Study On Fabrication Of AlON Transparent Ceramics With High Optical Quality By Carbothermal Reduction Nitridation Process

Transparent aluminum oxynitride(AlON) ceramics with cubic spinel structure have been considered as a potential structure-function integration material due to the excellent optical and mechanical properties comparable to the sapphire, which can make it be applied to infrared window material, dome, transparent armor, white LED and other optical elements.In this paper, single-phase AlON powders were synthesized by the carbothermal reduction and nitridation process using commercial γ-Al2O3 and nanocarbon black powders as starting materials. Then AlON transparent ceramics with high optical quality were fabricated by pressureless sintering and hot isostatic pressing(HIP) with the sintering additives of Y2O3 and MgO. First of all, the influences of ball-milling time of raw materials and the calcination temperature as well as the holding time of the two-step carbothermal reduction and nitridation process on phase composition, specific surface area and micromorphology of the calcined powders were investigated. The results show that prolonging ball-milling time can effectively break the coarse carbon particles into fine particles and increase the contact area of γ-Al2O3 and carbon, however, it has little influence on the phase composition of the resultant powders synthesized by the carbothermal reduction and nitridation method. In the two-step carbothermal reduction and nitridation process, the dwelling temperature of the first step has almost no influrence on the phase composition of the synthesized powders. And the synthesis temperature as well as the hoding time of the second step are the key points to obtain the single-phase AlON powder. The appropriate synthesis route is determined as calcining the 24 h ball-milled γ-Al2O3/C mixture at 1550 °C for 1 h and a following heat treatment at 1750 °C for 2 h under the flowing of N2.Then, the effects of ball-milling time on particle size distribution and morphology of the AlON powders, as well as the microstructure and optical property of AlON transparent ceramics were investigated. It is shown that the AlON powder ball-milled for 24 h has smaller particles, narrower particle size distribution and more regular shaped particle compared with the 12 h one, which is benefit for the improvement of optical transmittance of AlON transparent ceramics. With the sintering aids of 0.25 wt% MgO and 0.04 wt% Y2O3, highly transparent AlON ceramics with in-line transmittance above 80% from visible to infrared range were obtained through pressureless sintering at 1850 °C for 6 h.In order to further enhance the transmittance of AlON transparent ceramics, hot isostatic pressing(HIP) was employed. The influences of presintered temperature and the addition of sintering aids on microstructure and optical transmittance of AlON transparent ceramics were studied. It can be obtained that the optical properties of AlON samples improve markedly over the visible and infrared range after post-HIP treatment at 1825 oC for 3 h in 200 MPa Ar. For the AlON ceramic samples wih sintering aids, the in-line transmittance above 85% from visible to infrared range is obtained, which is infinitely close to its theoretical value. The utilization of HIP can realize the highly transparent AlON ceramics with fine grain size, which make it have good prospect of application in achieving the commercialized manufacture of large-size AlON transparent ceramics.