Morphology and ablation mechanism of Cf/Si3N4 composites ablated by oxyacetylene torch at 2200°C

Abstract

Si3N4 ceramic matrix composites reinforced by carbon fibres (Cf/Si3N4) were prepared by low pressure chemical vapour infiltration at 1250°C using SiCl4 and NH3 as precursor. The as prepared Cf/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 Cf/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 Cf/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.