## 3 Results and Discussion
### Microstructure of Coatings  
Figure 3 shows the optical microstructures of the bottom, middle, and top regions of the 316L coating and the composite coating. It can be seen from Figure 3a that the 316L coating is mainly composed of coarse columnar crystals, cellular crystals, and fine equiaxed crystals in the different regions. At the bottom of the 316L coating, the grow direction of coarse columnar crystals is perpendicular to the interface between the 316L coating and the substrate. Previous studies [25; 26] have found that the heat transfer direction of the molten pool is always along the direction of the maximum temperature gradient (\(G\)), which is perpendicular to the isotherm of the molten pool. In addition, during the solidification process, the temperature gradient at the bottom of the molten pool is the largest due to the good heat dissipation of the substrate. Moreover, the solidification rate (\(R\)) can be obtained according to Equation (1). \[R=V_{s}\cdot\cos\theta \tag{1}\]  
where, \(V_{s}\) is scanning speed, and \(\theta\) is the angle between the solidification front and the scanning direction. Therefore, the solidification rate of the single track decreases from the bottom to the top of the molten pool. The grain characteristics of the solidified coating are governed by a combination of \(G\) and \(R\)[27]. The large temperature gradient reduces the subcooling at the solidification front. The nucleation rate was effectively increased. However, the small solidification rate provides sufficient time for grain growth. As a result, coarse columnar crystals are formed at the bottom of the 316L coating, as shown in Figure 3. A visible remelting zone can be found in Figure 3. The grain morphology at the bottom of the single track is similar to that at the bottom of the coating, while the fine equiaxed crystals are concentrated at the top of the single track. As the liquid metal at the top of the melt pool is in contact with the cold shielding gas and air, the heat dissipation is mainly transferred to the air by convection and radiation. Therefore, the temperature gradient would be decreased. In addition, the increase in the solidification rate at the solidification front results in a decrease in G/R. This behavior promotes the generation of fine equiaxed crystals. When 10 wt% NbC and 10 wt% TiC particles are added to the 316L powder, the microstructure of the composite coating fabricated by laser cladding is shown in Figure 3b1-b3). The microstructure of the composite coating has evolved significantly compared to the 316L coating. Some gray spherical particles are evenly distributed at the grain boundaries at the bottom, middle, and top of the composite coating. As shown in Figure 4, the SEM image indicates that the composite coating is mainly composed of the matrix phase (P1), the strip precipitate phase (P2 and 3), and gray spherical particles (P4 and 5). To identify the composition of these particles, energy spectral analysis (EDS) tests were performed, and the results are listed in Table 3. The region P1 mainly contains Fe, Cr, and Ni elements, which can be identified as austenite according to the XRD results. As shown in Figure 4, regions P2 and P3 are rich in Nb elements, indicating that Nb is the main component. EDS results show that the atomic ratio of Nb and C in the P2 and P3 regions was close to 1:1. Combined with the XRD results of composite coating, these could be confirmed as NbC ceramic particles. Similarly, gray spherical particles (P4 and 5) at the grain boundary can be regarded as TiC particles. Due to the low thermal conductivity of ceramic particles, the heat conduction in the molten pool along the temperature gradient is inhibited by the uniform distribution of TiC and NbC ceramic particles during the solidification process. Therefore, the temperature gradient and \(G\)/\(R\) would be decreased. In addition, the unmelted ceramic particles increase the nucleation points in the molten pool and refine the grains.