of incidence and the orientation of the polarization of the laser light in Sect. 1.1.2.1, the average absorptivity of a sphere has been calculated in [03Kin]. For the calculation it has been assumed that a collimated laser beam is incident on the spherical particles. Taking into account the angle of incidence at the surface of the sphere, the total absorptivity of the sphere can be calculated. In Table 1.1.9 the absorptivity of a plane surface for perpendicular incidence of the beam is compared to the average absorptivity of a sphere. It can be seen, that the absorptivity is higher in the case of the sphere than at a plane surface [03Kin]. Table 1.1.9. Calculated absorptivity of Fe and Al for wavelengths of 1.064 µm and 10.6 µm [03Kin]. Wavelength

1.064 µm (Nd:YAG)

10.6 µm (CO2)

Substrate

Fe

Fe

Al

Al

Temperature [°C]

25

1227

25

1227

25

1227

25

1227

Absorptivity on a plane surface [%]

36.0

31.2

5.7

11.3

3.3

12.1

2.3

5.6

Absorptivity on a sphere [%]

37.7

33.1

6.6

13

4.1

14.4

3.0

7.0

Percentile changing [%]

4.9

6.1

15.6

15.6

25.6

19.0

28.3

24.8

In [96Haa] the absorption characteristic of a metal powder bed under CO2-laser irradiation was investigated. Calorimetric absorption measurements were carried out for different powder materials Al, Cu, Fe and Ti3Al with a particle size range between 75 µm and 200 µm. The experimental results showed that no dependence between absorption and particle size could be observed. The measured absorptivity at low laser intensities of the order of 1 to10 W/cm² ranged between 28 % and 43 %. The Ti3Al powders lead to the highest absorptivity, the Al powders to the lowest. As laser intensity was increased (laser power up to 230 W), the Cu and Fe powders showed strong signs of oxidation when irradiated in air (Fig. 1.1.18). Along with the oxidation, absorptivity of these powders rose up to 70 %. Neither oxidation nor increased absorptivity was observed when helium was used as shielding gas. 100

100

Copper

Iron 90

80

Absorptivity A [%]

80

Absorptivity A [%]

90

air argon helium

70 60 50

70 60 50

40

40

30

30 20 0

20 0

a

50

100 200 150 Laser power P [W]

250

b

air argon helium

50

100 200 150 Laser power P [W]

250

Fig. 1.1.18. Absorptivity at 10.6 µm versus laser power for (a) Fe and (b) Cu particles with a particle size < 100 µm [96Haa].


24


[Ref. p. 62

1.1.2.5 Non-linear absorption In the previous sections only “linear” absorption has been considered where the index of refraction is basically independent of laser intensity. With increasing electric field in the material also non-linear absorption mechanisms may become important. In this case the energy coupling will be a function of intensity. Two basic non-linear absorption mechanisms are impact ionization and photoionization like multiphoton and tunnel ionization. Impact ionization is the inverse process of Auger recombination. Collisons of high energetic free or conduction electrons with valence electrons can yield additional low-energetic conduction electrons. In a sufficiently high electric field these electrons will then be accelerated again such that upon the next collision the process can be repeated. This will increase the density of conduction electrons exponentially if losses due to recombination can be compensated. The resulting electron avalanche leads to a heavily absorbing microplasma and finally to an explosive material damage known as “optical breakdown” even in the case of initially weakly absorbing dielectrics. Impact ionization is almost independent of laser wavelength [87All1]. Such an avalanche is only possible if initially a sufficient density of seed electrons is available. For metals this is always true whereas in dielectrics conduction electrons have to be generated first. Such seed electrons can be provided e.g. either by energetically unfavorable but statistically possible highly excited electrons or by defects and impurities [98Shi]. This would result in a stochastic nature of material damage concerning both threshold and localization, which, in fact, has been observed experimentally [97Liu]. Another possible mechanism for seed electron generation in dielectrics is multiphoton absorption. Due to the lack of conduction electrons, insulators can only absorb photons by the electronic system at deep temperatures if the provided energy is larger than the bandgap. For typical laser wavelengths this is only possible if several photons are simultaneously absorbed. Since the probability for this process decreases rapidly with the number of necessary photons, multiphoton absorption is only important for very high intensities. In contrast to impact ionization it shows a strong dependence on laser wavelength and is nonstochastic. Furthermore, this absorption mechanism is basically instantaneous in time whereas typical build-up times for electron avalanches in insulators are of the order 0.1…1 ns [87All1]. For pulse lengths longer than about 1…10 ps, impact ionization alone is sufficient to explain the onset of material damage in insulators and plasma breakdown [99Tie, 96Stu, 98Ret, 96Du, 93Maz]. If pulse lengths are too short to induce optical breakdown by impact ionization from background carriers alone, multiphoton absorption can provide the necessary additional seed electrons. This behavior is confirmed in Fig. 1.1.19, which shows calculated values of fluence where material damage occurs (this threshold is a measure for the onset of effective absorption) as a function of pulse duration together with the contributions of impact and photoionization only. The observation of significant multiphoton absorption can be shifted towards shorter pulse lengths for higher background carrier density or more frequent electron collisions [99Tie, 96Du]. These theoretical predictions are in good agreement with experimental observations as shown in Fig. 1.1.20 for SiO2. For longer pulses the ablation threshold is roughly proportional to the square root of the pulse duration, which has also been predicted by simplified models [95Nie].


1.1 Fundamentals of laser-induced processes impact ne = 1015 cm-3 impact ne = 2.2 × 1012 cm-3 impact ne = 10 8 cm -3 multiphoton total

Fluence [J cm-2 ]

10

1 10 -14

10 -11 10 -13 10 -12 Pulse duration tH [s]

10 -10

Fig. 1.1.19. Damage threshold as a function of laser pulse length τH (measured at FWHM) for different background electron densities ne in SiO2 [99Tie]. Contributions of impact and multiphoton ionization have been denoted by symbols and dashed line, respectively.

Landolt-Börnstein New Series VIII/1C

100 Fluence [J cm-2 ]

Ref. p. 62]

25

Du et al. (800 nm) Stuart et al. (1053 nm) Lenzner et al. (800 nm) Tien et al. (800 nm)

10

1 10 -15 10 -14 10 -13 10 -12 10 -11 10 -10 10 - 9 10 - 8 Pulse duration tH [s]

Fig. 1.1.20. Experimental results for damage threshold in SiO2 taken from various references [99Tie, 96Stu, 96Du, 98Len].

62

References for 1.1

References for 1.1 (1/2) 1900Dru

Drude, P.: Ann. Physik 1 (1900) 566.

1903Hag Hagen, E., Rubens, H.: Ann. Physik 11 (1903) 873. 47Pri

Price, D.J.: Proc. Phys. Soc. London 47 (1947) 131.

55Rob

Roberts, S.: Phys. Rev. 100 (1955) 1667.

57Hul

Hulst, H. van de: Light Scattering by Small Particles. New York: Wiley, 1957.

59Rob

Roberts, S.: Phys. Rev. 114(1) (1959) 104.

62Eub 62Lan

Eubanks, A.G., Moore, D.G., Pennington, W.A.: J. Electrochem. Soc. 109 (1962) 211. Landolt, H., Börnstein, R.: 6. Aufl., Berlin: Springer, 1962.

65Seb

Seban, R.A.: J. Heat Transfer (1965) 173.

68Aga

Agababov, S.G: High Temperatures – High Pressures 6(1) (1968) 76.

69Mil

Miller J.C.: Phil. Mag. 20 (1969) 1115.

71Ash

Ashcroft, N.W., Sturm, K.: Phys. Rev. 3 (1971) 1898.

72Com 72Tou 72Uji

Comins, N.R.: Phil. Mag. 25(4) (1972) 817. Touloukian, Y.S., DeWitt, D.P.: The Macmillan Company, New: IFI/Plenum, 1972. Ujihara, K.: J. Appl. Phys. 43 (1972) 5.

75Jac

Jackson, J.D.: Classical Electrodynamics, 2nd ed., New York, NY: John Wiley & Sons, 1975.

76Wie

Wieting, T., Schriempf, T.: J. Appl. Phys. 47 (1976) 4009.

79Neu 79Tan

Wörner, B., Neuer, G.: High Temperatures – High Pressures 11(4) (1979) 383. Tanaka, T.: Jap. J. Appl. Phys. 18(6) (1979) 1043.

80Dec

Decker, D.L., Hodgkin, V.A.: Laser Induced Damage in Optical Materials 1980, Proceedings of a Symposium (NBS-SP-620), Washington, DC: NBS (1981) 190. Isetti, C., Nannei, E.: High Temperature 12 (1980) 307. Löfving, S.: Appl. Phys. Lett. 36(8) (1980) 632.

80Ise 80Loe 81Wea

Weaver, J.H., Krafka, C., Lynch, D.W., Koch, E.E.: Karlsruhe: Fachinformationszentrum Energie – Physik – Mathematik, 1981.

82Bar

Barbarino, S., Grasso, F., Guerriera, G., Musumeci, F., Scordino, A., Giulitti, D., Lucchesi, M.: Lettre al Nuovo Cimento 33(13) (1982) 417. Elson, J.M., Sung, C.C.: Appl. Opt. 21(8) (1982) 1496. Karlsson, B., Ribbing, C., Roos, A., Karlsson, T.H.: Physica Scripta 25 (1982) 826.

82Els 82Kar

References for 1.1 83Boh

63

83Pop

Bohren, C.F., Huffmann, D.R.: Absorption and Scattering of Light by Small Particles, New York: Wiley, 1983. Ordal, M.A., Long, L.L., Bell, R.J., Bell, S.E., Bell, R.R., Alexander, R.W., Jr., Ward, C.A.: Appl. Opt. 22(7) (1983) 1099. Popova, O.R.: High Temperature 21(2) (1983) 199.

84Arn

Arnold, G.: Appl. Opt. 23(9) (1984) 1434.

85Ord

Ordal, M.A., Bell, R.J., Alexander, R.W., Jr., Long, L.L., Querry, M.R.: Appl. Opt. 24(24) (1985) 4493. Palik, E.D.: Handbook of Optical Constants of Solids, Vol. 1, Orlando, FL: Academic Press, 1985.

83Ord

85Pal 86Bor

Born, M., Wolf, E.: Principles of Optics, 6th ed., Oxford: Pergamon Press, 1986.

87All1

Allmen, M. van: Laser-Beam Interactions with Materials, Materials Science 2, Berlin: Springer, 1987. Ruck, B.: Laser-Doppler-Anemometrie: Eine berührungslose optische Strömungsgeschwindigkeitsmeßtechnik, Stuttgart: AT-Fachverlag, 1987.

87Ruc 89Bru 89Lag

Brückner, M., Schäfer, J., Uhlenbusch, J.: J. Appl. Phys. 66(3) (1989) 1326. Lagain, P.: Contribution experimentale aux traitements de surface par laser avec apport de poudre, Ph.D. thesis, Université Aix-Marseille, France, 1989.

90Kri

Krishnan, S., Hansen, G.P., Hauge, R.H., Margrave, J.L.: High Temperature Science 29 (1990) 17. Prokhorov, A.M., Konov, V.I., Ursu, I., Mihailescu, I.N.: Laser heating of metals, Bristol: Hilger, 1990. Stern, G.: Proc. 3rd European Conf. on Laser Treatment of Materials ECLAT '90 (Erlangen, Germany). In: Bergmann, H., Kupfer, R. (eds.): Coburg: Sprechsaal Publ. Group (1990) 25.

90Pro 90Ste 91Fre 91Pal 91Yil

Frenk, A., Hoadley, A., Wagniere, J.: Metallurgical Transactions B 22B (1991) 139. Palik, E.D.: Handbook of Optical Constants of Solids, Vol. 2, Orlando, FL: Academic Press, 1991. Yilbas, B.S., Danisman, K., Yilbas, Z.: Meas. Sci. Technol. 2 (1991) 668.

92Mar

Marsden, C. F., Frenk, A., Wagniere, J.-D.: In: Mordike, B.L. (ed.): Laser Treatment of Materials, ECLAT '92, Oberusel: DGM Informationsgesellschaft Verlag (1992) 375.

93Dau 93Gas

Dausinger, F., Shen, J.: ISIJ International 30(9) (1993) 925. Gasser, A.: Oberflächenbehandlung metallischer Werkstoffe mit CO2-Laserstrahlung in der flüssigen Phase, Ph.D. thesis, TH Aachen, Germany, Wissensschaftsverlag, 1993. Mazhukin, V.I., Gusew, I.V., Smurov, I., Flamant, G.: In: Denney, P., Miyamoto, I., Mordike, B.L. (eds.): Proc. Laser Materials Processing Conf. ICALEO '93 (Orlando, FL), Orlando, FL: LIA 77 (1993) 213. Pedrotti, F.L., Pedrotti, L.S.: Introduction to optics, 2nd ed., Englewood Cliffs, NJ: PrenticeHall International, 1993. Stern, G.: Absorption kontinuierlicher CO2- und Nd:YAG-Laserstrahlung durch verschiedene Metalllegierungen, Anwendungen im Bereich der Laserbehandlung, Report for European Project EUREKA EU194, 1993.

93Maz 93Ped 93Ste

64 94Han 94Hue 94Jun 94Kar 95Dau 95Nie 95Ste

References for 1.1 Hansen, F., Duley, W.W.: J. Laser Appl. 6(3) (1994) 137. Hüttner, B.: J. Phys.: Condens. Matter 6 (1994) 2459. Jung, R.: Untersuchungen zur Erzeugung gleichmässiger Oxidschichten und deren Einfluss auf den Absorptionsgrad beim Festphasenhärten mit Laserstrahlung, Diploma thesis, Fraunhofer Institut für Lasertechnik, RWTH Aachen, Germany, 1994. Kar, A., Mazumder, J.: Phys. Rev. E 49(1) (1994) 410. Dausinger, F.: Strahlwerkzeug Laser: Energieeinkopplung und Prozeßeffektivität. Habilitation thesis, Univ. of Stuttgart, Germany. Stuttgart: B.G. Teubner Verlag, 1995. Niemz, M. H.: Appl. Phys. Lett. 66 (1995) 1181. Stern, G.: Société Française des Mécaniciens SFM (ed.): Proc. Deutsch-Französische Tagung Mechanik und Optik: Hochleistungslaser im Maschinenbau (Saint-Louis, France), Courbevoie: SFM (1995) 393.

96Du 96Haa 96Kel 96Sch1 96Stu

Du, D., Liu, X., Mourou, G.: Appl. Phys. B 63 (1996) 617. Haag, M., Hügel, H., Albright, C.E., Ramasamy, S.: J. Appl. Phys. 79(8) (1996) 3835. Kelly, R., Miotello, A.: Appl. Surf. Sci. 96–98 (1996) 205. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: J. Phys. D: Appl. Phys. 29 (1996) 1564. Stuart, B.C., Feit, M.D., Hermann, S., Rubenchik, A.M., Shore, B.W., Perry, M.D.: Phys. Rev. B 53 (1996) 1749.

97Gar

Garnov, S.V., Konov, V.I., Tsarkova, O.G., Dausinger, F., Raiber, A.: Proc. SPIE 2966 (1997) 149. Liu, X., Du, D., Mourou, G.: IEEE J. Quant. Electron. 33 (1997) 1706. Mayerhofer, R.: Mikromaterialbearbeitung mit Kupferdampflasern: Prozeßcharakterisierung und Werkstoffabhängigkeit des Abtrags, Ph.D. thesis, Univ. of Erlangen-Nürnberg, 1997. Nanai, L., Vajtai, R., George, T.: Thin Solid Films 298 (1997) 160.

97Liu 97May 97Nan 98Cal 98Len 98Luk 98Ret 98Sei 98Shi 99Rai 99Sei 99Tie

Callies, G., Schittenhelm, H., Berger, P., Hügel, H.: Appl. Surf. Sci. 127–129 (1998) 134. Lenzner, M., Krüger, J., Sartania, S., Cheng, Z., Spielmann, C., Mourou, G., Kautek, W., Krausz, F.: Phys. Rev. Lett. 80 (1998) 4076. Luk’yanchuk, B.S., Marine, W., Anisimov, S.I.: Laser Physics 8(1) (1998) 1. Rethfeld, B., Kaiser, A., Vicanek, M., Simon, G.: Proc. SPIE 3343 (1998) 388. Seibold, G., Brandner, M., Dausinger, F., Hügel H.: In: Mordike, B.L. (ed.): Proc. European Conf. on Laser Treatment of Materials ECLAT '98 (Hannover, Germany), Frankfurt: Werkstoff-Informationsgesellschaft (1998) 189. Shirk, M.D., Molian, P.A.: J. Laser Appl. 10 (1998) 18. Raiber, A.: Grundlagen und Prozeßtechnik für das Lasermikrobohren technischer Keramiken, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Seibold, G., Dausinger, F., Hügel, H.: In: Kujanpää, V., John, I. (eds.): 7th Nordic Conference in Laser Processing of Materials NOLAMP (Lappeenranta, Finland), Acta Universitatis Lappeenrantaensis 84 (1999) 526. Tien, A.-C., Backus, S., Kapteyn, H., Murnane, M., Mourou, G.: Phys. Rev. Lett. 82 (1999) 3883.

00Sch1

Schellhorn, M.: CO-Hochleistungslaser: Charakteristika und Einsatzmöglichkeiten beim Schweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000.

03Kin

Kindler, H.: Optische und gerätetechnische Entwicklungen zum Laserspritzen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2003.

Ref. p. 62]


25

1.1.3 Thermophysical and dynamical “response” 1.1.3.1 Condensed matter 1.1.3.1.1 Heat conduction As has been described in Sect. 1.1.2 in detail, laser energy is initially absorbed only by electrons leading to a non-thermal electronic distribution. Due to subsequent electron-electron collision the electrons then return to a thermal Fermi distribution within a characteristic relaxation time of the order τe ≈ 100 fs [98Ret]. Energy transfer to the lattice by electron-phonon interaction is usually much slower. Only for timescales considerably longer than the electron-phonon relaxation time τph, the system can be described by just one temperature value and classical heat conduction. For shorter laser pulses, at least the temperatures for electrons and phonons have to be regarded separately and, for pulse durations of the order of electronic relaxation times or shorter, it is not possible to characterize the system by any thermal distribution. 1.1.3.1.1.1 Fourier heat conduction The classical Fourier heat conduction can be applied for times of interest longer than about 100 ps because in such cases temperature equilibrium between electrons and phonons is reached (see also Table 1.1.12). The continuity equation for the heat flux can then be written in terms of the local material temperature T(x, t):

ρcp

∂ T ( x, t ) − ∇ ⋅ (λ th ∇T ( x, t ) ) = q& ( x, t ) . ∂t

(1.1.23)

Here ρ, cp, and λth represent density, specific heat capacity, and thermal conductivity respectively whereas q& has been included to account for a volumetric heat source. Together with the boundary condition

λ n ⋅ ∇TS = (1 − R ) I 0 − hc (TS − T∞ ) − ε σ (TS4 − T∞4 ) Landolt-Börnstein New Series VIII/1C

(1.1.24)

26

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

at the surface and an initial temperature distribution, this equation is sufficient to describe the transient temperature within a material. Herein n denotes a unit surface normal vector, ε the emissivity factor, σ the Stefan-Boltzmann constant, and T∞ is the ambient temperature. In this equation a surface heat source of intensity I0 has been included taking a reflectivity R at the surface into account. Typical values of intensities used in laser materials processing are in the order of 104…1014 W cm–2. Losses due to heat convection and re-radiation at the surface as described by the last two expressions in (1.1.24) can be roughly estimated: Using a typical convective heat transfer coefficient for a sonic jet of hc ≈ 200 W m–2 K–1, a difference of 3500 K between surface temperature TS and the flow would lead to a surface cooling intensity of 70 W cm–2 [97Mod]. On the other hand a black body of the same surface temperature would emit about 850 W cm–2 [70Chu]. Both fluences are considerably lower than the absorbed values and can, therefore, be neglected for most applications. Since (1.1.23) is linear in temperature for constant thermophysical properties it can be solved for many cases analytically [90Car, 72Dab, 96Eng]. A frequently used expression is the one-dimensional case for constant laser intensity, which can be either applied as a surface heat source as in (1.1.24) or a volumetric source according to Lambert-Beer’s law of absorption: q ( z , t ) = α (1 − R ) I 0 e −α z .

(1.1.25)

Due to the small values of lα both descriptions, the surface and the volume heat source, yield basically the same result for metals while for insulators (1.1.25) should be used. Depending on the ratio between the thermal diffusion length lth = 2 κ t and the absorption length lα = α–1 (where κ = λth/(ρcp) represents the thermal diffusivity) l th ≈ 2α κ t lα

(1.1.26)

one obtains for the time-dependent surface temperature: ⎧ 2 (1 − R) I 0 κ t ⎪ T∞ + ⎪ λth TS (t ) = ⎨ α ( 1 − R) I 0 ⎪ T∞ + t ρ cp ⎪⎩

for lth >> lα

,

for lth > k bT , a weaker dependence is found for hν > lth >> lα keyhole welding

Reference

steel

s

v d f / 4κ + 1.1

60

1.1.4 Simplified dependences in laser processes

[Ref. p. 62

For ablation, the onset of vaporization is taken as the start of material removal. Vaporization can be the dominant material removal process at very high pulse power, but even at processes dominated by melt expulsion is it necessary to create a driving force by the recoil pressure of vapor production and expansion. It is important to note, however, that in drilling the threshold increases with penetration depth [99Dau]. In most cases the threshold shows up in terms of power divided by focus diameter. For cutting, the minimum power scales with the sheet thickness s, mainly. Only in the case of ablation with short pulses, the intensity is the right measure for the threshold. If for the cases of ablation and welding other intensity distributions than Gaussian are considered, the factor B needs to be modified. Table 1.1.16 gives correction factors for top hat and Airy profiles, the latter being representative for unstable resonators, the first for beams homogenized e.g. transmitted by glass fibers. The correction factors depend on the dimensionality of heat flux and are given for the two limiting cases characterized by the ratio between the thermal diffusion length and the focal diameter. Table 1.1.16. Correction factors for B in case of non-Gaussian beams [00Sch1].

l th / d f >> 1

l th / d f 12

0

0.483

95Dau

~2

~0.76

95Dau

inert gas cutting

In ablation processes the calculation of the process enthalpy is impeded by the difficulty to know the phase composition and temperature of the removed material ranging from just molten to superheated at temperatures above boiling point, see Sects. 1.1.3.1.2.2 and 1.1.3.1.3.2. Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]


61

Table 1.1.17 gives summaries of the coefficients a1 and a2 taken from approximative models. The values for keyhole welding have been obtained from a two-dimensional model [73Swi, 95Dau]. It has to be distinguished between two velocity regimes, the confining value being determined by the so-called Peclet number which relates the velocity v to the melt width b and the thermal diffusivity κ . For inert gas cutting a linear approximation to the models of [93Sch2, 93Ros] was used to calculate the coefficients.

1.1.4.2.3 Factors determining efficiency The efficiency of laser processes is determined by the coupling rate η A (1.1.48) discussed in Sect. 1.1.4.1.2 and by the thermal efficiency η th according to:

η P = η A η th .

(1.1.56)

The thermal efficiency describes to what extent the absorbed power (released heat) produces the desired effect on the workpiece, e.g. the melt in welding:

η th ≡

vbsHP . ηA P

(1.1.57)

An analysis on the basis of analytic approximations [95Dau] showed that, for a given process, the thermal efficiency depends on the following parameter combination, mainly,

X ≡

η A PL ⋅ . λth TP s

(1.1.58)

It is worth to note, that the combination of material parameters in (1.1.58) is simply the inverse of the factor M in (1.1.52) which determines the threshold value. Figure 1.1.66 shows for four different metals the values of thermal efficiency calculated on the basis of [73Swi]. It demonstrates that e.g. for welding of aluminum at least 2 kW per millimeter of welding depth are necessary to achieve a satisfying efficiency level.

Thermal efficiency η th [%]

100

80

Experimental Al 6110; sandblasted Al 6110; untreated Al 5182; untreated

Calculated Ti Fe Al Cu

60

40

20

0

3 5 1 2 4 Power/welding depth P/s [kW/mm]


6

Fig. 1.1.66. Thermal efficiency in 2D-keyhole welding. Experimental data for several Al-alloys together with curves calculated according to [73Swi] using the following values of C = ηA/L⋅Tm [kW/mm]. Ti: 24, Fe: 10, Al: 4, and Cu: 1.5.

62

References for 1.1

References for 1.1 (2/2) 41Ros

Rosenthal, D.: Welding J. 20 (1941) 220s.

45Gou

Gouffé, A.: Rev. Optique 24 (1945) 1.

54Mae

Maecker, H., Peters, T.: Z. Physik 139 (1954) 448.

59Sed

Sedov, L.I.: Similarity and Dimensional Methods in Mechanics. London: Cleaver–Hume Press, 1959.

61Bar

Bartels, J., Borchers, H., Ten Bruggencate, P., Hausen, H., Hellwege, K.-H., Schäfer, K., Schmidt, E. (eds.): Landolt-Börnstein II/4, Berlin: Springer, 1961.

65Rai 65Rea 65Wha

Raizer, Yu P.: Sov. Phys. JETP 21(5) (1965) 1009. Ready, J.F.: J. Appl. Phys. 36 (1965) 462 Wharton, C.B.: Microwave Techniques. In: Huddlestone, R.H., Leonard, S.L. (eds.): Plasma Diagnostic Techniques. New York, NY: Academic Press, 1965.

66Zel

Zel’dovich, Ya B., Raizer, Yu P.: Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena I, New York, NY: Academic Press, 1966.

67Tan

Tanenbaum, B.S.: Plasma Physics, New York, NY: McGraw-Hill, 1967.

68Ani 68Bas

Anisimov, S.I.: Sov. Phys JETP 27(19) (1968) 182. Basov, N.G., Gribkov, V.A., Krokhin, O.N., Sklizkov, G.V.: J. Sov. Phys. JETP 27(4) (1968) 575.

69Hoh

Hohla, K., Büchl, K., Wienecke, R., Witkowski, S.: Z. Naturforsch. 24a (1969) 1244.

70Chu 70Hor 70Rai 70Stu

Chun, M.K., Rose, K.: J. Appl. Phys. 41 (1970) 614. Hora, H., Wilhelm, H.: Nuclear Fusion 10 (1970) 111. Raizer, Yu P.: Sov. Phys. JETP 31(6) (1970) 1148. Stull, D.R., Prophet, H.: JANAF Thermochemical Tables, US Dept. of Commerce, 1970.

71Kar

Karasev, I.G., Kirillov, V.M., Norskii, V.E., Samoilov, V.I., Ulyakov, P.I.: Sov. Phys. Techn. Phys. 15 (1971) 1523.

72Dab 72Pae

Dabby, F., Paek, U.-C.: IEEE J. Quant. Electron. 8 (1972) 107. Paek, U.-C., Gagliano, F. P.: IEEE J. Quant. Electron. 8 (1972) 112.

73Ani 73Het 73Mul

Anisimov, S.I., Rakhmatulina, A.Kh.: Sov. Phys. JETP 37(3) (1973) 441. Hettche, L.R., Schriempf, J.T., Stegman, R.L.: J. Appl. Phys. 44 (1973) 4079. Mulser, P., Sigel, R., Witkowski, S.: Physics Reports (Section C of Physics Letters) 6(3) (1973) 187. Pirri, A.: The Physics of Fluids 16(9) (1973) 1435. Prokhorov, A.M., Batanov, V.A., Bunkin, F.V., Fedorov, V.B.: IEEE J. Quant. Elec. 9(5) (1973) 503. Stegman, R.L., Schriempf, J.T., Hettche, L.R.: J. Appl. Phys. 44(8) (1973) 3675.

73Pir 73Pro 73Ste

References for 1.1

63

73Swi

Swift-Hook, D.T., Gick, A.E.F.: Welding J. 52 (1973) 492s.

74Ani 74Gob 74Low 74Mar

Anisimov, S.I., Kapeliovich, B.L., Perel’man, T.L.: Sov. Phys. JETP 39 (1974) 375. Gobrecht, H.: In: Bergmann-Schaefer: Lehrbuch der Experimentalphysik, Vol. I, Berlin: de Gruyter (1974) 712 Lowder, J.E., Pettingill, L.C.: Appl. Phys. Lett. 24 (1974) 204. Martynyuk, M.M.: Sov. Phys. Tech. Phys. 19 (1974) 793.

75Fow 75Nie

Fowler, M.C., Smith, D.C.: J. Appl. Phys. 46(1) (1975) 138. Nielson, P.E.: J. Appl. Phys. 46(10) (1975) 4501.

76All 76Ash

Allmen, M. van: J. Appl. Phys. 47 (1976) 5460. Ashcorf, N.W., Mermin, N.D.: Solid State Physics, Fort Worth: Saunders College Publishing, 1976. Klemens, P. G.: J. Appl. Phys. 47 (1976) 2165. Steverding, B., Dudel, H.P.: J. Appl. Phys. 47 (1976) 1940.

76Kle 76Ste 77Mal 77Rai

Malz, D., Vogler, K.: Exp. Technik Physik 25(6) (1977) 555. Raizer, Yu P.: Laser-induced Discharge Phenomena, Studies in Soviet Science, New York: Consultants Bureau, 1977.

78Pir 78Zhi

Pirri, A.N., Root, R.G., Wu, P.K.S.: AIAA Journal 16(12) (1978) 1296. Zhiryakov, B.M., Popov, N.I., Samokhin, A.A.: Sov. Phys. JETP 48 (1978) 247.

79Kni

Knight, C.J.: AIAA J. 17(5) (1979) 519.

80Mah 80Wen

Maher, W.E., Hall, R.B.: J. Appl. Phys. 51(3) (1980) 1338. Wende, B.: Das Plasma. In: Gobrecht, H. (ed.): Bergmann-Schaefer: Lehrbuch der Experimentalphysik Vol. IV: Aufbau der Materie, Part 2, 2nd ed., Berlin: de Gruyter, 1980.

83Zie

Ziesche, P., Lehmann, G.: Ergebnisse in der Elektronentheorie der Metalle, Berlin: Springer, 1983.

84Dan 84Pop

Dan‘shchikov, E.V., Lebedev, F.V., Ryazanov, A.V.: Sov. J. Plasma Phys. 10(2) (1984) 225. Poprawe, R.: Materialabtragung und Plasmaformation im Strahlungsfeld von UV-Lasern, Ph.D. thesis, Germany: TH Darmstadt, 1984. Woodroffe, J.: In: Onorato, M. (ed.): Proc. Fourth Intl. Symposium on Gas Flow and Chemical Lasers GCL '82 (Stresa, Italy), New York, NY: Plenum Press (1984) 97.

84Woo 85Bey 85Mat

Beyer, E.: Einfluß des laserinduzierten Plasmas beim Schweißen mit CO2-Lasern, Ph.D. thesis, TH Darmstadt, Germany, Düsseldorf: DVS, 1985. Matsunawa, A.: In: Albright, C. (ed.): Proc. Laser Welding, Machining, and Materials Processing, Proc. ICALEO '85 (San Francisco, CA), Berlin: Springer (1986) 41.

86Jel 86Kit 86Miy

Jellison, G.E., Lowndes, D.H., Mashburn, D.N., Wood, R.F.: Phys. Rev. B 34 (1986) 2407. Kittel, C.: Introduction to Solid State Physics, 6th ed., New York, NY: Wiley, 1986. Miyamoto, I., Maruo, H., Arata, Y.: Proc. SPIE 668 (1986) 11.

87All1

Allmen, M. van: Laser-Beam Interactions with Materials, Materials Science 2, Berlin: Springer, 1987.

64 87All2 87All3 87Cha 87Knu 87Lar 87Pee 87Pop 87Roc

References for 1.1 Allen, P.B.: Phys. Rev. Lett. 59(13) (1987) 1460. Allen, P.B.: Phys. Rev. B 36(5) (1987) 2920. Chan, C.L., Mazumder, J.: J. Appl. Phys. 62(11) (1987) 4579. Knudtson, J.T., Green, W.B., Sutton, D.G.: J. Appl. Phys. 61(10) (1987) 4771. Larciprete, R., Stuke, M.: Appl. Phys. B 42 (1987) 181. Peebles, H.C., Williamson, R.L.: In: Arata, Y. (ed.): Proc. Intl. Conf. on Laser Advanced Materials Processing LAMP '87 (Osaka, Japan), Osaka: High Temperature Society of Japan (1987) 19. Poprawe, R., Wehner, M., Brown, G., Herziger, G.: Proc. SPIE 801 (1987) 191. Rockstroh, T.J., Mazumder, J.: J. Appl. Phys. 61(3) (1987) 917.

88Aut 88Pet 88Sok 88Ste 88Wyb

Autric, M., Vigliano, P., Astic, D., Bournot, P., Dufresne, D.: Proc. SPIE 1020 (1988) 103. Petring, D., Abels, P., Beyer, E., Herziger, G.: Feinwerktechnik & Meßtechnik 96 (1988) 364. Sokolowski, W., Herziger, G., Beyer, E.: Proc. SPIE 1020 (1988) 96. Steen, W.M., Dowden, J., Davis, M., Kapadia, P.: J. Phys. D: Appl. Phys. 21 (1988) 1255. Wybourne, M.N.: In: Properties of Silicon, EMIS Data Review Series 4, London: INSPEC (1988) 37.

89Jos 89Sok

Joseph, D.D.: Rev. Mod. Phys. 61 (1989) 41. Sokolowski, W., Herziger, G., Beyer, E.: Proc. SPIE 1132 (1989) 288.

90Ade 90Ber 90Car 90Fab

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Schellhorn, M., Eichhorn, A.: Optics & Laser Technology 28(5) (1996) 405. Tsukamoto, S., Hiraoka, K., Asai, Y., Irie, H.: In: Duley, W.W., Shiabata, K., Poprawe, R. (eds.): Proc. Laser Materials Processing Conf. ICALEO '96 (Detroit, MI), Orlando, FL: LIA 81 B76. Witke, T.: Optische Plasmaspektroskopie und Kurzzeituntersuchungen an gepulsten Laser-, Bogen und Kanalfunkenplasmen, Ph.D. thesis, Univ. of Dresden, Germany, Aachen: Shaker Verlag, 1996. Aden, M., Kreutz, E.W., Schlüter, H., Wissenbach, K.: J. Phys. D: Appl. Phys. 30 (1997) 980. Hugenschmidt, M., Schmitt, R., Althaus, M.: Proc. SPIE 2869 (1997) 1017. Jandeleit, J., Russbüldt, P., Urbasch, G., Hoffmann, D., Treusch, H.G., Kreutz, E.W.: Proc. SPIE 3092 (1997) 481. Jüptner, W., Franz, Th., Sikau, J., Sepold, G.: Laser Physics 7(1) (1997) 202. Körner, C.: Theoretische Untersuchungen zur Wechselwirkung von ultrakurzen Laserpulsen mit Metallen, PhD thesis, Univ. of Erlangen-Nürnberg, 1997. Lacroix, D., Jeandel, G.: J. Appl. Phys. 81(10) (1997) 6599. Liu, X., Du, D., Mourou, G.: IEEE J. Quant. Electron. 33 (1997) 1706. Mazhukin, V. I., Nosov, V. V., Callies, G., Schittenhelm, H., Berger, P.: Proc. 15th IMACS World Congress (Berlin) Vol. III, Berlin: Wiss. u. Technik Verlag (1997) 271. Modest, M. F.: J. Laser Appl. 9 (1997) 137. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: Appl. Surf. Sci. 109/110 (1997) 493. Schellhorn, M., Eichhorn, A.: Proc. SPIE 3092 (1997) 522. Schellhorn, M., Eichhorn, A., Hohenberger, B.: Frühjahrstagung der DPG (Mainz, Germany), Verhandl. DPG (VI) 32 (1997) 221. Szymanski, Z., Kurzyna, J., Kalita, W.: J Phys. D: Appl. Phys. 30 (1997) 3153. White, G.K., Minges, M.L.: Int. J. Thermophysics 9 (1997) 1269. Willis, D.A., Xu, X., Poon, C.C., Tam, A.C.: In: Fabbro, R., Kar, A., Matsunawa, A. (eds.): Proc. Laser Materials Processing Conf. ICALEO '97 (San Diego, CA), Orlando, FL: LIA 83 (1997) A71. Zhigilei, L.V., Kodali, P.B.S., Garrison, B.J.: J. Phys. Chem. B 101 (1997) 2028. Zhigilei, L.V., Kodali, P.B.S., Garrison, B.J.: Chem. Phys. Lett. 276 (1997) 269. Amoruso, S., Armenante, M., Berardi, V., Bruzzese, R., Velotta, R., Wang, X.: Appl. Surf. Sci 127–129 (1998) 1017. Doyle, L.A., Martin, G.W., Al-Khateeb, A., Weaver, I., Riley, D., Lamb, M.J., Morrow, T., Lewis, C.L.: Appl. Surf. Sci. 127–129 (1998) 716. Duley, W.W.: Laser Welding, New York, NY: John Wiley & Sons, 1998. Fabbro, R., Chouf, K., Sabatier, L., Coste, F.: In: Beyer, E., Chen, X., Miyamoto, I. (eds.): Proc. Laser Materials Processing Conf. ICALEO '98 (Orlando, FL), Orlando, FL: LIA 85 (1998) F179. Herrmann, R.F.W., Gerlach, J., Campbell, E.E.B.: Appl. Phys. A 66 (1998) 35. Hermann, J., Vivien, C., Carricato, A.P., Boulmer-Leborgne, C.: Appl. Surf. Sci. 127–129 (1998) 645. Hügel, H., Berger, P., Dausinger, F.: In: Beyer, E., Chen, X., Miyamoto, I. (eds.): Proc. Laser Materials Processing Conf. ICALEO '98 (Orlando, FL), Orlando, FL: LIA 85 (1998) G141. Lunney, J.G., Jordan, R.: Appl. Surf. Sci. 127–129 (1998) 941. Matsunawa, A., Seto, N., Mizutani, M., Katayama, S.: In: Beyer, E., Chen, X., Miyamoto, I. (eds.): Proc. Laser Materials Processing Conf. ICALEO '98 (Orlando, FL), Orlando, FL: LIA 85 (1998) G151. Ohmukai, M., Takigawa, Y., Kurosawa, K.: J. Appl. Phys. 83 (1998) 3556. Pinho, G.P., Schittenhelm, H., Duley, W.W., Schlueter, A., Jahani, H.R., Mueller, R.E.: Appl. Surf. Sci. 127–129 (1998) 983.

68 98Pur 98Ret 98Sch1 98Sch2 98Sch3 98Vog 98Zef 99Agu 99Arn 99Bre 99Cal 99Dau 99Fuh 99Jeo 99Ker 99Rai 99Sch 99Sia 00Cha 00Sch1 00Sch2 01Bre 01Maz 01Sud

References for 1.1 Puretzky, A.A., Geohegan, D.B.: Appl. Surf. Sci. 127–129 (1998) 248. Rethfeld, B., Kaiser, A., Vicanek, M., Simon, G.: Proc. SPIE 3343 (1998) 388. Schittenhelm, H., Callies, G., Straub, A., Berger, P., Hügel, H.: J. Phys. D: Appl. Phys. 31 (1998) 418. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: J. Thermophys. Aeromech. 5(2) (1998) 255. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: Appl. Surf. Sci. 127–129 (1998) 922. Vogel, N., Kochan, N.: Appl. Surf. Sci. 127–129 (1998) 928. Zefferer, H.: Dynamik des Schmelzschneidens mit Laserstrahlung, PhD thesis, RWTH Aachen, Germany, Aachen: Shaker Verlag, 1998. Aguilera, J.A., Aragon, C.: Appl. Phys. A 69 (1999) S475. Arnold, N., Gruber, J., Heitz, J.: Appl. Phys. A 69 (1999) S87. Breitling, D., Schittenhelm, H., Berger, P., Dausinger, F., Hügel, H.: Appl. Phys. A 69 (1999) S505. Callies, G.: Modellierung von qualitäts- und effektivitätsbestimmenden Mechanismen beim Laserabtragen, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Dausinger, F., Abeln, T., Breitling, D., Radtke, J., Konov, V., Garnov, S., Klimentov, S., Kononenko, T., Tsarkova, O.: LaserOpto 31(3) (1999) 78. Fuhrich, T., Berger, P., Huegel, H.: In: Christensen, P., Denney, P., Miyamoto, I., Watkins, R. (eds.): Proc. Laser Materials Processing Conf. ICALEO '99 (San Diego, CA), Orlando, FL: LIA 87 (1999) E166. Jeong, S.H., Greif, R., Russo, R.E: J. Phys. D: Appl. Phys. 32 (1999) 2578. Kern, M.: Gas- und magnetofluiddynamische Maßnahmen zur Beeinflussung der Naht-qualität beim Laserstrahlschweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Raiber, A.: Grundlagen und Prozeßtechnik für das Lasermikrobohren technischer Keramiken, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Schittenhelm, H., Müller, J., Berger, P., Hügel, H.: In: Christensen, P., Denney, P., Miyamoto, I., Watkins, R. (eds.): Proc. Laser Materials Processing Conf. ICALEO '99 (San Diego, CA), Orlando, FL: LIA 87 (1999) E195. Siano, S, Pini, R., Salimbeni, R.: Appl. Phys. Lett. 74 (1999) 1233. Chang, C.-L.: Berechnung der Schmelzbadgeometrie beim Laserstrahlschweißen mit Mehrfokustechnik, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000. Schellhorn, M.: CO-Hochleistungslaser: Charakteristika und Einsatzmöglichkeiten beim Schweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000. Schittenhelm, H.: Diagnostik des laserinduzierten Plasmas beim Abtragen und Schweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000. Breitling, D., Schittenhelm, H., Berger, P., Dausinger, F., Hügel, H.: Proc. SPIE 4184 (2001) 534. Mazhukin, V.I., Nosov, V.V., Smurov, I., Nickiforov, M.G.: In: German Scientific Laser Society WLT (ed.): Proc. First Intl. WLT-Conf. on Lasers in Manufacturing 2001 (Munich, Germany), Stuttgart, Germany: AT-Fachverlag (2001) 199. Sudnik, W., Erofeew, W., Radaj, D., Berger, P., Hügel, H.: Simulation der Fügetechniken – Potentiale und Grenzen: Beiträge zum DaimlerChrysler-Technologiekolloquium 2001 (Wiesensteig, Germany). In: Pollmann, W., Radaj, D. (eds.): DVS-Berichte 214, Düsseldorf, Germany: Verl. für Schweißen und Verwandte Verfahren, DVS (2001) 94.

Ref. p. 101]

2.1 Surface treatment

75

2.1 Surface treatment H.W. BERGMANN

Based on the fundamentals of interaction between laser radiation and substrate material, which are described in Part 1, Chap. 1, the present chapter reports on the technical application of laser surface treatment. The great advantage of the laser beam compared to other heat sources for surface treatment is its flexibility which allows an effective and locally defined treatment with good process control. Generally, these processes can use gaseous, liquid or solid additives and supporting media like shielding gases and coatings for improvement of the absorption. Laser surface treatment is used to change the material properties (e.g. surface hardening or impulse strengthening), to change the material and the geometry of the workpiece (e.g. coating, thermal spraying or LPVD/LCVD – laser physical/chemical vapor deposition – processes) or to change the system properties (e.g., cleaning and smoothing, changing the stress state and structuring). Depending on the desired effects, the mechanisms are either thermal, mechanical or chemical. However, an exact separation of mechanisms is difficult, because all of them interact somehow (Fig. 2.1.1). The different laser surface techniques can be divided into laser macro processing and thin-film technologies (Fig. 2.1.2).

Laser surface treatment Laser macro processing

Thin layer technology

CO2-lasers, Nd:YAG-lasers

Q-switched Nd:YAG, Excimer

cw / ms-pulsed / 0.1 ms-pulsed layer thickness: 0.05 − 10 mm

pulse duration 0.1 − 200 ns layer thickness: 0.05 − 50 µm

Surface hardening Remelting / alloying Coating

Cleaning and smoothing Alloying Coating Shock hardening

Fig. 2.1.1. Different mechanisms of influence to the material.

Fig. 2.1.2. Subdivision of laser surface treatments.

2.1.1 Laser macro processing The technique of laser macro processing is limited to thin-layer techniques with an affected depth of typically 50 µm and is usually performed with CO2- or Nd:YAG-lasers, operating either in the cw or


76

2.1.1 Laser macro processing

[Ref. p. 101

millisecond pulsed mode (see Fig. 2.1.2). Examples for these processes are surface hardening of ironbased materials (solid-state hardening), remelting of cast iron or alloying techniques using a liquid phase.

2.1.1.1 Solid-state hardening Surface hardening of steels using laser radiation is a multipurpose process for generating hard, wearresistant layers on tools and components and is applicable to transformable iron-based materials [86Bra]. The surface of the workpiece is heated up to a temperature above the Ac3 temperature (end of austenization for hyper- and hypoeutectique steels) using the laser beam as a heat source (short-time austenitization, see also Sect. 2.1.1.1.2). The cold workpiece behaves as a very efficient heat sink and therefore no external quenching is necessary (self-quenching). Due to the rather short dwell time in the austenite phase region (several seconds), the peak temperature (control temperature) has to be higher than the austenitizing temperature used for conventional hardening to guarantee a complete transformation and a sufficient homogenization of the austenite. One of the great advantages of the laser beam hardening process is the relatively low distortion of the workpiece. A schematic drawing of the process is given in Fig. 2.1.3.

Fig. 2.1.3. Schematic drawing of the laser hardening process.

2.1.1.1.1 Physical basics When laser light is incident on a metallic workpiece a proportion of the radiation is reflected while the rest is absorbed by the surface and transformed into heat. This leads to heating of the workpiece in the absorption layer and, through heat conduction, to a heating of deeper layers [83Kou, 83Rot, 88Gei]. Controlling factors are the laser beam intensity, the interaction time and the material properties of the workpiece as already discussed in Part 1, Chap. 1. This can be summarized in two basic equations which describe on the one hand energy transfer and on the other mechanical changes which occur through thermal expansion: ∂T – ∇ (λ ∇ T) = Q ∂t with

(2.1.1)

T: temperature, λ: thermal conductivity, Q: heat source,


Ref. p. 101]

ρ with


∂ 2ui ∂t

2

–

∂ σij = ρ Fi ∂x j

77

(2.1.2)

ρ: density, ui: movement of a volume element, σij: component of the stress tensor, Fi: force acting on a volume element.

The material's reaction is thus a function of both, temperature and stress fields, as long as the heating rates are relatively moderate (power densities < 105 W/cm2). The changes caused by the temperature field and the thermodynamic properties of the material are bound to intrinsic time constants [94Kör]. The phase reactions that occur during the thermal cycle of the hardening process of steels are summarized in TTT (Time-Temperature-Transformation) and TTA (Time-Temperature-Austenization) diagrams [54Wev, 72Ros, 73Ohr, 76Ohr]. The efficiency of optical to thermal energy transformation depends on the optical properties of the material for the wavelength of the incident radiation. Within the range of wavelengths commercially available high-power lasers provide (CO2 lasers 10.6 µm, Nd:YAG lasers 1.064 µm, diode lasers 800…940 nm), metals normally show relatively low absorption coefficients (see e.g. Part 1, Chap. 1, Table 1.1.2 with the absorption coefficient α = 1/lα according to Part 1, Chap. 1, (1.1.5)). The absorption coefficients vary with increasing temperature and depend on the surface structure of the workpiece. Several possibilities exist to enhance the absorption [88Dau, 88Rud, 92Jas]: -

thermochemical reactions between the substrate and the surrounding gas atmosphere, absorbing coatings, use of polarized laser light.

2.1.1.1.2 Material science basics Heating of a steel above the Ac1 temperature (begin of austenization) leads to a phase transformation from the iron bcc (body centered cubic) crystal structure, called ferrite, into a fcc (face centered cubic) structure, called austenite. Between Ac1 and Ac3 (see above) (and Acm – complete austenization for hypereutectique steels, dissolution of carbides) the iron carbide decomposes and the carbon dissolves into the austenite which has a high solubility for carbon. The carbon concentration gradient established after a certain time is defined by diffusion coefficients which are both temperature and concentration dependent. At high heating rates, a maximum temperature well above the Ac3 or Acm point is necessary to get a homogeneous distribution of carbon. Due to the temperature gradient, this can only be reached within the outer surface layers. With increasing distance from the surface, decreasing effects of austenitization can be observed [91Fri, 91Lep]. When the laser irradiation ends, the surface layer is cooled down rapidly by heat conduction into the core of the workpiece, which remains cold. This effect is called self-quenching. The resulting transformed microstructure depends on the cooling rate, the chemical composition and the final temperature of the surface [85Wis]. Figure 2.1.4 shows calculated time-temperature curves for laser beam hardening with typical CO2 lasers. Typical hardness-depth profiles for laser beam hardening of a 0.45 % carbon steel are displayed in Fig. 2.1.5 [91Bac].


78


Fig. 2.1.4. Calculated time-temperature curves for laser beam hardening with a typical CO2 laser (cross section of the laser beam: 10 × 10 mm2, feed rate: 0.3 m/min.) [96Mor], z: depth below the surface, 1: z = 0 mm, 2: z = 0.5 mm, 3: z = 1.1 mm, 4: z = 2.0 mm, 5: z = 3.7 mm, 6: z = 6.6 mm, 7: z = 20 mm.

Fig. 2.1.5. Hardness-depth profiles hardening of 0.45 % carbon steel. temperatures (feed rate: 0.3 m/min.) 1200 °C, 2: Tc = 1250 °C, 3: Tc = 1350 °C.

[Ref. p. 101

for laser beam Different control [91Bac]: 1: Tc = 1300 °C, 4: Tc =

In addition to microstructural and property changes, the temperature field causes thermal expansion (according to the different expansion coefficients of ferrite and austenite) and volumetric changes due to phase transformation. This results in a transient stress field. The temperature dependent material properties (particularly the flow stress) and the hysteresis of the α → γ, γ → α phase transformation (α – body centered cubic (ferrite), γ – face centered cubic (austenite) iron) lead to elastic and plastic deformations and cause dimensional changes, distortions and residual stresses. A functional design and a locally defined heat treatment help to minimize distortions and to generate compressive residual stresses within the critical, high-strained areas of the workpiece. This helps to improve fatigue strength and therefore leads to an extended fatigue lifetime [96Mül].

2.1.1.1.3 Production-related aspects According to DIN EN ISO 11145, a system for laser beam hardening comprises a laser source, an optical arrangement for beam guiding and shaping, and a machine for manipulation of the workpiece (or the laser beam). For surface hardening of larger areas, high-power lasers in the multi-kilowatt range are necessary. The electrical overall efficiency and other properties for common laser types are given in Table 2.1.1. The laser-beam-hardening process can be performed either power- or temperature-controlled. In the first case, the output power of the laser and the time of irradiation is controlled (kept constant). In the second case, the temperature of the surface is measured within the hot zone of the laser spot (for example by pyrometric measurement) and, via a closed loop system kept at a predetermined level through control of the output power [88Ber, 93Gei]. To achieve an appropriate depth of the hardened zone, a power density of 104...105 W/cm2 and irradiation times between 0.1 s and 3 s are required. Different optical systems and components are available to generate a beam profile with the desired shape and intensity distribution [88Jüp, 90Pie, 90Rud, 90Tön1, 90Tön2, 92Ber, 94Agr]. Large areas of the surface can be hardened by movement of the laser spot (hardened tracks). If those tracks are overlapping, the first track is tempered by the second one, accompanied by changes in microstructure and properties. This should be avoided by choosing suitable hardening strategies [91Gei, 95Mül].


Ref. p. 101]


79

Table 2.1.1. Properties of common laser types for surface hardening. Laser type CO2 lasers

Properties gas laser, very high output powers available wavelength: 10.6 µm (absorption coating is necessary) beam guiding and shaping via cooled mirrors electrical overall efficiency < 12 %

Nd:YAG lasers

solid-state laser wavelength: 1064 nm beam guiding via glass fibers, beam shaping via lenses electrical overall efficiency < 5 %

Diode lasers

semiconductor laser, arrays of many single diodes wavelength: 800…940 nm electrical overall efficiency > 20 % small in size and weight

2.1.1.1.4 Time scheme of the irradiation The effect of pulsed irradiation on the resulting microstructures, properties and hardness depths has been discussed in [97Ber]. Different hardness-depth profiles attained using either cw or pulsed radiation with various frequencies are shown in Fig. 2.1.6. The results are generally similar, but using the pulsed mode, a slight increase of the hardening depth can be achieved. On the other hand, by using pulsed radiation with frequencies below 50 Hz, the risk of surface melting increases, while the desired compressive residual stress is not achieved [95Dom].

Fig. 2.1.6. Hardness-depth profiles for laser beam hardening in the cw and pulsed mode with different pulse frequencies (Nd:YAG-laser, feed rate: 0.3 m/min.) [97Ber]: 1: cw, 2: 100 Hz, 3: 50 Hz, 4: 25 Hz, 5: 12.5 Hz.

2.1.1.1.5 Observed degradations and their reasons Table 2.1.2 gives possible degradations which can be observed occasionally (also in classical surface hardening) and their metallurgical and/or process-related reasons.


80


[Ref. p. 101

Table 2.1.2. Degradations of laser-surface-hardened parts and their metallurgical and process-related reasons [96DIN1]. Characteristic feature 1 surface hardness too low

Reason conditional on the heat treatment Relevant fault during surface hardening 1.1

untransformed ferrite, pearlite, or too less carbides dissolved

a) b)

1.2 proportion of martensite too low 1.2.1 due to formation of bainite and/or a) pearlite and/or ferrite b) c) d) e) 1.2.2 due to retained austenite a) b) c) d)

1.3

e) martensite too soft (also locally a) defined) b) c)

austenitizing temperature too low time of irradiation too short

austenitizing temperature too low time of irradiation too short (self-)quenching effect too low severe oxidation of the surface decarburization austenitizing temperature too high (overheating) time of irradiation too long (oversoaking) not, too late or not adequately tempered not, too late or not adequately subzero-treated carburization due to coatings tempering temperature too high tempering time too long tempering because of overlapping tracks not tempered or too late tempered tempering temperature too low tempering time too short

1.4

proportion of retained austenite a) too high and/or too less carbide b) precipitated c)

2 surface hardness too high

2.1

martensite within the surface a) layer too hard b) c)

not tempered tempering temperature too low not tempered a sufficient number of times

3 insufficient depth of hardening

3.1

spatially incomplete austenitiza- a) tion b) proportion of martensite too low a)

austenitization temperature too low time of irradiation too short insufficient quenching rate

4 depth of harde- 4.1 ning too high

depth of austenitization too high

austenitizing temperature too high time of irradiation too long

5 too much distortion

(thermal and/or transformation a) induced) internal stresses too high or too asymmetric b) c)

3.2

5.1

a) b)

too rapidly or too unsteadily heated and austenitized unsuitable fixture of the workpiece unfavorable size/shape of the heattreated area


Ref. p. 101]


81

Table 2.1.2 continued. Characteristic feature 6 cracks

Reason conditional on the heat treatment Relevant fault during surface hardening 6.1

(thermal and/or transformation a) induced) internal stresses too high (fracture toughness ex- b) ceeded) c) d) e)

too rapidly or too unsteadily heated and austenitized not tempered tempering temperature too low tempering time too short overlapping tracks

7 deviations in the shape of edges and/or surfaces

7.1

partial melting

temperature too high time of irradiation too long power flux density of the beam too high overheating of an edge

a) b) c) d)

2.1.1.2 Remelting In contrast to solid-state hardening, the surface is heated up to a temperature above the melting point. The aim of laser surface melting is to generate a surface layer with better properties than the substrate material resulting from rapid solidification. The quenching rates that can be obtained can be as high as those achieved in completely different techniques, like melt spinning or powder atomization. These extremely high cooling rates determine the nature and stability range of phases, their distribution, size and morphology [96Mor]. The solidification process can be characterized by three important parameters: -

the quenching rate ε = dT/dt, the solidification velocity R = υ • n (velocity of the solidification front), the temperature gradient G = n ∇ T across the liquid/solid interface.

These parameters are related by the equation

ε = R G .

(2.1.3)

As Fig. 2.1.7 illustrates, these parameters depend on the shape of the melt pool [93Her], which is determined by the laser parameters, the material properties and the melt dynamics due to convection within the pool [84Böt, 86Rap, 90Kre]. Laser v C

A R

α

Gx

α

L

time of melt pool existence: τ = L/υ, temperature gradient: Gz = dT , Gx = dT ,

v

Gz

Fig. 2.1.7. Schematic drawing of the melting pool and definition of the characteristic parameters of the solidification process:

dz

dx

solidification velocity: R = υ cos α, B

TM

cooling rate: dT = ε = R G

dt

(cooling rate corresponds to quenching rate).


82


u eq

ia

x

d

rit

ic

10 =

4

ε

10 2

ed

d en

[Ref. p. 101

10

2

K/

s

0

rit de

nd

10 0

−2

10

Solidification velocity R [mm/s]

ic

10

10

−2

r

la

lu el

10

10

c −4

pl

an

ar

−4

10 −1

10 0 10 1 10 2 Temperature gradient G [K/mm]

10 3

Fig. 2.1.8. Influence of the solidification velocity R and the temperature gradient G on the solidification morphology [86Kur].

The actual values of the variables ε , R and G determine the resulting microstructure [81Kur, 83Cly, 83Sah], which can be epitaxial, dendritic, planar, cellular or even amorphous. The relationship is given in Fig. 2.1.8 [86Kur]. Remelting can be performed without additives on cast iron and steels and with or without additives on several nonferrous metals. If additives are used, they can either be pre-deposited on the surface of the workpiece (two-step process), see Fig. 2.1.9a, or they can be supplied continuously during the melting process (one-step process), see Fig. 2.1.9b. In the one-step process, the additives can either be supplied in backhand or forehand direction (Fig. 2.1.9c and d).


Ref. p. 101]


83

Fig. 2.1.9. Different methods of laser surface remelting: (a) two-step process, (b) one-step process, (c) backhand remelting, (d) forehand remelting.

2.1.1.2.1 Remelting of cast iron Iron-carbon alloys can solidify according to the metastable Fe-Fe3C phase diagram or the Fe-graphite equilibrium phase diagram. The relative stability of cementite (Fe3C) and graphite is effected by alloying elements. Generally, the stability of graphite increases with increasing content of carbon and silicon (gray cast iron). Rapid cooling favors the formation of cementite. This effect is used in laser surface treatment of cast iron. If the surface of gray cast iron is melted, the extraction of the heat into the bulk substrate leads to resolidification and a self-quenching effect of the surface layer. The melt solidifies as ledeburite, an eutectic mixture of Fe3C and austenite. The mechanical properties of this layer differ dramatically from those of the gray cast iron, especially the hardness and wear resistance is improved. Figure 2.1.10 gives an example for a hardness-depth profile of a laser-remelted camshaft made of GGL 25. The hardness of the resolidified layer reaches between 800 and 900 HV 0.1. A remarkable spread of the values within the transition area is due to microstructural effects (primary Fe-dendrites and ledeburite) [92Mül]. Typical hardness values for different alloyed cast irons obtained after laser surface melting are given in Table 2.1.3. Table 2.1.3. Hardness of various remelted cast irons in the as-quenched state [85Ber]. Cast-iron composition [wt.-%] C Si Mn 3.5 2.0 ... 3.5 0.5 ... 2.5 1.7 ... 3.0 2.4 6.0 2.1 5.0 ... 2.7 0.3 ...


Hardness [HV] P 1.0 2.0 ... ... ... ...

Cr ... ... ... 12.0 ... 28.0

Ni ... ... 33.0 ... ... ...

1100...1200 1100...1200 450...550 450...550 600...650 600...650

84


Fig. 2.1.10. Hardness-depth profile of a laser surface remelted camshaft of lamellar gray cast iron (GGL25) [92Mül]; material: lamellar gray cast iron (GGL25), laser output power: 8 kW, intensity profile: line (16 mm × 1.5 mm), feed rate: 0.8 m/min., shielding gas: CO2, pre-heating: 480 °C, after-process treatment: 580 °C/30 min.

[Ref. p. 101

Fig. 2.1.11. Influence of the laser output power and the feed rate on the depth of the molten zone [92Mül]; material: ductile cast iron (GGG60), intensity profile: line (15 mm), surface preparation: phosphate, initial surface roughness: Rz = 15 µm.

The depth of the melt typically varies between 0.5 and 1.5 mm. Figure 2.1.11 demonstrates the influence of the laser output power and the feed rate on the depth of the molten zone [92Mül]. The effect of the surface roughness before surface melting on the melting depth can be neglected. The roughness of the surface after laser surface melting is in the magnitude of about 10 µm and depends on process parameters and supply of the shielding gas [88Gil]. The properties of the heat-affected zone between the surface layer and the unaffected substrate depend on the microstructure of the matrix. This is an important factor in deciding upon the layer specifications to meet the requirements of the component in service.

2.1.1.2.2 Remelting of aluminum alloys One of the main difficulties in surface remelting of aluminum and aluminum alloys is the poor absorption of infrared laser light (see e.g. Part 1, Chap. 1, Table 1.1.3 and Fig. 1.1.1). The absorption can be improved by grinding or sand blasting of the surface, providing higher roughness, but usually coatings are used to get satisfactory results. Furthermore, use of shielding gas is necessary to prevent reactions between the melt and the atmosphere (for example oxygen in the atmosphere). A significant increase of the surface hardness and therefore of wear resistance can be achieved by the following mechanisms: -

grain refinement, precipitation hardening, dual phase hardening / dispersion hardening.

An example for laser surface remelting of nonferrous metals without using additives is the surface treatment of eutectic or near eutectic Al-Si-alloys (e.g. pistons for automotive engines). Due to rapid quenching, laser surface remelting results in a very close-grained microstructure, which has better wear resistance than the cast structure, although the hardness of the surface layer is not enhanced significantly [96Mor]. The range of the solid solubility in binary aluminum alloys can be extended significantly by rapid quenching. This leads to a combination of solid solution hardening and/or precipitation hardening. The strength of Al-alloys at room temperature and at elevated temperatures can be improved by precipitation


Ref. p. 101]


85

hardening. For an efficient precipitation hardening process a high solid solubility of the 2nd element in the Al-rich crystal at higher temperatures is desired, simultaneously with almost no solid solubility at room temperature. If in addition high-temperature strength is required, it is necessary that stable intermetallic phases with high melting point precipitate. Table 2.1.4 gives various examples for alloying elements, their solid solubility and the melting point of the intermetallic compounds, which can be precipitated.

Table 2.1.4. Solid solubility of transition metals in binary aluminum alloys and some of their intermetallic compounds. Alloying elements

Solid solubility Intermetallic Melting in rapid compound temperature of solidification the inter[at. %] metallic compound [°C] 1.9 5.0 6.0 CrAl7 725 940 Cr2Al11 1010 CrAl4 1170 CrAl3 4.4 FeAl3 1160 9.0 MnAl6 710 822 MnAl4 880 MnAl3 7.7 NiAl3 854 1133 Ni2Al3

Ce Co Cr

Solid solubility in conventional solidification [at. %] 0.01 0.01 0.38

Fe Mn

0.03 0.70

Ni

0.02

Ti Zr V

0.74 0.08 0.20

2.0 3.0 2.0

W Mo

0.02 0.07

1.9 1.0

TiAl3 ZrAl3

1340 1580

Comments

rarely used improvement of corrosion resistance

suitable for Al-Si-alloys, formation of extremely hard Mn-silicides very fine eutectic with good strength and ductility suitable alloying element suitable alloying element extremely high hardness, but inhomogeneous layers extremely high hardness, but inhomogeneous layers

Laser remelting allows to disperse hard particles into the surface layers. These particles have to meet the following conditions: -

thermal stability within the temperature range of application, chemical stability against the liquid metal (e.g. no oxides that can be reduced by aluminum), higher hardness than the wear partner of the component, no porosity (explosion under the laser irradiation), within a suitable density range (otherwise sedimentation or floating of the powders).

Table 2.1.5 gives examples for hard particles which can be dispersed in aluminum alloys.


86


[Ref. p. 101

Table 2.1.5. Properties of ceramic components for dispersion in an Al-matrix. Density [g/cm3]

Hardness [GPa]

Melting / decomposition temperature [°C]

Price / availability

Comments

3.97 5.8 2.3

18...23 10...15 10...13

2050 (melt.) 2700 (melt.) 1723 (melt.)

++ + +++

not chemically attacked not chemically attacked

SiC

3.2

20...30

2300 (decomp.)

++

dissociated, if the particle size is too small

TiC Cr2C3 B4C diamond

4.9 6.7 2.51 3.5

approx. 28 10...18 approx. 30 –

3140 (melt.) 1980 (melt.) 2450 (melt.) decomp. dep. on heating rate

0 – + ––

Si3N4

3.18

8...19

1900 (decomp.)

+

TiN AlN

5.4 3.2

16...20 approx. 12

2930 (melt.) 2235 (decomp.)

– ++

4.5

15...28

2900 (melt.)

–

Material

Oxides Al2O3 ZrO2 SiO2 Carbides

Nitrides reaction products around the particles

Borides TiB2

The appropriate particle size is between 5 µm and 50 µm (for a layer thickness of some 100 µm). If the particles are too large, they sometimes contain thermal cracks. Smaller particles are inefficient barriers to plastic deformation, hence wearing out of the surface can occur. A volume fraction of 10...30 % of hard particles within the surface layer has shown good results. 2.1.1.2.3 Remelting of titanium alloys Titanium alloys have inherent advantages of high strength combined with low density. Their tribological behavior is characterized by high friction coefficients and poor wear resistance. The hardness values are typically 300...350 HV. TiN, which is formed by the reaction Ti + ½ N2 → TiN, has a hardness of about 2400 HV. Thus it can be used to generate hard and wear-resistant surface layers on titanium and titaniumalloy components. Conventional solid-state treatments (e.g. annealing within a nitrogen atmosphere or plasma heat treatment) depend on diffusion processes and therefore on a parabolic law for the increase of the layer thickness. Within reasonable times, a layer thickness of some 10 µm can be reached. Laser remelting of the surface within a reactive atmosphere (“gaseous laser alloying”) is an alternative treatment for titanium and Ti-alloys. It can provide a layer thickness of typically 0.5...1 mm. If the layer is too thick, it tends to crack. The hardness of the surface depends on the laser parameters and on the composition of the atmosphere (Fig. 2.1.12). With pure nitrogen, a surface hardness of ∼900 HV can be reached [86Ber]. Table 2.1.6 Landolt-Börnstein New Series VIII/1C

Ref. p. 101]


87

gives examples of the properties of gaseous laser-alloyed TiAl6V4 (IMI318) for different thermochemical treatments. Dispersing of particles (e.g. TiC) into titanium alloys is also possible [90Gas], but the extreme reactivity of titanium melts restricts the number of appropriate powders.

Fig. 2.1.12. Hardness-depth profiles of laser-nitrided TiAl6V4: (a) Influence of the laser output power; CO2-laser, cw-mode, feed rate: 1 m/min., pre-heating: none, atmosphere: N2, 1: 700 W, 2: 600 W, 3: 500 W, 4: 400 W, 5: 300 W, 6: 200 W. (b) Influence of the atmosphere (nitrogen content); CO2-laser, cw-mode, feed rate: 1m/min., preheating: none, atmosphere: 1: 100 % N2, 2: 90 % N2, 3: 75 % N2, 4: 50 % N2, 5: 25 % N2, 6: 10 % N2.

Table 2.1.6. Properties of gaseous laser-alloyed TiAl6V4 (IMI318) for different thermochemical treatments. Treatment

Color

Microstructure

Phases

as received

silver

equiaxial grains

α', β-Ti

remelted under N2

golden

TiN-dendrites in α'-matrix

α'-Ti + TiN

dense layer

remelted under CH4 + Argon

gray

TiC-precipitates in α'-matrix

α'-Ti + TiC

inhomogeneous layer

repeatedly remelted under CH4 + Argon

silver

TiC-dentrides in α'-matrix

α'-Ti + TiC

homogeneous, dense layer

remelted under O2

dark gray

massive oxide

TiO + TiO2

inhomogeneous layer thickness


Comments

88


[Ref. p. 101

2.1.1.2.4 Remelting of magnesium alloys Due to their very low specific densities (1.75...1.85 g/cm3), magnesium alloys are of great interest for some technical application, but the poor wear and corrosion properties cause some restrictions. Table 2.1.7 summarizes the chemical composition of some commonly used Mg-alloys and the microhardness that can be reached by laser surface remelting without additives, due to grain refinement and microstructural changes [94Gal]. A significant increase of the surface hardness can be obtained by laser alloying with powders of elements that can form hard and stable intermetallic phases with magnesium. Table 2.1.8 gives some examples. The results of laser-alloying experiments are summarized in Table 2.1.9 [94Gal, 95Gal]. Table 2.1.7. Chemical composition (wt.%) of common Mg-alloys and their microhardness after laser surface remelting. Alloy

Condition Al

cp Mg Al 80 AZ 61 AZ 91 WE 54

as cast extruded extruded

Zn

0.8 6.0 9.0

Mn

1.0 1.0

Y

Zr

RE

Mg

2.9

99.9 rem rem. rem. rem.

0.3 0.3

cast, T6

5.1

0.5

Hardness of substrate [HV 0.1] 35

Hardness of laser track [HV 0.1] 45

60 70 95

75 115 75 (85 after precipitation hardening)

Table 2.1.8. Alloying elements and their intermetallic compounds with magnesium. Alloying element

Melting point [°C]

Max. solubility [at. %]

Intermetallic compound

Al Cu

660 1083

11.5 0.013

Ni Si

1453 1410

0.04 0.01

Mg17Al12 Mg2Cu MgCu2 Mg2Ni Mg2Si

Melting point of the compound [°C] 402 568 819 760 (peritectic) 1085

Table 2.1.9. Results of laser-alloying experiments on Mg-alloys. Substrate

Alloying element Al Al Al

Alloying content [at. %] 29 16 26

cp Mg cp Mg AZ 91

Hardness HV 0.1 200 100 180

cp Mg Al 80 AZ 61 AZ 91 WE 54

Cu Cu Cu Cu Cu

41 33 30 45 31

180 230 220 180 230

WE 54

Cu

38

220

Structure of the intermetallic compound compact eutectic fibers compact

Comments

compact oriented dendrites dendrites compact needle shaped dendrites needles

resulting microstructure strongly dependent on the alloying elements of the substrate

relatively low increase of hardness, above 34 at. % Al completely intermetallic compound due to extended solubility after rapid quenching


Ref. p. 101]


89

Table 2.1.9 continued. Substrate

Alloying element Ni Ni Ni Ni

Alloying content [at. %] 32 28 36 32

cp Mg Al 80 AZ 91 WE 54

cp Mg AZ 61 AZ 61 WE 54

Hardness HV 0.1 300 250 320 300

Si Si Si Si

23 22 28 27

260 220 300 280

Structure of the intermetallic compound oriented dendrites oriented dendrites dendrites needle shaped dendrites

Comments

dendrites dendrites dendrites dendrites

hardness values up to 750 HV are possible with a Si content of ∼55 at. % (primary Si with Si-Mg2Sieutectic)

hardness can be enhanced by higher Ni contents, but the resulting embrittlement leads to cracking

2.1.1.2.5 Observed degradations of surface-remelted components and their reasons Table 2.1.10 gives possible degradations which can be observed occasionally on surface-remelted components of cast iron and their metallurgical and/or process-related reasons. An overview of the degradations on surface-remelted components (with additives) of nonferrous alloys is given in Table 2.1.11. Table 2.1.10. Occasionally complained degradations on surface-remelted components of cast irons and their metallurgical and process-related reasons [96DIN2]. Characteristic feature 1 too many pores within the surface layer

Reason conditional on the heat treatment 1.1 overheating of the melt

Relevant fault during surface hardening a) time of irradiation too long or intensity too high

2 graphite within the 1.2 incomplete melting of the surface a) molten layer layer

time of irradiation too short or intensity too low

3 melted depth too low

3.1 line energy too low

a) b)

energy flux density too low feed rate too high

4 melted depth too high

4.1 line energy too high

a) b)

energy flux density too high feed rate too low

5 surface hardness too low

5.1 carbide content within the surface layer too low

a)

5.2 content of retained austenite too high within the surface layer

a)

solidification rate too high, due to too high energy flux density and too short time of irradiation inadequate cooling rate below the Ac1-temperature not or insufficiently tempered

b) 6 surface hardness too high


6.1 carbide content within the surface layer too high

a)

6.2 content of martensite too high within the surface layer

a)

solidification rate too low, due to too less energy flux density and too long time of irradiation cooling rate below the Ac1temperature too high

90


[Ref. p. 101

Table 2.1.10 continued. Characteristic feature 7 too much distortion

Reason conditional on the heat treatment 7.1 thermal and transformation induced stresses too high or too asymmetric

7.2 displacement of the melt too large

8 cracks

8.1 thermal and transformation induced stresses too high or too asymmetric (fracture toughness exceeded)

Relevant fault during surface hardening a) too rapidly or too unsteadily heated and remelted b) unsuitable fixture for the workpiece c) unfavorable size/shape of the heat-treated area a) inhomogeneous energy distribution b) inadequate position of the workpiece a) b) c) d) a)

8.2 too much deformation in the incompletely solidified state (hot cracking) b) c)


9.1 surface of the melt deforms too much

a) b) c) d) e)

pre-heating temperature too low too rapidly or too unsteadily heated and remelted too rapid or too unsteady cooling not or insufficiently tempered inadequate energy distribution of the beam unsuitable fixture of the workpiece unfavorable size/shape of the heat-treated area time of irradiation too long energy flux density too high inhomogeneous intensity distribution inadequate position of the workpiece unfavorable size/shape of the heat-treated area

Table 2.1.11. Occasionally complained degradations on surface-remelted components of nonferrous metals (with additives) and the metallurgical and process-related reasons [96DIN2]. Characteristic feature 1 too many pores in the surface layer

Reason conditional on the heat treatment 1.1 overheating of the melt

Relevant fault during surface hardening a) time of irradiation too long or intensity too high

2 washing-in of the substrate material

1.2 incomplete melting of the surface a) layer

time of irradiation too short or intensity too low

3 melted depth too low

3.1 line energy too low

energy flux density too low feed rate too high insufficient pre-deposition or feed rate of the additive material too low

3.2 content of additives too low

a) b) a)


Ref. p. 101]


91

Table 2.1.11 continued. Characteristic feature 4 melted depth too high

Reason conditional on the heat treatment 4.1 line energy too high 4.2 content of additives too high

5 surface hardness too low

5.1 content of hard particles within the surface layer too low

Relevant fault during surface hardening a) energy flux density too high b) feed rate too low a) excessive pre-deposition or feed rate of the additives too high a)

b) c)

6 surface hardness too high

6.1 content of hard particles within the surface layer too high

a)

b) c) 7 too much distortion

7.1 thermal and transformation induced stresses too high or too asymmetric

a) b) c)

7.2 displacement of the melt too large

a) b)

8 cracks

8.1 thermal and transformation induced stresses too high or too asymmetric (fracture toughness exceeded) 8.2 too much deformation in the incompletely solidified state (hot cracking)

a) b) c) a) b) c)


9.1 surface of the melt deforms too much

a) b) c) d) e)


inadequate solidification rate due to unsuitable choice of energy density and irradiation time inadequate cooling rate after the resolidification insufficient pre-deposition or feed rate of the additive material too low inadequate solidification rate due to unsuitable choice of energy density and irradiation time inadequate cooling rate after the resolidification excessive pre-deposition or feed rate of the additives too high too rapidly or too unsteadily heated and remelted unsuitable fixture of the workpiece unfavorable size/shape of the heat-treated area inhomogeneous energy distribution inadequate position of the workpiece too rapidly or too unsteadily heated and remelted too rapid or too unsteady cooling not or insufficient tempered inadequate energy distribution of the beam unsuitable fixture of the workpiece unfavorable size/shape of the heat-treated area time of irradiation too long energy flux density too high inhomogeneous intensity distribution inadequate position of the workpiece unfavorable size/shape of the heat-treated area

92


[Ref. p. 101

2.1.1.3 Laser cladding Laser cladding is used to generate a surface layer of an alloy different from the substrate material to improve the wear and/or corrosion properties of a component [86Maz, 89Wu]. Laser cladding can be performed either in a one-step process [90Lug1, 92Lug, 94Lug] or in a two-step process (pre-deposited material method) [90Mar1] (compare with Fig. 2.1.9a and b). Furthermore, the two-step process can be performed either in a free-cladding or in a shape-determined way using a crucible (Fig. 2.1.13) [94Lan, 95Lan].

Fig. 2.1.13. Different methods of laser cladding: (a) free cladding, (b) shape-determined cladding using a crucible.

In contrast to the remelting processes like alloying and dispersing (Sect. 2.1.1.2), the amount of molten substrate material is very low only ensuring a metallurgical bond between the substrate and the layer. The major part of the clad is built up by the deposited material. Therefore final shape of the workpiece is defined by the thickness and shape of the layer. Using the two-step process, different forms of pre-deposition can be taken into account, depending on the application, like -

powders, pastes, foils, wires, thermally sprayed coatings.

In the application, the one-step process is often preferred, because it is easier to control. The most common method of laser surface cladding is to inject the material in form of powder or wire [89Bur] through a nozzle into the laser/material interaction zone. If a powder is used, a certain amount of the powder, which does not hit the molten pool, gets lost. Powder efficiency and surface quality are very sensitive to the geometrical arrangement. The influence of the incident direction of the powder stream on the powder efficiency is discussed in Fig. 2.1.14. Alternatively, a coaxial powder feed nozzle can be used, promising best results and avoiding directional effects on the shape of the clad bead [96Lin].


Ref. p. 101]


93

100

N

oz

zl

e

−

Position of the nozzle

80

+

Efficiency [%]

Laser

40

10

θ1

60

Y axis

20

Sample displacement

a

0

10 20

60 50

−

θ 2-0°

Position of the nozzle

+

+θ 2

b

X axis

Efficiency [%]

− θ2

θ 2 -180°

30 40 50 60 70 80 90 θ 1 [degrees]

40 30 20 10 0 -180

-90

0 θ 2 [degrees]

90

180

Fig. 2.1.14. Influence of the incident direction of the powder stream on the powder efficiency [90Mar1]: (a) influence of angle θ1 (θ2 = 0°), (b) influence of angle θ2 (θ1 = 55°). Substrate: mild steel (St37), powder: Stellite 6, laser output power: 1500 W, beam diameter: 2 mm, speed: 0.8 m/min., step: 0.6 mm, powder flow rate: 6 g/min., gas flow rate: 1.3 l/min.

Using a mixture of different powders, in which one component has a high melting point and low absorption coefficient, a dual phase layer containing hard particles can be generated [84Tuc] (e.g. for cutting tools). Taking into account that both components may differ severely in density, it has to be considered that floating or sedimentation of the particles may occur. A vertical cladding position is an alternative method to prevent this effect [95Lan]. Crack-free cladding is essential for satisfactory corrosion resistance. The different thermal expansion coefficients of the substrate and the coating often result in tensile residual stresses within the cladded coating. As a consequence, cracking can occur even after the process but prior to the application of any additional load. Pre-heating of the component and/or post-annealing can help to reduce the residual stresses [86Dek]. Examples for laser cladding on different substrate materials and possible applications are given in Table 2.1.12.


94


[Ref. p. 101

Table 2.1.12. Examples of laser cladding on different substrate materials. Substrate material Cladding various NiCrBSi

mild steel

NiCrBSi NiCrTaB 50 vol.-% TiC, 50 vol.-% Stellite 6 Co-base alloy with TIC or WC FeCrAlY

CoCrAlY

mild steel (St37) mild steel (St37) mild steel (St37)

steels

Stellite 6 45Fe-55Cr Fe-12.5Cr-9.5P1.7Y Fe-12.5Cr-9.5P1.7Y Ni-25Cr-5.2Al2.7Si-0.5Y NiCrBSi

steels

NiCr(Nb,Ta)B

45NiCr6

Stellite 21

GGG40

Ni

14CrMoV6-9 NiCr20AlTi

Mo-alloy NiCrAlY

pure Ni IN738

mild steel (St52)

Al2O3/TiO2 (97/3) Al2O3/TiO2 (87/13) mild steel (St52) + Al2O3/TiO2 (97/3) bondcoat Ti (laser remelting) mild steel (St52) ZrO2/Y2O3 (92/8) X6CrNiTi18-10 Al2O3/TiO2 (97/3) X6CrNiTi18-10 Al2O3/TiO2 (87/13) X6CrNiTi18-10 PSZ: ZrO2/Y2O3 (92/8) X6CrNiTi18-10 Al2O3 X6CrNiTi18-10 Cr2O3 X6CrNiTi18-10 Cr2O3/SiO2/TiO2 (90/5/3)

Method two-step (pre-placed paste) blown powder blown powder blown powder

Comments Ref. superior wear properties to 79Bel plasma-sprayed coatings, esp. at high pressures

good metallurgical bonding WC superior to TiC

two-step (pre-placed powder) two-step (pre-placed powder) blown powder blown powder blown powder

84Tuc

excellent oxidation resistance 83Liu at 1000...1200 °C excellent oxidation resistance 83Liu at 1000...1200 °C, but cracking problems 90Mar2 90Mar2 90Mar2

blown powder

90Mar2

blown powder

90Mar2

one-step / two-step better corrosion resistance than austenitic steel and Nimonic 75 one-step / two-step better corrosion resistance than austenitic steel Nomonic 75 and NiCrBSi coatings two-step, repairing of dies coaxial-nozzle remelting of transport containers for plasma-sprayed nuclear application layer one-step extruder parts, 780...1130 HV one-step exhaust valves of large diesel engines one-step no sufficient bondage! one-step no sufficient bondage! one-step improved wettability of metal to ceramic

90Lug2

one-step one-step one-step one-step one-step one-step one-step

cracks and pores free of pores and cracks free of pores and cracks Y freely soluble in Zr2O3 cracks and pores 190 nm. For optical components to be used at 157 nm, CaF2 and MgF2 are the almost sole materials. Reflection optics becomes of increasing importance. In the EUV, reflection optics has to be used. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]

5.1 High-precision optical metrology for surfaces

411

5.1.2.2.2 Increasing the numerical aperture (NA) Increasing NA beyond NA = n implies a transition to the near field because of the low-pass characteristics of free-space propagation: Like optical imaging, free space propagation can be described in frequency space by multiplication with a transfer function, cf. (5.1.3). This follows from the convolution-type diffraction integrals. Derived from the first Rayleigh-Sommerfeld diffraction integral the transfer function for propagation along a distance z becomes 1/2 P (fx , fy , z) = exp +i2π n2 λ−2 − fx2 − fy2 z , (5.1.10)

, z)

2 1/2

10 0 10-10

2

P (f =(fx +fy )

Amplitude of

which results for spatial frequencies fx2 + fy2 > n2 λ−2 in an exponential damping. The amplitude of P (f = (fx2 + fy2 )1/2 , z) is depicted in Fig. 5.1.4.

10-20 10-30 0

fx l

1 / NA 2

3 3

2

1

z/l

0

Fig. 5.1.4. Amplitude (logarithmic scale) of the free-space transfer function depending on distance z and spatial frequency f .

5.1.2.2.3 Decreasing the prefactor κ A decreased constant κ in the optical resolution limit can be achieved by several means: – Non-linear imaging processes like two-photon and multiphoton confocal microscopy [98Str]. Then, linear system theory and the derived resolution limits are no longer valid. Because of the non-linearity, though, the image interpretation should be performed with care. The need for a preparation of the objects makes it difficult to apply these methods to technical surfaces. – Optimized illumination techniques as in off-axis interference lithography [99Bru], by use of phase-conjugating mirrors [99Uhl], or by use of particular pupil filters [97Sal]. – Model-based imaging and extrapolation of the spatial frequency spectrum: See Sect. 5.1.2.3.

5.1.2.3 Usage of a-priori information: From model-based imaging to threshold criteria With a-priori information we denote knowledge about the structure that is known literally before the actual measurement, i.e. in particular it is independent from the measurement. A-priori information plays an important role in microstructure metrology, where some information about the structure is usually available (for instance material or etching depth due to the fabrication process). Furthermore, the task of metrology on these structures is not to reveal the complete shape but to determine certain structure parameters like the linewidth of a groove. As a consequence, the

Landolt-B¨ ornstein New Series VIII/1C

412

5.1.3 Methods and instrumentation

[Ref. p. 439

Table 5.1.3. Surface measurements applying a-priori information. Method

A-priori information

Obtained accuracy (∆xR = Rayleigh resolution)

fit of PSF [00San]

instrument’s PSF

∆xR /3

fit of spatial frequency spectrum [93Kru]

object type; here: double lines of known phaseshift

λ/20

thresholds [87Nyy]

object = edge

λ/10

obtainable measurement precision becomes more important than the resolution and the signal-tonoise ratio gains great practical importance [86Cox]. Table 5.1.3 lists some measurement methods that use an increasing amount of a-priori information together with the obtained accuracy. In model-based imaging a mathematical model of the image is formulated and fitted to the image with the quantities of interest as free parameters. This was shown for object- [00San] and frequency-space [93Kru]. The incorporation of multiple scattering in the object has also been shown [98Che]. Techniques for extrapolation of the spatial frequency spectrum utilize the analyticity of the spectrum. An analytic function is completely determined by a contiguous section. Therefore, it is possible to reconstruct the complete spectrum from a band-limited transfer by analytic continuation [96God]. The a-priori information comprises merely the spatial restriction of the object. However, because of practical difficulties these methods are currently of limited practical use. Of considerable importance from an application-oriented point of view in this respect is the concept of the ambiguous image [93Sem]: A band-limited image might be caused by several possible objects of which the incorrect ones have to be excluded by application of a-priori information – a fact that provides an upper bound for the extrapolation, dependent on the signal-to-noise ratio. If additional a-priori information about the object is available (e.g. pure amplitude or phase object), this can also be incorporated in the reconstruction. For instance, the reconstruction of the spatial frequency spectrum using projection methods has been shown [97Fri]. Currently, the most important use of a-priori information is in the form of threshold- and extreme-value criteria which will be discussed in Sect. 5.1.3.2.1. Their application transforms the image interpretation into a localization problem. A threshold value, for instance, is used to determine the position of an opaque edge. It is a certain relative intensity in the image that coincides with the position of the structure edge. Extreme-value criteria make use of relative maxima or minima in the image. The main advantage of both is the possible simplicity of set-up and evaluation method: A conventional bright-field or confocal microscope together with a search for prescribed relative intensity values is sufficient. The strong dependence on the a-priori information, however, makes the methods susceptible to errors. To become more independent in this respect, generalized threshold criteria are applied that make use of additional values of the image (for instance dual-threshold criteria [87Nyy]).

5.1.3 Methods and instrumentation In this section, we give an overview of state-of-the-art set-ups for high-precision metrology of surface microstructures. The methods are roughly divided into 3 categories:


Ref. p. 439]


413

1. field-measuring microscopy, where the whole object field is imaged in parallel, 2. confocal microscopy where the image is obtained by scanning of a focus, and 3. near-field microscopy, where the distance between the imaging device and the structure is less than a wavelength. The term microscopy without prefix generally denotes far-field microscopy. If near-field microscopy is meant, it is explicitly stated. Pupil-measuring methods like scatterometry and conoscopy are not considered here. Well-known monographs on light microscopy are provided by M. Pluta [88Plu, 89Plu, 93Plu] for general instrumentation and aspects, T. Wilson [90Wil] for confocal microscopy., T.R. Corle and G.S. Kino [97Cor] for scanning microscopy in a broader context including interference- and near-field methods. As a starting point for work on scanning-probe microscopy the book edited by R. Wiesendanger and H.-J. G¨ utherodt [95Wie] is useful. A fundamental prerequisite for accurate microstructure metrology is the availability of diffraction-limited, high-NA microscope lenses in a wide wavelength region. Therefore, we start this section with a brief review of the state of the art of high-NA lens design.

5.1.3.1 High-NA lenses Microscope lenses are characterized by high numerical apertures and small fields of view [94Vol]. Virtually all modern microscope lenses are corrected at the image side to infinity, i.e. a tube lens is necessary to produce a real image. Consequently, the declared magnification refers to the focal length of the tube lens. This enables the manufactures to provide different magnifications by exchangeable tube lenses. Furthermore, additional optical components can be introduced between both lenses without affecting the image quality; at the same time the distance between both lenses can be varied. The user has to be careful, though, because different manufacturers use different standard focal lengths for their tube lenses (Zeiss: 164 mm, Olympus: 180 mm, Leica: 200 mm [94Vol]). Furthermore, part of the correction (in particular chromatic aberrations) might be left to the tube lens, so that, to be on the safe side, one should use matched microscope and tube lenses. A lens cut of a modern microscope/tube lens pair is shown in Fig. 5.1.5. For microstructure metrology, in particular high-aperture, dry (i.e. usable without immersion) objectives are of interest. They should 1. be corrected to the diffraction limit at highest NA’s, 2. be usable for a spectral region as large as possible, and 3. impose no depolarization to the transmitted field (apart from a geometrical depolarization due to different propagation angles in object and image space). Numerical apertures in object space of NAObj = 0.9 are common and high-end lenses provide NAObj = 0.95. In object space (n = 1), this corresponds to a solid angle of ±64 ◦ and ±72 ◦ , respectively. Because of compliance with the sine condition, the NAImg in image space is obtained from the magnification M by NAImg = M −1 NAObj .

(5.1.11)

State-of-the-art objectives have – for the correction wavelength – spherical aberration corrected to < λ/100 over the full pupil yielding Strehl ratios of 0.97 and more [94Vol].

Fig. 5.1.5. Ray path of a Leica 100/0.9 dry objective [00Lei]. Landolt-B¨ ornstein New Series VIII/1C

414


[Ref. p. 439

Microscope lenses for the VISible (VIS) wavelength region are roughly divided into two classes, depending on their correction state for longitudinal chromatic aberration. In the achromat, the C(656.3 nm) and F-line (486.1 nm) are corrected against the standard d-line (587.6 nm) and the secondary spectrum is partly corrected [94Shi]. In the apochromat the color correction is made down to the g-line (422.7 nm) and the secondary spectrum is almost removed. But this is not the only difference. Generally, an apochromat is in a higher performance class, considering NA and field of view, than an achromat [94Vol]. Recently, microscope lenses that are broad-band corrected down to the i-line (365 nm) have been introduced to the market. For still shorter wavelengths, single-line corrected lenses (for instance in the Deep UltraViolet (DUV) at 248 nm) are available. As dielectric interfaces are hit at oblique angles within a lens, some polarization losses of the transmitted field must be taken [88Han]. The designers try to minimize them together with reflection losses by anti-reflection coatings. Measurements at a manufacturer [99Wes] yielded maximum polarization losses (between parallel polarizers) at the rim of the pupil of a few % for high-NA dry, and up to 15 % for high-NA oil objectives.

5.1.3.2 Field-measuring microscopy The term “field-measuring methods” summarizes microscopical imaging techniques that capture the image as a whole, i.e. perform no scanning. Different field quantities are measured: intensity, phase, polarization, or a combination of them. Additionally, a spatial frequency filter may be introduced into the exit pupil of the microscope lens allowing for a spatial-frequency-dependent preprocessing of the image. A necessary prerequisite for all quantitative, high-precision imaging techniques is a well-defined illumination. It is provided by a set-up developed in 1893 by A. K¨ ohler [1893Koe, 1899Koe] resulting in the optical path sketched in Fig. 5.1.6. According to the illumination optical path, the source, the aperture stop – located in the entrance pupil of the condenser –, and the exit pupil are conjugate planes. For the imaging optical path these are the field stop, the object plane, and the image plane. Because every point in the aperture stop yields an incident plane wave in the object plane, unavoidable inhomogenities of the illumination are equalized.

Collector

Condenser

Objective lens

Tube lens

Source Field Aperture Object Exit pupil stop stop plane

Image

Fig. 5.1.6. Illumination and imaging optical path for K¨ ohler illumination.

Another often used illumination scheme is the critical illumination where the light source is imaged directly into the object plane. Because the degree of spatial coherence of the light incident on the object depends solely on the condenser numerical aperture, it is the same as in Köhler illumination [87Bor].

5.1.3.2.1 Intensity microscopy Intensity microscopy is probably the most simple and robust metrology tool: A bright-field or dark-field image is captured and a threshold or extreme-value criterion is applied to determine the Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]


415

1.25

Intensity

1.00 0.75

incoherent

0.50 coherent

0.25 0 -2

-1

a

Intensity

1.5

0 x NA / l

1

2

1

2

j = p/ 2

1.0

0.5 j=p 0 -2

-1

b

0 x NA / l

Fig. 5.1.7. Intensity images of an opaque halfplane (top) and a phase-shifting (ϕ0 ) half-plane (bottom).

position of the edges. To avoid non-axial aberrations, the sample is often put onto a scanning stage and only the central region of the image is used for the measurements [77Nyy]. For the intensity image of an opaque edge we get in the coherent and incoherent limit the well-known intensity distributions coherent limit: incoherent limit :

2 Icoh (x) = π−1 Si(NA k0 x) + 0.5 , −1 Iinc (x) = π−1 Si(2 NA k0 x) + [cos(2NA k0 x) − 1] (2π NA k0 x) + 0.5 (5.1.12)

with the sine integral Si(z) as defined in [70Abr] z sin t dt Si(z) = t 0

(5.1.13)

and k0 = 2πλ−1 . The results are shown in Fig. 5.1.7. The coherent threshold is 0.25 (because for the amplitude it must be 0.5 due to symmetry) and in the incoherent limit it becomes 0.5 (again due to symmetry). An extreme-value criterion indicates the edge position by a relative maximum or minimum. The coherent image of a phase edge of phase shift ϕ0 , for instance, is a well-known example:

2 Iphase edge (x) = 1 + (exp(iϕ0 ) − 1) π−1 Si(NAk0 x) + 0.5 .

(5.1.14)

The results for two different phase shifts are shown in Fig. 5.1.7. The incoherent image shows, of course, no modulation because the phase is lost. In the coherent image, the minimum drops to zero for a π phase shift (zero crossing of the amplitude). For phase shifts deviating from π it is shallower and vanishes for ϕ0 = m 2π. Of course, threshold criteria can also be applied to other types of microstructure images like interferograms or polarization microscopy images. Landolt-B¨ ornstein New Series VIII/1C

416


[Ref. p. 439

0.34

Threshold

0.32 0.30 0.28 0.26 0.24 0

0.2 0.4 0.6 0.8 Coherency parameter s

1.0

p-Pol.

0.8

.

1.0

Fig. 5.1.8. Threshold for an opaque edge dependent on the coherency parameter σ. Filled circles: values from [65Wat], line: interpolated.

l / l0

0.6 s-Pol.

0.4

1074 nm 1161 nm

0.2 1017 nm

0 -1500 -1000 -500

s-Pol. p-Pol. AFM-width

0 500 1000 1500 x [nm]

Fig. 5.1.9. Polarization effect for a bar on silicon (s-pol: E parallel edge, p-pol: E perpendicular edge) [97PTB].

The simplicity of the evaluation, however, is opposed by a strong dependence on experimental conditions – like coherence and polarization – and on structural parameters like edge shape and reflectivity. The dependence of the threshold for the image of an opaque edge on the coherency parameter σ=

NA of condenser NA of objective

(5.1.15)

is shown in Fig. 5.1.8 taken from [65Wat]. The threshold values up to σ = 1 could be described by a simple interpolation according to Ithreshold (σ) ≈

4

aj exp(2jσ)

(5.1.16)

j=0

with the empirically obtained coefficients: a0 = 2.5335 e − 1, a1 = −0.8549 e − 3, a2 = 6.3557 e − 3, a3 = −1.2634 e−3, a4 = 1.0259 e−4. Please note that this simple interpolation becomes increasingly doubtful for σ > 0.8. The dependence of the threshold values on inclination angle and phase shift for phase edges is shown in [94Kru] together with the development of a generalized line of thresholds to extract the edge properties. The threshold values are polarization-dependent as shown in [95Sro]. Figure 5.1.9 shows an example. The width of a bar of 1017 nm width and 376 nm height on silicon was measured by optical means with an imaging NA of 0.9 and an illumination NA of 0.71 at a wavelength of 525 nm. A polarization of the incident field parallel to the structure provided a width that differed by almost 90 nm to the width obtained for perpendicular polarization. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]


417

5.1.3.2.2 Microscopy with pupil filters Several methods were suggested and tried out for enhancing measurement accuracy by modifying the spatial frequency spectrum of the object. This is done experimentally by inserting an appropriate filter into the pupil plane. In dark-field microscopy, where the 0th order is removed by a corresponding stop, the edge position is indicated by a dark line surrounded by a bright halo [88You]. Using a non-linear crystal in combination with a polarizer as pupil filter frees the experimentalist from the need to know form and size of the 0th order in advance [98Reh]. A π/2 phase-shifting pupil filter for the 0th order realizes Zernike phase contrast. The phase distribution of week phase objects, i.e. for instance shallow topographies, is transformed into an intensity distribution that can be evaluated by threshold criteria [93Boz]. Other modifications of the spatial frequency spectrum include single-band transmission and performing the derivative by optical means using a pupil filter with linear amplitude gradient. However, the user should keep in mind that for all these methods the spatial frequency spectrum is altered, making a very careful image interpretation necessary.

5.1.3.2.3 Interference microscopy In interference microscopy, the phase of the image field is measured by superposition with a reference wave or by superposing two laterally sheared wave fields. For homogeneous materials the actual topography of the structure can be measured within the diffraction limit. Three types of set-ups are mainly in use for microstructure measurements in reflection (Fig. 5.1.10): Nomarsky interferometry [88Plu, 89Plu, 93Plu] as a lateral shearing method and Mirau and Linnik interferometry for measuring the phase-modulated object wave against an undisturbed reference wave. The major advantage of these methods is their suitability for high-numerical-aperture lenses (in particular Linnik interferometry (Fig. 5.1.10c)). Measurement and evaluation of the interferograms is usually performed according to the algorithms of phase-shifting interferometry because then no additional resolution limitation, as in the carrier frequency techniques, is imposed. The interferogram in the image plane stems from the field E O due to the object and accordingly a phase-shifted reference field E R (to simplify the notation, the spatial dependence is not written). Because interference microscopy can be combined with polarization techniques, we consider vector fields: 2

I∆ϕ = |E O + E R exp(i∆ϕ)| ,

(5.1.17)

which becomes in the usual form I∆ϕ = Ig (1 + γ cos(φ + ∆ϕ))

(5.1.18)

with phase φ, intensity Ig , and contrast γ a

b

c

Image

Nom. prism

Reference mirror

Obj. lens

Beam splitter Structure


Fig. 5.1.10. Schematic representation of methods for high-aperture interference microscopy: (a) Nomarski shearing interferometry, (b) Mirau interferometry, (c) Linnik interferometry.

418

5.1.3 Methods and instrumentation 2

2

Ig = |E O | + |E R | ,

γ=

2 |E O · E R | , Ig

[Ref. p. 439

φ = arg (E O · E ∗R ) .

(5.1.19)

As there are three unknown quantities, at least 3 different measurements – different phase shifts – are necessary to obtain the phase. For compensation of phase-shift errors the number of measurements is usually larger; a detailed description of the various techniques (and means to introduce the phase shift) can be found in [88Cre] and [90Swi]. With regard to stability and measurement time, the 8-phase-step method [96Smi] is useful. There, 8 images Ij (j = 1 . . . 8) are captured where the j’th component is phase-shifted by βj = jπ/2. Phase φ, intensity I0 = Ig , and contrast γ are obtained by √ Z2 + N 2 Z (5.1.20) tan φ(x, y) = and γ(x, y) = N 32 I0 with Z = 5 I2 − 15 I4 + 11 I6 − I8 ,

N = I1 − 11 I3 + 15 I5 − 5 I7 ,

and I0 =

1 8

8 j=1

Ij .

(5.1.21)

Many interferometrical techniques require a precise mechanical movement with nm accuracy. This can be achieved by piezo-driven flexure stages with capacitive sensors. Under suitable conditions, phase images, as obtained with interference microscopy, show phase singularities. These are localized points in a wave field, where the intensity is zero and the phase is not defined. Because of the continuity of the free-space wave field the phase isolines form to a starshaped pattern around a singularity. Phase singularities, also called phase dislocations, and optical vortices occur also in the reflected near field of small diffracting structures, down to sub-wavelength width. Figure 5.1.11 shows simultaneously the amplitude and the phase isolines of the reflected field caused by a plane wave of λ = 550 nm incident perpendicularly on a rectangular elevation (width: 300 nm, height: 160 nm) of a Si-substrate. The field was computed rigorously using an integralequation approach [97To1]. Where the amplitude drops down to zero, a singularity has formed. If the lens of an interference microscope is focused to a plane containing singularities, supersteep edges are observed. They stem from the π phase jump which is due to the change of sign of the complex field when crossing the singularity. Figure 5.1.12 shows a phase image of a rectangular bar of ≈ 200 nm width and 160 nm thickness on a silicon surface measured with a Linnik interference microscope with NA = 0.9, a magnification of 400 and λ = 549 nm [97To1]. Note the supersteep edges.

1.0 0.8 0.6 0.4

Amplitude

Phase

0.2 0 800 600 z [n 400200 m] 0 -500

0 x [nm]

500

Fig. 5.1.11. Phase singularity in the reflected near field of a sub-λ structure.


Ref. p. 439]


419

h [nm]

1

x[µ 0 m] -1

-2

0

2 ] y [ µm

1

4

3

Fig. 5.1.12. Supersteep edges in an interferometrical image.

-1000

-1000 b = 300 nm

z [nm]

-800

d = 120 nm

d = 90 nm b = 240 nm b = 210 nm

-600

-800

d= 150 nm

d= 180 nm

-400

z [nm]

250 200 150 100 50 0 2

-600 d = 90 nm

-400 d = 60 nm

d= 210 nm

-200 0 -500

-300

a

-100 0 100 x [nm]

300

d = 120 nm

d = 150 nm

-200 0 -500

500

b

-300

-100 0 100 x [nm]

300

500

Fig. 5.1.13. Calculated phase singularity position for bars on a Si-substrate of width 300 nm (*), 240 nm (x) and 210 nm (o) and increasing height for a perpendicularly incident plane wave with λ = 549 nm. (a) s-polarization, (b) p-polarization [97To2].

Phase singularities can be located in the image with high precision, therefore they might be used as indicators for the correct edge position, i.e. a kind of a self-adapting threshold criterion. The migration of the phase singularities in the near field for different structure widths is shown for both polarizations in Fig. 5.1.13: In both cases the phase singularities follow with increasing structure height generally an arc-shaped path through the near field above the structure (solid line, the dashed line corresponds to structures of identical height). The distance of the phase singularities agrees with the true structure width only for a particular structure thickness. Comparing Fig. 5.1.13a and b, we note that the phase singularities are generated in a larger distance above the structure for s- than for p-polarization. This is due to the higher scattering cross-section for s-polarization. Because of the doubled phase shift due to the reflection configuration, interference microscopy has an unambiguity range of λ/4. This limit can be extended by using a second wavelength [99Ped], oblique incidence [91Boe] or a combination of both [98Fra]. Both, multiple-wavelength and obliqueincidence approaches generate an enlarged effective wavelength Λ in z-direction, the unambiguity range is given now by Λ/4. For a plane wave incident at an angle ϑ the effective wavelength is determined by the component of the wavevector perpendicular to the surface Landolt-B¨ ornstein New Series VIII/1C

420


[Ref. p. 439

7

Focus position [µm ]

6 5 4 3 2 1 0 1

2 3 4 Lateral position [ µm]

5

Fig. 5.1.14. Intensity of a camera line depending on the focus position above a groove in silicon [00ITO].

Λ = λ cos ϑ .

(5.1.22)

For dual-wavelength interferometry the effective wavelength Λ is the beat wavelength Λ=

λ1 λ2 . |λ1 − λ2 |

(5.1.23)

In white-light interference microscopy (also called correlation microscopy [97Cor] or coherence radar) the limit is overcome entirely by using a continuous broad-band source, scanning the object vertically through the focus and evaluating the interferogram contrast [95Win]. A measurement example is shown in Fig. 5.1.14. Here, a 570 nm deep groove of 2 µm width was measured at 780 nm with ∆λ = 50 nm (FWHM). With a Mirau objective lens with a silicon-nitride beam-splitter membrane, the method was applied to the UV-wavelength of 325 nm [98Cha].

5.1.3.2.4 Polarization interferometry In particular, for linear microstructures the near and far fields depend strongly on the polarization. Consider for instance a small groove (width wavelength) in a good conductor: If the field is polarized parallel to the groove edges (TE-polarization) it can’t excite a propagating mode and therefore it may penetrate the groove only exponentially damped as an evanescent wave. For perpendicular polarization (TM-polarization) it penetrates as a propagating mode. Accordingly, the reflected field is scarcely modulated in TE, but definitely stronger in TM polarization. For silicon, as a strong dielectric, similar effects occur. This polarization dependence can be used to get images with a high edge sensitivity [00Tot]. To this end, the parallel and perpendicular polarized components are measured with respect to each other by means of polarization interferometry (Fig. 5.1.15): The structure is illuminated with circularly polarized light and imaged with a high-NA lens (250 × 0.95). An analyzer under an angle of 45 ◦ to the structure in front of the detector interferes both components. Prior to the interference they are phase-shifted by an adjustable amount using a Liquid-Crystal Phase Shifter (LCPS). To get accurate edge images, a two-step magnification system is used. The interferograms are captured by a CCD-camera. A single 9.2 × 8.4 µm2 pixel on the CCD corresponds to a 9.9 × 9.0 nm2 pixel in the object space. Image evaluation is performed according to the algorithms of phaseshift interferometry. Three images are obtained: phase, contrast and intensity, whereby phase and contrast are particularly sensitive to structure defects. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]


421

CCD Polarizer LCPS Lens

3.2 x

l /4 retarder

Tube lens BS

Laser diode Polarizer

{

Köhler illum.

Rotating ground glass

250/0.95

Lens Structure .

z piezo

Fig. 5.1.15. Set-up for polarization interferometry.

Frequently, polarization-optical methods allow a better imaging of sub-wavelength structures than intensity microscopy. This is demonstrated by two measurement examples obtained with λ = 635 nm. For instance, a 330 nm wide groove in silicon (depth: 160 nm) allows a clear edge detection in contrast and phase, but not in intensity (Fig. 5.1.16a–c). The measured width of 200 nm, though, is smaller than the actual width of 330 nm. As can be shown by rigorous numerical simulations, these deviations are due to systematical errors depending on structure depth. They may be calibrated if the depth is known. A second example (Fig. 5.1.16d–f) shows five neighbored trenches with 800 nm pitch etched 120 nm deep into silicon. Again, the contrast and phase image show an improved edge-detection capability compared with the intensity image.

5.1.3.3 Confocal microscopy In confocal microscopy, a point light source is imaged by a lens as a focus spot into the object plane. The spot is imaged by a second lens onto a pinhole detector. The pinhole acts as a kind of focus plane filter, because only light from the proper focus plane is transmitted without attenuation. Light from object parts in front or behind the focus plane is imaged defocused onto the pinhole yielding a strongly reduced detector signal, see Fig. 5.1.17. The name confocal denotes the fact that the foci of the illumination and imaging lens agree. In reflection mode, the focusing and imaging lens are the same. Optical sectioning is obtained by scanning the focus through the object. The depth response, and therefore, in a sense, the vertical resolution, depends on the kind of the investigated object. Two kinds of objects are of particular importance: point objects in biology, and surface patches in engineering. The resulting lateral and axial response and the corresponding Full Width Half Maxima (FWHM) are listed in Table 5.1.4, u and v are normalized lateral and axial coordinates according to v = k0 NA r

and

u = 2nk0 (1 − cos α) z

(5.1.24)

(α is the maximum acceptance angle, see Sect. 5.1.2.1). With decreasing NA, u in (5.1.24) approaches the small-angle formula u = nk0 z sin2 α. The lateral intensity response equals the square of the point spread function obtained for incoherent field microscopy. Therefore a 27 % smaller FWHM of the spot is obtained what results in an improved resolution of about 20 % for incoherent scatterers (as encountered in fluorescence Landolt-B¨ ornstein New Series VIII/1C

422


a

Intensity

4

Intensity

4

3

[Ref. p. 439

d

3

y [µ m ]

1 .0 0 .8

2

0. 8

2

0 .6 0 .4

1

0. 6 0. 4

1

0. 2 0

0 0 4

1

2

3

Contrast X [µ m ]

0

4

b

1

2

3

Contrast

4

3

y [µm ]

1. 0

4

e

3 0.8

0 .8 0 .6

2

0.6

2

0 .4 0 .2

1

1

0.2

0

0 0

1

2

3

Phase

4

0

4

c 4

1

2

3

Phase

4

3

y [ µm ]

0.4

4

f

3 1.5

2 2

1.0

2 0 -2

1

0

0.5 0

1

0 0

1

2

3

4

0

1

2

3

4

x [µm ]

x [µm ]

Fig. 5.1.16. Intensity (a), contrast (b), and phase image (c) of a small trench of 330 nm width and 160 nm depth in Si between two bars. Intensity (d), contrast (e), and phase image (f ) of 5 trenches of 120 nm depth and 800 nm pitch in Si.

Object

Pinhole

Det.

Fig. 5.1.17. Effect of pinhole detector.


Ref. p. 439]


423

Table 5.1.4. Lateral and axial response, and focus size in confocal microscopy in reflection. Objects

point

Lateral response intensity I(r) 4 J (v) 2 1v

Axial response intensity I(z) 4 sin (u/4) u/4

surface

–

sin (u/2) u/2

Lateral FWHM ∆r

Axial FWHM ∆z

λ 0.37 NA

0.64 λ n (1 − cos α)

–

0.44 λ n (1 − cos α)

2

Collector

Light source Condenser Imaging lens

CCD-camera

Beam splitter

Rotating Nipkow-disk

Tube lens

Microscope lens Object

Fig. 5.1.18. Confocal microscopy with Nipkow disk.

microscopy). For coherent scatterers, the point spread function is not given by the intensity but by the amplitude and a resolution equivalent to incoherent field microscopy is obtained. Yet another remarkable fact is that the halfwidth of the axial response is ≈ 50 % larger for isolated point objects than for surface objects. The reason for this is the reflection geometry. The measurement volume is elongated in axial direction. Confocal variants with further decreased measurement volume are multiphoton confocal microscopy [98Str] and 4π confocal microscopy [92Hel]. In the former the strong nonlinearity caused by the multiphoton excitation reduces the effective volume and in the latter the axial spot size is reduced by simultaneous illumination from back and front. Currently, both procedures seem difficult to apply for technical surfaces. For this field of application, the need for scanning either sample or optical system and the resulting slow measurement rate is a major drawback of confocal microscopy. A variant that allows fast, confocal measurements on technical surfaces uses a Nipkow disk which consists of pinholes arranged in a spiral shape (typical values are for instance 20 µm pinholes at 200 µm distance [98Jor]). The rotating disk is illuminated by a plane wave and acts as a scanning multiple-point light source that is imaged into the focal plane of the objective lens (Fig. 5.1.18); i.e., here not only one but many focal spots are imaged onto the object. After reflection or scattering, each Nipkow pinhole acts as its own detector pinhole. The depth-discriminated xy-information is imaged onto a CCD camera. During one rotation of the disk an xy-section of the sample is obtained in video real-time. By an additional z-scan of the specimen a stack of n camera frames is acquired each corresponding to a certain axial object Landolt-B¨ ornstein New Series VIII/1C

424

5.1.3 Methods and instrumentation b

zn z n -1 • • • z2 z1

Intensity

a

[Ref. p. 439

FWHM

z1

z

zn

yl

Fig. 5.1.19. Reconstruction of topography from frames: (a) frame stack, (b) single-pixel intensity.

xk

20 0

y [ µm]

15 0

1 00

50

0 0

50

100 15 0 x [µ m]

200

Fig. 5.1.20. Measurement example for confocal microscopy: wafer topography.

position (Fig. 5.1.19). The 3D-topography can be reconstructed with a resolution of about 1 % of the FWHM (Fig. 5.1.20). A reliable method to obtain the topographical height is to determine the center of gravity of intensity values above the FWHM. To increase the light yield, microlenses might be aligned above the holes with the focal spots precisely in the pinholes. A very stable setup with high luminous intensity can be obtained by just replacing the Nipkow pinholes by microlenses aligned in spiral shape [00Tiz]. However, lateral and vertical resolution may be slightly reduced. Even the vertical scanning becomes dispensable if chromatic length aberrations are utilized to get an image where the height information is coded in the color. The basic idea is that for an optical system with chromatic length aberrations, and spherical aberration corrected for each wavelength, each color has its own focal plane. For a multicolor light source, the confocal image of a topography becomes colored and every color indicates a certain height range [94Tiz]. The measurement range is given by the chromatic length aberration as large as 10 µm. Furthermore, a chromatic principle works with diffractive elements by sequential selection of the different wavelengths of the light source [00Tiz]. In an experimental set-up (Fig. 5.1.21) four stabilized semiconductor diode lasers were used as light sources. The diode lasers were focused by graded-index lenses into single-mode optical fibers and coupled together to form a single point source. In the microlens arrangement, 100 × 100 microlenses were used. They are a kind of complementary to the Nipkow-disk arrangement. While in the Nipkow-disk arrangement one objective and many pinholes are used, the microlens arrangement applies many objectives (i.e. the microlenses) and one pinhole. A tube lens images the microlens pupils onto a CCD-camera. In particular, diffractive microlenses have a large chromatic aberration together with an inverse proportionality between focal length and wavelength when operating in the first order. It was found by computer simulations that the optimal distance between two wavelength focal lengths matches the FWHM, hence the wavelengths chosen were 750 nm, 780 nm, 810 nm, and 840 nm.


Ref. p. 439]


4 Laserdiodes (l1... l4 )

Tube lens

425

Pinhole CCDcamera

Beam splitter 1 2 3 4

Diffractive microlens-array

I

Df(chro.) l

Fig. 5.1.21. Confocal chromatic microscopy with microlenses.

5.1.3.4 Near-field microscopy Diffraction, scattering and even reflection of radiation of frequency ν0 = c0 λ−1 at material structures have the potential to produce spatial frequencies that can’t propagate in the surrounding medium of refractive index n. These are bounded to the structure and form an exponentially damped field: the near field. It applies to all spatial frequencies that exceed the inverse free-space wavelength of the surrounding, i.e. |f | >

n . λ

(5.1.25)

The spatial-frequency-dependent damping constant is obtained from (5.1.10). If either source or detector are situated in the near field of the structure, spatial frequencies exceeding the limit (5.1.25) can also contribute to the image. By this means the numerical aperture is exceeded beyond the free-space limit of 1. The best known and manageable near field is the evanescent field stemming from total reflection. It is generated by a plane wave incident at angle α – beyond the critical angle of total reflection – from a high-index medium of refractive index n onto a plane interface to a low-index medium n0 . The evanescent field at the interface declines exponentially into the lower-index medium according to (5.1.10) 1/2 U (x, z) = exp −k0 n2 sin2 α − 1 z exp (i nk0 sin α x) . (5.1.26) A few degrees above the critical angle the penetration depth (1/e amplitude) z1/e =

λ

1/2

2π n2 sin2 α − 1

(5.1.27)

becomes already small compared to the wavelength. However, if a second interface is brought within this distance to the first, energy couples into the second medium. The evanescent wave at the interface excites a propagating wave in the second medium with an amplitude of approximately U (x, d). Figure 5.1.22 shows a rigorous numerical simulation of this process – the frustrated total reflection. Note the evanescent field in the air gap and the frustrated total-reflected field in the lower part. It is considerably stronger below the smaller air gap. Conversely, the interference contrast in the upper part is stronger for the larger gap, because there the reflected amplitude is almost unaffected. Because of the close analogy to electron tunneling, frustrated total reflection is called photon tunneling. Of course, the evanescent wave may also be absorbed or scattered at the interface. Landolt-B¨ ornstein New Series VIII/1C

426


[Ref. p. 439

incident wave

z[l]

glass

air glass

Fig. 5.1.22. Frustrated total reflection at an air (n = 1)–glass (n = 1.5) step in s-polarization according to a rigorous simulation. The gray scale presents the amplitude of the real part of the electric field.

x [l]

a

b Oil immersion lens

c

n n Transducer

Low-NA dry lens

High-NA dry lens

n0

Hemispherical lens

a a'

n

d

Aplanatic lens

Fig. 5.1.23. The three basic configurations for PTM [90Gue, 92Kin, 97Cor].

The near-field effects are deployed in Photon-Tunneling Microscopy (PTM) [90Gue, 92Kin] (also called frustrated total reflection microscopy or solid-immersion-lens microscopy [92Kin]). A thorough review can be found in [97Cor]. The various forms of PTM are depicted in Fig. 5.1.23. In Fig. 5.1.23a [90Gue] an immersion-type microscope lens is used to illuminate conically a glass-air interface (the “transducer”) under angles beyond the critical angle. An evanescent field forms at the bottom side of the transducer. Figure 5.1.23b [92Kin] uses a high-NA dry objective and a hemispherical lens. The need for a high-NA objective is relaxed in Fig. 5.1.23c [97Cor], where an aplanatic lens is used instead. The aplanatic lens is free of spherical aberration and also satisfies the sine condition exactly, i.e. it is free of coma. Furthermore, it provides a magnification of 2.3. The basic formulas are [94Shi] d=

n+1 r n

and β =

n sin α = 2.3 , sin α

(5.1.28)

where r is the radius of the sphere forming the aplanatic lens. Provided a calibration is performed that relates reflected intensity and gap width according to (5.1.27), three-dimensional images can be obtained. Using such a calibration, PTM was applied, for instance, for monitoring and measurement of latent resist images (i.e. photoresist patterns after exposure, but before development) [98Lid]. The lateral resolution of PTM is only moderately improved. It is determined by the xdependence in (5.1.26) and therefore increased by a factor of n compared to far-field microscopy. In Fig. 5.1.23a and b, the lateral resolution limit does not exceed the resolution obtained with an immersion fluid of refractive index n. In Fig. 5.1.23c the refractive power of the aplanatic lens Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]

a


2500 nm

Vitreous Optic nerve Iris Lens

Fig. 6.1.4. Cross-section of the human eye and optical properties [97Hen].

6.1.3.2 Threshold limit values and laser classification To avoid hazards related to laser radiation, threshold limits for exposure have been internationally established, below which it is believed to be safe. These limits, termed “Maximum Permissible Exposure” (MPE), are tabulated as a function of the wavelength and the exposure time. Due to the transmission properties of the eye and the severity of the retinal damages for radiation from 400 nm to 1400 nm, there are separate MPE’s for the eye and the skin (Fig. 6.1.5). In addition, source-size-dependent (collimated or extended source) and wavelength-dependent correction factors are included. MPE’s are given in the standard IEC 825 [96IEC] and standards derived from it. To simplify the hazard assessment related to a distinct laser system, the lasers are classified in the order in which the maximum intensity emitted by the laser can affect the human body. The threshold limit values are termed “Accessible Emission Limit” (AEL), see Fig. 6.1.5. Lasers are classified in 4 distinct laser classes, whereby Class 3 is subdivided into Class 3A and 3B. The AEL’s for each laser class are given in tabular form (IEC 825, CDRH) [96IEC], presented as a function of emission time and wavelength. Wavelength- and time-dependent correction factors are also included.

MPE

AEL

Maximum permissible exposure

Accessible emission limit

Skin

400nm-1400nm

Eye

Influencing variables: - wavelength - operational mode - irradiation geometry - time of irradiation

Laser classification

Fig. 6.1.5. Threshold limit values MPE and AEL [98Toe].


Ref. p. 477] Class 1: Class 2: Class 3A: Class 3B: Class 4:

6.1 Laser safety

465

Inherently safe under any condition. Very low emission of the laser device itself or enclosure of lasers with higher outputs. Covers only visible radiation. Possible eye hazard but safe for accidental momentary viewing. Eye protection normally by natural blink reflex (0.25 s). Eye hazard if magnifying viewing instruments are used to view the beam. For visible radiation, safe for momentary viewing (0.25 s). Hazard to the unaided eye. Viewing diffuse reflection may be safe. Exposure to concentrated beams can exceed the safety threshold limit for skin. Eye and skin hazard. Diffuse reflections may be hazardous. Possible fire hazard and danger of explosions.

6.1.3.3 Safety measures In dealing with the class of the laser system, particular technical as well as administrative safety measures have to be provided: – marking of lasers according to the laser classes (e.g. warning signs), – design of the laser (e.g. protective enclosures, safety interlocks, key control, warning light, beam attenuator), – definition of laser-controlled areas (for Class 3b and Class 4 lasers), – protective wear (e.g. protective eye wear), – education (e.g. laser safety officer, operator). Requirements for safety measures or measuring equipment for laser radiation are given in the standard IEC 825 and further standards more related to certain measures or devices (e.g. EN 31553, “Laser processing machines” [98EN1], DIN 5335 “Guards for laser workplace” [93DIN], EN 61040 “Power and energy measuring detectors” [92EN1] etc.). Besides, the qualification of certain safety measures has been subject of many investigations (e.g. screens for laser radiation [96Alu]). To prevent eye hazards in cases where there is the risk of being exposed to levels exceeding the MPE, protective filters have to be used. These filters reduce the irradiance to MPE levels or below by ensuring sufficient transmission in the visible range. These filters are used both within enclosures (Class 1) for viewing the laser material processing zone or for protective eye wear. Generally, two types of filters can be distinguished: absorbing filters and coated filters reflecting the radiation. The determination and calculation of necessary optical densities of the filters and further requirements on the properties of both for filters in enclosures and protective eyewear are addressed in e.g. IEC 825, EN 207, EN 208 [96IEC, 93EN1, 93EN2].

6.1.3.4 Hazard distances The assessment of hazards within a certain area exposed to laser radiation can be realized by determination of the distance over which a propagating laser beam is hazardous [80Sli, 89Sut, 97Hen]. The distance to the laser source, where the maximal irradiance of the laser beam is equal to the MPE, is termed “Nominal Ocular Hazard Distance” (NOHD). This concept includes different beam geometries (collimated beam/divergent beam, beam propagation beyond focal spot of focused beams, optical fibers and point or extended sources by direct or diffuse reflection). The area around a laser for which an eye hazard exists is called the “Nominal Ocular Hazard Area” (NOHA). The range of the NOHA can be narrowed down by suitable guards or screens. Laser hazards may not only arise from the direct beam, but also as a result of reflection of the beam from irradiated surfaces. The amount of reflected radiation depends on the reflectance Landolt-B¨ ornstein New Series VIII/1C

466

6.1.4 Secondary hazard potentials and safety measures

[Ref. p. 477

level of the surface. The geometry of the reflected beam is also influenced by the surface. Specular reflecting plane surfaces redirect the beam, whereas curved surfaces may increase or decrease the reflected beam divergence. Diffuse reflections, caused by mat surfaces, redistribute the reflected radiation in all directions. Due to the complete change of beam geometry, the irradiated area on the surface must be considered as a secondary source of radiation (according to area size as point or extended source).

6.1.3.4.1 Specular-reflected beam In cases where an accidentally misdirected beam of a > Class 3 laser is specularly reflected, exposures can easily exceed the MPE’s. Due to the accidental situation, exposure times will usually be only a few seconds (respond of beam control etc.). Care must be taken to have proper beam alignment and a beam control [93Hie].

6.1.3.4.2 Diffuse-reflected beam The level of diffuse scattered CO2 -laser radiation in a typical environment of laser processing with a 2.5 kW laser is found to be low, under normal conditions [93Hie]. Due to the fact that diffuse scattered radiation can last over a long time if protective measures are not taken, longtime exposure (8 h) has to be considered. Nevertheless, in cases of misalignment and a highly reflective workpiece surface, high levels of radiation can occur, so protection is recommended. For Nd:YAG lasers, protection against scattered light is essential, despite the lower reflectance of most materials for 1.06 µm, because of the hazardous effects on the eye and the accordingly low MPE for 1.06 µm. When using excimer lasers, the diffuse scattered radiation can easily exceed the MPE. Total enclosures with UV-absorbing windows are therefore recommended [93Hie].

6.1.4 Secondary hazard potentials and safety measures 6.1.4.1 Laser system and components 6.1.4.1.1 Electrical safety Lasers are generally electrically powered. Depending on the laser system, respectively the electrical components, there are different hazards. Especially pulsed laser systems using high voltage for excitation give rise to serious hazards. These lasers (CO2 ) use large capacitor banks to store the electric charge. Precautions must be taken, since high voltage remains even after the system has been switched off and disconnected from the main power supply. During maintenance, the safe discharge of capacitors (short-circuited, grounding) and the prevention of a direct contact between charged parts and the human body (e.g. interlocks) have to be ensured [80Sli, 89Sut, 97Hen]. Furthermore, the generation of electromagnetic fields can lead to potential hazards. Many laser systems use High electromagnetic Frequency fields (HF) for excitation, some also generate radiation in the Low Frequency (LF) and microwave region. In cases of insufficient shielding, electromagnetic radiation is emitted and can lead to health risks for operators. To prevent harmful radiation, parts emitting Radio Frequency (RF) radiation must be shielded by a safely grounded electrically conducting grid or net (Faraday cage). Exposure limits for LF/HF radiation are given Landolt-B¨ ornstein New Series VIII/1C

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467

in norms and standards (e.g. German DIN VDE 0848, Part2). Maximum permissible interference levels for technical components are given in the European standard EN 55011 [98EN2]. Requirements on the compatibility with other devices concerning electromagnetic radiation come from the Electromagnetic Compatibility Directive (89/336 EEC) and norms and standards (e.g. IEC 801) [89EEC3]. General requirements for the safety of electrical components of machines are addressed within EN 60204 “Safety of Machinery – Electrical Equipment for Machines” [93EN3].

6.1.4.1.2 Optical components Under normal operation conditions, hazards from materials used for laser optics do not arise. However, in cases of misalignment or pollution of the optics, absorption of the laser radiation can rapidly increase, resulting in damage of the optics [95Kre]. If optical components made of gallium-arsenide or zinc-selenide are used, absorption can lead to evaporation and bursting of the optics. Due to chemical reactions (> 200◦ C) with the ambient air, arseniuretted hydrogen (arsine) respectively hydrogen selenide can be generated. Both gases are toxic and have to be immediately extracted from the workplace to avoid health risks. The remaining particles (fumes, dust) contain toxic arsenic or arsenic oxides, respectively selenium or selenium oxides, and must also be handled with care (using personal protective equipment). Furthermore, a minor hazard results from the handling of arc and flash lamps used for optical pumping of solid-state lasers. During maintenance (replacement of lamps), exploding lamps can cause injuries, and protective wear is recommended.

6.1.4.1.3 Laser gases Laser systems can contain hazardous gases or liquids. In particular, excimer lasers shall be concentrated on, which use inert-halogen gas mixtures. In cases of leakage, the fluorine or chlorine can contaminate the workplace (as molecular halogen or hydrogen halogenide). Hazards can be easily assessed by calculating conditions for a spontaneous leakage in the halogen gas supply equipment. Assuming a laser cabin of 10 × 10 × 5 m3 (500 m3 ) the German threshold limit value for fluorine “Maximale Arbeitsplatz-Konzentration” (MAK) of 0.1 ml/m3 (also peak limit 0.1 ml/m3 according to TRGS 900) [03TRG] means a maximum of 50 ml fluorine in the workplace air. Typically the halogen gas mixture contains 5 % fluorine (rest He), which correlates with a maximum permissible leakage gas volume of 1 l. Therefore, the room or cabin where the gas supply and the laser installation are located should be equipped with gas sensors activating warning lights, emergency extraction systems and automatically controlling safety valves, which stop the gas supply. Leakage of the resonator (e.g. mirror sealings) generally causes a creeping pressure loss (about 1000 Pa/h). Since the halogen concentration is considerably lower than in the halogen gas supply and usually extraction systems are integrated in the laser system, this failure is of secondary meaning to a leakage in the gas supply. In comparison, gas leakage for CO2 lasers is less critical than for excimer lasers, due to the threshold limit value for CO2 in the workplace air, which is magnitudes higher (CO2 : 5000 ml/m3 ). Dye lasers can emit volatile substances in cases of leakage. The solvent fumes are highly inflammable, and ignition by heat, sparks or open flames must be avoided. Precautions must be taken, due to the respirability of the substances and their toxic effects on persons.


468


[Ref. p. 477

6.1.4.1.4 Handling devices The development of lasers with increasing output powers and excellent beam properties will result in constantly increasing processing velocities. Parallel to this, the technical progress of handling devices (2-D, 3-D) leads to higher positioning velocities. Hazards related to handling devices mainly result from these high feed rates. At present, typical feed rates of handling devices industrially used are several 10’s of meters per minute, and devices exceeding 100 m/min. are already entering the market [92Dau, 94Pre]. In cases of malfunctions or striking objects within the path of the devices, high deceleration values cause high mechanical stresses. This leads to high damage rates or – if people are affected – to severe injuries or the death of person. Moreover, insufficient protection of even slow moving components can cause severe bruises in cases of accidents. Due to the severity of possible accidents, special care must be taken to control both the function and position of the device as well as the area of movement, to avoid uncontrolled access of persons. Depending on the distinct application, sufficient covering of moving parts and mechanical barriers, light barriers, camera systems etc. which provide rapid emergency stops are adequate protective measures. Norms and standards related to handling components of machinery are arranged under the basic Machinery Directive and the EN 292 “Safety of Machinery”: – EN 418: “Safety of Machinery – Emergency Stop Equipment” [92EN2], – EN 349: “Safety of Machinery – Minimum Gaps to avoid Crushing or Parts of the Human Body” [93EN4], – EN 294: “Safety of Machinery – Safety Distances to Prevent Danger Zones being Reached by Lower Limbs” [93EN3] etc.

6.1.4.2 Secondary radiation The main emphasis shall be placed upon the non-coherent UV and blue-light radiation, due to their capability of causing eye hazards, and upon UV-radiation below 242 nm which generates the toxic gas ozone or other harmful substances. This secondary radiation is formed during laser processing by ionized substances (e.g. metal vapor or shielding gases), due to the very high temperatures in the interaction zone. Assessing the hazards related to secondary radiation, the bio-effects are more complex than for monochromatic radiation due to the broad frequency band from the UV to the IR. Focussing on the UV to blue-light radiation, the bio-effects are mostly photochemical in nature (photoceratitis, photoconjunctivitis, cataracts). Due to the low threshold limits in this range (TLV by ACGIH [98ACG]) and cumulative effects, the radiation (UV-C, -B, -A to blue light) may reach critical values within certain periods (Table 6.1.1) [95Hur]. Measurements for laser welding at a typical operating distance (0.5 m) show, in comparison to conventional arc welding, low total levels of radiation, but they can exceed levels typical for Tungsten Inert Gas (TIG) and Manual Metal Arc (MMA) welding. The secondary radiation increases with the laser power, and the choice of assist gas remarkably effects the radiation levels. As shown in Table 6.1.1, the maximum permissible exposure times for laser welding can be reached within seconds. Therefore, especially for high-power laser processing (welding, punching operations), UV-A and blue-light absorbing screens and protective eyewear are recommended [95Hur]. Ultraviolet radiation below 242 nm also generates ozone by dissociating oxygen in the interaction zone. Ozone is a toxic gas which has to be extracted out of the working area to avoid health risks (see also Sect. 6.1.4.4). Landolt-B¨ ornstein New Series VIII/1C

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Table 6.1.1. Secondary radiation from laser welding [95Hur]; UVeff : effective irradiance of all UVradiation, tUV : time to reach threshold limit for UV-radiation, blueeff : effective irradiance of blue light, tblue : time to reach threshold limit for blue light, butt: butt joint, BOP: bead on plate, T: T-seam, ms: mild steel, ss: stainless steel. Laserpower [kW] 9 9 9 8 2.5 2.5 2.5 2.5 9 7 6 10

Weld type butt BOP BOP butt butt butt butt butt T butt butt butt

Material

Shielding gas ms He ss He ss He ss Ar ms Ar ms He ss Ar ss He ms He AlMg5 He AlMg4.5Mn Ar/He Titanium He

UVeff [W/m2 ] 3.0 6.6 3.8 19.3 2.5 2.2 2.4 1.3 6.7 0.3 3.6 7.0

tUV [s] 10.0 4.5 8.0 1.6 12.0 13.6 12.6 22.7 4.5 100.0 8.3 4.3

blueeff [W/m2 ] 1.0 5.0

tblue [s] 100 20

5.6 0.6 0.4 0.5 0.7 1.2 0.1 0.8 2.0

18 180 220 220 400 83 1000 1300 50

6.1.4.3 Explosive atmospheres and fire hazards The use of high-power lasers always implies latent fire hazards. Demands on suitable materials for guards, screens, medical instruments, covering materials etc. have already been mentioned in Sect. 6.1.3. Care must be taken that no inflammable materials (except a minimum for use) are inside the area the laser beam can affect, under normal and faulty conditions. Special caution is required for laser applications in oxygen-enriched atmospheres, due to the higher ignition threshold of materials in this atmosphere. In this context, it shall also be pointed out that fire hazards can as well result from the filter systems for gases and particulates. Melt spatter or igniting organic gases can cause a smouldering fire of the filter material. To avoid hazards, deflector plates for melt spatter, a sufficient distance between process and filter and frequent cleaning of deposits from the extraction pipe are suitable measures. If explosive mixtures are inside the laser areas of Class 3B and Class 4 lasers, explosion protection must be ensured. Requirements are given in the explosion protection guidelines (e.g. EX-RL) [86ZH1].

6.1.4.4 Emission of gases and fumes 6.1.4.4.1 Characteristics of laser-generated air contaminants The physical basis of “Laser-Generated Air Contaminants” (LGAC) is the absorption of laser radiation in the interaction zone. Depending on the wavelength and the material properties, the energy is transformed into thermal energy or can cause direct disbonding. As a result, the material is partially melted, vaporized or sublimated and leaves the interaction zone at high velocities. Due to the thermodynamic conditions, the by-products can immediately condense into fumes or remain in the gas phase as Volatile Organic Compounds (VOC’s). If vaporization takes place in layers below the surface, the gas pressure mechanically “explodes” the material or tissue in the upper layers, and the fragments accelerate to reach high velocities.


470


[Ref. p. 477

The quantity, the kind (gaseous, particulate) and the chemical composition of the emissions generated depend mainly on the material processed, the process parameters and the process atmosphere [01Haf, 02Bar]. While VOC’s are generally airborne, the size and weight of aerosol particles determines their behavior. For usual air speeds at ambient air temperature at the workplace from 0.1 m/s to 3 m/s, particles smaller than 10 µm can be regarded as airborne [93VDI]. Molten pearls or larger fragments sediment out, due to their size and weight near the processing zone. Apart from these material-specific by-products, process-specific emissions like ozone and nitrogen oxides can be generated, independent of the material processed in certain temperature and radiation ranges. The toxic gas ozone is generated mainly by secondary radiation in the UV-range (120. . . 242 nm). Since it is thermodynamically unstable, ozone is reduced to oxygen at high temperatures. Moreover, aerosol particles generated by the laser process intensify the reduction as a catalyser. The toxic nitrogen oxides are generated at temperatures above 1000 K by oxidation of nitrogen. For industrial applications, three groups of applications can be identified, based on typical emission rates (Fig. 6.1.6) [98Haf2]. Cutting applications cause the highest emission rates. Applications with moderate emission rates are, e.g. welding, cladding and material removal. Emission rates below 1 mg/s are characteristic for laser marking and micro-machining. With the progressive use of laser systems with higher kW powers (greater than 10 kW) it can be expected that the emission rates will increase with increasing cutting speeds.

Aerosols Hydrocarbon Nitrogen oxides Ozone

100 10

0.1

10

-6

10

-7

Micromaching

-5

Marking (metals)

10

Marking (plastics)

-4

Cladding

10

Material removal

-3

Welding

10

Cutting (metals)

0.01

Cutting (plastics)

Rate of emissions [mg/s]

1

Fig. 6.1.6. Emission rates for different laser processes [98Haf2].

When laser processing metals, aerosols are the main emission products. Nitrogen oxides and ozone are two orders of magnitude smaller [98VDI]. Investigations of the fume particle morphology show that for all materials and tissues processed, the fume particles are of a spherical shape. Most of the particles, about 90 %, are smaller than 1 µm aerodynamic diameter (Figs. 6.1.7 and 6.1.8) [97VDI, 98VDI]. They exist as single particles or as agglomerates. The smallest single particles have geometric diameters of about 5. . . 10 nm. The properties of the particles are due to the formation process by vaporization and condensation.


Ref. p. 477]

6.1 Laser safety

30 nm

AlCuMg2 Ti99.4 RST37-2 X5CrNi189 St02Z275NA ST14.03

50

Relative mass frequency [%]

471

40

30

20

10

0 0

0.03

0.06

0.13

0.25

0.50

1.0

2.0

4.0

8.0

16

Aerodynamic particle diameter [µm]

Fig. 6.1.7. Size distributions of aerosols for cutting metals [98Haf2].

Pa6 PE,N PE PC PP,N PP PVC PS,N PMMA

Relative mass frequency [%]

60

40

1 nm

20

0 0

0.03

0.06

0.13

0.25

0.50

1.0

2.0

4.0

8.0

16

Aerodynamic particle diameter [µm]

Fig. 6.1.8. Size distributions of aerosols for cutting plastics [97Haf].

Because of their size, the fume particles are highly respirable (more than 80 % according ACGIH [98ACG, 93Rei]). When processing metals, fumes mainly consist of metal oxides and elements with low evaporation temperatures (e.g. Zinc, Manganese) are overrepresented compared to the chemical composition of the material processed [98VDI]. Landolt-B¨ ornstein New Series VIII/1C


300 250 200 150

Aerosole Hydrogenchlorid

100

80 PE, N PE PP, N PP PS, N PS/SB PA6 PC PVC PMMA

60

40

100 50 0

20

laser beam power

350

Total Hydrocarbons

thickness

120

cutting speed : v f = 2 m/min processing gas : synthetic air processing : 0.05 MPa gas pressure

[Ref. p. 477

material

400

Emission rate [mg/s]

Concentration in the exhaust air [mg/m 3 N ]

472

[mm] 3 3 6 3 4 3 3 3 3 3

[W] 750 750 750 750 630 630 775 630 450 500

0 PA 6

PE,N

PE

PC

PP

PP,N

PVC PS/SB

PS,N PMMA

Fig. 6.1.9. Emission rates for laser cutting of plastics [98Haf2]. The concentration in the exhaust air refers to normal conditions, i.e. 1013 hPa and 273.15 K.

During laser processing of organic materials (polymers, wood, tissue), apart from aerosol emissions, a large number of different volatile hydrocarbon compounds is also emitted [98VDI, 00Haf, 00Haf2]. As shown in Fig. 6.1.9, plastics can be roughly divided into two groups [02Bar]: – materials which mainly emit aerosols, – materials which are mainly pyrolyzed into gases. Hydrocarbon compounds are generated by chemical fragmentation of the organic molecules and the following chemical reactions. Fragments (e.g. belonging to the groups of olefines, acrylates, aldehydes, aromates etc.) usually refer to the chemical structure of the material processed [03Bar]. Polystyrene (PS) and polymethylmethacrylate (PMMA) typically represent polymers, which are broken down (retropolymerized) into their monomers styrene resp. methylmethacrylate. In most cases, stable compounds like aromatic substances (benzene, toluene, xylene) and polycyclic aromatic hydrocarbons (PAH) are generated [98VDI, 00Haf2, 01Haf]. Investigations of the particle morphology show that again most of the fume particles, about 90 %, are smaller than 1 µm (aerodynamic diameter). For some plastics, like PA 6 or woods, the particles are viscous and can easily stick together [95Fri, 97Haf, 98VDI, 01Haf]. To assess the hazards resulting from emission products and to plan safety measures, the user of a laser installation is obliged by laws to carry out a hazard analysis. According to this, all emissions have to be characterized and compared with threshold limit values for the contamination of the workplace. Especially, when processing organic materials, characterization can comprise more than 50 single compounds. Concerning the aerosols, the hazard potential of the LGAC’s arises from the respirability of the fume particles (particles in the range of 0.05. . . 1 µm remain from 40. . . 60 % in the alveoli and are taken in by the so-called cleaning cells “alveolar macrophages”) and their morphological as well as their chemical characteristics [95Mal]. Many substances emitted during laser processing are known as toxic (Mn, Zn) or as carcinogenic (Ni, Co, particle-bounded PAH) in industrial medicine. Especially when cutting stainless steel, the toxic soluble chromium (VI) and the carcinogenic insoluble chromium (VI) have to be considered [95Leu, 98VDI]. For medical applications, the possibility of virus-DNA-fragments, transported with particles, is a significant hazard potential, if they infect tissues [97Wol2, 97VDI]. Gases and volatile organic components (VOC) are inhaled and


Ref. p. 477]

6.1 Laser safety

473

affect the respiratory tract, respectively the mucous membranes. Due to their interactions with the organism, most of the substances emitted are allergic, toxic or carcinogenic. To avoid health risks for the laser operators, contamination of the workplace atmosphere must be reduced well below threshold limit values (TLV) given by occupational medicine. It is not possible to conclude the contamination of the workplace atmosphere from the emission characterizations. A value which allows the comparison of different processes and which gives answers to extraction systems to be installed is the “Nominal Hygienic air requirement Limit value” (N HL) [73IIW]. The N HL describes the amount of fresh air to be supplied to keep the threshold limit values for air contaminants: A B 3 N HL = m ˙ G · 3600 · + + ... [m /h] (6.1.1) T LVA T LVB with mG A, B, . . . T LVA , T LVB , . . .

emission rate [mg/s], emission rate of each component related to mG , threshold limit value of each component [mg/m3 ].

Assuming a homogeneous distribution of the LGAC’s in a typical workplace area with a volume of 1000 m3 , the air exchange rate for, e.g. laser cutting mild steel, is larger than 10, for stainless steel larger than 20 and for plastics more than 50 [00Haf3]. The air exchange based on natural ventilation (windows, doors) is about 0.5 [98Haf2]. Forced ventilation systems grant an exchange rate from 5 to 10. However, it has to be taken into account that a high exchange rate significantly increases the costs for heating in winter. From this data it can be concluded that direct capturing of the emissions near the point of generation is most efficient to minimize the work-air contamination [00Haf].

6.1.4.4.2 Extraction systems To avoid critical contamination of the workplace atmosphere, efficient extraction and additional room ventilation systems are necessary. In general capturing devices can be divided according to VDI 2262 into 3 types [93VDI]: – open design (e.g. capture hood, extractor tube), – semi-open design (e.g. extraction stand, extraction table), – enclosed design (e.g. encapsulation, working cabin). In all cases, the capturing device has to be positioned as near as possible to the point of the emission generation, to ensure entire removal of the emissions. Dependent on the laser application, different types of capture systems can be used [98Haf2]: – capture hoods, extractor tube, working-head-integrated system, – table extraction system, – total enclosures/cabins. Open systems like capture hoods can only be satisfactorily used for applications where there is a constant convectional flow (e.g. welding, ablating). Working-head-integrated capture systems are favorable for 3-D applications (e.g. welding, surface treatment) but the stand-off distance of the head to the workpiece has to be well-controlled to ensure both effective shielding and efficient extraction [98VDI]. Extractor tubes are typically used for medical applications. The design (funnel, half-funnel, tube) of the capture inlet depends on the procedure (endoscopic or open surgery) [97VDI]. Semi-open systems are used for applications where a directional emission flow is present. During laser cutting, most (greater than 90 %) of the emissions are released below the workpiece, due to Landolt-B¨ ornstein New Series VIII/1C

474


[Ref. p. 477

the processing gas stream [98Haf2]. Therefore, the table-integrated capture system allows a high capture efficiency for adequate extraction air volumes adjusted. Advantageous – to reduce volume flow and heat losses in winter – are small moving systems integrated in the table, which can be synchronously positioned to the movement of the laser head [00Haf]. Alternatively, if the building design permits, an enclosed working area with an emission extraction system provides the best solution, where the enclosure also isolates the operator from laser and secondary radiation hazards. It must be taken into account that the capturing efficiency also depends on the pressure loss of the filtering device. If the filter system is clogged, efficiency will drop rapidly, and contamination of the working atmosphere increases.

6.1.4.4.3 Filtration When passing the extracted air to the environment, high demands are put upon filtration systems, due to the complex mixtures of very small aerosols, gases and volatile organic components and their properties (toxicity, carcinogenity, infection). When processing organic materials, generally filtration must be divided into a pre-filtration system for aerosols and a filtration system for volatile organic components. An overview of qualified types of filtration techniques depending on industrial applications and materials processed is given in Table 6.1.2 [98Haf2]. Table 6.1.2. Proven techniques for different laser applications and materials processed [98Haf2]. Application cutting welding material removal cladding marking micromaching ∗

Material metals org. materials metals metals org. materials metals metals org. materials metals org. materials

Filtration of aerosols self-cleaning filter self cleaning/fabric filter, washer∗ self-cleaning filter self-cleaning filter self cleaning/fabric filter, washer∗ self-cleaning filter fabric filter fabric filter fabric filter fabric filter

Filtration of gases not necessary catalytic, biological, adsorptive∗ not necessary not necessary catalytic, biological, adsorptive∗ not necessary not necessary adsorption not necessary adsorption

Specific filtration technics necessary, depending on application and material processed.

For airborne non-adhesive aerosol emissions (fumes) resulting from the processing of metals, inorganic materials (ceramics, glasses) and some types of organic materials, self-cleaning surface filters provide high separation efficiencies (greater than 99.995 %) and stable pressure losses. For applications with high aerosol rates (e.g. cutting, welding) storage filters will be clogged within a short operation time, and filters have to be changed frequently. Storage filters are therefore only suitable for applications with low aerosol emission rates such as marking or micro-machining. If adhesive particles which can be generated during the cutting of plastics and woods are present, then self-cleaning surface filters will also become clogged. For these applications, special solutions where the filter is continuously pre-coated with lime powder have to be considered [98Haf2, 98VDI]. For medical applications, mostly fabric pre-filters with separation efficiencies greater than 99.995 (High-Efficiency Particulate Air (HEPA) filters) and secondary Ultra-Low-Penetrating Air (ULPA) filters are suitable for separating aerosols [97Wol1]. In all cases, the contaminated filters, respectively the separated material must be disposed of. Especially for medical applications when filters are contaminated with carcinogenic or infectious substances, special treatment of the filters (e.g. disinfection, handling) is needed [97Wol1, 97VDI]. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 477]

6.1 Laser safety

475

Laser processing of organic materials (plastics, wood, paper, human tissue) requires gas filtration to keep at or below permissible threshold limit values and to avoid bad odor. For these applications, no standard solutions can be used. In general, either – adsorption technique, – catalytic combustion or – biological filtration can be applied. The choice depends on the laser application, the properties of the filter system, the space available and the costs [00Haf2]. Adsorbing charcoal filters are mostly used, but there are effects (e.g. desorption during laser off-times, displacement of high-volatile compounds) which impair performance and which have to be controlled during operating and non-operating times [03Bar]. Systems based on catalytic combustion are useful for complex mixtures of VOC’s, but due to the energy costs, such systems are not suitable for a stand-alone laser installation. Biological air-cleaning systems based on the bio-trickling method (liquid nutrient solution) show good results for reducing LGAC’s and bad odor. The advantage of the bio-filtration systems is the low pressure loss in comparison with conventional bio-filters working with shredded bark, cleaning adaptability and the ease of control during non-operation [98Haf1, 00Haf2].

6.1.5 Risk assessment The assessment of (non-avoidable) safety and health risks at the workplace is a requirement on the employers which is defined in the framework of the European Directives (e.g. 89/391 EEC, 88/642 EEC) [89EEC1, 88EEC]. There are many different approaches to risk assessment (“WhatIf-Method”, “Failure Mode and Effect Analysis – FMEA”, “Hazard and Operability Study – HAZOP”, “Fault Tree Analysis – FTA” etc.) [95Gre, 97Hen]. Since the discussion of method details would go too far, some fundamentals shall be focused on. The risk factors to be considered in any assessment might include: – – – – – –

likelihood of incidents occurring which could cause harm, likelihood of harm being caused, should an incident occur, harm, extent of harm (number of people who could be affected), exposure period of hazard, exposure frequency.

Risk assessment should identify which factors have to be addressed to reduce the risk to an acceptable level. Records should be kept to document the factors for any particular hazard being assessed. They are the basis for structured judgement and for comparisons with future re-assessment. Risk assessment of a laser installation should both fulfill the regulatory duty and provide a useful basis for product or process improvements. It must include three major areas according to IEC 825-1: – capability of the laser/laser system to injure personnel, – environment in which the laser is used, – level of training of personnel operating the laser or those who may be exposed to this radiation.


476

6.1.6 Training and education

[Ref. p. 477

The laser risk assessment process should at least cover the following distinct stages: – establishing background information (laser, environment, norm-standards etc.), – identify hazards in distinct areas (laser process, laser installation, beam delivery), – perform risk assessment. In Europe, some methods have been tested for the suitability of laser-processing risk assessment [95Gre, 97Hen]. Moreover, software programs have been developed to facilitate risk management using computers [95Tyr].

6.1.6 Training and education Training and education in laser safety are of decisive importance to ensure safe utilization of laser technology. The international safety standard IEC 825 [96IEC] requires that all persons controlling laser systems of Class 3A, Class 3B and Class 4 (German VBG 93 [95VBG] additional: “operate Class 2 to Class 4 laser systems or have access to Class 3B or Class 4 laser safety areas (“Nominal Ocular Hazard Areas” (NOHAs))” have to be trained to an appropriate level. The training should include at least the following items: – – – – –

familiarization with systems operating procedures, proper use of hazard control procedures, warning signs etc., need for personal protection, accident reporting procedures, bioeffects of the laser upon the eye and the skin.

The standard also establishes the need for a Laser Safety Officer (LSO) within an organization where Class 3B and Class 4 laser systems are in use. According to the IEC 825, the LSO is the person, “who is knowledgeable in the evaluation and control of laser hazards and has responsibility for oversight of the control of laser hazards”. According to the VBG 93, the duties of the LSO are partially reduced (responsibilities are firstly focused on the employer) and defined as follows: – supervision of the operation of laser installations, – assistance of the employer with regard to safe operation and necessary safety measures, – co-operation with specialists for workplace safety and training on important affairs of laser protection. More detailed guidance on the duties of the LSO and safety management than in the IEC or EN standards is given by the American standard ANSI Z136.1. Also, the ANSI Z136.1 provides more guidance to the training of LSO and laser users. However, the ANSI Z136.1 tends towards describing the role of the LSO as a more executive one [96IEC, 93ANS]. In addition to the “laser-specific duties” of training and education, the framework of the European Directives 89/391/EEC (transferred to national norms) also obliges the employer to provide to employees adequate health and safety training. Beneath general instructions for workplace safety, training and education for dealing with hazardous substances (which can be generated during laser processing) should be an integral part [89EEC1]. Worldwide, many institutes are involved in the development and optimization of training and education, e.g. summarizing minimum criteria for LSO courses, optimization and harmonization of syllabuses for the education of laser staff, e.g. [93Bel, 97Ray, 97Smi].


References for 6.1

477

References for 6.1 72Wel

Welling, H., Gerhard, H., G¨ uttner, A.: Z. Phys. 253 (1972) 113.

73IIW

Swedish Delegation: Draft standard for the classification of electrodes for manual welding according to the kind and composition of their fumes, IIW-Document: VIII-543-73.

75Wel

Welling, H., Schr¨ oder, H.W., Wellegehausen, B.: Spectrosc. Lett. 8(9) (1975) 685.

80Che 80Sli

Chemikaliengesetz (ChemG) vom 16.09.1980, BGBl.I (1980) 1718. Sliney, D., Wolbarsht, M.: Safety with lasers and other optical sources, Plenum Press, 1980.

85CFR

CFR 50 (161) 1085: Food and Drug Administration/Center for Devices and Radiological Health: Performance standard for laser products, 1985.

86TAL

Technische Anleitung zur Reinhaltung der Luft (TA-Luft) vom 04.04.1986, GMBl (1986) 202. ZH 1/10: Explosionsschutz-Richtlinien (Ex-RL): Richtlinien f¨ ur die Vermeidung der Gefahren durch explosionsf¨ ahige Atmosphären. Carl Heymanns, 1986.

86ZH1

88ANS 88EEC

89EEC1 89EEC2 89EEC3 89OSH1 89OSH2 89Sut

ANSI Z-136.2: Safe use of optical fiber communications systems utilizing laser diode and LED sources, 1988. 88/642/EEC: Directive of the European Economic Community: Protection of employees against hazards by chemical, physical and biological substances at work, 1988. 89/391/EEC: Directive of the European Economic Community: Introduction of measures to encourage improvements in the safety and health of workers at work, 1989. 89/392/EEC: Directive of the European Economic Community: Machinery Directive, 1989. 89/336/EEC: Directive of the European Economic Community: Electromagnetic Compatibility Directive, 1989. Occupational Safety and Health Administration (OSHA): Respiratory protection, 29CFR 1910.134, 1989. Occupational Safety and Health Administration (OSHA): Toxic and hazardous substances, 29CFR, subpart Z, 1989. Sutter, E., Schreiber, P., Ott, G.: Handbuch Laser-Strahlenschutz, Berlin: Springer, 1989.

90BIM

Bundesimmissionsschutzgesetz (BImSchG) vom 14.05.1990, BGBl.I (1990) 880.

91OSH

Occupational Safety and Health Administration (OSHA): Guidelines for laser safety and hazard assessment, Pub 8.-1.7, 1991.

92Dau

Dausinger, F., H¨ ugel, H.: Nutzungspotentiale von Lasern in der Blechbearbeitung, Bänder/Bleche/Rohre, 7/1992. EN 61040: European Standard: Power and energy measuring detectors – Instruments and equipment for laser radiation, 1992. EN 418: European Standard: Safety of machinery – Emergency stop equipment, functional aspects, 1992.

92EN1 92EN2


478

References for 6.1

92EN3

EN 294: European Standard: Safety of machinery – Safety distances to prevent danger zones being reached by upper limbs, 1992.

93ANS 93Bel

ANSI Z-136.1: American National Standard: Safe use of lasers, 1993. Bellido, F., Montejo, J.F., Botts, M., Engstrom, H., Garcia, J., Green, M., Laitinen, H., Woolnough, R.: Proc. Industrial Laser Safety Forum ’93, United Kingdom (1993) 245. DIN 5335: German Standard: Screens for laser work places, 1993. EN 207: European Standard: Personal eye-protection, filters and eye-protection against laser radiation, 1993. EN 208: European Standard. Personal eye-protection, eye-protectors for adjustment work on lasers and laser systems, 1993. EN 60204-1: European Standard: Safety of machinery, Electrical equipment of machines, 1993. EN 349: European Standard: Safety of machinery – Minimum gasps to avoid crushing of parts of the human body, 1993. Hietanen, M., Schr¨ oder, K., Meijer, J.: Proc. Industrial Laser Safety Forum ’93, United Kingdom (1993) 115. Reist, P.C.: Aerosol science and technology, McGraw-Hill Inc., 1993. VDI 2262: VDI-Guideline: Workplace air, reduction of exposure to air pollutants, 1993. Welling, H., Mitschke, F., Steinmeyer, G.: In: Walter, H., Koroteev, N., Scully, M. (eds.): Frontiers in nonlinear optics – The Sergei Akhmanov Memorial Volume, IOP Publishing (1993) 240. Welling, H., Mitschke, F., Steinmeyer, G., Ostermeyer, M., Fallnich, C.: Appl. Phys. B 56 (1993) 124.

93DIN 93EN1 93EN2 93EN3 93EN4 93Hie 93Rei 93VDI 93Wel1

93Wel2

94Pre

Preißig, K.-U., Petring, D., Herziger, G.: Proc. SPIE 2207 (1994) 96.

95EN 95Fri

EN 292: European Standard: Safety of machinery, 1995. Friedrich, G., St¨ ahle, H.J.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 257. Green, M., Tozer, B.A.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 23. Hurup, K., Glandholm, A., Hietanen, M., Nandelstadh, P. von, Schr¨ oder, K.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 91. Kreutz, E.W., Dahmen, M., Hass, C.R., Wesner, D.A.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 83. Leummens, M., Herber, R.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 105. Malkusch, W., Rehn, B., Bruch, J., Hechler, B.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 115. Tyrer, J., Vassie, L.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 39. VBG 93: Unfallverh¨ utungsvorschrift der Berufsgenossenschaften: Laserstrahlung, Fassung von 10/95, 1995.

95Gre 95Hur 95Kre 95Leu 95Mal 95Tyr 95VBG

96Alu 96ANS 96EN 96IEC 96Nie 96Wel

Alunovic, M., Kreutz, E.W.: Abschirmungen an Laserarbeitspl¨ atzen, Wirtschaftsverlag NW, 1996. ANSI Z-136.3: Safe use of lasers in the health care environment, 1996. EN 60601-2-22: European Standard: Medical electrical equipment. Particular requirements for the safety of diagnostic and therapeutic laser equipment, 1996. IEC 845: International Electrotechnical Commission: Safety of laser products, 1996. Niemz, M.: Laser-Tissue Interactions, Berlin: Springer, 1996. Wellegehausen, B., Welling, H., Momma, C., Feuerhake, M., Mossavi, K., Eichmann, H.: Opt. Quantum Electron. 28 (1996) 267.


References for 6.1 97Haf 97Hen 97Ray 97Roc1 97Roc2 97Smi 97VDI 97Wol1 97Wol2 98ACG 98EN1 98EN2 98Gef 98Haf1 98Haf2 98Rob 98Toe 98VDI

00Haf 00Haf2 00Haf3

479

Haferkamp, H., Alvensleben, F. von, Seebaum, D., Goede, M., P¨ uster, T.: Proc. International Laser Safety Conference ’97, USA (1997) 209. Henderson, R.: A guide to laser safety, Chapman&Hall, 1997. Raymond, E.A., Tyrer, J.R.: Proc. International Laser Safety Conference ’97, USA (1997) 338. Rockwell, B.A., et al.: Proc. International Laser Safety Conference ’97, USA (1997) 159. Rockwell, R.J.: Proc. International Laser Safety Conference ’97, USA (1997) 564; Laser Accident Database, LaserNet Homepage, http://www.rli.com. Smith, J.F., Jones, J.E.: Proc. International Laser Safety Conference ’97, USA (1997) 354. VDI-TZ Physikalische Technologien: Bewertung von Abbrandprodukten bei der medizinischen Laseranwendung, 1997. W¨ ollmer, W.: Proc. International Laser Safety Conference ’97, USA (1997) 373. W¨ ollmer, W.: Proc. International Laser Safety Conference ’97, USA (1997) 383. American Conference of Governmental Industrial Hygienists (ACGIH), Threshold limit values for chemical substances and physical agents and biological exposure indices, 1998. EN 31553: European Standard: Laser and laser-related equipment, 1998. EN 55011: European Standard: Industrial, scientific and medical radio-frequency equipment, radio disturbance characteristics, 1998. Gefahrstoffverordnung (GefStoffV), Fassung vom 12.06.1998, BGBl.I (1998) 1286. Haferkamp, H., Alvensleben, F. von, Goede, M., Barcikowski, S., Daniel, S.: Z. Wasser, Boden, Luft (WLB), Februar 1998. Haferkamp, H., Bach, Fr.-W., Goede, M., P¨ uster, T., Seebaum, D.: Emissions generated during laser cutting and safety precautions, IIW-Document I-1040-96 41(3) 1998. Robinson, K.: Photonics Spectra, October 1998 (1998) 92. Tönshoff, H.K., Lubatschowski, H., Goede, M.: Vortragsband 25, Sicherheitsfachtagung Krankenhaus, Medizinische Hochschule Hannover (1998) 143. VDI-TZ Physikalische Technologien: Sicherheitstechnische und medizinische Aspekte bei der Laserstrahlmaterialbearbeitung, 1998. Haferkamp, H., Bunte, J., Barcikowski, S.: Furnier Magazin, Dezember 2000 (2000) 18. Haferkamp, H., Goede, M., P¨ uster, T., Barcikowski, S.: Proc. USC-TRG Conf. on Biofiltration 2000, Los Angeles, CA, USA, 19.–20. Oct. 2000 (2000) 115. Haferkamp, H., Goede, M., Barcikowski, S., Feld, A.: Joint International Congress and Exhibition “Electronics Goes Green 2000+”, Berlin, 11.–13. Sept. 2000, Proc. Vol. 1, Technical Lectures, (2000) 613.

01Haf

Haferkamp, H., Goede, M., Barcikowski, S.: LaserOpto 04/01, August 2001 (2001) 68.

02Bar

Barcikowski, S., Goede, M., Ostendorf, A.: Proc. HPL 2002, 1st International Symposium at LAMP 2002, 27.–31. May 2002, Osaka, Japan (2002) 459.

03Bar

Barcikowski, S., Bunte, J., Haferkamp, H., P¨ uster, T., Sattari, R.: Proc. Expo Laser 2003, March 2003, Ancona, Italy (2003) 124. Technische Regeln f¨ ur Gefahrstoffe (TRGS), Kissing: WEKA Fachverlag, Februar 2003.

03TRG


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Densities of Aromatic Hydrocarbons (Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology , Vol 8)

Adsorption on Surfaces and Surface Diffusion of Adsorbates (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Collisions of Electrons with Atomic Ions (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Elementary Particles, Nuclei and Atoms)

Science and Technology of Polymers and Advanced Materials

Strength of Materials (Materials Science and Technologies Series)

Handbook of Sputter Deposition Technology: Principles, Technology and Applications (Materials Science and Process Technology Series)

Handbook of Contamination Control in Microelectronics: Principles, Applications and Technology (Materials Science and Process Technology Series)

Laser Processing of Materials: Fundamentals, Applications and Developments (Springer Series in Materials Science)

Advanced membrane technology and applications

Advanced Dairy Science and Technology

Advanced Dairy Science and Technology

Composite Materials: Science and Applications

Composite Materials: Science and Applications

Selected Systems from Al-Fe-V to Al-Ni-Zr (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Photonic Crystal Fibers: Properties and Applications (Springer Series in Materials Science) (Springer Series in Materials Science)

Photonic Crystal Fibers: Properties and Applications (Springer Series in Materials Science) (Springer Series in Materials Science)

Photonic Crystal Fibers: Properties and Applications (Springer Series in Materials Science) (Springer Series in Materials Science)

Advanced Functional Materials

Functional Materials and Biomaterials

Laser physics and applications

Research and Trends in Data Mining Technologies and Applications

Functional Materials And Biomaterials

Functional Organic Materials: Syntheses, Strategies and Applications

Laser Applications (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Advanced Materials and Technologies)

Interactions of Photons and Electrons with Atoms (Numerical Data and Functional Relationships in Science and Technology - New Series)

Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology Part 2

Virial Coefficients of Pure Gases (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Advanced Piezoelectric Materials: Science and Technology

3d, 4d and 5d Elements, Alloys and Compounds (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Advanced Web Technologies and Applications

Smart Materials and New Technologies

Densities of Aromatic Hydrocarbons (Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology , Vol 8)

Adsorption on Surfaces and Surface Diffusion of Adsorbates (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Collisions of Electrons with Atomic Ions (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Elementary Particles, Nuclei and Atoms)

Science and Technology of Polymers and Advanced Materials

Strength of Materials (Materials Science and Technologies Series)

Handbook of Sputter Deposition Technology: Principles, Technology and Applications (Materials Science and Process Technology Series)

Handbook of Contamination Control in Microelectronics: Principles, Applications and Technology (Materials Science and Process Technology Series)

Laser Processing of Materials: Fundamentals, Applications and Developments (Springer Series in Materials Science)

Advanced membrane technology and applications

Advanced Dairy Science and Technology

Advanced Dairy Science and Technology

Composite Materials: Science and Applications

Composite Materials: Science and Applications

Selected Systems from Al-Fe-V to Al-Ni-Zr (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Photonic Crystal Fibers: Properties and Applications (Springer Series in Materials Science) (Springer Series in Materials Science)

Photonic Crystal Fibers: Properties and Applications (Springer Series in Materials Science) (Springer Series in Materials Science)

Photonic Crystal Fibers: Properties and Applications (Springer Series in Materials Science) (Springer Series in Materials Science)

Advanced Functional Materials

Functional Materials and Biomaterials

Laser physics and applications

Research and Trends in Data Mining Technologies and Applications

Functional Materials And Biomaterials

Functional Organic Materials: Syntheses, Strategies and Applications

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