Tribology at the atomic scale with density functional theory

Understanding the quantum mechanical origins of friction forces has become increasingly important in the past decades with the advent of nanotechnology. At the nanometer scale, the universal Amontons-Coulomb laws cease to be valid and each interface requires individual scrutiny. Furthermore, measurements required to understand friction at the atomic scale are riddled with artificial factors such as the properties of the friction force microscope, effect of the environment, and the type of the substrate. It therefore proves difficult to isolate the actual behavior of interfaces from these effects. Electronic structure methods are an indispensable tool in understanding the details of interfaces, their interactions with lubricants, the environment and the support. In particular, density functional theory (DFT) has given large contributions to the field through accurate calculations of important properties such as the potential energy surfaces, shear strengths, adsorption of lubricant materials and the effect of the substrate. Although unable to tackle velocity- or temperature-dependent properties for which classical molecular dynamics is employed, DFT provides an affordable yet accurate means of understanding the quantum mechanical origins of the tribological behavior of interfaces in a parameter-free manner. This review attempts to give an overview of the ever-increasing literature on the use of DFT in the field of tribology. We start by summarizing the rich history of theoretical work on dry friction. We then identify the figures-of-merit which can be calculated using DFT. We follow by a summary of bulk interfaces and how to reduce friction via passivation and lubricants. The following section, namely friction involving two-dimensional materials is the focus of our review since these materials have gained increasing traction in the field thanks to the advanced manufacturing and manipulation techniques developed. Our review concludes with a brief touch on other interesting examples from DFT tribology literature such as rolling friction and the effect of photoexcitation in tribology.