Mechanical and thermal properties of ceramics :

Abstract

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 used.The 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 property-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 ...

The influence of microstructure on the fracture statistics of polycrystalline diamond and polycrystalline cubic boron nitride

“…However, it is worth noting that for strength values far below the mean it gives a finite probability of negative strength values occurring [4]. This is an obviously unrealistic outcome which imposes limitations on the use of the normal distribution function.…”

“…The Weibull distribution has traditionally been employed to describe the strength distribution of brittle materials [2,3,4]. This theory assumes a direct correlation between the density of flaws distributed throughout the specimen and the subsequent strength distribution.…”

Mechanical properties of SiOx gas barrier coatings on polyester films

“…However, at saturation the fragment length is too small to measure its strength directly. Nevertheless it can be modelled using the Weibull distribution for the probability of failure of ceramics, based on the idea that failure at any flaw leads to total failure of the material [5,12,13]. The strength of coating fragments depends on the probability of having a critical size defect, which for a uniform defect distribution is related to the volume (i.e.…”

Strength-probability-time diagrams using power law and exponential kinetics models for fatigue

“…The above analysis is well known and has been published in textbooks [3]. However, it is outlined in detail here for the following reason.…”