Thermo-Elastohydrodynamics of Coated Polymer Gears
Final Report Abstract
In this joint research project, the tribological behavior of engineering polymers such as the PEEK, PEEK+GF30, PA66, and PA66+GF30 was analyzed in uncoated and PVD coated state. At first, a tailored pretreatment method was developed and adopted by the cooperation partner BUT. Furthermore, the DLC coatings were customized to the PEEK-based and PA66-based engineering polymers. The coating and compound analysis revealed promising properties for the tribological investigations. A high compound adhesion between the engineering polymers and the DLC coatings was fundamentally needed to enable further testing in tribological contacts. On this basis, the coated engineering polymers were first tested under tribological conditions. The analysis showed a higher coefficient of friction and a significant wear reduction due to the application of DLC coatings on the considered engineering polymers. Afterward, the DLC coatings were deposited on different engineering specimens for further tribological analysis. The tribological behavior of polymers in the hybrid polymer-steel contact was investigated on a twin-disk tribometer and by numerical calculations. It was shown that the interfacial friction is very small owing to the conformal contact geometry, caused by the deformation of the polymer. In contrast to the behavior of steel-steel contacts, the increase of the hydrodynamic pressure and the effective fluid viscosity is small. The correlation between the coefficient of friction and sliding velocity is linear, which indicates Newtonian fluid behavior that is characterized by the dominant lubricant viscosity under fluid film lubrication. Surface roughness of the polymer can result in mixed lubrication and strong influence of solid friction. Effects of the coating on the frictional behavior were especially observed under mixed lubrication conditions. The formation of a conformal contact geometry remains unaffected by the coating supported by numerical calculations of the equivalent contact stiffness of the coated polymer-steel pairing. In particular, the DLC-1 coated PEEK and PA66 show a high sensitivity to sum and sliding velocities during the conditioning procedure. Under these conditions, a significant increase in interfacial friction appears, compared to the uncoated variants. This behavior was also observed for the industrial ta-C coating, however, in a less pronounced manner. It is assumed that interactions at the molecular level of the coatings, triggered by the high sum or sliding velocities, are responsible for this behavior. At low sum and sliding velocities, the industrial ta-C coating, in particular, offers significant potential in terms of frictional behavior in mixed lubrication regime and the predominant high solid load portion. Despite the comparable low interfacial friction, strong internal heating due to damping and viscoelastic material behavior dominates the temperature distribution and is a function of loading frequency and normal load amplitude. In this context, polyamides show a significantly more pronounced damping behavior than PEEK, especially in the range of the glass transition temperature. Heating due to shearing and compression of the lubricant, on the other hand, is considered small due to the low hydrodynamic pressure. In the same way, the heating owing to solid interfacial friction is assigned a subordinate role in the polymer TEHL contact. In lubricated contacts with coated polymers, the influence of the coating on the bulk temperature behavior is small. The driving factor remains the heating due to damping losses, which are primarily affected by the polymer itself. Thermal insulation through the DLC was observed in numerical calculations of TEHL contacts even at small coating thicknesses, but with only a small influence on the tribological contact system. The results of this joint research project enabled a variety of insights into the pretreatment and coating deposition on engineering polymers as well as an analysis of the tribological behavior under differing tribological conditions. Nevertheless, more analyses based on the results of this project are needed to gain a deeper understanding of coated polymers and their effect on friction, wear, and temperature behavior. In particular, we would like to draw attention to the conditioning procedure of coated polymers, which supposedly has a decisive influence on the tribological properties. Promising results were obtained especially concerning the wear resistance of coated polymers at high sliding velocities providing great potential for application in polymer machine elements like polymer gears.
Publications
- Detailed analysis of film thickness in thermoplastic EHL contacts, VDI International Confernce on Gears, 18.-20. September 2019, Garching b. München; VDI-Berichte, Bd. 2355; Seite 1391-1400
Maier, E.; Sperka, P.; Lohner, T.; Stahl, K.
(See online at https://doi.org/10.51202/9783181023556-1391) - Lubrication of rolling – sliding polymer contacts 25th International Conference Engineering Mechanics 2019; 13. – 16. Mai 2019 in Svratka, Czech Republic
Křupka, J.; Šperka, P.; Křupka, I.; Hartl M.
(See online at https://doi.org/10.21495/71-0-219) - Steigerung der Leistungsfähigkeit technischer Kunststoffe durch DLC- Beschichtungen, Reibung, Schmierung und Verschleiß: Forschung und praktische Anwendungen: 60. GfT Tribologie-Fachtagung 23. bis 25. September 2019 in Göttingen
Bobzin, K.; Brögelmann, T.; Kalscheuer, C.; Thiex, M.
- A Numerical Study on Thermal Elastohydrodynamic Lubrication of Coated Polymers Tribology letter 68:71; 2020
Ziegltrum, A.; Maier, E.; Lohner, T.; Stahl, K.
(See online at https://doi.org/10.1007/s11249-020-01309-6) - Friction and Temperature Behavior of Lubricated Thermoplastic Polymer Contacts Lubricants 8:6; 2020
Reitschuster, S.; Maier, E.; Lohner, T.; Stahl, K.
(See online at https://doi.org/10.3390/lubricants8060067) - Steigerung der Leistungsfähigkeit technischer Kunststoffe durch DLC-Beschichtungen, Tribologie und Schmierungstechnik, 67, 15-24, 2020
Bobzin, K.; Brögelmann, T.; Kalscheuer, C.; Thiex, M.
- Tribological Potential of DLC coated plastics in Highly-Loaded Contacts 61. GfT Tribologie-Fachtagung Göttingen 2020-09-28 - 2020-09-30, 56/1-56/4, 2020 ISBN: 978-3-9817451-6-0
Bobzin, K.; Brögelmann, T.; Kalscheuer, C.; Thiex, M.; Stahl, K.; Lohner, T.; Maier, E.; Reitschuster, S.
