CRITICAL REVIEWS Friction and energy dissipation at the atomic scale: A review

Discussions of energy dissipation during friction processes have captured the attention of engineers and scientists for over 300 years. Why then do we know so little about either dissipation or friction processes? A simple answer is that we cannot see what is taking place at the interface during sliding. Recently, however, devices such as the atomic force microscope have been used to perform friction measurements, characterize contact conditions, and even describe the ‘‘worn surface.’’ Following these and other experimental developments, friction modeling at the atomic level—particularly molecular dynamics ~MD! simulations—has brought scientists a step closer to ‘‘seeing’’ what takes place during sliding contact. With these investigations have come some answers and new questions about the modes and mechanisms of energy dissipation at the sliding interface. This article will review recent theoretical and experimental studies of friction processes at the atomic scale. Theoretical treatments range from simple, analytical models of two-dimensional, coupled ball-spring systems at 0 K, to more complex MD simulations of three-dimensional arrays of hydrogenand hydrocarbon-terminated surfaces at finite temperatures. Results are presented for the simplest yet most practical cases of sliding contact: sliding without wear. Sliding without friction is seen in weakly interacting systems. Simple models can easily explain the energetics of such friction processes, but MD studies are needed to explore the dynamics ~excitation modes, energy pathways,...! of thermally excited atoms interacting in three-dimensional fields. These studies provide the first atomic-scale models for anisotropic friction and boundary lubrication. Friction forces at atomic interfaces must ultimately be measured at the macroscopic level; these measurements, which depend on the mechanical properties of the measuring system, are discussed. Two rather unique experimental studies of friction are also reviewed. The first employs a ‘‘surface force apparatus’’ to measure adhesion and friction between surfactant monolayers. The correlation of adhesion hysteresis and friction provides a new mechanism of friction; moreover, the interpretation for the effect—hysteresis from entanglement of the molecular chains during a phase transformation—implies that the dynamics are taking place at an accessible time scale ~seconds to minutes!. The second study extends the time domain at which friction can be measured to the nanosecond scale. A quartz-crystal oscillator is used to monitor the viscosity of monolayer liquids and solids against solid surfaces. Interfaces slip angstroms in nanoseconds. Modelers have suggested a variety of mechanisms for this atomic-scale friction process, from defect-mediated sliding to electron drag effects. The article ends by identifying the vast, barely charted time-space domain ~micro-to-pico time and length scales! in which experiments are needed to further understand the dynamic aspects of friction processes.