Linking microstructural evolution and macro-scale friction behavior in metals

Abstract

A correlation is established between the macro-scale friction regimes of metals and a transition between two dominant atomistic mechanisms of deformation. Metals tend to exhibit bi-stable friction behavior --low and converging or high and diverging. These general trends in behavior are shown to be largely explained using a simplified model based on grain size evolution, as a function of contact stress and temperature, and are demonstrated for pure copper and gold. Specifically, the low friction regime is linked to the formation of ultra-nanocrystalline surface films (10 to 20 nm), driving toward shear accommodation by grain boundary sliding. Above a critical combination of stress and temperature --demonstrated to be a material property --shear accommodation transitions to dislocation dominated plasticity and high friction. We utilize a combination of experimental and computational methods to develop and validate the proposed structure-property relationship. This quantitative framework provides a shift from phenomenological to mechanistic and predictive fundamental understanding of friction for crystalline materials, including engineering alloys.

Concurrent transitions in wear rate and surface microstructure in nanocrystalline Ni-W

“…The average grain size in this layer was measured to be 9 nm, which is also similar to the 10 nm grain size found in the original sample. These findings help support the model proposed by Argibay et al [38] since it seems that the surface layer converges to nearly the same average grain size for an identical level of applied surface stress. Moreover, the grain size distribution also spanned the range of 3 -40 nm, indicating that both fine-grained (d < 10 nm) and coarser-grained (d > 10 nm) films of Ni-19 at.% W evolve to an identical structure.…”

“…For the d = 45 nm sample, grain-boundary-mediated plasticity should be shut off and Figure 8 even shows evidence of mottled contrast inside many grains, providing evidence of dislocation plasticity. A major proposition in this area is that the steady-state grain size changes as a function of applied surface stress [38,39]. Additional experimental evidence of this claim would help to validate and inform this model.…”

“…Argibay et al [37] analyzed similar Ni-W coatings with d ~ 5 nm and suggested that the thickness and average grain size within the evolved layer is a function of applied stress, correlating to the friction behavior. More recently, Argibay et al [38] developed a model which predicts either coarsening or refinement in surface tribolayers as a function of the equilibrium dislocation splitting distance (re), with coarsening occurring when d < 2re and refinement when d > 2re. The exact grain size to which an evolved surface layer will eventually converge is then dependent on the applied surface stress.…”

Solids Under Extreme Shear: Friction‐Mediated Subsurface Structural Transformations

“…[25,26] In this context and for linking the mechanical properties, and consequently the microstructure, of a metal with its tribological properties Argibay and co-workers recently brought forward a feedback cycle. [27] The authors assume that different surface stresses will lead to a variation in how the material responds to shearing, i.e., by grain boundary sliding or by dislocation motion which therefore depend on the normal load during a tribological experiment. To visualize this concept, a feedback loop between grain size, friction coefficient and surface stress was developed which highlights the need to know the stress field under the sliding bodies.…”

Friction and Tribo-Chemical Behavior of SPD-Processed CNT-Reinforced Composites

“…In a tribological system, where two bodies are moving relative to each other with a certain speed under the application of a normal force (F N ) and along a certain distance with certain environmental conditions (e.g., temperature and humidity), there are other aspects, such as the surface finish [1][2][3][4] and the microstructure [5][6][7], that affect the wear, the friction and even the tribo-oxidation on the contact zone. Furthermore, it has been shown that intermittent tribological loading induces plastic deformation in a layer beneath the wear track in the tribologically transformed zone (TTZ) [8], which could result in grain refinement (in initially coarse-grained materials) [4,9] or grain coarsening (in initially nanocrystalline materials) [4,6,7,10,11].…”