## 3 Results and Discussion
### Tribological Properties  
Three-body abrasion test: The results of the measurements of the volume loss during the DSRW abrasion test is presented in Figure 11. The best three-body abrasion resistance was obtained with the CrN/AITiN coating followed by CrN/CrCN. The latter coatings are characterized with having better adhesive and cohesive properties as highlighted in Figure 9. CrN/CrCN/DLC gave the lowest level of protection against abrasive wear. Although CrN/CrCN had a high potential for reducing wear rate, applying DLC as a top layer decreased its resistance to abrasive wear. CrCN/TiSiCN coating showed medium resistance to abrasion when compared to the other three coatings studied. Figure 10: SEM micrographs of the micro cracks on coated carbide substrates after Daimler–Benz adhesion tests (**a**) CrN/CrCN/DLC, (**b**) CrN/AITiN, (**c**) CrN/CrCN, and (**d**) CrCN/TiSiCN. The typical wear patterns of each coating are shown in Figure 12. It is seen that the CrN/AlTiN and CrN/CrCN coatings are firmly attached to the substrate (Figure 12b,c). No coating failure was observed for these two coatings. On the other hand, the CrN/CrCN/DLC coating showed severe wear. Fatigue wear occurred, as seen in Figure 12a owing to the presence of cracks, which are perpendicular to the sliding direction. Furthermore, the coating showed obvious signs of delamination, which were most likely caused by the lower adhesion strength of CrN/CrCN/DLC to the substrate compared to other coatings. Therefore, abrasive sand particles could promote coating detachment. [PERSON] et al. [34] showed that DLC had lower abrasive wear resistance compared to TiN on steel substrate in DSRW tests. The behavior was attributed to the lower strength at the interface between the substrate-DLC film against shear stresses. Figure 11: DSRW abrasion test result for different coatings on carbide tips. Figure 12: SEM images of wear track on coating (**a**) CrN/CrCN/DLC, (**b**) CrN/AlTiN, (**c**) CrN/CrCN, and (**d**) Crocin/TiSiCN. D delamination within the coating layers and abrasion occurred on the surface coated with CrCN/TiSiCN due to the abrasive sand particles, as seen in Figure 12d. Dry sliding wear test: Figure 13 shows the wear rate of the four coatings studied along with the results obtained with uncoated substrates for different sliding speeds, i.e., 50, 100, and 250 mm/s and a constant normal load of 10 N. For CrN/CrCN and CrN/CrCN/DLC coatings, the wear rate as a function of sliding speed followed a similar behavior. As the sliding speed increased from 50 to 100 mm/s, the wear rate increased to its maximum value and remained constant with further increasing sliding speed. The wear rate of the uncoated sample, CrCN/TiSiCN, and CrN/TiAlN coatings increased by increasing the sliding speed. CrN/CrCN coating showed a lower wear rate compared to others. It is well established that the addition of carbon to the CrN system reduces the residual stress and friction coefficient. It also increases wear resistance as well as the adhesion of the coating to the substrate [35]. Moreover, CrN/TiAlN coating increased the wear resistance of the WC-Co substrate. As stated by [PERSON] et al. [36], higher wear resistance of hard films is associated with higher hardness. According to the results presented in Figure 13, the CrN/CrCN coating significantly decreased the wear rate of WC-Co substrates by 74%, 66% and 77% at sliding speeds of 50, 100 and 250 mm/s, respectively. Since the coefficient of friction (COF) of any given surface plays an important role on its wear resistance, the latter was characterized. Figure 14 presents the variation of the coefficient of friction of the different coatings studied as a function of sliding distance at sliding speeds of 50, 100, and 250 mm/s. All samples experienced an abrupt increase of its COF at the beginning of sliding. The initial formation of wear debris and its further involvement in the sliding contact is responsible for such an increase in friction coefficient [37].