# Tribological Analysis of Several Coatings under Flood and Cryogenic Cooling Conditions
## 4 Conclusions  
The objective of this work was to analyze the tribological conditions and performance of novel coatings specifically developed for cryogenic-assisted machining, in terms of apparent friction coefficient, volume of built-up material (adhesion), and temperature. This analysis was performed on two cooling/lubrication strategies [60]: liquid nitrogen (LN\({}_{2}\)) and flood (oil-water mixture). A pin-on-bar tribometer specially designed to reproduce the contact conditions in machining was used to conduct the tribological tests using uncoated and coated pins in contact with a workpiece made of Ti-6 Al-4V alloy, with several sliding \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**MWF**} & **Sliding Speed** & **Adhesion** & \multirow{2}{*}{\(\mu\)} & **Composite** \\  & **(m/min)** & **Volume (mm\({}^{3}\))** & & **Desirability** \\ \hline
**Flood** & 60 & 0.00507 & 0.259 & 0.864 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Optimized type of MWF and sliding speed to reduce the adhesion of Ti–6 Al–4V to the pins and the apparent friction coefficient. Figure 26: Influence of the (**a**) type of MWF and the sliding speed, and (**b**) their interactions, on the volume of the built-up layer of Ti–6 Al–4V alloy in the pins. speeds and two contact pressures. Four coatings developed in a previous research work [45] were selected for the tribological tests, namely, CrN, AlCrN, M2 and M4. The results show that, for both cooling/lubrication strategies, the coating has no relevant effect on the apparent friction coefficient. However, the apparent friction coefficient decreased as the sliding speed increased until about 60-80 m/min, and it then increased with the sliding speed until 100 m/min. These values were generally higher under LN\({}_{2}\) when compared to flood conditions. The increase in the apparent friction coefficient after 60-80 m/min was related to the increase in the adhesion of Ti-6 Al-4V alloy to the pins under both cooling/lubrication strategies, and this adhesion was higher for LN\({}_{2}\) cooling due to the absence of a lubricant. Increasing the contact pressure from 872 MPa to 1329 MPa decreased the apparent friction coefficient, although the adhesion was higher at 1329 MPa. The temperature measurements in the pins show that the temperature was lower for coated pins compared to uncoated ones. The lowest pin temperature under flood conditions was obtained with the AlCrN coating, while the lowest temperature under LN\({}_{2}\) cooling was obtained in the M4 coating. The DoE analysis permitted us to identify the sliding speed and cooling/lubrication strategy as the two main factors influencing the friction coefficient and the adhesion of Ti-6 Al-4V alloy to the pins, while the type of coating had no relevant effect. The apparent friction coefficient decreased with the increase in the sliding speed until around 60 m/min, then it increased up to 100 m/min. The adhesion volume was almost constant until a sliding speed of 60 m/min, and it then increased up to 100 m/min. LN\({}_{2}\) cooling led to a greater apparent friction coefficient and adhesion volume than the flood conditions, which is attributed to the lack of lubricant used in LN\({}_{2}\). Finally, the optimization analysis permitted 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 consequently reduce the apparent friction coefficient. This study is derived from the Ph.D. thesis [45] of the first author [PERSON], and all the experimental tests, results and analyses are from this study. Conceptualization, Y.Z., J.C.O. and C.N. ; methodology, Y.Z., J.C.O. and B.M. ; software, Y.Z. and J.C.O. ; validation, Y.Z., J.C.O. and B.M. ; formal analysis, Y.Z., J.C.O. and L.A.D. ; investigation, Y.Z. and B.M. ; resources, J.C.O. and C.N. ; data curation, Y.Z. ; writing--original draft preparation, Y.Z. and J.C.O. ; writing--review and editing, Y.Z., J.C.O. and C.N. ; visualization, Y.Z. and J.C.O. ; supervision, J.C.O. and C.N.