Effect of Laminated Structure on Mechanical Properties of Composition-modulated Co-Ni Laminated Plating

A composition-modulated Co-Ni laminated plating has been developed to extend the lifetime of molds used for continuous steel casting. We have investigated the relationship between the laminated structure and the mechanical properties of the plating films. The tensile strength of as-plated film increased with increasing thickness of the constituent layers, while the elongation did not depend on the thickness of the constituent layer and remained almost constant in the range from 3 to 5%. Heat treated at 400°C improved the tensile strength and the elongation. The improvement in the elongation was remarkable, and the elongation reached 13% in the film composed of layers with a thickness of 0.8 µm. The layer with low Ni content had an hcp structure, and that with high Ni content comprised two phases of the hcp and fcc structures in the as-plated state. By the heat treatment, the high Ni-content layer turned into the single fcc phase, while the low Ni-content layer kept the hcp phase, and accordingly, the film structure changed into the one where the lamination of the hcp and fcc layers was distinct. The fact that the fcc layers, which was easily deformed, were formed continuously in the lateral direction, was seemed to contribute to the significant improvement in the elongation after heat treatment.