TEMPERATURE STABILITY OF SAMARIUM-DOPED ALPHA-SIALON CERAMICS

Dense Sm-doped alpha-sialon ceramics along the tie line SmxSi12-4.5xAl 4.5xO1.5xN16-1.5x between Si3N4 and Sm2O3 . 9 AlN were prepared by hot -pressing at 1800 degrees C. The materials were subsequently heat-trea ted at different temperatures in the range 1300-1750 degrees C for dif ferent times. The samples were either cooled by turning the furnace of f, 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 fou nd necessary to apply this quenching technique in order to reveal the true phase relations at high temperatures. It was found from phase ana lysis 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 o nly stable at temperatures above 1650 degrees C, while at lower temper atures it transformed to the more stable beta-sialon modification toge ther 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 correl ate 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 deteriorat e: 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, r espectively.