Micro-Scale Friction and Wear at Orthorhombic BaSO₄(0 0 1) Surface

Abstract

To study relationship between friction and plastic deformation modes of single crystals, wear patterns were analyzed for BaSO 4 , an orthorhombic crystal having various cleavage faces and slip systems. Upon scanning a sapphire stylus along the b-axis at (0 0 1) face, formation of triangular deformation patterns extending outside the wear track were observed with optical microscopy. Atomic force microscopic analysis revealed that the triangular areas were elevated, and the two sides were formed by {2 1 0} cleavage steps, starting within the track, and continued by {1 1 0} slip steps outside the track. Upon scanning with heavier normal loads, lots of {0 1 1} slip steps were formed inside the triangles, presumably due to horizontal compression given by the stylus to the triangular areas isolated from the rest of the surface. Mechanism of the wear pattern formation was proposed. When the stylus was scanned along the a-axis, the wear tracks were segmented into short strips by {1 0 1} slips. Under heavier normal loads, the segment structures were partly destroyed by the stylus, presumably by (0 0 1) slip and cleavage, producing debris on both sides of the track. The changes in deformation patterns under heavier normal loads caused steeper increase in friction curves measured in the two directions. Different deformation modes were activated depending upon the scan directions, normal loads and locations relative to the stylus.