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

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.

Asperity-Level Origins of Transition from Mild to Severe Wear

Ramin Aghababaei, Tobias Brink, Jean-François Molinari

Wear is the inevitable damage process of surfaces during sliding contact. According to the well-known Archard’s wear law, the wear volume scales with the real contact area and as a result is proportional to the load. Decades of wear experiments, however, show that this relation only holds up to a certain load limit, above which the linearity is broken and a transition from mild to severe wear occurs. We investigate the microscopic origins of this breakdown and the corresponding wear transition at the asperity level. Our atomistic simulations reveal that the interaction between subsurface stress fields of neighboring contact spots promotes the transition from mild to severe wear. The results show that this interaction triggers the deep propagation of subsurface cracks and the eventual formation of large debris particles, with a size corresponding to the apparent contact area of neighboring contact spots. This observation explains the breakdown of the linear relation between the wear volume and the normal load in the severe wear regime. This new understanding highlights the critical importance of studying contact beyond the elastic limit and single-asperity models.

2018

Fractal surfaces in adhesive wear processes.

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

Wear of materials plays a key role in the durability of manifactured objects and it has thus an economical importance in our society. In spite of the efforts, a unified picture of the underlying physics of the phenomenon is still far from being reached, because of the numerous mechanisms and processes that constitute the complexity of wear. Moving one step closer to the understanding of the physics of wear would mean enhancing our understanding of all the related fields that it affects, like friction and contact mechanics. The evolution of the surfaces morphology upon sliding is one of the aspects that need further investigations. In fact, while it is known that newly created surfaces by means of crack propagation are self-affine, what happens to existing surfaces rubbed against one another is still unclear. What is known from experimental studies is that self-affinity is preserved for highly abrasive wear and that in other cases the worn surface undergoes asymmetric changes with respect of its original mean plane. Surface roughness is also linked to the wear rate, as rough surfaces are found to deteriorate more in the running-in, up to the point that they have been smoothed enough and the wear rate becomes smaller and constant (the opposite happening for surface that are initially too smooth). As atomistic simulations have been proved to be an effective means of investigation for the understanding of the physics of wear, we adopt a recently developed approach to investigate the surface roughness evolution under adhesive wear processes (i.e, when the material loss is mainly due to transfer of particles from one body to another). The simulated system is two-dimensional, with periodic boundary conditions along the horizontal direction to allow for continuous sliding at constant velocity of the top surface over the bottom one. In the early stages of contact, the two surfaces form a debris particle that continuously rolls between them, interacting with them by removing material and deforming them. The peculiarity of the performed simulations is that they are long enough to allow for an investigation of the surfaces geometry evolution over time. The simulations display the two-regime evolution of the surfaces, characterized by high wear rate at running- in and lower wear rate in later stages of the process. Once the running-in is over, the morphology of the sliding surfaces and of the rolling debris particle are analyzed by means of their PSD. All the analyzed surfaces appear self-affine, and their heights are positively correlated. This type of roughness is consistent with the ones found in faults and, by analogy with gradient percolation models, hints that short range interactions prevail on long range elasticity. In some cases it is also observed that, after the running-in, the debris particle can heavily perturb the process increasing again the roughness of the mating surfaces, similarly to the running-in conditions. Despite the fact that this is not the picture commonly expected, it can be explained analyzing the debris particle geometry and the stress state at the contact interface. In particular, roughness enhancement happens by removal of material from the opposing surfaces when the particle is approximately round, while an irregular shape favours the presence of stress singularities in the particle itself and thus the deposit of material from the particle on the opposing surfaces. In this presentation we analyze different initial setups (i.e. different morphologies, system size, materials), seeing how the final self-affine morphology is independent of the initial state and how the shape of the rolling particle can alter the wear process.

2018

Adhesive wear and interaction of tangentially loaded micro-contacts

Tobias Brink, Jean-François Molinari, Son-Jonathan Pham-Ba

Current engineering wear models are often based on empirical parameters rather than built upon physical considerations. Here, we look for a physical description of adhesive wear at the microscale, at which the interaction between two surfaces comes down to the contact of asperities. Recent theoretical work has shown that there is a critical micro-contact size above which it becomes energetically favorable to form a wear particle. We extend this model by taking into consideration the elastic interaction of multiple closely-spaced micro-contacts in 2D, with different sizes and separation distances. Fundamental contact mechanics solutions are used to evaluate the elastic energy stored by shearing the micro-contacts, and the stored energy is compared to the energy needed to detach a single joined debris particle or multiple debris particles under the micro-contacts. Molecular dynamics simulations are used to test the predictions of the outcome for various sets of parameters. Our model provides simple criteria to evaluate the energetic feasibility of the different wear formation scenarios. Those criteria can be used to rationalize the transition between mild and severe wear regimes and help define the notion of asperity.

2020

Fractal surfaces in adhesive wear processes.

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

2018