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Mechanisms behind the environmental sensitivity of carbon fiber reinforced polytetrafluoroethylene (PTFE)
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0003-0996-7827
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0003-3157-4632
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0002-4271-0380
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0001-6085-7880
2024 (English)In: Friction, ISSN 2223-7690, E-ISSN 2223-7704, Vol. 12, no 5, p. 997-1015Article in journal (Refereed) Published
Abstract [en]

Carbon fiber reinforced polytetrafluoroethylene (CF/PTFE) composites are known for their exceptional tribological performance when sliding against steel or cast iron in inert gas environments. Compared to experiments in humid air, about an order of magnitude lower wear rate and several times lower coefficient of friction have been reported for tests conducted in dry nitrogen and hydrogen. Moreover, trace moisture has been shown to affect the friction and wear significantly of this tribosystem, although a possible effect of oxygen cannot be ruled out due to uncertainties regarding the oxygen concentrations. While several studies have pointed out the environmental sensitivity of CF/PTFE, the understanding of the underlying mechanisms are very limited. The objective of this research is to investigate the individual and combined effect of oxygen and moisture on the tribological behavior of CF/PTFE sliding against steel. Additionally, this study aims to elucidate the underlying mechanisms that govern the environmental sensitivity of the system. Climate-controlled three-pin-on-disc experiments were conducted in nitrogen atmospheres at various concentrations of oxygen and moisture. The tribological results clearly demonstrate that both moisture and oxygen contribute to increased friction and wear. However, the adverse effect was much more pronounced for oxygen than moisture. A qualitative method was developed to estimate the tribofilm coverage on the CF/PTFE surface. Results showed strong correlation between high coverage of strongly adhered tribofilm and low wear rate. Moreover, a loosely adhered tribofilm was observed on top of the CF/PTFE surface in presence of moisture. FTIR analysis indicated that the loosely adhered tribofilm found in the moisture-enriched environment contained a significant amount of adsorbed water, which may explain the lower coefficient of friction in presence of moisture compared to oxygen. The adsorbed water in the loosely adhered tribofilm could be an indication of moisture-driven lubrication by the non-graphitic carbon in the tribofilm.

Place, publisher, year, edition, pages
Springer Nature, 2024. Vol. 12, no 5, p. 997-1015
Keywords [en]
polymer composite, tribofilm, tribochemistry, atmosphere
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
URN: urn:nbn:se:ltu:diva-101605DOI: 10.1007/s40544-023-0824-9ISI: 001113623800003Scopus ID: 2-s2.0-85178446936OAI: oai:DiVA.org:ltu-101605DiVA, id: diva2:1803581
Note

Validerad;2024;Nivå 2;2024-04-02 (hanlid);

Full text license: CC BY 4.0

Available from: 2023-10-09 Created: 2023-10-09 Last updated: 2024-04-02Bibliographically approved
In thesis
1. Tribology of carbon fiber reinforced PTFE composites in dry gas environments
Open this publication in new window or tab >>Tribology of carbon fiber reinforced PTFE composites in dry gas environments
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With hydrogen being one of the key pillars of decarbonization, the reliability and efficiency of hydrogen applications become increasingly important. In applications such as oil-free reciprocating hydrogen compressors and Stirling engines, the self-lubricating pistonrings and other dynamic seals are critical components in terms of efficiency, service life and proper functioning. These components are commonly made from carbon fiber reinforced PTFE (CF/PTFE) composites, which need to work under very dry conditions. Up tothis day, the available literature on the tribology of CF/PTFE in dry gas environments is scarce and the fundamental understanding of the governing mechanisms behind its low friction and wear in such environments is limited. The main aim of this research project is to increase the fundamental understanding of the tribology of CF/PTFE in dry gas environments, focusing on how gas impurities and counterface properties affect the tribological performance, as well as to investigate the governing mechanisms behind the ultralow wear and low friction in inert gas atmosphere. Since fluoropolymers are potential emitters of harmful PFAS during their lifecycle, restrictions towards the use of fluoropolymers are currently considered by the European Chemical Agency (ECHA). Therefore, a secondary aim is to evaluate the tribological performance of other potential carbon fiber reinforced polymers in dry gas environments.

To be able to study the tribology CF/PTFE composites in dry gas environments, a climate control system was developed to enable continuous monitoring and precise control of the moisture and oxygen content at ppm levels. Tribological investigations in gas environments with different levels of oxygen and moisture highlight the environmental sensitivity of CF/PTFE composites. CF/PTFE composites have superior tribological performance in high-purity gas environments, where small amounts of oxygen have a detrimental effect on friction and wear. Moisture also affects the tribological performance negatively. However, the effect of moisture is rather mild in comparison to oxygen and the coefficient of friction remain slow. Analysis of the tribofilms formed on the counterface and the CF/PTFE surface in high-purity and in moisture-enriched gas indicate that the mechanisms behind the low friction and wear are different for the two environments. In high-purity gas, the sliding takes place between an iron fluoride tribofilm on the counterface and a carbon-based tribofilm on the CF/PTFE surface. Molecular dynamics simulations corroborated this finding, where simulations showed a similar coefficient of friction between a non-graphitic carbonsurface and an iron fluoride surface as in the experiments. For the moisture-enriched environment, the low coefficient of friction could be related to the generation of carbon in the sliding interface, which becomes lubricious in the presence of moisture.

CF/PEEK was selected as a potential replacement for CF/PTFE due to its reported low friction when sliding against steel in a dry gas environment. From the evaluation of the tribological performance of CF/PEEK in dry gas environments, it could be concluded that CF/PEEK may be an appropriate replacement for CF/PTFE in moisture-rich environments. However, in moisture-deficient environments, CF/PEEK wears excessively.

The effect of counterface properties on the friction and wear of CF/PTFE composites has been studied by testing different materials, roughness and hardness individually. The counterface material had a distinct effect on friction and further changed the effect of the environment on friction and wear. Roughness only had a slight negative effect on the wear of the CF/PTFE composite during steadystate conditions for the tested range of roughness levels. Moreover, it was found that the coefficient of friction at steady-state conditions is unaffected by roughness, while the friction behavior during running-in varies significantly between a smooth and a rough countersurface. Furthermore, the transient wear of the PTFE composite was higher against a rough counterface than a smooth. Surface analysis from different stages of running-in was done to elucidate the formation of tribofilms and their different characteristics. For the rough counterface, a loosely adhered transfer film is transitionally formed at the beginning of sliding to enable the formation of a persistent transfer film. Contrarily, in the case of a smooth countersurface, the formation of a persistent transfer film is initiated from the start. Hardness did not show any significant effect on friction or wear during steady-state sliding.

In summary, the superior tribological performance of CF/PTFE when sliding against a steel counterface in high-purity gas can be related to the beneficial mechanical and tribochemical degradation of PTFE and carbon fiber in the absence of oxygen and moisture. Since the defluorination of PTFE is key to the formation of robust tribofilms in high-purity gas, PTFE cannot be easily replaced by another polymer that does not contain carbon–fluorine bonds. Contrarily, the tribological performance of carbon fiber reinforced polymers in moisture-rich environments is governed by the carbonfibers, where formed carbon-based tribofilms become lubricious in the presence of moisture. Due to the strong link between tribochemical reactions and the tribological performance of CF/PTFE, the chemical composition of the counterface has a significant effect on the friction and wear of the system.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2023
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-101607 (URN)978-91-8048-401-5 (ISBN)978-91-8048-402-2 (ISBN)
Public defence
2023-12-07, A3024, Luleå tekniska universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, P 44650-1
Available from: 2023-10-12 Created: 2023-10-12 Last updated: 2023-11-30Bibliographically approved

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Johansson, PontusMarklund, PärBjörling, MarcusShi, Yijun

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