OBTAINING OF COATINGS BASED ON THE MoSi2-HfB2 SYSTEM ON A C/C-SiC COMPOSITE FROM THE MoSi2-HfSi2-HfB2-SiB4 AND MoSi2-HfSi2-SiB4 POWDER COMPOSITIONS

The possibility of obtaining coatings on a substrate of C/C-SiC composite using the slurry-firing fusion technology of MoSi<sub>2</sub>-HfSi<sub>2</sub>-HfB<sub>2</sub>-SiB<sub>4</sub> and MoSi<sub>2</sub>-HfSi<sub>2</sub>-SiB<sub>4</sub> powder compositions at 1620°C and a rarefied argon pressure of ~ 100 Pa was studied. The coatings have a framework structure formed mainly by fragmentarily sintered MoSi<sub>2</sub> grains and HfB<sub>2</sub> particles uniformly distributed between them. The morphology of the synthesized HfB<sub>2</sub> particles is represented by polyhedral formations with sizes from 5 to 10 μm and nanowhiskers with a thickness of 100-200 nm and a length of 2-4 μm. A sequence of reaction interaction in the MoSi<sub>2</sub>-HfSi<sub>2</sub>-HfB<sub>2</sub>-SiB<sub>4</sub> and MoSi<sub>2</sub>-HfSi<sub>2</sub>-SiB<sub>4</sub> systems, determining the in situ synthesis of HfB<sub>2</sub> and SiC secondary phases, is proposed. Physicochemical calculations in the investigated systems are carried out. It is shown that the total porosity of the resulting coatings, depending on the composition, is ~ 53-54% of their volume. It is established that the greatest contribution to the formation of discontinuities is made by silicon evaporation and pyrolysis of the organic binder during heat treatment. The porosity from their contribution is 29-31% and 22-25% of the volume, respectively. Promising directions of porosity reduction of coatings formed by the considered technology are determined.