Two-step pressureless sintering of ultrafine powders for the preparation of high strength and transmittance AlON ceramics

AlON ceramics with high transmittance and strength are essential for transparent armor application. However, high-temperature sintering for long time would result in large grain sizes associated with low strength. In this paper, a low + high temperature two-step sintering method with ultrafine AlON powders was reported for the preparation of high-strength and high-transmittance AlON ceramics. Firstly, the high-purity AlON powders with high specific surface area of 29.77 m2/g was synthesized by solution combustion synthesis-nitridation method and subsequently ball-milling process. Thereafter a sample with 97% density and small grain size (less than 4 μm) can be obtained after sintered at 1750 °C for 3 h, in which the difference of kinetics between grain-boundary diffusion and grain-boundary migration was used to suppress the final stage of grain growth. Further increasing sintering temperature to 1850 °C resulted in the elimination of the residual porosity, and realizing transparent AlON ceramics with high optical transmission (85.4%) and high flexural strength (375.1 MPa) could be achieved.