Microstructure And Properties Of Ni-based Composite Coatings Prepared By Laser Cladding

Ni-based self-fluxing alloy is one of the most common cladding materials used by laser cladding because of its good wear resistance, corrosion resistance, and excellent wettability with the substrate materials. However, studies show that Ni-based alloy coatings can easily come to failures due to fatigue wear and adhesive wear under high load and non-lubricated conditions, which largely shorten the useful life of the components. In order to further improve the performance of the Ni-base alloy coatings, a certain amount of graphite/Ni, graphene and WC/Ni was added in the Ni-base alloy powder, respectively. And the corresponding composite coatings with good mechanical properties and friction and wear properties were prepared.In the present study, Ni-based alloy coatings with different content of graphite/Ni, grapheme, and WC/Ni were prepared by laser cladding on the surface of40CrNi2Si2MoV steel, respectively. The influence of additives on the microstructure and properties of the Ni-based alloy coatings were systematically investigated by X-ray diffraction, scanning electron microscopy, electron probe micro-analyzer, Vickers hardness tester, and friction wear testing machine.Graphite/Ni is a kind of composite powder material of a core-shell structure. The presence of the Ni coating layer can not only avoid the reaction between the core portion of the graphite phase and alloying elements in the laser molten bath, can also increase the wetting properties between the graphite phase and the Ni-based alloy matrix. As a kind of good solid lubricating materials with unique lamellar structure, Graphite phase in the core can significantly improve the friction and wear properties of the composite coatings. Experiments showed that, the composite coatings with different graphite/Ni content mainly consisted of y-Ni, Ni3B, M23C6, M7C3, and graphite phases, and the Cladding layer showed the typical hypoeutectic microstructure. When the graphite/Ni addition is beyond10vol.%, the spherical particles of graphite phase could be clearly observed in composite coatings. With increasing of the graphite/Ni content, the dendrites became gradually fine, and the hardness and the wear resistance increased gradually, while the anti-friction property present the trend of first decrease then increase, and reach the lowest values at the graphite/Ni content of6vol.%.Graphene was proved to be a material of the best mechanical behavior so far, and because of the inheritance of the lamellar structure of the graphite, graphene can also be used as the solid lubricant. Experiments showed that, the composite coatings with different graphene content mainly consisted of γ-Ni, Ni3B, M23C6and M7C3phases, no graphene was detected in the coatings, and the microstructure of the cladding layer was composed of the typical dendrite and eutectic. With increasing of the graphene content, the hardness and the wear resistance of the coatings were improved due to the increase of carbide hard phase quantity and the refined microstructure, while the anti-friction property showed a trend of decreasing gradually.WC/Ni is also a kind of composite powder material with core-shell structure. The presence of WC particles in the coatings, was undertaken primarily to improve the wear resistance of the composite coatings. Experiments showed that, the composite coatings with different WC/Ni content mainly consisted of y-Ni, WC, W2C, M23C6and M7C3phases, and the dendrites quantity decreased significantly with increasing the WC/Ni addition, while eutectic microstructure quantity increased obviously. Due to the partial dissolution of WC in laser cladding process, a large number of epitaxial whisker-like microstructure on the verge of the WC particles could be viewed. The existence of whisker-like microstructure could ensure to achieving metallurgical bonding strength between the WC particles and nickel-based alloy matrix, with the increase of the WC particles in the composite coatings, the hardness and wear resistance are improved, and also, the anti-friction property decreased gradually.By contrast, the composite coating with14vol.%graphite/Ni had the best anti-friction property, and the best wear resistance and the highest hardness belonged to the composite coating with20vol.%WC/Ni.