Preparation, Microstructure And Properties Of Microwave Sintered Biomedical Porous NiTi Alloy

The porous structure of the porous NiTi alloys is beneficial to the new bonetissue ingrowth, the firm fixation of the materials with host bone tissue, body fluidstransfering and exchanging of nutrients and effectively reducing the elastic modulusof the alloy to make it more suitable match the human hard tissue. Meanwhile, theporous NiTi alloys also exhihit excellent shape memory effect and superelasticity.Therefore, the porous NiTi alloys have become one of the research hotspots for therepair and replacement surgical implant materials on bones, joints, teeth and otherhard tissues. In this paper, the porous NiTi alloys were prepared by microwavesintering, the effects of process parameters and pore-forming agent on themicrostructure and mechanical properites of the porous NiTi alloys were studied aswell as discussion of the mechanism of microwave sintering of porous NiTi alloys. Inaddition, the effects of the porosities and pore sizes on the phase transition behavior,superelasticity, permeability and corrosion resistance of porous NiTi alloy wereinvestigated systematically.The results showed that the porous NiTi alloys were mainly composed of NiTi,Ti2Ni and Ni3Ti phase. When using the carbonyl Ni powders and without addingNH4HCO3, they could obtain the single phase NiTi free from the impurity phases.With the increasing of NH4HCO3contents and its particle size, the porosities and poresizes of the porous NiTi alloys increased, and the number of the Ti2Ni phase andNi3Ti phase also increased. With the increase of sintering temperatures, the porositiesdecreased and the number of Ti2Ni phase and Ni3Ti phase also decreased. With theincrease of Ni and Ti particle sizes, all of the porosities, pore sizes and the number ofimpurity phases increased; Moreover, the pressing pressures and holding timespresented few effects on the pore structure and the phase composition of porous NiTialloy in this experimental conditions.With increasing the porosities, the hardness, bending strength, elastic modulusand compressive strength of the porous NiTi alloys gradually reduced. With theincrease of the average pore sizes (from97μm to294μm), the hardness, compression strength and elastic modulus of the porous NiTi alloys presented "S" type changetrendency. When the pore size was about238μm, all of the properties reached themaximum values. The bending strength reduced first, and then increased. Theporosity and average pore size had great impact on the corrosion resistance,superelasticity and permeability of the porous NiTi alloys, while had little influenceon the phase transformation behavior. With the increase of porosities and pore sizes,the superelasticity of the porous NiTi alloys gradually reduced. When the pre-strainwas8%, the superelasticity of the sample with the porosity of22.1%could reach5.18%, while the superelasticity of the50.1%sample decreased to4.07%. Thepermeability of the porous NiTi alloys enhanced with increasing the porosity. Whenthe porosity was higher than50.1%, the contact angle quickly reduced with increasingthe time. With increasing the pore size, the permeability increased first, and thendecreased. When the average pore size was149μm, the permeability rapidly increased.When the pore size was bigger than238μm, the droplet on the surface of the sampledid not move because of the lack capillary action, leading to the decrease of thepermeability. With the increasing of porosities, the corrosion resistance of the porousNiTi alloys reduced. The corrosion resistance of the sample with porosity of62.7%decreased by two orders of magnitude compared to that of the22.1%sample. Asincreasing the average pore sizes, the corrosion resistance also reduced. The corrosionresistance of the sample with the average pore size of294μm decreased by one orderof magnitude compared to that of the97μm sample.