Dual-structured oxide coatings with enhanced wear and corrosion resistance prepared by plasma electrolytic oxidation on Ti-Nb-Ta-Zr-Hf high-entropy alloy

Abstract

The electrochemical oxidation behavior of high-entropy alloys (HEAs) is supposed to be different from that of traditional Ti-based alloys due to the presence of multi-principal elements. In this work, equi-atomic Ti-Nb-Ta-Zr-Hf bio-HEA (PEO-free) was subjected to plasma electrolytic oxidation (PEO) at 150 (PEO-150 V), 200 (PEO-200 V) or 250 V (PEO-250 V). Due to substrate oxidation, the PEO-formed coatings were composed of multiple metallic oxides in their complete oxidation states. Moreover, elements such as Ca, Si, Na and O from the electrolyte were also introduced into the coatings. Different from the amorphous nature PEO-150 V coating, the microstructures of PEO-200 V and PEO-250 V coating were characterized by Hf0.5Zr0.5O2 nano-crystals homogeneously dispersed in amorphous coating matrices, with PEO-250 V group (∼20 μm in diameter) possessing larger nano-crystals than PEO-200 V group (∼6 μm in diameter). The dry-sliding test identified an order of PEO-200 V > PEO-250 V > PEO-150 V > PEO-free group for the wear resistance. The corrosion resistance evaluated by electrochemical impedance spectroscopy followed the trend of PEO-150 V > PEO-200 V > PEO-250 V > PEO-free group. The in vitro experiments confirmed the favorable cyto-compatibility of PEO-treated surfaces. Together, the results indicate that the coating with appropriate amorphous/crystalline dual-structure on Ti-Nb-Ta-Zr-Hf bio-HEA surface is promising for biomedical applications.