Effect of Y2O3 rare earth oxide on the microstructure and growth kinetics of TiO2-based ceramic coatings fabricated by micro arc oxidation

This study investigates the influence of Y 2 O 3 addition on the microstructure, growth kinetics, and mechanical properties of TiO 2 -based ceramic coatings formed on Ti–6Al–4V alloy through the Micro Arc Oxidation (MAO) process. The experiments were carried out using Ti–6Al–4V plates coated in silicate-based electrolytes, with and without 1 g/L Y 2 O 3 , under a constant current density of 5 A/dm 2 and a voltage of 360 ± 20 V for different treatment times (1, 3, 7, and 10 min). The coatings were evaluated by weight gain measurements, SEM/EDS analyses, X-ray diffraction (XRD), and constant load scratch testing supported by profilometry. Results revealed that Y 2 O 3 significantly accelerated coating growth during the initial stages, leading to higher weight gains and denser, less porous morphologies compared to Y 2 O 3 -free coatings. XRD analysis indicated amorphous TiO 2 and anatase phases at short durations, while rutile formation became dominant with longer treatments. Kinetic modeling demonstrated that both coatings initially followed a linear reaction-controlled mechanism (n ≈ 1), with calculated growth constants of 3.44 × 10⁻ 4 mg/mm 2 ·s for Ti-MAO and 7.45 × 10⁻ 4 mg/mm 2 s for Ti–Y 2 O 3 –MAO. However, after approximately 420 s, the Y 2 O 3 coatings exhibited a marked reduction in growth rate due to electrical insulation effects of the thickened ceramic layer, resulting in multi-stage kinetics. Constant load scratch testing showed that Y 2 O 3 incorporation reduced groove depth and width by nearly 45%, improving adhesion strength and mechanical durability. These findings confirm that rare-earth oxide addition is an effective strategy for enhancing the performance of MAO coatings on titanium alloys.