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Aqueous gel-casting of hydroxyapatite
Materials Science and Engineering A 435–436 (2006) 198–203
Aqueous gel-casting of hydroxyapatite Biqin Chen, Zhaoquan Zhang, Jingxian Zhang, Manjiang Dong, Dongliang Jiang ∗ The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 6 April 2006; received in revised form 19 June 2006; accepted 3 July 2006
Abstract In the present work, an aqueous gel-casting of hydroxyapatite using a new polymeric system has been investigated. The effects of the dispersant, pH values on the dispersing ability of HA powder were studied and the rheological behavior of the slurries was investigated. The green body has a density of 1.621 g/cm3 and compressive strength as 32.6 ± 3.2 MPa. After pressureless sintering at 1250 ◦ C for 2 h the relative density of the final product is about 96.7%. Bending strength, elasticity modulus and hardness are 84.6 ± 12.6 MPa, 138 ± 7 GPa and 4.44 ± 0.35 GPa, respectively. SEM images show a compact and uniform microstructure; XRD and FTIR determined the phase and the radical preserved after sintering. © 2006 Elsevier B.V. All rights reserved. Keywords: Colloidal processing; Gel-casting; Hydroxyapatite; Suspensions; Sintering
1. Introduction Hydroxyapatite (HA) and calcium phosphate ceramics have been widely used for the replacement of the bone tissue [1–4]. Due to the similarity in composition and high biocompatibility with natural bone, HA can easily bound to bone. However, the main drawbacks of HA are poor mechanical properties and the difficulty to manufacture parts with desired shapes. Many efforts have been made to overcome these problems using colloidal processing techniques, such as slip-casting [5–7], tape-casting [8,9] and gel-casting [10–13]. Among these techniques, gel-casting has significant advantages over others, in terms of dimensional accuracy and complex shaping capabilities, and moreover, the uniform structure and high strength of HA ceramics by gelcasting will surely give high reliability to HA parts for clinical applications. Gel-casting, developed by Janney and co-workers [14–16], has been utilized in the forming of many sorts of ceramic material systems, such as alumina [17,18], silicon nitride [19], silicon carbide [20,21] and alumina–zirconia [22] composites. However, few reports about the use of this method for dense HA are found in the literature [10,11]. These reports used the traditional polymeric system that is conflicted with the preparation of high solid concentration HA slurries. The HA ceramics they got still
have poor mechanical properties and phase impurity due to high sintering temperature and long soaking time. In this work, HA powders with appropriate specific surface area and particle size were prepared. The effects of the dispersant, pH on the dispersing ability of HA powder were studied and the rheological behavior of the slurries in the new polymeric system was investigated. Enhanced mechanical properties of green pieces and sintered HA sample were fabricated. 2. Experimental procedure 2.1. Starting materials Calcium nitrate tetrahydrate and diammonium hydrogen phosphate were used as starting materials to prepare HA powders. Ammonium hydroxide was used to adjust pH value. All the regents used were chemically pure. In the gel-casting process, dimethylacrylamide (DMAA) was used as monomer, while N,N -methylenebisacrylamide (MBAM, Aldrich) as crosslinker. The initiator here we used 2,2 -azobis[2-(2-imidazolin2-yl)propane] (AZIP), a kind of water soluble azo-initiator. The dispersant was Lopon890, a polyacrylic acid sodium salt (PAANa, BK Giulini, MW = 4000–5000, pH 8.0–9.0). 2.2. HA preparation and characterization
∗
Corresponding author. Tel.: +86 21 5241 2606; fax: +86 21 5241 3122. E-mail address: [email protected] (D. Jiang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.028
HA powders were prepared by a wet-chemical synthesis technique, based on the precipitation of HA precursors from
B. Chen et al. / Materials Science and Engineering A 435–436 (2006) 198–203
aqueous solutions. Diammonium hydrogen phosphate solution was slowly added to calcium nitrate solution with the pH value at 10 adjusting by ammonium hydroxide solution. Then the HA precursors solution was aged for 24 h at 70 ◦ C, decanted, rinsed with deionized water and 95% ethanol. After filtration, the cake was dried at 100 ◦ C for 12 h and then calcined at 900 ◦ C for 2 h. Finally, the powder was wet-milled for 20 h to gain suitable particle size distribution and specific surface area. Specific surface area was calculated using the BET method from N2 adsorption isotherm obtained in a Micromeritics ASAP2010 analyzer. Particle size distribution and mean particle size were determined by sedimentation using a SICAS-4800 photo size analyzer. 2.3. Slurry preparation An aqueous solution containing 12 wt.% of DMAA and MBAM (Aldrich) in 13/1 ratio was used as dispersing medium. The slurries were prepared by adding HA powder to the premix solutions under mechanical agitation while keeping the temperature of the suspension at 0–5 ◦ C with a water bath. Zeta potentials of the HA particles at different pH values were measured with Zetaplus (Brookhaven Instruments Corporation). 0.001 M KCl was used as supporting electrolytes. NaOH and HCl solutions were used as pH adjustors. HA suspensions (5 vol.%) in the absence and presence of dispersant were made for sedimentation measurements. After mixing for 24 h, suspensions were poured into graded cylinders and the sedimentation volumes were recorded. Rheological measurements were performed on a stresscontrolled rheometry (SR-5 Rheomeric scientific instrument company, U.S.A.). Measurements were carried out at 20 ◦ C.
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ing machine, at a crosshead speed 2.5 mm/min. The flexural strength of HA ceramics was measured by three-point bending, using a span of 30 mm and a crosshead speed 0.5 mm/min. The indentation test was performed in a microhardness tester (IF, AkashIII, Japan), with a Vickers indenter, applying a load of 2.0 kg for 10 s. Interference lenses were used in the optic microscope to determine clearly the indentation. 3. Results and discussions 3.1. HA characterization It has been documented that HA powders with a specific area about 10 m2 /g would be appropriate for obtaining the stable suspension and subsequent sintering [6,26]. The initial powder was heat-treated at 900 ◦ C and then wet-milled for 20 h to make powder with specific surface area as 10.40 m2 /g. Fig. 1 shows the particle size distribution of the milled powder. The mean particle size is 0.60 m. In the future work, this powder will be used as the starting material for gel-casting pieces.
2.4. Gel-casting and sintering The suspension was degassed in order to eliminate the air bubbles trapped inside before casting into moulds (50 mm length, 6 mm width and 7 mm height). Then the moulds were put in water bath at 60 ◦ C for 30 min in order to gel the system. The gelled pieces were carefully dried to avoid cracking. The green blocks were heated to 600 ◦ C at a heating rate of 1.0 ◦ C/min to burn out the monomers and other volatiles, followed by the pressureless sintering at 1250 ◦ C for 2 h for densification purpose.
Fig. 1. Particle size distribution of the starting HA powder.
2.5. Characterization of gel-casting bodied and ceramics bodies The density of green pieces was determined by Hg intrusion porosimetry in a Micromeritics ASAP2010 porosimeter. The relative density and porosity of the sintered samples were determined by Archimedes’s method. The microstructure of the composites was observed on the fractured surface and polished surface by scanning electron microscopy (SEM) (EPMA8705Q, HII, Shimadzu, Japan). The average grain size was found by using the linear intercept method. The compressive strength of the green bodies was measured in Instron 5566 universal test-
Fig. 2. Zeta potential of starting HA powder in the absence and presence of dispersant (PAA-Na).
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3.4. Slurries preparation and characterization
Fig. 3. Influence of pH on the stability of 5 vol.% HA slurries in the absence and presence of dispersant after 7 days.
3.2. Surface properties of HA
3.4.1. Effect of dispersant content Fig. 4 shows viscosity curves and shear stress versus shear rate of 40 vol.% HA slurry containing various quantities of dispersant. The viscosity and the shear stress decreased with the increase amount of dispersant. The slurry containing 1.5 wt.% dispersant showed the lowest viscosity and shear stress. However, the higher content of dispersant caused the increase of viscosity and shear stress. Since the pH of the present slurry is about 9, PAA-Na of dispersant might be dissociated into ionic PAA and Na+ . The ionic PAA may be adsorbed to the particle surface and thus provide the electrosteric effect to stabilize the slurry, thereby resulting in a decrease in the viscosity of the slurry. When the added PAA-Na content is higher, the free ionic PAA would exist in the liquid medium. This may cause inter-locking of PAA polymer chain, which bridge the solid particles and caused viscosity increase. Therefore, 1.5 wt.% PAA-Na was chosen as optimum dispersant amount.
The electrophoretic mobility of the HA powder with and without dispersant were studied as a function of pH. The results are shown in Fig. 2. The isoelectric point (IEP) for the HA particles was not observed over the range of pH values evaluated. This result may suggest HA particles that have been prepared via precipitation, to be covered by a thin film of specifically adsorbed ions. In this paper, polyacrylic sodium salt (MW = 4000–5000) was used as dispersant. The addition of PAA-Na leads to an obvious increase of negative charge on HA particle surface, indicating the chemical adsorption type of it. In the presence of dispersant, the zeta potential curves showed big absolute value at pH > 7.5. 3.3. Sedimentation study The sedimentation volume percent as a function of pH is shown in Fig. 3. Without the dispersant, the slurry is stable in pH 10–12 range, at which the HA powder is stabilized electrostatically due to the high surface charge. At low pH range, due to dissolution of HA, the slurries are not stable. With the dispersant PAA-Na, the slurry can be stabilized in a wide pH range (pH 7–12). At pH < 7, HA powders were easily agglomerated, which was confirmed by the high sedimentation volume. The reason is that added PAA-Na was adsorbed on the surface of HA powders as neutral PAA in the acidic region [23], and the repulsive energy between HA powders was low. On the contrary, at pH > 7, the slurry is well stabilized characterized as a low sedimentation volume and high suspension volume. This slurry stability can be correlated to the electrosteric mechanism [24]. At high pH region, the dispersant was adsorbed as ionic PAA. The adsorbed ionic PAA increased the electrostatic repulsive energy between HA particles. Moreover, the high steric repulsive between the adsorbed polymers also enhanced the stability of the slurries. Due to the alkali of PAA-Na and initiator, the slurry was in the pH > 7 so we did not have to adjust the slurries’ pH value any more.
Fig. 4. Influence of dispersant content on (a) the viscosity and (b) the shear stress vs. shear rate of 40 vol.% HA slurries.
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3.4.2. Effect of solids loading Fig. 5 shows the rheological curves of slurries containing 40, 50 and 51.2 vol.% of HA powder. In gel-casting, the solids loading of the slurry becomes the green density of the cast part. Consequently, it is important to have as high a solids loading as possible in the slurry. On the other hand, low viscosity is beneficial both mixing and casting in slurry processing. It is, therefore, important to maintain slurry fluidity while optimum solids loading. The slurry containing 40 and 50 vol.% of HA powder showed a shear thinning behavior, whereas the 51.2 vol.% slurry showed a shear thinning behavior at low shear rate and showed shear thickening behavior at higher shear rate. And with the increase of the solids loading, the viscosity increased. The shear thinning behavior shows that viscosity is strongly dependent on the solids loading. But when the solids loading was above 51.2 vol.%, it was hard to get slurry with well rheological properties in the study. The poor rheological properties of the slurry will cause defects in the green body when casting. Optimum gel-casting slurry should have high solids concentration and low viscosity [14–16]. Finally, the 50 vol.% slurry with high enough solids loading and fluid enough suspension was used to prepare ceramic bodies.
Fig. 5. Properties of HA suspensions with different solids loading.
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3.5. Pieces characterization After casting, gelification and drying, well shaped green bodies were obtained. The pieces could be easily demoulded. Neither contraction nor cracking was observed and the pieces showed enough strength to be handled. The dried pieces were sintered at 600 ◦ C (1 ◦ C/min) for 1 h to burn out volatiles followed by densification with 2 h holding at 1250 ◦ C (5 ◦ C/min). In Table 1 the characteristics of green body and the sintered samples was showed. The relative green density is 51.4%, lower than the common pressing pieces, similar as the other colloidal process. On the contrary, the mechanical properties of green pieces are higher than other processes mentioned here. The green pieces have a compressive strength of 32.6 ± 3.2 MPa. The high strength, resulting from the cross-linking gel network in green body, leads to an easy green machining. The density of sintered body from 50 vol.% suspension is 3.057 ± 0.008 g/cm3 (96.7% of theoretic density); Bending strength, elasticity modulus and hardness is 84.6 ± 12.6 MPa, 138 ± 7 GPa and 4.44 ± 0.35 GPa, respectively. The mechanical properties of sintered pieces are
Fig. 6. Fracture and surface microstructure of HA pieces.
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Table 1 Properties of HA green pieces and sintered pieces
Green body Sintered body (50 vol.% slurry) a
Density (g/cm3 )
Flexural strength (MPa)a
Hardness (GPa)
Elasticity modulus (GPa)
1.621 3.057 ± 0.008
32.6 ± 3.2 84.6 ± 12.6
– 4.44 ± 0.35
– 138 ± 7.0
Green body was characteristic as compressive strength.
4. Conclusions A thermal treatment at 900 ◦ C follow by ball-milling for 20 h led to HA particles obtained by chemical precipitation with specific area as 10.40 m2 /g. With the dispersant, the HA slurry can be stabilized in a wide pH range (pH 7–12). The amount of the dispersant and the solids loading are the important factors that determine the rheological behavior and viscosity of the slurries. The optimum amount of the dispersant PAA-Na is 1.5 wt.% of the added powder, enables to prepare fluid suspension containing 50 vol.% solids loading with satisfied rhelogical properties suitable for gel-casting. After gel-casting, drying and sintering at 1250 ◦ C, HA samples can be densified to a relative density of 96.7%. The mechanical properties of the obtained pieces are satisfying, with the flexural strength, hardness and the elasticity modulus as 84.6 ± 12.6 MPa, 138 ± 7 GPa and 4.44 ± 0.35 GPa, respectively. Acknowledgement The authors thank the National Science Foundation of China (Grant No. 50232010) for the grants that support this research. References
Fig. 7. (a) XRD patterns and (b) FTIR spectra of staring HA powder and sintered body.
higher than that from the similar process [10,11], while within the range of reported for HA [4]. Fracture and surface micrographs (SEM) of pieces prepared after sintering are shown in Fig. 6. The surface is characteristic of compacted ceramics materials. As the surface micrograph showed, the mean grain size is determined to be 2.13 m by using the linear intercept method. The fracture micrograph shows that the fracture mode is mainly transgranular. XRD patterns and FTIR spectra after sintering compared with starting powder are shown in Fig. 7a and b, respectively. The XRD did not show the presence of the other phases, indicating that HA phase is well preserved after being sintered, which is in agreement with the literature [27]. FTIR spectra also showed the bands of OH− P–O–H and PO4 3− radical before and after sintering mean that the active radical of HA are preserved [25].
[1] L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487–1510. [2] S.V. Dorozhkin, M. Epple, Angew. Chem. Int. Ed. 41 (2002) 3130– 3146. [3] T. Kokubo, H.-M. Kim, M. Kawashita, Biomaterials 24 (2003) 2161– 2175. [4] W. Suchanck, M. Yoshimura, J. Mater. Res. 13 (1998) 94–117. [5] R. Rao, S.K. Kannan, J. Am. Ceram. Soc. 84 (2001) 1710–1717. [6] L.M. Rodr’ıguez-Lorenzo, M. Vallet-Reg’ı, J.M.F. Ferreira, Biomaterials 22 (2001) 1847–1852. [7] X.M. Yao, S.H. Tan, J. Mater. Sci.: Mater. Med. 16 (2005) 161–165. [8] I.H. Arita, D.S. Wilkinson, V.M. Castazo, J. Mater. Sci.: Mater. Med. 6 (1995) 19–23. [9] J.X. Zhang, M. Maeda, N. Kotobuki, M. Hirose, H. Ohgushi, D.L. Jiang, M. Iwasa, Mater. Chem. Phys., in press. [10] S. Padilla, R. Garc´ıa-Carrodeguas, M. Vallet-Reg´ı, J. Eur. Ceram. Soc. 24 (2004) 2223–2232. [11] S. Padilla, M. Vallet-Reg´ı, M.P. Ginebra, F.J. Gil, J. Eur. Ceram. Soc. 25 (2005) 375–383. [12] H.R. Ramay, M. Zhang, Biomaterials 24 (2003) 3293–3302. [13] P. Sepulveda, F.S. Ortega, Murilo D.M. Innocentini, V.C. Pandolfelli, J. Am. Ceram. Soc. 12 (2000) 3021–3024. [14] M.A. Janney, Method for forming ceramic powders into complex shapes. U.S. Patent 4,894,194 (1990). [15] O.O. Omatete, M.A. Janney, R.A. Strehlow, Am. Ceram. Soc. Bull. 70 (1991) 1641–1649. [16] O.O. Omatete, M.A. Janney, R.A. Strehlow, J. Am. Ceram. Soc. 17 (1997) 407–413. [17] P. Sepulveda, J.G.P. Binner, J. Eur. Ceram. Soc. 19 (1999) 2059–2066.
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[23] V.A. Hackley, J. Am. Ceram. Soc. 9 (1997) 2315–2325. [24] Y. Bao, A.M.R. Senos, M. Almeida, L.J. Gauckler, J. Mater. Sci.: Mater. Med. 13 (2002) 639–643. [25] P.N. Kumta, C. Sfeir, D.-H. Lee, D. Olton, D. Choi, Acta Biomater. 1 (2005) 65–83. [26] F. Leli`evre, D. Bernache-Assollant, T. Chartier, J. Mater. Sci.: Mater. Med. 7 (1996) 489–494. [27] A.J. Ruys, M. Wei, C.C. Sorrell, M.R. Dickson, A. Brandwood, B.K. Milthorpe, Biomaterials 16 (1995) 409–415.
Aqueous gel-casting of hydroxyapatite Biqin Chen, Zhaoquan Zhang, Jingxian Zhang, Manjiang Dong, Dongliang Jiang ∗ The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 6 April 2006; received in revised form 19 June 2006; accepted 3 July 2006
Abstract In the present work, an aqueous gel-casting of hydroxyapatite using a new polymeric system has been investigated. The effects of the dispersant, pH values on the dispersing ability of HA powder were studied and the rheological behavior of the slurries was investigated. The green body has a density of 1.621 g/cm3 and compressive strength as 32.6 ± 3.2 MPa. After pressureless sintering at 1250 ◦ C for 2 h the relative density of the final product is about 96.7%. Bending strength, elasticity modulus and hardness are 84.6 ± 12.6 MPa, 138 ± 7 GPa and 4.44 ± 0.35 GPa, respectively. SEM images show a compact and uniform microstructure; XRD and FTIR determined the phase and the radical preserved after sintering. © 2006 Elsevier B.V. All rights reserved. Keywords: Colloidal processing; Gel-casting; Hydroxyapatite; Suspensions; Sintering
1. Introduction Hydroxyapatite (HA) and calcium phosphate ceramics have been widely used for the replacement of the bone tissue [1–4]. Due to the similarity in composition and high biocompatibility with natural bone, HA can easily bound to bone. However, the main drawbacks of HA are poor mechanical properties and the difficulty to manufacture parts with desired shapes. Many efforts have been made to overcome these problems using colloidal processing techniques, such as slip-casting [5–7], tape-casting [8,9] and gel-casting [10–13]. Among these techniques, gel-casting has significant advantages over others, in terms of dimensional accuracy and complex shaping capabilities, and moreover, the uniform structure and high strength of HA ceramics by gelcasting will surely give high reliability to HA parts for clinical applications. Gel-casting, developed by Janney and co-workers [14–16], has been utilized in the forming of many sorts of ceramic material systems, such as alumina [17,18], silicon nitride [19], silicon carbide [20,21] and alumina–zirconia [22] composites. However, few reports about the use of this method for dense HA are found in the literature [10,11]. These reports used the traditional polymeric system that is conflicted with the preparation of high solid concentration HA slurries. The HA ceramics they got still
have poor mechanical properties and phase impurity due to high sintering temperature and long soaking time. In this work, HA powders with appropriate specific surface area and particle size were prepared. The effects of the dispersant, pH on the dispersing ability of HA powder were studied and the rheological behavior of the slurries in the new polymeric system was investigated. Enhanced mechanical properties of green pieces and sintered HA sample were fabricated. 2. Experimental procedure 2.1. Starting materials Calcium nitrate tetrahydrate and diammonium hydrogen phosphate were used as starting materials to prepare HA powders. Ammonium hydroxide was used to adjust pH value. All the regents used were chemically pure. In the gel-casting process, dimethylacrylamide (DMAA) was used as monomer, while N,N -methylenebisacrylamide (MBAM, Aldrich) as crosslinker. The initiator here we used 2,2 -azobis[2-(2-imidazolin2-yl)propane] (AZIP), a kind of water soluble azo-initiator. The dispersant was Lopon890, a polyacrylic acid sodium salt (PAANa, BK Giulini, MW = 4000–5000, pH 8.0–9.0). 2.2. HA preparation and characterization
∗
Corresponding author. Tel.: +86 21 5241 2606; fax: +86 21 5241 3122. E-mail address: [email protected] (D. Jiang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.028
HA powders were prepared by a wet-chemical synthesis technique, based on the precipitation of HA precursors from
B. Chen et al. / Materials Science and Engineering A 435–436 (2006) 198–203
aqueous solutions. Diammonium hydrogen phosphate solution was slowly added to calcium nitrate solution with the pH value at 10 adjusting by ammonium hydroxide solution. Then the HA precursors solution was aged for 24 h at 70 ◦ C, decanted, rinsed with deionized water and 95% ethanol. After filtration, the cake was dried at 100 ◦ C for 12 h and then calcined at 900 ◦ C for 2 h. Finally, the powder was wet-milled for 20 h to gain suitable particle size distribution and specific surface area. Specific surface area was calculated using the BET method from N2 adsorption isotherm obtained in a Micromeritics ASAP2010 analyzer. Particle size distribution and mean particle size were determined by sedimentation using a SICAS-4800 photo size analyzer. 2.3. Slurry preparation An aqueous solution containing 12 wt.% of DMAA and MBAM (Aldrich) in 13/1 ratio was used as dispersing medium. The slurries were prepared by adding HA powder to the premix solutions under mechanical agitation while keeping the temperature of the suspension at 0–5 ◦ C with a water bath. Zeta potentials of the HA particles at different pH values were measured with Zetaplus (Brookhaven Instruments Corporation). 0.001 M KCl was used as supporting electrolytes. NaOH and HCl solutions were used as pH adjustors. HA suspensions (5 vol.%) in the absence and presence of dispersant were made for sedimentation measurements. After mixing for 24 h, suspensions were poured into graded cylinders and the sedimentation volumes were recorded. Rheological measurements were performed on a stresscontrolled rheometry (SR-5 Rheomeric scientific instrument company, U.S.A.). Measurements were carried out at 20 ◦ C.
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ing machine, at a crosshead speed 2.5 mm/min. The flexural strength of HA ceramics was measured by three-point bending, using a span of 30 mm and a crosshead speed 0.5 mm/min. The indentation test was performed in a microhardness tester (IF, AkashIII, Japan), with a Vickers indenter, applying a load of 2.0 kg for 10 s. Interference lenses were used in the optic microscope to determine clearly the indentation. 3. Results and discussions 3.1. HA characterization It has been documented that HA powders with a specific area about 10 m2 /g would be appropriate for obtaining the stable suspension and subsequent sintering [6,26]. The initial powder was heat-treated at 900 ◦ C and then wet-milled for 20 h to make powder with specific surface area as 10.40 m2 /g. Fig. 1 shows the particle size distribution of the milled powder. The mean particle size is 0.60 m. In the future work, this powder will be used as the starting material for gel-casting pieces.
2.4. Gel-casting and sintering The suspension was degassed in order to eliminate the air bubbles trapped inside before casting into moulds (50 mm length, 6 mm width and 7 mm height). Then the moulds were put in water bath at 60 ◦ C for 30 min in order to gel the system. The gelled pieces were carefully dried to avoid cracking. The green blocks were heated to 600 ◦ C at a heating rate of 1.0 ◦ C/min to burn out the monomers and other volatiles, followed by the pressureless sintering at 1250 ◦ C for 2 h for densification purpose.
Fig. 1. Particle size distribution of the starting HA powder.
2.5. Characterization of gel-casting bodied and ceramics bodies The density of green pieces was determined by Hg intrusion porosimetry in a Micromeritics ASAP2010 porosimeter. The relative density and porosity of the sintered samples were determined by Archimedes’s method. The microstructure of the composites was observed on the fractured surface and polished surface by scanning electron microscopy (SEM) (EPMA8705Q, HII, Shimadzu, Japan). The average grain size was found by using the linear intercept method. The compressive strength of the green bodies was measured in Instron 5566 universal test-
Fig. 2. Zeta potential of starting HA powder in the absence and presence of dispersant (PAA-Na).
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3.4. Slurries preparation and characterization
Fig. 3. Influence of pH on the stability of 5 vol.% HA slurries in the absence and presence of dispersant after 7 days.
3.2. Surface properties of HA
3.4.1. Effect of dispersant content Fig. 4 shows viscosity curves and shear stress versus shear rate of 40 vol.% HA slurry containing various quantities of dispersant. The viscosity and the shear stress decreased with the increase amount of dispersant. The slurry containing 1.5 wt.% dispersant showed the lowest viscosity and shear stress. However, the higher content of dispersant caused the increase of viscosity and shear stress. Since the pH of the present slurry is about 9, PAA-Na of dispersant might be dissociated into ionic PAA and Na+ . The ionic PAA may be adsorbed to the particle surface and thus provide the electrosteric effect to stabilize the slurry, thereby resulting in a decrease in the viscosity of the slurry. When the added PAA-Na content is higher, the free ionic PAA would exist in the liquid medium. This may cause inter-locking of PAA polymer chain, which bridge the solid particles and caused viscosity increase. Therefore, 1.5 wt.% PAA-Na was chosen as optimum dispersant amount.
The electrophoretic mobility of the HA powder with and without dispersant were studied as a function of pH. The results are shown in Fig. 2. The isoelectric point (IEP) for the HA particles was not observed over the range of pH values evaluated. This result may suggest HA particles that have been prepared via precipitation, to be covered by a thin film of specifically adsorbed ions. In this paper, polyacrylic sodium salt (MW = 4000–5000) was used as dispersant. The addition of PAA-Na leads to an obvious increase of negative charge on HA particle surface, indicating the chemical adsorption type of it. In the presence of dispersant, the zeta potential curves showed big absolute value at pH > 7.5. 3.3. Sedimentation study The sedimentation volume percent as a function of pH is shown in Fig. 3. Without the dispersant, the slurry is stable in pH 10–12 range, at which the HA powder is stabilized electrostatically due to the high surface charge. At low pH range, due to dissolution of HA, the slurries are not stable. With the dispersant PAA-Na, the slurry can be stabilized in a wide pH range (pH 7–12). At pH < 7, HA powders were easily agglomerated, which was confirmed by the high sedimentation volume. The reason is that added PAA-Na was adsorbed on the surface of HA powders as neutral PAA in the acidic region [23], and the repulsive energy between HA powders was low. On the contrary, at pH > 7, the slurry is well stabilized characterized as a low sedimentation volume and high suspension volume. This slurry stability can be correlated to the electrosteric mechanism [24]. At high pH region, the dispersant was adsorbed as ionic PAA. The adsorbed ionic PAA increased the electrostatic repulsive energy between HA particles. Moreover, the high steric repulsive between the adsorbed polymers also enhanced the stability of the slurries. Due to the alkali of PAA-Na and initiator, the slurry was in the pH > 7 so we did not have to adjust the slurries’ pH value any more.
Fig. 4. Influence of dispersant content on (a) the viscosity and (b) the shear stress vs. shear rate of 40 vol.% HA slurries.
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3.4.2. Effect of solids loading Fig. 5 shows the rheological curves of slurries containing 40, 50 and 51.2 vol.% of HA powder. In gel-casting, the solids loading of the slurry becomes the green density of the cast part. Consequently, it is important to have as high a solids loading as possible in the slurry. On the other hand, low viscosity is beneficial both mixing and casting in slurry processing. It is, therefore, important to maintain slurry fluidity while optimum solids loading. The slurry containing 40 and 50 vol.% of HA powder showed a shear thinning behavior, whereas the 51.2 vol.% slurry showed a shear thinning behavior at low shear rate and showed shear thickening behavior at higher shear rate. And with the increase of the solids loading, the viscosity increased. The shear thinning behavior shows that viscosity is strongly dependent on the solids loading. But when the solids loading was above 51.2 vol.%, it was hard to get slurry with well rheological properties in the study. The poor rheological properties of the slurry will cause defects in the green body when casting. Optimum gel-casting slurry should have high solids concentration and low viscosity [14–16]. Finally, the 50 vol.% slurry with high enough solids loading and fluid enough suspension was used to prepare ceramic bodies.
Fig. 5. Properties of HA suspensions with different solids loading.
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3.5. Pieces characterization After casting, gelification and drying, well shaped green bodies were obtained. The pieces could be easily demoulded. Neither contraction nor cracking was observed and the pieces showed enough strength to be handled. The dried pieces were sintered at 600 ◦ C (1 ◦ C/min) for 1 h to burn out volatiles followed by densification with 2 h holding at 1250 ◦ C (5 ◦ C/min). In Table 1 the characteristics of green body and the sintered samples was showed. The relative green density is 51.4%, lower than the common pressing pieces, similar as the other colloidal process. On the contrary, the mechanical properties of green pieces are higher than other processes mentioned here. The green pieces have a compressive strength of 32.6 ± 3.2 MPa. The high strength, resulting from the cross-linking gel network in green body, leads to an easy green machining. The density of sintered body from 50 vol.% suspension is 3.057 ± 0.008 g/cm3 (96.7% of theoretic density); Bending strength, elasticity modulus and hardness is 84.6 ± 12.6 MPa, 138 ± 7 GPa and 4.44 ± 0.35 GPa, respectively. The mechanical properties of sintered pieces are
Fig. 6. Fracture and surface microstructure of HA pieces.
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Table 1 Properties of HA green pieces and sintered pieces
Green body Sintered body (50 vol.% slurry) a
Density (g/cm3 )
Flexural strength (MPa)a
Hardness (GPa)
Elasticity modulus (GPa)
1.621 3.057 ± 0.008
32.6 ± 3.2 84.6 ± 12.6
– 4.44 ± 0.35
– 138 ± 7.0
Green body was characteristic as compressive strength.
4. Conclusions A thermal treatment at 900 ◦ C follow by ball-milling for 20 h led to HA particles obtained by chemical precipitation with specific area as 10.40 m2 /g. With the dispersant, the HA slurry can be stabilized in a wide pH range (pH 7–12). The amount of the dispersant and the solids loading are the important factors that determine the rheological behavior and viscosity of the slurries. The optimum amount of the dispersant PAA-Na is 1.5 wt.% of the added powder, enables to prepare fluid suspension containing 50 vol.% solids loading with satisfied rhelogical properties suitable for gel-casting. After gel-casting, drying and sintering at 1250 ◦ C, HA samples can be densified to a relative density of 96.7%. The mechanical properties of the obtained pieces are satisfying, with the flexural strength, hardness and the elasticity modulus as 84.6 ± 12.6 MPa, 138 ± 7 GPa and 4.44 ± 0.35 GPa, respectively. Acknowledgement The authors thank the National Science Foundation of China (Grant No. 50232010) for the grants that support this research. References
Fig. 7. (a) XRD patterns and (b) FTIR spectra of staring HA powder and sintered body.
higher than that from the similar process [10,11], while within the range of reported for HA [4]. Fracture and surface micrographs (SEM) of pieces prepared after sintering are shown in Fig. 6. The surface is characteristic of compacted ceramics materials. As the surface micrograph showed, the mean grain size is determined to be 2.13 m by using the linear intercept method. The fracture micrograph shows that the fracture mode is mainly transgranular. XRD patterns and FTIR spectra after sintering compared with starting powder are shown in Fig. 7a and b, respectively. The XRD did not show the presence of the other phases, indicating that HA phase is well preserved after being sintered, which is in agreement with the literature [27]. FTIR spectra also showed the bands of OH− P–O–H and PO4 3− radical before and after sintering mean that the active radical of HA are preserved [25].
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