Spectroscopic study of micro-discharges during plasma electrolytic oxidation of Al-Zn-Si alloy

Abstract

Plasma electrolytic oxidation (PEO) process of Al-Zn-Si alloy in water solution containing 4 g L-1 sodium metasilicate at constant current density of 400 mA cm−2 was investigated. The species present in PEO micro-discharges and their ionization stages were identified using optical emission spectroscopy technique. The obtained PEO spectrum consists of atomic/ionic lines originating from the elements present both in the substrate (Al, Zn) and the electrolyte (Na, O, H). Apart from atomic and ionic lines, AlO band at 484.2 nm was also detected. Plasma electron number density diagnostics was performed from the Hβ line shape. The electron temperature of 4000±400 K was estimated by measuring the relative line intensities of zinc atomic lines at 481.05 and 636.23 nm. In addition, surface morphology, chemical and phase composition of oxide coatings were investigated by SEM-EDS and XRD. Oxide coating morphology is strongly dependent of PEO time. The elemental components of PEO coatings are Al, Zn, O and Si. The oxide coatings are partly crystallized and mainly composed of gamma phase of Al2O3.