Friction and wear of PTFE composites at cryogenic temperatures1
Introduction
New technologies lead to a growing interest in material investigations for low temperature applications, in particular polymers and composites as an alternative to metals. Good experiences were made thereby with polymers, particularly with PTFE and PTFE filled composites [1], [2]. It is well known that the material properties of polymers are strongly temperature-dependent. Young’s modulus and hardness are much higher at low temperatures compared to room temperature [3], [4], whereas the already low heat conductivity continues to decrease. A consequence of this is the change in wear mechanism of the polymers at low temperatures. While at ambient temperature adhesive wear dominates, abrasive wear is observed at low temperatures [1], [5].
Tribological performances of polymers depend significantly on the temperature at the friction contact. That applies also to very low temperature tribosystems, especially for polymers with low heat conductivity.
More recently, some tribological studies in cryogenic environment [6], [7], [8], [9] showed that both friction and wear are reduced due to the change in adhesion and deformation characteristics of the polymer materials. However, it was stated that this trend is not linear down to extreme low temperatures, but strongly depends on the cooling medium [9]. At T=4.2 K in LHe, friction and wear increased compared to the values measured at T=77 K in LN2.
Concerning chemical analyses, XPS studies of filled PTFE matrix materials at room temperature [10], [11], [12] showed tribochemical reactions between the PTFE and the metal of the counterface as well as between PTFE and metallic fillers. The same results were observed with EDX (Energy Dispersive X-ray analyses) at room temperature in helium gas [13]. Polymer decompositions and triboreactions were also found for PCTFE in LN2 [5].
The aim of this study is to investigate the tribological behaviour of polymer-composites at low temperatures, with special attention given to the elementary processes at the friction contact. This paper describes experiments carried out in parallel at room temperature and in cryogenic environments and analyses of the polymer transfer occurring during these tribological tests at different temperatures as well as in different media. Transfer films and the surface of the pins were examined, and chemical surface analyses were performed.
Section snippets
Materials
The materials investigated here are two types of PTFE matrix composites chosen from experiments carried out at room temperature [14]. Table 1 presents the composition of these PTFE composites. Figure 1 shows the structural formula of the polymers. The processing of these PTFE compounds takes place via a sinter process at a temperature between 360°C and 380°C. The materials were delivered as compression-moulded cylinders, which were cut into pins (surface of contact 4×4 mm). To shorten the
Friction measurement
Figure 2 shows the coefficient of friction of two PTFE composites at RT in air, at T=77 K in He-gas and LN2 and at T=4.2 K in LHe, for the two sliding speeds v=1 m/s and v=0.2 m/s. The graphs clearly demonstrate the influence of temperature but also of cooling properties of the inert environment on the results.
The coefficient of friction of PTFE composites decreases from room temperature to T=77 K, with a minimal value in LN2. However, this decrease is not linear down to extreme low temperature
Influence of thermal properties of environments and materials
As described in [8], [9], the coefficient of friction of PTFE composites decreases from room temperature to T=77 K in LN2. This can be explained by the increased hardness of polymers at low temperature and a decrease in friction due to deformation. However, the temperature of the cryogen is not the significant parameter to understand and explain the tribological behaviour of these composites—and generally speaking of materials—at low temperature. The determinant factor is the
Conclusion
This work presents investigations on the tribological behaviour of PTFE composites against steel at cryogenic temperatures. It can be stated that thermal properties of the cryogenic medium have a significant influence on the tribological performance of the polymer composites. The generation of a gaseous film around the friction contact decreases significantly the cooling ability of the environment. Therefore, the effect of the low temperatures on the material properties was more clearly
Acknowledgements
The experiments were executed by Mr O. Berndes and M. Heidrich. Surface analyses of the samples were carried out in the BAM laboratories VIII.11, VIII.12 and VIII.23. Materials samples were provided by the companies Norton and UPM GmbH. This work is subsidised by the Germany Research Association (DFG) under the project numbers Hu 791/2-1 and Fr 675/31-1.
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- 1
Extended version of the paper presented at the 2nd World Tribology Congress, Vienna, 3–7 September 2001.