Microstructure and mechanical properties of hydroxyapatite thin films grown by RF magnetron sputtering

Hydroxyapatite (HA) thin films for applications in bone prosthesis fabrication were obtained by the radio-frequency magnetron sputtering technique. The depositions were performed from pure HA targets on Si and Ti–5Al–2.5Fe alloy substrates. In some experiments a buffer layer of TiN was introduced by pulsed laser deposition before HA coating. The films were deposited in low-pressure argon or Ar–O2 mixtures at substrate temperatures ranging from 70 to 550 °C. We observed that the films grown at temperatures below 300 °C were prevalently amorphous and contain a small amount of crystalline material. The initial crystalline structure of the target is reconstructed after 1 h films annealing at 550 °C in ambient air. Both the films directly deposited at 550 °C and the ones obtained at room temperature and after that annealed at this temperature—mostly contain the HA phase and exhibit good mechanical characteristics. Weaker peaks of CaO secondary phase are visible in the X-ray diffraction patterns of the films directly grown at 550 °C. Traces of TiO2 were detected at the interface with the metallic substrate in case of structures grown without the TiN interlayer. All films were smooth, with an average surface roughness of 50 nm. The films grown on TiN interlayer are harder, have higher elasticity modulus values and an increased mechanical resistance at the indenter penetration.