Plasma electrolytic oxidation of copper in an aluminate based electrolyte with the respective additives of Na3PO4, NaH2PO4 and NaH2PO2

Abstract

Plasma electrolytic oxidation of copper is first investigated in an aluminate-based electrolyte using trisodium phosphate (TP), sodium dihydrogen phosphate (DP) and sodium hypophosphite (HP), respectively, as additives. Black coatings can be successfully formed in the electrolytes of DP and HP. Detailed characterizations show that an initial thin layer of aluminum phosphate and alumina (AAP) was formed on cooper in DP electrolyte, which helps the fast establishment of plasma discharges, leading to successful coating formation. The AAP layer also formed in the electrolyte with TP as additive, but the AAP layer may be too thin to ensure stable plasma discharges. The coating formation mechanism is drastically different in HP: pitting corrosion along with loose Cu(OH)2 deposition occurs at initial stage, which may be a dual effect of local acidification and the destruction of the passive layer by the reducing hypophosphite ions. However, the deposited Cu(OH)2 and the subsequent formation of AAP layer may re-passivate the copper surface and provide an insulating layer for plasma discharging and coating formation. Optical emission spectroscopy (OES) and Mott-Schottky tests have also been used to investigate the features of plasma discharges and the semiconductor properties of the coatings, respectively.