Preparation and characterization of C/Si₃N₄ composites by chemical vapor infiltration

C/Si₃N₄ composites were prepared by chemical vapor infiltration (CVI) from SiCl₄-NH₃-H₂ system. The element and phases of the as-processed sample were characterized by EDS and XRD. The phase of the as-processed sample is an amorphous silicon nitride. The phase of the sample heat treated at 1350°C is also an amorphous silicon nitride. The XRD indicates that the formation of α-Si₃N₄ and β-Si₃N₄ occurs after heat treatment at 1450°C. The influences of temperature, time and gas flux on densification, weight gain and microstructure were investigated. The results show that the gases infiltrate and react in preforms equably, the carbon preforms can be infiltrated uniformly and the uniform C/Si₃N₄ composites were prepared when the process parameters are as follows: temperature 900°C, SiCl₄ flow 30 mL/min, H₂ flow 100 mL/min, NH₃ flow 80 mL/min, time 120 h and total pressure l000 Pa.