Improvement of the hydroxyapatite mechanical properties by direct microwave sintering in single mode cavity

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Improvement of the hydroxyapatite mechanical properties by direct microwave sintering in single mode cavity A. Thuault a,∗ , E. Savary a,b,1 , J.-C. Hornez b,1 , G. Moreau b,2 , M. Descamps b,2 , S. Marinel a,3 , A. Leriche b,4 b

a CRISMAT, UMR CNRS 6508, 6 boulevard Maréchal Juin, 14050 Caen, France Laboratoire des Matériaux Céramiques et Procédés Associés, EA 2443 Université de Valenciennes et du Hainaut-Cambrésis, boulevard Général De Gaulle, 59600 Maubeuge, France

Received 11 September 2013; received in revised form 20 December 2013; accepted 24 December 2013

Abstract The main purpose of this study consists in investigating the direct microwave sintering of hydroxyapatite (HA) in a single mode cavity. Firstly, stoichiometric HA powders were synthesized by a coprecipitation method from diammonium phosphate and calcium nitrate solutions and shaped by slip-casting. Then, using the one-step microwave process, dense pellets with fine microstructures were successfully obtained in short sintering timespan. A parametric study permitted to determine the influence of powder grain size, sintering temperature and dwell time on the sintered samples microstructures. The Young’s modulus (E) and hardness (H) were measured by nanoindentation and the values discussed according to the microstructure. Finally, the resulting mechanical properties determined on the microwave sintered samples (E = 148.5 GPa, H = 9.6 GPa, σ compression = 531.3 MPa and KIC = 1.12 MPa m1/2 ) are significantly higher than those usually reported in the literature, whatever the sintering process, and could allow the use of HA for structural applications. © 2014 Elsevier Ltd. All rights reserved. Keywords: Microwave sintering; Hydroxyapatite; Microstructure; Mechanical properties

1. Introduction Hydroxyapatite (HA, Ca10 (PO4 )6 (OH)2 ) is widely used in dental, maxillofacial and orthopedic surgery as a scaffolding material to repair and restrict damaged parts of the human skeleton.1–3 Indeed, its chemical composition is close to the one of the mineral part of the human bone. Moreover, this material has good osteoconduction.4–6 However, its weak mechanical ∗

Corresponding author. Tel.: +33 231451377; fax: +33 231951309. E-mail addresses: [email protected], [email protected] (A. Thuault), [email protected] (E. Savary), [email protected] (J.-C. Hornez), [email protected] (G. Moreau), [email protected] (M. Descamps), [email protected] (S. Marinel), [email protected] (A. Leriche). 1 Tel.: +33 327511828; fax: +33 327531667. 2 Tel.: +33 327531670; fax: +33 327531667. 3 Tel.: +33 231451369, fax: +33 23 951309. 4 Tel.: +33 327531666; fax: +33 327531667.

properties limit its use for load bearing applications. Therefore, one way to overcome this limitation consists in decreasing the grain size while achieving high density. For this purpose, numerous sintering processes have been used such as hot pressing,7 post-hot isostatic pressing,4,8 spark plasma sintering (SPS)9 and different microwave configurations. Among the last ones, it is worth mentioning the hybrid microwave furnaces,10,11 the multimode microwave ovens12–14 or the domestic ones.15–19 Hoepfner et al.20 already reported the use of a single mode microwave cavity in which the sample was surrounded by a susceptor (yttria-doped zirconia). However, to the best of our knowledge, the direct sintering of hydroxyapatite in a single mode cavity has never been investigated so far. Indeed, hydroxyapatite easily absorbs microwave radiation. So that, there is no need to use any susceptors to heat the sample by infrared radiation like in a conventional furnace. In fact, the use of susceptors reduces the benefits of using microwave heating as it screens in a large extent the microwave fields. Consequently the potential microwave effect on the final microstructures of the sintered samples will be lowered.

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Microwave process is a highly efficient technique for ceramic elaboration thanks to its specific features such as the high heating rates, the very short irradiation times as well as the volumic heating of the sample.21,21–23 Those characteristics allow to get dense materials with tailored microstructures. Many studies report the fabrication of dense samples with fine grains.24,25 The main goal of our study consists in elaborating highly dense hydroxyapatite materials with fine grain size so as to improve the mechanical properties. Firstly, fine and pure hydroxyapatite powders were synthesized by a precipitation method and characterized in terms of chemical composition, crystallite size and sinterability. Secondly, the slip-casted pellets were sintered by direct microwave process in a single mode cavity and subsequently characterized in terms of density, microstructure and mechanical properties. Lastly, the results were discussed according to the sintering parameters. 2. Experimental 2.1. Synthesis and characterization of the powders Hydroxyapatite (HA) powders were prepared by aqueous precipitation technique26,27 using a diammonium phosphate solution (NH4 )2 HPO4 (Carlo Erba, France) and a calcium nitrate solution Ca(NO3 )2 ·4H2 O (Brenntag, France). The pH of the solution was kept at a constant value of 8.0 by a continuous addition of ammonium hydroxide at a constant temperature of 50 ◦ C.28 The crystalline phase composition was characterized by X-ray powder diffraction (Philips X’Pert diffractometer) using the Cu K␣ radiation.29 The absence of calcium oxide in HA was checked by the phenolphthalein test.30 The as-synthesized powders exhibit a high specific surface area (>60 m2 /g), which does not permit to obtain stable slurry for the slip casting process. Therefore, a post synthesis thermal treatment has been implemented to slightly increase the average particle size of HA powder. Two calcination temperatures were tested (800 and 900 ◦ C) to obtain optimal grain sizes suitable to get high quality sintered products with microstructure as fine as possible. In order to break up agglomerates and reduce the particle size to its optimal value, calcined powders at 800 and 900 ◦ C were ground respectively 96 and 48 h by using a HPDE milling jar and yttrium stabilized zirconia balls. This grinding step leads to the elaboration of powders with clearly distinct specific surface areas determined at 25.3 m2 /g (800 ◦ C) and 10.7 m2 /g (900 ◦ C) by the BET method (Micrometrics, Flow Sorb 3). These figures have been used to estimate the values of the grains diameters, respectively of 75 and 177 nm by using Eq. (1) assuming spherical and mono-dispersed powders: d=

6000 ρS

(1)

assuming spherical and mono-dispersed powders, in which ρ is the theoretical density (3.16 g/cm3 ), and S, the specific surface area. These results are consistent with the SEM observations of those two powders (Fig. 1). HA powders were subsequently shaped by slip casting process. Aqueous slurries were prepared with a powder

Fig. 1. SEM micrographs of HA powders calcined at (a) 800 ◦ C; (b) 900 ◦ C.

concentration equal to 65 wt% and the powder dispersion was obtained by adding a commercial organic dispersant agent (Darvan C, R.t. Vanderbilt Co.) in amount equal to 1.5–3 wt% of the powder content according to the specific surface area of the powder. Slurries were casted into cylindrical plaster mold (Φ = 7 mm, h = 3.5 mm). 2.2. Microwave sintering The microwave heating system consists of a microwave generator (SAIREM GMP 20 KSM, 2.45 GHz), that delivers a variable power up to 2 kW along a standard WR340 rectangular waveguide (section of 86.36 × 43.18 mm2 ) equipped with a circulator and ended by a TE10m rectangular microwave cavity. The cavity can be tuned in both modes TE10m (m = 4 or 5) by adjusting the length between the coupling iris and the short circuit piston.21,31 The resonance is manually tuned with a stub and the temperature is adjusted by varying the incident power. The sample was placed in the middle of the cavity into an alumina–silicate box (Fiberfrax Duraboard® ) which is microwave transparent and ensures a good thermal insulation of the sintered sample (Fig. 2). It can be noticed that any susceptor was placed around the pellet leading to a pure direct microwave heating.

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Fig. 2. Thermal insulation device.

In this study, the cavity was tuned in TE105 mode involving mostly the electric microwave field (E mode).32 The temperature was measured by using an infrared pyrometer (Ircon, Modline 5, 350–2000 ◦ C) vertically positioned on the top of the cavity and focused on the surface of the sample.31 Thermo-mechanical analyses (TMA Setaram) were carried out, in air, in order to study the thermal behavior of the powders versus temperature up to 1300 ◦ C (Fig. 3). A dwell time of 60 min at 1300 ◦ C was applied and heating/cooling ramps were fixed at 150◦ /h. It appears that the temperature at the inflection point on the curves is quite similar for both powders (about 1200 ◦ C). According to these results, five sintering temperatures were chosen: 1190, 1210, 1230, 1250 and 1270 ◦ C. Practically, a power of 250 W was applied to initiate the sample heating (about 10 min are needed to reach 350 ◦ C). Then, the sample is heated up by raising the power up to 300–320 W to reach the sintering temperature within 5 min. Three dwell times were fixed (5, 15 and 30 min) to discriminate the influence of this parameter on the sintered samples microstructures. The assembly was then cooled down to room temperature (RT) within 3 min. A typical thermal cycle is presented Fig. 4.

Fig. 3. TMA of pellets made from powders calcined at (a) 800 ◦ C; (b) 900 ◦ C.

determined on the discharge load versus depth curve by the Oliver and Pharr method.33 Compression tests were carried out using a universal testing machine (Instron 5569) working in this case at a constant strain

2.3. Sintered samples characterization The crystalline phases were identified by X-ray diffraction (XRD) using Cu K␣ radiation (Philips X’Pert diffractometer) on bulk samples. The density of sintered samples was determined by Archimedes’ method in distilled water. The sintered specimens were coated in a carbon resin (Struers Polyfast® ), polished (Struers Tegra-Pol 31® ) and observed by scanning electron microscopy (Zeiss Supra 55). Young’s modulus and hardness were determined by using a nanoindenter (MTS XP) equipped with a Berkovich tip, for a 50 ␮m indentation. A continuous stiffness measurement method was used to calculate the mechanical properties throughout the penetration of the tip into the material. During those tests, microunloads enabled the measurement of the hardness and Young’s modulus on the entire penetration depth. Young’s modulus was

Fig. 4. Thermal cycle recorded for a sintering temperature of 1210 ◦ C and a dwell time of 5 min.

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Fig. 6. Evolution of the sintered samples density as a function of the sintering parameters. Fig. 5. XRD patterns of powders calcined at 800 and 900 ◦ C and bulk samples sintered in microwave and conventional furnaces.

rate value of 10−3 s−1 and equipped with a 50 kN capacity load cell. Finally, the KIC values were determined by using Eq. (2).  2/5 P E KIC = ξ (2) H dl1/2 where KIC is the fracture toughness, ζ is a coefficient depending on the indenter geometry (ζ = 0.0089), E and H are Young’s modulus and hardness respectively (GPa), P is the applied load (N), l is the crack length (mm) and d is the half diagonal length of the indents (mm).

1230 ◦ C and decreases above this temperature. This diminution could be explained by dedensification mechanisms due to Ostwald ripening or coalescence phenomena. Moreover, the dwell time parameter, within the investigated range, does not significantly change the final density values. To conclude, it is highlighted that the best sintering conditions are: a sintering temperature of 1230 ◦ C and a short dwell time of 5 min for a powder calcined at 800 ◦ C. The obtained density levels of about 99.6% are slightly higher than the values reported after conventional sintering (about 97–98%).4,8 It may be mentioned that non-thermal effects such as electromigration and/or ponderomotive force may act as additional driving forces for diffusion mechanisms.34 3.2. Sintered samples microstructure

3. Results and discussion Samples were sintered by microwave in single mode cavity at 1190, 1210, 1230, 1250, and 1270 ◦ C and three dwell times were applied for each temperature (5, 15 and 30 min) (Fig. 5). First of all, it appears that whatever the sintering process (microwave or conventional) the HA phase is conserved. 3.1. Sintered samples density The evolution of the density both as a function of the sintering parameters and powders calcination temperature is represented in Fig. 6. First of all, it can be noticed that, except for the sample sintered during 5 min at 1190 ◦ C and made from the powder calcined at 900 ◦ C, all the specimens exhibit a density higher than 96% which confirms that the microwave process is relevant for the sintering of HA materials. Whatever the sintering parameters, samples prepared from powder calcined at 800 ◦ C present higher densities than samples prepared from powder calcined at 900 ◦ C, which confirms the better reactivity of the powder calcined at lower temperature. It is noted that, for both powder calcination temperatures, the density increases with the sintering temperature up to

Sintered samples microstructures were characterized by SEM micrographs analyses. Grain size measurements were carried out by using a linear intercept technique. In this paper the grain size is assimilated to the average linear intercept length between two grains in order to compare the present results with those usually reported in the literature. Fig. 7 represents the evolution of the grain size both as a function of the sintering parameters (temperature and dwell time) and powder calcination temperature. Whatever the initial powder grain size, the sintered sample grain size strongly increases for temperature higher than 1230 ◦ C (Fig. 8a and b). In the case of samples prepared from powder calcined at 800 ◦ C, grain size appears to be submicronic (around 560 nm) when sintering temperature is 1190 ◦ C (Fig. 8a). If the whole sintering temperature range is considered, the powder calcination temperature does not seem to have a significant effect on the grain size (Fig. 8c and d). However, the dwell time at sintering temperature clearly defines the sintered samples grain size (Fig. 8a and d). As expected, the increase of the dwell time leads to the increase of the energy supplied to the sample that permits the grain boundaries to move. As a consequence, a grain growth can be observed. It appears that grain sizes obtained after

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Table 1 HA grain size as a function of the sintering method for powders calcined at 800 ◦ C. Conventional sintering4,8 Sintering temperature Grain size (␮m)

(◦ C)

1130 0.79

1200 1.3

Microwave sintering (5 min) 1250 2.55

1190 0.56 ± 0.04

1210 0.84 ± 0.03

1250 1.23 ± 0.15

Finally, the grain growth law was investigated in isothermal conditions using Eq. (3). Gn − Gn0 = kt

(3)

where t is the dwell time, G is the average grain diameter at t, G0 is the initial value of G at t = 0, n is the grain growth exponent and k is the kinetic coefficient. The n exponent was only determined for the higher temperature (1250 ◦ C) in order to limit the influence of the densification on the results. The experimentally determined value (n = 2.4) is close to the theoretical value (n = 2) usually obtained in the case of an ideal system controlled by diffusion. 3.3. Sintered samples mechanical properties Fig. 7. Evolution of the sintered samples grain size as a function of the sintering parameters.

microwave sintering are significantly lower than those obtained after conventional sintering (Table 1).4,8 This is explained by that fact that microwave sintering requires shorter thermal treatments.

Mechanical properties (Young’s modulus and hardness) of the microwave sintered samples were determined by nanoindentation (Fig. 9). The general trend is that sintered samples mechanical properties increase with a temperature up to 1230 ◦ C and slightly decrease above this temperature. Moreover, a clear influence of the initial powder grain size (calcination temperature) on

Fig. 8. SEM micrographs of microwave sintered samples (a) calcined at 800 ◦ C and sintered 5 min at 1190 ◦ C; (b) calcined at 800 ◦ C and sintered 5 min at 1270 ◦ C; (c) calcined at 900 ◦ C and sintered 30 min at 1190 ◦ C; (d) calcined at 800 ◦ C and sintered 30 min at 1190 ◦ C.

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Fig. 9. Representation of the evolution of the microwave sintered samples (a) Young’s modulus; (b) hardness, both of them as a function of the sintering parameters.

the mechanical properties is highlighted. The higher values of Young’s modulus observed for lower calcination temperature are related to the sintered samples density. The powder calcined at 800 ◦ C is more reactive and this leads to higher density values. Since porosity can be considered as material damage, sintered samples made of powder calcined at 800 ◦ C possess a higher Young’s modulus than those made of powder calcined at 900 ◦ C, which is consistent with the results reported by Asmani et al.35 Whatever the powder calcination temperature, the Young’s modulus of the sintered samples follows the same trend. Thus, according to the density, the Young’s modulus increases within a temperature up to 1230 ◦ C and decreases above this temperature. Dwell time does not have a significant influence on the Young’s modulus sintered samples, since this parameter does not affect the density (Fig. 6). We have previously reported that the dwell time has no significant influence on the density but strongly determines the grain size. As the dwell time at high temperature seems not to change the Young’s modulus, we can assume that the grain size does not seem to affect its values which is consistent with the literature for grain size higher than 100 nm.36–38

Hardness seems to depend on the density since the presence of pores can contribute to decrease the locally measured value.39 Thus, samples prepared from powder calcined at 800 ◦ C present higher hardness values than those made of powder calcined at 900 ◦ C. In addition, following the evolution of density, hardness increases with the temperature up to 1230 ◦ C. Above this temperature, the decrease of the density and, most importantly, the strong increase of the grain size lead to the decrease of the hardness values. The noticeable influence of the grain size on the hardness values is represented by the clear effect of the dwell time on hardness values. Both high density and small grain size of the samples sintered in one-step by microwave involve higher mechanical properties (E = 148.5 GPa and H = 9.6 GPa) than those synthesized with the same method and sintered by post-hot isostatic pressing (two-step method) (E = 122 GPa and H = 6.1 GPa).4,8 Direct microwave sintering enables increasing the Young’s modulus of about 20% and the hardness of about 55% which is a significant improvement. In addition, compression tests were carried on five sintered samples. The sintering has been carried out according to the optimal conditions in order to obtain the higher hardness and Young’s modulus values, that is to say samples made of powder calcined at 800 ◦ C and sintered during 5 min at 1230 ◦ C. It appears that a maximal compressive strength of about 531.3 ± 42.2 MPa is obtained, which is significantly higher than the values usually reported in the literature for dense HA conventionally sintered (about 430 ± 95 MPa).40 The improvement of the compressive strength is mainly due to the considerable increase of the sintered samples density (97% in the case of conventionally sintered samples). Moreover, the KIC values were determined by using Eq. (2) on ten indentation prints. Thus, for the same sintering conditions as previously KIC = 1.12 ± 0.07 MPa m1/2 , which is slightly higher than the values usually reported in the literature (0.92 MPa m1/2 ) and can be explained by the fine grain size which limits the crack propagation.4,8 This opens the way to the use of hydroxyapatite for structural applications.

4. Conclusion Powders grain sizes of 75 and 177 nm were obtained by calcination at 800 and 900 ◦ C respectively. Direct microwave process applied to HA powders, appears to be an efficient method to obtain high density materials (99.6%). Moreover, grain growth can be limited by applying very short irradiation times (less than 20 min), which leads to the production of fine microstructures (grain size of about 1 ␮m). The influence of the sintering parameters such as dwell time and temperature on microstructures were investigated and discussed according to the mechanical properties. The optimal sintering conditions leads to Young’s modulus, hardness and compressive strength values (E = 148.5 GPa, H = 9.6 GPa, σ compression = 531.3 MPa and KIC = 1.12 MPa m1/2 ) strongly improved compared to other densification techniques. The very low energy consumption, the very short times of thermal treatment and the improved mechanical properties open a promising way to the use of one-step

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