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Fabrication of complex shaped alumina parts by gelcasting on 3D printed moulds
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication of complex shaped alumina parts by gelcasting on 3D printed moulds Sindi Sithembiso Ndinisaa,b,∗, David James Whitefielda,b, Iakovos Sigalasa,b a b
School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa DST-NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg, South Africa
A R T I C LE I N FO
A B S T R A C T
Keywords: Gelcasting 3D printing Alumina Solid loading Moulds
Alumina ceramic components were produced using gelcasting and 3D printing techniques to generate the end product. The 3D printed mould made from (acrylonitrile butadiene styrene) ABS filament provides a convenient demoulding method by dissolution of the mould using acetone as a solvent. This process enables low cost production of complex shaped ceramic components. The effect of the suspension solid loading on the properties and microstructure of complex shaped alumina parts was investigated. The produced ceramic components had densities up to 99.0%, hardness of 18 GPa, flexural strength of 374 MPa and a fracture toughness of 3.8 MPa√m after sintering in air for 3 h, in good agreement with published values.
1. Introduction Additive Manufacturing (AM) has demonstrated an ability to produce custom parts with complex geometries and proven to be of specific use in the rapid production of functional prototypes. AM is based on Computer Aided Design (CAD) which gives a high degree of freedom for verification of designs prior to manufacturing parts. CAD allows for simple, rapid, and economic design alterations with minimal time investments and relatively low cost [1]. AM can be used to achieve complexity in products without negatively affecting production rates as well as maintaining the quality of the products [1,2]. The simplicity of the 3D printing process is the main benefit over traditional shaping methods [3]. The increase in demand for advanced ceramics has highlighted some shortcomings of the current processing techniques. Custom made parts with complex shapes are difficult and occasionally impossible to produce by traditional methods. Due to ceramic's high hardness and low fracture toughness, the machining of dense ceramic parts is difficult, time consuming and expensive. There is a rising need to find alternate solutions to manufacturing challenges that require custom solutions. The demand for low cost techniques has motivated innovative concepts for powder processing and moulding techniques for the production of complex shaped ceramics [4]. Direct material AM methods such as Selective Laser Sintering (SLS) and Three Dimensional Printing (3DP) have been used in the preparation of complex shaped ceramics and metals. Complex shaped parts
∗
with high densities have been produced. However, the equipment used is complex, costly and a protective atmosphere is required. Additive manufacturing can be applied to ceramic forming by producing complex shaped expendable moulds that can be used for investment type casting procedure [5–7]. Investigations into the colloidal processing of ceramics have been utilised and these have been found to improve their reliability [8]. Gel casting is a widely accepted colloidal forming method. It has been proven to be an effective and low cost near net shape method in the fabrication of complex shaped three-dimensional ceramic parts [9]. This method has been widely accepted due to its versatility and the ease by which it is adopted into this range of applications. It involves filling a non-porous mould with a ceramic suspension, followed by the in situ polymerisation of the suspension. A green body with sufficient strength is formed, removed from the mould and sintered to form a dense ceramic part. Progress has been made on the optimisation of gel casting to produce ceramic parts with high reliability [4,8,10,11]. These developments have widened the application of gel casting as a ceramic forming method. The non-porous moulds used in gelcasting include materials such as aluminium, plastics and wax. Wax has been successfully used as an expendable mould. However, the production of complex shapes was generally achieved through split moulds and this process proved costly for low production volumes. With a focus on small production volumes and complex geometries, AM provides versatility, high precision and repeatability [5]. The materials used for the mould include Polylactic
Corresponding author. School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa. E-mail address: [email protected] (S.S. Ndinisa).
https://doi.org/10.1016/j.ceramint.2019.10.021 Received 12 June 2019; Received in revised form 19 September 2019; Accepted 2 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sindi Sithembiso Ndinisa, David James Whitefield and Iakovos Sigalas, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.021
Ceramics International xxx (xxxx) xxx–xxx
S.S. Ndinisa, et al.
Acid (PLA) and ABS plastics. These are excellent materials as they are soluble in inexpensive solvents, such as water and acetone. Thus, easier demoulding of complex shaped objects can be accomplished. The aim of this work was to investigate the casting behaviour and fabrication of complex shaped alumina ceramic part by combining 3D printing and gelcasting methods. This would solve problems in existing applications and could lead to new ones. The materials used for casting is an alumina suspension that was cast into 3D printed moulds. Once gelled, the part will be demoulded by dissolving the mould in acetone. The produced parts were thermally processed and characterised by determining their relative densities, mechanical properties and their microstructures, as examined by Scanning Electron Microscopy (SEM). 2. Materials and methods 2.1. Preparation of suspensions Fig. 1. Rheological behaviour of alumina suspensions prepared with 0.3 wt% Isobam®.
Commercially available alumina powder (Alfar Aesar) with an average particle size of 0.8 μm was used as the starting powder. A copolymer of Isobutylene and maleic anhydride (Isobam®) was used as the dispersant and gelling agent while distilled water was used as the dispersion medium. The viscosities of alumina powders with and without the dispersant were measured by a rheometer (Anton Paar). The rheological analyses gave an indication of the optimal processing parameters for gelcasting. The gelling agent, Isobam®, was dissolved in distilled water to form the premix solution. Slurries with a solid loading of up to 60 vol% were prepared by adding the alumina powder into the premix solution. Slurries were mixed in a 250 ml stainless steel milling bowl with a zirconium oxide lining and an inside diameter of 7 cm. Alumina grinding balls with a diameter of 2 mm were used as the milling media. The powder and the milling balls used for the milling were carefully weighed to achieve a ball to powder mass ratio of 1:3. The slurries were milled for 24 h s to break up the agglomeration of the particles and ensure perfect mixing using a planetary ball mill at a milling speed of 300 rpm. The resulting slurries were degassed in a desiccator connected to a vacuum pump to remove any trapped air. 2.2. 3D printing, casting and thermal processing Various solid models were constructed using computer aided design software. Cura® software was used to slice the model and produce its negative which acted as the mould pattern. A three dimensional printer was used to print the mould by extrusion of a plastic filament of acrylonitrile butadiene styrene (ABS). The slurry was poured into the 3D printed ABS plastic moulds. The slurry containing moulds were left at room temperature for gelation to occur. After the polymerisation of the slurry was completed, the green samples were removed from the moulds by dissolving the mould in acetone. The green samples were dried in an environmental chamber with a controlled humidity of 90.0% and the temperature was kept at 50 °C. They were then heated to 600 °C for 1hr to remove the polymer in a special debindering muffle furnace and sintered in air at a temperature of 1650 °C for 3 h s.
Fig. 2. Photographs of a) the 3D designed mould, b) green gelled part and c) sintered part. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
polished surfaces with a diameter of 16 mm and a thickness of 2 mm. An interactive Web-Mathematica tool, developed by the Institute of Structural and Functional Ceramics, Montan University, Leoben [13,14] was used to calculate the TRS values. Polished and fracture surfaces of the samples were examined by SEM after the surfaces were thermally etched at 1350 °C for 30min and coated using carbon and gold-palladium.
2.3. Characterisation The Archimedes’ method was used to measure the density and porosity of the parts that were fabricated. The hardness and fracture toughness were determined by Vickers indentation onto mirror finished surfaces. Five indentations were made using a 5kgf load for 15 s. The values of the diagonals were measured and the hardness was calculated. The fracture toughness was determined by direct crack measurement method using the Anstis [12] equation. The flexural strength was determined by using the ball on three balls (B3B) [13] using a Tinius Olsen H50KT press to determine the force at which each specimen broke. The tests were performed on
3. Results and discussion 3.1. Suspension characteristics Rheological properties of gelcasting suspensions are important as 2
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In this study the fabrication of complex shaped parts was achieved by casting in 3D printed ABS plastic moulds. Fig. 2 shows a) the desired final shape, b) the gelled gelcast part and c) the part after sintering in air at 1650 °C. Gelation of Isobam® slurries occurs when the suspension is at rest in room temperature. The gelled part (Fig. 2 (b)) was removed from the mould by dissolving the latter away in acetone. The formed parts contained both thick and thin sections as shown in Fig. 3. This shows the capabilities of the combined 3D printing and gelcasting method because the density was found to be uniform at each sectional level. This is in agreement with Young et al. [11], that the forming of an object by gelcasting is only limited by the availability of the moulding tool. Dimensional shrinkage is a typical phenomenon related to the casting processes and occurs during drying. Therefore the drying is regarded as the rate limiting step. Adequate drying conditions were experimentally determined and drying in a chamber with controlled humidity (90.0%) and a temperature of 50 °C was found to be favourable. The drying shrinkage was found to be 4.0% for both the 50 and 60 vol% solids loading, contrary to parts gelcast using 40 vol% solids loading which showed shrinkage of 14.0%. This illustrates that the drying shrinkage is a function of the solids loading. Thus, the drying shrinkage can be minimized by increasing the solids loading. However, a high solid loading is unfavourable as it leads to an increase in viscosity and potentially increases the occurrence of defects.
Fig. 3. Complex shaped alumina parts produced by gelcasting on 3D printed moulds and sintered at 1650 °C.
they regulate the properties of the formed green body. The rheological behaviour of alumina suspensions was determined as a function of the solids loading. The study was performed on slurries containing 40–60 vol% alumina powder and 0.3 wt% Isobam® after ball milling for 24 h s. Fig. 1 illustrates the rheological flow curves for alumina suspensions as a function of solids loading ranging from 40 to 60 vol%, over a shear rate of 1 to 100s−1. The apparent viscosity (η) is plotted against the shear rate (γ). The apparent viscosity of all suspensions was found to exhibit a shear thinning behaviour over a wide range of shear rate. The viscosity and shear thinning behaviour at high shear rates increases with an increase in the solid loading as observed in the viscosity curve. This phenomenon was also reported by Young et al. [11], and Qin et al. [15], thus the general behaviour of an alumina suspension was to exhibit shear thinning and an increase in viscosity with increasing solids loading. The high viscosity at low shear rates (thixotropic behaviour) for the suspensions containing 50 and 60 vol% solids loading indicates a flocculated state of particles within the dispersing medium. This phenomenon was reported by Liu et al., [10]. However, all viscosities were low enough to ensure complete filling of the mould.
3.3. Effect of de-airing A defect that was observed in the gelcast parts was entrapped air bubbles. This is a common problem that is inherent to solvent based processing techniques. Fig. 4 reveals elliptic shaped pores in the microstructure with diameters up to 200 μm; these are a result of poor deairing prior to casting the suspension. The entrapped air bubbles adversely affect the density and the mechanical properties of the parts produced. Methods of eliminating entrapped air bubbles during slurry preparation were investigated. Ultrasonication, defoaming agents and vacuum de-airing were utilised to eliminate entrapped air in the slurry. These methods used in combination were shown to be effective in eliminating entrapped air. With sufficient deairing, fewer pores were observed at low magnification and the parts were found to have relative densities up to 99.0%. However at higher magnifications both intergranular and intragranular pores were observed in all the ceramic parts produced, Fig. 5. A homogeneous microstructure was observed in Fig. 5 a for the part gelcast using 40 vol% solids loading. The 50 vol% solids loading has a few large grains surrounded by smaller grains and the microstructure for the 60 vol% solid loading sample has more larger grains that are surrounded by a number of smaller grains. Moreover, the presence of intergranular and intragranular pores can be observed for the 50 and 60 vol% solids loading (Fig. 5b and c). The pores can be attributed mainly to grain growth which was expected for pressure-less sintering; this was in agreement with German [16].
3.2. Gelcasting Gelcasting requires the use of non-porous moulds to achieve the desired shape. However the mould design and mould material play a significant role during the fabrication of a complex shape. The mould should be designed such that it allows shrinkage to occur but minimise the resulting stresses during gelation. The mould should also facilitate the release of air bubbles when casting the slurry in the presence of air.
Fig. 4. SEM micrographs illustrating the effect of de-airing on the microstructure of sintered alumina gelcast prior to deairing. 3
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Fig. 5. SEM micrographs of polished surface of sintered (1650 °C) specimens made from suspensions containing a) 40, b) 50 and c) 60 vol% solids loading. Table 1 Properties of the fabricated parts as a function of solid loading (samples sintered at 1650 °C). Solids content
40 vol%
50 vol%
60 vol%
Density of green body (g/cm3) Density of sintered body (g/cm3) Relative density (%) Shrinkage (%)
2.13 3.93 99.0 ± 0.8 19.5
2.49 3.92 98.7 ± 0.2 16.8
2.59 3.89 98.0 ± 0.5 18.0
Fig. 7. The effect of solids loading on the flexural strength and hardness of sintered alumina parts.
3.4. Effect of solids loading on sintered body The solid loading of a suspension able to meet the requirements for ensuring the filling of the mould was as high as 60 vol%. However, the solid content in the slurry has a critical effect on the properties of the sintered parts. Table 1 summarises the effect of the solids content on the alumina suspensions prepared using 0.3 wt % Isobam®, gelcast in 3D printed moulds and sintered in air at 1650 °C using a dwell time of 3 h s. The green density, measured from the volume and mass of the green bodies, was found to increase as the solids loading increased; this was due to the amount of powder in the green compact. However, the degree of densification is mainly related to the particle packing, the solids loading in the starting gelcasting suspension and the ratio of the surface energy of the powder particles to the interfacial energy of the sintered body. The higher the ratio, the more compact the sintered body can be densified at a particular set of sintering conditions [16]. Fig. 6 shows the effect of the solids loading on the green and sintered relative densities of the fabricated alumina parts. The sintered density is dependent on the packing of the powder. The sintered density slightly decreases from 99.0% at 40 vol% to 98.7% at 50 vol%. A further decrease at 60 vol% is observed and this is the lowest relative
Fig. 6. The effect of solids loading on the relative (sintered and green) densities of sintered alumina (referenced to the density of alumina taken as 3.97 g cm3). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 2 Mechanical properties of gelcast alumina sintered at 1650 °C. Solids content
40 vol%
50 vol%
60 vol%
Relative density (%) Hardness (GPa) Fracture toughness (MPa.√m) Flexural strength (MPa)
99.0 ± 0.8 18 ± 1 3.8 ± 0.7 374 ± 66
98.7 ± 0.2 16 ± 2 3.7 ± 0.5 378 ± 34
98.0 ± 05 15 ± 2 2.7 ± 0.7 283 ± 17
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Fig. 8. SEM micrographs of fractured surface of sintered specimens at 1650 °C: a) 40 vol%, b) 50 vol% and c) 60 vol% solids loading.
The de-airing technique and solids loading played a critical role in the final relative density and mechanical properties of the parts produced. The densities and mechanical properties of the parts produced are comparable to parts fabricated by using conventional methods. The obtained relative densities obtained depend on the solids loading, sintering temperature and deairing. Part densities were up to 99.0%, hardness of 18 GPa, flexural strength of 374 MPa and a fracture toughness of 3.8 MPa√m. By using a 40 and 50 vol% solids loading suspension, complex shaped parts such as an impeller and a pyramid with relative densities above 99.0% were produced. These results demonstrate the capabilities of the combined 3D printing and gelcasting method. Thus, the forming of an object by gelcasting is only limited by the availability of the fabrication tool.
density obtained. The low densification at 60 vol% may be a result of the solids loading increasing beyond the maximum allowable solids content in the slurry. This may also be attributed to the higher viscosity at high solids loading and the hindrance of optimum particle packing arrangement in the suspension. High viscosity leads to insufficient deairing and hinders the flow of the slurry thus entrapping air and forming defects. The defects lead to stress concentrations in the final part, and have negative effects on the resulting mechanical properties [17]. 3.5. Mechanical properties Hardness and fracture toughness measurements were performed on the sintered parts by means of the Vickers indentation method using a loading force of 5kgf. The flexural strength was determined using the ball on three balls testing method [13] and these are summarised in Table 2. The effect of solids loading on the flexural strength and hardness of alumina parts was examined in Fig. 7. The hardness and flexural strength of the sintered alumina parts decreased as the solids loading increased. This is supported by the relative density as shown in Fig. 6. As the relative density increases, so does the hardness and flexural strength of the alumina parts. The parts produced using 40 and 50 vol% solids loading showed the highest flexural strength. The higher flexural strength on these samples was due to their high relative densities of about 99.0% which demonstrates a better packing of the ceramic particles in the green body. Thus, the mechanical properties show a strong dependence on the density of the sintered body and this is in agreement with Chantikul et al., [18]. Microstructural and surface defects were found to be the course of crack initiation during flexural strength tests. The grain size in the 40 vol% solids loading (Fig. 8(a)) acted as an internal defect that initiated the fracture, therefore, slightly lowering the fracture strength. The 60 vol% samples had lower fracture flexural strength than all the samples due to the aggregation of smaller pores that were found just below the surface. The high solids loading led to difficulties in de-airing the suspension, which increased the volume of trapped air pockets, thus pores were the origin of fracture. Moreover, agglomerates resulted as a result of the high viscosity. The agglomerates tend to have large, inter agglomerated porosity that accumulates and forms defects in the green body. The formed defects always act as crack-initiating defects, thus no healing occurs during sintering [19].
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge the Department of Science and Technology (DST), National Research Foundation (NRF) and the Centre of Excellence in Strong Materials (CoE) - DST-NRF CoE in Strong Materials (South Africa) for financial support and Lukas Janse Van Rensburg for designing and 3D printing the moulds. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.10.021. References [1] J. Klein, M. Stern, G. Franchin, G. Kayser, C. Inamura, S. Dave, J.C. Weaver, P. Houk, P. Colombo, M. Yang, N. Oxman, “Additive manufacturing of optically transparent glass”, 3D Print. Addit. Manuf. 2 (3) (2015) 92–105. [2] B. Utela, D. Storti, R. Anderson, M. Ganter, “A review of process development steps for new material systems in three dimensional printing (3DP)”, J. Manuf. Process. 10 (2008) 96–104. [3] A. Zocca, C.M. Gomes, E. Bernardo, R. Muller, J. Gunster, P. Colombo, “LAS glassceramic scaffolds by three-dimensional printing”, J. Eur. Ceram. Soc. 33 (2013) 1525–1533. [4] W.M. Sigmund, N.S. Bell, L. Bergstrom, “Novel powder processing methods for advanced ceramics”, J. Am. Ceram. Soc. 83 (7) (2000) 1557–1574. [5] S. Wang, A.G. Miranda, C. Shih, “A study of investment casting with plastic patterns”, Mater. Manuf. Process. 25 (12) (2010) 1482–1488. [6] P. J. Whalen, V. R. Jamalabad, J. P. Pollinger, M. Agarwala and S. C. Danforth, “Gelcasting molding with fugitive molds”. United States of America Patent 5824250, 20 October 198. [7] A. Ortera, C. D'Ängelo, S. Gianella, D. Gaia, “Cellular ceramics produced by rapid prototyping and replication”, Mater. Lett. 80 (2012) 95–98. [8] F. Chabert, “Cross-linked polyvinyl alcohol as a binder for gelcasting and green machining”, J. Am. Ceram. Soc. 91 (10) (2008) 3138–3146. [9] A.A. Babaluo, M. Kokabi, A. Barati, “Chemorheology of alumina-aqueous acrylamide gelcasting systems”, J. Eur. Ceram. Soc. 24 (2004) 635–644. [10] X. Liu, Y. Huang, J. Yang, “Effect of rheological properties of the suspension on the mechanical strength of Al2O3-ZrO2 composites prepared by gelcasting”, Ceram. Int. 28 (2) (2002) 159–164. [11] A.C. Young, O.O. Omatete, M.A. Janney, P.A. Menchhofer, “Gelcasting of alumina”, J. Am. Ceram. Soc. 74 (3) (1991) 612–618.
4. Conclusion Dense complex shaped alumina ceramics with relative densities between 98.0 and 99.0% of the theoretical density were obtained using a gelcasting method combined with additive manufacturing. The gelling of an alumina suspension was achieved by using a water soluble copolymer of Isobutylene and Maleic Anhydride (Isobam®). Complex shaped parts were produced by moulding suspensions containing 40, 50 and 60 vol% alumina and 0.3 wt% Isobam® into 3D printed ABS moulds. This process reduces the time and cost of machining dense parts to achieve complex shapes. The properties of the produced alumina parts are highly depended on the preparation of the gelcasting suspension. 5
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[16] R.M. German, “Strength evolution in debinding and sintering”, Proceedings of the 3rd International Conference on Science, Technology & Applications of Sintering, 2003. [17] J. Stampfl, H. Liu, S.W. Nam, K. Sekamoto, H. Tsuru, S. Kang, A.G. Cooper, A. Nickel, F.B. Prinz, “Rapid prototyping and manufacturing by gelcasting metallic and ceramic slurries”, Mater. Sci. Eng. A 334 (2002) 187–192. [18] P. Chantikul, S.J. Bennison, B.R. Lawn, “Role of the grain size in the strength and Rcurve properties of alumina”, J. Am. Ceram. Soc. 73 (8) (1990). [19] P. Greil, “Generic principles of crack-healing ceramics”, Journal of advanced ceramics 1 (4) (2012) 249–267.
[12] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, “A critical evaluation of indentation techniques for measuring fracture toughness; I, direct crack measurements”, J. Am. Ceram. Soc. 64 (1981) 533–538. [13] A. Borger, P. Supancic, R. Danzer, “The ball on three balls test for strength testing of brittle discs- Stress distribution in the disc”, J. Eur. Ceram. Soc. 22 (2002) 1425–1436. [14] M. Universitat, “Ball on 3 balls test (B3B) - strength testing”, [Online] http://www. isfk.at/de/960/ , Accessed date: 2 October 2018. [15] X. Qin, Y. Zhou, Y. Yang, X. Shu, S. Shimai, S. Wang, “Gelcasting of transparent YAG ceramics by a new gelling system”, Ceram. Int. 40 (2014) 12745–12750.
6
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication of complex shaped alumina parts by gelcasting on 3D printed moulds Sindi Sithembiso Ndinisaa,b,∗, David James Whitefielda,b, Iakovos Sigalasa,b a b
School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa DST-NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg, South Africa
A R T I C LE I N FO
A B S T R A C T
Keywords: Gelcasting 3D printing Alumina Solid loading Moulds
Alumina ceramic components were produced using gelcasting and 3D printing techniques to generate the end product. The 3D printed mould made from (acrylonitrile butadiene styrene) ABS filament provides a convenient demoulding method by dissolution of the mould using acetone as a solvent. This process enables low cost production of complex shaped ceramic components. The effect of the suspension solid loading on the properties and microstructure of complex shaped alumina parts was investigated. The produced ceramic components had densities up to 99.0%, hardness of 18 GPa, flexural strength of 374 MPa and a fracture toughness of 3.8 MPa√m after sintering in air for 3 h, in good agreement with published values.
1. Introduction Additive Manufacturing (AM) has demonstrated an ability to produce custom parts with complex geometries and proven to be of specific use in the rapid production of functional prototypes. AM is based on Computer Aided Design (CAD) which gives a high degree of freedom for verification of designs prior to manufacturing parts. CAD allows for simple, rapid, and economic design alterations with minimal time investments and relatively low cost [1]. AM can be used to achieve complexity in products without negatively affecting production rates as well as maintaining the quality of the products [1,2]. The simplicity of the 3D printing process is the main benefit over traditional shaping methods [3]. The increase in demand for advanced ceramics has highlighted some shortcomings of the current processing techniques. Custom made parts with complex shapes are difficult and occasionally impossible to produce by traditional methods. Due to ceramic's high hardness and low fracture toughness, the machining of dense ceramic parts is difficult, time consuming and expensive. There is a rising need to find alternate solutions to manufacturing challenges that require custom solutions. The demand for low cost techniques has motivated innovative concepts for powder processing and moulding techniques for the production of complex shaped ceramics [4]. Direct material AM methods such as Selective Laser Sintering (SLS) and Three Dimensional Printing (3DP) have been used in the preparation of complex shaped ceramics and metals. Complex shaped parts
∗
with high densities have been produced. However, the equipment used is complex, costly and a protective atmosphere is required. Additive manufacturing can be applied to ceramic forming by producing complex shaped expendable moulds that can be used for investment type casting procedure [5–7]. Investigations into the colloidal processing of ceramics have been utilised and these have been found to improve their reliability [8]. Gel casting is a widely accepted colloidal forming method. It has been proven to be an effective and low cost near net shape method in the fabrication of complex shaped three-dimensional ceramic parts [9]. This method has been widely accepted due to its versatility and the ease by which it is adopted into this range of applications. It involves filling a non-porous mould with a ceramic suspension, followed by the in situ polymerisation of the suspension. A green body with sufficient strength is formed, removed from the mould and sintered to form a dense ceramic part. Progress has been made on the optimisation of gel casting to produce ceramic parts with high reliability [4,8,10,11]. These developments have widened the application of gel casting as a ceramic forming method. The non-porous moulds used in gelcasting include materials such as aluminium, plastics and wax. Wax has been successfully used as an expendable mould. However, the production of complex shapes was generally achieved through split moulds and this process proved costly for low production volumes. With a focus on small production volumes and complex geometries, AM provides versatility, high precision and repeatability [5]. The materials used for the mould include Polylactic
Corresponding author. School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa. E-mail address: [email protected] (S.S. Ndinisa).
https://doi.org/10.1016/j.ceramint.2019.10.021 Received 12 June 2019; Received in revised form 19 September 2019; Accepted 2 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sindi Sithembiso Ndinisa, David James Whitefield and Iakovos Sigalas, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.021
Ceramics International xxx (xxxx) xxx–xxx
S.S. Ndinisa, et al.
Acid (PLA) and ABS plastics. These are excellent materials as they are soluble in inexpensive solvents, such as water and acetone. Thus, easier demoulding of complex shaped objects can be accomplished. The aim of this work was to investigate the casting behaviour and fabrication of complex shaped alumina ceramic part by combining 3D printing and gelcasting methods. This would solve problems in existing applications and could lead to new ones. The materials used for casting is an alumina suspension that was cast into 3D printed moulds. Once gelled, the part will be demoulded by dissolving the mould in acetone. The produced parts were thermally processed and characterised by determining their relative densities, mechanical properties and their microstructures, as examined by Scanning Electron Microscopy (SEM). 2. Materials and methods 2.1. Preparation of suspensions Fig. 1. Rheological behaviour of alumina suspensions prepared with 0.3 wt% Isobam®.
Commercially available alumina powder (Alfar Aesar) with an average particle size of 0.8 μm was used as the starting powder. A copolymer of Isobutylene and maleic anhydride (Isobam®) was used as the dispersant and gelling agent while distilled water was used as the dispersion medium. The viscosities of alumina powders with and without the dispersant were measured by a rheometer (Anton Paar). The rheological analyses gave an indication of the optimal processing parameters for gelcasting. The gelling agent, Isobam®, was dissolved in distilled water to form the premix solution. Slurries with a solid loading of up to 60 vol% were prepared by adding the alumina powder into the premix solution. Slurries were mixed in a 250 ml stainless steel milling bowl with a zirconium oxide lining and an inside diameter of 7 cm. Alumina grinding balls with a diameter of 2 mm were used as the milling media. The powder and the milling balls used for the milling were carefully weighed to achieve a ball to powder mass ratio of 1:3. The slurries were milled for 24 h s to break up the agglomeration of the particles and ensure perfect mixing using a planetary ball mill at a milling speed of 300 rpm. The resulting slurries were degassed in a desiccator connected to a vacuum pump to remove any trapped air. 2.2. 3D printing, casting and thermal processing Various solid models were constructed using computer aided design software. Cura® software was used to slice the model and produce its negative which acted as the mould pattern. A three dimensional printer was used to print the mould by extrusion of a plastic filament of acrylonitrile butadiene styrene (ABS). The slurry was poured into the 3D printed ABS plastic moulds. The slurry containing moulds were left at room temperature for gelation to occur. After the polymerisation of the slurry was completed, the green samples were removed from the moulds by dissolving the mould in acetone. The green samples were dried in an environmental chamber with a controlled humidity of 90.0% and the temperature was kept at 50 °C. They were then heated to 600 °C for 1hr to remove the polymer in a special debindering muffle furnace and sintered in air at a temperature of 1650 °C for 3 h s.
Fig. 2. Photographs of a) the 3D designed mould, b) green gelled part and c) sintered part. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
polished surfaces with a diameter of 16 mm and a thickness of 2 mm. An interactive Web-Mathematica tool, developed by the Institute of Structural and Functional Ceramics, Montan University, Leoben [13,14] was used to calculate the TRS values. Polished and fracture surfaces of the samples were examined by SEM after the surfaces were thermally etched at 1350 °C for 30min and coated using carbon and gold-palladium.
2.3. Characterisation The Archimedes’ method was used to measure the density and porosity of the parts that were fabricated. The hardness and fracture toughness were determined by Vickers indentation onto mirror finished surfaces. Five indentations were made using a 5kgf load for 15 s. The values of the diagonals were measured and the hardness was calculated. The fracture toughness was determined by direct crack measurement method using the Anstis [12] equation. The flexural strength was determined by using the ball on three balls (B3B) [13] using a Tinius Olsen H50KT press to determine the force at which each specimen broke. The tests were performed on
3. Results and discussion 3.1. Suspension characteristics Rheological properties of gelcasting suspensions are important as 2
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In this study the fabrication of complex shaped parts was achieved by casting in 3D printed ABS plastic moulds. Fig. 2 shows a) the desired final shape, b) the gelled gelcast part and c) the part after sintering in air at 1650 °C. Gelation of Isobam® slurries occurs when the suspension is at rest in room temperature. The gelled part (Fig. 2 (b)) was removed from the mould by dissolving the latter away in acetone. The formed parts contained both thick and thin sections as shown in Fig. 3. This shows the capabilities of the combined 3D printing and gelcasting method because the density was found to be uniform at each sectional level. This is in agreement with Young et al. [11], that the forming of an object by gelcasting is only limited by the availability of the moulding tool. Dimensional shrinkage is a typical phenomenon related to the casting processes and occurs during drying. Therefore the drying is regarded as the rate limiting step. Adequate drying conditions were experimentally determined and drying in a chamber with controlled humidity (90.0%) and a temperature of 50 °C was found to be favourable. The drying shrinkage was found to be 4.0% for both the 50 and 60 vol% solids loading, contrary to parts gelcast using 40 vol% solids loading which showed shrinkage of 14.0%. This illustrates that the drying shrinkage is a function of the solids loading. Thus, the drying shrinkage can be minimized by increasing the solids loading. However, a high solid loading is unfavourable as it leads to an increase in viscosity and potentially increases the occurrence of defects.
Fig. 3. Complex shaped alumina parts produced by gelcasting on 3D printed moulds and sintered at 1650 °C.
they regulate the properties of the formed green body. The rheological behaviour of alumina suspensions was determined as a function of the solids loading. The study was performed on slurries containing 40–60 vol% alumina powder and 0.3 wt% Isobam® after ball milling for 24 h s. Fig. 1 illustrates the rheological flow curves for alumina suspensions as a function of solids loading ranging from 40 to 60 vol%, over a shear rate of 1 to 100s−1. The apparent viscosity (η) is plotted against the shear rate (γ). The apparent viscosity of all suspensions was found to exhibit a shear thinning behaviour over a wide range of shear rate. The viscosity and shear thinning behaviour at high shear rates increases with an increase in the solid loading as observed in the viscosity curve. This phenomenon was also reported by Young et al. [11], and Qin et al. [15], thus the general behaviour of an alumina suspension was to exhibit shear thinning and an increase in viscosity with increasing solids loading. The high viscosity at low shear rates (thixotropic behaviour) for the suspensions containing 50 and 60 vol% solids loading indicates a flocculated state of particles within the dispersing medium. This phenomenon was reported by Liu et al., [10]. However, all viscosities were low enough to ensure complete filling of the mould.
3.3. Effect of de-airing A defect that was observed in the gelcast parts was entrapped air bubbles. This is a common problem that is inherent to solvent based processing techniques. Fig. 4 reveals elliptic shaped pores in the microstructure with diameters up to 200 μm; these are a result of poor deairing prior to casting the suspension. The entrapped air bubbles adversely affect the density and the mechanical properties of the parts produced. Methods of eliminating entrapped air bubbles during slurry preparation were investigated. Ultrasonication, defoaming agents and vacuum de-airing were utilised to eliminate entrapped air in the slurry. These methods used in combination were shown to be effective in eliminating entrapped air. With sufficient deairing, fewer pores were observed at low magnification and the parts were found to have relative densities up to 99.0%. However at higher magnifications both intergranular and intragranular pores were observed in all the ceramic parts produced, Fig. 5. A homogeneous microstructure was observed in Fig. 5 a for the part gelcast using 40 vol% solids loading. The 50 vol% solids loading has a few large grains surrounded by smaller grains and the microstructure for the 60 vol% solid loading sample has more larger grains that are surrounded by a number of smaller grains. Moreover, the presence of intergranular and intragranular pores can be observed for the 50 and 60 vol% solids loading (Fig. 5b and c). The pores can be attributed mainly to grain growth which was expected for pressure-less sintering; this was in agreement with German [16].
3.2. Gelcasting Gelcasting requires the use of non-porous moulds to achieve the desired shape. However the mould design and mould material play a significant role during the fabrication of a complex shape. The mould should be designed such that it allows shrinkage to occur but minimise the resulting stresses during gelation. The mould should also facilitate the release of air bubbles when casting the slurry in the presence of air.
Fig. 4. SEM micrographs illustrating the effect of de-airing on the microstructure of sintered alumina gelcast prior to deairing. 3
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Fig. 5. SEM micrographs of polished surface of sintered (1650 °C) specimens made from suspensions containing a) 40, b) 50 and c) 60 vol% solids loading. Table 1 Properties of the fabricated parts as a function of solid loading (samples sintered at 1650 °C). Solids content
40 vol%
50 vol%
60 vol%
Density of green body (g/cm3) Density of sintered body (g/cm3) Relative density (%) Shrinkage (%)
2.13 3.93 99.0 ± 0.8 19.5
2.49 3.92 98.7 ± 0.2 16.8
2.59 3.89 98.0 ± 0.5 18.0
Fig. 7. The effect of solids loading on the flexural strength and hardness of sintered alumina parts.
3.4. Effect of solids loading on sintered body The solid loading of a suspension able to meet the requirements for ensuring the filling of the mould was as high as 60 vol%. However, the solid content in the slurry has a critical effect on the properties of the sintered parts. Table 1 summarises the effect of the solids content on the alumina suspensions prepared using 0.3 wt % Isobam®, gelcast in 3D printed moulds and sintered in air at 1650 °C using a dwell time of 3 h s. The green density, measured from the volume and mass of the green bodies, was found to increase as the solids loading increased; this was due to the amount of powder in the green compact. However, the degree of densification is mainly related to the particle packing, the solids loading in the starting gelcasting suspension and the ratio of the surface energy of the powder particles to the interfacial energy of the sintered body. The higher the ratio, the more compact the sintered body can be densified at a particular set of sintering conditions [16]. Fig. 6 shows the effect of the solids loading on the green and sintered relative densities of the fabricated alumina parts. The sintered density is dependent on the packing of the powder. The sintered density slightly decreases from 99.0% at 40 vol% to 98.7% at 50 vol%. A further decrease at 60 vol% is observed and this is the lowest relative
Fig. 6. The effect of solids loading on the relative (sintered and green) densities of sintered alumina (referenced to the density of alumina taken as 3.97 g cm3). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 2 Mechanical properties of gelcast alumina sintered at 1650 °C. Solids content
40 vol%
50 vol%
60 vol%
Relative density (%) Hardness (GPa) Fracture toughness (MPa.√m) Flexural strength (MPa)
99.0 ± 0.8 18 ± 1 3.8 ± 0.7 374 ± 66
98.7 ± 0.2 16 ± 2 3.7 ± 0.5 378 ± 34
98.0 ± 05 15 ± 2 2.7 ± 0.7 283 ± 17
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Fig. 8. SEM micrographs of fractured surface of sintered specimens at 1650 °C: a) 40 vol%, b) 50 vol% and c) 60 vol% solids loading.
The de-airing technique and solids loading played a critical role in the final relative density and mechanical properties of the parts produced. The densities and mechanical properties of the parts produced are comparable to parts fabricated by using conventional methods. The obtained relative densities obtained depend on the solids loading, sintering temperature and deairing. Part densities were up to 99.0%, hardness of 18 GPa, flexural strength of 374 MPa and a fracture toughness of 3.8 MPa√m. By using a 40 and 50 vol% solids loading suspension, complex shaped parts such as an impeller and a pyramid with relative densities above 99.0% were produced. These results demonstrate the capabilities of the combined 3D printing and gelcasting method. Thus, the forming of an object by gelcasting is only limited by the availability of the fabrication tool.
density obtained. The low densification at 60 vol% may be a result of the solids loading increasing beyond the maximum allowable solids content in the slurry. This may also be attributed to the higher viscosity at high solids loading and the hindrance of optimum particle packing arrangement in the suspension. High viscosity leads to insufficient deairing and hinders the flow of the slurry thus entrapping air and forming defects. The defects lead to stress concentrations in the final part, and have negative effects on the resulting mechanical properties [17]. 3.5. Mechanical properties Hardness and fracture toughness measurements were performed on the sintered parts by means of the Vickers indentation method using a loading force of 5kgf. The flexural strength was determined using the ball on three balls testing method [13] and these are summarised in Table 2. The effect of solids loading on the flexural strength and hardness of alumina parts was examined in Fig. 7. The hardness and flexural strength of the sintered alumina parts decreased as the solids loading increased. This is supported by the relative density as shown in Fig. 6. As the relative density increases, so does the hardness and flexural strength of the alumina parts. The parts produced using 40 and 50 vol% solids loading showed the highest flexural strength. The higher flexural strength on these samples was due to their high relative densities of about 99.0% which demonstrates a better packing of the ceramic particles in the green body. Thus, the mechanical properties show a strong dependence on the density of the sintered body and this is in agreement with Chantikul et al., [18]. Microstructural and surface defects were found to be the course of crack initiation during flexural strength tests. The grain size in the 40 vol% solids loading (Fig. 8(a)) acted as an internal defect that initiated the fracture, therefore, slightly lowering the fracture strength. The 60 vol% samples had lower fracture flexural strength than all the samples due to the aggregation of smaller pores that were found just below the surface. The high solids loading led to difficulties in de-airing the suspension, which increased the volume of trapped air pockets, thus pores were the origin of fracture. Moreover, agglomerates resulted as a result of the high viscosity. The agglomerates tend to have large, inter agglomerated porosity that accumulates and forms defects in the green body. The formed defects always act as crack-initiating defects, thus no healing occurs during sintering [19].
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge the Department of Science and Technology (DST), National Research Foundation (NRF) and the Centre of Excellence in Strong Materials (CoE) - DST-NRF CoE in Strong Materials (South Africa) for financial support and Lukas Janse Van Rensburg for designing and 3D printing the moulds. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.10.021. References [1] J. Klein, M. Stern, G. Franchin, G. Kayser, C. Inamura, S. Dave, J.C. Weaver, P. Houk, P. Colombo, M. Yang, N. Oxman, “Additive manufacturing of optically transparent glass”, 3D Print. Addit. Manuf. 2 (3) (2015) 92–105. [2] B. Utela, D. Storti, R. Anderson, M. Ganter, “A review of process development steps for new material systems in three dimensional printing (3DP)”, J. Manuf. Process. 10 (2008) 96–104. [3] A. Zocca, C.M. Gomes, E. Bernardo, R. Muller, J. Gunster, P. Colombo, “LAS glassceramic scaffolds by three-dimensional printing”, J. Eur. Ceram. Soc. 33 (2013) 1525–1533. [4] W.M. Sigmund, N.S. Bell, L. Bergstrom, “Novel powder processing methods for advanced ceramics”, J. Am. Ceram. Soc. 83 (7) (2000) 1557–1574. [5] S. Wang, A.G. Miranda, C. Shih, “A study of investment casting with plastic patterns”, Mater. Manuf. Process. 25 (12) (2010) 1482–1488. [6] P. J. Whalen, V. R. Jamalabad, J. P. Pollinger, M. Agarwala and S. C. Danforth, “Gelcasting molding with fugitive molds”. United States of America Patent 5824250, 20 October 198. [7] A. Ortera, C. D'Ängelo, S. Gianella, D. Gaia, “Cellular ceramics produced by rapid prototyping and replication”, Mater. Lett. 80 (2012) 95–98. [8] F. Chabert, “Cross-linked polyvinyl alcohol as a binder for gelcasting and green machining”, J. Am. Ceram. Soc. 91 (10) (2008) 3138–3146. [9] A.A. Babaluo, M. Kokabi, A. Barati, “Chemorheology of alumina-aqueous acrylamide gelcasting systems”, J. Eur. Ceram. Soc. 24 (2004) 635–644. [10] X. Liu, Y. Huang, J. Yang, “Effect of rheological properties of the suspension on the mechanical strength of Al2O3-ZrO2 composites prepared by gelcasting”, Ceram. Int. 28 (2) (2002) 159–164. [11] A.C. Young, O.O. Omatete, M.A. Janney, P.A. Menchhofer, “Gelcasting of alumina”, J. Am. Ceram. Soc. 74 (3) (1991) 612–618.
4. Conclusion Dense complex shaped alumina ceramics with relative densities between 98.0 and 99.0% of the theoretical density were obtained using a gelcasting method combined with additive manufacturing. The gelling of an alumina suspension was achieved by using a water soluble copolymer of Isobutylene and Maleic Anhydride (Isobam®). Complex shaped parts were produced by moulding suspensions containing 40, 50 and 60 vol% alumina and 0.3 wt% Isobam® into 3D printed ABS moulds. This process reduces the time and cost of machining dense parts to achieve complex shapes. The properties of the produced alumina parts are highly depended on the preparation of the gelcasting suspension. 5
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[16] R.M. German, “Strength evolution in debinding and sintering”, Proceedings of the 3rd International Conference on Science, Technology & Applications of Sintering, 2003. [17] J. Stampfl, H. Liu, S.W. Nam, K. Sekamoto, H. Tsuru, S. Kang, A.G. Cooper, A. Nickel, F.B. Prinz, “Rapid prototyping and manufacturing by gelcasting metallic and ceramic slurries”, Mater. Sci. Eng. A 334 (2002) 187–192. [18] P. Chantikul, S.J. Bennison, B.R. Lawn, “Role of the grain size in the strength and Rcurve properties of alumina”, J. Am. Ceram. Soc. 73 (8) (1990). [19] P. Greil, “Generic principles of crack-healing ceramics”, Journal of advanced ceramics 1 (4) (2012) 249–267.
[12] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, “A critical evaluation of indentation techniques for measuring fracture toughness; I, direct crack measurements”, J. Am. Ceram. Soc. 64 (1981) 533–538. [13] A. Borger, P. Supancic, R. Danzer, “The ball on three balls test for strength testing of brittle discs- Stress distribution in the disc”, J. Eur. Ceram. Soc. 22 (2002) 1425–1436. [14] M. Universitat, “Ball on 3 balls test (B3B) - strength testing”, [Online] http://www. isfk.at/de/960/ , Accessed date: 2 October 2018. [15] X. Qin, Y. Zhou, Y. Yang, X. Shu, S. Shimai, S. Wang, “Gelcasting of transparent YAG ceramics by a new gelling system”, Ceram. Int. 40 (2014) 12745–12750.
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