Micromechanical properties of anomalously electrodeposited nanocrystalline Nickel-Cobalt alloys: a review

The microhardness values of the anomalously electrodeposited nanocrystalline Nickel-Cobalt (eNiCo) alloys reported in the literature varies in the range of 290 HV-700 HV, where a given alloy composition possess variable microhardness values, which considerably limits the prospective uses of these alloys. Therefore, this review systematically analyzes the influence of the electrodeposition operating conditions on the resultant micromechanical properties of the eNiCo alloys. The electrodeposition operating conditions reviewed in this paper includes the electrolyte metal content, bath pH value, organic bath additives, electrodeposition mode (direct current and pulse plating mode) and the bath temperature. The anomalous co-deposition models and the material strengthening phenomena are also considered. The literature review shows that when the alloy composition is varied exclusively by varying the electrolyte cobalt content, then the microhardness value increases with an increase in the alloy cobalt content up to 45 wt%-55 wt%. However, when certain additives and combination of operating conditions (pulse plating mode, forced bath convection etc) are used, finer grain deposits are obtained, which consecutively results in deposits with higher microhardness values independent of the alloy composition. The comprehensive literature review is concluded by further research aspects, which should be investigated to exploit the micromechanical properties of the eNiCo alloy system to a higher extent for MEMS and microforming applications.