Microstructure And Properties Of Hydroxyapatite And Hydroxyapatite/Titanium Composite Coatings Prepared By Warm And Cold Spraying Processes

Titanium and its alloys were commonly used as artificial bone graft materials due to their well mechanical and biological properties as well as corrosion resistance.The bioactivity of the titanium alloys can be improved the bioactivity of the titanium alloys through fabricating the bioceramic coatings on its surface without reducing its mechanical properties.Depositing hydroxyapatite(HA)coating on titanium alloy surface by plasma spraying was commonly used method for fabricating implants.Nevertheless,it is liable to occurs the unexpected decomposition of HA which deteriorate their biological properties.In this thesis,the warm spray(WS)and cold spray(CS)processes were used to decrease the temperature of flame to prohibit the decomposition of HA during coating formation.In addition,the titanium particles were mixed with HA powder to enhance the mechanical properties of deposited HA coatings and to produce the HA/Ti coatings with well mechanical and biological properties.The individual HA particle was deposited by using WS process in which the propane is used as fuel gas,the oxygen was used as burning assistant gas,the nitrogen was used as powder feed and cooling gas.The individual agglomerated spherical HA particle was deposited on polished and acetone cleaned stainless substrate surface using WS with the spraying parameters: the pressure and the flow rate of C3H8 were 0.40 MPa and 15 slpm,respectively.The pressure and flow rate of oxygen were 0.5 MPa and 60 slmp;The pressure and flow rate of nitrogen were 0.6 MPa and 40 slpm;The spray distance was 30 mm.In addition,the individual agglomerated spherical HA particle was also deposited on polished and acetone cleaned 316 L stainless steel,Ti6Al4 V and HA substrate surface,by using CS process with the spraying parameters: pressure of working gas(nitrogen)and powder feed gas(nitrogen)was 2.0 MPa and 2.2 MPa,respectively,spray distance was 20 mm,and the gas temperature in gun chamber was 300,450 and 600? respectively.The surface and cross sectional morphologies of deposited HA splats were examined by scanning electron microscope(SEM).It is demonstrated that HA splats deposited by WS and CS presented a pie shape with a hump in the center and some radial stripes on the periphery as well as some ring grooves and tiny granules on the surface,The cross section of splats exhibited an ellipse shape with an unsymmetrical up and bottom surface as well as inhomogeneous pore distribution.The HA coating and HA/Ti composite coatings were sprayed on 316 L stainless steel substrate by WS using the HA,HA90%Ti10%(H9T1)?HA70%Ti30%(H7T3)?HA50%Ti50%(H5T5)and HA30%Ti70%(H3T7)powders under the optimized spraying parameters.In addition,the HA coating was also prepared by CS under the spray parameters:pressure of working gas(nitrogen)and powder feed gas was 2.0 MPa and 2.4 MPa,respectively,spray distance was 20 mm,gas temperature in gun chamber was 300?±30?,the transverse speed of gun was 15 mm/s.The microstructure,phase constitution,chemical composition and structure of the original powders and coatings were researched by using SEM,energy dispersive spectrdmeter(EDS),X-ray diffraction(XRD),fourier transform Infrared Spectroscopy(FTIR),respectively.The crystal structure of original HA powder and CS HA coating was also researched by using transmitted electron microscopy(TEM).The mechanical properties and electrochemical corrosion properties were researched through estimation the micohardness,elastic modulus,tensile strength,abrasive wear weight loss and polarization curve of the coatings.The biological property of the coatings was also researched by in vitro test using immerse in simulated body fluid(SBF).It is shown that the HA coating fabricated by WS and CS exhibited comparatively dense microstructure with no obvious lamellae structures and with no obvious unbound areas or cracks in coating substrate interface.The phase constitutions,and chemical structures of the HA coatings were consistent with that of original powder and the composition of the HA coating was comparable to original powder.The crystal structure of CS HA coating was the same as that of original powder though the crystal size reduced somewhat.There occurred the oxyhydroxyapatite(OHA)in the WS spryed HA/Ti composite coating.The microhardness,elastic modulus,tensile strength,abrasive wear weight loss against on Al2O3 of WS deposited HA?H9T1?H7T3?H5T5 and H3T7 coatings were 0.32?0.43?0.59?0.97 and 1.41 GPa,1.37?2.69?5.05?12.14 and 23.28 GPa,17.29?22.33?25.45?28.27 and 34.76 MPa,2.9?1.7?1.4?1.2 and 1mg respectively.The microhardness,tensile strength,abrasive wear weight loss against on Al2O3 of CS deposited HA were 59.0±5.6 kg/mm2?11.1±1.2 MPa and 2.24 mg respectively.The wear weight loss of CS HA and WS HA coating against on Polycarbonate(PC)and on Polyurethane(PU)was not detectable.The CS HA coating exhibited comparatively lower corrosion potentialdifference and corrosion current indicating the well electrochemical corrosion resistance.There appeared some tiny apatite particles on the surface of HA and HA/Ti composite coatings after immersed in SBF indicating their well apatite formation ability.