Preparation And Formation Mechanism Of Composite Coating By Laser Cladding YSZ@Ni Nanoparticles

YSZ@Ni core-shell nanoparticles are used to prepare composite coatings because pure YSZ nanoparticles are agglomerate readily and are easily lost during laser cladding. In this study, the effect of different materials on microstructures of the coatings were investigated and the melting behavior of YSZ@Ni nanoparticles was discussed. Then, the microstructure evolution law and formation mechanism of laser cladding coatings were explored. At last, the influence of laser power, scanning speed,and powder bed depth on the microstructure and microhardness of the coatings were analysed.Firstly, the microstructure of the coatings prepared by laser cladding YSZ nanoparticles and YSZ/Ni powders were not uniform. However, the coating microstructures prepared by the laser cladding YSZ@Ni core-shell particles were uniform, dense, and well combined with the matrix. Ni powders were proved to be beneficial to improve the compatibility and wettability between YSZ and Ni based alloys. In addition, pure YSZ nanoparticles was lost seriously during laser cladding,the YSZ/Ni powders was the second, and the YSZ@Ni nanoparticles was the best.Secondly, YSZ@Ni core-shell nanoparticles completely melted during laser cladding, and they were divided into core-YSZ and shell-Ni layer with a stratification feature. The monolayer coating comprised flower-like YSZ aggregates, nano-scale YSZ particles, proeutectic YSZ phases, and YSZ/FeCr2O4 eutectic structure. The multilayer coating structure was more homogeneous, mainly consisting of YSZ/FeCr2O4 eutectic structure. The microhardness of the ceramic layer of the multilayer coating was greater than that of the monolayer coating.Thirdly, by increasing scanning speed and keeping other parameters constant, the thickness of the ceramic coating decreased and it also had a tendency to be layered;by increasing laser power, the depth of the molten pool increased and formed a diffusion layer consisted of Cr and O; when the powder bed depth was less than 0.3mm, the formed coating was too thin. When the powder bed depth increased to 0.8mm, the ceramic coating thickness was up to 120 μm. If the powder bed depth continued to increase, the coating quality was again poor. By comparative analysis,the optimal processing parameters for the preparation of composite coatings were as follows: P=1500 W, V=600 mm/min, powder bed depth=0.3 mm, D=5 mm.