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Improving the homogeneity and reliability of ceramic parts with complex shapes by pressure-assisted gel-casting
Materials Letters 58 (2004) 3893 – 3897 www.elsevier.com/locate/matlet
Improving the homogeneity and reliability of ceramic parts with complex shapes by pressure-assisted gel-casting Y. Huanga, L.G. Maa, H.R. Leb,*, J.L. Yanga a
Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China b Department of Engineering, University of Cambridge, Trumpington St., Cambridge CB2 1PZ, United Kingdom Received 30 March 2004; received in revised form 10 April 2004; accepted 24 August 2004 Available online 11 September 2004
Abstract Forming process plays an important role in the homogeneity and reliability of advanced ceramic products. Inspired by the surprising observation of pressure-induced solidification in colloidal forming processes, the authors have developed a new ceramics forming process, colloidal injection moulding, with the potential of forming large ceramic parts of complex shapes such as engine turbines and biomedical implants. This process combines the advantages of the conventional injection moulding and gel-casting processes of ceramics. Not only the efficiency but also the homogeneity of the green body is greatly improved owing to the pressure-induced solidification. It is also found that the inner stress generated by non-uniform solidification due to temperature gradient is responsible for the origination of microstructural defects in green body during colloidal-forming processes. A new strategy is taken to reduce the inner stress by adjusting the stiffness of the green body using a moderator additive. Ceramic products with complex shape such as engine turbines have successfully been fabricated with reduced defects. D 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Reliability; Gel casting; Injection moulding
1. Introduction Advanced structural ceramic materials are promising candidates for aerospace systems, engine components, machine tools, etc., due to their excellent high-temperature endurance; wear resistance and stability in corrosive environment. However, the nature of ceramics is brittle. The mechanical properties of ceramics are extremely sensitive to microstructural defects so that the performance cannot be predicted accurately. This has become the major obstacle to the successful application of advanced ceramics in engineering and attracted great attention over the last several decades. It is well known that ceramic
* Corresponding author. Tel.: +44 1223 332784; fax: +44 1223 332662. E-mail address: [email protected] (H.R. Le). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.08.015
materials are usually made from abundant elements in the earth crust such as silicon, aluminium, oxygen, nitrogen in natural or synthetic forms via powder metallurgy process. The fabrication process usually involves powder preparation, forming and sintering. Previous studies have indicated that microstructural defects such as delamination, microcracks and large pores in the product are usually associated with the non-uniformity in green body caused at the forming or drying stage. These defects will become fracture origins and hence reduce the strength and reliability of the product [1]. Therefore, one main objective in a forming process is to produce uniform green body free of structural defects. Recently, colloidal forming processes such as gel casting [2] and direct coagulation casting (DCC) [3], in which ceramic powder is dispersed in a water-based suspension and held in a gel network, have attracted great interest. In theory, the uniformity of green body can be improved if the
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suspension is perfectly dispersed. In practice, DCC forming process is usually quite slow and inevitably, delamination occurs while the suspension destabilises [4,5]. As a consequence, the strength of green body soproduced is low. Gel-casting process has been applied to the fabrication of Al2O3- and Si3N4-based ceramics by the authors [6,7]. Both the flexural strength and Weibull modulus have been improved by optimising the rheological properties of the suspension in gel-casting process and subsequent sintering process. The drawback of gelcasting is that the solidification of the suspension is usually induced by the mould temperature; thus, the solidification is not uniform in large complex parts due to the existence of temperature gradient, resulting in inner stress at the forming stage and microcracks on subsequent drying and de-bindering processes. Conventional injection moulding is applied to form complex ceramic parts such as engine turbines. The process involves plastic flow of a mixture of organic binders and ceramic powder at elevated temperature and subsequent solidification upon cooling in the mould. Though the forming process is efficient and suitable for automation on large-scale production, delamination usually occurs in green bodies of complex shapes leading to cracks on the de-bindering process to remove the large amount of organic binders. On the contrary, using colloidal forming processes can improve the uniformity of green bodies and reduce structural defects but at the cost of efficiency. An optimal approach would be a combination of injection moulding and gel-casting processes, bringing together their advantages. Based on this concept, a new ceramics-forming process, colloidal injection moulding, is developed by the authors [8]. It was an exciting moment when the authors observed the unexpected solidification of slurry at the exit valve of a storage tank in a fridge [9]. This implies that gelation has occurred under pressure even at low temperature. In this paper, we investigate the effect of hydrostatic pressure on the solidification process and homogeneity of the green bodies in colloidal injection moulding process and a new strategy to minimise the inner stress and microstructural defects in green body.
2. Experimental procedure The experimental process is outlined in Fig. 1. A premix solution of monomers was prepared in deionised water with a concentration of 14 wt.% of acrylamide, C2H3CONH2 (AM) and 0.6 wt.% of N,NV-methylenebisacrylamide, (C2H3CONH)2CH2 (MBAM). The ceramic powders with 1 wt.% poly(methacrylic-acid-ammonium) as dispersant were mixed in the premix solution at a solid loading of about 50 vol.% using ball milling. The slurry was added with 0.1–0.5 vol.% of initiator, a 5 wt.% aqueous solution of ammonium persulfate (NH4)2S2O8 and 0.05–0.25 vol.% of an organic catalyst, N,N,NV,NV-tetramethylethylenediamine. The suspension was stirred slowly and ultrasonically agitated to achieve high homogeneity and then kept in a fridge to avoid solidifying prior to forming. Finally, the suspension was injected into a hot steel mould so that the slurry solidified quickly to become a preform with the polymerisation of the monomers. The preform was demoulded and dried at 70 8C to remove the water and organic contents. The dried green bodies were then sintered in appropriate conditions for each material system. Each sintered body was cut and ground into specimens of 3436 mm for the bending strength measurements and 4630 mm for single edge notched bending (SENB) measurements of fracture toughness.
3. Results 3.1. Effect of hydrostatic pressure on solidification A pressure induction unit was then added to apply an external pressure after injection. Gelation curves of Al2O3 slurry under different pressures with a mould temperature of 25 and 36 8C are shown in Fig. 2. Under increased pressures, the onset time of gelation becomes shorter and the gelation speed increases significantly. At higher temperature, the influence of the pressure becomes more significant and the gelation finishes almost immediately. The pressure-induced solidification will have distinct advantages over temperature-induced solidification.
Fig. 1. A flowchart of colloidal injection moulding of ceramics.
Y. Huang et al. / Materials Letters 58 (2004) 3893–3897
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the solidification on this process is associated with the surface temperature of the mould. The gradient of the temperature will inevitably cause non-uniform solidification and inner stress in the green body. A new strategy is developed to minimise the inner-stress in green bodies prepared in this process. 3.3. Controlling the inner stress in the green body
Fig. 2. The effect of external pressure on the gelation of Al2O3 slurry with a solid loading of 50 vol.%; Ts is the starting temperature.
3.2. Homogeneity of the green bodies An alumina (Al2O3) green body of turbine shape with a diameter of 105 mm was prepared by this process. The spatial distribution of density is compared with that by conventional process using the same injection moulding system in Fig. 3. The value in each part represents the density in g/cm3. The relative standard deviation of density for colloidal injection moulding is 0.2% compared to 0.7% for conventional injection moulding. The results show that colloidal injection moulding process significantly improves the homogeneity in density of the green body. Nevertheless,
Fig. 3. Comparison of density distribution in Al2O3 green bodies of turbine prepared by (a) conventional injection moulding and (b) colloidal injection moulding on the same system.
The inner stress in green body is often responsible for the initiation of microcracks on subsequent drying and debindering processes. During colloidal injection moulding or other gel-casting processes, monomers in ceramic suspension polymerise and then form gel networks to hold the ceramic particles. The solidification speed increases with increasing temperature for a given composition. Non-uniform solidification occurs due to the temperature gradient in the ceramic suspension and results in the development of inner stress in the green body. Theoretical analysis has indicated that the magnitude of the inner stress increases with the stiffness of the green body. Based on this finding, a fraction of the monomers is replaced by a moderator, hydroxylethylacrylate in the suspension during gel-casting or colloidal injection moulding to control the stiffness of the gel network. Test bars with dimensions of 6642 mm were prepared with 50 vol.% solid loading and various amount of the moderator from 0 to 100 wt.% in the total monomers. The samples were dehydrated at about 70 8C in an electrical furnace. The flexural strength and elastic modulus of the green body are evaluated using three-point bending. The results are shown in Fig. 4 against the amount of the moderator. Both the flexural strength and elastic modulus of green body decrease simultaneously with the amount of the moderator. This reveals that the strength of polymer network is reduced when the harder polymer chain is relaxed by the incorporation of the shorter chain molecules of the moderator. Five alumina discs, a, b, c, d and e, were produced under the same conditions. The diameter of the samples is about 50 mm and the thickness is about 4 mm. The
Fig. 4. Strength and elastic modulus of alumina green bodies versus the amount of moderator.
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identifiers of the disc samples are marked on top of the corresponding amount of the moderator in Fig. 4. Fig. 5 shows the surface patterns of the alumina green bodies after drying and de-bindering to remove all the water and organic binders. Radial cracks are found on samples (c, d and e), while samples (a and b) with higher amount of moderator are free of visible cracks. It is believed that the inner stress was initiated at the forming stage and magnified during drying process when the preform becomes harder. The intrinsic strength of the green body after drying and de-bindering is mainly determined by the nature of the ceramic powder and the solid loading. The observations of the surface patterns confirm that both the inner stress and the elastic modulus of the green body decrease with the amount of the moderator. Choosing a proper amount of the moderator can effectively reduce the occurrence of cracks on the subsequent processes. 3.4. Improved reliability of sintered product The repeatability of the mechanical properties of final products has been greatly improved. A typical zirconiatoughened alumina (ZTA-50) and several grades of silicon nitride-based ceramics (Si3N4) were formed by colloidal injection moulding. The ZTA was sintered at 1580 8C in air for 2 h. The Si3N4 green bodies were further cold isostatic pressed to improve the green density prior to a two-stage gaspressure sintering process. The first stage was performed at 1750 8C for 1.5 h under a nitrogen pressure of 0.3 MPa and the second stage at 1900 8C under a pressure of 6 MPa in N2 for various holding time from 1 to 2.5 h. The flexural strength was measured by standard three-point bending method for ceramics. Weibull modulus was derived from 20 such tests on each material. As shown in Fig. 6, the Weibull modulus of ZTA-50 materials is as high as 24 and the optimum of Si3N4-based materials 34 for the optimum holding time (2 h). As a reference, the typical value of these materials prepared by conventional forming processes is
Fig. 5. Surface patterns of alumina green bodies after drying and debindering.
Fig. 6. Strength and Weibull modulus of ceramics; r is the flexural strength, F is the probability of rupture, r m is the mean flexural strength, m is the Weibull modulus.
about 15. Therefore, the repeatability of the mechanical properties has greatly been improved. The microstructures of the sintered ZTA-50 and Si3N4 bodies were observed on SEM as shown in Fig. 7. The micrograph of ZTA-50 confirms that the ZrO2 particles are uniformly dispersed. Similarly in the sintered Si3N4, the elongated h-Si3N4 grains are also well distributed. The improved uniformity of the
Fig. 7. Microstructure of the sintered bodies by SEM, (a) ZTA-50 and (b) Si3N4.
Y. Huang et al. / Materials Letters 58 (2004) 3893–3897
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microstructure is responsible for the increased repeatability of the fracture strength.
These processing strategies have led to the improvement of the reliability of the products.
4. Discussions
Acknowledgments
A new ceramics-forming process, colloidal injection moulding is developed by combining the conventional injection moulding and gel-casting of aqueous ceramic suspension. This process has the capability to form ceramics parts of complex shape and great potential to improve the homogeneity and reliability of the ceramic products via pressure-induced solidification.
This research work was supported by b863 ProgramQ (2001AA337060) and b973 ProgramQ of the People’s Republic of China (Grant No. G2000067203 and No. G2000067204). The authors are grateful for the grants.
(1)
[1] F.F. Lange, Powder processing science and technology for increased reliability, J. Am. Ceram. Soc. 72 (1989) 3 – 15. [2] A.C. Young, O.O. Omatete, M.A. Janney, A. Strehlow, Gelcasting of alumina, J. Am. Ceram. Soc. 74 (1991) 612 – 618. [3] L.J. Gauckler, T.J. Graule, F.H. Baader, Ceramic forming using enzyme catalyzed reactions, Mater. Chem. Phys. 61 (1999) 78 – 102. [4] B. Balzer, M.K.M. Hruschka, L.J. Gauckler, Coagulation kinetics and mechanical behavior of wet alumina green bodies produced via DCC, J. Colloid Interface Sci. 216 (1999) 379 – 386. [5] J. Yang, Y. Huang, L.P. Meier, H. Wyss, et al., Direct coagulation casting via increasing ionic strength, Key Eng. Mater. 224-2 (2002) 631 – 636. [6] X. Liu, Y. Huang, J. Yang, Effect of rheological properties of the suspension on the mechanical strength of Al2O3–ZrO2 composites prepared by gel casting, Ceram. Int. 28 (2002) 159 – 164. [7] L. Zhou, Y. Huang, Z. Xie, A. Zimmermann, F. Aldinger, Preparation of Si3N4 ceramics with high strength and high reliability via a processing strategy, J. Eur. Ceram. Soc. 22 (2002) 1347 – 1355. [8] J. Yang, L. Su, L. Ma, Y. Huang, Colloidal injection moulding of ceramics, Key Eng. Mater. 224-2 (2002) 667 – 671. [9] L. Su, J. Yang, L. Ma, C. Dai, Y. Huang, Colloidal injection moulding of ceramics induced by pressure, Rare Met. Mater. Eng. 31 (2002) 129 – 132.
(2) (3)
(4)
The pressure-induced solidification occurs simultaneously within the slurry so that the homogeneity of the green body can be improved and that the inner stress reduced. The hydrostatic pressure helps to suppress the structural defects such as delamination or cracks. The solidification process can be achieved at room temperature so that the process can be simplified and easily automated for continuous production on large batch. The pressure can be controlled precisely to optimise the speed of solidification and minimise the structural defects.
The inner stress in green bodies caused by non-uniform solidification in gel-casting processes can be significantly reduced by the addition of a moderator in the ceramic suspension. Combining this strategy with the colloidal injection moulding process is a promising solution to forming inner-stress free complex shape ceramic parts.
References
Improving the homogeneity and reliability of ceramic parts with complex shapes by pressure-assisted gel-casting Y. Huanga, L.G. Maa, H.R. Leb,*, J.L. Yanga a
Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China b Department of Engineering, University of Cambridge, Trumpington St., Cambridge CB2 1PZ, United Kingdom Received 30 March 2004; received in revised form 10 April 2004; accepted 24 August 2004 Available online 11 September 2004
Abstract Forming process plays an important role in the homogeneity and reliability of advanced ceramic products. Inspired by the surprising observation of pressure-induced solidification in colloidal forming processes, the authors have developed a new ceramics forming process, colloidal injection moulding, with the potential of forming large ceramic parts of complex shapes such as engine turbines and biomedical implants. This process combines the advantages of the conventional injection moulding and gel-casting processes of ceramics. Not only the efficiency but also the homogeneity of the green body is greatly improved owing to the pressure-induced solidification. It is also found that the inner stress generated by non-uniform solidification due to temperature gradient is responsible for the origination of microstructural defects in green body during colloidal-forming processes. A new strategy is taken to reduce the inner stress by adjusting the stiffness of the green body using a moderator additive. Ceramic products with complex shape such as engine turbines have successfully been fabricated with reduced defects. D 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Reliability; Gel casting; Injection moulding
1. Introduction Advanced structural ceramic materials are promising candidates for aerospace systems, engine components, machine tools, etc., due to their excellent high-temperature endurance; wear resistance and stability in corrosive environment. However, the nature of ceramics is brittle. The mechanical properties of ceramics are extremely sensitive to microstructural defects so that the performance cannot be predicted accurately. This has become the major obstacle to the successful application of advanced ceramics in engineering and attracted great attention over the last several decades. It is well known that ceramic
* Corresponding author. Tel.: +44 1223 332784; fax: +44 1223 332662. E-mail address: [email protected] (H.R. Le). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.08.015
materials are usually made from abundant elements in the earth crust such as silicon, aluminium, oxygen, nitrogen in natural or synthetic forms via powder metallurgy process. The fabrication process usually involves powder preparation, forming and sintering. Previous studies have indicated that microstructural defects such as delamination, microcracks and large pores in the product are usually associated with the non-uniformity in green body caused at the forming or drying stage. These defects will become fracture origins and hence reduce the strength and reliability of the product [1]. Therefore, one main objective in a forming process is to produce uniform green body free of structural defects. Recently, colloidal forming processes such as gel casting [2] and direct coagulation casting (DCC) [3], in which ceramic powder is dispersed in a water-based suspension and held in a gel network, have attracted great interest. In theory, the uniformity of green body can be improved if the
3894
Y. Huang et al. / Materials Letters 58 (2004) 3893–3897
suspension is perfectly dispersed. In practice, DCC forming process is usually quite slow and inevitably, delamination occurs while the suspension destabilises [4,5]. As a consequence, the strength of green body soproduced is low. Gel-casting process has been applied to the fabrication of Al2O3- and Si3N4-based ceramics by the authors [6,7]. Both the flexural strength and Weibull modulus have been improved by optimising the rheological properties of the suspension in gel-casting process and subsequent sintering process. The drawback of gelcasting is that the solidification of the suspension is usually induced by the mould temperature; thus, the solidification is not uniform in large complex parts due to the existence of temperature gradient, resulting in inner stress at the forming stage and microcracks on subsequent drying and de-bindering processes. Conventional injection moulding is applied to form complex ceramic parts such as engine turbines. The process involves plastic flow of a mixture of organic binders and ceramic powder at elevated temperature and subsequent solidification upon cooling in the mould. Though the forming process is efficient and suitable for automation on large-scale production, delamination usually occurs in green bodies of complex shapes leading to cracks on the de-bindering process to remove the large amount of organic binders. On the contrary, using colloidal forming processes can improve the uniformity of green bodies and reduce structural defects but at the cost of efficiency. An optimal approach would be a combination of injection moulding and gel-casting processes, bringing together their advantages. Based on this concept, a new ceramics-forming process, colloidal injection moulding, is developed by the authors [8]. It was an exciting moment when the authors observed the unexpected solidification of slurry at the exit valve of a storage tank in a fridge [9]. This implies that gelation has occurred under pressure even at low temperature. In this paper, we investigate the effect of hydrostatic pressure on the solidification process and homogeneity of the green bodies in colloidal injection moulding process and a new strategy to minimise the inner stress and microstructural defects in green body.
2. Experimental procedure The experimental process is outlined in Fig. 1. A premix solution of monomers was prepared in deionised water with a concentration of 14 wt.% of acrylamide, C2H3CONH2 (AM) and 0.6 wt.% of N,NV-methylenebisacrylamide, (C2H3CONH)2CH2 (MBAM). The ceramic powders with 1 wt.% poly(methacrylic-acid-ammonium) as dispersant were mixed in the premix solution at a solid loading of about 50 vol.% using ball milling. The slurry was added with 0.1–0.5 vol.% of initiator, a 5 wt.% aqueous solution of ammonium persulfate (NH4)2S2O8 and 0.05–0.25 vol.% of an organic catalyst, N,N,NV,NV-tetramethylethylenediamine. The suspension was stirred slowly and ultrasonically agitated to achieve high homogeneity and then kept in a fridge to avoid solidifying prior to forming. Finally, the suspension was injected into a hot steel mould so that the slurry solidified quickly to become a preform with the polymerisation of the monomers. The preform was demoulded and dried at 70 8C to remove the water and organic contents. The dried green bodies were then sintered in appropriate conditions for each material system. Each sintered body was cut and ground into specimens of 3436 mm for the bending strength measurements and 4630 mm for single edge notched bending (SENB) measurements of fracture toughness.
3. Results 3.1. Effect of hydrostatic pressure on solidification A pressure induction unit was then added to apply an external pressure after injection. Gelation curves of Al2O3 slurry under different pressures with a mould temperature of 25 and 36 8C are shown in Fig. 2. Under increased pressures, the onset time of gelation becomes shorter and the gelation speed increases significantly. At higher temperature, the influence of the pressure becomes more significant and the gelation finishes almost immediately. The pressure-induced solidification will have distinct advantages over temperature-induced solidification.
Fig. 1. A flowchart of colloidal injection moulding of ceramics.
Y. Huang et al. / Materials Letters 58 (2004) 3893–3897
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the solidification on this process is associated with the surface temperature of the mould. The gradient of the temperature will inevitably cause non-uniform solidification and inner stress in the green body. A new strategy is developed to minimise the inner-stress in green bodies prepared in this process. 3.3. Controlling the inner stress in the green body
Fig. 2. The effect of external pressure on the gelation of Al2O3 slurry with a solid loading of 50 vol.%; Ts is the starting temperature.
3.2. Homogeneity of the green bodies An alumina (Al2O3) green body of turbine shape with a diameter of 105 mm was prepared by this process. The spatial distribution of density is compared with that by conventional process using the same injection moulding system in Fig. 3. The value in each part represents the density in g/cm3. The relative standard deviation of density for colloidal injection moulding is 0.2% compared to 0.7% for conventional injection moulding. The results show that colloidal injection moulding process significantly improves the homogeneity in density of the green body. Nevertheless,
Fig. 3. Comparison of density distribution in Al2O3 green bodies of turbine prepared by (a) conventional injection moulding and (b) colloidal injection moulding on the same system.
The inner stress in green body is often responsible for the initiation of microcracks on subsequent drying and debindering processes. During colloidal injection moulding or other gel-casting processes, monomers in ceramic suspension polymerise and then form gel networks to hold the ceramic particles. The solidification speed increases with increasing temperature for a given composition. Non-uniform solidification occurs due to the temperature gradient in the ceramic suspension and results in the development of inner stress in the green body. Theoretical analysis has indicated that the magnitude of the inner stress increases with the stiffness of the green body. Based on this finding, a fraction of the monomers is replaced by a moderator, hydroxylethylacrylate in the suspension during gel-casting or colloidal injection moulding to control the stiffness of the gel network. Test bars with dimensions of 6642 mm were prepared with 50 vol.% solid loading and various amount of the moderator from 0 to 100 wt.% in the total monomers. The samples were dehydrated at about 70 8C in an electrical furnace. The flexural strength and elastic modulus of the green body are evaluated using three-point bending. The results are shown in Fig. 4 against the amount of the moderator. Both the flexural strength and elastic modulus of green body decrease simultaneously with the amount of the moderator. This reveals that the strength of polymer network is reduced when the harder polymer chain is relaxed by the incorporation of the shorter chain molecules of the moderator. Five alumina discs, a, b, c, d and e, were produced under the same conditions. The diameter of the samples is about 50 mm and the thickness is about 4 mm. The
Fig. 4. Strength and elastic modulus of alumina green bodies versus the amount of moderator.
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identifiers of the disc samples are marked on top of the corresponding amount of the moderator in Fig. 4. Fig. 5 shows the surface patterns of the alumina green bodies after drying and de-bindering to remove all the water and organic binders. Radial cracks are found on samples (c, d and e), while samples (a and b) with higher amount of moderator are free of visible cracks. It is believed that the inner stress was initiated at the forming stage and magnified during drying process when the preform becomes harder. The intrinsic strength of the green body after drying and de-bindering is mainly determined by the nature of the ceramic powder and the solid loading. The observations of the surface patterns confirm that both the inner stress and the elastic modulus of the green body decrease with the amount of the moderator. Choosing a proper amount of the moderator can effectively reduce the occurrence of cracks on the subsequent processes. 3.4. Improved reliability of sintered product The repeatability of the mechanical properties of final products has been greatly improved. A typical zirconiatoughened alumina (ZTA-50) and several grades of silicon nitride-based ceramics (Si3N4) were formed by colloidal injection moulding. The ZTA was sintered at 1580 8C in air for 2 h. The Si3N4 green bodies were further cold isostatic pressed to improve the green density prior to a two-stage gaspressure sintering process. The first stage was performed at 1750 8C for 1.5 h under a nitrogen pressure of 0.3 MPa and the second stage at 1900 8C under a pressure of 6 MPa in N2 for various holding time from 1 to 2.5 h. The flexural strength was measured by standard three-point bending method for ceramics. Weibull modulus was derived from 20 such tests on each material. As shown in Fig. 6, the Weibull modulus of ZTA-50 materials is as high as 24 and the optimum of Si3N4-based materials 34 for the optimum holding time (2 h). As a reference, the typical value of these materials prepared by conventional forming processes is
Fig. 5. Surface patterns of alumina green bodies after drying and debindering.
Fig. 6. Strength and Weibull modulus of ceramics; r is the flexural strength, F is the probability of rupture, r m is the mean flexural strength, m is the Weibull modulus.
about 15. Therefore, the repeatability of the mechanical properties has greatly been improved. The microstructures of the sintered ZTA-50 and Si3N4 bodies were observed on SEM as shown in Fig. 7. The micrograph of ZTA-50 confirms that the ZrO2 particles are uniformly dispersed. Similarly in the sintered Si3N4, the elongated h-Si3N4 grains are also well distributed. The improved uniformity of the
Fig. 7. Microstructure of the sintered bodies by SEM, (a) ZTA-50 and (b) Si3N4.
Y. Huang et al. / Materials Letters 58 (2004) 3893–3897
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microstructure is responsible for the increased repeatability of the fracture strength.
These processing strategies have led to the improvement of the reliability of the products.
4. Discussions
Acknowledgments
A new ceramics-forming process, colloidal injection moulding is developed by combining the conventional injection moulding and gel-casting of aqueous ceramic suspension. This process has the capability to form ceramics parts of complex shape and great potential to improve the homogeneity and reliability of the ceramic products via pressure-induced solidification.
This research work was supported by b863 ProgramQ (2001AA337060) and b973 ProgramQ of the People’s Republic of China (Grant No. G2000067203 and No. G2000067204). The authors are grateful for the grants.
(1)
[1] F.F. Lange, Powder processing science and technology for increased reliability, J. Am. Ceram. Soc. 72 (1989) 3 – 15. [2] A.C. Young, O.O. Omatete, M.A. Janney, A. Strehlow, Gelcasting of alumina, J. Am. Ceram. Soc. 74 (1991) 612 – 618. [3] L.J. Gauckler, T.J. Graule, F.H. Baader, Ceramic forming using enzyme catalyzed reactions, Mater. Chem. Phys. 61 (1999) 78 – 102. [4] B. Balzer, M.K.M. Hruschka, L.J. Gauckler, Coagulation kinetics and mechanical behavior of wet alumina green bodies produced via DCC, J. Colloid Interface Sci. 216 (1999) 379 – 386. [5] J. Yang, Y. Huang, L.P. Meier, H. Wyss, et al., Direct coagulation casting via increasing ionic strength, Key Eng. Mater. 224-2 (2002) 631 – 636. [6] X. Liu, Y. Huang, J. Yang, Effect of rheological properties of the suspension on the mechanical strength of Al2O3–ZrO2 composites prepared by gel casting, Ceram. Int. 28 (2002) 159 – 164. [7] L. Zhou, Y. Huang, Z. Xie, A. Zimmermann, F. Aldinger, Preparation of Si3N4 ceramics with high strength and high reliability via a processing strategy, J. Eur. Ceram. Soc. 22 (2002) 1347 – 1355. [8] J. Yang, L. Su, L. Ma, Y. Huang, Colloidal injection moulding of ceramics, Key Eng. Mater. 224-2 (2002) 667 – 671. [9] L. Su, J. Yang, L. Ma, C. Dai, Y. Huang, Colloidal injection moulding of ceramics induced by pressure, Rare Met. Mater. Eng. 31 (2002) 129 – 132.
(2) (3)
(4)
The pressure-induced solidification occurs simultaneously within the slurry so that the homogeneity of the green body can be improved and that the inner stress reduced. The hydrostatic pressure helps to suppress the structural defects such as delamination or cracks. The solidification process can be achieved at room temperature so that the process can be simplified and easily automated for continuous production on large batch. The pressure can be controlled precisely to optimise the speed of solidification and minimise the structural defects.
The inner stress in green bodies caused by non-uniform solidification in gel-casting processes can be significantly reduced by the addition of a moderator in the ceramic suspension. Combining this strategy with the colloidal injection moulding process is a promising solution to forming inner-stress free complex shape ceramic parts.
References