Multiscale simulations of chemical reactions and friction at material interfaces

Author / Creator

Li, Zhuohan, 1993- author

Available as

Online

Physical

Summary

In this work, we have studied the coupling of chemical reactions and mechanical response(friction) at material interfaces. Specifically, our focus is on the effect of interfacial chemical bonding o...

In this work, we have studied the coupling of chemical reactions and mechanical response(friction) at material interfaces. Specifically, our focus is on the effect of interfacial chemical bonding on the static and kinetic friction. It is believed that the interfacial bonds can enhance the contact quality of the material interfaces, resulting in a wide range of unusual frictional response. However, there are still many scientific problems that are widely debated, since all the desired information is buried at the interfaces and is not easy to be retrieved. Here, we used multiple simulation techniques to investigate the physico-chemical processes at the static and the sliding material interfaces in multi length- and time-scale. Our theoretical studies provide new physical insights of how the atomistic chemical reactions can affect the frictional response of the material interfaces at much larger length scale. We have built multi-scale contact ageing model to investigate the effects of interfacial chemical bonding on the contact time-dependent static friction at various experimental conditions and contact geometries. At nanoscale single-asperity level, our multi-physics kinetic Monte Carlo (kMC) model predicted that ageing is linearly dependent on the normal loads in a low-load regime, but the nonlinearity becomes apparent at higher loads. This is because the number of available reaction sites is proportional to contact area, which increases sublinearly with contact load according to the contact mechanics. We also discovered a nonmonotonic temperature dependence of ageing with a peak near room temperature. This is due to the two competing thermal effects on the reaction kinetics of bond formation and breaking. The former is enhanced, and the latter is decreased by increasing the temperature, where both of them are thermally activated processes. At larger length scale, we investigated the effect of surface roughness on the contact ageing behavior by simulating the interfacial chemical bonding at randomly rough interfaces. We discover that surface roughness affects the ageing behavior primarily by modifying the real contact area and the local contact pressure, whereas the effect of contact morphology is relatively small. We also thoroughly investigated the features of the energy barrier distributions that results in linear time dependence of the contact ageing at short time scale (<1 s). We demonstrate that ageing at these timescales requires the existence of a particular range of reaction energy barriers for interfacial bonding. Specifically, linear ageing behavior consistent with experiments requires a narrow peak close to the upper bound of this range of barriers. We have built a kinetic multi-bond model for investigating the dynamical behavior of sliding interface with interfacial chemical bond formation and breaking. We observe a logarithmic trend of decreasing friction with sliding velocity (i.e., velocity-weakening) at low velocities and a transition to increasing friction with velocity at higher velocities (i.e., velocity-strengthening). We propose a physically based kinetic model for the nanoscale memory effect, the "activationpassivation loop" model, which accounts for the activation and passivation of chemical reaction sites and the formation of new chemical bonds from dangling bonds during sliding. Our results provide one possible physical mechanism for the memory-effect at nanoscale chemically active sliding interface. We have investigated the atomistic origin of the mechanochemical response, i.e., change in the reaction energy landscape under the applied stress, of material interfaces using density functional theory (DFT) calculations. Our results show that the mechanochemical response can be decomposed into the contribution from the interface itself (deformation of interfacial bonds) and a contribution from the underlying solid. The relative contributions depend on the stiffness of these regions and the contact geometry, which affects the stress distribution within the bulk region. We demonstrate that, contrary to what is commonly assumed, the contribution to the activation volume from the elastic deformation of the surrounding bulk is significant and, in some case, may be dominant. We also show that the activation volume and the mechanochemical response of interfaces should be finite due to the effects on the stiffness and stress distribution within the nearsurface bulk region

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