Clean grain boundaries in aluminium nitride ceramics densified without additives by a plasma-activated sintering process

High-purity aluminium nitride (AlN) powders of submicron size were consolidated to almost theoretical density in minutes at 1800°C using a plasma-activated sintering (PAS) process. The structure and microchemistry of grain boundaries in this polycrystalline material (average grain size, about 0.43 μm), sintered without additives commonly used in the densification of ceramics, was examined by high-resolution electron microscopy and high-spatial-resolution electron-energy-loss spectroscopy (EELS). The grain boundaries were ‘clean’ down to atomic-scale resolution and revealed AIN grain-to-grain contacts throughout the material; EELS data show no evidence for significant oxygen at grain boundaries or at grain triple junctions. A low density of oxygen-containing inversion domain boundaries were found in the grains, and they are thought to be formed to accommodate small amounts of oxygen (about 0.82 at.%) during the PAS consolidation process. The observed clean grain boundaries suggest that the plasma activation step in the PAS process efficiently removes impurities and/or oxide layers prior to rapid densification.