MORPHOLOGY OF GRAIN-BOUNDARY PHASES IN ALUMINUM NITRIDE CERAMICS WITH Y2O3 ADDITIVE

AlN ceramics with Y2O3 as a sintering aid often show unique inhomogeneous appearances such as mesh-like or spotted pattern. In order to clarify the origin of the inhomogeneity, the morphology of grain boundary phases is studied on AlN ceramics with 2 to 3.5 wt% Y2O3 sintered at 1870-degrees-C. By XRD analysis the grain boundary phases were observed to compose of Y4Al2O9 single-phase, mixtures of Y2O3 and Y4Al2O9 or mixtures of Y4Al2O9 and YAlO3. Microscopic observations suggested that the migration of the grain boundary phases above 1840-degrees-C to a distance of order of 10 to 100-mu-m is the origin of the inhomogeneous appearances. The migration causes segregation of the grain boundary phases, pore formation at the grain junctions and exudation of the grain boundary phases from the surface and results in the inhomogeneous appearances. Driving force of the migration of the grain boundary phases is considered to be capillary force, because the grain boundary phases are found not to wet the AlN grain. The wettability was presumed to change from wetting in first stage of the sintering to non-wetting in the final stage. The unique inhomogeneity is considered to be due to the non-wetting.