Evaluation and comparison of hydroxyapatite coatings deposited using both thermal and non-thermal techniques

This paper compares the properties of hydroxyapatite (HA) coatings, obtained using two different deposition technologies on Ti–6Al–4V substrates. The deposition techniques evaluated were: atmospheric plasma spray (APS, thermal treatment) and a novel micro-blasting technique known as CoBlast (non-thermal treatment). The HA coatings were examined with respect to their morphology, crystallinity and adhesion, while the phase concentration of the metallic substrates was also analysed. In vitro cell proliferation and cell morphology studies using MG-63 osteoblastic cells were carried out on the HA coated substrates obtained using the two deposition techniques, with untreated titanium grade 5 (Ti–6Al–4V) substrates utilised as a control. XRD analysis of the CoBlast deposited HA coatings demonstrated that it was comprised of the same crystalline HA as the precursor powder. For the APS HA coatings however, additional calcium phosphate phases were observed, and these were attributed to phase changes caused by the high plasma deposition temperatures. The APS treated samples also exhibited evidence of substrate modification, with substrate conversion to a β-rich surface at the HA/substrate interface observed in the XRD analysis. CoBlast HA coatings, with an average thickness of approx. 2.5 μm, were found to have higher tensile adhesion values (33.6 and 35.7 MPa), when compared with the 5 MPa obtained for the approx. 26.9 μm thick APS coatings using a modified tensile adhesion test. This lower adhesion tensile value is most likely due to the increased residual stress generated in the HA coating during thermal plasma processing. The cell response studies on the four surfaces tested indicate that the HA surfaces exhibited higher levels of cell proliferation than the untreated titanium after 5 days, with the CoBlast surfaces displaying statically significant increases in cell proliferation.