New findings on properties of plasma electrolytic oxidation coatings from study of an Al–Cu–Li alloy

Abstract

Coatings have been produced on an 2A97 Al–Cu–Li alloy by plasma electrolytic oxidation (PEO) using dilute and concentrated sodium silicate solutions with pulsed electrical regimes. Brownish red coatings were formed, especially in the dilute silicate solution. The colour is attributed to cuprite, formed by oxidation of copper in the alloy. The main phases of the coatings formed in the dilute and concentrated electrolytes are γ-Al2O3 and mullite, respectively. A bi-layer coating is formed in the dilute electrolyte, with large pores between the layers. The outer layer contains a small amount of δ-Al2O3 and α-Al2O3. Coatings formed using both electrolytes provided good corrosion resistance. The wear resistance was superior for the coating formed in the dilute electrolyte, since the outer layer is compact and of high strength and after the removal of the outer layer, a more adherent inner layer, with a hardness of 9–12GPa, provides excellent protection. Based on the coating microstructure, a modified model of coating growth was proposed.