Understand anisotropy dependence of damage evolution and material removal during nanoscratch of MgF₂ single crystals

Abstract

To understand the anisotropy dependence of the damage evolution and material removal during the machining process of MgF2 single crystals, nanoscratch tests of MgF2 single crystals of different crystal planes and directions were systematically performed, and surface morphologies of scratched grooves under different conditions were analyzed. The experimental results indicated that the anisotropy had a significant influence on the damage evolution in the machining process of MgF2 single crystals. The stress field model induced by the scratch was developed by considering the anisotropy, which indicated that median cracks induced by the tensile stress initiated and propagated at the front of the indenter during the loading process. Lateral cracks induced by tensile stress initiated and propagated on the subsurface during the unloading process. In addition, surface radial cracks induced by the tensile stress were easy to generate during the unloading process. The stress change led to the deflection of the propagation direction of lateral cracks, so that the lateral cracks propagated to the workpiece surface and resulted in the brittle removal in the form of chunk chips. The plastic deformation parameter indicated that the more slip systems were activated, the more easily plastic deformation occurred. The cleavage fracture parameter indicated that the cracks propagated along the activated cleavage planes, and the brittle chunk removal was caused by the subsurface cleavage cracks propagating to the crystal surface. Under the same processing parameters, the scratch of the (001) crystal plane along the [100] crystal-orientation is most conducive to achieving the plastic machining of MgF2 single crystals. The theoretical results agreed well with the experimental results, which will not only enhance the understanding of anisotropy dependence of damage evolution and removal process during machining of MgF2 crystals, but also provide a theoretical foundation for achieving the high-efficiency and low-damage processing of anisotropy single crystals.

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