The role of CaO additive on sintering of aluminum nitride ceramics

Abstract AlN with 0.5–8 wt% CaO was pressureless sintered between 1100 °C and 1800 °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 AlN with CaO additive. It was observed that the additive content is the main parameter controlling the densification behavior during liquid phase sintering (LPS) of AlN 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 °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 AlN at a temperature as low as 1650 °C.