We report a hybrid wear-reducing surface by sliding steel counterfaces with pre-textured micro-pits pattern against polymeric solid lubricants. When being slid against a freshly prepared pin of the ...same polymer material, the hybrid surface significantly reduced wear rate of the polymer and steel counterface by up to 69% and 550% compared with the untextured unconditioned surface for the four different polymers surveyed in this study. Polymer wear generally decreased with increased iteration round of preconditioning while counterface wear was more dependent on transfer film and wear track topography. Worn counterface profilometry analysis suggested the preconditioned hybrid surface improved debris retention within the micro-pits which might mitigate wear by acting as debris reservoirs at the sliding interface and promoting transfer film formation.
Graphical abstract
A transient and unusually high friction coefficient always appears when the tire–road interface changes from wet to dry and has been known as the tacky regime. This paper aims to study the mechanism ...of such friction peak that appears in a hydrophilic contact between an oxidized silicon nitride (ox-Si
3
N
4
) sphere and a sheet of oxidized polydimethylsiloxane (ox-PDMS) film. The results found, in the tacky regime, the friction coefficient increased by 23% from the dry sliding condition. A threshold technique was used to quantify the real solid–solid contact area (
A
r
) and contact area with a water film less than 110 ± 2 nm thick (
A
f
) during the tacky transition. A strong linear relation between the transient friction coefficient and the water film area ratio (
A
f
/(
A
f
+
A
r
)) is found. We proposed that the transient frictional force (1) consists of the solid–solid shear force and the capillary shear force, (2) could be analyzed with a simple rule-of-mixture model, and (3) is dominated by thin, high shear strength water films within the apparent contact area. Based on the proposed model, capillary bridges within the apparent contact area have a shear strength ~ 2 times of the solid–solid contact and contribute 40% to the transient friction force.
Graphical Abstract
The friction peak that occurs in tire-road sliding when the contact changes from wet to dry was previously attributed to capillary cohesion, van der Waals attraction, and surface roughness, but the ...detailed mechanisms have yet to be revealed. In this study, friction and static contact experiments were conducted using a custom-built
in situ
optical microtribometer, which allowed us to investigate the evolution of the friction, normal load, and contact area between a polydimethylsiloxane (PDMS) film and a silicon nitride ball during water volatilization. The friction coefficient increased by 100%, and the normal force dropped by 30% relative to those in the dry condition during the wet-to-dry transition. In static contact experiments, the probe indentation depth increased, and the normal load decreased by ∼60% as the water evaporated. Combining the friction and static contact results, we propose that the large friction peak that appeared in this study can be attributed to the combined effects of increased adhesive capillary force and increased plowing during the wet-to-dry transition.
Polymer wear against a metal surface depends on both the voluntary, pre-defined roughness and the involuntary, in-situ generated roughness by the polymer wear. Using highly defined roughness ...manufactured on stainless counterfaces with laser-surface-texturing (LST), a new roughness directionality parameter (IRMS), two established ultralow wear polymers (5 wt% α-Al2O3−PTFE and 20 wt% PEEK-PTFE), reciprocating pin-on-flat tribometry and two sets of relevant data in the literature, we systematically studied the variance of polymer wear and transfer with the initial and in-situ generated counterface roughness. While no consistent relation was found between roughness and wear rate, strongly linear relations were noticed between worn surface directionality and polymer wear volume at the low wear transients. The results support the working hypothesis and suggest the involuntary roughness effects could be 1) dominant as they easily destabilize the transfer film, 2) quantified using the newly defined roughness directionality.
•Decoupling of the effects of initial and in-situ generated roughness on wear.•Quantification of in-situ generated roughness using a single unified metric.•Inconsistent relation between roughness and wear attributed to in-situ roughening.•In-situ roughening destabilizes transfer film and increases wear.
Friction and unloading experiments of an ox-Si3N4 ball against a PDMS film were conducted using an in-situ optical microtribometer. The interface was first wetted with 0.1 mL water and allowed to dry ...out during the sliding. A transient friction coefficient 60% greater than in dry condition always occurred in accordance with discretely distributed nanoscaled water bridges, increased pull-off force, stick-slip amplitude, and contact deformation. In summary, this study provides experimental support for the hypothesis that multi-asperity capillary bridges could significantly impact the macroscale frictional behavior of solid-solid contacts.
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Prior studies suggest mechanical interlocking, preferential adhesion of polar polymers, and PTFE tribochemistry all contribute to the wear reduction in polar polymer-filled PTFE composites; whereas ...their relations remain poorly understood. This study aims to decouple the relevant mechanisms by investigating wear and tribofilm environmental sensitivities of a 5 wt% polyamide-imide (PAI) filled PTFE composite. The results highlight (1) strong correlation between a carboxylate-salt-rich transfer film and ultralow wear, and (2) a tribochemical interface gradient that discourages transfer film removal. We proposed that PTFE tribochemistry induces ultralow wear by significantly increasing (1) transfer film adhesion through a virtuous cycle of tribochemical accumulation and wear reduction and (2) transfer film cohesion by promoting a surface energy interface gradient beneficial to low wear.
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•Studying wear and tribofilm environmental sensitivities of a PAI-PTFE composite.•a virtuous cycle of tribochemistry and low wear increases transfer film adhesion.•a tribochemical surface energy interface gradient increases transfer film cohesion.•Increased transfer film adhesion and cohesion induce PAI-PTFE ultralow wear.•PTFE tribochemistry dominates wear reduction in the model material.
Effects of various counterface roughness components (i.e., magnitude, density, direction) on polymer wear and transfer film are strongly coupled and difficult to isolate and test. Previous works ...found counterface roughness grooves and ridges perpendicular to the direction of sliding were generally beneficial to debris retention and tribofilm formation, but the effect was competing with a decrease in debris retention caused by in-situ generated furrows parallel to the direction of sliding. To further study the effect of roughness directionality on polymer wear, a model polymer solid lubricant (5 wt% Al2O3-PTFE) was slid against counterfaces with laser-textured grooves parallel to the direction of reciprocated sliding and a 2 × 3 variable matrix of groove depth and density. The results found: (1) all textures yielded higher polymer wear than the untextured surface did, (2) counterface with the deepest, densest grooves had the highest polymer wear rate and transient wear volume (wear volume during run-in sliding), (3) shallow groove textures generally produced less wear of the counterface and polymer, stronger tribofilm IR signature and more continuous and complete tribofilms than deep groove textures did. A new roughness parameter (IRa) was defined as a single, unified and comprehensive metric for counterface roughness directionality. By comparing with a previous study, the results found a general correlation between IRa and polymer transient wear volume and suggested strong beneficial effects of perpendicular grooves on debris retention and parallel grooves on debris ejection. A simple framework was postulated to illustrate the interrelations between texture parameters, roughness directionality and debris mobility. It was further supported by analyzing results of a closely relevant work. Although being strongly limited to the model polymer solid lubricant, this study provides perhaps some useful tools for advancing our understanding of the complex relations between roughness directionality, debris mobility (e.g., ejection, retention) and wear of polymer solid lubricant.
•Laser textured counterfaces help illucidating relations between roughness and wear.•A single, unified and comprehensive metric for roughness directionality is defined.•Roughness directionality correlates both qualitatively and quantitatively with wear.
The adhesive processes of loose debris production and preferential transfer to a high surface energy counterface were modeled using an asperity contact based framework and statistical hypotheses ...about adhesion, average debris size, and the preferential load support by existing transfer film. Transfer film area fraction evolution of a well-studied ultralow wear polymer solid lubricant strongly supports the model’s prediction of geometric growth. Further analyses suggest the model’s strong predictability is mathematically independent of debris size, which is also in accordance with leading adhesive wear theories on the critical size of asperity junction and loose debris.
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•Propose and validate first quantitative model of adhesive transfer film formation.•Bridges the gap between asperity and macroscale adhesive wear events.•Tribofilm geometric growth related to statistical variation of interface properties.•Model mathematically in accordance with leading adhesive wear theories.
Capillary adhesion contributes to friction of sliding multi-asperity contact in humid environment by increasing the solid-solid contact area; whereas the increased adhesion from capillary meniscus is ...hard to measure due to elastic recovery of contacting asperities. Here, we report the first direct measurement of increased interfacial adhesion from mesoscopic menisci in a macroscale multi-asperity contact comprising soft elastomer and glass surfaces with a diminishing water film. Results also found capillary adhesion and its contribution to a high transient friction coefficient decrease with increased interfacial roughness, which is consistent with prior literature hypotheses.
Polymeric solid lubricants rely on the deposition of a debris-formed transfer film on the usually metallic counterface to reduce wear and friction. Debris retention at the sliding surface is ...determined by the interactions between debris and roughness asperities which are complex functions of debris size and roughness profile. Recent works found pre-existing, sandpaper-lapped counterface roughness grooves perpendicular to the sliding direction could significantly improve debris retention and reduce the wear rate of an alumina PTFE solid lubricant by 70%. In this paper, we aimed to test the independent effects of roughness groove depth and density on debris retention and wear performance of a well-studied alumina PTFE solid lubricant using laser-textured counterfaces with independently varied groove depth and interval. All grooves in this study were engraved perpendicular to the sliding direction. The results suggested both groove depth and interval have self-competing effect on wear due to the in situ grounding and roughness alignment of the counterface topography during sliding and may compete or cooperate with each other in determining solid lubricant wear. A new surface directionality parameter was defined to quantify the in situ counterface roughness alignment which increased proportionally with increased composite wear volume at low-wear transition. A conceptual framework was proposed to illustrate the relations between counterface texture, transient wear volume, surface directionality and counterface abrasion.
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