Ralf Riedel (Editor)

Handbook of Ceramic Hard Materials

@WILEY*VCH

Related titles from WILEY-VCH

M. Swain (Ed.) Structure and Properties of Ceramics ISBN 3-527-26824-3 R. J. Brook (Ed.) Processing of Ceramics Part I: ISBN 3-527-26830-8 Part 11: ISBN 3-527-29356-6J. Bill, F. Wakai, F. Aldinger Precursor-Derived Ceramics ISBN 3-527-29814-2

Ralf Riedel (Editor)

Handbook of Ceramic Hard Materials

@WILEY-VCHWeinheim . New York . Chichester . Brisbane . Singapore . Toronto

Editor: Prof. Dr. Ralf Riedel Fachgebiet Disperse Feststoffe Fachbereich Materialwissenschaft Technische Universitit Darmstadt PetersenstraRe 23 64287 Darmstadt Germany

This book was carefully produced. Nevertheless, authors, editor and publisher do not warrant the information contained therein to be free of errorb. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No. Applied for A catalogue record for this book is available from the British Library Deutschc Bibliothek Cataloguing-in-Publication Data: A catalogue record for this publication is available from Die Deutschc Bibliothek ISBN 3-527-29912-6

C WILEY-VCH Verlag GmbH, D-69469 Weinheiin (Federal Republic of Germany), 2000Printed on acid-free and chlorine-free paper All rights reserved (including those of translation in other languages). N o part of this book may be reproduced in any form by photoprinting, microfilm, or any other means nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically maked as such, are not to be considered unprotected by law. Composition: Alden Bookset, Oxford Printing: betz-druck, Darmstadt Bookbinding: Buchbinderei Osswald, NeustadtiWstr. Printed in the Federal Republic of Germany~ ~

This book is dedicate to

Ute, Vincent, Lorenz and Marlene

Preface

With increasing demand for improved efficiency of engines, plants and production processes, ceramics have gained great importance as structural engineering materials in recent years. Within the group of the so called advanced materials, carbon in form of diamond or diamond-like structures, carbides, nitrides and borides have reached an outstanding position due to their excellent hardness and thermo-chemical and thermo-mechanical properties. The distinct covalent bonding of the aforementioned structures positively influences their hardness and their tribological behavior. Moreover, a series of oxides such as stishovite, a high pressure modification of silica, or boron sub-oxides have been recently discovered to exhibit high hardness apart from the well known alumina. There is presently much effort in basic science and applied research to work on novel ceramic hard materials denoted as super- or ultra-hard materials that can compete with the hardness of conventional diamond. Aim and scope of the research in this field is to develop hard materials with superior mechanical and chemical properties and with similar hardness. Moreover, calculations of properties of hypothetical carbon nitrides like C3N4 indicated that there might be compounds exhibiting even higher hardness values than that of diamond. The low-temperature synthesis of diamond and cubic boron nitride on the one hand as well as the successful research on new carbon nitrides on the other hand have caused an enormous impact around the world on both the basic science and the technological development of these novel ultra-hard materials. With the present book we wish to review comprehensively and concisely the state of the art concerning the structure, synthesis, processing, properties and applications of ceramic hard materials in general. In particular, the synthesis, modeling and properties of novel hard materials like binary carbon nitrides, ternary boron carbonitrides and others are also addressed. It is the aim of this reference book not only to reflect the state of the art and to give a sound review of the literature, but to delineate the underlying concepts and bearing of this interdisciplinary field. With the present edition we wish to show that the field of hard materials research and development has to be recognized into the wider context of chemistry, physics as well as materials science and engineering. The book is organized in two volumes and three parts, covering the structure and properties of ceramic hard materials (Volume 1, Part I), synthesis and processing (Volume 1, Part 11) as well as the typical fields of applications (Volume 2, Part 111). Volume 1 starts with an introduction into novel ultra hard ceramics including diamond and diamond-like carbon, carbon nitrides and silicon nitrides as well as boron containing carbides, nitrides and carbonitrides. Here we wish to recognize the great fundamental and technological challenge of developing new superhard

VIII

Preface

materials which can compete with the hardest counterparts such as diamond and cubic boron nitride. In dealing with properties, the first Chapter in Part I is then devoted to the structure of crystalline and amorphous ceramic hard materials. The structural features are responsible in particular for the intrinsic materials properties such as melting point and hardness. It has been found that in many cases the hardness of a crystalline substance correlates with its melting point. Therefore, detailed knowledge of the 3dimensional arrangement of the atoms is required to understand the materials behavior under certain conditions. More details of the individual crystal structures with respect to a 3dimensional view can be found on our hard materials homepage under the web address www.hardmaterials.de. Phase transitions and materials synthesis under high pressure in laser heated diamond cells is the topic of the continuing Chapter. The materials behavior under high pressure and temperature is of fundamental interest for the synthesis of hard materials since many of the ultra-hard substances like diamond, cubic boron nitride or stishovite are formed naturally or synthetically under these harsh conditions. The next three Chapters are concerned with the mechanical behavior and corrosion of ceramic hard materials and their relation to microstructure. This correlation is an important feature since hardness is not only governed by the intrinsic atomic structure of the respective material but also to a great extend by its polycrystalline nature. Therefore, the grain morphology and grain boundary chemistry play a decisive role in the materials response under environmental or mechanical load. In the following Chapter transition metal carbides, nitrides and carbonitrides are discussed with a focus on their structure and bonding, thermodynamic behavior as well as on their physical and mechanical properties. Part I is then completed by two Chapters which deal with the theoretical design of novel sp2-bonded carbon allotropes and novel superhard materials based on carbon and silicon nitrides. These Chapters tribute to the fact that with proceeding computerization the number of calculated novel solid structures that led to the prediction of new materials with hardness comparable to or exceeding that of diamond has increased enormously in recent years. Part I1 continues with the synthesis and processing of ceramic hard materials. Since the conventional powder technological synthesis and processing of ceramics has been treated in a large number of published review articles here we concentrate on novel synthetic routes that provide ceramic hard materials. Consequently, six Chapters report on i) directed metal oxidation, ii) self-propagating high temperature synthesis, iii) hydrothermal synthesis of diamond, chemical vapor deposition of diamond (iv) and cubic boron nitride (v) films and finally vi) the polymer to ceramic transformation. All these processes are particularly suitable for the formation of refractories with high hardness. Part I1 is then closed by a Chapter on nano structured superhard materials. In the course of this work high hardness is achieved by microstructural control rather than by the synthesis of a distinct crystal structure. In Volume 2 ceramic hard materials are highlighted in the light of their applications. Chapter 1 of Part I11 concisely reviews the history of diamond and diamondlike super abrasive tools while Chapter 2 and 3 are concerned with the application of chemical vapor deposited diamond and diamond-like carbon films. These sections

Prejace

IX

include the synthesis of optical grade CVD diamond windows and discuss their physical and mechanical properties. The most important and wide-spread ceramic hard materials are based on alumina. Chapter 4 reports on the processing developments to increase the hardness of alumina based ceramics for grinding and cutting applications. Silicon carbide and silicon nitride materials are the most technologically important non-oxide compounds and have gained great significance in the field of cutting ceramics and are treated in Chapters 5 and 6. Boron-based ceramics are a further group of either established or candidate materials with extreme hardness. Therefore, Chapter 7 deals with boron carbide or transition metal borides like titanium diboride and their distinct properties and applications. In Chapter 8, classical hard metals comprised of tungsten carbide as the hard phase and cobalt as the binder phase are discussed. Volume 2 is finally completed by a data base (Chapter 9) containing approximately 130 hard materials including carbides, nitrides, borides, silicides and oxides. The data base references the crystal structure, physical properties like melting point and density, mechanical properties (Youngs modulus, micro hardness) and oxidation resistance of the respective compounds. Future developments of novel hard materials such as the recently discovered intermetallic phase A1MgBl4will be updated on our internet homepage www.hardmaterials.de. In closing these introductory remarks, I would like to emphasize that the special chance to place a summary of the outstanding expertise on the field of present hard materials research and development would not have been possible without the great enthusiasm and commitment of all the colleagues who contributed in the writing of this two volume set. I am grateful for their enormous efforts in compiling a fascinating series of articles imparting depth insight into the individual fields of modern hard materials research. Finally, I wish to thank the Wiley-VCH Editors Peter Gregory and Jorn Ritterbusch for encouraging me in the preparation of this book and for their continuous support throughout the editorial process. Ralf Riedel March 2000 Darmstadt

Foreword

One of the clearest hierarchies in materials science and engineering is provided by the property of hardness. There are, of course, many properties where remarkable differences exist between groups of materials. An example is provided by electrical conductivity where a ratio of 10l8can be readily found; with electrical conductivity, however, the different materials do not come into direct competitive opposition. In the case of hardness, the very value of this property lies in the ability of one material to demonstrate a higher place in the hierarchy than another; the one material is used in effect to overpower the other. The existence of this hierarchy, which has been long recognised in the traditional measurement scale for the property, has direct relation to applications. In any use of materials it is important to be able to shape them to be fit for purpose; where the shaping process involves some type of machining, as it most commonly does, then the property of hardness becomes the unambiguous figure of merit. It is for these reasons that there has been long standing and productive interest in hard materials, in their design, in their fabrication, in their use, and in the underlying science and engineering. It is thorougLlv in keeping with this tradition of research relevant to application that the present book brings together a set of authoritative reviews of the progress which has been made. The organisation of the book is a direct reflection of the logic which has been used in developing hard materials. One of the great attractions of the subject has been the close link that exists between hardness on the one hand and the bonding and structure of the material on the other. The link between these two has proved to be one of the best foundations on which to base materials development. The link is a central theme in the first part of the book where fine examples are given of the rich contribution which has been made and which continues to be made by fundamental studies of bonding and structure to materials performance. It has long been recognised that the very aspect of their extreme resistance to deformation would make it a particular challenge to manufacture hard materials in reliable and cost-effective ways. It is here that the materials community has shown itself to be imaginative and forward looking in seeking innovative fabrication routes. These are well presented in the second part of the book where specific attention is given to the paths which can be used to assemble materials of precisely defined form without sacrifice of their characteristic mechanical resilience. The most striking aspect of hard materials, however, is the direct link to applications. This link has brought an unusual degree of purpose to materials development which has enjoyed the benefits of being conducted in full recognition of the target to be reached. It has also meant that the progress made in research can be rapidly evaluated since the testing procedures relate so directly to the end use. The third

XI1

Foreword

part of the book accordingly gives close accounts of the performance of the different classes of hard materials in the applications context. The contributors to this text are to be congratulated on bringing their many disciplines to bear on this central theme. Materials science is well known to undergo fashions as materials are developed and discarded and indeed as sectors of application grow and decline. The one requirement which will remain is that the forming and shaping of materials will always be necessary whatever the eventual sector of application. We can accordingly be confident that the long history of hardness studies, not least in the last two hundred years from the carbon tool steels, to high speed steels, to stellite, to tungsten carbide, to cermets, to ceramics, and now to diamond, boron, nitride and other special systems, will be continued with informed imagination and with creative innovation. The present book is a splendid platform on which to base such future development. Richard Brook January 2000 Oxford, UK

Contents

List of Contributors XXVII List of Symbols XXXIII List of Abbreviations XXXIX Introduction: Novel Ultrahard Materials A . Zerr and R. Riedel Introduction XLV Hard Materials XLVI Hardness XLVII Carbon-based Hard Materials L Diamond LII Diamond-like and Amorphous Carbon LV Novel Hypothetical Three-dimensional Carbon Phases Fullerenes LIX Carbon Nitride (C3N4) LIX Boron-based Hard Materials LXIV Boron Nitrides LXIV Boron-rich Boron Nitrides LXVII Nitrogen-rich Boron Nitride LXVIII Boron Carbonitrides (B,C,N,) LXVIII Boron Suboxides LXXI . Silicon-based Materials LXXI Concluding Remarks LXXII Acknowledgement LXXIII References LXXIII Part I Structures and Properties Structural Chemistry of Hard Materials W . Jeitschko, R. Pottgen, and R.-D. Hoffmann Introduction 3 Diamond and Diamond-Related Structures 5 The Crystal Structure of Diamond 5 The Isoelectronic Compounds c-BN and S i c 6 Crystal Chemistry of Borides and Boron Carbides 8 The Structures of Transition Metal Carbides 12 Silicides and Silicide Carbides of Transition Metals 20

LVI

11.11.2

1.2.1 1.2.2 1.31.4

1.5

XIV 1.6 1.6.1 1.6.2 1.6.3 1.7 1.7.1 1.7.2 1.8

Contents

Nitrides 23 Nitrides of Main Group Elements 24 Transition Metal Nitrides 25 Perspectives: Nitridosilicates 29 Oxide Ceramics 30 Hard Ceramics of Main Group Elements 30 Transition Metal Oxides 32 Amorphous Hard Materials 36 References 37 Phase Transitions and Material Synthesis using the C02-Laser Heating Technique in a Diamond Cell A . Zerr, G. Serghiou, and R . Boehler Introduction 41 Technique of C02-Laser Heating in a Diamond Anvil Cell 42 Sample Assemblage in a Diamond Anvil Cell 42 Pressure Conditions in the Sample Volume 43 Experimental Set-up for C02-Laser Heating in a Diamond Anvil Cell 44 Temperature Determination 45 Temperature Stabilization 45 Radial Temperature Gradients 48 Raman and Fluorescence Spectroscopic Analysis of Samples in a Diamond Anvil Cell 48 Determination of Melting Temperatures at High Pressures 49 Melting of Cubic BN at 10 GPa 49 Melting Temperatures of Materials Relevant to the Earths Lower Mantle 51 Phase Diagrams, Decomposition Reactions, and Stability of Solids at High Pressures and Temperatures 54 Coesite-Stishovite Phase Boundary 55 High Pressure and Temperature Phase Diagram and Decomposition Reactions in a Ternary System 56 Stability of a Perovskite Oxide with Respect to its Component Oxides 59 C02-laser Heating Experiments on Organic Compounds 60 Conclusion 62 Acknowledgments 62 References 62 Mechanical Properties and their Relation to Microstructure D. Sherman and D . Brandon Introduction 66 Applications and Engineering Requirements 66

2

2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 2.3 2.3.1 2.3.2 2.4 2.4.1 2.4.2 2.4.3 2.5 2.6

3

3.1 3.1.1

Contents

XV

3.1.2 3.1.3 3.1.4 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.4 3.5 3.5.1 3.5.2 3.5.3 3.6

Bulk Components 68 Coatings 70 Engineering Requirements 70 Principal Mechanical Properties 7 1 Elastic Modulus 71 Strength 72 Fracture Toughness 74 Hardness 79 Mechanical Testing of Hard Materials 81 Elastic Modulus 81 Fracture Strength 8 1 Fracture Toughness 83 Hardness 84 Indentation Toughness 86 Erosion, Wear and Scratch Tests 89 Microstructural Parameters and Mechanical Properties 9 1 Failure Mechanisms 94 Creep Behavior 94 Mechanical Fatigue 95 Ballistic Properties 97 Conclusions 98 References 99Nanostructured Superhard Materials S. Veptek

4

4.1 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2 4.3.3 4.4 4.4

Introduction 104 Concept for the Design of Superhard Materials 109 Nanocrystalline Materials 110 Heterostructures 114 Preparation and Properties of Superhard Nanocrystalline Composites 116 Preparation 116 Properties of the ncM,N/aSi3N4 Composites 119 Other Superhard Nanocomposites and the General Validity of the Design Principle 124 Discussion of the Possible Origin of the Hardness and Stability of the Nanostructure 128 Conclusions 133 Acknowledgments 134 References 134Corrosion of Hard Materials K . G. Nickel and Y. G. Gogotsi

5

5.1

Introduction

140

XVI 5.2 5.3 5.3.1 5.3.2 5.4 5.4.1 5.4.2 5.4.3 5.4.4 5.4.5 5.5 5.5.1 5.5.2 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.5

Contents

Corrosive Media 140 Corrosion Modes 141 Active and Passive Corrosion 141 Homogeneity and Location of Attack: Internal, External and Localized Corrosion 141 Corrosion Kinetics 142 Physical Boundary Conditions 142 Active Corrosion Kinetics 143 Basic Passive Corrosion Kinetics 145 Kinetic Breaks 147 Complex Kinetics 148 Corrosion Measurement 150 Experimental Methods 150 Corrosion Data 151 Materials 154 Diamond and Diamond-like Carbons 154 Carbides 155 Nitrides 166 Carbonitrides 173 Titanium Diboride 176 References 177 Interrelations Between the Influences of Indentation Size, Surface State, Grain Size, Grain-Boundary Deformation, and Temperature on the Hardness of Ceramics A . Krell Introduction 183 The Assessment of Residual Porosity and Flaw Populations: A Prerequisite for any Hardness Investigation 184 Theoretical Considerations 185 The Role of the Lattice and of Grain Boundaries in the Inelastic Deformation at an Indentation Site in Sintered Hard Materials 185 Quantitative Understanding the Load Effect on the Hardness: Theoretical Considerations Compared with Single Crystal Data 188 Influences of the Grain Size and the State of the Surface 191 The Grain Size Influence on the Load Effect of the Hardness: Modeling Experimental Results 191 The Effect of the Grain Size and the Surface State in Ceramics when Recorded by Different Measuring Approaches 193 Comparing the Grain Size Effect and the Indentation Size Effect: The Role of Grain Boundaries at Room Temperature 195 The Effects of Temperature on the Hardness of Ceramics 198 Summary 199 References 20 1

6

6.1 6.2 6.3 6.3.1 6.3.2 6.4 6.4.1 6.4.2 6.5 6.6 6.7

Contents

XVII

77.1 7.2 7.2.1 7.2.2 7.3 7.4 7.4.1 7.4.2 7.5 7.5.1 7.5.2 7.5.3 7.5.4 7.5.5 7.6 7.6.1 7.6.2 7.6.3 7.6.4 7.6.5 7.6.6 7.6.7 7.7 7.7.1 7.7.2 7.7.3

Transition Metal Carbides, Nitrides, and Carbonitrides W. Lengauer Introduction 202 General Features of Structure and Bonding 205 General Structural Features 205 General Features of Bonding 206 Preparation 207 Characterization 2 10 Chemical Analysis 2 10 Physical Microanalysis 21 1 Thermodynamics 2 12 Stability of Carbides 212 Nitrogen Partial Pressure of Nitrides 212 Phase Equilibria of Important Carbide Systems 213 Transition Metal-Nitrogen Systems and Structure of Phases 216 Carbonitride Systems 221 Properties of Important Transition Metal Carbides, Nitrides, and Carbonitrides 224 Melting Points 224 Color 224 Thermal and Electrical Conductivities 225 Thermal Expansion 228 Diffusivities 229 Elastic Properties 23 1 Microhardness 234 Industrial Applications 238 Cemented Carbides and Carbonitrides 238 Deposited Layers 241 Diffusion Layers 246 Acknowledgments 248 References 248 New Superhard Materials: Carbon and Silicon Nitrides J . E. Lowther Introduction 253 Modeling Procedures 254 Semi-empirical Approaches 254 Tight-binding Schemes 255 Ab initio Pseudopotential Approach 256 Transition Pressures and Relative Stability 256 Carbon Nitride 257 Crystalline Structures 258 Graphitic Structures 259 Amorphous Structures 261

8 8.1 8.2 8.2. I 8.2.2 8.2.3 8.2.4 8.3 8.3.1 8.3.2 8.3.3

XVIII 8.3.4 8.4 8.4.1 8.4.2 8.4.3 8.5

Contents

Relative Stability 263 Silicon Carbon Nitride 264 j3SiC2N4 265 Near-cubic Forms of SiC2N4 266 Relative Stability 268 Conclusions 268 Acknowledgements 269 References 269Effective Doping in Novel sp2 Bonded Carbon Allotropes G. Jungnickel, P. K. Sitch, T. Frauenheim, C. R. Cousins, C. D. Latham, B. R. Eggen, and M . I. Heggie

9

9.1 9.2 9.3 9.4 9.5 9.6

Introduction 271 Lattice Description 274 Computational Methods 276 Static Properties 278 Electronic Properties 279 Conclusions 282 Acknowledgments 283 References 283Synthesis and Processing Directed Metal Oxidation V. Jayaram and D. Brandon

Part I1

1 1.1 1.2 1.2.I 1.3 I .4 1.5 1.5.1 1.5.2 1.5.3 1.5.4 1.5.5 1.5.6 1.6 1.6.1 1.6.2 1.6.3 1.6.4 1.6.5 1.6.6

Historical Background 289 Oxidation and Oxide Formation 290 Initial Oxidation 291 Related Ceramic Processing Routes 293 Directed Metal Oxidation Incubation 295 Directed Metal Oxidation Growth 300 Introduction 300 Directed Metal Oxidation Composites from Al-Mg Alloys 300 Directed Metal Oxidation Growth from other Aluminum Alloys 304 Microstructural Scale 305 Growth into Particulate Preforms 307 Growth into Fibrous Preforms 309 Mechanical Properties 310 Elastic Modulus 310 Strength and Toughness 3 11 Thermal Shock 313 High Temperature Strength 313 Wear Properties 314 Mechanical Properties of Fiber-reinforced DMO Composites 3 14

Contents

XIX

1.7 1.8 1.9 1.9.1 1.9.2 1.9.3

Corrosion of Directed Metal Oxidation Composites 3 16 Other Properties 316 Applications 3 I6 Wear Resistant Components 317 Ceramic Composite Armor 3 17 Thermal Barriers and Heat Sinks 318 References 3 18 Self-propagating High-Temperature Synthesis of Hard Materials Z . A . Munir and U. Anselmi-Tumburini Introduction 322 Mechanistic Characterization of the Process 327 Effect of Experimental Parameters 33 1 Synthesis of Dense Materials 342 Synthesis by Field-Activated Self-propagating High-temperature Synthesis 348 Selected Recent Examples of Synthesis of Hard Materials 356 Acknowledgment 368 References 368 Hydrothermal Synthesis of Diamond K. G. Nickel, T. Kruft, and Y. G. Gogotsi Introduction 374 Evidence from Nature 376 Hydrothermal Synthesis 377 C-H-0 System 377 Hydrothermal Treatment of S i c 382 Outlook 387 Acknowledgments 387 References 387 Chemical Vapor Deposition of Diamond Films C.-P. Klages Introduction 390 Preparation Methods for Diamond Films 391 Hot-filament Chemical Vapor Deposition 392 Microwave-plasma-based Methods 397 Preparation of Special Forms: Textured and Heteroepitaxial Films 400 Thermochemistry and Mechanism of Chemical Vapor Deposition Diamond Growth 407 Transformation of Graphite to Diamond at Low Pressures 407

22.1 2.2 2.3 2.3 2.4 2.6

33.1 3.2 3.3 3.3.1 3.3.2 3.4

4

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1

xx4.3.2 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.5

Contents

Reactive Species in Diamond Chemical Vapor Deposition, the Role ofCH3 408 Properties and Applications of Chemical Vapor Deposited Diamond 410 Diamond Coated Cutting Tools 41 1 Thermal Conductivity of Chemical Vapor Deposited Diamond: Thermal Management Applications 412 Electrical Properties and Electronic Applications 413 Electrochemical Use of Chemical Vapor Deposited Diamond 415 Summary 417 References 4 17Vapor Phase Deposition of Cubic Boron Nitride Films K. Bewilogua and F. Richter

5

5.1 5.2 5.2.1 5.2.2 5.2.3 5.3 5.4 5.4.1 5.4.2 5.5 5.5.1 5.5.2 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.7

Introduction 420 Empirical Results 421 Deposition Methods 421 Morphology and Structure of cBN Films 423 Film Adhesion 427 Models of cBN Formation 427 Sputter Deposition of cBN Films 429 Sputter Deposition with Conducting Targets 430 Deposition by d.c. Magnetron Sputter with a Hot Boron Target 43 1 Discrimination between Nucleation and Growth Phase 433 Detection of hBNxBN Transition 433 RF Magnetron Sputtering 435 Properties of cBN Films 440 Mechanical and Tribological Properties 440 Optical Properties 440 Electrical Properties 441 Other Properties 441 Summary and Outlook 442 References 442Polymer to Ceramic Transformation: Processing of Ceramic Bodies and Thin Films G. D. Soraru and P. Colombo

6

6.1 6.2 6.3 6.3.1

Introduction 446 Processing of Monolithic Components 450 Preparation and Characterization of SiAlOC Ceramic Bodies by Pyrolysis in Inert Atmosphere 452 Experimental Procedure 452

Contents

XXI

6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.5 6.6 6.6.1 6.7 6.8 6.9 6.10 6.10.1 6.10.2 6.1 1

Results 453 Characterization of the Pre-ceramic Precursors 453 Characterization of the Pre-ceramic Components 454 Characterization of the Ceramic Components 455 Mechanical Characterization at High Temperature 457 Discussion 458 Preparation and Characterization of SiAlON Ceramics by Pyrolysis in Reactive Atmosphere 460 Experimental 460 Results and Discussion 460 Processing of Thin Ceramic Films 463 Experimental 463 Results and Discussion 464 Conventional Conversion Process: Annealing in Controlled Atmosphere 464 Nonconventional Conversion Process: Ion Irradiation 467 Conclusions 472 Acknowledgments 473 References 473

Part 1 1 1 11.1 1.1.1 1.1.2 1.1.3 1.1.4 1.1.5 1.1.6 1.1.7 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.2.7 1.3 1.3.1 1.3.2 1.3.3

Materials and Applications Diamond Materials and their Applications Edited by R. J . CaveneySuperabrasive tools: A Brief Introduction 479 Introduction 479 Early History 479 Synthetic Diamond 48 1 Cubic Boron Nitride 482 Polycrystalline Diamond and Cubic Boron Nitride 482 Chemical Vapor Deposited Diamond 484 Outline of Chapter 485 The Crystallization of Diamond 485 The Carbon Phase Diagram 485 Diamond Crystallization at High Pressure 487 High Pressure Apparatus 490 The Synthesis of Particulate Diamond Abrasives 49 1 Growth of Large Synthetic Diamonds 496 Novel Diamond Synthesis Routes 504 Cubic Boron Nitride Crystallization 5 10 Polycrystalline Diamond and Cubic Boron Nitride 5 12 Natural Polycrystalline Diamond 5 12 Synthetic Polycrystalline Diamond 512 Mechanisms involved in Polycrystalline Diamond Manufacturing Process 513

XXII 1.3.4 1.4 1.4.1 1.4.2 1.4.3 1.4.4 1.4.5 1.4.6 1.4.7 1.5 1.5.1 1.5.2 1.5.3 1.5.4 1.5.5

Contents

Polycrystalline Cubic Boron Nitride 5 18 New Ultrahard Materials 521 Introduction 521 Hardness 521 C3N4 523 Boron Rich Nitride 526 Boron Carbonitrides 526 Boron Suboxides 526 Stishovite 526 Industrial Applications of Diamond and cBN 527 Introduction 527 Abrasive Application 528 Machining of Stone and Concrete 540 Applications of Polycrystalline Ultra-hard Materials Applications of Single Crystal Diamond 559 Acknowledgments 566 References 566

548

2 2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.3 2.3.1 2.3.2 2.4 2.4.1 2.4.2 2.5 2.5.1 2.5.2 2.5.3 2.5.4 2.6 2.6.1 2.6.2

Applications of Diamond Synthesized by Chemical Vapor Deposition R. S. Sussmann Introduction 573 Properties of Chemical Vapor Deposited Diamond 574 Material Grades 574 Optical Properties 576 Strength of Chemical Vapor Deposited Diamond 580 The Young Modulus 581 Thermal Conductivity 582 Dielectric Properties 583 Optical Applications 583 Chemical Vapor Deposited Diamond for Passive Infrared Windows in Aggressive Environments 584 Windows for High-power Infrared Lasers 589 Windows for High Power Gyrotron Tubes 597 Window Requirements 598 The Development of Chemical Vapor Deposited Diamond Gyrotron Windows 599 Thermal Management of Laser Diode Arrays 606 Laser Diode Arrays: General Issues 607 Modelling of Submount Heat Resistance 607 Flatness of Submount 610 Thermal Stress 610 Cutting Tools, Dressers and Wear Parts 61 1 Cutting Tools Trends 61 1 Cutting Tool Application of Chemical Vapor Deposited Diamond 612

Contents

XXIII

2.6.3 2.6.4

Chemical Vapor Deposited Diamond Dressers 616 Chemical Vapor Deposited Diamond Wear Parts 617 References 6 19Diamond-like Carbon Films C.-P. Klages and K . Bewilogua

33.1 3.2 3.2.1 3.2.2. 3.2.3 3.3 3.3.1 3.3.2 3.4. 3.4.1 3.4.2 3.5. 3.5.1 3.5.2 3.5.3 3.5.4

Introduction 623 Preparation Methods for Diamond-like Carbon Films 623 Hydrogenated Amorphous Carbon (a-C: H) 623 Hydrogen Free Amorphous Carbon (ta-C) 627 Metal-containing Amorphous Hydrocarbon 629 Microstructure and Bonding of Diamond-like Carbon 630 Amorphous Carbon and Hydrogenated Amorphous Carbon Metal-containing Amorphous Carbon Films 634 Physical Properties of DLC Films 637 Electrical and Optical Properties 637 Mechanical Properties 639 Applications of DLC Films 640 Adhesion of DLC Films 640 Tribology of DLC Coatings 642 Tribological Applications 644 Other Applications 644 References 645Ceramics Based on Alumina: Increasing the Hardness for Tool Applications A . Krell

630

4

4.1 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2 4.3.3 4.4 4.4.1

Recent Trends in the Application of Ceramic Tool Materials 648 Technological Essentials for Producing Hard and Strong Tool Ceramics 650 Typical Defects in Ceramics Tool Materials: The State of The Art 651 Recent Trends in Ceramic Technologies Related to Tool Ceramics 653 Tool Materials with Undefined Cutting Edge: Sintered Grinding Materials 658 Technical Demands for Grinding Materials 660 661 Advanced Commercial Products: Sol/gel-derived Corundum Sintered Alumina Grits Produced by Powder Processing Approaches 665 New Trend for Cutting Hard Workpieces: Submicrometer Cutting Ceramics for Tools with Defined Cutting Edge 666 Demands for Cutting Materials Used for Turning Hard Workpieces 667

XXIV4.4.2 4.4.3 4.4.4 4.5

Contents

Carbide Reinforced Composite Ceramics Based on A1203 669 Single Phase Sintered Corundum 670 Comparative Cutting Studies with Submicrometer Ceramics: A1203 and Composites Reinforced with Ti(C,N) and Ti(C,O) 670 Summary 680 References 68 1Silicon Carbide Based Hard Materials K. A . Schwetz

5

5.1 5.1.1 5.1.2 5.2 5.3 5.3.1 5.3.2 5.3.3 5.4 5.4.1 5.4.2 5.4.3 5.5 5.6 5.7

lntroduction 683 History 683 Natural Occurrence [7] 684 Structure and Phase Relations of S i c 685 Production of S i c 688 The Acheson/ESK Process 688 Other Production Methods 69 1 Dense S i c Shapes 699 Properties of Silicon Carbide 719 Physical Properties 719 Chemical Properties 720 Tribological Properties 723 Quality Control 734 Toxicology and Occupational Health 736 Uses of Silicon Carbide 736 Acknowledgments 740 References 740Silicon Nitride Based Hard Materials M . Herrmann, H. Klemm, Chr. Schubert

6

6.1 6.2 6.3 6.4 6.4.1 6.4.2 6.5 6.5.1 6.5.2 6.5.3 6.5.4 6.6

Introduction 749 Crystal Structure and Properties of the Si3N4Modifications 753 Densification 755 Microstructural Development 758 Microstructural development of P-Si3N4materials 758 Microstructural development of a-SiALON materials 768 Properties of Si3N4Materials 771 Mechanical properties at room temperatures 771 High-temperature properties of silicon nitride materials 777 Wear resistance of Si3N4materials 782 Corrosion resistance of Si3N4 786 Conclusions/Further potential of silicon nitride materials 792 Acknowledgements 795 References 795

Contents

xxv

77.1 7.2 7.2.1 7.2.2 7.3 7.3.1 7.3.2 7.4 7.4.1 7.4.2 7.4.3 7.4.4 7.4.5 7.5 7.5.1 7.5.2 7.5.3 7.6 7.7 7.8 7.8.1 7.8.2 7.8.3 7.8.4 7.9

Boride-Based Hard Materials R. Telle, L. S . Sigl, and K. Takagi Introduction 802 Chemical Bonding and Crystal Chemistry of Borides 803 Chemical Bonding of Borides 803 The Crystal Structure of Borides 804 Phase Systems 812 Binary Phase Diagrams of Technically Important Systems 813 Ternary and Higher Order Systems 818 Boron Carbide Ceramics 837 Preparation of Boron Carbide 837 Sintering of Boron Carbide 839 Properties of Boron Carbide 851 Chemical Properties and Oxidation of Boron Carbide 855 Boron Carbide-Based Composites 857 Transition Metal Boride Ceramics 874 Preparation of Transition Metal Borides 875 Densification of Transition Metal Borides 876 Properties of Transition Metal Borides Ceramics 878 Multiphase Hard Materials Based on Carbide-Nitride-Boride-Silicide Composites 888 Boride-Zirconia Composites 888 Cemented Borides 895 Boron Carbide-Based Cermets 895 Titanium Diboride-Based Cermets 897 Cemented Ternary Borides 919 Potentials and Applications 927 Future Prospects and Fields of Application 933 References 936 The Hardness of Tungsten Carbidecobalt Hardmetal 946 S . Luyckx 946 Introduction 946 The Hardness of the Two Component Phases 947 The Hardness of Tungsten Carbide 947 The Hardness of Cobalt 948 Factors Affecting the Hardness of WC-Co Hardmetal 950 Cobalt Content and Tungsten Carbide Grain Size 950 Grain Size Distribution and Cobalt Mean Free Path 952 Binder Composition and Carbon Content 952 Porosity 953 Effect of Temperature 953 Relationships between Hardness and Other Hardmetal Properties 960

8 8.1 8.2 8.2.1 8.2.2 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.3.5 8.4

XXVI8.4.1 8.4.2 8.5

Con ten /s

Relationship between Hardness and Toughness 962 Relationship between Hardness and Abrasive Wear Resistance Conclusions 963 Acknowledgments 963 References 964

962

99.1 9.2 9.3

Data Collection of Properties of Hard Materials G. Berg, C. Fviedrich, E. Broszeit, and C. BevgerIntroduction 965 Profile of Properties 965 Organization and Contents of the Data Collection Acknowledgement 967 Refercnces 99 1

966

Index 997

List of Contributors

U. Anselmi-Tamburini Dipartimento di Chimica Fisica Universita di Pavia 27100 Pavia Italy M. W. Bailey De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 21 35 South Africa G. Berg Fachgebiet und Institut fur Werkstofiunde der TU Darmstadt und Staatliche Materialpru fungsanhalt Grafenstrasse 2 D-64283 Darmstadt Germany C. Berger Fachgebiet und Institut fur Werkstofiunde der TU Darmstadt und Staatliche Materialpriifungsanhalt Grafenstrasse 2 D-64283 Darmstadt GermanyK. Bewilogua Fraunhofer Institut fur Schicht und Oberflachentechnik (IST) Bienroder Weg 54 E D-38 108 Braunschweig Germany

R. Bohler Max-Planck-Institute for Chemistry Saarstrasse 23 D-55020 Mainz Germany

D. Brandon Department of Materials Engineering Technion - Israel Institute of Technology Haifa 32000 Israel J. R. Brandon De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2 135 South AfricaE. Broszeit Fachgebiet und Institut fur Werkstofiunde der TU Darmstadt und Staatliche Materialprufungsanhalt Grafenstrasse 2 D-64283 Darmstadt Germany

R. C. Burns De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 21 35 South Africa

XXVIII

List o Contributors f

R. J. Caveney De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa

G. J. Davies De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa B. R. Eggen School of Chemistry, Physics and Environmental Sciences University of Sussex Falmer Brighton BNl 9QJ UK D. Fister HC Starck Gmbh Kraftwerkweg 3 D-79725 Laufenburg Germany T. Frauenheim Fachbereich Physik Universitat/Gesamthochschule Paderborn D- 33095 Paderborn Germany C. Friedrich Fachgebiet und Institut fur Werkstoffkunde der TU Darmstadt und Staatliche Materialprufungsanhalt Grafenstrasse 2 D-64283 Darmstadt GermanyY. G. Gogotsi Institut fur Angewandte Mineralogie Universitat Tubingen Wilhelmstrasse 56 D-72074 Tubingen Germany

S. E. Coe De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South AfricaJ. L. Collins De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa P. Colombo Universita di Bologna Dipartimento di Chimica Applicata e Scienza dei Materiali viale Risorgimento 2 1-40I36 Bologna Italy M. W. Cook De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 21 35 South Africa C. R. Cousins Department of Physics University of Exeter Stocker Road Exeter EX4 4QL UK

List of Contributors

XXIX

J. 0. Hansen De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2 135 South Africa M. Hoffmann Fakultat fur Maschinenbau Institut fur Werkstoffkunde I1 Universitat Karlsruhe Kaiserstrasse 12 Postfach 6980 D-76 128 Karlsruhe GermanyR. D. Hoffmann Westfalische Wilhelms-Universitat Miinster Anorganisch-Chemisches Institut Wilhelm-Klemm-Strasse 8 D-48 149 Miinster Germany

G. Jungnickel Fachbereich Physik Universitat/Gesamthochschule Paderborn D- 33095 Paderborn Germany C. P. Klages Fraunhofer Institut fur Schicht und Oberflachentechnik (IST) Bienroder Weg 54 E D-38108 Braunschweig Germany T. Kraft lnstitut fur Angewandte Mineralogie Universitat Tubingen Wilhelmstrasse 56 D-72074 Tubingen Germany A. Krell Fraunhofer Institute for Ceramic Technologies and Sintered Materials Winterbergstrasse 28 D-0 1277 Dresden GermanyC. D. Latham Department of Physics University of Exeter Stocker Road Exeter EX4 4QL UK

M. I. Heggie School of Chemistry, Physics and Environmental Sciences University of Sussex Falmer Brighton BN1 9QJ UK V. Jayaram Department of Metallurgy Indian Institute of Science Bangalore India W. Jeitschko Westfalische Wilhelms-Universitat Miinster Anorganisch-Chemisches Institut Wilhelm-Klemm-Strasse 8 D-48149 Miinster Germany

W. Lengauer Institute for Chemical Technology of Inorganic Materials Vienna University of Technology Getreidemarkt 9/161 A-1060 Vienna Austria

XXX

List of Contributors

J. E. Lowther Department of Physics University of Witwatersrand Johannesburg South AfricaS. Luyckx School of Process and Materials Engineering University of the Witwatersrand Johannesburg 2050 South Africa

R. Pottgen Westfalische Wilhelms-Universitat Munster Anorganisch-Chemisches Institut Wilhelm-Klemm-Strasse 8 D-48149 Munster Germany

Z. A. MunirFacility for Advanced Combustion Synthesis Department of Chemical Engineering and Materials Science University of California Davis CA 95616 USA K. G. Nickel Universitat Tubingen Applied Mineralogy Wilhelmstrasse 56 D-72074 Tubingen Germany

F. Richter Technische Universitat ChemnitzZwickau Institut fur Physik D-09107 Chemnitz GermanyR. Riedel Fachbereich Materialwissenschaft Technical University of Darmstadt Petersenstrasse 23 D-64287 Darmstadt Germany

K. A. Schwetz Advanced Ceramics Lab Elektroschmelzwerk Gmbh Max-Schaidhauf-Strasse 25 D-87437 Kempten GermanyP. K. Sen De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa G. Serghiou Max-Planck-Institute for Chemistry Saarstrasse 23 Mainz Germany

S. Ozbayraktar De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa C. S. J. Pickles De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa

List of Contributors

XXXI

D. Sherman Department of Materials Engineering Technion - Israel Institute of Technology Haifa 32000 Israel M. Sibanda De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa

K. Takagi Toyo Kohan Co. Ltd. Tokyo Japan R. Telle Institut fur Gesteinshuttenkunde RWTH Aaachen MauerstraBe 5 D-52056 Aachen Germany S. Veprek Institute for Chemistry of Inorganic Materials Technical University Munich Lichtenbergstrasse 4 D-85747 Garching b. Munich Germany C. J. H. Wort De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa A. Zerr Fachgebiet Disperse Feststoffe Technical University of Darmstadt Petersenstrasse 23 D-64287 Darmstadt Germany

I. Sigalas De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2 135 South AfricaP. K. Sitch Fachbereich Physik Universitat/Gesamthochschule Paderborn D- 33095 Paderbron Germany G. D. Soraru Universita di Trento Dipartimiento di Ingegneria dei Materiali Via Mesiano 77 1-38050 Trento Italy R. S . Sussmann De Beers Industrial Diamond Division Pty Ltd Diamond Research Lab PO Box 1770 Southdale 2135 South Africa

List of Symbols

a!

aa!

P PYYi"/S

a a?P, x

r(n)

rs

tan 6 Ac AG!98 ASint AT A X& &

irl rl

08

206 66 1

x x x x x x x

62

A

absorption coefficient atomic attraction constant growth parameter power absorption coefficient polytypes or phases atomic repulsion constant geometrical factor rake angle secondary ion yield orientational surface energy surface energy width of X-ray reflection microplastic deformability dielectric loss factor concentratation difference Gibbs free energy interfacial entropy temperature change change of size or mass elastic strain emissivity strain rate degree of conversion to nitride fraction of reaction completed angle constant relating tensile strength and hardness X-ray scattering angle entering angle thermal conductivity thermal conductivity of rectants thermal conductivity of products empirical parameter relating bulk modulus and inclination angle layer thickness, mean free path polarity of bond thermal conductivity wavelength X-ray wavelength mean free path

degreesJ

JK or "C

degrees J cm-2 s-I bond length degrees m

K-'

m m

XXXIVP P

List o Symbols f

v7 r

P P PPmU U

ffU O

ffb

ffijffS

UY 77-

4 4, w, 74 4XW

(40a a aa0

7

C/Ti ratio coefficient of friction Poisson ratio complementary energy J density kg m-3 dislocation density resistivity theoretical density of product conductivity electrical conductivity stress Pa median failure stress Pa fracture strength in bending, modulus of rupture local stress field Stefan-Boltzmann constant J cm-* sC1 KC4 yield stress GPa annealing time help time S angle between crack and tensile stress ternary phases azimuthal angle between polarization vector and substrate direction constraint factor electron affinity wear coefficient average diffusion distance in time r vacancy in crystal structure indent size, half length of diagonal crack length depth of cut equilibrium bond distance critical flaw size depth of lateral crack on erosion heat capacity coefficients crystal unit cell parameters lattice constants area contact area material constantm m mm m m m

acr ai

a, b a, 6 , c

nm m2 m2

A A A ABAB, ABCABC A,B,H,HA b B B (B4C)

stacking sequences stacking positions Burgers vector bulk modulus GPa designation of a dissolved species in, e. g., a liquid designation of a non-stociometric compound (solid solution)

List of Symbols

XXXV

Ci CP d d d d d d d d

d P

D D D D

D ODH, DC

e E E E E O E" E C E F

4f f f f

Ei EirnaxEP

hF

radius of radial crack m m s-' velocity of light in vacuo interfacial concentration m radius of lateral crack on erosion proportionality constant specific heat per unit volume concentration of impurities heat capacity J g-' K-' bond length A degree of dilution diameter of Brinell impression m m diameter of Vickers impression grain size m height of beam m layer thickness rad spacing of powder diffraction rings pore diameter diameter of Brinell indenter m diameter of median crack m diffusivity size of particles diffusion factor diffusion coefficients at high and low temperature unit electron charge activation energy J binding energy J Young's modulus theoretical Young's modulus E/(l-v2) J composite potential energy Fermi energy J band gap J ion energy J maximum ion energy J potential energy mm mine' feed rate Hz frequency volume fraction Hz Weibull safety factor volume fraction of Z N force statistical failure probability ion flux J external work (linear elasticity) vapor phase shear modulus

XXXVI

List o Symbols f

i, a

I IBB

4IRBji

J k kk0 hog kL

k* kr KKKIC KICOJ

1 1 1 1

LLm,, m

M M M A 4Mi

n n

strain energy release rate toughness, fracture energy, work of fracture convective heat transfer coefficient indentation depth Planck constant hardness enthalpy enthalpy of formation at 298 K Brine11 hardness Meyer hardness Knoop hardness plastic hardness Vickers hardness average applied compressive stress in hardness GPa fluxes of impinging ions and deposited atoms rank order of test result intensity of black body radiation peak height intensity of real body radiation current density mass flux Boltzmann constant constant for layer growth reaction constant logarithmic rate constant linear rate constant parabolic rate constant reaction coefficient proportionality constant stress intensity factor fracture toughness diffusion path length length of sample long diameter of Knoop impression span of beam defined load maximum load Weibull modulus mode of deformation (I, 11, or 111) metal Mohs hardness weight molecular mass of impurity number order of reaction

N m-l J cm-2 s - ~ K-'

GPa GPa GPa GPa test

m2s-l or kg2m-4s-'

Paris exponent for fatigue refractive index stress exponent number of tests average coordination number no. of constraints for coordination number r average coordination number grain size exponent momentum porosity fixed load gas pressure porosity pressure ambient and high pressure confining pressure in powder pressure of nitrogen partial pressure of O2 Porod scattering vector scattering vector activation energy heat of reaction resonance factor heat of transport radius distance from crack tip interatomic distance radius of curvature equilibrium interatomic distance gas constant film growth rate average roughness radius of plastic zone solid phase elastic recovery stoichiometric ratio entropy thickness of window time ChI4? delay time time for wave propagation temperature ternary phase ambient temperature temperature limits

N

Y oGPa Pa Pa Pa

J J g-'J

m m

m

m

Y oJmol-' K-'S S S

K or "C K or "C K or "C

XXXVIII

List of Symbols

UP

V

adiabatic combustion temperature critical temperature and pressure eutectic temperature absolute melting temperature substrate temperature displacement stoichiometry factor wave velocity internal energy substrate bias voltage elastic strain energy plasma potential surface free energy average velocity machining velocity volume wavenumber longitudinal velocity of sound volume of pores volume volume fraction symbol for vacancy in a chemical formula applied substrate bias voltage volume lost in erosion impact molar volume of metal orientational growth rate molar volumes of product and reactants sample thickness RF power carbon-to-metal ratio coordinate layer thickness liquid phase lattice directions Miller indices

K or "C K or "C K or "C m

J J Jm min-' m3 cm-' m s-' m3 V m3

v

mW

m

List of Abbreviations

3PB 4PB ACC AES AFM APW AR ASEA ASTM b.c.c. b.c.t. BB BET 3C, 4H, 6R CAD CALPHAD cBN CCD CED CMC COOP CN CVD CVI

cw

d.c. DAC DCC DF-TB DH DIN dlC DLC DMO DOS DTA ECH ECR

three-point bend four-point bend amorphous covalent ceramics Auger electron spectroscopy atomic force microscope augmented plane wave antireflection Swedish company American Society for Testing and Materials body-centered cubic body-centered tetragonal black body Brunauer-Emmett-Teller method for determining porosity polytype notations of Sic (C: cubic, H: hexagonal, R: rhombohedral) cathodic arc deposition calculation of phase diagrams model cubic boron nitride charge coupled device cutting edge displacement ceramic matrix composite crystal orbital overlap population coordination number chemical vapor deposition chemical vapor infiltrated continuous wave direct current diamond anvil technique direct coagulation casting density-functional tight-binding method methy ldiethoxysilane Deutsche Industrie Norm diamond-like carbon diamond-like carbon directed metal oxidation density of states differential thermal analysis electron cyclotron heating electron cyclotron resonance

XL

List of Abbreviations

EDAX EELS EP EPMA EPR ERD ERDA ESCA ESK EXAFS f.c.c. FEPA FTIR FWHM FZK GA-XRD

g c

GEC GFRP GGA h hBN HF-CVD HIP HK HOMO HOPG HPHT HPL HPMS HR HR-TEM HSS HV IBAD ICSD ICDD IED IR ISE IS0 ITER JAERI JFM JIS KFM

energy-dispersive analysis of X-rays electron energy loss spectroscopy electroplated electron probe microanalysis electron paramagnetic resonance elastic recoil detection elastic recoil detection analysis electron spectroscopy for chemical analysis Elektroschmelzwerk Kempten extended X-ray absorption fine structure face-centered cubic Federation Europeen des Fabricants de Produits Abrasifs Fourier transform infrared full width at half maximum Forschungzentrum Karlsruhe glancing angle XRD glassy carbon General Electric Company, USA glass-fiber reinforced plastic generalized gradient approximation hexagon a1 hexagonal boron nitride hot filament CVD hot isostatic pressing Knoop hardness highest occupied molecular orbital highly ordered pyrolytic graphite high-pressure high-temperature high-pressure laminate high-pressure microwave source Rockwell hardness high-resolution TEM high-speed steel Vickers hardness ion-beam assisted deposition inorganic crystal structure database international center for diffraction data ion energy distribution infrared indentation size effect International Standards Organization international thermonuclear experimental reactor Japan atomic energy research institute Johnson figure of merit Japanese Standards Keyes figure of merit

List o Abbreviations f

XLI

LAS LDA LDA LIDT LPI LPSSiC LPSSS LRO LSF LSI LWIR MAK MAS-NMR Me-DLC MMC MOR MOSFET MS MSIB MTES MTF MW MWP-CVD NASA ncTi02 NDE Nd-YAG NEA NICALON NIRIM NMR NRA ORNL PA-CVD PAlC PCS p.p.m. PBC PC pcBN PcD PCS PCT PTC PTES PTFE

lithium aluminosilicate local density approximation laser diode array laser induced damage threshold liquid polymer infiltration liquid-phase sintered S i c low-pressure solid-state source long-range order line spread function liquid silicon infiltration longer wavelength infrared Maximal zulassige Arbeitsplatz Konzentration magic angle spinning NMR metal-DLC hybrid metal matrix composite modulus of rupture metal-oxide silicon field effect transistor mass spectroscopy mass-selected ion beam methy ltriethoxy silane modulation transfer function microwave microwave plasma CVD National Aeronautics and Space Administration (USA) nanocrystalline titania nondestructive evaluation neodymium-yttrium-aluminum-garnet laser negative electron affinity branded Si-C-0 composite fiber from Nippon Carbon National Institute for Research in Inorganic Materials (Japan) nuclear magnetic resonance nuclear reaction analysis Oak Ridge National Laboratory plasma assisted CVD polyaluminocarbosilane polycarbosilane parts per million periodic bond chain potential cycling polycrystalline boron nitride polycrystalline diamond . polycarbosilane Patent Cooperation Treaty polytitanocarbosilane phen y ltriethoxy silane polytetrafluoroethylene

XLII PP PVD r RBAO RBM rBN RBS RBSN RF RSF RSSC S.C. s.c.cm. SAD SAXS

List of Abbreviations

scs

SEM SERR SHS SIALON SiCAlON Si-DLC SIF SIMS SNMS SP SRO STM ta-C taC TD TCNE TEM TGA TH TRS TZP UHP UPS

uv VAMAS vcVEC VLS

vs

wBN

polymer pyrolisis physical vapor deposition rhombohedral reaction bonded aluminum oxide reaction bonded mullite rhombohedral boron nitride Rutherford back-scattering reaction bonded silicon nitride radio frequency reduced spatial frequency reaction sintered silicon carbide simple cubic standard cubic centimeters small angle diffraction small-angle X-ray scattering Textron process Sic fibers with C core and C surface scanning electron microscope strain energy release rate self-propagating high-temperature synthesis Si-A1-0-N (silicon aluminum oxynitride) fiber SiC-AlN-Al20C composite fiber Si-DLC hybrid stress intensity factor secondary ion mass spectrometry secondary neutron mass spectrometry sintered powder short-range order scanning tunnel microscopy hydrogen-free amorphous carbon tetrahedral amorphous carbon theoretical density tetracyanoethylene transmission electron microscope thermogravimetric analysis triethoxysilane transverse rupture strength (= MOR) tetragonal zirconia polycrystals ultahigh purity ultraviolet photoelectron spectroscopy ultaviolet Versailles Agreement on Materials and Standards vapor phase formation and condensation process valence electron concentration vapor-liquid-solid process vapor-solid reaction wurtzitic boron nitride

L s of Abbreviations it

XLIII

XANES XPS XRD YAG YLF

X-ray absorption near edge structure X-ray photoelectron spectroscopy X-ray diffraction yttrium aluminium garnet, yttrium aluminate, Y2A15012 yttrium-lithium-fluorite

Introduction: Novel Ultrahard MaterialsA. Zerr and *R. Riedel

IntroductionThe synthesis of new materials with hardness comparable to or even harder than diamond is of considerable fundamental and technological interest and is a great challenge to chemists, physicists, and materials scientists. Most of the known ultrahard materials, including diamond and cubic boron nitride, were first synthesized in the 1950s and industrially manufactured using high pressure-high temperature processes [I-31. Extensive research in this domain continues and recently a few new materials have been synthesized or rediscovered as superhard ones (Si02-stishovite [4], cubic Si3N4 [5]). New vapor deposition methods (CVD, PVD, laser ablation etc.) which allow the deposition of diamond, cubic boron nitride and other hard materials films at low temperature and low pressure (i.e. often under metastable conditions) on a variety of substrates have been developed since the early 1980s [&lo]. Diamond and cBN (cubic boron nitride) combine excellent mechanical, chemical, and physical properties. However, owing to its instability at high temperatures, diamond cannot be used, for example, as a cutting tool for steel. Moreover, with increasing temperatures diamond and cBN weaken due to the onset of the transformation to the graphite structure so that above 1100C in a nonoxidizing atmosphere boron carbide B4C(with a hardness of about 30 GPa) has been identified as the hardest material [I I]. For this reason and because of the need to replace expensive diamond in many other applications, new hard materials with comparable or even superior properties are required. Theoretical work on the carbon nitride C3N4predicted that this compound could have a hardness comparable to or even greater than that of diamond [12,13]. The synthesis of polycrystalline C3N4films was first reported in 1992 [I41 and in 1993 [ 151and has been under extensive investigationsince then, resulting in hundreds of publications on this subject. However, in a few recent publications there has been doubt the thermodynamic stability of theoretically predicted ultrahard C3N4 phases [ 161 and whether their hardness would be comparable to that of diamond [ 171. In consequence, research on the low-temperature synthesis of diamond and cubic boron nitride [18],the search for other possible candidate compounds as well as microstructure design of known materials and their composites [ 19-21] (heterostructures, whiskers, nanocomposites) remain the subject of experimental and theoretical efforts. Microstructure design is addressed in this book by D. Sherman and D. Brandon in Part I, by S. Vepfek in Part I1 and by A. Krell in Part 111. This introduction deals with some of the latest experimental and theoretical developments in the field of novel boron- and carbon-based ultrahard materials as well as with new observations on a class of silicon-based compounds which previously were not classified as ultrahard.* This is a revised version of the paper published earlier [ 1521.

XLVI

Introduction: Novel Ultrahurd Materials

Hard MaterialsGenerally, hard materials are solids with high hardness in the range 8-10 on the Mohs scale of hardness, given by the sequence of minerals which can be scratched by the next (Table 1). Usually, common hard materials are subdivided into compounds with metallic (like TIN or WC), ionic (Al2O3>, covalent bounding or (diamond, Si3N4)[22]. The definition of ultrahard materials is that their hardness values are comparable to that of diamond. Another definition often used in the literature is that the hardness of such materials exceeds 40GPa. Among all known single phase compounds diamond, cBN and probably boron carbides (Bl3C2-Bl2C3)satisfy the latter definition. Accordingly one can expect that novel ultrahard compounds will be found in the isothermal ternary phase diagram BC-N given schematically in Fig. 1. In particular, the carbon nitrides (e.g. C3N4) and the boron carbonitrides (B,C,NZ) have been discussed as substitute materialsTable 1. Hardness ranking of minerals and some prominent synthetic ceramic materials according to F. Mohs. In the case of the synthetic materials microhardness values are given in units of the Knoop scale. The microhardness variations result from variations in the grain size, the load of indentation, the phase composition and the used densification techniques. Modified after [ 1521.Minerals/ synthetic materials Talcum Hexagonal boron nitride* Gypsum Calcite Fluorite Apatite Feldspar Quartz Topaz p-Silicon nitride* Corundumt Titanium nitride* Silicon carbide* %-Silicon nitride Titanium carbide* Boron carbide' Titanium diboride* Boron suboxides Stishovite* Cubic boron nitride' Diamond Formula Mohs hardness Microhardness,' Knoop 100 Wal 0.15-0.30 Microhardness variations for polycrystals' [GPa]

Mg,[(OH)z/Si4O,oI hBN CaS04 2 H 2 0 CaCO, CaF2 Ca5[(F3OH)/(PO&I K[AlSi,O,] Si02 A1z[F2/Si041 BSi3N4t g 3

1

2 3 4 5 6 7 8 179 21 26 26-35 28 14-26 14-29 20-38 2648 13-32 20-38 19-35

}

Sic %Si3N4 TIC B4C TiBz B,O Si02 cBN C

}

3030-59 33 45 75-100

10

' The microhardness values are taken from published sources [7] and [102].The microhardness varations are taken mostly from previous work [25].* Synthetic material.

Synthetic material or natural mineral.

Hard Materials

XLVII

Figure 1. Schematic of the isothermal ternary B C - N phase diagram at temperatures below the decomposition of the stoichiometric compounds and the regimes of composition of some solid solutions discussed. For explanation of the compounds see text. First published in [I 521 and reproduced with permission.

for diamond. One can also expect that metastable dense high pressure phases of the compounds based on silicon or oxygen can belong to ultrahard materials. One of these is the high pressure-high temperature stishovite phase of Si02 with a hardness exceeding 33 GPa [4], which is almost an order of magnitude above that of quartz, the ambient pressure phase of Si02 (Table 1). The second one is cubic Si3N4with the spinel structure, whose hardness may be comparable to that of Si02-stishovite [5].

HardnessHardness is one of the quantitative parameters that describe resistance of a material towards plastic (irreversible) deformation. Plastic deformations begin when the shear component of the stress applied to a material exceeds some value called the yield stress. There are many ways to create a plastic deformation and consequently many ways to define and to measure resistance of a material towards such deformations. Hardness can be determined in several ways:-

Scratching methods (Mohs, Martens), Grinding methods (Rosiwal). Indenting methods (Vickers, Knoop, Brinell, Rockwell, Shore).

For example, the Vickers hardness, Hv, is defined as the applied load P divided by the surface area of the impression, while the Knoop hardness H K is derived from the load P divided by the projected area of the impression: 1854.4P Hv = d2 14 229 PHK=12

XLVIII

Introduction: Novel Ultrahard Materials

where d is the diagonal of the square-based diamond pyramid of the Vickers indenter and I is the long diagonal of the rhombus-based diamond pyramid of the Knoop indenter. In the case of applied test loads below 1.96N the determined hardness is defined as microhardness and depends on the applied amount of load [23]. Generally, the unit of the hardness measured is given in kgmm-* or in GPa (1000 kgmmP2 = 9.81 GPa). The Vickers or Knoop hardness can be also expressed in terms of Mohs hardness, M , by the following expression [23]:

H v = 3.2M3.

(3)

However, this expression cannot be applied for superhard materials like diamond. Each testing method yields different hardness values for one material. Thus, in order to compare hardness values of different materials, the specific test method and the test conditions have to be described carefully. Moreover, the hardness of single crystals depends for many compounds on the crystallographic plane tested and for the Knoop indentor additionally on the orientation of the indentors long axis relative to the crystallographic axis of the examined crystal. For example, in experiments on single-crystalaSi3N4the Vickers hardness measured on different crystallographic planes varied by more than 34% and the Knoop hardness measured on the same crystallographic plane by 7% [24]. Hardness of polycrystalline materials is strongly influenced by: residual stresses, toughening phases, microstructural textures, the grain size, the applied load and the porosity as well as by the structure and composition of grain boundaries. As a consequence, the hardness of polycrystals may significantly differ from that of the single crystal counterpart. For example, due to the above reasons the measured microhardness of polycrystalline TIC varies between 1200 and 3250Hv or by more than 270% and that of polycrystalline Si3N4between 1500 and 4800 (Hv)or by more than 300% [25]. In Table 1, the microhardness values and their variations for some prominent synthetic ceramic materials are listed ranked according to the Mohs scale. The above topics are discussed in more detail in Part I by D. Sherman and D. Brandon, and by A. Krell. There are few theoretical attempts to describe hardness quantitatively in terms of the elastic bulk or shear moduli of an ideal solid. The elastic moduli and the corresponding hardness of a compound or element are strongly related to its molar volume, to its chemical bonding, and to its crystal structure. Basically, it can be stated that the higher the hardness the lower the molar volume and the more covalent the bonding of the material. Liu and Cohen suggested in their paper on hypothetical PC3N4 [12] that on the microscopic level, for ideal systems, hardness is determined by the bulk modulus. To estimate the bulk modulus they used an empirical model developed earlier [26], where the bulk modulus scales as a homopolar energy gap divided by the volume of the bond charge. The resulting relation gives theodependence of the bulk modulus B (in GPa) as a function of the bond length d (in A) and the empirical parameter A: 1971 - 220X (4) B= d3.5

Hurd Materials

XLlX

Table 2. Calculated equilibrium volumes, bulk moduli, and cohesive energies of some postulated ultrahard materials. The calculated and/or experimental values of diamond, lonsdaleite, cubic BN, and !3Si3N4are given for comparison. The experimental volume and density values are derived = from crystal structure data. CH.6 = all sp2-bonded hexagonal carbon phase 26,73,74]; Cb,c,t,.4 all sp*-bonded body-centered tetragonal carbon [26,72,76]; BNh.c.t.-4 all sp -bonded body centered =

1

tetragonal BN [119]. First published in [152] and reproduced with permission. Material Crystal symmetry cubic hexagonal cubic hexagonal hexagonal hexagonal tetragonal tetragonalVo

nm3/ atom] 5.67 (exp) 6.47 (exp) 5.90 (exp) 10.42 (exp) 6.25 6.29 6.72 7.37

Density [g/cm3] 3.51 (exp) 3.52 (exp) 3.45 (exp) 3.19-3.20 (exp) 3.49 3.17 2.97 2.82

B, [GPa]

Ecoh

Refs

[eVI 8.17/atom (calc) 7.37/atom (exp) 8.14/atom (calc) 13.2/BN pair (exp) 74.3/unit cell (calc) 81.5/unit cell 9.06/atom 8.47/atom 13.36/BN pair

Cdiarnond

Clonsdalelte cBN

444 (calc) 435 (exp) 440 (calc) 367 (calc) 369 (exp) 265 (calc) 256 (exp) 427 (calc) 372 (calc) 362 (calc) 268 (calc)

X is 0 for homopolar solids of group IV elements, and 1/2, 1, and 2 for heteropolar solids of group 111-IV, 111-V, and 111-VI elements, respectively. From this, it is evident that increasing ionicity results in a decrease in B. This expression has been further improved to account for non-octet compounds such as Si3N4 by using the average coordination number N, # 4 [26]:

B=-

(N,.) (1971 - 220X) 4 d3.5

In Table 2, some experimental and calculated structural properties such as the equilibrium volume, bulk modulus, and cohesive energies of candidate materials with potentially ultrahigh hardness are summarized. The equilibrium volume corresponds to the minimum of the total crystal energy as a function of volume [26]. The cohesive energy of a crystal represents the difference between the energy of the isolated constituent atoms or molecules and the energy of the crystal at absolute zero temperature [27]. It is evident from Table 2 that the higher the cohesive energy and the lower the equilibrium volume the higher the bulk modulus of the material. On the other hand materials deform plastically only when subjected to shear stress. According to Frenkel analysis, strength (yield stress) of an ideal crystalline solid is proportional to its elastic shear modulus [28,29]. The strength of a real crystal is controlled by lattice defects, such as dislocations or point defects, and is significantly smaller then that of an ideal crystal. Nevertheless, the shear stress needed for dislocation motion (Peierls stress) or multiplication (Frank-Read source) and thus for plastic deformation is also proportional to the elastic shear modulus of a deformed material. Recently Teter argued that in many hardness tests one measures plastic deformation which is closely linked to deformation of a shear character [17]. He compared Vickers hardness data to the bulk and shear

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Introduction: Novel Ultrahard Materials

Figure 2. Scattering of the Vickers hardness for a common set of hard materials when compared with bulk modulus (gray region) and shear modulus (black region). Following this comparison Teter suggested that the shear modulus is a significantly better qualitative predictor of hardness than the bulk modulus [ 171.

moduli from over thousand measurements and found that the shear modulus of polycrystalline aggregates is a significantly better qualitative predictor of hardness than the bulk modulus (Fig. 2). Experimental bulk moduli can be obtained from the measurement of lattice parameters and volumes as a function of pressure [30]. The single crystal elastic moduli can be measured using the Brillouin spectroscopy, inelastic neutron scattering, ultrasonic measurements or the Schaefer-Bergmann method [311. Once the single crystal moduli are known one can derive the bulk B and shear G moduli of a polycrystalline material [32].

Carbon-based Hard MaterialsThe most common allotropes of elemental carbon are graphite and diamond. Graphite crystallizes in a sheet structure with hexagonal symmetry and sp2 hybridization of the trigonally coordinated C-atoms whereas diamond exhibits a cubic lattice related to the zinc blende structure with sp3 hybridization of the tetrahedrally coordinated C-atoms. Less common carbon polymorphs are lonsdaleite with the hexagonal wurtzitic structure 1331, and the rhombohedral polymorph called pdiamond (3R polytype of wurzitic structure) [34,35]. The rhombohedral polymorph, which was reported as birefringent lamellae in natural diamonds, has not yet been obtained in significant amounts needed for detailed studies [35]. Graphite,

Carbon-based Hard Materials

LI

Figure 3. The different carbon modifications: (a) hexagonal graphite; (b) cubic diamond; and (c) hexagonal lonsdaleite. First published in [ 1521 and reproduced with permission.

diamond, and lonsdaleite are, however, well investigated and their structures are shown in Fig. 3. In lonsdaleite, comprised of the same tetrahedral configuration as in diamond, the planes of six-membered carbon rings are in the chair and boat conformation and are stacked in an ABAB sequence while in diamond, these planes exhibit the chair conformation exclusively and are arranged in an ABCABC sequence. The recent discovery of the c 6 0 molecule shown in Fig. 4, which is the most prominent representative of the fullerene family, led to a new type of carbon allotrope. In fullerenes, the carbon atoms form spherical clusters comprised of six- and five-membered rings of the general composition C20+2m (m = 0 , 2 , 3 . ..) where m is the number of hexagons. The c 6 0 fullerene has been predicted by Kroto and Smalley since 1985 [36-38] and was isolated first in macroscopic quantities by Kratschmer et al. in 1990 [39-41]. At present, the spherical carbon clusters and their derivatives are discussed with respect to their application in material science [42,43]. Furthermore, a variety of new polymeric carbon networks have been postulated to exhibit interesting materials properties [41,44]. Recently, carbon nitrides such as the hypothetical compound C3N4 have been also considered for the synthesis of ultrahard materials [ 14,151.

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Introduction: Novel Ultrahard Materials

Figure 4. Molecular model of the fullerene C60.The gray carbon atoms illustrate one of the 12 pentagons present in C6,,. First published in [152] and reproduced with permission.

DiamondBesides the aesthetic appearance of diamond in the form of gemstones, diamond is the hardest material known, and is, therefore, on the top of the Mohs scale and has the lowest molar volume (3.4cm3) of any material. Owing to this unique property, diamond is used for many technological applications such as abrasion, cutting, and polishing. Diamond also has the highest thermal conductivity (2000 W m-' K-') of any material at room temperature, four times as high as the value of Cu or Ag, is a good electrical insulator, it has a small dielectric constant and exhibits a high electron/hole mobility. Boron doped diamond (Typ IIb) exhibiting a p-type conductivity has been known for a long time. Production of diamond with a reasonable n-type conductivity was one of the most difficult tasks in diamond synthesis. Koizumi and coworkers recently obtained diamond films exhibiting n-type conductivity by using microwave enhanced plasma CVD with phosphine PH3 as a donor source for homoepitaxial deposition of n-type diamond 1451. Therefore, diamond will be a most important candidate material for future applications in electronic devices. In addition, diamond has low coefficients of friction and thermal expansion, high chemical and corrosive resistance towards most acids and oxidizing substances, it is transparent to visible and infrared light, withstands ionizing radiation and can, therefore, also be used as window or lens material or as a protective coating for this application. Conventionally, diamond is synthesized at high pressure and high temperature according to the method developed by the General Electric Company in the 1950s

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[l, 461. Synthetic diamond is now commercialized and is utilized for cutting, grinding, and polishing. Recent developments in the high-pressure synthesis technique allow growth of diamond single crystals up to 25 carats in weight [47]. The high-pressure synthesis of diamond is described in more detail by M. W. Bailey et al. in Part 111, while hydrothermal synthesis is discussed in Part I1 by K. G. Nickel et al. Since the mid-1980s the vapor deposition synthesis of diamond films has attracted increasing scientific and industrial interest. At present, vapor-grown diamond products are commercially viable as thin-film-coated cutting tools and freestanding thick-film cutting tools, as substrates for thermal management application and radiation detectors, as optical windows and for production of high-range audio-speaker diaphragms. Commercialization of the vapor-grown diamond became possible due to the cost reduction below $5/carat as a result of the dramatic increase in growth rate and of improvements in energy-use efficiency [481. Advanced diamond films are synthesized using the gas-phase decomposition of volatile carbon sources such as methane (CH4), acetone (H3C-CO-CH3), carbon monoxide (CO), acetylene (HC-CH), or adamantane (CI0Hl6)and is conducted under temperature and pressure conditions where graphite is the stable polymorph (Fig. 5). The most important methods for producing CVD-diamond under metastable conditions are:-

Microwave Plasma Assisted Chemical Vapor Deposition (PACVD) [49,50], Heated Filament Assisted Chemical Vapor Deposition (HFCVD) [5 13,

Figure 5. Pressure and temperature conditions of the diamond synthesis: (a) shock wave production of diamond; (b) high temperature, high pressure regime for the synthesis of diamond; (c) catalytic region for diamond formation; (d) chemical vapor deposited diamond; and (e) transformation of CG0into diamond. The most recent review of the P, T phase diagram of carbon can be found elsewhere [151].

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Introduction: Novel Ultrahard Materials

Direct Current (DC) Plasma Jet Deposition [52,53],and Oxygen-Acetylene Torch [54].

The chemical-vapor deposition of diamond films and their applications are reviewed by C.-P. Klages and by R. S. Sussmann et al. in Parts I1 and 111, respectively. To date the most effective CVD method (with the greatest mass deposition rate) is based on the hydrogen/hydrocarbon gas mixtures. In this method diamond is formed for kinetic reasons according to the simplified reaction:Cdiamond f 2H2. In a typical process, the hydrocarbon precursor containing more than 95% H2-gasis passed through a plasma (700-1000C) or over a heating filament ( T M 2000") at less than atmospheric pressure. The reaction product, solid carbon, is then deposited on a substrate heated at 800-1000C and contains both graphite and diamond. In the presence of H2-gas, the formation of unsaturated carbon nuclei and hence the growth of graphite is suppressed by atomic hydrogen formed under these conditions and is due to the reaction of the solid carbon with hydrogen radicals (H') giving volatile hydrocarbons. The reaction rate of Cgraphite H' is about 20 times as with high as the rate of diamond. Therefore, the formation of diamond is promoted. Growth rates of up to 0.9 mm h-' [55] or 20 carats per hour [48] have been achieved on a variety of substrates making the CVD-diamond a highly interesting material for technological applications. Diamond films have also been deposited from hydrogen-free gas mixtures such as C60/Ar using microwave apparatus. The deposition rates are, however, significantly lower. Additionally, use of hydrogen-poor plasmas results in nanocrystalline (3-10nm) diamond films in contrast to micrometer sized crystals from the hydrogen-rich plasmas [56]. There are continuous theoretical attempts to describe the mechanism of CVDdiamond synthesis including mechanisms of surface reactions, diamond nucleation, and film growth. To achieve this aim various phenomenological or first-principles models, molecular dynamics and Monte Carlo simulations have been used [57,58]. While the dominant substrate for low pressure growth of diamond films is single crystal silicon, several different materials such as Ta, Mo, W, Cu, Au, Ni, Sic, SO2, and Si3N4have been coated with polycrystalline diamond or diamond-like deposits. The nucleation rates and the adhesion of the generated films vary with the type of substrate material, which is related to the formation of intermediate carbide layers such as Sic or WC. Identification of vapor grown diamond is accomplished by X-ray diffraction or electron diffraction using a transmission electron microscope (TEM) and by Raman spectroscopy. Diamond coatings show the characteristic first-order Raman peak for diamond at 1332cm-' [59]. In many cases, an additional peak located at ~ 1 5 5cm-' occurs in the Raman spectrum which is attributed to a highly disordered 0 carbon phase (diamond-likecarbon) [60]. This diamond-like carbon contains sp3 and sp2 hybridized C-atoms which do not coincide with graphite. Recently a new simple method for the synthesis of diamond, called metallic reduction-pyrolysis-catalysis, was reported. In this method diamond powder was CH4

H2

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obtained as a product of reaction of carbon tetrachloride with metallic sodium in an autoclave at 700C, where CC14 served as a carbon source [18]: CC14700C + 4NaCl- Catalyst

C

+ 4NaCl.

As a metal catalyst an Ni-Mn-Co alloy (70 :25 :5 wt%) was used. About 2% of the carbon yield was well crystallized diamond and the residue was most probably in an amorphous form, as was evident from X-ray powder diffraction and Raman spectroscopic measurements.

Diamond-like and Amorphous CarbonIn the course of the research into the synthesis of diamond under metastable conditions, a new class of materials, diamond-like carbon and hydrocarbon phases, have been discovered. The diamond-like hydrocarbons (aC :H) are generated by the R F self-bias method, a technique derived from R F sputtering, developed by L. Holland [61,62]. The molecular ions, C,H;, derived from the particular hydrocarbon used in the plasma, disintegrate upon colliding with the substrate surface resulting in the formation of diamond-like hydrocarbon films [63]. The main structural feature of diamond-like hydrocarbons is the presence of both sp3- and sp2-carbon. Solid-state NMR-investigations revealed that the material contains sp3-carbon atoms of the form -C-H or H-C-H [6]. No quaternary carbon atoms could be detected while methyl groups, -CH3, were found to be present in small amounts. In addition, resonant Raman spectroscopy showed small 7r-bonded clusters of sp2 sites [64]. It was found that conducting the R F self-bias experiment with hydrocarbons in the presence of H2 results in production of small diamond clusters. This experimental result indicates that aC : H could be considered as an intermediate compound formed during the transformation of hydrocarbons to diamond in the plasma processes [65]. The mechanical properties of diamond-like hydrocarbon films strongly depend on the hydrogen content. Increasing the ratio of sp3 sites results in decreased hardness values and lower wear resistance [66]. This phenomenon is attributed to the fact that hydrogen is monovalent and cannot contribute to formation of a covalently bonded and highly cross-linked carbon network as found in the diamond structure. However, microhardness values in the range 30-50 GPa have been measured in aC :H films, values significantly higher than the hardness of Sic (20-38 GPa). In contrast, diamond-like carbon (aC) is free of hydrogen or contains only little hydrogen and does not represent microcrystalline diamond [67,68]. The preparation of aC has been accomplished by magnetron sputtering [68]. Again, the structure and properties of the aC phase is related to the ratio of the sp3/sp2 sites. However, hydrogen is not responsible for the stabilization of the sp3-hybridizedcarbon atoms. Here, the mechanical constraints resulting from random covalent networks, which can be calculated using the constraint-counting method developed by J. C. Phillips [69], can be reduced by the generation of medium-range or long-range order by clustering or crystallization.

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Introduction: Novel Ultrahard Materials

Another class of novel carbon material is amorphic carbon which can be obtained by laser ablation [70]. This amorphous phase contains sp3 -bonded nodules of pure carbon embedded in a matrix of other types of carbon phases. The mean particle size of the nodules is 20 nm, the density of the films is between 1.85 and 2.89 g crnp3.The internal stresses derived by distortions of the bonding angles in random networks are counterbalanced by the high surface to volume ratios of the nanoparticles. Substrate discs 30mm in diameter were coated with uniform layers of amorphous carbon by laser ablation of a graphite feedstock. Maximum growth rates of about 0.5 pm h-' and film thicknesses of up to 5 ym have been achieved on different substrate materials such as Si, Ge, ZnS, Cu, stainless steel, quartz, glass, and plastics. One outstanding property of laser ablation in comparison with the CVD process for the preparation of carbon films is that the substrate temperature does not exceed 35C during the whole deposition procedure. In contrast to the CVD diamond, which develops polycrystalline columnar structures, the laser-ablated amorphous carbon is deposited in the form of self-seeding nodules. Diamond-like carbon films are discussed in more detail in Part I11 by C.-P. Klages and K. Bewilogua.

Novel Hypothetical Three-dimensional Carbon PhasesA dense carbon phase with a calculated density of 4.1 g cmP3was predicted by N. N. Matyusenko and V. E. Strel'nitzkii in 1979 [71]. Due to this high density value, ultrahigh hardness of this carbon material is expected. In addition, several different hypothetical three-dimensional polymeric carbon networks with interesting materials properties have been proposed. The most relevant ones with respect to the potential of high hardness are the following carbon networks. (i) In 1983, R. Hoffmann published a metallic allotrope of carbon in which layers of infinite polyene chains are connected by bonds parallel to the c-axis. Each layer is rotated by 90" about the c-axis (Fig. 6a). The unit cell of the crystal structure is primitive body centered tetragonal and contains four atoms (b.c.t.-4 structure). In this network, carbon is present in the form of trigonal sp2 atoms. However, in contrast to graphite where the carbon atoms are arranged in a two-dimensional sheet structure, the trigonal carbon atoms of the b.c.t.4 structure form a three-dimensional network [72]. (ii) A similar carbon allotrope was proposed by M. A. Tamor and K. C. Hass in 1990. In this structure, the chains of the trigonal sp2 carbon atoms are rotated by 60" rather than 90" between the layers. The three-dimensional all sp2 phase of carbon has a hexagonal Bravais lattice with six atoms in the primitive unit cell and is known as the H-6 structure (Fig. 6b). However, theoretical calculations indicate that H-6 carbon is unstable with respect to the transformation to diamond. This instability is probably due to the short distances between carbon chains [73,74]. (iii) The 'super diamond' structure shown in Fig. 7 can be derived from tetraethynylmethane C(C2H)4. In principle, the polymeric network of the super diamond could be obtained by oxidative coupling of the methane derivative [41,75].

Curbon-based Hard Materials

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Figure 6. (a) Model of a section of the hypothetical polymeric network of the body centered tetragonal structure (b.c.t.4) of carbon and BN suggested by R. Hoffmann et al. [72]. (b) Model of a section of the hypothetical hexagonal structure (H-6) of carbon postulated by M. Tamor and K. Hass [73,74]. In both structures, each carbon atom is considered to be trigonally coordinated and sp2-bonded, exclusively. First published in [ 1521 and reproduced with permission.

(iv) The carbon skeleton of allene, 2HC=C=CH2, could also be utilized for the formation of a three-dimensional carbon phase [41]. In this case, the oxidative coupling of allene could provide a carbon structure with orthogonally arranged chains of sp2 carbon atoms (Fig. 8). A similar carbon phase could be obtained [75].Here, by the coupling of tetraethynylallene, (HCGC)~C=C=C(C=CH)~ the carbon chains are connected by butadiene units instead of single bonds. Both the b.c.t.-4 and the H-6 phase of carbon are discussed in terms of intermediate structures formed during the chemical vapor deposition of diamond. Furthermore, these types of structures could play an important role in diamond-like phases. The calculations of the structural properties of the b.c.t.-4 and H-6 carbon phase using

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Introduction: Novel Ultrahard Materials

Figure 7. Hypothetical super-diamond structure suggested earlier [41,75]. First published in [ I 521 and reproduced with permission.

the first-principles pseudopotential total-energy method revealed high bulk moduli B of 350 GPa and 372 GPa, respectively [26,76]. The extraordinary high values of B are in the range of cBN, the second hardest material known. Presently, the synthesis of new carbon phases by the coupling of unsaturated molecular organic compounds such as allenes or alkynes is being intensively investigated in several laboratories.

Figure 8. Hypothetical allene structure of carbon postulated earlier [41,75]. First published in [I521 and reproduced with permission.

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LIX

FullerenesIt has been shown that fullerenes or their derivatives can exhibit very interesting chemical, electrical, magnetic, and mechanical properties. Besides, for example, the superconductivity, which has been experimentally verified for the alkali metal fullerides M 3 C 6 0 with M = K or Rb [77], fullerenes can serve as a starting material for diamond synthesis [78,79] and may exhibit high hardness themselves under high pressure conditions [go]. In 1992 M. Regueiro et al. reported on the transformation of c 6 0 molecules into polycrystalline diamond by nonhydrostatic compression at room temperature [78]:

& c60

P > 20 GPa/20"C

' Cdiarnond.

In contrast c 6 0 withstands hydrostatic pressure up to 20 GPa [81]. However, the football molecules seem to be unstable towards uniaxial or shear stresses whereas they are stable under isotropic stress where the spherical molecules are homogeneously deformed. In a dense arrangement of c 6 0 spheroids, 48 of the 60 carbon atoms have a quasi-tetrahedral coordination which is required in the diamond structure. Only small structural rearrangements are then necessary for the transformation into diamond [78]. R. S. Ruoff and A. L. Ruoff proposed that c 6 0 is stiffer than diamond [go]. This result has been estimated from the calculated bulk modulus of individual c 6 0 molecules. The calculations revealed a bulk modulus B = 843 GPa which is nearly twice the experimental value of 441 GPa determined for diamond. Since solid c 6 0 forms a van der Waals crystal with f.c.c. lattice and the distance between the single c 6 0 molecules is about l.Onm, B would be relatively small under normal conditions. However, when the individual carbon spheres are compressed until they touch each other, the bulk modulus of the crystal would become in the range of that of the molecule. Taking into account a volume fill