Kelvin probe microscopy measurements of surface potential change under wear at low loads

Kelvin probe microscopy has been used to detect wear precursors at ultralow loads. Samples studied include single crystal silicon (1 0 0), single crystal silicon (1 0 0) lubricated with fully bonded Z-DOL (a perfluoropolyether), gold, aluminum and alumina. The effect of load and number of cycles on surface potential change under ultralow loads has been investigated. Influence of the scan parameters and the reproducibility of the experimental results have been studied. It is found that the measured change in surface potential is strongly affected by the scan parameters as well as the shape of the probe. Negative wear depth on single crystal silicon (1 0 0), and single crystal silicon (1 0 0) lubricated with fully bonded Z-DOL during early stages of sliding has been observed. (The scratched area was raised rather than depressed as compared to the unworn surrounding region.) Chemical analysis of unworn and worn regions of silicon and silicon lubricated with fully bonded Z-DOL was performed to reveal possible mechanisms of changes in surface potential. It is believed that the removal of a thin contaminant layer, natural oxide layer or lubricant during a few wear cycles gives rise to the initial change in surface potential. It is further believed that structural changes which precede generation of wear debris and/or measurable wear scars occur under ultralow loads in the top few nanometers of the sample, which are primarily responsible for the measured changes in surface potential.