Core–shell structure from the solution–reprecipitation process in hot-pressed AlN-doped SiC ceramics

A core–shell structure was observed in a series of 6H–SiC ceramics doped with 5–15 mol.% of AlN and hot-pressed at 2050 °C. Analytical electron microscopy revealed not only a significant AlN solution in the shell areas but also a detectable amount of solutes in the core regions. This core–shell structure was formed via the solution–reprecipitation process, which is promoted by the addition of 0.5 wt.% Y2O3 and the residual oxides. The low and equilibrated AlN solution in the core was due to the diffusion from the liquid before the shell started to grow. At high doping levels, AlN-based 2H-phase started to reprecipitate while the AlN solution in the shell of 6H–SiC grains was depressed. Silica in the residual oxides was depleted by a gasifying reaction, leaving the residual oxides to form Y3Al5O12 phase in small triple pockets and an alumina-based film at the grain boundaries.