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.