Role of transfer films and interfacial cracking in metallic sliding wear

Abstract

The origin of wear particles in metallic sliding contacts remains debated. Classical views based on cold-welded junctions suggest that plastic yielding of the real contact area should lead to large wear coefficients, in apparent contradiction with the small values typically measured for metals. Here we argue that this discrepancy can be resolved if most junctions do not directly produce wear particles, but instead cause metal transfer and the formation of a weakly bound transfer film. Wear then occurs intermittently when fragments of this film detach due to crack propagation at the interface between the transfer film and the underlying bulk metal. We perform unlubricated reciprocating sliding experiments on nominally smooth stainless steel, brass, and aluminum. For steel on steel, the wear mass loss shows an initial stage with negligible mass change up to a sliding distance of $\sim 2.4 \ {\rm m}$, followed by a linear regime. Transfer-film formation in dissimilar-metal contacts is evidenced by optical imaging, net mass gain of the steel slider, and energy-dispersive X-ray spectroscopy, and the collected debris is flake-like. These observations support a transfer-film-controlled wear mechanism associated with cold-welded junctions.

Figures (5)

• Figure 1: Schematic illustration of two wear mechanisms in metallic sliding contacts. (a) Delamination-type wear where subsurface crack nucleation and propagation lead to the formation of platelet-like debris. (b) Wear mechanism proposed in this study, where a weakly bound transfer film is formed by metal transfer at cold-welded junctions, and crack propagation at the interface between the transfer film and the underlying bulk metal causes fragments of the film to detach.
• Figure 2: (a) Wear mass loss and (b) friction coefficient as a function of the sliding distance for a steel-on-steel contact.
• Figure 3: Optical images of a steel slider after sliding $64.8 \ {\rm m}$ on (a) an aluminum substrate and (b) a brass substrate. For steel sliding on brass, the transfer film appears as yellow regions on the steel surface.
• Figure 4: Scanning Electron Microscopy (SEM) images of wear particles collected from the substrate for identical-metal sliding contacts: (a) steel on steel, (b) brass on brass, and (c) aluminum on aluminum. In all cases, the wear particles are flake-like, with lateral dimensions ranging from a few micrometers up to $\sim 100 \ {\rm \mu m}$.
• Figure 5: Energetic criterion for the formation of wear particles in asperity contacts. To remove a particle of linear size $d$, sufficient elastic deformation energy must be stored in its vicinity to break the bonds required to form a free particle. The bond-breaking energy scales as $U_{\rm c} \approx \gamma d^2$, where $\gamma$ is the cohesive energy per unit surface area, while the elastic energy scales as $U_{\rm el} \approx (\tau^2/E) d^3$, where $\tau$ is the shear stress in the asperity contact region. Hence, particle removal is possible only if $d > \gamma E/\tau^2$, so only sufficiently large asperity contact regions give rise to wear particles.