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150.H. M. Kandil, J. D. Greiner, and J. F. Smith. Single-Crystal Elastic Constants of Yttria-Stabilized Zirconia in the Range of 20° to 700°C. J. Am. Cer. Soc. 6(5):341–346, 1984.

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158.V. Mandorf and J. Hartwig. High Temperature Properties of Titanium Diboride. High Temperature Materials II (G. Ault, W. Barclay, and H. Munger, eds.). Interscience, New York, 1963, pp. 455–467.

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162.G. de With. High Temperature Fracture of Boron Carbide: Experiments and Simple Theoretical Models. J. Mat. Sci. 19:457–466, 1984.

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170.R. H. J. Hannink and M. J. Murray. Elastic Moduli Measurements of Some Cubic Transition Metal Carbides and Alloyed Carbides. J. Mat. Sci. 9:223–228, 1974.

171.T. K. Gupta. Strength Degradation and Crack Propagation in Thermally Shocked Al2O3. J. Am. Cer. Soc. 55(5):249–253, 1972.

172.T. K. Gupta. Critical Grain Size for Noncatastrophic Failure in Al2O3 Subjected to Thermal Shock. J. Am. Cer.Soc. 56(7):396–397, 1973.

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181.P. F. Becher. Abrasive Surface Deformation of Sapphire. J. Am. Cer. Soc. 59(3–4):143–145, 1976.

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183.W. D. Scott and K. K. Orr. Rhombohedral Twinning in Alumina. J. Am. Cer. Soc. 66(1):27–32, 1983.

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189.F. Schmid and D. C. Harris. Effects of Crystal Orientation and Temperature on the Strength of Sapphire. J. Am. Cer. Soc. 81(4):885–893, 1998.

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7

Grain Dependence of Hardness,

Compressive Strength, Wear,

and Related Behavior at

Elevated Temperatures

I.INTRODUCTION

This chapter complements the preceding one by completing the review of the grain dependence of mechanical properties of monolithic ceramics as a function of temperature. Hardness, compressive strength, wear, and related behavior are addressed, while the preceding chapter addressed crack propagation, toughness, and tensile strength. This chapter also compares the limited data on both tensile and compressive failure in the same material, i.e. the overall trend for the large difference between the two to disappear at higher temperatures, as in ductile metals, but for some differences to remain.

Unfortunately there is again a significant lack of data first simply as a function of only temperature, and especially also as a function of grain parameters (mainly size), though the extent of this limitation varies with the particular property. However, besides such direct data, there are three other sources of some information on the grain dependence as a function of temperature. First is the substantial data on such dependence of properties at 22°C reviewed in Chapters 4 and 5, and the fact that properties generally undergo gradual changes with increasing T, so grain dependences are also expected to change gradually. Second is information on the grain dependence of one property versus temperature and the correlations of this property with other related ones, e.g. of E and H

428

Grain Dependence at Elevated Temperatures

429

with each other and with other mechanical properties as discussed in Chapters 2–6. Third is information on the temperature dependence of these properties for single crystals, since these reflect both the limit of G and of the orientation dependence of textured polycrystalline bodies.

Thus hardness, compressive strength (σC), wear, and related behavior are also affected by temperature-driven changes in underlying properties such as E and by the onset and increase in various deformation, e.g. creep, processes. However, there are other basic effects due to the inherent reduction of stresses for plastic deformation as temperature (T) increases due to two counter effects. On the one hand, increased plasticity will reduce local fracture and hence cracking associated with hardness indentations, compressive stressing, wear, and erosion. On the other hand, increased plasticity also means reduced hardness and more penetration of asperities into mating wear surfaces as T increases. Further, basic changes can occur in the mechanisms of failure, and hence in their G dependence. Thus, for example, increased plasticity in particulates causing erosion, or in the surfaces they impact, can change erosion, e.g. shifting of the angle for maximum erosion from 90° (i.e. normal) to the eroding surface for brittle processes to 30° for ductile erosion processes [1,2]. At higher temperatures, increased bonding (e.g. via welding) and increased chemical reaction can also become important factors in erosion, and especially wear. Similarly, repeated stressing, i.e. fatigue testing as temperatures increase, can be compounded by changes from mainly or exclusively brittle fracture to increasing effects of various nonelastic processes (again with an inverse effect of strain rate). Besides affecting properties, increased plastic deformation and reduction of brittle fracture as temperature increases also affects strength tests. Thus while alignment and interface stresses are important issues in compressive testing of brittle materials, they become less critical as the degree of plastic deformation increases as T increases. Similarly, though receiving almost no study, hardness-related cracking should decrease, i.e. as T increases.

While changes in properties considered in this and the previous chapter are generally gradual with increasing temperature, three factors should be noted. First, there can be important changes at modest temperatures, e.g. such as shown for the tensile strength of Al2O3 (Fig. 6.12), which are indicated as correlating with changes of hardness, and especially compressive strength, of Al2O3 (Fig. 7.6). Second, there can be substantial and rapid changes in properties as a function of temperature in the limited, but important, cases where properties have been measured through a phase transformation, e.g. as mainly seen in the more extensive H–T data. Third, though not commonly noted, as macroplasticity occurs in ceramics at higher temperatures, the differences in tensile versus compressive strengths decrease, especially for single crystals, as does the H/σC ratio.