Two types of interactions can occur during the sliding of two surfaces with respect to each other: the microscopic interaction where primarily plastic deformation and displacement of the interlocking surface asperities are required, and the more macroscopic interaction where the asperities of the harder material either plow grooves in the surface of the softer one via plastic deformation or result in fracture, tearing or fragmentation, Figure 1. Plowing of one or both surfaces can also occur by wear particles trapped between them, and truly macroscopic.
plowing of the softer material by the harder, with the dimensions of the plowed groove being orders of magnitude greater than those of the asperities on either surface. Plowing deals with relatively large-volume deformations and small strains, whereas the shearing mechanism and local asperity interactions involve very thin, interfacial regions (a fraction of a nanometer thick) and large strains. During any relative motion, adhesion and asperity interactions are always present. The plowing contribution may or may not be significant; its magnitude depends on the surface roughnesses and relative hardnesses of the two surfaces, and on the size, shape and hardness of any wear debris and reaction products trapped between them. If one of the sliding surfaces is harder than the other, the asperities of the harder surface may penetrate and plow into the softer surface and produce grooves if shear strength is exceeded. Plowing into the softer surface may also occur as a result of impacted wear particles. In addition, interaction of two rather rough surfaces may result in mechanical interlocking on a micro- or macroscale. During sliding, interlocking would result in plowing of one of the surfaces. Because of the plowing displacement, a certain lateral (friction) force is required to maintain motion. Plowing not only increases the friction force, it creates wear particles, which in turn increase subsequent friction and wear. Before the onset of sliding between two surfaces, µd largely controls the coefficient of static friction
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