Suppression of Viscous Grain-Boundary Sliding in a Dilute SiAION Ceramic

Abstract

Aluminum nitride (AIN) and alumina (Al 2 O 3 ) additives were blended into a high-purity silicon nitride (Si 3 N 4 ) powder, and optimized processing conditions were applied for promoting a sintering process, which was shown to partly remove the amorphous silica-rich (SiO 2 -rich) remains from grain boundaries. Such an approach led to the formation of a dilute SiAION structure. The microstructure of the obtained SiAION material consisted of two distinct types of matrix phases, namely, a main network of equiaxed (dilute) β-SiAlON grains, with a minor fraction of (oxygen-rich) acicular O'-SiAION grains. As a consequence of the resulting solid solution of aluminum plus oxygen in the Si 3 N 4 lattice, many grain boundaries were free from any amorphous interlayer, as revealed by high-resolution electron microscopy (HREM) observation. Other than some boundaries that were still wetted, small amounts of residual glass were noted at some triple-grain pockets. However, the average size of such glass pockets was only a few nanometers. The observed microstructural characteristics were critical for the anelastic response of the present SiAION material, as compared with undoped Si 3 N 4 , whose grains were continuously encompassed by an amorphous SiO 2 film. Internal-friction data that were collected up to very high temperatures showed that, in the present SiAION material, no anelastic relaxation peak was monitored. Thus, viscous sliding along grain boundaries was inhibited. In addition, the partial elimination of the amorphous SiO 2 film also implied that no continuous path for oxygen diffusion was available along grain boundaries. Because of this important circumstance and despite the possible softening effect due to solid solution, the viscoelastic (background) component of the internal-friction data shifted toward higher temperatures and the creep resistance of the material was improved.