Electroplating and Characterization of Invar Fe–Ni Alloy Films from Plating Baths Containing Organic Acids

Abstract

Practical plating baths and plating conditions for preparing Fe–Ni alloy electroplated films with compositions in the Ni content range 36–100 mass% have been proposed. The addition of saccharin in sulfate-chloride bath is indispensable for preparation of Fe-30–45 mass% Ni alloy electroplated films, i.e., those in invar composition region. The addition of tartaric acid or malonic acid to Fe–Ni alloy plating baths (containing saccharin) prevents a reduction in the toughness of the plated film by masking Fe3+ generated by oxidation. It was possible to suppress the internal stress in invar Fe–Ni alloy electroplated films to low levels. Resultantly, the invar electroplated films obtained from these optimized plating baths show promise for electroforming. On the contrary, the addition of citric acid, which forms a more stable complex with Fe3+, causes very large internal stress in the resultant invar Fe–Ni alloy electroplated films, making them unsuitable as MEMS materials. It was confirmed that Fe–Ni alloy electroplated films with Ni contents of 45–100 mass% comprise a single-phase alloy having an fcc structure but the invar Fe-36–42 mass% Ni alloy electroplated film is a metastable bcc phase or a mixture of bcc and fcc phases. Therefore, to impart low thermal expansion characteristics to as-plated invar alloy films, it is necessary to induce phase transformation to an fcc phase, which is the equilibrium phase at high temperature, by the means of heat treatment.