E.Α. Pavlatou, N. Chronopoulou, A. Karantonis, C. Sarafoglou, D.I. Pantelis
Nanocoatings-International Conference on Functional Nanocoatings (2013), 8-9 July, Politecnico of Milano, Milano, Italy (oral)
Electrodeposition is one of the most widely-used methods for preparing metal/alloy coatings and nanostructured composite coatings. Among various process technologies, electrodeposition has advantages such as cost-effectiveness relative to other techniques like spray and sputtering processes. The development of production technology of composite electrocoatings, such as metal matrix reinforced with nano-particles, is primarily aimed at improving the mechanical, tribological and corrosion behavior of metal matrix. Conventionally, ceramic powders such as alumina, silicon carbide, and diamond were used as reinforcing materials for Ni-based nanocomposite coatings. Recently, carbon nanotube (CNT) has been applied as a new reinforcing agent for composite coatings due to its excellent mechanical properties and high thermal conductivity. As Ni exhibits high wear resistance, good ductility, and ferromagnetism, Ni-CNT composite coatings have potential applications not only for wear-resistance coatings and microelectromechanical systems (MEMS), but also for corrosion-resistance coatings. Moreover, Ni matrix composites reinforced by carbon nanotubes have been studied for both improved mechanical properties and for potential applications as anticathode material in dye-sensitized solar cells. This study concerns the electrolytic co-deposition of hybrid structures composed by multiwalled carbon nanotubes (MWCNTs) and Al2O3 microparticles with nickel by applying both direct and pulsed current conditions. In this work a Watts type bath in absence of additives was used with a constant pH=3.5 and temperature 50±1°C. The electrodeposition of Ni/CNT-Al2O3 composites was carried out on a rotating disk electrode (RDE) by using various velocities in the range of 200 up to 1200 rpm. Pure Ni deposits were also produced under the same experimental conditions, as reference state for comparison reasons. The surface morphology, the crystallographic orientation and the grain size of nickel matrix were investigated as a function of the concentration of CNT-Al2O3 nanoparticles in the bath as well as of the type of applied current (e.g. direct or pulse). The imposed current pulses exhibited frequency of 0.01 up to 1000 Hz, while the duty cycle was kept constant and equal to 50%. The corrosion resistance of composite coatings was studied by conducting potentiodynamic tests and applying electrochemical impedance technique. The results of this work demonstrated that the imposition of pulse current conditions at high frequencies and reduced amounts of hybrid particles in the bath results to the production of composite coatings with enhanced wear resistance and relatively low corrosion rates. The simultaneous presence of CNT-Al2O3 particles in the bath and the application of high current pulses results to the production of nano-structured coatings with a mixed preferred crystalline Ni orientation ([110] +[211]) compared to the pure Ni coatings that presented a [110] texture.