Insights into tribology from in situ nanoscale experiments Tevis D.B. Jacobs , Christian Greiner , Kathryn J. Wahl , and Robert W. Carpick 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. Specif cally, 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. Introduction materials modif cation to an existing system, friction and wear Tribology underlies the performance, safety, and reliability performance are not currently predictable. Furthermore, the of nearly every mechanical system on land, at sea, and in lack of direct observation of the sliding surfaces is a central space. The understanding and harnessing of tribological phe- obstacle to predicting performance and preventing failures. nomena holds promise to address the 11% of annual energy In situ techniques in tribology reveal the buried interfaces, as consumption in transportation, utilities, and industrial appli- discussed in the 2008 issue of MRS Bulletin on the topic. 5 The cations lost due to friction and wear. 1 This is in addition to ensuing decade has brought signif cant advances both at the the opportunities to save billions of dollars annually lost to larger scales (e.g., References 6 – 17 ) and at the nanoscale (e.g., downtime of industrial equipment, 2 to eliminate billions of References 18 – 22 ); the latter is the focus of the present article. 3 tons of CO2 emissions annually, and to signif cantly reduce In situ nanotribology decreases the size scale of experiments human suffering caused by the failure of medical devices to improve resolution and control over sliding conditions, such as implants. 4 with the goal of identifying and describing the underlying Tribology depends on the physical, chemical, electrical, physical mechanisms ( Figure 1 ). The fundamental under- and system properties of the sliding materials, such as mechani- standing that is achieved from in situ nanoscale investigations cal stiffness and strength, thermal and electrical conductance, can be generalized and harnessed at all length scales to improve hydrodynamic behavior, surface topography, material com- tribological performance. Further, in situ experiments can be patibility, temperature, sliding speed, and gas/f uid environ- directly matched with atomistic simulations, enabling atomic- ment; in biological settings, a host of additional properties scale understanding of results. By establishing and quantify- come in to play. Thus, the key parameters of interest such as ing the physical relationships that relate structure, processing, the friction coeff cient and wear rate are not material param- and properties, the goal is to enable the rational design of eters, but rather, are system properties that vary with opera- components, devices, and systems to improve tribological tional conditions. For newly engineered systems, or after a 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 Downloaded478 MRSfrom BULLETIN https://www.cambridge.org/core • VOLUME 44 • JUNE 2019. University • www.mrs.org/bulletin of Pittsburgh, on 11 Jun 2019 at 14:42:52, subject to the Cambridge Core terms of use, © available 2019 Materials at Research Society https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2019.122 INSIGHTS INTO TRIBOLOGY FROM IN SITU NANOSCALE EXPERIMENTS Figure 1. While tribological phenomena are complex and multiscale, fundamental understanding can be gained by reducing the system size to improve the control and measurement of local conditions. In situ nanotribology reveals the fundamental physical mechanisms and enables their systematic interrogation with the goal of informing design and optimization of larger-scale systems. Instrumentation for in situ nanotribology and aluminum; the in situ capability allowed Bowden and In situ nanotribology experiments include the use of micros- colleagues to observe that plastic deformation was negligible copy or an analysis technique to characterize interfacial pro- below a stress approaching the ideal strength of the metals cesses during sliding. One primary approach is to use specialty tested; they then applied the instrumentation in a transmission specimen holders to directly observe the tribological testing electron microscope (TEM).26 Kato and co-workers27 conducted of nanoscale bodies with an electron microscope.23 This enables in situ SEM-based wear studies of steel in 1988, directly Ångström-scale resolution of morphology and surface topog- observing a transition from plowing to wedge-forming to cut- raphy. Simultaneously, the crystallographic structure, com- ting as the degree of penetration increased. Shortly thereafter, position, and bonding chemistry can be characterized using Spence and co-workers developed a scanning tunneling analytical techniques such as nanobeam diffraction, energy- microscope operating in situ inside a TEM, observing nanoscale dispersive x-ray spectroscopy (EDS), and electron energy- compression of surfaces by the tip.28,29 loss spectroscopy (EELS). Another primary approach is to use More recently, both commercial and custom-built TEM- atomic force microscopy (AFM) to perform both sliding and based instruments have revealed a wide range of phenomena characterization. AFM enables precise control over the load- for several different material classes. For instance, in situ TEM ing and sliding conditions, with simultaneous characterization investigation of contact between noble metals demonstrated of the surface’s morphology and properties.24 The environment “cold welding” of asperities upon contact,30,31 followed by can be varied widely, covering vacuum, ambient, controlled liquid-like behavior during separation (discussed in more detail atmospheres, and liquid environments, including conditions later). In the case of two-dimensional (layered) materials that are biologically compatible. (e.g., graphite, MoS2), which are important solid lubricants, in Another type of investigation (termed quasi-in situ) inter- situ TEM nanotribology has revealed rolling,32 exfoliation,33,34 rupts sliding to analyze surfaces. This includes sliding an and material transfer35 during sliding. Another transformative AFM probe for some distance before removing it for high- aspect of in situ electron microscopy is the ability to reveal resolution imaging of the tip or sample, or sliding a pin on chemistry and bonding of the materials at the interface. For a disk followed by the sectioning of surfaces for electron example, EELS analysis revealed changes in the hybridization microscopy analysis. Throughout this article, we discuss these state of carbon-based materials during in situ contact. Merkle quasi-in situ investigations as leading indicators, setting up and co-workers36 demonstrated a sliding-induced increase in the key scientifc questions that will be answered by true the sp2-to-sp3 ratio of diamond-like carbon; conversely, Janei in situ experiments. and co-workers37 demonstrated a decrease in sp2 content of soot particles after in situ compression. Revealing the buried interface: Key results and Other recent work has leveraged the ability to work in future directions biocompatible fuids, where in situ work has demonstrated Observations of novel phenomena links between tribological behavior and biological response.38 In situ tribology experiments were frst demonstrated in the This includes the effects of shear forces on the growth and 1960s by the famous tribologist Bowden25 who developed proliferation of cells, as elucidated by approaches such as the an in situ scanning electron microscope (SEM)-based inden- direct observation of contact between a soft hydrogel cap and tation apparatus and studied small contacts of gold, copper, a layer of corneal epithelial cells imaged via fuorescence Downloaded from https://www.cambridge.org/core. University of Pittsburgh, on 11 Jun 2019 at 14:42:52, subject to theMRS Cambridge BULLETIN Core • VOLUME terms of 44 use, • JUNE available 2019 • at www.mrs.org/bulletin 479 https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2019.122 INSIGHTS INTO