A novel two-step sintering strategy for highly transparent AlON ceramics by combination of ultra-fast high-temperature sintering followed by conventional sintering

By employing ultra-fast high-temperature sintering (UHS) followed by conventional sintering (CS), AlON ceramics with high transmittance (≥80%) over a wide wavelength range of 350-4900 nm were successfully fabricated via holding for 2 min at 1850 ℃ of UHS and subsequent 120 min at 1880 ℃ of CS. The relative density, pore size and grain size were compared among the samples prepared by UHS, UHS+CS and CS to investigate the mechanism behind achieving excellent transparency through UHS+CS. After UHS, AlON with a high relative density of 97.43% was obtained, much higher than the 88.51% of the sample prepared at 1850 ℃ via CS. Further CS of the UHSed AlON increased its relative density to 99.71%, with fewer small pores (≤0.37 µm) observed on the fracture surface. These properties are noticeably better than those of the sample directly sintered by CS (relative density of 99.30%, pore size of 0.45-2.40 µm). The high relative density of the UHSed sample, along with the predominance of small grains and non-equilibrium grain boundaries induced by UHS, contributes to the rapid densification of AlON during subsequent CS. Thereby resulting in the excellent transparency and high hardness (HV=19.57±0.23 GPa) of the final product. Consequently, UHS+CS is an effective and efficient strategy for fabricating highly transparent AlON ceramics, with UHS being a crucial step.