Characterization and ablation resistance of ZrB₂-xSiC gradient coatings deposited with HPPS

To improve the oxidation protection of carbon fiber-reinforced carbon (C/C) composites, a four-layer ZrB₂-xSiC gradient coating was designed and deposited on a C/C substrate by high-power plasma spraying (HPPS). The phases and microstructures of the spray powders and gradient coating were characterized. The results showed that as the SiC content increased, the molar ratio of ZrB₂ to SiC approached the eutectic point gradually, more eutectic structures can be produced in the spray powders; thus, the porosity of the gradient coating decrease accordingly from the outer to the inner layer. The anti-ablation property of the ZrB₂-xSiC gradient coating and ZrB₂-50 vol% SiC (Z5S5) and ZrB₂-20 vol% SiC (Z8S2) single-component coatings were investigated by the oxyacetylene flame (OAF) test. The mass ablation rate of the ZrB₂-xSiC gradient coating surged from 1.63 × 10^-4 to 2.46 × 10^-3 g/s because the evaporation speed of the ablative product (SiO₂) increased rapidly as the ablation temperature increased from 1900 °C to 2200 °C, respectively. Compared with those of the Z5S5 and Z8S2 coatings, the gradient coating had a better ablation resistance owing to the formation of a dense ZrO₂ top-layer structure and decreased thermal stresses.