Toward rational design of ceramic coatings generated on valve metals by plasma electrolytic oxidation: The role of cathodic polarisation

Abstract

Plasma electrolytic oxidation (PEO) has attracted significant attention as a promising eco-friendly technology for the deposition of ceramic-like oxide coatings with superior wear- and corrosion resistance as well as biomedical, catalytic, magnetic, dielectric and optical properties. However, the industrial implementation of PEO has been hindered by a poor understanding of the mechanisms underlying plasma-assisted electrochemical coating formation. This study compares the characteristics of PEO processes on different valve metals in a dilute aqueous solution of NaOH and Na2SiO3 under pulsed unipolar and bipolar polarisation conditions. The focus is given to the influence of the nature of the valve metal on the transition to the soft sparking regime characterised by reduced energy consumption whilst yielding sintered coating materials enriched with crystalline phases. In-situ dynamic voltammetry analysis was employed to study the effects of the net current ratio and local negative charge injection using diagnostic pulses embedded in the polarisation signal. It has been demonstrated that soft sparking is a characteristic feature of valve metals such as Mg, Al, Zr and Ta that form insulating surface oxides with negligible electronic conductivity, whereas Ti and Nb form semiconducting oxides, which do not exhibit a characteristic voltage drop, although some features of spark softening were detected. This generalisation paves a way for implementation of a model known as ‘digital twin’ to the rational design of the PEO processes for deposition of the functional ceramics.