Ablation resistant behavior of silicide modified HfB2-SiC coating on graphite by spark plasma sintering: Role of MeSi2 addition

The silicides (MeSi2, Me = Ta, Zr, W, Mo) with strong Si supply capability are promising for improving the loose and porous HfO2 skeletons from ablation resistant coatings for carbon-based materials. Herein, MeSi2 modified HfB2-SiC coatings prepared by spark plasma sintering were evaluated under oxyacetylene ablation condition (2.4 MW/m2) to analyze the protection mechanism. Experimental and theoretical analyses show that, MoSi2 can be considered as the optimal Si additive and HfB2-SiC-MoSi2 coating exhibits the lowest linear variation rate of -0.19 mu m/s. The by-product retention of WOx and ZrO2 derived from WSi2 and ZrSi2 will cause rapid thickening of Hf-Si-O layer, while the by-product (Hf6Ta2O17) from TaSi2 leads to severe consumption of HfO2 skeleton (0.29 mu m/s). The disparate mechanisms of MeSi2 addition are jointly decided by the fluctuation of Hf-O bond length (0.219-0.229 nm) in Me modified HfO2 solid solutions, as well as the differently dynamic consumption of MeOx by-products.