Strengthening of Ceramics: Treatments: Tests, and Design Applications PDF
Preview Strengthening of Ceramics: Treatments: Tests, and Design Applications
STRENGTHENING OF -CERAMICS MANUFACTURING ENGINEERING AND MATERIALS PROCESSING A Series of Reference Books andTextbooks SERIES EDITORS Geoffrey Boothroyd George E. Dieter DepartmentofMechanicalEngineering Dean, CollegeofEngineering UniversityofMassachusetts UniversityofMaryland Amherst,Massachusetts CollegePark, Maryland 1. Computers in Manufacturing, U. Rembold, M. Seth, andJ.S. Weinstein 2. Cold Rolling of Steel, William L. Roberts 3. Strengthening of Ceramics: Treatments, Tests, and Design Applications, Henry P. Kirchner OTHERVOLUMES IN PREPARATION. STRENGTHENING OF CERAMICS Treatments, Tests, and Design Applications Henry P. Kirchner CeramicFinishing Company StateCollege, Pennsylvania MARCEL DEKKER, INC. New York and Basel Library of Congress Cataloging in Publication Data Kirchner, Henry Paul. Strengthening of ceramics. (Manufacturing engineering and materials processing 3) Bibliography: p. Includes index. 1. Ceramic materials. I. Title. II. Series. TA455.C43K57 666 79-17514 ISBN 0-8247-6851-5 COPYRIGHT © 1979 by MARCEL DEKKER, INC. ALL RIGHTS RESERVED Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo- copying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 10016 Current printing (last digit): 10 9 8 7 6 5 4 3 2 PRINTED IN THE UNITED STATES OF AMERICA PREFACE Considerable resources have been devoted to diverse attempts to strengthen ceramic materials. In the majority of cases the results achieved have been marginal, leading to considerable frustration. Based on earlier success with strengthening of glass and glass- ceramics, using approaches involving treatments to induce compressive surface stresses, substantial progress was made in strengthening con- ventional polycrystalline ceramics and oxide single crystals. This monograph describes these advances. The methods described were de- veloped, in large part, at Ceramic Finishing Company, but results from other laboratories that have come to my attention are included. The scope of this monograph is limited to treatments for polycrystal- line ceramics and oxide single crystals. Despite the limited scope it was necessary to omit much important data. This information is available in the references. Designers will find that, in addition to the descriptions of individual treatments and the resulting improvements in strength, information on potential applications, limitations of the treatments, design considerations, and costs are presented. The principal advantages of compressive surface layer treatments are improved strength, decreased penetration of surface damage and strength degradation, and improved impact, thermal shock and delayed fracture performance. The highest strengths have, thus far, been achieved by thermal (quenching) treatments. These treatments are limited, to some extent, by thermal 'shock damage in the larger and more complex shapes. Coatings, chemical treatments and treatments to induce phase transformations are alternative processes. Although the achievable strengths using these treatments are somewhat lower, a much wider range of shapes and sizes can be treated. Potential applications should be sought in the following areas: bearings, cutting tools, gas turbines, radomes, IR domes, armor, ex- trusion dies, pump parts, lighting envelopes, laser windows, leading edges, other wear resistance parts, and so forth. Progress in the iii iv PREFACE application of "tempered" glasses and glass-ceramics can provide some guidance as to what to expect. It should be noted that the payoff for improvements in many of these applications may be very large because the ceramic part is the critical link in the operation of the complete device. Operations of complete machines may succeed or fail depending on the success or failure of these individual ceramic parts. In these circumstances, substantial improvements in strength may be essential to reliable performance. Subcritical crack growth is the principal factor affecting the reliable use of many ceramics in load bearing applications. The original flaws gradually increase in size to the point that stress intensification may cause catastrophic failure. To reduce subcriti- cal crack growth, one can reduce the load or redesign the part so that the stress is less. However, in many cases neither means is satis- factory. Use of compressive surface stresses provides another alter- native. These surface stresses, because they subtract from normal tensile stresses in the surface where most of the severe flaws are, reduce the stress intensity factor at flaws in stressed members, thus decreasing the subcritical crack growth rate, Proof testing is an accepted means for assuring the strength of ceramic parts by removing parts weaker than a particular strength level from the distribution. The most critical proof test parameter is the difference between the proof test stress and the use stress. The greater the difference can be, the greater will be the effective- ness in removing weak parts from the lot, especially when the sub- critical crack growth during loading and unloading from proof testing is considered. Because the compressive surface stresses raise the nominal stresses at which surface flaws act to cause failure, treated specimens can be proof tested at stresses that are much higher than otherwise possible. For a given use stress this means a larger dif- ference between the proof stress and the use stress and therefore a smaller probability of failure of a partin service. More widespread use of ceramics can aid greatly in improvement of efficiencies of energy conversion equipment and in weight reduc- tion in aerospace applications. Improvements in energy efficiency can occur by means such as increased inlet temperatures leading to increased Carnot efficiencies and by reduction in the requirements for cooling air in gas turbine engines. Weight reduction can be accomplished by substitution of ceramics for refractory metals, re- duction of the thickness of sections through the use of the greater stiffness of some ceramics, and by other means. Weight reductions in aerospace applications can be especially important because less fuel is needed leading to still further weight reductions. The in- creased strength and reliability of ceramics with compressive surface stresses will aid in adapting ceramics to these applications. It is a pleasure to acknowledge the contributions of my assoc- iates at Ceramic Finishing Company, especially Robert M. Gruver and others, many of whom have been coauthors of the various earlier de- scriptions of this work, and the many research workers at other organizations who have contributed to our knowledge of this field. I am also grateful to the sponsors of the research including espec- ially the Naval Air Systems Command and the Office of Naval Research, PREFACE v and the technical contract monitors, Charles F. Bersch and Arthur M. Diness. H~lpful suggestions were made by Richard C. Bradt and John B. Wachtman, Jr. who reviewed the original manuscript. It is also a pleasure to acknowledge the typing done by Shirley Wittlinger and Elaine Smile~. Henry P. Kirchner April, 1979
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