Effect of Anisotropy of Potassium Dihydrogen Phosphate Crystals on Its Deformation Mechanisms Subjected to Nanoindentation

Potassium dihydrogen phosphate (KDP) is an important nonlinear material due to its excellent physical and optical properties. However, it is also a difficult-to-machine material due to its complex anisotropic microstructure. To better understand the deformation mechanisms under external stresses, this paper aims to carry out systematic nanoindentation simulations using molecular dynamics (MD). To facilitate the structural characterization of KDP, a machine learning-based method was developed. The results showed that the subsurface damage is obviously anisotropic. On the (001) surface, both tetragonal and monoclinic phases appear simultaneously and part of the monoclinic phase transfers to the tetragonal phase. The generated phases close to the surface undergo amorphization and are squeezed out to form pile-ups. On the (100) surface, however, an orthorhombic phase emerges directly from the original structure rather than transforming through the monoclinic phase. No amorphization happens and no pile-ups appear in this case. The first "pop-in" in the load-displacement curve of nanoindentation signified the emergence of phase transformation under the combined hydrostatic and shear stresses. After unloading, the recovery of the deformed KDP is also anisotropic. The maximum recovery takes place when the indentation is on the (100) surface.