Influence of Graphene Oxide and Palladium Incorporation on Hydrothermally Grown Hydroxyapatite Coatings on β-Ti30Zr5Mo Alloy: Surface Chemistry, Corrosion and Biological Performance

Hydroxyapatite coatings containing graphene oxide and palladium were hydrothermally deposited on a β-type Ti30Zr5Mo alloy in order to establish how the organization of graphene oxide and the incorporation of palladium influence the structural, electrochemical, and biological performance of the coating system. Six compositions—hydroxyapatite, hydroxyapatite with 1 wt% palladium, hydroxyapatite with 2 wt% palladium, hydroxyapatite/graphene oxide, and the corresponding graphene oxide composites containing 1 and 2 wt% palladium—were compared using X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy, potentiodynamic polarization, cell-density analysis with A7R5 cells, and bioluminescence imaging of Staphylococcus aureus Xen36. The apatite framework was retained in every composition, and Raman spectroscopy confirmed that the carbon phase derived from graphene oxide survived hydrothermal processing. X-ray photoelectron spectroscopy of the graphene oxide coating containing 2 wt% palladium revealed oxygenated carbon environments and predominantly oxidized, oxygen-coordinated palladium species rather than metallic palladium. Electrochemical behavior depended strongly on composition. The hydroxyapatite coating containing 2 wt% palladium showed the lowest corrosion current density, 1.46 ± 0.02 µA cm−2 (0.0127 ± 0.0002 mm year−1), and a mean total resistance of 15.73 ± 0.13 kΩ cm2, close to that of the unmodified hydroxyapatite coating (1.95 ± 0.04 µA cm−2; 16.52 ± 0.40 kΩ cm2). The hydroxyapatite/graphene oxide coating showed the weakest barrier behavior, with a corrosion current density of 4.21 ± 0.07 µA cm−2 (0.0366 ± 0.0006 mm year−1) and a mean total resistance of only 8.50 ± 0.15 kΩ cm2. Microscopic and electrochemical evidence indicated that the expected barrier effect of graphene oxide was lost because the graphene oxide-containing layer was structurally heterogeneous. This layer contained folded, sheet-like domains and pathways that remained accessible to the electrolyte, and restacking or aggregation of the sheets induced by calcium ions is proposed as a plausible origin of this morphology. Adding 1 wt% palladium partially restored coating coverage and raised the mean total resistance to 14.97 ± 0.20 kΩ cm2 (2.15 ± 0.04 µA cm−2), whereas 2 wt% palladium lowered the interfacial charge-transfer resistance and reduced the mean total resistance to 11.73 ± 0.30 kΩ cm2 (2.91 ± 0.06 µA cm−2), even though the outer part of the coating remained intact. All coatings supported higher short-term A7R5 cell densities than the uncoated substrate, and the highest value, 330.2% of the control, was obtained for the graphene oxide coating containing 2 wt% palladium. The same graphene-oxide/palladium coatings, however, produced considerably stronger bioluminescent responses from Staphylococcus aureus than the remaining compositions. Overall, the hydroxyapatite coating containing 2 wt% palladium offered the most balanced combination of electrochemical and biological performance. The results show that the behavior of these coatings is governed by the organization of graphene oxide, by the interfacial chemistry associated with palladium, by the pathways available to the electrolyte, and by buried charge-transfer processes, rather than by nominal additive content alone. Their architecture is characterized by folded sheet-like domains and accessible electrolyte pathways, with Ca2+-mediated restacking/aggregation proposed as a plausible origin. Incorporation of 1 wt% Pd partially restored coating coverage and increased the mean total resistance to 14.97 ± 0.20 kΩ cm2, whereas increasing Pd to 2 wt% reduced the interfacial charge-transfer resistance and decreased the mean total resistance to 11.73 ± 0.30 kΩ cm2 despite retention of the outer-coating barrier. All coatings supported higher short-term A7R5 cell densities than the uncoated substrate, with HA/GO-2Pd reaching 330.2% of the control. However, the GO/Pd coatings produced substantially higher Xen36 bioluminescent responses than the remaining compositions. Overall, HA-2Pd provided the most balanced electrochemical and biological performance, while the results demonstrate that the behavior of HA/GO/Pd coatings is governed by GO organization, Pd-associated interfacial chemistry, electrolyte-access pathways, and buried interfacial charge-transfer processes rather than nominal additive content alone.