Study Of Plasma Sprayed Hydroxyapatite Coatings

Bioceramics have been widely used as biomedical materials due to their good biocompatibility,biological affinity,biological activity,high stability and sufficient mechanical properties.A micron-scale hydroxyapatite coating on the surface of bio-inert metal prosthesis can satisfy the mechanical properties as well as the biological activity of the implant.However,pure hydroxyapatite coatings prepared by plasma spraying technology often have poor mechanical properties and serious amorphous phases,which greatly affects the long service life of the coatings.In this paper,pure hydroxyapatite coating and hydroxyapatite/yttria stabilized zirconia composite coating were prepared on Ti-6Al-4V metal surface by plasma spraying,and their material properties were verified by various experiments.The results show that impurity phases exist in both coatings after spraying,such as tricalcium phosphate,tetracalcium phosphate and calcium oxide.X-ray diffraction patterns show that the amorphization of the coatings is very high,which is formed by the thermal decomposition of hydroxyapatite.Hydrothermal treatment greatly accelerated the material exchange process between coating and solution.After hydrothermal treatment at 150? for 4 hours,the diffraction peaks of the decomposed phases in the two coatings disappeared completely.With the prolongation of heating time,hydroxyapatite microcrystals appeared on the surface of the two coatings,but the morphology of the two coatings was different from that of the simulated body fluid treated hydroxyapatite microcrystals.After spraying,the crystallinity of pure hydroxyapatite coating is 6.9%,while the value of hydroxyapatite/yttria stabilized zirconia composite coating is 4.27%.The crystallinity of the two coatings is proportional to the heating time.The maximum crystallinity of the two coatings is 93.76% and 77.35% respectively after 8 hours.The results show that hydrothermal treatment is an effective method for coating crystallization.After 8 hours of hydrothermal treatment,self-healing phenomena appeared on both coatings,which is attributed to the continuous growth of hydroxyapatite microcrystals,and it plays a role in filling microcracks.It was observed from the cross-section structure that the cross-section micro-cracks of the two coatings gradually diffused with the prolongation of the heating time.The crack level of hydroxyapatite/yttria stabilized zirconia composite coating is lower than that of pure hydroxyapatite coating,which indicates that the addition of yttria stabilized zirconia can enhance the microstructure of the coating.The Young's modulus of the coating is positively correlated with the microhardness and heating time at the early stage of hydrothermal treatment.After hydrothermal treatment for 4 hours,the Young's modulus of the pure hydroxyapatite coating reached the maximum of 8.754 GPa.After 6 hours,the maximum value of hydroxyapatite/yttria stabilized zirconia composite coating was 14.621 GPa.Both of the Young's modulus and microhardness begin to decline after they reached the maximum values.The comprehensive effect of residual stress,crack propagation and other factors are responsible for this phenomenon.Overall,the values of hydroxyapatite/yttria stabilized zirconia composite coatings are larger than those of pure hydroxyapatite coating.The results show that Yttria stabilized zirconia can improve the mechanical properties of the coating.Owing to the higher melting point of composite coating,the surface morphology of hydroxyapatite/yttria stabilized zirconia composite coating is smoother than that of pure hydroxyapatite coating.After immersion in simulated body fluid for the same time intervals,X-ray diffraction patterns showed that impure and amorphous phases gradually dissolved and recrystallized to hydroxyapatite micron-granules with the prolongation of immersion time.The macroscopic surface features of the coatings after sprayed was white flocculent precipitation.The surface micro-morphology of the two coatings was also analyzed by scanning electron microscopy(SEM).After 5 days of immersion,clear "hardening" occurred on the surface of the two coatings,which was the result of accelerated dissolution,the release of residual stresses from the inner coatings and the expansion of surface cracks.After 20 days immersion,the surface of the two coatings was almost completely covered by spherical crystallization of hydroxyapatite.At the same soaking time point,the crystallization rate of hydroxyapatite on the surface of pure hydroxyapatite is faster than that of hydroxyapatite/yttria stabilized zirconia composite coating,which is determined by the mass fraction of hydroxyapatite.The coating with higher mass fraction completes the process of phase dissolution ? euuilibrium ? precipitation with a shorter soaking time.Overall,Hydrothermal treatment is an effective method of crystallization and the addition of yttria stabilized zirconia can improve the mechanical properties of the coating without affecting its biological activity.