## 3 Results and Discussion
### Tribological Properties
The dominant wear mechanisms for the uncoated substrate were transgranular cracks in the WC grains and their brittle fracture, which was attributed to the cyclic loading during the sliding wear test (Figure 17e). Figure 18 shows the SEM micrographs of typical worn surfaces of different coatings and uncoated WC-Co substrates at the sliding speed of 250 mm/s. As presented in Figure 18a, CrN/CrCN/DLC coating is delaminated from the substrate within the wear track. The surface is worn due to the severe consequences of the ploughing mechanism. The worn surface is covered with wear debris. Entrapment of debris particles in the wear track led to high abrasion stress, causing the wear mechanism to shift from two-body to three-body abrasion and ultimately producing deep grooves. Severe delamination, crack and abrasive wear accounted for the high wear rate of this sample, as shown in Figure 9.3 It is seen that the CrN/CrCN coating was worn due to microcutting (Figure 18c). Wear tracks of the CrN/CrCN and CrN/AlTiN coatings were relatively shallow, and smoother compared to other samples. The wear mechanism of these coatings did not change by increasing the sliding speed. Fatigue is responsible for the formation of cracks in the CrCN/TiSiCN coating, which resulted in the coating chipping away from the surface (Figure 18d). Stresses in a coating may cause crack propagation and plastic deformation via local coating fracture, thus increasing the wear rate [46]. Slight abrasive wear was observed due to the presence of wear debris within the track (Figure 18d). Moreover, tribofilms formed on the surfaces coated with CrN/AlTiN and CrCN/TiSiCN. For the uncoated sample, cracks occurred in most WC grains, which were predominantly transgranular, followed by grain crushing. Increasing sliding speed resulted in increased cobalt binder extrusion and widespread grain crushing, which led to a higher wear rate of this sample compared to what was observed at lower sliding speeds (Figure 13). The fragmented grains served as three-body abrasive hard particles promoting the abrasive wear mechanism (Figure 18e). \begin{table}
\begin{tabular}{c c c} \hline
**Element** & **\% Mass** & **\% Atomic** \\ \hline C & 7.83 & 13.34 \\ \hline N & 12.79 & 18.70 \\ \hline O & 41.11 & 52.60 \\ \hline Al & 1.07 & 0.81 \\ \hline Si & 0.95 & 0.69 \\ \hline W & 2.93 & 0.33 \\ \hline Ti & 12.50 & 5.34 \\ \hline Cr & 20.81 & 8.19 \\ \hline \end{tabular}
\end{table} 
Table 5: Qualitative EDS analysis of zone 1 covering with oxide films and adhesive layers in Figure 17d. Figure 19 shows the worn surface of the alumina counterparts at the sliding speed of 50 mm/s for the series of specimens studied. Grooves and tribofilms can be seen on the balls sliding against the coated substrates. The grooves are due to the three-body abrasion taking place during the friction test. The balls that were in contact with the CrN/CrCN/DLC and CrN/CrCN coated samples showed smoother surface. The characterization of these balls showed the presence of chromium as well as a large amount of carbon from the coatings. The latter was responsible for lowering COF as it constitutes an easy-to-shear tribofilm on the surface of the balls. It was reported that the transfer layer facilitates sliding by lessening the contact between the counterparts [47]. Elements such as Ti, Cr, and C were detected on the balls that were used against the CrCN/TiSiCN and CrN/AlTiNi coatings. Their presence confirmed that adhesive wear was active during the test. Furthermore, the EDS spectrum of the counter body used to characterize the wear resistance of the  
Figure 18: SEM micrographs of the wear track surface of different coatings at the sliding speed of 250 mm/s (**a**) CrN/CrCN/DLC, (**b**) CrN/AlTiN, (**c**) CrN/CrCN, (**d**) CrCN/TiSiCN, and (**e**) uncoated WC-Co substrate. CrCN/TiSiCN coating shows the presence of significant weight fraction of W (16.62 wt%) along with Co (Figure 19f), which proves that the coating was removed during the sliding test.