Introduction Transparent aluminum oxynitride (AlON) ceramics have superior optical transparency properties, high strength and hardness, having great potential applications as window, dome, and other optical components requiring high strength combined with optical transparency. Recent efforts are made to fabricate transparent AlON ceramics, i.e., synthesis of high purity AlON powder, optimization of the particle size distribution of AlON powder, and doping with various additives in different amounts. In these works, sintering additives, such as Y2O3, Y2O3/La2O3, Y2O3/La2O3/MgO, etc., are commonly used to accelerate densification process of AlON. However, a long dwelling duration of ≥6 h is generally required to eliminate pores to obtain a high transmittance in pressureless sintering. Although the dwelling duration of pressureless sintering AlON ceramics can be shortened to 2.5 h, it is crucial to carefully match the doping amount of Y2O3 with the particle size of AlON powder. The excessive Y2O3 doping can result in a significant decomposition of AlON into α-Al2O3 and AlN, and the massive formation of α-Al2O3 can lead to particle aggregation/coarsening, and even separation between components during the early stage of sintering, which in turn retards the subsequent densification process. Studies on using CaCO3 as an additive to fabricate AlON ceramics indicate that it should be a promising way to fabricate highly transparent AlON ceramics via suppressing AlON decomposition during heating with sintering additive. In this paper, La2O3 was respectively doped to AlON powders with median particle size (D50) of 1.1 μm and 2.0 μm to prepared transparent AlON ceramics via pressureless sintering (PS) at 1 880 ℃ for 2.5 h. The effect of La2O3 doping amount on the transmittance of AlON ceramics was investigated, and the phase composition, microstructure and densification process of samples during heating were monitored to investigate the fast densification mechanism of AlON. Methods Pure AlON powder and La2O3 powder were used as starting materials. The AlON powders doped with 0-0.50% La2O3 were ground in absolute ethyl alcohol in a grinding mill with Si3N4 balls, and AlON powder with D50 of 1.1 μm and 2.0 μm were obtained, referred to as P1.1 and P2.0, respectively. The ground powders were loaded into a pellet of 13 mm in diameter at 50 MPa and then cold isostatically pressed at 120 MPa to prepare green bodies. The green bodies were pressureless sintered in N2 in a graphite furnace at a heating rate of 20 ℃/min. One group of samples were sintered at 1 880 ℃ for 2.5 h to prepare transparent AlON ceramics, and another group samples were respectively heated to 1 400-1 900 ℃ (no dwelling) to investigate the phase transformation, microstructure evolution and densification process during heating. The transparent AlON ceramics were ground and mirror polished on the both sides to a thickness of 2 mm for the optical transmittance measurement. The polished samples were hot etched at 1 640 ℃ for 40 min to determine their microstructure.Results and discussion After a 2.5 h dwell at 1 880 ℃, all the La2O3 doped AlON ceramics prepared (i.e., P1.1 and P2.0) show higher transmittance than their undoped counterparts, and the transmittances at 3 850 nm enhance from 17% and 0% to 80% and 84%, respectively. Moreover, the doping of La2O3 broadens the transmission wavelength range of the fabricated samples at 2 500-6 000 nm for P1.1 and the optical range at 220-6 000 nm for P2.0. Doping amount of La2O3 has a similar effect on the transmittance of AlON ceramics prepared by P1.1 and P2.0. When the doping amount of La2O3 ≤0.15% in P1.1 and ≤0.40% in P2.0, increasing the doping amount of La2O3 contributes to the enhanced transmittance of the AlON ceramics. Doping 0.15%-0.25% and 0.40% La2O3 into P1.1 and P2.0, respectively, results in the transmittances of AlON ceramics up to 80%-84% at 3 850 nm. The high transmittance of fabricated AlON ceramics is mainly due to thehigh relative density. For the two highly transparent AlON ceramics prepared by P1.1 doped with 0.20% La2O3 and P2.0 doped with 0.40% La2O3, their average grain sizes are 97.3 μm and 60.2 μm, and their Vickers hardness values are (16.00±0.31) GPa and (16.36±0.31) GPa.For the samples prepared by P1.1 doped with 0.20% La2O3 and by P2.0 doped with 0.40% La2O3, although AlON undergoes decomposition and reformation during heating. α-Al2O3 content in the samples does not exceed 25.1%. Moreover, a lower amount of α-Al2O3 appears in the sample P2.0, compared to the sample P1.1 at 1 400-1 700 ℃. Furthermore, compared to the samples without additive doping, the two samples above exhibit lower α-Al2O3 contents at ≤1 600 ℃. It is indicated that the decomposition of AlON during heating is slightly suppressed by doping La2O3 as an additive. No obvious particle aggregation/coarsening or no composition decomposition occur on the fracture surface of the samples due to the reduced α-Al2O3 content in the samples. It is beneficial for the subsequently densification process. Conclusions AlON ceramics with a high infrared transmittance of 80%-84% at 3 850 nm were fabricated via pressureless sintering at 1 880 ℃ for 2.5 h after doping 0.15%-0.25% and 0.40% La2O3 to AlON powders with D50 of 1.1 μm and 2.0 μm, respectively. During heating, La2O3 could suppress the decomposition of AlON and lead to a lower amount of α-Al2O3 being decomposed from AlON. This could reduce the particle aggregation/coarsening and even the separation of composition at the early stage of sintering. The obtained exceptional microstructure was beneficial for the subsequent fast densification. Moreover, La2O3 could provide sufficient sintering kinetics to effectively eliminate pores at the later stage of sintering, thus promoting the densification of AlON.