Fabrication And Bioactivity Investigation Of Bioceramic Coating On The Surface Of Titanium Alloy By Laser Cladding

Based on high power CO₂ laser,the bioactive modified treatment of Ti-6Al-4V alloy was realized.The gradient bioactive ceramic coating was fabricated on the surface of Ti-6Al-4V alloy by laser cladding.The composite coating was investigated by optical microscopy(OM),scanning electron microscope(SEM), fluorescence microscope,X-ray diffraction(XRD),electron probe microanalysis (EPMA),thermal gravimetry and differential scanning calorimetry(TG-DSC), Inductively Coupled Plasma(ICP) and microhardness(HV) instrumentation.The main research results are as follows.Firstly,The processing parameters of laser cladding were optimized at the experimental condition.In the condition of laser output power 2.5kW and laser beam size 15mm×1mm,the hydroxyapatite could be probably formed at laser scanning speed of 120～160mm/min.Based on microstructure observation,microhardness and analytical calculations about heat transmission and mass transfer during laser cladding,the optimal processing parameters of laser cladding were as follows:laser output power 2.5kW,laser scanning speed 140mm/min,and laser beam size 15mm×1mm.Secondly,the microstructure of laser-cladded composite coating was investigated.The results indicated that the coating was metallurgically bonded to the titanium alloy substrate by the action of laser.Gradient component was varied from coating to substrate.The content of titanium was gradually decreased and the calcium and phosphorus were gradually increased.The laser-cladded composite coating contained such bioactive phases as hydroxyapatite(HA),β-tricalcium phosphate(β-TCP) and calcium titanate(CaTiO₃),etc.The ability of bioactive ceramic phases formation with clad powders of Ca/P=1.4 was better than other ratio. Besides granular and rod-like morphology of the classical HA morphology,the morphology of flocculent,honeycomb and micropore existed on the surface of composite coating.The appearances of multiform morphologies on the coating made the areas of coating extremely increased.And more contact area was provided for the growth and biofixation of new bone tissue.The pores(aperture:clad belt 4～9μm; clad edge 30～80μm) existed in local zone made the host bone grow into materials by the microcirculation of tissue fluid and combined together with chimerism.These may be in favor of the osseous tissue to grow along.Thirdly,the effect of 0.2～0.8wt.%rare earth oxide CeO₂ on laser-cladded bioactive ceramic coating and the mechanism was also investigated by SEM,XRD, TG-DSC and microhardness.The results indicated that the microstructure of coating of 0.4～0.6 wt.%rare earth oxide CeO₂ was homogeneous.Moreover,the addition of 0.4～0.6 wt.%CeO₂ could reduce the tearing tendency and accelerate the formation of HA andβ-TCP bioactive phases.According to the kinetics of reaction model,the activation energy of the system of 0.4～0.6wt.%CeO₂ addition was smaller than the system of without rare earth oxide on the temperature region of bioactive ceramic phase formation.Therefore,the molecule of reaction could easily form activated molecule of effective collision.The bioactive ceramic phases could form in an extensively range of temperature.The stabilization of high temperature was enhanced and the formation of HA bioactive phases was accelerated.The Simulated Body Fluid(SBF) test indicated that the composite coating at the experimental condition was of bioactivity by SEM,XRD,Energy Dispersive X-ray analysis(EDX) and Inductively Coupled Plasma(ICP).After soaking in SBF,the flocculent precipitates were increased on the surface of coating and the apatite phases were gradually nucleated and grew.The morphology of precipitates was mainly composed of flake-like and the globular,flocculent and acicular morphology were also appeared.Moreover,the appearance of a small quantity of crystal whisker (diameter about 400nm,slenderness ratio about 40～100) in local zone sufficiently benefited intensify toughness.The diffraction peak of apatite extremely intensified and the CaO distinctly decreased when the coating soaked in SBF.It indicated that the coating had the ability of depositing apatite quickly.However,the untreated substrate was not formed the apatite precipitates after a long soaking period of 14days.The surface was merely formed trace of soaking and some precipitation of salts.The cell cutured in vitro test indicated that cell proliferation on the surface of coating exhibited ascending tendency.The osteoblast proliferation quantity of coating was higher than the untreated substrate and the cultured plate.The cell growth on the surface of coating was more luxuriance and the cytotoxicity grade was zero.The cells were well attached and spread on the surface of the coating exhibiting spindle-shaped morphologies.The cell pasted the wall closely.And the morphology was complete and full and the pseudopod obviously spread to outward.The state of viable cell was observed by fluorescence microscope.The results indicated that the viable cell commendably spread on the bioceramic coating,keeping their healthy spindle-shaped morphologies.Furthermore,the SEM morphology was corresponding to the MTT colorimetry results of cell proliferation count.The coating was fine accepted by osteoblast and had well cell biocompatibility.