Influence of current density on nano-Al2O3/Ni+Co bionic gradient composite coatings by electrodeposition

Metal and nano-ceramic nanocomposite coatings were prepared on the gray cast iron surface by the electrodeposition method. The Ni-Co was used as the metal matrix, and nano-Al₂O₃ was chosen as the second-phase particulates. To avoid poor interface bonding and stress distribution, the gradient structure of biology materials was found as the model and therefore the gradient composite coating was prepared. The morphology of the composite coatings was flatter and the microstructure was denser than that of pure Ni-Co coatings. The composite coatings were prepared by different current densities, and the 2-D and 3-D morphologies of the surface coatings were observed. The result indicated that the 2-D structure became rougher and the 3-D surface density of apices became less when the current density was increased. The content of nanoparticulates reached a maximum value at the current density of 40 mA·cm⁻², at the same time the properties including microhardness and wear-resistance were analyzed. The microhardness reached a maximum value and the wear volume was also less at the current density of 40 mA·cm⁻². The reason was that nano-Al₂O₃ particles caused dispersive strengthening and grain refining.