Heat treatment of ultrasonic electrodeposited Ni-W/ZrO2 nanocomposites

Metal matrix composite (MMC) coatings of zirconium oxide (ZrO2) in nanocrystalline Ni-W alloy were obtained by ultrasound assisted direct current electrodeposition from aqueous plating electrolyte. In the present study, effects of the heat treatment (in the range of 400 degrees C-800 degrees C, in argon atmosphere) on the microstructure and physicochemical properties of hard Ni-W/ZrO2 coatings deposited on steel substrates were investigated. It was found that thermal treatment considerably affected the microstructure of Ni-W/ZrO2 coatings, including phase compositions, surface morphology and grain size. Microstructure features of annealed composite coatings were correlated to tribological and mechanical properties. A significant improvement in hardness (12 GPa) and wear index (0.56 mm(3)/Nm) of Ni-W/ZrO2 nanocomposites was achieved by thermal treatment at 500 degrees C compared to as-deposited coatings (8 GPa and 1.5 mm(3)/Nm, respectively). At higher temperature (about 600 degrees C), a tendency towards phase separation was observed and iron diffusion from the substrate to the composite coating layer became evident.