Research On Mo-Si Anti-Oxidation Coating Of C/C Composites

A new kind of anti-oxidation coating system was designed in this paper, the basic structure of which is SiC+B₄C/Si+MoSi₂/Si, the composition and structure were optimized also. The microstructure and anti-oxidation property of the anti-oxidation coating system, which was fabricated by fused slurry method were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and differential scanning calorimetry (DSC).The composition and structure of the anti-oxidation coating system were optimized in order to reduce the thermal cracks which were induced during the fabrication and application because of the mismatch of coefficient of thermal expansion between the C/C composites and the coating. The calculation indicated that: (1) the substrate was imposed with compressive stress and the coating with tensile stress during the decrease of temperature, and the thermal stress of the substrate is far lower than that of the coating, and the values of thermal stresses from high to low in the coating are SiC inner layer, MoSi₂/Si outer layer and B₄C/Si middle layer, and the maximal thermal stresses of the substrate and each layer situated in the borders. (2) The maximal thermal stress of the coating increases with the increase of the volume fraction of B₄C and MoSi₂, and decreases with the increase of the thickness of the coating.The anti-oxidation coating was fabricated by means of fused slurry method, and the process is as follows: the samples were dipped into 6wt. %Al-Si slurry, B₄C-Si slurry and Mo-Si slurry to be pre-coated, respectively, and then were sintered at 1450℃under vacuum atmosphere. The observation and analysis of the microstructure indicate that the SiC border reactive layer is bonded tightly to the C/C substrate, and the microstructure of the middle layer is B₄C particles dispersed in continuous silicon phase, and white MoSi₂ particles were dispersed in grey continuous silicon phase in the outer layer. There were net cracks and pores at the surface of the coating, and the surface of the coating is SiO2 film after high temperature oxidation experiments. There are still net cracks in the coating after optimization, but the size of the cracks were decreased obviously.