Synthesis of composite AlN-AlON-Al2O3 powders and ceramics prepared by high-pressure sintering

The process of carbothermal reduction of Al2O3 in a nitrogen flow at temperatures of 1450 and 1600oC has been studied. It was established that the powder synthesized at 1450oC is a mixture of Al2O3 and AlN particles and that the powders synthesized at 1600oC are mixtures of AlN, AlON, and Al2O3. Amorphous and crystalline AlON films form on the surface of Al2O3 and AlN particles. Both synthesized powders contain unreacted residual carbon. In the stage of high-pressure sintering of specimens at a temperature of 1750oC under a pressure of 7 GPa for 110 s, AlON decomposes into AlN and Al2O3. Residual graphite transforms into a crystalline graphite phase, which decreases the strength and increases the fracture toughness. The highest hardness of 19.5 GPa was achieved in the ceramics obtained from the powder synthesized at 1600oC for 4 h, and the largest fracture toughness of ~7.2 MPa m1/2 was achieved in the ceramics obtained from the powder synthesized at 1450oC for 4h.