Preparation and characterization of Si₃N_4(P)/Si ₃N₄ composites by chemical vapor infiltration

Silicon nitride particle reinforced silicon nitride composites (Si ₃N_4(p)/Si₃N₄ composite) were prepared by chemical vapor infiltration (CVI) process from SiCl ₄-NH₃-H₂-Ar system. X-ray fluorescence (XRF) results show that the main elements in the composites were Si, N, and O. X-ray diffraction (XRD) results show that the composite is composed of α-Si ₃N₄, amorphous Si₃N₄ and amorphous SiO₂. Also there are small amounts of β-Si₃N ₄ and free Si. The amorphous Si₃N₄ would transform into crystal phase Si₃N₄ after heat treatment at 1600°C for 2h. The scanning electron microscope (SEM) results show that the composites are compact especially at the interface of conglomerations. The morphology of CVI Si₃N₄ is similar to cauliflower, which can be associated with lirtle-liquid-drop growth mechanism. Micro-cracks exist due to the thermal expansion mismatch between the silicon nitride particles and CVI Si₃N₄. The density of the preform is 1.63g.cm ^-3, and the pore ratio is 47.18%. After CVI process, the density of the preform is 2.30g.cm^-3, and the pore ratio is 12.03%.