Preparation, Structures And Properties Of Ni-W-P-CNTs Composite Coatings By Electroplating

Electrodeposition, as one of the commonly used surface engineering technologies, has been widely utilized in the fields of daily production, manufacturing, r epair, etc. As electrodeposited Cr and its alloys have excellent properties, such as good wear resistance, good corrosion resistance, etc., they had been extensively applied for decades. However, the traditional chromium plating process has been forbidden and abandoned because the Cr6+ is poisonous and not environmental friendly. In recent years, Ni-W-P alloy coating has been expected to replace the electrodeposited Cr-based composite coatings for their excellent properties. In this research, Ni-W-P coatings were prepared through the electrodeposited method. Ni-W-P-CNTs composite coatings were also obtained on the basis of Ni-W-P coatings. Optical microscope, scanning electron microscope(SEM), differential thermal analyzer(DTA), energy spectrum analyzer(EDS), X-ray diffraction(XRD), etc. were applied to analyze the morphologies, organizational structures and compositions of the coatings. In the meantime, through testing microscopic hardness, wear, corrosion, etc., the physical and chemical properties of the coatings were analyzed and studied.In this research, the plating solution was composed of Na2WO4, Ni SO4, Na H2PO2 and Na3C6H5O7(60 g/L, 50 g/L, 20 g/L and 80 g/L respectively), the current density was 4A/dm2, the PH was between 7 and 8, the electroplating time was 3 hours and the electroplating temperature was 70 oC. The research results showed that Ni-W-P coatings of high qualities could be obtained under the conditions. On the basis of Ni-W-P coatings, Ni-W-P-CNTs composite coatings were also prepared with the existence of dispersing agent(TNWDIS), ultrasonic dispersion and air agitation.Microstructure morphology observation showed that the typical microstructure of the obtained coatings was cellular, the thickness was mostly between 20um~30um and the surface was compact and continuous. EDS results showed that the Ni-W-P coatings were mainly composed of Ni(68.61%), which was followed by P and W. The XRD results showed that the structures of the coatings were amorphous or mixed crystal. The structures of Ni-W-P coatings turned into crystal and some second phases(e.g., Ni-P, Ni-W, etc.) separated out after heat treatment. In the process of heat treatment, the microscopic hardness and wear resistance increased significantly while the corrosion resistance decreased slightly. However, the corrosion resistance improved sharply after heat treatment at a relatively high temperature.The carbon nano-tubes(CNTs) appeared to be vimineous and coacervated fibers under scanning electron microscope. The agglomeration of CNTs would be improved if the solution was ultrasonically dispersed and mechanically agitated with the existence of TNWDIS dispersing agent. Compared with the Ni-W-P coatings, the CNTs improved the microscopic hardness and the wear resistance significantly, decreased the friction coefficient dramatically and sacrificed the corrosion resistance of the Ni-W-P-CNTs composite coatings slightly. Besides, the crystallization temperature of Ni-W-P-CNTs composite coatings was lower than that of Ni-W-P coatings. Heat treatment also had the similar impacts on the structures and the properties of the Ni-W-P-CNTs composite coatings as it did on the Ni-W-P coatings. However, the corrosion resistance of the Ni-W-P-CNTs composite coatings was not as good as that of the Ni-W-P coatings after heat treatment at the same temperature.