Preparation And Properties Of Metal, Alloy And Composite Coatings By Cathode Plasma Electrolytic Deposition

Cathode plasma electrolytic deposition (CPED) is an advanced surface engineering technique, which is a hybrid of conventional electrolysis and atmospheric plasma processing. At present, there are mainly two problems that will restrict the development of this technique. On the one hand, the nonuniform micro-arc discharge on the gaseous envelope may lead to form rough and porous coatings on the cathode surface. On the other hand, owing to the high cathodic current density in the depositing process, the coatings can’t be deposited on the samples with large area.Based on the problems above, a series of work were carried out on the fundamental researches, and the preparation, microstructure and properties of the novel metal, alloy and composite coatings as follows:(1) The basic law of cathode plasma electrolytic depositing metal coatings was revealed. Owing to the massive hydrogen bubbles and plasma arcs on the surface of the cathode under the high current density in CPED process, the ion transfer process is restricted severely. The deposited coatings will always be rough or dendritical with the conventional composition of the electrolyte. However, the current efficiency and depositing rate can be reduced with lower main salt concentration and higher acid content in the electrolyte, which will contribute to obtain dense and uniform metal coatings by CPED. The current density-voltage (Ic-U) curves in the depositing process were also measured. It indicates that there are two essential conditions required in the plasma discharging process:the formation of a continuous gaseous envelope on the surface of the cathode, and the rising of the depositing voltage to the critical value. The addition of micro-beads can promote the formation of the gaseous envelope and decrease the cathodic current density in the depositing process significantly. Moreover, the mechanical attrition effect of the micro-beads can contribute to the surface uniformity of the deposited coatings. Finally, a three-dimentional electrolyte-dripping device and a electro-bath with mechanical attrition device were proposed according to the above, to realize the deposition of coatings on samples with large area.(2) The novel cobalt coating and trivalent chromium coating were prepared by CPED. In the study of Co coating, it is found that the best depositing voltage is about 10-20V above the critical value, for a relatively lower voltage will result in that the deposits can’t be modified by the micro-arcs sufficiently. It will also do harm to the samples if excessively higher voltage is applied since the micro-arcs are too dense and severe. In the study of Cr coating, it indicates that the deposited coating is crack-free with the thickness up to 30μm. Both of the coatings exhibit nanocrystalline structure, metallurgical adhesion with the substrate, high hardness, favorable corrosion and wear resistance, and high temperature oxidation resistance.(3) A series of alloy and composite coatings were prepared by CPED, such as M-Cr alloy coatings and the coatings with the dispersion of rare earth oxide nanoparticles. The element distribution is uniform in these coatings, and no cluster can be found. In the protection of specific materials, the Ni-20wt.%Cr alloy and Ni-20wt.%Cr-Y2O3 nanocomposite coatings were prepared on T91 boiler steels for high temperature resistance, and the Ni-Al2O3 and Ni-SiO2 nanocomposite coatings were prepared on Cu or Al wires.As an environmental friendly technique, CPED will have a widespread application in the future.