Flow Field Analysis Of Hap Cold Spraying Process And Hap Coating Evaluation

With the continuous advancement of the medical standards, artificial boneprosthesis of single material no longer meets the needs of clinical transplantation dueto its failure and defects after clinical application, artificial bone prosthesis with abioactive coating is widely used. Medical titanium and titanium alloy coated withHydroxyapatite（Hap） coating holds dominant position in these materials because ofits outstanding bioactivity and mechanical property. At present, the methods offabricating Hap coating have some defects such as thin coating thickness, lowbonding strength, bioactivity decline, etc. This thesis fabricated Hap coating withself-developed cold spraying equipment, sprayed with high-pressure air in normaltemperature in fabricating process, took thermal treatment with substrate heatingapparatus and the heating temperature lower than300℃, the coating fabricated withsuch method has several advantages such as higher density, higher bonding strength, no thermal decomposition of coating material which ensures excellent bioactivity,etc.In this thesis, the knowledge of gas dynamics was utilized to simplify the flowin the laval nozzle as compressible gas one-dimensional steady flow, to analyze7different flow status in the laval nozzle in different pressure ratio conditions and toanalyze shock wave phenomenon and airflow parameters relations before and aftershock wave. Numerical analysis of gas-solid two phase flow in the spraying processwas conducted with the CFD software ANSYS Fluent12.1, distributions of airflowaxial velocity in the flow field under the conditions of different gas pressure,different spraying distance and different substrate temperature were obtained,discrete phase particles were added into the flow field, then the distributions ofparticle axial velocity in the flow field under the conditions of different gas pressure,different spraying distance, different substrate temperature and different powder sizewere obtained, the pre-shock particle axial velocity was defined as particledepositing velocity, then the variations of particle depositing velocity in differentspraying process parameters were gained.In this thesis, a set of orthogonal experiments were designed using gas pressure, spraying distance, substrate temperature and spraying time as experimental factors,using coating thickness as evaluation index of the experimental results. Theinteractions between gas pressure and spraying distance, between gas pressure andspraying time, between spraying distance and spraying time were taken intoconsideration, thickness data was handled with analysis of variance, the relationshipsof the experimental factors influencing coating thickness were gained: spraying time> gas pressure> spraying distance> substrate temperature; the best combination ofexperimental factors is: gas pressure is0.7MPa; spraying distance is9mm; sprayingtime is105s; substrate temperature is160℃.In this thesis, single factor experiments of gas pressure, spraying distance,spraying time and substrate temperature with Hap powder of three different powdersizes （2.5μm,5μm and10μm） were conducted, coating thickness measurement, phaseanalysis, microstructure characterization and morphology analysis on the Hapcoatings were conducted using Keyence Laser range finder （LRF）, scanning electronmicroscopy （SEM）, X-ray diffraction（XRD）. The coating thickness increases;erosion morphology occurs on the coating surface and becomes more and moreobvious because of the impact of high-speed particles and the micro cracks on the surface expend as the gas pressure increasing. As the substrate temperatureincreasing, the coating thickness remains almost the same; the micro cracks expendbut not that obvious; the thermal decomposition of Hap coating material doesn’toccur and the crystallinity of Hap coating improves. As the spraying distanceincreasing, the coating thickness increases at first, reaches the maximum at sprayingdistance of9mm, then decreases. As the spraying time increasing, the coatingthickness increases with it. In the single factor experiment of powder size, as thepowder size increasing, the coating thickness trends in decreasing, the distribution ofthe micro cracks on the coating surface expands but not that much.Hap-TiO₂composite coating was fabricated due to the different coefficients ofthermal expansion between Hap powder and Ti substrate which results in microcracks during spraying process, so TiO₂powder was utilized to neutralize thedifference of the coefficients. Hap-TiO₂composite coating sample was fabricatedaccording to the different relative contents of TiO₂powder from5%to20%, as therelative content of TiO₂powder increasing, the coating thickness decreases, thecoating surface shows porous and particle accumulating morphology. As the relativecontent of TiO₂powder increasing gradually, the coating thickness decreases and porosity of the coating increases because of the velocity of TiO₂powder is less thanits critical velocity which effects the deposition of TiO₂powder on the substrate, butthe micro cracks on the coating surface trends in reducing. This method of reducingthe micro crack defect on coating surface is advisable when increasing the gaspressure.