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troduction Tribology underlies the performance, safety, and reliability of nearly every mechanical system on land, at sea, and in space. The understanding and harnessing of tribological phenomena holds promise to address the 11% of annual energy consumption in transportation, utilities, and industrial applications lost due to friction and wear.1 This is in addition to the opportunities to save billions of dollars annually lost to downtime of industrial equipment,2 to eliminate billions of tons of CO2 emissions annually,3 and to significantly reduce human suffering caused by the failure of medical devices such as implants.4 Tribology depends on the physical, chemical, electrical, and system properties of the sliding materials, such as mechanical stiffness and strength, thermal and electrical conductance, hydrodynamic behavior, surface topography, material compatibility, temperature, sliding speed, and gas/fluid environment; in biological settings, a host of additional properties come in to play. Thus, the key parameters of interest such as the friction coefficient and wear rate are not material parameters, but rather, are system properties that vary with operational conditions. For newly engineered systems, or after a

materials modification to an existing system, friction and wear performance are not currently predictable. Furthermore, the lack of direct observation of the sliding surfaces is a central obstacle to predicting performance and preventing failures. In situ techniques in tribology reveal the buried interfaces, as discussed in the 2008 issue of MRS Bulletin on the topic.5 The ensuing decade has brought significant advances both at the larger scales (e.g., References 6–17) and at the nanoscale (e.g., References 18–22); the latter is the focus of the present article. In situ nanotribology decreases the size scale of experiments to improve resolution and control over sliding conditions, with the goal of identifying and describing the underlying physical mechanisms (Figure 1). The fundamental understanding that is achieved from in situ nanoscale investigations can be generalized and harnessed at all length scales to improve tribological performance. Further, in situ experiments can be directly matched with atomistic simulations, enabling atomicscale understanding of results. By establishing and quantifying the physical relationships that relate structure, processing, and properties, the goal is to enable the rational design of components, devices, and systems to improve tribological performance.

Tevis D.B. Jacobs, Department of Mechanical Engineering and Materials Science, University of Pittsburgh, USA; [email protected] Christian Greiner, Karlsruhe Institute of Technology (KIT), Institute for Applied Materials, Germany; [email protected] Kathryn J. Wahl, Chemistry Division, US Naval Research Laboratory, USA; [email protected] Robert W. Carpick, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, USA; [email protected] doi:10.1557/mrs.2019.122

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• VOLUME 44 •