Surface/Subsurface Damage Detection And Analysis Of KDP Crystal Grinding

KDP crystal (Potassium dihydrogen phosphate, KH₂PO₄) which developed in the forty age of the twentieth century is one of the excellent non-linear optical crystal materials. It's widely used in high tech fields, 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, such as laser frequency conversion, electro-optic modulation, optical speediness switch, inertia control fusion and so on. However, KDP crystal has several disadvantages such as soft, fragile, hygroscopic, anisotropy, and thermally sensitive, make it became one of the most difficult machining material. Under present machining conditions, the surface damage will be existed in all the machining methods. Therefore, in order to come true the low damage, high efficiency and ultra-precision machining of KDP crystal, the research of the damage mechanism, the analysis and detection of the damage are necessary.Firstly, use for reference the methods used in the damage measurement on hard and brittle material, a set of method for detecting the KDP crystal surface/subsurface damage was found, which are based on a lot of experiments and the optimization and combination of detection techniques. The micro-hardness and fracture toughness of different lattice orientation on KDP crystal (001) lattice plane were measured by indention experiments; the surface damage of KDP crystal induced by grinding were observed by optical equipment, such as Olympus, SEM and so on; the size, density and distributing of dislocations on different crystal lattice plane were measured by selecting etching method; the subsurface damage induced by grinding were measured by cross section method; at last, the relationship between the grinding parameters and the SSD of the KDP crystal were researched.The results indicate that: there are obvious anisotropy characteristic on different orientation of the KDP crystal surface; obvious nick, crash and crack on KDP crystal surface induced by grinding; there are obvious difference of dislocations density and size on different crystal lattice plane, base on the different shear elasticity value and machining conditions; besides the effect of granularity of grinding wheel, the order of the effect degree to the SSD are wheel feed, wheel speed and the worktable speed of grinding machine. In conclusion, adopting the equality finer grinding wheel granularity, low wheel feed, high worktable and grinding wheel rotate speed, along [110] orientation, can get better quality surface.