*3.1. Experimental Setup*

#### 3.1.1. Materials

The investigated substrate was medical UHMWPE [34] (Chirulen® GUR 1020, Mitsubishi Chemical Advanced Materials, Vreden, Germany). The specimens to be coated were flat disks, which have been used for mechanical characterization (see [35]). The UHMWPE disks had a diameter of 45 mm and a height of 8 mm. Before coating, the specimens were mirror-polished in a multistage polishing process (Saphir 550-Rubin 520, ATM Qness, Mammelzen, Germany) and cleaned with ultrasound (Sonorex Super RK 255 H 160 W 35 Hz, Bandelin electronic, Berlin, Germany) in isopropyl alcohol.

## 3.1.2. Coating Deposition

Monolayer a-C:H coatings were prepared on UHMWPE under two-fold rotation using an industrial-scale coating equipment (TT 300 K4, H-O-T Härte- und Oberflächentechnik, Nuremberg, Germany) for physical vapor deposition and plasma-enhanced chemical vapor deposition (PVD/PECVD). The recipient was evacuated to a base pressure of at least 5.0 × <sup>10</sup>−<sup>4</sup> Pa before actual deposition. The recipient was not preheated before deposition on UHMWPE to avoid the deposition-related heat flux into UHMWPE. The specimens were then cleaned and activated for 2 min in an argon (Ar, purity 99.999%)+-ion plasma with a bipolar pulsed bias of −350 V and an Ar flow of 450 sccm. The deposition time of 290 min was set to achieve a resulting a-C:H coating thickness of approximately 1.5 to 2.0 μm. Using reactive PVD, the a-C:H coating was deposited by medium frequency (MF)-unbalanced magnetron (UBM) sputtering of a graphite (C, purity 99.998%) target under Ar–ethyne (C2H2) atmosphere (C2H2, purity 99.5%). During this process, the cathode (dimensions 170 × 267.5 mm) was operated with bipolar pulsed voltages. The negative pulse amplitudes correspond to the voltage setpoints, whereas the positive pulses were represented by 15% of the voltage setpoints. The pulse frequency *f* of 75 kHz was set with a reverse recovery time *RRT* of 3 μs. A negative direct current (DC) bias voltage was used for all deposition processes. The process temperature was kept below 65 ◦C during the deposition of a-C:H functional coatings on UHMWPE. In Table 1, the main, varied deposition process parameters are summarized. Besides the reference coating (Ref), the different coating variations (C1 to C9) of a centrally composed full factorial 23 experimental design are presented in randomized run order. In this context, the deposition process parameters shown here for the generation of different coatings represent the basis for the machine learning process.


**Table 1.** Summary of the main deposition process parameters for a-C:H on UHMWPE.
