# Performance Investigation of Coated Carbide Tools in
## 2 Materials and Methods
### Perpendicular Impact Test  
In general, perpendicular impact experiments on coated tools are conducted to evaluate their mechanical durability, adhesion strength, and resistance to dynamic loads, which are critical for manufacturing applications [22]. It is worth mentioning that in milling procedures, cutting tools experience intermittent cutting forces due to the cyclic engagement and disengagement with the workpiece. These impact forces can lead to micro-chipping, coating delamination, and premature tool failure. In particular, milling involves repeated mechanical shocks as the tool enters and exits the workpiece. Hence, perpendicular impact tests could replicate these conditions in a controlled environment to analyze how coatings withstand cyclic loading [23]. Moreover, the repeated impacts help assess the fatigue resistance of both the coating and substrate, so a material that could withstand multiple impact cycles without significant degradation is more suitable for milling applications. Therefore, by performing perpendicular impact tests, manufacturers can optimize coating selection, improve milling tool longevity, and enhance machining efficiency, ultimately leading to cost savings and better surface quality in industrial milling operations. Taking into account the above-mentioned, an impact tester was exploited in order to identify the maximum impact force that the investigated coated cutting insert can endure without failing [23]. As presented in Figure 3, the ceramic ball is repeatedly driven into the examined coating under an adjustable maximum force. Using a PID controller, the output voltage of a variable transformer is regulated via a DC motor in order to maintain consistent impact force peaks throughout the entire experiment, enabling that way the characterization of the coating's fatigue endurance stress. In order to quantify the fatigue endurance stress of the investigated coated carbide inserts, the imprint depths during the impact test were recorded across various load conditions. Specifically, the fatigue fracture behavior of the coating was examined after \(10^{6}\) impact cycles at different impact force amplitudes, as exhibited in Figure 4a. To determine the portion of the remaining imprint depth associated with plastic deformation \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & & & \\ \hline Composition & C & Si & Cr & Mn & Ni \\ \cline{2-6} In weight \% & 0.37 & 0.3 & 2.0 & 1.4 & 2.0 \\ \hline \multirow{3}{*}{Mechanical Properties} & Tensile & Yield & Hardness & Elastic & Fracture \\  & Strength (MPa) & Strength (MPa) & (HRB) & Modulus (GPa) & Elongation A [\%] \\ \cline{2-6}  & 900 & 1020 & 310 & 205 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties and chemical composition of the AISI P20 Steel [20]. in the substrate, impact tests were initially conducted for \(10^{4}\) cycles, a level at which no coating failure is expected [19]. The difference in imprint depths between \(10^{6}\) and \(10^{4}\) cycles represents the coating failure depth (CFD), a critical indicator of coating degradation, which was determined to be approximately 0.5 \(\upmu\)m [19]. The bottom part of Figure 4b presents characteristic results regarding imprint depths at different impact cycles, while the upper part displays a typical 3D imprint derived from the perpendicular impact test. The imprint depth measurements were carried out via the usurf confocal microscope of NanoFocus AG. It is evident from Figure 4a that at an impact load amplitude of 800 N, the imprint depths after \(10^{4}\) and \(10^{6}\) cycles are nearly identical, indicating that there is no significant coating damage due to the low CFD value. However, at a higher impact load of 950 N, the remaining imprint depth after \(10^{6}\) cycles exceeded that of \(10^{4}\) cycles, achieving a CFD equal to the criterion, suggesting the onset of micro-fatigue mechanisms that could lead to coating fracture and removal.