The field of materials tribology has entered a phase of instrumentation and measurement that involves accessing and following the detailed chemical, structural, and physical interactions that govern friction and wear. Fundamental tribological research involves the development of new experimental methods capable of monitoring phenomena that occur within the life of a sliding contact. Measuring friction phenomena while the process is ongoing is a major improvement over earlier techniques that required the surfaces to be separated and analyzed, thereby interrupting the friction-causing event and modifying surface conditions. In the past, MRS Bulletin has highlighted how in situ approaches can greatly enhance our understanding of materials structure, processing, and performance. This issue highlights in situ approaches as applied to materials tribology, namely, the study of contacting surfaces and interfaces in relative motion.

The primary role of a lubricant is to control the friction and wear of rubbing surfaces to optimize the operation of a component by forming an interfacial film separating the surfaces. Lubrication research seeks to develop new lubricant formulations and to optimize component life and performance. To do this, we must understand the mechanisms of film formation and film properties and the way these relate to operating conditions. In many engineering components, the lubricant film is subjected to severe mechanical and thermal stresses as it passes through the loaded zone. These severe conditions can result in molecular alignment or conformational change and the formation of new chemical species, which will impact the lubrication performance of the fluid. The lubricant response within the contact is often transient and thus has proved difficult to study by conventional surface analytical methods. One alternative is to replace one of the surfaces by a transparent window and use molecular microspectroscopy (infrared or Raman) to analyze the film within the contact zone formed during rubbing. This article reviews the development and application of in-contact molecular spectroscopy for the study of lubricant properties within the rubbing interface for both conventional and biolubrication systems. This technique has been used to study molecular conformation, chemical composition, and pressure distribution in the high-pressure region of the contact zone. However, challenges remain, including detecting very thin films, obtaining depth profile information, and applying these methods more generally to biotribology.

Directly seeing into a moving contact is a powerful approach to understanding how solid lubricants develop low-friction, long-lived interfaces. In this article, we present optical microscopy and spectroscopy approaches that can be integrated with friction monitoring instrumentation to provide real-time, in situ evaluation of solid lubrication phenomena. Importantly, these tools allow direct correlation of common tribological events (such as variations in friction and wear) with the responsible sliding-induced mechanical and chemical phenomena. We demonstrate the utility of in situ approaches with applications to a variety of thin-film solid lubricants.

For many years, a fundamental problem in contact mechanics, both tribology and indentation problems, has been the inability to see what is taking place—the buried-interface problem. Over the past few years, there have been developments whereby it has become possible to perform contact mechanics experiments in situ within a transmission electron microscope. These new experiments have been enabled by both the miniaturization of sensors and actuators and improvements in their mechanical stability and force sensitivity. New information is now becoming available about the nanoscale processes of sliding, wear, and tribochemical reactions, as well as microstructural evolution during nanoindentation such as dislocation bursts and phase transformations. This article provides an overview of some of these developments, in terms of both the advances in technical instrumentation and some of the novel scientific insights.

Fundamental processes of wear include the rupture of single chemical bonds and the displacement of atoms or small clusters by mechanical action. Experimental studies of such processes have become feasible with the development of scanning probe microscopy. The small volume affected in these experiments overlaps with the size scale of large atomistic simulations, making a direct comparison possible. The complexity of real-world wear processes is reduced in most nanometer-scale experiments, for example, by probing surfaces of single crystals or by establishing and maintaining carefully controlled environments, including ultraclean conditions. The studies address the onset and topography of wear, the formation of debris structures, the interplay of mechanical and chemical action, the role of ultrathin films, the role of crystal defects in wear processes, and temporal and thermal effects.

The dynamics of frictional motion have been studied for hundreds of years, yet many aspects of these important processes are not understood. First described by Coulomb and Amontons as the transition from static to dynamic friction, the onset of frictional motion is central to fields as diverse as physics, tribology, mechanics of earthquakes, and fracture. We review recent studies in which fast (real-time) visualization of the true contact area along a rough spatially extended interface separating two blocks of like material has revealed the detailed dynamics of how this transition takes place. The onset of motion is preceded by a discrete sequence of rapid cracklike precursors, which are initiated at shear levels that are well below the threshold for static friction. These precursors systematically increase in spatial extent with the applied shear force and leave in their wake a significant redistribution of the true contact area. Their cumulative effect is such that, just prior to overall sliding of the blocks, a highly inhomogeneous contact profile is established along the interface. At the transition to overall motion, these precursor cracks trigger both slow propagation modes and modes that travel faster than the shear wave speed. Overall frictional motion takes place only when either the slow propagation modes or additional shear cracks excited by these slow modes traverse the entire interface. Surprisingly, in the resulting stick–slip motion, the surface contact profile retains the profile built up prior to the first slipping event. These results suggest a fracture-based mechanism for stick–slip motion that is qualitatively different from other descriptions.