Tribology—the study of contacting, sliding surfaces—seeks to explain the fundamental mechanisms underlying friction, adhesion, lubrication, and wear, and to apply this knowledge to technologies ranging from transportation and manufacturing to biomedicine and energy. Investigating the contact and sliding of materials is complicated by the fact that the interface is buried from view, inaccessible to conventional experimental tools. In situ investigations are thus critical in visualizing and identifying the underlying physical processes. This article presents key recent advances in the understanding of tribological phenomena made possible by in situ experiments at the nanoscale. Specifically, progress in three key areas is highlighted: (1) direct observation of physical processes in the sliding contact; (2) quantitative analysis of the synergistic action of sliding and chemical reactions (known as tribochemistry) that drives material removal; and (3) understanding the surface and subsurface deformations occurring during sliding of metals. The article also outlines emerging areas where in situ nanoscale investigations can answer critical tribological questions in the future.

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.

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.

Spider silk is a material with unique mechanical properties under tension. In this study, we explore the anisotropic mechanical properties of spider silk using instrumented indentation. Both quasistatic indentation and dynamic stiffness imaging techniques were used to measure the mechanical properties in transverse and longitudinal sections of silk fibers. Quasistatic indentation yielded moduli of 10 ± 2 GPa in transverse sections and moduli of 6.4 ± 0.5 GPa in longitudinal sections, demonstrating mechanical anisotropy in the fiber. This result was supported by dynamic stiffness imaging, which also showed the average reduced modulus measured in the transverse section to be slightly higher than that of the longitudinal section. Stiffness imaging further revealed an oriented microstructure in the fiber, showing microfibrils aligned with the drawing axis of the fiber. No spatial distribution of modulus across the silk sections was observed by either quasistatic or stiffness imaging mechanics.

Six nanocomposite coatings of yttria-stabilized zirconia (YSZ), Au, diamond like carbon and MoS2 have been studied by in situ tribometry. The coatings all had a nominal MoS2 content of between 14 and 18 mol%, while the concentrations of the other components were varied. Steady state friction coefficients in both dry (0.04–0.05) and wet sliding conditions (0.06–0.08) were similar for all coatings. However, by in situ tribometry, wear mechanisms were found to differ depending on coating composition and test environment humidity. Friction spiking in high YSZ coatings was identified with local plowing and scoring of the track followed by an extrusion of transfer film material. Trends in coating hardness and modulus are correlated to composition and coating wear processes.

Instrumented indentation and micro-Raman spectroscopy were used to investigate the mechanical and structural properties of Bombyx mori silk films. Twelve different films were prepared from B. mori silk fibroin protein using a variety of post-deposition processing treatments (e.g. soaking in methanol, soaking in water, stretching, and/or enzymatic etching). The results show that different treatments lead to changes in both the conformation of the silk fibroin protein and the mechanical properties of the films.

Instrumented indentation and micro-Raman spectroscopy were used to investigate the mechanical and structural properties of Bombyx mori silk films. Twelve different films were prepared from B. mori silk fibroin protein using a variety of post-deposition processing treatments (e.g. soaking in methanol, soaking in water, stretching, and/or enzymatic etching). The results show that different treatments lead to changes in both the conformation of the silk fibroin protein and the mechanical properties of the films.