TRIBOLOGICAL BEHAVIOUR OF PTFE COMPOSITES AGAINST STEEL AT CRYOGENIC TEMPERATURES G. MULET, W. HÜBNER Federal institute for materials research and testing (BAM), Unter den Eichen 44-46, D-12203 Berlin, GERMANY; e-mail: geraldine.mulet@bam.de P. KLEIN, K. FRIEDRICH Institute for composite materials Ltd. (IVW), University of Kaiserslautern, Erwin-Schrödinger-Strasse, D-67663 Kaiserslautern, GERMANY; e-mail: klein@ivw.uni-kl.de SUMMARY This paper presents some investigations on tribological behaviour of PTFE composites against steel at cryogenic temperatures. The results showed that the friction coefficient decreases with temperature down to 77K, but did not follow a linear evolution down to extreme low temperatures. It could 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 show the presence of iron fluorides.

Keywords: PTFE composite, cryogenic temperature, transfer film, SEM, XPS.

1 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. Previous experiences indicated improvements in tribological performance of polymers at low temperatures due to an increase in hardness and young’s modulus [1,2]. A consequence of this is the change of the wear mechanism of the polymers at low temperatures. While at ambient temperature adhesive wear dominates, abrasive wear is observed at low temperatures [2,3]. Tribological performances of polymers depend significantly on the temperature at the friction contact. This applies also to cryogenic tribosystems, especially for polymers with low thermal conductivity.

More recently, some tribological studies in cryogenic environment [4,5] showed that both the friction and the 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 up to extremely low temperatures, but strongly depends on the cooling medium [6].

Concerning chemical analyses, many XPS studies of filled PTFE matrix materials at room temperature [7,8] showed tribochemical reactions between the PTFE and the metal of the counterface as well as between PTFE and metallic fillers. Same results were observed with EDX (Energy-Dispersive X-ray spectroscopy) at room temperature in helium gas [9]. Polymer decompositions and triboreactions were also found for PCTFE in LN2 [3].

For a better understanding of the tribological behaviour of PTFE-composites at low temperatures, tests were carried out in parallel at room temperature and in

cryogenic environments. The investigations were focussed on the analysis of the polymer transfer occurring during tribological tests at different temperatures as well as in different media. Transfer films were examined, and chemical surfaces analyses were performed.

2 EXPERIMENTS

2.1 Materials The materials investigated here are two types of PTFE matrix composites chosen from the experiments carried out at the IVW [10]: PTFE matrix with 30 % bronze and 10 % carbon fibres and PTFE matrix filled with an aromatic polyester Ekonol® (10 % to 20 %) and carbon fibres (5 % to 20 %). The polymer material was used as pin. The counterface is a disc from steel 100Cr6.

2.2 Tribometers Tribological tests were carried out with a pin on disc assembly in the tribometers CT2 and CT3 described in [10]. At room temperature in air (RT), in liquid nitrogen (LN2, T=77 K) and in liquid helium (LHe, T=4.2 K), the tribometer CT2 was used. The investigations at T=77 K in helium gas (He-gas) were carried out in CT3. The test parameters were:

Normal load : FN= 50 N

Sliding speed : v = 1 m/s and 0.2 m/s

Sliding distance: s = 2000 m

During the tribological test, the frictional force was measured to determine the coefficient of friction. In this experiment, the weight loss of the pin could not be measured since the wear was too small after 2000 m. Consequently, a sensor was built up in CT2 to be able to measure the linear wear. After the experiments the

transfer films were examined with optical and Scanning Electron Microscopy (SEM) as well as with Atom Force Microscopy (AFM). X-ray Photo electron Spectroscopy (XPS) was performed to examine the surface composition of the transfer film and the chemical reactions between the PTFE matrix material and the steel counterface.

0

0,10,2

0,30,4

0,5

Bronze andCF

Ekonol andCF

Bronze andCF

a: v=1m/s b: v=0.2m/s

fric

tion

coef

ficie

nt

Air, RT He-Gas, 77K LN2, 77K LHe, 4.2K

Figure 1: friction coefficient of PTFE matrix composites

at: a) v=1 m/s and b) v=0.2 m/s 3 RESULTS

3.1 Tribological tests The figure 1 shows the coefficient of friction of two PTFE composites at RT in air, at T=77 K in He-gas and in LN2 as well as at T=4.2 K in LHe, for the two sliding speeds v=1 m/s (fig. 1a) and v=0.2 m/s (fig. 1b). The graphs clearly show the influence of the temperature but also of the environment on the results. As seen in [5,6], the tests performed in LN2 give the best results. At a higher speed, the coefficient of friction is the same at T=77 K (He-gas) and at T=4.2 K (LHe). The temperature at the friction contact is high enough to affect the cooling efficiency of the medium. Therefore, the same experiments were performed with a lower speed (v=0.2 m/s). Fig. 1b shows indeed that in this latter case, the coefficient of friction is lower at T=4.2 K in LHe than at T=77 K in He-gas.

3.2 Wear measurements Figure 2 shows the linear wear of several PTFE matrix composites at RT and in LN2. The parameters chosen in this experiment are F=50 N and 0.2 m/s. The graph illustrates the good performance of the polymer composites at low temperatures compared to RT. Linear wear depends significantly on the material composition. The material with 30 % bronze and 10 % carbon fibres gives the lowest wear at RT. The materials with Ekonol® however have a very small linear wear in LN2, quite difficult to detect after 2000 m. (The mass loss calculated from these linear wear measurements would lay between 0.07 mg and 1.2 mg). Further measurements will be carried out to estimate the wear behaviour after a longer running period.

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30%Bronze,10%CF

10%Ekonol,15%CF

20%Ekonol,15%CF

20%Ekonol,5%CF

20%Ekonol,20%CF

linea

r wea

r (µm

)

T=RT/air T=77K/LN2

Figure 2: Linear wear measurement of PTFE matrix

composites at RT and in LN2

3.3 Analyses of the transfer film

3.3.1 Topography Investigations of the transfer film formed in air and He-gas at room temperature, in LN2, in He-gas at T=77 K as well as in LHe were carried out.

For v=1 m/s, the profile measurements and the optical microscopy show the important influence of the temperature as well as the cryogenic medium on the topography. At room temperature in air, the transfer film consists of a homogeneous film while at T=77 K in He-gas, the transfer is inhomogeneous and consists of long PTFE lumps.

a) b) c)

Figure 3: SEM pictures of PTFE matrix composite transfer film on steel at: a) 77K in He-gas, b) 77 K in LN2 and c) 4.2 K in LHe.

The polymer transfer seems to be deformed (fig. 3a). In LN2 the transfer is much thinner and consists of scattered polymer particles (fig. 3b). In LHe the transfer film is thicker compared with that at LN2. The transfer film is brittle and shows many small wear debris at the edge as well as in the centre of the film (fig. 3c). At v=0.2 m/s, the topography of the disc is quite different of these for v=1 m/s. Significant abrasive scarves are present in LN2 as well as in LHe.

Figure 4: AFM picture of PTFE matrix composite

transfer film at 4.2 K in LHe

3.4 Chemical analyses of the transfer The spectra of the fluorine peak F(1s), carbon C(1s) of the polymer material as well as iron Fe(2p) of the counterface were determined at the surface of the transfer by XPS. At room temperature in air, the F(1s) spectrum shows two peaks: one at 688.6 eV, characteristic of the fluoride in PTFE, and another at 684.8 eV, which indicates the presence of a metal fluoride. In the Fe(2p) spectrum, two overlapping peaks were noticed. The peak of the iron fluoride is at the same position as the one of ferric oxide (711 eV). Since by other authors [10,14] FeF2 was found, it can be deduced that iron fluoride is also present in our experiment. Similar results were obtained at T=77 K in LN2. The Fe(2p) spectrum shows however two peaks at 711 eV and at 714.7 eV, which indicate the presence of FeF2 and FeF3 (714.7 eV).

F (1s)687,7

684,8

Binding Energy (eV)

Figure 5: F(1s) peak at T=4.2 K

At T=4.2 K the spectrum of F(1s) shows a larger percentage of the metal fluoride (peak with 684.8 eV) with the corresponding C(1s) peak of PTFE (292 V). The Fe(2p) spectrum has a higher peak at 714.7 eV which suggests the presence of FeF3 (fig. 5).

4 DISCUSSION As described in [5,6], the coefficient of friction decreases from room temperature to T=77 K in LN2. This can be explained by the increased hardness of polymers at low temperature. The importance of the cooling medium is clearly demonstrated with the results at 77 K between He gas and LN2. This decrease with temperature however is not observed down to extreme low temperature for high sliding speed. Indeed, a higher coefficient of friction is measured at T=4.2 K and v=1 m/s. Under this condition, the friction heat promotes a temperature rise at the contact area, especially for polymers with low thermal conductivity. Moreover, liquid nitrogen has a better heat transfer capacity as LHe, which allow a better cooling effect. The lower coefficient of friction and only small material transfer in LN2 confirm this assumption. In He-gas and liquid helium, no differences in the coefficient of friction appear at high speed v=1 m/s. In this case, the entire contact is certainly surrounded by gas, which separates the hot contact zone from the surrounding cryogenic liquid. That means that the heat at the friction contact is similar for each test in He-environment (RT, 77 K, 4.2 K), and thus the same temperatures lead naturally to the same coefficients of friction. Although similar friction values were found in He-gas and LHe at v=1 m/s, surfaces analyses presented some differences (fig. 3a, 3c). In our investigations the transfer film is characterised at T=4.2 K by small cracks and wear particles. Obviously the transfer film formed during the test is affected by the low temperature, possibly after the test or between the passes.

At v=0.2 m/s, the friction heat at the contact is lower as for v=1 m/s. The coefficient of friction in LN2 and LHe are similar, both disc surfaces present strong abrasive wear scarves.

The surface analysis of the disc shows that PTFE matrix composites transfer down to very low temperatures. The fact that there are no deep scarves at v=1 m/s but some at v=0.2 m/s at the surface of the disc, points out that the wear process is mainly adhesive in the first case and abrasive in the latter case. Low speed reduces the heat generated at the friction contact. Better cooled down, the polymer hardness increases, which causes more abrasive wear.

That corresponds with preceding studies where adhesive wear was observed in [1] whereas abrasive wear was found in [2] for experiments running at very low speeds (0.006 m/s), in order to keep the friction temperature low and to make thus the very low-temperature characteristics of the material visible.

The XPS analyses correspond with previous works carried with PTFE composites at RT [8,13] which described chemical reactions between the fluorine atom and the metal counterface. In this study, the presence of iron fluoride was detected down to LHe. It was also observed that the percentage of the fluor from PTFE decreases at the surface of the transfer in liquid helium, and respectively, the percentage of the metal fluoride

increases. That could be due to the fact that iron fluorides build up at the contact of the steel, within the first few runs of the test. PTFE transfers then on the top of this triboreaction layer. Similar structured layers of the transfer film were observed at room temperature by other workers [8,14]. At T=4.2 K the transfer film is characterised by small cracks and wear debris, which could belong to the upper PTFE layer and to the iron fluoride layer. In this study the relation between the presence of metal fluorides and the tribological performance was not obvious. However, fluoridation of metal surface with formation of metal fluorides layers are used in recent space applications studies to improve the tribological performance of cryogenic turbo pump bearing [15].

5 CONCLUSION This work presented some investigations on tribological behaviour of PTFE composites against steel at cryogenic temperatures. It could be stated that the cryogenic medium has a significant influence on the tribological performance of the polymer composites. The effect of the low temperatures on the material properties was more clearly detected at low sliding speed, with a change in wear mechanism from adhesive to abrasive. Chemical analyses show the presence of iron fluorides down to 4.2K. The XPS results suggested that these fluorides lay directly at the surface of the disc and are covered by a layer of PTFE. No influence of the metal fluorides on the tribological performance could be determined here, but results from other works suggest to pursue these investigations.

Further studies will be carried out in particular at low speeds to evaluate the influence of the cryogenic environment and elementary friction mechanisms will receive a special attention.

6 ACKNOWLEDGEMENTS The experiments were executed by the Mr. O. Berndes and M. Heidrich. Surfaces 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 German Research Association (DFG) under the project numbers Hu 791/2-1 and Fr 675/31-1.

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