Detonation Spraying of Functionally Graded Hydroxyapatite/Titanium Coatings on Ti–6Al–4V Alloy

In this work, two configurations of Ti/HAp functionally graded coatings were fabricated on Ti–6Al–4V alloy substrates using detonation spraying. The coatings differed in the number and sequence of Ti and hydroxyapatite (HAp) deposition cycles, resulting in distinct gradient architectures: Configuration 1 incorporated a sharper transition from the Ti-rich base to the HAp-rich surface, whereas Configuration 2 featured a smoother and more gradual compositional gradient. The microstructure and elemental distribution were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Both configurations exhibited well-defined gradient layering, with titanium concentrated near the coating–substrate interface and an increased Ca and P content toward the upper bioceramic region. Raman spectroscopy confirmed the preservation of hydroxyapatite as the main phase, showing a characteristic 961 cm−1 band. Adhesion strength measured according to ASTM C633-13 was 45.78 ± 4.4 MPa for Configuration 1 and 52.32 ± 6.7 MPa for Configuration 2, both significantly exceeding the minimum required 15 MPa. The findings demonstrate that detonation-sprayed Ti/HAp gradient coatings provide strong adhesion and stable bioceramic surfaces, making them promising for metal implant applications.