24th International Colloquium Tribology - January 2024 125 Investigation of Polymer Solid Lubricated Steel-Bronze Contacts for Worm Gears Applications Konstantinos Pagkalis 1* , Manuel Oehler 1 , Thomas Schmidt 2 , Michaela Gedan-Smolka 2 , Stefan Emrich 3 , Michael Kopnarski 3 , Oliver Koch 1 1 RPTU Kaiserslautern-Landau, Chair of Machine Elements, Gears and Tribology Gottlieb-Daimler Str. Geb. 42, D-67661 Kaiserslautern, Germany 2 Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, 01069 Dresden, Germany 3 Institut für Oberflächen-und Schichtanalytik GmbH, Trippstadter Str. 120, 67663 Kaiserslautern, Germany * Corresponding author: konstantinos.pagkalis@rptu.de 1. Introduction In drive technology, machine elements are usually lubricated with oil or grease. However, there are cases where these types of lubrication cannot be used. In medical area, in vacuum or in high temperature applications oil and grease lose their efficiency. Hence solid lubrication is essential. PTFE is employed over a wide temperature range (-250°C to +260°C) where it has the advantage that it displays high chemical resistance and very good friction properties. However, its poor mechanical properties and low adhesion to metal surfaces are considered as a drawback. On the other hand, polyamide (PA) exhibits good mechanical properties and wear resistance but has high friction against steel. The advantageous properties are connected via a combination of the polymer matrices. So it made sense to blend these two compounds to get the optimum performance. However, physical blends of PA-PTFE have worse mechanical properties compared to PA [1]. The reason for this is that the polar polyamide matrix and the non-polar PTFE filler are not compatible. Hence, the chemical coupling of PA with irradiated PTFE compounds occurs by reactive melt extrusion. Using high energy radiation PTFE can be modified in the presence of air to obtain perfluoroalkyl radicals and functional groups (-COF and -COOH) while C-C and C-F bonds break. A covalent bond between polyamide and irradiated PTFE can be formed from COOH functional groups, which further improves the adhesion and compatibility between PTFE and hydrophillic/ polar polymer matrix. T. D. Nguygen et. al. found that on block on ring polymer-steel contact, PA66 with chemically bonded (cb) PTFE (MP1100) provided the lowest friction when compared to pure unmodified PA66 while PA66-MP1200-cb provided worse wear behaviour than other PA- PTFE-cb components due to its worse mechanical properties [2]. Moreover, Franke et al. demonstrated that friction and wear behaviour of polyamides PA6, PA66 and PA12 could be enhanced with a mass fraction of PTFE-cb between 3.3 and 50 wt-% [3]. It can be deducted that Polymer-PTFE-cb dry lubricants can enhance tribological behaviour and mechanical properties of polymer-metal contacts. It is therefore investigated if and how the novelty chemically bonded polymer compounds produced by reactive melt extrusion can be beneficial compared to commercial compounds on a steel/ bronze contact in terms of tribological effectiveness. 2. Experimental studies Tests are conducted in 3-disc test rig and worm gearbox setup [1, 4]. The tribological system in both cases includes the polymer compound acting as the sacrificial element in contact with a steel of 16MnCr5, while bronze CuSn12Ni-GC is in contact with the steel. The 3-disc test rig consists of a test unit and three electric motors, which can be independently driven to set an arbitrary slide-to-roll ratio while load is applied vertically. Tests are conducted at a 270 MPa steel/ bronze mean contact pressure, slide-to-roll ratio equal to 50- %, 3- h test duration. The worm gearbox setup (centre distance a-=-32-mm, gear ratio i-=-29, 5000 cycles, 10-h test duration, mean contact pressure 179-MPa and 5-Nm output torque for steel/ bronze contact) has a drive motor connected to the steel worm while the two other motors are connected to the bronze and polymer wheel respectively. The wear of the compounds is determined by measuring mass before and after the experiments. All PA-PTFE compounds have a ratio of 80-wt-% PA and 20-wt-% PTFE. A commercially available physically mixed PA66-PTFE compound (ALCOM) is used as state of the art. Different types of PA (PA12 and PA66) and irradiated PTFE MP1100E were extruded in melt to obtain novel PA-PTFE-cb compounds. There are two different PA12 polymer matrices that are used with the modified PTFE, PA12L and PA12Z. MP1100E type is thermally post-treated during industrial manufacturing to remove the perfluorooctanoic acid (PFOA) it contains to meet the EU requirements regarding the limit values of this compound. This research investigates the effect of those chemically bonded dry lubricants on a steel/ bronze contact in a 3-disc tribometer, on a situation that resembles worm gears. Thus, it can be understood which polymer provides the best results in terms of tribofilm effectiveness (friction, transfer film formation and wear of the components) and then to be tested on the worm gearbox setup. 3. Results Figure 1 displays the results at the three-disc set-up regarding (a) friction coefficient of the steel-bronze contact (b) the wear coefficient of the bronze with the polymer combinations described above while Figure 2 compares the wear mass of the bronze obtained from 3-disc test rig and worm gearbox setup respectively for PA12 and PA12+modified PTFE compounds. 126 24th International Colloquium Tribology - January 2024 Investigation of Polymer Solid Lubricated Steel-Bronze Contacts for Worm Gears Applications Figure 1: (a) COF of steel-bronze contact (b) Bronze wear for the polymer combinations in 3-disc set-up. Figure 2: (a) Bronze wear mass for PA12L and PA12L + MP1100E-cb tested at (a) gearbox (b) at 3-disc setup. Figure 1 indicates that pure unmodified PA12 and PA66 give higher friction and bronze wear compared to the corresponding PA-PTFE compounds. This can be expected as there is no PTFE which acts supportively. Besides, PA12L (NH2 terminated) with MP1100E shows better wear results than PA12Z (non-regulated) with the same modified compound. PA12L has more NH2 groups than PA12Z and so can form more amide groups which can enhance adhesion and hence reduce wear. With regard to PA66, the physically mixed (pm) commercial compound ALCOM provides slightly higher friction and bronze wear compared to PA66-PTFE-cb however the differences are not significant. Measurements of near-surface chemical properties indicate that no PTFE-specific chemical bonds are detected on the tribologically stressed surfaces of the steel and bronze discs when using the commercial dry lubricant PA66+20 wt-% ALCOM. In contrast, the specific CF2bonds are identified in the transfer film of the dry lubricant compound with MP1100E, especially on the bronze discs. The allocation of PTFE on the disc surfaces could explain the comparatively better performance in tribological tests when using the dry lubricant with MP1100E. Moreover, Figure 2 shows that the initial results of worm gearbox setup show that there is a correlation on the wear behavior of the bronze for both setups used. 4. Conclusion The chemical coupling of PA12L with irradiated PTFE MP1100E reduces both the coefficient of friction in the steel/ bronze contact and the wear of the bronze disc in the 3-disc tribometer. Moreover, compared to commercially available polymer compounds made from PA66 and physically bonded PTFE components, the combination of PA66 and radiation-modified PTFE (MP1100E) tends to be tribologically superior in terms of friction and wear. Thus, the reactive melt extrusion process for irradiated PTFE provides better results than the commercial compound. Surface analysis studies show, that on the tribologically stressed surfaces of the bronze discs, transfer films with PTFE-typical CF 2 compounds are formed. References [1] L. Simo Kampa, T-D. Nguyen, S. Emrich, M. Oehler, T.-Schmidt, M. Gedan-Smolka, M. Kopnarski, B. Sauer, The effect of irradiated PTFE on the friction and wear behavior of chemically bonded PA46-PTFE-cb and PA66-cb compounds: Wear 502-503, 2022. [2] T-D. Nguyen, M. Gedan-Smolka, L. Simo Kamga, B.- Sauer, S. Emrich, M. Kopnarski, B. Voit Chemical Bonded Oil-P TFE-PA 66 Composites as Novel Tribologically Effective Materials: Part 1, Solid State Phenomena, 320, pp. 113-118, 2021. [3] R. Franke, D. Lehmann, K. Kunze, Tribological behaviour of new chemically bonded PTFE polyamide compounds, Wear 262, 242-252, 2007. [4] L. Simo Kamga, M. Oehler, O. Koch, and B. Sauer. „Untersuchungen zum thermischen Verhalten eines mit PTFE trockengeschmierten Schneckengetriebes mithilfe vom Experiment und Simulation“ 63. Tribologie-Fachtagung. Gottingen: Gesellschaft fur Tribologie e.V, S. 45/ 1-45/ 5, 2022.-