## 2 Materials and Methods  
For this research work, steel substrates (AISI D2) with the dimensions of 25 mm in diameter and a thickness of 8 mm were used to deposit the films. Before the depositions, the substrates were mirror-polished and cleaned with ethanol and acetone in an ultrasonic bath lasting 15 min each. Once the preparation of the substrates was finished, they were glued using thermal and electric conductive silver glue onto an aluminum substrate holder rotating with a constant speed of 23.5 rpm around the central axis of the deposition chamber during the depositions. The distance between the substrates and the targets was 80 mm in the deposition chamber. A rotary and turbomolecular pump was used to reach the base pressure of below 3 \(\times\) 10\({}^{-4}\) Pa before the depositions. To clean the surfaces of the substrates and, consequently, enhance the adhesion, etching treatment was applied by ion gun with a voltage and current of 36 V and 20 A, respectively, when a bias of \(-\)150 V was used for the substrates for 40 min. To further improve the adhesion of the DLC films to the substrates, a Cr adhesion layer was deposited by DCMS (Huttinger PFG 7500 DC, Ditzingen, Germany) in pure Ar plasma at the pressure of 0.8 Pa and \(-\)80 V of substrate biasing, and it was followed by the deposition of the CrN supporting layer that was deposited when introducing an N\({}_{2}\) content of 33% in the discharge gas and the same deposition conditions as the Cr deposition. To deposit DLC films, a graphite (99.95%) target with dimensions of 150 \(\times\) 150 \(\times\) 10 mm was used in the Ar and Ar + Ne discharge gases at 0.8 Pa by DOMS (HIPIMS, Zpulser Inc., Mansfield, MA, USA), with the following parameters: ton = 6 \(\mu\)s, T = 130 \(\mu\)s, D = 1690 \(\mu\)s and average power (P\({}_{\text{a}}\)) equal to 1.3 KW. In this work, three distinct DLC coatings were produced. One DLC film was deposited using pure Ar plasma. For the deposition of the other two DLC films, before ignition of the plasma, Ne was introduced to the discharge gas with contents of 25% and 50% simultaneously, with reducing the Ar flux to maintain the deposition pressure at 0.8 Pa. In Table 1, the respective deposition conditions for the DLC coatings are presented. A CrN coating was deposited for comparison purposes by using the mentioned DCMS power supply with a power of 1.2 KW in the Ar + N\({}_{2}\) discharge gas that had a flow rate of 68 SCCM (flow ratio of Ar: N\({}_{2}\) was equal to 1:3) with a pressure of 0.3 Pa.  
For tribological testing, a block on the ring assembly was used (Figure 1); more details are presented in Reference [18]. The principle of this test consists of placing a sample (coated substrate) in contact (90\({}^{\circ}\) in relation to the geometric center of the ring) with the outer surface of the ring. In this assembly, there are two load cells, one working as the input (normal force cell, F\({}_{\text{N}}\)) and another as the output, where the friction force (F\({}_{\text{f}}\)) is measured (tangential force cell), which, by dividing F\({}_{\text{f}}\) by F\({}_{\text{N}}\), the coefficient of friction (\(\upmu\)) is obtained. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Pressure** & **Ne** & **V\({}_{\text{p}}\)** & **I\({}_{\text{p}}\)** & **P\({}_{\text{p}}\)** & **F\({}_{\text{i}}\)** \\ \hline (Pa) & (\%) & (V) & (A) & (KW) & (Hz) \\ \hline  & 0 & 1384 & 76.1 & 105 & 247 \\ \hline
0.8 & 25 & 1365 & 90 & 123 & 261 \\ \hline  & 50 & 1349 & 96 In this assembly, there is a lubricant reservoir for keeping the contact lubricated with the lubricating oil. The counter body is a grey cast iron EN GJL-300 ring (typical material used in cylinder liners) with the dimensions of \(\mathcal{O}116\)\(\times\) 12 mm. To approximate as closely as possible the working conditions existing in ICE, the ring was honed on the outer surface to ensure lubricated contact with PR [19].