Double-layer oxidation protective SiC/Cr–Al–Si coating for carbon–carbon composites

Double-layer oxidation resistant SiC/Cr–Al–Si coating for carbon–carbon composites was prepared by a two-step method of pack cementation and slurry. The phase composition, micro-structure and oxidation resistance of mono-layer SiC coating and double-layer SiC/Cr–Al–Si coating were studied by XRD, SEM, and isothermal oxidation test at 1773 K in air, respectively. Mono-layer SiC coating consists of SiC and Si, which can protect carbon–carbon composites for several hours depending on its thin thickness and porous structure. After adding outer Cr–Al–Si alloy coating, the as-prepared double-layer SiC/Cr–Al–Si coating with a dense surface and about 150 μm thickness showed improved oxidation resistance, which can protect carbon–carbon composites for 197 h with only 0.079 wt.% mass gain. The failure of the double-layer SiC/Cr–Al–Si coating is thought to occur due to the excessive depletion of the outer Cr–Al–Si alloy coating and the appearance of defects in the SiO2 glass layer.