The Effect Of Microstructure Of AlON Transparent Ceramics On The Optical And Mechanical Property

AlON transparent ceramics can be used as infrared window,domes and transparent armor for its excellent optical and mechanical properties.Surmet Company of the United States has successfully fabricated 610mm×914mm AlON slabs and various conformal windows,and carried out research and application of AlON transparent ceramics in aerospace and bulletproof transparent armor.The research of domestic AlON is in its infancy,and there is a big gap in the size,optical performance?transmittance and optical uniformity?and mechanical properties between foreign products and ours.The main reason for these gaps is that we don't have deep study on the microstructure of the material.First,in the powder aspect,we haven't thoroughly studied the properties of AlON powder.Secondly,we don't know enough about the microstructure of the material,such as twins and grains in the aluminum oxynitride ceramics.In response to the above questions,we deeply studied the decomposition and soild solution process of aluminum oxynitride powder in the low temperature stage and the effect of decomposition on the optical properties of the final sintered ceramics.We preliminary explore the effect of the new sintering aid system Y₂O₃-La₂O₃-CaO and Y₂O₃-La₂O₃-MnO on the sintering properties of AlON.Evolution disciplinary mechanics strengthening mechanism of twins in the transparent ceramics of aluminum oxynitride are studied.Polytype reinforced AlON ceramic is successfully prepared,and the flexural strength is improved by 20%.The main research work is as follows:?1?The decomposition of AlON powders at low temperatureWhen the temperature is lower than 1640?C,AlON is decomposed into Al₂O₃ and AlN,and when the temperature is higher than 1640°C,the decomposed Al₂O₃ and AlN are re-solidified into an AlON phase.In this process of phase decomposition and synthesis,the heating rate system will have a significant effect on the degree of phase decomposition and synthesis.The slow temperature rate will increase the degree of decomposition of AlON,and the Al₂O₃ and AlN grains produced by decomposition will develop well.At 1640°C,the difficulty of re-solidification of AlON is increased;on the contrary,rapid temperature rate will make AlON decomposition low,and the Al₂O₃ and AlN particles produced by decomposition are fine and uniformly mixed.When the temperature is higher than 1640°C,it can be easily re-solidified into an AlON phase.And the conversion efficiency of Al₂O₃ and AlN is also inconsistent during the re-solidification of Al₂O₃ and AlN to AlON.The phase decomposition and resynthesis process of AlON during the heating process results in non-uniform nitrogen and oxygen content in the final sintered ceramic grains.In the microstructure,high-density nano-domains are formed,and the differernt refractive indices of micro-domain structures are the causes for the phenomenon of extinction under the orthogonal polarizing microscope.It appears as a phenomenon of water ripple on the macroscopic.Controlling the appropriate heating rate is of great importance for our preparation of high optical quality AlON.?2?Exploration of new sintering aidsAiming at solving the problems of uneven grain size and high sintering temperature of AlON ceramics sintered with Y₂O₃-La₂O₃ sintering aid system,the new sintering aid system Y₂O₃-La₂O₃-CaO and Y₂O₃-La₂O₃-MnO were respectively investigated for the sintering of aluminum oxynitride transparent ceramics.Firstly,the Y₂O₃-La₂O₃-CaO sintering aid system can effectively reduce the grain size to about80?m,the grain size is even and small,and the water ripple phenomenon is obviously weakened.The Y₂O₃-La₂O₃-MnO sintering aid system can effectively reduce the sintering temperature by 100°C,and the grain size of sintered samples is about 110?m.?3?AlON twinning strengthening mechanismScanning electron microscopy electron backscatter diffraction?EBSD?,scanning microscopy?SEM?and high resolution transmission electron microscopy?HRTEM?was used to explore the growth of twin grain boundaries in AlON ceramics.It was found that increasing the sintering temperature and prolonging the holding time can effectively increase the content of twins in the material.At the same time,the mechanism of mechanical strengthening of twins was revealed.First,the twin boundaries can change the direction of crack propagation and cause deflection of cracks,consuming the driving force of crack propagation;second,the twin boundaries can effectively block the dislocation motion,causing dislocation plugging and increasing the resistance of the dislocation motion,thereby ultimately increasing the strength and hardness of the aluminum oxynitride transparent ceramic.?4?AlON multiphase ceramicsAccording to the Al₂O₃-AlN phase diagram,a self-reinforced AlON composite ceramic was successfully developed by adding AlN powder to AlON to form an AlON polytype inside the ceramic matrix.The AlON polytype is rod-shaped interspersed inside the grain or between the grains.During the ceramic fracture process,it plays the role of pulling out toughening,bridging and causing crack deflection.At the same time,we have also studied the microstructure of the rod-shaped polytype.and it has been found that the rod is composed of an AlON polytype and an amorphous matrix.