Ultrasound-assisted electrodeposition of nickel: Effect of ultrasonic power on the characteristics of thin coatings

• Electrodeposition of thin Ni coatings from a Watts bath under 32–38 kHz ultrasound
• Influence of ultrasonic power on the characteristics of thin Ni coatings
• Effect of ultrasonic power on surface finish and grain orientation and structure
• Hardening effect by ultrasound probably due to grain refinement and work hardening
• Bubble structures forming on the surface strongly affect the properties of coatings.

The effect of ultrasonic power on the characteristics of low-frequency ultrasound-assisted electrodeposited Ni coatings from an additive-free Watts bath has been evaluated by different methods. XRD analysis showed that, while mechanical agitation favoured the electrocrystallization of Ni in the [211] direction, ultrasound promoted the electrodeposition of Ni with a [100] preferred orientation. FIB-SEM images of the surface of Ni deposits not only indicated that the surface structure agreed to some extent with the XRD results, but also that ultrasound refined, to a certain extent, some of the grains of the surface of the coatings. FIB-SEM images of the cross-section of the coatings confirmed this effect of ultrasound on the microstructure of the deposits. Such change in the microstructure of Ni, along with work-hardening by ultrasound, resulted in an increase in the hardness of the deposits. The characteristics of the deposits depended on the ultrasonic power employed, and it was found that Ni coatings electrodeposited using an ultrasonic power of 0.124 W/cm3 presented the higher proportion of crystals with a [100] preferred orientation, the highest degree of grain refinement in the surface and the highest microhardness values. Nevertheless, these deposits also presented visible erosion marks on the surface of the coatings due to the formation of transient bubble structures near the surface of the cathode during the electrodeposition. These erosion marks might be considered the main drawback to the use of ultrasound during the electrodeposition.