Research On Nano-mechanical Behavior And Fly-cutting Subsurface Damage Evaluation Of KDP Crystals

Potassium dihydrogen phosphate?KH₂PO₄?crystals,abbreviated as KDP crystals,have excellent physical properties,including frequency doubling effect,photoelectric effect,piezoelectric effect,easy phase matching,superior transparency in a wide range of the optical spectra and good optical homogeneity.Furthermore,large-size KDP crystals?420 mm × 420 mm?can be obtained by a rapid growth technique.Therefore,KDP crystals are widely used in the fields of laser frequency conversion,electro-optic modulation,acousto-optic modulation,high-speed Q-switching,parametric oscillation and piezoelectric transducer.In particular,it is irreplaceable for large-size KDP crystals as switching and frequency doubling components in the laser ignition facility of inertial confinement fusion?ICF?.To output high laser energy,KDP crystal components must own an ultra-smooth surface and a damage-free subsurface.However,fracture and crack damages often occur in the surface/subsurface of KDP crystals during ultra-precision machining because the material hardness is low and brittleness is high.This significantly reduces the laser damage threshold?LDT?of KDP crystal components.Thus,to avoid the fracture damage of KDP crystals,the materials have to be removed in a plastic mode.To date,it is yet unclear to related problems such as the plastic deformation mechanism of KDP crystals and the critical criterion of brittle-plastic transition,which restricts the development of the novel ultra-precision cutting technology of KDP crystals.As a result,this thesis takes the KDP crystal nano-mechanical properties as a research object.Advanced nano-mechanical property test method and material microstructure characterization technology are used to deeply study a series of problems,including the KDP crystal plastic deformation mechanism,plastic removal behavior and machining-induced damage.The pop-in and creep phenomena during nanoindentation indicate that KDP crystals can be plastically deformed at micro/nano scale.In the stage of the plastic deformation of KDP crystals,the subjected stress increases first,then decreases and again increases with increasing indented depth.In other words,the stress has a nonlinear relationship with the indented depth.A loading rate has little effect on the plastic deformation of KDP crystals;but load and indenter shape have significant effects on the plastic deformation of KDP crystals.Dislocation slip is the main mechanism of the plastic deformation of KDP crystals.When the stress concentration generated by dislocation pileup exceeds the strength limitation of KDP crystals,the material plastic deformation changes into brittle fracture.This work establishes a theoretical foundation to machine KDP crystals plastically.Nanoscratching experiments show that KDP crystals can be plastically removed at micro/nano scale.The main mechanism of the plastic removal of KDP crystals is dislocation movement.Lateral force increases with increasing normal force,but the larger the normal force,the more difficult the plastic removal of KDP crystals.The influence of indenter shape on lateral force has a close relationship with a normal force.The effect of normal force on the friction coefficient of KDP crystals is small under a Berkovich indenter;however,the friction coefficient increases with the increase of the normal force under spherical and conical indenters.The friction coefficient produced by a spherical indenter is the smallest when normal force is low;on the other hand,when the normal force is large,the friction coefficient produced by a Berkovich indenter is the smallest.The friction force mainly derives from the elastic-plastic deformation of KDP crystals during nanoscratching.This work provides a guidance to understand the cutting mechanism of KDP crystals.The nano-mechanical properties of KDP crystals on a fly-cutting surface are lower than those on a damage-free surface.The elastic-plastic deformation of KDP crystals is easier on a fly-cutting surface than that on a damage-free surface.In addition,the elastic modulus and hardness of KDP crystals on a fly-cutting surface are also lower than those on a damage-free surface.This means that the KDP crystal microstructure has been damaged on a fly-cutting subsurface.Otherwise,the anisotropy of mechanical properties of KDP crystals is reduced after fly-cutting.The lateral force produced by nanoscratching is also low on the machined surface,indicating that the KDP machinability is improved in subsurface.This work provides a new idea to improve the ultra-precision cutting process of KDP crystals.Grazing incidence X-ray diffraction?GIXD?is an effective method to detect the subsurface damage of KDP crystals.The subsurface damage of KDP crystals can be divided into two types: one is only plastic deformation zone;the other is a combination of plastic deformation and crack,and the plastic deformation zone is above the crack zone.The formation mechanism of the subsurface damages of KDP crystals can be explained that dislocation nucleation and motion lead to the plastic deformation damage under an external stress.When a tensile stress exceeds the KDP strength limitation,cracks occur below the plastic deformation zone.Subsurface damage depth can be measured based on the strain difference between damage zone and the KDP matrix when the subsurface is only plastically deformed.When the subsurface has cracks,subsurface damage depth can be measured based on the grazing incidence angle of the last drift peak.This work is significant to develop damage-free machining technology of KDP crystals.