Fused silica is widely used in optical systems and high-end equipment because of its excellent optical properties, low thermal expansion coefficient, and good thermal and chemical stability. However, fused silica is a typical hard and brittle material, and the surface or subsurface damage introduced during its grinding can strongly affect its service performance. In repeated grinding processes, the material subjected to a subsequent grinding pass is no longer in an undamaged initial state. The damage generated by the previous machining process changes the surface morphology, subsurface densification state, and local network structure of the material, which may further affect the contact behavior between the abrasive grain and fused silica in the subsequent grinding process. Therefore, clarifying how the initial damage state affects subsequent grinding is important for developing high-efficiency and low-damage machining strategies. In this work, a molecular dynamics model of single abrasive grain grinding of fused silica is established to investigate the influence mechanism of initial damage on subsequent grinding. First, the grinding process of undamaged fused silica under different grinding depths is simulated, and the changes in the grinding force, surface morphology, subsurface densification, and short-range structural parameters are analyzed. The ratio of grinding depth to abrasive grain radius is defined as the normalized grinding depth (λ). Based on the grinding results under different grinding depths, three pre-damaged material states with gradually increasing damage degrees are constructed. These pre-damaged models are then used as the initial material states for subsequent grinding under the same grinding condition, so that the influence of the initial damage state on the material removal, grinding force, surface profile, and subsurface structural evolution could be compared. Furthermore, ultrasonic vibration-assisted grinding is introduced into the subsequent grinding process of pre-damaged fused silica to analyze its influence on the suppression of machining-induced damage. In the ultrasonic vibration-assisted grinding simulation, a periodic vibration is applied along the normal direction of the abrasive grain, so that the contact mode between the abrasive grain and the material is changed from continuous contact to periodic intermittent contact. The simulation results show that increasing the grinding depth enhances the normal indentation and tangential sliding interaction between the abrasive grain and fused silica, and the material damage degree increases with the grinding depth. For the pre-damaged material generated at λ=0.20, the initial damage degree is relatively low, and a relatively obvious additional subsurface densification appears during subsequent grinding. For the pre-damaged material generated at λ=0.60, the changes in surface profile and internal structure during subsequent grinding are relatively small, indicating that the pre-existing densified region can accommodate part of the subsequent mechanical loading and reduce additional structural rearrangement under the same subsequent grinding condition. For the pre-damaged material generated at λ=1.00, material removal becomes more pronounced, whereas subsurface structural heterogeneity is further enhanced, indicating that excessive initial damage may promote material removal while increasing non-uniform subsurface structural modification. Compared with conventional grinding, ultrasonic vibration-assisted grinding significantly reduces the average normal force during subsequent grinding. The average normal force of the three damage states decreases by 37.4%, 39.8%, and 41.0%, respectively. The reduction in the normal force indicates that ultrasonic vibration weakens the continuous normal indentation of the abrasive grain into the material. Stress and strain analyses further show that abrasive grain-workpiece recontact produces localized transient stress concentrations, whereas the subsequent separation and unloading shorten the duration of high-stress loading. At the same time, ultrasonic vibration-assisted grinding mitigates the decrease of the Si—O—Si bond angle associated with network densification and reduces the densification degree of the subsurface layer after machining. These results show that the initial damage state changes the grinding force, surface morphology, and subsurface damage evolution during subsequent grinding, and the subsequent grinding result does not vary monotonically with the increase in the initial damage degree. Under the investigated conditions, the pre-damaged material generated at λ=0.60 exhibits a better balance between material removal and damage control among the three damage states considered. Ultrasonic vibration-assisted grinding reduces the dependence of the normal force on the initial damage state, while periodic separation and unloading suppress the persistent accumulation of shear deformation and subsurface densification.
Key words
molecular dynamics simulation /
fused silica /
ultrasonic vibration-assisted grinding /
single abrasive grain /
grinding
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Funding
Joint Funds of the National Natural Science Foundation of China（U24A20126）
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