Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams

Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si3N4 formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al2O3 and Y2O3 were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si2N2O and whisker α-Si3N4 phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si3N4 as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.