Sol-gel spin coating is a promising process to obtain hydroxyapatite (HA) thin films. It is an alternative route to the hydroxyapatite deposition techniques usually employed to cover orthopaedic or dental titanium implant surfaces. The sol–gel (SG) parameters leading to a pure and crystalline HA coatings on Ti substrate were determined. They allow to reach a stoichiometric hydroxyapatite composition (ideal Ca/P atomic ratio 1.67) and a control of the growth of the crystalline phases. The samples, when observed by Scanning Electron Microscopy (SEM), exhibit grains of ca. 200 nm, well adapted for cell proliferation. The crystallisation of the HA films was thoroughly studied by X-Ray diffraction (XRD). The aim of this paper is to validate the sol–gel method as a processing method allowing the control of the mechanical state of the films and, in particular, of the residual stresses (RS) at metal–ceramic interfaces. These stresses were determined on titanium substrates. While the uncoated Ti substrates were in a compressive residual state, the coated ones were in a low tensile state. These results suggest that the sol–gel process is indeed a processing route to obtain HA coated Ti implants.
Keywords: Sol–gel; Residual stresses; Hydroxyapatite
Fig. 2. (a) X-ray diffraction pattern (2θ scan) of SG HA coated Ti plotted using a constant incidence angle ω = 3° and (b) selected-area electron diffraction (SAED) of polycrystalline HA (the indexation corresponds to HA PDF file #9-432)  Powder Diffraction File #9-432, International Centre of Diffraction Data (ICDD), Newton Square, PA, USA .
Fig. 3. SEM image of the HA spin-coated sol–gel film after heat treatment at temperature of 500 °C. Some parts of the coating consist of small grains (200 nm) (zone (a)), with other parts show bigger agglomerates of ca. 400 nm (zone (b)).
Characteristic data of titanium and hydroxyapatite: crystalline structure, space group, lattice parameters, elastic modulus (E), Poisson’s ratio (ν), radio crystallographic constants (S1 and S2) and coefficient of thermal expansion (α).