Heterophase ceramics in the Hf–Si–Mo–B system obtained by a combination of SHS and hot pressing methods

Abstract

The paper focuses on obtaining heterophase powder ceramics and consolidated ceramics based on borides and silicides of hafnium and molybdenum by combining the methods of self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic SHS powders HfB2–HfSi2–MoSi2 were obtained according to the scheme of magnesium-thermal reduction from oxide raw materials where the combustion wave is characterized by temperatures of 1750–2119 K and high mass combustion rates of 8,4– 9,3 g/s. The structure of synthesized SHS powders consists of relatively large MoSi2 grains up to 10 μm in size and submicron elongated HfB2 grains located mainly inside the MoSi2 grains and rounded Si precipitates. The composition with a lower concentration of boron contains a large number of polyhedral HfSi2 grains with a size of less than 10 μm. The resulting powders are characterized by an average particle size of ~6 μm with a maximum size up to 26 μm. Phase compositions of ceramics consolidated by the HP method and SHS synthesized powders are identical. The microstructure of compact samples consists of faceted HfB2 elongated grains 0,5– 10,0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size and silicon interlayers. Consolidated ceramics has a high structural and chemical homogeneity, low residual porosity of 1,1–1,7 %, high hardness of 11,7–12,6 GPa and thermal conductivity of 62–87 W/(m·K).