Calcium phosphate coatings: Morphology, micro-structure and mechanical properties

Many biomedical implant coating systems consist of micro-deposited calcium phosphate droplets thermally sprayed onto a commercially pure Ti (CP Ti) substrate. In this study, the morphology of solidified droplet “splats” was examined using Scanning Electron Microscopy (SEM). The topography of splats sprayed onto substrates at room temperature (25 °C) and preheated to 100 °C and 300 °C was investigated. The splat shape was found to be strongly dependent on substrate preheating temperature. A homogeneous deposit density of amorphous calcium phosphate in splats deposited onto the cold substrate was confirmed by micro-Raman spectroscopy, whereas a very early stage of re-crystallization was detected using Transmission Electron Microscopy (TEM) for splats deposited onto 300 °C preheated substrates. TEM in conjunction with Focused Ion Beam (FIB) revealed the splatsʹ ultra micro-structure. Correlation of Atomic Force Microscopy (AFM) with these results enabled links between different types of micro-structures and true splat contacts with the substrates to be shown. Splats deposited onto the substrate at 300 °C showed generally well-adherent interfaces. The established presence of a thin layer of native oxide on this polished and preheated surface could serve to enhance the splat-substrate adhesion. Nano-indentation revealed that splats deposited onto the substrates at room temperature and 100 °C have similar hardness and elastic modulus values; however, preheating the substrate up to 300 °C improved these micro-mechanical properties. These combined findings promote further understanding of the extrinsic properties of the bulk calcium phosphate coating.