Effects of CaCO3 content on the densification of aluminum nitride

The effect of adding up to 13.4 wt.% CaCO3 on the densification behavior of aluminium nitride (AlN) was investigated during pressureless sintering between 1100 and 2000 °C. The presence of second-phases, weight losses, Ca contents, and microstructures of sintered samples were correlated with the densification curves. Two microstructural aspects determined the densification of aluminum nitride with CaCO3: second-phase evolution path and formation of large pores. Additions of small amounts of CaCO3 caused the formation of higher melting point calcium aluminates (mainly CA2) that increased the temperature at which liquid-phase sintering process started, but once activated rapid densification was observed. For larger CaCO3 amounts, liquid-phase started to form at lower temperature, but the initial densification was slow, diminishing the advantage of lower C12A7 related eutectic temperature. Irrespective of the initial CaCO3 content, all second-phase evolution paths converged to CA phase above 1600 °C, suggesting that during sintering of AlN with CaO at high temperatures, a liquid phase with composition of CA phase is more stable than others compositions. The effect of this composition changing on densification is discussed. Large pores were formed in the sites originally occupied by large particles of CaCO3 and retarded the bulk densification in samples with high additive contents.