The chemical nature of metal-polytetrafluoroethylene tribological interactions as studied by X-ray photoelectron spectroscopy

doi:10.1016/0043-1648(79)90115-7

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Volume 54, Issue 2, June 1979, Pages 211-215

https://doi.org/10.1016/0043-1648(79)90115-7 Get rights and content

Abstract

Metallic fluorides are shown by X-ray photoelectron spectroscopy to be present at the metal-polymer interfaces after stainless steel and nickel samples are rubbed with polytetrafluoroethylene under atmospheric conditions.

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Counterface roughness is known to affect the tribological behavior of carbon fiber reinforced PTFE. However, the effect of roughness in trace moisture environments has not yet been extensively investigated. In this study, the tribological behavior and tribofilm formation were evaluated for a carbon fiber reinforced PTFE composite sliding in a trace moisture environment against 34CrNiMo6 steel counterfaces with different roughness. Tribotests were conducted with a three-pin-on-disc tribometer at a sliding velocity of 2.2 m/s and in a nitrogen environment with moisture content controlled to 11 ppm. Generally, smoother counterfaces gave lower wear, both during running-in and steady-state. Contrarily, the coefficient of friction was only affected by roughness during running-in. Surface analysis from different stages of running-in were done to elucidate the formation of tribofilms and their different characteristics. For the rough countersurface, a loosely adhered transfer film is transitionally formed at the beginning of sliding to enable the formation of a persistent transfer film. Contrarily, for the case of a smooth countersurface, the formation of a persistent transfer film is initiated from the start. Similarly for the rough and smooth countersurface, a micrometer thick tribofilm with excellent low friction properties is observed on the PTFE composite after running-in.
Tribological properties of PAO40@SiO<inf>2</inf>/PTFE/aramid fabric composites subjected to heavy-loading conditions
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The content of F-C bond in the transfer film associated with the shaft contacting with Fabric-0% appeared to be significantly higher than that of Fabric-6% SiO2 and Fabric-6% PSMS. More importantly, metal-F (685 eV) bonds [33–36] were observed for the transfer films in relation to Fabric-6% SiO2 and Fabric-6% PSMS, but were almost invisible when rubbing with Fabric-0%. This phenomenon inferred that the silica particles could promote the tribo-chemical reaction between PTFE and the shaft surface to form the metal-F bonds.
PAO40@SiO₂ microcapsules (PSMS) were prepared by adsorbing Poly α-olefin lubricating oil 40# (PAO40) with hollowed SiO₂ particles. The tribological properties of polytetrafluoroethylene (PTFE)/aramid fabric composites at different microcapsules contents were evaluated by the ring-block tests. The friction coefficient and wear rate of PTFE/aramid fabric composites containing 6 wt% PSMS (Fabric-6% PSMS) were reduced by 42.9% and 31.5% respectively, as opposed to those of neat PTFE/aramid fabric composites (Fabric-0%). The service life of Fabric-6% PSMS under radial pressure of 240 MPa was increased by 5.3 times as compared with that of Fabric-0%. The wear mechanisms of Fabric-6% PSMS revealed that SiO₂ particles promoted the formation of metal-F bonds on the shaft surfaces and further improved the abrasive resistance of transfer films.
Easy plasma nano-texturing of PTFE surface: From pyramid to unusual spherules-on-pyramid features
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Hence, though PTFE likely loses fluorine, the impinging metal atoms capture F in fluoride form and as a matter of fact the total F/C atomic ratio stays fairly constant. The formation of metal fluoride, in particular FeF2, is confirmed by the position of the maximum of the Fe 2p3/2 peak, 712 eV, compatible with the presence of such species (see Fig. 7) [22,24,25]. It should be here highlighted that, looking at the shape of the Fe2p region, iron is likely present in various oxidation states, even metal, at 706 eV.
Oxygen plasma etching of polytetrafluoroethylene has been investigated as a way to sculpture nano-features on its surface and producing a super-hydrophobic material in one step.
It has been found that, depending on the experimental conditions, the shape of the structures engraved in the polymer surface passes from pyramidal to unique pyramids with well-defined spherules on the top. The latter are obtained without any mask/lithographic option.
In a previous paper the water repellent character of such etched polytetrafluoroethylene surface has been well documented and explained by some of the authors. In this paper, instead, some insights in the formation mechanism of such unusual morphology are provided. Chemical and morphological characterization indicate in the iron surface contamination the clue for the formation of such complex agglomerates.
Ultralow wear Perfluoroalkoxy (PFA) and alumina composites
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PTFE has been shown to transfer to a metal countersample after only a single sliding cycle [51,52]. During these transfer events, carbon–carbon bonds are broken, presenting an opportunity for tribochemistry [5,6,8,11,17,46,47,51–60]. Recently, the mechanism behind this remarkable material has been investigated using various research tools including XPS, FTIR, AFM, nano-indentation, 3D X-ray microtomography, and environmental/vacuum testing [8–12,17].
Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1–5×10⁻⁴ mm³/Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel (P=6.3 MPa, v=50.8 mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10⁻⁴ mm³/Nm, and dropped to 4.0×10⁻⁸ mm³/Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well.
Formation of metal-F bonds during frictional sliding: Influence of water and applied load
2016, Applied Surface Science
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Two evident F peaks at around 689.0 eV and around 685.3 eV are seen on the worn Al2O3 surface under dry sliding, whilst hardly any peak is observed on the worn Al2O3 surface sliding in water-lubricated conditions. The 685.3 eV peak points at the existence of metalF bonds, while the peak at 689.0 eV is assigned to CF bonds in PTFE [3,19,20]. The detected 689.0 eV peak suggests the existence of PTFE transfer films on Al2O3 ball surface, while the detected 685.3 eV peak indicates the formation of AlF bonds on Al2O3 ball surface.
Effects of water lubrication and applied load on the formation of PTFE transfer films and metalF bonds during sliding when PTFE filled composites sliding against steel and Al₂O₃ are investigated. In water-lubricated conditions, XPS analysis reveals that a thin layer of water molecules at the sliding interface inhibits the formation of PTFE transfer films and AlF bonds on the Al₂O₃ ball, leading to a detrimental effect on the tribo-performance. Under various normal loads in dry sliding condition, it is found that the smearing of PTFE onto the wear surface of the composite and the transfer of PTFE onto the surface of the steel counterpart are stimulated by a high load. During sliding, the contact pressure is found to be the driving force of the reaction between steel and PTFE. It is concluded that under various loads, the total amount of PTFE transfer films has a larger impact on the friction behavior than the formation of FeF bonds.
Experimental study on tribological properties of carbon/polytetrafluoroethylene hybrid fabric reinforced composite under heavy loads and oil lubrication
2016, Tribology International
Carbon fabric reinforced composites have drawn much attention currently. Special carbon fabric reinforced composites were fabricated with surface layers modified by carbon/PTFE hybrid fabrics. The tribological properties of these composites under heavy loads were evaluated. The results showed that tribological properties of the composite were improved with increasing the PTFE content. The friction coefficient, volume wear and temperature rise of the composite with 6 yarns of PTFE were reduced by 40%, 91% and 64% respectively in comparison with the composite without PTFE. After testing, PTFE could be observed over the entire wear groove surface, including the bottom.

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The chemical nature of metal-polytetrafluoroethylene tribological interactions as studied by X-ray photoelectron spectroscopy

Abstract

Wear

Wear

Anal. Chem.

Wear

Appl. Spectrosc.

Friction and Lubrication of Solids

Friction and Lubrication of Solids

Plain and filled plastics materials in bearings: a review

Nature (London)

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