Interfacial Microstructure Evolution Due to Strain Path Changes in Sliding Contacts

Abstract

We performed large-scale molecular dynamics (MD) simulations to study the transient softening stage that has been observed experimentally in sliding interfaces subject to strain path changes. The occurrence of this effect can be of crucial importance for the energy efficiency and wear resistance of systems that experience changes in the sliding direction, such as bearings or gears in wind parks, piston rings in combustion engines, or wheel-rail contacts for portal cranes. We therefore modeled the sliding of a rough counterbody against two polycrystalline substrates of face-centered cubic (fcc) copper and body-centered cubic (bcc) iron with initial near-surface grain sizes of 40 nm. The microstructural development of these substrates was monitored and quantified as a function of time, depth, and applied pressure during unidirectional sliding for 7 ns. The results were then compared to the case of sliding in one direction for 5 ns and reversing the sliding direction for an additional 2 ns. We observed the generation of partial dislocations, grain refinement, and rotation as well as twinning (for fcc) in the near-surface region. All microstructures were increasingly affected by these processes when maintaining the sliding direction but recovered to a great extent upon sliding reversal up to applied pressures of 0.4 GPa in the case of fcc Cu and 1.5 GPa for bcc Fe. We discuss the applicability and limits of our polycrystalline MD model for reproducing well-known bulk phenomena such as the Bauschinger effect in interfacial processes.

Effect of Temperature on the Deformation Behavior of Copper Nickel Alloys under Sliding

“…Our simulations were performed using the open-source MD code LAMMPS [32], which has become the de-facto standard for meshless simulations in academic tribological research. The polycrystalline MD model measures 85 × 85 × 40 nm 3 , contains approximately 25 million atoms, and was prepared as explained in [33,34]. Boundary constraints were applied to the lower 3 Å of the model, where a "sacrificial layer" of ∼10 nm grains was attached to the "working layer" of ∼40 nm grains so none of the occurring mechanisms would be externally constrained.…”

Elucidating the Onset of Plasticity in Sliding Contacts Using Differential EBSD Tomography

“…1) measures 85 × 85 × 40 nm 3 , contains approximately 25 million atoms, and was prepared as detailed further in Refs. [32,38]. The fcc CuNiX samples, where X is the atomic percentage of Ni (0%, 5%, 25%, 60%, and 100%), interact according to an embedded atom potential with parameters taken from [39].…”

“…Changes of the microstructure, especially subtle ones, already occur during running-in and sometimes even after a single sliding pass of the counterbody [17,31,32]. Since it is known that running-in greatly determines the subsequent tribological behavior of the material, it is highly important to understand the microstructural evolution happening during this stage.…”

Insights into tribology from in situ nanoscale experiments

“…Postmortem analyses inherently allow different interpretations for how the observed structures have been achieved. For instance, MD simulations 101 have suggested the importance of the Bauschinger effect, where the back-stresses of dislocations that are induced during loading in the forward direction serve to aid the activation of dislocation slip in the reverse direction. Complete understanding of real-time processes requires in situ nanoscale analysis.…”