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 (SiCw) were introduced into the HfB2-SiC-Si/SiC coatings via chemical vapor deposition (CVD). Effects of SiCw on isothermal oxidation and thermal shock resistance for the HfB2-SiC-Si/SiC coatings were investigated. Results showed that the SiCw-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 SiCw. Meanwhile, after 50 times thermal cycling, the weight loss of the SiCw-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 SiCw-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 SiCw-HfB2-SiC-Si/SiC coatings could be ascribed to the SiCw, 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 SiCw could effectively alleviate thermal stress generated by temperature variation. Furthermore, the SiCw-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.