Recent Surface Forces Apparatus (SFA) experiments have shed new light on the roles of surface roughness and surface deformations, both at the molecular and microscopic scales, in determining adhesion and friction. Depending on the roughness, but also on other factors such as the stiffness and viscoelasticity of surfaces, these effects can cause the effective adhesion force to be orders of magnitude lower or higher than the thermodynamic (equilibrium) value, which in turn determines the frictional behaviour. The interactions of 'ideal', i.e., molecularly smooth, undamaged, surfaces are now quite well understood, both theoretically and experimentally. However, this is not the case for rough surfaces, which are difficult to define and characterize, and for which rigorous theories and reliable experimental data are lacking. Yet, most real or engineering' surfaces are rough on the nanometer to micrometer scale. This significantly affects their short-range interactions, their tribological behavior (adhesion, friction and lubrication) and their failure mechanisms (fracture and wear). Even when great care is taken to ensure smoothness, it is generally very difficult to produce a surface with a roughness of less than 25A except over small areas (usually for research purposes) and it is known that even a few nm of roughness can reduce the adhesion force between hard surfaces by more than an order of magnitude, and totally change their tribological behavior. To date there is no general theory for the interactions of rough surfaces nor any systematic experiments that identify the main trends; for example, how does the adhesion force or friction coefficient of a particular surface depend on its roughness? Indeed, one of the major problems is that roughness itself is not well defined. Simply quoting the RMS roughness of a surface is too simplistic, whereas describing it rigorously in terms of its Fourier components is too impractical. This whole issue has recently come to be appreciated as central to many tribological questions. We have recently performed measurements, using the SFA and scanning lateral probe techniques, on various surfaces having different roughness in an attempt to uncover some general trends and principles that describe the dependence of adhesion, friction and wear on roughness. We find, for example, that there is a generic, approximately exponential, relationship for the repulsive force between two rough surfaces, and that for non-adhering surfaces the friction force is proportional to the load (not the real contact area) at the molecular level. These and other, such as the dynamic, aspects of the tribological interactions between rough surfaces are described and discussed.
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