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 degrees 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 degrees 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 degrees 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 degrees 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/cm(2) 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 degrees C, respectively. The spectral emissivity of the coating epsilon(lambda) 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 lambda = 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.