Study On Grinding Mechanism And Process During Ultra-precision Grinding Of Laser Crystals Of Rare-earth Oxide

Laser crystals of rare-earth oxide are the preferred materials for making core components of solid-state lasers.Surface and subsurface damage,including brittle fracture and cracks,will easily occur during the manufacturing process,which will seriously affect the output power and service life of the lasers.At present,the mechanical properties and stress-strain relationship of these materials at ultra-precision processing scale are not systematically studied,and the surface and subsurface formation mechanisms of these materials during the machining process are not clear.Therefore,achieving the high efficiency and high surface integrity processing is a bottleneck problem in the field of the element fabrication of solid lasers.In this paper,according to the research ideas of quasi-static indentation scratch experiments to grinding experiments under high strain conditions,and mechanical properties of materials and material removal and formation mechanisms to ultra-precision processing technology,the nanoindentation tests,nanoscratch tests,ultra-precision grinding tests were systematically performed for two typical laser crystals of rare-earth oxide,garnet and lutetium oxide.The mechanical properties of laser crystals of rare-earth oxide at ultra-precision processing scale were obtained.The surface and subsurface formation mechanisms of these materials during the ultra-precision process were revealed.The ultra-precision grinding process of laser crystals of rare-earth oxide was acquired,which broke through the difficult problem of high-efficiency and low-damage processing of laser components.Nanoindentation experiments of laser crystals of rare-earth oxide were performed.The mechanical properties including elastic recovery rate,nano hardness,elastic modulus and fracture toughness at ultra-precision scale were acquired.Based on the force-displacement curve,the stress-strain curve of laser crystals of rare-earth oxide was obtained.Quasi-static varied-depth nanoscratch experiments of laser crystals of rare-earth oxide were performed.Based on the elastic-plastic contact theory,theoretical models of elastic-to-plastic transition depth and brittle-to-ductile transition depth in nanoscratch process were established.The TEM results indicated that the plastic deformation of GGG single crystals induced by the nanoscratch at quasi-static concitions was dominated by the transformation from sin gle crystal to polycrystalline nanocrystalline and amorphous.The results can provide theoretical support for researching the surface and subsurface formation mechanisms during grinding of laser crystals of rare-earth oxide.A prediction model of scratch depth considering strain rate effect was established.Nanoscratch experiments of laser crystals of rare-earth oxide at different scratch velocities were carried out to verify the reliability of the model.The results indicated that the higher the strain rate resulted in crystal planes slipping along more directions.These slipping planes were jointly subjected to the indenter load,which can restrain the generation of long slip surfaces,reduce the subsurface damage depth,and improve the depth of brittle-to-ductile transition for laser crystals.The research results can lay a theoretical foundation for the study of the effect of strain rate on the ground surface and subsurface formation mechanism of laser crystals of rare-earth oxide.Ultra-precision grinding experiments of laser crystals of rare-earth oxide were systematically performed,and the ductile ground surface/subsurface without brittle fracture and crack damage was achieved.The ductile grinding mechanism of laser crystals of rare-earth oxide was revealed.When the contact stress between the workpiece and abrasives reaches the stress required for crystal plane slipping of only one direction,the material tended to slip along cleavage planes of single direction,accompanied by atomic-level plastic defects such as dislocations,stacking faults and distortions of atomic planes;when the contact stress reaches the stress required for crystal plane slipping of multiple directions,the material tended to plastic flow in the form of polycrystalline nanocrystallines and amorphous.A theoretical model of ductile grinding force of laser crystals of rare-earth oxide was established,which took the combined effect of strain rate effect,random distribution of abrasive tip radius and elastic-plastic transformation mechanism of workpiece materials into account for the first time.The predicted values of the model were in good agreement with the experimental results.The model provided a theoretical support for further understanding the interaction action between abra sives and workpiece and the surface/subsurface formation mechanisms of the materials.A theoretical model of grinding surface topography and roughness was established,which considered the randomness of abrasive tip radius,abrasive protrusion height and abrasive location,and abrasive trajectory.The ultra-precision grinding process experiments of laser crystals of rare-earth oxide were performed to verify the reliability of the model.During the ultra-precision grinding experiments of laser crystals of rare-earth oxide,the value of surface roughness increased linearly as the abrasive size increases,and the influence of grinding parameters on the value of surface roughness was not significant.The theoretical model gave an insight into the material formation and removal behaviours at ultra-precision machining scale,and provided a theoretical support for the selection of high-efficiency ultra-precision grinding parameters for laser crystals of rare-earth oxide.Ultra-precision grinding experiments of laser crystals of rare-earth oxide assisted by lubrication of graphene-oxide coolant were systematically performed.The ductile deformation mechanism of GGG single crystal during graphene oxide assisted lubrication grinding was revealed.The results indicated that the plastic deformation of GGG crystals during GO lubrication assisted grinding was affected by interlayer slipping and filling actions of graphene oxide,and dominated by nanocrystalline polycrystallization and amorphous transformation which were induced by the slippage of crystal planes.Compared with conventional grinding,the surface roughness of workpiece Sa and friction coefficient decreased by about 25% and 30%,respectively.The research results provided a new theoretical basis and technical support for high-efficiency and low-damage ultra-precision grinding of laser crystals of rare-earth oxide.