Thermal properties of the nitrogen-rich Ca-alpha-Sialons

摘要

Nitrogen-rich Ca-alpha-Sialon (Ca_xSi_(12-2x)Al_(2x)N_(16) with x = 0.2, 0.4, and 0.8, 1.2 and 1.6) ceramics were prepared from the mixtures of Si_3N_4, AlN and CaH_2 powders in a hot press at 1800 deg C using a pressure of 35 MPa and a holding time of 4h, and then were investigated with respect to reaction mechanism, phase stability and oxidation resistance. In addition the sample with x = 1.6 was prepared in the temperature range 600-1800 deg C using a pressure of 35 MPa and a holding time of 2 h. The alpha-Sialon phase was first observed at 1400 deg C but the alpha-Si_3N_4 and AlN phases were still present at 1700 deg C. Phase pure Ca-alpha-Sialon ceramics could not be obtained until the sintering temperature reached 1800 deg C. The phase pure nitrogen-rich Ca-alpha-Sialon exhibited no phase transformation in the temperature range 1400-1600 deg C. In general, mixed alpha/beta-Sialon showed better oxidation resistance than pure alpha-Sialon in the low temperature range (1250-1325 deg C), while alpha-Sialons with compositions located at alpha/beta-Sialon border-line showed significant weight gains over the entire temperature range tested (1250-1400 deg C). The phases formed upon oxidation were characterized by X-ray, SEM and TEM studies.