Oxidation Resistance of a Si–TiSi(2)–MoSi(2)–TiB(2)–CaSi(2) Coating on a C(f)/C–SiC Substrate in High-Speed High-Enthalpy Air Plasma Flows

The results of a study on the development and testing of a heat-resistant coating in a Si–TiSi(2)–MoSi(2)–TiB(2)–CaSi(2) system to protect C(f)/C–SiC composites from oxidation and erosional entrainment in high-speed flows are presented here. The coating was formed using firing fusion technology on the powder composition. Oxidation resistance tests were carried out under static conditions in air at 1650 °C and under conditions of interaction with high-speed air plasma flows, with Mach numbers M = 5.5–6.0 and enthalpy 40–50 MJ/kg. The effectiveness of the protective action of the coating was confirmed at surface temperatures of T(w) = 1810–1820 °C for at least 920–930 s, at T(w) = 1850–1860 °C for not less than 510–520 s, at T(w) = 1900–1920 °C for not less than 280–290 s, and at T(w) = 1940–1960 °C for not less than 100–110 s. The values of the rate of loss of the coating mass and the rate constant of heterogeneous recombination of atoms and ions of air plasma on its surface were estimated. The performance of the coating was ensured by the structural-phase state of its main layer, and the formation and evolution on its surface during operation of a passivating heterogeneous oxide film. This film, in turn, is composed of borosilicate glass with titanium and calcium liquation inhomogeneities, reinforcing TiO(2) microneedles and in situ Si(2)ON(2) fibers. It was shown that at T(w) ≥ 1850–1860 °C, the generation of volatile silicon monoxide was observed at the “oxide layer–coating” interface, followed by the effects of boiling and breakdown degradation of the oxide film, which significantly reduced the lifespan of the protective action of the coating.