Effect of wear particles and roughness on nanoscale friction

Frictional contacts lead to the formation of a surface layer called the third body, consisting of wear particles and structures resulting from their agglomerates. Its behavior and properties at the nanoscale control the macroscopic tribological performance. It is known that wear particles and surface topography evolve with time and mutually influence one another. However, the formation of the mature third body is largely uncharted territory, and the properties of its early stages are unknown. Here we show how a third body initially consisting of particles acting as roller bearings transitions into a shear-band-like state by forming adhesive bridges between the particles. Using large-scale atomistic simulations on a brittle model material, we find that this transition is controlled by the growth and increasing disorganization of the particles with increasing sliding distance. Sliding resistance and wear rate are, at first, controlled by the surface roughness, but upon agglomeration wear stagnates and friction becomes solely dependent on the real contact area in accordance with the plasticity theory of contact by Bowden and Tabor.

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

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

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

Fractal surfaces in adhesive wear processes.

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

2018