Damage Detection Of Different Machined Surface And Damage Mechanism Analysis Of KDP Crystal

KDP(Potassium dihydrogen phosphate,KH₂PO₄) crystal,based on its biggish electro-optic and non-linear optical coefficient,higher laser damage threshold,lower optical absorption coefficient,higher optical uniformity and suitably transparent wave band,is widely used in high tech fields,such as laser frequency conversion,electro-optic modulation, optical speediness switch,inertia control fusion and so on.With the development of science and technology,KDP crystal requires larger and thinner,machining accuracy and surface integrity also requires higher.However,it's well-known that KDP crystal is one of most difficult machining materials because it has several disadvantages such as soft,fragile, hygroscopic and anisotropy.Such as cutting,turning,grinding and polishing are common methods in KDP crystal mahining.Grinding is one of machining method in KDP crystal processing.Because grinding is a complicated mechanical process,many factors can influence the surface quality,so it is necessary to study the removal mechanisms of KDP crystal grinding and the damage detection of machined surface in order to improve the quality of grinding.Based on lots of references read and reference the method used on hard and brittle materials,the machining damage of KDP crystal was investigate in this paper.Such as optical microscope and SEM were used to detect the surface machined by cutting,lapping,polishing and grinding;cross section,selective etching and angle polishing were used to study the subsurface damage generated by cutting lapping and grinding.Besides,the influence of anisotropy and feed to quality of grinding surface were studied,scratch experiment was taken to investigate the damage mechanism of KDP crystal.The results indicated that:the surface quality and subsurface damage were different on differents machined surface;both the surface quality and the scratch damage of surface layer were different with the change of crystal orientation.The depth of subsurface damage of cutting by diamond wire and lapping by #3000 abrasive papers was 85.59μm and 19.32μm, respectively.When the polishing slurry and the cleaning method of polished surface were suitable,surface roughness reached 1.556nm.When KDP crystal was machined by rotational grinding with #600 grinding wheel,the subsurface damage depth was 7.41μm and 8.96μm corresponding to the feed was 10μm and 40μm.Anisotropy was showed by the ground surface quality machined in different orientation in method of surface grinding on(001) crystal plane.Scratch experiment on(001) plane of KDP crystal showed that:with the increase of radial force,surface deformation changed from the region of mainly plastic to the region of mainly brittle along the scratch;subsurface damage changed from dislocation to middle crack and transverse crack so that the damage of crystal was increased and made material removal in brittle mode.The orientation of scratch crack propagate,friction coefficient on surface and the shape of crack,depth of damage,etching pit of dislocation in subsurface were different with the change of scratch direction.