COMPREHENSIVE HARD MATERIALS VOLUME 2

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COMPREHENSIVE HARD MATERIALS

VOLUME 2

CERAMICS

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COMPREHENSIVE HARD MATERIALS

VOLUME 2

CERAMICS

EDITOR-IN-CHIEF

VINOD K. SARIN

College of Engineering, Boston University Boston, MA, USA

EDITED BY

LUIS LLANES

Departament de Ciència dels Materials i Enginyeria Metal $ lúrgica Universitat Politècnica de Catalunya $ Barcelona Tech

Barcelona, Spain

DANIELE MARI

Laboratoire de Physique de la Matière Complexe

Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

AMSTERDAM•WALTHAM•HEIDELBERG•LONDON•NEW YORK•OXFORD PARIS•SAN DIEGO•SAN FRANCISCO•SINGAPORE•SYDNEY•TOKYO

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PREFACE

Comprehensive Hard Materials,first reference work of its types, is intended to merge together the myriad facets of this class of materials in a three volume series that reflect the abundance of established and novel uses across industry, from tribology to electronics and from transportation to energy. The range, growth, and innovations of the hard/superhard materials field has become commonplace within materials science as is evident from the vibrant growth of research across and within hardmetals (cemented carbides), ceramics, and superhard materials. Furthermore, the global nature of material science is also reflected by this work and contributors have been drawn from a panorama of global research hubs, with many chapters addressing the specific resource and application challenges within basic research, development, and emergent industrial markets. It is in the true spirit of collaboration across the range of these otherwise hermetically sealed areas of research that this work has been conceived and built. The reader will find original articles covering many relatively unexamined but converging research spaces and the content fully examines the various relationships that focus on fundamentals, properties, characterization, and applications of such materials. It was the objective of the editorial team to assemble a work that would provide strategic insights into hard/superhard materials.

The ﬁrst volume is devoted to hardmetals, generally known as cemented carbides in the US. This class of materials is composed of unique combinations of carbides and nitrides of transition metals ranging from the fourth to the sixth group of the periodic table and ferrous metals such as Co, Ni, Fe. Since the discovery of WC–Co by Schröter in 1922, they have epitomized a century of industrial applications mainly as cutting tools and wear resistant parts. Probably, without them many challenging enterprises in the ﬁeld of rock drilling, mechanics or just wire manufacture would not have been possible. Starting from a historical viewpoint the volume contains detailed aspects of manufacturing, composition/structure/properties, and applications.

The aim was to deﬁne the state of the art in hardmetal manufacturing together with a thorough illustration of the most recent progress obtained by scientiﬁc research.

Volume 2 is devoted to hard ceramicsda unique group of materials with a long-standing technical success history but subjected to permanent scientific and technological evolution. Representative ceramics within this subgroup of hard materials are alumina, silicon nitride, silicon carbide, their composites, in the form of bulk material and coatings. Also included are transition metal nitrides, carbides, borides, oxides, and mullite-based environmental barrier coatings. The intention was not only to project the substantial progress achieved in this field, but also to illustrate how scientific research findings are continuously transferred into industrial and technological advances. The overall aim of this volume was to present a critical review of established knowledge and recent advances of hard ceramics covering both their materials science and engineering aspects.

Volume 3 is devoted to superhard materialsdmaterials resistant to a change of shape under applied force, and with a Vickers hardness value exceeding 40 GPa. The strongest of these are composed out of boron, carbon, nitrogen, and oxygen, and can be classiﬁed into two categories: intrinsic compounds and extrinsic compounds.

The intrinsic group includes diamond as hardest material, cubic boron nitride (c-BN), carbon nitrides and ternary compounds such as B–N–C. Extrinsic materials are those that show super hardness determined by microstructure rather than composition, such as nanocrystalline diamond. These materials are of great interest in many industrial applications such as abrasives, polishing and cutting tools, wear-resistant, and protective coatings. Furthermore, diamond is emerging as an attractive material for new device applications in theﬁeld of power electronics, room temperature quantum computing, biosensing, biointerfaces, microelectromechanical systems, color centers and high energy radiation, and particle detectors to name a few. The aim of volume 3 was to present all the critical areas related to superhard materials with an emphasis on recently achieved progress in research, development, and applications.

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The Elsevier publication team, especially Paul Prasad Chandramohan, has worked tirelessly to make this publication possible and available in two formats:

l An online version via the Science Direct platform that will guarantee easy worldwide distribution.

l A hard copy version that will be made available based on orders received.

This dual concept readily lends itself to maintaining this reference work current via additions and modiﬁcations.

Elsevier (Donna de Weerd-Wilson) has discussed this concept with the editors and is in the process of moving on it in the near future.

The idea for this multivolume major reference work originated through discussions at the International Conferences on the Science of Hard Materials. From the outset, David Sleeman and Graham Nisbet (Elsevier) identified not only an overwhelming qualitative demand for extensive, international, innovative, methodo- logically sound coverage of the subject matter, but also keen interest in a quantitative form as well. Given this encouragement the volume editors Daniele Mari, Luis Miguel Llanes, Christoph E. Nebel, numerous expert contributors and reviewers are responsible for the creation of this definitive publication to encompass the full breadth and considerable depth of the hard/superhard materialsfield. The editors, contributors, and publishers invested 5 years of time and effort culminating in this, Comprehensive Hard Materials,and we are proud to present it to our colleagues and the community it is intended to serve.

Vinod K. Sarin Boston University Boston, MA 02215 USA x Preface

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EDITOR-IN-CHIEF

Prof. Saringraduated from MIT (Boston, MA) with an Sc.D in material science.

He then joined Sandvik’s Coromant Research Center, Stockholm, Sweden. Four years later he came back to the US, joined Adamas Carbide and then GTE, where he started an R&D group on hardmetals and ceramics, and was promoted to senior research scientist (Fellow) at GTE. After nearly 20 years of industrial experience he joined academia (Boston University) as professor in the College of Engineering. He has been the recipient of several technical, achievement, and industrial awards, invited distinguished scientist at the Max Planck Institute (Germany), University of Linkoping (Sweden), Visiting Professor, University of Lund (Sweden), Visiting Scholar/Professor Universitat Politècnica de Catalunya (Spain), and a global industrial R&D consultant. He singly or jointly holds over 80 patents, has edited several books, has authored or coauthored several chapters, and over a hundred technical publications. He is a member of the editorial board of several journals, and has chaired several technical sessions and orga- nized several conferences on hard materials.

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VOLUME EDITORS

Daniele Marigraduated in physics in 1986, and in 1991 obtained his PhD at the Federal Institute of Technology (EPFL) in Lausanne, Switzerland, with a study on high temperature properties of hardmetals. From 1992 to 1993, he continues his research at the Massachusetts Institute of Technology working in the ﬁeld of shape memory alloys. In 1993, he joins the company Amysa Yverdon SA (Switzerland) as director of R&D and creates Advanced Composite &

Microwave Engineering with activities in theﬁelds of the electromagnetic heating and materials. In 2004, he returns to academia at EPFL with a research group specialized in mechanical spectroscopy. He is the co-author of more than 70 scientiﬁc papers and is presently deputy director of the physics section at EPFL.

He is a member of the Editorial Board of the International Journal of Refractory Metals and Hard Materials. He has been co-chairman of the last two Interna- tional Conferences on the Science of Hard Materials (ICSHM).

Luis Llanesis a full professor and current chairman in the Department of Materials Science and Metallurgical Engineering, as well as board member of the Research Center of Nanoengineering in the Technical University of Cata- lonia (UPC). He graduated in materials engineering from Simon Bolivar University and received his PhD degree in 1992 from the University of Penn- sylvania. He carries out his research activities within the Structural Integrity, Micromechanics and Reliability of Engineering Materials (CIEFMA) group at UPC. His research interests include mechanical integrity and damage assess- ment of structural materials, particularly hardmetals, advanced ceramics, stainless steels, and hard coatings. He is the co-author of more than 70 papers published in top journals in theseﬁelds. In addition, he has a track record of technology transfer and consultancy with hard materials industries. He is member of the Editorial Board of the International Journal of Refractory Metals and Hard Materials as well as of the Steering Committee of the European Hard Materials Group within the European Powder Metallurgy Association. He has been co-chairman of the last three ICSHM.

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Chrsitoph E. Nebel graduated in electrical engineering at the University of Stuttgart where he also got his PhD in 1989. He became a postdoc at the Xerox Research Center, Palo Alto, USA between 1990 and 1992 funded by the Alexander v. Humboldt Foundation, Germany. He joined the team of Martin Stutzmann at the Walter Schottky Institute of the Technical University Munich in 1993 where he habilitated in 1998. In 2004, he joined the Diamond Research Center of the National Institute of Advanced Industrial Science and Technology, Japan, where he was leading the “Bio-Functionalized Device Team”. In 2008, he went back to Germany to become head of the Micro- and Nano-Sensor Department of the Fraunhofer-Institute for Applied Solid State Physics in Freiburg.

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LIST OF CONTRIBUTORS

Jocelyn Achard

Laboratoire des Sciences des Procédés et des Matériaux (LSPM), UPR3407 CNRS, Université Paris 13, France;

Laboratoire d’Excellence Sciences and Engineering for Advanced Materials and devices (SEAM), Villetaneuse, France

Igor Aharonovich

School of Physics and Advanced Materials,

University of Technology Sydney, Ultimo, New South Wales, Australia

Marc Anglada

Departament de Ciència dels Materials i Enginyeria Metal$lúrgica, Universitat Politècnica de

Catalunya$Barcelona Tech, Barcelona, Spain Thomas Babinec

Departments of Electrical Engineering and Applied Physics, Stanford University, CA, USA

Soumendra N. Basu

Division of Materials Science and Engineering, Department of Mechanical Engineering, Boston University, Boston, MA, USA Bikramjit Basu

Materials Research Center, Indian Institute of Science, Bangalore, India

Carmen Baudín

Instituto de Cerámica y Vidrio, CSIC, Madrid, Spain

I. Bello

Center of Super-Diamond and Advanced Films (COSDAF), and Department of Physics and Materials Sciences, City University of Hong Kong, Tat Chee Avenue, Hong Kong SAR, P.R. China

Eleni Berdermann

GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany

Raul Bermejo

Institut für Struktur- und Funktionskeramik, Montanuniversität Leoben, Leoben, Austria

Vladimir Blank

Technological Institute for Superhard and Novel Carbon Materials, Moscow, Russia

Vadim Brazkin

Institute for High Pressure Physics, Russian Academy of Sciences, Troitsk, Moscow, Russia

Ovidiu Brinza

Y.M. Chong

Marvin L. Cohen

Department of Physics, University of California at Berkeley, Berkeley, CA, USA; Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

Christoph Czettl

CERATIZIT Austria GmbH, Reutte, Austria Robert Danzer

Institut für Struktur- und Funktionskeramik, Montanuniversität Leoben, Leoben, Austria Goffredo de Portu

National Research Council of Italy (CNR), Chemical Science & Materials Technology Department, Institute of Science and Technology for Ceramics (ISTEC), Faenza, Italy

Nadira Derkaoui

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Eric F. Drake

Rice University, Houston, Texas, USA Yasuaki Einaga

Department of Chemistry, Keio University, Yokohama, Japan

Peter Ettmayer

Vienna University of Technology, Institue of Chemical Technologies and Analytics, Vienna, Austria

Z. Zak Fang

Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT, USA

Gilbert Fantozzi

INSA de Lyon, MATEIS (UMR CNRS 5510), Villeurbanne, France

S.A. Firstov

Department of Physics of Strength and Plasticity of Materials, Frantsevich Institute for Problems of Materials Science, Kiev, Ukraine

Rainer Gadow

University of Stuttgart, Institute for Manufacturing Technology of Ceramic Components and Composites, Stuttgart, Germany

A.J. Gant

National Physical Laboratory, Teddington, Middlesex, TW, UK

Mark G. Gee

Randall M. German

Department of Mechanical Engineering, College of Engineering, San Diego State University, San Diego, CA, USA

Alix Gicquel

Brahma Raju Golla

Engineering Ceramics Research Group, Korea Institute of Materials Science, Changwon, Gyeongnam, Korea Stefano Guicciardi

National Research Council of Italy (CNR), Chemical Science & Materials Technology Department, Institute of Science and Technology for Ceramics (ISTEC), Faenza, Italy

Stuart Hampshire

Materials and Surface Science Institute, University of Limerick, Limerick, Ireland

Khaled Hassouni

Bin He

Mathias Herrmann

Fraunhofer-Institut für Keramische Technologien und Systeme IKTS, (www.ikts.fraunhofer.de) Dresden, Germany

Jonathan Hird

Schlumberger Gould Research, Cambridge, United Kingdom

Tetsuo Irifune

Geodynamics Research Center, Ehime University, Matsuyama, Japan; Earth-Life Scinece Institute, Tokyo Institute of Technology, Tokyo, Japan

Thomas J. Jewett

Global Tungsten and Powders Corporation, Towanda, Pennsylvania, USA

Emilio Jiménez-Piqué

Departament de Ciència dels Materials i Enginyeria Metal$lúrgica, Universitat Politècnica de Catalunya$ Barcelona Tech, Barcelona, Spain

Shinhoo Kang

Department of Materials Science and Engineering, Seoul National University, Gwanak-ku, Seoul, Korea

Hiromitsu Kato

Energy Technology Research Institute, AIST, Japan

Frank Kern

University of Stuttgart, Institute for Manufacturing Technology of Ceramic Components and Composites, Stuttgart, Germany

Hagen Klemm

Fraunhofer-Institut für Keramische Technologien und Systeme IKTS, (www.ikts.fraunhofer.de) Dresden, Germany

xvi List of Contributors

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Hans Kolaska

Fachverband für Pulvermetallurgie e.V., Hagen, Germany

Igor Konyashin

Element Six Hard Materials, Element Six GmbH, Städeweg, Burghaun, Germany

Mark C. Koopman

Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT, USA

Aaron D. Krawitz

University of Missouri, Columbia, Missouri, USA Jochen Kriegesmann

Department of Materials Technology, Glass and Ceramics, University of Applied Sciences Koblenz, Höhr-Grenzhausen, Germany

Anke Krueger

Institute for Organic Chemistry, Julius-Maximiliaus University würzburg, Am Hubland, 97074 Würzburg, Germany

Boris Kulnitskiy

Sabine Lay

Grenoble Institute of Technology, Saint-Martin d’Hères, France

Shuit-Tong Lee

Luis Llanes

Departament de Ciència dels Materials i Enginyeria Metal$lúrgica, Universitat Politècnica de Catalunya$ Barcelona Tech, Barcelona, Spain

Kian Ping Loh

Department of Chemistry, National University of Singapore, Singapore

Guillaume Lombardi

John E. Lowther

School of Physics and DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg, South Africa

Andriy O. Lyakhov

Department of Geosciences, Stony Brook University, Stony Brook, NY, USA

Alexander Lyapin

Institute for High Pressure Physics, Russian Academy of Sciences, Troitsk, Moscow, Russia

Daniele Mari

Laboratoire de Physique de la Matière Complexe, Ecole Polytechnique Fédérale Lausanne, Lausanne,

Switzerland

Julián Martínez-Fernández

Dpto. Física de la Materia CondensadadICMS, Universidad de SevilladCSIC, Sevilla, Spain Pankaj K. Mehrotra

Kennametal Inc., Latrobe, PA, USA Jan Meijer

Nuclear Solid State Physics, University Leipzig, Leipzig, Saxony, Germany

Lutz-Michael Berger

Fraunhofer Institute IWS, Dresden, Germany

Armelle Michau

Claude Michotte

CERATIZIT Luxembourg S. à. r. l., Route de Holzem, Mamer, Luxembourg

K.P. Mingard

Rosa M. Miranda

UNIDEMI, Departamento de Engenharia Mecânica e Industrial, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal

Pilar Miranzo

Institute of Ceramics and Glass, CSIC. Kelsen, 5, Madrid, Spain

Jean-Michel Missiaen

Grenoble Institute of Technology, Saint-Martin d’Hères, France

Christian Mitterer

Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben, Austria

List of Contributors xvii

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Michael Moseler

Fraunhofer Institute for Mechanics of Materials IWM, Freiburg, Germany

Christoph E. Nebel

Fraunhofer-Institute for Applied Solid State Physics, Freiburg, Germany

Steffen Nowotny

Fraunhofer Institut IWS, Dresden, Germany Artem R. Oganov

Department of Geosciences, Center for Materials by Design, Institute for Advanced Computational Science, Stony Brook University, Stony Brook, NY, USA; Moscow Institute of Physics and Technology, Dolgoprudny city, Moscow Region, Russian Federation; Northwestern Polytechnical University, Xi’an, China

Hugo M. Ortner

Darmstadt University of Technology, Department of Material Sciences: Materials Characterization, Darmstadt, Germany

Maria Isabel Osendi

Institute of Ceramics and Glass, CSIC. Kelsen, 5, Madrid, Spain

Lars Pastewka

Fraunhofer Institute for Mechanics of Materials IWM, Freiburg, Germany

Arvid Eric Pasto

Retired from Sparks, NV, USA Sébastien Pezzagna

Nuclear Solid State Physics, University Leipzig, Leipzig, Saxony, Germany

S.S. Ponomarev

Department of Physics of Strength and Plasticity of Materials, Frantsevich Institute for Problems of Materials Science, Kiev, Ukraine

Michail Popov

Leo Prakash

Kyocera Unimerco Tooling A/S, Sunds, Denmark Joaquin Ramírez-Rico

Dpto. Física de la Materia CondensadadICMS, Universidad de SevilladCSIC, Sevilla, Spain Pascal Reynaud

INSA de Lyon, MATEIS (UMR CNRS 5510), Villeurbanne, France

John Robertson

Department of Engineering, Cambridge University, Cambridge, UK

B. Roebuck

National Physical Laboratory, TW, UK Gregory S. Rohrer

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA, USA Catherine Rond

Laboratoire des Sciences des Procédés et des

Matériaux (LSPM), UPR3407 CNRS, Université Paris 13, France; Laboratoire d’Excellence Sciences and Engineering for Advanced Materials and devices (SEAM), Villetaneuse, France

Malika Saâdaoui

LERSIM Ecole Mohammadia d’Ingénieurs, Université Mohamed V Agdal, Rabat, Morocco

Vinod K. Sarin

Division of Materials Science and Engineering, Department of Mechanical Engineering, Boston University, Boston, MA, USA Uwe Schleinkofer

CERATIZIT Austria GmbH, Reutte, Austria Matthias Schreck

Universität Augsburg, Institut für Physik, Augsburg, Germany

Alex V. Shatov

8 Parkside Drive, Jamesburg, NJ, USA Olga A. Shenderova

International Technology Center, Raleigh, NC, USA;

Adámas Nanotechnologies, NC, USA François Silva

Laboratoire des Sciences des Procédés et des

Matériaux (LSPM), UPR3407 CNRS, Université Paris 13, France; Laboratoire d’Excellence Sciences and Engineering for Advanced Materials and devices (SEAM), Villetaneuse, France

Raj P. Singh Gaur

Vladimir L. Solozhenko

LSPM–CNRS, Université Paris 13, Villetaneuse, France

Jörg Spatzier

Fraunhofer Institut IWS, Dresden, Germany xviii List of Contributors

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Hitoshi Sumiya

Electronics Materials R&D Laboratories, Sumitomo Electric Industries Ltd, Itami, Japan

Alexandre Tallaire

Takashi Taniguchi

Advanced Key Technology Division, National Institute for Materials Science, Tsukuba, Ibaraki, Japan Shi Wun Tong

Department of Chemistry, National University of Singapore, Singapore

Yadir Torres

Universidad de Sevilla, Department of Mechanical and Materials EngineeringdETSI, Sevilla, Spain

Vladimir Turkevich

Synthesis and Sintering of Superhard Materials at High Pressures and High Temperatures, Institute for

Superhard Materials of the Ukrainian National Academy of Sciences, Kiev, Ukraine

Hongtao Wang

Kennametal Inc., Rogers, AR, USA

Maxime Wartel

Dow Whitney

Retired from Department of Materials Science and Engineering at the University of Florida, Gainesville, Florida, USA

Christoph Wild

Diamond Materials GmbH, Freiburg, Germany Thomas A. Wolfe

Eckhard Wörner

Diamond Materials GmbH, Freiburg, Germany Wenjun J. Zhang

Qiang Zhu

Department of Geosciences, Stony Brook University, Stony Brook, NY, USA

List of Contributors xix

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SECTION I

INTRODUCTION

2.01 Fundamental Aspects of Hard Ceramics

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2.01 Fundamental Aspects of Hard Ceramics

2.01.1 Introduction 4

2.01.2 Structure and Property Relationships 4

2.01.2.1 Bonding 5

2.01.2.2 Crystal Structure 6

2.01.3 Processing and Fabrication of Ceramics 7

2.01.3.1 Shaping and Forming 7

2.01.3.1.1 Basic Ceramic Forming Processes 7

2.01.3.1.2 Near-Net-Shape Processes 7

2.01.3.2 Thermal Treatment of CeramicsdSintering 9

2.01.3.2.1 Solid-State Sintering ₁₀

2.01.3.2.2 Liquid-Phase Sintering ₁₂

2.01.3.2.3 Reaction Sintering ₁₅

2.01.3.2.4 Microwave Sintering ₁₆

2.01.3.2.5 Pressure-Assisted Sintering ₁₆

2.01.3.3 Finishing Processes 16

2.01.4 Microstructure 17

2.01.4.1 Grain Boundary Structure and Modiﬁcation 18

2.01.4.1.1 Addition of Dopant Elements to Modify Grain Boundary Energy ₁₈

2.01.4.1.2 Addition of Small Particles at Grain Boundaries ₁₈

2.01.5 Mechanical Properties 19

2.01.5.1 Strength and Fracture Resistance 19

2.01.5.2 Time Dependence of Strength 20

2.01.5.3 Hardness 21

2.01.6 Some Examples of Hard Ceramics 22

2.01.6.1 Silicon Nitride 22

2.01.6.1.1 Reaction-Bonded Silicon Nitride ₂₂

2.01.6.1.2 Dense Silicon Nitride ₂₂

2.01.6.1.3 SiAlONs ₂₅

2.01.6.2 Silicon Carbide 25

2.01.6.3 Borides 26

2.01.7 Summary 26

References 26

Nomenclature

Al₂O₃Alumina, aluminium oxide which ina- Al₂O₃adopts a hexagonal structure but also exists as other structural modiﬁcations, notablyg- alumina which adopts the cubic spinel structure B₄CBoron Carbide

BNBoron Nitride, which exists as both cubic (with pseudo-diamond crystal structure) and hexagonal (with pseudo-graphite crystal structure) forms

Hf B₂Hafnium Diboride LiFLithium Fluoride

MoSi₂Molybdenum Disilicide NbCNiobium Carbide

SiCSilicon Carbide, which exists as many different structural polytypes. The basic forms are

b-SiC which adopts a cubic zinc blende (3C) structure anda-SiC which is hexagonal (6H).

Si₃N₄Silicon Nitride, which has three conﬁrmed crystallographic modiﬁcations,aandb, both hexagonal, and a high pressure produced cubic spinel phase,g-Si₃N₄. Silicon nitride is also a general term for a family of ceramics which are named according to their fabrication or sintering process:

RBSNReaction Bonded Silicon Nitride, formed by nitriding silicon powder compacts in the

temperature range 1100–1450^oC. Theﬁnal material contains 15–25% porosity.

SSNDense Sintered (or Pressureless Sintered) Silicon Nitride, formed by sintering a silicon

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nitride powder compact containing densiﬁcation additives (e.g. Y₂O₃+ Al₂O₃) under 0.1MPa N₂ atmosphere at 1750–1900^oC. The process involves Liquid Phase Sintering.

HPSNHot-Pressed Silicon Nitride, formed by sintering a silicon nitride powder compact containing densiﬁcation additives (e.g. Y₂O₃+ Al₂O₃) under an applied pressure in a graphite die heated by induction at 1750–1900^oC. The process involves Liquid Phase Sintering aided by the applied pressure.

HIPSNHot-Isostatically Pressed Silicon Nitride, formed by sintering a silicon nitride powder compact containing densiﬁcation additives (e.g.

Y₂O₃+ Al₂O₃) under a gas pressure applied through an impermeable vitreous membrane to encapsulate the compact. Theﬁnal process involves Liquid Phase Sintering.

GPSSNGas Pressure Sintered Silicon Nitride, formed by sintering a silicon nitride powder compact containing densiﬁcation additives (e.g.

Y₂O₃+ Al₂O₃) under a gas pressure (MPa). The process involves Liquid Phase Sintering.

SRBSNSintered Reaction Bonded Silicon Nitride, formed by nitriding a silicon powder compact containing densiﬁcation additives (e.g. Y2O₃+

Al₂O₃) in the temperature range 1100–1450ôC, followed by pressureless sintering under 0.1MPa N₂atmosphere at 1750–1900ôC. Thefinal process involves Liquid Phase Sintering.

SiAlONsSolid solutions of Al and O in the silicon nitride structure.b’-SiAlON is formed when O² replaces N³in theb-Si₆N₈structure while, at the same time, Si⁴⁺is replaced by Al³⁺to maintain charge neutrality.a-sialons (a’) contain Al³⁺and O²and are based on thea-Si₁₂N₁₆unit cell with accommodation of other ions, M¼Li⁺, Ca²⁺, Y³⁺

or other rare earth lanthanide ions (Ln³⁺), in two interstitial sites in the unit cell.

SiO₂Silica, silicon dioxide TaB₂Tantalum Diboride TaCTantalum Carbide TaNTantalum Nitride TiB₂Titanium Diboride TiCTitanium Carbide

TiO₂Titania, titanium dioxide

WCTungsten Carbide, usually liquid phase sintered with cobalt (WC-Co) to give a Cermet (ceramic-metal) composite.

ZrB₂Zirconium Diboride, sometimes used in a composite ceramic with SiC (ZrB₂–SiC).

2.01.1 Introduction

Ceramics are inorganic, nonmetallic materials that are produced from minerals or synthetic particulate materials that are formed into a green shape. Consolidation is typically achieved via a high-temperature process to reduce porosity and densify and strengthen the compact. Traditional ceramics include pottery and whitewares as well as industrial products such as clay ware, building materials, refractories, abrasives and cements. Advanced ceramics are generally pure oxides or nonoxides (nitrides, carbides, borides, etc.) or composites of these materials. They include materials that have been developed with designed microstructures for (1) structural applications (hard ceramics), making use of their mechanical, tribological or high-temperature properties for cutting and drilling tools, bearings, seals, dies, engine components and coatings on other materials to improve corrosion resistance or thermal stability and (2) functional applications, such as electronic, dielectric or mag- netic components, biomedical implants or devices,ﬁlters, membranes or catalyst supports.

Ceramics are hard, brittle, strong in compression, weaker in tension, wear resistant, can withstand high temperatures (1200–1700C), and have good resistance to chemical corrosion. These properties are, as with other materials, highly dependent on their structure, both at the atomic level (crystal structure) and on the micro- and macrostructural levels. These in turn are crucially dependent on the overall chemical composition and the material fabrication conditions which need to be carefully controlled in order to optimize the desired properties.

Since ceramics are manufactured for use in wear-resistant applications and possess high hardness, most parts have to be formed as near-net shape as possible by compacting powders into a body which is then sintered at high temperatures. During sintering the body shrinks, the grains bond together and a dense material is produced.

2.01.2 Structure and Property Relationships

Ceramics are made up of metallic and nonmetallic elements, the atoms of which are held together by chemical bonds that can be covalent and/or ionic. The type of bonding and the atomic arrangements (crystal structure) 4 Fundamental Aspects of Hard Ceramics

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have a strong inﬂuence on mechanical properties including elastic modulus and hardness. For high elastic modulus, the bond strength between atoms and stiffness of the bond must be high. Some values for Young’s modulus of some engineering ceramics are given inTable 1 along with their melting or decomposition temperatures, which also require high bond strengths. Ceramics have much higher melting points than other materials, the lowest in the range 1600–1700C, which is still higher than iron and steels.

2.01.2.1 Bonding

The types of bonds present in ceramics are ionic, covalent or a combination of the two.Pauling (1948)proposed a relationship between the“ionic character”of a bond and the difference in electronegativities of the two elements. Electronegativity is the tendency of an atom to attract electrons toward itself. Table 2(a) shows the electronegativity values for elements in theﬁrst two rows of the periodic table. The greater the difference between electronegativity values, the more ionic character the bond will possess. Conversely, the smaller the differences in electronegativity values the more “covalent character” the bond will possess. Table 2(b) compares the differences in electronegativity values between elements for different compounds and the approximate fraction of ionic or covalent character of the bonds. If the electronegativity difference is less thanw1.7, the bond is more covalent with atoms sharing the bonding electrons. Thus, boron carbide, silicon carbide and silicon nitride (Si₃N₄) have more covalent bonding whereas alumina has a higher proportion of ionic character.

Table 1 Young’s modulus and melting point of some engineering ceramics

Ceramic Young’s modulus (GPa) Melting point (C)

Titania TiO2 230 1605

Silica SiO2(fused) 72 1715

Chromic oxide Cr2O3 270 1990

Alumina Al2O3 390 2050

Silicon nitride Si3N4 310 1910 (sublimates)

Boron nitride c-BN w400 2970 (sublimates)

Tantalum nitride TaN – 3090 (sublimates)

Silicon carbide SiC 410 2500

Boron carbide B4C 470 2750

Tungsten carbide WC 650 2850

Titanium carbide TiC 460 3120

Niobium carbide NbC 370 3500

Tantalum carbide TaC 530 3850

Titanium diboride TiB2 – 3230

Tantalum diboride TaB2 680 3000

Hafnium diboride HfB2 – 3250

Table 2(a) Electronegativity values for elements in theﬁrst two rows of the Periodic table

I II III IV V VI VII

Li0.98 Be

1.57 B

2.04 C

2.55 N

3.04 O

3.44 F

Na 3.98

0.93 Mg

1.31 Al

1.61 Si

1.90 P

2.19 S

2.58 Cl

3.16

Table 2(b) Electronegativity differences and percentage of ionic/covalent character of the bonds for representative ceramic materials

Compound LiF Al₂O₃ SiO₂ Si₃N₄ SiC B₄C C (Diamond)

Electronegativity difference 3.0 1.83 1.54 1.14 0.65 0.51 0

% ionic character 90 63 50 30 11 6 0

% covalent character 10 37 50 70 89 94 100

Fundamental Aspects of Hard Ceramics 5

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2.01.2.2 Crystal Structure

The type of bonding and the relative sizes of atoms, as well as balance of electrostatic charges, all have an influence on the actual arrangement of atoms in ceramic crystal structures. In covalent crystals, the bond is directional and the atoms have small coordination numbers (4). For example, SiC exists in many poly- morphic forms calledpolytypeswhere the coordination between Si and C is always tetrahedral and the structures differ only in the sequence of identical layers of Si atoms coordinated to four C atoms.b-SiC adopts the cubic zinc blende structure shown inFigure 1(a), in which the Si atoms are arranged in a face-centered cubic or cubic close-packed type of lattice and the C atoms occupy half of the tetrahedral sites in the cube with each atom surrounded by four atoms of the other element (Fisher & Barnes, 1990; Pandey & Krishna, 1975). Using Ramsdell (1947)notation, which specifies the total number of stacking layers within a cubic (C), hexagonal (H) or rhombohedral (R) unit cell, b-SiC can be classified as 3C-SiC in which there are three Si–C bilayers with stacking sequence ABCABC. The other common polymorphs have a hexagonal structure, related to the wurtzite crystal structure shown inFigure 1(b)and, for historical reasons, are collectively referred to asa-SiC (Pandey &

Krishna, 1975). The difference between them is in the stacking planes of the Si–C bilayers along thec-axis. The wurtzite structure is 2H-SiC with stacking sequence AB–AB–A, 4H-SiC has a stacking sequence of ABCB-A, and 6H-SiC has a stacking sequence of ABCACB-A (Ching et al., 2006).

Si₃N₄has three confirmed crystallographic modifications,aandb(Hardie & Jack, 1957), both hexagonal, and a high pressure produced cubic spinel phase,g-Si₃N₄(Zerr et al., 1999). Ina- andb-Si₃N₄, all Si atoms are tetrahedrally bonded to N and all N atoms are in an approximate planer configuration. Octahedral bonding of Si to N is very rare and apparently only forms at high temperatures and pressures. The bonding leads to a framework of SiN₄tetrahedra (slightly distorted) joined by sharing nitrogen corners so that each nitrogen is common to three tetrahedra. Theb-structure is composed of puckered rings of alternating Si and N atoms which can be considered as layers with a stacking sequence ABAB as shown in Figure 2(a) (Hampshire, Park, Thompson, & Jack, 1978) and forming long continuous channels in the c-direction. Thec-dimension of the a-Si3N₄structure is approximately twice that ofb, where the layers of atoms inbare linked along the [001]

direction in the sequence ABAB, the a-structure has the sequence ABCDABCD. The CD layer, shown in Figure 2(b)(Hampshire et al., 1978), is similar to the AB layer except that it is rotated by180on thec-axis. The long continuous channels in thec-direction ofbare therefore closed off to form two large interstices. Ing-Si₃N₄, Si occupies both the tetrahedral A site and the octahedral B site in the spinel structure (Zerr et al., 1999).

In alumina, the oxygen atoms form an almost close-packed hexagonal arrangement with aluminum atoms occupying two-thirds of the octahedral sites (coordination of six) within the structure (Ishizawa, Miyata, Minato, Marumo, & Iwai, 1980). Alumina also exists as other structural modiﬁcations, notablyg-alumina that adopts the cubic spinel structure (Lee, Cheng, Heine, & Klinowski, 1997).

Boron-containing compounds have much more complex structures involving“clusters”of B atoms (Yakel, 1975). In B₄C, a central C–B–C chain is coordinated to a number of B12structural units making up a rhombohedral unit cell.

(a) (b)

A

B

Figure 1 (a) Cubic zinc blende (3C) structure adopted byb-SiC. (b) Hexagonal wurtzite (2H) structure similar toa-SiC (6H).

6 Fundamental Aspects of Hard Ceramics

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While the intrinsic properties of a material are dependent on chemistry, bonding and crystal structure, the major properties of ceramics, particularly strength and fracture resistance are much more inﬂuenced by microstructure.

The type of microstructure obtained for a ceramic depends on a number of factors:

l Shape, size, and size distribution of initial powder particles l Various steps used for shaping

l Conditions used for thermal treatment (temperature, time, atmosphere, pressure, etc.) l Mechanisms of densiﬁcation

l Chemical reactions or crystal-phase transitions.

2.01.3 Processing and Fabrication of Ceramics

2.01.3.1 Shaping and Forming

2.01.3.1.1 Basic Ceramic Forming Processes

Most ceramic forming processes prior toﬁring may be classiﬁed into three types (Heinrich, 2004):

1. Casting, including slip casting and tape casting.

2. Plastic forming including extrusion and injection molding.

3. Pressing including die pressing and isostatic pressing.

In these processes, the initial starting powders are prepared by suspending in aqueous or organic solvents. In the case of casting, these suspensions are directly processed into aﬁnal thin-walled complex shape with a uniform thickness using a suitable porous mold. A slip is prepared by ball milling appropriate powders with binders, plasticizers or deﬂocculants in water or an organic solvent. The liquid from the slip is absorbed into the mold by capillary action depositing a layer of the ceramic powder on the internal wall of the mold. Higher green densities can be achieved using pressure casting.

Plastic forming requires partly dewatered or higher viscosity feeds. Injection molding is suitable for high-volume processing of complex-shaped parts and precision components with a high level of reproducibility since it is a near-net shape technique. The powder has to be mixed with the appropriate polymer vehicle which becomes ﬂuid during the injection process. The disadvantage is that the polymer has to be burned out before sintering.

In pressing, almost completely dry free-flowing granules produced by spray drying are used. Die pressing, which involves the uniaxial compaction of this type of granulated powder in a metal die, is by far the most frequently used forming process for advanced ceramics. Isostatic pressing involves the shaping of powder compacts in a flexible container subject to either a pressurized liquid, the wet-bag process, or a compressed gas, the dry-bag process. Both methods allow a uniform compaction to form a green body with homogeneous density that re- tains the general shape of theflexible container and the profile of any internal tooling.

2.01.3.1.2 Near-Net-Shape Processes

In addition to slip casting and injection molding, other near-net-shape forming methods have been developed, based on the principles of colloid science, in order to produce complex-shaped components and these include the following.

Figure 2 Silicon nitride crystal structure showing (a) ABAB layers ofb-silicon nitride and (b) ABCD layers ofa-silicon nitride.

Hampshire et al. (1978).

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2.01.3.1.2.1 Direct Coagulation Casting (DCC)

This process (Graule, Baader, & Gauckler, 1994) allows consolidation of high solids content ceramic slurries into uniform, defect-free powder compacts by coagulation. If a polar liquid is used as the liquid vehicle, the viscosity of the ceramic suspension at a given solids loading depends on the particle surface charge, usually described by the zeta potential. This can be greatly inﬂuenced by changing the pH of the suspension. In general, the viscosity is maximum in the pH range close to the isoelectric point (IEP) of a given system. If a chemical reaction occurs within the suspension, such as the enzymatic decomposition of urea, then the pH value of the aqueous suspension can be shifted closer to the IEP, which causes coagulation and setting of the highly loaded ceramic suspension.

2.01.3.1.2.2 Hydrolysis-Assisted Solidification

This is a similar process to DCC (Kosmac, Novak, & Sajko, 1997) using an added powder that undergoes hydrolysis which results in a shift of pH toward the IEP with an increase in viscosity which can be exploited for solidifying a cast or molded ceramic green body in a way similar to that in the DCC process.

2.01.3.1.2.3 Gel Casting

This process (Omatete, Janney, & Strehlow, 1991) uses concentrated ceramic powder slurry in a solution of organic monomers, which is poured into a mold and gelled by in situ polymerization to form a green ceramic body. The monomer solution provides a low-viscosity vehicle to transport theﬂuid slurry into the mold and the polymer gel holds the ceramic powder in the desired shape. Vinyl monomers are used and, because they un- dergo a free-radical chain polymerization reaction, the setting is very rapid. Drying is then carried out to remove the solvent (water) followed by burnout of the polymer andﬁring of the ceramic.

2.01.3.1.2.4 Colloidal-Based Methodologies

Colloidal-based methodologies for ceramics fabrication (Lewis, 2000) enable control over the structural evolution within the ceramic suspension leading to much improved homogeneity. This requires careful control of the different stages of ceramics fabrication, especially powder synthesis, suspension preparation and rheology, consolidation into the desired component shape and removal of the solvent phase (drying) to produce theﬁnal microstructure duringﬁring that gives optimal performance.

2.01.3.1.2.5 Solid Freeform Fabrication

Solid freeform fabrication describes a series of moldless manufacturing methods (Halloran, 1999) that incorporate the use of computer-aided design (CAD) to create a 3D computer model of the desired part. This model is then separated into thin layers and a computer-controlled device is used to build the component up by sequential deposition, layer by layer. These methods are outlined below.

2.01.3.1.2.5.1 Direct Inkjet Printing

Inkjet printing is a noncontact dot matrix printing method (Derby, 2011) where droplets of ink are ejected from a printhead nozzle directly to a speciﬁed position on a substrate.

2.01.3.1.2.5.2 Three-Dimensional Printing (TDP) or 3-DP

This process is under development forﬂexible production of prototype parts and components directly from a CAD model. Three-dimensional printing or 3-DP takes a relatively long time to form a ceramic green body and has limitations when processing ceramic particles<1mm diameter.

2.01.3.1.2.5.3 Fused Deposition of Ceramics

This process uses ceramic–polymer feedstock that are extruded into thinﬁlaments and used as feed material for fabrication of three-dimensional green ceramic objects (Agarwala, Bandyopadhyay, van Weeren, Safari, &

Danforth, 1996) using a commercial fused depositional modeling (FDM)^Ôsystem. The polymer or wax acts as a carrier and binder for the ceramic particles as the materialﬂows out of the heated extruder head. The green ceramic object thus created is then subjected to conventional binder removal and sintering processes, to produce fully dense ceramic components.

2.01.3.1.2.5.4 Stereolithography

Stereolithography involves dispersion of a concentrated suspension of ceramic particles within a photo- polymerizable liquid solution. The ceramic suspension is photocured, layer by layer, converting the solution to 8 Fundamental Aspects of Hard Ceramics

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a polymer binder and forming a net-shape ceramic green body (Grifﬁth & Halloran, 1996) without the need for molds. Binder burnout is then required before sintering.

2.01.3.2 Thermal Treatment of CeramicsdSintering

The termsinteringis used to refer to the process that consolidates powder compacts into strong and usually dense polycrystalline materials via heating. When an aggregate of particles is assembled into a shape, they do notﬁll space and the maximum packing density is usually<60% of full density, that is>40% porosity. Many changes occur during sintering. The compact usually shrinks and the particles change shape with average grain size increasing.

Sintering is possible only if the atoms can diffuse to form the necks that effectively bond the particles together. The transport of matter can occur in the vapor phase, in the liquid phase, by diffusion in the solid state, or through the viscousﬂow of a glass. Most mechanisms are activated thermally because the action of temperature is necessary to overcome the potential barrier between the initial state of higher energy (compacted powder) and theﬁnal state of lower energy (consolidated material).

The driving force for sintering is the reduction in surface energy within the compact and therefore one prime requirement is a high surface area powder or one with aﬁne particle size.German (1996)states“With particles there is an inherent driving force for massﬂow, and the smaller the particle size, the greater the driving force.”

The reduction in total interfacial energy occurs via densiﬁcation and also coarsening or grain growth. Ifgis the speciﬁc surface or interfacial energy andAis the total surface or interfacial area within a powder compact, then the total interfacial energy of the compact isgA. The reduction in this total energy can be expressed by (Kang, 2005) the following equation:

DðgAÞ ¼ DgAþgDA (1)

The change in interfacial energy,Dg, is due to densiﬁcation and the change in interfacial area,DAoccurs because of grain coarsening. Dgis related to replacement of solid/vapor interfaces or surfaces by solid/solid interfaces. The overall change in total interfacial energy occurs through densiﬁcation and grain growth (coarsening) as shown schematically inFigure 3(Kang, 2005).

It is only when the temperature is sufficiently high in the range 0.5–0.8 Tm (where Tm is the melting point in Kelvin) that sintering occurs at a practical rate. The mechanisms by which the important atom movements occur determine thefinal densification and microstructural features of the ceramic.

Figure 3 Basic phenomena (densiﬁcation and grain coarsening) occurring under the driving force for sintering,D(gA).

Kang (2005), by kind permission.

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2.01.3.2.1 Solid-State Sintering

Densiﬁcation and strengthening during sintering in the solid state relies wholly on diffusion of atoms with no formation of liquid. Three stages have been identiﬁed:

1. Initial stage, during which necks begin to form at the contact points between adjacent particles. Neck formation is driven by the energy gradient resulting from the different curvatures of the particles and the necks.

2. Intermediate stage, during which the growing necks merge to form grains within a polycrystalline body with an interconnecting cylindrical pore network (density ranging from 70% to 93% theoretical) forming along the grain boundaries (GBs). At the end of this stage, the pore surfaces are smoother and pores become isolated from each other.

3. Final stage, during which isolated pores gradually reduce in volume and grain growth becomes signiﬁcant.

Figure 4shows the schematic curve of densiﬁcation of a powder compact with time during the three stages of sintering.

There are a number of material transport mechanisms that may operate during solid-state sintering of a polycrystalline ceramic and these are shown schematically inFigure 5and are classiﬁed below.

2.01.3.2.1.1 Surface Transport Processes

Surface transport processes, which change the pore shape but not the pore volume and lead to strengthening but no shrinkage or densiﬁcation, include the following:

1. Surface diffusion, D_s, which involves the diffusion of atoms along surfaces where the source is a particle surface and the sink is a neck. It only occurs to a depth of approximately one to two atomic distances (0.3–0.5 nm) (Rahaman, 2003). This type of diffusion is initiated at lower temperatures due to its lower activation energy and is usually the dominant mass-transport mechanism during the early stages of neck

Figure 4 Schematic curve of densiﬁcation of a powder compact with time during the three stages of sintering.Kang (2005), by kind permission.

Figure 5 Material transport paths during sintering.Kang (2005), by kind permission.

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growth, as the compact is heated up to the sintering temperature. It becomes less dominant as neck formation between particles increases.

2. Vapor transport, D_g, which allows the material to be transported from one source surface to another sink surface by evaporation of atoms and condensation at another site, through the pore network. This can also take place because of vapor pressure differenceDp.

3. Lattice diffusion, D_l, which allows the material to be transported from a particle surface to the neck area by diffusion through the bulk lattice with no increase in densiﬁcation of the material.

2.01.3.2.1.2 Bulk Transport Processes

Bulk transport processes, associated with GBs or the crystal lattices, allow movement of material so that particle centers approach each other, resulting in shrinkage, reductions in pore volume and densiﬁcation and include the following:

1. GB diffusion, D_b, which allows the atoms from the GB to be transported along the GB to a neck region or pore surface, driven by the high level of disorientation of the atoms along the GB. Even though GBs are quite narrow, they are very active transport paths.

2. Bulk lattice diffusion, D_l, which allows the atoms from the GB to be transported from the GB through the lattice to a neck or pore surface and so the boundary acts as a site for vacancy annihilation.

3. Plasticﬂow,h, which is only active where the initial dislocation density is very high and therefore is not usually operative in ceramics.Viscousﬂow,h, may operate in ceramics where a viscous liquid phase is formed.

This will be considered underSection 2.01.3.2.2.

2.01.3.2.1.3 Densification Kinetics

Kuczynski (1949)analyzed the rate of growth of the neck during the initial stage for various mechanisms of transport of material into the neck from the particle surface andHerring (1951)included the effects of particle size. For all mechanisms, a general relationship follows:

ðx=rÞⁿ ¼ K₁t=r^m (2) whereris the particle radius,xis the radius of the interparticle contact area,tis the time andn,mandK₁are constants. Plots of log (x/r) versus logtor logrshould result in linear plots with slopes ofnorm.

During the second intermediate stage, depending on the mechanism of material transport, shrinkage may occur according to

DL=Lo ¼ K₂t^y (3)

whereDLis the change in length,L_ois the original length,tis the time andyandK₂are constants. A plot ofDL/L_o versus logtshould be linear with slopey. The mechanism of material transport is then deduced from the slope

as follows: _n _m _y

Surface diffusion,Ds 7 4 No shrinkage

Evaporation/condensation,Dg 3 2 No shrinkage

Volume (bulk) diffusion,Dl 5 3 0.4/0.5

Grain boundary diffusion,Db 5 3 0.3/0.33

Viscous/plasticﬂow,h 2 1 1

During the intermediate stage, when relative density is in the range 0.85–0.93, porosityP is decreasing in volume but pores are still interconnected. If no grain growth occurs, then

DP=P_o ¼ K₃t (4)

whereDPis the change in volume fraction porosity,P_ois the original volume fraction porosity,tis time andy andK₂are constants. If grain growth occurs according to

D³D³_o ¼ K₄t (5)

whereDis the grain size after timetandD_ois the original grain size, then

DP=P_o ¼ K₅logt (6)

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More than one of the above matter transport mechanisms may operate at the same time. For example, a compact may increase density by a diffusion process but at the same time, an evaporation/condensation process may round pores. A shrinkage measurement (isothermally with time) would reveal only matter transport by diffusion whereas vapor or surface diffusion would decrease the driving force for all material transport and would thus decrease the rate of shrinkage.

The activation energies for surface, GB and lattice diffusivity increase in that order (Barsoum, 2002). Thus the type of matter transport mechanism is highly dependent on the sintering temperature. Particle size, packing, degree of agglomeration, presence of impurities, sintering atmosphere, etc. can affect the mechanism of sintering, the extent of pore/GB interactions (Brook, 1969) and whether densiﬁcation or coarsening occurs.

Figure 6shows schematic curves of relative density versus time and the effects of temperature, pressure and particle size (Kang, 2005).

2.01.3.2.1.3.1 Fast Firing

Fastfiring (Harmer & Brook, 1981) was proposed as a sintering method in order to enhance densification and suppress grain growth and involved heating a powder compact at much higher heating rates than in conventional sintering. This has the effect of attaining higher temperatures very rapidly where the activation energy for densification,Q_r, is higher thanQ_g, the activation energy for grain growth. The ratio of densification rate to grain growth rate increases with increasing temperature above an intermediate temperature T_int, but below this temperature, grain growth will be faster than densification as shown inFigure 7. Fastfiring thus allows a powder compact to reach temperatures aboveT_intvery quickly thus minimizing grain coarsening during heating.

2.01.3.2.2 Liquid-Phase Sintering

Liquid-phase sintering is a more complex process than solid-state sintering and involves the partial melting of one or more components and therefore takes place at a temperature above the solidus for the particular composition. Changes in temperature result in changes in the nature of both the solid and the liquid phases.

The parameters which control densiﬁcation include grain size and shape, pore size and shape, liquid volume and viscosity, solubility of the solid, wetting of the solid by the liquid, phase distribution and phase-boundary energies.

Figure 6 Effect of sintering parameters (T, temperature;P, pressure;L, particle size) on densiﬁcation.Kang (2005), by kind permission.

Log rate dρ/dt or dG/dt

grain growth Q_g/R Qp/R Densification

Qg/R

Q_p/R high T low T

1/T Tint

Figure 7 Densiﬁcation rate or grain growth rate as a function of temperature. Adapted fromHarmer and Brook (1981).

COMPREHENSIVE HARD MATERIALS VOLUME 2

COMPREHENSIVE HARD MATERIALS

VOLUME 2

CERAMICS

COMPREHENSIVE HARD MATERIALS

VOLUME 2

CERAMICS

EDITOR-IN-CHIEF

VINOD K. SARIN

College of Engineering, Boston University Boston, MA, USA

EDITED BY

LUIS LLANES

Departament de Ciència dels Materials i Enginyeria Metal $ lúrgica Universitat Politècnica de Catalunya $ Barcelona Tech

Barcelona, Spain

DANIELE MARI

Laboratoire de Physique de la Matière Complexe

Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

CONTENTS

PREFACE

EDITOR-IN-CHIEF

VOLUME EDITORS

LIST OF CONTRIBUTORS

SECTION I

INTRODUCTION

2.01 Fundamental Aspects of Hard Ceramics

2.01 Fundamental Aspects of Hard Ceramics

2.01.1 Introduction

2.01.2 Structure and Property Relationships

(a) (b)

2.01.3 Processing and Fabrication of Ceramics