Effect of thermal treatment on structure, phase and mechanical properties of hydroxyapatite thin films grown by RF magnetron sputtering

Highlights: • Coatings with Ca/P ratio close to hydroxyapatite stoichiometry value were obtained. • H and E were improved in 5% and 9% respectively for HA coatings heat treated at 400 °C • Adhesion strength was enhanced in 38% in HA coatings with annealing at 400 °C. -- Abstract: Hydroxyapatite (HA) thin films with a Ca/P ratio of 1.69 and approximately 500 nm in thickness were deposited by radio frequency magnetron sputtering on Ti-6Al-4 V substrates at 100 °C and a working pressure of 0.5 Pa. The obtained coatings were thermally treated at 400 and 700 °C in argon atmosphere and characterized in terms of elemental and phase composition, morphology, structure and mechanical properties by energy dispersive x-ray spectroscopy, scanning electron microscopy, atomic force microscopy, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction and nanoindentation. It was found that the thermal treatments increase the crystallinity although induce residual tensile stresses formation in the HA coatings, which are greater at high temperatures. Nevertheless, enhances in the mechanical properties: hardness (H), elastic modulus (E) and adhesion strength for the heat treated at 400 °C samples compared to the as-deposited coatings were obtained. Values of 7.45 ± 0.6 GPa (H), 115 ± 10 GPa (E) and a critical load of 175 ± 4 mN were measured for those thermally treated samples at 400 °C.