Preparation And Characteristics Of Biocoatings On Ti6Al4V By High Velocity Flame Spraying

High velocity flame spraying was adopted in this study to prepare biocoating on Ti6Al4V substrate. Bio-coating was designed on the basis of simulating the structure, the function and the chemical composition of the natural bone. Ti6Al4V with a surface biomemtic coating, i.e., coating materials of artificial bone, in which a bottom layer and a surface layer were integrated, was prepared by high velocity flame spraying. The coating has characteristics of gradient change of linear expansion coefficient, controlled porosity, bioactivity and crystal size. Both the bottom layer and the working layer have the function of artificial bone, insuring the service life and the stability of implant at initial stage.The coating material for the bottom layer was mainly pure titanium powder with addition of glaze glass (G). The coating material for the working layer was mainly HA powder with doped bioactive glass (BG). Effects of the powder size and the additive content on the characteristics of the coatings were investigated. The bonding strength of the coatings was tested by tensile testing. The residual macro-stress in the coatings was characterized by grazing incidence X-ray diffraction. Polarization curves were measured to study the corrosion behavior of the coatings in simulated body fluid (SBF) environment. By the use of TG-DSC, crystallization, docomposition and heat treatment temperature of the powder and coatings were determined. The crystallization activation energies of powders were also calculated. The microstructure and the interface characteristics of the coatings were examined by means of XRD, SEM, TEM and AFM. The growth of bone-like apatite on the surface of coatings and the biological behavior of coating materials and osteoblast were investigated in vitro. The osseointegration behavior on the implants inserted into dogs was studied in vivo.The bonding strength of biocoating was affected by the granularity of the spraying powder, the addition content of G and BG, and the crystallization process. The bonding strength of the bottom layer (80wt.%Ti-20wt.%G) and the working layer(80 wt.% HA-20wt.%BG), were 52MPa and 33MPa, respectively, meeting the standard requirements for dental implants. The residual compressive stress in the coatings has relationship to the sprayed materials and the particle size. The spraying impact stress plays an important role in the generation of the residual stress. The addition of G and BG powder adjusts the linear coefficient of thermal expansion and reduces the heat stress in the coating. The fracture modes are manly spalling, and breaking in and between particles.Powder feeding mode, granularity and additive content of the powder and crystallization treatment affect the structural and interfacial characteristics of the coatings. The crystallization dynamic calculation shows that G and BG materials have a low active energy of crystallization. The addition of 20 wt.% G into Ti increases the activation energy of pure G. The addition of 20 wt.% BG into HA reduces the crystallization activation energy further. After crystallization treatment, there exists a transitional zone with 3μm in width, in which microsize and nanosize particles were precipitated, at the interface between substrate and the bottom layer, achievingthe metallurgical bonding between the substrate and the coating through oxidation, diffusion reaction and glass soakage. Anatase TiO2, rutile TiO2, and Na2Ti6O13 were precipitated in cracks in Ti/G bottom layer, healing up the cracks. The bioactivity of the coating surface was increased by the precipitation of HA and Na2Ca(PO4) with size from nanometer to micron.The study on the growth of bone-like apatite in vitro shows that TiO2 in bottom glaze glass can provide nucleation site for apatite, and bone-like apatite with needle-like shape can be formed. BG in the working layer increases the dissolution trend of the coating, contributing to the growth of apatite. The coating, with 8Ti2G as bottom layer and 8H2B as working layer, has electrochemical corrosion resistance. Both 8Ti2G and 8H2B have no disturbance and inhibition for the biological behavior of osteoblasts, and can facilitate osteoblasts attachment and proliferation. The osseointegration ability of dog's implant was followed by 8H2B>HA>8Ti2G. BG in 8H2B shows the high activity and the fast dissolution, making Ca.and P ions migrating towards interface. Then, the micropores were formed, providing the channels and spaces for osteoblasts, which contributs to the formation of CHA on the implant materials surface. Therefore, a chemical bonding can be formed, increasing the stability of the implants at initial stage.