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Vol. 9. Issue 1.
Pages 242-252 (January - February 2020)
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Vol. 9. Issue 1.
Pages 242-252 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.051
Open Access
Processing of Ti50Nb50−xHAx composites by rapid microwave sintering technique for biomedical applications
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Chander Prakasha, Sunpreet Singhb, Animesh Basakc, Grzegorz Królczykd, Alokesh Pramanikc, Luciano Lambertie, Catalin I. Pruncuf,g,
Corresponding author
c.pruncu@imperial.ac.uk

Corresponding author at: Mechanical Engineering, Imperial College London, Exhibition Rd., London SW7 2AZ, UK.
a School of Mechanical Engineering, Lovely Professional University, Phagwara, Punjab 144411, India
b Production Engineering, Guru Nanak Dev Engineering College, Ludhiana 141006, India
c School of Civil and Mech. Eng., Curtin University, GPO Box U1987, Perth, WA 6845, Australia
d Opole University of Technology, 76 Proszkowska St., 45-758 Opole, Poland
e Dipartimento di Meccanica, Matematica e Management, Politecnico di Bari, Via Orabona 4, 70126 Bari, Italy
f Mechanical Engineering, Imperial College London, Exhibition Rd., London SW7 2AZ, UK
g Mechanical Engineering, School of Engineering, University of Birmingham, Birmingham B15 2TT, UK
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Tables (6)
Table 1. Nomenclature of MA-SPS process parameters and corresponding levels considered in this study.
Table 2. Observed mean and S/N ratio values of porosity, compressive strength, and elastic modulus of the Ti50Nb50−xHAx sintered composite according to L-9 OA.
Table 3. Results of ANOVA analysis for the parameter combination yielding the highest structural porosity.
Table 4. Results of ANOVA analysis for the parameter combination yielding the highest compressive strength.
Table 5. Results of ANOVA analysis for the parameter combination yielding the lowest Young’s modulus.
Table 6. Optimal best conditions for individual output response characteristics with confirmation test.
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Abstract

The main objective of this research is to fabricate porous mechanical-tuned (low elastic modulus and high strength) Ti-based composites with improved bioactivity for orthopaedic applications. Another objective is to demonstrate the potential of microwave sintering and temporary space alloying technique to synthesize porous Ti-based composites. In this study, porous Ti50Nb50−xHAx (x=0, 10 and 20) composite was fabricated for orthopaedic applications using a powder metallurgical and rapid microwave sintering (PM-RMS) process. Effects of key PM-RMS parameters on the structural porosity, compressive strength, and elastic modulus of built composite were then analysed. The microstructure, pore characteristics, and mechanical properties were investigated in detail. Using high hydroxyapatite (HA) content (20%), short sintering time (5min), and high compacting pressure (200MPa) appears to be the best condition among those studied in terms of yielding a high degree of structural porosity (21%) and low elastic modulus (25GPa) in the sintered composite. Since size of pores in the synthesized composite is in the range of 20–30μm, structural porosity not only reduces elastic modulus but also enhances bio-activity of sintered composite. The combination of highly porous structure, low elastic modulus, high compressive strength, improved corrosion resistance, and enhanced bioactivity makes porous Ti-Nb-HA composites fabricated by microwave sintering process potential and promising candidates for orthopedic applications.

Keywords:
Orthopaedic implants
Porous Ti50Nb50−xHAx composite
Rapid microwave sintering
Porosity
Compressive strength
Elastic modulus
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1Introduction

Over the last few decades, titanium-based alloys have extensively been used as biomaterials for orthopaedic implants due to their exceptional biomechanical integrity and excellent bio-activity [1]. However, Ti-based alloys have relatively high elastic modulus (110GPa) as compared to bone tissues (10–20GPa). This can lead to stress-shielding that impedes bone remodeling and results in bone resorption and implantation failure [2–4]. To overcome these problems, researchers focused their interest towards the fabrication of porous β-type Ti-alloys [5]. Structural porosity reduces the stiffness of Ti-alloys and results in a reduction of elastic modulus [6,7]. The formation of a β-phase crystalline structure with stabilizing elements (such as Nb and Sn) has also been reported to reduce the elastic modulus of Ti alloys to about 40GPa [8–15]. This porous structure not only solves the problem of stress shielding but also provides suitable conditions for the tissue growth around the implant [16].

Hydroxyapatite (HA, Ca10 (PO4)6(OH)2) can be used as an alloying element to promote osseointegration and bioactivity, and to accelerate the bone in-growth support provided by the implant [17–20]. However, there is still a requirement for better designs and manufacturing strategies to produce porous structures rather than the naturally occurring dense ones, transforming the α-Ti to β-Ti microstructure, and tailoring the mechanical integrity to better match the patient’s host tissue [5]. A number of β-type Ti-alloys using HA and other stabilizing elements can be fabricated, including Ti-HA [21–23], Ti-Nb [12,13,15,24,25], Ti-Nb-Zr [8,9,15], Ti-Sn-Nb [26,27], Ti-Ni 25,29], Ti-Nb-Zr-x(HA) [30], Ti-Nb-HA [31], and Ti-Nb-Ta-Zr-xHaP [32] alloys. Amongst these materials, HA and Nb-based Ti-alloys gathered much interest from those developing orthopaedic implants, because they possess both low elastic modulus and high HA and Nb content, improving the corrosion resistance of the alloys [21,28,30,31].

Powder metallurgy (PM) is the most widely used method for building porous scaffolds, as it is both cost-effective and allows more control over pore size, pore density, and morphology; it also achieves near-net shape production [33]. PM consists of alloying metallic powders and then sintering the alloyed powder mixture to facilitate the customization of its material composition, mechanical properties, and architectural geometry [34,35].

In earlier decades, various sintering techniques such as convention sintering [36], hot isostatic pressing (HIP) [37], and self-propagating high-temperature synthesis (SHS) [38,39] were utilised. However, these techniques suffer from several disadvantages in terms of high temperature requirements and long sintering times. These parameters influence the uniformity of the microstructure and usually result in coarse grain growth that counters the desired mechanical properties and biomedical performance [40,41]. More recently, microwave sintering (MS) processes produced more uniformly controlled microstructures and porosity in Ti-based alloys [42]. The heating gradient (including heating temperature and heating rate) and time determine microstructure and grain growth [43,44]. Furthermore, since the microwave heating process involves rapid transformation of electromagnetic energy into heat, fine grain growth and a uniform microstructure are easy to be obtained, thus creating a densified, compact material with lower porosity defect rates [45–48]. Microwave sintering is therefore very suited for sintering metallic and ceramic powders in biomedical and other industrial applications [49–52].

Choy et al. [53] synthesized porous Ti-CaP composites using mechanical alloying (MA) and the MS process for biomedical applications. They found that the sintering of the composite including HA formed beta-tricalcium phosphate (β-TCP) and calcium titanate (CaTiO3). Furthermore, they reported that the as-fabricated composites exhibit high porosity and excellent mechanical properties (compressive strength and elastic modulus of 212MPa and 12GPa, respectively). These mechanical properties and the bone-like apatite growth of the Ti-CaP composite confirmed its suitability for the orthopaedic industry [54]. Bovand et al. further synthesized Ti-HAX (X=10–30%) alloys using MS to study the effect of HA content on structural porosity and hardness of the resulting composite, and higher proportions of HA were found to increase the porosity and hardness of the composite [55].

Biomimetic low-modulus porous Ti-6Al-4V/TiC composite was also developed using the MA and MS process for biomedical applications. The developed composite exhibited a 27% porous structure, 90MPa compressive strength, 2.9GPa compressive modulus, and 556HV micro-hardness. This suggested that the material could potentially be used for load bearing applications [56]. Xu et al. fabricated porous Ni-Ti shape memory composite using the MS technique and studied the effect of pore size on its microstructure, mechanical properties and corrosion resistance [57]. The application of MS for the synthesis of Ti-Nb shape memory composite was also studied, and parametric optimization of the microwave process was carried out to enhance mechanical properties of sintered alloys [58]. Xu et al. further fabricated porous Ti-15MO alloys using PM and MS, achieving 16–50% porosity. However, thery reported that this high degree of structural porosity decreased the mechanical properties of the resulting composite [59].

The available literature does not indicate how to select rapid microwave sintering processes to optimize mechanical properties, geometry and biological performance of Ti50Nb50-xHAx (x=0, 10 and 20). In this work, much effort was put in assessing effects of process parameters on structural porosity, compressive strength, elastic modulus and morphological features of the developed composite. In addition, as-prepared samples were characterized in order to assess microscopic changes occurring in material processing.

2Material and methods

Powders of titanium-Ti (99% purity, ≤25μm), niobium-Nb (99% purity, ≤25μm), and hydroxyapatite-HA (99% purity, ≤2μm) were procured from Intelligent Materials Private Limited, India; these were used to synthesise the Ti50Nb50-xHAx composite. Fig. 1 shows the morphology of the raw powders before blending. The powders were mixed to achieve the desired composition of Ti 50%wt, Nb 50-x %wt, and HA x %wt (x=0, 10, and 20%). The powder mixture was blended in a high emery planetary ball mill (Fritsch, Pulverisette 7) using a tungsten carbide vial and balls, per the processes adopted in previous research work [60–62]. The powder-to-balls ratio was 1:10. Blending of the composite composition was performed at 200rpm for about 12h at ambient conditions. In order to prevent agglomeration and cold welding of the powders, 2% stearic acid was also added to the mixture. After blending, the powder mixture was then compacted into performs of 10×20 (φ×L) mm, at variable pressures, using a mounting. Table 1 presents the nomenclature of the process parameters and the corresponding levels considered in the experiments.

Fig. 1.

SEM micrographs of raw powders before mechanical alloying (a) Ti, (b) Nb, and (c) HA.

(0.55MB).
Table 1.

Nomenclature of MA-SPS process parameters and corresponding levels considered in this study.

Process parameters  Symbol  Units  Levels 
HA content  HA  wt.%  0, 10, 20 
Sintering time  Ts  Min  5, 10, 15 
Compacting pressure  Pc  MPa  100, 150, 200 

The green compact was then sintered using the rapid microwave sintering process. The experimental set-up for rapid microwave sintering process was developed in-house using a domestic IFB-microwave of 25L capacity (1400W, 2.45HZ). The sintering was carried out for 5min and the surface temperature reached 1500°C. Fig. 2 presents a schematic diagram of the in-house experimental setup developed for rapid microwave sintering, while Fig. 3 presents the process steps for the synthesis of the Ti50Nb50-xHAx composite and the sintered dense and porous alloys.

Fig. 2.

Schematic of the in-house developed experimental setup of rapid microwave sintering process.

(0.22MB).
Fig. 3.

Fabrication route for synthesis of the Ti50Nb50-xHAx (x=0, 10, 20) composite and sintered dense and porous alloys.

(0.9MB).

After sintering, microstructure, morphology, elemental and phase composition, structural porosity, and mechanical properties of the sintered Ti50Nb50-xHAx composite were analysed in detail. Field emission scanning electron microscopy (FE-SEM-7600F by JEOL) and energy dispersive spectroscopy (EDS) were used for the morphological and elemental composition investigations. The working distance for FE-SEM and EDS analysis was 13–15mm. X-ray diffraction (XRD; X’pert-PRO) was used for evaluating phase composition for the setting of CuKα radiation ∼45kV, current intensity ∼40mA, incident radiation ∼2θ. The structural porosity percentage (P) was determined using the formula

P=(1m/v*ρ)×100
where, m, v and ρ are the mass, volume, and theoretical density of the sintered composite, respectively.

Compressive strength and elastic modulus of the sintered composite were determined via compression testing. Cylindrical specimens of dimensions 5×10mm (Diameter×Height) were cut from the sintered alloys. A uniaxial compression test was then conducted at room temperature on the hydraulic UTM testing machine (Model-25, Tinius Olsen) at 0.05mm/min loading rate. The elastic modulus was calculated based on the initial stage of the stress-strain curve using dedicated software. Deformation was determined from load-displacement curve provided by the testing machine.

3Results and discussion

The results obtained from the RMS process, based on Taguchi’s method L-9 OA, are summarized in Table 2. Structural porosity, compressive strength and elastic modulus were measured to quantify the effects of rapid microwave sintering (RMS) process parameters. In Taguchi’s method, it is possible to convert output responses into a signal/noise (S/N) ratio serving as output response measurement index. For orthopaedic implant applications, the composite should possess a highly porous structure, low elastic modulus and high compressive strength. Therefore, structural porosity was analyzed in terms of ‘higher-the-better’, elastic modulus was set as ‘lower-the-better’, and compressive strength was a ‘higher-the-better’ type output response characteristic for the development of a Ti50Nb50-xHAx (x=0, 10, 20) composite. Using Taguchi’s method, the optimal combination of process parameters was determined for the desired level of structural integrity including a highly porous composite structure, good compressive strength, and a low elastic modulus. In the present study, five replicas were performed for each selected combination of parameters.

Table 2.

Observed mean and S/N ratio values of porosity, compressive strength, and elastic modulus of the Ti50Nb50−xHAx sintered composite according to L-9 OA.

Sr. no  HA content  Sintering pressure (MPa)  Sintering time (Min)  PorosityCompressive strength (MPa)Young’s modulus (GPa)
        Mean  S/N ratio  Mean  S/N ratio  Mean  S/N ratio 
100  13.00  21.99  751.4  57.52  50  −33.98 
150  10  5.00  13.89  500.3  53.98  64  −36.12 
200  15  8.00  18.45  258.1  48.24  60  −35.56 
10  100  10  12.00  21.34  823.7  58.32  54  −34.65 
10  150  15  5.00  14.50  463.8  53.33  70  −36.90 
10  200  17.00  25.50  582.6  55.31  34  −30.63 
20  100  15  14.00  22.63  391.4  51.85  50  −33.98 
20  150  16.00  23.95  522.3  54.36  40  −32.04 
20  200  10  20.00  25.33  300.9  49.57  36  −31.13 
3.1Effects of process parameters on sintered material properties3.1.1Analysis of structural porosity

Fig. 4 presents the variations of average structural porosity for Ti50Nb50-xHAx (x=0, 10, 20) sintered composite with respect to RMS process parameters. It can be seen that average structural porosity tends to increase for higher HA contents.

Fig. 4.

Effect of process parameters on structural porosity. (A1, A2, A3) denote HA content levels; (B1, B2, B3) denote sintering time levels; (C1,C2.C3) denote compacting pressure levels.

(0.13MB).

For x=0 (no HA), densification was extreme and average structural porosity decreased to 8.6%. The lowest structural porosity was 5% at HA=0%, sintering pressure=150MPa, and sintering time=10min. The 20% HA content resulted in a much more porous composite (Ti50Nb30HA20) with an average structural porosity of 16.66%. The highest structural porosity was 20% at HA=20%, sintering pressure=200MPa, and sintering time=10min.

This may be caused by the concourse of two reasons: (i) HA particles are unstable at high temperature and disintegrate during sintering; (ii) HA particles are sacrificed and eroded during grinding and mechanical polishing processes [30]. In general, structural porosity decreased as compacting pressure and sintering time increase. These are the driving forces that remove pores and consolidate the mixture, respectively [31,32]. The average structural porosity hence followed this trend. At high temperature, HA is transformed into β-TCP and CaO, thus increasing porosity. Table 2 shows that average structural porosity of the composite with no HA decreased from 13% to 5% by increasing sintering time from 5min to 10min, but then increased to 8% for the very high sintering time. This happened because sintering temperature increases with sintering time, burning the green compact and making structure more porous. Average structural porosity decreased from 16% to 9% as compacting pressure passed from 100MPa to 200MPa. As per the observed results from the main effect and S/N ratio plots, the third level of HA content (HA=20%), the third level of sintering pressure (Pc=200MPa) and shortest sintering time (Ts=5min) yield the highest structural porosity (22.56%) in the Ti50Nb50-xHAx (x=0, 10, 20%) sintered composite. Significance level and contribution of each process parameter to the optimal amount of structural porosity were determined using ANOVA. Table 3 shows that HA content is the most significant factor affecting structural porosity (44.53% contribution), followed by compacting pressure (28.53% contribution) and sintering temperature (26.93%). Statistical errors related to unattributed contribution have a marginal effect on variations in porosity (0.004% contribution).

Table 3.

Results of ANOVA analysis for the parameter combination yielding the highest structural porosity.

Source  DF  Seq-SS  Adj-SS  Adj-MS 
HA  99.55  99.55  49.77  112  0.009  44.53 
Pc  62.88  62.8  31.44  70.75  0.014  28.53 
Ts  60.22  60.22  30.11  67.75  0.015  26.93 
Residual error  0.88  0.88  0.44      0.004 
Total  223.55           
3.1.2Analysis of compressive strength

Fig. 5 presents the variation of compressive strength for Ti50Nb50-xHAx (x=0, 10, 20%) sintered composite with respect to RMS process parameters. The compressive strength of Ti50Nb50-xHAx (x=0, 10, 20%) sintered composite became higher by increasing HA content up to 10% but it decreased for the very high HA content. This occurred because HA powder has a small atomic diameter and crystal size, allowing it to easily fill the gaps formed by the Ti and Nb powders, and finally improving mechanical properties of the composite. However, under a combination of higher compacting pressure and longer sintering time, HA changes its crystallographic structure and transforms into β-TCP and CaO, thus forming a porous structure. This limits improvements in mechanical properties.

Fig. 5.

Effect of process parameters on compressive strength. (A1, A2, A3) denote HA content levels; (B1, B2, B3) denote sintering time levels; (C1, C2, C3) denote compacting pressure levels.

(0.14MB).

The maximum compressive strength (876MPa) of Ti50Nb50-xHAx sintered composite was achieved for x=10% HA content, 100MPa compacting pressure and 5min sintering time. ANOVA results presented in Table 4 indicate that compacting pressure was the most significant factor affecting compressive strength (40.06% contribution), followed by sintering time (33.74% contribution) and HA content (25.15% contribution). Errors again affect variations in compressive strength just marginally (1.04%).

Table 4.

Results of ANOVA analysis for the parameter combination yielding the highest compressive strength.

Source  DF  Seq-SS  Adj-SS  Adj-MS 
HA  71,849  71,849  35,924  24.55  0.039  25.15 
Ts  114,427  114,427  57,214  39.1  0.025  40.06 
Pc  96,370  96,370  48,185  32.93  0.029  33.74 
Residual error  2927  2927  1463      1.03 
Total  285,572           
3.1.3Analysis of elastic modulus

Fig. 6 presents the variation of elastic modulus for Ti50Nb50-xHAx (x=0, 10, 20%) sintered composite with respect to RMS process parameters. Elastic modulus significantly depends on the structural porosity developed in the sintered composite. Table 2 shows that Young's modulus of the sintered composite decreases with the increase in HA content. This is due to the fact that for higher HA contents, structural porosity increases thus reducing the stiffness of the structure. The elastic modulus decreased overall from 60GPa to 40GPa (these values correspond to the average of measured values for HA=0 and HA=20%, respectively).

Fig. 6.

Effect of process parameters on elastic modulus. (A1, A2, A3) denote HA content levels; (B1, B2, B3) denote sintering time levels; (C1, C2, C3) denote compacting pressure levels.

(0.17MB).

Looking in more detail at the data of Table 2, it can be seen that elastic modulus tends to increase with sintering time although some oscillations in data trend were seen due to the complexity of the process. Since HA particles remain stable at 1050°C, higher sintering temperature and compaction pressure reduce structural porosity thus increasing density of sintered composite. Longer sintering times make surface temperature increase. Hence, HA becomes unstable and disintegrates, thus leading to have a more porous structure with low elastic modulus. Elastic modulus increased for higher compacting pressures. Enhancing compactness of the green compact led to have a highly dense compact sintered composite. Similar behaviour was recently reported in the literature for the Ti-Zr-Nb system [63].

The lowest value of elastic modulus (25GPa) of sintered material observed in the present experiments was obtained for HA=20%, Ts=5min and Pc=200MPa. Table 5 shows the ANOVA results for compressive strength, suggesting that compacting pressure is the most significant factor affecting elastic modulus variation (41.65% contribution), followed by HA content (31.68% contribution) and sintering time (25.74% contribution). Statistical error again makes little contribution (0.92%) to elastic modulus variations.

Table 5.

Results of ANOVA analysis for the parameter combination yielding the lowest Young’s modulus.

Source  DF  Seq-SS  Adj-SS  Adj-MS 
HA  99.56  99.56  49.78  34.46  0.03  31.68 
Ts  80.89  80.89  40.44  28.00  0.03  25.74 
Pc  130.89  130.89  65.44  45.31  0.02  41.65 
Residual error  2.89  2.89  1.44      0.92 
Total  314.22           
3.2Analysis of surface morphology and elemental and phase composition

As mentioned in the previous sections, optimal settings of process parameters to obtain maximum structural porosity, highest compressive strength, and lowest elastic modulus, correspond to different characteristics of the sintered composite. Table 6 outlines the different types of alloys fabricated using the optimal settings with Taguchi’s method. Statistical predictions were confirmed by experimental measurements.

Table 6.

Optimal best conditions for individual output response characteristics with confirmation test.

Sr. no  Input process parametersOutput responsecharacteristics
  HA  Ts  Pc  PredictedMeasured
        Porosity (%)  Compr. strength (MPa)  Young’s modulus (GPa)  Porosity (%)  Compr. strength (MPa)  Young’s modulus (GPa) 
20  200  21  383.16  24.55  19  400  25 
10  100  15  876.63  43.55  17  880  46 
10  15  150  468.93  68.55  21  475  70 

Three types of alloys were fabricated with the various HA percentage contents, as presented in Fig. 7. A highly dense composite (Ti50Nb50) was fabricated without adding any HA content.

Fig. 7.

(a) SEM image of the Ti50Nb50 composite; (b) EDS spectrum of the Ti50Nb50 composite; (c) SEM image of the Ti50Nb40HA10 composite; (d) EDS spectrum of the Ti50Nb40HA10 composite; (e) SEM image of the Ti50Nb30HA20 composite; (f) EDS spectrum of the Ti50Nb30HA20 composite.

(1.76MB).

The possible best conditions for the synthesis of dense Ti50Nb50 composite included the first level HA content (HA=0%), third level sintering time (Ts=15min), and second level compacting pressure (Pc=150MPa); 2–5% structural porosities were obtained in the synthesized composite. Fig. 7(a–b) shows the morphology and associated EDS spectrum of the resulting Ti50Nb50 composite, respectively. The metallographic structure is dense (see Fig. 7(a)) and pore size ranged between 2 and 5μm. EDS analysis revealed the presence of Ti and Nb elements in the sintered Ti50Nb50 composite (see Fig. 7(b)). These results confirmed that no contamination occurred during element alloying or ball milling processes. When HA content was added in the Ti50Nb50-xHAx (x=0, 10, 20%) composite porous structure was obtained. The possible best condition for the synthesis of porous Ti50Nb40HA10 composite was the second level of HA content (HA=10%), third level of sintering time (Ts=15min), and third level of compacting pressure (Pc=200MPa). Fig. 7(c–d) shows the morphology and EDS spectrum of Ti50Nb40HA10 composite. It can be clearly seen that the structure was porous and a high degree of structural porosity (18.22%) was observed. The size of pores in the synthesized composite was in the range of 5–10μm. The EDS spectrum shows the presence on Ti and Nb elements along with Ca, P, and O elements analysis, which confirm that HA disintegrated at high sintering temperature and compacting pressure. Highly porous composite (Ti50Nb30HA20) was synthesized when 20% HA content was alloyed. The possible best condition for the synthesis of Ti50Nb30HA20 composite was the third level of HA content (HA=20%), first level of sintering time (Ts=5min), and third level of compacting pressure (Pc=200MPa). Fig. 7(e–f) shows the morphology and EDS spectrum of Ti50Nb30HA20 composite. It can be clearly seen that the structure was porous and a high degree of structural porosity (22.55%) was observed. The size of pores in the synthesized composite was in the range of 20–30μm. The high intensity of Ca and P in the EDS spectrum confirms the presence of content of HA in the sintered composite.

Fig. 8 shows the XRD pattern of the sintered Ti50Nb50-xHAx (x=0, 10, 20) alloys. Peaks of Nb and Ti β-phases can clearly be observed for all alloys. The XRD pattern of sintered Ti50Nb50 composite presents α-Ti, β-Ti and Nb phases only. The Nb element is a β-phase stabilizer, assisting the transformation of α-Ti to β-Ti. However, HA is unstable at high-temperature, leading to crystallographic and morphological changes in the structure. The alloying of HA content thus obstacles transformation of α-Ti to β-Ti as HA surrounds the Ti and Nb powder particles, generating a lower diffusion rate between the alloying elements. From the XRD pattern of the Ti50Nb30HA20 sintered composite, it can be seen that the intensity of β-phases is lower than those of Ti50Nb40HA10 and Ti50Nb50 alloys. Moreover, at higher sintering temperatures, the HA disintegrated into Ca, P, and O elements, thus generating various biocompatible phases. Ca3(PO4)2, Ti5P3, and CaO formed during the sintering of HA with Ti elements is thought to benefit osseointegration performance [19,30–32].

Fig. 8.

XRD patterns of sintered Ti50Nb50, Ti50Nb40HA10 and Ti50Nb30HA20 alloys.

(0.18MB).
4Conclusions

In this study, porous low elastic and high strength Ti50Nb50-xHAx (x=0, 10, 20%) alloys were successfully synthesized using the powder metallurgical process. The study also provided early evidence of the influence of several process variables on the properties of synthesized composite. Higher HA content (20%), short sintering time (Ts=5min), and high compacting pressure (Pc=200MPa) were found to be the best optimal conditions, where the sintered composite possessed high degree of structural porosity (21%), good compressive strength (383MPa), and low elastic modulus (25GPa). The high degree of structural porosity reduced stiffness but degraded mechanical properties of the composite, in particular compressive strength and elastic modulus. HA decomposed at high sintering temperature and compacting pressure, forming various biocompatible phases in the Ti50Nb50-xHAx composite. Various biocompatible phases such as Ca3(PO4)2 (known as TCP), Ti5P3, and CaO were formed during the sintering process. These are beneficial to the promotion of tissue growth and improve osseointegration between such composite-based implants and natural bone.

In summary, the combination of highly porous structure, low elastic modulus, high compressive strength, improved corrosion resistance, and enhanced bioactivity makes the porous Ti-Nb-HA composites fabricated in this research by microwave sintering process potential and promising candidates for orthopedic applications.

Conflict of interest

The authors have declared no conflict of interest.

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