Microstructure And Properties Of Graded Porous Pure Titanium Fabricated By Spark Plasma Sintering

As the first-generation medical titanium material used in clinical practice,pure titanium has ideal biocompatibility,corrosion resistance and mechanical properties.But the elastic modulus of the material is still larger than the human bones,which leads to the effect of"stress shielding",thus the implant fails.Many studies have showed that a porous structure can effectively reduce the elastic modulus,and the porous structure can provide a large space for the growth of bone tissue,which contributes to the fixation of the implant and improves the biocompatibility of the material.However,as the porosity increases,mechanical properties,such as compressive strength of a porous material,could be also significantly reduced.In order to solve the above problem,from the point of view of human bone bionics,spark plasma sintering(SPS)method was used in to obtain porous titanium with a continuous gradient of porosity in order to maintain the lower elastic modulus of pure titanium in this tesis.The perpose of the study is to improve both the mechanical properties and the biocompatibility.And the results achieved will provide a theoretical basis for the development and application of medical gradient porous titanium materials.In this study,pure titanium powder was selected as raw material,and NH4HCO3 was used as space holder,which was systematically studied using a optical microscope(OM),a scanning electron microscope(SEM),X-ray diffraction(XRD),and the density and compression properties were also tested.The effects of SPS parameters(sintering temperature,sintering pressure,etc.)and gradient design for the effects of porosity and mechanical properties.In addition,the characteristics of pores and the pore formation mechanism of gradient porous titanium were investigated,and the characteristics of fracture of gradient porous titanium were also revealed.The mechanism of the improvement in the mechanical properties of gradient porous titanium was disscussed.The results show that the porosity in the gradient of porous titanium obtained by SPS shows a continuous gradient without apparent separation of the interface.In the gradient(10%+30%)porous titanium,the pore size and porosity in the side with 10%additions of NH4HCO3 is higher than thatwith single-layer 10%NH4HCO3,and the size and porosity in the side with 30%additions of NH4HCO3 is lower than that with single-layer 30%NH4HCO3.NH4HCO3 volatilizes during the sintering process,and the gas diffusion flattens the porosity change at the interlayer interface between eacher layer with different addition of NH4HCO3.On the other hand,the vibration of Ti particles under the action of SPS pulse current also promotes the continuous gradient change of porosity.The characteristics of the pores show that the large pores and the small pores coexist.The shape of the pores is irregular,and there is a connection between the large pores.The mechanism of the formation of large pores is mainly dominated by the formed after the volatilization of the space holder.The pore volumeis determined by the amount of the initially added NH4HCO3.Small pores continuously grow with the growth of sintered neck,and the pores are connected by pores.Shrinkage is formed,and the pore volume is related to the degree of compaction of the sintered body.Within the range of SPS parameters in this study,the strength of gradient porous titanium gradually increases with the increasing of SPS sintering temperature and sintering pressure.The average porosity gradually decreases.When sintering temperature is 750? and sintering pressure is 4MPa,gradient porous titanium with certain porosity and good matching of strong plasticity can be obtained.Moreover,the elastic modulus of porous titanium designed with a gradient of 10%+30%is 9.43GPa,and its compressive strength reaches 1167MPa.Compared with the mechanical properties of 30%single-layer porous titanium(8.52GPa,857MPa),the compressive strength is increased by 36.2%.The compressive strength of gradient porous titanium increases with the increasing of the mumbers of gradient layers,because the gradient change of porous titanium with three-layer gradient design is more flat,which improves the uneven plastic deformation and improves the overall mechanical properties of gradient porous titanium.Similarly,the compressive strength of porous titanium designed with asymmetric gradient is higher than that of porous titanium designed with symmetrical gradient.Under stress loading,cracks in gradient porous titanium initiates at weak pore walls in high porosity regions and extend to low porosity regions.However,the deformation resistance of low porosity region is high.Therefore,the continuous gradient porous pure titanium can improve the mechanical properties.