Solids Under Extreme Shear: Friction‐Mediated Subsurface Structural Transformations

Abstract

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...

Evidence of Inverse Hall-Petch Behavior and Low Friction and Wear in High Entropy Alloys

“…. It is widely reported that deformation due to sliding contact of metals is confined to a thin surface layer 28,29,[31][32][33][34][35][36][37] , typically about 100 nm, whereas the volume that participates in supporting the normal force is much larger 38 even at relatively mild contact stresses like those used here. The bulk hardness or flow stress can be converted into a shear strength using the approximation…”

Early deformation mechanisms in the shear affected region underneath a copper sliding contact

“…These microstructural modifications alter the properties of the material in the subsurface area and in turn affect friction and wear [7][8][9] . However, the underlying elementary deformation mechanisms are not sufficiently clear to allow for systematic modifications or even predictive simulation 10 . Atomistic simulations in nanocrystalline structures have demonstrated that the inhomogeneities of plastic deformation in the different grains and grain rotation play a significant role in tribologically induced deformation and wear 11 .…”

Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts: Alloy Dependent Sliding Surfaces and Their Effects on Wear and Friction

“…According to the EBSD results, the lattice orientation of grains exhibiting hardening is substantially different from neighbouring orientations. Thus, the number of their potential gliding planes with low Schmidt factors [29] differs a lot. The number of available slip planes is regarded as the main reason for the inhomogeneity of strain accumulation along the contact surface.…”