## 3 Results and Discussion
### Microstructure and Mechanical Performance of Repaired Gear
#### 3.5.3 Wear Performance  
Wear performance is one of the most important macroscopic characteristics of the LC layer, which can be evaluated by friction coefficient, volume loss, and worn surface morphology [48; 49; 50]. The sample of the frictional wear test was obtained from the repaired gears, as shown in Figure 17. The friction coefficient curves of the substrate and gear teeth are shown in Figure 17. It shows that the friction coefficient is unstable initially, but it attained a stable value after about 10 min. This implies that the friction coefficient of the cladding layer and the substrate are similar. The average friction coefficient of the gear teeth is 0.49, which is 1.06 times of the substrate. Moreover, a stable coefficient of friction allows for the orderly removal of material from the deposited layer, resulting in a robust fatigue life.  
The volume loss of substrate and repaired teeth are shown in Figure 18a,b. The section area of repaired teeth is 1674.983 \(\upmu\)m\({}^{2}\) and the section area of the substrate is 1675.596 \(\upmu\)m\({}^{2}\). It indicates that wearing capacity is similar. Furthermore, obvious plastic deformation at the edge of the wear track is observed in the wear surface of the 20 CrMnTi substrate, as shown in Figure 18(b1,b2). To reveal the wear mechanism, larger magnification optical images of each sample are shown in Figure 19. The large plastic deformation of the substrate would lead to rapid material removal and the formation of abrasive particles. As the slide continues, the abrasive particles micro-cut the substrate, and finally a large groove is  
Figure 16: Microhardness and indentation morphologies of the gear teeth.  
formed on the worn surface, as shown in Figure 19(a1). There would be thermal effects and residual stresses in the deposited layer due to rapid cooling during LC [51; 52]. The high and concentrated stress would promote the propagation of micro-cracks, as shown in Figure 19(a3,b3). As shown in Figure 19(a1,a2), the spallings and grooves are precipitated from the worn surface. Therefore, the wear mechanism of sample a is mainly abrasive wear [48]. The substrate peeled off many flakes or scalp flakes, so the worn surface is uneven, as shown in Figure 19(b2). The flakes of the substrate have greater volume, and they are easy to focus on peeling off, especially in the grooves. These flakes with no plastic deformation at the edge and some fatigue cracks are found inside the flakes, indicating that the wear is a typical sign of fatigue spalling. Therefore, the wear mechanism of sample a is mainly adhesive wear and fatigue wear [53; 54]. As shown in Figure 19(a3,b3), most of the cracks in the substrate extended very seriously and gathered at the spalling and flakes. The cracks in the cladding layer were few, and their expansion of cracks is not serious. In summary, the wear performance of the cladding layer and the substrate is similar.  
Figure 17: The friction coefficient of the substrate, Ni-based coating.  
Figure 18: 3D morphologies of wear tracks of (**a1**) 3D worn surface of Ni60 gear teeth, (**a2**) corresponding section profile in zone a1, (**b1**) 3D worn surface of 20 CrMnTi substrate, (**b2**) corresponding section profile in zone (**b1**).