Hydroxyapatite – Al2O3 reinforced poly– (lactic acid) hybrid coatings on magnesium: Characterization, mechanical and in-vitro bioactivity properties

Abstract

Hydroxyapatite – Al2O3 reinforced poly– (lactic acid) hybrid coating was synthesized on magnesium in this research, which was carried out to obtain improved interface strength and higher corrosion properties with a lower dissolution rate of magnesium. Compared to uncoated magnesium substrates, HA and HA/Al2O3 coatings were synthesized by dissolving PLA in chloroform and adding reinforcement materials. Coatings were produced on magnesium using the spin-coating technique of the synthesized solutions. Characterization (FTIR, SEM/EDX, XRD, and contact angle analyses), mechanical (tape tests and micro-Vickers hardness), and in-vitro bioactivity (immersion tests) features of the hybrid coatings were investigated. XRD results showed that no phase transformation occurred in the coatings. In addition, it was determined that the PLA coating was effective in covering the surfaces of Mg substrates. With the reinforcement of HA, CaPs were formed on the surface, but the adhesion strength was reduced (2B). With the addition of HA/Al2O3, the surface properties are preserved, and the adhesion strength increased (4B). In electrochemical corrosion behavior, a decrease was observed in corrosion current density (Icorr) values with the coatings compared to the uncoated Mg alloy (249.848, 1.659, 0.246, and 0.136 µA·cm−2 for uncoated, PLA coated, PLA-HA coated, and PLA-HA/Al2O3 coated, respectively), and the corrosion resistance increased. Especially, the Icorr value obtained with the Al2O3 reinforcement was measured quite low compared to previous studies using PLA-HA composite coatings, and it was determined that the corrosion resistance was increased. As a consequence of SBF immersion tests, while more CaPs were found on the surfaces of PLA–HA coatings, deep crack formation was found on the surface. Although fewer CaPs were observed with HA/Al2O3 additive, no cracks that would increase adhesion were formed and it was predicted that the degradation rate of Mg could be controlled. In the hardness measurements, it was found that the hardness value increased from 137.7 ± 5.7 to 268.7 ± 3.6 HV with the addition of HA/Al2O3 so it can be applied in wear-sensitive areas. In the pH changes during the SBF immersion tests, the average of the PLA–HA/Al2O3 coatings was 7.4 during the 15-day period. It has been determined that this value will not cause any adverse reactions in tissue interaction during the use of Mg in the body. Moreover, according to H2 evolution measurements, all coated samples have lower H2 gas evolution than uncoated Mg. As a result, it was thought that Mg will strengthen its place in potential implant applications thanks to hybrid coating compositions such as PLA–HA/Al2O3.