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 composition range 0.3 less than or equal to x less than or equal to 0.7. Sintered 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 temperature interval and during heat treatment times up to 30 days. Unlike corresponding neodymium- and samarium-doped alpha-sialons, dysprosium-doped alpha-sialon does not decompose into beta-sialon and rare-earth-rich grain-boundary phase(s) at temperatures below 1550 degrees C. The 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-xAlxO3+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 glassy phase-free material. The Dy-M'-phase formed had the maximum aluminium substitution, i.e. x approximate to 0.7. Dysprosium-doped alpha-sialon exhibited very high hardness (H-v10=22 GPa) and a fracture toughness of 4.5 MPam(1/2), and the hardness and toughness decreased only slightly 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 significantly.