Stick-slip oscillations: Dynamics of friction and surface roughness

While its classical model is relatively simple, friction actually depends o n both the interface properties of interacting surfaces and on the dynamics of the system containing them. At a microscopic level, the true contact ar ea changes as the surfaces move relative to each other. Thus at a macroscop ic level, total friction and normal forces are time-dependent phenomena. Th is paper introduces a more detailed friction model, one that explicitly con siders deformation of and adhesion between surface asperities. Using probab ilistic surface models for two nominally flat surfaces, the stick-slip mode l sums adhesive and deformative forces over all asperities. Two features di stinguish this approach from more traditional analyses: (i) Roughness distr ibutions of the two interacting surfaces are considered to be independent, (ii) Intersurface contacts occur at both asperity peaks, as in previous mod els, and on their slopes. Slope contacts, in particular, are important beca use these oblique interactions produce motion normal to the plane of slidin g. Building the model begins by analyzing local friction forces as composit es of resistance to elastic deformation and shear resistance arising from a dhesion between asperity surfaces. By extending the expressions obtained fo r normal and tangential friction forces over the macroscopic surfaces, the model then describes the stick-slip behavior frequently observed in dynamic systems and permits simulating a rigid body on a moving platform. Numerica l results for several surface and system parameters illustrate both time-de pendent and time-averaged frictional forces. These analyses also show that, although total averaged friction remains constant with respect to sliding velocity for the cases considered, the relatively small deformation compone nt exhibits resonancelike behavior at certain speeds. Stick-slip occurs onl y within a narrow range around these critical speeds of a system. External damping can prevent stick-slip motion, and both deformative and adhesive fr ictional forces must be present for it to occur at all. (C) 1999 Acoustical Society of America. [S0001-4966(99)03701-7].