Hydroxyapatite/Al 2 O 3 /diopside ceramic composites and their behaviour in simulated body fluid

Preparation and characterisation of HA/Al₂O₃/diopside ceramic composites

Hydroxyapatite of high purity (99%) and small grain size (1–3 μm), produced by the Department of Materials Science and Engineering in Shandong University, was used as the starting material. Commercial small grain size (0·5–1 μm) 99·9% pure Al₂O₃ powder and diopside [MgCa(SiO₃)₂], composed of 55 wt-%SiO₂, 24 wt-%CaO and18 wt-%MgO, were used as additives. The raw materials were blended with each other according to their proportions listed in Table 1. Milling was carried out for 100 h in alcohol using a vibratory ball mill with cemented carbide balls, and then metal mill media impurities were removed by washing in 10 mol.-% hydrochloric acid. After drying, densification of the powder was achieved in a graphite die by uniaxial hot pressing at 1320°C (heating rate, 20°C min⁻¹) at a pressure of 20 MPa in an N₂ atmosphere for 60 min.

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Table 1

Starting compositions and mechanical properties of HA/Al₂O₃/diopside ceramic composites

Specimens	Compositions/wt-%			Mechanical properties
	HA	Al2O3	Diopside	Vickers hardness/GPa	Flexural strength/MPa	Fracture toughness/MPa m1/2
H0	100	0	0	5·8±1·3	26±3	1·0±0·22
H1	68	30	2	5·3±1·2	162±3	2·4±0·16

The sintered bodies were cut into bars, and then standard testpieces (3×4×36 mm) were ground and polished with diamond paste. Three-point bending mode was used to measure the bending strength using an electronic universal experimental instrument (Jinan Test Co., Ltd) with a span of 20 mm at a crosshead speed of 0·5 mm min⁻¹. At least 12 specimens were tested for each series of composition in air at room temperature. Hardness was measured on the polished surfaces with a load of 9·8 N for 5 s using a microhardness tester (Shanghai Hengyi Electronic Testing Instrument Corporation). Fracture toughness measurements were performed using the indentation method. The indentations on polished surfaces were generated by a Vickers microhardness tester with a diamond pyramid indenter at a load of 196 N and a loading time of 30 s. The formula proposed by Cook and Lawn9 was used to calculate the final fracture toughness. Data of hardness and fracture toughness were determined using at least 10 indentations on polished surfaces with an R_a of 0·1 μm for each specimen.

Ability of HA/Al₂O₃/diopside ceramic composites to form apatite layer

Simulated body fluid containing ion concentrations similar to those in human blood plasma was prepared according to the method described by Kokubo.10 Briefly, reagent grade CaCl₂, K₂HPO₄.3H₂O, NaCl, KCl, MgCl₂.6H₂O, NaHCO₃ and Na₂SO₄ were dissolved in distilled water and adjusted to pH 7·25. Pure HA and HA/Al₂O₃/diopside ceramic bars were soaked in SBF at 37°C for 9 days. After soaking, the bars were dried at 100°C for half day. Microstructures of the specimens were studied on the original polished surfaces and on the soaked bar surfaces by scanning electron microscopy (Hitachi S-570). Fourier transform infrared spectroscopy (FTIR; Vector) analysis was adopted to identify the appearance of OH^– and . The element analysis in the microzone on the surfaces of the soaked pure HA and HA/Al₂O₃/diopside ceramic composites was detected by an electron probe microanalyser (EPMA; JXA-8800R).

Microstructures of HA/Al₂O₃/diopside ceramic composites

Scanning electron microscopy images of the H₀ specimen before and after soaking in SBF for 9 days are shown in Figs. 1 and 2. Those of the H₁ specimen before and after soaking in SBF for 9 days are shown in Figs. 3–5 respectively. The polished surfaces of H₀ and H₁ specimens before soaking in SBF were smooth (Figs. 1 and 3). Compared with the polished surfaces of H₀ and H₁ specimens before soaking in SBF, there were obvious bright mineral layers precipitated on the polished surfaces of H₀ and H₁ specimens after soaking in SBF for 9 days (Figs. 2, 4 and 5). The mineral layers that precipitated on the polished surfaces of H₀ specimen showed a typical ‘cauliflower’ morphology of HA crystals (Fig. 2). There were some crystals with cuboidal shape precipitated on the polished surfaces of H₁ specimen (Figs. 4 and 5), which suggested that they may be sodium chloride. The precipitation of some cuboidal shaped crystals may be mainly because SBF was not rinsed off. Some crystals with cauliflower shape were also found in Fig. 5, which indicated that the introduction of Al₂O₃ and diopside in the HA matrix could not only improve the strength and toughness of the composites but also maintain its ability to form an apatite layer. In order to detect the phases of the bright mineral layers formed on the composite surfaces, the specimens were characterised by FTIR and EPMA respectively.

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Figure 1

Image (SEM) of H₀ specimen before soaking in SBF (×12 000)

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Figure 2

Image (SEM) of H₀ specimen after soaking in SBF for 9 days (×500)

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Figure 3

Image (SEM) of H₁ specimen before soaking in SBF (×8000)

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Figure 4

Image (SEM) of H₁ specimen after soaking in SBF for 9 days (×4000)

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Figure 5

Image (SEM) of H₁ specimen after soaking in SBF for 9 days (×6000)

Fourier transform infrared spectroscopy analysis of HA/Al₂O₃/diopside ceramic composites

Fourier transform infrared spectra of the H₀ and H₁ specimens after soaking in SBF for 9 days were analysed by FTIR (Vector, Germany) (Figs. 6 and 7). The vibration peak of hydroxyl bonds located at 3575, 3444, 3440, 1646, 634, 603 and 601 cm⁻¹, and that of at 1644 and 1470 cm⁻¹. The vibration peak of ranged from 1124 to 946 cm⁻¹. The analysis of FTIR spectra made it sure that the phase of bright mineral layers, formed on the surfaces of the H₀ and H₁ specimens after soaking in SBF for 9 days, were mainly HAP or hydroxycarbonate apatite (HCA), which had a similar mineral composition to human bones.11,12

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Figure 6

Fourier transform infrared spectra of H₀ specimen after soaking in SBF for 9 days

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Figure 7

Fourier transform infrared spectra of H₁ specimen after soaking in SBF for 9 days

Electron probe microanalyser analysis of HA/Al₂O₃/diopside ceramic composites

Energy dispersive spectroscopy spectra of the H₀ and H₁ specimens after soaking in SBF for 9 days are shown in Figures 8 and 9 respectively. Ca and P appeared on the surfaces of H₀ and H₁ specimens after soaking in SBF for 9 days, which indicated that the phase of bright mineral layers were mainly HAP or HCA.

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Figure 8

Energy dispersive spectroscopy spectra of H₀ specimen after soaking in SBF for 9 days

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Figure 9

Energy dispersive spectroscopy spectra of H₁ specimen after soaking in SBF for 9 days

It was reported that silicon played an important role for the nucleation and growth mechanism of the apatite-like layer on bioactive composites.13,14 In the present study, silicon was introduced by adding diopside in the composites, so an interchange was suggested to take place between the Ca²⁺ ions of the HA/Al₂O₃/diopside ceramic composites and the H₃O⁺ of the SBF, which could promote the formation of Si–OH groups and induce apatite nucleation. The nuclei thus formed and grew at the expense of the ions in the solution that has been saturated with respect to apatite. Therefore, silicon seemed to be very important in maintaining the in vitro bioactive behaviour of HA/Al₂O₃/diopside ceramic composites.