Abstract
The effect of adding up to 13.4 wt.% CaCO 3 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 CaCO 3: second-phase evolution path and formation of large pores. Additions of small amounts of CaCO 3 caused the formation of higher melting point calcium aluminates (mainly CA 2) that increased the temperature at which liquid-phase sintering process started, but once activated rapid densification was observed. For larger CaCO 3 amounts, liquid-phase started to form at lower temperature, but the initial densification was slow, diminishing the advantage of lower C 12A 7 related eutectic temperature. Irrespective of the initial CaCO 3 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 CaCO 3 and retarded the bulk densification in samples with high additive contents.
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