Improvement of the Resistance of the ZrSi₂–MoSi₂–ZrB₂–ZrC Coating to Oxidation and Ablation in a High-Speed High-Enthalpy Air Plasma Flow

The previously considered composition of the powder mixture in the ZrSi₂–MoSi₂–ZrB₂–Si system is corrected toward decreasing the content of the relatively low-melting phases ZrSi₂ and MoSi₂ and increasing the fraction of the refractory phase ZrB₂. A heat-resistant coating is formed on the C/C–SiC composite by the firing facing of the powder mixture at 1750°C under an argon expansion pressure of 150–200 Pa. The phase composition of the coating includes (mol %): 23.2 ZrSi₂, 16.8 MoSi₂, 46.0 ZrB₂, and 14.0 ZrC. The synthesis of the secondary phase ZrC is carried out in situ by the reaction in the ZrSi₂–C system. Oxidation and ablation resistance tests are carried out under flow at surface and surface heating conditions in a T_w temperature range of 1300–2350°C with an air plasma flow at a speed of 4.7–4.8 km/s and a stagnation enthalpy of 48–50 MJ/kg. The performed correction of the composition is shown to provide an enhancement of the protective ability of the coating at T_w = 2200°C by 2.5 times (up to 170 s), as well as an increase in the maximum permissible level of working temperatures from T_w = 2200 to 2350°C. At the same time, the average values of the specific mass loss and mass removal rate of the coating decrease by 23 and 14% and amount to 3.9 mg/cm² and 13.1 mg cm^–2 h^–1, respectively. The rate constants of heterogeneous recombination of air plasma atoms and ions on the coating surface are estimated: K_w = 2 ± 1, 5 ± 2, 9 ± 3, 14 ± 3, and 19 ± 2 m/s at T_w = 1300–1450, 1500–1750, 1800–1950, 2000–2150, and 2200–2350°C, respectively. The spectral emissivity of the coating ε_λ is found to decrease from 0.69 ± 0.02 in the initial state to 0.41 ± 0.02 after the fire tests in the wavelength range λ = 600–900 nm at room temperature. The main factors limiting the protection effect resource of the coating are shown to be the through oxidation of the ZrSi₂ matrix and evaporation of the zirconium-modified borosilicate glass leading to an increase in the fraction of the ZrO₂ phase with high anionic conductivity and catalytic activity in the oxide film.