Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO2–Na2CrO4 electrolyte

Abstract

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti2AlNb alloy in various NaAlO2 electrolytes containing 2 g L−1, 4 g L−1, and 6 g −1 Na2CrO4 additive, respectively. The influence of Na2CrO4 additive in NaAlO2 electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na2CrO4 additive in the NaAlO2 electrolyte not only promotes the formation of γ-Al2O3 phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr3O and (Al0.948Cr0.052)2O3 phases. The PEO ceramic coatings formed in NaAlO2 electrolytes with 2 g L−1 and 4 g L−1 Na2CrO4 additive show better oxidation resistance than those PEO coatings formed in a NaAlO2 basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO2 electrolyte with 4 g L−1 Na2CrO4 additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO2 basic electrolyte without any Na2CrO4 additive.