Influence of Al₂O₃ addition in NaAlO₂ electrolyte on microstructure and high-temperature properties of plasma electrolytic oxidation ceramic coatings on Ti₂AlNb alloy

Plasma electrolytic oxidation (PEO)has become an effective surface modification method to form ceramic coatings on orthorhombic Ti₂AlNb alloy due to its high productivity, economic efficiency and ecological friendliness. In the present work, different contents of Al₂O₃ additive were introduced into a NaAlO₂ basic electrolyte with the aim of improving the high-temperature properties of PEO ceramic coatings on Ti₂AlNb alloy. The influence of Al₂O₃ addition in NaAlO₂ electrolyte on microstructure, adhesive strength, isothermal oxidation behavior and tribological properties of PEO coatings at elevated temperatures was investigated by means of X-ray diffraction, scanning electron microscopy, the pull-off method, isothermal oxidation tests as well as ball-on-disc friction and wear tests. The introduction of Al₂O₃ into the basic electrolyte promotes the formation of Al₂TiO₅ phase in the PEO coatings. Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings with Al₂O₃ addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L⁻¹ Al₂O₃ additive exhibits the least mass gain of only 1.33 mg cm⁻². The micro-hardness of the PEO coatings prepared with Al₂O₃ additive is improved due to the presence of Al₂TiO₅ phase, however, the adhesive strength of PEO coating is slightly weakened due to the formation of defects inside ceramic coating. The wear resistance of Ti₂AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coatings prepared with Al₂O₃ addition is comparable to those formed in the basic electrolyte at both room temperature and 600 °C.