Fabrication of highly transparent AlON ceramics by inhibiting decomposition of AlON

In consolidation of AlON, the extensive decomposition of AlON into α-Al2O3 and AlN during sintering is one of the major reasons that significantly retard its densification process. To accelerate densification of AlON, an ultra-low concentration of La2O3 (0.03 wt.%) and Y2O3 (0.02-0.04 wt.%) was co-doped as sintering additives to inhibit decomposition of AlON during sintering for fast fabrication of highly transparent AlON ceramics. The phase transformation, microstructure evolution and densification process of the samples undoped and doped additive during heating were studied to investigate the densification mechanism. The results reveal that, compared to the samples undoped additive and single-doped with La2O3, the α-Al2O3 content of the samples co-doped with La2O3 and Y2O3 is significantly deduced at ≤1600 °C, which indicates that the decomposition of AlON is effectively inhibited by co-doping La2O3 and Y2O3. The reduction of α-Al2O3 along with the deceleration effect of La3+ on mass transfer, contributes to more small particles being conserved at high temperatures. Base on this, at later stage of sintering, benefiting from the enhanced mobility of grain boundary by Y3+, AlON ceramics with high relative density (99.79 %) and excellent transparency were obtained via pressureless sintering at 1880 °C for 2.5 h by co-doping 0.03 wt.% La2O3 and 0.04 wt.% Y2O3. The maximum transmittance of the fabricated AlON ceramics is up to 81.6 % at 3600 nm, and a transmittance of 76.0 % was measured at 400 nm. The ultra-low doping amount of La2O3 and Y2O3 and the high relative density should be the main reasons that lead to the outstanding transparency of the obtained AlON. Additionally, the fabricated AlON ceramics also exhibit a high Vickers hardness of 18.89 ± 0.18 GPa.