Migration of liquid phase and grain growth in low-temperature sintering of AlN

The formation and the migration of the liquid phase towards the surface of AlN ceramics produced via low-temperature sintering of mixture of AlN, YF_3 and CaF_2 powders, as well as the mechanism and the effects of this phenomenon in the final products were experimentally investigated. The liquid phase migrates towards the surface and solidified at the grain boundaries during cooling. Therefore, dense surface and porous bulk microstructure are obtained, which causes warping effect. The rapid shrinkage occurs at the beginning of sintering and the poor wetting of the liquid at the grain boundaries as long as the dihedral groove angle is over 72.5° favor migration. The main phases at the surface are AlN and Y_2O_3, causing bluish coloring of the surface. However, Y_2O_3 probably resulted from carbothermal secondary reaction of yttrium aluminates (Y_3Al_5O_(12), Y_4Al_2O_9) migrates at the surface of AlN with carbon from the graphite-heating element under nitrogen atmosphere.