TOFSIMS and XPS characterisation of strontium in amorphous calcium phosphate sputter deposited coatings

Strontium (Sr) substituted calcium phosphate (CaP) coatings have stimulated a lot of interest as Sr is known to provide osteoconductive properties. Most of the work to date with respect to Sr substituted coatings has focused on the creation of crystalline Sr-substituted hydroxyapatite (HA), as opposed to amorphous coatings. The work reported here utilises radio frequency magnetron sputtering to deposit both amorphous CaP and Sr-containing CaP coatings and their characterisation using X-Ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The CaP coating had a high Ca/P atomic ratio at 1.81 ± 0.02. The amorphous Sr containing CaP coating was shown to contain Sr with a Ca/P and (Ca + Sr)/P atomic ratios of 1.18 ± 0.05 and 1.49 ± 0.05, respectively. Peak-fitting of the overlapping Sr3d and P2p region also showed the presence of previously unreported doublets for each element. The ToF-SIMS results also highlighted that Sr was homogeneously distributed across the surface of the Sr containing coating via detailed chemical mapping experiments. This study has shown that sputtering can be used to deposit Sr-containing CaP coatings and that the use of surface analytical techniqes is important for understanding their uppermost surface properties.