Sintering behavior of Al2O3–AlN solid solution in pressureless direct solid-state reaction sintering for transparent AlON ceramics

The intricate preparation process and prohibitively high manufacturing costs have long constrained the research and application of transparent ceramics. This study presents a straightforward, low-cost pressureless direct solid-state reaction sintering method, through which highly transparent AlON ceramics with optical properties comparable to those produced by conventional two-step methods were successfully fabricated. In this study, we found that AlN played a critical role in the sintering behavior of the Al 2 O 3 -AlN solid solution. The evolution of density and phase composition in the Al 2 O 3 -AlN solid solution with temperature revealed that the density reaches its maximum at ∼1250 °C, followed by a gradual decrease and eventual stabilization, which is due to volume expansion caused by the progressive formation of the AlON phase at ∼1650 °C. Before the complete phase transformation, the density of the solid solution decreased with increasing AlN content. Pure AlON phases were achieved when the Al 2 O 3 :AlN molar ratios were 9:3, 9:4, and 9:5, with similar densities. Microstructural examination and particle size distribution analysis revealed that variations in AlN content also affected the average grain size of the solid solution, demonstrating a non-monotonic trend characterized by an initial decrease (from ∼197.6 to ∼8.5 μm), a subsequent increase (to ∼135.8 μm), and a final decrease (to ∼7.3 μm). In contrast, hardness measurements demonstrated that the Vickers hardness decreased linearly with the increase of AlN content, from ∼19.5 to ∼13.2 GPa. Transmittance assessment indicated that the AlN content exerted a significant influence on the sintering temperature requisite for fabricating AlON transparent ceramics, with an increase in AlN content necessitating a correspondingly higher processing temperature. Finally, AlON ceramics with high optical transmittance of ∼82.3 % at 850 nm were achieved at temperatures of 1880 °C and 1900 °C for compositions with Al 2 O 3 to AlN ratios of 9:4 and 9:5, respectively.