Micromechanical Origins of Adhesive Wear Mechanisms: From Asperity Smoothing to Debris Creation

The adhesive wear process consists of several physical phenomena including plasticity and fracture which occur at different length and time scales. Despite the critical importance of the adhesive wear process in all engineering applications, it is still described via classical, yet fully empirical, models [1,2] since the microscopic principles have not been yet understood. Using novel coarse-grained atomistic simulations , we for the first time capture the debris formation during the adhesive collision between surface asperities [3]. A systematic set of atomistic simulations reveals a characteristic length scale that controls the adhesive wear mechanisms (i.e. asperity smoothing versus debris formation) at the asperity level. This length scale provides a critical adhesive junction size where bigger junctions produce wear debris by fracture while smaller ones smooth out plastically. This finding explains why wear debris has not been observed in previous atomistic simulations of adhesive wear, where the junction is too small and/or weak to form debris by fracture. Based on this observation, we formulate a simple analytical model that predicts the transition in the asperity-level adhesive wear mechanisms in both the simulation results and atomic force microscope (AFM) wear experiments. We show that the proposed framework opens up a new research path to implicitly model and explore the wear process and revisit classical models. References: [1] J. F. Archard (1953) Journal of Applied Physics [2] E. Rabinowicz (1961) Journal of Applied Physics [3] R. Aghababaei, D.H. Warner and J.F. Molinari (2016) Nature Communications

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

On the debris-level origins of adhesive wear

Ramin Aghababaei, Jean-François Molinari, Derek Warner

Every contacting surface inevitably experiences wear. Predicting the exact amount of material loss due to wear relies on empirical data and cannot be obtained from any physical model. Here, we analyze and quantify wear at the most fundamental level, i.e., wear debris particles. Our simulations show that the asperity junction size dictates the debris volume, revealing the origins of the long-standing hypothesized correlation between the wear volume and the real contact area. No correlation, however, is found between the debris volume and the normal applied force at the debris level. Alternatively, we show that the junction size controls the tangential force and sliding distance such that their product, i.e., the tangential work, is always proportional to the debris volume, with a proportionality constant of 1 over the junction shear strength. This study provides an estimation of the debris volume without any empirical factor, resulting in a wear coefficient of unity at the debris level. Discrepant microscopic and macroscopic wear observations and models are then contextualized on the basis of this understanding. This finding offers a way to characterize the wear volume in atomistic simulations and atomic force microscope wear experiments. It also provides a fundamental basis for predicting the wear coefficient for sliding rough contacts, given the statistics of junction clusters sizes.

Natl Acad Sciences2017

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