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Effect of TiO2 doping on densification and mechanical properties of hydroxyapatite by microwave sintering
Ceramics International 45 (2019) 13647–13655
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Effect of TiO2 doping on densification and mechanical properties of hydroxyapatite by microwave sintering
T
Junfeng Niea, Jian Zhoua,∗, Xiaoguang Huanga, Lin Wangb, Guizhen Liua, Jiping Chengc a
State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan, 430070, China c Materials Research Institute, The Pennsylvania State University, University Park, PA16802, USA b
ARTICLE INFO
ABSTRACT
Keywords: TiO2-HAP Microwave sintering Mechanical properties Multiphase bioceramics
Hydroxyapatite (HAP) possesses excellent bioactivity/osteointegration properties. Nevertheless, its inferior flexural strength and fracture toughness limit its use in human weight-bearing parts. We investigated a microwave sintering technology which can be effectively used to develop titanium dioxide-hydroxyapatite (TiO2-HAP) ceramics with different amounts of TiO2 (0.8,1.6,2.4,3.2,4.0,4.8,5.6 and 6.4 wt%), which contribute to extremely high flexural strength (90–130 MPa) along with a good combination of elastic modulus and fracture toughness. The results of the Rietveld refinement show that multiphase bioceramics (HAP, β-TCP) can be achieved by doping nano-TiO2 under microwave sintering. Despite the fact that the main phases of the sintered TiO2-HAP ceramics are HAP and β-TCP, X-ray diffraction confirms the formation of the CaTiO3 and CaTi2O4(OH)2 phases. Furthermore, the sintering reactions to form these phases are discussed and the results show that an appropriate amount of nano-TiO2 can not only effectively inhibit the growth of grain, but also change the fracture mode and increase the relative density. Finally, it is found that doping nano-TiO2 by microwave heating is an effective technique for producing HAP/β-TCP composite load-bearing implants in clinical applications without coarsening the size of grain.
1. Introduction Hydroxyapatite (Ca10(PO4)6·(OH)2) is one of the most significant biomaterials because of its distinctive ability in promoting osseointegration in both bone graft substitute and biomimetic coating of prosthetic implants [1–3]. The elders or patients with osteopenia, with the acceleration of osteoporosis and osteomalacia, pathological fractures may eventually need to completely replace their damaged bone. Titanium (Ti) is a recognized biomaterial for bone substitute [4] and has been utilized in various clinical applications because of its excellent corrosion resistant, favorable mechanical properties and biocompatibility [5]. Unfortunately, Ti, as a bioinert material, cannot powerful bond with bone tissues and it is likely to become incompact after a longterm usage, accordingly lose the implant effect [6]. The composition and structure of HAP are similar to that of the human skeleton system, and HAP is highly bioactive, non-toxic, non-inflammatory, and possesses a non-immunogenic property; therefore, it can be an ideal alternate material for bone replacement and reconstruction [7–9]. Although hydroxyapatite possesses an excellent biological activity, its poor mechanical strength limits the use in human weight-bearing parts,
∗
thus it is mainly used as coatings or bone grafts for other non-loadbearing areas [10,11]. In order to improve the mechanical properties of HAP, researchers have done a lot of research in various aspects, such as adding dopants (metal, ceramic, polymer) or using advanced sintering technology to control the microstructure of sintered materials [12]. Microwave sintering technology is superior to the conventional sintering methods in that the material is heated to the sintering temperature by the dielectric loss of the material, rather than gradually transferring heat to the interior of the material by heating the surface of the material [13,14]. In comparison with the traditional sintering technology, the microwave sintering technology possesses the advantages of uniform heating temperature field, small thermal stress, fast heating rate, high energy utilization, no pollution, and low sintering temperature. Microwave sintering technology is capable of avoiding the serious growth of crystal grains caused by the slow heating rate and high sintering temperature in the conventional sintering. The obvious advantages of microwave sintering technology in energy saving and improving ceramic mechanical properties make this technology popular in recent years, especially in the field of bioceramics [15]. The fracture toughness and flexural strength of human cortical bone are
Corresponding author. E-mail address: [email protected] (J. Zhou).
https://doi.org/10.1016/j.ceramint.2019.04.007 Received 9 March 2019; Received in revised form 24 March 2019; Accepted 1 April 2019 Available online 05 April 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
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2–12 MPa m1/2 and 100–150 MPa, respectively [16,17]. Therefore, the ideal bone repair materials should have similar flexural strength and fracture toughness values. In the current work, we combine the advantages of nano-HAP and nano-TiO2 to develop TiO2-doped HAP bioceramics with potential applications in human weight-bearing parts. For this purpose, we investigate how the microwave sintering technology can be effectively used to produce dense HAP/β-TCP multiphase bioceramicas as well as to demonstrate the excellent combination of flexural strength and fracture toughness that one can obtain in HAP/βTCP ceramics. The phases of the sintered TiO2-HAP ceramics are quantitatively analyzed by using the Rietveld refinement. The microstructures of the samples are comprehensively characterized by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The effects of different TiO2 doping amounts on the mechanical properties and microstructure of microwave sintered TiO2-HAP are studied. The mechanical properties are close to the reasonable flexural strength and fracture toughness of the human cortical bone. 2. Methods & materials 2.1. Processing In this work, commercially pure (AR) nano-TiO2 powder (provided by Aladdin, China) and HAP powder (provided by Nanjing Emperor Nano Material, China) were used as the starting materials. Titanium dioxide was rutile and had a particle size of 25 nm. The hydroxyapatite particles had a length of 150 nm and a width of 20 nm. Various concentrations of nano-TiO2 were first added to the nanocrystal HAP, and then the mixed powder was dispersed in 70 mL of absolute ethanol under a sonication of 40 kHz for a predetermined time. Furthermore, the compound was wet ground for 5 h with zirconia balls (diameter: 3 mm, BPR = 2:1) and ethanol as the grinding medium. The generated seriflux was dried at 70 °C for 24 h using a vacuum oven. The desiccative filter mass was crushed and sifted to prepare TiO2-doped hydroxyapatite powder. A 4 wt% aqueous solution of polyvinyl alcohol was used as a binder. The binder was added dropwise to the powder, uniformly mixed, and sieved to granulate. The prepared particles were uniaxially compacted at 10 MPa to form a rectangular parallelepiped (50 mm × 8 mm × 5 mm). Subsequently, the green-pressing was subjected to cold isostatic pressing (CIP) at a pressure of 285 MPa (Aviation Industry Chuanxi Machine Factory, China). After cold isostatic pressing, the green body was placed in a tubular electric furnace, and the binder in the green body was removed by gradually raising the temperature to 400 °C at a heating rate of 1 °C/min. The microwave sintering was performed utilizing a microwave sintering facility equipped with a variable power output magnetron source capable of operating from 0.2 kW to 1.4 kW at 2.45 GHz. Microwave furnace was operated at 220 V and microwave leakage intensity less than 2 mW/cm2. The cavity was not only bulky and but also overmoded, guaranteeing mixing of the microwave modes and generating a symmetrical field distribution. The amount of TiO2 doping in this experiment were 0.8 wt%,1.6 wt%,2.4 wt %,3.2 wt%,4 wt%, 4.8 wt%,5.6 wt% and 6.4 wt%. The heating rate was 23.75 °C/min, the sintering temperature was 1250 °C, and the holding time was 40 min. All samples were polished to 1.5 μm by utlizing silicon carbide (SiC) sandpapers of diverse grit sizes. Before further testing, each polished sample was cleaned in absolute ethanol using an ultrasonic cleaner.
samples after sintering. Various surface bonds in samples before and after sintering were identified by Fourier transform infrared (FTIR) spectroscopy (Nexus, USA). The absorbances of each sample were measured in the range of 4000–400 cm−1 with KBr pellet as a reference. Consequently, cuboids of sintered TiO2-doped HAP were ground into fine grits and mingled with the dried KBr powder. The different functional groups were identified by plotting the absorbance against the wavenumber. The detailed microstructure of TiO2-doped HAP after microwave sintering was characterized by field emission scanning electron microscopy (SEM; Zeiss Ultra Plus, Carl Zeiss, Germany), SEM was secondary electron (SE) mode, and the accelerated voltage of SEM was 5 kV. The elemental composition of the different phases on the selected area was determined by energy dispersive spectroscopy (EDS; X-Max 50, Oxford Instruments, UK) and EDS was attached to the SEM. Before the SEM-EDS analysis, each sample was coated with platinum to enhance the conductivity of the sample during the observation period. 2.3. Mechanical properties The anatomical location in clinical applications determines the mechanical and physical properties required for bone implant materials, and the basic requirements for ideal biomaterials in orthopedic applications are a suitable combination of density, hardness, high flexural strength, and high fracture toughness [18]. For the purpose of characterizing the designed TiO2-doped HAP, the relative density [19], elastic modulus and flexural strength were measured by the Archimedes method, the static bending test method and the three-point bending techniques, respectively. The Vickers hardness and fracture toughness (KIC) of the samples were obtained by using a standard microindentation device (Wolpert, USA). The widely used static bend test method was utilized in the current analysis and the following formula was used to compute the elastic modulus of test pieces:
E=
The sintered TiO2-doped HAP was subjected to X-ray diffraction (XRD) to identify the phases existing before and after sintering. The XRD instrument (Empyrean, Netherlands) was operated at 40 kV and 40 mA with Cu Kα radiation (λ = 1.54184 Å), a scanning rate of 1° min−1, a step size of 0.01°, and 2θ changed from 10° to 80°. The COD standard was applied to index the various phases presented in the
L3 P × 1000 f
(1)
Where ΔP is the load increment, Δf is the deflection increment, L is the span, B is the width of the test pieces and H is the thickness. In the formula, if all length units are mm and the unit of ΔP is N, the unit of the calculated elastic modulus is GPa. For the Vickers hardness test, the shape of the diamond indenter was a square-based pyramid, and the angle, ψ, between the two opposite edges was equal to 136°.The load of the indentation was 500 g and this load was maintained on the surface of the test pieces for 15 s. For statistical significance, at least five indentation experiments were performed on every test pieces and the average of Vickers hardness was calculated. The hardness was calculated by the following formula in the test [20]:
VHN =
P P P = 2 =1.8544 2 ATAC d /2·sin( /2) d
(2)
Where P is the applied load and d is the diagonal of the indentation. When the unit of P is N and the unit of d is mm, the unit of VHN is represented by MPa. The real area of contact between the test piece and the indenter is expressed by ATAC. In the three-point bending test, the loading speed of the indenter was 0.5 mm min−1 and the span was 30 mm. The flexural strength was obtained using the following equation [21]. max
2.2. Microstructural characterization
4H 3B
=
3 Pmax L 2 wh2
(3)
The maximum load in the test is expressed by Pmax, and the span is represented by L. w and h are the width and thickness of the test pieces, respectively. The indentation fracture toughness was calculated using the formula derived by Niihara [22]:
K Ic = 0.203
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c a
1.5
(HV )(a)0.5
(4)
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Fig. 1. (a)XRD spectrums of the sintered pure HAP, 1.6 wt%TiO2-HAP, 2.4 wt%TiO2-HAP,3.2 wt%TiO2-HAP and 4 wt%TiO2-HAP. (b and c) XRD spectrums of sintered 4 wt%TiO2-HAP at two various 2θ ranges demonstrates the existence of CaTiO3 and CaTi2O4(OH)2.
The radial crack size measured by the center of the indentation is represented by c,i.e. c = a + L, where a and L are the semi-diagonal of indentation and length of crack, respectively. HV represents the Vickers hardness of the test pieces. 3. Results and discussion 3.1. Phase assemblage and quantitative phases analysis X-ray diffraction (XRD) patterns of the pure HAP, 1.6 wt%TiO2HAP, 2.4 wt%TiO2-HAP, 3.2 wt%TiO2-HAP and 4 wt%TiO2-HAP are shown in Fig. 1(a). Compared with the TiO2-undoped HAP after sintering, the β-TCP phase formed by HAP decomposition increases with the increase of TiO2-doped amount. Therefore, nano-TiO2 has a role in promoting the decomposition of HAP during microwave sintering. For purpose of further analyzing the XRD results, the spectrograms obtained from as-sintered 4 wt%TiO2-HAP over two different 2θ ranges are showed in Fig. 1(b) and (c). XRD patterns of the 4 wt%TiO2-HAP ceramics demonstrate the existence of β-TCP, CaTiO3 and CaTi2O4(OH)2. The chemical reaction between nano-HAP and nano-TiO2 can be described by the following equations:
Ca10 (PO4 )6 (OH )2
3Ca3 (PO4) 2 + CaO + H2 O
Ca10 (PO4 )6 (OH )2 + TiO2 Ca10 (PO4)6 (OH )2 + 2TiO2
3Ca3 (PO4 ) 2 + CaTiO3 + H2 O CaTi2 O4 (OH )2 + 3Ca3 (PO4 )2
Compared to pure HAP, a slight shift in some peak positions of the TiO2-doped samples can be found by detailed observation. When the ionic radius of the dopant is smaller, the shift is to the right [23]. Since
the 4 wt% TiO2-HAP exhibits an excellent combination of flexural strength and fracture toughness, Rietveld refinement is performed on representative 4 wt% TiO2-HAP and pure HAP. In this work, XRD data were collected by slow scanning and refined by using the GSAS software [24]. By using Rietveld refinement, not only the content of each phase in the ceramics is obtained but also the change in the unit cell parameters of each phase is analyzed [25]. Fig. 2 shows the results of the Rietveld analysis performed on the specimens. The results of quantitative analysis of sintered 4 wt% TiO2-HAP are listed in Table 1. It can be seen that the sintered 4 wt% TiO2-HAP contains 45.87 wt% β-TCP, which indicates that a HAP/β-TCP multiphase ceramic is produced by doping nano-TiO2. β-TCP can degrade in organism and is a typical biological material. In addition, the biocompatibility and osteoconductivity of β-TCP are excellent. If β-TCP biomaterials are implanted in a body, the formation of new bone tissue will accelerate [26]. However, the inferior physical and mechanical properties of β-TCP limit the application of β-TCP in the main load-bearing parts of the human body. β-TCP cannot be used alone as a major load-bearing implant in clinical applications [27]. Consequently, HAP/β-TCP multiphase bioceramics with excellent mechanical properties produced in this work are encouraging and are expected to achieve clinical applications. Table 2 shows the unit cell parameters of pure HAP and 4 wt %TiO2-HAP acquired by Rietveld refinement. Compared with pure HAP, the unit cell parameters a and c of the HAP phase in the 4 wt% TiO2-HAP ceramics become larger and the unit cell volume of the HAP increases. The smaller radius of Ti4+ (0.605 Å) during sintering may enter the HAP lattice to form a structure of interstitial solid solution, resulting in a larger HAP cell. The lattice is distorted so that defects in the sintered body are increased, thereby promoting the diffusion of the solid matter and contributing to the improvement of the sinterability.
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Fig. 2. Rieteveld refining of microwave sintered samples at 1250 °C for 40 min (a) Pure HAP (b) 4 wt% TiO2-HAP. Black line: observed intensity; Red line: calculated intensity. Green line: the background of the refinement. Blue line: The difference between the observed intensity and the calculated intensity. . (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Quantitative analysis results of sintered 4 wt% TiO2-HAP obtained by the Rietveld refinement. phase
HAP
β-TCP
CaTiO3
content/wt.%
44.89
45.87
9.24
Moreover, the microwave sintered specimens and the starting powders are subjected to particular FTIR analysis, but only representative 4 wt%TiO2-HAP is reported here. The FTIR spectrum of the 4 wt%TiO2-HAP powder before sintering and the 4 wt% TiO2-HAP ceramic after sintering is reported in Fig. 3. The weak sharp peak at
3571 cm−1 is the stretching band of OH− in the apatite and the weak peak at 632 cm−1 is the libration band of OH−, both of which are characteristic bands of HAP [28]. Nevertheless, the interaction of nanoHAP with rutile-TiO2 nanoparticles causes some bands of HAP to move. For instance, OH− stretching band at 3571 cm−1 moves to lower wavenumber, whereas OH− libration band at 632 cm−1 shifts to higher wavenumber. In addition, the absorption peaks of 963 cm−1 (ν1), 1047 cm−1 (ν3), 603 cm−1 (ν4) and 567 cm−1 (ν4) correspond to the characteristic signal bands of PO43− in HAP [29–31]. In the 650900 cm−1 range, the specimens display quite weak bands which can be attributed to the ν4 and ν2CO32− modes [32,33]. The absorption peaks of 1421 cm−1 and 876 cm−1can be attributed to ν3CO32− modes [18].
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Table 2 Unit cell parameters and unit cell volume of each phase in sintered pure HAP and 4 wt% TiO2-HAP. phase
a/Å
b/Å
c/Å
α
β
γ
Volume/Å3
HAP(Pure HAP) HAP (4 wt%TiO2-HAP) β-TCP(4 wt%TiO2-HAP) CaTiO3(4 wt%TiO2-HAP)
9.421343 9.425511 10.40318 5.397897
9.421343 9.425511 10.40318 5.427043
6.900527 6.902843 37.48627 7.639716
90° 90° 90° 90°
90° 90° 90° 90°
120° 120° 120° 90°
530.443 531.09 3513.458 223.803
Fig. 3. Representative FTIR spectra of powder and sintered 4 wt%TiO2-HAP specimen.
The presence of carbonate ions may result from the atmospheric CO2 entering the structure during synthesis and sintering of the samples. The hydrogen bond, formed between the adsorbed water and the OH− group of the hydroxyapatite, results in a broad OeH stretching band between 3500 and 3200 cm−1. Moreover, the weak peak at 1631 cm−1 is caused by the lattice water presented in the sample [31]. It is worth noting that the weak bands of the sintered 4 wt% TiO2-HAP ceramic at 555 cm−1,571 cm−1 and 946 cm−1 are related to the secondary β-TCP phase in HAP [34–36]. In conclusion, in the FTIR spectrum of the specimens after sintering, the absorption peaks associated with the βTCP phase and the HAP phase are found, clearly demonstrating the coexistence of the β-TCP and HAP phases in the 4 wt% TiO2-HAP ceramics. 3.2. Microstructure development Grain growth in the microstructure is revealed by acquiring SEM figures in secondary electronic mode. The grains are tightly arranged in pure HAP and TiO2-doped HAP, and no significant pores are observed, indicating that the prepared ceramics are dense. The results show that the addition of TiO2 can change the grain size, and the grain size declines as the weight percentage of TiO2 increases (Fig. 4(a)–(c) and Fig. 5(a)-(c)). When the sintering temperature is 1250 °C, the average grain size of the pure HAP is approximately 801 nm, while for the samples with the TiO2 doped amount of 1.6 wt% and 4.0 wt%, the average grain size dramatically decreases to 741 nm and 397 nm, respectively. Nevertheless, the average grain size of the microwave-sintered pure HAP is relatively fine compared with the average grain size acquired by conventional pressureless sintering techniques. For example, the grain size of HAP under the conventional sintering technology of 1250 °C is as large as 2.03 μm [37], but the grain size of HAP
measured under microwave sintering conditions in the current work is about 801 nm. At the same time, the average grain size of the microwave sintered sample can be further reduced by doping moderate amount of TiO2. It is well known that materials having a smaller grain size generally have superior mechanical properties compared to the same materials having a larger grain size [22]. The reduction of the grain size inside the ceramic materials not only increases the grain boundary area but also makes the grain boundary more tortuous, which is more unfavorable for crack propagation. Consequently, the sample having a smaller grain size can be obtained by adding nano-TiO2, which exhibits more excellent mechanical properties. In the light of the theory of crystal growth, the grain growth process is primarily realized by crystal boundary migration [38]. Microwave sintering can prevent grain growth during high temperature sintering compared to conventional pressureless sintering [22]. In addition, according to the GaussAmp fitting of the grain size distribution, the grain size of 4 wt%TiO2HAP is more uniform than that of pure HAP, whereas the uniformity of grain size of 1.6 wt%TiO2-HAP is decreased. The SEM topographies of the fracture surfaces of pure HAP and TiO2-doped HAP sintered at 1250 °C are shown in Fig. 6. It can be clearly seen that most of the grain boundaries in pure HAP are incognizable, indicating that the fracture mode of pure HAP is a mixture of transgranular fracture and intergranular fracture, and the larger grain mainly exhibits transgranular fracture. This indicates that the interface strength is higher than the strength of some grains in pure HAP. However, the proportion of transgranular fracture in 1.6 wt%TiO2-HAP is further increased compared to pure HAP, as indicates by the arrow in Fig. 6(b). When the doped amount of TiO2 reaches 4 wt%, the fracture mode of the sample is completely transgranular fracture, indicating that nano-TiO2 improves the bonding strength of the ceramic grain boundary. The pinning effect of nano-TiO2 on the growth of ceramic grains can achieve the purpose of transferring and dispersing the load [39], so that the crack propagation mode changes from transgranular and intergranular mixing mode to complete transgranular fracture, which significantly enhances the flexural strength and fracture toughness of the ceramics. The elemental composition of 4 wt%TiO2-HAP is characterized by EDS and shown in Fig. 7. Calcium, titanium, oxygen and phosphorus elements are detected in the corresponding doped samples, confirming that Ti can dope in the HAP structure. Moreover, the element mapping of the 4 wt%TiO2-HAP indicates that the various elements are uniformly distributed in the samples. The Ca/P atomic ratio of the 4 wt %TiO2-HAP is 1.72 according to the EDS results. Evidently, the value of the Ca/P atomic ratio is slightly higher than 1.67 for the stoichiometric HAP [40], which indicates that the sample is phosphorus-deficient apatite. It is possible that phosphorus vaporizes during microwave sintering. The mechanical and biological properties of calcium phosphate bioceramics are determined by the Ca/P ratio, which is an extremely important parameter [41]. It is noteworthy that relatively high Ca/P can promote adhesion of osteoblast on calcium phosphate [42]. 3.3. Mechanical properties A typical SEM micrograph of the Vickers indentation acquired on the surface of a 4.0 wt% TiO2-HAP specimen sintered at 1250 °C for 40 min is shown in Fig. 8. In the current test, we measured the lengths of the cracks from the four vertices and diagonal lines of the
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Fig. 4. Secondary electron SEM micrograph of specimens surface: (a) pure HAP (b) 1.6 wt%TiO2-HAP (c) 4 wt%TiO2e HAP.
indentation, and calculated the fracture toughness and hardness of the specimens. The mechanical properties of pure HAP and TiO2-doped HAP designed in this research will be demonstrated as follows. The various mechanical properties of the tested specimens are listed in Table 3. Taking the theoretical densities of pure HAP (3.16 g cm−3) and TiO2 (rutile, 4.25 g cm−3) into account, we calculated the theoretical densities of the sintered specimens by the rules of the mixture. Fig. 9 depicts the relationship between the relative density of the specimens and the amount of doped-TiO2. In this experiment, the sintered density of pure HAP is 95.94% pth. Nevertheless, the sintered density of the TiO2doped HAP (Doping amount = 0.8 wt%,1.6 wt%,2.4 wt%, 4.8 wt %,5.6 wt% and 6.4 wt%) is found to be lower. If the HAP material is sintered to the same density, the microwave sintering takes less than 3% of the total time required for conventional pressureless sintering [43]. The difference between the initial particles sizes of HAP and TiO2 can result in the density of some TiO2-doped HAP ceramics that is slightly lower than that of pure HAP under the same sintering conditions. The particle size of TiO2 is 25 nm. On the other hand, the length and width of the HAP particles are 150 nm and 20 nm, respectively. Therefore, although HAP host materials with superfine particle sizes would be more susceptible to densification, TiO2 with larger particle sizes would restrain speedier densification. It can be noted that the maximum density value of 98.26% is measured for 4.0 wt%TiO2 dopedsample. The addition of TiO2 nanoparticles significantly can decrease irregular porosity of the specimens [44]. Fig. 10 reveals the effect of amount of doped-TiO2 on the flexural strength and fracture toughness (KIC) of microwave sintered ceramics. The results indicate that the flexural strength and fracture toughness of the microwave-sintered samples increase initially and then decrease with increasing the dopedTiO2 amount. When the doped amount of TiO2 is 4.0 wt%, the flexural strength and fracture toughness of the sample reach the maximum,
which are 1.46 MP m1/2 and 129.91 MPa, respectively. Note that the flexural strength of cortical bone was reported to be 150–170 MPa in the literature [45]. Therefore, the measured values are close to the reasonable flexural strength of the human cortical bone. In addition, it is observed that the average flexural strength of TiO2-doped samples increases from the average value of 84.99 MPa–129.91 MPa with the increase of relative density, and thereafter decreases almost linearly to 99.04 MPa with the decrease of relative density when the amount of doped-TiO2 is 6.4 wt%. Actually, the maximum KIC values for HAP reported in most of the literature varied between 0.96 MPa m1/2 and 1 MPa m1/2 [22]. Consequently, the high level of 1.46 MPa m1/2 acquired for TiO2-doped HAP in this research is worth inspiring. It is believed that the reformative sinterability of nano-TiO2 particles with restricted grain growth in the process of microwave sintering causes these increases in flexural strength and fracture toughness in the designed specimens. Fig. 11 shows the influences of TiO2 doping amount on the elastic modulus and VHN of microwave sintered specimens. The elastic modulus of the specimens will increase as the TiO2 doping amount increases, and reaches the highest point at a specific TiO2 doping amount, and then declines as the TiO2 doping amount further increases to 4.0 wt %. It can be noted that the maximum elastic modulus value of 94.33 GPa is measured for 3.2 wt%TiO2 doped sample. VHN decreases from 4.91 to 3.98 GPa when 1.6 wt%TiO2 are added to the pure HAP matrix. However, further TiO2 addition up to the content of 3.2 wt% improves the VHN values (4.75 GPa), and minimum values (3.82 GPa) are observed when 6.4 wt% TiO2 are added. This result indicates that the Vickers hardness values are slightly affected by a small addition of TiO2 to pure HAP. However, the further addition of TiO2 is detrimental to this property, mainly due to the effect of TiO2 on the grain size at the sintering temperature.
Fig. 5. Grain size distribution: (a)pure HAP(b)1.6 wt%TiO2-HAP(c)4 wt%TiO2e HAP.
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Fig. 6. SEM micrograph of the fracture surface: (a)pure HAP(b)1.6 wt%TiO2-HAP(c)4 wt%TiO2e HAP.
4. Conclusions The current research involves the phase change, densification, sintering and mechanical properties as well as the microstructure of pure HAP and TiO2-doped HAP ceramics prepared by microwave sintering techniques. Doping nano-TiO2 is beneficial to the densification and mechanical properties of hydroxyapatite under microwave sintering conditions. At 1250 °C and a holding time of 40 min, the relative density of 4 wt% TiO2-HAP rapidly sintered using microwaves is over 98%, and the fine-grained microstructure is kept. Compared with TiO2-doped HAP sintered by microwave technology, significant grain coarsening can be observed in pure HAP, and the average grain size of the 4 wt %TiO2-HAP is 397 nm. At the same time, the uniformity of the grain size of the HAP ceramics can be improved by doping 4 wt%TiO2. Doping 4 wt%TiO2 can completely convert the ceramics into
Fig. 8. Representative Vickers indentation acquired on the surface of TiO2-HAP sample sintered at 1250 °C.
Fig. 7. EDS micrographs of the 4 wt%TiO2-HAP sample. The distributions of 4 elements (Ca, P, O and Ti) were detected by the area EDS analysis.
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Table 3 Summary of the mechanical properties measured with the investigated xTiO2-doped HAP (x = 0, 0.8 wt%, 1.6 wt%,2.4 wt%,3.2 wt%, 4.0 wt%,4.8 wt%,5.6 wt %,6.4 wt%). Sample
Relative density/%
Flexural strength/MPa
Fracture toughness/(MPa·m1/2)
Vickers Hardness/GPa
Elastic modulus/GPa
Pure HAP 0.8 wt% 1.6 wt% 2.4 wt% 3.2 wt% 4.0 wt% 4.8 wt% 5.6 wt% 6.4 wt%
95.94 ± 2.64 91.30 ± 2.16 92.99 ± 1.79 93.57 ± 3.42 97.23 ± 1.58 98.26 ± 1.17 94.1 ± 3.36 93.79 ± 2.46 91.85 ± 2.94
73.84 ± 7.59 84.99 ± 5.03 101.99 ± 10.28 109.24 ± 8.73 123.27 ± 7.05 129.91 ± 14.33 121.57 ± 12.61 115.47 ± 7.41 99.04 ± 18.48
0.85 _ _ 0.92 1.39 1.46 1.07 _ _
4.91 4.35 3.98 4.29 4.75 4.69 4.22 4.62 3.82
63.77 ± 8.54 80.54 ± 4.9 79.54 ± 3.97 80.77 ± 4.8 94.33 ± 5.03 83.26 ± 6.55 89.1 ± 2.07 79.4 ± 2.58 82.37 ± 2.31
± 0.08 ± ± ± ±
0.06 0.13 0.1 0.08
± ± ± ± ± ± ± ± ±
0.06 0.37 0.32 0.28 0.22 0.34 0.29 0.31 0.44
Fig. 11. The change of elastic modulus and Vickers hardness with TiO2 doping amount.
Fig. 9. The effect of TiO2 doping on the relative density of the samples.
seems reasonable to use a suitable TiO2-doped HAP for load-bearing implants in clinical applications. Acknowledgements This work was supported by National Key R&D Program of China (2017YFC1103800), Fundamental Research Funds for the Central Universities (WUT: 2018III006CG). References
Fig. 10. The change of flexural strength and fracture toughness with TiO2 doping amount.
transgranular fracture compared to the transgranular and intergranular mixing in pure HAP. Under the condition that the microwave sintering temperature is 1250 °C and the holding time is 40 min, no decomposition of pure HAP is observed. However, under the same sintering conditions, the decomposition of HAP is intensified with the increase of the doping amount of TiO2. HAP/β-TCP multiphase bioceramics with excellent mechanical properties are successfully obtained by doping 4 wt%TiO2 during microwave sintering. Compared with pure hydroxyapatite, microwave sintered 4 wt%TiO2-HAP exhibits good mechanical properties. Average flexural strength of up to 129.91 MPa is measured with 4 wt%TiO2-HAP samples. The maximum fracture toughness of 1.46 MPa m1/2 and the maximum elastic modulus of 94.33 GPa are acquired for TiO2-HAP microwave-sintered at 1250 °C, respectively. It
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Effect of TiO2 doping on densification and mechanical properties of hydroxyapatite by microwave sintering
T
Junfeng Niea, Jian Zhoua,∗, Xiaoguang Huanga, Lin Wangb, Guizhen Liua, Jiping Chengc a
State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan, 430070, China c Materials Research Institute, The Pennsylvania State University, University Park, PA16802, USA b
ARTICLE INFO
ABSTRACT
Keywords: TiO2-HAP Microwave sintering Mechanical properties Multiphase bioceramics
Hydroxyapatite (HAP) possesses excellent bioactivity/osteointegration properties. Nevertheless, its inferior flexural strength and fracture toughness limit its use in human weight-bearing parts. We investigated a microwave sintering technology which can be effectively used to develop titanium dioxide-hydroxyapatite (TiO2-HAP) ceramics with different amounts of TiO2 (0.8,1.6,2.4,3.2,4.0,4.8,5.6 and 6.4 wt%), which contribute to extremely high flexural strength (90–130 MPa) along with a good combination of elastic modulus and fracture toughness. The results of the Rietveld refinement show that multiphase bioceramics (HAP, β-TCP) can be achieved by doping nano-TiO2 under microwave sintering. Despite the fact that the main phases of the sintered TiO2-HAP ceramics are HAP and β-TCP, X-ray diffraction confirms the formation of the CaTiO3 and CaTi2O4(OH)2 phases. Furthermore, the sintering reactions to form these phases are discussed and the results show that an appropriate amount of nano-TiO2 can not only effectively inhibit the growth of grain, but also change the fracture mode and increase the relative density. Finally, it is found that doping nano-TiO2 by microwave heating is an effective technique for producing HAP/β-TCP composite load-bearing implants in clinical applications without coarsening the size of grain.
1. Introduction Hydroxyapatite (Ca10(PO4)6·(OH)2) is one of the most significant biomaterials because of its distinctive ability in promoting osseointegration in both bone graft substitute and biomimetic coating of prosthetic implants [1–3]. The elders or patients with osteopenia, with the acceleration of osteoporosis and osteomalacia, pathological fractures may eventually need to completely replace their damaged bone. Titanium (Ti) is a recognized biomaterial for bone substitute [4] and has been utilized in various clinical applications because of its excellent corrosion resistant, favorable mechanical properties and biocompatibility [5]. Unfortunately, Ti, as a bioinert material, cannot powerful bond with bone tissues and it is likely to become incompact after a longterm usage, accordingly lose the implant effect [6]. The composition and structure of HAP are similar to that of the human skeleton system, and HAP is highly bioactive, non-toxic, non-inflammatory, and possesses a non-immunogenic property; therefore, it can be an ideal alternate material for bone replacement and reconstruction [7–9]. Although hydroxyapatite possesses an excellent biological activity, its poor mechanical strength limits the use in human weight-bearing parts,
∗
thus it is mainly used as coatings or bone grafts for other non-loadbearing areas [10,11]. In order to improve the mechanical properties of HAP, researchers have done a lot of research in various aspects, such as adding dopants (metal, ceramic, polymer) or using advanced sintering technology to control the microstructure of sintered materials [12]. Microwave sintering technology is superior to the conventional sintering methods in that the material is heated to the sintering temperature by the dielectric loss of the material, rather than gradually transferring heat to the interior of the material by heating the surface of the material [13,14]. In comparison with the traditional sintering technology, the microwave sintering technology possesses the advantages of uniform heating temperature field, small thermal stress, fast heating rate, high energy utilization, no pollution, and low sintering temperature. Microwave sintering technology is capable of avoiding the serious growth of crystal grains caused by the slow heating rate and high sintering temperature in the conventional sintering. The obvious advantages of microwave sintering technology in energy saving and improving ceramic mechanical properties make this technology popular in recent years, especially in the field of bioceramics [15]. The fracture toughness and flexural strength of human cortical bone are
Corresponding author. E-mail address: [email protected] (J. Zhou).
https://doi.org/10.1016/j.ceramint.2019.04.007 Received 9 March 2019; Received in revised form 24 March 2019; Accepted 1 April 2019 Available online 05 April 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
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2–12 MPa m1/2 and 100–150 MPa, respectively [16,17]. Therefore, the ideal bone repair materials should have similar flexural strength and fracture toughness values. In the current work, we combine the advantages of nano-HAP and nano-TiO2 to develop TiO2-doped HAP bioceramics with potential applications in human weight-bearing parts. For this purpose, we investigate how the microwave sintering technology can be effectively used to produce dense HAP/β-TCP multiphase bioceramicas as well as to demonstrate the excellent combination of flexural strength and fracture toughness that one can obtain in HAP/βTCP ceramics. The phases of the sintered TiO2-HAP ceramics are quantitatively analyzed by using the Rietveld refinement. The microstructures of the samples are comprehensively characterized by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The effects of different TiO2 doping amounts on the mechanical properties and microstructure of microwave sintered TiO2-HAP are studied. The mechanical properties are close to the reasonable flexural strength and fracture toughness of the human cortical bone. 2. Methods & materials 2.1. Processing In this work, commercially pure (AR) nano-TiO2 powder (provided by Aladdin, China) and HAP powder (provided by Nanjing Emperor Nano Material, China) were used as the starting materials. Titanium dioxide was rutile and had a particle size of 25 nm. The hydroxyapatite particles had a length of 150 nm and a width of 20 nm. Various concentrations of nano-TiO2 were first added to the nanocrystal HAP, and then the mixed powder was dispersed in 70 mL of absolute ethanol under a sonication of 40 kHz for a predetermined time. Furthermore, the compound was wet ground for 5 h with zirconia balls (diameter: 3 mm, BPR = 2:1) and ethanol as the grinding medium. The generated seriflux was dried at 70 °C for 24 h using a vacuum oven. The desiccative filter mass was crushed and sifted to prepare TiO2-doped hydroxyapatite powder. A 4 wt% aqueous solution of polyvinyl alcohol was used as a binder. The binder was added dropwise to the powder, uniformly mixed, and sieved to granulate. The prepared particles were uniaxially compacted at 10 MPa to form a rectangular parallelepiped (50 mm × 8 mm × 5 mm). Subsequently, the green-pressing was subjected to cold isostatic pressing (CIP) at a pressure of 285 MPa (Aviation Industry Chuanxi Machine Factory, China). After cold isostatic pressing, the green body was placed in a tubular electric furnace, and the binder in the green body was removed by gradually raising the temperature to 400 °C at a heating rate of 1 °C/min. The microwave sintering was performed utilizing a microwave sintering facility equipped with a variable power output magnetron source capable of operating from 0.2 kW to 1.4 kW at 2.45 GHz. Microwave furnace was operated at 220 V and microwave leakage intensity less than 2 mW/cm2. The cavity was not only bulky and but also overmoded, guaranteeing mixing of the microwave modes and generating a symmetrical field distribution. The amount of TiO2 doping in this experiment were 0.8 wt%,1.6 wt%,2.4 wt %,3.2 wt%,4 wt%, 4.8 wt%,5.6 wt% and 6.4 wt%. The heating rate was 23.75 °C/min, the sintering temperature was 1250 °C, and the holding time was 40 min. All samples were polished to 1.5 μm by utlizing silicon carbide (SiC) sandpapers of diverse grit sizes. Before further testing, each polished sample was cleaned in absolute ethanol using an ultrasonic cleaner.
samples after sintering. Various surface bonds in samples before and after sintering were identified by Fourier transform infrared (FTIR) spectroscopy (Nexus, USA). The absorbances of each sample were measured in the range of 4000–400 cm−1 with KBr pellet as a reference. Consequently, cuboids of sintered TiO2-doped HAP were ground into fine grits and mingled with the dried KBr powder. The different functional groups were identified by plotting the absorbance against the wavenumber. The detailed microstructure of TiO2-doped HAP after microwave sintering was characterized by field emission scanning electron microscopy (SEM; Zeiss Ultra Plus, Carl Zeiss, Germany), SEM was secondary electron (SE) mode, and the accelerated voltage of SEM was 5 kV. The elemental composition of the different phases on the selected area was determined by energy dispersive spectroscopy (EDS; X-Max 50, Oxford Instruments, UK) and EDS was attached to the SEM. Before the SEM-EDS analysis, each sample was coated with platinum to enhance the conductivity of the sample during the observation period. 2.3. Mechanical properties The anatomical location in clinical applications determines the mechanical and physical properties required for bone implant materials, and the basic requirements for ideal biomaterials in orthopedic applications are a suitable combination of density, hardness, high flexural strength, and high fracture toughness [18]. For the purpose of characterizing the designed TiO2-doped HAP, the relative density [19], elastic modulus and flexural strength were measured by the Archimedes method, the static bending test method and the three-point bending techniques, respectively. The Vickers hardness and fracture toughness (KIC) of the samples were obtained by using a standard microindentation device (Wolpert, USA). The widely used static bend test method was utilized in the current analysis and the following formula was used to compute the elastic modulus of test pieces:
E=
The sintered TiO2-doped HAP was subjected to X-ray diffraction (XRD) to identify the phases existing before and after sintering. The XRD instrument (Empyrean, Netherlands) was operated at 40 kV and 40 mA with Cu Kα radiation (λ = 1.54184 Å), a scanning rate of 1° min−1, a step size of 0.01°, and 2θ changed from 10° to 80°. The COD standard was applied to index the various phases presented in the
L3 P × 1000 f
(1)
Where ΔP is the load increment, Δf is the deflection increment, L is the span, B is the width of the test pieces and H is the thickness. In the formula, if all length units are mm and the unit of ΔP is N, the unit of the calculated elastic modulus is GPa. For the Vickers hardness test, the shape of the diamond indenter was a square-based pyramid, and the angle, ψ, between the two opposite edges was equal to 136°.The load of the indentation was 500 g and this load was maintained on the surface of the test pieces for 15 s. For statistical significance, at least five indentation experiments were performed on every test pieces and the average of Vickers hardness was calculated. The hardness was calculated by the following formula in the test [20]:
VHN =
P P P = 2 =1.8544 2 ATAC d /2·sin( /2) d
(2)
Where P is the applied load and d is the diagonal of the indentation. When the unit of P is N and the unit of d is mm, the unit of VHN is represented by MPa. The real area of contact between the test piece and the indenter is expressed by ATAC. In the three-point bending test, the loading speed of the indenter was 0.5 mm min−1 and the span was 30 mm. The flexural strength was obtained using the following equation [21]. max
2.2. Microstructural characterization
4H 3B
=
3 Pmax L 2 wh2
(3)
The maximum load in the test is expressed by Pmax, and the span is represented by L. w and h are the width and thickness of the test pieces, respectively. The indentation fracture toughness was calculated using the formula derived by Niihara [22]:
K Ic = 0.203
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c a
1.5
(HV )(a)0.5
(4)
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Fig. 1. (a)XRD spectrums of the sintered pure HAP, 1.6 wt%TiO2-HAP, 2.4 wt%TiO2-HAP,3.2 wt%TiO2-HAP and 4 wt%TiO2-HAP. (b and c) XRD spectrums of sintered 4 wt%TiO2-HAP at two various 2θ ranges demonstrates the existence of CaTiO3 and CaTi2O4(OH)2.
The radial crack size measured by the center of the indentation is represented by c,i.e. c = a + L, where a and L are the semi-diagonal of indentation and length of crack, respectively. HV represents the Vickers hardness of the test pieces. 3. Results and discussion 3.1. Phase assemblage and quantitative phases analysis X-ray diffraction (XRD) patterns of the pure HAP, 1.6 wt%TiO2HAP, 2.4 wt%TiO2-HAP, 3.2 wt%TiO2-HAP and 4 wt%TiO2-HAP are shown in Fig. 1(a). Compared with the TiO2-undoped HAP after sintering, the β-TCP phase formed by HAP decomposition increases with the increase of TiO2-doped amount. Therefore, nano-TiO2 has a role in promoting the decomposition of HAP during microwave sintering. For purpose of further analyzing the XRD results, the spectrograms obtained from as-sintered 4 wt%TiO2-HAP over two different 2θ ranges are showed in Fig. 1(b) and (c). XRD patterns of the 4 wt%TiO2-HAP ceramics demonstrate the existence of β-TCP, CaTiO3 and CaTi2O4(OH)2. The chemical reaction between nano-HAP and nano-TiO2 can be described by the following equations:
Ca10 (PO4 )6 (OH )2
3Ca3 (PO4) 2 + CaO + H2 O
Ca10 (PO4 )6 (OH )2 + TiO2 Ca10 (PO4)6 (OH )2 + 2TiO2
3Ca3 (PO4 ) 2 + CaTiO3 + H2 O CaTi2 O4 (OH )2 + 3Ca3 (PO4 )2
Compared to pure HAP, a slight shift in some peak positions of the TiO2-doped samples can be found by detailed observation. When the ionic radius of the dopant is smaller, the shift is to the right [23]. Since
the 4 wt% TiO2-HAP exhibits an excellent combination of flexural strength and fracture toughness, Rietveld refinement is performed on representative 4 wt% TiO2-HAP and pure HAP. In this work, XRD data were collected by slow scanning and refined by using the GSAS software [24]. By using Rietveld refinement, not only the content of each phase in the ceramics is obtained but also the change in the unit cell parameters of each phase is analyzed [25]. Fig. 2 shows the results of the Rietveld analysis performed on the specimens. The results of quantitative analysis of sintered 4 wt% TiO2-HAP are listed in Table 1. It can be seen that the sintered 4 wt% TiO2-HAP contains 45.87 wt% β-TCP, which indicates that a HAP/β-TCP multiphase ceramic is produced by doping nano-TiO2. β-TCP can degrade in organism and is a typical biological material. In addition, the biocompatibility and osteoconductivity of β-TCP are excellent. If β-TCP biomaterials are implanted in a body, the formation of new bone tissue will accelerate [26]. However, the inferior physical and mechanical properties of β-TCP limit the application of β-TCP in the main load-bearing parts of the human body. β-TCP cannot be used alone as a major load-bearing implant in clinical applications [27]. Consequently, HAP/β-TCP multiphase bioceramics with excellent mechanical properties produced in this work are encouraging and are expected to achieve clinical applications. Table 2 shows the unit cell parameters of pure HAP and 4 wt %TiO2-HAP acquired by Rietveld refinement. Compared with pure HAP, the unit cell parameters a and c of the HAP phase in the 4 wt% TiO2-HAP ceramics become larger and the unit cell volume of the HAP increases. The smaller radius of Ti4+ (0.605 Å) during sintering may enter the HAP lattice to form a structure of interstitial solid solution, resulting in a larger HAP cell. The lattice is distorted so that defects in the sintered body are increased, thereby promoting the diffusion of the solid matter and contributing to the improvement of the sinterability.
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Fig. 2. Rieteveld refining of microwave sintered samples at 1250 °C for 40 min (a) Pure HAP (b) 4 wt% TiO2-HAP. Black line: observed intensity; Red line: calculated intensity. Green line: the background of the refinement. Blue line: The difference between the observed intensity and the calculated intensity. . (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Quantitative analysis results of sintered 4 wt% TiO2-HAP obtained by the Rietveld refinement. phase
HAP
β-TCP
CaTiO3
content/wt.%
44.89
45.87
9.24
Moreover, the microwave sintered specimens and the starting powders are subjected to particular FTIR analysis, but only representative 4 wt%TiO2-HAP is reported here. The FTIR spectrum of the 4 wt%TiO2-HAP powder before sintering and the 4 wt% TiO2-HAP ceramic after sintering is reported in Fig. 3. The weak sharp peak at
3571 cm−1 is the stretching band of OH− in the apatite and the weak peak at 632 cm−1 is the libration band of OH−, both of which are characteristic bands of HAP [28]. Nevertheless, the interaction of nanoHAP with rutile-TiO2 nanoparticles causes some bands of HAP to move. For instance, OH− stretching band at 3571 cm−1 moves to lower wavenumber, whereas OH− libration band at 632 cm−1 shifts to higher wavenumber. In addition, the absorption peaks of 963 cm−1 (ν1), 1047 cm−1 (ν3), 603 cm−1 (ν4) and 567 cm−1 (ν4) correspond to the characteristic signal bands of PO43− in HAP [29–31]. In the 650900 cm−1 range, the specimens display quite weak bands which can be attributed to the ν4 and ν2CO32− modes [32,33]. The absorption peaks of 1421 cm−1 and 876 cm−1can be attributed to ν3CO32− modes [18].
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Table 2 Unit cell parameters and unit cell volume of each phase in sintered pure HAP and 4 wt% TiO2-HAP. phase
a/Å
b/Å
c/Å
α
β
γ
Volume/Å3
HAP(Pure HAP) HAP (4 wt%TiO2-HAP) β-TCP(4 wt%TiO2-HAP) CaTiO3(4 wt%TiO2-HAP)
9.421343 9.425511 10.40318 5.397897
9.421343 9.425511 10.40318 5.427043
6.900527 6.902843 37.48627 7.639716
90° 90° 90° 90°
90° 90° 90° 90°
120° 120° 120° 90°
530.443 531.09 3513.458 223.803
Fig. 3. Representative FTIR spectra of powder and sintered 4 wt%TiO2-HAP specimen.
The presence of carbonate ions may result from the atmospheric CO2 entering the structure during synthesis and sintering of the samples. The hydrogen bond, formed between the adsorbed water and the OH− group of the hydroxyapatite, results in a broad OeH stretching band between 3500 and 3200 cm−1. Moreover, the weak peak at 1631 cm−1 is caused by the lattice water presented in the sample [31]. It is worth noting that the weak bands of the sintered 4 wt% TiO2-HAP ceramic at 555 cm−1,571 cm−1 and 946 cm−1 are related to the secondary β-TCP phase in HAP [34–36]. In conclusion, in the FTIR spectrum of the specimens after sintering, the absorption peaks associated with the βTCP phase and the HAP phase are found, clearly demonstrating the coexistence of the β-TCP and HAP phases in the 4 wt% TiO2-HAP ceramics. 3.2. Microstructure development Grain growth in the microstructure is revealed by acquiring SEM figures in secondary electronic mode. The grains are tightly arranged in pure HAP and TiO2-doped HAP, and no significant pores are observed, indicating that the prepared ceramics are dense. The results show that the addition of TiO2 can change the grain size, and the grain size declines as the weight percentage of TiO2 increases (Fig. 4(a)–(c) and Fig. 5(a)-(c)). When the sintering temperature is 1250 °C, the average grain size of the pure HAP is approximately 801 nm, while for the samples with the TiO2 doped amount of 1.6 wt% and 4.0 wt%, the average grain size dramatically decreases to 741 nm and 397 nm, respectively. Nevertheless, the average grain size of the microwave-sintered pure HAP is relatively fine compared with the average grain size acquired by conventional pressureless sintering techniques. For example, the grain size of HAP under the conventional sintering technology of 1250 °C is as large as 2.03 μm [37], but the grain size of HAP
measured under microwave sintering conditions in the current work is about 801 nm. At the same time, the average grain size of the microwave sintered sample can be further reduced by doping moderate amount of TiO2. It is well known that materials having a smaller grain size generally have superior mechanical properties compared to the same materials having a larger grain size [22]. The reduction of the grain size inside the ceramic materials not only increases the grain boundary area but also makes the grain boundary more tortuous, which is more unfavorable for crack propagation. Consequently, the sample having a smaller grain size can be obtained by adding nano-TiO2, which exhibits more excellent mechanical properties. In the light of the theory of crystal growth, the grain growth process is primarily realized by crystal boundary migration [38]. Microwave sintering can prevent grain growth during high temperature sintering compared to conventional pressureless sintering [22]. In addition, according to the GaussAmp fitting of the grain size distribution, the grain size of 4 wt%TiO2HAP is more uniform than that of pure HAP, whereas the uniformity of grain size of 1.6 wt%TiO2-HAP is decreased. The SEM topographies of the fracture surfaces of pure HAP and TiO2-doped HAP sintered at 1250 °C are shown in Fig. 6. It can be clearly seen that most of the grain boundaries in pure HAP are incognizable, indicating that the fracture mode of pure HAP is a mixture of transgranular fracture and intergranular fracture, and the larger grain mainly exhibits transgranular fracture. This indicates that the interface strength is higher than the strength of some grains in pure HAP. However, the proportion of transgranular fracture in 1.6 wt%TiO2-HAP is further increased compared to pure HAP, as indicates by the arrow in Fig. 6(b). When the doped amount of TiO2 reaches 4 wt%, the fracture mode of the sample is completely transgranular fracture, indicating that nano-TiO2 improves the bonding strength of the ceramic grain boundary. The pinning effect of nano-TiO2 on the growth of ceramic grains can achieve the purpose of transferring and dispersing the load [39], so that the crack propagation mode changes from transgranular and intergranular mixing mode to complete transgranular fracture, which significantly enhances the flexural strength and fracture toughness of the ceramics. The elemental composition of 4 wt%TiO2-HAP is characterized by EDS and shown in Fig. 7. Calcium, titanium, oxygen and phosphorus elements are detected in the corresponding doped samples, confirming that Ti can dope in the HAP structure. Moreover, the element mapping of the 4 wt%TiO2-HAP indicates that the various elements are uniformly distributed in the samples. The Ca/P atomic ratio of the 4 wt %TiO2-HAP is 1.72 according to the EDS results. Evidently, the value of the Ca/P atomic ratio is slightly higher than 1.67 for the stoichiometric HAP [40], which indicates that the sample is phosphorus-deficient apatite. It is possible that phosphorus vaporizes during microwave sintering. The mechanical and biological properties of calcium phosphate bioceramics are determined by the Ca/P ratio, which is an extremely important parameter [41]. It is noteworthy that relatively high Ca/P can promote adhesion of osteoblast on calcium phosphate [42]. 3.3. Mechanical properties A typical SEM micrograph of the Vickers indentation acquired on the surface of a 4.0 wt% TiO2-HAP specimen sintered at 1250 °C for 40 min is shown in Fig. 8. In the current test, we measured the lengths of the cracks from the four vertices and diagonal lines of the
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Fig. 4. Secondary electron SEM micrograph of specimens surface: (a) pure HAP (b) 1.6 wt%TiO2-HAP (c) 4 wt%TiO2e HAP.
indentation, and calculated the fracture toughness and hardness of the specimens. The mechanical properties of pure HAP and TiO2-doped HAP designed in this research will be demonstrated as follows. The various mechanical properties of the tested specimens are listed in Table 3. Taking the theoretical densities of pure HAP (3.16 g cm−3) and TiO2 (rutile, 4.25 g cm−3) into account, we calculated the theoretical densities of the sintered specimens by the rules of the mixture. Fig. 9 depicts the relationship between the relative density of the specimens and the amount of doped-TiO2. In this experiment, the sintered density of pure HAP is 95.94% pth. Nevertheless, the sintered density of the TiO2doped HAP (Doping amount = 0.8 wt%,1.6 wt%,2.4 wt%, 4.8 wt %,5.6 wt% and 6.4 wt%) is found to be lower. If the HAP material is sintered to the same density, the microwave sintering takes less than 3% of the total time required for conventional pressureless sintering [43]. The difference between the initial particles sizes of HAP and TiO2 can result in the density of some TiO2-doped HAP ceramics that is slightly lower than that of pure HAP under the same sintering conditions. The particle size of TiO2 is 25 nm. On the other hand, the length and width of the HAP particles are 150 nm and 20 nm, respectively. Therefore, although HAP host materials with superfine particle sizes would be more susceptible to densification, TiO2 with larger particle sizes would restrain speedier densification. It can be noted that the maximum density value of 98.26% is measured for 4.0 wt%TiO2 dopedsample. The addition of TiO2 nanoparticles significantly can decrease irregular porosity of the specimens [44]. Fig. 10 reveals the effect of amount of doped-TiO2 on the flexural strength and fracture toughness (KIC) of microwave sintered ceramics. The results indicate that the flexural strength and fracture toughness of the microwave-sintered samples increase initially and then decrease with increasing the dopedTiO2 amount. When the doped amount of TiO2 is 4.0 wt%, the flexural strength and fracture toughness of the sample reach the maximum,
which are 1.46 MP m1/2 and 129.91 MPa, respectively. Note that the flexural strength of cortical bone was reported to be 150–170 MPa in the literature [45]. Therefore, the measured values are close to the reasonable flexural strength of the human cortical bone. In addition, it is observed that the average flexural strength of TiO2-doped samples increases from the average value of 84.99 MPa–129.91 MPa with the increase of relative density, and thereafter decreases almost linearly to 99.04 MPa with the decrease of relative density when the amount of doped-TiO2 is 6.4 wt%. Actually, the maximum KIC values for HAP reported in most of the literature varied between 0.96 MPa m1/2 and 1 MPa m1/2 [22]. Consequently, the high level of 1.46 MPa m1/2 acquired for TiO2-doped HAP in this research is worth inspiring. It is believed that the reformative sinterability of nano-TiO2 particles with restricted grain growth in the process of microwave sintering causes these increases in flexural strength and fracture toughness in the designed specimens. Fig. 11 shows the influences of TiO2 doping amount on the elastic modulus and VHN of microwave sintered specimens. The elastic modulus of the specimens will increase as the TiO2 doping amount increases, and reaches the highest point at a specific TiO2 doping amount, and then declines as the TiO2 doping amount further increases to 4.0 wt %. It can be noted that the maximum elastic modulus value of 94.33 GPa is measured for 3.2 wt%TiO2 doped sample. VHN decreases from 4.91 to 3.98 GPa when 1.6 wt%TiO2 are added to the pure HAP matrix. However, further TiO2 addition up to the content of 3.2 wt% improves the VHN values (4.75 GPa), and minimum values (3.82 GPa) are observed when 6.4 wt% TiO2 are added. This result indicates that the Vickers hardness values are slightly affected by a small addition of TiO2 to pure HAP. However, the further addition of TiO2 is detrimental to this property, mainly due to the effect of TiO2 on the grain size at the sintering temperature.
Fig. 5. Grain size distribution: (a)pure HAP(b)1.6 wt%TiO2-HAP(c)4 wt%TiO2e HAP.
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Fig. 6. SEM micrograph of the fracture surface: (a)pure HAP(b)1.6 wt%TiO2-HAP(c)4 wt%TiO2e HAP.
4. Conclusions The current research involves the phase change, densification, sintering and mechanical properties as well as the microstructure of pure HAP and TiO2-doped HAP ceramics prepared by microwave sintering techniques. Doping nano-TiO2 is beneficial to the densification and mechanical properties of hydroxyapatite under microwave sintering conditions. At 1250 °C and a holding time of 40 min, the relative density of 4 wt% TiO2-HAP rapidly sintered using microwaves is over 98%, and the fine-grained microstructure is kept. Compared with TiO2-doped HAP sintered by microwave technology, significant grain coarsening can be observed in pure HAP, and the average grain size of the 4 wt %TiO2-HAP is 397 nm. At the same time, the uniformity of the grain size of the HAP ceramics can be improved by doping 4 wt%TiO2. Doping 4 wt%TiO2 can completely convert the ceramics into
Fig. 8. Representative Vickers indentation acquired on the surface of TiO2-HAP sample sintered at 1250 °C.
Fig. 7. EDS micrographs of the 4 wt%TiO2-HAP sample. The distributions of 4 elements (Ca, P, O and Ti) were detected by the area EDS analysis.
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Table 3 Summary of the mechanical properties measured with the investigated xTiO2-doped HAP (x = 0, 0.8 wt%, 1.6 wt%,2.4 wt%,3.2 wt%, 4.0 wt%,4.8 wt%,5.6 wt %,6.4 wt%). Sample
Relative density/%
Flexural strength/MPa
Fracture toughness/(MPa·m1/2)
Vickers Hardness/GPa
Elastic modulus/GPa
Pure HAP 0.8 wt% 1.6 wt% 2.4 wt% 3.2 wt% 4.0 wt% 4.8 wt% 5.6 wt% 6.4 wt%
95.94 ± 2.64 91.30 ± 2.16 92.99 ± 1.79 93.57 ± 3.42 97.23 ± 1.58 98.26 ± 1.17 94.1 ± 3.36 93.79 ± 2.46 91.85 ± 2.94
73.84 ± 7.59 84.99 ± 5.03 101.99 ± 10.28 109.24 ± 8.73 123.27 ± 7.05 129.91 ± 14.33 121.57 ± 12.61 115.47 ± 7.41 99.04 ± 18.48
0.85 _ _ 0.92 1.39 1.46 1.07 _ _
4.91 4.35 3.98 4.29 4.75 4.69 4.22 4.62 3.82
63.77 ± 8.54 80.54 ± 4.9 79.54 ± 3.97 80.77 ± 4.8 94.33 ± 5.03 83.26 ± 6.55 89.1 ± 2.07 79.4 ± 2.58 82.37 ± 2.31
± 0.08 ± ± ± ±
0.06 0.13 0.1 0.08
± ± ± ± ± ± ± ± ±
0.06 0.37 0.32 0.28 0.22 0.34 0.29 0.31 0.44
Fig. 11. The change of elastic modulus and Vickers hardness with TiO2 doping amount.
Fig. 9. The effect of TiO2 doping on the relative density of the samples.
seems reasonable to use a suitable TiO2-doped HAP for load-bearing implants in clinical applications. Acknowledgements This work was supported by National Key R&D Program of China (2017YFC1103800), Fundamental Research Funds for the Central Universities (WUT: 2018III006CG). References
Fig. 10. The change of flexural strength and fracture toughness with TiO2 doping amount.
transgranular fracture compared to the transgranular and intergranular mixing in pure HAP. Under the condition that the microwave sintering temperature is 1250 °C and the holding time is 40 min, no decomposition of pure HAP is observed. However, under the same sintering conditions, the decomposition of HAP is intensified with the increase of the doping amount of TiO2. HAP/β-TCP multiphase bioceramics with excellent mechanical properties are successfully obtained by doping 4 wt%TiO2 during microwave sintering. Compared with pure hydroxyapatite, microwave sintered 4 wt%TiO2-HAP exhibits good mechanical properties. Average flexural strength of up to 129.91 MPa is measured with 4 wt%TiO2-HAP samples. The maximum fracture toughness of 1.46 MPa m1/2 and the maximum elastic modulus of 94.33 GPa are acquired for TiO2-HAP microwave-sintered at 1250 °C, respectively. It
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