## 3 Results and Discussion
### Stribeck Curves  
The Hersey parameter was calculated by the viscosity of the lubricating oil at different temperatures (Table 4) times the applied sliding speed during the tests and divided by the normal force that is the only constant parameter in this study. This was not completely right, as there was always some variation in the normal load during the tests due to residual misalignment of the external surface of the ring. This residual misalignment was so insignificant that the normal load was considered constant.  
Figure 3 shows the Stribeck curves that were obtained by the values of the coefficient of friction from the different tests performed at distinct speeds and temperatures for different coating films. The standard deviation values were very small for all points and tended to decrease with increasing the sliding speed; hence, only the average data set was demonstrated. By analyzing these curves, three lubrication regimes could be obtained, which were the boundary, mixed and hydrodynamic lubrication. The hydrodynamic lubrication occurred at a higher sliding speed and viscosity of the lubricant at lower temperatures. As the temperature of the tests increased, and the sliding speed decreased, and the lubrication regime shifted to mixed lubrication, where the coefficient of friction started to increase due to the mechanical contact of higher asperities caused by the lower thickness of the lubricant film. Finally, it reached the boundary lubrication regime, which occurred at lower sliding speeds and the viscosity of the lubricant at higher temperatures. In this regime, the mechanical contact of the asperities was promoted, and the thickness of the lubricant could not overcome the roughness of the contact surfaces; hence, the coefficient of friction significantly increased.  
\begin{table}
\begin{tabular}{c c c c c c} \hline Temp. & (\({}^{\circ}\)C) & 25 & 60 & 80 & 100 \\ \hline Viscosity & (mPa.s) & 56.063 & 15.579 & 7.632 & 3.695 \\ \hline \end{tabular}
\end{table}
Table 4: Viscosity values for the different temperatures used for the tests lubricated with PAO 8.  
Figure 2: Viscosity–temperature graph for PAO 8 oil.  
Comparing the Stribeck curves for different films showed that DLC films have a lower COF compared to CrN in the boundary lubrication regime due to their smoother surface. However, the reduction of COF compared to CrN was 9.5% for the Ar-DLC film, whereas, for the Ne-DLC films, this reduction reached 22.8% (COF < 0.1) compared to CrN. By increasing the Hersey parameter and approaching the mixed and hydrodynamic lubrication regimes, the values of COF for Ar-DLC and CrN were close to each other, but for the Ne-DLC films, the value of COF was still much lower in these regimes, achieving ultralow friction of less than 0.001, which can be a major efficiency improvement in contact between the PR and cylinder liner. This means adding Ne to the plasma not only caused the deposition of DLC films with lower COF in the boundary lubrication regime but also caused a better tribological behavior in the mixed and hydrodynamic lubrication regimes, where the amount of friction was determined mostly by the lubricant. Therefore, this was related to solid-liquid interfacial properties, which are vital for lubricated contacts, and they will be discussed in detail in the next section.