# Tribological Analysis of Several Coatings under Flood and Cryogenic Cooling Conditions
## 3 Results and Discussion  
### Results of the Tribological Tests  
Based on the tangential and normal forces obtained from the tribological tests, the apparent friction coefficient was determined. Figure 18 shows the friction coefficient as a function of the sliding speed for different coatings, for both MWF (flood and LN\({}_{2}\)) and for an average contact pressure of 872 MPa. This figure shows that the apparent friction coefficient decreased as the sliding speed increased until about 60-80 m/min, then it increased as the sliding speed increased up to 100 m/min. Similar results were also obtained by [PERSON] et al. (2019) using an in-process (cutting) open tribometer to determine the friction coefficient between the Ti-6 Al-4V alloy and 6 mm-diameter pins coated with an AlTiN coating, using several pin axial (normal) forces (between 5 N and 400 N), and under dry and lubricated (oil) conditions. Similar to the present work, they also found that the apparent friction coefficient showed a minimum value for intermediate sliding speeds when 100 N of normal force was applied. According to [PERSON] et al. (2019), the increase in the friction coefficient with higher sliding speeds may be due to the higher shear forces at higher relative speeds, and a stronger adhesion of the workpiece material to the pins. This strong adhesion at higher speeds was observed in the present tribological tests, as shown in Figure 19. This figure shows a strong increase in the volume of the layer of build-up of the Ti-6 Al-4V alloy in the pins with sliding speeds greater than 60-80 m/min, under both flood and LN\({}_{2}\) coolant/lubricant conditions. Nevertheless, the uncoated pin seemed to present a lower adhesion of Ti-6 Al-4V alloy under LN\({}_{2}\) cooling conditions (Figure 19b), which can be explained by its lower surface roughness (Figure 14). This volume was higher when LN\({}_{2}\) was applied, compared to the flood, due to the non-existent lubricant action of LN\({}_{2}\) compared to the flood. The coating type does not have a significant influence on \begin{table}
\begin{tabular}{c c} \hline \hline
**Factors** & **Levels** \\ \hline
**Sliding speed (m/min)** & 10, 20, 40, 60, 80 and 100 \\
**Coating Type** & No coating (uncoated pin), CrN, AlCrN, M2 and M4 \\
**MWF Type** & LN\({}_{2}\) and mixture of oil-water \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Factors and levels used in the multilevel factorial DoE. \begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Sliding Speed (m/min)** & **Coating Type** \\ \hline
1 & 10 & Uncoated \\
2 & 20 & Uncoated \\
3 & 40 & Uncoated \\
4 & 60 & Uncoated \\
5 & 80 & Uncoated \\
6 & 100 & Uncoated \\
7 & 20 & Uncoated \\
8 & 60 & Uncoated \\
9 & 100 & Uncoated \\
10 & 20 & M2 \\
11 & 60 & M2 \\
12 & 100 & M2 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Additional tests for an average contact pressure of 1329 MPa. the friction coefficient, as for each sliding speed, the variance in the friction coefficient was within about 0.05 for coated and uncoated pins under the flood condition, regardless of the coating type. The low variance may originate from the different roughnesses of the pins, but the relevance of this is low (Figures 14-16). Under the LN\({}_{2}\) condition, relatively higher variances in both the friction coefficient and the adhesion volume were presented for different coating types; these may be due to the different tribological mechanisms for each coating type. The contact conditions and the degrees of delamination of the coating from the pins are different, both of which need to be studied further. As far as the average contact pressure is concerned, increasing the pressure from 872 MPa to 1329 MPa reduces the apparent friction coefficient, as shown in Figure 20 for uncoated and coated pins (M2) and both MWF. In particular, the apparent friction coefficient decreased monotonically with the sliding speed when a higher pressure was used and for both MWFs. Similar results were also obtained by [PERSON] et al. [21].