| Hydroxyapatite (HA), due to its remarkable biocompatibility and its high chemical similarity to biological apatite present in human hard tissues, is a widely used biomaterial in clinical applications to repair human hard tissue bone and dentin. However, the successful application of HA ceramics in load-bearing areas is limited because of its low mechanical properties. Up to date, the reinforcement of HA with ductile titanium (Ti) particles via hot pressing has recently attracted much attention. But its high costs and limits on shaping make this method less attractive from both the technical and economical point of view. It is necessary to develop a cost-effective route of fabricating HA/Ti composites with improved strength and fracture toughness. The purpose of this project is, therefore, to synthesize a novel iron-containing HA/titanium composite in an attempt to develop HA-based composites with improved strength and toughness via low-temperature pressureless sintering in vacuum, including the synthesis of HA powders, preparation of the composites, mechanical properties and biological behavior of the composites. The present study might open up a new approach for the development of bio-composites with improved mechanical properties for the load-bearing applications.HA powder was prepared by two kinds of reagent groups which included Ca(NO3)2-P2O5ethanol solution and Ca(OH)2-H3PO4. In the process using Ca(NO3)2-P2O5as starting materials, the influence of citric acid (CA) on the formation process of HA nanopowder, phase purity and particle size were investigated. The results showed that the addition of CA improved the gel’s stability and changed the transformation process of HA from precursors due to the chelating effect of CA. The content of CaO phase in HA nanopowder obtained after addition of CA was reduced substantially in comparison with the powder prepared without using CA. Moreover, CA was also found to be able to suppress the agglomeration of powder particles, leading to the finer HA powders with high dispersibility. In the process using Ca(OH)2-H3PO4as starting materials, the influence of pH value, ball milling and calcination temperature on HA preparation were investigated. The results showed that the obtained HA powders were pure and highly crystallized when pH value was controlled at10.5.It was also found that the particle size was smaller after ball milling. The temperature of750℃was considered as the optimal calcination temperature. To prepare HA powders using Ca(OH)2-H3PO4as starting materials was finally chosen since this method is more cost-effective and easier to control. Besides, the sintering behavior of HA powders prepared by precipitation method was investigated. A relative density of99%could be obtained when the HA powders were pressureless sintered in vacuum at1000℃.A new iron-containing HA/Ti composite was synthesized using obtained HA powders and Ti-Fe particles as starting materials via pressureless sintering in vacuum at a relatively low temperature. The pure Ti and Fe powders, with a ratio of Ti-33w%Fe based on Ti-Fe phase diagram, were mixed by ball-milling to prepare Ti-Fe particles, which were then used to prepare HA/Ti-Fe composites. The influence of the addition of iron on the phase composition and microstructure of the composites were investigated via XRD, SEM and TEM. The results revealed that the addition of iron lessened the decomposition of HA and interaction between HA and Ti as well, which made the desirable Ti phase remain in the matrix. A unique core/shell microstructure of irregularly shaped Ti-Fe reinforcing particles was obtained, which consisted of outer shell of titanium and inner core of iron. This led to a favorable microstructure and good interfacial bonding with the HA matrix.In correlation with the preparation and microstructure of the HA based composites, the mechanical properties of the composites, and especially the underlying strengthening and toughening mechanisms were studied. It was found that the relative density, hardness, and Young’s modulus decreased with increasing amount of Ti-Fe particles, while the flexural strength, fracture toughness, and fatigue resistance increased significantly by the addition of iron. The flexural strength of the composite with5%of Ti-Fe particles which was sintered at1000℃reached the maximum value of93MPa. The fracture toughness of the composite with15%of Ti-Fe particles reached1.3MPa·m1/2. This represented an increase of the flexural strength and fracture toughness by42%and128%in comparison with pure HA, respectively. Such remarkable improvement in the mechanical properties of the composites was mainly due to the presence of more ductile Ti-Fe particles in the form of core/shell structure, coupled with their good bonding with the HA matrix. In the composites the major toughening mechanisms were crack bridging, crack branching and crack deflection. The influence of sintering temperature on the microstructure and mechanical properties of the HA based composites were investigated, too. It was found that the variation of sintering temperature had little effect on the formation of the novel core/shell microstructure, but influenced the mechanical properties. The relative density, flexural strength, fracture toughness and cyclic fatigue resistance were enhanced with increasing sintering temperature. But the decomposition of HA became severe when sintering temperature reached1050℃. Therefore, the optimal sintering temperature was1000℃.The fractal dimension of fracture surfaces of the HA based composites was evaluated using SEM stereoscopy coupled with a3D surface analysis. And the potential relationship between fracture toughness and fractal dimension in the HA based composites was discussed. The fracture surfaces of the HA based composites were observed to exhibit basically fractal characteristics. Both fracture surface roughness and fractal dimension increased with increasing amount of Ti-Fe reinforcing particles. Though no straightforward relation between the flexural strength and fractal dimension could be established due to the presence of porosity, the fracture toughness increased linearly with the square root of fractal dimensional increment (i.e., follows the Mecholsky-Mackin relationship well) due to the presence of more ductile Ti-Fe particles along with the effect of porosity in brittle materials.Finally, the in-vitro biological behavior of the newly-developed HA based composites was studied by immersing the composites in the simulated body fluid (SBF). The addition of iron was found to have a significant influence on the in-vitro biological behavior of the composites. The present results suggested that the stability of the composites in the physiological solution was markedly improved. Precipitation occurred on the surface of HA/5%(Ti-33w%Fe) composite, showing a good combination of physiostability with bioactivity, while HA/15%(Ti-33w%Fe) composite exhibited superior physiostability since there was no obvious change on the surface of HA/15%(Ti-33w%Fe) composite. |
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