Powder Metallurgy Progress, Vol.8 (2008), No 4 367

MICROSTRUCTURE AND PROPERTIES OF HYDROXYAPATITE CERAMICS

A. S. Fomin, V. V. Smirnov, S. V. Kucev, D. Ferro, S. M. Barinov

Abstract To develop nano-sized HA-ceramics technology, a comparative analysis of ceramics fabricated from HA-powders of different BET surface area (BETSA) – 10 (micro-HA, MHA), 70 and 110 (nano-HA, or NHA) m2/g and with the use of sintering additive based on carbonate salts in amount from 0 to 7 wt. % was carried out. Samples were formed as beams under 100 MPa pressure and sintered at 600, 650, 700 and 750°С. With an increase of SSA and the sintering additive content, linear shrinkage essentially increases. Without the sintering additive no sintering was observed in HA-samples over the whole temperature range. Strength varied from 1 to 118 MPa. The strength of NHA-samples was higher by 4-6 times than MHA-samples for the same sintering temperature and sintering additive content. According to SEM, grains size in ceramics was from 30 to 200 nm. More detailed SEM investigation of ceramic microstructures showed that sintering additive promotes the formation of compact ceramic structure. It was shown that during sintering secondary crystallization of HA occurs. Liquid phase formation was confirmed by features of melted zones in the grain boundaries. Secondary HA-crystals are rod shaped, which is typical for crystallization from the melt. Keywords: hydroxyapatite, ceramics, nano-sized materials

INTRODUCTION Hydroxyapatite is widely used in surgery for bone defects substitution [1-3]. But

there is a lack of commercially available micro-HA Materials based on nano-HA offer the possibility of overcoming such drawbacks [2]. Development of nano-sized hydroxyapatite ceramics is of current importance in modern materials science. Such materials, as expected, will possess both higher mechanical and osteoinductivity properties. In this study we investigated the influence of sintering additive based on lithium and potassium carbonates on sintering of HA-ceramics from powders of different sizes.

EXPERIMENTAL The synthesis of nano-sized HA powder was carried out at room temperature by

the following reaction (equation 1): 10Ca(NO3)2 + 6(NH4)2 HPO4 + 8NH3(aq) + 2H2O = Ca10(PO4)6(OH)2 + 20NH4NO3 (1).

200 ml of ammonia was added to 200 ml of a 1M Ca(NO3)2 solution. Then, 200 ml of (NH4)2HPO4 (0.6M) was added dropwise over 10 min. The resulting mixture was stirred for 2 h and allowed to age for 24 h. The pH of the medium was maintained above 10.5. After Alexander Sergeevich Fomin, Valery Vyacheslavovich Smirnov, Sergey Vladimirovich Kucev, Sergey Mironovich Barinov, Baikov’Institute of Metallurgy and Materials Sciences, Russian Academy of Sciences, Moscow, Russia Daniela Ferro, Institute of Nanostructured Materials, Consiglio Nazionale delle Recerche, Roma, Italy

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that, the precipitate was separated on a Buchner funnel and dried at 120°C. The powder was screened through a 60 µm sieve, disaggregated by titration in toluene or ethanol to obtain HA of nano-scale or in water, to obtain HA of micro-scale, and calcined at 400°C for 2 h. For X-ray analysis the necessary amount of powders was calcined at 900°C for 2 h.

Powder samples were studied by X-ray powder diffraction (XRD, Shimadzu XRD-6000), by the BET method of measuring specific surface areas (BET, Tristar Micromeretics) with N2 as an adsorbent and by scanning electron microscopy (SEM, LEO 1450 VP).

HA powders of different sizes were mixed with a sintering additive based on both lithium and potassium carbonates in amount from 0 to 7 wt.%. Green samples were formed by uniaxial compacting under a pressure of 100 MPa. The samples were then sintered at temperatures of 600, 650, 700 and 750ºС.

RESULTS AND DISCUSSION XRD patterns of the calcined powders correspond to JCPDS #9-437 for HA.

Estimated unit cell parameters calculated via lines (300), (410) (parameter a) and (002), (004) (parameter с) were a = 9.432 and c = 6.881 Å. No additional phases were revealed. Depending on the titration medium, BETSA for samples was estimated as 10 m2/g (water), 70 m2/g (ethanol), and 110 m2/g (toluene). Particles size corresponds to BET results – 100-300 nm (water), 20-40 nm (ethanol), and 10-20 nm (toluene). Estimated size of hydroxyapatite particles was calculated from theoretical density of HA (3.15 g/m2) and with assumption that HA particles are spherically shaped.

After sintering at different temperatures only HA phase was found in the ceramic samples with and without sintering additive. With temperature increase, HA-peaks became more narrow and their intensity increased.

Temperature dependence of linear shrinkage is shown in Fig.1. One can see that in the chosen temperature range linear shrinkage is rather intensive for samples with sintering additives. Linear shrinkage grows with an increase of BETSA value.

Fig.1. Temperature dependence of linear shrinkage.

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Temperature dependence of open porosity (determined via hydrostatics method) generally corresponds to that of linear shrinkage. It is shown that for samples with high additive content and with high BETSA value open porosity equals to 0,5-1,5% for sintering temperatures 700-750ºС. Open porosity of samples without sintering additives is higher than 40%.

Compressive strength (CS) of samples, measured with UTS-100 (Testsysteme GmbH) machine in accordance to standards ASTM C 1161 and DIN 51110, is rather different. The CS values are in a range 1-118 MPa. CS of samples from nano-HA are stronger than of micro-HA for the same sintering temperature and additive content. At additive content of 2 wt.% CS value exceeds that of samples without additive by factor of 4-6.

Fig.2. SEM pictures of ceramic structure from powder with BETSA 10 m2/g after

sintering at 700°С without sintering additive (a) and with sintering additive content 7 wt.% (b).

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Fig.3. SEM pictures of ceramics structure from powder with BETSA 110 m2/g after

sintering at 700°С with sintering additive content 2 wt.%.

SEM microstructures of ceramics formed from micro-HA with and without sintering additive are shown in Fig.2. One can clearly see that sintering mechanism with and without sintering additive is rather different. With the sintering additive, grain boundaries are fused. This shows that the sintering additive promotes formation of a liquid phase. One can see that without the additive ceramics seem not to be sintered, the open porosity is higher than 50%. With the additive, compact structure of material is formed, the open porosity being near 3%. It is evident that secondary crystallization processes take place. Secondary HA-crystals are rod shaped, which is typical for crystallization from the melt. CS of sample with the additive is 90 MPa. Figure 3 shows the microstructure of ceramics sintered at 700ºC nano-scale HA (110 m2/g) with 2 wt.% sintering additive. One can see that that such ceramics is nano-sized, with grain size 30-50 nm. CS of such ceramics is 45 MPa and open porosity 26%.

Fig.4. SEM pictures of ceramics structure from powder with BETSA 110 m2/g after

sintering at 700°С with sintering additive content 7 wt.%.

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When sintering the same powder, but with higher additive content (7 wt.%), detected were uncommon microstructure states: powder nano-particles form some kind of matrix and joined into hexagonal or more complex aggregates (Fig.4). A self-reinforcement process resulted in high strength values – 118 MPa with open porosity near 10 %.

CONCLUSIONS HA-powders of micro- and nano-scale were synthesized. HA-ceramics from HA-

powders of different sizes, with and without sintering additive based on lithium and potassium carbonates were studied. It was shown that sintering mechanisms of ceramics with and without sintering additive are different. Sintering additive promotes the formation of a liquid phase. This results in reduction of sintering temperature and increase of mechanical properties.

Acknowledgements This work was supported by the Russian Foundation for Basic Research (project

no. 06-03-32192).

REFERENCES [1] Aoki, H.: Science and Medical Applications of Hydroxyapatite. Tokyo : JAAS, 1991 [2] Barinov, SM., Komlev, VS.: Biokeramika na osnove fosfatov kal’tsiya (Bioceramics

Based on Calcium Phosphates). Moscow : Nauka, 2005 [3] Veresov, AG., Putlyaev, VI., Tret’yakov, YuD.: Ross. Khim. Zh., vol. 44, 2000, no. 36,

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MICROSTRUCTURE AND PROPERTIES OF HYDROXYAPATITE CERAMICS A. S. Fomin, V. V. Smirnov, S. V. Kucev, D. Ferro, S. M. Barinov  Abstract Keywords: hydroxyapatite, ceramics, nano-sized materials INTRODUCTION EXPERIMENTAL RESULTS AND DISCUSSION Fig.1. Temperature dependence of linear shrinkage.Fig.2. SEM pictures of ceramic structure from powder with BETSA 10 m2/g after sintering at 700(С without sintering additive (a) and with sintering additive content 7 wt.% (b).Fig.3. SEM pictures of ceramics structure from powder with BETSA 110 m2/g after sintering at 700(С with sintering additive content 2 wt.%.Fig.4. SEM pictures of ceramics structure from powder with BETSA 110 m2/g after sintering at 700(С with sintering additive content 7 wt.%.

CONCLUSIONS Acknowledgements REFERENCES