Abstract
Sintering effects and mechanical properties of bovine hydroxyapatite/CaTiO3 composites are investigated for different CaTiO3 ratios. Results indicate that densities of the biocomposites increase while total porosities of those decrease with increasing sintering temperature. Moreover, modulus of elasticity and microhardness increase with increasing sintering temperature. However, values of both toughness and fracture toughness of the biocomposites generally rise with increasing sintering temperature except for 1300°C.
1. Introduction
Calcium phosphate bioceramics, namely, hydroxyapatite (HA, Ca10(PO4)6(OH)2), are well-known materials for bone reconstruction. HA is mostly used to fill in and to construct bone and teeth in medicine. Generally, HA is mixed or coated by other minerals and materials to rehabilitate its brittleness and low strength [1–3]. CaTiO3 is auxiliary to form apatite and prevents HA from dissolving in acidic tissue environment. Moreover, materials including CaTiO3 do not give any negative reaction with tissues and connect better to osteoblast cells with respect to HA and titanium. For these reasons, it is thought that CaTiO3-HA composites can show good biocompatibility, bioactivity, osteoconductivity, and mechanical performance [4–10]. More CaTiO3 addition than 60% to HA raises AC conductivity and the dielectric constant of CaTiO3-HA composites [11]. Linh et al. have found that 30 wt% CaTiO3 with 70 wt% HA has a better biocompatibility and mechanical strength with respect to varying HA contents (0%, 50%, 70%, and 100%) [12]. Studies indicate that (i) BHA is more economic than synthetic HA [13–15] and (ii) characterization and understanding of mechanical behavior of BHA are still an open area in biomaterials world [16]. In particular, there is scarcely any study related to influence of sintering temperature on mechanical properties of CaTiO3-HA composites in the literature.
The aim of this study is to determine the sintering effects and mechanical properties of bovine hydroxyapatite/CaTiO3 composites with different weight of CaTiO3. For this purpose, HA bioceramic material produced from bovine bones (i.e., bovine hydroxyapatite, BHA) is considered.
2. Material and Methods
BHA obtained from compact section of a bovine femur may be a biomechanically preferable material for medical applications, especially orthopaedic applications [17]. It is generally known that small pieces of cleared femur bone are treated with NaOH to remove organic compounds. After completing deproteinization process [18], the BHA powder is produced by the method of [19]. Final BHA particle size is around 100 μm. Cylindrical BHA/CaTiO3 samples are put together by mixing dry BHA and CaTiO3 (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany). The catalog number for CaTiO3 is #372609. According to the British Standard BS 7253, pellets (approximately 10 mm in diameter and 12 mm in height) are prepared by using uniaxial cold pressing in hardened steel dies under 350 MPa.
Due to the fact that sintering temperature may increase solubility and mechanical strength of general calcium phosphates [20], 4 h may be a long soak time for HA to decrease solubility with respect to high crystallinity. In addition to soak time effect, however, in order to decrease the effect of thermal stress on HA, heating rate is not to be exceeding 5°C/min [21]. Therefore, while one study on HA considers heating rate of 2°C/min for 2 h [22], others regard that of 1°C/min for 3 h [21] or that of 4°C/min for 4 h [23]. Consequently, this study regards that the pressed samples are sintered in an open atmospheric furnace at different temperatures, specifically 1000, 1100, 1200, and 1300°C, for 4 h under a heating rate of 4°C/min. For each temperature, 12 cylindrical samples of hydroxyapatite and hydroxyapatite/CaTiO3 biocomposites are produced. Three testing groups are prepared as BHA (100 wt% BHA), 5BHA (95 wt% BHA and 5 wt% CaTiO3), and 10BHA (90 wt% BHA and 10 wt% CaTiO3).
Density of the sintered samples is determined by the Archimedes method. Porosity measurements are carried out with a mercury porosimeter (Quantachrome Corporation, Poremaster 60, USA). Crystalline phases identification is realised with an X-ray diffractometer (Rigaku Ultima-IV, Japan) with scanning rate of 0.5 degrees/min and scanning interval of 20–60 degrees. Microstructural analysis is completed by a scanning electron microscope (QUANTA 400F Field Emission SEM, USA) for 1000 and 10000 magnifications. From the micromechanical point of view, measurements of Vickers microhardness and modulus of elasticity are carried out in a microhardness testing machine (CSM Instrument, Switzerland), using 100 g load applied for 20 s (dwell time). Microhardness testing machine is also used to get the traces of the indents and the length of the fractures to make the fracture toughness calculations under 10 kg load applied for 20 s. For the macromechanical tests, a universal tensile testing machine (Zwick, Germany) is utilized to obtain maximum compressive strength and toughness under a loading rate of 0.5 mm/min.
The experimental evaluations are analyzed with a general purpose statistical data analysis program WINKS SDA (Version 6.0.91 Professional Ed., TexaSoft, Houston, TX). In these statistical analyses, independent group t-test/ANOVA with comparison type Newman-Keuls and any difference at the 5% level are considered.
3. Results and Discussion
Table 1 represents component and compound name for abbreviations used in X-ray diffractogram figures. The X-ray diffractograms of BHA composites sintered at 1000, 1100, 1200, and 1300°C are shown in Figure 1. Increment of sintering temperature seems not to affect the amount of HA. CO and CP compounds come up at 1200°C and 1300°C. COH takes place at 1100°C and disappears later. The X-ray diffractograms of 5BHA composites sintered at 1000, 1100, 1200, and 1300°C are shown in Figure 2. The amount of HA stays like same with increasing sintering temperature. CT reduces after 1200°C and TO takes place.
The X-ray diffractograms of 10BHA composites sintered at 1000, 1100, 1200, and 1300°C are shown in Figure 3. It is seen that C4P and TO come up at 1300°C and CT decreases. From all the X-ray diffractograms, it is determined that composite mixtures are pure (just including BHA and CaTiO3 components).
Mean density and total porosity values of sintered samples are given in Figures 4 and 5. From these two figures, one can say that densities of samples increase with increasing sintering temperature and the highest density belongs to 10BHA followed by 5BHA and BHA composites in the decreasing order. The ranking of porosity from the highest to the lowest is at the order of BHA, 5BHA, and 10BHA. The total porosities reduce with increasing sintering temperature for three different compositions. As a result, it is obvious that density is supposed to rise while porosity decreases.
The microstructures of sintered BHA, 5BHA, and 10BHA samples are given in Figures 6–8. Generally, it is determined that the mixtures of CaTiO3 and BHA are homogeneous and the increment of sintering temperature increases fusion among the mixture grains and diminishes porosity. There are the wide pores among grains at 1000°C for all composites. One can observe that the needle-like crystalline aggregates in Figure 6(d). It may be discussed that becoming more crystalline particles is due to the soaking time of 4 h. In Figures 7 and 8, CaTiO3 pieces can be seen at 1000°C. The small pores among grains are still available and the fusing and merging among grains rise at 1100°C. The pores among the fused and merged grains disappear at 1200°C. Finally, the small merged grains unite with each other and create bigger grains at 1300°C.
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(d)
Figures 9 and 10 present variations of modulus of elasticity and Vickers microhardness of BHA, 5BHA, and 10BHA ceramic samples with respect to sintering temperature. Modulus of elasticity rises with the increment of sintering temperature. The ranking from the highest to lowest according to elasticity modulus values is at the order of BHA, 5BHA, and 10BHA except for 1100°C before. The behavior of microhardness is supposed to show resemblance to that of modulus of elasticity as in this study. BHA belonging to the highest microhardness values is followed by 5BHA and 10BHA in the ranking like modulus of elasticity. Statistically, results of analysis of modulus of elasticity are as follows. (i) At the 0.05 significance level, groups BHA at 1000 and 1100°C are not significantly different, (ii) all four groups of 5BHA are significantly different from each other, and (iii) groups 10BHA at 1000 and 1100°C are not significantly different. And the results of statistical analysis of Vickers hardness are as follows. (i) At the 0.05 significance level, groups BHA at 1000 and 1100°C are not significantly different and (ii) groups of 5BHA and 10BHA at 1000 and 1100°C are not significantly different.
Variation of toughness of the sintered ceramics is represented in Figure 11. The toughness values rise with increasing sintering temperatures except for 10BHA at 1300°C. The highest toughness values mostly belong to BHA followed by 10BHA and 5BHA. The addition of 5% and 10% CaTiO3 to BHA seems to decrease toughness. It is seen that higher toughness values can be obtained with more CaTiO3 addition than 10% to BHA. Statistically, results of analysis of toughness are as follows: (i) At the 0.05 significance level, all four groups of BHA are significantly different from each other, (ii) groups 5BHA at 1100 and 1200°C are not significantly different, and (iii) all four groups of 10BHA are significantly different from each other.
Characteristic crack pattern and mean fracture toughness values of sintered samples are given in Figures 12 and 13, respectively. Fracture toughness values are calculated by using (1), where , , , , , and are fracture toughness (MPa·m0.5), constant ( = 0.016), modulus of elasticity (N/m2), Vickers hardness, N/m2, load (kg·m/s2), and fracture length (m), respectively [24].Figure 13 indicates that (i) the fracture toughness increases with increasing sintering temperature except for 1300°C; (ii) the ranking of fracture toughness from the highest to the lowest is in the order of BHA, 10BHA, and 5BHA; BHA possesses the highest fracture toughness at 1200°C; (iii) there are reductions of BHA and 10BHA after 1200°C in contrast to 5BHA. It is discussed that the fracture toughness of composites including more than 10% of CaTiO3 addition may exceed the fracture toughness of BHA. Results of statistical analysis of fracture toughness are as follows: (i) At the 0.05 significance level, groups of BHA at 1000, 1100, and 1300°C are not significantly different, (ii) groups 5BHA at 1100 and 1200°C are not significantly different, and (iii) groups 10BHA at 1000 and 1300°C are not significantly different.
Thus, with respect to the consolidation of 10BHA, good sintering regime can be observed at 1200°C, which is reflected in the high values of toughness and fracture toughness. The grains of 10BHA gradually grow at 1300°C, where that material with large grains is more fragile than the materials of all the other groups. Moreover, for all groups, due to the fact that there is no observation of intergranular/intragranular crack growth, increasing sintering temperature in the range of 1100–1300°C increases values of modulus of elasticity and Vickers hardness. Therefore, it can be discussed that 10BHA in the range of 1100–1200°C sintering temperature may be suitable for the biomedical applications with possible granular crack existence.
4. Conclusions
The total porosities reduce with increasing sintering temperature for three different compositions which are ranking from the highest to the lowest at the order of BHA, 5BHA, and 10BHA. On the contrary, the densities of samples increase with increasing sintering temperature. Increment of sintering temperature seems not to affect the amount of HA. CO and CP compounds come up at 1200°C and 1300°C in BHA X-rays. CT reduces after 1200°C and TO takes place in 5BHA X-rays. C4P and TO come up at 1300°C and CT decreases in 10BHA X-rays and all composite mixtures are pure. The mixtures of CaTiO3 and BHA are homogeneous and the increment of sintering temperature increases fusing and melting among the mixture grains and diminishes porosity.
Modulus of elasticity and microhardness rise with the increment of sintering temperature. The ranking to the lowest from the highest is at the order of BHA, 5BHA, and 10BHA for both. The fracture toughness and toughness of composites generally increase with increasing sintering temperature except for 1300°C. The ranking for both from the highest to the lowest is in the order of BHA, 10BHA, and 5BHA. It is supposed that more CaTiO3 than 10% be added in BHA to exceed the fracture toughness and toughness values of BHA.
Competing Interests
The authors declare that they have no competing interests regarding the publication of this paper.