Abstract

This work investigates the influence of the additive content on the beta- to alpha-SiC phase transformation in hot-pressed LPS-SiC ceramics. Powder mixtures containing beta-SiC, lwt-% alpha-SiC and 10, 15 and 20wt-% of additives, AlN and Y2O3, at a constant molar ratio of 80:20, were hot-pressed at 1900degreesC under a pressure of 25 MPa and further heat-treated at 2000degreesC during 1, 2, 4 and 8 h in nitrogen atmosphere. The phases present in the sintered and annealed samples were determined by X-ray diffraction. The X-ray diffraction patterns revealed only the presence of SiC-polytypes, predominantly the 6H polytype; no other crystalline phases have been detected. The results of the quantitative analysis of the SiC polytypes showed that with increasing AlN content the phase transformation rate is reduced and that the ratio of 6H to 15R SiC polytype decreases. Furthermore, grain growth is also strongly reduced by increased AlN contents.