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

Abstract

Indentation experiments and friction measurements were performed at three low index faces of fluorite (CaF 2) 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.

Micro-Scale Friction and Wear at Orthorhombic BaSO₄(0&thinsp;0&thinsp;1) Surface

“…The steps were most probably created by horizontal compression applied by the stylus at the apex of each triangle. Similar activation of slips by horizontal compression was clearly observed in the case of CaF 2 crystal [17].…”

“…However, the step signs, upward or downward, can be determined only with height images. We have previously used deflection images to show cracks and large slip steps formed at CaF 2 surfaces [17]. Since the steps formed in the wear processes often have heights of several hundred nm, the height range was set at 0.5-2.0 μm full-scale in AFM imaging.…”

Minimization of CaF2 spherical surface deformation caused by annealing

“…The essential difference between the deformed and the annealed states lies in the dislocation content and arrangement. 18 The observed deformation of the lens surface also resembles, by its orientation and symmetry, the slip steps and cleavage steps observed on the deformed or indented CaF 2 (111) surfaces [21][22][23] and their possible step-crossing patterns. 24 The periodically varying regions of highs and lows after annealing coincide with a characteristic surface shape after mechanical processing of lens shapes (Fig.…”