*2.3. Tribological Analysis*

The measurement of the COF was performed in a ball-on-disc (BoD) setup. We used a universal mechanical tester (UMT TriboLab, Bruker Corporation, Billerica, MA, USA) to perform the tribological evaluation. The system is equipped with a linear drive allowing linear reciprocating translation and a second drive unit to apply a specific load of the ball on the base plate. The used force sensor allows measuring the load FL and the lateral force FX up to 5 N. The resulting COF is calculated by FX/FL. We measured the friction behaviour between the 100Cr6 steel substrate and either tribometer balls of chrome steel (100Cr6) or tungsten carbide, both with a diameter of 6.3 mm. The resulting wear on the substrate is captured using laser scanning microscopy (VK-X200, KEYENCE DEUTSCHLAND GmbH, Neu-Isenburg, Germany). Therefore, the track width (point-to point) of the wear tracks on the substrate is measured by a threefold rating. Beside the dry friction test, the COF is measured in a lubricated environment by a thin film of engine oil (5W40).

Due to the elastic deformation of the 100Cr6 steel and tungsten carbide balls, different contact areas appear. Using a Hertzian deformation model of a sphere in contact with a flat sample, the diameter of the contact area is calculated in Table 1.


**Table 1.** Diameter of contact areas in the specific ball-on-disc configurations.

An overview of the applied tribologcial test conditions is given in Table 2 for both sliding regimes, dry and under lubrication. Due to the decreased wear generation using lubrication, higher load forces and numbers of test cycles are possible.

**Table 2.** Summarized test conditions of tribological evaluation.

