## 3 Research Results and Discussion
### Fatigue Test  
The increase in fatigue life with CFUBMS coating varies depending on the coating parameters, type of coating, coating layer, and even thickness. Additionally, it is possible to reduce residual stresses induced by CAD with the coating interlayer's optimized design. The relationship between surface roughness and fatigue damage is also directly related to the initiation of cracks under fatigue conditions. In the case of fatigue, the crack begins with a burr, a notch, or a scratch on the surface. Therefore, fatigue damage is directly related to surface roughness [50; 51]. The aim of the PVD system is to increase the tensile and fatigue strength of TiN, CrN, ZrN, TiAlN, TiCrN, and TiZrN-coated metals and alloys. At the same  
Figure 4: EDAX graphs of multilayer Cr/CrN/TiCrN/TiCrCN nitride films coated on AZ91 and AA6063. Figure 5: XRD graphs of multilayer Cr/CrN/TiCrN/TiCrCN nitride films coated on AA6063 and AZ91. time, it is possible to further improve the mechanical properties and adhesion strength of the coatings by adding intermediate layers [52]. In that respect, scientific research has attempted to obtain better fatigue strength by improving the roughness using different methods, especially [PERSON] et al. (2024), who considerably increased the fatigue strength of AZ91 with multilayer Ti/TiN/TiVN coating in their research [34]. Upon reviewing the studies in which single-layer or multilayer PVD coatings displayed different behaviors in terms of fatigue strength, TiN coatings increased fatigue life in some materials while reducing it in others. The study by [PERSON] and [PERSON] (2001) stressed that the fatigue limit increased with a significant increase in hardness, a better surface quality, and a decrease in superficial tensile residual stress in TiN-coated AISI D2 [29]. [PERSON] et al. (2010) assumed that the possible surface residual stresses arising in the CrN grown by the PVD procedure, which significantly improved the fatigue resistance of AISI 302, mainly contributed to the improvement [31]. CrN coatings significantly increased fatigue life [31; 32]. Nevertheless, some studies have shown that TiN and CrN coatings have reduced fatigue strength in aluminum alloys. In their study, [PERSON] et al. (2005) evaluated the fatigue strength of different CrN-coated-with-PVD specimens produced from H11 or 6082 alloys. The fatigue limit of steel-coated specimens improved in comparison with uncoated ones, and an increase of 15% was observed. However, for coated aluminum specimens, no improvement was determined in the fatigue limit compared to uncoated ones; on the contrary, a decrease was observed [32]. In the study conducted by [PERSON] and [PERSON] (2011), a significant reduction of 94% in fatigue life was recorded for TiN coated on 7075-T6 alloy by PVD compared to the base material [33]. Figure 6 shows the comparative logarithmic S-N curves of AA606 and AZ91 materials and coated specimens. The load was added until the specimens broke or until \(1.0\times 10^{7}\) cycles were completed. The curves demonstrated a sharp decrease in the 90-70 MPa range. The fatigue strength of uncoated AZ91 was 70.26 MPa, but after nitride coating it increased to 78.15 MPa. The coating increased the fatigue limit value and life by approximately 11.22%. The fatigue strength of AA 6063 decreased from 79.71 MPa to 71.9 MPa with the Cr/CrN/TiCrN/TiCrCN coating. The nitride coating reduced the fatigue limit value by 9.79%. In light of the studies in the literature [34], it can be resulted that the fatigue strength and life may have increased with the considerable improvement in strength and adhesion strength in AZ91 Mg alloy with the reduced surface roughness in our multilayer nitride coating. Although the present study did not measure residual stresses, it is thought that  
Figure 6: Specimens’ S-N curves. the gain in the fatigue limit of the coating substrate system originates from compressive residual stresses in the coating that prevent the propagation of fatigue cracks.