Participation of {1 0 0}<0 1 1> Slip System in Sliding Friction at (0 0 1), (1 1 1) and (1 1 0) Surfaces of Fluorite (CaF2) Crystal

概要

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Indentation experiments and friction measurements were performed at three low index faces of fluorite (CaF2) crystal. Deformation patterns were analyzed by observing step structures with atomic force microscopy (AFM). Upon indentation at (0 0 1) surface, {1 0 0}<0 1 1> slip steps were formed at limited parts of the surface near the compressed area, where horizontal shear stress is added to vertical one. Upon indentation at (1 1 1) surface, the surface was split into 6 sectors separated by {1 1 1} cleavage lines. Steps formed on each sector were also explained by the {1 0 0}<0 1 1> slip. The signs of steps suggest that the steps in three alternate sectors were formed by vertical compression, and the steps in the other sectors were formed mainly by horizontal compression. The slip mechanism was explained by a simple mechanical model. Upon scratching the (0 0 1) surface in [1 0 0] direction, only steps in [1 0 0] direction, not in [0 1 0] direction, were formed outside the wear track. Scratch on (1 1 1) surface activated the slip in three possible directions selectively, depending upon the scan directions of the stylus. Slip did not occur easily at (1 1 0) surface, where slip can occur in five directions close to each other. Frictional anisotropy was discussed in relation to the slip system.