Biological apatites contain several elements as traces. In this work, Magnesium and fluorine co-substituted hydroxyapa tites with the general formula Ca9Mg(PO4)6(OH)2-yFy, where y = 0, 0.5, 1, 1.5 and 2 were synthesized by the hydrother mal method. After calcination at 500℃ , the samples were pressureless sintered between 950℃ and 1250℃ . The substi tution of F - for OH - had a strong influence on the densification behavior and mechanical properties of the materials. Below 1200℃ , the density steeply decreased for y = 0.5 sample. XRD analysis revealed that compared to hydroxyl fluorapatite containing no magnesium, the substituted hydroxyfluorapatites decomposed, and the nature of the decom position products is tightly dependent on the fluorine content. The hardness, elastic modulus and fracture toughness of these materials were investigated by Vickers’s hardness testing. The highest values were 622 ± 4 GPa, 181 ± 1 GPa and 1.85 ± 0.06 MPa.m1/2, respectively.
Hydroxyapatite (HA) is widely used in orthopedic and reconstructive surgery thanks to its excellent bioactivity and biocompatibility with the human body [
Hence, it is thought worthwhile to synthesize and characterize Mg/F-co-substituted hydroxypatites. The present study deals with the sintering of these materials and the investigation of their mechanical properties, all the more as only few studies have dealt with this kind of materials [21,22].
Analytical grades Ca(NO3)2∙4H2O, Mg(NO3)2∙6H2O, (NH4)2HPO4 and NH4F were used as starting materials. Appropriate amounts according to the stoichiometric formulas of Ca10(PO4)6(OH)F and Ca9Mg(PO4)6(OH)2-yFy with y = 0, 0.5, 1, 1.5 and 2, were weighed, respectively and dissolved into 5 cm3 of deionised water under vigorous stirring. The pH of the mixed solution was adjusted to 9 by adding a concentrated ammonia solution. After that, the mixed solution was transferred to a Teflon vessel (model 4749 Parr Instrument) and sealed tightly. The autoclave was oven-heated at 180˚C for 6 h, and then cooled to room temperature naturally. The collected precipitates were washed with deionised water and dried at 70˚C overnight. After drying, the powders were calcined under argon flow at 500˚C for 1 h with a heating rate of 10˚C∙min−1.
In the following sections, the compositions Ca10(PO4)6(OH)F, Ca9Mg(PO4)6(OH)2, Ca9Mg(PO4)6(OH)1.5F0.5, Ca9Mg(PO4)6(OH)F, Ca9Mg(PO4)6(OH)0.5F1.5 and Ca9Mg(PO4)6F2 will be named as HFA, MHA, MHF0.5A, MHF1A, MHF1.5A and MFA, respectively.
The (Ca + Mg)/P molar ratios in the as-prepared powders were evaluated by a chemical analysis [23,24]. The fluoride content was measured using a fluoride selective electrode (Ingold, PF4-L).
The XRD patterns of the as-prepared and calcined powders were collected on a Philips X-pert diffractometer operating with Cu-Ka radiation (l = 1.5406 Ǻ) for a 2θ range from 20˚ to 55˚. The scan step was 0.02˚ and the integration time was 1 s per step. The crystalline phases were identified by comparing the experimental XRD patterns to the standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS cards).
The 31P magic angle spinning nuclear magnetic resonance (31P MAS NMR) spectra were performed on a Brucker 300 WB spectrometer. The 31P observational frequency was 121.49 MHz with a spin speed 8 kHz. The 31P shift is given in parts per million (ppm) referenced to an aqueous solution of 85 wt% H3PO4.
The specific surface area (SSA) of the as-synthesized and calcined powders was measured with a Belsorp 28 SP apparatus using the BET method, while nitrogen was utilized as an adsorbed gas.
To carry out sintering experiments, the calcined powders were uniaxially pressed under 45 MPa into pellets in a 13 mm diameter steel die, then the pellets were sintered under argon flow in a temperature ranging from 950˚C to 1250˚C with 50˚C in interval for various times.
The sintered samples were characterized based on relative density, X-ray diffraction analysis and microstructural analysis using a scanning electron microscope (PHILIPS XL 30).
Both green and sintered densities (dex) of compacts were determined through dimension and weight, and the relative density was calculated using the formula:
The theoretical density for each composition (Ca9Mg(PO4)6(OH)2-yFy) was calculated taking in account its molecular weight (W), the number of units per unit cell (1) and the volume of the unit cell, according to the following equation:
where A is Avogadro’s number, and a and c are the lattice parameters.
The mechanical properties were investigated on pellets of 13 mm in diameter sintered at different temperatures for 1 h. The sintered samples were polished to mirror finish prior to mechanical investigation using various grade silicon carbide papers (grade 800 - 1200) and a 0.2 mm diamond paste.
The hardness was checked with Vickers’ indentation technique using a Matsuzawa Seiki digital micro-hardness Tester (Japan). Five samples were used for each hardness data point, and for each sample, ten indentations were performed at an applied load of 200 g for 15 s. Thus, the reported hardness Hv is the average of the fifty values calculated according to the equation [
where P is the indentation load and d is the length of the diagonal of the indentation.
The Elastic modulus (Young’s modulus) was estimated from an empirical relationship reported by Marshall et al. [
where Hv is Vickers’ indentation, a is the length of the shorter diagonal, b is the length of the longer diagonal determined using a Knoop’s indenter and b' is the crack length.
In Equation (4)
with
The comparison of Vickers and Knoop’ hardness for ceramics material reported by several authors [27,28] showed that the average value of the ratio HK/HV is 1.105.
Under these conditions, the value of a according to the Knoop indentation is determined according to the following equation:
where l is the length of Vickers’ indenter diagonal.
So, Equation (4) may be written as follows:
The fracture toughness (KIC) was determined using the indentation technique, and following the relationship given below [
where E is Young’s modulus; Hv, Vickers’ hardness; P, the applied load; c, the crack length indentation and a, the length of Vickers’ indenter diagonals.
The quantitative chemical analyses of the samples are listed in
The X-ray diffraction patterns of the as-prepared powders are shown in
The 31P MAS NMR spectra of the as-prepared samples are shown in
kind of compounds [
As can be seen from
After calcination at 500˚C, the XRD patterns of Mg/Fco-substituted hydroxyapatites showed only the apatite reflections, and neither a decomposition sign nor an appearance of a new crystalline phase, resulting from the crystallization of an amorphous phase in the as-prepared powders was detected by XRD (
the as-prepared powder was single-phased, b-MTCP would result from the decomposition of MHA. Thus, these findings show the destabilizing effect of Mg on the hydroxyapatite, and the stabilizing role of the fluorine on the Mg-substituted hydroxyapatite.
The values obtained for the SSA of the powders calcined at 500˚C are summarized in
T ˚C
(a)
0.0 0.5 1.0 1.5 2.0 y(F)
(b)
maximum of about 96% at around 1050˚C - 1100˚C, and then decreased. For MFA, the maximum density was observed at 1050˚C, while for the two other samples it was attained at 1100˚C, indicating that MFA densified better than the partially fluoridate samples, and obviously hydroxyapatite. Above 1200˚C, the sintered samples of the latter group were distorted, making impossible the determination of their density. The difference in the curve shape of the densities suggests that the mechanisms responsible for the densification are different for the two groups of composition.
According to
The effect of time on the relative density of the sintered samples was examined by fixing the temperature at 1050˚C (