*3.1. Coatings' Characterization*

For the coatings' characterization, the thickness of the three applied coatings was analyzed by preparing the unused tools for SEM analysis. Both the T1 and T3 tools were considered in the characterization of the AlCrN coating, as they were coated with the same coating using the same deposition parameters. Regarding the thickness measurements, these were performed in the same area for all the tested tools. Thickness values were obtained near each tool's rake and clearance face (obtained from the tools' cross-sections). The average thickness value was then determined. It is worthy to note that there were no significant changes detected for the analyzed coatings' thicknesses.

The average thickness values registered for each of the coatings can be observed in Table 5.


**Table 5.** Average thickness values for the analyzed coatings.

Although the AlCrN coating is a multi-layered coating, in the polished samples the interface between layers was not observed. This was due to the polishing process itself, as it smoothed the sample's cross-section, homogenizing the coating and making it look like a monolithic coating. However, if the coating was fractured, this multi-layered structure was observed, as seen in Figure 2a. Also in Figure 2, SEM measurements of the TiAlSiN coating thickness can be observed. This is an example of a measurement performed for all the coatings' thicknesses. In Figure 2a, the Al layers are the lighter ones, whereas the Cr-rich layers are the darker ones. Regarding their thickness, due to the SEM equipment's limitations, this was not determined.

**Figure 2.** Coatings' thickness analysis: multi-layered structure of the AlCrN coating (**a**); measurement of TiAlSiN (**b**).

The coatings' mechanical properties were also evaluated, namely, hardness and the Young's modulus, by conducting ultra-micro hardness tests on the TiAlN, TiAlSiN and AlCrN coatings using a dynamic ultra-micro hardness tester, the Fischerscope H100. As previously mentioned, the coatings were deposited onto flat substrate samples that were coated simultaneously with the tools. Ten tests were conducted on each of the deposited coatings, determining values for hardness and the reduced Young's modulus. The average of these values was calculated and is presented in Table 6. In Figure 3, the hardness values of the coatings are presented as a column graph, comparing these with the hardness value of the base material.

**Table 6.** Average hardness, Young's modulus, H/E and H3/E2 ratio values obtained for the produced coatings.


**Figure 3.** Hardness values of the tested coatings and base material (UNS S32101).

As seen in Table 6, the H/E and H3/E2 ratios are displayed. These ratios provide valuable information regarding the coatings' wear behavior [53], with the H/E ratio being a good indicator of the coating's wear resistance, with higher values being preferred (closer to 1; however, the presented values are quite satisfactory for all analyzed coatings). The H3/E2 ratio is related to the coating's ability to resist plastic deformation, which impacts the coating's wear behavior as well [54] (for this ratio, the higher value is also preferred, with ratios closer to 1 indicating very good plastic deformation resistance).

Still regarding the coating's wear behavior, another indicator of this is the ratio between the Young's modulus value of the coating and the substrate. Usually, a higher E value in the substrate is preferred (when compared to the coating's E value), as this can delay the onset of plastic deformation in the substrate and avoid subsequent coating cracking and chipping [55]; as such, lower values of this ratio are preferred. Substrate/coating systems that have this ratio closer to 1 are more susceptible to phenomena such as cracking and coating delamination. The substrate's Young's modulus was determined to be 611 GPa (average value). In Table 7, the ratio between the coatings (Ec) and the substrate's (Es) Young's modulus value can be observed.

**Table 7.** Ec/Es ratio determined for each of the tested coatings.


### *3.2. Cutting Force Analysis*

Cutting forces provide valuable information regarding the machining process, primarily about process stability and tool wear. Using the methodology presented in the previous chapter, cutting forces were also registered during all the conducted tests; however, due to the used parameters, primarily a low depth of cut, the cutting forces produced did not present high values.

Analyzing the cutting force graphs obtained from each of the conducted tests, a common trend was noticed. Independent of feed rate, the cutting forces registered an increase in value for the Fx, Fy and Fz components, with the increase in the Fz component being quite significant for all the cases. This increase was registered at around 2 m of the cutting length for all the tested tools. It was also registered that the values for these components increased until the end of the test, at 4 m of the cutting length.

This increase in cutting force value may be attributed to the wear sustained by the cutting tools; however, this cutting force value variation was quite low (as previously mentioned). These low values were attributed to the selected cutting parameters, and more concretely, the low value of the axial depth of cut. Due to this fact, these values were omitted in further analyses.
