Coating and active surface of Ni–Zn alloys studied by atomic force microscopy

The topology analysis of a real material surface undertaken here runs into a new approach of the active surface parameter, Sa, based on the scaling behaviour of the interface width, w. This approach is experimentally illustrated from the coating of a copper substrate with 2 μm-thick Ni–Zn alloy electodeposits investigated varying their growth rate, r, located below 0.50 μm/s. The imaging of their surface by atomic force microscopy (AFM) enables the following of their Hurst exponent, α, and their correlation length, Lc, versus r. It is shown that their interface growth may proceed from the Kardar–Parisi–Zhang (KPZ) dynamics, the restricted surface diffusion effect increasing with the increase of r. Both, α and Lc stabilise from a critical rate rc′=0.16 μm/s. The study evidences a peculiar surface microstructure of the Ni–Zn alloy favourable to catalytic activity at another critical point rc″=0.08 μm/s which corresponds to a Ni (80 at.%)-rich specimen leading to the maximal active surface obtained Sa=132 nm.