Construction and growth of black PEO coatings on aluminum alloys for enhanced wear and impact resistance

Abstract

Black coatings with superior optical properties have attracted considerable attention for the surface treatment of aluminum alloys. This study focuses on the development of black coatings with superior mechanical properties on 6061Al alloy using plasma electrolyte oxidation (PEO) technique. Two different electrolytes are designed, and the effects of different electrolyte components on the microstructure, composition, growth mode, and mechanical properties of black coatings are investigated. The results show that the inward-outward growth behavior and mechanical properties of coatings are strongly influenced by the deposition originating from the electrolyte. The deposition of P– and V–containing coatings in the mixed electrolyte composed of phosphate and metavanadate proceeds easily, forming outer deposition layers with loose structures. This makes the growth of coatings largely dependent on the mode of outward growth and decreases the adhesion strength. Conversely, in the mixed electrolyte of phosphate and dichromate, the deposition of species from the electrolyte is difficult, resulting in the initial growth of coatings that relies on the oxidation of the substrate. Such a growth mode is characterized by inward growth, which endows coatings with excellent wear and impact resistance. Furthermore, the coloring mechanism is discussed, and it is concluded that the doping of vanadium (V) or chromium (Cr), especially Cr, into alumina significantly contributes to the black appearance of the coatings.