This work focuses on the friction and temperature behavior of thermo-elastohydrodynamically lubricated (TEHL) contacts under rolling-sliding conditions. For this purpose, a twin-disk test rig is used with a hybrid setup of plain and fiber-reinforced polyamide (PA) 66 and polyetheretherketone (PEEK) disks paired with case-hardened steel disks and three different lubricants. Experimental investigations include various lubrication regimes by varying sum velocity and oil temperature as well as load and slip ratio. The measured friction in thermoplastic TEHL contacts is particularly very low in the area of high fluid load portion, which refers to the large deformation of the compliant polymer surface. Newtonian flow behavior mainly determines fluid friction. The low thermal effusivity of polymers insulates the contact and can further reduce the effective lubricant viscosity, and thus the fluid friction. For low sum velocities, solid friction influences the tribological behavior depending on the solid load portion. Although the interfacial contact friction is comparably small, material damping strongly contributes to power losses and increases bulk temperature, which in turn affects the TEHL contact. Thus, loading frequency and the resulting bulk temperature are identified as one of the main drivers of power losses and tribological behavior of lubricated thermoplastic polymer contacts.

The application of high-performance polymer gears in power transmissions is increasingly gaining focus in current research and industrial practice. The well-known advantages of thermoplastics such as cost-efficient production and low density compete with moderate strength and limited operating temperatures. However, the properties of thermoplastics can be used effectively to address the challenges of more efficient powertrains with higher power density, reduced fuel consumption, and reduced manufacturing costs. This work focuses on the implementation of high-performance polymer gears in the powertrain of a light electric vehicle. For demonstration purposes, a series transmission of a prototype of a light electric vehicle is modified and investigated considering the specific properties of polymers and steel materials. Due to its material properties, the high-performance polymer VESTAKEEP® 5000 G was used for the investigations. Different constructional modifications to the serial transmission as well as modifications of the micro geometry of the gears were applied to improve the load distribution and provide sufficient load carrying capacity to the polymer gears. Assuming usual driving cycles and lifetime, the calculations according to VDI 2736 [1] and RIKOR [2] indicate a sufficient load-carrying capacity for the polymer gears used inside. Under the given assumptions, the use of polymer gears in a powertrain of light electric vehicles appears to be possible.

The purpose of this study was to increase the limited wear resistance of polymers in highly stressed tribological contacts. Therefore, diamond-like carbon (DLC) coatings were firstly developed and deposited on polymers. The development of the low temperature coating process was conducted on an industrial scale coating unit. Due to the use of polyether ether ether ketone (PEEK) and polyamide as substrate material, the coatings were tailored in accordance with different mechanical properties of the polymers. Furthermore, analyses regarding morphology, roughness as well as compound adhesion between coatings and polymers were conducted. The tribological analyses were accomplished for reciprocating sliding kinematics on pin-on-plate tribometer under dry conditions as well as boundary lubricated conditions. The results prove the ability of the coatings to undergo high plastic deformations without delamination. In addition, the possible formation of a composite layer of DLC and PEEK in the wear track due to high energy input under dry conditions was shown. Furthermore, the transformations of sp2 chain into ring bindings as well as a change of size and distribution of the sp2 clusters during dry contact conditions were observed.

The goal of this work is to evaluate the potential of diamond-like carbon (DLC) coatings on thermoplastic polymers for friction and wear reduction in highly stressed rolling–sliding contacts. Therefore, hydrogen-containing DLC coatings were deposited on the polymer surface by a low-temperature high power pulsed magnetron sputtering (HPPMS) physical vapor deposition (PVD) process. The rolling-sliding contact between coated polyamide 66 (PA66) or coated polyether ether ketone (PEEK) against case-hardened steel 16MnCr5 is investigated in a twin-disk tribometer at normal loads up to FN = 1,000 N, sum velocities between 1 m/s ≤ vΣ ≤ 16 m/s and slip ratios up to s = 50%. Results show a friction reduction with the application of DLC on the considered polymers compared to uncoated polymers under specific lubrication conditions. High solid losses caused by the polymer’s internal damping properties dominate the temperature behavior of the polymer, even when coated with DLC. Regarding the wear behavior, DLC coatings show potential especially under severe mixed lubrication conditions with high-solid load portion and sliding. The knowledge gained about coated polymers can be used to improve the overall tribological performance in terms of friction and wear of thermoplastic machine elements like gears.

Gears made of polymers increasingly gain relevance in current research and industrial practice. Due to their advantages such as low density and the possibility of cost-efficient production in large quantities, more and more efforts are made to substitute steel gears with polymer wherever possible. However, the usage of polymer gears in current vehicle systems is still almost exclusively limited to applications with restricted power transmission, like actuators. To extend the field of application to power transmissions as well, in this study a transmission was developed for the usage in a small electric vehicle of the 7Le power class considering the application of polymer gears. Compared to the serial transmission, one of the gear stages was completely substituted with polymer gears. The load-carrying capacity of the newly designed hybrid polymer-steel transmission was investigated for functional verification on a specially developed test rig as well as inside a modified demonstrator vehicle under real-life conditions. Assuming usual operating conditions, the load-carrying capacity of the polymer gears appears suitable for the aimed application on the test rig as well as inside the demonstrator vehicle. Moreover, with increased power and under continuous sustained loading, high performance of the polymer stage as well as a stable temperature behavior can be observed. Under the given circumstances, polymer gears can be used as part of a powertrain in small electric vehicles.