Influence of rare-earth oxide additives on the oxidation resistance of Si3N4–SiC nanocomposites

The influence of different rare earth oxide additives (La2O3, Nd2O3, Sm2O3, Y2O3, Yb2O3 and Lu2O3) on the oxidation behaviour of carbon derived Si3N4–SiC micro-nanocomposites has been investigated. All investigated composites exhibited predominately parabolic oxidation behaviour indicated diffusion as the rate limiting mechanism. Except the Si3N4–SiC composite sintered with Lu2O3 the rate-limiting oxidation mechanism for all other materials was an outward diffusion of the additive cations along the grain boundary towards the surface. Such diffusion of cation has been strongly suppressed in the Lu-doped composite because of the beneficial effect of stable grain boundary phase and the presence of the SiC particles predominately located at the grain boundaries of Si3N4. Nanoparticles at the grain boundaries act as the obstacles for migration of cations of the additives resulting in superior oxidation resistance of Si3N4–SiC–Lu2O3 where the rate-limiting step is inward diffusion of oxygen through the oxide layer to the bulk ceramics.