Damage Mechanism And Experimental Study On Ultra Precision Grinding Of Sapphire

Sapphire crystal has unique lattice structure,excellent optical performance,good chemical stability,especially its high strength,hardness,wear resistance.Therefore,sapphire crystal has been widely used in national defense,semiconductor,biomedical and optical fields.These applications impose very high surface and subsurface quality requirements for sapphire components,such as nano-level surface roughness and no surface/subsurface damage for sapphire radome.Sapphire is a typical hard and brittle material,especially its high hardness,second only to diamond,which brings the ultraprecision manufacturing of sapphire components extremely challenge.This thesis focuses on ultra-precision manufacturing of sapphire and studies the mechanism and process of ultra-precision grinding,so as to reveal the damage mechanism and damage control mechanism in sapphire grinding and provide a technical foundation for the ultra-precision manufacturing of sapphire components.The research of this thesis includes the damage behavior and damage mechanism of sapphire material during processing,the characterization of damage evolution of sapphire based on acoustic emission technology and on-line prediction model of grinding surface roughness,and the critical conditions of sapphire ultra-precision grinding and the influence of abrasive grain morphology and abrasive wear on the critical conditions of sapphire ultra-precision grinding,etc.In this thesis,to reveal the mechanism of sapphire damage,the removal mode and damage behavior of sapphire material during cutting were studied based on single grit scratching experiment firstly.The initiation mechanism and location of cracks for sapphire were analyzed by high resolution transmission electron microscope.Based on the elastic stress field model,the damage mechanism of sapphire and the brittle damage evolution pattern of sapphire were studied.This study provides a theoretical basis for damage control in ultra-precision grinding of sapphire.Based on acoustic emission(AE)technology,the evolution of brittle damage of sapphire material during the process of single grit scratching was studied,and the AE signal characteristics at different damage stages were analyzed.The theoretical model of AE signal waveform generated by crack propagation was established.The waveform characteristics of AE signals with crack propagation length were discussed.The fractal and frequency characteristics of AE signals generated during sapphire processing were analyzed.Besides,according to the fractal and frequency characteristics of the signal,the damage behavior of sapphire during grinding was analyzed.Based on the above researches,the characterization of AE signal fractal dimension on damage evolution was studied,the relationship between crack propagation and surface roughness in the process of grinding was analyzed,and then the on-line characterization model of grinding surface roughness based on the variation coefficient of fractal dimension of AE signals was established.The crack initiation and propagation are important factors affecting the machining quality.The damage control condition of sapphire material during grinding was studied based on the crack propagation mechanism.In this paper,the grinding force model of single grain grinding was established,then the critical cutting depth of crack initiation in grinding process was analyzed,and the size model of crack propagation was established.The calculation method of the critical length of crack growth in grinding process was given,and the critical condition of sapphire ultra-precision grinding mode was studied in combination with the crack growth size model,which was verified by experiments.The influence of grinding parameters and grinding wheel parameters on the critical condition of ultra-precision grinding mode was analyzed.Based on SPH-FEM finite element simulation and single grit scratching experiment,the influence of grain morphology and grain wear on the damage behavior and removal mode of sapphire material during grinding was studied.The influence of grain wear on the critical condition of ultra-precision grinding was analyzed,and a method to determine the critical cutting depth of ultra-precision grinding considering grinding-wheel wear was given.Finally,the influence of grinding wheel parameters on the surface quality was studied through grinding experiments,and the grinding wheel parameters suitable for sapphire grinding were given.On this basis,the ultra-precision grinding experiment of sapphire was designed,and the non-brittle damage ultra-precision grinding of sapphire was realized.