35
Surfaces and Interfaces of Ceramic Materials
Surfaces and Interfaces of Ceramic Materials
NATO ASI Series Advanced Science Institutes Series
A Series presenting the results of activities sponsored by the NA TO Science Committee, which aims at the dissemination of advanced scientific and technological knowledge, with a view to strengthening links between scientific communities.
The Series is published by an international board of publishers in conjunction with the NATO Scientific Affairs Division
A Life Sciences B Physics
C Mathematical and Physical Sciences
o Behavioural and Social Sciences E Applied Sciences
F Computer and Systems Sciences G Ecological Sciences H Cell Biology
Series E: Applied Sciences - Vol. 173
Plenum Publishing Corporation London and New York
Kluwer Academic Publishers Dordrecht, Boston and London
Springer-Verlag Berlin, Heidelberg, New York, London, Paris and Tokyo
Surfaces and Interfaces of Ceramic Materials
edited by
Louis-C. Dufour Reactivite des Solides, UA 23 CNRS, Universite de Bourgogne, Dijon, France
Claude Monty Physique des Materiaux, CNRS, Laboratoires de Bellevue, Meudon, France
and
Georgette Petot-Ervas Ingenierie des Materiaux et des Hautes Pressions, CNRS, Universite de Paris-Nord, Villetaneuse, France
Kluwer Academic Publishers
Dordrecht / Boston / London
Published in cooperation with NATO Scientific Affairs Division
Proceedings of the NATO Advanced Study Institute on Surfaces and Interfaces of Ceramic Materials CAES-CNRS, lie d'Oleron, France September 4-16, 1988
Library of Congress Cataloging in Publication Data
NATO Advanced Institute on Surfaces and Interfaces of Ceramic Materials (1988 : Ile d'Oleron. France)
Surfaces and interfaces of ceramic materials I edited by Louis-C. Dufour. Claude Monty. Georgette Petot-Ervas.
p. cm. -- (NATO ASI series. Series E. Applied sciences; vol. 173)
'Proceedlngs of the NATO Advanced Institute on Surfaces and Interfaces of Ceramic Materials. CAES-CNRS. Ile d'Dleron. France. September 4-16. 1988."
"Published in cooperation with NATO Scientific Affairs Division."
ISBN-13: 978-94-010-6957-1 DOl: 10.1007/978-94-009-1035-5
e-ISBN -13: 978-94-009-1035-5
1. Ceramic materials--Surfaces--Congresses. I. Dufour. LouiS -Claude. II. Monty. Claude. III. Petot-Ervas. Georgette. IV. North Atlantic Treaty Organization. Scientific Affairs Division. V. Title. VI. Series: NATO ASI series. Series E. Applied sciences; no. 173. TA455.C43N37 1988 620.1·4--dc20 89-20048
Published by Kluwer Academic Publishers, P.O. Box 17, 3300 AA Dordrecht, The Netherlands.
Kluwer Academic Publishers incorporates the publishing programmes of D. Reidel, Martinus Nijhoff, Dr W. Junk and MTP Press.
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All Rights Reserved © 1989 by Kluwer Academic Publishers Softcover reprint of the hardcover 1st edition 1989
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CONTENTS
PREFACE ..... . . .... XI
ACKNOWLEDGEMENTS. . XV
LIST OF P ARTICIP ANTS . XVII
Introductory Lecture
THE MATERIALS SCIENCE OF CERAMIC INTERFACES
RJ. Brook. . . . . . . . . . . . . . . . . . . . . . . . . . . . XXIII
1 : STRUCTURE AND MICROSTRUCTURE
Tutorial Lectures
ELECTRON SPECTROSCOPIC DETERMINATION OF THE ELECTRONIC, GEOMETRIC AND CHEMISORPTION PROPERTIES OF OXIDE SURFACES V.E. Henrich. . . . . . . . . . . . . . . . . . . . . . . ... 1
STRUCTURE AND MICROSTRUCTURE OF INTERFACES IN CERAMIC MATERIALS C. B. Carter . . . . . . . . . . . . . . . . . . . . . . . . . 29
INTERGRANULAR PHASES IN POLYCRYSTALLINE CERAMICS D.R. Clarke . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Invited Lectures
INVESTIGATION OF THE FRACTAL STRUCTURE OF THE PORE-GRAIN INTERFACE IN ALUMINA CERAMICS F. Brouers and A. Ramsamugh . . . . . . . . . . . .. .... 81
STRUCTURE AND PROPERTIES OF GRAIN BOUNDARY IN MgO BICRYSTALS E. Yasuda and S. Kimura . . . . . . . . . . . . . . .
INTERFACES IN DIRECTIONALLY SOLIDIFIED OXIDE-OXIDE EUTECTICS
. .. 93
G. Dhalenne and A. Revcolevschi. . . . . . . . . . . . . . . . . . 109
vi
GRAIN BOUNDARIES IN HEXAGONAL CARBIDE CERAMICS J. Vicens, S. Lay, E. Laurent-Pinson and G. Nouet. ....... 115
Contributed Papers
INTERFACES BETWEEN PIGEONITE, CLINOAMPHIBOLE AND AUGITE W. Skrotzki and W.F. Muller. . . . . . . . . . . . . . .. .. 141
A VIBRATIONAL STUDY OF TETRACY ANOETHYLENE ADSORBED ON MAGNESIA J.J. Hoagland and K.W. Hipps . . . . . . . . . . . . . . .. .. 155
INVESTIGATION OF SURFACE HYDROXYLS ON CORDIERITE AEROGEL BY FT-IR SPECTROSCOPY M.1. Baraton, T. Merle-Mejean, P. Quintard and V. Lorenzelli .. 165
FT-IR CHARACTERIZATION OF HIGH SURFACE AREA SILICON NITRIDE AND CARBIDE G. Ramis, G. Busca, V. Lorenzelli, M.I. Baraton, T. Merle-Mejean and P. Quintard . ......... ......... . . . 173
2: SEGREGATION AND TRANSPORT PROPERTIES
Tutorial Lectures
THEORY OF DOPANT SEGREGATION IN CERAMIC OXIDES KG. Egdell and W.e. Mackrodt . . . . . . . . . . .
SURFACE AND GRAIN BOUNDARY SEGREGATION IN METAL OXIDES
185
J. Nowotny . . . . . . . . . . . . . . . . . . . . . . .. .. 205
SURFACE DIFFUSION AND SURFACE ENERGIES OF CERAMICS with application to the behavior of volatile fission products in ceramic nuclear fuels Hj. Matzke . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
GRAIN BOUNDARY DIFFUSION IN CERAMICS A. Atkinson and e. Monty . . . . . . . . . . . . . . . . . . . . 273
Invited Lectures
SEGREGATION AT CERAMIC SURFACES AND EFFECTS ON MASS TRANSPORT J.M. Blakely and S.M. Mukhopadhyay. . . . . . . . . . . . . . . 285
THE ROLE OF GRAIN BOUNDARIES AND INTERFACES ON SUPERCONDUCTIVITY D. Dimos and D.R. Clarke . . . . . . . . .
GROWTH AND MASS TRANSPORT IN CERAMIC TYPE PROTECTIVE SCALES ON METALS
vii
301
W.W. Smeltzer . . . . . . . . . . . . . . . . ....... 319
DYNAMIC SEGREGATION IN MULTICOMPONENT OXIDES UNDER CHEMICAL POTENTIALGRADIENTS G. Petot-Ervas . . ........ 337
Contributed Papers
STUDY OF NON STOICHIOMETRIC PURE AND ZR-DOPED YTTRIA SURFACES BY X-RAY PHOTOELECTRON SPECTROSCOPY AND SCANNING ELECTRON MICROSCOPY M. Gautier, J.P. Duraud, F. JolIet, N. Thromat, P. Maire and C. Le Gressus . . . . . . . . . . . . . . . . . . . . . . . . . 351
SEGREGATION IN Zr0z-Y 203 CERAMICS G.S.AM. Theunissen, AJ.A Winnubst and AJ. Burggraaf
MICROSTRUCTURAL CHANGES AND DYNAMIC SEGREGATION NEAR AN INTERFACE FORMED DURING THE REDUCTION OF (Fel_x_yCay)O (Extended Abstract)
365
J. Kusinski, S. Jasienska, A Riviere and C. Monty. . . . . . . . . . 373
3: CERAMIC-METAL INTERFACES
Tutorial Lectures
THERMODYNAMICS AND CHEMISTRY OF CERAMIC-METAL INTERFACES J.T. Klomp . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
CERAMIC-METAL INTERFACES M.G. Nicholas . . . . . . . 393
Invited Lectures
SMALL PARTICLES AND THIN FILMS OF METALS ON CERAMIC OXIDES L-C. Dufour and M. Perdereau . . . . . . . . . . . . . . . . . . 419
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MODELLING OF METAL-OXIDE INTERFACE BEHAVIOUR DURING OXIDE SCALE GROWTH CONTROLLED BY CATION DIFFUSION B. Pieraggi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449
Contributed Paper
INTERFACIAL TENSION AND CONTACT ANGLE IN IMMISCIBLE SYSTEMS BY CAPILLARY PRESSURE MEASUREMENTS (Extended Abstract) L. Liggieri, E. Ricci, N. Rando and A Passerone .
4: ROLE OF SURFACES AND INTERFACES IN ELABORATING CERAMICS MATERIALS
Tutorial Lectures
CHARACTERIZATION, PROPERTIES AND PROCESSING OF CERAMIC POWDERS
455
T.A. Ring . . . . . . . . . . . . . . . . . . . . . . . . . 459
SOME CURRENT ISSUES ON POLYCRYSTALLINE STRUCTURES AND GRAIN GROWTH A Mocellin . . . . . . . . . . . . . . . . . . . . . . . . . . . 485
Invited Lectures
THE ROLE OF SURFACES IN CERAMIC PROCESSES H. Schubert . . . . . . . . . . . . . . . . . . . . . . . . . . . 507
SOME ASPECTS OF THE INFLUENCE OF PARTICLE SIZE ON PROPERTIES AND BEHAVIOUR OF A DIELECTRIC MATERIAL: EXAMPLE OF BARIUM TITANATE J-C. Niepce. . . . . . . . . . . . . . . . . . . . . . . . . . . . 521
Contributed Papers
VARIATION WITH PROCESSING CONDITIONS OF BULK AND GRAIN BOUNDARY PTCR PHENOMENA IN DOPED BaTi03 D.C. Sinclair and AR West .................... 535
COPPER-CORDIERITE COSINTERING V. Oliver, J. Guille, J. C. Bernier, B.S. Han, J. Werckmann, J. Faerber, P. Humbert and B. Carriere . . .. . . . . . . . . . . . 545
GRAIN BOUNDARY PHENOMENA IN THE EARLY STAGES OF SINTERING OF MO OXIDES
ix
A.M.R Senos, M.R Santos, A.P. Moreira and J.M. Vieira .... 553
SINTERING OF Nd203 AND CERAMIC STABILITY TO HYDRATION J.M. Heintz, P. Poix and J. C. Bernier . . . . . . . . . . .... 565
CONTROL OF CARBON-SILICATE INTERFACE AREA IN THE PREPARATION OF SiMON CERAMIC PRECURSORS FROM CLAYS F. Kooli and F. Bergaya . . . . . . . . . . . . . . . .. .... 575
PREPARATION OF AN ALUMINA-ZIRCONIA SOL FOR PRODUCING MICROSPHERES L. Montanaro . . . . . . . . . . . . . . . . . . . . . . . 587
5: INTERACTION OF CERAMICS WITH ENVIRONMENT
Tutorial Lectures
SOLID-GAS AND SOLID-SOLID INTERACTIONS OF CERAMIC OXIDES AT HIGH TEMPERATURES H.J. Grabke . . . . . . . . . . . . . . . . . . . . . . . . 599
GROWTH OF CERAMIC LAYERS FROM V APOR PHASE F. Teyssandier. . . . . . . . . . . . . . . . .
REACTIONS OF CERAMIC OXIDES WITH AQUEOUS SOLUTIONS (INCLUDING DISSOLUTION)
..... 625
M.A. Blesa . . . . . . . . . . . . . . . . . . . . . . . . . . . 639
Invited Lecture
DISSOLUTION MECHANISMS OF OXIDES AND TITANATE CERAMICS -ELECTRON MICROSCOPE AND SURFACE ANALYTICAL STUDIES P.S. Turner, C.F. Jones, S. Myhra, F.B. Neall, D.K. Pham and RSt.C. Smart . . . . . . . . . . . . . . . . . . . . . . . . 663
Contributed Papers
THE ROLE OF GRAIN BOUNDARY MODIFICATIONS IN THE THERMAL DECOMPOSITION OF Mn-FERRITES J.H. Boy and G.P. Wirtz ...................... 691
x
TEMPTATIVE MODELING OF SURFACE REACTIVIlY WITH OXIDIZINGREDUCING MIXTURES ON RUTILE Ti02-() (Extended Abstract) F. Morin and L-C. Dufour . . . . . . . . . . . . . . . . . .. 701
6: PROPERTIES RELATED TO SURFACES AND INTERFACES IN SPECIFIC CERAMIC MATERIALS
Tutorial Lectures
N ANOPHASE CERAMICS, MEMBRANES AND ION IMPLANTED LAYERS AJ. Burggraaf, K Keiser and B.A van Hassel . . . . . . . . .. . 705
SURFACE DETERMINED PROPERTIES OF SILICATE GLASSES A.A. Kruger . . . . . . . . . . . . . . . . . .. ....... 725
IMPORTANCE OF INTERFACIAL STRENGTH ON FRACTURE TOUGHNESS OF BRITTLE MATRIX COMPOSITES P. Pirouz, G. Morscher and J. Chung .
Invited Lecture
PRECIPITATION TOUGHENING AND PRECIPITATION HARDENING IN Y203-STABILIZED Zr02 CRYSTALS
. ...... 737
A Dominguez-Rodriguez and AH. Heuer . . . . . . . . . . . . . 761
Contributed Papers
RESIDUAL STRESSES IN POROUS PLASMA-SPRAYED ALUMINA COATING ON TITANIUM ALLOY FOR MEDICAL ApPLICATION (Extended Abstract) H. Carrerot, J. Rieu and A Rambert . . . . . . . . . .. .... 777
W-TiN-SiC MATERIAL FOR HIGH TEMPERATURE ApPLICATION L.R. Wolff . . . . . . . . . . . . . . . . . . . . . . . . . . . .779
COLLOIDAL FILTRATION OF HYDROPHOBIC ALUMINA. INFLUENCE OF SEDIMENTATION ON FILTRATION KINETICS AP. Philipse and H.P. Veringa . . . . . . . . . . . . . . . . . . 789
SUBJECT INDEX . . . . . . . . . . . . .. ............ 799
PREFACE
This book contains the proceedings of the NATO Advanced Study
Institute on Surfaces and Interfaces of Ceramic Materials, held on the Oleron island, France, in September 1988. This Institute was organized in nine months after receiving the agreement of the NATO Scientific Affairs
Division. Despite this very short time, most of the lecturers contacted have accepted our invitation to prepare a specific talk.
The meeting was held at "La Vieille Perrotine" on the Oleron island. This
holiday village of the French CNRS is located near the Ocean in a natural area which contributed to create a very pleasant atmosphere favourable to develop interaction between the 91 participants in this Institute.
First of all, the Institute was aimed at diffusing the foremost results on the characterization of and the role played by surfaces, grain boundaries and interfaces in preparation and overall properties of ceramic materials, mainly
of oxide ceramics. Through its interdisciplinary character, the Institute was
also aimed at developing interaction between scientists and engineers interested in basic and practical aspects of processing and use of ceramics. Lastly, the Institute has emphasized the fundamental importance and the key role of the external and internal surfaces in the advanced technology of these materials. 42 hours of courses were given by 23 lecturers and 13 additional
hours on more limited subjects by 13 invited lecturers. 8 hours were devoted to three round table discussions on segregation, ceramic-metal bonding and
sintering processes. Moreover, a poster session, preceded by a short oral introduction by the authors, enabled the students to present their own works.
Most of the lectures and papers presented during this Advanced Study
Institute are gathered in this book which is divided in six sections according
to the following description:
1/ STRUCTURAL AND MICROSTRUCTURAL ASPECTS OF SURFACES AND
INTERFACES OF CERAMIC MATERIALS, INCLUDING ELECTRONIC AND
CHEMISORPTION PROPERTIES. Although progress is still needed, many
theoretical calculations are now available and reliable to predict the
xi
xii
thermodynamical stability of both free and doped surfaces and interfaces *. Modern high resolution spectroscopy and microscopy make it possible to explore and understand the crystallographic, electronic and chemical structure of surfaces and interfaces on an atomic scale. These investigations
reveal the fundamental role played by defects and additives or impurities in the properties of surfaces and interfaces of these materials. Also the theory of fractals is considered a very interesting mathematical approach to treat some specific problems, particularly wetting, elaboration by sol-gel route or sintering of ceramics **.
2/ SEGREGATION PHENOMENA AND TRANSPORT PROPERTIES. One
important point emphasized here concerns the role of segregation now better
analyzed and controlled in the equilibrium conditions. After thermal
treatment, this phenomenon is often involved in modifying, sometimes drastically, the expected behaviour of the ceramic materials and its study
therefore is to be considered a priority. Recent approaches on the role of both
surface and grain boundary diffusions in the overall properties of ceramic protective layers, solid electrolytes, sensors and electrical ceramics, including new high Tc oxide superconductors, are presented.
3/ THERMODYNAMIC AND STRUCTURAL ASPECTS OF CERAMIC-METAL
INTERFACES. Another question largely involved in many industrial applications concerns the ceramic-metal bonding. The way to improve the bond strength of ceramic-metal interfaces by subtle changes in chemical composition or stoichiometry at interfaces is discussed. Recent data on preparation and properties of small particles and very thin films of metals on oxides is also presented.
* For instance , see references in: (a) Henrich V.E., The surfaces of metal oxides, Rep. Prog. Physics, 48 (1985) 1481-1541; (b) Egdell R.G. and Mackrodt W.c., this book ; (c) the papers published by Stoneham A.M., Tasker P.W. and coworkers (e.g. , ref. in the chapter 1, The theory of ceramic surfaces, in 'Surface and Near-Surface Chemistry of Oxide Materials', Nowotny J. and Dufour L.c., (Eds), Elsevier, Amsterdam, 1988) ** Some references in this field may be found in: (a) Mandelbrot B., 'The Fractal Geometry of Nature', Freeman, San Francisco, 1982; (b) Cherbit G. (Ed.), 'Fractals, Dimensions Non Entieres et Applications', Masson, Paris, 1987; (c) Feder J., 'Fractals', Plenum Pub. Corp., New York, 1988; (d) Botet R. and Julien R., A theory of aggregating systems of particles: the clustering of clusters process, Ann. Phys. Fr., 13 (1988) 153-221
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4/ ROLE OF SURFACES AND INTERFACES IN ELABORATING CERAMIC
MATERIALS. Current issues on processing of ceramic powders, particularly from mono sized particles and on grain growth in sintered ceramics are analysed in this section.
5/ INTERACTION OF CERAMIC MATERIALS WITH ENVIRONNMENT, INCLUDING SOLID-GAS AND SOLID-LIQUID REACTIONS. Several chapters
present the most recent development on the role of surfaces and interfaces in the interactions of ceramics with gas at high temperature and with liquids such as acidic solutions. The key role of the surface defects as well as the
formation, in specific conditions, of thin protective amorphous films is discussed in the processes of dissolution of ceramic oxides.
6/ PROPERTIES RELATED TO SURFACES AND INTERFACES OF SPECIFIC CERAMIC MATERIALS AND COMPOSITES, INCLUDING MECHANICAL PROPERTIES.
Here attention is more particularly concentrated on the relationship between microstructural properties of the interfaces and strength of composites, on the surface properties of vitreous silicates and the elaboration of ceramic membranes.
The aim of an Advanced Study Institute is to collect the knowledge gained
in a scientific field and to make this information easily accessible to those interested. We trust that this book will show that the subject treated is timely
and important for basic research as well as in the development of modern technology for ceramic materials. This research area is now in fast expansion and has an ever increasing impact on the way to improve ceramics and composite materials for catalysis, metallurgy, energetics, electronics, space science,.. No doubt that, in short future, this scientific branch becomes one of the most active and rapidly developing of contemporary Materials Science.
Louis-C. Dufour,
Claude Monty
Georgette Petot-Ervas
ACKNOWLEDGEMENTS
The Organizers of this NATO Advanced Study Institute gratefully acknowledge support and co-sponsorship from the following Institutions and Companies:
- NATO Scientific Mfairs Division which provided the main support. Our sincere gratitude goes particularly to Dr. L.V. da Cunha and Dr. G. Sinclair for their encouragement and assistance.
- Centre National de la Recherche Scientifique (CNRS), Paris. Particular thanks are due to Prof. M. Fayard, Head of the Department of Chemistry.
- European Research Office of the US Army Research, Development and Standardization Group, London. We particularly thank Dr W.e. Simmons, Chief of the Materials Sciences Branch.
- Riber SA., Rueil-Malmaison, France
The Organizing Committee is very grateful to lecturers and participants who have contributed to the success of this meeting and cooperated in the preparation of this book
Our thanks are also addressed to: - Prof. J-e. Colson, University of Burgundy, for his encouragement
and for providing facilities - Mr G. Lutrot and his staff, for their efficient assistance during the
meeting at the CNRS Village "La Vieille Perrotine" on the Oleron island - Drs K. Chhor and J.L. Lirhman, CNRS Villetaneuse, Dr. P. Bracconi,
CNRS Dijon and Mr M. Vareille, University of Burgundy, for their kind assistance.
- the "Groupe Fran~ais de Cerami que" and the French "Club Surfaces et Interfaces" which have kindly allowed us to use their address listings
- the Editors of "The Bulletin of the American Ceramic Society", "l'Actualite Chimique", "Journal de Physique", "Reactivity of Solids" and "Journal de Chimie Physique" who announced the meeting in their Journal.
The Members of the Organizing Committee: Drs Louis-e. Dufour, Director; Alan Atkinson; Claude Monty ;
Janusz Nowotny; Georgette Petot-Ervas; Werner Weppner
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PARTICIPANTS
Mr. Jean-Andre ALARY
Mr. S. ALPERINE
Dr. Alan ATKINSON
Mr. J(Ilrgen B. BILDE-S0RENSEN
Dr. SaraA. BILMES
Dr. Michel BISCONDI
Prof. JackM. BLAKELY
Dr. Miguel A. BLESA
Prof. GOOter BORCHARDT
Dr. Gilles BORDIER
Mr. Jeffrey H. BOY
Prof. Richard]. BROOK
Prof. Fran~is BROUERS
Prof. A.J. BURGGRAAF
CEGEDUR Pechiney, Centre de Recherche et de Developpement, B.P. 27,38340 VOREPPE (France)
Office National d'Etudes et de Recherches Aerospatiales (ONERA), B.P. 72, 92322 CHATILLON CEDEX (France)
Materials Development Division, AERE Harwell, HARWELL, Oxfordshire OXII ORA (UK)
Metallurgy Department, RiSf/l National Laboratory, Postbox 49, DK 4000 ROSKILDE (Denmark)
Physikaiisches Institut III, UniversitlU Diisseldorf, Universitlitstrasse 1,4000 DUSSELDORF 1 (FRG)
Ecole NationaIe Superieure des Mines, 158, Cours Fauriel, 42 023 ST ETIENNE CEDEX (France)
Dept of Materials Science and Engineering, Coli of Engineering, Cornell University, Bard HaIl, ITHACA, NEW YORK 14853 (USA)
Comision Nacional de Energia Atomica, Dpto "Quimica de Reactores", A venida del Libertador, 8250, 1429 BUENOS AIRES (Argentina)
Institut fiir Allgemeine Metallurgie, Technische Univ.ClausthaI, Robert-Koch-Str. 42, Postfach 1253, 3392 CLAUSTHALZELLERFELD (FRG)
DPC / STS, Blitiment 125, Centre d'Etudes Nucleaires de Saclay, 91191 GIF/SUR/YVETTE CEDEX (France)
University of Illinois, Coli. Engng, Dept MatSci. & Engng, Ceramics Division, 204 Cer.Blg, 105 South Goodwin Av., URBANA, ILLINOIS 61801 (USA)
Max-Planck Institut fiir Metallforschung, Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, 7000 STUTTGART 80 (FRG)
Institut de Physique, Universit6 de Liege, B6, Sart Tilman, B-4000 LIEGE (Belgium)
Twente University of Technology, Dpt Chern. Engng /Lab Inorg. Chemistry. & Materials Science, P.O. Box 217, 7500 AE ENSCHEDE (The Netherlands)
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Prof. C. Barry CARTER Dept. of Materials Science and Engineering, College of Engineering, Cornell University, Bard Hall, ITHACA, NEW YORK 14853 (USA)
Mrs. Dominique CHATAlN Lab. de Thermodynamique et Physico-Chimie Metallurgiques (LTPCM), Domaine Universitaire, BP 75, 38402 ST MARTIN D'HERES (France)
Dr. DavidR. CLARKE I.B.M., Thomas J. Watson Research Center, Materials Science Department, P.O. Box 218, YORKTOWN HEIGHTS, NEW YORK 10598 (USA)
Ms. Charlotte CLAUSEN Metallurgy Department, Ri~ National Laboratory, P.O. Box 49, DK 4000 ROSKlLDE (Denmark)
Mr. Michel COURBIERE 42, rue Marguerite, 69100 VILLEURBANNE, (France)
Mr. Julian DE ANDRES CERATEN S.A., Carpinteros 10, Pol Industrial Los Angeles, 28906 GETAFE (MADRID) (Spain)
Dr. Duane DIMOS I.B.M., Thomas J. Watson Research Center, Materials Science Department, P.O. Box 218, YORKTOWN HEIGHTS, NEW YORK 10598 (USA)
Mr. Jan DIPHOORN DSM Research BV, Department MT Ker, P.O. Box 18,6160 MD GELEEN (The Netherlands)
Prof. Arturo DOMINGUEZ- Depto de Fisica de la Materia Condensata, RODRIGUEZ Universidad de Sevilla, Apdo 1065, SEVILLA (Spain)
Dr. Louis C. DUFOUR Reactivite des Solides, UA 23 CNRS, Universite de Bourgogne, BP. 138,21004 DIJON CEDEX (France)
Dr. Russell G. EGDElL Department of Chemistry, Imperial Coil. of Science and Technology, SOUTH KENSINGTON, LONDON SW7 2AZ (UK)
Mr. Jose V. EMlLIANO R. Coronel Artur Whittacker 85413690 DES CAL V ADO SP (Brasil)
Mr. Claude ESNOUF INSA, B1itiment 502 I GEMPPM, 69 621 VILLEURBANNE CEDEX, (France)
Dr. Martine GAUTIER Centre d'Etudes Nucleaires Saclay, DPCI SPCN /SES, 91191 GIF-SUR-YVETTE CEDEX (France)
Mr. Dominique GESLIN Centre de Recherches P6chiney, Service RD-R, B.P. 54,13541 GARDANNE CEDEX (France)
Mr. Jacques GILLOT Societe de Ceramiques Techniques, B.P. 1,65460 BAZET, (France)
Prof. Daniele GOZZl Dipartimento di Chimica, Universita "La Sapienza", Piazzale Aldo Moro 5, 00 185 ROMA (Italy)
Prof. Hans I.
Mr. HelmutB.
Mr. Fred&ic
Dr. Riza
Dr. lean-Marc
Prof. VictorE.
Mr. Ioseph
Prof. Ian
Prof. Stanislawa
Dr. Hallt
Prof. Gretchen
Dr. I.T.
Prof. Per
Mr. Fathi
Dr. Albert A.
Dr. Mongi
Dr. Sylvie
GRABKE
GROBMEIER
GRUY
GORBUz
HEINTZ
HENRICH
HOAGLAND
IANOWSKI
IASIENSKA
KALEBOZAN
KALONfl
KLOMP
KOFSTAD
KOOLl
KRUGER
LABIDI
LARTIGUE
Max-Planck Institut fiir Eisenforschung, Max-Planck Strasse I, Postfach 140260,4000 DUSSELDORF 1 (FRG)
Institut fiir Reaktorwerkstoffe, KFA Iiilich GmbH, Postfach 1913, D-5170 mLlCH 1 (FRG)
Ecole Nationale Supeneure des Mines, 158, cours Fauriel, 42023 SAINT -ETIENNE, (France)
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Metallurgical Engineering Department, Middle East Technical University, P.K. 06531, ANKARA (Turkey)
Chimie du Solide CNRS, Universite de Bordeaux I, 351, cours de la Liberation, 33405 TALENCE CEDEX (France)
Applied Physics, Yale University, Becton Center, P.O. Box 2157 Yale Station, NEW HAVEN, CONNECTICUT 06520-2157 (USA)
Washington State University, Chemistry Dept, PULLMAN, WASHINGTON 99164-4630 (USA)
Director, St Staszic University of Mining and Metallurgy, uJ. Szymanowskiego I-lOA, 30047 KRAKOW (poland)
Academy of Mining and Metallurgy, Institute of Metallurgy, al. Mickiewicza, 30, 30059 KRAKOW (poland)
Department of Metallurgical Engineering, Middle East Technical University, In~nii Bulvary, 06531 ANKARA (Turkey)
Opt. Mat. Science and Engineering. / Room 13 5094, Massachusetts Institute of Technology, 77, Massachusetts Avenue, CAMBRIDGE, MA 02139 (USA)
Centre for Technical Ceramics, C/O University of Technology, P.O. Box 513, 5600 MB EINDHOVEN (The Netherlands)
Department of Chemistry, University of Oslo, P.B. 1033 BLINDERN,0315 OSLO 3 (Norway)
CNRS/ CRSOCI, lB, rue de la Ferollerie, 45071 ORLEANS CEDEX 2, (France) Battelle Pacific Northwest Laboratories, P.O. Box 999, RICHLAND, WA 99352 (USA)
FacuIte des Sciences de Tunis, Departement de Physique, LE BELVEDERE 1001, (Tunisia)
Metallurgie S b"ucturale, B1itiment 413, Universite Paris XI, 91405 ORSAY CEDEX (France)
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Dr. Colin LEACH Imperial College London, Department Materials -Room B 513, LONDON SW72BP, (UK)
Mr. Libero LIGGIERI Istituto. Chimica Fisica Applicata dei Materiali, C.N.R. GENOVA, Via Lungobisagno Istria 34, 16141 GENOVA (Italy)
Mr. PedroM. MANTAS Dept. Eng. Ceramica e do Vidro, Universidade de Aveiro, 3800 A VEIRO, (portugal)
Dr. Hj. MA1ZKE CEC - JRC,! European Inst.for TransuraniumElements, Postfach 2340, D 7500 KARLSRUHE (FRG)
Ms. Caroline MAUNIER Centre dEtudes Nucleaires Saclay, CENS /SRMP BAtiment 20, 91191 GIF-SUR-YVETIE, (France)
Dr. Andrea MENNE Max-Planck Institut FestkOrperforschung, Heisenbergstrasse I, Postfach 80 06 65, D-7000 STUTTGART 80 (FRG)
Prof. Alain MOCELLIN Ecole Polytechnique Federale, Dept des Materiaux, Lab. de Ceramique, 34, Chemin de Bellerive, CH-l007 LAUSANNE (Switzerland)
Mrs. Laura MONTANARO Politecnico di Torino, Dipto Scienza dei Materiali, Corso Duca Degli Abruzzi 24, 10129 TORINO (Italy)
Dr. Regina C. MONTEIRO Universidade. Nova de Lisboa, Fac.Ciencias Tecnol., Dpto Ciencias Materiais, Quinta da Torre, 2825 MONTE DE CAPARICA (portugal)
Dr. Claude MONTY CNRS, Physique des Materiaux, I, place Aristide Briand, BELLEVUE, 92195 MEUDON CEDEX (France)
Dr. Fran~ois MORIN Chimie des Materiaux, Institut de Recherches d'Hydro-Quebec, C.P. 1000, VARENNES, Quebec, JOL 2PO (Canada)
Mrs. Maria MUOLO Istituto Chim. Fisica Applicata dei Materiali, C.N.R. Luigia Lungobisagno lstria, 34,16141 GENOVA (Italy)
Dr. MichaelG. NICHOLAS Metals Technology Centre, Harwell Lab., UKAEA, DIDCOT,OXON OXll ORA (UK)
Prof. Jean-C. NIEPCE THOMSON-LCC,!Centre de Saint-Apollinaire, Av.du Colonel Prat, 21850 ST APOLLINAIRE (France)
Dr. Trois NORBY Department of Chemistry, University of Oslo, P.B.1033 BLINDERN, N- 0315 OSLO 3 (Norway)
Dr. Gerard NOUET LERMAT-CNRS/lSMRA, Boulevard du Marechal Juin, 14032 CAEN CEDEX (France)
Dr. Janusz NOWOTNY Max-Planck Institut FestkOrperforschung, Heisenbergstrasse I, Postfach 800665, 7000 STUTTGART 80 (FRG)
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Prof. Man:el PERDEREAU R~ctivite des Solides, UA 23 CNRS, Universite de Bourgogne, BP 138, 21004 DUON CEDEX (France)
Dr. Georgette PETOT-ERVAS LIMHP/CNRS, Centre Univ.Paris-Nord, Avenue Jean-Baptiste Clement, 93430 VlLLETANEUSE (France)
Dr. Albert P. PH1LlPSE National Ceramic Centre, NKA, Netherlands Energy Research Foundation, ECN, Postbus I, 1755 ZG PETTEN (The Netherlands)
Dr. Bernard PIERAGGI Metallurgie Physique, UA 445 CNRS, E.N.S.C. Toulouse, 118, rue de Narbonne, 31077 TOULOUSE CEDEX (France)
Prof. P. PIROUZ Dept of Metallurgy and Materials Science, Case Western Reserve University, CLEVELAND, OHIO 44106 (USA)
Prof. Pierre QUINTARD Lab. Spectrometrie Vibrationnelle, UA 320 CNRSI Ceramiques Nouvelles, Facultes des.Sciences, 123 Av. Albert Thomas, 87060 LIMOGES (France)
Prof. Alexandre REVCOLEVSCHI Universite de Paris-Sud, Chimie des Solides, Batiment414,91405 ORSAY CEDEX (France)
Prof. Jean RIEU Ecole des Mines de Saint-Etienne, 158, Cours Fauriel, 42023 SAINT-ETIENNE CEDEX (France)
Prof. Terry A. RING Ecole Poly technique Federaie /Lab. de Technologie des PoudresJC/O Bfltiment de Chimie, 1015 LAUSANNE-ECUBLENS (Switzerland)
Mr. Roberto SANGIORGI Istituto Chim. Fisica Applicata dei Materiali, C.N.R., Lungobisagno !stria, 34,16141 GENOVA (Italy)
Prof. Bernard SAPOVAL Physique Matiere Condensee IGR 38 CNRS, Ecole Polytechnique, Route de Saclay, 91128 PALAISEAU CEDEX (France)
Dr. Helmut SCHUBERT Max-Planck Institut filr Metallforschung, Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, 7000 STUTTGART 80 (FRG)
Dr. Ana Maria SEGADAES Departamento de Engenharia. Ceramica e do Vidro, Universidade de Aveiro, 3800 A VEIRO, (portugal)
Dr. Ana Maria SENOS Departamento de Engenharia Ceramica e do Vidro, Universidade de Aveiro, 3800 A VEIRO, (Portugal)
Mr. DerekC. SINCLAIR University of Aberdeen, Department of Chemistry, Meston Walk, OLD ABERDEEN AB9 2UE (UK)
Dr. W. SKROTZKI Institut Geologie und Dynarnik der Lithosphlire, Universitllt GOttingen, Goldschmidtstrasse 3, D-34oo GOTTINGEN (FRG)
Prof. Walt W. SMELTZER Institute for Materials Science, McMaster University, 1280 Main Street West, HAMILTON, Ontario, LSS 4MI (Canada)
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Dr. Francis TEYSSANDIER Institut Science et de Genie des Materiaux et Procedes, CNRSI Universite de Perpignan, Av. de Villeneuve, 66025 PERPIGNAN CEDEX (France)
Mr. G.S.A.M. TIlEUNISSEN University of Twente, Fac. of Chemical Engineering, P.O. Box 217, 7500 AE ENSCHEDE (The Nethedands)
Dr. Peter S. TURNER Division of Science and Technology, Griffith University, NATHAN, BRISBANE, Queensland 4111 (Australia)
Dr. Colette VIGNAUD CNRS LP 15, Physique des Liquides et Electrochimie, 51 Bvd A. Blanqui, 75013 PARIS (France)
Dr. We:rr;;::r WEPPNER Max-Planck Institut FestkOrperforschung, Heisenbergstrasse 1, Postfach 80 06 65, 7000 STUTTGART 80 (FRG)
Mr. Jocques WERCKMANN C.N.R.S./I.P.C.M.S.,Surfaces et Interfaces, Universite Louis Pasteur, 4, rue Blaise Pascal, 67000 STRASBOURG (France)
Dr. A.R. WEST University of Aberdeen, Department of Chemistry, Meston Walk, OLD ABERDEEN AB9 2UE (UK)
Dr. L.R. WOLFF Center for Technical Ceramics (CTK), Eindhoven University of Technology, P.O. Box 513, 5600 MB EINDHOVEN (The Netherlands)
Prof. Eiichi YASUDA Tokyo Institute of Technology, Research Lab of Engineering Mat., 4259 NAGATSUTA, MIDORI, YOKOHAMA 227 (Japan)
THE MATERIALS SCIENCE OF CERAMIC INTERFACES
R.J. BROOK Max-Planck-Institute for Metals Research Powdermetallurgy Laboratory He isenber!Jstr. 5 D-7000 Stuttgart 80 West Germany
ABSTRACf. The significance of surfaces and interfaces for the processing and application of ceramics has lon~ been recognised. In this review attention is given to phenomena where the eXIstence of surfaces is responsible for important phenomena in processing (free surfaces and the sintering of powders) and properties (surface cracks and the influence on mechanical strength). Attention is also given to two subject areas where an improved understanding of interface behaviour would be beneficial. The first relates to the significance of segregation at grain boundaries and at free surfaces for the development of ceramic microstructures; the second relates to the structure and behaviour of grain boundaries in the presence of second phases, a factor important both for the processing and for the high temperature properties of ceramic materials.
1. Introduction
As preliminary to a conference where much of the discussion will involve close analysis of the chemical and physical properties of surfaces and interfaces in ceramic materials, it may be helpful to review the subject from the point of view of the ceramist, namely from the point of view of someone wishing to fabricate and use this set of materials for applications and seeking to understand surfaces and interfaces as a means of improving .eroduct performance. This approach has the potential benefit that it allows identification of the critical questions f1S seen from the viewpoint of someone working with the processing or use of ceramfcs; it should also be helpful to the wider communIty in revealing any prejudices or preoccupations that may have arisen among those working with the development of ceramics.
In the opening section a brief review is given of certain topics where the ceramist has long recognised that interfaces or surfaces playa critical role. These include the densification of powders by sintering and the mechanical properties of ceramics at ambient temperatures. In subsequent sections two topics are treated where the current understanding of grain boundary or surface structure is incomplete and where further progress should prove rewarding for the processing or application of these materials. The two themes are the segregation of dopants or impurities at grain boundaries in single phase ceramic materials and secondly the
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L.-C. Dufour et al. (eds.), Surfaces and Interfaces of Ceramic Materials, xxiii-xxxvii. © 1989 by Kluwer Academic Publishers.
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distribution and properties of second phases in the grain boundaries of multiphase ceramics. The two themes therefore relate directly to the structure and behaviour of solid/solid grain boundaries and of solid/liquid/solid boundaries.
2 Surfaces and Interfaces and the Processing, Properties and Application of Ceramic;
One approach to materials science and engineering is to recognise a causal sequence between processing, the resulting microstructure, and the properties that are then associated with this microstructure; applications must then be satisfied on the basis of the properties that have been developed. Recognition of this logical sequence allows one to argue that a given application will require a given set of material properties which must then be attained by specific processin~ designed to achieve a tnlcrostructure capable of yielding these properties. The SIgnificance of surfaces and interfaces in each of these stages has long been recognised by the ceramics communityl.
The processing of ceramics is most commonly based upon the heat treatment of powders. The energy present in a powder as a consequence of its high surface area provides a driving force for structural change which results in the densification of the powder and the production of a finished solid component. The central importance of this sintering process in the fabrication of ceramics has made it a much studied subject.
At the macroscopic level, a requirement for successful densification is that the surface energy in the powder be greater than the interfacial energy of the resulting polycrystalline material. This then ensures that densificatIOn, the removal of porosity and the replacement of free surfaces by grain boundaries is favourable. One explanation, e.g., for the difficulties in the sintering of SiC lies in the view2 that this energy change may be insufficient to drive densification. At the microscopic level, it is important to recognise and control the atom movements that provide the mechanism for the microstructural change. Here attention is given to the role of curved surfaces (Fig. 1) in establishin~ the chemical potential gradients in the system which then drive the atom diffuSIOn. The argument is that since the diffusion of atoms from point to point in the powder bed is likely to be the slow process in the sequence of events leading to microstructural change, all other steps in the sequence can be considered to be at virtual eqUilibrium. Where curved surfaces occur in the system, the ability of the surfaces to act as rapid sources or sinks for atoms ensures that they are in equilibrium with the local partial pressures in an adjacent gas phase or with the local point defect concentration in an adjacent solid phase.
The local pressures and concentrations are readily calculated3• The first step is to recognise the pressure difference, ap, that occurs across a curved surface at equilibrium. The energy balance for an infinitesimal volume change dV undergone by a spherical pore of radius r in a solid is
ap dV = tdA (1)
where 'Y is the specific surface energy and dA the change in pore surface area. For the spherical geometry, therefore,
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L\.P = ~ r (2)
The second step is to consider two systems, one where we have a planar interface at equilibrium between two phases, A and B (these could also be two differently oriented grains of the same phase meeting at a grain boundary), and one where we have the two phases in local equilibrium with a curved interface. Molar quantities of A and B are then transferred from the first system to the second. Since both systems are themselves at eq,uilibrium, the energy change on transferring A must equal that on transferring B, I.e.,
(3)
where n is the molar volume and dP the difference in the pressures acting normal to the interfaces in the two systems.
Combining (2) and (3) and taking the example for the second system of a spherical interface concave towards phase B (Fig. 2),
dPB = P rnA n ~ nB]
If phase A is an ideal gas and phase B a solid, then with nA » nB,
dPB =~ r Similarly with
dPA = - P [UB ~BnA]
dP - - ~ nB PA A - r RT
(4)
(5)
(6)
(7)
From (7), local variations in surface curvature cause local variations in local gas partIal pressure giving rise to diffusion and atom transport in the gas. Similarly, the differences in local normal pressures at differently curved surfaces in the solid (5), give rise to chemical potential gradients which in turn lead to atom diffusion in the solid.
Much effort has ~one into the conversion of these atom fluxes into the form of the corresponding mIcrostructural changes that are observed in the sinterin~ of powders; notwithstanding the complexities of these changes, it must be recogmsed that the understanding of the process is now on a sound footing.
An important distinction that must be recognised in the resulting microstructural changes is the one between densifying and non - densifying processes. If densification is to occur then material must be removed from points between the grain centres of adjacent grains to allow shrinkage. The only reversible source for atoms that exists between the two centres is that provided by the grain boundary between the grains; diffusion processes which Involve the movement of atoms from grain boundaries to adjacent pores therefore result in an approach of the grain centres and in densificatlOn (Fig. 3). Movement of atoms between other sources and sinks, say from one part of the pore surface to another part of the surface does not result in densification; the changes then produce a
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coarsening of the microstructure in which the surface energy is reduced by reduction of the total surface area rather than by replacement of this surface by grain boundary. A major ambition in sinterin~ IS therefore to control the processing conditions, namely, powder particle SIZe, temperature, composition, applied pressure, and processin~ time, so that the densification process is favoured and the coarsening process is mhibited (Fig.4). The important covalent ceramic systems SiC and SiJN4 are examples where the control of this balance has been recognised as a major problem.
The role of surfaces has also been recognised as of vital importance in mechanical properties4. When a stress is imposed on a material, elastic strain energy is introduced as the atoms are pulled from their equilibrium positions. To a first approximation the only mechamsm capable of absorbing this energy during the fracture of a ceramic is the formation of new surface. If, in the propagation of a crack, the rate of elastic strain energy release exceeds the rate of surface energy demand, then the crack will grow. That is, when
d [u 2] d ilL 2E L3 ~ ilL (2'Yf) L2 (8)
Here u is the applied stress, E the elastic modulus, If the eneq~y required to form unit area of the new fracture surfaces and L a dimension scahng WIth the size of the crack. The fact that the elastic strain energy scales as the volume of material containing the atoms under stress while the surface energy scales as the area of new surface means that flaws above a certain size
L > 4;if (9)
are unstable and lead to rapid brittle failure. The critical r6le of flaw size in determining the applied stress, u, which can
be tolerated (the strength) has placed ~reat importance on the identification of structural flaws and on their size reductIon. A variability of flaw size produces a corresponding variability in mechanical stren~h. In order to achieve reliable and consistent strength values, therefore, a major ambition in current processing technology is to achieve control of the flaw population in the material5.
The importance of the If term, namely the energy required to bring about extension of the fracture surface, has resulted in efforts to develop mechanisms capable of raising this energy. In addition to the purely thermodynamic surface energy, contributions can be found from local plastic deformation and from a variety of mechanisms introduced by dehberate microstructural design (microcracking6, crack branching or deflection7, plastic deformation in metallic inclusions8, branching or deflection or pullout in fibre containing systems9, additional strain energy terms as in transformation toughening10). For many of these mechanisms, further interfacial energy terms become significant as, for example, in the case of the interface between fibrous inclusions and a matrix where the course of the crack path through the material can be influenced by modification of the interface characterll.
A further feature is the importance of interactions between the newly formed faces of the crack and the environment; these can provide additional contributions to crack extension. The general tendency to look more closely at the detailed micromechanics of crack development places more weight on the nature
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and behaviour of the interfaces and surfaces involved. The two examples which have been introduced, one from processing and one
from properties, are but representative of the central role which surface and interface character play in ceramics. To indicate the range of problems, two other examples can be very briefly mentioned. The great refinement that has occurred in recent years in the preparation of powders (Fig. 5) and in the control of the rheology of powder suspensions in connection with ceramic forming p'rocessesl2 has placed great emphasis on the nature and properties of the powder/fluid interface and on its control with additives. A further example from the mechanical properties area would be that of high temperature creep; the existence of a viscous phase in the grain boundaries of ceramics (Fig. 6) can be a major source of weakening under load at high temperaturesl3. The nature and distribution of such second phases is therefore a major research theme.
Insofar as the above processing and properties issues relate to all ceramics, the impact of surfaces and interfaces on fabrication and applications is. widespread. Specific examples can, however, be identified where surface related properties become the central aspect of the application. These would include uses which depend upon the wear resistance of the material such as cutting toolS14 or medical implant materialsl5. The role of a surface in wear resistance is self-evident but full understanding of the wear process has been difficult to achieve: in general, behaviour is extremely specific to the application concerned.
The uses of functional ceramics which find application as electrical or magnetic components also provide many examfles where surfaces and interfaces play a SpeCifIC role. Deliberate engineering 0 the interface structure is, e.g., mvolved in the development of boundary layer capacitors16 and of positive temperature coefficient devices17. Again, the behaviour of the surface can be very specific to the system involved and generalizations are difficult.
To summarize, It is im{>Ortant to recognise that surfaces and interfaces have long been seen by the ceramlC community as critical elements in the processing and use of these materials. There are issues which are common to all ceramics such as those discussed above related to sintering and to mechanical strength. There are, in addition, examples which specifically exploit surface phenomena. The almost universal importance of surfaces and interfaces for the use of ceramics has made the commuruty as alert to the impact of surface phenomena for these materials! as for any other materials class.
3. Structure of Interfaces in Ceramics
As a basis for understanding the behaviour and properties of surfaces and grain boundaries in ceramics, attention, as in other aspects of solid state science, is first given to structure. Excellent progress has been made in the understanding of mterface structure in recent years and this has been well covered in reviewsl8.
The structure of surfaces has been thoroughly studied, notably with the technique of LEED, and systematic descriptionsl~ of surface geometry are now available much as has long been the case for internal crystal structure. A feature of such descriptions is the careful attention given to the preparation and measurement conditions since these interact so profoundly with the detailed observations.
The structure of grain boundaries is understood at a considerably less
xxviii
detailed level. Proposals for specific structures20 can be developed on the basis of molecular dynamics calculations; particularly favourable orientations can then be sought and matched with the relative frequency of their occurrence to assess the extent of agreement between experimentally determined and theoretically predicted structures.
Major advances in understanding have been achieved on the basis of the rapid developments in the instrumentation available for surface and interface studies. High resolution transmission electron microscopy and more recently scanning tunneling electron microscopy21 allow detailed study of the local physical structures at interfaces; the growing array of surface analysis methods22 allows exceptional depth resolution of chemical characteristics normal to an interface. The combination of methods offers remarkable opportunities for the determination of the physical and chemical character of the interfaces whose behaviour is to be controlled and utilised. As seen in the following sections, however, the very complexity of the processes wherein interfaces play a role leaves room for uncertainties in interpretation and sets major tasks for future research.
4. Segregation and Boundary Migration
The wish to control the extent of structural coarsening during the heat treatment of ceramics has placed emphasis on the methods that can be made available for controlling grain boundary mobility. Of these, the most promising exploits the segregation of dopant or impurity atoms to the grain boundaries; such segregated species then act to rest ram or inhibit boundary motion (the impurity drag effect)23. The underlying idea is that segregation arises from the existence of low energy sites for the segregant at the boundary, either as a consequence of size differences24 and the existence of less ordered, more accommodating sites at the boundary, or as a consequence of space charges25 and the resulting electrostatic interactions between charged dopant/impurity species and the boundary; movement of the boundary then requires movement of the segregated atoms if they are to remain in the low energy sites. The boundary mobility is then not determined by the simple jump of atoms from one side of the boundary (the shrinking ~rain) to the other side (the growing grain) but by the associated long distance diffusion of the cloud of segregated atoms.
The observations of segregation are now many26 and evidence exists both for size difference27 and for electrostatic interaction"28 as the responsible factor. Attempts have been made at a systematic classification29; generally segregation is to be expected except where dopant and host atoms have both the same size and the same charge (or where a similarly sized atom of different charge is accompanied by similar sized atoms of compensating charge difference).
The basis for electrostatic segregation25 lies in the fact that the individual formation energies for the compensating intrinsic defects in ionic systems are likely to be different, e.g., the formation energy of one vacancy type is less than that of the other in Schottky disorder in a binary ionic compound. The existence of the boundary allows this difference to be reflected in a locally enhanced concentration of the lower energy defect, the resulting space charge being offset by the compensating charge at the boundary itself. Distribution of charged dopants in the resulting electrical field in the boundary region then appears as segregation.
Studies of grain growth in Ce02 can illustrate30 the position. Ce4+ (920
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pm) and Gd3+ (938) are of similar ion size. Segregation of Gd3+ is, however, to be expected on electrostatic wounds (Fig. 7). If for Ce02, anion Frenkel disorder is taken as the intrinsic dIsorder type and if Ef(Vo) < Er(Oi) then a defect distribution as in (a) is ex~ted (positive spacecharge offset by a negative surface layer rich in 0 2-). AddItion of Gd2D3 raises the Vo concentration (b) to compensate for Gd' ce so that the vacancy concentration rises above the value which it would have were charge balance requirements relaxed (dotted line). The concentration can relax to this value at the surface establishing a positive surface layer and negative space charge within which distribution of the Gd' ce takes place giving rise to segregation. Grain growth in Gd - doped Ce02 is reduced in comparison with that in the undoped material. The effect of Nb2D5 addition to the doped material (Fig. 8) is to enhance grain growth at small concentrations (Fig. 7d) where the co-doping allows defect concentrations to reach the values which they would adopt in the absence of any need for charge compensation and where segregation is accordingly absent; at larger concentrations, Nb ce must itself be compensated by Oi (Fig. 7c) and segregation of Nb ce then occurs in the resulting space charge field. Although Nb5+ has a different ion size (690 pm), the evidence provided by the wain growth behaviour for the existence of charge compensation among the additives underlines the importance of electrostatic factors. Similar behaviour has been found in the more com(>lex example of BaTi0331.
The difficulties which currently anse when attempts are made to predict the influence of additives or of dopants on the mobility of boundaries are several. They include:
(1) Segregation can, as noted, arise from different causes. Distribution in the segregation regions can accordingly take different forms.
(2) Segre~ation at interfaces can be highly anisotropic with different degrees of segregation ansing at crystallographically different interfaces32.
(3) The direct link between degree of segregation and boundary kinetics33 is difficult to make experimentally. Grain growth studies34 indicate that the influence of segre~ated additives can be very considerable; direct measurements are, however, diffIcult and are complicated by the existence of instabilities, i.e., dependence of the mobility on the local driving force for migration.
(4) Mobility can be influenced by other factors such as second phases or attached porosity35.
The present position is that the widespread occurrence of segregation as a phenomenon has been demonstrated as has the ability of a segregated dopant to Influence boundary mobility. Systematic study of behaviour, particularly on a quantitative basis, is still very much required in order that more rational use of this method for the control of coarsening can be made. The opportunities for interesting scientific and technolo~cal contributions arising from studies of segregation and its consequences are mdeed considerable.
5. Special Boundaries and Liquid Films
The influence of liquid phases at high temperature in ceramics has been of longstanding interest not only for such propertIes as creep13 and (following cooling too ambient temJ?C?rature) thermal conductivity36 but also because of the prevalence of liqUId phase sintering as a processing method (where a grain boundary liquid is used to accelerate diffusion during densification) as a processing method.
xxx
An important issue concerns the stability37 of liquid phases in grain boundaries, i.e., the extent to which boundaries remain wetted by a liquid as the temperature is reduced from that used during processing, and the dependence of this stability on the orientation of the grains formin~ the boundary. Although discussions of boundary energy as a function of orientatlOn have not taken place to the extent recognised for solid/solid boundaries, a number of indications suggest that the anisotropy can be substantial. These include:
(1) The notable structural differences occurring at solid/liquid/solid interfaces of different orientation (Fig. 6) imply energy differences for the two sides of the boundary; correspondingly, different energies are expected for boundaries where one of the grains can take up a favoured orientation (upper grain in Fig. 6).
(2) Liquid infiltrated grain boundaries are known to be favourable38 for the occurrence of abnormal grain growth (where a few grains grow rapidly within a fine-grain matrix). As shown by computer modelling39, the existence of special boundaries of low energy is also favourable to this development. For Al2<)3, isotropic grain structures (Fig. 9) develop40 either in the 1?ure material (free from grain boundary films) or m materials in which a liquid IS present but with less anisotropic interfacial energies (work on MgO-doped material or at high temperature). Otherwise, anisotropic grain structures readily develop. The indications {rom such work are (Fig. 10) that anisotropy of the boundary energy with grain orientation is more extreme in liquid contaimng systems than in those where no liquid boundary phase occurs. Where additives promote anisotropy41, e.~. Cao, then the tendency to produce faceted grains and abnormal growth IS enhanced.
The influence of temperature, of orientation and of chemistry makes the interaction between grain boundaries and liquid second phases a complex and fascinating issue. The methods for controlling second phase distribution, amount and character (additives42, distribution of additives43, crystallisation44, evaporation45, solid solution formation with the grains46) are numerous and the theme of grain boundary engineering has developed on the basis of the different possibilities. The widespread importance of boundary films makes this an area where intensive and systematic studies will continue to prove fruitful.
6. Concluding Remarks
These introductory notes can perhaps serve to emphasize the number of research questions in ceramics where further progress will depend upon an improved ability to understand, design, construct, and exploit surfaces and interfaces. There are many other examples as in sensors or catalysts or in wear or corrosion where this is equally true.
The special feature that unites all such themes is the opportunity that now exists for rapid advance. The combination of sophisticated instrumentation, of refined modellin~, and of better characterised chemical systems provides an excellent foundation for progress in establishing a quantitative and comprehensive understanding of phenomena where surfaces and interfaces are involved. The significance of such work should not be underestimated: the search for deliberate and predictive control over the processing and properties of ceramics has been a long one; imaginative and systematic work on surfaces and interfaces is one of the most promising ways to further progress.
xxxi
Figure 1. Differences in surface curvature drive the atomic movements which cause microstructural change during the densification of a powder. (Courtesy J. - L. Chermant).
A A
:~q!~ I
System 1 System 2
Figure 2. The gas ,eressure over the curved surface is raised by dP A in system 2 relative to system 1 (Eq. 7); the normal stress in the solid is raised by dPB for the curvature shown (Eq. 5).
xxxii
Figure 3. Atom movements from grain boundaries to pores (black) allow the shrinkage of the grain/grain inter-centre distance which represents densification. (CoO sample; 2~m marker; courtesy S.P. Howlett)
Figure 4. Completed densification in alumina (courtesy E.W. Roberts). The grain boundary curvature drives grain to grain atom movements which produce coarsenin~; as a result of the boundary motion, small grains disappear and the average SIze of the remainder increases.
xxxiii
Figure 5. Chemical methods of powder production have resulted in particles specifically adapted for the sinterin~ process. A difficulty with ordered arrays is that minor departures from size uniformity can produce large scale faulting; the existence of disordered regions between different regions is also a serious source of inhomogeneity. (Si02 spheres; courtesy S.l. Milne)
Figure 6. Interface structure between three grains of silicon nitride (courtesy D.R. Clarke). The presence of an amorphous boundary phase allows enhanced deformation under load at high temperature.
xxxiv
I ··1
~ ; :::::====V""'·· =
~,N.~-...ee !, Nb~e
v~ .• = ... = .. =. ~=~" .. f--_~V 0:....-, 0
~ _________ I-__ O_i"_
Vo ~.......... . ............. .
V O~ ..............................•
I
( D ) ( b ) ( c ) ( d )
Figure 7. Defect concentrations in Ce02 adjacent to an external surface or grain boundary (Courtesy Y.S. Zhen30). The schematic plots show the situation: a) for the undoped material assuming anti - Frenkel disorder and a lower for-mation energy for the vacancy than for the interstitial b) for material doped with gadolinium oxide c) for material doped with niobium oxide d) for co-doped material. The dashed lines show the concentrations that would exist for the host defects on the basis of the individual formation energies, i.e., if requirements for charge balance in the bulk of the crystal could be relaxed.
ol~ --~~~--~~--~~-o 2 4 ,6 8 10 12 14
Nb20S Additive (mole%)
Figure 8. Grain size in cerium oxide after firing for 5 hours at 1600· C (Courtesy Y.S. Zhen30). The materials contain 7 mol%Gd203.
Figure 9. alumina40•
>. 0'1 La> c a>
c1 u c1
'+L-
a> +-'
C .......
xxxv
< ~
,. < , » < .... ..,,> no liquid v"
11.,.21' < ,\/,> '\":> with MgO
with liquid
liquid+MgO --...... ,.- --, ,.,..-....... ,,-- with liquid
..... v" at high temp.
Orientation
The dependence of grain shape on processing conditions in
E 3.0 t f ~ 2A \ \ 3A CTl with liquid
~ 2.5 Anisotropic tgo\\ IsotropIc a> \ \ ~ 2 0 •• -rrm.,. 12BI \ \ 13BI :; . [KJ ~_~ 't: Isotropic -_. ~ 1.5 • E without liquid .......
1. 0 '-___ -L-__ ---.JL--__ -'--__ ---l
1500 1600 1700 1800 1900 Sintering temperature, ·C
Figure 10. The pronounced shape anisotropy of grains in liquid-containing systems suggests that the orientation dependence of the solid/liquid interfacial eneq~y is greater than that for solid/solid boundaries40• The degree of anisotropy is qualItatively indicated to match the experimental results shown in Fig. 9.
xxxvi
References
1. 2. 3.
4. 5. 6.
7. 8.
9.
10. 11.
12.
13.
14. 15. 16. 17. 18a.
b.
19.
20. 21. 22. 23. 24.
25. 26. 27. 28. 29. 30.
31. 32. 33.
W.D. Kingery, J.Am.Ceram.Soc., 57 (1), 1-8 (1974). S. Prochazka and RM. Scanlan, J.Am.Ceram.Soc., 58 (1-2),72 (1975). Y.E. Geguzin, 'Physik des Sinterns', VEB Deutscher Verlag fUr Grundstoffindustrie, Leipzig (1973). S.N. Freiman, Am.Ceram.Soc.Bull. 67 (2) 392-402 (1988). F.F. Lange, J.Am.Ceram.Soc., 72 (1),3-15 (1989). N. Claussen, J. Steeb and RF. Pabst, Am. Ceram. Soc. Bull. 56 (6),559-562 (1977). K.T. Faber and AG. Evans, Acta Metall. 31 (4), 577-584 (1983). V.D. Kostic, P.S. Nicholson and RG. Hoagland, J.Am.Ceram.Soc., 64 (9), 499-504 (1981). D.B. Marshall, B.N. Cox and AG. Evans, Acta Metall. 33, 2013 - 2021 (1985). AG. Evans and RM. Cannon, Acta Metall. 34 (5), 761- 800 (1986). M.D. Thouless, O. Sbaizero, L.S. Sigl and AG. Evans. J.Am. Ceram. Soc., 72 (4) 525-532 (1989). E. Barringer, N. Jubb, B. Fegley, RL. Pober and HK. Bowen, pp. 315-333 in 'Ultrastructure Processin~ of Ceramics, Glasses, and Composites', ed. L.L. Hench and D.R Ulnch, Wiley, New York (1984). S.M. Wiederhorn, B.J. Hockey, RF. Krause, Jr., and K. Jakus, J.Mat.Sci. 21, 810-824 (1986). J.G. Baldoni and S.T. Buljan, Am.Ceram.Soc.Bull. 67 (2) 381-387 (1988). P. Vincenzini, ed., 'Ceramics in Surgery', Elsevier, Amsterdam (1983). R Mauczok and R Wernicke, Philips Tech. Rev. 41, 338 (1983/4). H. Ihrig, Adv. in Ceram. 7, 117 (1984). J. Nowotny and L.C. Dufour, eds., 'Surface and Near Surface Chemistry of Oxide Materials', Elsevier, Amsterdam (1988). L.c. Dufour and J. Nowotny, eds., 'External and Internal Surfaces in Metal Oxides', Mat.Sci.Forum 29, 1-303 (1988). J.M. Maclaren, J.B. Pendry, P.J. Rous, D.K. Saldin, G.A Somorjai, M.A van Hove and D.D. Vredensky, 'Surface Crystallographic Information Service: Handbook of Surface Structures', Reidel, Dordrecht (1987). D.M. Duffy and P.W. Tasker, Adv. in Ceram. 10,275-289 (1984). W. Hirschwald, pp. 61- 187 in Ref. 18a. S. Hoffmann, Surface and Int. Analysis, 9, 3-20 (1986). J.W. Cahn, Acta Metall. 10, 789 (1962). D.R McLean, 'Grain Boundaries in Metals', Oxford University Press, Oxford (1957). K.L. Kliewer and J.S. Koehler, Phys.Rev., 140 (4A), 1226 (1965) P. Wynblatt and RC. McCune, pp. 247-279 in Ref. 18a. RF. Cook and AG. Schrott, J.am.Ceram.Soc. 71 (1) 50-58 (1988). W.D. Kingery, Pure AfPl.Chem., 56 (12), 1703-1714 (1984). A.J. Burggraaf and A. .A. Winnubst, pp. 449-477 in Ref. 18a. Y.S. Zhen, 'Oxygen Ion Conduction in Doped Rare Barch Oxides', Ph.D. Thesis, UniversIty of Leeds (1988). RJ. Brook, W.H. Tuan and L.A. Xue, Ceramic Trans. 1,811-823 (1988). S. Baik and c.L. White, J.Am.Ceram.Soc. 70 (9) 682-688 (1987). A.M. Glaeser, H.K. Bowen and RM. Cannon. J.Am. Ceram. Soc. 69 (2), 119-125 (1986). (But see in Y.M. Chiang and W.D. Kingery,
34. 35. 36.
37. 38. 39.
40. 41.
42. 43.
44.
45. 46.
xxxvii
~:~£eJjmli~iI Ii.~)Anm~l,7l~&ram.Soc. 68, C: 22-24 (1985). J. ROdel and AM. Glaeser, J.Am.Ceram.Soc. 70 (8), C: 172 (1987). Y. Kurokawa, K. Utsumi and H. Takamizawa, J.Am.Ceram.Soc. 71, 588-594 (1988). D.R. Clarke, Ann.Rev.Mat.Sci. 17,57-74 (1987). F.M.A Carpay and AL. Stuijts, Sci.Ceram. 8, 23 (1976). D.J. SroIOVICZ, G.S. Grest and M.P. Anderson, Acta Metall. 33, 2233-2247 (1985). B. Hong, S. - J.L. Kang and RJ. Brook, for publication. W.A Kaysser, M. Sprissler, C.A Handwerker and J.E. Blendell, J.Am.Ceram.Soc. 70 (5),339 (1987). J. White, Mat.Sci.Res. 6, 81-108 (1973). R. Riedel, G. Passing and G. Petzow, Proc.2nd Int.Conf.Powd.Proc.Sci., Berchtesgaden, DKG Verlag, Koln (1989). M.H. Lewis, G. Leng- Ward and S. Mason, Brit.Ceram.Proc. 39, 1-13 (1987). R W. Rice, Proc.Br.Ceram.Soc. 12, 99 (1969). K.H. Jack, J.Mater.Sci. 11, 1135-1158 (1976).
