Effect of free silicon content in inner SiC-Si coating on thermal cyclic ablation behaviour and microstructure of ZrC/SiC-Si coating

To investigate the influence of free silicon content in inner SiC-Si coating to thermal cycle ablation behaviour outer CVD-ZrC coating, SiC-Si inner coatings with different free silicon content were prepared by pack cementation on the surface of carbon/carbon (C/C) composites. These three kinds of coatings are named as PC-5, PC-7.5 and PC-10, of which the inner coatings are prepared with three different raw material powders (mass ration Si:C:Al₂O₃, PC-5 (5:1:0.5), PC-7.5 (7.5:1:0.5), PC-10 (10:1:0.5)). It can be observed that a gradient distribution tendency exists between the ZrC and SiC layers both in specimens (PC-7.5 and PC-10). Among three kinds of samples, PC-7.5 exhibited the best cyclic ablation resistance. The amount of Zr-Si-O glass on the surface of ablated specimens increases with free silicon content in inner coating. Interestingly, when the Si element in inner coating was excessive, Zr-Si-O glass would exhibit a spherical structure accompanied with the ablation resistance decreasing. Meanwhile convex structure ZrO₂ was formed after the ablation process of ZrC/SiC-Si coating. The amount of convex ZrO₂ structures gradually decreased with the increase of silicon content. The size of ZrO₂ grains increased with the increase of the silicon content or the time of cyclic ablation.