Dilatometric and microstructural study of particle and functionally graded composites based on hydroxyapatite and crystalline bioglass

Science of Sintering, 55 (2023) 289-306 ________________________________________________________________________ _____________________________ *) Corresponding author: [email protected] https://doi.org/10.2298/SOS221028017D UDK: 53.086; 666.3.019; 622.785 Dilatometric and Microstructural Study of Particle and Functionally Graded Composites Based on Hydroxyapatite and Crystalline Bioglass Daniel Drdlik1,2*), Katarina Drdlikova1, Karel Maca1 1CEITEC BUT, Brno University of Technology, Purkynova 123, 612 00 Brno, Czech Republic 2Institute of Materials Science and Engineering, Brno University of Technology, Technicka 2, 616 00 Brno, Czech Republic Abstract: Hydroxyapatite (HA) and bioglass (BG) ceramics have become of prime importance in bone tissue engineering. Besides the appropriate composition, the microstructure of bone replacement plays a crucial role. In the present work, particle composites and functionally graded material (FGM) based on HA and BG prepared by electrophoretic deposition were thoroughly characterised in terms of the preparation method, sintering process, phase composition and microstructure. The sintering was monitored by high-temperature dilatometry in two directions, the sintering rates were calculated, and the overall sintering process was discussed. The SEM showed the continuous change in the microstructure of FGM with gradual interconnected porosity favourable for bio-applications. The fundamental fractographic analysis proved the crack development in FGM related to the sintering process, and the recommendations for the reduction of the crack development were given. The phase transformations during thermal treatment were analysed using X-ray diffraction analysis and deeply discussed. Keywords: Hydroxyapatite; Bioglass; Functionally graded material; Sintering; Microstructure. 1. Introduction Functionally graded materials (FGMs) are a special kind of composite in which the material properties vary smoothly and continuously from one surface to the other. The variation of the properties is attained by the gradual change of the material composition. FGMs attract great attention today in the materials science and engineering society because conventional composites do not achieve the necessary properties appropriate for many challenging applications [1]. FGMs are successfully applied in automobile, military, energy, machinery, aerospace or medical areas where a continuous chemical and structural change over the material is demanded [2]. In particular, regarding the medical area, the most important examples of FGMs are directly located in a human body. Bones or teeth have a gradual change in their structure and properties. The most common strategy for hard tissue regeneration after some kind of trauma is to heal the bone by autograft or allograft. In the case of allograft, the utilisation of synthetic bioactive material with low risk of immune reaction is a highly discussed topic in the D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 290 literature. Hydroxyapatite (HA) is being considered as a candidate for hard tissue substitution because of its 65% chemical similarity of the mineral constituent with the natural bone and excellent biocompatibility, bioactivity and cytocompatibility [3, 4]. Although the biocompatibility of HA is high, it can be improved further. It has been proposed that the addition of silicon to the apatite structure influences the material in biological environments [5]. The enhancement of HA bioactivity is due to the substitution of SiO44for PO43ions that affect HA surface chemistry or local bioavailable Si release [6]. The favourable results of the substitution were reported by several authors [7-9]. Therefore, the using of Bioglass® (BG) together with HA seems to be justified. Moreover, the addition of more bioactive BG [10] into HA composite can accelerate its dissolution rate because the dissolution rate of HA is relatively low [11], corresponding to ionic substitutions into the HA lattice and crystallinity [12]. Many studies have been made to describe HA/BG composite systems [5, 13-16]. The authors mainly focused their efforts to show manufacturability, microstructure and properties of the composites. However, it is hard to maintain the BG in an amorphous structure. The BG has a high crystallisation ability and it crystallises by a surface crystallisation mechanism, so that the material is highly crystalline even a low temperature (600-750 °C) [17]. Crystalline phase may affect its bioactivity and dissolution profile [18, 19]. Therefore, tailoring of manufacturing method with applied pressure [13, 14, 16] or thermal conditions [13, 14] has to be done. Probably for these very reasons, only a few studies are available in the literature aiming on FGM from HA and BG [20-24]. Most of these studies deal preferably with coatings instead of bulk material. Moreover, all these works report specific types of multi-layered functionally graded HA/BG rather than FGM with continuous microstructural change. It is attributed to the fabrication techniques used, i.e. Spark Plasma Sintering (SPS) [20], plasma spraying [21, 22], pulsed laser deposition [23] or enamelling technique [24]. Another suitable method for preparation of non-composite [25, 26] or composite materials (particle [27] and fibre reinforced composites [28] or laminates [29]) including FGM [27, 30] is electrophoretic deposition (EPD). It is a versatile technique allowing fabrication of complicated objects from suspensions containing charged ceramic particles through their movement in an electric field applied between two electrodes. Contrary to previously mentioned methods, EPD offers the possibility of preparing FGM with continuous change of composition without any sharp phase interphase. Although, the EPD is widely used to prepare HA and BG composites [31, 32], but it has not yet been used to fabricate the FGM from these biomaterials. However, products of electrophoretic deposition are subsequently processed by conventional sintering, therefore crystallisation of BG in HA/BG composite is inevitable. Crystallisation of BG could consequently be suppressed by combining EPD with following SPS or Hot Isostatic Press methods (HIP). Another challenging issues associated with the HA/BG FGM materials is coefficients of thermal expansion (CTE) of HA and BG and sintering shrinkage if the prepared green bodies do not have the same density and undergo phase changes during sintering. In the first case, the CTE of HA and BG are 15.9 and 9.7 ×10−6 K−1, respectively, indicating the possible development of cracks during cooling from the sintering temperature. In the second case, the structural change during sintering, i.e. variation of grain or pore morphology and size, phase transformations are accompanied by volume change (a shrinkage). Generally speaking, the shrinkage of BG is considerably high (60 % at 1050 °C [17]) compared to HA (13 % at 1050 °C [33]), which may lead to crack development in the FGM as well. This situation is very well documented in the work of Luginina et al. [20]. Therefore, a deep description and understanding of the sintering process of FGM based on HA and BG are needed. Present work as the first describes the preparation of bulk FGM based on HA and BG with continuous phase change using EPD. To accomplish this goal, the various bulk (several millimetres thick) HA/BG particle composites had to be prepared and evaluated as well. Since D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 291 conventional sintering of the prepared composites was chosen, the article brings original information about densification, crystallisation of bioglass and phase changes in HA/BG and FGM composites during sintering determined by high-temperature dilatometry and X-ray diffraction analysis. 2. Materials and Experimental Procedures Two commercial powders, i.e. hydroxyapatite (HA, 99,95%, Nanografi Nanotechnology, Turkey) and bioglass 45S5 (BG, Schott AG, Germany) with mean particle size of 50 nm and 10 µm, respectively, were used to prepare functionally grade and particle composite materials as well as standards (pure HA or BG). The fine HA particles were chosen in order to use the lowest possible sintering temperature. The suspensions designed to prepare FGM consisted of 15 wt % HA or BG, 0.85 wt % of monochloroacetic acid (Merck, Germany) and 84.15 wt % of 2-propanol (Lachner, Czech Republic). The suspensions related to fabrication of the two standards and three particle composites had the same compositions of ingredients, but the ceramics loads had different weight ratios of powders (HA:BG): 100:0, 75:25, 50:50, 25:75, 0:100. After mixing all components, the suspensions were homogenised mechanically and ultrasonically for 30 minutes. The electrical conductivity of the suspensions was measured using a SevenCompact Conductivity S230 conductometer (Mettler Toledo AG, Switzerland). Fig. 1. Scheme of the flow rate of suspension change during fabrication of FGM. The EPD apparatus was assembled from a glass cell, two stainless steel electrodes placed in vertical positions with 26 mm distance between them and a stable power source with applied constant current of 5 mA (current density of 0.27 mA/cm2). The depositions of the pure ceramics and particle composites were interrupted each 5 minutes and the suspensions were stirred to avoid a sedimentation of the particles at an electrophoretic cell bottom (described elsewhere [25, 26, 28]). At the same time, a weight yield on the deposition electrode was monitored to obtain data about deposition kinetics required for preparation of FGM. The deposition time was up to 40 minutes to obtain robust set of kinetic data and thick enough deposits. Based on that, the FGM could be prepared using the continuous mode of EPD. The suspension containing only BG was drained continuously from the glass cell, and the fresh suspension with HA was fed into at the same rate. The flow rate was increased during the deposition process to respect the evolution of the deposition kinetics given by D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 292 materials change in the suspension, thickness and resistivity of the growing deposit. The schemes of the EPD apparatus and flow rate during the deposition (23 minutes) are given in Fig. 1. The electrodes covered by deposits were allowed to dry in a desiccator for 24 hours at 25 °C. After then the several millimetres thick deposits were removed from electrodes, and the organic residues were burnt out at 600 °C for 1 hour. The samples were sintered at 1050 °C for 2 hours with heating rate of 10 °C/min, whereas the linear dimensional changes were recorded using a high-temperature dilatometer L70/1700 (Linseis, Germany) in longitudinal (parallel with electrode) and transversal (in the direction of deposition) direction. The samples for longitudinal direction had the shape of the prismatic bar with dimensions (length × width × thickness): 10×4×1-5 mm. The samples for transversal direction had the shape of a cylinder with dimensions (diameter × thickness): 5×1-5 mm. The thickness of the measured sample depended on the thickness of the deposit. The theoretical shrinkage based on the knowledge of green (ρgb) and final (ρ) densities of the ceramic body was calculated using equation [34]: 𝜀𝜀𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 1 −�𝜌𝜌𝑔𝑔𝑔𝑔 𝜌𝜌 3. (1) The density and pore size distribution of the samples were determined using the Archimedes method (EN 623-2) and an automatic pore size analyser (PoreMaster 60, Quantachrome Instruments, USA) equipped with low (1-340 kPa) and high (0.1-330 MPa) pressure stations with a measurable range of pores of 0.004-1000 µm, respectively. The phase composition of the samples was determined using X-ray diffraction analysis (XRD, SmartLab, Rigaku, Japan). Based on the phase compositions, the theoretical density of individual composites was calculated. The microstructures were observed using a Lyra3 scanning electron microscope (SEM, FEG/FIB, Tescan, Czech Republic). 3. Results and Discussion 3.1 Electrophoretic deposition To determine a correct manufacturing process for preparation of FGM, the deposition kinetics of the pure HA and BG ceramics and their mixtures were studied first. Fig. 2a represents dependence of electrical conductivity on BG amount in the suspension. It is clearly evident that the electrical conductivity of the suspension increased with the BG amount. The suspension containing HA powder only had the electrical conductivity of 0.2 µS/cm. The addition of 25 and 50 wt % of BG in suspensions led to the electrical conductivity increase to 4.8 and 11.5 µS/cm, respectively, which was the highest measured value. The subsequent increase in BG concentration resulted in the electrical conductivity of the suspensions on the similar level. This behaviour can be explained in two ways. An adsorption capacity of individual material compositions in suspensions had a different ability to bind the MCCA molecules on the surface of ceramic particles and thus cause their dissociation [25]. The second phenomenon that may have occurred in BG-containing suspensions is slight dissolution of the BG due to the presence of MCAA because the conductive ion release from BG occurs significantly faster at lower pH [35]. The electrical conductivity of the suspensions markedly affected the EPD kinetics. The kinetic curves are given in Fig. 2b. It can be seen from the figure that the fastest deposition was achieved at the lowest electrical conductivity of the suspension. A high voltage is generated between the electrodes in suspension with low electrical conductivity, which causes the particles to move faster. Moreover, this also results in a very rapid depletion of the ceramic particles from the suspension and the deposition is completed in a relatively short time. In the extreme case, the deposited particle applied pressure on the surface of the D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 293 already created deposit, which can be separated from the electrode due to the development of cracks [36, 37]. In our case, HA:BG 100:0 and HA:BG 75:25 depositions had to be terminated soon, i.e. 10 and 15 minutes. The results described in Fig. 2b also corresponds to the measured thickness of the deposits, which decreased with increasing electrical conductivity of the suspension from 5.3 mm (HA:BG 100:0) to 0.9 mm (HA:BG 0:100). It is clear from the figure that the kinetic curves obtained from suspensions containing HA:BG 50:50 to HA:BG 0:100 are very similar, which corresponds to the measurement of the electrical conductivity shown in Fig. 2a. Fig. 2. Dependence of electrical conductivity on bioglass amount in suspension (a) and dependence of deposited weight on time of deposition during preparation of pure ceramics and HA:BG composites (b). Based on the knowledge of the deposition kinetics for various HA and BG compositions it was possible to estimate an increment of ceramic deposit on the electrode (described in detail for different materials in [29]) and subsequently adjust the correct flow rate of the suspension during the preparation of FGM. 3.2 Physical properties of green bodies The dependence of relative density on BG amount in the pure and composite green bodies is shown in Fig. 3a and Table I. The relative density increased with concentration of the BG in the HA structure from 25.8 % t.d. up to 47.5 % t.d. It is evident that the value of the relative density was affected mainly by the value of the electrical conductivity of the suspensions from which the pure HA and BG ceramics and HA:BG composites were prepared (see Fig. 2a). The higher values of the electrical conductivity in the suspensions led to the slower deposition rates resulting to better arrangement of ceramic particles to the most advantageous positions in the deposit, thereby increasing the density of the samples [38]. D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 294 Fig. 3. Dependence of relative density on bioglass amount in annealed deposits (a) and pore size distribution of pure ceramics and HA:BG composites (b). Tab. I Summary of relative densities and shrinkages of HA:BG composite and FGM samples measured in longitudinal (L) and transversal (T) directions. Sample Relative density (%) Shrinkage (%) ρgb (at 600 °C) ρL (at 1050 °C) ρT (at 1050 °C) εL calc εL εT calc εT 100:0 25.8 ±0.1 38.1 ±0.1 37.9 ±0.2 12.2 14.2 12.0 16.1 75:25 36.7 ±0.1 50.7 ±0.0 48.9 ±0.2 10.2 16.4 9.1 22.1 50:50 44.0 ±0.1 54.0 ±0.1 54.0 ±0.1 6.6 19.1 6.6 24.2 25:75 43.5 ±0.1 95.0 ±0.3 92.1 ±0.7 22.9 32.6 22.1 38.6 0:100 47.5 ±0.1 92.7 ±0.4 91.4 ±0.6 20.0 59.0 19.6 67.7 FGM - - - - 21.2 - 38.9 Green density of the samples did not change linearly with concentration. An explanation could be the different size of the HA and BG particles causing different porosity of deposits, whereby the small HA particles could fill the large pores between the big BG particles and thus reduce the overall porosity of the composite. The pore size distribution of these samples is displayed in Fig. 3b. The pure HA ceramic sample (HA:BG 100:0) showed a trimodal pore distribution with most frequent pore sizes of 0.04, 0.08 and 0.16 μm. As the content of the BG in the HA increased, the most frequent pore sizes increased as well. The HA:BG 75:25 composite also showed a trimodal pore size distribution with most frequent pore sizes of 0.03, 0.13 and 0.25 μm. Bimodal pore size distributions with most frequent pore sizes of 0.15, 0.21 μm and 0.19, 0.36 μm were measured for HA:BG 50:50 and HA:BG 25:75 samples, respectively. The measurement of HA:BG 0:100 pure ceramics showed an unimodal D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 295 pore size distribution with most frequent pore size of 1.21 μm. In contrast to the pure and composite materials, a broad distribution of pores in FGM was determined. This result can be expected as a superposition of performed measurements on the aforementioned samples. The broad size distribution in FGM may imply significant influence on physical, microstructural and mechanical (not studied here) properties. 3.3 Dilatometric evaluation The sintering shrinkage (ε) of standards, HA:BG composites and FGM, measured in the longitudinal direction, is shown in Fig. 4a. An enormous difference can be seen between sintering shrinkage of pure HA (HA:BG 100:0 sample) and BG (HA:BG 0:100 sample) in both longitudinal and transversal directions. The temperature 1050 °C was selected not to exceed the melting point of the BG. At this temperature, the shrinkage of the BG in longitudinal direction was 59.0 %. A very similar result was reported before in the work of Bretcanu et al. [17]. Table I shows the calculated shrinkage values using Eq. 1. It should be noted that this equation is valid for isotropic shrinkage. Despite the obvious anisotropy of the studied materials, the calculation was performed in individual directions in order to demonstrate the contribution of more than one process to shrinkage. The calculated sintering shrinkage for pure BG is 20.0 % (see Table I) indicating the existence of more internal processes during thermal treatment including first and second glass transition, glass in glass phase separation and crystallisation processes [39, 40]. It is clear from the Table 1 that these processes were applied in all studied materials. Contrary to BG, the shrinkage of the HA was only 14.2 % as a consequence of the low sintering temperature and the low value of green body density. The shrinkage curve of the FGM was close to the measured HA:BG 50:50 composite with similar final shrinkages of 19.1 % and 21.2 %, as could be expected for correctly designed FMG. Fig. 4. Linear shrinkage of standards, HA:BG composites and FGM in (a) longitudinal and (b) transversal direction; sintering rate of standards, HA:BG composites and FGM in (c) longitudinal and (d) transversal direction. D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 296 The sintering shrinkage of standards, HA:BG composites and FGM, measured in transversal direction, is shown in Fig. 4b. It can be seen from Fig. 4b and Table 1 that shrinkage is slightly higher in the transversal direction than in the longitudinal direction for all measured samples. The visibly different sintering shrinkages in the longitudinal and transversal directions compared with density values in Table I, more evident in samples containing the BG, indicate sintering shrinkage anisotropy. The preferential packing of particles during EPD causes probably this phenomenon [41]. Concerning EPD, a similar behaviour was observed recently for the sintering characterisation of deposited alumina [42]. The explanation is strengthened by the fact that higher anisotropy was observed for big nonspherical BG particles. The two significant sintering steps are often identified during sintering of the pure BG up to 1050 °C. The first densification step is seen to start at 530 °C (T1) and to end at 620 °C (T2), while the second densification step starts at 850 °C (T3), followed by a slight inflexion at 1000 °C attributed to a softening point when viscous flow starts [17]. Contrary to the literature data, our investigations for the same heating rate (10 °C/min) in longitudinal direction showed slight temperature deviations at the beginnings of the densifications steps and their shifting with composition change. The first densification step for HA:BG 100:0 sample started at 490 °C (T1) and ended at 610 °C (T2), while the second densification step started at 770 °C (T3). The shrinkage during the first step was 7.0 %. It should be noted that no inflexion was observed. The typical sharp sintering steps were also observed in HA:BG 25:75 sample, but the beginnings of the steps were recorded at higher temperatures, i.e. T1 was 505 °C, T2 was 620 °C, and the second densification step started at 900 °C with reduced shrinkage of 2.2 % in the first step. Higher content of the HA in the HA:BG 50:50 sample eliminated sharpness of typical plateau (between T2 and T3). In this sample, the densification started at 550 °C, followed by smooth curve descent with substantial bend at 990 °C. The record of HA:BG 75:25 sample shows no plateau with identified densification temperature at 590 °C. The curve of pure HA is smoothly descending as can be expected for this material with beginning of the densification at 640 °C. However, it is clear from this result that the second sintering phase did not develop for pure HA, which was mainly due to the low green body density and low sintering temperature. The situation for the samples measured in transversal direction was similar to the longitudinal direction. In sample HA:BG 0:100, the first step started at 520 °C and ended at 700 °C. The shrinkage of 21.2 % during this step was much higher when compared to the longitudinal direction. The second densification step started at 790 °C. The beginning of densification of HA:BG 25:75 sample was determined at 530 °C, and the end of the first step was recorded at 640 °C accompanied by shrinkage of 2.8 %. The T3 was 900 °C. From this point of view, the densification in HA:BG 50:50 and HA:BG 75:25 samples began at temperatures about 10°C higher than those measured in longitudinal direction and HA:BG 100:0 sample revealed no change. To conclude, the higher HA content shifts the starts and ends of the individual sintering steps of BG to higher temperatures and reduces the typical plateau. The derivate plot vs the temperature for longitudinal and transversal directions is given in Fig. 4c and Fig. 4d, respectively. It was found that the sintering rates of individual materials are similar in both directions. The sintering shrinkage rate of HA:BG 100:0 sample began to increase at 640 °C. The curve had no visible maximum. Such a situation is expected because the maximum densification rate occurs at 1080 °C, as Palard et al. [43] reported for pure HA having an analogous surface area. Moreover, this fact is reflected in all plots of measured samples containing HA, where no primary maxima of sintering rates were observed. Therefore, for samples HA:BG 75:25, HA:BG 50:50 and HA:BG 25:75, the only secondary maxima around 670 °C, 600 °C and 590 °C attributed to the first densification step of BG accompanied by its phase transformation were obtained, respectively. The pure BG sample (HA:BG 0:100) reached the secondary maxima of the densification rate at around D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 297 585 °C. Moreover, one more maximum at 650 °C is present in the transversal direction (see Fig. 4d), probably corresponding to the small inflexion visible on the shrinkage curve in Fig. 5b. The FGM started to shrink at 600 °C without distinct primary maximum and one measurable secondary maximum at 815 °C in the transversal direction. Based on the results, it is evident that BG accelerated the sintering rate in the composite materials. Fig. 5. X-ray diffraction patterns of HA and BG powders. 3.4 XRD analysis Fig. 5 shows diffraction patterns of commercial HA and BG powders. The red line is a typical pattern for amorphous bioglass with a single flat peak 2θ = 32 ° and other types of glass [44, 45]. The measurement is in good agreement with previously published data [17, 40]. On the contrary, the hydroxyapatite powder exhibited sharp, well-defined peaks, patently indicating the high crystallinity [46]. No secondary phases or impurities were detected during both analyses. Fig. 6. X-ray diffraction patterns of standards and HA:BG composites annealed at 600 °C. D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 304 37. D. Drdlik, M. Slama, H. Hadraba, K. Drdlikova, J. 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Ohji, Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide, Journal of the American Ceramic Society 84(11) (2001) 2638-2644. 64. F. Pecqueux, F. Tancret, N. Payraudeau, J.M. Bouler, Influence of microporosity and macroporosity on the mechanical properties of biphasic calcium phosphate bioceramics: Modelling and experiment, Journal of the European Ceramic Society 30(4) (2010) 819-829. Сажетак: Керамике на бази хидроксиапатита (ХА) и биостакла (БС) су од највеће важности у инжењерству коштаног ткива. Поред одговарајућег састава, микроструктура коштане замене игра кључну улогу. У овом раду, композити честица и функционално градирани материјал на бази ХА и БС припремљени електрофоретским таложењем су детаљно окарактерисани у смислу методе припреме, процеса синтеровања, фазног састава и микроструктуре. Синтеровање је праћено високотемпературном дилатометријом у два правца, израчунате су брзине синтеровања и дискутовано је о целокупном процесу синтеровања. СЕМ је показао континуирану промену у микроструктури са постепеном међусобно повезаном порозношћу погодном за био-апликације. Фундаменталном фрактографском анализом доказан је развој прслине у материјалу у вези са процесом синтеровања и дате су препоруке за смањење развоја прслине. Фазне трансформације током термичке обраде анализиране су анализом дифракције рендгенских зрака и детаљно размотрене. Кључне речи: хидроксиапатит; биостакло; функционални материјали; синтерованје; микроструктура. D. Drdlik et al.,/Science of Sintering, 55(2023)289-306 ___________________________________________________________________________ 306 © 2023 Authors. Published by association for ETRAN Society. 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