| Hydroxyapatite(HA) coatings and Ti-HA graded coatings were produced on commercially pure titanium by plasma spraying. The surface morphology, microstructure, mechanical properties and biological behavior of the coatings were studied by using scanning electron microscopy(SEM), transmission electron microscopy(TEM), electron probe microanalysis(EPMA), X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FTIR), Raman spectroscpoy and valence electron theoretical calculations. The results show that under the involved conditions, melting and flatting of HA particles are weakened with the increase of spraying distance. At higher power level, amorphization of HA intensifies and CaO appears under all distances with TCP (Tricalcium phosphate) coming forth in its β-structure. As compared to non-graded coating, more decomposed phases such as TCP and TCPM(Tetracalcium phosphate monoxide) are produced in graded coating and their XRD peaks show perfectly, which is caused by diffusion of Ti into HA. Post –heat –treatment of (600~700℃)×2h can make the sprayed HA regain high crystallinity and OH-content equivlant to start powder. Meanwhile, HA in graded coating retrieved its crystallizing integrality without reaction products of HA and Ti. The maximum compressive shear strength of the coating-substrate sprayed at 30 kW is 16.6MPa, which increases to 19.9MPa after heat treatment. The shear bond strength of graded coatings varies from 24.9 to 30.6 MPa, 50~80% higher than that of non-graded coating. Increased atomic diffusion and stress relaxation from interlayers play a dominant role in the increase of interface bond strength of both heat treated and graded coatings. HA coatings cracked trimly within coating or at the coating-substrate interface, but for the heat treated and graded coatings, dot-like exfoliation occurs within surface coating and interlayers. This also reflects that heat treatment and graded components distribution can enhance the bond strength. Deformed Ti and HA particles alternately distribute in graded coating, more interlayers can make greater stress relaxation effect. Continuous metallurgical bond of interface forms between the substrate Ti and adjacent Ti in coating. Simultaneously, elements in coating exhibit the alternant distribution,in addition some overlapped Ca and Ti emerged. Respective network of Ti and HA formed in coating could retard the cracks from expanding. Hollow morphology of HA spheres was found in HA+Ti paticles plasma sprayed into water, which is resulted from gaseous P2O5 and H2O generated from molten HA, and is the main reason why HA particles are larger than Ti particles sprayed into water. At the same time, the hollow construction would affect multi-side properties of the coating. Lower cooling rate in water improves crystallization and the effect of Ti catalyzing the decomposition of HA. The reaction product Ti1.7P of Ti and HA was observed and its coalescence could take place during flying. The coexisting zone of crystalline and amorphous HA, and that of HA and TCP, as well as TCPM, CaO and CaTiO3 were observed by using TEM. Rapid cooling and complex lattice array play an important role in the formation of amorphous HA. TiP2O7 and CaTiO3 were found in graded coating. These two compounds are unbeneficial to lattice integrity, but could improve the bonding of Ti with HA. HA nucleation within TCPM and its growth in tape-like shape were found in heat treated coating, and similarity of crystal lattice is the intrinsic necessity for transformation from TCPM to HA. Dot-like, needle-like, efflorescent and aggregated precipitates with a size of 100~300nm appeared on the surface of heat treated HA coating. Ca2+ concentration firstly rises and then falls indicating dissolution and precipitation simultaneously happened on HA coatings immersed in distilled water. The surface of the as-sprayed coatings were covered by evenly distributed HA particulates with a size of about 100nm. Their size stabilizes with the increase of immersion time, but the size of the aggregates gradually becomes larger. The higher stability of the heat treated coating was verified by Ca2+ concentration and surface morphology changes during immersion. Perfect osseointegration formed on the heat treated HA coatings after 28 and 84 days implantation,new bone grew into pores on coating surface and spreaded along the gap. Isolated bone deposition on HA coating surface in medullary cavity should be able to state the conduction and induction effect of HA for bone formation. The valence electron theoretical calculations showed that there exist lattice planes with continuous electron density among the several selected biocompatible materials, which is consistent with the requirement of Cheng`s theory, and by which the validity of EET(the Empirical Electron Theory in Solids and Molecules) and Cheng`s theory applied in biomaterials was verified. The proposed osssointegration mechanism is the existence or the build-up of the interface with continuous electron density between implant materials and host bone.
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