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Tribological, mechanical and electrochemical properties of nanocrystalline copper deposits produced by pulse electrodeposition

Nanocrystalline metals and alloys with grain sizes smaller than 100 nm have attracted extensive interest due to their improved mechanical, physical and chemical properties. Although electrodeposition has been one of the methods for synthesizing nanocrystalline materials, properties of nanocrystalline electrodeposits are less evaluated, especially for tribological applications or potential applications in nanoscale devices such as MEMS and NEMS. In this work, nanocrystalline and microcrystalline copper deposits were produced by pulse and direct current electrodeposition processes respectively. Effects of deposition parameters, such as the peak density, frequency, current-on time and current-off time of the pulse current (PC), on the grain size were investigated for the purpose of process optimization. The grain size of nanocrystalline coatings was determined using x-ray diffraction and atomic force microscopy (AFM). Mechanical and tribological properties of the deposits were investigated using nanoindentation, nanoscratch and microscratch techniques. It was demonstrated that the nanocrystalline film was markedly superior to regularly grained film made by direct current (DC) plating; the nanocrystalline deposit shows higher hardness, lower friction coefficient and lower wear rate. The surface electron stability and chemical reactivity of the deposits were also evaluated by measuring their electron work function (EWF). Results indicate that the nanocrystalline surface is more electrochemically stable than the DC-plated one. This increased stability result is attributed to the formation of a stronger and more adherent passive film on the nanocrystalline copper, confirmed by potentiodynamic polarization and electrical contact resistance measurements.