Frictional force was measured on directional calcite (1 0-1 4) surface, where triangular carbonate ions are all tilted to [4 2-1] direction. With the normal load < 49 mN, observed frictional asymmetry, +[4 2-1] >-[4 2-1], was the same as that measured in nm-scale. In the higher normal load range, a new component was added to the friction in-[4 2-1] direction, and the asymmetry was reversed. Only in the direction of stronger friction, characteristic triangular deformation patterns were formed. Microscopic observation after indentation experiments revealed that the patterns were formed by slip on c (0 0 0 1) faces on one side of the stylus, followed by cleavage on r{1 0-1 4} faces. A mechanical model was proposed to explain why the deformation occurs only in one quadrant of the surface. The result of scratch perpendicularly in [0 1 0] direction, also supports the mechanism of deformation. The results show when and how the contribution of plastic deformation is added to intrinsic surface friction and affects the frictional asymmetry. The frictional force in the-[4 2-1] scratch increased linearly with the total lengths of the traces of c-slip.
Frictional force was measured using Bowden-Leben apparatus with a sapphire stylus at directional (1 0 -1 4) surface of magnesite crystal. Deformation patterns formed during the scratch in ± [4 2 -1] directions were observed with optical microscopy. With the normal load up to 460 mN, no trace of slip or cleavage was observed and the friction coefficient remained at 0.0085. Frictional asymmetry due to the tilt of carbonate ions was not recognized in this range. At higher normal loads, new components were added to the frictional force, with the appearance of traces of slip, and then, cleavage on {1 0 -1 4} faces. In contrast with the case of calcite, traces of the slip remained within the width of the stylus. Traces of the cleavage, on the other hand, reached far outside the wear track, and asymmetry was recognized in the pattern formation. In indentation experiments, the cleavage occurred in four directions, but slip on (0 0 0 1) was not recognized. In contrast with isostructural but softer calcite crystal, the difficulty in the (0 0 0 1) slip with magnesite keep the friction coefficient low toward higher normal loads.
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