Femtosecond Laser-Induced Phase Transformation on Single-Crystal 6H-SiC.

Silicon carbide (SiC) is widely used in many research fields because of its excellent properties. The femtosecond laser has been proven to be an effective method for achieving high-quality and high-efficiency SiC micromachining. In this article, the ablation mechanism irradiated on different surfaces of 6H-SiC by a single pulse under different energies was investigated. The changes in material elements and the geometric spatial distribution of the ablation pit were analyzed using micro-Raman spectroscopy, Energy Dispersive Spectrum (EDS), and an optical microscope, respectively. Moreover, the thresholds for structural transformation and modification zones of 6H-SiC on different surfaces were calculated based on the diameter of the ablation pits created by a femtosecond laser at different single-pulse energies. Experimental results show that the transformation thresholds of the Si surface and the C surface are 5.60 J/cm2 and 6.40 J/cm2, corresponding to the modification thresholds of 2.26 J/cm2 and 2.42 J/cm2, respectively. The Raman and EDS results reveal that there are no phase transformations or material changes on different surfaces of 6H-SiC at low energy, however, decomposition and oxidation occur and then accumulate into dense new phase material under high-energy laser irradiation. We found that the distribution of structural phase transformation is uneven from the center of the spot to the edge. The content of this research reveals the internal evolution mechanism of high-quality laser processing of hard material 6H-SiC. We expect that this research will contribute to the further development of SiC-based MEMS devices.

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.