Sol-infiltrated multilayer SiC/yttrium silicate coatings for imparting enhanced oxidation resistance to C-SiC composites

Abstract

Yttrium silicate coatings are applied on C-SiC composites for advanced aerospace structural applications at high temperatures. The Air Plasma Spray (APS) method is widely used for the deposition of yttrium silicate coatings. However, the oxidation resistance offered by the APS-deposited coating is limited by the inherent porosity and micro-cracks in the coating. The present study demonstrates the effective sealing of the pores and micro-cracks in the APS coating by impregnation with yttrium silicate sol followed by sintering at 1400 °C. The cyclic oxidation behavior is comparatively assessed at 1300 and 1500 °C in air for the C-SiC composite samples applied with multilayer coatings comprising combinations of CVD SiC/APS yttrium silicate/sol infiltrated yttrium silicate coatings. The lone SiC coated and the SiC/APS yttrium silicate coated samples exhibited rapid decrease in weight. On the contrary, the SiC/APS coated samples infiltrated with yttrium silicate sol exhibited significantly improved oxidation resistance. The weight loss during oxidation was prevented after infiltration of the yttrium silicate sol at a higher vacuum level of 1 mbar, which effected in better healing of the pores in the APS coating and formation of a dense yttrium silicate scale on the surface.