Anisotropy in wear processes measured by scanning probe microscopy

Wear of smooth amorphous carbon overcoats was performed in continuous sliding contact in the range of 10⁴–10⁷ cycles by a diamond or diamond-like carbon counterbody. The overcoats on superpolished hard disks were examined using friction and topographic scanning probe microscopy. An optimal Fourier (Wiener) filter was designed which preferentially filtered noise out of images of the smooth films. Second derivatives of the z-height were calculated parallel and perpendicular to the wear direction. The crosscorrelation of a friction force image and the corresponding z-height image shows that the correlation length is also extended in the wear direction. The local friction is independent of the z-height before wear, but increases with z-height upon wear. A model for the z-height and friction force evolution in contact sliding is discussed and compared with data.