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Aqueous colloidal processing of nickel powder
Acta mater. 49 (2001) 645–651 www.elsevier.com/locate/actamat
AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER ´ NCHEZ-HERENCIA*, A. J. MILLA ´ N‡, M. I. NIETO and R. MORENO A. J. SA Instituto de Cera´mica y Vidrio (CSIC), Ctra. de Valencia Km. 24,300, Arganda del Rey, 28500 Madrid, Spain ( Received 25 May 2000; received in revised form 9 October 2000; accepted 15 October 2000 )
Abstract—Nickel micronic powders are processed by a colloidal route using aqueous suspensions. The optimum amount of dispersant is selected by means of rheological tests. The slurries show a plastic behaviour that retards sedimentation. Dispersed slurries are mixed with a gelling agent (-carrageenan) and cooled in order to provide the sample with strength enough for handling. Dynamic and static sintering studies are performed on green samples at temperatures ranging from 700 to 1400°C in flowing argon atmosphere. Porous and dense materials fabricated by this route are characterized by SEM and mercury porosimetry. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Powder processing; Injection moulding; Sintering; Nickel
1. INTRODUCTION
For many applications metals are processed by simple deformation methods, in which the metal is melt and the piece is shaped by casting into moulds, laminated, extruded, etc. [1]. These methods are suitable for producing dense compacts. When porous bodies are to be obtained powder processing routes must be used [2, 3]. Powder processing of metals has been traditionally performed using forming techniques where the starting powders are in the dry state or as a viscous paste. The first case refers to cold pressing techniques (axial or isostatic) and hot pressing [4]. Pastes are used in methods like extrusion or injection moulding, where high viscosity mixtures are forced through a die or injected in a mould cavity [5]. Metal injection moulding (MIM) has attracted a great interest for the production of complex shaped parts where machining and costs are significantly reduced or avoided [6]. However the large concentrations of polymers (15–50 vol%) in the blend is the responsible for the important drawbacks of this technique. Firstly, the high viscosity of the blend (10–1000 Pa•s) makes necessary a high pressure to be applied (50–100 Pa). Secondly, the burn out process is long (typically 100 h) and difficult to control. These problems are similar to those found in cer-
* To whom all correspondence should be addressed. Tel.: ⫹34-91-871-1800; fax: ⫹34-91-871-05-50. E-mail address: [email protected] (A. J. Sa´nchez-Herencia) ‡ Permanent address: Dpto. Materiales, IUT Dr. Federico Rivero Palacios, Caracas, Venezuela.
amic processing, where the development of low cost methodologies for the production of defect free materials is a major objective for both the research and the industry. Much effort has been devoted by ceramists to improve the materials properties through the control and manipulation of the colloid chemistry of powder suspensions [7]. Many ceramic shaping methods are based in the preparation of stable, welldispersed slurries with high solid content. Stability occurs when the particles are kept apart each other throughout the medium and requires the development of repulsive forces stronger than the van der Waals forces, always present, which are attractive and very strong, specially near the contact distance [8]. Rheology constitutes a powerful technique for the evaluation of the stability of a suspension. The measurement of flow curves for different dispersant concentrations allows to determine the value at which a minimum viscosity is obtained and thus, to evaluate the best dispersing conditions for further processing steps. This can be made by control rate (CR) measurements, where the slip is forced to flow at an increasing velocity gradient or shear rate and the resulting torque or stress is measured. This is useful for a simple characterization, specially in the high shear region. However, at low rates (i.e. those existing at rest in tanks or into moulds during casting) poor information is obtained in control rate mode. For the low shear region a more accurate analysis can be made by measuring the rheological behavior at control stress (CS) mode, where a stress is applied and the resulting deformation or flow is measured [9]. The solid loading of the slurry should be as high as poss-
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 3 4 7 - 5
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´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
ible, while maintaining a viscosity low enough to ensure the flow capability during shaping (mixing and milling, pumping, pouring into a mould, etc). However, the viscosity exponentially increases with the solid loading until a maximum is reached that corresponds to a maximum packing fraction, where the slurry behaves as a solid. The rheological characterization is widely used in ceramic processing. However, metal powder processing has not reached a parallel progress. The high density of metals and the fact that submicronic metal powders are hardly available have discouraged many metallurgists to process them by colloidal techniques. Nevertheless, this difference in densities has been successfully used to generate continuous metal–ceramic functionally graded materials [10]. Aqueous gel casting has been successfully employed in shape forming of ceramics. This technique consists in the preparation of a concentrated, low viscosity suspension containing a polysaccharide that gels on cooling. This new technology can be applied for shaping any powdered suspension as the gel is formed through the linkage between water molecules and does not depend on the nature of the particles. Agarose has been mostly studied as the gelling agent [11]. Recently the potential of carrageenans as gelling binder for manufacturing ceramic products such as Al2O3 or Si3N4 has been demonstrated [12, 13], giving the additional advantage of economy to the process because carrageenan is much less expensive than agarose. Colloidal processing has demonstrated to be a very efficient and low cost route for shaping ceramics. The objective of this work is to apply the colloidal approach to the field of powder metallurgy. Aqueous gel casting by adding carrageenans is proposed as a new shaping route for metallic powders. Nickel has been selected because it is a well-known material and has important applications in metal/ceramic systems, either from a structural point of view, i.e. high toughness ceramics [14], thermal barrier coatings [15], reinforced layered composites [16], etc.; or in electronic devices as solid oxide or molten carbonate fuel cells [3]. 2. EXPERIMENTAL
As starting material a commercially available nickel powder (INCO T110, Canada) was used. It has a mean particle size of 2.5 µm, a surface area of 1.0 m2 g⫺1 and a density of 8.7 g cm⫺3. Figure 1 shows the particle size distribution and a SEM micrograph of this powder. Particle size distribution was measured with a laser analyser (Mastersizer, Malvern, UK). Surface area was measured by one point N2 adsorption (Monosorb, Quantachorme, USA). Powder density was measured with a helium picnometer (Multipicnometer, Quantachorme, USA). Suspensions of this powder were prepared in deionized water to a solid concentration of 77 wt%
Fig. 1. (a) Particle size distribution and (b) SEM micrograph of the starting powder.
(corresponding to a 27 vol%). For stabilizing the slurries an ammonium salt of a polyacrylic acid (Duramax D-3005, Rohm and Haas, USA) was used. Slurries with concentrations of polyelectrolite ranging from 0 to 2.0 wt% (referred to dry solids) were prepared using a 400 W sonication probe (IKA U400S, Germany) for 2 min. After dispersion the slurries were mechanically stirred for 4 h in order to ensure stabilization. The rheological behaviour of all prepared slurries was performed with a rheometer (Haake RS50, Germany) capable to operate at either control rate or control stress modes. The sensor system consisted on a double cone rotor and a stationary plate, corresponding to a Searle measuring system. The chamber is protected with a covering plate to reduce evaporation phenomena. For characterizing the slurry stability the flow curves were determined in control rate mode (CR). Measurements were performed by increasing the shear rate from 0 to 1000 s⫺1 in 5 min, maintaining at 1000 s⫺1 for 1 min and returning to 0 in 5 min. Temperature was maintained constant at 25°C during these experiments. CS measurements were performed by increasing the stress from 0 to a value higher than the yield point estimated by direct observations of the CR flow curves (i.e 40–50 Pa). The up and the down curves were measured without an intermediate step. For both ramps the measuring time was 5 min.
´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
647
Fig. 2. Flow curves registered in the CR mode for the slurries with different concentrations of dispersant (wt% referred to solids).
Table 1. Yield point and viscosity values for the slurries with different contents of dispersant wt.% of dispersant Yield point (Pa) Viscosity at 100 s
Herschel–Bulkley fit Stress/strain plot ⫺1
(mPa•s)
0.0
0.1
0.2
12.5 25 549
7.5 17 427
1.4 6 347
0.6 0 3 239
0.8 1.0 5 264
Fig. 3. Strain vs. stress (in log/lin scale) for slurries with different dispersant contents. The yield point corresponds to the strain jump at a given deformation.
A new gelling binder was used for shaping, namely carrageenan. Carrageenans constitute a family of large molecular weight polysaccharides whose monomers are sulphated derivatives of d-galactose. A comercial -carrageenan (Secogel TC, Hispanagar, Spain) was used as described in literature [12]. For a maximum efficiency the binder powder was dissolved in water by heating at 90°C. Carrageenan solutions of 2 wt% were maintained at 60°C and added to the nickel aqueous slurries heated also at that temperature. The rheological properties of nickel slurries with
and without gelling addition were studied as a function of temperature in order to evaluate the gelling behaviour on cooling. The total concentration of carrageenans was as low as 0.5 wt% referred to solids. The slurries containing the gelling agent were cast on a water-cooled metallic mould with dimensions 10⫻10⫻60 mm. After gelation samples were removed from the mould and left in air for drying until no weight lost was detected (24–48 h). Since the concentration of organics is very low, no debinding process was necessary. Dry green casts were sintered
648
´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
Fig. 4. Viscosity at 100 s⫺1 vs. temperature for the slurries with and without carrageenans. For the slurry without carrageenan heating and cooling is plotted. For the slurry with carrageenan only the cooling ramp is plotted, as it is prepared at 60°C.
at temperatures ranging from 700 to 1400°C under Ar flowing atmosphere. Dynamic sintering was also studied using a dilatometer (Netzsch, Germany) up to 1200°C in Ar atmosphere. The microstructure of specimens in the green state and sintered at 700°C was observed by scanning electron microscopy (SEM) on fracture surfaces, whereas samples sintered at higher temperatures were observed on polished surfaces after chemical etching. Density was measured by the Archimedes method in water for dense compacts while mercury porosimetry (Micrometrics, Autopore II 9220, USA) was used to determine the pore size distribution in green and porous sintered samples. 3. RESULTS AND DISCUSSION
3.1. Rheological characterization of slurries In Fig. 1 it can be observed that there is a difference in the particle sizes measured with the particle size analyzer and the observed in the SEM micrograph. This difference indicates the presence of strong agglomerates, mainly due to the thermal decomposition process used to obtain these powders. Suspensions were prepared in water by ultrasonic mixing without dispersant and with eight different dispersant concentrations ranging from 0.1 to 2.0 wt%. The flow curves of some of these slurries obtained by CR measurements can be seen in Fig. 2. In this plot only the increasing shear rate curves are shown. The slurry without dispersant shows the highest viscosity as well as some thixotropy. Both viscosity and thixotropy decrease when dispersant is added until a minimum viscosity is reached for 0.6 wt%. At this concentration the suspended particles are surrounded by the polyelectrolyte chains so that it can be assumed that there is a complete coverage. When an excess of dispersant is added, it cannot adsorb onto the particles and remains free in the liquid medium.
This has two effects: (a) a decrease in the electrical double layer at the particle surface as a consequence of the increased concentration of electrolyte and (b) the interaction between particles through the free polymer chains, that can promote agglomerates by a polymeric bridging mechanism [17]. In fact, Fig. 2 shows that for dispersant concentrations higher than 0.6 wt% the viscosity slightly increases. All the suspensions reveal a shear thinning behaviour. The experimental data were analyzed using different regression models. The best fitting was obtained for the Herschel–Bulkley model. The variation of viscosity as a function of deflocculant content is reported in Table 1 for a shear rate of 100 s⫺1. The term plasticity is widely used in engineering applications referring to the existence of a minimum yield stress that must be exceeded before the slurry starts to flow. The yield point calculated by fitting the CR flow curves is shown in the same table. More accurate values are provided from CS measurements by plotting a diagram of strain vs. stress in log/lin scale as shown in Fig. 3. In this plot it can be seen that a minimum variation in stress promotes a strong deformation that increases even more than five orders of magnitude before the slurry starts to flow. The strong deformation within a minimum stress range gives a clear image of the plasticity of slurries [18]. By this method it was observed that slurries behave as plastic in all the cases. The yield stress values obtained in CS mode are also reported in Table 1. The yield stress value decreases as the dispersant concentration increases, so indicating a more fluid response. A minimum yield value is reached for a dispersant concentration of 0.6 wt%, in good agreement with the viscosity values. In the case of the nickel powders considered in this study, the high density of the powder could lead to sedimentation phenomena. For this reason some plasticity is helpful because the viscosity at rest is high,
´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
649
Fig. 5. Pore size distribution and SEM micrograph of the green samples obtained.
then avoiding settling. With a small agitation, viscosity is low enough to allow an easy casting or injection process with these slurries [19]. Carrageenans were added to the slurry maintaining a temperature of 60°C. Carrageenan solutions are fluid at high temperatures but when the temperature decreases below the glass transition temperature (Tg) a rigid gel develops allowing the particle to consolidate as a solid body. Figure 4 shows the variation of viscosity for the slurry with and without carrageenans. The gel point is 34°C. The suspension with gelling additive is poured at 60°C into a metallic mold cooled by flowing water. This allows the body to consolidate in a few seconds (5–20 s) and then the samples can be removed from the mold and left in air to dry. Except in the gelling region it can be seen that the viscosity decreases as the temperature increases for the slurry with and without carrageenans, this being a consequence of the decreasing viscosity of water. The low viscosity registered between 40 and 60°C indicates a good homogenization of the mixture, since neither agglomeration due to water evaporation nor polymeric bridging occurs [20].
The nickel slurry was mixed with the carrageenan solution at 60°C and then cast into a water cooled bar shaped mold. After 20 s, samples were removed and dried at room conditions. After complete drying, the shrinkage was of 17% and the density was 3.7 g cm⫺3 (41.6% of theoretical). XRD performed on the cast samples did not reveal the presence of NiO. The pore size distribution of green samples with its corresponding SEM microstructure is shown in Fig. 5. 3.2. Sintering studies Static and dynamic sintering studies were performed with the green bodies. Figure 6 shows the dilatometry of the sample under Argon flowing atmosphere. Two peaks are clearly observed in the derivative curve of the shrinkage, also shown in Fig. 6. The first one, located at 550°C corresponds to the neck formation of the finer portion of particles detected by SEM of the starting powders. The second peak, centered at ⬇900°C, corresponds to the maximum shrinkage during sintering. Densification has finished at 1000°C and only grain growth is expected to occur after this temperature.
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´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
Fig. 6. Sintering dilatometry curve of the nickel sample under argon flowing atmosphere. The maximum shrinkage correspond to 900°C.
The sample sintered at 700°C has a density of 5.02 g cm⫺3 (56.4%) showing an open porosity of 36%. All other temperatures promote a density higher than 92%th without open porosity. Figure 7 shows the pore size distribution and the microstructure observed by SEM of the sample treated at 700°C. As can be
observed, only the finest fraction of nickel particles has started to sinter, while the biggest has only developed necks among them. The pore size distribution shows a multimodal pore size distribution with two major fractions, centered at 0.5 µm and the biggest centered at 0.007 µm. Between these two
Fig. 7. SEM micrograph of a porous nickel sample sintered at 700°C for 1 h (a) with its corresponding pore size distribution (b).
´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
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slurries is described. Compacts are formed by gel casting 77 wt% nickel aqueous slurries containing only 0.5 wt% of carrageenan at a temperature of 50– 60°C, which acts as a gelling agent on cooling. In this way green compacts can be near-net shaped and handled. The gel cast samples have been pressureless sintered in argon atmosphere and the porosity of the final compact can be controlled by controlling the sintering conditions. Grain sizes of the samples depend on the sintering temperature. For very high sintering temperatures a high intragranular porosity is observed due to the exaggerated grain growth. The use of the gelcasting technique in water for shaping powdered suspensions opens a simple, low cost route for the production of complex shaped metallic parts with a controlled porosity. As the gelling mechanism involves the bonding through the water molecules and does not depend on the surface properties of the powders, this technique can be easily transferred to a broad variety of materials. Acknowledgements—This work has been supported by Comunidad de Madrid (Spain) under project 07N/0038/1999 and CICYT MAT97 0676. A.J.M. thanks CONICIT (Venezuela) for the concession of a grant.
REFERENCES
Fig. 8. SEM micrograph of a dense nickel sample sintered at 900°C (a) and 1400°C (b) for 1 h.
peaks some small fractions of porosity are observed at 0.02 and 0.05 µm. The total shrinkage for the complete densification process was of 25%. 3.3. Microstructural characterization Samples treated at temperatures ranging from 850 to 1400°C had a final density higher than 92%th, indicating that sintering has progressed to a great extent. Figure 8 shows the microstructure obtained by SEM on polished and chemically etched samples sintered at 900°C (a) and 1400°C (b). The average grain size of these samples was 30 µm and 60 µm, respectively. Grains of both samples are equiaxial, and some remaining intragranular porosity can be observed. This porosity is higher for samples sintered at 1400°C as a consequence of the exaggerated grain growth, so that pores due to triple points are trapped within the large grains that are growing. 4. CONCLUSIONS
The manufacture of porous and dense nickel compacts by a colloidal route from high solid content
1. Kalpakjian, S., Manufacturing Processes for Engineering Materials, 2nd ed. Addison-Wesley, Reading, MA, 1991. 2. German, R. M., Powder Metallurgy Science, 2nd ed. Metal Powder Industries Federation, Princeton, NJ, 1994. 3. Antolini, E., Ferretti, M. and Gemme, S., J. Mater. Sci., 1996, 31, 2187. 4. Bruck, H. A. and Rabin, B. H., J. Mater. Sci., 1999, 34, 2241. 5. German, R. M., Powder Injection Moulding. Metal Powder Industries Federation, Princeton, NJ, 1989. 6. Bufford, B., Microwaves & RF, 1999, 38, 120. 7. Lange, F. F., J. Am. Ceram. Soc., 1989, 72, 3. 8. Moreno, R., Am. Ceram. Soc. Bull., 1991, 71, 229. 9. Gutie´rrez, C., Sa´nchez-Herencia, A. J. and Moreno, R., Bol. Soc. Esp. Ceram. Vidrio, 2000, 39, 105. 10. Chu, J., Ishibashi, H., Hayashi, K., Takebe, H. and Morinaga, K., J. Ceram. Soc. Jpn., 1993, 101, 841. 11. Fanelli, J. A., J. Am. Ceram. Soc., 1989, 72, 1833. 12. Milla´n, A.J., Sa´nchez-Herencia, A.J., Nieto, M.I., Moreno, R. Spanish Patent, ES 200000561, 2000. 13. Millan, A.J., Nieto, M.I., Moreno, R. J. Am. Ceram. Soc., 2000, in press. 14. Tuan, W. and Brook, R. J., J. Eur. Ceram. Soc., 1990, 6, 31. 15. Takebe, H., Teshima, T., Nakashima, M. and Morinaga, K., J. Ceram. Soc. Jpn., 1992, 100, 387. 16. Cheng, Z. and Melchoslsky, J. J., J. Am. Ceram. Soc., 1995, 76, 1258. 17. Hunter, R. J., Foundations of Colloidal Science. Oxford Science Publishers, Oxford, UK, 1986. 18. Barnes, H. A. and Walters, K., Rheol. Acta, 1985, 24, 323. 19. Sa´nchez-Herencia, A. J., Milla´n, A. J., Nieto, M. I. and Moreno, R., Adv. Mater., 2000, 12, 1192. 20. Milla´n, A. J., Bravo, M., Moreno, R. and Nieto, M. I., Bol. Soc. Esp. Ceram. Vidrio, 1999, 38, 93.
AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER ´ NCHEZ-HERENCIA*, A. J. MILLA ´ N‡, M. I. NIETO and R. MORENO A. J. SA Instituto de Cera´mica y Vidrio (CSIC), Ctra. de Valencia Km. 24,300, Arganda del Rey, 28500 Madrid, Spain ( Received 25 May 2000; received in revised form 9 October 2000; accepted 15 October 2000 )
Abstract—Nickel micronic powders are processed by a colloidal route using aqueous suspensions. The optimum amount of dispersant is selected by means of rheological tests. The slurries show a plastic behaviour that retards sedimentation. Dispersed slurries are mixed with a gelling agent (-carrageenan) and cooled in order to provide the sample with strength enough for handling. Dynamic and static sintering studies are performed on green samples at temperatures ranging from 700 to 1400°C in flowing argon atmosphere. Porous and dense materials fabricated by this route are characterized by SEM and mercury porosimetry. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Powder processing; Injection moulding; Sintering; Nickel
1. INTRODUCTION
For many applications metals are processed by simple deformation methods, in which the metal is melt and the piece is shaped by casting into moulds, laminated, extruded, etc. [1]. These methods are suitable for producing dense compacts. When porous bodies are to be obtained powder processing routes must be used [2, 3]. Powder processing of metals has been traditionally performed using forming techniques where the starting powders are in the dry state or as a viscous paste. The first case refers to cold pressing techniques (axial or isostatic) and hot pressing [4]. Pastes are used in methods like extrusion or injection moulding, where high viscosity mixtures are forced through a die or injected in a mould cavity [5]. Metal injection moulding (MIM) has attracted a great interest for the production of complex shaped parts where machining and costs are significantly reduced or avoided [6]. However the large concentrations of polymers (15–50 vol%) in the blend is the responsible for the important drawbacks of this technique. Firstly, the high viscosity of the blend (10–1000 Pa•s) makes necessary a high pressure to be applied (50–100 Pa). Secondly, the burn out process is long (typically 100 h) and difficult to control. These problems are similar to those found in cer-
* To whom all correspondence should be addressed. Tel.: ⫹34-91-871-1800; fax: ⫹34-91-871-05-50. E-mail address: [email protected] (A. J. Sa´nchez-Herencia) ‡ Permanent address: Dpto. Materiales, IUT Dr. Federico Rivero Palacios, Caracas, Venezuela.
amic processing, where the development of low cost methodologies for the production of defect free materials is a major objective for both the research and the industry. Much effort has been devoted by ceramists to improve the materials properties through the control and manipulation of the colloid chemistry of powder suspensions [7]. Many ceramic shaping methods are based in the preparation of stable, welldispersed slurries with high solid content. Stability occurs when the particles are kept apart each other throughout the medium and requires the development of repulsive forces stronger than the van der Waals forces, always present, which are attractive and very strong, specially near the contact distance [8]. Rheology constitutes a powerful technique for the evaluation of the stability of a suspension. The measurement of flow curves for different dispersant concentrations allows to determine the value at which a minimum viscosity is obtained and thus, to evaluate the best dispersing conditions for further processing steps. This can be made by control rate (CR) measurements, where the slip is forced to flow at an increasing velocity gradient or shear rate and the resulting torque or stress is measured. This is useful for a simple characterization, specially in the high shear region. However, at low rates (i.e. those existing at rest in tanks or into moulds during casting) poor information is obtained in control rate mode. For the low shear region a more accurate analysis can be made by measuring the rheological behavior at control stress (CS) mode, where a stress is applied and the resulting deformation or flow is measured [9]. The solid loading of the slurry should be as high as poss-
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 3 4 7 - 5
646
´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
ible, while maintaining a viscosity low enough to ensure the flow capability during shaping (mixing and milling, pumping, pouring into a mould, etc). However, the viscosity exponentially increases with the solid loading until a maximum is reached that corresponds to a maximum packing fraction, where the slurry behaves as a solid. The rheological characterization is widely used in ceramic processing. However, metal powder processing has not reached a parallel progress. The high density of metals and the fact that submicronic metal powders are hardly available have discouraged many metallurgists to process them by colloidal techniques. Nevertheless, this difference in densities has been successfully used to generate continuous metal–ceramic functionally graded materials [10]. Aqueous gel casting has been successfully employed in shape forming of ceramics. This technique consists in the preparation of a concentrated, low viscosity suspension containing a polysaccharide that gels on cooling. This new technology can be applied for shaping any powdered suspension as the gel is formed through the linkage between water molecules and does not depend on the nature of the particles. Agarose has been mostly studied as the gelling agent [11]. Recently the potential of carrageenans as gelling binder for manufacturing ceramic products such as Al2O3 or Si3N4 has been demonstrated [12, 13], giving the additional advantage of economy to the process because carrageenan is much less expensive than agarose. Colloidal processing has demonstrated to be a very efficient and low cost route for shaping ceramics. The objective of this work is to apply the colloidal approach to the field of powder metallurgy. Aqueous gel casting by adding carrageenans is proposed as a new shaping route for metallic powders. Nickel has been selected because it is a well-known material and has important applications in metal/ceramic systems, either from a structural point of view, i.e. high toughness ceramics [14], thermal barrier coatings [15], reinforced layered composites [16], etc.; or in electronic devices as solid oxide or molten carbonate fuel cells [3]. 2. EXPERIMENTAL
As starting material a commercially available nickel powder (INCO T110, Canada) was used. It has a mean particle size of 2.5 µm, a surface area of 1.0 m2 g⫺1 and a density of 8.7 g cm⫺3. Figure 1 shows the particle size distribution and a SEM micrograph of this powder. Particle size distribution was measured with a laser analyser (Mastersizer, Malvern, UK). Surface area was measured by one point N2 adsorption (Monosorb, Quantachorme, USA). Powder density was measured with a helium picnometer (Multipicnometer, Quantachorme, USA). Suspensions of this powder were prepared in deionized water to a solid concentration of 77 wt%
Fig. 1. (a) Particle size distribution and (b) SEM micrograph of the starting powder.
(corresponding to a 27 vol%). For stabilizing the slurries an ammonium salt of a polyacrylic acid (Duramax D-3005, Rohm and Haas, USA) was used. Slurries with concentrations of polyelectrolite ranging from 0 to 2.0 wt% (referred to dry solids) were prepared using a 400 W sonication probe (IKA U400S, Germany) for 2 min. After dispersion the slurries were mechanically stirred for 4 h in order to ensure stabilization. The rheological behaviour of all prepared slurries was performed with a rheometer (Haake RS50, Germany) capable to operate at either control rate or control stress modes. The sensor system consisted on a double cone rotor and a stationary plate, corresponding to a Searle measuring system. The chamber is protected with a covering plate to reduce evaporation phenomena. For characterizing the slurry stability the flow curves were determined in control rate mode (CR). Measurements were performed by increasing the shear rate from 0 to 1000 s⫺1 in 5 min, maintaining at 1000 s⫺1 for 1 min and returning to 0 in 5 min. Temperature was maintained constant at 25°C during these experiments. CS measurements were performed by increasing the stress from 0 to a value higher than the yield point estimated by direct observations of the CR flow curves (i.e 40–50 Pa). The up and the down curves were measured without an intermediate step. For both ramps the measuring time was 5 min.
´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
647
Fig. 2. Flow curves registered in the CR mode for the slurries with different concentrations of dispersant (wt% referred to solids).
Table 1. Yield point and viscosity values for the slurries with different contents of dispersant wt.% of dispersant Yield point (Pa) Viscosity at 100 s
Herschel–Bulkley fit Stress/strain plot ⫺1
(mPa•s)
0.0
0.1
0.2
12.5 25 549
7.5 17 427
1.4 6 347
0.6 0 3 239
0.8 1.0 5 264
Fig. 3. Strain vs. stress (in log/lin scale) for slurries with different dispersant contents. The yield point corresponds to the strain jump at a given deformation.
A new gelling binder was used for shaping, namely carrageenan. Carrageenans constitute a family of large molecular weight polysaccharides whose monomers are sulphated derivatives of d-galactose. A comercial -carrageenan (Secogel TC, Hispanagar, Spain) was used as described in literature [12]. For a maximum efficiency the binder powder was dissolved in water by heating at 90°C. Carrageenan solutions of 2 wt% were maintained at 60°C and added to the nickel aqueous slurries heated also at that temperature. The rheological properties of nickel slurries with
and without gelling addition were studied as a function of temperature in order to evaluate the gelling behaviour on cooling. The total concentration of carrageenans was as low as 0.5 wt% referred to solids. The slurries containing the gelling agent were cast on a water-cooled metallic mould with dimensions 10⫻10⫻60 mm. After gelation samples were removed from the mould and left in air for drying until no weight lost was detected (24–48 h). Since the concentration of organics is very low, no debinding process was necessary. Dry green casts were sintered
648
´ NCHEZ-HERENCIA et al.: AQUEOUS COLLOIDAL PROCESSING OF NICKEL POWDER SA
Fig. 4. Viscosity at 100 s⫺1 vs. temperature for the slurries with and without carrageenans. For the slurry without carrageenan heating and cooling is plotted. For the slurry with carrageenan only the cooling ramp is plotted, as it is prepared at 60°C.
at temperatures ranging from 700 to 1400°C under Ar flowing atmosphere. Dynamic sintering was also studied using a dilatometer (Netzsch, Germany) up to 1200°C in Ar atmosphere. The microstructure of specimens in the green state and sintered at 700°C was observed by scanning electron microscopy (SEM) on fracture surfaces, whereas samples sintered at higher temperatures were observed on polished surfaces after chemical etching. Density was measured by the Archimedes method in water for dense compacts while mercury porosimetry (Micrometrics, Autopore II 9220, USA) was used to determine the pore size distribution in green and porous sintered samples. 3. RESULTS AND DISCUSSION
3.1. Rheological characterization of slurries In Fig. 1 it can be observed that there is a difference in the particle sizes measured with the particle size analyzer and the observed in the SEM micrograph. This difference indicates the presence of strong agglomerates, mainly due to the thermal decomposition process used to obtain these powders. Suspensions were prepared in water by ultrasonic mixing without dispersant and with eight different dispersant concentrations ranging from 0.1 to 2.0 wt%. The flow curves of some of these slurries obtained by CR measurements can be seen in Fig. 2. In this plot only the increasing shear rate curves are shown. The slurry without dispersant shows the highest viscosity as well as some thixotropy. Both viscosity and thixotropy decrease when dispersant is added until a minimum viscosity is reached for 0.6 wt%. At this concentration the suspended particles are surrounded by the polyelectrolyte chains so that it can be assumed that there is a complete coverage. When an excess of dispersant is added, it cannot adsorb onto the particles and remains free in the liquid medium.
This has two effects: (a) a decrease in the electrical double layer at the particle surface as a consequence of the increased concentration of electrolyte and (b) the interaction between particles through the free polymer chains, that can promote agglomerates by a polymeric bridging mechanism [17]. In fact, Fig. 2 shows that for dispersant concentrations higher than 0.6 wt% the viscosity slightly increases. All the suspensions reveal a shear thinning behaviour. The experimental data were analyzed using different regression models. The best fitting was obtained for the Herschel–Bulkley model. The variation of viscosity as a function of deflocculant content is reported in Table 1 for a shear rate of 100 s⫺1. The term plasticity is widely used in engineering applications referring to the existence of a minimum yield stress that must be exceeded before the slurry starts to flow. The yield point calculated by fitting the CR flow curves is shown in the same table. More accurate values are provided from CS measurements by plotting a diagram of strain vs. stress in log/lin scale as shown in Fig. 3. In this plot it can be seen that a minimum variation in stress promotes a strong deformation that increases even more than five orders of magnitude before the slurry starts to flow. The strong deformation within a minimum stress range gives a clear image of the plasticity of slurries [18]. By this method it was observed that slurries behave as plastic in all the cases. The yield stress values obtained in CS mode are also reported in Table 1. The yield stress value decreases as the dispersant concentration increases, so indicating a more fluid response. A minimum yield value is reached for a dispersant concentration of 0.6 wt%, in good agreement with the viscosity values. In the case of the nickel powders considered in this study, the high density of the powder could lead to sedimentation phenomena. For this reason some plasticity is helpful because the viscosity at rest is high,
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Fig. 5. Pore size distribution and SEM micrograph of the green samples obtained.
then avoiding settling. With a small agitation, viscosity is low enough to allow an easy casting or injection process with these slurries [19]. Carrageenans were added to the slurry maintaining a temperature of 60°C. Carrageenan solutions are fluid at high temperatures but when the temperature decreases below the glass transition temperature (Tg) a rigid gel develops allowing the particle to consolidate as a solid body. Figure 4 shows the variation of viscosity for the slurry with and without carrageenans. The gel point is 34°C. The suspension with gelling additive is poured at 60°C into a metallic mold cooled by flowing water. This allows the body to consolidate in a few seconds (5–20 s) and then the samples can be removed from the mold and left in air to dry. Except in the gelling region it can be seen that the viscosity decreases as the temperature increases for the slurry with and without carrageenans, this being a consequence of the decreasing viscosity of water. The low viscosity registered between 40 and 60°C indicates a good homogenization of the mixture, since neither agglomeration due to water evaporation nor polymeric bridging occurs [20].
The nickel slurry was mixed with the carrageenan solution at 60°C and then cast into a water cooled bar shaped mold. After 20 s, samples were removed and dried at room conditions. After complete drying, the shrinkage was of 17% and the density was 3.7 g cm⫺3 (41.6% of theoretical). XRD performed on the cast samples did not reveal the presence of NiO. The pore size distribution of green samples with its corresponding SEM microstructure is shown in Fig. 5. 3.2. Sintering studies Static and dynamic sintering studies were performed with the green bodies. Figure 6 shows the dilatometry of the sample under Argon flowing atmosphere. Two peaks are clearly observed in the derivative curve of the shrinkage, also shown in Fig. 6. The first one, located at 550°C corresponds to the neck formation of the finer portion of particles detected by SEM of the starting powders. The second peak, centered at ⬇900°C, corresponds to the maximum shrinkage during sintering. Densification has finished at 1000°C and only grain growth is expected to occur after this temperature.
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Fig. 6. Sintering dilatometry curve of the nickel sample under argon flowing atmosphere. The maximum shrinkage correspond to 900°C.
The sample sintered at 700°C has a density of 5.02 g cm⫺3 (56.4%) showing an open porosity of 36%. All other temperatures promote a density higher than 92%th without open porosity. Figure 7 shows the pore size distribution and the microstructure observed by SEM of the sample treated at 700°C. As can be
observed, only the finest fraction of nickel particles has started to sinter, while the biggest has only developed necks among them. The pore size distribution shows a multimodal pore size distribution with two major fractions, centered at 0.5 µm and the biggest centered at 0.007 µm. Between these two
Fig. 7. SEM micrograph of a porous nickel sample sintered at 700°C for 1 h (a) with its corresponding pore size distribution (b).
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slurries is described. Compacts are formed by gel casting 77 wt% nickel aqueous slurries containing only 0.5 wt% of carrageenan at a temperature of 50– 60°C, which acts as a gelling agent on cooling. In this way green compacts can be near-net shaped and handled. The gel cast samples have been pressureless sintered in argon atmosphere and the porosity of the final compact can be controlled by controlling the sintering conditions. Grain sizes of the samples depend on the sintering temperature. For very high sintering temperatures a high intragranular porosity is observed due to the exaggerated grain growth. The use of the gelcasting technique in water for shaping powdered suspensions opens a simple, low cost route for the production of complex shaped metallic parts with a controlled porosity. As the gelling mechanism involves the bonding through the water molecules and does not depend on the surface properties of the powders, this technique can be easily transferred to a broad variety of materials. Acknowledgements—This work has been supported by Comunidad de Madrid (Spain) under project 07N/0038/1999 and CICYT MAT97 0676. A.J.M. thanks CONICIT (Venezuela) for the concession of a grant.
REFERENCES
Fig. 8. SEM micrograph of a dense nickel sample sintered at 900°C (a) and 1400°C (b) for 1 h.
peaks some small fractions of porosity are observed at 0.02 and 0.05 µm. The total shrinkage for the complete densification process was of 25%. 3.3. Microstructural characterization Samples treated at temperatures ranging from 850 to 1400°C had a final density higher than 92%th, indicating that sintering has progressed to a great extent. Figure 8 shows the microstructure obtained by SEM on polished and chemically etched samples sintered at 900°C (a) and 1400°C (b). The average grain size of these samples was 30 µm and 60 µm, respectively. Grains of both samples are equiaxial, and some remaining intragranular porosity can be observed. This porosity is higher for samples sintered at 1400°C as a consequence of the exaggerated grain growth, so that pores due to triple points are trapped within the large grains that are growing. 4. CONCLUSIONS
The manufacture of porous and dense nickel compacts by a colloidal route from high solid content
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