Friction and wear of PTFE composites at cryogenic temperatures

The use of polymers and polymer composites in various tribological situations has become state of the art. Nevertheless, new developments are still under way to explore new fields of application for these materials and to tailor their properties for more extreme loading conditions. Some of these developments can be followed when searching through the 56 references given at the end of this review. The present overview describes, in particular, some of the authors' approaches in designing polymeric composites in order to operate under low friction and low wear against steel counterparts. Special emphasis is focused on thermoplastics and thermosets reinforced with special fillers (including nanoparticles). An attempt is made to predict their wear properties and to carry out systematic parameters studies by the use of artificial neural networks. Further information will be given on the fibre orientation dependence of wear of continuous carbon fibre-polymer matrix composites, and on attempts to predict...

Wear of polymer composites

The temperature dependence of the kinetic friction between a highly oriented pyrolytic graphite surface and a silicon nitride probe tip has been evaluated through atomic force microscopy measurements performed under an ultra-high vacuum environment over the temperature range 140–750 K. As temperature increases from 140 to 400 K a sharp decrease in friction is observed. A relatively weaker dependence on temperature is observed in the friction measured between 400 and 750 K. Collectively, these results obtained from a fundamental interface are consistent with an activated mechanism of energy dissipation during sliding.

Thermally Activated Friction

Both the academic society and the industry are hunting for new energy forms for the future. However, the world should not forget the conventional technologies that contribute to the sustainable society by technical innovations. Among them, lubrication plays a significant role in energy saving and in low CO2 emission by increasing the fuel efficiency and by prolonging the service life of machines. With the advance of novel synthetic approaches, and nanoscience and technologies, novel lubrication oils and additives and their formulations are being developed to reduce friction and wear, and novel surface treatment routes and surface coatings are invented and provide more efficient lubrication. These technologies create tremendous chances for machines to work more efficiently with low energy consumption. Here we review the recent progresses and challenges associated with some novel lubrication techniques that include novel surface treatment (such as texturing, high-performance nanocomposite coatings, adapting coating), tribology design (solid and liquid lubrication), energy-conserving engine oil and novel lubricants and formula (such as ionic liquids, low S, P content additives) which are to be adopted to enhance the fuel efficiency to achieve energy saving and low carbon emission. There is increased demand to replace fossil lubricants by degradable green lubricants. Specially designed coatings can reduce drag significantly during navigation of both airplanes and ships. All these aspects will be also reviewed in the paper.

Lubricating a bright future: Lubrication contribution to energy saving and low carbon emission

Solid lubricants used in aerospace applications must provide low friction and a predictable operation life over an extreme range of temperatures, environments and contact conditions. PTFE and PTFE composites have shown favorable tribological performance as solid lubricants. This study evaluates the effect of temperature on the friction coefficient of neat PTFE, a PTFE/PEEK composite and an expanded PTFE (ePTFE)/epoxy coating. These experiments evaluate friction coefficient over a temperature span which, to the investigators’ knowledge, has not been previously examined. Results show a monotonic increase in friction coefficient as sample surface temperature was decreased from 317 to 173 K for all three samples. The frictional performance of these and other published solid lubricant polymers was modeled using an adjusted Arrhenius equation, which correlates the coefficient of friction of the polymer materials to their viscoelastic behavior. A model fit of all the polymer data from 173 to 450 K gives an activation energy of 3.7 kJ/mol. This value suggests that breaking of van der Waals bonds is the likely mechanism responsible for the frictional behavior over this temperature range.

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Cryogenic Friction Behavior of PTFE based Solid Lubricant Composites

In the cryogenic temperature range common lubricants solidify and therefore, they cannot be applied. Thus, for tribosystems in low temperature environment technical solutions based on good dry lubricating properties of the materials themselves must be found. In this concern polymers are of great interest, because they show comparatively high mechanical strength at low density, and they often possess favourable friction and wear properties at low temperatures. 



This work presents investigations on the tribological behaviour of carbon fibre (CF) filled PTFE and PEEK composites in a cryogenic environment (liquid nitrogen; T = −196 °C). In general, these materials clearly exhibit lower friction and wear properties at low temperatures than at room temperature, which can be explained mainly by the temperature dependence of the mechanical strength. With increasing amount of carbon fibres in both composites, friction and wear increase. The optimal composition was found to be at 5 wt.% CF for the friction and 15 wt.% CF for the wear values. At low temperatures, the percentage of carbon fibres in PTFE and PEEK composites has not such an important influence as it has at room temperature.



Reibung und Verschleis von kohlefasergefullten Polymeren bei Raum- und Tieftemperaturen



Im Temperaturbereich der Kryotechnik verfestigen herkommliche Schmiermittel und konnen daher nicht verwendet werden. Daher mussen fur Tribosysteme in Tieftemperaturumgebung technische Losungen gefunden werden, die auf guten Trockenreibungseigenschaften der eingesetzten Werkstoffe beruhen. Polymerwerkstoffe sind hierfur von besonderem Interesse, da sie neben relativ hohen Festigkeitswerten bei sehr niedriger Dichte haufig gute Reibungs- und Verschleiseigenschaften bei tiefen Temperaturen aufweisen. 



In dieser Arbeit werden Untersuchungen an Kohlenstofffaser-(CF-)gefullten PTFE- und PEEK-Materialien hinsichtlich ihres tribologischen Verhaltens in einem kryogenen Umgebungsmedium (Flussigstickstoff; T = −196 °C) vorgestellt. Es zeigte sich, dass bei diesen Werkstoffen allgemein sowohl die Reibung als auch der Verschleis bei tiefen Temperaturen erheblich geringer sind als bei Raumtemperatur, was hauptsachlich durch die Temperaturabhangigkeit der mechanischen Festigkeit zu erklaren ist. Mit steigendem Faseranteil nehmen Reibung und Verschleis bei beiden Werkstoffgruppen zu. Ein Optimum wurde bei 5 Gew.% CF fur die Reibung und 15 Gew.% CF fur den Verschleis gefunden. Bei tiefen Temperaturen hat der Anteil der Kohlenstofffasern wesentlich geringeren Einfluss als bei Raumtemperatur.

Friction and Wear of Carbon Fibre Filled Polymer Composites at Room and Low Temperatures

Outstanding features favour the application of polymers and polymer composites in low-temperature technology. The booming hydrogen technology is a challenge for these materials, which are considered as seals and bearings in cryogenic pumps. In the present study, three types of thermoplastics, i.e., polyetheretherketone (PEEK), polyetherimide (PEI) and polyamide 6,6 (PA6,6), and one epoxy were considered as matrix materials. Micron-sized fillers, i.e., short carbon fibres, graphite flakes, and PTFE powders, were incorporated into these polymers together with nano-sized TiO2 particles. Optimised compositions of each matrix were selected from our previous works at room temperature in order to be studied at very low temperature conditions. In particular, frictional tests were carried out with polymer composite pins against polished steel surfaces under constant load over a certain distance in liquid hydrogen and liquid nitrogen. Afterwards, worn surfaces were analysed by using scanning electron microscopy (SEM). It was found out that the tribological properties in liquid hydrogen are dominated by the matrix materials, in particular thermoplastics perform generally slightly better than thermosetting resins.

Sliding performance of polymer composites in liquid hydrogen and liquid nitrogen

New technologies lead to a growing interest in material investigations for low temperature applications, in particular polymers and composites as an alternative to metals. It is well known that tribological performances of polymers improve at low temperatures due to an increase in hardness and young's modulus. This results in particular in a change of the wear mechanism, from adhesive at room temperature to abrasive at lower temperature. However, this change in material properties is not systematic but depends on the polymer structure and on the composites. This paper presents the tribological behaviour of PA6, PI, PTFE filled PI and bronze filled PTFE composites in cryogenic environment. Transfer films were analysed by means of SEM, AFM, as well as XPS. One aspect shown in this study is the important role of the test parameters and the influence of temperature and medium on the tribological behaviour of the materials. The effect of friction heat at the contact area on the cooling action of the cryogen liquid is notably observed in this investigation.

P. Klein

Papers

Friction and wear of PTFE composites at cryogenic temperatures

Friction and Wear of Carbon Fibre Filled Polymer Composites at Room and Low Temperatures

Sliding performance of polymer composites in liquid hydrogen and liquid nitrogen

Der Einfluss kryogener Medien auf den Materialtransfer im Tribosystem Polymerkomposit/Stahl