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YAG wafers are the most host laser crystals used for high-power lasers, which are usually machined by grinding to meet the required accuracy for laser components. Warping deformation induced by the residual stress is one of the main damages for YAG wafers after the grinding process, which will seriously decrease the service accuracy and life of the lasers. Developing theoretical model of warping deformation is of great significance to achieving the ultra-precision machining of YAG wafers. The cutting depth of single abrasive and grinding force in self-rotating grinding were investigated by considering the kinematic trajectory of abrasives, brittle-to-ductile transition, elastic mechanics, elastic deformation of the grinding wheel and strain rate effect. A theoretical model of warping deformation in self-rotating grinding of YAG wafers was developed based on the cutting depth and grinding force. The influence of subsurface damage and residual stress on warping deformation was analyzed based on the theoretical model and finite element simulation. Self-rotating grinding tests of YAG wafers were performed, and the results showed that the warping deformation decreased as the wheel rotational speed increased, and increased as the abrasive size, workpiece rotational speed and feed speed increased. The experimental results agreed well with the simulated results of the theoretical model, indicating that the theoretical model can accurately predict the warping deformation induced by self-rotating grinding process. This work will not only enhance the understanding of the essence of the wafer warping induced by ultra-precision machining, but also provide a guide for optimizing the processing parameters in self-rotating grinding of YAG wafers.

Graphical abstract, Highlights • The mappings between micro cracks and diffraction pattern curves were revealed based on GIXD and MD. • Surface and subsurface defects of CaF2 crystals induced by abrasive machining were evaluated and characterized accurately. • MD simulated results agreed well with the experimental results. • This work provided a novel technology for nondestructive testing of defects of single crystals at nano- and micro- scales., Introduction Surface and subsurface defects were easily induced during abrasive machining process of optical crystals due to their high brittleness. Accurate characterization of these defects is the prerequisite for obtaining optical components with high surface integrity. Objectives This work aims to evaluate subsurface defects of CaF2 single crystals induced by abrasive machining, the mappings between micro cracks and diffraction pattern curves, and the influence of micro cracks on lattice structures. Methods Molecular dynamics simulation, grazing incidence X-ray diffraction experiments and cross-sectional TEM detection were used in this work. Results In grazing incidence X-ray diffraction (GIXD) detection experiments on lapping and polished specimens, shifts phenomenon of the peak position under the non-theoretical grazing incidence angle indicated that the subsurface was damaged to a certain extent. The micro cracks of the subsurface were evaluated by the consistent characteristic of “peak drift” of the diffraction pattern curve in both experiments and simulations. In addition, cross-sectional TEM results showed that regular micro cracks were found on the subsurface, which agreed well with the simulation results. Conclusion The subsurface defects of CaF2 single crystals induced by abrasive machining, and the influence of micro cracks on lattice structures can be evaluated by molecular dynamics simulation. The simulation results revealed the mappings between the micro cracks and diffraction pattern curve, which demonstrated that a phenomenon of “peak drift” occurred near the diffraction angle of a specific crystal plane. This work provided a novel technology for the nondestructive testing of defects of single crystal materials at nano- and micro- scales.

Constant- and varied-depth nanoscratching tests of GGG single crystal were carried out at different scratching velocities. The morphologies of the scratched grooves and chips were analysed using scanning electron microscope. The experimental results indicated that higher scratching velocity led to shallower penetration depth, shallower residual depth, and larger continuous chips. Increasing the scratching velocity could effectively improve the plasticity and reduce the brittle-to-ductile transition depth of GGG single crystal. Based on the contact stress and contact area between the analysed sample and Berkovich indenter, a model for predicting the penetration depth was developed, which took into account the strain rate effect and elastic recovery of materials. The model was verified using constant- and varied-depth nanoscratching tests, and the predicted and experimental results were in good agreement. Subsurface damage underneath the ductile surface was characterised using transmission electron microscope. The TEM results demonstrated that higher scratching velocity led to the slipping planes appearing in more directions, which prevented the generation of long slipping plane and reduced the depth of the damage layers. The plastic deformation of GGG at the scratching velocity of 100 μm/s was dominated by poly-crystalline nanocrystallites and amorphous phases, and was similar to that at the low scratching velocity. This study provided a fundamental understanding of the strain-rate dependence of surface/subsurface deformation mechanisms of GGG during ultra-precision machining.

Gallium nitride single crystal (GaN) is difficult to achieve high-efficiency and low-damage machining due to anisotropy, high hardness and brittleness. Nanoscratch tests of GaN single crystals was conducted on (0001) plane along different zone axes, and the anisotropy dependence of material removal and deformation behaviors were investigated systematically. The results showed that crack-free plastic deformation of GaN crystals could be acquired along different zone axes, which was dominated by phase transition, polycrystalline nanocrystals, amorphous transition, as well as close-to-atomic scale damages including stacking faults, dislocations and lattice distortions. As the stress increases, special surface radial cracks with the same orientations caused by shear stress will appear on the groove surface, and striated and step-shaped brittle fractures can be induced by the propagation and intersection of the cracks. The processing along [11–20] zone axis was more conducive to achieve the plastic removal and deformation, deeper penetration depth and less phase transformation. The higher stress along [1-100] zone axis induced more phase transition from hexagonal system to cubic system. This work will enhance the understanding of the anisotropy dependence of material removal and damage mechanisms, and provide a guide for achieving high-efficiency and low-damage machining of GaN crystals.

Despite being the most promising third-generation semiconductor materials, the deformation and removal mechanisms of gallium nitride (GaN) single crystals involved in the ultra-precision machining process are not well revealed and few investigations on the grinding of GaN crystals have been reported, which hinders the development of high-efficiency and ultra-precision manufacturing of GaN components. Self-rotating grinding tests of GaN crystals were performed, and the results indicated that abrasive size had a significant influence on the surface morphology and roughness, in comparison with wheel rotational speed and feed speed. As the abrasive size decreased from 18 μm to 1.6 μm, the brittle fracture-dominated surface gradually changed to a full-plastic surface without brittle fractures and cracks. An ultra-smooth surface with a roughness of 1 nm in Sa was acquired using #8000 grinding wheels and a spark-out time of 10 min, which indicated that the machining technology of “grinding instead of polishing” of GaN crystals was achieved in this work. The plastic deformation mechanism of GaN crystals induced by ultra-precision machining was investigated using a cross-sectional TEM method and MD simulation, and both experimental and simulation results indicated that the plastic deformation involved in the scratching process was caused by the formation of polycrystalline nanocrystals, high-angle lattice misorientations, and close-to-atomic-scale defects, including stacking faults, dislocations and serious lattice distortions, along with a small amount of amorphous and phase transitions. There was an obvious delamination phenomenon in the plastic deformation zone. This research enhances the understanding of the deformation and damage mechanisms of GaN crystals involved in the ultra-precision machining process and is of significance for achieving the high-efficiency and high-accuracy manufacturing of GaN components.

KDP crystal is the primary nonlinear crystal for making core components of inertial confinement fusion and laser frequency doublings. However, as a soft-brittle material, KDP crystal is difficult to machine due to its easy deliquescence, high brittleness, and sensitive to high temperature. A theoretical model of temperature field during fly-cutting of KDP crystals assisted by coolant lubrication was established based on cutting-heat generation theory and thermal transmission theory. This model considered the thermal exchange between the air and work material, the thermal exchange between the fluid and work material, and anisotropy of KDP crystals. The predicted results of the model indicated that oil coolant could effectively increase the thermal diffusivity and decrease the friction coefficient between cutting tools and work materials, thus decreasing the cutting temperature during the fly-cutting process. The highest cutting temperature in coolant lubrication cutting decrease by 9.42–36.70% compared with that in dry cutting. The fly-cutting experiments of KDP crystals assisted by coolant lubrication were performed to verify the model, which indicated that experimental results agreed well with predicted temperatures and the average error of the highest temperature was approximately 7.78%. Both simulated and experimental results indicated that increasing feed speed and cutting depth or decreasing the cutting speed would result in the obvious increase of the highest cutting temperature.