Frictional force microscopic anisotropy on (001) surfaces of alkali halides and MgO

In frictional force microscopic measurement of anisotropy at (001) surfaces of NaCl, NaF, KCl, LiF and MgO crystals, significantly lower friction was observed when the probe was scanned in the [110], rather than the [100] direction in air at 25% relative humidity. In more humid conditions, the anisotropy was lost with NaCl, KCl and NaF, most probably due to lubrication by water which condensed beneath the tip. The anisotropy was explained by corrugation in charge-dipole interaction potentials between the ions at the sample surface and the probe. Due to much larger corrugation along the [100] direction where anions and cations alternate, greater horizontal force is required to scan the stylus in that direction. A good correlation was recognized between the ratio of potential gradients and the ratio of frictional coefficients in the two directions.