Microhardness and thermal stability of pulse-plated Zn-Ni alloy coatings

The microhardness and thermal stability of Zn-Ni alloy coatings with Ni content up to 62 wt.% produced by the square wave pulse-plating technique were investigated. The microhardness of these coatings was observed to increase with increasing Ni content. Several factors which contribute to the hardness such as solid solution hardening, effects owing to different phases and grain size were discussed in order to explain the hardness behaviour. Two methods, namely conventional furnace annealing and in-situ annealing in a transmission electron microscope were considered in this work to study the thermal stability of Zn-14 wt.% Ni and Zn-55 wt.% Ni alloys, respectively. The results indicated that microcrystalline Zn-14wt.% Ni alloy coatings were thermally stable up to a temperature of 200 degrees C for 24 h, however electron beam exposure of a nanocrystalline Zn-55 wt.% Ni alloy deposit for a few minutes in the transmission electron microscope resulted in considerable grain growth.