Tribological behaviour of galvanic gold coatings reinforced with silica nanoparticles

Electroplated composite Au/SiO2 coatings on a nickel substrate have been produced by dispersing an emulsion of silica nanoparticles (20 nm in diameter) in an industrial hard-gold bath. The tribological behaviour of the Au/SiO2 coatings with different SiO2 content were investigated under sliding conditions against an alumina counterpart at different loads and sliding velocities. The tribological induced transformation of the coated surfaces were analysed using 3D-profilometry, scanning electron microscopy (SEM), focused-ion beam (FIB) cross sectioning and Auger electron spectroscopy (AES). The composite coatings provide higher hardness and ten-fold longer lubricating lifetime compared to the original hard gold coating. Hardness was identified as a key factor reducing the adhesive component of friction and increasing the resistance of gold against smearing. These effects allow to postpone the exposure of the nickel substrate and thus the loss of the self-lubricating character of the coating.