Kinetics and High-Temperature Oxidation Mechanism of Ceramic Materials in the ZrB2-SiC-MoSi2 System

This study is devoted to the fabrication of the ZrB2-SiC-(MoSi2) compact ceramics according to hybrid technology (self-propagating high-temperature synthesis (SHS) + hot pressing), as well as to investigating its phase composition, structure, and high-temperature oxidation kinetics. Reaction mixtures are prepared according to the following scheme: mechanical activation (MA) of Si + C powders; wet admixing of Zr, B, and Si + C MA-mixture powders; and drying mixtures in a drying oven. The ZrB2-SiC SHS composite powder is formed in a reactor in a combustion mode by elemental synthesis. Compact samples with a homogeneous structure and low residual porosity not exceeding 1.3% are formed by hot pressing the SHS powder. Two compositions are selected for testing, notably, the first one calculated for the formation of ZrB2 + 25% SiC; the second composition is similar to the first one, but with the addition of 5% of the MoSi2 commercial powder. The microstructure of the samples is presented by dispersed dark gray rounded SiC grains distributed among light faceted ZrB2 grains. The sample with the MoSi2 additive has a more finely dispersed structure. The high-temperature oxidation of the samples at 1200A degrees C results in the formation of SiO(2)aEuro'ZrB2-(B2O3) complex oxide films on their surface with a thickness on the order of 20-30 mu m, which serve as an efficient diffusion barrier and lower the oxidation rate. Their structure also contains ZrSiO4 complex oxide after prolonged holding (longer than 10 h). In addition, an insignificant weight loss of the samples is observed after 10 h testing, which is caused by the volatilization of gaseous oxidation products (B2O2, CO/CO2, MoO3). The sample with the MoSi2 additive shows better resistance to oxidation.