**4. Conclusions**

In the present work, an analysis of the wear behavior of four milling tools used in machining operations of DSS was presented. Four tools with different geometries and coatings were evaluated. One tool had two flutes (T1) and three tools had four flutes (T2, T3 and T4). The tested coatings were TiAlN (T2), AlCrN (T1 and T3) and TiAlSiN (T4). Additionally, the influence of cutting length and feed rate was evaluated based on the tools' wear and production quality (machined surface quality). Furthermore, the wear of the tested tools was analyzed, and the present wear mechanisms identified.

Regarding the tested tools' performance, it was found that T1 (two-flute, AlCrN) and T2 (TiAlN) tools were not the most suited to machine this DSS alloy, as they presented severe wear for the highest feed rate conditions. Their flank wear was heavily promoted by an increase in cutting length. This can be related to the coating's properties in the case of T2, as this coating did not present the best mechanical properties (of all evaluated coatings). However, in the case of T1, the increased wear and lack of performance can be attributed

to the fact that this tool only had two cutting edges. The AlCrN coating's properties were quite good, presenting high values of H/E (0.11) and H3/E2 (0.395) (when compared to the other analyzed coatings) [53,54], which are representative of a good coating wear behavior. However, the AlCrN-coated tools presented a high Ec/Es ratio value (0.581, the highest of all analyzed coatings), indicating that it is quite prone to phenomena such as coating cracking and delamination [55].

As for the performances of T3 (four-fluted, AlCrN) and T4 (TiAlSiN), these tools exhibited the best performances, with the T3 tool producing the best results in terms of machined surface quality (for highest feed rate values). However, this tool presented more wear when compared to T4. Furthermore, for the rest of the test conditions (75% and 100% feed rate value), the T4 tool produced the best machined surface quality. As for flank wear, the T4 tool clearly outperformed the rest of the tested tools. This can be attributed to, not only the four-fluted geometry, but also to the mechanical properties of the TiAlSiN coating, which presented a high value of H/E (0.081, which is very close to 0.1) and a high H3/E2 value (0.142; these ratios' values were the second highest of all the analyzed coatings, being very close to the AlCrN coatings), and a lower Ec/Es ratio value (0.435), when compared to the AlCrN coating (in fact, this value was the lowest for all the analyzed substrate/coating systems).

Regarding the identified wear mechanisms, these were common among all evaluated tools, albeit at different levels of severity. These were: abrasive wear, adhesive wear and coating delamination/spalling. The coating failure method seemed to be common to all tools, as the workpiece material adheres to the machining grooves left by the substrate's grinding process. This adhesion promotes abrasive wear and further material buildup in these areas, eventually resulting in the spalling of the tool coating.

**Author Contributions:** V.F.C.S.: investigation, formal analysis and writing—original draft; F.J.G.S.: conceptualization, methodology, project administration, resources, supervision and writing—review and editing; R.A.: coating development, formal analysis, resources and visualization; G.P.: formal analysis, validation and writing—review and editing; A.B.: formal analysis, validation and writing review and editing; J.S.F.: investigation and supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** The present work was conducted and funded under the scope of the project ON-SURF (ANI|P2020|POCI-01-0247-FEDER-024521, co-funded by Portugal 2020 and FEDER, through the COMPETE 2020-Operational Programme for Competitiveness and Internationalisation.

**Data Availability Statement:** No data is made available regarding this work.

**Acknowledgments:** F.J.G. Silva thanks INEGI—Instituto de Ciência e Inovação em Engenharia Mecânica e Engenharia Indústria for its support. The authors would like to thank Rui Rocha from CEMUP (Porto, Portugal), due to his active collaboration in getting the best SEM pictures and helping with his critical analysis of some phenomena. The authors also would like to thank Ing. Ricardo Alexandre for his extremely important role in providing all the coatings through the TEandM company, and Eng. Nuno André for providing the substrate material and uncoated tools through the Inovatools company.

**Conflicts of Interest:** The authors declare no conflict of interest.
