Thermal fatigue behavior of four kinds of alumina ceramics, having different fracture toughness/ bending strength ratio, was investigated. After the testpieces pre-cracked by Knoop indentation were heated in a constant temperature, they were quenched in water, and their propagated crack lengths were measured. Since the heat transmission condition varied from natural convection to nucleant boiling in the range of this experimental temperature, the heat transfer coefficient was corrected before illustrating the (KImax/KIC)-V curve. The thermal fatigue behavior of each alumina ceramics was discussed quantitatively based on the fatigue parameters n and A, which were derived from the crack propagation rate equation V=A(KI/KIC)n. For the alumina ceramics in which transgranular fracture was mainly observed, the n values were hardly different but logA values decreased with increasing particle size. This indicated that the crack propagation rate became low. For those in which more larger particles or pores were observed, the logA values and the crack propagation rate were quite different. Those different behaviors on thermal fatigue may be explained by different propagation mechanisms at crack tips. The control of microstructure is important for thermally used ceramics, because the microstructure determines the crack propagation pass.

Rumpf's equation for the tensile strength of a powder bed was modified to indicate the intergranular fracture strength of a bed of pressed granules. The cohesive force between compacted granules was assumed to be proportional to the corresponding contact area. This assumption was investigated by directly measuring the cohesive force between two granules of alumina. The contact area was estimated from a relation for the deformation of a granule under a contact force; two parameters, b and m, are used. The modified equation (Stgα Dg2(m+y)‐b‐1(1–ɛ/ɛ)1‐m–γpm+γ) predicts how the tensile strength (Stg) of a compact is affected by the compaction pressure (P), intergranular porosity (ɛ), and granule size (Dg).

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