AFM Imaging, Roughness Analysis and Contact Mechanics of Magnetic Tape and Head Surfaces

Roughness measurements of a magnetic tape, a biaxially oriented poly (ethylene terephthalate) (PET) substrate, a tape head and a rigid-disk slider were made by an atomic force microscope (AFM) and a non-contact optical profiler (NOP). The lateral resolution of the surface topographs ranges from 1 μm (for NOP) down to 1 nm (for AFM). The AFM images show submicron features of the surface that are characteristic of the manufacturing processes. Some of the statistical roughness parameters conventionally used in theories of contact mechanics showed strong dependence on instrument resolution. This suggests that, firstly, roughness measured by NOP at resolutions larger than 1 μm cannot be used to study tribology at sub-micrometer scales and, secondly, a scale-independent characterization by fractal geometry is necessary. Fractal analysis of the tape surface reveals two regimes of roughness demarcated by a scale of 0.1 μm corresponding to the size of magnetic particles. The fractal behavior explains the dependence of the rms height, slope and curvature on the instrument resolution. The predictions of real area of contact suggest that nanometer-scale asperities tend to deform plastically whereas micrometer-scale ones deform elastically.