Transient liquid-phase sintering of AlN ceramics with CaF2 additive

Transient liquid-phase (TLP) sintering of CaF2 additive on the densification behaviors and microstructural development of AlN ceramics are investigated. It is found that 1 wt% CaF2 can effectively promote densification process. Increasing content of CaF2 results in finer grain size and slower densification during intermediate sintering stage. XRD results show that grain-boundary phase of CaAl4O7 is formed at 1150 °C from reactions of AlN–CaF2–Al2O3. With further temperature increasing, the grain-boundary phases of CA2 and CaAl12O18, which were formed from the reaction between CaF2 and oxide layers, experienced transformations firstly into CaAl4O7 above 1600 °C and into CaAl2O4 at higher temperature. SEM and TEM results show that formed grain-boundary phases can evaporate from sintering bodies during further soaking, leaving clean grain boundaries. The efficiency of TLP sintering mechanism is further manifested by the preparation of transparent AlN ceramics with good combination properties.