Morphology and ablation mechanism of C f /Si 3 N 4 composites ablated by oxyacetylene torch at 2200°C

Abstract

Si₃N₄ ceramic matrix composites reinforced by carbon fibres (C _f /Si₃N₄) were prepared by low pressure chemical vapour infiltration at 1250°C using SiCl₄ and NH₃ as precursor. The as prepared C _f /Si₃N₄ composites were ablated to determine the mechanism of the ablation resistance and oxidisation resistance by oxyacetylene torch at 2200°C. 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 X-ray diffraction. The results indicated that the matrix of the C _f /Si₃N₄ composites was composed of the amorphous Si₃N₄ and nanometre α-Si₃N₄. A central ablation region and a ring oxidisation region appeared on the surface of the as ablated C _f /Si₃N₄ composites. Sublimation of the Si₃N₄ matrix and oxidation of the carbon fibres are the main ablation behaviours in the central region. Oxidation of the Si₃N₄ matrix and deposition of SiO₂ particles are the main ablation behaviour in the ring region. A large number of SiO₂ 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 Si₃N₄ matrix, Si₃N₄ matrix was cracked under the thermal impact of the oxyacetylene flame. With the passive oxidation of the as cracked surface, the continuous SiO₂ liquid was formed in the ring region. Subsequently, some residual Si₃N₄ particles were covered by transparent SiO₂ layer to form an amber-like microstructure.