Study On Plasma-Sprayed Hydroxyapatite/Nano-Zirconia Graded Coatings

Hydroxyapatite (HA) coating, HA-ZrO₂ graded coatings, HA/ZrO₂ intermediate coatings were deposited onto Ti-6Al-4V substrate by atmospheric plasma spraying (APS) and HA+ZrO₂ composite coatings were produced by micro-plasma spraying (MPS) in this study. The surface morphology, microstructure, bond strength and biological behavior of the coatings were particularly studied by using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) before and after heat treatment.The results showed that under the involved conditions, melting and flatting of HA particles were weakened when increasing the spraying distance. At higher level power of input, amorphization of HA intensified and CaO, tricalcium phosphate (TCP), tetracalcium phosphate (TTCP) appeared under all the spraying distances. The characteristics of OH- bands and non-degenerate symmetric stretching of the PO43- groups in HA disappeared in all the coatings. All these changes suggested the distortion of the crystal structure and the dehydroxylation. MPS prepared HA+ZrO₂ composite coatings consisted of primitive tetragonal phase ZrO₂, a quantity ofα-TCP and a little CaO.The bond strength of the HA coating, HA/ZrO₂ intermediate coatings and HA-ZrO₂ graded coatings respectively were 28.31MPa,41.56MPa,33.00MPa after using the optimized spraying parameters. With regard to post heat-treatment, the heat treated pure HA coating showed the highest bond strength value (36.47MPa) when heating up to 650°C, while the HA/ZrO₂ intermediate coatings and HA-ZrO₂ graded coatings showed the highest bond strength value (50.86MPa and 39.72MPa) when heating up to 750°C. The higher bond strength of the HA-ZrO₂ graded coatings and HA/ZrO₂ intermediate coatings might due to the nano-zirconia bond coating as an intermediate layer between hydroxyapatite coating and titanium alloy substrate. XRD and FTIR analysis revealed a partial recovery of the crystalline HA structure due to the post heat treatment of 550℃for 3 hours. The peak of the CaO disappeared and the crystallinity of the coating was 61.6%. The graded coatings surface was covered with a newly formed"dune-like"apatite layer consisting of small granular precipitates after 2-week immersion in simulated body fluid (SBF) solution. The 550℃×3h heat-treated coatings with high solubility presented excellent bioactivity. Post-heat treatment of 750℃for 3 hours made the sprayed HA regain high crystallinity (86.2%) and OH- content equivalent to the feedstock. When immersing in SBF for 56 days, the 750℃×3h heat-treated coatings had little apatite precipitation layer in contrast to the 550℃×3h sample. The increased crystalline HA content and dense structure of the 750℃×3h heat-treated coatings led to low solubility and long-term implant stability.When micro-plasma sprayed HA+ZrO₂ composite coatings soaking in SBF for 7 days, some clustered ball-like particles were formed on the composite coatings. After immersion duration of maximum 28 days, the number of these ball-like particles increased, and the surface was totally covered by the newly formed layer consisting of small granular structures. Our results suggested that the HA+ZrO₂ composite coatings prepared by this method was also bioactive.