High-temperature oxidation and plasma torch testing of MoSi2-HfB2-MoB ceramics with single-level and two-level structure

Influence of the composition and structure of the heterophase ceramics in system MoSi2-HfB2-MoB on the thermal conductivity, kinetics and mechanisms of oxidation at 1200 degrees C and 1650 degrees C, including the plasma torch testing at 2000 degrees C, was investigated. Dense ceramics with heterogeneous single-level structure (SLS) and two-level structure (TLS) were obtained by the hot pressing of powders produced by self-propagating high-temperature synthesis (SHS) in combustion mode. TLS ceramics have a lower thermal conductivity as compared to the SLS ceramics with similar elemental composition. Addition of hafnium diboride leads to the increase of mass change during the oxidation due to the formation of HfO2 and HfSiO4, which are denser than SiO2. In the case of the SLS ceramics, at 1200 degrees C a two-layered oxide film is formed. The upper layer is comprised of amorphous Hf-doped oxide layer, and the lower layer is crystalline crystoballite alpha-SiO2. TEM investigation of samples oxidized at 1650 degrees C revealed the formation of HfSiO4 precipitates in the alpha-SiO2 matrix. In the case of the TLS ceramics, regardless of oxidation temperature two-layered oxide film consists of upper SiO2 and lower HfSiO4 TLS ceramics demonstrated the highest oxidation resistance under the plasma torch and kept its structural integrity during 180 s at 2000 degrees C.