Properties of silicon-based ceramics produced by hot isostatic pressing ultrafine powders

The densification of ultrafine laser-synthesized SiC (below 20 nm) and of mixtures of ultrafine SiSiC with conventional carbon powders has been performed by encapsulated hot isostatic pressing (HIP). Density and grain size as functions of temperature, HIP pressure, additive content and porosity distribution of the green body are discussed for the case of poor SiC samples. The results of measurements of thermomechanical properties and the influence of the grain size are presented. Hardness shows a significant increase with decreasing grain size, especially in specimens with a high density. Hot hardness measurements up to 1100°C on a specimen with a grain size of about 300nm revealed a relatively high hardness level compared with conventional SiC ceramics in the intermediate temperature range. Indentation fracture toughness slightly decreased with decreasing grain size. Thermal conductivity of the fine-grained SiC-based samples are lower than the values of conventional SiC ceramics, indicating the influence of the fine grain size on the phonon scattering. Measurements at temperatures up to 1500°C reveal that the reduction in the thermal conductivity in fine-grained SiC is significantly reduced at higher temperatures.