The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot‐pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot‐pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density.

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The Young's and shear moduli of polycrystalline monoclinic hafnium oxide were determined by the sonic resonance method at room temperature as a function of volume fraction porosity. The Spriggs equation empirically described the data and the zero porosity moduli as 283.6 GN/mZ and 109.2 GN/m2 for Young's and shear moduli, respectively. The Debye temperature calculated from the elastic constants was 484°K. Several specimens had anomalously low moduli values, attributed to microcracking. Grain size and internal friction measurements showed that the microcracking occurred in specimens with grain sizes >2 to 3 pm and was characterized by high internal friction.

Elasticity, internal friction, and thermal expansion were measured at temperatures up to 1500°C to characterize the micro‐cracking behavior of HfO2. Crack‐free HfO2 showed consistent, typical changes in these properties with temperature. Micro‐cracked HfO2 showed hysteresis in the elastic moduli and thermal expansion on thermal cycling. Internal friction increased with microcrack healing. Repeated thermal cycling and high‐temperature anneals markedly changed the microcrack system and a time‐ and atmosphere‐dependent aging effect in microcrack formation was noted.