Influence of annealing temperature on RF magnetron sputtered calcium phosphate coatings

The effect of different annealing temperatures on the characteristics of thin calcium phosphate coatings fabricated by radiofrequency magnetron sputtering was studied. Annealing of the as-sputtered films was necessary to change the amorphous coating to a crystalline coating. The films were annealed for 2 and 4 h at 400, 600, 800,1000 and 1200 °C under dry argon or argon and water vapour flow. After annealing, the structure and the chemical composition of these films were characterized with incident light microscopy, Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), and Fourier transform infrared absorption spectrometry (FTIR). Incident light microscopy showed cracks in the coatings annealed at a higher temperature than 400 °C. RBS revealed that the as-sputtered coatings had a high Ca/P ratio which decreased with increasing annealing temperature. After annealing at a temperature of 600 °C or more the XRD showed crystalline hydroxyapatite (HA) coatings. However, the second phase, present in the coatings, changed from tetra-calcium phosphate to calcium oxide to β-tricalcium phosphate with increasing annealing temperature. FTIR measurements showed the existence of OH- and PO- bonds in all coatings, although the PO- bonds varied for different annealed coatings, from the PO- bonds due to HA to PO- bonds due to other calcium phosphates. From the results of this study we suggest that 600 °C is probably the best annealing temperature to obtain a better characterization and understanding of the coating.