| Titanium alloy has excellent comprehensive mechanical properties,but its biological compatibility and osteogenic inductivity are both poor.Titanium alloy can be eroded which leads to the release of toxic ions,such as Al and V,when it is implanted into human body to directly contact with body fluids for a long time.Hydroxyapatite(Ca10(PO4)6(OH)2,HA)has good biocompatibility and osteogenic inductivity,but it also has flaws,such as brittleness and low strength.When Ca10(PO4)6(OH)2 is coated on the surface of titanium alloy to form HA/Ti composite,which not only have the high strength and toughness as that of titanium alloy,but also have excellent biological compatibility as that of hydroxyapatite.However the physical property(such as elastic modulus,thermal expansion coefficient) difference between titanium alloy and hydroxyapatite leads to poor interfacial bonding strength between HA coating and titanium matrix which limits its application in medical operations.In this paper,nanometer powder of hydroxyapatite was first prepared by co-precipitation process,then porous oxide films on titanium alloy were obtained by anode micro-arc oxidation of titanium alloy in acidic and alkaline electrolyte salt solution respectively,HA /porous TiO2/Ti bio-composites were finally prepared by using electrophoretic deposition and electrochemical deposition combined with hydrothermal treatment.With scanning electron microscopy,X-ray diffraction,photoelectron spectroscopy,transmission electron microscopy and some other micro-analytical tools,the material compositions,phases s,grain size and morphologies of nanometer powder of hydroxyapatite,porous oxide film resulted from micro-arc oxidation and hydroxyapatite coatings were systematically examined.The shear bonding strength and tensile bonding strength of HA/Porous TiO2/Ti bio-composites were measured using a universal electronic tensile testing machine.The formation mechanisms of the materials during each step in the process were also discussed.For co-precipitation reaction of Ca(NO3)2 and(NH4)2HPO4 to prepare nanometer hydroxyapatite, the reaction products were CaHPO4,β-Ca3(PO4)2 and Ca10(PO4)6(OH)2 successively with increaseing the amount of ammonia.The amount of ammonia added was 7.5ml/250ml,that means a pH value of 11.3 before the reaction and of 9.4 after the reaction,gave the optimal result.The morphologies of HA powder are pin and sheet in shape.Within the temperature range of (25℃-75℃) and the reactant concentration range of 0.25 M-0.75 M,temperature and reactant concentration had little effect on the purity,grain size,particle size and morphology of formed Ca10 (PO4)6(OH)2 powder.After heat treated at 800℃,there was little change of HA powder phase composition,but grain size increased and morphology transformed to spherical shape.After heat treated at 900℃,HA powder was decomposed and mostly transformed intoβ-Ca3(PO4)2.When 10%H3PO4 was used as electrolyte and the initiation voltage of Micro-arc oxidation was 180V,the orifice size of the porous oxide film was 400-800nm and the phases of the film were rutile and anatase TiO2 co-existence.When 10%H3PO4 was used as electrolyte and the initiation voltage of micro-arc oxidation was 80V,the same crater-like surface morphology was obtained,and the orifice size was 100-200nm.When voltage was raised to 90V,the composition of oxide film were also rutile and anatase TiO2 co-existence.When voltage was raised to 120V,rutile was the main phase.When 0.2%HF was used as electrolyte and the initiation voltage of micro-arc oxidation was 80V,the surface of the film was uneven with shallow craters about 100～200nm deep.The thickness of titanium oxide film was gradually increasing,with increasing voltage.Using sodium dihydrogen phosphate as electrolyte,porous TiO2 membrane could be prepared on titanium alloy surface by micro-arc oxidation method,the size of apertures on the membrane ranged from 1 to 6μm.The structure of porous oxide film was amorphous.The optimal process parameters: electrolyte concentration is 0.3M,oxidation voltage of micro-arc oxidation voltage is 280V, oxidation time is 300s,initial temperature of the electrolyte is 10℃.Using butanol as dispersion agent,the distance between cathode and anode was 15mm,with electrophoretic deposition under normal temperature and atmosphere pressure,uniform and compact coating was achieved with the following optimal parameters:the concentration of HA suspension was 15.0g/L,deposition Voltage was 250V,deposition time was 180s.As the sintering temperature increased,the shear bonding strength between the coating and the substrate also increased.The optimal coating was achieved when heat treated at 800℃,beyond 800℃Ca10(PO4)6(OH)2 begined to decompose.Compared with HA/Ti composite coatings by electrophoretic deposition under the same process parameters,single HA coating shear bonding strength was 4.67MPa when sintered at 800℃,while HA/TiO2/Ti composite coatings was 7.42MPa.Electrochemical deposition could not get pure HA coating.CaHPO4·2 H2O and a small amount of HA appeared in low voltage,and Ca(OH)2 coating was obtained when voltage was more then 15V. Through 10h hydrothermal treatment in NaOH solution,CaHPO4·2 H2O coating prepared by electrochemical deposition in low voltage was wholly changed into Ca10(PO4)6(OH)2(HA) coating. The composition and phase structure of Ca(OH)2 prepared by high voltage electrochemical deposition had no change during the hydrothermal treatment.The best electrochemical deposition process parameters:deposition voltage was 8V,the concentration of electrolyte both of(NH4)2HPO4 and Ca(NO3)2 are 0.05M.When heat treated at 800℃,the shear bonding strength between the coating and titanium alloy substrate was 12.37MPa,and its tensile bonding strength was 11.28MPa.
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