Mechanical Behaviors Of Bk7 Optical Glass During Nano/micro Scratching Under Different Shaped Indenters And Different Scratch Directions

Optical glass is widely used in precision fields such as aerospace,micro-electro-mechanics,display and touch fields with good physical chemical characteristics and unique optical properties.Surface cracks and subsurface damage are extremely easy to occur during machining since optical glass is a typical hard-brittle material,which is a great challenge for processing and manufacturing.Textured grinding wheels have been widely used in the precision and microstructure processing of hard-brittle materials.In order to explore the influence of the shape and arrangement direction of the abrasive grains in textured grinding wheels on the grinding quality,mechanical behaviors of BK7 optical glass during nano/micro scratching under different shaped indenters and different scratch directions were systematically researched in this paper,including force and displacement response(P?h),elastic recovery rate,friction characteristic,densification,crack resistance,ductile-to-brittle transition(DBT)and brittle removal.The characteristics of BK7 optical glass including the elastic modulus,the micro-nano hardness,and the fracture toughness were systematically measured and analyzed based on the nano-indentation test.Compared with single crystal silicon,the ability to resist contact load and crack propagation is relatively weaker for BK7 optical glass under the same conditions.Based on micro-nano scratch of optical glass,the mechanical response and micro-nano hardness models of different shapes(including Vickers and Berkovich indenter)and different scratch directions(including face-forward and edge-forward)were established.A predictive model of critical depth for ductile-to-brittle transition was established considering the indenter geometry,scratch directions and the elastic recovery rate of the material.Experimental research on the P?h response,elastic recovery rate,material removal volume of BK7 optical glass under face-forward and edge-forward scratching using Vickers indenter and Berkovich indenter was investigated.The theoretical prediction values of the scratching depth and material removal volume were well matched with the experimental results.On this basis,the theoretical model of micro-nano scratching friction coefficient under different scratch directions was established and compared with the experimental results.The relative error between the predicted value of the theoretical model and the experimental friction coefficient value is within 9%.During the scratching process,the scratch direction has a profound effect on the densification of the material,and the densification has an important effect on the crack resistance of the material.Using Vickers and Berkovich indenters under different scratch directions with the same normal load to scratch BK7 optical glass,the samples after scratching were annealed at high temperature.The width,depth,collapse volume,pile-up height and volume of the grooves before and after annealing were measured,and the densification was quantitatively evaluated by the volume ratio of annealing recovery V_R.The relationships between densification deformation,residual stress field and crack resistance of optical glass under different scratch directions were theoretically speculated.The experimental results showed that under the same scratch load,the densification of the optical glass during scratching by Vickers indenter was greater than that caused by Berkovich indenter,and the densification of edge-forward scratching is greater than that caused by face-forward scratching.The greater the densification,the smaller the residual tensile stress,and the less likely it is to cause the initiation and propagation of subsurface cracks,thereby helping to improve the crack resistance of the material.Based on the study of material densification and crack resistance,the effects of indenter shape and scratch direction on ductile-to-brittle transition and brittle removal of optical glass were thoroughly investigated.The critical depth of ductile-to-brittle transition for BK7 optical glass was obtained by the focused ion beam(FIB)-scanning electron microscope(SEM)method,and then the effects of different shapes of indenters and different scratch directions with the same were explored under linear loading scratch mode.The experimental results showed that the critical load of crack generated by Vickers indenter was larger than that of Berkovich indenter,and the critical depth and load of ductile-to-brittle transition for optical glass under edge-forward scratching was larger than that under face-forward scratching,which was consistent with the aforementioned effect of densification on crack resistance.In addition,the critical depth of ductile-to-brittle transition by conical indenter is larger than that by Vickers indenter and Berkovich indenter under face-forward scratching,but it is smaller than that by the Vickers indenter under edge-forward scratching.The measured value of the critical depth for ductile-to-brittle transition was compared with the predicted value of the theoretical model.The average error of the predicted result is 7.29%.The material removal of the BK7 optical glass scratched by Vickers indenter under a large load and different scratch directions was studied,and the stress field of the BK7 optical glass under different scratch directions was obtained by finite element analysis.The face-forward scratch is more likely to introduce the initiation and propagation of lateral cracks to the surface due to the larger residual stress,while the edge-forward scratch is more likely to cause the chip to discharge from both sides of the groove due to the larger COF.The experimental results matched the theoretical COF model and FEM simulation well.