The presence of roughness on a wide range of length scales has profound effects on contact and friction between experimental surfaces. The talk will present results for the load dependence of contact area, contact stiffness and friction for nonadhesive and adhesive surfaces with a self-affine fractal geometry. Simulations retain atomic structure for a few layers of atoms near the surface and use a Greens function method for an elastic continuum to determine the long-range elastic response. Under a broad range of conditions the area of intimate contact Ac between nonadhesive surfaces and the normal stiffness kN are both proportional to the applied load. These quantities are relatively insensitive to the atomic scale structure of surfaces, and even local plasticity. In contrast, atomic structure changes friction and tangential stiffness by orders of magnitude. For weak adhesion and strong roughness the linear relation between area, stiffness and load is retained, but the ratio of area to load increases. A simple scaling law predicts the change in ratio and the conditions for a finite pull-off force.

Area, stiffness, friction, and adhesion in multi-asperity contacts

244th National Meeting (2012)

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