Microstructural stability of duplex alpha-beta-sialon ceramics

Duplex Y-, Yb- and Nd-doped alpha-beta-sialon ceramics were fabricated from starting powder compositions within the two-phase alpha'-beta' area. The microstructures contained high aspect ratio beta-sialon grains homogeneously distributed in an alpha-sialon matrix and only smaller volume fractions of intergranular phases. The morphology of the intergranular regions was determined by the overall chemistry and the doping element. The beta'/alpha' + beta' weight ratio as well as the alpha and beta-sialon substitution levels were dependent upon the alpha' stabilizing cation. Sialons with an beta'/alpha' + beta' weight ratio of around 0.5 had a favourable combination of high hardness, HV10 around 21 GPa, and indentation fracture toughness, 5.0-5.5 MPa m(1/2). The toughness of these duplex sialons was however reduced to below 4 MPa m(1/2) after long-term heat treatment at 1450 degrees C. The alpha' stabilizing which were stable during heat treatment at 1450 degrees C, while the Nd alpha' structures decomposed into Al-substituted melilite, Nd2Si2AlO4N3, beta-sialon and the 21R sialon polytype at this temperature. The results imply that duplex alpha-beta-sialons stabilized by Y and Yb have a high potential as engineering ceramics. (C) 1997 Elsevier Science Limited.