3.3.1. Wear Measurements and Characterization

As previously mentioned, the results obtained from the performed wear measurements will be described here, starting with the flank wear measurements conducted during SEM analysis on the tools' top surfaces (localized flank wear, VB3, according to [40]). These results can be observed in Table 7.


**Table 7.** Average flank wear (VB) values measured on the tools' top surfaces during SEM analysis.

According to the values displayed in Table 7, the values of VB3 (maximum localized flank wear) were higher for a longer cutting length. This was to be expected; however, the increase was quite significant, meaning that after only a short distance, the tools suffered a lot of flank wear. This was due to the properties of Inconel 718 causing high abrasive damage that led to tool chipping [51]. Additionally, the values that were obtained for tests conducted at 100 m/min were slightly lower than those obtained at a 125 m/min cutting speed. This indicates that an increase in cutting speed also caused an increase in the VB values.

As with the previous section, graphs displaying the values presented in Table 7 are provided for ease of comparison. As seen in Figure 7, there was a significant increase in the VB value from the tests conducted ata5m cutting length to those conducted at a 15 m cutting length, with the value of the latter being almost five times higher than that of the former [8]. Regarding the tests conducted at a 5 m cutting length, a common trend was identified: the VB values seemed to be higher for cutting tests carried out at 75% feed per tooth, and this value decreased with an increase in the feed per tooth parameter. This trend was identified for tests conducted at both 100 m/min and 125 m/min cutting speeds. As for the tests conducted at a 15 m cutting length, no clear trend could be identified. However,

the flank wear seemed to be more intense for the tests conducted at higher feed-per-tooth values.

From Figure 7, it can be observed that the cutting length (as mentioned above) and cutting speed exerted clear influences. As previously mentioned, the feed per tooth parameter exerted an influence in the tests conducted ata5m cutting length. Regarding the influence of cutting speed on the registered flank wear values, it seems that an increase in cutting speed resulted in a more severe level of flank wear. When comparing the tests conducted at 5 m and 15 m cutting lengths, the tests carried out at 125 m/min produced higher values of flank wear. Regarding the tests conducted at a 15 m cutting length, there seemed to be no clear influence of cutting speed on the sustained flank wear. In the case of the tools tested at a 125 m/min cutting speed, the highest amount of wear was identified for the 150% fz condition, whereas in the case of the tests conducted at 100 m/min, it seems that the worst case of flank occurred under the 100% fz condition. Although an increase in cutting speed usually promotes a smoother cutting behavior for the cutting tools, it can be observed that, in this case, it produced a more accentuated flank wear.

Regarding the type of flank wear identified for tools tested at 100 m/min and 125 m/min, although the maximum localized flank wear was less intense in the case of tests conducted at 100 m/min, the wear mark was considerably wider. In this case, the sustained wear had a more prominent area than that in the tests conducted at 125 m/min. For these tested tools, the wear appeared to be more localized. An example can be observed in Figures 8 and 9, where a top view and a rake face view can be observed for tools tested at 100 m/min and 125 m/min.

In Figure 8, two images obtained through SEM analysis of two tools' top surfaces are displayed. In Figure 8a, a tool that was tested at 100 m/min witha5m cutting length and 0.07 mm/tooth (100% fz) is displayed. In Figure 8b, a tool that was tested at 125 m/min is shown (with the remaining machining parameters staying the same as those for the tool depicted in Figure 8a). In the case of Figure 8b, the wear is slightly more accentuated when compared with Figure 8a.

Figure 9 displays two rake faces of tools tested under the same conditions as those shown in Figure 8. Although the wear is quite similar, it seems that the rake face wear sustained by the tools tested at 125 m/min was more intense. Of course, these are the tools tested at a 5 m cutting length. As previously mentioned, the impact of the cutting length seemed to increase the wear significantly. This is shown in Figure 10 (15 m cutting length), where the tools' top views (Figure 10a) and rake faces (Figure 10b) are shown.

**Figure 8.** Top view of tools tested at S100F100L5 (**a**) and S125F100L5 (**b**).

Figure 10 clearly shows the impact of cutting length on tool wear. Longer cutting lengths contributed to higher levels of tool wear, which was mainly because the tools would machine more material and be subject to adverse cutting conditions for longer periods of time (when compared with shorter cutting lengths). It was observed that the wear mark was wider for tools tested at lower feed rates and cutting speed values, and deeper for tools tested at higher feed rates and cutting speeds. This was because the tool was subjected to higher attrition wear at lower cutting speeds and feed-per-tooth values, causing more severe wear and altering the tools' geometry. This, in turn, promoted a different chip formation mechanism that seemed to accentuate the tool wear even more.

Regarding the influence of tool wear on the machined surface's quality, it seems that the tools with more severe wear produced lower machined surface quality. However, this was particularly observed for the tools tested at 125 m/min and tested at a 15 m cutting length, although the value of fz usually influenced the machined surface quality, with a higher value resulting in worse machined surface quality and better results in terms of tool wear (measured flank wear). However, this tendency was only observed for tools tested at a 5 m cutting length, with the fz parameter not greatly influencing the measured tool wear.

**Figure 10.** Top view of tools tested at S100F100L15 (**a**); rake face view (RF1) of tools tested at S100F100L15 (**b**).

#### 3.3.2. Tool Wear Mechanism Analysis

In this subsection, the identified wear mechanisms will be presented, evaluated, and discussed. They were common for all tested tools, with the ones tested at a 15 m cutting length having more severe levels of these mechanisms and overall wear (as expected) [8]. However, these wear mechanisms, as mentioned, were the same for all tested conditions.

The predominant wear mechanism was abrasion; in Figure 11, this wear is evident due to the smooth appearance caused by material abrasion on the tool's surface. Abrasive wear is also evident in Figure 10, where the loss of material caused by Inconel 718 abrasion during testing can be seen. Abrasive wear usually leads to the loss of tool material, even causing some alterations regarding geometry. This can also be seen in the tools' coating, as presented later in the manuscript. Abrasive wear is usually observed when machining Inconel 718 [7], which was observed in all the tested tools, albeit at different intensities. As previously mentioned, the tools tested at a 15 m cutting length exhibited considerably higher levels of wear than those tested at 5 m. The former tools exhibited more evidence of abrasive wear, as can be observed in Figure 11.

**Figure 11.** Rake face (RF2) of a tested S125F100L15 tool exhibiting abrasive wear.

Although Inconel 718 is known to adhere to cutting tools [23], the registered material adhesion was not abundant. That is, light workpiece material adhesion was registered on the tools' flanks, edges, and rake faces, as shown in Figure 12. Furthermore, there seemed to be a higher level of adhesion to the tools' uncoated areas (substrate) than to the tool coatings themselves (as shown in the marked area of Figure 12). Further, regarding material adhesion, the formation of a built-up edge was also registered, albeit in a primordial stage (not developed), which can be observed in Figure 13.

**Figure 12.** Material adhesion on the rake face surface (RF1) of a tested S125F100L5 tool.

**Figure 13.** Initiation of the formation of a built-up edge (BUE) on a tool's top surface, exhibiting material adhesion on the exposed substrate (shown on a tested S100F75L15 tool).

The adhered material's composition was assessed by performing EDS analyses, proving that it was indeed Inconel 718. This adhesion may eventually lead to adhesive wear. It was detected on some of the tools' faces, such as the rake face depicted in Figure 12. Adhesive wear can lead to premature tool failure, as it promotes the removal of tool coatings and substrates, with are removed with the adhered material on the tools' surface.

A considerable amount of chipping, mainly of the tools' substrate was registered in the tested tools, particularly those tested at a 15 m cutting length (as seen in Figure 10). Indeed, machining Inconel 718 can be quite aggressive to the cutting tools, promoting high levels of

wear [7,24]. This chipping hinders the production quality that can be achieved with these tools, as well as negatively impacts the overall machining performance [10,17]. This is due to the change in the cutting tools' geometry, altering the chip formation mechanism and, thus, promoting a higher amount of wear, cutting forces, and machined surface roughness values [9,10].

The tool coatings seemed to hold quite well. The TiN layer improved the coatings' crack resistance and, as such, no major cracking was identified in the tested coated tools [17]. Furthermore, the coating's adhesion was deemed to be quite good, as there was no major evidence of coating delamination, with the main coating wear mechanism being abrasion, as shown in Figure 14. As shown in Figure 11, the abrasion smoothed out and marked the tool's surface. In this case, the coating was eroded due to the machined material being "dragged" across the tools' surface.

**Figure 14.** Coating abrasion shown on a tested S100F75L15 tool rake face (RF1).
