Optimization Of Optical Properties Of AlON Transparent Ceramics

AlON has stable cubic phase structure,optical isotropy and high theoretical transmittance.AlON have better mechanical properties than other transparent materials.The excellent comprehensive properties make AlON ceramics have greater application potential in extreme environments.However,AlON transparent ceramics still faces the problem of declining optical performance with the increase of size.The residual pores are the most important factor affecting the optical quality of AlON ceramics.It is well-known that HIP sintering can effectively improve optical quality by removing pores.However,there are still some problems in HIP sintering of AlON ceramics.First of all,the HIP sintering temperature is very high,which is mostly between 1850 and 1900°C.Secondly,there is residual stress in HIPed AlON ceramics.In view of the above problems,the influence of SiO₂,SiO₂-Y₂O₃ and Si₃N₄ sintering additive on the HIP densification behavior,microstructure evolution and properties of AlON ceramics was studied in this paper.The main research work is as follows:（1）Effect of SiO₂ on the densification behavior and properties of AlON ceramicsIn this paper,AlON transparent ceramics with high optical quality were prepared by using lab-synthesized purely AlON powder.The effects of SiO₂ morphology and content,pre-sintering temperature,soaking time and HIP temperature on the microstructure evolution,mechanical and optical property of HIPed AlON ceramics were studied.It was found that the addition limit of SiO₂ in AlON was lower than10wt%.SiO₂ can promote sintering by forming liquid phase under high temperature.In addition,SiO gas formed by decomposing from SiO₂ during the sintering process.Highly transparent AlON ceramics with loss coefficient 0.005 cm^-1 at 2000 nm was produced by pre-sintering under N₂ atmosphere at 1910°C for 4 h followed by HIP in argon at 1810°C for 3 h.（2）The densification mechanism of HIP of AlON ceramicsFor HIPed AlON ceramics doped with SiO₂ and pre-sintered under N₂ atmosphere,the residual stress is reduced significantly without degrading the optical properties after annealing.The general densification mechanism of hot isostatic pressing sintering is that the residual pores in the pre-sintered body are compressed into nano-pores at high temperature and high pressure,thus improving the transmittance.However,under the condition of annealing under normal pressure,the compressed pores will expand at the grain or grain boundary,resulting in the decrease of transmittance.Hence,another two AlON ceramics were prepared.For the first sample,the pre-sintering atmosphere was changed to Ar instead of N₂,while keeping all parameters the same.The second was doped with Y₂O₃ and La₂O₃,while pre-sintering,HIP and annealing conditions were the same as those of the present work.It was found that the transmittance of the two samples all dropped after annealing.The HIP densification mechanism of AlON doped with SiO₂ under high temperature was proposed based on the observation of microstructure and the detection results of escaping gas.SiO₂ was reduced to SiO gas under high temperature and low oxygen partial pressure.The pores including SiO and N₂ were compressed and diffused into AlON matrix during HIP process and not able to expand when annealed under normal pressure,which could explain the maintance of the high transmittance after annealing in present work.（3）Reaearch on new sintering additive for AlON transparent ceramicsAdding a little Y₂O₃ can form liquid phase with residual Al₂O₃ of raw powders to promote grain growth and eliminate grain boundary pores.The 4mm-thick AlON transparent ceramics prepared by doping 0.5wt%SiO₂ and 0.04wt%Y₂O₃ has a transmittance of 83%at 600nm.Si₃N₄ can promote the sintering of AlON ceramics and realize the rapid densification at a lower temperature.Highly transparent AlON ceramics（4mm-thick）doped with 0.1wt%Si₃N₄with transmittance 84.3%at 600nm was obtained.However,when the doping amount of Si₃N₄ additives reaches or exceeds0.5wt%,the central area of HIPed AlON ceramics will turn black.EDS results show that the appearance of black is due to the enrichment of Si element.