Structure of hydroxyapatite coatings produced by electrodeposition on stainless steel and titanium nickelide substrates

Abstract

Problem. Hydroxyapatite (HA, chemical formula Ca10(PO4)6(OH)2) is widely used as a biocompatible and bioactive ceramic due to its structural and chemical similarity to the main mineral component of human hard tissues. A disadvantage of hydroxyapatite is its low mechanical properties (strength, crack resistance, and impact resistance), which undoubtedly limits its use as a structural material but not as a bioactive coating on metal implant surfaces. Electrodeposition is one of the most promising methods for forming bioactive calcium-phosphate coatings on metal implants due to the ability to control the composition, structure, and thickness of the coating at low process temperatures. Aim. The aim of this work is to identify patterns in the formation of the composition and structure of hydroxyapatite coatings produced by electrodeposition on 12X18H10T stainless steel and titanium nickelide substrates, and to optimize the deposition modes. Methods. The influence of electrolyte parameters (Ca2+ and PO4 3 ion concentration, pH, H2O2 and NaOH additives), temperature (23–85 °C), potential and deposition time on the stoichiometry, crystallinity, texture and crystallite size of HA coatings was studied using X-ray fluorescence (XRF), X-ray diffraction (XRD), and IR-spectroscopy. Results. It was shown that the brushite phase (CaHPO4·2H2O) is formed at room temperature regardless of the electrolyte composition. Increasing the temperature to 65 °C or higher, when combined with optimization of pH (4.1–6.0) and potential (−1.7…−3.4 V), allows the synthesis of coatings with a composition close to stoichiometric hydroxyapatite. The regimes ensuring the production of single-phase, highly-crystalline HA coatings with a texture coefficient of up to 2.6, which corresponds to the structure most conducive to osseointegration, were experimentally determined. Conclusions. The best coating characteristics (high crystallinity CI=9.1, crystallite size along the c-axis ~70 nm, c/a ratio=0.730) were achieved for the sample synthesized at T=75 °C, pH=5.0, and U=−1.7 V