# Tribological Analysis of Several Coatings under Flood and Cryogenic Cooling Conditions
## 3 Results and Discussion
### DoE Analysis and Discussion  
In this section, an analysis of the DoE presented in Section 2.5 is performed, with the following objectives:  
1. To identify the relevant factors affecting the apparent friction coefficients and the volume of built-up layer of Ti-6 Al-4V alloy in the pins;
2. To determine the influence of these relevant factors on the apparent friction coefficient and the volume of the built-up layer of Ti-6 Al-4V alloy in the pins.  
This DoE analysis was performed using the _MinitabTM_ statistical software (Minitab(r) 20). The first step was to identify the significant factors affecting the apparent friction coefficient and the volume of the built-up layer of Ti-6 Al-4V alloy in the pins, as well as their interactions, using Pareto analysis. As shown in Figure 24, the significant factors are the type of MWF (flood vs. LN\({}_{2}\)), the sliding speed, and their interaction. The type of coating showed no significance, confirming what was already mentioned in the previous section. As shown in Figures 18 and 19, the apparent friction coefficient varied within 0.05 under flood conditions, and within 0.1 under cryogenic conditions, for all pins. Similar small variations can be found in the built-up layer of Ti-6 Al-4V.  
Figure 25 shows a graphical representation of the influence of the significant factors (MWF, sliding speed and their interactions) on the apparent friction coefficient. Figure 25a shows that the apparent friction coefficient was higher for cryogenic (LN\({}_{2}\)) cooling when compared to flood conditions. This can be explained by the fact that no lubricant was used during cryogenic cooling with LN\({}_{2}\). This figure also shows a decrease in the apparent friction coefficient with sliding speed up to 60 m/min, followed by an increase up to 100 m/min. This confirms the previous results, and this behavior can be attributed to the increase in the adhesion after 60 m/min. Figure 25b shows the interaction between the type of MWF and the sliding speed. This figure shows that, for most of the sliding speeds (except for 100 m/min), the apparent friction coefficient was higher for cryogenic (LN\({}_{2}\)) cooling compared to flood conditions.  
Figure 23: Temperature in the pins with different coatings (average contact pressure of 872 MPa and a sliding speed of 60 m/min).  
Figure 26 shows a graphical representation of the influence of the significant factors on the volume of the built-up layer of Ti-6 Al-4V alloy in the pins. This volume can be used to estimate the degree of adhesion of Ti-6 Al-4V alloy in the pins. Figure 26a shows that the adhesion was higher for cryogenic (LN\({}_{2}\)) cooling when compared to flood conditions. Again, this can be explained by the fact that no lubricant was used during cryogenic cooling with LN\({}_{2}\). This figure also shows that adhesion was almost constant up until a sliding speed of 60 m/min, followed by a fast increase in adhesion beyond 60 m/min until 100 m/min. Figure 26b shows that adhesion was higher under cryogenic (LN\({}_{2}\)) cooling when compared to flood conditions, regardless of the sliding speed, which confirms the previous results.  
Figure 24: Pareto analysis of the factors affecting the (**a**) apparent friction coefficient and (**b**) the volume of the built-up layer of Ti–6 Al–4V alloy in the pins.  
To determine the optimal type of MWF and sliding speed to reduce the adhesion of Ti-6 Al-4V to pins, and thus the apparent friction coefficient, an optimization analysis was performed. The results are presented in Table 10, and allow us to conclude that flood conditions and a sliding speed of 60 m/min should be used to reduce the adhesion of Ti-6 Al-4V to the pins and thus the apparent friction coefficient.  
Figure 25: Influence of the (**a**) type of MWF and sliding speed, and (**b**) their interactions, on apparent friction coefficient.