## 3 Results and Discussion  
### Microstructure Analysis  
In order to identify the phase composition of the coatings with different Mo contents, XRD analysis were conducted on the coating. The XRD spectra are presented in Figure 2. With the increase of Mo content, the phases in the coating are changed and the intensity of the diffraction peak becomes weaker. The phases in the Mo-free coating are \(\alpha\)-Fe, M\({}_{2}\)B (M = Fe, Ni), M\({}_{23}\)(C, B)\({}_{6}\), and M\({}_{7}\)(C, B)\({}_{3}\). Another \(\alpha\)-Fe peak appears near the main peak when Mo is added into the coating. The intensity of the peak gradually increases with the increase of Mo content. When the Mo content is 4.0 wt. %, Mo\({}_{2}\)C phase appears in the coating. With the increase of Mo content, the amount of M\({}_{7}\)(C, B)\({}_{3}\) decreases gradually, and the amount of M\({}_{2}\)B, M\({}_{23}\)(C, B)\({}_{6}\), and Mo\({}_{2}\)C gradually increases. Figure 3 shows the metallographic microstructure of the coating. It can be seen from the bottom part of the coating (Figure 3a,c) that a light line is formed between the coating and the substrate. The light line is the fusion that grows mainly in the form of planar grains. The fusion line indicates a good metallurgical bonding between the substrate and the coating. During the solidification process, the microstructure of the coating is mainly determined by the ratio (G/R) of the temperature gradient (G) and solidification rate (R) [35]. The temperature gradient (G) is high and the solidification rate (R) is very low at the bottom of the molten pool, so the ratio of G/R is relatively large and the solidification structure grows as planar  
Figure 1: Schematic diagram of wear tester. 1—Rotating ring, 2—The coating, 3—45 steel substrate, 4—Load. grains [36]. With the increase of distance from the fusion line, the temperature gradient (G) decreases and the solidification rate (R) increases, so that the G/R value is reduced, and the microstructure of the coating is transformed from planar grains to cell grains, dendrites, and equiaxed grains [37]. Figure 2: _Cont._  
Figure 4 shows the scanning electron micrographs of the coating with different Mo contents. It can be seen from Figure 4 that the coating is mainly composed of a netlike eutectic structure, small granular eutectic structure, and dendrite matrix. During the solidification process, the dendrite matrix is firstly precipitated from the liquid phase, and then the eutectic structure between the dendrites is formed through the eutectic reaction in the remaining liquid phase. The size of the dendrite is obviously refined with the increase of Mo content from 0 to 6.0 wt. %, as shown in Figure 4. The sizes of the dendrite are measured by Image Pro-plus 6.0 software, and the average sizes of dendrites in the coatings of Mo-free, 2.0 wt. % Mo, 4.0 wt. % Mo, and 6.0 wt. % Mo are 7.9 \(\upmu\)m, 6.5 \(\upmu\)m, 4.4 \(\upmu\)m, and 3.9 \(\upmu\)m, respectively. This may result from the addition of Mo, which affects the nucleation process of the coating [38]. On the one hand, the Mo element can prevent the growth of austenite, which can refine the structure of the coating. On the other hand, part of Mo can dissolve in the coating, which results in lattice  
Figure 3: Optical micrographs of the coating: (**a**) The bottom part of the Mo-free coating; (**b**) The upper part of the Mo-free coating; (**c**) The bottom part of the 4.0 wt. % Mo coating; (**d**) The upper part of the 4.0 wt. % Mo coating. Figure 2: XRD spectrums of coatings: (**a**) Mo-free; (**b**) 2.0 wt. % Mo; (**c**) 4.0 wt. % Mo; (**d**) 6.0 wt. % Mo. distortion. The compositions of netlike eutectic structure 1, the dendrite structure 2, and the granular eutectic structure 3 with the addition of 4.0 wt. % Mo content (Figure 4c) are analyzed by EPMA point scanning. The results are shown in Table 2. The EPMA results show that the contents of Cr, B, and C in the eutectic structure are relatively high.