Research Of The Preparation Of Nano Hydroxyapatite Powder And Coating

Nano hydroxyapatite(HA)coating prepared by plasma spraying method has been successfully used on the surface of metal implants such as bone and teeth to improve osteoconductivity and biocompatibility.However,there are still two outstanding problems that need to be further solved in the application of the plasma sprayed nano HA coating,such as the low purity and crystallization degree of nano HA powder for plasma spraying and the poor bond strength of HA coating,which have become an obstacle to the wider applications of the plasma sprayed nano HA coating in medical clinic.Therefore,this paper focuses on the development of nano HA powder with high purity and crystallization degree and HA coating with high bond strength to provide a new way to solve these problems.Firstly,nano HA with high purity and crystallization degree is synthesized by controlling hydrothermal process conditions and adding surfactants,and the control mechanism of the growth and morphology of nano HA is studied.Then,nano HA is treated by spray drying,plasma hollowing and spheroidization to obtain the agglomerated nano HA(AHA)powder and hollow spherical nano HA(HSHA)powder for plasma spraying,and a new HSHA powder with a thin shell structure(TS-HSHA)is prepared using the control technology of shell thickness,and the hollowing mechanism of HSHA powder in plasma flame and the control factors of its shell thickness are investigated.Finally,TS-HSHA coating with high bond strength is prepared by plasma spraying method,and the mechanism of strong bond between TS-HSHA coating and the substrate is discussed.The results are as follows:The effects of hydrothermal temperature,hydrothermal time,ionic and nonionic surfactants on the crystal structure and morphology of nano HA prepared by hydrothermal method are studied.The results show that the increase of hydrothermal temperature will lead to a significant increase in the hydrothermal reaction rate constant k and a decrease in the solubility of HA crystal,which will accelerate the hydrothermal reaction process and the growth of HA crystal,and lead to an increase in the crystallization degree and particle size of nano HA.Besides,increasing hydrothermal time can make HA crystal react and grow more fully,which will improve the crystallization degree of nano HA.Anisotropic rod-like HA particle with large aspect ratio will be obtained by adding sodium alkyl sulfonate(SAS)anionic surfactant to the hydrothermal reaction solution,while isotropic spherical and short rod-like HA particles will be obtained by adding dimethyl distearylammonium chloride(DODMAC)cationic surfactant.The mechanism bases on:OH-Ca₆ cationic growth unit of HA crystal a,b-axis or Ca-P₆O₂₄ anionic growth unit of HA crystal c-axis can be attracted by different ion groups with negative or positive charge produced by the dissoidation of anionic or cationic surfactant,which in turn affects the growth of HA crystal in the direction of a,b-axis or c-axis,and eventually leads to the formation of HA particles with different morphology.Polyethylene glycol(PEG)nonionic surfactant does not attract the growth unit of HA crystal,and it mainly plays the role of increasing solubility and dispersion,which promotes the hydrothermal reaction and reduces the agglomeration degree between HA particles.The hydrothermal process parameters are optimized by orthogonal experiment,and PEG surfactant is added to prepare HA with crystallization degree of 93.8%.The observation results of transmission electron microscope(TEM)show that as-prepared HA is a nano-sized rod-like crystal particle with a length of 25?200 nm,a diameter of 10?30 nm,an average aspect ratio of 5:1,and excellent dispersion between particles.Meanwhile,it can be observed that the boundary of nano HA crystal is clear and complete,and the crystal surface pattern is straight,indicating that there are fewer crystal defects.X-ray diffraction(XRD)peaks of nano HA conform to HA standard date card(JCPDS No.09-0432),and no other absorption peaks of the functional groups other than HA and H₂O structures are found in infrared spectral(IR)pattern of nano HA,which show that nano HA has a high purity.The existence of lattice hydroxyls and the substitution of CO₃^2-in the crystal structure of nano HA are studied.The results show that there are lattice hydroxyls in the lattice of nano HA.At the same time,lattice hydroxyls cause the increase of cell volume of nano HA.CO₃^2-substituted HA is obtained by inputing CO₂ into the hydrothermal reaction solution.It is found that the substitution mode of CO₃^2-substituted HA is AB mixed-type with more B-type through IR analysis.Nano HA is made into a slurry for spray drying.The effects of the solid content,physical dispersion process and dispersants on the rheological behavior,dispersion and stability of nano HA slurry are studied.The results show that the viscosity of nano HA slurry increases with the increase of the solid content and the decrease of the shear rate,which indicates the rheological behavior of pseudoplastic fluid.Dispersed and stable nano HA slurry can be obtained by mechanical stir,ultrasonic and the addition of ammonium salt polymer(PMAA-NH₄)dispersant with a molecular weight of 3000?3600.AHA powder is prepared by spray drying of nano HA slurry after the process parameters of spray drying are optimized by orthogonal experiment.The heat treatment process of AHA powder is determined through thermogravimetric analysis(TG).HSHA powder is obtained by plasma hollowing and spheroidization of AHA powder.Based on the simulation of the temperature distribution of plasma flame and the melting degree of powder particles,the formation mechanism of HSHA powder,the effect factors of hollowing and the shell thickness of HSHA powder are studied.The results show that the plasma hollowing and spheroidization process of AHA powder mainly includes the stages of surface melting to form a liquid shell wrapped in internal gas,totally melting to form hollow droplet,cooling and solidifying to form HSHA powder.The hollowing efficacy of HSHA powder is influenced by the initial porosity of powder,the melting degree of powder and the air tightness of droplet.The temperature of HA droplet is the key factor determining the shell thickness of HSHA powder.A new TS-HSHA powder with a mean shell thickness of only 3.2?m is developed firstly by using the shielding device of temperature distribution during the plasma hollowing and spheroidization process.The mechanism of the preparation of TS-HSHA powder bases on:after hollow HA droplet flies away from plasma flame,the shielding device of temperature distribution will play an important role in preventing the heat losses and maintaining sufficient ambient temperature above the solidification temperature of HA,which prevents the droplet from solidifying prematurely and keeps it heated,and thus leads to continuous expansion of hollow HA droplet and reduction of its shell thickness.Ti-6Al-4V substrate is grit-blasted before plasma spraying.It is found that the bond strength of HA coating firstly increases and then decreases with the increase of blasting pressure and time.AHA,HSHA with a thick shell and TS-HSHA coatings are prepared by plasma spraying method on Ti-6Al-4V substrate,and the effects of different particle structure of HA powder on the bond strength of coating are studied.The results show that the mean bond strength of AHA,HSHA with a thick shell and TS-HSHA coating is 14.3,19.3 and 23.8MPa,respectively.The mean bond strength of TS-HSHA coating is 40%and 19%higher than that of AHA and HSHA with a thick shell coating,respectively.The mechanism of higher bond strength of TS-HSHA coating bases on:A phenomenon of counter liquid jetting can occur when hollow HA droplet impacts the substrate surface.This phenomenon causes TS-HSHA droplet after fuller spread and solidification on the substrate surface to produce stronger mechanical adhesion and form more uniform splat with fewer defects,which improves the bond strength of TS-HSHA coating.Furthermore,TS-HSHA powder is melted more sufficiently in plasma flame because of its thinner shell,which also contributes to enhancing the adhesion between the coating and the substrate.In addition,it is found that TS-HSHA coating is stable and its solubility is small in distilled water through dissolving experiment.