Si 3 N 4 ceramic matrix composites reinforced by carbon fibres (C f /Si 3 N 4 ) were prepared by low pressure chemical vapour infiltration at 1250uC using SiCl 4 and NH 3 as precursor. The as prepared C f /Si 3 N 4 composites were ablated to determine the mechanism of the ablation resistance and oxidisation resistance by oxyacetylene torch at 2200uC. The morphology and microstructure of the composites were examined by scanning electron microscopy. The phase compositions of the composites were confirmed by energy dispersive X-ray spectroscopy and Xray diffraction. The results indicated that the matrix of the C f /Si 3 N 4 composites was composed of the amorphous Si 3 N 4 and nanometre a-Si 3 N 4 . A central ablation region and a ring oxidisation region appeared on the surface of the as ablated C f /Si 3 N 4 composites. Sublimation of the Si 3 N 4 matrix and oxidation of the carbon fibres are the main ablation behaviours in the central region. Oxidation of the Si 3 N 4 matrix and deposition of SiO 2 particles are the main ablation behaviour in the ring region. A large number of SiO 2 liquid droplets produced during ablation were retained and formed spherical solid particles on the surface of the ring region after ablation. For the mismatch of the coefficient of thermal expansion of the carbon fibres and the Si 3 N 4 matrix, Si 3 N 4 matrix was cracked under the thermal impact of the oxyacetylene flame. With the passive oxidation of the as cracked surface, the continuous SiO 2 liquid was formed in the ring region. Subsequently, some residual Si 3 N 4 particles were covered by transparent SiO 2 layer to form an amber-like microstructure.

Aircrafts hold the outstanding mastery of the sky in modern wars, however the laser beam weapons can carry out laser attacking to aircrafts. The purpose of the present paper is to research on a new type laser protective material. Polycarbosilanes (PCS)/divinylbenzene mixtures containing ZrO 2 powders were brushed to the surface of the aluminum alloy plates and then cured at 150°C for 6 h. The PCS-coated plates were ablated by laser for 3 s. The phase identification of as-ablated powders was examined by X-ray diffraction. The results indicated that the as-ablated powders of cured PCS were composed of major phase β- SiC and smaller amounts of free carbon. The PCS composite coating played a certain role of laser ablation resistance. The effect of added ZrO 2 powders on the pyrolysis of PCS-coating under laser ablation is conspicuous.

The BN/Si 3 N 4 composites were prepared by LPCVD using SiCl 4 -NH 3 -Hr-Ar as precursor at 950°C. The microstructure of the BN/Si 3 N 4 composites was examined by scanning electron microscopy. The phase identification of the nanometer Si 3 N 4 was confirmed by X-ray diffraction. The morphology and growth mechanism of the Si 3 N 4 matrix were investigated using chemical reaction kinetics and supersaturation-condensation-fusion mechanism. Because of the mismatching of the coefficient of the thermal expansion and the poor wettability between the coating and the substrate, the coatings could be easily flaked away. The internal surface of the Si 3 N 4 shell was very smooth, which proved the poor wettability. The external surface of the Si 3 N 4 shell was composed of the amorphous spheroid particles, which proved the S-K growth model.

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