Reaction Sintering Mechanism And Densification Behavior Of Aluminum Oxynitride

Aluminum oxynitride（AlON）transparent ceramics are called"one of the most important defense materials in the 21st century"by the US military due to its excellent mechanical and optical properties.Among the existing preparation for AlON ceramics,reaction sintering between Al₂O₃ and AlN is an ideal one for industrialization scale production in the future because its production process is simple and the cost is low.However,research progress on reaction sintering of AlON transparent ceramics is slow.The reaction sintering mechanism between Al₂O₃ and AlN has not been clearly understood since 1979.Moreover,its optical quality has not been significantly improved although efforts have made on the viewpoint of sintering additives.In this study,high-purity Al₂O₃ and AlN powders were used as raw materials to prepare AlON transparent ceramics by reaction sintering.The densification behavior and microstructure evolution of Al₂O₃-AlN green body during sintering were studied.Based on these results,the effects of sintering additives on the sintering process and optical properties of AlON transparent ceramics were explored.Combined with optimizing the ball milling and holding time,high transmittance and low haze transparent AlON ceramics were prepared.Besides,the high temperature oxidation behavior of AlON powder was also studied.The main research work is as follows:（1）Reaction sintering mechanism of AlONReaction sintering process of Al₂O₃-AlN was studied in a wide composition range（10～50 mol%AlN）.It was found that the process could be divided into four stages for the AlON solid solution range of 25～42 mol%AlN（1950℃）.The densification of Al₂O₃ happened in the low temperature sintering stage（<1650℃）.In the temperatue rang from 1650℃ to 1750℃,AlN reacted with partial Al₂O₃ to form a nitrogen-rich AlON solid solution,accompanied by the decrease in density,volume and lattice expansion with the diffusion of AlN in the nitrogen-rich AlON solid solution.Subsequently,the remaining Al₂O₃ diffused into the nitrogen-rich solid solution to reach the nominal composition with temperature increasing,accompanied by lattice contraction.With further increasing temperature,AlON ceramic densification began,along with grain growing up and porosity decreasing.Furthermore,it is also found that the grain sizes and densities of AlON ceramics decreased with the increase of its AlN content.The lower the AlN content,the easier the sintering densification of the sample proceeded,and its thermal corrosion resistance was worse.（2）Effect of sintering aids on the sintering and optical properties of AlON ceramicsDuring reaction sintering,MgAl₂O₄ aid promoted the reaction process between Al₂O₃ and AlN and reduced the formation temperature of AlON by inhibiting the grain growth of Al₂O₃.Y₂O₃ aid played a role in densification of AlON ceramic which improved the mobility of grain boundaries and promoted the pore elimination.Co-doping of MgAl₂O₄ and Y₂O₃ as aids improved on optical quality of the ceramic was significant from 8%（for MgAl₂O₄-doped）or 35%（for Y₂O₃-doped）to 60%.Based on the co-doping of MgAl₂O₄ and Y₂O₃,adding a small amount of BN or H₃BO₃ could inhibit the growth of grains.The optical quality of AlON ceramics was further improved when the amount of BN is 0.02～0.05 wt.%,and H₃BO₃ is 0.06～0.12 wt.%.With 0.1wt.%Y₂O₃-0.4 wt.%MgAl₂O₄-0.12 wt.%H₃BO₃ doped and 30h ball-milling,a transparent AlON ceramic（4mm thick）with 83.8%total transmittance and 3.7%haze were obtained by sintering at 1950℃ for 30 h.This is a breakthrough in the history of reactive sintering of AlON.（3）The oxidation mechanism of AlON powderDuring the oxidation process,the Al-N coordination structure of AlON was oxidized into the Al-ON groups containing lone pair electrons,which leads to the increase of the mass and cell parameters with the increase of temperature.When all Al-N coordination was completely oxidized,the mass of the oxidation product reaches the maximum weight.With the further increase of temperature,the product is unstable and decomposed intoα-Al₂O₃ and N₂.