Role of Surface Roughness in Tribology: From Atomic to Macroscopic Scale

The practical importance of friction cannot be underestimated: from the creation of fires by rubbing sticks together, to the current efforts to build nano devices, friction has played an important role in the whole history of technology of mankind. Friction is a complex multiscale phenomenon that depends both on the atomic interactions inside the contacts, on the macroscopic elastic and plastic behavior of the solids in contact, and on the unavoidable, stochastic roughness characterizing real surfaces. Tribology, the science of friction, has developed much in recent years, but many questions are still open. This thesis addresses the role of surface roughness in tribology from atomic to macroscopic scale with the aid of numerical calculations. We have studied several features of the contact between rough surfaces, such as the area of contact, the interfacial separation, the adhesive and frictional properties, and leakage of sealed fluids. We have also studied the wetting behavior of nanodroplets on randomly rough surfaces. In order to study contact mechanics accurately it is necessary to consider an elastic solid whose thickness is comparable to the largest wavelength of the surface roughness. In principle, one should simulate a system with a very large amount of atoms, even for a relatively small system. A fully atomistic model is impracticable, and we have developed a multiscale molecular dynamics approach: the atomistic description is employed where necessary, at the nanocontacts and on the surfaces, while a coarse-grained picture allows us to simulate the correct long-range elastic response. The area of contact between rough surfaces and the interfacial separation, with and without adhesion, have been analyzed. The real area of contact plays a crucial role in the friction, adhesion and wear. The interfacial separation is relevant to capillarity, leak-rate of seals and optical interference. Numerical simulations showed that at small squeezing pressure in the absence of adhesion, the area of contact depends linearly on the squeezing pressure, and the interfacial separation depends logarithmically on squeezing pressure. The sliding of elastic solids in contact with both flat and the rough surfaces, has been studied. We found a strong dependence of sliding friction on the elastic modulus of solids, and this is one of the main origins of the instability while sliding. For elastically hard solids with planar surfaces with incommensurate surface structures, extremely low friction (superlubricity) has been observed, which increases very abruptly as the elastic modulus of the solids decreases. Even a relatively small surface roughness or a low concentration of adsorbates can eliminate the superlubricity. The wetting behavior of nanodroplets on rough hydrophilic and hydrophobic surfaces has been studied. The problem is relevant for the fields of nano-electro-mechanics and of nano fluid dynamics, both of which are of great current interest. No contact angle hysteresis has been detected for nano-droplets on hydrophobic surfaces due to thermal fluctuations. The contact angle increases with the root-mean-square roughness of the surface and is almost independent of the fractal dimension of the surface. We have found that thermal fluctuation are very important at the nanoscale. On hydrophilic surfaces, however, thermal fluctuations do not remove the hysteresis of the contact angle