Emergence of self-affine surfaces during adhesive wear

Friction and wear depend critically on surface roughness and its evolution with time. An accurate control of roughness is essential to the performance and durability of virtually all engineering applications. At geological scales, roughness along tectonic faults is intimately linked to stick-slip behaviour as experienced during earthquakes. While numerous experiments on natural, fractured, and frictional sliding surfaces have shown that roughness has self-affine fractal properties, much less is known about the mechanisms controlling the origins and the evolution of roughness. Here, by performing long-timescale molecular dynamics simulations and tracking the roughness evolution in time, we reveal that the emergence of self-affine surfaces is governed by the interplay between the ductile and brittle mechanisms of adhesive wear in three-body contact, and is independent of the initial state.

Wear at the asperity level: An atomistic viewpoint

Ramin Aghababaei, Jean-François Molinari, Derek Warner

Tribological problems are particularly difficult to comprehend. Different physical mechanisms (including the environment, plastic deformation, third body interactions, phase transformations, recrystallization) interact at disparate length and time scales. Here, we will present some of our recent attempts to understand the origins of wear mechanism at the atomistic level. Macroscopically, the wear process, in terms of debris formation, is well described by the well-known Archard model [1]. As suggested, the wear rate is linearly proportional to the ratio of the applied force to the material hardness with a constant factor called wear coefficient which represents the probability of wear debris formation. This coefficient is the most important yet least understood parameter in the field of tribology. Since the physical origins of the debris formation process are still unclear, this coefficient cannot be estimated theoretically and must be only determined experimentally. In order to understand the physical origins of the debris formation process, we have performed a systematic set of molecular dynamics simulations employing a new family of interatomic potentials. These potentials allow us to independently modify physical properties such as hardness and surface energy where other parameters remains constant. Using that, we will discuss different wear mechanisms at the asperity level ranging from a gradual smoothing to a debris formation. Additionally, we elaborate the steady-state sliding regime in terms of frictional force, debris size and surface roughness and explore several influencing parameters such as applied load, sliding velocity and inter-facial adhesion [2]. The proposed framework opens up a new research path to implicitly model and explore the wear process and revisit classical models.

2015

Surface roughness evolution in atomistic simulations of adhesive wear: from asperity collision to three-body configuration.

Ramin Aghababaei, Tobias Brink, Enrico Milanese, Jean-François Molinari

Surface roughness is relevant to all the phenomena and processes that take place at the interface between two bodies, like adhesion, contact, friction, and wear. Understanding the relation between surface roughness evolution and wear is therefore key in several fields: from the optimization of the design of manufactured objects to the understanding of fault slip during earthquakes. Experimental evidence and field observation show that worn surfaces are self-affine. The complexity of the wear phenomenon makes the understanding of the underlying physics a challenge though, and complicates the quest for simplified, yet sufficiently accurate models capable of describing the wear process. The same reason does not make numerical modeling simple either, as continuum approaches cannot capture the aforementioned complexity, while discrete approaches are limited by the computational cost necessary to investigate length and time scales relevant to engineering applications. A recently developed atomistic simulations approach 3 permitted us to overcome the length-scale limitation. This allowed us to gain significant insights on the physics of surface roughness evolution during adhesive wear processes. Our results show that the evolution of the surface morphology can be split in two different phases: running-in and long-term sliding. In the first phase, two surfaces come into contact at the asperity level. We find that a material-dependent critical length scale governs the ductile-to-brittle transition: if the junction formed by the two colliding asperities is smaller than this critical length, the asperities deform plastically, otherwise they break. This is a fundamental turning point for the surface roughness evolution: if no junction is large enough, the surfaces will smooth continuously until they weld together. On the contrary, when the junction size is sufficiently large, a debris particle is formed by fracture, thus creating roughness. The evolution of the surface roughness after running-in is characterized by a different configuration. In our simulations, the debris particle formed upon the initial contact is constrained to roll between the two surfaces and the system transitions to a three-body configuration. The changes in the morphology are then governed by the contact between the debris particle and the surface. Over long time-scales, this leads the worn surfaces to exhibit a self-affine morphology. In this presentation we will also address the relation between tangential work and wear volume and the interaction between multiple asperities at the onset of wear.

2018

Creation and evolution of roughness on silica under unlubricated wear

Jean-François Molinari, Son-Jonathan Pham-Ba

Friction and wear are important phenomena occurring in all devices with moving parts. While their origin and the way they evolve over time are not fully understood, they are both intimately linked to surface roughness. Guided by pin-on-disc experiments, we present the steps giving rise to the formation of surface roughness on silica, first by the creation of roughly spherical wear particles whose size is related to a critical length scale governing the transition between ductile and brittle behavior, then by the accumulation of these small particles into a larger third body layer, or gouge. We show that, for the explored range of loading conditions, the surface roughness evolves toward a common steady state under unlubricated wear regardless of the initial surface roughness, hinting toward the possible predictability of roughness evolution in wearing components. The friction coefficient is shown to be related to the surface roughness and its time evolution is discussed as well.

2020

Surface roughness evolution in atomistic simulations of adhesive wear: from asperity collision to three-body configuration.

Ramin Aghababaei, Tobias Brink, Enrico Milanese, Jean-François Molinari

2018