Thomas Gradt

Friction and wear of PTFE composites at cryogenic temperatures

This paper presents investigations on the tribological behaviour of PTFE composites against steel at cryogenic temperatures. The results showed that the friction coefficient decreases with temperature down to 77 K, but did not follow a linear evolution further down to extreme low temperatures. It can be stated that the cryogenic environment has a significant influence on the tribological performance of the polymer composites. The effect of low temperatures was more clearly detected at low sliding speed, where friction heat is reduced. A change in wear mechanism from adhesive to abrasive was observed in this case. SEM and AFM analyses showed that the PTFE matrix composites investigated under these experimental conditions have transferred material onto the disc down to very low temperatures. Chemical analyses indicate the presence of iron fluorides.

Low temperature tribometers and the behaviour of ADLC coatings in cryogenic environment

Abstract In a cryogenic environment components with interacting surfaces in relative motion (tribosystems) like bearings, seals and valves often generate undesired heat and experience high wear. Because conventional lubricants like oils or greases cannot be used in this temperature range, solid lubricants or materials with good frictional properties in unlubricated operation must be used. To test the suitability of conventional and advanced materials for tribosystems in this extreme environment and to obtain reliable materials data, model friction tests at low temperatures are performed. In most of the tests the samples are in ball-on-disc-configuration but various methods for cooling, loading, and measurement are applied. As an example the test results for amorphous carbon coatings in the temperature range 10–77 K are presented. These coatings can be suitable for cryogenic tribosystems, but their behaviour strongly depends on the composition and deposition method.

Friction and wear at low temperatures

For investigations of the friction and wear behaviour of materials under cryogenic conditions a special test device (Cryotribometer) has been constructed. The friction tests are performed with samples in a pin-on-disc configuration in a He-gas environment at temperatures between 8 and 77 K. Most of the tests were performed with filled and unfilled polymers like polyterafluorethylene (PTFE), polyamide (PA), polyoxymethylene (POM) and polyimide (PI) against steel. Compared to room temperature most polymers have an improved frictional behaviour in dry sliding at low temperatures, due to their increasing hardness and mechanical strength. Some experiments with hard carbon coatings indicate that the running-in and wear behaviour of these materials is influenced mainly by temperature.

Influence of the temperature on the tribological behaviour of PEEK composites in vacuum environment

This paper describes tribological experiments carried out with polyetheretherketone (PEEK), filled with carbon fibres and solid lubricants (polytetrafluoroethylene (PTFE), graphite or MoS2), against steel discs. Oscillating sliding tests were performed in high vacuum environment in the temperature range between -40°C and +160°C. Results indicate that MoS2 filled PEEK show the best tribological performance in vacuum. Particularly, in the lower temperature range and at higher loads the friction behaviour is improved by the MoS2 content.

Frictional Behavior of Material Couples in Superconducting MRI Magnet Systems at 4.2 K

Windings in superconducting magnets operating at 4.2 K are highly susceptible to quenching caused by small frictional heat inputs. Small movements in the magnet system are inevitable during ramping due to the increasing electromagnetic forces. Friction pairs of polymer based materials have been investigated at 4.2 K to gain an understanding of their sliding behavior in conditions representative of superconducting MRI systems. The results indicate that polymer-polymer pairs experience unstable sliding behavior with repeated stick-slip whereas polymer-aluminum couples have stable sliding behavior up to high contact pressures of 20 MPa.

Temperature behaviour of permeation of helium through Vespel and Torlon

Abstract Gases such as helium can diffuse through polymers in considerable amounts. The increasing use of polymers in cryogenic engineering may make it necessary to estimate the magnitude of this effect to judge whether it may be dangerous in vacuum insulations and the like. This paper reports measurements on the solubility and the diffusion constant of helium in two widely used polymers as a function of temperature. A continuous flow cryostat and a helium leak detector were the key elements of the experimental set-up. The results show that while the permeation flow of helium through polymers can be considerable at ambient temperature, it ceases to be a problem at temperatures of 200 K or below. For certain applications, however, this effect may be a serious problem and must be considered.

Friction and Wear of Polymer Materials at Cryogenic Temperatures

Polymers are extensively used for sliding systems in cryogenic applications because of their favourable friction and wear behaviour in the absence of external lubrication. Since important new technologies are based on applications under extreme conditions, such as at low temperatures, new requirements on material properties, in particular regarding their operability and reliability, must be met. Up to now, most tribological investigations have been carried out in inert cryogens or cryogenic gas (He, N2). Few experiments have been performed in vacuum environment at cryogenic temperatures. Rarely were testing in reactive media, such as LH2 or LOX. Due to the wide range of operating conditions in cryogenic applications, it is difficult to state general rules. Therefore, this chapter tends to give an overview on theories and experimental studies on polymer tribology at cryogenic temperatures.

Friction and Wear Testing at Cryogenic Temperatures

Publisher Summary In cryogenic environment components with interacting surfaces in relative motion (tribosystems) like beatings, seals and valves cannot be lubricated conventionally by using oils or greases. Therefore, they are critical in respect to wear and frictional heat generation. To obtain reliable data of the properties of conventional and advanced materials in cryogenic tribosystems, model wear tests at low temperatures are performed. Tests with polymer-steel couples verified that some polymers have a favorable tribological behavior at low temperatures. Coatings of amorphous carbon are on principle suitable for cryogenic tribosystems, but their behavior depends strongly on the composition and deposition method. The results of the tribological model tests of polymers show that the basic wear mechanisms of polymers, deformation and adhesion, are still acting at low temperatures but the increasing mechanical strength give rise to a reduced wear. Tests with hard carbon coatings show that they are appropriate for tribosystems in cryogenic environment. Temperature dependent low and high friction states show that further investigations on this material are necessary.