Asperity (materials science) | EPFL Graph Search

In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness" (from the Latin asper—"rough"), has implications (for example) in physics and seismology. Smooth surfaces, even those polished to a mirror finish, are not truly smooth on a microscopic scale. They are rough, with sharp, rough or rugged projections, termed "asperities". Surface asperities exist across multiple scales, often in a self affine or fractal geometry. The fractal dimension of these structures has been correlated with the contact mechanics exhibited at an interface in terms of friction and contact stiffness. When two macroscopically smooth surfaces come into contact, initially they only touch at a few of these asperity points. These cover only a very small portion of the surface area. Friction and wear originate at these points, and thus understanding their behavior becomes important when studying materials in contact. When the surfaces are subjected to a compressive load, the asperiti

A mechanistic understanding of the wear coefficient: From single to multiple asperities contact

Ramin Aghababaei, Lucas Henri Galilée Frérot, Jean-François Molinari

Sliding contact between solids leads to material detaching from their surfaces in the form of debris particles, a process known as wear. According to the well-known Archard wear model, the wear volume (i.e. the volume of detached particles) is proportional to the load and the sliding distance, while being inversely proportional to the hardness. The influence of other parameters are empirically merged into a factor, referred to as wear coefficient, which does not stem from any theoretical development, thus limiting the predictive capacity of the model. Based on a recent understanding of a critical length-scale controlling wear particle formation, we present two novel derivations of the wear coefficient: one based on Archard’s interpretation of the wear coefficient as the probability of wear particle detachment and one that follows naturally from the up-scaling of asperity-level physics into a generic multi-asperity wear model. As a result, the variation of wear rate and wear coefficient are discussed in terms of the properties of the interface, surface roughness parameters and applied load for various rough contact situations. Both new wear interpretations are evaluated analytically and numerically, and recover some key features of wear observed in experiments. This work shines new light on the understanding of wear, potentially opening a pathway for calculating the wear coefficient from first principles.

2018

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

Ramin Aghababaei, Jean-François Molinari, Derek Warner

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

2017

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