On the Debris-Level Origins of Adhesive Wear

On the debris-level origins of adhesive wear Ramin Aghababaeia,b, Derek H. Warnerc, and Jean-François Molinaria,b,1 aInstitute of Civil Engineering, Ecole´ Polytechnique Fed´ erale´ de Lausanne, CH 1015 Lausanne, Switzerland; bInstitute of Materials Science and Engineering, Ecole´ Polytechnique Fed´ erale´ de Lausanne, CH 1015 Lausanne, Switzerland; and cSchool of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853 Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved June 1, 2017 (received for review January 17, 2017) Every contacting surface inevitably experiences wear. Predicting contact junctions with sizes above a critical junction size, which is the exact amount of material loss due to wear relies on empiri- a function of bulk and interfacial properties. This finding opens cal data and cannot be obtained from any physical model. Here, the possibility of quantifying the amount of detached materials we analyze and quantify wear at the most fundamental level, i.e., in the form of debris particles and studying the origins of macro- wear debris particles. Our simulations show that the asperity junc- scopically observed wear relations. tion size dictates the debris volume, revealing the origins of the Inspired by this finding (19), this report aims to address how long-standing hypothesized correlation between the wear vol- much material is detached during sliding contact, by focusing on ume and the real contact area. No correlation, however, is found the quantification of wear and the above-mentioned wear rela- between the debris volume and the normal applied force at the tions at the most fundamental level, i.e., wear debris particles. debris level. Alternatively, we show that the junction size controls It shows that the volume of individual wear debris can be evalu- the tangential force and sliding distance such that their product, ated, without any empirical factor. The contacting asperity junc- i.e., the tangential work, is always proportional to the debris vol- tion size is found to be the governing factor, controlling the tan- ume, with a proportionality constant of 1 over the junction shear gential force and sliding distance for debris creation such that strength. This study provides an estimation of the debris volume the tangential work required to create a debris particle is always without any empirical factor, resulting in a wear coefficient of proportional to its volume. Discrepant microscopic and macro- unity at the debris level. Discrepant microscopic and macroscopic scopic wear observations and models are then contextualized on wear observations and models are then contextualized on the the basis of this understanding. basis of this understanding. This finding offers a way to char- ENGINEERING acterize the wear volume in atomistic simulations and atomic Results force microscope wear experiments. It also provides a fundamen- A large set of multimillion-atom simulations of sliding contact tal basis for predicting the wear coefficient for sliding rough con- were performed, differing in system size, boundary conditions, tacts, given the statistics of junction clusters sizes. asperity shape and size, and bulk and interfacial properties. The simulations spanned two distinct geometrical configurations, Archard’s wear law j adhesive wear j friction j wear debris particle j i.e., a single asperity in contact with an atomistically flat surface nanotribology and two asperities on opposing surfaces. As shown in our recent study (19), there exists a critical junction size, with larger asper- he study of material loss at sliding surfaces, known as “wear,” ity junctions forming wear debris particles and smaller junctions Thas over two centuries of history (1). Substantial progress smoothing plastically. Accordingly, the configuration of all simu- occurred in the mid-1900s with a systematic series of wear exper- lations discussed here was chosen so that the junction size leads iments that showed, within a certain range of applied load, (i) the to wear debris formation. A set of 2D (19) and newly developed wear volume (i.e., total volume of wear debris) is independent 3D model interatomic potentials and a recently developed dia- of apparent area of contact (2, 3), (ii) the wear rate (i.e., wear mond potential (20) are used. The formation process of 3D wear volume per sliding distance) is linearly proportional to the macro- particles is simulated and analyzed. Details of the potentials and scopic load acting normal to the interface, i.e., Archard’s wear law (3, 4), and (iii) the wear volume is proportional to the fric- Significance tional work (i.e., the product of frictional force and sliding distance), which was first hypothesized by Reye in 1860 (5) and inter- Wear causes a huge amount of material and energy losses mittently discussed and observed experimentally (3, 5–8). The annually, with serious environmental, economic, and indus- first observation is commonly rationalized by arguing that the trial consequences. Despite considerable progress in the 19th wear process is a direct result of contact between elevated sur- century, the scientific understanding of wear remains mainly face asperities and is consequently associated with the real area of empirical. This study reveals the long-standing microscopic ori- contact (2, 3). The second observation can then be understood by gins of material detachment from solids surface, at the most noting that the real area of contact is observed to be proportional fundamental level, i.e., wear particles. It discloses that the to the macroscopic normal load (2, 9). The third observation fol- detached particle volume can be estimated without any empir- lows from the first two if one assumes a wear volume proportional ical factor, via the frictional work. This study unifies previously to sliding distance and a tangential force proportional to normal disconnected and not understood experimental observations. force (i.e., Amontons’ first law of friction) (10). The results open the possibility for developing new wear mod- Despite the passage of more than 50 years, these wear rela- els with drastically increased predictive ability, with applica- tions remain fully empirical, and their microscopic origins are tions to geophysics, physics, and engineering. still unclear (11). Single-asperity wear simulations (12–14) and atomic force microscope (AFM) wear experiments (15–17) chal- Author contributions: R.A. designed research; R.A. carried out simulations; R.A. ana- lenge the origins of wear debris formation by reporting a gradual lyzed data; R.A., D.H.W., and J.-F.M. participated in the discussions; J.-F.M. supervised atom-by-atom asperity smoothing. This observation further chal- the research; and R.A., D.H.W., and J.-F.M. wrote the paper. lenges a long-standing question posed by Archard (4, 18): When The authors declare no conflict of interest. does an asperity collision lead to the formation of a wear debris This article is a PNAS Direct Submission. particle? This question was recently addressed (19) by the identi- 1To whom correspondence should be addressed. Email: jean-francois.molinari@epfl.ch. fication of a critical length scale that controls debris formation at This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. asperity contacts. It was shown that debris particles form only at 1073/pnas.1700904114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1700904114 PNAS Early Edition j 1 of 6 Downloaded by guest on September 29, 2021 corresponding physical properties and critical junction size are tions with different asperity shapes. On the whole, the behavior presented in Methods (see also SI Appendix, Fig. S1 and Table observed in the simulations is consistent with the classical pic- S1). In all cases, the simulations represent the dry adhesive slid- ture of adhesive wear hypothesized from experimental observa- ing of identical materials at a constant temperature to reduce the tions (21–24). Recent small-scale wear experiments on ceram- complexity of the model and subsequent analysis. Details of the ics and rocks (25, 26) confirm the formation of cylindrical and simulations are given in Methods and SI Appendix, Fig. S2 and spherical wear debris particles. We make the distinction that this Table S2. work is focused on wear by fracture-induced debris formation, as Universal features are apparent in all simulations despite the opposed to surface folding and delamination (23) mechanisms variety of parameters and configurations examined (Fig. 1). Ini- that may occur at different scales and/or under different wear tially, a strong adhesive bond (junction) forms between contact- conditions, e.g., abrasive wear. ing asperities. Subsequent sliding leads to the buildup of tangential forces and stored elastic energy (Fig. 1 E–H). During Wear Is Predictive at Debris Level. Inspired by Archard’s wear this phase, the junction grows by localized inelastic deforma- model (4) [V = k(N × S)=H ], we first examine the relationship tion (SI Appendix, Fig. S3), until crack nucleation and growth between the debris particle volume, V , and the product of the ensues at the two corners of the junction loaded in tension. With applied normal force, N , and sliding distance, S (Fig. 2A), with subsequent sliding, the forces transmitted across the junction H being the material hardness and k being a proportionality con- decrease as the cracks grow, ultimately creating a debris parti- stant (i.e., the wear coefficient). S is taken as the sliding dis- cle. SI Appendix, Fig. S4 depicts a similar observation in simula- tance at which the tangential force returns to zero (Fig. 1). In A E B F C G D H Fig. 1. Debris formation at the asperity level. Snapshots at different sliding distance S of multimillion-atom simulations of debris formation and corresponding forces evolutions in (A) asperity-flat with 2D model potential, (B) colliding asperities with 2D model potential (P1), (C) colliding asperities with 3D model potential (P4), and (D) colliding asperities with diamond potential (20).

Load more