3.3. tribological behavior
3.3.1. Rotating wear test
3.3.1.1. The friction coefficients. Friction coefficient behaviors for the TiN coatings applied with and without Ar gas are shown in Fig. 10.
The evolution of friction coefficient was studied as a function of time during 90 min. For the coatings applied without Ar (Fig. 10(a)), it has been shown that both TiN coatings coated with 14% and 25% of duty cycle display similar behavior. The friction coefficient reached directly a value of 0.12 and then remained constant during the entire test. Similar results were found by Tannoa et al. [31] when focusing on the relation between the friction coefficient of the TiN coatings with crystallite orientation. However, for the coating coated with 25% duty cycle, the first part of the curve represents a rapid increase until approximately 0.9. Concerning the second part, it represents a typical relative steady state wear regime. The same phenomenon can be seen in Fig. 10(b) for the coatings applied using 14%, 20% and 25% duty cycle with Ar gas distinguished with obvious fluctuation in the first part of the curve. In the second part, the friction coefficient was stable for the three coatings and was between 0.82 and 0.95. Similar shape friction for four coatings can be related to the wear debris gener- ated from the olive seed because it leads to a rapid increase in the friction coefficient.
3.3.1.2. Wear resistance. Fig. 11 shows the evolution of the wear depth as function of duty cycle for both coatings applied with DC bias and pulse bias voltage.
It is worthy to note that the best wear resistance was observed for both coatings applied with DC 150 V and pulse bias 20% duty cycle. This can be explained by the fact that it is characterized by the lowest number of droplets that can affect the wear resistance as previously observed. For the TiN coatings coated without Ar, it can be seen that the best wear resistance was for the coating with 20% duty cycle. Nonetheless, for those coated using Ar gas, it was for 14% duty cycle. The addition of Ar into the hard coatings does not have a significant positive effect on the wear behavior of the TiN coatings. In addition, there is no significant evolution of the wear resistance when increasing the duty cycles. These results are consistent with the finding of Aliofkhazraei et al. [32]. When comparing the wear depth of the substrate with the TiN coating applied with DC À150 V as well as with those applied with pulse bias voltage, a good contribution for the TiN coating applied in terms of wear resistance can be noticed. These results are similar with the findings of Cozza et al. [33] who have explained this fact by the effect of the high residual stress in the coatings applied with pulse bias voltage which can affect directly the wear behavior. As previously discussed, the wear tests using olive seed as counterbody have shown that the wear depth for the stainless steel could be easily quantified. Meanwhile, for the TiN coatings the wear damage was not significant. Thus, it will be difficult to make a comparison between the studied coatings. For this reason, in the following section reciprocating wear tests have been con- ducted using alumina ball in order to accelerate the wear rate evolution under different applied loads. The reason for choosing reciprocating wear test is that olive pomace translation movement presents the lowest percentage of displacement compared to the rotational movement. So, we choose the low conditions that can make wear in the olive-oil extraction devices.