Porous silicon carbide (SiC) ceramics were fabricated by an oxidation‐bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation‐derived SiO2 glass. Because of the crystallization of amorphous SiO2 glass into cristobalite during sintering, the fracture strength of oxidation‐bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 μm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation‐bonded SiC ceramics were observed to exhibit an excellent oxidation resistance.

Porous ZrO2 ceramics were fabricated by compacting a fine ZrO2 powder, followed by pressureless sintering. Two unidirectional pressures of 30 and 75 MPa were used to prepare the green compacts. The strength and the fracture toughness of porous ZrO2 specimens sintered from the compacts prepared by 75 MPa were substantially higher than those by 30 MPa, especially for the specimens with low porosity. However, the corresponding Young's moduli were identical. This caused the strain to failure of these porous bodies to increase significantly with increasing compaction pressure. Microstructural analyses showed that a number of voids and small flaws existed in the green compacts prepared by the lower pressure, due to the agglomeration of fine ZrO2 grains. It was revealed that the ZrO2 agglomeration resulted in a localized nonuniform shrinkage and degraded the mechanical properties of porous ZrO2 ceramics.

The thermal shock behavior of isotropic and anisotropic porous Si3N4 was evaluated using the water‐quenching technique. The critical temperature difference for crack initiation was found to be strongly dependent on the ratio of fracture strength to elastic modulus. Because of a very high strain‐to‐failure, anisotropic porous Si3N4 showed no macroscopic cracks and was able to retain its strength even at a quenching‐temperature difference of ∼1400°C.

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