The Effect of Crystallographic Orientation on Material Removal Behavior of (001) Plane KDP Crystal in Nano-Scratch Test

Abstract

KDP crystal has been widely used in harmonic generation and optical parametric oscillators due to its good nonlinear optical properties and high laser damage threshold. However, because of its inherent properties, such as fragility, hygroscopic, anisotropy and low rigidity etc., KDP crystal is regarded as one of the most difficult machining materials. Nanoscratch tests were conducted in [100], [110] and [010] orientation of (001) plane KDP crystal at room temperature under a ramp loading condition from 40μN to 200 mN using with a nanomechanical test system in scratch mode to study the effects of crystallographic orientations on plastic deformation and brittle deformation features of KDP crystal. A spherical monocrystalline diamond indenter was employed in this study. Penetration depths and residual deformations of the scratch tracks were collected during the scratch process. Morphology characteristics of the scratch grooves in different scratch directions, including plastic deformation features and brittle deformation features, were observed by scanning electron microscopy. The experimental results clearly showed that there exist two distinct material deformation modes of each scratch process: plastic deformation mode, and brittle deformation mode. Comparative studies of surface depth profiles and scratch groove features induced in different crystallographic orientations revealed that the anisotropy of (001) plane KDP crystal has significant effects on the deformation features. It also presented that [110] orientation of (001) plane KDP crystal has the maximal critical load and critical depth, and can produce the highest proportion plastic deformation, which imply that [110] orientation can get adequate surface quality of KDP crystal for diamond cutting, grinding, and milling etc..