Frictional force microscopic detection of frictional asymmetry and anisotropy at (101(1)over-bar4) surface of calcite

Frictional asymmetry due to the tilt of carbonate ions at (10(1) over bar4) surface of calcite (CaCO(3)) was detected with frictional force microscopy. The asymmetry signal was recorded by scanning the probe across the boundary between two surfaces of oppositely placed crystals in [42(1) over bar] direction. Observed asymmetry was in striking contrast with previous results for tilted S=O bonds of CaSO(4)(100), lower friction being observed when the probe was scanned against the tilt. The difference was explained by the tilt angles of C=O and S=O bonds, which are described as mass-spring systems, from surface normal. More tilted C=O bonds of calcite support the normal load mainly by the reaction against bending of the springs, while less tilted S=O bonds support the load by the reaction against compression of the springs. As was expected, the asymmetry signal was strongest along the [42(1) over bar] direction, while no asymmetry was observed along the [010] direction. Compared to the case of CaSO(4), the asymmetry signal at the calcite surface took a large proportion of the total friction. The anisotropy in the total friction was also studied at the same surface, weakest total friction being observed along the [010] direction. Travelling straight in that direction, the tip atom at the probe surface frequently meets bridge sites where they can interact with two oxygen atoms of the carbonate ions. The result was in accordance with a similar experiment with a corrugated CaSO(4)(001) surface.