Research On Hydroxyapatite Coatings Deposited By High-velocity Oxy-fuel Spray

Hydroxyapatite (HA) coatings deposited on titanium alloy substrates using air plasma spray (ASP) is being widely used in medical bone prostheses and substitutions.However,due to the extremely high temperature of the plasma jet, the bond strength is low,and the degradation of HA involves the formation of tricalcium phosphate (TCP), tetracalcium phosphate (TTCP) will cause the coating dissolution and desquamation. High volecity oxy-fuel (HVOF) spray has extraordinary high particle velocity and comparatively low temperature and can be an appropriate method to produce HA coating.HA coatings was prepared by HVOF and the effects of three flow rates on the properties of phases composites, melting state of sprayed HA particles and surface morphology of coatings were investigated in this paper. The results showed that the increase of fuel flow rates could enhance the melting degree in the surface of HA particles, the calculation of melt fraction of HA particle revealed that the melt fraction increased a little from 12.79% to 18.00% as the fuel rate risen. The reason is that larger flow rate result in higher temperature of the flame as well as accelerated the velocity of HA particles and reduced the time of particle during in the flame. Qualitative phase analysis with X-ray diffraction (XRD) showed that the coating is mainly crystalline HA and the crystallinity of HA decreased from 91.4% to73.8% when the flow rates increased, it is caused by the different melt fraction between the HA particles. As the scanning electronic microscopy (SEM) observation shown, HVOF sprayed coatings exhibited three typical morphologies and microstructures after the HA particles impinged on the substrates. HA/ bioactive glass (BG) coatings were produced to improve the bioactivity of the coating. The effect of BG addition on properties of as-sprayed coatings in terms of phase composites, surface morphology and bioactivity were investigated. XRD showed that the coatings were mainly crystalline HA and exhibited no secondary phase. The addition of BG had no effect on phase's composites. As the SEM showed, the spherical BG particles studded in the HA coating. The content of BG in the surface of coating is less than the content of the input powder. Soaked in simulate body fluid (SBF) after 7 days showed only the coating with 20wt% BG precipitated bone-like apatite, it means BG increased the coating's bioactivity.Since the HA coating desquamated in the SBF, it was coated with polylactic acids polymer(PLA)in order to improve the stability of the coating in water. The effect of PLA concentration and coating times were investigated.The result showed that: the residual area of coating without PLA was less than 1% after ultrasoniced 30 seconds in water while it is more than 8% with PLA coated. Because PLA is hydrophobic, it protected HA coating from water filtered. The PLA concentration and coating times both significantly affected the coating's stability. The films invalidated in the form of punctate desquamation, it is probably because the films is porous.Low concentration resulted in flaky films, easy to be broken when the coating times was low, but if it was coated too many times the bond strength between PLA films and HA coating became weaker, and the film would apart from coating.The suitable coating time was three, when the PLA concentration is 3g/100ml. In the condition of 1g/100ml, the effect was better when coated five times.