Tribology is the science and engineering of interacting surfaces under load and shear, it includes the study of adhesion, friction, lubrication, and wear. In the last decades, tribology has become a highly interdisciplinary field, which attracts attention from physicists, chemists, engineers, materials scientists, and biologists. The renewed interest in tribology has been fueled by several factors, among which the advent of new experimental and computational techniques that allow for fundamental studies on the origin of friction, and an increased attention to the enormous adverse effects of friction on global energy loss and the environment.
We apply first-principles-based simulations to study key microscopic mechanisms that affect the macroscopic frictional behavior and help the design of improved materials to reduce friction.
Fundamental understanding on frictional phenomena
Relevant information on the frictional behavior of an interface can be obtained by the analysis of the potential energy surface (PES) that describes the surface adhesion during sliding [12, 22, 27]. For example, we predicted a pressure-induced friction collapse in systems with an anticorrugating PES . And identified the collective atomistic mechanisms that trig the onset of frictional slip, which we described as a nucleation process . We also showed that shrinking the size of a body in contact with a substrate increases its commensurability and resistance to sliding .
The pioneering application of ab initio molecular dynamics to simulate sliding interfaces allowed us to highlight the effects of mechanical stresses in promoting chemical reactions and the tribo-catalytic conversion of molecules in thin films [29, 59].
We unraveled the fundamental nature of adhesive and friction forces by correlating them with the electronic charge displacements at the interface of two mating surfaces .
Lubricant materials and tribochemistry
The function of many lubricant materials is to reduce interfacial adhesion. In solid lubrication this is achieved by covering the surfaces by inert layers or coatings, while in boundary lubrication slippery “tribofilms” are formed in situ through tribochechemical reactions, i. e. reactions assisted by mechanical stresses, involving lubricant additives.
Simulations based on quantum mechanics offer the possibility to increase our basic knowledge in mechanochemistry and lubricant mechanisms. We applied them to investigate the tribochemistry of:
i) 2D materials, such as graphene [26, 27, 33, 39, 42, 56], MoS2 [30, 33, 41, 66], Phosphorene ;
ii) hard coatings, such as diamond and DLC [17, 22, 24, 29, 36];
iii) extreme pressure additives, such as organophosphorus complexes, MoDTC and ZDDP [43, 45, 58, 63, 65].