The structure of an RF-magnetron sputter-deposited silicate-containing hydroxyapatite-based coating investigated by high-resolution techniques

A biocompatible nanostructured silicate-containing hydroxyapatite-based (Si-HA) thin coating was deposited by radio-frequency (RF) magnetron sputtering on silicon and titanium substrates. The morphology of the Si-HA coating was pore-free, dense and followed the topography of the underlying substrates. Energy-dispersive X-ray spectroscopy (EDX) gave molar Ca/P and Ca/(P + Si) ratios of 1.78 and 1.45, respectively. According to XRD-analysis, the coating was nanocrystalline with a crystallite size in the range of 10–50 nm. The ultrastructure of the coating was analyzed by high-resolution transmission electron spectroscopy (HRTEM) combined with fast Fourier transform (FFT) analysis. The average crystallite size calculated by the Rietveld method was in good agreement with the HRTEM results. Moreover, HRTEM-observations indicated the presence of atomic layer misorientations originating from imperfections between the nanocrystals in the coating. The average coating nanohardness (11.6 ± 1.7 GPa) was significantly higher than that of the uncoated Ti substrate (4.0 ± 0.3 GPa), whereas no significant difference between the Youngʹs modulus of the coating (125 ± 20 GPa) and the substrate (115 ± 10 GPa) was found. Immersion of the coated substrates in simulated body fluid (SBF) led to the deposition of an apatite layer.