reducing energy consumption is a seemingly unlikely candidate: tribology. Tribology is the study of interacting surfaces in relative motion, including friction, wear and lubrication. In the transportation sector, a third of the energy consumed is lost by overcoming friction. [2] As far back as 1977, it was estimated that 11% of the energy used by the transportation, the industrial and the utilities sectors could be saved by new developments in tribology. [3] Although the term "tribology" was only coined in the 1960s, [4] tribological technologies date back to antiquity. Starting a fire by rubbing two pieces of wood together was only possible due to frictional heating. Transportation of the massive stone building blocks of the pyramids required lubricated contacts. [5] Since ancient times, scientific interest in friction and wear has come in cycles, with luminaries like Leonardo da Vinci contributing to the field. [6] In the 1960s tribology rose again to the forefront of government-funded research. Due to limitations in the instrumentation available at the time, the broad interest in tribology then gradually waned. The invention of the atomic force microscope in 1986 [7] and its application to friction in 1987 [8] may be viewed as the origin of a recent renaissance in tribology. [9,10] State of the Art in Materials TribologyGiven tribology's long history and tremendous societal impact, it is somewhat surprising how little mechanistic understanding is available in this field. It has long been recognized, for example, that the complex nature of tribological processes makes it extremely challenging to link nanoscale phenomena to the macroscopic world of gears and engines. [9] On the meso and macro length scales, the elementary mechanisms governing friction and wear, especially for metallic materials, therefore remain elusive. A thorough understanding of the microstructure-properties relation, the key concept of materials science, has not yet been established. Part of the reason is that the microstructure of the material under the contact is highly dynamic [11,12] and its evolution can usually not be observed in situ. This current lack of knowledge makes a strategic tailoring of a material's frictional properties, e.g., during the manufacturing process, very difficult to impossible. Many Tribological contacts consume a significant amount of the world's primary energy due to friction and wear in different products from nanoelectromechanical systems to bearings, gears, and engines. The energy is largely dissipated in the material underneath the two surfaces sliding against each other. This subsurface material is thereby exposed to extreme amounts of shear deformation and often forms layered subsurface microstructures with reduced grain size. Herein, the elementary mechanisms for the formation of subsurface microstructures are elucidated by systematic model experiments and discrete dislocation dynamics simulations in dry frictional contacts. The simulations show how pre-existing dislocations transform into prismatic dis...

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Dislocation-grain boundary interaction plays a key role in the plasticity of polycrystalline materials. Capturing the effect of discrete dislocations interacting with a grain boundary in continuum models is not yet achieved. To date several approaches exist, but they have shortcomings in capturing the influence of dislocation-dislocation interaction across a grain boundary and the parameters which control grain boundary yield are phenomenologically motivated. In this work we show that grain boundary yielding is not inherently connected to physical dislocation transmission and that a realistic model needs to incorporate the interaction of dislocations across grain boundaries to capture the true strain distribution in the individual grains. By comparing discrete dislocation dynamics simulations of a single crystal with an artificial grain boundary to continuum dislocation dynamics results, a clear influence on the strain profile from the elastic interaction of dislocations belonging to different grains is shown. Our results demonstrate that continuum models like gradient plasticity need to extend their grain boundary modeling to incorporate dislocation interactions because a single yield criterion is not sufficient