Influence of pulse parameters on the microstructure and microhardness of nickel electrodeposits

Square-wave cathodic current modulation was used to electrodeposit fine-grained nickel from an additive-free and saccharin-containing Watts bath. The influence of pulse on-time, off-time, peak current density and saccharin on the grain size, surface morphology, crystal orientation, and microhardness was determined. The study showed that at constant off-time and peak current density, the crystal size of the deposits was found initially to decrease with pulse on-time before it started to increase with further increase in on-time. The crystal orientation progressively changed from a (111) texture at the on-time of 0.1 ms to a strong (200) texture at an on-time of 8 ms. An increase in the pulse off-time at constant on-time and peak current density resulted in a progressive increase in crystal size. However, the crystal orientation remained unaffected with increasing off-time. An increase in peak current density resulted in considerable refinement in crystal size of the deposits. The crystal orientation progressively changed from an almost random distribution at the lowest peak current density of 0.2 A/m2 to a strong (200) texture at a peak current density of 2.0 A/m2. The nanocrystalline nickel with grain size in the order of 30 nm can be produced from saccharin-containing Watts' baths. In contrast, when using an organic-free Watts' bath and similar pulse-plating conditions, the grain size can only be refined down to about 80-100 nm. The microhardness of deposits is related with grain size: when the grain size is large, the microhardness is consistent with Hall-Petch law (HPL); when the grain size is ultrafine, “nano-effect” would be generated, the microhardness is against HPL.