| Due to its favorable biocompatibility and mechanical properties, hydroxyapatite (HA)-coated Ti–6A1–4V alloy has been accepted as one of the most promising implant materials for orthopaedic and dental applications. it was restricted to be applied widely for its weak bond strength between Ti–6A1–4V alloy and HA resulted from difference of physical and chemical properties between them. In this thesis bioactive gradient HA coatings were fabricated successfully by plasma spray .In this way the coating with high strength and satisfactory biocompatibility were expected. The microstructure ,chemical composition and adhesion strength of bioactive gradient coating were investigated by means of X-ray diffraction (XRD) ,scanning electron microscopy (SEM),EDS and tensile test . The coating surface morphology and its resultant product after immersion in SBF were investigated by FTIR,EDS and SEM.In my study field of residual stress of bioactive gradient coating is simulated by ANSYS. The result showed that residual stress on interface of gradient HA coating is much lower than that of single HA coating. With the increasing of thickness of coating ,residual stress is increased, while spraying temperature had little effect on residual stress.On the residual stress simulating and gradient coating design ,the gradient coating with high strength and satisfactory biocompatibility was fabricated. Well bonded interfaces were found in multilayer gradient coatings. Elements of P,O,Ca and Ti distributed alternately in medium layer. Net work of HA and Ti formed in medium layer could improve its bonding strength. Bonding strength of gradient coatings was 32.17MPa which is much higher than single HA coating which was 20.8 MPa. The texture of gradient coatings which was gradiently altered from the dense bottom layer to porous top layer was benefit to the long-term stability and bioactivity of coatings. In order to investigate their biological property gradient coatings were immersed in SBF solution for 2 weeks and 4 weeks. After 4 weeks immersion the surface of gradient coatings were covered by carbonate substitute apatite (Ca10-x(PO4)6-x(CO3)x(OH)2). The result revealed that the coatings show excellent biocompatiblity. Heat treatment under 800℃resulted in the enhancing of relative crystalline HA , increasing of cracks on the surface and formation of granular of crystal HA . Those changes had great effect on the behavior of coatings in SBF. The carbonate substitute apatite was granular on coatings after heat treatment while it was lamellar on coatings before heat treatment.
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