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.