PREPARATION AND PROPERTIES OF STABLE DYSPROSIUM-DOPED ALPHA-SIALON CERAMICS

Dense ceramics with overall compositions DyxSi12-4.5xAl4.5xO1.5xN16-1. 5x, where 0.2 less than or equal to x less than or equal to 1.0, along the Si3N4-Dy2O3 . 9AlN tie line were prepared by hot-pressing at 1800 degrees C. The dysprosium-doped alpha-sialon phase formed in the comp osition range 0.3 less than or equal to x less than or equal to 0.7. S intered materials of different compositions were post-heat-treated at temperatures in the range 1300-1750 degrees C for different times and it was shown that the Dy-alpha-sialon phase is stable over a large tem perature interval and during heat treatment times up to 30 days. Unlik e corresponding neodymium- and samarium-doped alpha-sialons, dysprosiu m-doped alpha-sialon does not decompose into beta-sialon and rare-eart h-rich grain-boundary phase(s) at temperatures below 1550 degrees C. T he alpha-phase can coexist with a liquid phase at temperatures greater than or equal to 1550 degrees C and with the Dy-M'-phase (Dy2Si3-xAlx O3+xN4-x) at lower temperatures. When heat treated at 1450 degrees C, any residual liquid grain-boundary phase reacted with minor amounts of the alpha-sialon phase and devitrified to Dy-M'-phase, yielding a gla ssy phase-free material. The Dy-M'-phase formed had the maximum alumin ium substitution, i.e. x approximate to 0.7. Dysprosium-doped alpha-si alon exhibited very high hardness (H-v10=22 GPa) and a fracture toughn ess of 4.5 MPam(1/2), and the hardness and toughness decreased only sl ightly after devitrification of the glassy phase. Some elongated alpha -sialon grains were formed at high x values in glassy phase-containing materials, but their presence did not affect the toughness significan tly.