A resistance-wire type strain gauge assembly affixed to the reverse side of the copper plating substrate was used to measure the real-time variation of the internal strain developed during zinc-cobalt alloy deposition by periodic reverse current electrolysis. Zinc content of the alloy deposits obtained at E=−1.06 V (vs. Ag/AgCl/saturated KCl) was 86.0 mol%, and these deposits always exhibited a compressive internal strain and a zinc-rich γ-phase structure. The internal strain, however, sharply shifted towards the tensile direction as soon as part of the zinc was selectively leached from the alloy deposits. This shows that the formation of compressive strain depends not only on the crystal structure of the deposits, but also related to the slightly greater zinc content compared with the stoichiometric amount of zinc that would be found in true γ-phase Co
5Zn
21. On the other hand, during periodic reverse current electrolysis conditions of cathode pulse Tc=5 s and anode pulse (Ta) 0.1 to 5 s, the internal strain cyclically changed from the compressive direction during the cathode deposition of the zinc-rich γ-phase alloy at Ec=−1.06 V, to the tensile direction during the anodic part (Ea=−0.70 V or −0.90 V) of the cycle, in which selective electrolytically dissolution of zinc from the alloy deposits occurred. Especially, under the plating condition of Ec=−1.06 V, Ea=−0.70 V, Tc=5.0 s and Ta=0.23 s, we obtained the stress-free zinc-cobalt alloy deposits.
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