Abstract

The densification behavior and microstructural development of sialon ceramics have been investigated for compositions with a constant n-value of 1.0 and m-values between 0.4 to 1.0. The cc-phase was stabilized by adding Nd2O3 and Yb2O3, respectively. All Nd-based compositions could be completely densified by gas-pressure sintering at 1800degreesC and 10 MPa pressure whereas Yb-based sialons require small amounts of excess in Yb2O3. Microstructurall analysis indicates an increase of the grain aspect ratio with an increasing excess of rare earth additives for all investigated compositions. Additionally it has been found that Nd2O3 always leads to more elongated grains compared to Yb-containing samples of the same composition. The observations are explained by the formation of a Nd-rich interface between sialon grains and the glassy phase which does not exist for systems with smaller rare earth cations. The influence of an excess in rare earth oxides on hardness and fracture toughness has been finally determined. An increasing additive content reduces the hardness, but increases the fracture toughness due to a more elongated microstructure and a weaker interface by matrix and residual glass.