Optimal design and fabrication of hydroxyapatite–Ti asymmetrical functionally graded biomaterial
Introduction
Hydroxyapatite (HA)–Ti system functionally graded biomaterials (FGMs) with the gradual distribution of components can eliminate the macroscopic interface existing between the HA coatings and titanium alloys, which make full use of the excellent bioactivity of HA and the high strength and toughness of Ti metal [1], [2], [3], [4], [5], [6], [7], [8]. So far, both HA–Ti asymmetrical FGM with a asymmetrically distributing compositional profile varying gradually from Ti side to HA side [2], [3], [5], [6] and HA–Ti symmetrical FGM with a symmetrically distributing compositional profile [7], [8] have been developed successfully by our group using powder metallurgical method. The mechanical properties, fracture behaviors and the relaxing characteristics of thermal residual stress of these FGMs have also been reported correspondingly.
As discussed previously, the change of ambient temperature could lead to the existence of additional residual thermal macroscopic stress and strain in each graded layer besides the thermo-elastic deformation of the whole FGM due to the heterogeneous characteristic along the thickness direction of FGM [9], [10], [11]. The sintering temperatures are far higher than the utilizing ones (or the body temperatures) of HA–Ti FGMs. As a result, the yielding of residual thermal macroscopic stress in the FGM could not be avoided, which will play an important role on the preparation and properties of HA–Ti FGMs. In our previous studies, it was found that HA–Ti symmetrical FGM with a compositional profile pre-designed has always many microcracks on the surfaces [7], while the perfect HA–Ti symmetrical FGM with the optimum graded composition can be prepared by the thermal stress relaxation design and structure optimization [8]. Although there are no microcracks on the surface of HA–Ti asymmetrical FGM, the compositional profile pre-designed is not the optimum for the relaxation of thermal residual stress in the FGM due to the presence of the maximal residual tensile stress in Ti–60vol.% HA region of the FGM with the poor strength and toughness [6]. Thus the optimal design of HA–Ti asymmetrical FGM should be made.
In this paper, the thermo-elastic properties of HA–Ti composites with various mixing ratios corresponding to each graded layer of HA–Ti asymmetrical FGM were tested firstly, which are essential for the structure optimization of the FGM. Then the optimal design of HA–Ti asymmetrical FGM was made. Finally, HA–Ti asymmetrical FGM with the optimum graded composition was fabricated by hot pressing. The residual thermal stresses in the sintered FGM sample were tested by X-ray stress analyzer (XSA).
Section snippets
Optimal design model of FGM
Fig. 1 shows the analysis model for FGM plate with a thickness of h, which consists of n layers of infinite macroscopic boards. To analyze the elastic properties of FGM plate simply and conveniently, the following hypotheses should be made: (1) The hypothesis of consistent deformation between layers: each layer bonds tightly and is deformed consistently with its contiguous layers; (2) The hypothesis of invariable straight normal: the normal on the median plane of FGM plate keeps vertical before
Raw materials and powder processing
The raw materials used were titanium powders and HA powders. The chemical composition of the titanium was (wt.%): Ti 99.3, Fe 0.039, O 0.35, N 0.035, C 0.025, Cl 0.034, H 0.024 and Si 0.0018. The HA was prepared by the reaction between Ca(NO3)2 and (NH4)2HPO4. Its Ca/P ratio was 1.67±2.0%. Sizing by means of Laser Particle Sizer (OMEC LS-POP(III)) showed the Ti particles had a average size of 45.2 μm (93.64 wt.% of Ti particles were in the range 37.0–60.0 μm), whereas the average size of HA
Thermo-elastic properties of HA–Ti composites
It is necessary for the structure optimization of HA–Ti asymmetrical FGM to acquire the thermo-elastic properties of the uniform HA–Ti composites with various mixing ratios corresponding to each graded layer of the FGM, such as thermal expansion coefficient, elastic modulus and Poisson's ratio.
Fig. 2 shows thermal expansion coefficient of HA–Ti composites with various mixing ratios corresponding to each graded layer of the FGM at different temperatures. It could be found that thermal expansion
Conclusions
The study of optimal design and fabrication of HA–Ti asymmetrical FGM leads to the following important conclusions.
(1) Thermal expansion coefficients of HA–Ti composites increase with the rise of testing temperature or the content of HA ceramic and aren't affected by the allotropic transformation of α→β-Ti phase at 882.5 °C, while there is no direct corresponding relationship between elastic modulus and the mixing ratio of HA–Ti composites.
(2) The optimal graded compositional distribution
Acknowledgements
The authors are grateful to Prof. S.D. Wang, Analysis and Testing Center, Southeast University, Nanjing, PR China for his kind help in part of the experimental work.
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