# Tribological Analysis of Several Coatings under Flood and Cryogenic Cooling Conditions
## 3 Results and Discussion
### Results of the Tribological Tests
However, this reduction in the apparent friction coefficient was not followed by a reduction in the volume of the layer of build-up of Ti-6 Al-4V alloy in the pins; in fact, the opposite was observed, as shown in Figure 21.  
Figure 18: Apparent friction coefficient according to sliding speed and for different coatings, for (**a**) flood and (**b**) LN\({}_{2}\) (average contact pressure of 872 MPa). Figure 19: Volume of layer of build-up of Ti–6 Al–4V alloy in the pins according to the sliding speed and for different coatings, for (**a**) flood and (**b**) LN\({}_{2}\) (average contact pressure of 872 MPa). Figure 20: _Cont._  
Figure 21: Volume of layer of build-up of Ti–6 Al–4V alloy in the pins as a function of the sliding speed for two average contact pressures (872 MPa and 1329 MPa), for (**a**) flood conditions using coated (M2) and uncoated pins and (**b**) for LN\({}_{2}\) conditions using uncoated pins. Figure 20: Apparent friction coefficient according to the sliding speed for two average contact pressures (872 MPa and 1329 MPa), for (**a**) flood conditions using coated (M2) and uncoated pins, and (**b**) for LN\({}_{2}\) conditions using uncoated pins. The apparent friction coefficients between the uncoated pins and the Ti6-Al-4V alloy with an average contact pressure of 1329 MPa and under LN\({}_{2}\) cooling conditions were compared with those obtained by [PERSON] et al. [57], as shown in Figure 22 for different sliding speeds. The present work yielded apparent friction coefficients about 20% higher than those found by [PERSON] et al. [57]. This difference may be due to the different contact conditions (pressure, temperature, etc.) between the present tribological tests and those performed by [PERSON] et al. [57], because the LN\({}_{2}\) (including the flow characteristics), pin diameter and normal forces applied were not the same in the present tribological test. Despite these differences, both curves of the apparent friction coefficient followed the same trend in relation to sliding speed. As far as the coating is concerned, and regardless of which MWF was used in the tribological tests, Figures 18 and 19 show no clear effects of the coating on the apparent friction coefficients and volumes of the built-up layer of Ti-6 Al-4V alloy in the pins, respectively. As far as the temperature in the pins is concerned, some differences between the coatings can be seen in Figure 23. This figure shows the temperatures in the pins with different coatings and for both MWFs, at an average contact pressure of 872 MPa and a sliding speed of 60 m/min. These temperatures were extracted from the curves of the temperature as a function of time, acquired during the tribological tests, and correspond to a cutting time of 3.5 s for all the coatings shown in Figure 23.  
This temperature was higher and more positive for flood conditions when compared to LN\({}_{2}\) cooling, wherein the temperature was negative. Moreover, this temperature was higher for uncoated pins, regardless of the MWF. The presence of a lower temperature in the pins depended on the coating and MWF used in the tribological tests. Under flood conditions, the lowest temperature was observed for the AlCrN coating, followed by M2 and M4. Under LN\({}_{2}\) cooling conditions, the lowest temperature was observed for the M4 coating, followed by CrN, AlCrN and M2. Nevertheless, from the thermal perspective, the coated pins performed better that the uncoated ones, because their temperatures were lower than that of the uncoated one. This means that the heat generated by the friction conducted to the pins would be lower in coated pins when compared to uncoated ones. Figure 22: Apparent friction coefficient as a function of the sliding speed obtained in the current tribological tests and by [PERSON] et al. [57] (uncoated pin; average contact pressure of 1329 MPa; LN\({}_{2}\) cooling).