Heretophase Ceramics in the Hf–Si–Mo–B System Fabricated by the Combination of SHS and Hot Pressing Methods

Abstract

This work is devoted to the fabrication of heterophase powder and consolidated ceramics based on hafnium and molybdenum borides and silicides by combining self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic HfB₂–HfSi₂–MoSi₂ SHS powders are fabricated according to the magnesium-thermal reduction flowsheet from oxide raw materials, in which the combustion wave has temperatures of 1750–2119 K and rather high mss combustion rates of 8.4–9.3 g/s. The structure of synthesized SHS powders consists of relative coarse MoSi₂ grains up to 10 μm in size, submicron elongated HfB₂ grains mainly located inside the MoSi₂ grains, and rounded Si precipitates. The composition with a lower boron concentration contains numerous polyhedral HfSi₂ grains smaller than 10 μm in size. The resulting powders have an average particle size of ~6 μm with a maximal size up to 26 μm. The phase compositions of the ceramics consolidated by the HP method and synthesized SHS powders are identical. The microstructure of compact samples consists of faceted elongated HfB₂ grains 0.5–10.0 μm in size, polyhedral HfSi₂ and MoSi₂ grains up to 8–10 μm in size, and silicon interlayers. The consolidated ceramics have 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).