24th International Colloquium Tribology - January 2024 101 Friction Reducing Effect of Lubricants Applied to Organic Fibres Igor Velkavrh 1* , Nicole Dörr 2 1 V-Research GmbH, Stadtstrasse 33, 6850 Dornbirn, Austria 2 AC2T research GmbH, Viktor Kaplan-Strasse 2/ c, 2700 Wiener Neustadt, Austria * Corresponding author: igor.velkavrh@v-research.at 1. Introduction Organic fibres suffer from wear and hence fibre rupture when sliding against each other and hard surfaces. To achieve a better understanding of the lubrication mechanism of these fibres, better understanding of its tribological behaviour is needed; however, only a limited number of friction and wear studies have been reported in the literature [1]. Within the present study, a similar approach was applied to determine to what extent the use of a lubricant can have a positive effect, i.e., friction reducing effect, on the tribological properties of tribosystems with fibres. Therefore, the aim of the present study was to establish a methodology for identifying the contact conditions at which the friction reducing effect occurs. Focus was put on temperature and load. 2. Experimental methods Dry, i.e., lubricant-free, fibres were coated with bio-based and synthetic lubricants. Tests with dry and coated fibres were performed on an oscillating tribometer (SRV ® 4, Optimol Instruments GmbH, Germany) with organic fibres wrapped around a steel cylinder. Sliding against a tungsten carbide plate was performed at different contact pressures and temperatures. During the tribological tests, coefficient of friction, normal force, contact resistance and surface temperature were continuously measured. After the tests, the appearance of the organic fibres and the tribofilm on the tungsten carbide plate were characterized. In Table 1, parameters applied in the tribological tests are listed. Table 1: Parameters applied in the tribological tests. Parameter Settings Normal force (N) 10 for 2 min (running in), afterwards 45 Hertzian contact pressure (MPa) max. 210/ mean 170 Temperature (°C) 21-°C for 2 min (running in), afterwards heating to 250-°C Stroke (mm) 4 Oscillation frequency (Hz) 1 Sliding velocity (m/ s) max. 0.0126/ mean 0.008 Test duration Until the rupture of organic fibres (observed in an increase of the coefficient of friction and a drop in the contact resistance) 3. Results and Discussion Figure 1: Coefficient of friction, surface temperature and normal force in dependence of testing time. Figure 2: Tribofilm on the tungsten carbide plate after the friction test. In Figure 1, coefficient of friction, surface temperature and normal force in dependence of testing time are presented. During the running-in phase performed during the first 2 minutes of test at ambient temperature under a normal load of 10 N, coefficient of friction was around 0.3. After the increase of normal load to 45 N, coefficient of friction instantaneously decreased to 0.2. Dependence of the coefficient of friction on the nominal contact pressure is typically observed for rubber materials and can be related to (among other) the saturation of the contact area (and friction force) due to high nominal squeezing pressure, non-linear viscoelasticity, adhesion, and frictional heating [2]. Afterwards, as temperature was increased coefficient of friction further decreased and reached a minimum of around 0.06 at a temperature of around 170-°C. 102 24th International Colloquium Tribology - January 2024 Friction Reducing Effect of Lubricants Applied to Organic Fibres This low-friction phase lasted for around 2-minutes, until afterwards coefficient of friction started to increase and became unstable indicating the rupture of the fibres and the removal of the lubricating film from the tribocontact. In Figure 2, a micrograph of the tribofilm on the tungsten carbide plate after the friction test is presented. A relatively thick viscous film formed on the surface. Obviously, the sufficiently high contact pressures and temperatures promoted the extraction of the lubricating fibre components and the applied lubricant from the fibres. 4. Conclusions The lubricating mechanisms of organic fibres depend strongly on load and temperature. Correlations between friction reduction with lubricants and the durability of the fibres (or the prevention of fibre rupture) were found. Furthermore, it was elaborated at which parameters friction reduction can be triggered. Acknowledgements Parts of the presented work were funded by the Austrian COMET Program (Project InTribology, no. 872176) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH) in collaboration with V-Research GmbH. References [1] N. Ismail, M. B. de Rooij, E. G. de Vries, N. H. Mohd Zini, D. J. Schipper, Friction between single aramid fibres under pre-tension load, Tribology International 137 (2019) 236-245. [2] G. Fortunato, V. Ciaravola, A. Furno, M. Scaraggi, B. Lorenz, B. N. J. Persson, On the dependency of rubber friction on the normal force or load: theory and experiment, Tire Science and Technology (2017) 45 (1): 25-54.