Hardness variation and annealing behavior of a Cr-Ni multilayer electroplated in a trivalent chromium-based bath

A Cr-Ni multilayer on a brass substrate was produced in a Cr(III)-Ni(II) plating bath with pulse--current electroplating. After electroplating, the multilayers were annealed at 400, 550, and 650 °C for 30 min, respectively. The hardness variation and microstructures of as-plated and annealed Cr-Ni multilayers were studied. Experimental results show that the as-plated multilayers were constructed of alternating layers of amorphous 150 nm Cr-rich and nanocrystalline 25 nm Ni-rich layers. After annealing at 400 °C, aggregation of nanosized grains in the amorphous Cr-rich layer was observed, but the crystalline Ni-rich layers did not exhibit any obvious changes. The Cr-Ni multilayer is fully crystallized and shows the highest hardness (1050 Hv) after annealing at 550 °C. This hardness increase is attributed to the presentation of fine crystalline grains and diamond membranes in the Cr-Ni multilayer. Diffusion of Zn from the brass substrate could be detected at temperatures above 550 °C. Moreover, abundant Zn could be found in Ni-rich phases owing to high affinity between Ni and Zn.