## 3 Results and Discussion
### Tribological Properties
The values of the COF of the CrN/CrCN and CrN/CrCN/DLC coatings are very similar, and they provided the lowest COF values among the coatings studied. After the initial sharp increase, the COF of the CrN/CrCN coating reached a stable state at all sliding speeds. The COF of CrN/CrCN/DLC increased gradually, decreased afterward, and finally stabilized for all sliding speeds. The COF of CrN/CrCN/DLC coating decreased as the sliding speed increased. This observation is most likely due to a higher contact temperature between the pin material and the coating, which is said to accelerate the trend of graphitization of the DLC layer during the wear test [19]. Figure 13: Wear rate of uncoated and coated carbide substrates. The friction coefficient of the WC-Co sample was quite similar to those of the CrN/CrCN and CrN/CrCN/DLC coatings at the sliding speed of 50 mm/s. However, a sudden increase in COF occurred for the uncoated specimen at a higher sliding speed. This behavior is related to the worn surface of the uncoated sample. CrN/AlTiN has the highest COF followed by the CrCN/TiSiCN coating. The latter had a gradual increase of its COF followed by an abrupt increase to finally reach a stable regime at higher sliding speeds. The lower COF of the CrCN/TiSiCN coating at the beginning of the test is caused by the accumulation of carbon particles alongside the wear track. These particles serve as a solid lubricant, lowering the friction coefficient [38]. Moreover, the following increase of the COF of the CrCN/TiSiCN coating is due to materials being removed from the worn surface. The higher friction coefficient of the CrN/AlTiN coating is probably due to the presence of a large number of droplets that formed at the surface of the specimen during the evaporation of the AlTi cathode [14]. Figure 15 shows SEM micrographs of typical worn surfaces of coatings and uncoated WC-Co substrate for the tests performed at sliding speed of 50 mm/s. Tribofilms are observed within the wear tracks of the CrN/CrCN/DLC, CrN/AlTiN, and CrCN/TiSiCN coatings. EDS analyses of zones A-C (in Figure 15a,b,d) show the presence of oxygen (Figure 16a-c). This indicates the occurrence of tribo-oxidation, thus the formation of an oxide layer during the test. The worn surfaces were covered with a tribofilm having fish scale-like pattern. The fish-scale morphology shows that the tribofilm underwent shear deformation during the test [39]. The tribofilms for the CrN/AlTiN coating were found to be present on almost the entire wear track, except for its edges. They were highly dense and bonded well, thus protecting the surface and leading to a lower wear rate. Figure 14: Variation of friction coefficient in terms of sliding distance at sliding speed of (**a**) 50 mm/s, (**b**) 100 mm/s, and (**c**) 250 mm/s. Traces of Al in the tribofilms characterized by EDS analysis confirms the transfer of material from the ball to the CrN/CrCN/DLC and CrCN/TiSiCN coatings (Figure 16a,c), which is the sign of adhesive wear. Parallel grooves indicating abrasive wear were observed on the wear surfaces of all coatings. These grooves were caused by the wear debris detached from the coating, leading to three-body abrasion. According to [PERSON] et al. [40], the primary wear mechanisms of cemented carbide coated with CrN/AlTiN during the friction test were abrasive and adhesive wear. The presence of W identified in the EDS spectra of Figure 16a, c, reveals that the CrN/CrCN/DLC and CrCN/TiSiCN coatings were detached from the substrates in some areas, which was the main reason for the higher wear rates of  
Figure 15: SEM micrographs of the wear track surface of different coatings at the sliding speed of 50 mm/s (**a**) CrN/CrCN/DLC, (**b**) CrN/AlTiN, (**c**) CrN/CrCN, (**d**) CrCN/TiSiCN, and (**e,f**) uncoated WC-Co substrate. these coatings as reported in Figure 13. As claimed by [PERSON] et al. [41], the dominant wear mechanisms of a TiSiCN coating under a slow sliding speed are abrasive and adhesive wear. However, according to [PERSON] et al.