Thermal decomposition and mechanical properties of hydroxyapatite ceramic

YANG Chun(杨 春)^{1, 2}, GUO Ying-kui(郭英奎)¹, ZHANG Mi-lin(张密林)²

1. College of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, China;

2. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

Received 10 February 2009; accepted 4 May 2009

Abstract: Pure hydroxyapatite (HAP) ceramic and HAP composite ceramic with B₂O₃ were prepared by isostatic press forming and pressureless sintering. The relationships between thermal decomposition ratio and mechanical properties for pure HAP ceramic and the composite ceramic were investigated by means of FTIR, X-ray diffraction and three-point bending method. The results indicate that the decomposition ratio of pure HAP ceramic increases with ascending the sintering temperature and nearly reaches 80% at    1 350 ℃. For the HAP composite ceramic, the thermal decomposition is inhibited obviously due to the addition of B₂O₃. The added B atoms incorporate into the crystal lattice of HAP to form solid solution, resulting in an enlargement in the crystal spacing and an improvement in the binding strength of HAP crystal cell. Thermal decomposition ratio of HAP decreases but bending strength and fracture toughness are enhanced for HAP composite ceramics. However, when the added B₂O₃ is more than 5% (mass fraction), HAP decomposition is promoted and a steady β-TCP is formed due to the fact that when B atoms with higher negative electricity are combined with O, sp2 and a full-air p are formed, and those voids have a strong trend to intake of the outer electrons. So, it is very possible to occupy the place where HAP loses OH^- or PO₄^3-.

Key words: B₂O₃; hydroxyapatite ceramic; thermal decomposition; bending strength; fracture toughness

1 Introduction

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAP) is the principal inorganic component of bones and teeth, which gets 72% and 97%, respectively. HAP possesses more excellent biocompatibility and interactive bioactivity than biomedical Ti alloys, silastic and carbon materials. It is the most typical bioactive material[1-3] that the traditional metallic materials can not match. At present, HAP is mainly applied in repair and replacements of hard tissues, such as oral implants, alveolar ridge strengthening, ear ossicle and vertebra repairs, and it also has great potential in biomimetic area[4-6]. However, HAP can easily be composed at a certain temperature, which results in poor sintering characteristics and mechanical properties for HAP ceramic[7-8], that is why prepared HAP ceramics usually possess low bending strength and fracture toughness, far away from the clinical demand. Therefore, how to inhibit the decomposition of HAP ceramics is a hotspot that needs to be solved. Normally, ZrO₂ is added into HAP ceramics to inhibit HAP thermal decomposition and improve the bending strength and fracture toughness[9-10]. But, the improvement was very limited. At the same time, the preparation cost of hot-pressed sintered materials is too high to apply in clinic.

B₂O₃ is an evaporable oxide at high temperature because of its low melting point (450 ℃), so it can be used as a applicable steam dopant[11]. Besides, it is also used as combustion improver to decrease sintering temperature[12]. So far, there is less report on the effect of B₂O₃ addition on mechanical properties of HAP ceramics.

In the present study, HAP ceramic with B₂O₃ as additive was prepared by cold iso-press molding and pressureless sintering on the basis of previous work[13], aiming at the presence manner of B₂O₃ in the HAP and its effects on thermal decomposition and mechanical properties.

2 Experimental

HAP powders were synthesized according to sol-gel method[14]. B₂O₃ was from Shunyi Weixin Chemical Plant of Beijing, China and its main chemical constitutions are given in Table 1.

Table 1 Main chemical constitutions of B₂O₃ powder (mass fraction, %)

HAP with B₂O₃ addition up to 20% (mass fraction) was ball ground for 50 h with Al₂O₃ beads as the grinding media. The ground powders were screened with a 275 mesh screener, and then preformed at 15 MPa. The preformed cake-like bodies were further isostatic pressed with a pressure of (250±2) MPa at ambient temperature, followed by pressless sintering at ambient atmosphere and covered with pure HAP powder. The sintering curve is shown in Fig.1. The sintering temperature of HAP composite ceramics was determined according to pure HAP ceramic.

Fig.1 Sketch map of sintering curve for HAP ceramic

We soaked four pieces of HAP composite ceramics of 20 mm×7 mm×3 mm in 100 mL simulated body fluid (SBF), which is referenced to No.9 SBF synthesized by KOKUBO[13], taking human body programming as base. The contents of different ions are listed in Table 2. And then these HAP ceramics were cultivated continually for 14 d in water bath at (37±1) ℃, and in SBF for another day. The contents of Ca²⁺ and HPO₄^2- were tested at the 1st, 3rd, 7th, 10th, and 14th day, respectively. The biological activities of HAP ceramics were preliminarily evaluated by analyzing the change of the contents of Ca²⁺ and HPO₄^2- and deducing the possible action in the solution.

Table 2 Comparison of ion species and concentration between simulated body fluid and human body plasma (mmol/L)

The bending strength and fracture toughness were measured by three-point bending method and single edge-notched beam (SENB) on DCS-500 Universal Testing Machine from Chinese Academy of Science. The samples were made into 3 mm×4 mm×36 mm in size and 1.0 mm in notch depth. The speed of pressure head was 0.1 mm/min. Phase identification was investigated by Y500-type X-ray diffractometer with Cu K_α as radiation source, at a accelerating voltage of 40 kV, a current of 50 mA and a scan speed of 0.06 (?)/s. The scanning range was from 20? to 45?. The resultant HAP volume fraction was determined by the relationship between the phase content and the corresponding peak area. HAP thermal decomposition ratio was detained by the difference of HAP volume fractions for biscuit and for sintered HAP ceramic. The FTIR analysis was performed by AVATAR 370 Fourier infrared spectrometer from Nicolet Company, United States.

3 Results and discussion

Fig.2 shows the XRD patterns of pure HAP ceramics sintered at 1 200, 1 250, 1 300 and 1 350 ℃, respectively. HAP ceramics sintered at different temperatures were all decomposed into β-TCP. The decomposition ratio increased with ascent of sintering temperature, as shown in Fig.3. It needs to be pointed out that when the sintering temperature was 1 350 ℃, this ceramic was decomposed badly with a decomposition ratio near 80%.

Fig.2 XRD patterns of HAP ceramics sintered at different temperatures for 2 h

Fig.3 HAP decomposition ratio as function of sintering temperature

Fig.4 shows the XRD patterns of HAP composite ceramic with B₂O₃ addition sintered at 1 250 ℃. It suggests that HAP was decomposed into β-TCP. Fig.5 gives the decomposition ratio of HAP calculated according to Fig.4. Fig.5 indicates that the decomposition ratio of HAP ceramic increases with ascending the content of B₂O₃, and B₂O₃ inhibits thermal decomposition of HAP at higher temperature obviously compared with pure HAP ceramic. When the content of B₂O₃ is over 5%, the thermal decomposition ratio of HAP is the lowest.

Fig.4 XRD patterns of HAP ceramics with different addition contents of B₂O₃

Fig.5 Effect of B₂O₃ content on HAP decomposition ratio

Fig.6 illustrates the effect of B₂O₃ addition on bending strength and fracture toughness for HAP composite ceramics. Pure HAP ceramic sintered at 1 250 ℃ possesses an average bending strength of 30 MPa and a fracture toughness of 0.4 MPa·m^1/2. They were improved obviously by B₂O₃ addition. For HAP composite ceramic with 5% B₂O₃, the bending strength and fracture toughness reached their maximum of 125 MPa and 1.35 MPa·m^1/2, respectively. Further adding B₂O₃ caused the decrease of bending strength and fracture toughness. When B₂O₃ addition was up to 15%, the bending strength and fracture toughness were decreased to 86.3 MPa and 1.07 MPa·m^1/2, respectively.

Fig.6 Relationships between fracture toughness and fracture toughness with B₂O₃ content for HAP composite ceramics

By comparing Fig.2 with Fig.4, it is found that B₂O₃ addition makes the main peaks of HAP shift left. The interplanar spacing (d₂₁₁) for the main peak of the samples was calculated according to Bragg equation, as shown in Table 3. It is indicated that the d₂₁₁ for HAP ceramics with B₂O₃ is larger than that for pure HAP ceramic, and gets the maximum value when 5% B₂O₃ is added due to the fact that HAP belongs to P6₃/m space group and hexagonal system structure. There are octahedral void and tetrahedral void in the main hexagon frame O^2-. B₂O₃ has a melting point of 450 ℃ and has become vapor when being sintered at 1 250 ℃, which may go into grain boundary of HAP. A part of positive ion B³⁺ in B₂O₃ molecule may insert into the octahedron void of HAP and grow into another framework as shown in Fig.7, which contributes to B³⁺ with small semidiameter. Therefore, the incorporation of B³⁺ into HAP crystal cell will further enlarge the crystal spacing, and make the diffraction peaks of HAP shift left, which benefits for improving the bending strength and fracture toughness of HAP.

Table 3 Effect of B₂O₃ content on interplanar spacing of (211) crystal plane for HAP

Fig.7 Possible location of B³⁺ in HAP crystal

The interspace of HAP octahedron bears a strong interaction with the atoms around. Once B³⁺ goes into the interspace of the octahedron, the neighboring octahedron is difficult to take in B³⁺ any more. This is consistent with research in Refs.[14-16], showing the limited adulterant in HAP composites. Therefore, in the present study, 5% B₂O₃ addition is the best to inhibit the catabolism of HAP. When B₂O₃ content is more than 5%, HAP decomposition is promoted and a steady β-TCP is formed due to the fact that there is a strong trend to intake of outside electrons because B atoms have strong negativity. B—O bonds are most covalent bonds as heterozygous orbit and B still has a empty p orbit with lost electron. As a result, it is very likely to take the position of OH^- or PO₄^3- tetrahedron lost by HAP.

Fig.8 shows FIRT spectra of samples with different B₂O₃ contents, in which diffusion peaks at 1 650 cm^-1 and 3 500 cm^-1 belong to absorption peaks of absorbed water. Absorption peaks at 3 571 cm^-1 and 632 cm^-1 correspond to flex vibrational peak (ν_ρ) and swing vibrational peak (ν_δ) for hydroxyl, 962 cm^-1 corresponds to symmetry flex vibrational peak (ν₁) for PO₄^3-, and 608 cm^-1 and 561 cm^-1 correspond to bend vibrational peak (ν₄) for PO₄^3-. Absorption peaks at 1 014 cm^-1 and 1 102 cm^-1 correspond to unsymmetry flex vibrational peak (ν₃) for PO₄^3-. From Fig.8, it is found that absorption peaks for HAP with B₂O₃ are stronger than those for the pure HAP. With increasing B₂O₃ content, absorption peaks for OH^- become weak and wide, leading to the three vibrational peaks for PO₄^3- (962 cm^-1, 1 014 cm^-1 and  1 102 cm^-1) combining together and confusing. These changes suggest that addition of 5% B₂O₃ can effectively restrain HAP decomposition, and further increasing B₂O₃ leads to a decline in stability of HAP conformed by XRD analysis.

Fig.8 FTIR patterns for samples with different contents of B₂O₃

The contents of Ca²⁺ and HPO₄^2- were measured by chemical titration at 1st, 2nd, 7th, 10th, and 14th day respectively (Table 4), when HAP composite ceramics were soaked in SBF. From Table 4, we can see, with increasing the time, the content of Ca²⁺ tends to increase, while that of HPO₄^2- tends to decrease. It is attributed to the fact that Ca²⁺ migrates from HAP to solution until being saturated. Ca and P happened to sediment on the surface, resulting in appearing an amorphous apatite layer structure, as well as the content of HPO₄^2- declining because HPO₄^2- in the SBF was consumed. This suggests that the prepared HAP ceramic materials behave a better biological activity.

Table 4 Concentration of Ca²⁺ and HPO₄^2- at different time

The HAP composite ceramics that were soaked in SBF for 14 d were detected by FIRT(Fig.9). There are five peaks in Fig.9. Two at 1 040 cm^–1 and 957 cm^–1 belong to amorphous calcium phosphate(ACP); and the other two peaks at 870 cm^-1 and 742 cm^-1 correspond to NO₃^-; and the rest peak at 962 cm^-1 may be a overlapping peak corresponding to NO₃^- and phosphate [17]. These results lead to the conclusion that after the HAP composite ceramics are soaked in SBF for 14 d, a new ACP structure is generated, which plays an important role in normal metabolism and repair of bone tissue.

Fig.9 FTIR spectrum for sample after being soaked in SBF for 14 d

4 Conclusions

1) The decomposition ratio for pure HAP ceramic increases with ascending sintering temperature. It reaches nearly 56% when being sintered at 1 250 ℃. The average bending strength and fracture toughness are 30 MPa and 0.4 MPa·m^1/2, respectively.

2) When 5% B₂O₃ is added into HAP ceramic and the mixture is sintered at 1 250 ℃, the thermal decomposition of HAP composite ceramics is inhibited effectively. The average bending strength and fracture toughness are improved to 125 MPa and 1.35 MPa·m^1/2, respectively.

3) B³⁺ could incorporate into HAP crystal structure and form a solid solution, enlarging the crystal spacing and enhancing the binding force of HAP crystals, resulting in a great decrease in HAP decomposition ratio and excellent improvement in both bending strength and fracture toughness. When B₂O₃ addition is over 5%, HAP decomposition is promoted and a stable HAP β-TCP is formed because electron defective structure of B—O occupies the tetrahedron position where HAP loses OH^- or PO₄^3-.

4) After the HAP composite ceramics are soaked in SBF for 14 d, a new ACP structure is formed, which performs a better biology activity and plays an important role in normal metabolism and repair of bone tissue.

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(Edited by YANG Hua)