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

Abstract

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.

Fractal Analysis on Machined Surface Morphologies of Soft-Brittle KDP Crystals Processed by Micro Ball-End Milling

“…However, there are some important issues that urgently need in-depth investigation before performing this micro-milling repair approach into the actual engineering application in ICF facilities [10]. One challenge is that as KDP optics are soft and brittle [11][12][13], brittle cracks are quite easily involved in micro-milling processes, deteriorating the machined surfaces, which might shorten the service performance and life of the repaired KDP optics [8,14,15]. Thus, it is significant to carry out a comprehensive evaluation of the repaired KDP surface morphologies.…”

On a Novel Modulation Cutting Process for Potassium Dihydrogen Phosphate with an Increased Brittle–Ductile Transition Cutting Depth

“…A kinematic model and wavelet analysis method have been constructed to predict the surface roughness under various combinations of cutting process parameters by incorporating the ductile plasticity and brittle crack characteristics [25,26]. Building on the insights gained from the fundamental nano-indentation, the optimal ductile material removal with shallow subsurface damage was identified on the (001) plane [27]. In addition, an elevated temperature and large negative tool rake angles are favorable for increasing the fracture toughness and suppressing the crack propagation for achieving the nanometer surface roughness of KDP crystals [28][29][30].…”