Oxidation protection and mechanism of the HfB2-SiC-Si/SiC coatings modified by in-situ strengthening of SiC whiskers for C/C composites

To improve the oxidation resistance and alleviate the thermal stress of the HfB₂-SiC-Si/SiC coatings for C/C composites, in-situ formed SiC whiskers (SiC_w) were introduced into the HfB₂-SiC-Si/SiC coatings via chemical vapor deposition (CVD). Effects of SiC_w on isothermal oxidation and thermal shock resistance for the HfB₂-SiC-Si/SiC coatings were investigated. Results showed that the SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent oxidation resistance for C/C composites with only 0.88% weight loss after oxidation for 468?h at 1500?°C, which was markedly superior to 4.86% weight loss for coatings without SiC_w. Meanwhile, after 50 times thermal cycling, the weight loss of the SiC_w-HfB₂-SiC-Si/SiC coated samples was 4.48%, which showed an obvious decrease compared with that of the HfB₂-SiC-Si/SiC coated samples. The SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent adhesion to the C/C substrate and had no penetrating cracks after oxidation. The improved performance of the SiC_w-HfB₂-SiC-Si/SiC coatings could be ascribed to the SiC_w, which effectively relieved CTE mismatch and remarkably suppressed the cracks through toughening mechanisms including whiskers pull-out and bridging strengthening. The above results were confirmed by thermal analysis based on the finite element method, which demonstrated that SiC_w could effectively alleviate thermal stress generated by temperature variation. Furthermore, the SiC_w-HfB₂-SiC-Si/SiC coating can provide a promising fail-safe mechanism during the high temperature oxidation by the formation of HfSiO₄ and SiO₂, which can deflect cracks and heal imperfections.