Review of the role of graphene and its derivatives in enhancing the performance of plasma electrolytic oxidation coatings on titanium and its alloys

The plasma electrolytic oxidation (PEO) surface modification process is able to improve the surface characteristics of titanium and titanium alloys by forming TiO2 coatings. Implementing the PEO process enhances corrosion and wear protection, tunes optical properties, and more importantly, modifies the biological activities of titanium and also its alloys. The surface features can be further enhanced by adding proper additives in the form of particles, sheets, powders, or compounds into electrolytes, forming TiO2 composite coatings with modified properties compared to the basic TiO2 coatings. With a wide range of desirable physical, mechanical, chemical, thermal, electrical, optical, and biological characteristics, graphene (G) and G-derivatives (i.e., graphene oxide (GO) and reduced graphene oxide (rGO)) are among the top additive choices used in PEO composite coatings production. The porous nature of PEO coatings provides a suitable place for the accumulation of G and GO that can change the features of the surface significantly. This article reviews the results of studies on the properties of PEO coatings modified by G, GO, and rGO formed on titanium and also its alloys. The addition of G and G-derivatives influences the growth process, microstructure, surface morphology, and phase composition of the formed coatings. Particularly, the hardness of the coatings is increased, corrosion and wear resistance are enhanced, optical properties and optoelectronic behavior are tuned, and last but not least bioactivity and biocompatibility of the coatings are modified after the addition of G and G-derivatives.