Study On Precision Grinding And Polishing Process Of Polycrystalline YAG

YAG is widely used as laser gain medium due to its good thermal conductivity,anti-illumination stability and a series of excellent mechanical and physical properties as well as stable chemical optical properties.With the continuous improvement of performance requirements for YAG devices,The size of single crystal YAG crystals is limited,and thus it cannot exactly meet the requirements of preparing large output power YAG devices.Although the size of polycrystalline YAG is not limited,existing of grain boundary step in the processing process have a strong impact on the surface quality and the performance of the laser.Therefore,in order to improve the output power of YAG laser,ultra-precision machining of polycrystalline YAG is essential.In this paper,the grinding and polishing process is used to process polycrystalline YAG.The major research contents are as follows:The effect of abrasive particle size on surface roughness,surface morphology and material removal rate was analyzed by the grinding of polycrystalline YAG with fixed abrasive particle.Firstly,diamond abrasive particles with the particle size of 18 μm were selected in the coarse grinding stage,and diamond abrasive particles with the particle size of 9μm and 5 μm were selected in the fine grinding process.Then the influence of loading pressure and rotation speed on the crystal surface damage was studied,and finally the optimum grinding process was determined.The chemical mechanical polishing experiments of polycrystalline YAG were carried out.Firstly,the evolution process of surface morphology and surface roughness with time during polishing stage were studied.In addition,the effects of acid and alkalinity,loading pressure,abrasive particle concentration and particle size on crystal surface quality were investigated.Finally,we found that the grain boundary steps are the main factor affecting surface quality in polishing,and the formation mechanism and evolution of grain boundary steps in polishing process were explained.A mathematical model of grain boundary step formation in chemical mechanical polishing of polycrystalline YAG is proposed,and the formation and evolution mechanism of grain boundary steps were analyzed.The relationship between the abrasive particle size,abrasive particle concentration and the height difference between grain direction was quantitatively revealed and a verification experiment was conducted.The experimental results agree well with the theoretical model.The mechanism of height difference between crystals under acidic and alkaline conditions was analyzed with using XPS detection technology.It was proved that aluminum silicate and yttrium silicate with lower hardness were produced in both reactions.Meantime,the material removal experiment of single crystal YAG was carried out to verify the difference of chemical reaction rate under different acid and alkali conditions.In order to achieve the balance of chemical and mechanical action so as to effectively inhibit the grain boundary steps,a mixture of alumina abrasive and silica sol polishing slurry is used for chemical mechanical polishing.After process optimization experiment,the surface roughness reached the minimum Sa 0.28 nm with the concentration of 3 wt% alumina and 10 wt%colloidal silica.No scratches and other defects were found on surface,and it reached the leading world level.