Effect of repeated nanoindentations on the deformation of potassium dihydrogen phosphate crystals

In many surfacing processes such as precision mechanical polishing, grinding and cutting, a material inevitably experiences repeated interactions with a surfacing tool, which often brings about significant surface integrity problems of the material. This paper aims to explore the deformation mechanisms of potassium dihydrogen phosphate (KDP) crystals subjected to repeated nanoindentations. The molecular dynamics (MD) method was employed to investigate the atomic scale distortion and possible phase transformation under the external stresses due to the indenter-KDP interactions. A machine learning-based method was used to facilitate the structural characterization. The results showed that the response of KDP to the repeated nanoindentations is significantly anisotropic, depending on the atomic lattice orientation to the indentation direction. On the (001) surface of KDP, the indentation will first create an irreversible amorphous phase which will remain stable in the subsequent indentation cycles. While indenting on the (100) surface, however, the amorphous phase emerged in the first indentation will transfer to a combination of orthorhombic and monoclinic phases during the subsequent indentation cycles.