Fabrication of soluble salt-based support for suspended ceramic structure by layered extrusion forming method

Materials and Design 183 (2019) 108173

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Fabrication of soluble salt-based support for suspended ceramic structure by layered extrusion forming method Guanjin Li, Shiyan Tang, Li Yang, Lei Qian, Fuchu Liu, Zitian Fan ⁎, Kang Zuo, Qingsong Wei, Wenming Jiang ⁎ State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• A water soluble slurry was used as an easily removed support material. • Support material is suitable for different sinter temperatures (from 900 to1300 °C). • The salt-based support material can be removed easily after sintering.

a r t i c l e

i n f o

Article history: Received 6 May 2019 Received in revised form 30 August 2019 Accepted 30 August 2019 Available online 31 August 2019 Keywords: Layered extrusion forming Salt-based slurry Magnesium sulfate Rheology Soluble support material

a b s t r a c t The support material is necessary to prevent the collapse of the suspended structure when forming complex ceramic parts via layered extrusion forming (LEF, a type of additive manufacturing) method, and needs to be removed easily after sintering. In this work, salt-based slurries were prepared with magnesium sulfate monohydrate (MSM) and polyvinyl pyrrolidone (PVP) in order to solve the problem of extrusion and removal of the support material. The effect of the solid content on the rheological performance of slurries and the printing quality of specimens was studied. The results showed that the salt-based slurry containing 60 wt% MSM and 10 wt% PVP powder possessed shear-thinning behavior and the corresponding samples held sufficient bending strength and superior surface quality, which satisfied the requirements for the support structures in LEF method. After calcination at different temperature, the salt-based support structure could be easily removed, which offers an alternative support material choice for the LEF method. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction The additive manufacturing [1,2] (AM) technology provides a new direction for fabrication of complex ceramic parts, such as stereo lithography [3] (SL), three-dimensional printing [4] (3DP), and layered ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Fan), [email protected] (W. Jiang).

extrusion forming [5–10] (LEF). The SL and 3DP methods possess an advantage of higher accuracy, but they require expensive equipment and raw material. In contrast, the LEF method possesses the advantages of miniaturization of equipment and lower cost, and is more and more widely used in medicine, ceramics, manufacturing as well as other industries. In the LEF method, the raw powder materials are processed into a uniformly dispersed and fluid slurry, and then extruded continuously

https://doi.org/10.1016/j.matdes.2019.108173 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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G. Li et al. / Materials and Design 183 (2019) 108173

by pushing the piston (pressure or mechanical force). At present, the LEF method possesses a number of achievements in applying to fields of bioceramics [11,12], piezoelectric ceramics [13,14] and some uncomplicated ceramic structural parts [6,8]. However, when forming suspended ceramic part (as shown in Fig. 1(a)) without support material, the collapse of the suspended structure will occur, as shown in Fig. 1(b). Consequently, it is necessary to set up the support during the forming process of the suspended structure, as shown in Fig. 1(c). To ensure the integrity of the desired suspended structure, the support structure should have sufficient bending strength for load-bearing and high surface quality to reduce the roughness of the connected surface. Meanwhile, the support material should be removed easily by stripping, dissolution or burn out methods. Until now, some materials have been reported to form support structure, but there are still facing problems, such as the stability during sintering with the main material, influence on the main material, removal mode and so on. It is reported that calcium carbonate [15] can be used as a support material, which is removed by putting into water or acid after calcification at 1500 °C. Moreover, graphite [16] is reported to be used as support materials and can be removed by sintering (about 500 °C). The graphite support material provides an alternative way for relatively low-temperature sintering because the support material may burn out before the ceramization of the main material and may lead to the collapse of the main material. Besides, due to soluble inorganic salt material possesses superior stability, water solubility and wide range of sintering temperature, it is also regarded as an alternative choice for the support material in the LEF method. However, the superior soluble inorganic salt particles are easy to agglomerate in the air and difficult to preserve, the magnesium sulfate monohydrate with relatively low water solubility and high melting point is a more appropriate choice. The salt melting points and solubilities of some inorganic salts were listed Table 1. In this study, in order to develop the water-soluble support materials suitable for LEF process, MgSO4·H2O was preferred as the main material of slurry. The absolute ethyl alcohol was used as dispersion medium, as a result, the polyvinyl pyrrolidone (PVP-k60) was used as the binder. By adjusting the content ratio of each component, the effects of the solid content of the MgSO4·H2O on the rheological performance of the saltbased slurries and the printing deviation, the depth of laminated striation of printing specimens were investigated and the optimized slurry component was obtained. A suspended ceramic structure using Al2O3 as bone material was fabricated with salt-based support, and after sintering the salt-based support was removed by soaking in water or acid solution.

Table 1 Melting points and solubilities of some inorganic salts. K2CO3 NaNO3 CaCl2 NaCl KCl Melting point/°C Solubility (20 °C, water, g/100 mL)

891 110.0

306.8 87.0

782 74.5

801 36.0

KNO3 MgSO4 NaCO3

770 334 34.0 31.2

1124 25.5

851 21.8

2. Materials and method 2.1. Materials The magnesium sulfate monohydrate (MgSO4·H2O, MSM, d50 = 25 μm, melting point is about 1124 °C) was chosen as the base material. The PVP-k60 (polyvinyl pyrrolidone [17], Qingdao Usolf Chemical Technology Co., Ltd., China) was adopted as the plasticizer as well as the binder system. The anhydrous ethanol (EtOH, ≥99.5 wt% purity, Sinopharm Chemical Reagent Co., Ltd., China) was applied as the dispersible medium for the salt-based slurry. The 1-octanol (Sinopharm Chemical Reagent Co., Ltd., China) was used as the antifoaming agent. The methyl silicone oil (Sinopharm Chemical Reagent Co., Ltd., China) and the glacial acetic acid (Sinopharm Chemical Reagent Co., Ltd., China) was used. The alumina powders (Al2O3, d50 = 5 μm) and methylcellulose (≥99.5 wt% purity, Sinopharm Chemical Reagent Co., Ltd., China) was used. 2.2. Slurry preparation PVP was added to anhydrous ethanol by stirring until completely dissolving, and then the solutions with binder and dispersant were obtained. The salt-based slurry was prepared by mixing the pre-prepared PVP solution, MSM powder and 1-octanol via planetary ball milling at the rotate speed of 270 rpm for 9 h. The compositions of the saltbased slurry were listed in Table 2. 2.3. Forming process and characterization A forming process was carried out by a self-constructed device, which contained an extruded system and a mechanical control system. Fig. 2 depicts the forming process of the salt-based slurry. The printing process was carried out in ambient air at room temperature. Firstly, the pre-prepared slurry was placed into the syringe and was compacted to remove the internal bubbles. Then, the main materials were printed when the first syringe moved along the X/Y direction

Fig. 1. Schematic diagram of printing of (a) suspended structure, (b) without support material and (c) with support material.

G. Li et al. / Materials and Design 183 (2019) 108173 Table 2 Composition of the salt-based slurry with the corresponding Herschel-Bulkley model parameters. The optimum constitution is shown in bold. An (−) means the values are outside the measurement range. 1-octanol (g)

PVP-k60 (wt%)

MgSO4 (wt%)

EtOH (wt%)

σy (Pa)

K (pa·sn)

n

0.1

10

52 54 56 58 60 62

38 36 34 32 30 28

38 254 327 919 1399 –

35.7 354.8 419.6 1305.8 1574.5 –

0.73 0.17 0.24 0.10 0.18 –

and the slurry was continuously extruded under air pressure. Moreover, the support materials were printed through the second syringe. The suspended structure was prepared layer-by-layer and each layer the main material and the supporting material are printed in the designed area by setting the path of the syringes and the time node of pressurization. Considering the particle size and printing efficiency, a nozzle with an inner diameter of 0.41 mm was used. The extrusion speed of slurry was adjusted by air pressure in the range of 0–5 kg/cm2. Some key processing parameters were listed in Table 3. After formation, the specimens were dried at 60 °C for 12 h and followed with sintering at 1300 °C for 2 h. The rheological performance was tested by rheometer (DHR-2, TA Instruments, USA). The fixture gap was 1000 μm. The shear rate tested ranged from 0.1 to 200 s−1. The printing deviation was calculated according to the Formula (1). ε ¼ ðh0 −h1 Þ=h0

ð1Þ

3

Table 3 Processing parameters were set for LEF method. Parameters

Value

Description

Nozzle diameter Layer height Air pressure Printing speed

0.41 mm 0.35 mm 0–5 kg/cm2 20 mm/s

The diameter of the syringe outlet Layered print height Propelling force of extrusion of slurry Movement speed of syringe

where ε is the printing deviation of salt-based specimens, h0 is the designed printing height of salt-based specimens, whose value is 0.21. And h1 is the dried printing height of salt-based specimens, as shown in Fig. 3. The depth of laminated striation was calculated according to the Formula (2). φ ¼ ðxmax −xmin Þ=2

ð2Þ

where φ is the depth of laminated striation of salt-based specimens, xmax and xmin are the widest and narrowest lengths of the cross section of the specimens, respectively, as shown in Fig. 3. The three-point bending strength of different salt-based specimens (64×16×4 mm) formed by the LEF method was tested by Electro Puls all-electric dynamic and fatigue test systems (ElectroPuls E1000, Instron, USA). The support span was 40 mm and the loading speed was 0.5 mm/min. The subtraction of support structure was confirmed by putting the sintered suspended part into acid solution or water. The microstructure of salt-based specimen was observed using an environmental scanning electron microscope (ESEM, Quanta 200, FEI, Netherlands). 3. Result and discussion In the LEF method, the support material was evaluated by the printing quality, bending strength, and removal method. The rheological property holds a great impact on the printing quality of the slurry. The viscosities of salt-based slurry with different solid contents were depicted in Fig. 4(a). It was obvious that all kinds of slurries showed characteristics of shear-thinning behavior (n b 1) [15,17] and the viscosities of slurries increased along the increase of the solid content. The slurry with higher viscosity was subject to greater resistance when flowing, which made it difficult to extrude under the same extrusion pressure. According to the experiment, it was difficult to extrude from the needle when the solid content of the slurry exceeded 62 wt%. The solid curves of stress versus shear rate were shown in Fig. 4 (b) and fitted to the Herschel-Bulkley model [12,18,19], which was

Fig. 2. Schematic diagram of the forming process of the salt-based slurry.

Fig. 3. Schematic diagram of measured parameters of printing deviation and the depth of laminated striation.

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Fig. 4. Plots of (a) log viscosity vs log shear rate plots of varying slurries, (b) stress versus shear rate plots of varying slurries.

Fig. 5. Cross section of the specimens printed by slurries with different solid contents.

given by the Formula (3), and model parameters were listed in Table 2 (correlation factor R2 was about 0.9). σ ¼ σ y þ k γ n

bonded by PVP-k60 and solid loading varying from 52 wt% to 62 wt% in sequence. It could be observed that all the specimens with the solid content of 52 wt%, 54 wt% and 56 wt%, completely collapsed. However,

ð3Þ

where σ is the stress, σy is the yield stress of the slurry, k is the consistency index, γ is the shear rate during extrusion, and n is the flow index. The shear rate γ during smooth extrusion was calculated by the Formula (4) [17]:   3 γ ¼ 32Q = πd

ð4Þ

where d is the needle diameter and Q is the volumetric flow rate when printing. In this study, the volumetric flow rate could be simplified to the ratio of printing volume to total time. According to calculation, the average shear rate during smooth extrusion was about 35 1/s. The viscosity and stress of slurry of 62 wt% MSM with PVP-k60 was not plotted because the values were beyond the measurement range. It was obvious that the yield stress of the slurry increased with the increase of the MSM solid content. The yield stress of slurry with the MSM solid content of 60 wt% reached a peak value of 1399 Pa. The high yield stress enables the slurry to maintain its shape after extrusion and eventually form the parts required [7,16,20,21]. Therefore, these slurries could easily flow during extrusion and maintain the shape after extrusion, which was the ideal characteristic for the LEF method [22]. Fig. 5 describes the cross sections of the specimens printed by slurries with varying compositions. All the specimens in Fig. 5(a)–(f) were

Fig. 6. Schematic of the collapse of slurries with (a) different yield stress and (b) different solid content.

G. Li et al. / Materials and Design 183 (2019) 108173 Table 4 Printing height, printing deviation and the depth of laminated striation of various specimens. The optimum constitution is shown in bold. Constitution of slurries

PVP-k60

Printing height/h1 (mm) Printing deviation/ε the depth of laminated striation/φ (mm)

58 wt%

60 wt%

62 wt%

1.71 18.57% 0.068

1.90 9.52% 0.065

1.92 8.57% 0.1225

Fig. 7. ESEM image of 60 wt% MSM and PVP-k60.

with the continual increase of solid content, the printing laminated striation became more obvious, which would significantly influence the side quality of the printed specimen [23]. The yield stress of slurry and the volatilization of the dispersion medium (EtOH) play an important role in the printing process. As shown in Fig. 6(a), the initial yield strength of the slurry determined the ability for the slurry to maintain its shape on the substrate at the moment of depositing on the substrate. The collapse could occur to the low yield stress slurry [7]. As shown in Fig. 6(b), after extrusion, the EtOH in the slurry evaporates rapidly in the air, resulting in an increase in the solid content of the slurry, which improves the yield strength of the slurry and enables it to support the printing of the next layer. When the slurry possesses a lower solid content, the volatilization time of EtOH becomes longer, so that the yield strength of the last layer cannot meet the requirements before printing the next layer, which would make the slurry unable to withstand the weight of the next layer and cause the collapse.

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In addition, the specimen with high solid content possessed a faster drying rate and thus a more pronounced printing laminated striation. Table 4 shows the printing height (h1), printing deviation (ε) and depth of laminated striation (φ) of the specimens with the solid content of 58 wt%, 60 wt% and 62 wt%. An increase of the printing height with the increase of solid content of slurry was observed and meant a decrease of collapse of specimen. The specimen with solid content of 62 wt% possessed a minimum printing deviation (8.57%). An increase of the depth of laminated striation with the increase of solid content meant worse surface quality. It is obvious that the slurry with 60 wt% solid content possessed a lower printing deviation (9.52%) and laminated striation depth (0.065), which guarantees the surface quality and size control for the support structures in the LEF method. The grid structure specimen was prepared by the salt-based slurry containing 60 wt% solid content. Fig. 7 shows that the salt filament can withstand a certain load and maintain a circular cross section. The result of three-point bending test showed that the bending strength of the specimens with the solid content of 60 wt% reaches 9.94 MPa, which is sufficient for the support material. In general, MgSO4·H2O loses its binding water at about 400 °C and decomposes into magnesium oxide at 900–1000 °C. The latter will lose more mass and produce a certain volume contraction. Therefore, there are different support removal methods for different sintering temperatures. When the sintering temperature is below 1100 °C, the sample consists mainly of magnesium sulfate and thus can be dissolved in water (90 °C). As shown in Fig. 8(a)–(d), the specimen underwent soften process in the earlier 30 min, dissolving process in the following 15 min and completely dissolved after another 15 min. The dissolution rate of specimen can be accelerated by proper stirring. When sintering temperature beyond 1100 °C (such as alumina), even though salt-based support had a volume contraction, the alumina had produced initial strength and the shrinkage of magnesium sulfate had little effect on the sample. Fig. 9(a)–(d) show the dissolution of salt-based support material in acid solution (0.1 mol/L of glacial acetic acid), which was used to support a suspended aluminum oxide structure and sintered at 1300 °C. The three-dimensional model with a suspended structure was shown in Fig. 9(a). Fig. 9(b) shows the printing process using a dual extrusion apparatus, which adopts alumina slurry as the main material and salt-based slurry as the support material. The alumina slurry consisted of alumina powders, 2% methyl cellulose aqueous solution and glacial acetic acid [10]. After printing, the specimen was dried at 60 °C for 12 h. And then, the specimen was put into the sintering furnace, degreased at 500 °C for half an hour, sintered at 1300 °C for 2 h, then cooled with the furnace. Fig. 9(c) shows that the sintered specimen is soaked in acid solution (0.1 mol/L of glacial acetic acid). As shown in Fig. 9(d), the product with a suspended structure was obtained until the support was removed. After sintering, the supports possessed an extremely low strength and a large shrinkage and could be removed easily. It could be concluded that after sintering at

Fig. 8. Printed salt-based specimen was dissolved in water of an initial temperature of about 90 °C.

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G. Li et al. / Materials and Design 183 (2019) 108173

Fig. 9. Preparation of suspended structure and dissolution of salt-based support material: (a) the 3D model, (b) formation process of the salt-based support and main ceramic structure, (c) salt-based supports being dissolved in acid solution, and (d) the finished product after removing the support material.

Table 5 Performance comparison of salt-based support, graphite-based support and CaCO3 support. Type of support material

Sintering temperature

Bending strength (MPa)

Drying shrinking percentage

Removal method

Salt-based support Graphite-based support [16] CaCO3 support [15]

Both low and high sintering temperature (1300 °C) Above 800 °C 1100 °C

9.94 1.04 –

0.8% 4.0% –

Dissolved by water or acid dissolution Burn out during sintering at about 600 °C Dissolved by water or acid dissolution

different temperatures, salt-based slurries could be removed by water and acid solution. Some important parameters of salt-based, graphite-based and CaCO3 support were listed in Table 5. As a comparison, graphite-based support disappears at relative low temperature, meanwhile the strength of the main material is insufficient, which may lead to collapse. CaCO3 support possesses a lower decomposition temperature and is easy to react with Al2O3. For MSM supports, water can be selected for removal at low sintering temperature (b1100 °C), and at high sintering temperature, the weak acid solution is chosen. Furthermore, the magnesium sulfate possesses a higher decomposition temperature and stability, and less impact on the main material. Therefore, the salt-based support possesses high printing accuracy, low drying shrinkage and wide application range, which directly demonstrates that the MgSO4 as a support material shows great prospect in LEF method. For improve the print quality the use of lower layer height and appropriate process can effectively reduce the lamination effect, also a thinner needle can be used for extruding low viscosity paste to improve the quality of printing. The surface quality can also be improved by grinding before sintering.

CRediT authorship contribution statement Guanjin Li: Conceptualization, Investigation, Software, Writing original draft. Shiyan Tang: Methodology, Validation. Li Yang: Investigation, Software, Methodology. Lei Qian: Software, Validation. Fuchu Liu: Project administration, Data curation. Zitian Fan: Supervision, Funding acquisition. Kang Zuo: Investigation, Validation. Qingsong Wei: Project administration. Wenming Jiang: Writing - review & editing. Acknowledgements This work was supported by the National Natural Science Foundation of China (NFSC, Grant No. 51775204), the Innovation Fund of Huazhong University of Science and Technology (NO. 2018JYCXJJ020), the fund of the State Key Laboratory of Solidification Processing in NWPU (No. SKLSP201821), and Project 2017CFB488 supported by the Natural Science Foundation of Hubei Province, China. The authors show gratitude to the Analytical and Testing Centre and the Research Project of State Key Laboratory of Materials Processing and Die & Mould Technology, HUST.

4. Conclusion References A salt-based slurry suitable for the LEF process was designed and optimized using MSM as the main material and PVP-K60 as binders. The effect of the solid content on the rheological performance of slurry, printing deviation, depth of laminated striation was investigated. It could be concluded that both the yield strength and the viscosity of the slurry increased as the solid content increased. Besides, the higher the molecular weight of PVP led to the higher yield strength and viscosity of the slurry under the same compositions. Furthermore, the optimal slurry formulation was 60 wt% MgSO4, 10 wt% PVP-k60 and balanced anhydrous ethanol. The yield stress of the MSM solid content of 60 wt % with PVP-k60 reached a peak value of 1399 Pa. Under this composition, the printed specimens possessed a higher bending strength (9.94 MPa) and better printing quality (the printing deviation is 9.52% and side surface quality is 0.065 mm). Finally, the printed specimens possessed an applicable bending strength and could be quickly dissolved in water or acid solution, which indicated that the salt-based slurry could be used as a soluble support material for the LEF method.

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