Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process

TLDR

The study examined how process parameters such as current density and treatment time affect plasma discharge behavior during pulsed‑dc plasma electrolytic oxidation of commercial pure aluminium, using optical emission spectroscopy in the 285–800 nm range. Oxide coatings were produced on Al 1100 alloy by pulsed‑dc PEO, and plasma electron concentrations and temperatures were determined from Stark shifts and line intensity ratios, with temperature profiles, surface morphology, and composition used to interpret discharge behavior. Plasma elements were identified, and distinct coating morphologies and compositions were attributed to three discharge types—substrate/coating interface, upper layer, and coating top layer—where high spike peaks in intensity and temperature indicated interface‑originated discharges and baseline fluctuations reflected electrolyte interface discharges.

Abstract

In this study, a plasma electrolytic oxidation (PEO) process was used to produce oxide coatings on commercially pure aluminium (1100 alloy) at a pulsed dc power mode. The effects of process parameters (i.e. current density and treatment time) on the plasma discharge behaviour during the PEO treatment were investigated using optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285–800 nm). The elements present in the plasma were identified. Stark shifts of spectral lines and line intensity ratios were utilized to determine the plasma electron concentrations and temperatures, respectively. The plasma electron temperature profile, coating surface morphology and coating composition were used to interpret the plasma discharging behaviour. The different coating morphologies and compositions at different coating surface regions are explained in terms of three types of discharge, which originate either at the substrate/coating interface, within the upper layer, or at the coating top layer. The high spike peaks on the plasma intensity and temperature profiles corresponded to discharges originated from the substrate/coating interface, while the base line and small fluctuations were due to discharges at the coating/electrolyte interface.