Micro-Arc Oxidation of Grade 2 Titanium in SiO2-Modified Electrolytes Containing Rice Husk-Derived Silica: Coating Formation, Microstructure and Physicomechanical Properties

This study investigates the effect of amorphous silicon dioxide (SiO2) derived from rice husk on the structure and properties of micro-arc oxidation (MAO) coatings formed on commercially pure Grade 2 titanium. The coatings were produced in a phosphate electrolyte with SiO2 concentrations of 0, 5, and 10 g/L. The effects of SiO2 content on the microstructure, porosity, tribological characteristics, and corrosion resistance of the coatings were investigated. The results showed that the addition of SiO2 reduced the porosity from 7.07 ± 0.5% for the coating without additives to 6.27 ± 0.4% and 6.10 ± 0.43% at concentrations of 5 and 10 g/L, respectively. The coating formed at an SiO2 concentration of 10 g/L exhibited a stable coefficient of friction and a 27% reduction in wear rate compared with the coating produced without SiO2 addition. X-ray diffraction analysis revealed that anatase TiO2 was the predominant crystalline oxide phase. Increasing the SiO2 concentration was accompanied by a decrease in the average coating thickness from 34.39 ± 3.56 to 29.16 ± 4.06 μm. Electrochemical tests in a 3.5 wt.% NaCl solution showed that the coating containing 10 g/L SiO2 exhibited the lowest corrosion current density (1.9 × 10−9 A/cm2) and the highest polarization resistance (2.71 × 106 Ω·cm2). These findings demonstrate the potential of rice husk-derived amorphous silicon dioxide for modifying MAO coatings on titanium to improve their functional properties.