To prevent carbon/carbon (C/C) composites from oxidation, a dense and multiphase inlaid SiCMoSi 2 -CrSi 2 coating was prepared by pack cementation. The coatings were characterised by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The effect of pack composition on the oxidation properties was studied on low density C/C composite, and then optimum composition was finalised. The optimum composition was the one containing 5 wt%Cr and oxidation behaviour of high and low density C/C composites coatings with this composition was studied. Oxidation test shows that, when the Cr content was 5 wt-%, the SiCMoSi 2 -CrSi 2 coating possessed excellent oxidation resistant ability. After application of the SiCMoSi 2 -CrSi 2 multiphase coating, the low density C/C composite showed a mass loss of only 1?36% after oxidation at 1873 K for 43 h, while the mass loss of the coated high density sample was up to 7?45% after oxidation at 1873 K for 8 h.

A comparative study has been carried out on microstructure, ultrahigh temperature tensile properties, and erosion behavior of non-equilibrated four-directional reinforced carbon/carbon composites, prepared using polyacrylonitrile based carbon fibers as the reinforcement and coal-tar pitch as the matrix, to assess the effect of microstructural changes on the properties of the composites in an ultrahigh temperature environment. X-ray diffraction and scanning electron microscopic analyses suggest that the carbon matrix is highly graphitized, which is composed of mosaic and domain structures. The composites retain superior tensile properties with a visco-plastic behavior at ultrahigh temperature, which is consistent with the observation of mixed microstructural changes, crack propagation, and deflection as well as fiber debonding. The ablation behavior of non-equilibrated four-directional reinforced carbon/carbon composites is mainly dominated by thermo-chemical ablation and mechanical erosion, and different microstructures exhibit distinguished erosion mechanisms.