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Unformatted text preview: UNITED STATES DEPARTMENT OF COMMERCE Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director Mechanical and Thermal Properties of Ceramics Proceedings of a Symposium Held at Gaithersburg, Maryland April 1-2, 1968 Edited by J. B. Wachtman, Jr. Institute for Materials Research National Bureau of Standards Washington, D.C. 20234 Sponsored by The American Ceramic Society The American Society for Testing and Materials and The National Bureau of Standards ,^ , National Bureau of Standards, Special Publication 303 Issued For sale May 1969 by the Superintendent of Documents, U.S. Government Printing Price $4. 25 Office, Washington, D.C. 20402 nnnuiwL mma JHl of srMBARDS t S &c Abstract The proceedings of the Symposium on Mechanical and Thermal Properties of Ceramics, held at the National Bureau of Standards in Gaithersburg, Maryland, on April 1-2, 1968, are presented. The symposium was cosponsored by the American Ceramic Society, the American Society for Testing and Materials, and the National Bureau of Standards. Attention was concentrated upon properties primarily of structural and/or high temperature use including melting points, thermal expansion, thermal conductivity, thermal radiation properties, elasticity, viscoelasticity, inelastic deformation, and fracture. The dependence of these properties upon composition and microstructure was surveyed and the importance of controlling these features of character to insure reliability of ceramics was emphasized. The procedures by which American ceramists formulate standards and specifications to assist in insuring reliability were summarized. Key Words: Ceramics; elasticity; fracture; inelastic deformation; melting point; standards; thermal conductivity; thermal expansion; thermal radiation; viscoelasticity. Library of Congress Catalog Card ii Number: 68-61960 19«8 ioo Foreword A major responsibility of the NBS Institute for Materials Research on the properties is to insure the which are urgently needed to meet the requirements of the Nation's scientists and engineers. To obtain reliable data of this kind not only must the experimental procedures be tailored to permit the required measurement accuracies, and the results correctly analyzed, but also the sample materials must be sufficiently uniform and well characterized to permit measurements amenable to meaningful interpretation and practical use. The reliability of data needed for structural or high-temperature use of ceramics is an important example. Sensitivity of strength data to test conditions and complications introduced by high measurement temperatures plague the measurements and make experimental procedures a prime concern. Also, several important properties, including thermal conductivity, inelastic deformation, and strength, are sensitive to small variations in microstructure and composition. To provide a basis for understanding the dependence of ceramic properties on various characterizing features, a Symposium on Mechanical and Thermal Properties of Ceramics was held at the National Bureau of Standards laboratories in Gaithersburg, Md., in April 1968. The meeting was jointly sponsored by the American Ceramic Society, the American Society for Testing and Materials, and NBS. The National Bureau of Standards is pleased to have the opportunity of making the papers presented at the Symposuim available in this publication. In this way we hope to assist availability of those basic data of materials those attending the conference in optimizing the use of ceramics, to make the conference results available to a wider audience, and to show appreciation for the essential contributions of the participants and the cosponsor organizations. A. V. Astin, Director m Introduction a collection of the proceedings of the symposium, Mechanical and Thermal Properties of Ceramics, held at the National Bureau of Standards laboratories at Gaithersburg, Maryland, on April 1-2, 1968. Approximately 210 scientists and engineers participated. The proper selection and effective use of materials is one of the fundamental factors upon which advances in technology depend. Ceramic materials have a long history of effective use in many applications for which their hardness, stiffness, strength, resistance to corrosion, thermal and electrical insulating qualities, high melting points, optical properties, semiconducting properties, dielectric properties, magnetic properties, piezoelectric properties, nuclear properties, or other special qualities are appropriate. The ceramic industry in the United States continues to grow and is approaching the ten billion dollar a year level. Every type of material, including ceramics, is challenged, however, both by competition from other materials in its traditional areas of use and by the requirements for improved properties needed in new This is applications. The development of new materials and the improvement of properties depend partly upon an empirical approach in which the composition and manufacturing procedure are systematically varied and correlation with resulting properties is attempted. This empirical will probably continue indefinitely to be an important part of materials development and need not conflict with the use of materials science. The latter is helpful in predicting areas where improvement appears possible, choosing the parameters to be varied, designing an efficient processing property study, and interpreting the results. Ceramics are frequently manufactured by a heating process which appears deceptively simple but which usually involves a number of competing reactions and transport processes occurring at different rates. Optimizing a particular property requires a starting composition and heat treatment which produces the final composition and microstructure (usually denoted by the word "character") needed for that property. In principle one should begin with the properties needed, determine the character required from an understanding of the propertycharacter relationship, and design the processing from an understanding of the chemistry and mass transport which are involved. In practice the understanding of both areas is limited so that empirical procedures, guided by some degree of fundamental understanding, are generally method — used. current status of ceramic processing was surveyed by the Materials Advisory Board ad hoc Committee on Ceramics Processing, chaired by Professor J. A. Pask, and their conclusions are available in Ceramic Processing, National Academy of Sciences Publication 1576 (1968). The importance of an improved understanding and use of the property-character relationship by users of ceramics as well as producers is emphasized. In particular, the user should understand that many properties depend on other aspects of character in addition to gross chemical composition. The engineer who specifies that a part be made of "alumina" is leaving the strength very poorly specified; commercial alumina ceramics range in strength from values below 2000 psi for certain refractories to values approaching 100,000 psi for certain tool bits and other special purpose ceramics. The present symposium is aimed primarily at the users of ceramics but it is hoped that producers will also find a systematic review of prop- The erty-character relationships useful. Properties of ceramics in general form too broad and diverse a subject to be effectively treated in this symposium; the properties selected for treatment are those related to mechanical and thermal behavior. Ceramics are frequently used to exploit their thermal properties and also as structural members in which case their deformation and ultimate fracture under load are of obvious importance. In many applications ceramics are used to exploit other properties, but mechanical and thermal properties are usually still important in determining the size, weight, and cost of the parts involved. Ceramics are, of course, particularly well suited to high temperature applications but the conditions of high temperature use frequently create thermal gradients with accompanying thermal stress. The symposium accordingly includes IV thermal factors not only because of their direct engineering importance, but also because of their role in limiting load-bearing capacity at high temperatures; i.e., the melting points, thermal expansion, thermal conductivity, and thermal radiation properties are surveyed. The practical utilization of high-performance ceramics requires more than their successful production on a laboratory scale and favorable economic factors. Such ceramics must be reliable in production, that is, their properties must be uniform within each piece and reproducible from piece to piece. Small lots of special ceramics frequently show significant variation in structure-sensitive properties including strength. This variation has raised questions concerning the permissible stress levels in structural applications and is currently inspiring research aimed at determining to what extent this variability in strength results from variability in processing, variability in testing, or from the intrinsic nature of brittle materials. This research aims at the ultimate establishment of a degree of processing control, testing accuracy, and design understanding to permit ceramics to be used routinely and reliably at high stress levels. Such use depends on acceptance of testing standards by the producers and users of ceramics. This symposium accordingly opens with a survey of the activities of the American Ceramic Society and the American Society for Testing and Materials, two of the principal institutions through which American ceramists communicate and formulate standards. The needs for high-performance ceramics are next discussed and the importance of reliability is stressed. The thermal properties are then treated, being taken first because of their role in determining stresses resulting from heat flow. Finally, the mechanical properties which determine response to stress, whether of thermal or mechanical origin, are discussed. Throughout the discussion of properties the emphasis is placed on the property-character relationship; i.e., on the features of composition and microstructure upon which the properties depend. Some property tabulations are given to illustrate property ranges, but exhaustive tabulation of numerical values has not been attempted. References are given to many existing tabulations which, excellent as some are, need to be used carefully with an understanding of how variation in character of the ceramic under consideration can cause properties to vary from the handbook values. Each of the following papers was written as a separate entity and can be read without reference to the others. A few comments on some of the highlights of the papers and on the interrelation of the various properties should, however, give a helpful perspective on the general subject of the symposium. An important point concerning high-performance ceramics is made in the paper on the American Ceramic Society and Ceramic Materials Technology. The example of ceramic insulators for spark plugs is quoted to make the point that when large volume requirements exist ceramics can be and are routinely produced at modest cost to meet high-performance specifications. This example raises the question of how well reliably expertise developed in producing a specialized ceramic can be transferred to the production purely emof other ceramics to meet somewhat different high-performance requirements. pirical production process developed around a specific raw material and product may not be a A good guide in developing a new product; understanding of the property-character relationship and of the process of character development during processing are needed. Standard test methods for both character determination and property measurement are therefore important to aid in developing this understanding in relation to commercial manufacturing of ceramics. The process by which United States Standards are developed and the role of the American Ceramic Society and the American Society for Testing and Materials in the development of standards for ceramics is accordingly described. Commercial development of new ceramics will occur only if justified by the demand; the paper on "Properties of Ceramics for Structural and/or High Temperature Use; Need for Control, Measurement and Compilation" discusses new markets open to ceramics and emphasizes the requirement of reliability and the consequent need for understanding the characterproperty relationship and for good testing procedures. The discussion of thermal properties begins with a paper on melting points. This might incorrectly be thought to be a simple subject but the engineer unfamiliar with phase equilibria who attempts to use ceramics at high temperatures may be in for a surprise. Thus, for example, Ti0 2 melting at about 1840 °C, and CaO, melting at about 2570 °C, might appear to be safe for use at 1500 °C. Anyone who has tried using CaO and Ti0 2 in contact with each other at 1500 °C or many similar combinations of ceramics above their eutectic temperature is referred to the paper on melting points for an explanation of what happened to his ceramic parts. , v One of the factors limiting use of ceramics at high temperature, as mentioned previously, the development of stress accompanying thermal gradients. Thermal shock parameters involving elastic moduli and thermal expansion have been used as a rough guide to the "thermal shock resistance" of materials, but it has long been recognized that a complete analysis of the heat flow and temperature distribution for each particular device is needed. High-speed computers make this feasible so that modern designs are increasing based on an analysis which requires the thermal radiation properties (emissivity, etc.) and thermal conductivity to calculate the temperature distribution and then requires the thermal expansion and elastic moduli (plus equations giving plastic response if it is involved) in order to calculate the stress distribuis tion. Fracture of polycrystalline ceramics as a result of stress of thermal or mechanical origin usually occurs suddenly and completely, at least at temperatures below the range of appreciable plastic deformation. The need to understand the characteristics of brittle fracture and to design for the use of ceramics with this in mind is apparent. The fracture paper in this symposium deals with this subject from the point of view of fracture mechanics as a branch of continuum mechanics as well as from the point of view of the effects of defects and environment. Finally, the subject of mechanical testing is treated in a separate paper because of its difficulty and importance. J. B. Wachtman, Jr. Acknowledgment The editor wishes to thank the following for their assistance in organizing and holding the symposium: R. T. Cook, National Bureau of Standards P. M. Corbett, Glidden-Durkee Division, SCM Corp. J. J. Diamond, National Bureau of Standards G. T. Leighty, National Bureau of Standards J. L. Pentecost, W. Pv. Grace & Co., Washington Research Center D. W. Robertson, General Refractories Company VI Contents Page Foreword Ill Introduction IV The American Ceramic J. S. ASTM: Its Function F. J. Mardulier ASTM J. Society and Ceramic Materials Technology 1 Owens Activities in C. Richmond and Philosophy 5 Ceramics 9 Properties of Ceramics for Structural and/or High Temperature Use: Need for Control, Measurement, and Compilation C. S. Bersch 13 Melting Points of Ceramics 19 S. J. Schneider Thermal Expansion of Ceramics 41 R. K. Kirby Thermal Conductivity of Ceramics 63 D. R. Flynn Thermal Radiation Properties of Ceramic Materials J. C. Elastic Deformation of J. B. Wachtman, Jr. Ceramics and Other Refractory Materials Viscoelasticity of Glasses P. B. de Macedo M. 139 169 Deformation of Oxide Ceramics Inelastic R. 125 Richmond 189 Spriggs Plastic Deformation of Carbides G. E. Hollox 201 Fracture of Ceramics S. M. Wiederhorn 217 Strength Testing of Ceramics—A Survey L. Mordfin and M. J. Kerper 243 Subject Index 263 vn The American Ceramic Society and Ceramic Materials Technology President, James S. Owens The American Ceramic Champion Spark Plug Company, Ceramic Society Division, Detroit, Michigan 48211 United States, through the continued growth of the use of of many new products and applications, has a sales volume approaching ten billion dollars a year. The American Ceramic Society, which was founded 69 years ago and represents all segments of the industry, has grown into the world's largest ceramic technical organization and is still growing at the rate of 50 percent per decade. It has diversified into ten Divisions to serve the economic, technological and scientific interests associated with ceramics. High performance ceramic materials with unique properties inherently dependent upon microstructure have been developed for specific applications that require either limited quantities or high volume mass production. Careful production control to insure a high level of performance is receiving close attention. The ceramic industry traditional ceramics in the and the development Key Words: Ceramics; ceramic industry; American Ceramic Society; microstructure; properties; special ceramics. It is an honor to helps to make possible new generations of computers and other complex, yet compact, electronic equipment; spark plug and jet igniter insulators; ceramic protective coatings for metals; brick, glass, and tile structural and decorative ceramics; glass containers; and dinnerware. Since the success of many applications of ceramic materials depends on their mechanical be invited to take part with you in this significant Symposium on Mechanical and Thermal Properties of Ceramics, and it is a pleasure to do so. Since its founding, The American Ceramic Society has been extremely interested in encouraging and supporting the continuation of the technical education of the members of the ceramic industry and of those associated with and thermal properties, this Symposium is an important and timely one. The American Ceramic Society had its inception seventy years ago in February 1898 at the 12th Annual Convention of the National Brick Manufacturers' Association in Pittsburgh where the first scientific paper of its kind was presented This meeting is an excellent example of such continuing education. Our Society is happy and proud to sponsor this Symposium through its Baltimore-Washington Section in cooperation with the Middle Atlantic District of the American Society for Testing and Materials and the National Bureau of Standards. It is particularly fitting that this program should be held here at the National Bureau of Standards since the Bureau has done outstanding research in ceramics and this Symposium was organized by J. B. Wachtman, Jr., who is Chief of the Physical Properties Section of the Bureau and is an officer of the Baltimore-Washington Section of The American Ceramic Society. The ceramic industry is a large one with estimated sales this year of about ten billion dollars. Ceramics include the inorganic nonmetallic solid materials. They have great importance for a wide variety of industrial, commercial and consumer uses, and to our Government for both national defense and civilian purposes. Their broad range of uses includes, for example, refractory ceramics for application at very high temperatures, in nuclear and aerospace technology, and in industrial furnaces and kilns; electronic ceramics in miniaturized microelectronic circuitry that it. before that Association. As a result, there developed in the next two days the idea for a separate society for the presentation of technical papers on ceramics that culminated a year later on February 6, 1899 in the official chartering of The American Ceramic Society in Columbus, Ohio, with 15 charter members. These i...
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