Frictional Asymmetry and Wear Pattern Formation by Slip and Cleavage Detected at Directional r{1 0 -1 4} Face of Calcite (CaCO3)

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.