Deposition and Evaluation of Hydroxyapatite-Containing Plasma Electrolytic Oxidation Coatings on 3D-Printed and Bulk Ti6Al4V Alloys

Abstract

Dental and orthopaedic implants are implemented to restore mobility and quality of life on a global scale, but failures and revision surgeries are frequent. The problem is how to develop coating materials that will favour rapid integration of the bone and allow it not to degrade over a prolonged period of time. The standard material used in the implants is titanium alloy Ti6Al4V, but the bioinert surface slows down the bonding of the bone. Plasma electrolytic oxidation (PEO) has been proposed as a viable solution to enhance the bioactivity and osseointegration of Ti6Al4V by producing porous TiO2 layers, which can add calcium and phosphate species to serve as nucleation sites of hydroxyapatite (HAp). AM Ti6Al4V is currently employed extensively in orthopaedic and dental implants to allow patient-specific shapes and porous lattices. As-built surface regulates the process of osseointegration, hence it requires bioactive modification. Its porous and rough topography, along with its unique microstructure, may alter the behaviour of PEO discharge and coating chemistry compared to wrought material. The hypothesis considered during this project was that coating formation and early bioactivity may depend significantly on substrate type (bulk versus AM Ti6Al4V) and particle-assisted electrolytes (NaOH/Na2HPO4; ±2 g/L HAp). The design was a complete factorial 2 x 2 design with a constant current density (1480 A/m²) and treatment time (11 min, 54 s). SEM and EDS analyses were used to examine surface morphology and chemistry, while XRD was used to determine the phase composition. Bioactivity was assessed through immersion in simulated body fluid (SBF) over 1, 7, and 14 days. Findings indicated definite substrate effects. Total thickness by eddy current gauge after PEO was comparable on AM and bulk Ti (~26–27 µm). Cross-sectional SEM showed a more developed porous layer on AM surfaces, while bulk oxides were smoother and more uniform. HAp addition to the electrolyte promoted the cross-sectional porous layer thickness on bulk Ti by ~76% (9.27 to 16.36 µm) and increased the addition of Ca and P on both substrates, while the total gauge thickness remained ~26–27 µm. EDS showed that on AM Ti, Ca-P nucleation proceeded rapidly (up to 8 at% Ca and 12 at% P within 1 day), but on bulk Ti, the uptake was slower. The initial, Ca-deficient apatite was also indicated by Ca/P ratios (0.63-0.73, stoichiometric hydroxyapatite = 1.67). SEM showed different bioactivity mechanisms: faceted crystals grew on PEO-coated AM Ti on day 7, whereas bulk Ti developed a fine granular surface with subsequent plate-like outgrowths. XRD revealed weak anatase and rutile shoulders and a broad low-angle peak at 11 -12 2θ, typical of amorphous/nanocrystalline Ca0P; discrete HAp peaks at 31.7-34.1 2θ were not detected with Bragg Brentano geometry. These results indicate that AM substrates promote faster nucleation and coalescence of Ca-P phases, whereas bulk substrates promote slow and uniform deposition. HAp-containing PEO improved the incorporation of Ca and P (Ca–P species) into the porous oxide and surface chemistry consistent with the ~11–12° 2θ nano CaP peak and the absence of sharp HAp peaks at 14 days. It offers comparative information on the influence of substrate state on coating growth, bioactivity, and kinetics of Ca-P structural rearrangements. The broader implication is that AM implants can provide quicker short-term bioactivity when modified with PEO coatings; however, their uniformity and crystallinity must be optimized to make a clinical translation. Further studies are required to evaluate the effects of longer immersion times, mechanical adhesion, corrosion resistance and in vivo behavior. The present research work will help in designing an effective bioactive surface for future patient-specific implants.