Study On Preparation And Properties Of The Laser Cladding Ni-Cr-Mo And Ni-Mo Series Coatings

As a new surface modification technology, laser cladding plays an important role in preparing abrasion-resistant and corrosion-resistant cladding coatings. It has great potentials for civil and military applications. In order to improve the corrosion resistance of marine and boiler parts, so as to extend their service life, Ni-Cr-Mo and Ni-Mo series coatings were prepared on the surface of the steel using alloyed powders by the Nd:YAG laser in this paper. All the coatings had homogeneous structure and metallurgical bonding with the substrates. The microstructure and the phase composition of the laser cladding coatings were analyzed by means of SEM (Scanning Electron Microscope), EDS (Energy Disperse Spectroscopy) and XRD (X-ray Diffraction). The microhardness curves of the coatings were examined by a nanoindenter instrument. The corosion-resistance of the coatings were characterized through electrochemical experiments and high temperature chlorine corrosion tests.The results showed that Mo was in the form of approximately spherical particles in the Ni-Mo coatings. With increasing content of Mo, cracks and pores as well as the grain size increased, while the surface hardness and the corrosion resistance declined due to the intergranular corrosion caused by molybdenum carbides in Mo-30 wt.% Ni coating. Thus, the Mo-40 wt.% Ni coating had more excellent performance than the Mo-30 wt.% Ni coating. The Mo-30 wt.% NiCrBSi cladding coating had higher hardness than the Ni-Mo coatings due to hard phases formed by B and Si during laser cladding process. In addition, the corrosion-resistance of the Mo-30 wt.% NiCrBSi coating was higher than that of the Ni-Mo coatings, attributing to a dense passive film formed by Cr.By comparing the laser cladding Ni-Cr-Mo coating and the C22 alloy with the same chemical composition, it was found that the Ni-Cr-Mo coating had significantly smaller grain size and higher hardness than C22 alloy, indicating fine grain strengthening effect by laser cladding technique. XRD results showed very similar phase composition in these two samples, explaining that laser cladding technique had little effect on the chemical composition of the original materials. Electrochemical experimental results showed that the corrosion resistance of C22 was slightly higher than that of Ni-Cr-Mo coating at room temperature, while the chlorine corrosion tests indicated that the laser cladding coating had a better high temperature corrosion resistance. Furthermore, the Ni-Cr-Mo alloy coating had better resistance to seawater corrosion than the Ni-Mo coatings, mainly because of the presence of appropriate content of Cr which could form protective films.