Temperature stability of samarium-doped alpha-sialon ceramics

Dense Sm-doped alpha-sialon ceramics along the tie line SmxSi12-4.5xAl4.5xO1.5xN16-1.5x between Si3N4 and Sm2O3 . 9 AlN were prepared by hot-pressing at 1800 degrees C. The materials were subsequently heat-treated at different temperatures in the range 1300-1750 degrees C for different times. The samples were either cooled by turning the furnace off, yielding a cooling rate (T-col) approximate to 50 degrees C min(-1) or quenched (T-col greater than or equal to 400 degrees C min(-1)) by quickly moving the samples to a connected cooling chamber. It was found necessary to apply this quenching technique in order to reveal the true phase relations at high temperatures. It was found from phase analysis of quenched samples that the alpha-sialon phase is formed for 0.3< x <0.61, in equilibrium with a liquid phase. The alpha-sialon was only stable at temperatures above 1650 degrees C, while at lower temperatures it transformed to the more stable beta-sialon modification together with an Al-containing Sm melilite (M') Phase. The z-value of beta-sialon as well as the Al content of the M'-phase were found to correlate with the degree of alpha-sialon decomposition. This decomposition affects also the microstructure of the ceramics, i.e. small and almost equi-axen beta-sialon grains form together with the M'-phase crystals located at the grain boundaries. The mechanical properties deteriorate: both hardness and fracture toughness of the materials decrease from 21 to 20 GPa and 4.5 to 2.5 MPa m(1/2) for unaged and aged samples, respectively.