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Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology
Journal of the European Ceramic Society 39 (2019) 3380–3386
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology
T
⁎
Xinyuan Lv, Fang Ye, Laifei Cheng , Shangwu Fan, Yongsheng Liu Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, 710072, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC whisker-reinforced SiC composites Spray drying Binder jetting Chemical vapor infiltration
Spray drying, binder jetting and chemical vapor infiltration (CVI) were used in combination for the first time to fabricate SiC whisker-reinforced SiC ceramic matrix composites (SiCW/SiC). Granulated needle-shaped SiCW was spray dried into SiCW spherical particles to increase flowability and thereby increase printability. Then, binder jetting was employed to print a novel SiCW preform with two-stage pores using the SiCW spherical particles. The subsequent CVI technology produced pure, dense, and continuous SiC matrix with high modulus and strength. Consequently, SiCW/SiC with appropriate mechanical properties was obtained. Finally, the challenges of the novel method and the ways to improve the mechanical properties of SiCW/SiC are discussed.
1. Introduction Ceramic matrix composites (CMCs), especially silicon carbide CMCs (CMC-SiC) are irreplaceable structural materials in aerospace field due to their excellent properties such as high temperature resistance, corrosion resistance, high strength, high hardness, and high modulus [1,2]. CMC-SiC is mainly reinforced by particles, whiskers, short-cut fibers and continuous fibers, among which whiskers and continuous fibers are considered to have outstanding toughening effect. The fiber preform structure of continuous fibers-reinforced CMCs (CFCCs) mainly includes 2D, 2.5D, 3D, and 3D needled (3DN) braided structures, which inevitably leads to the anisotropy of CFCCs [3–6]. The CFCCs with 2D braided structure have low interlaminar shear strength, so their applications have been limited to the structures that sustain only in-plane stress field [3]. 3D and 3DN braided structures also have limitations, such as low in-plane strengths for the 3D braided fibers and shorter tensile fatigue life and lower compression strengths for the 3DN structure [3]. In contrast, SiC whisker-reinforced SiC matrix composites (SiCW/ SiC) are isotropic. Mahfuz [7] prepared SiCW/SiC by hot pressing SiC powders and SiC whiskers at 1750 °C and 3500 psi. She [8] prepared SiCW/SiC using hot isostatic pressing combined with sintering at 1850 °C. While the prepared SiCW/SiC had good properties, a higher preparation temperature was required for the hot press sintering technology. Hua [9–11] prepared SiCW/SiC by cold pressing combined with chemical vapor infiltration (CVI). Although CVI technology requires a lower temperature, it is difficult to prepare complex shape parts using
⁎
cold pressing. Chen [12] prepared SiCW/SiC by gel-casting combined with precursor infiltration and pyrolysis (PIP). However, the SiC matrix prepared by PIP has a low crystallization degree, and its structural stability at high temperatures needs to be improved. Three-dimensional printing (3D printing), namely additive manufacturing, is a material forming technology based on CAD models, which is suitable for preparing parts with complex shapes. Numerous ceramic parts have been fabricated by the 3D printing method of binder jetting [13–19]. The raw materials used for binder jetting are ceramic powders, whose flowability can determine the printability of parts [13,14]. It is known that SiCW has poor flowability due to its needle-like morphology, making it difficult to directly use SiCW for binder jetting. Spray drying is one of the most convenient and effective methods for granulation of ceramic materials. In this technique, ceramic slurry is sprayed into spherical droplets by a high-speed rotating atomizer, and the spherical droplets are rapidly dried at a high temperature to obtain spherical particles [20]. Spray drying is expected to improve the flowability of SiCW. The SiCW preform prepared using binder jetting combined with spray drying would have two-stage pores, i.e., large pores between the spherical particles and small pores within the spherical particles. CVI is especially suitable for depositing matrix in the preform with two-stage pores in comparison with PIP and reaction melt infiltration (RMI). Moreover, the ceramic matrix obtained by CVI has dense microstructure and good properties. Therefore, the subsequent introduction of SiC matrix into SiCW perform can be carried out by CVI. Based on the above considerations, a novel method has been
Corresponding author. E-mail address: [email protected] (L. Cheng).
https://doi.org/10.1016/j.jeurceramsoc.2019.04.043 Received 26 February 2019; Received in revised form 22 April 2019; Accepted 23 April 2019 Available online 27 April 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Journal of the European Ceramic Society 39 (2019) 3380–3386
X. Lv, et al.
Fig. 1. Schematic diagram of the fabrication procedure for SiCW/SiC.
2.4. CVI
proposed in this paper for the preparation of SiCW/SiC composites based on 3D printing and CVI technology. The process includes three steps (Fig. 1): granulation of SiCW into SiCW spherical particles by spray drying; preparation of SiCW spherical particles into SiCW preform using binder jetting; and finally, densification of SiCW preform by CVI to obtain SiCW/SiC.
The SiC matrix was deposited in the green bodies using self-developed CVI deposition furnace for 120 h, 240 h, 360 h, 480 h, 600 h, 720 h, and 840 h. The organics in the green bodies were burned out in the heating stage. Methyltrichlorosilane (MTS: CH3SiCl3), hydrogen (H2) and argon (Ar) gas were used as the precursor, carrier, and dilution gas, respectively. The flow rate of Ar was 350 mL/min, and the input gas ratio of MTS, H2, and Ar was 1:40:40. The total pressure was 5000 Pa. The deposition temperature was 1000 °C. The as-obtained SiCW/SiC composites were used for testing and analysis.
2. Materials and methods 2.1. The preparation of ceramic slurry SiCW (Beantown Chemical, UK) with a mean diameter of 1.5 μm, a mean length of 18 μm and a purity of A.R. ≥99% was used in this study. The ceramic slurry was prepared by dispersing SiCW in deionized water with polyethylene glycol (PEG, Mr = 400, Tianjin Kemiou Chemical Reagent Co., Ltd., China) as the dispersant, tetramethylammonium hydroxide (TMAH, Tianjin Fuchen Chemical Reagent Co., Ltd., China) as the pH adjuster, and dextrin (MW = 504, Tianjin Dingshengxin Chemical Co., Ltd., China) as the binder. The ceramic slurry contained 25 vol. % SiCW, 2 wt. % PEG, 4 vol. % TMAH, and 10 wt. % dextrin (relative to deionized water content).
2.5. Characterization Microstructure of the samples was observed using secondary electrons of scanning electron microscopy (SEM, S-4700, Japan). The particle size distribution was determined by processing approximately 3000 SiCW spherical particles by image processing software (Image-Pro Plus). The bulk density, tap density and flowability of SiCW spherical particles were measured by Hall flow meter (ST-1002, Xiamen East Instrument Co., Ltd., China). The bulk density and open porosity of SiCW/SiC were measured by Archimedes’ method. Three-point bending test was employed to measure the flexural strength of the samples (3 mm × 4 mm × 40 mm) with the support distance of 30 mm and a cross-head speed of 0.5 mm/min, using an electromechanical universal testing machine (CMT5504, MTS SYSTEMS, Co., Ltd., China). Fracture toughness was measured using a single-edge notched beam (SENB) test with a span of 30 mm and a cross-head speed of 0.05 mm/min using 3 mm × 4 mm × 40 mm bars. A low-speed cutting machine was used to prepare the SENB samples with a notch width of 0.2 mm, a notch depth of 2 mm, and a notch curvature radius of 38 mm. In addition, 5 samples were tested for each group of samples for flexural strength and fracture toughness. Pore size distribution was measured by mercury intrusion method (AutoPore IV, Micromeritics Instrument Co., Ltd., USA). The elastic modulus of SiC matrix was measured by a nanoindenter (TI980, Hysitron Co., Ltd., USA) at 9 mN for 2 s. For some samples, detailed features of the microstructure were revealed by plasma etching (RES102, LEICA Co., Ltd., Germany) conducted for 1.5 h.
2.2. Spray drying The prepared ceramic slurry was fed into a centrifugal spray dryer to be granulated at an atomizer speed of 2400 r/min, an inlet temperature of 330 °C–350 °C, an outlet temperature of 100 °C–120 °C, and a feeding speed of 20 ml/min. Then, the SiCW spherical particles were obtained.
2.3. 3D printing The as-prepared SiCW spherical particles were used as the raw materials for 3D printing (ProJet360, 3D Systems Inc., California, USA). Specimens of size 3 mm × 4 mm × 40 mm were printed with layer thickness of 100 μm and binder saturation of 100%/200%. The green bodies were dried in situ for 4–10 h and then removed. 3381
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Fig. 2. Morphology of multiple SiCW spherical particles (a), single SiCW spherical particle (b), and surface of SiCW spherical particle (c); particle size accumulation and frequency distribution of SiCW spherical particles (d).
3. Results and discussion
3.2. 3D printed SiCW preform and SiCW/SiC
3.1. Granulated SiCW spherical particles by spray drying
Using the SiCW spherical particles, SiCW preform and SiCW/SiC composites were obtained by binder jetting and CVI, respectively. The microstructures of SiCW preform and SiCW/SiC are shown in Fig. 3. Fig. 3a and b show the fracture morphology of a novel and unique preform microstructure, which is stacked by numerous SiCW spherical particles, and each sphere consists of numerous SiCW. This multi-stage preform microstructure creates a two-stage void with large inter-particle pores of 10–20 μm and small intra-particle pores of 1–3 μm. The internal microstructure of the SiCW spherical particles as seen in Fig. 3b indicates that the distribution of SiCW was relatively uniform. Overall, the SiCW spherical particles were stacked uniformly without external assistance. The volume fraction of SiCW preform was estimated by the bulk density of SiCW spherical particles according to Eq. (1).
Fig. 2 shows the morphology and particle size distribution of SiCW spherical particles obtained using spray drying, and Table 1 lists their properties. Specifically, it was observed from the SEM image in Fig. 2a that the agglomeration rate was very high and the SiCW powder was almost entirely granulated into spherical particles. The SEM image in Fig. 2b shows that the SiCW particles had spherical shape. The SEM image in Fig. 2c shows that the SiCW were randomly oriented in the sphere, which made the sphere isotropic. From the particle size distribution shown in Fig. 2d, it can be seen that the SiCW spherical particles displayed a single peak and narrow particle size distribution, indicating that the particle size was concentrated. More importantly, Table 1 shows that the Hausner ratio of SiCW spherical particles was 1.25. The powder classification according to Hausner ratio is as follows: the Hausner ratio > 1.4 for non-flowing and cohesive powders, and it is 1–1.25 for free-flowing powders [21]. Therefore, it was confirmed that the SiCW spherical particles had acceptable flowability and were suitable for binder jetting.
Vp ≈ ρb ρth × 100%
where Vp is the volume fraction of SiCW preform, ρb is the bulk density of SiCW spherical particles and ρth is the theoretical density of SiCW. In this work, Vp is 17.7 vol. %. The SEM images in Fig. 3c–f show the fracture morphology of SiCW/ SiC (CVI 720 h). It can be seen in Fig. 3c and d that each SiCW spherical particle was coated by SiC matrix, which separated the pores into the inter-particle and intra-particle ones. All the intra-particle pores were closed pores and most of the inter-particle pores were also closed pores. The inter-particle closed pores were large and dispersed, while the intra-particle closed pores were small and concentrated. Moreover, it can be observed in Fig. 3e and f that after CVI-SiC, the exterior of the SiCW spherical particle was relatively denser and the interior of the
Table 1 The properties of SiCW spherical particles. Bulk density (g/cm3)
Tap density (g/cm3)
Hausner ratio (ρBulk/ρTap)
Flowability (ml/ s)
d10/d50/ d90 (μm)
0.566
0.708
1.25
3
6/25/62
(1)
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Fig. 3. Fracture morphology of SiCW preform (a); internal microstructure of SiCW spherical particles (b); fracture morphology of SiCW/SiC (c), fracture morphology of one SiCW spherical particle after CVI-SiC (d), exterior (e) and interior (f) of the SiCW spherical particles after CVI-SiC.
increased slowly, and finally increased rapidly again. The same trend was observed for the decrease in open porosity. In the first stage, the growth of SiC matrix occurred mainly intra particles. The CVI precursor had good gas permeability in SiCW preform due to its two-stage void microstructure, causing the open porosity to decrease rapidly from 82.3 vol. % to 22.27 vol. %. The closed porosity at this point was 6.74 vol. % which was almost entirely derived from the intra-particle pores. In the second stage, SiCW/SiC presented a single-stage void microstructure, and the growth rate of SiC matrix began to decrease. During this process, the inter-particle open pores were gradually filled by SiC matrix, on account of the pore size distribution of SiCW/SiC, as shown in Fig. 5. The closed porosity at this stage increased only slightly by 1.13 vol. % due to the bottleneck effect of CVI. In the third stage, the open porosity decreased rapidly from 6 vol. % to 1 vol. % and the closed porosity increased significantly to 11.4 vol. %. This could be attributed to the encrustation of deposits on the surface during CVI, which converted most of the open pores into closed pores. Fig. 6 shows the relationship between the density, flexural strength
SiCW spherical particle had a small amount of closed pores, indicating that the distribution of SiCW spherical particles had a slight radial gradient. This is possibly because, during the spray drying, the binder migrates to the surface of SiCW droplet with water as the water evaporates. Therefore, a discontinuous film is formed, which hinders the timely discharge of water vapor inside the SiCW spherical particles and thus causes internal expansion [20,22]. The open porosity and density of SiCW/SiC are shown in Fig. 4. The closed porosity of SiCW/SiC was calculated using the open porosity and density of SiCW/SiC according to Eq. (2).
pc = (1 − ρb ρth − po ) × 100%
(2)
where pc, po, ρb and ρth are the closed porosity, open porosity, bulk density and theoretical density of SiCW/SiC, respectively. In Fig. 4, the different density values in horizontal axis corresponded to different CVI deposition times. As the density increased, the closed porosity increased and the open porosity decreased. This variation process can be divided into three stages. The closed porosity increased rapidly at first, then 3383
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Fig. 4. Relationship between density, open porosity and closed porosity of SiCW/SiC.
Fig. 7. Load-displacement curves of SiCW/SiC with different densities.
Fig. 5. Pore size distribution of SiCW/SiC with different densities and CVI time. Fig. 8. Load-depth curves plotted by the nano-indentation of five points (located in the SiC matrix).
and fracture toughness of SiCW/SiC. Before the encrustation of deposits, both the flexural strength and fracture toughness of SiCW/SiC increased with density. After the encrustation of deposits, both the flexural strength and fracture toughness of SiCW/SiC were significantly reduced. The flexural strength of SiCW/SiC was 200 MPa and fracture toughness was 3.4 MPa m1/2. Obviously, both the flexural strength and fracture toughness are lower than other SiCW/SiC reported in the literature. There are mainly two reasons for this. The first is the volume fraction of SiCW (17.7 vol. %) is not high enough. The second is that the surface of SiCW/SiC exists a lot of stress concentration points because of its higher surface roughness. The higher surface roughness of SiCW/SiC is attributed to the inevitable high powder bed roughness caused by the large particle size of the SiCW spherical particles. As well as the liquid binder flows to the place where it does not need to be bonded when it infiltrates into the powder bed, however it is difficult to control the infiltration behaviors of the liquid binder. The load-displacement curves in Fig. 7 indicate that the fracture mode of SiCW/SiC was brittle fracture. Meanwhile, the nano-indentation curves in Fig. 8 was used to calculate the elastic modulus of SiC matrix as 458 GPa. Therefore, preparation a low-modulus interface may be required to regulate the modulus matching relationship between SiC matrix and SiCW
Fig. 6. Relationship between density, flexural strength and fracture toughness of SiCW/SiC.
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Fig. 9. SEM images of whisker pull-out (a), crack deflection (b), whisker bridging (c), and crack branching (d).
Fig. 10. Pictures of (a) SiCW green body and (b) SiCW/SiC with gear shape.
investigations and it might be necessary to prepare an interface in the SiCW/SiC or improve the surface roughness of SiCW/SiC to further improve its mechanical properties.
(> 550GPa). The fracture morphology in Fig. 9 revealed four main toughening mechanisms of SiCW, which were whisker pull-out, crack deflection, whisker bridging, and crack branching. Fig. 10 presents the pictures of a pair of meshing gears, which illustrates the ability of this method to fabricate complex shape parts.
Acknowledgements This work was supported by the National key R&D Program of China No. 2017YFB1103500 and National Natural Science Foundation of China (Grant No.51632007, 51602258, 51521061, 51672218, 51872229, 51802263), the 111 Project of China (B08040).
4. Conclusions In conclusion, a novel method was developed this work for the preparation of SiCW/SiC with appropriate mechanical properties by combining spray drying, binder jetting, and CVI. The combination of these three techniques provided several benefits. Through spray drying, the ceramic powder having poor flowability with an irregular shape, needle shape or sheet shape was aggregated into a spherical shape. Spherical particles are the ideal raw material for binder jetting. The products from binder jetting do not require molds and complicated machining. Moreover, geometrically-complex products can be rapidly prepared by binder jetting, thereby saving the cost and shortening the production period. Furthermore, the ceramic matrix prepared by CVI was pure, dense and continuous, with high modulus and strength. The introduction of ceramic matrix densified the preform. Therefore, the novel method is expected to have extensive practical applications. In this work, the flexural strength and fracture toughness of SiCW/SiC were as high as 200 MPa and 3.4 MPa m1/2, respectively. In the future, the preparation of large-scale products would require more
References [1] H. Ren, L. Zhang, K. Su, Q. Zeng, L. Cheng, G. Kang, L. Hui, Thermodynamics investigation of the gas-phase reactions in the chemical vapor deposition of silicon borides with BCl3 –SiCl4 –H2 precursors, Struct. Chem. 25 (5) (2014) 1369–1384. [2] S. Wu, L. Cheng, N. Dong, L. Zhang, Y. Xu, Flexural strength distribution of 3D SiC/ SiC composite, J. Mater. Eng. Perform. 15 (6) (2006) 712–716. [3] X.W. Yin, L.F. Cheng, L.T. Zhang, N. Travitzky, P. Greil, Fibre-reinforced multifunctional SiC matrix composite materials, Metall. Rev. 62 (3) (2016) 117–172. [4] N.E. Prasad, S. Kumari, Fracture behaviour of 2D-weaved, silica–silica continuous fibre-reinforced, ceramic–matrix composites (CFCCs), Eng. Fract. Mech. 71 (18) (2004) 2589–2605. [5] E. Lara-Curzio, M. Singh, High-temperature interlaminar shear strength of HiNicalonTM fiber-reinforced MI-SiC matrix composites with BN/SiC fiber coating, J. Mater. Sci. Lett. 19 (8) (2000) 657–661. [6] M. Jenkins, M. Mello, Fabrication, processing, and characterization of braided, continuous SiC fiber-reinforced/CVI SiC matrix ceramic composites, Adv. Manuf.
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373–385. [15] Z. Zhou, F. Buchanan, C. Mitchell, N. Dunne, Printability of calcium phosphate: calcium sulfate powders for the application of tissue engineered bone scaffolds using the 3D printing technique, Mater. Sci. Eng. C 38 (1) (2014) 1–10. [16] J.A. Gonzalez, J. Mireles, Y. Lin, R.B. Wicker, Characterization of ceramic components fabricated using binder jetting additive manufacturing technology, Ceram. Int. 42 (9) (2016) 10559–10564. [17] L. Rabinskiy, A. Ripetsky, S. Sitnikov, Y. Solyaev, R. Kahramanov, Fabrication of porous silicon nitride ceramics using binder jetting technology, Materials Science and Engineering Conference Series, (2016) 012023. [18] Y. Ma, X. Yin, X. Fan, L. Wang, P. Greil, N. Travitzky, Near-net-shape fabrication of Ti3SiC2-based ceramics by three-dimensional printing, Int. J. Appl. Ceram. Technol. 12 (1) (2015) 71–80. [19] X. Yin, N. Travitzky, P. Greil, Three-dimensional printing of nanolaminated Ti3AlC2 toughened TiAl3-Al2O3 composites, J. Am. Ceram. Soc. 90 (7) (2010) 2128–2134. [20] A. Stunda-Zujeva, Z. Irbe, L. Berzina-Cimdina, Controlling the morphology of ceramic and composite powders obtained via spray drying – a review, Ceram. Int. 43 (15) (2017) 11543–11551. [21] V.N. Daggupati, G.F. Naterer, K.S. Gabriel, R.J. Gravelsins, Z.L. Wang, Effects of atomization conditions and flow rates on spray drying for cupric chloride particle formation, Int. J. Hydrogen Energy 36 (17) (2011) 11353–11359. [22] S.J. Lukasiewicz, Spray- drying ceramic powders, J. Am. Ceram. Soc. 72 (4) (1989) 617–624.
Process. 11 (1) (1996) 99–118. [7] H. Mahfuz, D.P. Zadoo, F. Wilks, M. Maniruzzaman, S. Jeelani, Fracture and flexural characterization of SiCW/SiC composites at room and elevated temperatures, J. Mater. Sci. 30 (9) (1995) 2406–2411. [8] J.H. She, D.L. Jiang, S.H. Tan, J.K. Guo, Hot isostatic pressing of particle and whisker-reinforced silicon carbide matrix composites, Key Eng. Mater. 108–110 (1995) 45–52. [9] Y. Hua, L. Zhang, L. Cheng, Z. Li, J. Du, Microstructure and high temperature strength of SiCW/SiC composites by chemical vapor infiltration, Mater. Sci. Eng. A 527 (21) (2010) 5592–5595. [10] Yunfeng Hua, Litong Zhang, Laifei Cheng, Jing Wang, Silicon carbide whisker reinforced silicon carbide composites by chemical vapor infiltration, Mater. Sci. Eng. A 428 (1) (2006) 346–350. [11] Y. Hua, L. Zhang, L. Cheng, Z. Li, J. Du, Microstructure and mechanical properties of SiCP/SiC and SiCW/SiC composites by CVI, J. Mater. Sci. 45 (2) (2010) 392–398. [12] N. Chen, L. Cheng, Y. Liu, F. Ye, M. Li, Z. Gao, L. Zhang, Microstructure and properties of SiCW/SiC composites prepared by gel-casting combined with precursor infiltration and pyrolysis, Ceram. Int. 44 (1) (2017). [13] J. Suwanprateeb, R. Sanngam, T. Panyathanmaporn, Influence of raw powder preparation routes on properties of hydroxyapatite fabricated by 3D printing technique, Mater. Sci. Eng. C 30 (4) (2010) 610–617. [14] A. Butscher, M. Bohner, C. Roth, A. Ernstberger, R. Heuberger, N. Doebelin, P.R. von Rohr, R. Müller, Printability of calcium phosphate powders for three-dimensional printing of tissue engineering scaffolds, Acta Biomater. 8 (1) (2012)
3386
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology
T
⁎
Xinyuan Lv, Fang Ye, Laifei Cheng , Shangwu Fan, Yongsheng Liu Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, 710072, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC whisker-reinforced SiC composites Spray drying Binder jetting Chemical vapor infiltration
Spray drying, binder jetting and chemical vapor infiltration (CVI) were used in combination for the first time to fabricate SiC whisker-reinforced SiC ceramic matrix composites (SiCW/SiC). Granulated needle-shaped SiCW was spray dried into SiCW spherical particles to increase flowability and thereby increase printability. Then, binder jetting was employed to print a novel SiCW preform with two-stage pores using the SiCW spherical particles. The subsequent CVI technology produced pure, dense, and continuous SiC matrix with high modulus and strength. Consequently, SiCW/SiC with appropriate mechanical properties was obtained. Finally, the challenges of the novel method and the ways to improve the mechanical properties of SiCW/SiC are discussed.
1. Introduction Ceramic matrix composites (CMCs), especially silicon carbide CMCs (CMC-SiC) are irreplaceable structural materials in aerospace field due to their excellent properties such as high temperature resistance, corrosion resistance, high strength, high hardness, and high modulus [1,2]. CMC-SiC is mainly reinforced by particles, whiskers, short-cut fibers and continuous fibers, among which whiskers and continuous fibers are considered to have outstanding toughening effect. The fiber preform structure of continuous fibers-reinforced CMCs (CFCCs) mainly includes 2D, 2.5D, 3D, and 3D needled (3DN) braided structures, which inevitably leads to the anisotropy of CFCCs [3–6]. The CFCCs with 2D braided structure have low interlaminar shear strength, so their applications have been limited to the structures that sustain only in-plane stress field [3]. 3D and 3DN braided structures also have limitations, such as low in-plane strengths for the 3D braided fibers and shorter tensile fatigue life and lower compression strengths for the 3DN structure [3]. In contrast, SiC whisker-reinforced SiC matrix composites (SiCW/ SiC) are isotropic. Mahfuz [7] prepared SiCW/SiC by hot pressing SiC powders and SiC whiskers at 1750 °C and 3500 psi. She [8] prepared SiCW/SiC using hot isostatic pressing combined with sintering at 1850 °C. While the prepared SiCW/SiC had good properties, a higher preparation temperature was required for the hot press sintering technology. Hua [9–11] prepared SiCW/SiC by cold pressing combined with chemical vapor infiltration (CVI). Although CVI technology requires a lower temperature, it is difficult to prepare complex shape parts using
⁎
cold pressing. Chen [12] prepared SiCW/SiC by gel-casting combined with precursor infiltration and pyrolysis (PIP). However, the SiC matrix prepared by PIP has a low crystallization degree, and its structural stability at high temperatures needs to be improved. Three-dimensional printing (3D printing), namely additive manufacturing, is a material forming technology based on CAD models, which is suitable for preparing parts with complex shapes. Numerous ceramic parts have been fabricated by the 3D printing method of binder jetting [13–19]. The raw materials used for binder jetting are ceramic powders, whose flowability can determine the printability of parts [13,14]. It is known that SiCW has poor flowability due to its needle-like morphology, making it difficult to directly use SiCW for binder jetting. Spray drying is one of the most convenient and effective methods for granulation of ceramic materials. In this technique, ceramic slurry is sprayed into spherical droplets by a high-speed rotating atomizer, and the spherical droplets are rapidly dried at a high temperature to obtain spherical particles [20]. Spray drying is expected to improve the flowability of SiCW. The SiCW preform prepared using binder jetting combined with spray drying would have two-stage pores, i.e., large pores between the spherical particles and small pores within the spherical particles. CVI is especially suitable for depositing matrix in the preform with two-stage pores in comparison with PIP and reaction melt infiltration (RMI). Moreover, the ceramic matrix obtained by CVI has dense microstructure and good properties. Therefore, the subsequent introduction of SiC matrix into SiCW perform can be carried out by CVI. Based on the above considerations, a novel method has been
Corresponding author. E-mail address: [email protected] (L. Cheng).
https://doi.org/10.1016/j.jeurceramsoc.2019.04.043 Received 26 February 2019; Received in revised form 22 April 2019; Accepted 23 April 2019 Available online 27 April 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Journal of the European Ceramic Society 39 (2019) 3380–3386
X. Lv, et al.
Fig. 1. Schematic diagram of the fabrication procedure for SiCW/SiC.
2.4. CVI
proposed in this paper for the preparation of SiCW/SiC composites based on 3D printing and CVI technology. The process includes three steps (Fig. 1): granulation of SiCW into SiCW spherical particles by spray drying; preparation of SiCW spherical particles into SiCW preform using binder jetting; and finally, densification of SiCW preform by CVI to obtain SiCW/SiC.
The SiC matrix was deposited in the green bodies using self-developed CVI deposition furnace for 120 h, 240 h, 360 h, 480 h, 600 h, 720 h, and 840 h. The organics in the green bodies were burned out in the heating stage. Methyltrichlorosilane (MTS: CH3SiCl3), hydrogen (H2) and argon (Ar) gas were used as the precursor, carrier, and dilution gas, respectively. The flow rate of Ar was 350 mL/min, and the input gas ratio of MTS, H2, and Ar was 1:40:40. The total pressure was 5000 Pa. The deposition temperature was 1000 °C. The as-obtained SiCW/SiC composites were used for testing and analysis.
2. Materials and methods 2.1. The preparation of ceramic slurry SiCW (Beantown Chemical, UK) with a mean diameter of 1.5 μm, a mean length of 18 μm and a purity of A.R. ≥99% was used in this study. The ceramic slurry was prepared by dispersing SiCW in deionized water with polyethylene glycol (PEG, Mr = 400, Tianjin Kemiou Chemical Reagent Co., Ltd., China) as the dispersant, tetramethylammonium hydroxide (TMAH, Tianjin Fuchen Chemical Reagent Co., Ltd., China) as the pH adjuster, and dextrin (MW = 504, Tianjin Dingshengxin Chemical Co., Ltd., China) as the binder. The ceramic slurry contained 25 vol. % SiCW, 2 wt. % PEG, 4 vol. % TMAH, and 10 wt. % dextrin (relative to deionized water content).
2.5. Characterization Microstructure of the samples was observed using secondary electrons of scanning electron microscopy (SEM, S-4700, Japan). The particle size distribution was determined by processing approximately 3000 SiCW spherical particles by image processing software (Image-Pro Plus). The bulk density, tap density and flowability of SiCW spherical particles were measured by Hall flow meter (ST-1002, Xiamen East Instrument Co., Ltd., China). The bulk density and open porosity of SiCW/SiC were measured by Archimedes’ method. Three-point bending test was employed to measure the flexural strength of the samples (3 mm × 4 mm × 40 mm) with the support distance of 30 mm and a cross-head speed of 0.5 mm/min, using an electromechanical universal testing machine (CMT5504, MTS SYSTEMS, Co., Ltd., China). Fracture toughness was measured using a single-edge notched beam (SENB) test with a span of 30 mm and a cross-head speed of 0.05 mm/min using 3 mm × 4 mm × 40 mm bars. A low-speed cutting machine was used to prepare the SENB samples with a notch width of 0.2 mm, a notch depth of 2 mm, and a notch curvature radius of 38 mm. In addition, 5 samples were tested for each group of samples for flexural strength and fracture toughness. Pore size distribution was measured by mercury intrusion method (AutoPore IV, Micromeritics Instrument Co., Ltd., USA). The elastic modulus of SiC matrix was measured by a nanoindenter (TI980, Hysitron Co., Ltd., USA) at 9 mN for 2 s. For some samples, detailed features of the microstructure were revealed by plasma etching (RES102, LEICA Co., Ltd., Germany) conducted for 1.5 h.
2.2. Spray drying The prepared ceramic slurry was fed into a centrifugal spray dryer to be granulated at an atomizer speed of 2400 r/min, an inlet temperature of 330 °C–350 °C, an outlet temperature of 100 °C–120 °C, and a feeding speed of 20 ml/min. Then, the SiCW spherical particles were obtained.
2.3. 3D printing The as-prepared SiCW spherical particles were used as the raw materials for 3D printing (ProJet360, 3D Systems Inc., California, USA). Specimens of size 3 mm × 4 mm × 40 mm were printed with layer thickness of 100 μm and binder saturation of 100%/200%. The green bodies were dried in situ for 4–10 h and then removed. 3381
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Fig. 2. Morphology of multiple SiCW spherical particles (a), single SiCW spherical particle (b), and surface of SiCW spherical particle (c); particle size accumulation and frequency distribution of SiCW spherical particles (d).
3. Results and discussion
3.2. 3D printed SiCW preform and SiCW/SiC
3.1. Granulated SiCW spherical particles by spray drying
Using the SiCW spherical particles, SiCW preform and SiCW/SiC composites were obtained by binder jetting and CVI, respectively. The microstructures of SiCW preform and SiCW/SiC are shown in Fig. 3. Fig. 3a and b show the fracture morphology of a novel and unique preform microstructure, which is stacked by numerous SiCW spherical particles, and each sphere consists of numerous SiCW. This multi-stage preform microstructure creates a two-stage void with large inter-particle pores of 10–20 μm and small intra-particle pores of 1–3 μm. The internal microstructure of the SiCW spherical particles as seen in Fig. 3b indicates that the distribution of SiCW was relatively uniform. Overall, the SiCW spherical particles were stacked uniformly without external assistance. The volume fraction of SiCW preform was estimated by the bulk density of SiCW spherical particles according to Eq. (1).
Fig. 2 shows the morphology and particle size distribution of SiCW spherical particles obtained using spray drying, and Table 1 lists their properties. Specifically, it was observed from the SEM image in Fig. 2a that the agglomeration rate was very high and the SiCW powder was almost entirely granulated into spherical particles. The SEM image in Fig. 2b shows that the SiCW particles had spherical shape. The SEM image in Fig. 2c shows that the SiCW were randomly oriented in the sphere, which made the sphere isotropic. From the particle size distribution shown in Fig. 2d, it can be seen that the SiCW spherical particles displayed a single peak and narrow particle size distribution, indicating that the particle size was concentrated. More importantly, Table 1 shows that the Hausner ratio of SiCW spherical particles was 1.25. The powder classification according to Hausner ratio is as follows: the Hausner ratio > 1.4 for non-flowing and cohesive powders, and it is 1–1.25 for free-flowing powders [21]. Therefore, it was confirmed that the SiCW spherical particles had acceptable flowability and were suitable for binder jetting.
Vp ≈ ρb ρth × 100%
where Vp is the volume fraction of SiCW preform, ρb is the bulk density of SiCW spherical particles and ρth is the theoretical density of SiCW. In this work, Vp is 17.7 vol. %. The SEM images in Fig. 3c–f show the fracture morphology of SiCW/ SiC (CVI 720 h). It can be seen in Fig. 3c and d that each SiCW spherical particle was coated by SiC matrix, which separated the pores into the inter-particle and intra-particle ones. All the intra-particle pores were closed pores and most of the inter-particle pores were also closed pores. The inter-particle closed pores were large and dispersed, while the intra-particle closed pores were small and concentrated. Moreover, it can be observed in Fig. 3e and f that after CVI-SiC, the exterior of the SiCW spherical particle was relatively denser and the interior of the
Table 1 The properties of SiCW spherical particles. Bulk density (g/cm3)
Tap density (g/cm3)
Hausner ratio (ρBulk/ρTap)
Flowability (ml/ s)
d10/d50/ d90 (μm)
0.566
0.708
1.25
3
6/25/62
(1)
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Fig. 3. Fracture morphology of SiCW preform (a); internal microstructure of SiCW spherical particles (b); fracture morphology of SiCW/SiC (c), fracture morphology of one SiCW spherical particle after CVI-SiC (d), exterior (e) and interior (f) of the SiCW spherical particles after CVI-SiC.
increased slowly, and finally increased rapidly again. The same trend was observed for the decrease in open porosity. In the first stage, the growth of SiC matrix occurred mainly intra particles. The CVI precursor had good gas permeability in SiCW preform due to its two-stage void microstructure, causing the open porosity to decrease rapidly from 82.3 vol. % to 22.27 vol. %. The closed porosity at this point was 6.74 vol. % which was almost entirely derived from the intra-particle pores. In the second stage, SiCW/SiC presented a single-stage void microstructure, and the growth rate of SiC matrix began to decrease. During this process, the inter-particle open pores were gradually filled by SiC matrix, on account of the pore size distribution of SiCW/SiC, as shown in Fig. 5. The closed porosity at this stage increased only slightly by 1.13 vol. % due to the bottleneck effect of CVI. In the third stage, the open porosity decreased rapidly from 6 vol. % to 1 vol. % and the closed porosity increased significantly to 11.4 vol. %. This could be attributed to the encrustation of deposits on the surface during CVI, which converted most of the open pores into closed pores. Fig. 6 shows the relationship between the density, flexural strength
SiCW spherical particle had a small amount of closed pores, indicating that the distribution of SiCW spherical particles had a slight radial gradient. This is possibly because, during the spray drying, the binder migrates to the surface of SiCW droplet with water as the water evaporates. Therefore, a discontinuous film is formed, which hinders the timely discharge of water vapor inside the SiCW spherical particles and thus causes internal expansion [20,22]. The open porosity and density of SiCW/SiC are shown in Fig. 4. The closed porosity of SiCW/SiC was calculated using the open porosity and density of SiCW/SiC according to Eq. (2).
pc = (1 − ρb ρth − po ) × 100%
(2)
where pc, po, ρb and ρth are the closed porosity, open porosity, bulk density and theoretical density of SiCW/SiC, respectively. In Fig. 4, the different density values in horizontal axis corresponded to different CVI deposition times. As the density increased, the closed porosity increased and the open porosity decreased. This variation process can be divided into three stages. The closed porosity increased rapidly at first, then 3383
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Fig. 4. Relationship between density, open porosity and closed porosity of SiCW/SiC.
Fig. 7. Load-displacement curves of SiCW/SiC with different densities.
Fig. 5. Pore size distribution of SiCW/SiC with different densities and CVI time. Fig. 8. Load-depth curves plotted by the nano-indentation of five points (located in the SiC matrix).
and fracture toughness of SiCW/SiC. Before the encrustation of deposits, both the flexural strength and fracture toughness of SiCW/SiC increased with density. After the encrustation of deposits, both the flexural strength and fracture toughness of SiCW/SiC were significantly reduced. The flexural strength of SiCW/SiC was 200 MPa and fracture toughness was 3.4 MPa m1/2. Obviously, both the flexural strength and fracture toughness are lower than other SiCW/SiC reported in the literature. There are mainly two reasons for this. The first is the volume fraction of SiCW (17.7 vol. %) is not high enough. The second is that the surface of SiCW/SiC exists a lot of stress concentration points because of its higher surface roughness. The higher surface roughness of SiCW/SiC is attributed to the inevitable high powder bed roughness caused by the large particle size of the SiCW spherical particles. As well as the liquid binder flows to the place where it does not need to be bonded when it infiltrates into the powder bed, however it is difficult to control the infiltration behaviors of the liquid binder. The load-displacement curves in Fig. 7 indicate that the fracture mode of SiCW/SiC was brittle fracture. Meanwhile, the nano-indentation curves in Fig. 8 was used to calculate the elastic modulus of SiC matrix as 458 GPa. Therefore, preparation a low-modulus interface may be required to regulate the modulus matching relationship between SiC matrix and SiCW
Fig. 6. Relationship between density, flexural strength and fracture toughness of SiCW/SiC.
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Fig. 9. SEM images of whisker pull-out (a), crack deflection (b), whisker bridging (c), and crack branching (d).
Fig. 10. Pictures of (a) SiCW green body and (b) SiCW/SiC with gear shape.
investigations and it might be necessary to prepare an interface in the SiCW/SiC or improve the surface roughness of SiCW/SiC to further improve its mechanical properties.
(> 550GPa). The fracture morphology in Fig. 9 revealed four main toughening mechanisms of SiCW, which were whisker pull-out, crack deflection, whisker bridging, and crack branching. Fig. 10 presents the pictures of a pair of meshing gears, which illustrates the ability of this method to fabricate complex shape parts.
Acknowledgements This work was supported by the National key R&D Program of China No. 2017YFB1103500 and National Natural Science Foundation of China (Grant No.51632007, 51602258, 51521061, 51672218, 51872229, 51802263), the 111 Project of China (B08040).
4. Conclusions In conclusion, a novel method was developed this work for the preparation of SiCW/SiC with appropriate mechanical properties by combining spray drying, binder jetting, and CVI. The combination of these three techniques provided several benefits. Through spray drying, the ceramic powder having poor flowability with an irregular shape, needle shape or sheet shape was aggregated into a spherical shape. Spherical particles are the ideal raw material for binder jetting. The products from binder jetting do not require molds and complicated machining. Moreover, geometrically-complex products can be rapidly prepared by binder jetting, thereby saving the cost and shortening the production period. Furthermore, the ceramic matrix prepared by CVI was pure, dense and continuous, with high modulus and strength. The introduction of ceramic matrix densified the preform. Therefore, the novel method is expected to have extensive practical applications. In this work, the flexural strength and fracture toughness of SiCW/SiC were as high as 200 MPa and 3.4 MPa m1/2, respectively. In the future, the preparation of large-scale products would require more
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