Hydroxyapatite-coated Ti–6Al–4V : Part 2: the effects of post-deposition heat treatment at low temperatures

The present investigation explores the effects of a 90-h post-deposition annealing treatment at 400°C in air on the crystallographic and chemical properties of a plasma-sprayed hydroxyapatite (HA) coating, the thickness and composition of the interfacial oxide layer, and the fatigue behaviour of the underlying Ti–6Al–4V substrate. X-ray diffraction analysis revealed that significant recovery of the crystalline HA structure occurred as a result of the treatment, however, as compared with results obtained through treatment at higher temperatures, recovery obtained through use of the present treatment was incomplete. X-ray photoelectron spectroscopy analysis showed no changes in the constituents of the oxide layer, with the oxide species TiO2, Al2O3, V2O5, V2O3, and VO2 present on both the as-sprayed and the heat-treated substrates. A change in film thickness was observed, however, as evidenced by a change in colour from opaque bronze to dark purple. The fatigue resistance of the substrate was found to be significantly reduced by the heat treatment, with the lives of heat-treated coupons with coatings of all thicknesses closely resembling those of as-sprayed coupons with thick HA coatings and uncoated stress-relieved coupons presented in Part 1 of this study. Stress relief was identified as the most likely cause of these reductions.