Pyrolytic Si_3N_4 has been deposited on a graphite substrate, using a mixture of SiCl_4, NH_3 and H_2. The pyrolysis is performed with deposition temperatures of 1100 to 1500℃, total gas pressures of 5 to 300 Torr, and flow rates of H_2=700, NH_3=60 and SiCl_4 (1iq.)=0.8 cm^3 min^. Massive amorphous and crystalline pyrolytic forms of Si_3N_4 are prepared at a maximum thickness of 4.6 mm. The effects of deposition conditions on some properties of the deposited products and the dependence of formation of amorphous or crystalline deposits on deposition temperature and total pressure were investigated. The surface and cross-sectional structures show growth cones and oriented crystals which are strongly dependent on the deposition conditions. The thin deposits are translucent ; the thick deposits vary in colour from white to black. The silicon content is close to the theoretical composition and independent of the deposition conditions, while the oxygen content increases with decreasing deposition temperature and total pressure. No segregation of silicon and nitrogen at cone boundaries was found
