Nano-tribological and wear behavior of boric acid solid lubricant

One uses atomic force microscopy to explore the surface morphology, frictio nal characteristics (ie.,friction mapping) and surface adhesive forces of b oric acid-treated aluminum surfaces. In earlier studies, boric acid was sho wn to be an effective solid lubricant providing 0.02 to 0.1 friction coeffi cients to the sliding surfaces of metallic and ceramic materials. In this s tudy, the authors mainly focused on the friction and wear behavior of cleav ed single crystals of boric acid. Tests were performed on uncoated and bori c acid-coated surfaces of aluminum substrates, as well as the bulk boric ac id material. The nano-scale wear of the boric acid crystals manifested itse lf in the displacement of atomic planes which, in turn, resulted in the for mation of thr wont area grown by a spiral-growth mechanism. In addition, ne w crystals in the vicinity of the sliding contact areas were formed. These crystals formed by a spiral dislocation mechanism. The nanoscale friction m easurements showed that the friction coefficient of a Si3N4 tip against the boric acid single crystal was in the range of 0.07 to 0.13, depending on t he sliding direction with respect to the crystallographic orientation. The friction coefficients of the boric acid-coated aluminum substrates varied b etween 0.11 and 0.19. While these values were consistent with earlier findi ngs and further demonstrated the lubricity of boric acid, they were higher than the previous measurements (i.e., 0.011-0.1) obtained on a pin-on-disk machine using steel or ceramic pins. An explanation is provided for this ph enomenon.