Oxidation Behaviour of Yttrium and Rare-Earth Doped α-Sialon Ceramics at 1350°C

A remarkably high oxidation resistance was observed for ytterbium doped α-sialon ceramics at 1350°C, i.e. substantially higher than that of its Y-, Nd- and Sm-analogues. It was noticed that a liquid phase was formed in the oxide scale of Nd-, Sm- and Y-doped samples and that an equilibrium between this liquid phase and the crystalline phases cristobalite and mullite was established. Our findings thus suggest this liquid phase facilitates the inward diffusion of oxygen and outward diffusion of cations such as Nd and Al. The α→β sialon phase transformation which readily occurs in the Nd- and Sm-samples makes these samples more susceptible to oxidation than the Y-doped one. In the Yb-α-sialon system no liquid phase is formed at 1350°C, and accordingly only crystalline phases are formed in the oxide scale. The diffusion processes are also different.