Effects of cobalt content on mechanical and corrosion properties of electroless Ni-Co-P/TiN nanocomposite coatings

Ternary Ni-Co-P/TiN (20 nm) nanocomposite coatings with Co content in the range of 0–23 wt% were successfully co-deposited on Al alloys by electroless plating. The morphology, composition, microstructure, micro-hardness, wear behavior, and corrosion properties of the coatings were tested using SEM, EDS, XRD, Vickers hardness, ball-on-disc wear tester, and electrochemical workstation, respectively. The effect of Co content on the tribological and corrosion properties of Ni-Co-P/TiN coatings was studied. It is found that all the coatings have a compact and rough granular structure with sub-micro globules non-uniformly distributed on the surface. The grain size and the content of Ni and P decrease, but the micro-hardness, adhesion force, friction coefficient, and corrosion resistance correspondingly enhance with the increase of Co content in Ni-Co-P/TiN coatings. The improved corrosion resistance of Ni-Co-P/TiN coating is due to the passivation behavior and small grain caused by the introduction of Co. Based on the excellent properties on the wear and corrosion resistance, ternary Ni-Co23-P/TiN coating with high Co content, may be applied as an optimum protective coating for Al alloys.