Study On Preparation, Structure And Properties Of Hydroxyapatite Based Bioceramic Composite Coatings

Fabricating bioactive ceramic coatings on the surface of metal or alloy can form hardtissue implants of fine performance, it combines the better strength and toughness of metallicsubstrates with the better bioactivity and biocompatibility of ceramic coatings. Comparedwith conventional methods, biomimetic synthesis fits various matrix materials with differentsurface mo.rphology and characteristic. And it provides an approach to fabricate organic/inorganic composite bioactive ceramic coatings. Especially some given drugs can beintegrated into the coatings, which best accords with the convalescence of patients during thecourse of clinical treatment. But fabricating apatite coatings by biomimetic synthesis needs along period and the adhesion strength between the substrate and coating is lower. In this paper,in order to increase the growth rate of calcium-phosphate coatings and improving the bondingstrength between matrix and coatings, hydroxyapatite based bioceramic composite coatingswere fabricated on 316L stainless steel and NiTi alloy substrates by optimizing thebiomimetic synthesis technology. And the structure, morphology, composition, adhesionstrength, corrosion resistance and bioactivity of coatings were investigated by SEM, EDS,XRD, FTIR, AAS, Constant Potential Apparatus and Scratch test etc.Firstly, influence of the composition, concentration and pH value of simulated bodyfluid(SBF) on the phase structure, crystallization and growth rate of calcium and phosphatecoatings were investigated. Calcium and phosphate ions' nucleation and growth on the surfaceof 316L stainless steel were induced by Ca(NO₃)-2·4H₂O and P₂O₅ in simulated bodyfluid(SBF). When the concentrations of Ca(NO)₂·4H₂O and P₂O₅ were increased, the Ca/Pratio of the coatings became nearer to 1.67 o f pure HAP. When 5×SBF concentrated simulatedbody fluid prepared with citric acid was used as buffering agent, hydr0xyapatite coatingsformed more quickly on NiTi alloy surface by accelerated biomimetic synthesis. Furthermore,organic/inorganic composite bioactive ceramic coating was prepared by combining 5×SBFwith chitosan solution. The results indicated that the corrosion resistance and bioactivity ofthe coatings became better after chitosan solution was added in the 5×SBF.During the course of biomimetic synthesis, the activation effect of pre-treatment on thesurface of substrate will decided the interphase nucleation ability of calcium and phosphateions on the surface of substrate. But the activation effect of conventional method is not very well and its reappearance probability is too low. In the paper, TiO₂-SiO₂-HAP composite filmswere prepared on NiTi alloy substrates by mixing 80TiO₂-20SiO₂ Sol and Ca-P Sol withdifferent ratio. The bioactivity of films was very well, but with the increasing of ratio of Ca-PSol in the films, film appeared a novel "island" structure which was composed of circle ofcrack, and the corrosion resistance and strength of film decreased. While coating the NiTialloy substrate layer by layer with TiO-2-SiO₂-HAP Sol of different ratio and multilayeredgradient films being formed, the crack on the films became micro-pore and the bioactivity ofthe films kept well, but the corrosion resistance and strength of the fihns were obviouslyimproved. Thus owing to the outstanding bioactivity of TiO₂-SiO₂-HAP gradient film,TiO₂-SiO₂-HAP/HAP bilayer film were prepared by substituting TiO₂-SiO₂-HAP gradientfilm for pre-treatment technology in the course of biomimetic synthesis. It was found that thecorrosion resistance and adhesion strength was further improved.