Machining-induced damage mechanisms in KDP crystals
Potassium dihydrogen phosphate (KH2PO4) crystals, abbreviated as KDP crystals, are widely used in some high-tech fields, including optical uniformity, laser frequency conversion, high-speed Q-switching, because KDP possesses excellent physical properties, such as superior photoelectric/piezoelectric capability, outstanding frequency-doubling effect and high nonlinear conversion efficiency. In particular, large-size KDP crystals are irreplaceable as switching and frequency doubling components in the laser ignition facility of inertial confinement fusion (ICF). However, the manufacture of KDP components is challenging because the material is soft, brittle, sensitive to the variations in temperature, humidity, or stress. These properties of KDP make this material one of the most difficult-to-process material. As a result, many basic issues related to the damage-free machining of KDP crystals are still unsolved. This thesis aims to reveal the machining-induced deformation mechanisms of KDP with the aid of molecular dynamics (MD) analyses. To this end, a reliable potential function will be established to conduct some large-scale MD simulations. The MD analyses include simple loading conditions (tension and compression) and complex loading conditions (nanoindentation and nanoscratching). The deformation mechanisms, failure behaviour and structure identification will then be revealed. The modelling method and structural characterisation methods will be optimised to assist the simulation and analysis. The major findings of this thesis are as follows: (1) To complete the MD simulation of KDP, this thesis pioneers a reliable interaction potential function that can be employed to analyse the deformation mechanism of KDP at the nano scale. The initial parameters of this potential function are provided by the polymer consistent force field (PCFF), then are optimised based on parameters such as atomic lattice constants and Young’s moduli for different crystal orientations. The reliability of this potential function is verified using experimentally measured and ab initio predicted data. (2) To effectively perform MD simulations of KDP, this thesis proposed a model creation process using MATLAB, which allows the created models for the simulations to break through physical memory constraints. In addition, to perform a good structural analysis of the simulation results, this thesis creates an effective method based on machine learning to identify phenomena including phase transitions, amorphous phases and slipping in the KDP structure. This method overcomes the requirement of a manual setup. (3) The deformation and failure mechanisms of KDP under simple loading conditions are investigated. The atomic structure is found to have a significant influence on the anisotropy of KDP, and the rotations of its PO4 groups are its main deformation mechanism. This leads to some failure mechanisms, including ductility and brittleness. In addition, some trigger and stable conditions for phase transformation and lattice slipping are identified. (4) By conducting nanoindentation simulation tests, it is demonstrated that the occurrence of the first pop-in represents the generation of phase changes at the nano scale, and the phase change under the indenter is shown to be triggered by a combination of hydrostatic and shear stresses. In different atomic lattice surfaces of KDP, the phase transitions initiated are different. The (001) surface has shallower subsurface layer damage and the (100) surface has better surface recovery. Nanoscratching simulation tests also show that the (001) surface has a much lower removal rate. Nanoindentation experiments confirm the existence of phase transformations. Dislocations and lattice misalignment structure (LMS) were found at greater depths.
Molecular Dynamics simulation, Potassium Dihydrogen Phosphate, Potential Function, Deformation Mechanisms, Structural Characterization, Nanoindentation / Nanoscratching
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