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.