From the context of a contemporary understanding of the phenomenological origins of friction and wear of materials, we review insightful contributions from recent experimental investigations of three ...classes of materials that exhibit uniquely contrasting tribological behaviors: metals, polymers, and ionic solids. We focus on the past decade of research by the community to better understand the correlations between environment parameters, materials properties, and tribological behavior in systems of increasingly greater complexity utilizing novel synthesis and in situ experimental techniques. In addition to such review, and a half-century after seminal publications on the subject, we present recently acquired evidence linking anisotropy in friction response with anisotropy in wear behavior of crystalline ionic solids as a function of crystallographic orientation. Although the tribological behaviors of metals, polymers, and ionic solids differ widely, it is increasingly more evident that the mechanistic origins (such as fatigue, corrosion, abrasion, and adhesion) are essentially the same. However, we hope to present a clear and compelling argument favoring the prominent and irreplaceable role of in situ experimental techniques as a bridge between fundamental atomistic and molecular processes and emergent behaviors governing tribological contacts.
The tribological properties of most high functioning tribological materials, including diamond, graphite, molybdenum disulfide, and polytetrafluoroethylene, depend strongly on environmental moisture. ...A particularly wear-resistant alumina–polytetrafluoroethylene (PTFE), for example, loses its capacity for ultralow wear in dry environments because a moisture-dependent tribochemical degradation product is necessary to anchor and stabilize its protective transfer films. A recent study Onodera et al., J. Phys. Chem. C, 2017, 121, 14589–14596 on a PEEK–PTFE composite suggested that the poly(etheretherketone) (PEEK) filler particles anchor PTFE transfer films to metallic surfaces via physical interactions that are, theoretically, insensitive to environmental moisture. This study tested the hypothesis that the physical nature of transfer film adhesion by PEEK–PTFE increases its wear tolerance to changes in environmental moisture. The optimal 20 wt % PEEK–PTFE composite exhibited the same ultralow-wear rates (8 × 10–8 ± 1 × 10–8 mm3/Nm) and low friction coefficients (0.18 ± 0.02) in dry nitrogen (0.05% RH) and humid air (30% RH). The results demonstrate that this unusually wear-resistant solid lubricant material is also unusually insensitive to environmental moisture. Compared to the well-studied alumina–PTFE system, whose ultralow-wear rates correlate strongly to the prominence of carboxylate peaks in infrared (IR) spectra, carboxylate peaks were either greatly attenuated or absent in the IR spectra of PEEK–PTFE following ultralow-wear sliding in both humid and dry environments. The results are consistent with the prediction from the Onodera group that the ultralow-wear rates of PEEK–PTFE can be retained in dry environments because the strong physical interactions between the PEEK filler and the counterface reduce or eliminate its dependence on water-dependent tribochemistry for transfer film adhesion.
Ultra-low-wear PTFE nanocomposites rely heavily on water-dependent tribochemistry, which reinforces surfaces by anchoring tribochemically-modified chains to nanofillers and the countersurface. In a ...recent study, we showed that trace nanofillers (0.1 wt%) reduced the wear rates of an already low wear PEEK-PTFE blend by 40-fold with minimal tribochemistry. Interestingly, wear rates increased by 2-fold at 5 wt% nanofillers despite increased tribochemical accumulation. This observation raises questions about the tribochemical and mechanical roles of nanofillers in this material system, particularly at the surface. This paper aimed to isolate these effects. Specifically, we varied environmental humidity to promote or inhibit favorable tribochemical accumulation while maintaining subsurface stability via PEEK reinforcement. When we discouraged tribochemical reinforcement using a dry environment, high loadings of nanofillers had severely detrimental effects on wear rates and tribofilm stability. For example, the addition of 5 wt% nano-alumina to 5 wt% PEEK-PTFE increased wear rates by > 100-fold in the dry environment. By contrast, the addition of trace amounts (0.1 wt%) of nano-alumina had no detrimental effect on wear rate (10−7 mm3/Nm) or tribofilm stability. These results suggest that the mechanical effects of nanofillers were primarily destabilizing rather than stabilizing and that these effects increased with filler loading. In humid environments, however, these adverse effects of nanofillers, particularly at loadings >1 wt%, were offset by the favorable competing effect of tribochemical accumulation. Trace nanofiller loadings (∼0.1 wt%) optimized surface reinforcement at both environmental extremes because they provided the tribochemical benefits of the nanofillers while minimizing their mechanical costs.
•Tested the hypothesis that trace nanofillers reduce PTFE wear by non-tribochemical means.•Rejected the hypothesis and demonstrated detrimental mechanical effects from nanofillers.•Showed that trace nanofillers optimized PTFE wear performance in dry and humid environments.•Trace nanofillers provide tribochemical benefits while minimizing mechanical costs.
In dry sliding conditions, polytetrafluoroethylene (PTFE) composites can form thin, uniform, and protective transfer films on hard, metallic counterfaces that may play a significant role in friction ...and wear control. Qualitative characterizations of transfer film morphology, composition, and adhesion to the counterface suggest they are all good predictors of friction and, particularly, wear performance. However, a lack of quantitative transfer film characterization methods and uncertainty regarding specific mechanisms of friction and wear control make definitive conclusions about causal relationships between transfer film and tribological properties difficult. This paper reviews the state of the art in the solid lubricant transfer film literature and highlights recent advances in quantitative characterization thereof.
A particular nanosized alumina (α-Al2O3) filler reduces the wear rate of polytetrafluoroethylene (PTFE) by nearly 4 orders of magnitude in ambient environments through the formation of stable ...interfacial tribofilms. One key to the unusual success of this system is the tribochemical degradation of the polymer and subsequent formation of carboxylate salts, which directly bond the PTFE to both the alumina nanofiller and the metallic counterface. However, previous studies have shown that the exceptional wear resistance of these cross-linked, stable, and well-anchored surfaces vanishes when slid against surfaces of slightly higher surface energy. In this paper, we elucidate the effects of these interfacial gradients within the native ultralow wear composite-on-transfer film system using interrupted wear tests and intermittent surface analysis. As anticipated, the transition from high wear to ultralow wear was accompanied by small adherent debris, tribochemical formation of carboxylates, increased surface energy, and increased adhesion. Interestingly, we observed significant differences on either side of the interface during low wear sliding; compared to the running films on the composite surface, the transfer films on the counterface exhibited consistently greater tribochemical degradation, surface energy, and adhesion to a model alumina probe. This interfacial gradient, we propose, is a necessary feature of the ultralow wear system and functions by setting the direction and driving force for transfer wear. In this case, the interfacial gradient stabilizes the transfer film and minimizes the driving force for running film wear.
This paper presents a PEEK filled PTFE composite that exhibits low friction and ultra-low wear. It is hypothesized that a synergistic effect shuts down the dominant wear mechanism of each ...constituent. The friction coefficient and wear rate of this composite material on lapped stainless steel were evaluated for samples with PEEK wt.% of 0, 5, 10, 20, 30, 40, 50, 70 and 100 using a linear reciprocating tribometer. Tests were performed in filtered, standard laboratory conditions with a nominal contact pressure of 6.35
MPa, a speed of 50
mm/s and total sliding distances ranging from 0.5
km for the unfilled PTFE to 140
km for a 20
wt.% PEEK filled sample. The friction coefficients, averaged over an entire test, ranged from
μ
¯
=
0.111
for a 50
wt.% composite to
μ
¯
=
0.363
for unfilled PEEK. Wear rates ranged from
K
=
2.3
×
10
−9
mm
3/(Nm) for a 20
wt.% PEEK sample to
K
=
6
×
10
−4
mm
3/(Nm) for unfilled PTFE. Scanning electron microscopy revealed a unique interfacial connection between the PTFE and PEEK that is likely responsible for the ultra low wear rates observed in these experiments.
Polychlorinated biphenyls (PCBs) are a primary contaminant of potential concern at the Newtown Creek superfund site. Measurements of PCBs in hundreds of samples of sediment (surface and cores) within ...Newtown Creek and at nearby reference locations were obtained from the Remedial Investigation (RI) databases. This data set was analyzed using Positive Matrix Factorization (PMF). A weight-of-evidence approach was used to attribute the PMF-generated fingerprints to sources. The PMF analysis generated eight factors (fingerprints or sources) that represent primary sources, such as Aroclors, as well as secondary sources, including the East River and Combined Sewer Outfalls (CSOs). In addition to the high-production volume Aroclors (1016/1242, 1248, 1254, and 1260), some less-widely used Aroclors (1232 and 1268) were found in Newtown Creek sediment. Aroclor 1268 is disproportionately abundant in the deepest sediments, while PCBs likely from CSOs are relatively more abundant in surface sediment.
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•PCB congeners were measured in 490 sediment samples from a Superfund Site.•This data was analyzed using Positive Matrix Factorization (PMF) to understand sources.•Monsanto's Aroclors were responsible for most of the PCBs found in sediment.•The secondary sources of the East River and Combined Sewer Outfalls were identified and quantified.
The interstitial fluid within articular cartilage shields the matrix from mechanical stresses, reduces friction and wear, enables biochemical processes, and transports solutes into and out of the ...avascular extracellular matrix. The balanced competition between fluid exudation and recovery under load is thus critical to the mechanical and biological functions of the tissue. We recently discovered that sliding alone can induce rapid solute transport into buried cartilage contact areas via a phenomenon termed tribological rehydration. In this study, we use in situ confocal microscopy measurements to track the spatiotemporal propagation of a small neutral solute into the buried contact area to clarify the fluid mechanics underlying the tribological rehydration phenomenon. Sliding experiments were interrupted by periodic static loading to enable scanning of the entire contact area. Spatiotemporal patterns of solute transport combined with tribological data suggested pressure driven flow through the extracellular matrix from the contact periphery rather than into the surface via a fluid film. Interestingly, these testing interruptions also revealed dynamic, repeatable and history-independent fluid loss and recovery processes consistent with those observed in vivo. Unlike the migrating contact area, which preserves hydration by moving faster than interstitial fluid can flow, our results demonstrate that the stationary contact area can maintain and actively recover hydration through a dynamic competition between load-induced exudation and sliding-induced recovery. The results demonstrate that sliding contributes to the recovery of fluid and solutes by cartilage within the contact area while clarifying the means by which it occurs.
Healthy articular cartilage is a remarkable bearing material optimized for near-frictionless joint articulation. Because its limited self-repair capacity renders it susceptible to osteoarthritis ...(OA), approaches to reinforce or rebuild degenerative cartilage are of significant interest. While exogenous collagen crosslinking (CXL) treatments improve cartilage's mechanical properties and increase its resistance to enzymatic degradation, their effects on cartilage lubrication remain less clear. Here, we examined how the collagen crosslinking agents genipin (GP) and glutaraldehyde (GTA) impact cartilage lubrication using the convergent stationary contact area (cSCA) configuration. Unlike classical configurations, the cSCA sustains biofidelic kinetic friction coefficients (μk) via superposition of interstitial and hydrodynamic pressurization (i.e., tribological rehydration). As expected, glutaraldehyde- and genipin-mediated CXL increased cartilage's tensile and compressive moduli. Although net tribological rehydration was retained after CXL, GP or GTA treatment drastically elevated μk. Both healthy and "OA-like" cartilage (generated via enzymatic digestion) sustained remarkably low μk in saline- (≤0.02) and synovial fluid-lubricated contacts (≤0.006). After CXL, μk increased up to 30-fold, reaching values associated with marked chondrocyte death in vitro. These results demonstrate that mechanical properties (i.e., stiffness) are necessary, but not sufficient, metrics of cartilage function. Furthermore, the marked impairment in lubrication suggests that CXL-mediated stiffening is ill-suited to cartilage preservation or joint resurfacing.
Articular cartilage's remarkable low-friction properties are essential to joint function. In osteoarthritis (OA), cartilage degeneration (e.g., proteoglycan loss and collagen damage) decreases tissue ...modulus and increases permeability. Although these changes impair lubrication in fully depressurized and slowly slid cartilage, new evidence suggests such relationships may not hold under biofidelic sliding conditions more representative of those encountered in vivo. Our recent studies using the convergent stationary contact area (cSCA) configuration demonstrate that articulation (i.e., sliding) generates interfacial hydrodynamic pressures capable of replenishing cartilage interstitial fluid/pressure lost to compressive loading through a mechanism termed tribological rehydration. This fluid recovery sustains in vivo-like kinetic friction coefficients (µk<0.02 in PBS and <0.005 in synovial fluid) with little sensitivity to mechanical properties in healthy tissue. However, the tribomechanical function of compromised cartilage under biofidelic sliding conditions remains unknown. Here, we investigated the effects of OA-like changes in cartilage mechanical properties, modeled via enzymatic digestion of mature bovine cartilage, on its tribomechanical function during cSCA sliding. We found no differences in sliding-driven tribological rehydration behaviors or µk between naïve and digested cSCA cartilage (in PBS or synovial fluid). This suggests that OA-like cartilage retains sufficient functional properties to support naïve-like fluid recovery and lubrication under biofidelic sliding conditions. However, OA-like cartilage accumulated greater total tissue strains due to elevated strain accrual during initial load application. Together, these results suggest that elevated total tissue strains—as opposed to activity-mediated strains or friction-driven wear—might be the key biomechanical mediator of OA pathology in cartilage.
Osteoarthritis (OA) decreases cartilage's modulus and increases its permeability. While these changes compromise frictional performance in benchtop testing under low fluid load support (FLS) conditions, whether such observations hold under sliding conditions that better represent the joints’ dynamic FLS conditions in vivo is unclear. Here, we leveraged biofidelic benchtop sliding experiments—that is, those mimicking joints’ native sliding environment—to examine how OA-like changes in mechanical properties effect cartilage's natural lubrication. We found no differences in sliding-mediated fluid recovery or kinetic friction behaviors between naïve and OA-like cartilage. However, OA-like cartilage experienced greater strain accumulation during load application, suggesting that elevated tissue strains (not friction-driven wear) may be the primary biomechanical mediator of OA pathology.
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