Formation of Ca, P, and Zn-doped ZrO2/TiO2 coating layer via plasma electrolytic oxidation and magnetic sputtering: Improving surface characteristics and biocompatibility of Ti-6Al-4V alloy

The study demonstrated the Ca, P, and Zn-doped ZrO2/TiO2 coating layer via plasma electrolytic oxidation (PEO) and radiofrequency magnetron sputtering (RF-MS) for improving surface characteristics and biocompatibility of Ti-6Al-4V alloy. The process involved sandblasting (SB) to create micro-scale irregularities on the alloy surface, followed by RF-MS and PEO. The coating's morphology, elemental composition, phase composition, and chemical analysis were examined by FE-SEM with energy-dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman analysis. The dual surface treatment resulted in the formation of composite coatings with TiO2, ZnO, ZrO2, and HA presence and displayed anisotropic multi-scale porous morphology while the surface texturing created by the SB process facilitated mechanical interlocking during the oxide layer growth. In the SB-Zr + PEO-Zn sample, the phase analysis indicated the strain-induced stabilization of t-ZrO2 in the PEO coating layer due to the incorporation of Ca2+ ions, and the formation of zinc-substituted hydroxyapatite was observed that showed enhanced roughness and wettability. Also, reduced hardness and elastic modulus (comparable to that of cortical bone) were observed due to transformation toughening under stress from t-ZrO2 to m-ZrO2. The presence of TiO2, ZrO2, ZnO, and HA within the coating, all act as protective barriers in effectively reducing corrosion rates. The biological analysis demonstrated better biocompatibility, including promoting cell proliferation, cell adhesion, osteoblast differentiation, ALP activity, and mRNA expression of several osteogenic markers of the cells cultured on the SB-Zr + PEO-Zn sample. These results demonstrated the higher potential of the PEO coating system with higher biocompatibility and osseointegration ability for dental implants.