The role of CaO additive on sintering of aluminum nitride ceramics

AIN with 0.5-8 wt% CaO was pressureless sintered between 1100 degrees C and 1800 degrees C. The results of density, second phases, microstructure, and weight loss were used to identify the possible mechanisms that may cause a significant delay on the densification of AIN with CaO additive. It was observed that the additive content is the main parameter controlling the densification behavior during liquid phase sintering (LPS) of AIN with CaO. The positive effects of increasing the CaO content were associated with the decrease in the temperature of liquid phase formation and the increase of the liquid-phase fraction during sintering. The negative effects were associated with the high viscosity of the liquid-phase below 1600 degrees C, the formation of large pores, and the gas entrapment in closed pores. The balance between positive and negative effects results in an intermediate additive content (around 2 wt% CaO), which is best to promote densification of AIN at a temperature as low as 1650 degrees C.