Effect of adhesion on material removal during adhesive wear

The process of surface degradation and material removal due to adhesive force between rubbing surfaces (i.e., adhesive wear) involves an apparent contradiction. The material exchange between sliding surfaces requires a strong adhesion between surface asperities, whereas the detachment of final wear particles demands a weak adhesion between worn fragments and sliding surfaces. Here, using a coarse-grained numerical technique, we study the complete process of wear particle formation (i.e., nucleation, evolution, and detachment) during adhesive sliding contact. We show that discrepant experimental and theoretical wear observations can be attributed to different stages of the wear particle formation. In particular, we address the opposite contribution of adhesion into the formation and detachment processes of wear particles. Our simulations reveal that reducing adhesion diminishes the adhesive wear in three ways: (i) reducing the probability of wear particle formation, (ii) increasing the required energy per unit volume of removed materials (i.e., decreasing the energy efficiency for particle formation), and (iii) alleviating the growth of formed wear fragments.

Figure 1

(a) Bulk and interfacial interatomic potentials, used in this study. The cut off radius is reduced by modifying the tail of the Morse potential [51], to embrittle the system and to allow simulating the crack nucleation and propagation at the atomic level [9]. To model adhesion, a similar set of potentials with the bond energy differing by a scalar (0.4–1) is used. This factor, referred to the adhesion coefficient, represents the degree of interfacial adhesion with respects to the cohesion strength of sliding solids (i.e., ${\gamma }_{\mathrm{adh}}/{\gamma }_{\mathrm{coh}}$). (b) The idealized setup for simulating the wear particle formation at the asperity level adhesive contact. See Refs. [37, 38] for more information about the simulation setup. Bulk and surface atoms are coloured in gray and red, respectively. While a same bulk potential is applied, two gray colors are used to distinguish the top and bottom solids. (c) and (d) show the result of a new algorithm that enables on-the-fly detection of surface atoms and reassignment of a weaker adhesion potential for modeling the effect of adhesion reduction due to surface contamination. See Sec. 2 for more information about the algorithm.

Figure 2

Snapshots of different stages of wear particle formation in the presence of different interfacial adhesion: (a) ${\gamma }_{\mathrm{adh}}={\gamma }_{\mathrm{coh}}$, (b) $0.8{\gamma }_{\mathrm{coh}}$, and (c) $0.6{\gamma }_{\mathrm{coh}}$. As shown in (d), no particle is formed with a lower interfacial adhesion ${\gamma }_{\mathrm{adh}}=0.4{\gamma }_{\mathrm{coh}}$. The morphology of individually removed fragments during the process of wear particle formation in the reference and final configurations are shown in (a8), (a9), (b8), (b9), (c8), and (c9). An isolated fragment is identified when an area is entirely closed by surface atoms. See movie of these simulations in Ref. [52].

Figure 3

The onset of snow ball particle growth mechanisms for different adhesion cases. Tracking the rotation of the interface line between the contacting asperities, it can be seen that the particle rolling is initiated at different stages of sliding for different adhesion parameters. (a) In the presence of low adhesion (${\gamma }_{\mathrm{adh}}=0.6{\gamma }_{\mathrm{coh}}$), wear particle rolling occurs after several contact events, once the size of the junction between the sliding surfaces becomes larger than the critical junction size required to form a particle. The particle formation, however, occurs at the first contact for higher adhesion parameters [(b) and (c)]. It can be seen that the higher the adhesion parameter, the smaller the wear particle at the onset of rolling. The onset of rolling is identified when the interface line between two asperities starts to rotate.

Figure 4

Quantitative analysis of the evolution of (a) tangential force $F$, (b) energy $W$, (c) the particle volume $V_p$, and (d) the required energy per unit volume of removed material $W/V_p$. (e) presents the volume of individual wear fragment ($V_f$), normalized by the total volume of the particle at steady rolling state. (f) shows the fraction of contaminated surface atoms (i.e., atoms which being part of free/fracture surfaces) inside the formed wear particles ($V_c{V}_p$). The unit thickness is assumed for volume quantities.

Figure 5

(a) Schematic of stress state at the contact between the rolling third body particle and sliding surface. The angle of subsurface crack propagation in adhesive wear simulations with (b) full (${\gamma }_{\mathrm{adh}}={\gamma }_{\mathrm{coh}}$) and (c) reduced (${\gamma }_{\mathrm{adh}}=0.6{\gamma }_{\mathrm{coh}}$) interfacial adhesion, showing that the stronger the interfacial adhesion, the deeper the crack propagation angle and the larger the amount of removal material. (d) Evolution of the angle of maximum principal stress (${\theta }_I$) at the trailing edge of the rolling wear particles.