Effect of ionic liquid as a surfactant in hydroxyapatite coatings for improvement corrosion resistance of Ti-6Al-4V substrates for implant applications

Research on hydroxyapatite (HAP) coatings for bone tissue applications has been investigated for decades due to their significant osteoconductive and bioactivity properties. HAP closely resembles the mineral component of human bone, making it ideal for biomedical applications such as implants. This study investigates the synthesis of hydroxyapatite nanoparticles (HAP-NPs) via the microemulsion method, which is essential for creating HAP coatings on the Ti-6Al-4V substrate. A variety of surfactants, including ionic liquid (IL; 2-hydroxyethyl ammonium octanoate ([HEA]OC)) and sodium lauryl sulfate (SLS), were employed to control the nucleation and crystal growth of HAP-NPs. The synthesized nanoparticles were dispersed in isopropanol and applied to the Ti-6Al-4V using the spin coating technique for creating HAP coatings. The samples were characterized using techniques such as FTIR, HNMR, XRD, FESEM, EDS, and AFM. Tafel and Electrochemical impedance spectroscopy (EIS) analyses were used to evaluate the corrosion resistance of the coatings in a simulated body fluid (SBF). The use of IL as a surfactant led to a significant reduction in nanoparticle size from 37.32 nm to 24.80 nm, which is critical for enhancing the coating's properties. Surface roughness decreased dramatically from 84.28 nm to 12.48 nm, indicating a smoother coating that can improve the adhesion strength of coatings. Electrochemical tests demonstrated improved corrosion resistance of coatings, with the charge transfer resistance (R_ct) for the HAP-0.4 IL coating reaching 7.84 MΩ × cm², compared to 0.11 MΩ × cm² for the bare Ti-6Al-4V. [HEA]OC as a surfactant improved the protective quality and uniformity of the HAP-IL coatings and reduced the surface roughness. These results indicate that modifying the surface of Ti-6Al-4V with HAP-IL coatings is a promising approach for enhancing its performance in biomedical applications.