The effect of KF on the structural evolution of natural hydroxyapatite during conventional and microwave sintering

The effect of KF on the structural evolution of natural hydroxyapatite during conventional and microwave sintering

Ceramics International 46 (2020) 1189–1194 Contents lists available at ScienceDirect Ceramics International journal homepage:

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Ceramics International 46 (2020) 1189–1194

Contents lists available at ScienceDirect

Ceramics International journal homepage:

The effect of KF on the structural evolution of natural hydroxyapatite during conventional and microwave sintering


D. Belamria,∗, A. Harabib, N. Karbouaab, N. Benyahiaa a b

Laboratory of Photonic Physics and Multifunctional Nanomaterials, Mohamed Khider University, Biskra, 07000, Algeria Ceramic Laboratory, Mentouri University, Route de Aîn el Bey Constantine, 2500, Algeria



Keywords: Hydroxyapatite Natural Conventional oven Microwave Sintering Structure Decomposition Additions

In this work, we studied the effect of microwaves and addition KF on the structural development of natural hydroxyapatite (bovine bone) during sintering using two types of furnaces: conventional: 1100–1300 °C and microwave: 900–1200 °C. Chemical as well as mechanical evolution monitoring of the material is ensured by Xray diffraction, infrared and micro-hardness tests. Thanks to these characterization methods, we have shown that KF plays an important role in the stability of hydroxyapatite during sintering and mechanically enhances it with the emergence of the new phase resulting from the interaction between KF and hydroxyapatite called “Fluorapatite".

1. Introduction Hydroxyapatite (Hap) “Ca5(PO4)3OH” is one of the best known materials for biocompatibility in living environments, the structure closest to those of teeth and bones, chemical formula is “Ca8.3(PO4)4.3(HPO4CO3)1.7(OH,CO33)" [1–3]. The appearance of biodegradable phases during heat treatment, such as TCP Tricalcium Phosphate from the chemical formula “Ca0.3(PO3)4″, β-TCP and α-TCP is still seen as a negative point in the structural and mechanical behaviour of Hydroxyapatite (Hap) in vivo [4,5]. The coexistence of this type of phase with Hap weakens it by generating cavities within the material after its rapid and easy degradation in vivo [6,7]. To solve this problem, Wojciech Suchanek experimented with several additions at the same time using the conventional furnace, including the KF, which aims to reduce the oxidation temperature to avoid decomposition and thus stabilize the structure [8]. Another sintering method is used in this work, using a microwave [9,10]. This method of high sintering rate and heating is justified in a homogeneous vibratory mode. This type of heating can make a difference by encouraging the conversion of OH− to F− ions that can easily move through the columns in the hydroxyapatite structure, resulting in the new phase Fluorapatite “Ca5 (PO4) 3F” which condenses better between 900 and 1100 °C [26]. As a result, the effect of evaporation, which leads the partial decomposition of Hap, will be greatly reduced by replacing F− ions at OH− ions locations. The aim of this work is, on the one hand, to show the effect of

microwaves on the structural evolution of natural hydroxyapatites (bovine bone) during sintering and, on the other hand, the structural stability due to substitution fluoride ions F− instead of hydroxyl particles OH− forced to be evaporated under the effect of a heat treatment. 2. Experimental techniques 2.1. Preparation of samples The initial stage of the preparation of Hap raw material from the femoral parts of the bovine bone, which recovered in large quantities (aged from 15 to 20 months), is to eliminate any fat on its surface and reduce the odours of gases emitted during the calcinations by using a gas torch [11–13]. The bones are then broken into small pieces to ensure effective calcinations in conventional ovens at 800 °C in order to reduce internal stress and obtain a good crystallisation (shrinking peaks). At this temperature, the water evaporates completely and the fat is carbonated as well as residual organic matter [14,15]. Then, we grounded the material obtained for 2 h using alumina balls (Al2O3) with different diameters to obtain a very fine granular powder (0,1 μm–1μm) in order to enlarge the contact surface between them and increase the density. Finally, we prepared pellet samples by compressing the powder from the grinding process under pressure of about 75 MPa. This value prevents us from introducing internal strain during pressure while at the

Corresponding author. E-mail address: [email protected] (D. Belamri). Received 6 February 2019; Received in revised form 15 July 2019; Accepted 10 September 2019 Available online 10 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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same time ensuring a large contact surface between all grains. Regarding the addition of KF with a low quantity (0.5% of the mass), we reused the crusher for 1 h in order to obtain a good homogeneity in all.

Table 2 Variation of crystallite size (CS) and FWHM for (002) direction as function of temperatures of samples sintered in microwave oven.

2.2. Sintering methods Regarding heat treatments, two methods were carried out. The first one is conventional heating and the second one is microwave heating. The purpose of this choice is; on the one hand, to compare the two methods, on the other hand, refer to reference works made in conventional ovens [8] as well as in microwave devices [16]. For the traditional method, we used NABERTHERM furnaces that heat up to 1400 °C, KANTHALAPM type, with a relatively slow heating rate only 6 °C/min. We have chosen, among other things, three great temperatures such as sintering temperatures: 1100, 1200, 1300 °C (possible partial decomposition of hydroxyapatite into other phases in the temperature range [1100–1300 °C] due to the phenomenon of nano-tomicro transition [21,23]). This temperature range can give us an overview of the structural behaviour of our natural raw materials, which are subject to different heat treatments. For the second microwave method, we used the LG MC–805 AR oven, with the dimensions (500 [depth], 322 [height], 530 [width]), modified and suitable for the laboratory [17]. The operating frequency is 2.45 GHz, with adjustable capacity (20, 40, 60, 80, 100%) of the total power 850 Watts. Equipped with a heating time controller ranging from 10 s to 90 min and thermocouples below the sample, this furnace can rise to 1600 °C. The furnace is combined with a special ceramic device around the sample, which maintains the heat obtained at the sample level [17]. The purpose of this type of treatment is to discover the structural attitude in an electromagnetic field under specific conditions and compare it with the reference behaviour in a conventional oven.

CS k = 0.9

Crystallite size. Correction factor to account for particle shapes. Full width at half maximum (FWHM) of (211) peak. Wavelength of the radiation (KαCu). Bragg angle.

(1.54056 A°)


2 theta

FWHM (°)


Raw material 800°C 1100°C 1200°C 1300°C

002 002 002 002 002

26,1048 26,1059 25,9356 25,9307 25,8855

0,0886 0,0886 0,1181 0,1181 0,1181

92,0403 92,0405 69,0262 69,0255 69,0192


Raw material 1050 °C 1100°C 1150°C 1200°C

002 002 002 002 002

26,1048 26,1008 26,1518 25,9261 25,5162

0,0886 0,1574 0,1574 0,1574 0,1574

92,0403 51,8088 51,8141 51,7906 51,7483

We have identified the resulting phases of each heat treatment. Analysis of the calcined powder at 800 °C confirmed that it was hydroxyapatite (Hap) (Fig. 1). The majority of patterns peaks are determined by the file number A.S.T.M (24–0033) represents Ca5(PO4)3OH. We also found that the unwanted CaO phase, which can generally be determined at 37.4° [11] and can cause the appearance of TCP at a low temperature [19], is almost unidentifiable in the starting powder. 3.2. Conventionally sintered samples In this section, we heat treated the sample without addition for a 2 h holding period at each sintering temperature. The heating rate in the conventional oven is relatively low. This encourages the increased grain size and thus gives more freedom to particle movement [25]. The X-ray diffraction patterns obtained in (Fig. 2) clearly show partial hydroxyapatite decomposition (Hap) of approximately 1200 °C with the appearance for a new transition β-TCP whose peaks can be determined by the file number A.S.T.M (06–0426). Known for its fast and easy formation [20], the appearance of the βTCP phase may be due to the des-hydroxylation process [21–23]. This promote the metastable atomic configuration in the growth of large grains, through surface atomic diffusion mechanism, at the expense of small grains [24,29]. Fig. 3 shows the reduction in grain sizes between the temperatures 800–1100 °C, due to the sintering process. Then, a slight stability of the particle size between the temperatures of 1100–1300 °C corresponds to the partial appearance of the easily degradable phase β-TCP, which may be the direct cause of the reduction of the hydroxyapatite X-ray diffraction peaks. Infrared spectra (Fig. 4) show the gradual disappearance of the large band of H2O at 3470 Cm-1. This confirms evaporation during heat treatment.

Table 1 Variation of crystallite size (CS) and FWHM for (002) direction as function of temperatures of samples sintered in conventional oven. Temperatures

FWHM (°)

3.1. Raw materials

k. . cos


2 theta

3. Results and discussion

To determine the phase, we used the “D8–Advanced” diffraction meter in the angular range between (2θ = 10 and 80°). The X-ray diffraction patterns are experimentally obtained and then compared with the (ASTM) database. For micro-structural analysis, micro-strain was assumed to be negligible and the entire diffraction peak broadening attributed to crystallite size (see Tables 1and 2). Scherrer equation was used to calculate crystallite size [18]:



In the case of (FTIR), the instrument used in this study is the “FTIR–8400 S″ infrared spectrometer which allows a frequency scan between 4000 and 400 cm−1. Measurements can be made either in transmittance or absorption (see Table 3). Finally, for mechanical evaluation, we used the Vickers (Hv) Wolpert Wilson precision Instruments–402UD test to monitor structural transformations and a possible precipitation hardening through micro-hardness measurements.

2.3. Methods of analysis

CS =


3.3. Microwave sintered samples The samples were heated using different concentrations, 0.5% KF and without adding in the microwave oven for 15 min, at different temperature levels with a non-linear heating rate (Fig. 5). The phases determination, resulting from each treatment, is shown by X-ray diffraction patterns in (Fig. 6) and (Fig. 8). 1190

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Table 3 Infrared absorption spectroscopy for HAP; bands and assignments [25]. Bands PO4





472 573 602 962 1050 1090 ~2000 ~2080 633 3570 ~1630 ~3470

Double degenerated bending mode, (ν2a), of the O–P–O bonds of the phosphate group. Triply degenerated bending mode, (ν4b), of the O–P–O bonds of the phosphate group. Triply degenerated bending mode, (ν4a), of the O–P–O bonds of the phosphate group. Nondegenerated symmetric stretching mode, (ν1), of the P–O bonds of the phosphate group. Triply degenerated asymmetric stretching mode, (ν3b), of the P–O bond of the phosphate group. Triply degenerated asymmetric stretching mode, (ν3a), of the P–O bond of the phosphate group. Harmonic overtone 2, ν3 or combination band ν1 + ν3b. Harmonic overtone or combination band. Librational mode, (νL), of the hydroxyl group, OH−. Stretching mode, (νS), of the hydroxyl group, OH−. Absorbed water molecule.

Fig. 1. The X–ray diffraction patterns of the powder calcined at 800 °C compared with raw material.

Fig. 2. X-ray diffraction patterns of the sampled samples without adding (800, 1100, 1200, 1300 °C) traditionally.

3.3.1. Sample without adding In (Fig. 6), we noted that for this type of sintering, the development of the β-TCP phase is slowed considerably compared to the conventional method. On the other hand, we observe the development of a ring diffuse between the angles (10–19) degrees which may correspond to a formation of the amorphous phase. According to Simsek's work, another phase evolves at the same time as the diffuse ring corresponds to the “Brushite” CaH(PO4).H2O phase is specified by file number A.S.T.M (72–0713) [16]. Consists of potential chemical reaction:

According to Sutton, the microwave sintering preferred phases with an impressive dielectric loss factor (ε‴) [9]. Therefore, the amorphous and Brushite phases represent most stable phases compared to the phases of hydroxyapatite (Hap) or β-TCP in the microwave oven due to these high dielectric loss factors. This type of electromagnetic wave sintering is also characterized by heating homogeneity due to the simultaneous vibrations of the molecules of the material. As a result, the temperature gradient phenomenon within the sample is not dominant compared to conventional sintering. Thus, the surface diffusion mechanism will necessarily be replaced by another mechanism, that granule diffusion mechanism [10].

Ca5(PO4)3OH → CaHPO4 + Ca3(PO4) + CaO 1191

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Fig. 3. The effect of the temperature on crystallite size in a conventional oven.

Fig. 4. Infrared spectra without the addition of samples sintered (800, 1100, 1200, 1300 °C) traditionally.

Fig. 5. Temperature variation as a function of the heating time in seconds in the microwave oven.

This means that there is a good chance that the new phase of Brushite precipitates at the grains boundaries of the mother phase, and that the original phase structure, which is hydroxyapatite, is transformed into an amorphous phase according to the X-ray diffraction pattern at 1200 °C. As a result, this transformation reduces slightly the size of the hydroxyapatite grains while raising the temperature from 1100 °C as shown in Fig. 7. It should also be noted that there is a small increase in grain size between 1050 and 1100 °C which may be due to a stress relaxation phenomenon which leads to good crystallisation before the amorphous phase reduce the size of the crystallites again.

3.3.2. Sample with addition Significant reduction in X-ray diffractions patterns peaks intensity for both phases (Brushite, β-TCP) as shown in Fig. 8. This can be explained by the formation of the liquid phase, due to the presence of ions (F−) and (K+), which facilitate the movement of atoms and consequently the sintering [15,28]. This phenomenon favours the formation of the phase Fluorapatite Ca5(PO4)3F after the occupation of OH− sites, which evaporated under heat treatment effect, by the fluorine ions F− [26,27]. This mobility may prevent the formation of the phase of β-TCP. We also observed a minority phase that may be KCaPO phase according to Wojciech [8]. 1192

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Fig. 6. The X-ray diffraction patterns of samples without addition sintered at (1050, 1100, 1150, 1200 °C) in the microwave oven compared with raw material.

Fig. 7. The effect of the temperature on crystallite size in a microwave oven.

Fig. 8. Comparison of the X-ray diffraction patterns of the sample (at 0.5% KF, without addition) sintered both at 1150 °C by microwave.

3.4. Micro hardness

KF, which has a melting temperature of 858 °C, which ensures the good transport of particles thus get rid of voids in the material and increase the bulk density. According to studies conducted by Benayed et al. [30], the densification of the “Fluorapatite” phase is important in this temperature range with possible rainfall in the grain joints that can mechanically strengthen the structure.

After studying the structural stability of the two samples (0.5% KF, without addition) sintered by microwave, we studied mechanical behaviour by micro hardness (Fig. 9). Of this curve, we observed that the mechanical attitude improves only in the temperature range between 1025 and 1200 °C. This phenomenon can be explained by the presence of liquid phase


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Fig. 9. Partial hardness of the samples, 0,5% KF and without addition, as a function of microwave sintering temperature.

4. Conclusions

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The partial decomposition of hydroxyapatite (bovine bone) is due to the rapid and easy formation of the unstable β-TCP phase for conventional sintering, and privileged formation of the Brushite and amorphous phases by electromagnetic wave sintering. The addition of KF stabilizes the hydroxyapatite (Hap) structure and mechanically strengthens it by the formation of the Fluorapatite phase following an occupation of the OH− ions sites, evaporated under the effect of temperature, by fluorine F− ions. On the other hand, the characteristics of the microwave sintering are quite different compared to the conventional oven. The heating rate and the simultaneous vibrations of the material particles by the microwave sintering have a significant impact on the evolution of the structure as well as on the microstructure [31]. This result is confirmed by the X-ray diffraction patterns of the two sintering methods (Figs. 2 and 6), and by the calculation of the FWHM for each temperature, which gives us the approximate size of the grains (Figs. 3 and 7). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// References [1] J. Black, Ceramics and composites, Orthopaedic Biomaterials in Research and Practice, Churchill Livingstone, Inc, New York, NY, 1988, p. 191. [2] M. Jarcho, Calcium Phosphate ceramics as hard tissue prosthetics, Clin. Orthop. Relat. Res. 157 (1981) 259. [3] D.D. Lee, C. Rey, M. Aiolova, Synthesis of Reactive Amorphous Calcium Phosphates, United States Patent 5676976. (1997). [4] L. Cerroni, R. Filocamo, M. Fabbri, C. Piconi, S. Caropreso, S.G. Condò, Growth of Soteoblast-like Cells on Porous Hydroxyapatite Ceramics: an in Vitro Study Biomolecular Engeneering vol. 19, (2002), p. 119 2–6. [5] B.S. Chang, C.K. Lee, K.S. Hong, H.J. Youn, H.S. Ryu, S.S. Chung, K.W. Park, Osteoconduction at Porous hydroxyapatite with various pore configurations, Biomaterials 21 (2000) 1291. [6] Y. Chen, X. Miao, Thermal and chemical stability of fluorohydroxyapatite ceramics with different fluorine contents, Biomaterials 26 (11) (2004) 1205. [7] C.P.A.T. Klein, A.A. Driessen, K. Groot, A. Van der Hooff, Biodegradation behaviour of various calcium phosphate materials in bone tissue, J. Biomed. Mater. Res. 17 (5) (1983) 769. [8] S. Wojciech, Y. Masahiro, Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants, J. Mater. Res. 13 (1) (1998) 94. [9] W.H. Sutton, Microwave processing of ceramic materials, Am. Ceram. Soc. Bull. 68 (2) (1989) 376. [10] Y. Fang, D.K. Agrawal, D.M. Roy, R. Roy, Microwave sintering of hydroxyapatite ceramics, J. Mater. Res. 9 (1) (1994) 180.