| Hydroxyapatite coating on titanium substrate is wildly used as an important material of the artificial bone implant materials in clinical trials because it combines titanium’s good mechanical properties with hydroxyapatite’ s good biological activity. But there is some problems that the interfacial bonding strength is low and HA falls off the base metal. This paper simulate stress field change, optimizes coatings design, produces bioceramic coating by laser cladding, and analysis morphology, structure and composition of gradient coatings, detect the electrochemical impedance spectroscopyand bonding strengthof coatings to improve bonding strength of coatings and matrix and supports its biocompatible.The stress field in HA/Ti bioceramic coating with different levels and thicknesses is simulated by finite element software ADINA. The hierarchical structure of HA/Ti coatings including thickness, composite gradiented are designed to get bioactive gradient coatings with continuously variable properties and decrease interfacial residual stresses. The results indicate that at the same thickness the interfacial stress changes slowly from single HA coating to double HA coatings, multilayer coatings, and the stress decreases with increasing of coatings, the coating stress and surface stress decreases 59 MPa and 84.2 MPa respectively, but it raises with increasing of thickness at the same gradient coatings. The composite of gradient coating selected by the experiment is HA, the mixture of 50 wt%TiO2 and 50 wt%HA, TiO2 from surface to matrix.Pure HA coatings, HA/TiO2 coatings and HA/HT/T gradient coatings are produced by substrate pretreatment,laser cladding and posttreatment processes in two different ways including the fore-put powder and the coaxial powder feeding respectively base the finite element simulation, and explores the suitable technology parameters. The micro- morphology, elements composites, interface structure, electrochemical impedant performance and interface bonding strength, etc of HA coatings with different structures are analyzed by SEM, XRD, EDS, XPS, metal ographic, electrochemical impedance spectroscopy, adhesive tensile test, etc.Coaxial powder feeding is more suitable for processing HA/Ti coatings compared with fore-put powder. The coatings made by coaxial powder feeding laser cladding are metallurgy bonding of atomic diffusion and the thickness of coatings could be controlled, especially from base to surface the HA/HT/T gradient coatings show a gradient structure feature of changing from loose to close. The electrochemical properties of every zone are stable, and the Ca/P values of HA/Ti is similar to HA, but the three coatings Ca/P value is higher and the uniformity is bad by the fore-put powder.The adhesive tensile test result shows that the interfacial bonding strength increases from pure HA coating to HA/HT/T gradient coating, and the gradient coating structure can improve inconsistency of thermal expansion coefficient, decrease residual stresses, and increase interfacial bonding strength, with agree to simulated results.The simulated body fluid tests and cytotoxicity tests are designed to study the biocompatibilities of coatings. The results indicate that there is biomineralization reation between SBF and coatings and the value of Ca/P is not decrease but increase, for the surface topography is arranged in order more, and form villi or branched- like protrusions and bone- like apatite after the reaction between coatings and SBF, so the biocompatibilities of SBF and coatings is good. The cytotoxicity tests results shows that the mouse fibroblastscell adhere and proliferateabnormally in the HA/HT/T coatings, and the HA/HT/T coatings has no adverse effects. The two experiments prove the good biocompatibilities of the HA/HT/T coatings. |
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