Effect of ultra-precision fly-cutting on the surface integrity of potassium dihydrogen phosphate crystals

The surface integrity of a potassium dihydrogen phosphate (KDP) crystal significantly affects the laser damage threshold of the material. However, the detection of the surface integrity of KDP crystals is difficult due to the material's special properties including soft, brittle, and sensitive to external environments (e.g., humidity, temperature, and applied stress). This results in conventional characterization methods, such as transmission electron microscopy (TEM) and scanning electron microscope (SEM), which cannot be used to study the mechanisms of surface/subsurface damages ofKDP crystals. This paper investigates the ultra-precision fly-cutting effect on the surface integrity of KDP crystals. To explore the fundamentals, nanoindentation was used. The results demonstrated that the elastic-plastic deformation of a KDP crystal occurs more easily on a machined surface than on a cleaved (damage-free) surface. The elastic modulus and hardness of the former surface are lower than that of the latter. Additionally, fly-cutting reduces the anisotropy of the elastic modulus and hardness. To explore the mechanisms behind such variations, a novel method to characterize subsurface damage was proposed by using the grazing incidence X-ray diffraction (GIXD) technique. It was identified that the damages induced by fly-cutting are dislocations and lattice misalignments.