Mechanism And Experimental Study Of Surface/subsurface Damage Of KDP Crystals

Due to its superior nonlinear coefficient and laser damage threshold,KDP crystal is an important optical element in laser fusion device,but its soft and brittle characteristics make it easy to produce surface defects in ultra-precision cutting,which seriously reduces the laser damage threshold.Therefore,it is imperative to repair the defects to prolong the life of crystal.At present,systematic research on crystal damage mechanism and damage range is lack and the existing defect repair efficiency is low,it is easy to cause secondary damage of crystal.Consequently,in this paper,the surface/subsurface damage of KDP crystal(001)plane is taken as the research object,simulation and experiment are carried out on the changes of micromechanical properties,damage mechanism,damage influence range and other issues,so as to provide reference for crystal damage repair.The elastoplastic deformation mechanism and micromechanical properties of KDP crystal(001)plane are investigated by nano-indentation and nano-scratching experiments.The experimental results show that the plastic deformation mechanism of the crystal is basically not affected by the numerical load,and the pop-in indicates the dislocation slip of the crystal.Due to the start of multiple slip systems,the deformation tends to be complicated with the load increasing.Through further analysis,it is found that the elastic modulus and hardness of the crystal have indentation size effect and anisotropy,while the mean value of the friction coefficient has little difference when the load increases,and the mean value of the friction coefficient does not change under different loading methods and loading speeds,indicating that the deformation mechanism of the crystal is independent of the loading methods and loading speeds.The KDP crystals with different surface qualities are obtained by changing the cutting parameters,and the change of their micromechanical properties,the characteristics and distribution of their surface defects are observed.After cutting,the crystal structure is damaged,resulting in a significant decrease in the elastic modulus and hardness values,and the average friction decreases slightly while has a wide range of fluctuations.It is indicated that the resistance of the crystal to elastoplastic deformation is weakened.The simulation model of pits and scratches on crystal surface is established based on SPH method.It is found that plastic deformation occurs when the stress on the crystal surpasses its yield limit,and when the stress value can break through the dislocation packing,the crystal cracks occur.The appearance of surface crack is related to the growth of subsurface lateral crack.The larger the load,the larger the damage range.The change of mechanical properties and subsurface damage around the prefabricated defect on KDP surface are obtained by experiments.The results show that the measured elastic modulus and hardness firstly increase and then become stable with the increasing distance from the prefabricated defects and the measurement point.It is indicated that the surface defects have an effect on the microstructure of the crystal.The crystal subsurface damage consists of the microcrack layer and the strain layer.The diffraction pattern corresponding to the microcrack layer appears polycrystalline phenomenon,and the diffraction pattern corresponding to the strain layer appears drift diffraction peak phenomenon.The larger surface roughness,the deeper subsurface damage.Moreover,the surface/subsurface damage of the defect gradually increases when the prefabrication load increases.