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3D SiC containing uniformly dispersed, aligned SiC whiskers: Printability, microstructure and mechanical properties
Journal of Alloys and Compounds 809 (2019) 151824
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
3D SiC containing uniformly dispersed, aligned SiC whiskers: Printability, microstructure and mechanical properties Huiwen Xiong a, Lianzhong Zhao a, Hehao Chen a, Xiaofeng Wang a, b, **, Kechao Zhou a, Dou Zhang a, * a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China School of Materials Science and Engineering, Central South University, Changsha, 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2019 Received in revised form 10 August 2019 Accepted 11 August 2019 Available online 12 August 2019
3D SiC-based lattices with aligned SiC whiskers were prepared via direct ink writing of SiCw/polycarbosilane (PCS)-based suspensions. Suspensions with different solid loading were prepared and used to investigate the morphology and printability of 3D SiCw/SiC lattices. Filaments with the morphologies of accumulation, coiling, meandering and straight were achieved with different printing height and speed. Higher printing height favored the formation of coiled filaments with longer periodic structure and increased amplitude. 3D lattices with straight filaments and good retention of shape were achieved only when applying a suitable printing parameters for SiCw/PCS-based suspensions with a solid loading of 62.3 vol%. The resulted 3D lattices with the porosity around 62% showed impressive bending and compression strength of 33.2 ± 5.2 MPa and 30.6 ± 4.3 MPa, respectively. The satisfactory properties of 3D SiCw/SiC lattices were contributed by the pullout of the aligned SiC whiskers. Furthermore, this 3D technique via the Polymer-derived Ceramics (PDCs) route allows the fabrication of large sized samples, which opens new opportunities for the fabrication of 3D structures with unique mechanical and functional properties in practical dimensions. © 2019 Elsevier B.V. All rights reserved.
Keywords: 3D SiC Direct ink writing Preceramic polymer Polycarbosilane
1. Introduction Additive manufacture of ceramics attracts lots of attentions, owing to its high efficiency, high accuracy of complicated structure and ability for tailoring the microstructure and composition with multifunctional properties [1e3]. To date, various 3D printing methods are developed, including selective laser sintering (SLS) [4], binder-based three dimensional printing (3DP) [5], stereolithography (SL) technology [6] and direct ink writing (DIW) [7,8]. DIW is a method based on carefully designed suspensions, which are extruded through a nozzle and constructed on a plate according to the pre-designed CAD models [7,9]. Compared with other methods, DIW shows advantages for fabricate ceramics with high flexibility, low cost and large-scale production [7,9,10]. SiC-based ceramics are considered to be potential materials for
* Corresponding author. ** Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China. E-mail addresses: [email protected] (X. Wang), [email protected] (D. Zhang). https://doi.org/10.1016/j.jallcom.2019.151824 0925-8388/© 2019 Elsevier B.V. All rights reserved.
many structural and wear applications, owing to their high chemical stability in extreme environments, high hardness, low density and satisfactory heat transfer performance [11e13]. However, pure SiC ceramic is hard to be sintered because of the strong covalent bonds between Si and C [14]. Although partial sintering aids with low melting points can accelerate the densification process, the obtained composites exhibit high relative density and improved mechanical strength at the expense of their high temperature properties [14,15]. Polymer derived ceramics (PDCs) are pyrolyzed at a low sintering temperature and exhibit impressive properties at ultra high temperature with respect to crystallization, oxidation, creep and phase separation temperature [16e18]. Moreover, preceramic polymers are, well known for the fabrication of the ceramic fibers and ceramic matrix composites [18], and recently attract attention for their application in preparing 3D ceramics via the DIW method [19e21]. Ceramics are usually very brittle and sensitive to the defects in the composites [13]. Therefore, for application as the load-carrying components, SiC fibre or carbon fibre reinforced SiC ceramics (SiCf/ SiC or Cf/SiC) are two typical composites with improved strength and fracture toughness [22]. Braided SiC or Carbon fibres are filled
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with SiC via the chemical vapor infiltration (CVI) or polymer impregnation pyrolysis (PIP) process. Compared with these conventional methods, DIW exhibits features of time-saving, better integrity, stronger design-ability, and better portability for other ceramic systems [7,8,10]. In order to achieve 3D SiC with satisfactory mechanical properties using DIW, short carbon fibers were used as the reinforced phases, which oriented along with the extruded direction due to the shear stress from the walls of nozzle [22e24]. However, micron SiCf or carbon fibres with millimeters in length were hardly to be extruded through a submillimeter nozzle. SiC whiskers exhibit high strength, and have been widely used as the reinforcements in ceramics [25e27]. Hence, SiC whiskers should be an ideal choice for preparing 3D-SiCw/SiC composites via the DIW. In this work, polycarbosilane (PCS) was used as the preceramic polymer of SiC. SiCw/ PCS-based suspensions with different solid loading were prepared and used for fabricating 3D SiCw/SiC ceramics. Although SiCw/SiCp reinforced 3D SiC were reported in previous work [28], the content of whisker was only 5 wt% (in related to that of the polycarbosilane (PCS)). Suspensions containing 20 wt% whiskers with aspect ratio over 20 were not very easy to extruded through a small nozzle with a diameter of 160 mm. Meanwhile, high amount of SiC whisker reinforced SiC was a attracting composites, owing to their well known strength and sometimes could be considered as a novel SiCf/ SiC composites [26]. Here, the printing process using PCS/n-hexane based suspensions containing 20 wt% SiCw exhibited smoothly and continuously, and capable for fabricating 3D structure over tens of layers. The reasons for the continuously printing of suspension with 60% solid content without clogging the nozzle and deformation (or slightly deformation) are divided into two parts. On the one hand, the suitable viscosity of the suspensions enable the successful extrusion, with rapid evaporation of the solvent. The rapid evaporation of n-hexane results in higher solid content of the suspension and also quickly increased elastic modulus of the filament without shear stress. On the other hand, addition of DVB plays a key role in the smooth printing without clogging, which avoids the sticking of residue suspensions around the nozzle. The printability of SiCw/ PCS-based suspensions was discussed, in related to the printing parameters and viscosity properties of suspensions. The microstructure, composition and mechanical properties of 3D SiC were investigated.
purchased from Sanzheng Polymer Materials Co., Ltd (Shanghai, China). Divinylbenzene (DVB) was provided by Shanghai Aladdin Bio-Chem Technology Co., Ltd (Shanghai, China). 2.2. Preparation of the SiCw/PCS-based suspensions Table 1 shows the composition of suspensions with various solid loading ranging from 52.2 vol% to 68.7 vol%. Here, 20 wt% SiCw (in related to the PCS) was added in order to obtaining high amount of whisker-reinforced composites. Fig. 1 shows the route of fabrication process for 3D SiCw/SiC-based composites and details can be found in previous work [29]. The suspensions were prepared as follows: Typically, 8g PCS, 1.6g SiCw, 15 g N-hexane and 0.02 g CH-3 were weighted and put into a Teflon bottle with a volume of 200 ml. Then, white ZrO2 balls with a diameter of 5 mm were added into the bottle; the weight of balls was kept at 125g. After that, the bottle was sealed and ball milled for 24 h, with a rotation speed of 80 r/ min. After milling, several short whiskers were formed and most of the whiskers have a aspect ratio over 20. Then, the suspensions were poured into a beaker and magnetic stirred in fume hoods. Meanwhile, 0.08g DVB was added into the beaker. Here, a small amount of DVB plays an important role in smooth printing and avoids the clogging of the nozzle. Hence, suspensions with solid
2. Experimental section 2.1. Materials Polycarbosilane (PCS, PCS161001, Suzhou cerafil. Ltd., China) with an average molecular weight of 1500 g/mol was utilized. Nhexane (Sinopharm chemical reagent Co. Ltd., China) was used as the solvent. b-SiC whiskers (SiCw, Sinet Advanced Materials Co., Ltd., China) were used as the reinforcements. The whiskers have an average diameter of 0.2 mm and aspect ratio over 30. The dispersant of SiC whiskers was an ester compound named CH-3, which was
Fig. 1. Schematic illustration of the fabrication process of 3D-SiCw/SiC composites using PCS-based suspensions. DVB means the divinylbenzen, which is used for achiving a smooth printing process for SiCw/PCS-based suspensions.
Table 1 Nominal composition of the SiCw/PCS-based suspensions with different solid loading (vol %). Samples
Polycarbosilane (PCS)
N-hexane
SiC whiskers (SiCw)
Solid loadinga (vol%)
A B C D E
63.8 58.3 56.1 54.0 48.9
31.8 37.7 40.0 42.3 47.8
4.4 4.0 3.9 3.7 3.3
68.7 62.3 59.9 57.7 52.2
a Calculated by the volume ratio of V(PCSþSiCw)/V(N-hexaneþPCSþSiCw). Density of PCS, SiCw and n-hexane are 1.1, 3.2 and 0.66 g/cm3, respectively. A rather small amount of DVB is not considered here, which is added after the ball milling process. The volume of CH-3 is neglected and its mass content equals 2% of the whiskers.
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loading ranging from 52.2 vol% to 68.7 vol% were achieved with the evaporation of N-hexane. Then, the suspension was moved into a syringe with a volume of 10 ml. In order to remove the residue gases in the suspension, the syringe was put into a centrifuge with a rotation speed of 2000 r/min for 1 min. Finally, the suspension sealed in the syringe was ready for DIW. 2.3. Direct ink writing The suspensions were extruded from the syringe under the nitrogen pressure and deposited on a glass plate, using a deposition machine (DR2203, EFD Inc., East Province, RI). Two types of syringes were used with the diameters of 160 mm and 210 mm of the nozzle. Here, the nitrogen pressure was ranging from 50 psi to 70 psi. The printing height meant the distance from the nozzle tip to the glass plate, which was adjusted from 0.1 mm to 0.8 mm, respectively. The printing speed was the horizontal movement speed of the nozzle, which was ranging from 8 mm/s to 30 mm/s. The printing process was as follows: the filaments were extruded out from the nozzle, which were then deposited on the glass plate. The geometric structure was determined by the predesigned CAD mode. After the formation of the first layer, the nozzle was lifted along the Z direction; the second layer was then formed with a spanning feature. After the deposition of filaments layer by layer, 3D PCS-based lattices were achieved. After that, the lattices were dried over night in the ambient condition before separating from the glass plate. Then, the 3D PCS-based lattices were cured in air at 200 C for 4 h. The pyrolysis process was as follows: the samples were firstly heated to 450 C with a heating rate of 2 C/min and held for 2 h. The sintering atmosphere was pure Ar. Then, the temperature went up to 700 C by 1 C/min and held for 2 h, in order to accomplish the ceramic conversion. After that, the samples were heated to 800e1400 C for 2 h, with a fast heating rate of 5 C/min and maintained for 2 h. Finally, the black SiCw/SiC composites were obtained by heating the cured lattices at 1200 C for 1 h. 2.4. Characterization The viscosity and elastic modulus of suspensions were tested at 25 C using a rotational rheometer (AR2000EX, TA instruments, USA). The shear rate ranged from 0.1 to 500 1/s and the shear stress was from 0.1 Pa to 1000 Pa. The TG and DSC of the cured lattices were explored in pure Ar using a thermal analyzer (Setsys evolution
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TGA-DTA/DSC, Setlaram instrument Ltd., France), with a heating rate of 10 C/min. The macrophotograph of extruded filaments or 3D SiCw/SiC lattices was taken using a stereomicroscope (SZX10, Nikon Co. Ltd., Japan). The SEM and TEM of SiCw/SiC lattices were obtained using a field emission scanning electron microscope (Nova NanoSEM 230, FEI, USA) and a transmission electron microscope (Technai G2 20S-Twin, FEI, USA). The Fourier transform infra-red (FTIR) spectra was tested using an infrared spectrometer (Nicolet 6700, Thermo Scientific, USA). The XRD results was obtained using a X ray diffraction (D/max 2550, Riguka, Japan). The density was measured by the Archimedes method. The bending strength and compression strength were tested using a MTS biomechanical testing machine (23 MTS Insight, MTS, USA), according to the Chinese standard of GB/T 9341-2008 and GB/T 47401999, respectively. The testing samples have a size of 25 mm 6 mm 7.5 mm, with a porosity around 62 vol%. 3. Results and discussion 3.1. Viscosities of the SiCw/PCS-based suspensions Fig. 2 shows the rheological properties of SiCw/PCS suspensions with solid loading from 68.7 vol% to 52.2 vol%, respectively. All suspensions exhibit the shear thinning behavior, which ensures the successful extrusion process. With increasing the solid loading from 52.2 vol% to 68.7 vol%, the viscosities at zero shear rate increase from 6.6 Pa S to 2375 Pa S, respectively. For the high solid loading of 68.7 vol%, viscosity property of SiCw/PCS/n-hexane suspension belongs to the feature of Carreau model of pure polymer [30]. With decreasing the solid loading, the effect of SiC whiskers plays a more important role in rheological behavior, resulting in shear thinning behavior of typical ceramic suspension. The test condition for the viscosity and elastic modulus of all suspensions was the same. Fig. 2(a) and (b) are not measured simultaneously, which are grouped differently. Compared to the suspensions without SiCw, the SiCw/PCS suspensions exhibit higher viscosity and also much higher elastic modulus. Fig. 2(b) shows the elastic modulus of suspensions as a function of shear stress. The elastic modulus is greatly affected by the solid loading of SiCw/PCS/ n-hexane suspension. Compared with the ceramic suspensions [31], the elastic modulus of PCS-hexane system is rather small, indicating the deformation of filaments after extrusion. With increasing the solid loading from 52.2 vol% to 68.7 vol%, the elastic modulus (G0 ) at 0.1 Pa of the shear stress (t) sharply increases from
Fig. 2. (a) Viscosities of the suspensions with different solid loading as a function of shear rate. The black data of 64.3 vol% belong to that of pure PCS-based suspension without whiskers. (b) Logelog plot of the related elastic modulus as a function of increasing shear stress. The black data of G0 without whiskers are not shown, owing to their rather small value (less than 0.1).
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2.41 Pa to 204 Pa, respectively. For suspensions with the solid loading of 62.3 vol%, the elastic modulus of suspension increased when the stress increased to 1000 Pa. The reason is that the break of the balance during the elastic modulus test when with a high shear stress. The sealed surface covered by methyl-silicone oil can sometime be destroyed, resulting in the increased solid loading of suspension caused by the rapid evaporation of n-hexane. When the shear stress (t) is lower than the shear yield stress (ty), the elastic modulus (G0 ) is relative stable, owing to the twining and aggregation of polymer molecules [32]. However, higher shear stress over the ty results in the disaggregation of polymer molecules, then sharply decreases the elastic modulus (G0 ). From the rheological study of silicon carbide suspensions, the most suitable solid loading for the direct ink writing should be around 62.3 vol%, with moderate viscosity and also high elastic modulus. Hence, the rapid evaporation of n-hexane on the surface of extruded filaments will greatly increase the G’ in situ, making the 3D shaping of SiCw/PCSbased filaments possible. Both the whiskers and the polymers affect the viscosity and elastic modulus of the suspensions, especially for the solid loading of 59.9 vol%. Therefore, effects from both whiskers and polymers lead to the different changes for suspensions of 59.9 vol%. We hold that the whisker alignment or interlocking plays a more important role for the suspensions with a relative low solid loading. In turn, the feature of the polymer shows up: the twinning and aggregation. In addition, in suspension with the solid loading of 59.9 vol%, the small peak generation around 10e100 Pa should be related to the twist of the SiCw dispersed in the solution. With a low shear stress of 10 Pa, the moving speed of SiCw on the surface and inside the suspension differs, leading to an increased friction resistance and also different orientation of the whiskers. Therefore, the friction
resistance and twist of SiC whiskers should be the reason for the sudden increase of elastic modulus. 3.2. Printability Fig. 3 shows the views of printed filaments using 59.9 vol% suspension with different printing height and speed. The diameter of the nozzle is 210 mm. The printing height and speed are ranged from 0.1 mm to 0.8 mm, and 14 mm/s to 30 mm/s, respectively. The morphology of the filaments can be divided into four typical, which are accumulation, coiling, meandering and straight, respectively. Low printing speed and height lead to the accumulation of the filaments. High printing height favors the coiling morphology. With the increasing of printing speed or height, the filaments are deposited based on a larger circle. Hence, the increased displacement along the XeY plane lead to the formation of views of meandering. When the speed reaches the critical value of 24 mm/s, the extrusion rods seem straight with various printing height. Further increasing the speed over 24 mm/s leads to straight filaments with decreased diameters. It is noteworthy that printing speed over 30 mm/s or height higher than 0.8 mm make the extruded rods out of shape. In fact, the relationship between specific rate of flow and printing speed can be expressed as follows [33]:
Q ðPÞ ¼
pnðaDÞ2 4
(1)
of which the Q(P) is the extrusion volume per unit time related to extrusion pressure, v is the printing speed, a is the expansion coefficient of suspension during extrusion and D is the diameter of the
Fig. 3. The diagram of the printed filaments' patterns with various selections of printing speed and printing height (top view), the optical images of four typical printed filaments' patterns (below view), scale bars are 2 mm. The gas pressure is 65 psi. The diameter of the nozzle is 0.21 mm and the solid loading of SiCw/PCS-based suspension is 59.9 vol%.
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nozzle, respectively. Hence, the printing speed can be calculated for a given suspension with certain gas pressure and size of nozzle. This calculated speed here is the “intrinsic printing speed”, which plays an important role in obtaining the straight filaments for the DIW process. Only when the printing speed matches the specific rate of flow can the straight shape of the filaments be presented. For the SiCw/PCS-based suspension with a solid loading of 59.9 vol%, the intrinsic printing speed, as shown in Fig. 3, is around 24 mm/s when the gas pressure and diameter of the nozzle are 65 psi and 210 mm, respectively. When the printing speed is lower than the intrinsic value, various morphology of filaments can be achieved via the adjustment of the printing height. However, when the printing speed is higher than 24 mm/s, the filaments are straight or out of shape with various printing height. Overall, the morphology of printed filaments is determined by the properties of suspensions, printing parameters and shape of nozzle. Four typical morphology of filaments, i.e. accumulation, coiling, meandering and straight, can be obtained here, via the controlling of printing speed and height. The printed size and morphology of 3D SiCw/SiC lattices are investigated using two CAD structure with the size and span length of (30 mm 6 mm 3 mm, 1 mm) and (31 mm 7 mm 3 mm, 0.5 mm), respectively. Fig. 4 (a) and (b) are the 3D models and Fig. 4 (c) and (d) are the corresponding SEM images. 3D lattices with regular structure and spanning features are confirmed, which indicates that the SiCw/PCS-based suspensions are suitable for the DIW process. Fig. 4(e) shows the measured size of 3D SiCw/SiC lattices. The diameter of the filaments is around 0.26 mm, which is higher than the size of nozzle (0.21 mm). Therefore, the SiCw/PCSbased suspensions act like the pure polymer suspension, exhibiting the expanding feature after the extrusion [28].
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the absorb water. At around 380 C, a weight loss of 3.54% occurs, owing to the releasing of H2 and CH4 during the self-linking of PCS [18,35]. However, no peaks below 600 C can be found in the DSC curve. The ceramic conversion takes place at 697.4 C, with a fast decreasing rate in weight. Further increasing temperature from 800 C to 1200 C, the weight loss is less than 2%. Two peaks at 987.8 C and 1157.8 C can be observed in the DSC curve. The reaction at 987.8 C indicates the side chains (C-H2 bending in Si-CH2Si) are further decomposed, which agrees well with our previous work [29]. The crystallization of SiC occurs at 1157.8 C and nano SiC grains are formed in amorphous SiC matrix. After pyrolyzed at 1200 C, the mass content of SiCw in the final 3D SiC composites after pyrolyzed at 1200 C is around 24 wt%. Fig. 5(c) shows the XRD patterns of pure SiCw, cured and pyrolyzed SiCw/PCS lattices. Only b-SiC phase is found in all patterns. After pyrolyzed, the peaks of b-SiC are widened, indicating the amorphous or nano SiC derived from the cured PCS. Fig. 5(d) shows the linear shrinkage, weight loss and density of 3D SiCw/SiC lattices fired. With increasing the pyrolysis temperature from 600 C to 1400 C, the data mentioned above increases from 1.8%, 9.5% and 1.51 g/cm3 to 19.3%, 19.6% and 2.64 g/cm3, respectively. The pyrolyzed 3D SiC maintains their sound geometric structure and pore size even with a high linear shrinkage of 19.6%. The low heating rate of 1 C/min from 450 C to 700 C is very important and favors the uniform shrinkage of 3D structure during the ceramic conversion. Here, pyrolysis temperature of 1200 C is suitable for obtaining SiCw/SiC composites with satisfactory properties. Although higher temperature than 1200 C will increase the density, more micro pores can be formed due to the carbothermal reduction reaction occurring between Si-O and carbon in the matrix [36]. 3.4. Microstructure and composition
3.3. Crosslinking and pyrolysis Fig. 5(a) and (b) show the infrared spectra and TG/DSC results. After curing at 200 C for 4 h in air, the intensity of Si-H group decreases greatly. The Si-H group is oxidized in air and the 3D net work of PCS macro-molecules is gradually formed, which helps the 3D lattices keep their complex shape during the pyrolysis process [34]. After sintering at 1200 C, only Si-O, Si-C and residue O-H of water are observed, indicating the complete ceramic conversion. The weight loss of 2.55% around 200 C should be the escaping of
Fig. 6 shows the macro-photos of 3D SiCw/PCS lattices before and after the pyrolysis process. The suspensions are extruded through the nozzle under gas pressure and maintain the structure owing to the increased viscosity without the shear stress. Moreover, an image of the cross-section demonstrates that no sagging exists in the z-direction (see Fig. 9(d)). Over twenty layered structure are successfully printed, indicating the well printability of SiCw/PCS-based suspensions. After heat-treatment, 3D ceramics with sound structure are obtained, with regular lattice (as shown in
Fig. 4. (a) and (b) Two CAD structure of the 3D structures with the size of 30 mm 6 mm 3 mm and 31 mm 7 mm 3 mm, respectively. The diameter of nozzle is 210 mm. The inter-layer spacing are 0.2 mm with different span distance of 1 mm and 0.5 mm, respectively. (c) and (d) The corresponding SEM morphology of pyrolyzed 3D structures of (a) and (b). (e) The measured size of the printed filaments based on the SEM images.
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Fig. 5. (a) Infrared spectra of mixture, cured and pyrolyzed SiCw/SiC lattices at 1200 C. (b) TG and DSC curves of cured lattices. The heating rate is 10 C/min and the atmosphere is Ar. (c) XRD patterns of SiC whiskers, mixture, and 3D SiC scaffolds after sintering at 1200 C. (d) The linear shrinkage, weight loss and density of cured 3D SiCw/SiC lattices.
Fig. 6(c)). Fig. 7 shows the microstructure of 3D SiCw/SiC lattices with different printing height or using suspensions with different solid loading. With increasing the printing height from 0.20 mm to 0.60 mm, the morphology of filaments changed from straight into meandering and coiling. Higher printing height favors the formation of coiled lattices with longer periodic structure and increased amplitude. Besides the effect of printing parameters of height or speed, the viscosity of suspensions greatly affects the final morphology of filaments and determines the final printability. Fig. 8 displays the morphology of filaments with different printing height. Owing to the vibration during the printing, the filaments in the air always exhibit the morphology of a screw cone. Therefore, higher printing height leads to a larger circle path of the filaments. During the extrusion process with a certain printing speed, the morphology of the filaments shows periodic appearance with different amplitude and unit length. Fig. 7(def) display the SEM images of 3D SiCw/SiC lattices using suspensions with solid loading of 57.7 vol%, 59.9 vol% and 62.3 vol%, respectively. Suspensions with shear thinning behavior ensure the extrusion process, owing to their low viscosity under shear stress. Nevertheless, an unsatisfactory viscosity at zero shear stress leads to the deformation of the filaments, as shown in Fig. 7(d and e). Although the quick evaporation of n-hexane increases the elastic properties of suspension, the initial poor elastic modulus (G’z10) of the two suspensions (Fig. 2(b)) cannot maintain the shape after the extrusion. In order to investigate the distribution of SiC whiskers, the enlarged views on the surface of filaments are provided, as shown in Fig. 7(hej). The deformation of the filaments after
extrusion induces the rearrangement of the SiCw, resulting in the weak direction. Moreover, 3D SiCw/SiC lattice with highly aligned SiCw are achieved, as shown in Fig. 7(j). SiCw/SiC lattices with straight filaments are used to reveal the microstructure and composition, as shown in Fig. 9. Regular 3D structure with spanning feature are achieved using SiCw/PCS-based suspension with the solid loading of 62.3 vol%. After sintering of the lattices via a 160 mm-type nozzle, the interval and diameter of filaments are around 130 mm and 120 mm, respectively. High aligned SiCw can be found in Fig. 9(c), owing to shear stress along the extrusion direction and also the satisfactory viscosity for maintaining the shape of filaments. Fracture morphology of lattices is advantage for investigating the distribution of whiskers along the radial direction. No pores or cracks are observed on the surface or in the filaments. Therefore, dense SiCw/SiC composites can be achieved via the DIW of SiCw/PCS-based suspensions. Meanwhile, whiskers inside the filaments also orient the same direction perpendicular to the page. The EDS results reveal the formation of SiCxOy phase derived from the PCS. The partial oxygen loading in SiC matrix is caused by the curing of PCS in air. Combined with the XRD results in Fig. 5, nano b-SiC grains are precipitated in amorphous SiC matrix. Hence, 3D SiCw/SiC lattices are composed of submicron b-SiCw and nano b-SiC particles dispersed in amorphous SiCxOy matrix. 3.5. Mechanical properties Fig. 10 shows the bending and compression strength of 3D SiCw/ SiC lattices, and the fracture morphology of a broken filaments. The
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Fig. 6. Photos of the printed and pyrolyzed 3D SiCw/PCS lattices at 1200 C. The complex structure are maintained after the heat-treatment. SiCw/PCS-based suspension with mass loading of 62.3 vol% is used. The printing height and speed are 0.15 mm and 14 mm/s, respectively. The diameter of nozzle is 160 mm. The gas pressure is 65 psi.
bending and compression strength of 3D-lattices are 33.2 ± 5.2 MPa and 30.6 ± 4.3 MPa, respectively. Two set of load-displacement curves in Fig. 10 (a) refer to the same samples by DIW of
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suspensions with solid loading of 62.3 vol%. The fracture of filaments can be evaluated by the final strength of lattices divided by the effective fracture area ratio of fractured filaments. Based on Fig. 9, the bending strength of a single filament was evaluated to be around 105 MPa. During the three point bending test, only the filaments across over the span pillars are broken up, as shown in Fig. 9(d). Fig. 10(b) shows the fracture morphology of the pyrolyzed filaments. Whiskers are pulled out during the fracture and lead to satisfactory strength. Therefore, much more energy is consumed during the bending test, resulting in satisfactory strength. During the compression test, the non-linear stage occurs indicating the supporting strength from the whiskers. This phenomena is usually known as pseudo-ductility, which is also reported in fibrereinforced composites [24,37]. General, the curve of bending strength exhibits the feature of brittle failure. Only a limited amount of whiskers are pulled out, resulting in limited strength of the 3D SiC. On the contrary, for the compression test, the effect of whiskers works, resulting in graceful failure of the curve. According to the morphology in Fig. 9(a), the porosity of 3D SiCw/SiC lattices is around 62.3%. The mechanical properties of 3D SiCw/SiC lattices are compared with SiC-based composites in other works, as shown in Table 2. When using the same method of DIW, strength of the SiCw/SiC lattices in this work is much higher than that of Cf/SiC with a higher porosity of 75%. The Cf/SiC composite via DIW and PIP show a much higher strength over 100 MPa, owing to a much lower porosity of 25.9e34.8%. For composite with the similar porosity of around 60%, mechanical properties of SiCw/SiC lattices here show advantage over those of the SiC-based composites prepared via foam replication or powder metallurgy. Nevertheless, carbon fibre or whisker-reinforced SiC by freeze casting show higher strength but a slightly lower compression strength. Therefore, the prepared 3D SiCw-SiC porous ceramics by DIW show potential application, such as catalysis supports, filters, or thermal insulators, which require porous structures able to combine structural stability with high surface area.
Fig. 7. SEM images of SiCw/SiC lattices under different printing height or using suspensions with different solid loading. The diameter of nozzle is 160 mm and the printing speed is 14 mm/s (a-c, f) Lattices using SiCw/PCS-based suspensions with the solid loading of 62.3 vol%. The printing height of (aec) and (f) is 0.60 mm, 0.50 mm, 0.40 mm and 0.30 mm, respectively. (def) Lattices using SiCW/PCS-based suspensions with solid loading of 57.7 vol%, 59.9 vol% and 62.3 vol%. Low viscosity of the suspensions leads to the deformation of filaments after deposition. (hej) Enlarged views of the red squares in (def). Suspensions with increased viscosity are beneficial for obtaining filaments with aligned whiskers. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 8. Schematic illustration of the morphology of extruded filaments with different printing height. Higher printing height results in the larger circle path of filaments.
Fig. 9. Microstructure and composition of SiCw/SiC lattices using SiCw/PCS-based suspensions with the solid loading of 62.3 vol%. The diameter of nozzle is 160 mm and the printing height and speed is 0.3 mm and 0.14 mm/s, respectively. The layer distance is kept at 0.15 mm. (aec) Surface morphology with different magnifications. (def) Fracture morphology with different magnifications; inset of (e1) is the EDS result. (h) TEM views of SiCw and SiC matrix. (i, j) High resolution TEM of SiCw and PCS-derived SiC.
4. Conclusions 3D SiC ceramics containing aligned SiC whiskers were fabricated via the direct ink writing of SiCw/PCS-based suspensions. Printability was evaluated by exploring the morphology evolution and dimensions of filaments concerning different printing height and speed. Microstructure and mechanical properties of SiCw/SiCbased lattices were discussed. Conclusions was summarized as follows: (1) With the adjustment of print height and speed, four-types morphology of filaments were obtained, which were
accumulation, coiling, meandering and straight, respectively. SiCw/PCS/N-hexane suspensions acted like the pure polymer suspensions, exhibiting the expanding feature after the extrusion from the nozzle. (2) Higher printing height changed the morphology of 3D SiCw/ SiC lattices from straight to meandering, and then coiling. Low solid loading of SiCw/PCS-based suspension caused the deformation of the filaments, resulting in the weak direction of SiCw. 3D lattices with straight filaments and good retention of shape could be achieved only when applying a suitable print parameters for SiCw/PCS suspensions with proper viscosity.
H. Xiong et al. / Journal of Alloys and Compounds 809 (2019) 151824
9
Fig. 10. (a) Bending and compressive strength curves of 3D SiCw/SiC lattices with the solid loading of 62.3 vol%. The printing speed and height are 14 mm/s and 0.3 mm, respectively. The typical morphology of tested samples can be referred to images of Fig. 8. (b) Fracture morphology of a filament, in which the whisker pullout is confirmed.
Table 2 Comparision of properties of SiC-based composites using different methods. Composition Starting materials
Methods
Porosity (%) Bending strength (MPa) Compression strength (MPa) Ref.
SiC SiC Si3N4/SiC Cf/SiC Cf/SiOC Cf/SiC
Foam replication Powder metallurgy Freeze casting Freeze casting DIW DIW þ PIP (5 times)
63.7 57 43 58 75 25.9e34.8
9.74 13 80.5 63 ± 21 3.8 ± 1.2 (84e123)±10
19.8 ± 0.7 26 e 20 ± 11 4 e
[38] [39] [40] [41] [23] [24]
DIW
62
33.2 ± 5.2
30.6 ± 4.3
This work
SiCw/SiC
SiC @ silicone resin powders Nano b-SiC powders ɑ-SiC powders, Al2O3, Y2O3 Carbon fibre, ɑ-SiC powders poly(methyl-silsesquioxane), carbon fibre, SiC powders Short carbon fibre, ɑ-SiC powders Polycarbosilane, b-SiC whiskers
(3) For the 3D SiCw/SiC lattices with the porosity of 62%, the bending and compression strength were 33.2 ± 5.2 MPa and 30.6 ± 4.3 MPa, respectively. The satisfactory properties of 3D SiCw/SiC lattices were contributed by the pullout or broken of the highly aligned whiskers.
[9]
[10] [11]
Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51672311 and 51704335), Science and Technology Project of Hunan Province, China (No. 2016WK2022) and supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. This research was also supported by the Open Sharing Fund for the Large-scale Instruments and Equipments of Central South University. References [1] A. Zocca, P. Colombo, C.M. Gomes, J. Gunster, Additive manufacturing of ceramics: issues, potentialities, and opportunities, J. Am. Ceram. Soc. 98 (2015) 1983e2001. [2] J.W. Halloran, Ceramic stereolithography: additive manufacturing for ceramics by photopolymerization, Annu. Rev. Mater. Res. 46 (2016) 19e40. [3] J. Wilkes, Y.C. Hagedorn, W. Meiners, K. Wissenbach, Additive manufacturing of ZrO2-Al2O3 ceramic components by selective laser melting, Rapid Prototyp. J. 19 (2013) 51e57. [4] K. Shahzad, J. Deckers, Z.Y. Zhang, J.P. Kruth, J. Vleugels, Additive manufacturing of zirconia parts by indirect selective laser sintering, J. Eur. Ceram. Soc. 34 (2014) 87e95. [5] B.Y. Nan, X.W. Yin, L.T. Zhang, L.F. Cheng, Three-dimensional printing of Ti3SiC2-based ceramics, J. Am. Ceram. Soc. 94 (2011) 969e972. [6] H.Y. Xing, B. Zou, S.S. Li, X.S. Fu, Study on surface quality, precision and mechanical properties of 3D printed ZrO2 ceramic components by laser scanning stereolithography, Ceram. Int. 43 (2017) 16340e16347. [7] J.A. Lewis, Direct ink writing of 3D functional materials, Adv. Funct. Mater. 16 (2010) 2193e2204. [8] C. Zhu, T.Y. Han, E.B. Duoss, A.M. Golobic, J.D. Kuntz, C.M. Spadaccini,
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[23] G. Franchin, L. Wahl, P. Colombo, Direct ink writing of ceramic matrix composite structures, J. Am. Ceram. Soc. 100 (2017) 4397e4401. [24] Y. Xia, L. Zhang, J. Cao, K. Miao, J. Li, D. Li, Microstructure and mechanical property of Cf/SiC core/shell composite fabricated by direct ink writing, Scr. Mater. 165 (2019) 84e88. [25] G. Franchin, H.S. Maden, L. Wahl, A. Baliello, M. Pasetto, P. Colombo, Optimization and characterization of preceramic inks for direct ink writing of ceramic matrix composite structures, Materials 11 (2018) 515e529. [26] Y. Luo, S. Zheng, S. Ma, C. Liu, X. Wang, Mullite-bonded SiC-whiskerreinforced SiC matrix composites: preparation, characterization, and toughening mechanisms, J. Eur. Ceram. Soc. 38 (2018) 5282e5293. [27] X. Zhang, L. Xu, S. Du, W. Han, J. Han, C. Liu, Thermal shock behavior of SiCwhisker-reinforced diboride ultrahigh-temperature ceramics, Scr. Mater. 59 (2008) 55e58. [28] X. Zhao, J. Yang, H. Xin, X. Wang, L. Zhang, F. He, Q. Liu, W. Zhang, Improved dispersion of SiC whisker in nano hydroxyapatite and effect of atmospheres on sintering of the SiC whisker reinforced nano hydroxyapatite composites, Mater. Sci. Eng. C 91 (2018) 135e145. [29] H. Xiong, H. Chen, L. Zhao, Y. Huang, K. Zhou, D. Zhang, SiCw/SiCp reinforced 3D-SiC ceramics using direct ink writing of polycarbosilane-based solution: microstructure, composition and mechanical properties, J. Eur. Ceram. Soc. 39 (2019) 2648e2657. [30] J.P. Lewicki, J.N. Rodriguez, C. Zhu, M.A. Worsley, A.S. Wu, Y. Kanarska, J.D. Horn, E.B. Duoss, J.M. Ortega, W. Elmer, R. Hensleigh, R.A. Fellini, M.J. King, 3D-printing of meso-structurally ordered carbon fibre/polymer composites with unprecedented orthotropic physical properties, Sci. Rep. 7 (2017) 43401. [31] J. Liao, H. Chen, H. Luo, X. Wang, K. Zhou, D. Zhang, Direct ink writing of zirconia three-dimensional structures, J. Mater. Chem. C 5 (24) (2017) 5867e5871.
[32] X. Cheng, Steady-state rheological characterizations of polycarbosilane melts, Polym. Mater. Sci. Eng. 25 (7) (2009) 53e56. [33] H. Yuk, X. Zhao, A new 3D printing strategy by harnessing deformation, instability, and fracture of viscoelastic inks, Adv. Mater. 30 (2018) 1704028. [34] M. Fukushima, P. Colombo, Silicon carbide-based foams from direct blowing of polycarbosilane, J. Eur. Ceram. Soc. 32 (2) (2012) 503e510. [35] C. Stabler, E. Ionescu, M. Graczyk-Zajac, I. Gonzalo-Juan, R. Riedel, Silicon oxycarbide glasses and glass-ceramics:“All-Rounder”materials for advanced structural and functional applications, J. Am. Ceram. Soc. 101 (2018) 4817e4854. [36] K. Kakimoto, F. Wakai, Synthesis of SieCeO bulk ceramics with various chemical compositions from polycarbosilane, J. Am. Ceram. Soc. 82 (1999) 2337e2341. ~ on, V.G. Rocha, S. Barg, M.D. Farin ~ as, T.G. Alvarez[37] C. Ferraro, E. Garcia-Tun Arenas, G. Sernicola, F. Giuliani, E. Saiz, Light and strong SiC networks, Adv. Funct. Mater. 26 (2016) 1636e1645. [38] H. Tian, Q. Ma, Y. Pan, W. Liu, Effects of inert filler addition on the structure and properties of porous SiOC ceramics derived from silicone resin, Ceram. Int. 38 (2012) 5039e5043. [39] P. Wan, Z. Wu, H. Zhang, L.Y. Gao, J. Wang, Porous nano-SiC as thermal insulator: wisdom on balancing thermal stability, high strength and low thermal conductivity, Mater. Res. Lett. 4 (2016) 104e111. [40] Q. Liu, F. Ye, Z. Hou, S. Liu, Y. Gao, H. Zhang, A new approach for the net-shape fabrication of porous Si3N4 bonded SiC ceramics with high strength, J. Eur. Ceram. Soc. 33 (2013) 2421e2427. [41] J. Cao, Z. Lu, K. Miao, H. Zhao, Y. Xia, F. wang, B. Lu, Fabrication of highstrength porous SiC-based composites with unidirectional channels, J. Am. Ceram. Soc. (2019), https://doi.org/10.1111/jace.16332.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
3D SiC containing uniformly dispersed, aligned SiC whiskers: Printability, microstructure and mechanical properties Huiwen Xiong a, Lianzhong Zhao a, Hehao Chen a, Xiaofeng Wang a, b, **, Kechao Zhou a, Dou Zhang a, * a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China School of Materials Science and Engineering, Central South University, Changsha, 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2019 Received in revised form 10 August 2019 Accepted 11 August 2019 Available online 12 August 2019
3D SiC-based lattices with aligned SiC whiskers were prepared via direct ink writing of SiCw/polycarbosilane (PCS)-based suspensions. Suspensions with different solid loading were prepared and used to investigate the morphology and printability of 3D SiCw/SiC lattices. Filaments with the morphologies of accumulation, coiling, meandering and straight were achieved with different printing height and speed. Higher printing height favored the formation of coiled filaments with longer periodic structure and increased amplitude. 3D lattices with straight filaments and good retention of shape were achieved only when applying a suitable printing parameters for SiCw/PCS-based suspensions with a solid loading of 62.3 vol%. The resulted 3D lattices with the porosity around 62% showed impressive bending and compression strength of 33.2 ± 5.2 MPa and 30.6 ± 4.3 MPa, respectively. The satisfactory properties of 3D SiCw/SiC lattices were contributed by the pullout of the aligned SiC whiskers. Furthermore, this 3D technique via the Polymer-derived Ceramics (PDCs) route allows the fabrication of large sized samples, which opens new opportunities for the fabrication of 3D structures with unique mechanical and functional properties in practical dimensions. © 2019 Elsevier B.V. All rights reserved.
Keywords: 3D SiC Direct ink writing Preceramic polymer Polycarbosilane
1. Introduction Additive manufacture of ceramics attracts lots of attentions, owing to its high efficiency, high accuracy of complicated structure and ability for tailoring the microstructure and composition with multifunctional properties [1e3]. To date, various 3D printing methods are developed, including selective laser sintering (SLS) [4], binder-based three dimensional printing (3DP) [5], stereolithography (SL) technology [6] and direct ink writing (DIW) [7,8]. DIW is a method based on carefully designed suspensions, which are extruded through a nozzle and constructed on a plate according to the pre-designed CAD models [7,9]. Compared with other methods, DIW shows advantages for fabricate ceramics with high flexibility, low cost and large-scale production [7,9,10]. SiC-based ceramics are considered to be potential materials for
* Corresponding author. ** Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, China. E-mail addresses: [email protected] (X. Wang), [email protected] (D. Zhang). https://doi.org/10.1016/j.jallcom.2019.151824 0925-8388/© 2019 Elsevier B.V. All rights reserved.
many structural and wear applications, owing to their high chemical stability in extreme environments, high hardness, low density and satisfactory heat transfer performance [11e13]. However, pure SiC ceramic is hard to be sintered because of the strong covalent bonds between Si and C [14]. Although partial sintering aids with low melting points can accelerate the densification process, the obtained composites exhibit high relative density and improved mechanical strength at the expense of their high temperature properties [14,15]. Polymer derived ceramics (PDCs) are pyrolyzed at a low sintering temperature and exhibit impressive properties at ultra high temperature with respect to crystallization, oxidation, creep and phase separation temperature [16e18]. Moreover, preceramic polymers are, well known for the fabrication of the ceramic fibers and ceramic matrix composites [18], and recently attract attention for their application in preparing 3D ceramics via the DIW method [19e21]. Ceramics are usually very brittle and sensitive to the defects in the composites [13]. Therefore, for application as the load-carrying components, SiC fibre or carbon fibre reinforced SiC ceramics (SiCf/ SiC or Cf/SiC) are two typical composites with improved strength and fracture toughness [22]. Braided SiC or Carbon fibres are filled
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with SiC via the chemical vapor infiltration (CVI) or polymer impregnation pyrolysis (PIP) process. Compared with these conventional methods, DIW exhibits features of time-saving, better integrity, stronger design-ability, and better portability for other ceramic systems [7,8,10]. In order to achieve 3D SiC with satisfactory mechanical properties using DIW, short carbon fibers were used as the reinforced phases, which oriented along with the extruded direction due to the shear stress from the walls of nozzle [22e24]. However, micron SiCf or carbon fibres with millimeters in length were hardly to be extruded through a submillimeter nozzle. SiC whiskers exhibit high strength, and have been widely used as the reinforcements in ceramics [25e27]. Hence, SiC whiskers should be an ideal choice for preparing 3D-SiCw/SiC composites via the DIW. In this work, polycarbosilane (PCS) was used as the preceramic polymer of SiC. SiCw/ PCS-based suspensions with different solid loading were prepared and used for fabricating 3D SiCw/SiC ceramics. Although SiCw/SiCp reinforced 3D SiC were reported in previous work [28], the content of whisker was only 5 wt% (in related to that of the polycarbosilane (PCS)). Suspensions containing 20 wt% whiskers with aspect ratio over 20 were not very easy to extruded through a small nozzle with a diameter of 160 mm. Meanwhile, high amount of SiC whisker reinforced SiC was a attracting composites, owing to their well known strength and sometimes could be considered as a novel SiCf/ SiC composites [26]. Here, the printing process using PCS/n-hexane based suspensions containing 20 wt% SiCw exhibited smoothly and continuously, and capable for fabricating 3D structure over tens of layers. The reasons for the continuously printing of suspension with 60% solid content without clogging the nozzle and deformation (or slightly deformation) are divided into two parts. On the one hand, the suitable viscosity of the suspensions enable the successful extrusion, with rapid evaporation of the solvent. The rapid evaporation of n-hexane results in higher solid content of the suspension and also quickly increased elastic modulus of the filament without shear stress. On the other hand, addition of DVB plays a key role in the smooth printing without clogging, which avoids the sticking of residue suspensions around the nozzle. The printability of SiCw/ PCS-based suspensions was discussed, in related to the printing parameters and viscosity properties of suspensions. The microstructure, composition and mechanical properties of 3D SiC were investigated.
purchased from Sanzheng Polymer Materials Co., Ltd (Shanghai, China). Divinylbenzene (DVB) was provided by Shanghai Aladdin Bio-Chem Technology Co., Ltd (Shanghai, China). 2.2. Preparation of the SiCw/PCS-based suspensions Table 1 shows the composition of suspensions with various solid loading ranging from 52.2 vol% to 68.7 vol%. Here, 20 wt% SiCw (in related to the PCS) was added in order to obtaining high amount of whisker-reinforced composites. Fig. 1 shows the route of fabrication process for 3D SiCw/SiC-based composites and details can be found in previous work [29]. The suspensions were prepared as follows: Typically, 8g PCS, 1.6g SiCw, 15 g N-hexane and 0.02 g CH-3 were weighted and put into a Teflon bottle with a volume of 200 ml. Then, white ZrO2 balls with a diameter of 5 mm were added into the bottle; the weight of balls was kept at 125g. After that, the bottle was sealed and ball milled for 24 h, with a rotation speed of 80 r/ min. After milling, several short whiskers were formed and most of the whiskers have a aspect ratio over 20. Then, the suspensions were poured into a beaker and magnetic stirred in fume hoods. Meanwhile, 0.08g DVB was added into the beaker. Here, a small amount of DVB plays an important role in smooth printing and avoids the clogging of the nozzle. Hence, suspensions with solid
2. Experimental section 2.1. Materials Polycarbosilane (PCS, PCS161001, Suzhou cerafil. Ltd., China) with an average molecular weight of 1500 g/mol was utilized. Nhexane (Sinopharm chemical reagent Co. Ltd., China) was used as the solvent. b-SiC whiskers (SiCw, Sinet Advanced Materials Co., Ltd., China) were used as the reinforcements. The whiskers have an average diameter of 0.2 mm and aspect ratio over 30. The dispersant of SiC whiskers was an ester compound named CH-3, which was
Fig. 1. Schematic illustration of the fabrication process of 3D-SiCw/SiC composites using PCS-based suspensions. DVB means the divinylbenzen, which is used for achiving a smooth printing process for SiCw/PCS-based suspensions.
Table 1 Nominal composition of the SiCw/PCS-based suspensions with different solid loading (vol %). Samples
Polycarbosilane (PCS)
N-hexane
SiC whiskers (SiCw)
Solid loadinga (vol%)
A B C D E
63.8 58.3 56.1 54.0 48.9
31.8 37.7 40.0 42.3 47.8
4.4 4.0 3.9 3.7 3.3
68.7 62.3 59.9 57.7 52.2
a Calculated by the volume ratio of V(PCSþSiCw)/V(N-hexaneþPCSþSiCw). Density of PCS, SiCw and n-hexane are 1.1, 3.2 and 0.66 g/cm3, respectively. A rather small amount of DVB is not considered here, which is added after the ball milling process. The volume of CH-3 is neglected and its mass content equals 2% of the whiskers.
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loading ranging from 52.2 vol% to 68.7 vol% were achieved with the evaporation of N-hexane. Then, the suspension was moved into a syringe with a volume of 10 ml. In order to remove the residue gases in the suspension, the syringe was put into a centrifuge with a rotation speed of 2000 r/min for 1 min. Finally, the suspension sealed in the syringe was ready for DIW. 2.3. Direct ink writing The suspensions were extruded from the syringe under the nitrogen pressure and deposited on a glass plate, using a deposition machine (DR2203, EFD Inc., East Province, RI). Two types of syringes were used with the diameters of 160 mm and 210 mm of the nozzle. Here, the nitrogen pressure was ranging from 50 psi to 70 psi. The printing height meant the distance from the nozzle tip to the glass plate, which was adjusted from 0.1 mm to 0.8 mm, respectively. The printing speed was the horizontal movement speed of the nozzle, which was ranging from 8 mm/s to 30 mm/s. The printing process was as follows: the filaments were extruded out from the nozzle, which were then deposited on the glass plate. The geometric structure was determined by the predesigned CAD mode. After the formation of the first layer, the nozzle was lifted along the Z direction; the second layer was then formed with a spanning feature. After the deposition of filaments layer by layer, 3D PCS-based lattices were achieved. After that, the lattices were dried over night in the ambient condition before separating from the glass plate. Then, the 3D PCS-based lattices were cured in air at 200 C for 4 h. The pyrolysis process was as follows: the samples were firstly heated to 450 C with a heating rate of 2 C/min and held for 2 h. The sintering atmosphere was pure Ar. Then, the temperature went up to 700 C by 1 C/min and held for 2 h, in order to accomplish the ceramic conversion. After that, the samples were heated to 800e1400 C for 2 h, with a fast heating rate of 5 C/min and maintained for 2 h. Finally, the black SiCw/SiC composites were obtained by heating the cured lattices at 1200 C for 1 h. 2.4. Characterization The viscosity and elastic modulus of suspensions were tested at 25 C using a rotational rheometer (AR2000EX, TA instruments, USA). The shear rate ranged from 0.1 to 500 1/s and the shear stress was from 0.1 Pa to 1000 Pa. The TG and DSC of the cured lattices were explored in pure Ar using a thermal analyzer (Setsys evolution
3
TGA-DTA/DSC, Setlaram instrument Ltd., France), with a heating rate of 10 C/min. The macrophotograph of extruded filaments or 3D SiCw/SiC lattices was taken using a stereomicroscope (SZX10, Nikon Co. Ltd., Japan). The SEM and TEM of SiCw/SiC lattices were obtained using a field emission scanning electron microscope (Nova NanoSEM 230, FEI, USA) and a transmission electron microscope (Technai G2 20S-Twin, FEI, USA). The Fourier transform infra-red (FTIR) spectra was tested using an infrared spectrometer (Nicolet 6700, Thermo Scientific, USA). The XRD results was obtained using a X ray diffraction (D/max 2550, Riguka, Japan). The density was measured by the Archimedes method. The bending strength and compression strength were tested using a MTS biomechanical testing machine (23 MTS Insight, MTS, USA), according to the Chinese standard of GB/T 9341-2008 and GB/T 47401999, respectively. The testing samples have a size of 25 mm 6 mm 7.5 mm, with a porosity around 62 vol%. 3. Results and discussion 3.1. Viscosities of the SiCw/PCS-based suspensions Fig. 2 shows the rheological properties of SiCw/PCS suspensions with solid loading from 68.7 vol% to 52.2 vol%, respectively. All suspensions exhibit the shear thinning behavior, which ensures the successful extrusion process. With increasing the solid loading from 52.2 vol% to 68.7 vol%, the viscosities at zero shear rate increase from 6.6 Pa S to 2375 Pa S, respectively. For the high solid loading of 68.7 vol%, viscosity property of SiCw/PCS/n-hexane suspension belongs to the feature of Carreau model of pure polymer [30]. With decreasing the solid loading, the effect of SiC whiskers plays a more important role in rheological behavior, resulting in shear thinning behavior of typical ceramic suspension. The test condition for the viscosity and elastic modulus of all suspensions was the same. Fig. 2(a) and (b) are not measured simultaneously, which are grouped differently. Compared to the suspensions without SiCw, the SiCw/PCS suspensions exhibit higher viscosity and also much higher elastic modulus. Fig. 2(b) shows the elastic modulus of suspensions as a function of shear stress. The elastic modulus is greatly affected by the solid loading of SiCw/PCS/ n-hexane suspension. Compared with the ceramic suspensions [31], the elastic modulus of PCS-hexane system is rather small, indicating the deformation of filaments after extrusion. With increasing the solid loading from 52.2 vol% to 68.7 vol%, the elastic modulus (G0 ) at 0.1 Pa of the shear stress (t) sharply increases from
Fig. 2. (a) Viscosities of the suspensions with different solid loading as a function of shear rate. The black data of 64.3 vol% belong to that of pure PCS-based suspension without whiskers. (b) Logelog plot of the related elastic modulus as a function of increasing shear stress. The black data of G0 without whiskers are not shown, owing to their rather small value (less than 0.1).
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2.41 Pa to 204 Pa, respectively. For suspensions with the solid loading of 62.3 vol%, the elastic modulus of suspension increased when the stress increased to 1000 Pa. The reason is that the break of the balance during the elastic modulus test when with a high shear stress. The sealed surface covered by methyl-silicone oil can sometime be destroyed, resulting in the increased solid loading of suspension caused by the rapid evaporation of n-hexane. When the shear stress (t) is lower than the shear yield stress (ty), the elastic modulus (G0 ) is relative stable, owing to the twining and aggregation of polymer molecules [32]. However, higher shear stress over the ty results in the disaggregation of polymer molecules, then sharply decreases the elastic modulus (G0 ). From the rheological study of silicon carbide suspensions, the most suitable solid loading for the direct ink writing should be around 62.3 vol%, with moderate viscosity and also high elastic modulus. Hence, the rapid evaporation of n-hexane on the surface of extruded filaments will greatly increase the G’ in situ, making the 3D shaping of SiCw/PCSbased filaments possible. Both the whiskers and the polymers affect the viscosity and elastic modulus of the suspensions, especially for the solid loading of 59.9 vol%. Therefore, effects from both whiskers and polymers lead to the different changes for suspensions of 59.9 vol%. We hold that the whisker alignment or interlocking plays a more important role for the suspensions with a relative low solid loading. In turn, the feature of the polymer shows up: the twinning and aggregation. In addition, in suspension with the solid loading of 59.9 vol%, the small peak generation around 10e100 Pa should be related to the twist of the SiCw dispersed in the solution. With a low shear stress of 10 Pa, the moving speed of SiCw on the surface and inside the suspension differs, leading to an increased friction resistance and also different orientation of the whiskers. Therefore, the friction
resistance and twist of SiC whiskers should be the reason for the sudden increase of elastic modulus. 3.2. Printability Fig. 3 shows the views of printed filaments using 59.9 vol% suspension with different printing height and speed. The diameter of the nozzle is 210 mm. The printing height and speed are ranged from 0.1 mm to 0.8 mm, and 14 mm/s to 30 mm/s, respectively. The morphology of the filaments can be divided into four typical, which are accumulation, coiling, meandering and straight, respectively. Low printing speed and height lead to the accumulation of the filaments. High printing height favors the coiling morphology. With the increasing of printing speed or height, the filaments are deposited based on a larger circle. Hence, the increased displacement along the XeY plane lead to the formation of views of meandering. When the speed reaches the critical value of 24 mm/s, the extrusion rods seem straight with various printing height. Further increasing the speed over 24 mm/s leads to straight filaments with decreased diameters. It is noteworthy that printing speed over 30 mm/s or height higher than 0.8 mm make the extruded rods out of shape. In fact, the relationship between specific rate of flow and printing speed can be expressed as follows [33]:
Q ðPÞ ¼
pnðaDÞ2 4
(1)
of which the Q(P) is the extrusion volume per unit time related to extrusion pressure, v is the printing speed, a is the expansion coefficient of suspension during extrusion and D is the diameter of the
Fig. 3. The diagram of the printed filaments' patterns with various selections of printing speed and printing height (top view), the optical images of four typical printed filaments' patterns (below view), scale bars are 2 mm. The gas pressure is 65 psi. The diameter of the nozzle is 0.21 mm and the solid loading of SiCw/PCS-based suspension is 59.9 vol%.
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nozzle, respectively. Hence, the printing speed can be calculated for a given suspension with certain gas pressure and size of nozzle. This calculated speed here is the “intrinsic printing speed”, which plays an important role in obtaining the straight filaments for the DIW process. Only when the printing speed matches the specific rate of flow can the straight shape of the filaments be presented. For the SiCw/PCS-based suspension with a solid loading of 59.9 vol%, the intrinsic printing speed, as shown in Fig. 3, is around 24 mm/s when the gas pressure and diameter of the nozzle are 65 psi and 210 mm, respectively. When the printing speed is lower than the intrinsic value, various morphology of filaments can be achieved via the adjustment of the printing height. However, when the printing speed is higher than 24 mm/s, the filaments are straight or out of shape with various printing height. Overall, the morphology of printed filaments is determined by the properties of suspensions, printing parameters and shape of nozzle. Four typical morphology of filaments, i.e. accumulation, coiling, meandering and straight, can be obtained here, via the controlling of printing speed and height. The printed size and morphology of 3D SiCw/SiC lattices are investigated using two CAD structure with the size and span length of (30 mm 6 mm 3 mm, 1 mm) and (31 mm 7 mm 3 mm, 0.5 mm), respectively. Fig. 4 (a) and (b) are the 3D models and Fig. 4 (c) and (d) are the corresponding SEM images. 3D lattices with regular structure and spanning features are confirmed, which indicates that the SiCw/PCS-based suspensions are suitable for the DIW process. Fig. 4(e) shows the measured size of 3D SiCw/SiC lattices. The diameter of the filaments is around 0.26 mm, which is higher than the size of nozzle (0.21 mm). Therefore, the SiCw/PCSbased suspensions act like the pure polymer suspension, exhibiting the expanding feature after the extrusion [28].
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the absorb water. At around 380 C, a weight loss of 3.54% occurs, owing to the releasing of H2 and CH4 during the self-linking of PCS [18,35]. However, no peaks below 600 C can be found in the DSC curve. The ceramic conversion takes place at 697.4 C, with a fast decreasing rate in weight. Further increasing temperature from 800 C to 1200 C, the weight loss is less than 2%. Two peaks at 987.8 C and 1157.8 C can be observed in the DSC curve. The reaction at 987.8 C indicates the side chains (C-H2 bending in Si-CH2Si) are further decomposed, which agrees well with our previous work [29]. The crystallization of SiC occurs at 1157.8 C and nano SiC grains are formed in amorphous SiC matrix. After pyrolyzed at 1200 C, the mass content of SiCw in the final 3D SiC composites after pyrolyzed at 1200 C is around 24 wt%. Fig. 5(c) shows the XRD patterns of pure SiCw, cured and pyrolyzed SiCw/PCS lattices. Only b-SiC phase is found in all patterns. After pyrolyzed, the peaks of b-SiC are widened, indicating the amorphous or nano SiC derived from the cured PCS. Fig. 5(d) shows the linear shrinkage, weight loss and density of 3D SiCw/SiC lattices fired. With increasing the pyrolysis temperature from 600 C to 1400 C, the data mentioned above increases from 1.8%, 9.5% and 1.51 g/cm3 to 19.3%, 19.6% and 2.64 g/cm3, respectively. The pyrolyzed 3D SiC maintains their sound geometric structure and pore size even with a high linear shrinkage of 19.6%. The low heating rate of 1 C/min from 450 C to 700 C is very important and favors the uniform shrinkage of 3D structure during the ceramic conversion. Here, pyrolysis temperature of 1200 C is suitable for obtaining SiCw/SiC composites with satisfactory properties. Although higher temperature than 1200 C will increase the density, more micro pores can be formed due to the carbothermal reduction reaction occurring between Si-O and carbon in the matrix [36]. 3.4. Microstructure and composition
3.3. Crosslinking and pyrolysis Fig. 5(a) and (b) show the infrared spectra and TG/DSC results. After curing at 200 C for 4 h in air, the intensity of Si-H group decreases greatly. The Si-H group is oxidized in air and the 3D net work of PCS macro-molecules is gradually formed, which helps the 3D lattices keep their complex shape during the pyrolysis process [34]. After sintering at 1200 C, only Si-O, Si-C and residue O-H of water are observed, indicating the complete ceramic conversion. The weight loss of 2.55% around 200 C should be the escaping of
Fig. 6 shows the macro-photos of 3D SiCw/PCS lattices before and after the pyrolysis process. The suspensions are extruded through the nozzle under gas pressure and maintain the structure owing to the increased viscosity without the shear stress. Moreover, an image of the cross-section demonstrates that no sagging exists in the z-direction (see Fig. 9(d)). Over twenty layered structure are successfully printed, indicating the well printability of SiCw/PCS-based suspensions. After heat-treatment, 3D ceramics with sound structure are obtained, with regular lattice (as shown in
Fig. 4. (a) and (b) Two CAD structure of the 3D structures with the size of 30 mm 6 mm 3 mm and 31 mm 7 mm 3 mm, respectively. The diameter of nozzle is 210 mm. The inter-layer spacing are 0.2 mm with different span distance of 1 mm and 0.5 mm, respectively. (c) and (d) The corresponding SEM morphology of pyrolyzed 3D structures of (a) and (b). (e) The measured size of the printed filaments based on the SEM images.
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Fig. 5. (a) Infrared spectra of mixture, cured and pyrolyzed SiCw/SiC lattices at 1200 C. (b) TG and DSC curves of cured lattices. The heating rate is 10 C/min and the atmosphere is Ar. (c) XRD patterns of SiC whiskers, mixture, and 3D SiC scaffolds after sintering at 1200 C. (d) The linear shrinkage, weight loss and density of cured 3D SiCw/SiC lattices.
Fig. 6(c)). Fig. 7 shows the microstructure of 3D SiCw/SiC lattices with different printing height or using suspensions with different solid loading. With increasing the printing height from 0.20 mm to 0.60 mm, the morphology of filaments changed from straight into meandering and coiling. Higher printing height favors the formation of coiled lattices with longer periodic structure and increased amplitude. Besides the effect of printing parameters of height or speed, the viscosity of suspensions greatly affects the final morphology of filaments and determines the final printability. Fig. 8 displays the morphology of filaments with different printing height. Owing to the vibration during the printing, the filaments in the air always exhibit the morphology of a screw cone. Therefore, higher printing height leads to a larger circle path of the filaments. During the extrusion process with a certain printing speed, the morphology of the filaments shows periodic appearance with different amplitude and unit length. Fig. 7(def) display the SEM images of 3D SiCw/SiC lattices using suspensions with solid loading of 57.7 vol%, 59.9 vol% and 62.3 vol%, respectively. Suspensions with shear thinning behavior ensure the extrusion process, owing to their low viscosity under shear stress. Nevertheless, an unsatisfactory viscosity at zero shear stress leads to the deformation of the filaments, as shown in Fig. 7(d and e). Although the quick evaporation of n-hexane increases the elastic properties of suspension, the initial poor elastic modulus (G’z10) of the two suspensions (Fig. 2(b)) cannot maintain the shape after the extrusion. In order to investigate the distribution of SiC whiskers, the enlarged views on the surface of filaments are provided, as shown in Fig. 7(hej). The deformation of the filaments after
extrusion induces the rearrangement of the SiCw, resulting in the weak direction. Moreover, 3D SiCw/SiC lattice with highly aligned SiCw are achieved, as shown in Fig. 7(j). SiCw/SiC lattices with straight filaments are used to reveal the microstructure and composition, as shown in Fig. 9. Regular 3D structure with spanning feature are achieved using SiCw/PCS-based suspension with the solid loading of 62.3 vol%. After sintering of the lattices via a 160 mm-type nozzle, the interval and diameter of filaments are around 130 mm and 120 mm, respectively. High aligned SiCw can be found in Fig. 9(c), owing to shear stress along the extrusion direction and also the satisfactory viscosity for maintaining the shape of filaments. Fracture morphology of lattices is advantage for investigating the distribution of whiskers along the radial direction. No pores or cracks are observed on the surface or in the filaments. Therefore, dense SiCw/SiC composites can be achieved via the DIW of SiCw/PCS-based suspensions. Meanwhile, whiskers inside the filaments also orient the same direction perpendicular to the page. The EDS results reveal the formation of SiCxOy phase derived from the PCS. The partial oxygen loading in SiC matrix is caused by the curing of PCS in air. Combined with the XRD results in Fig. 5, nano b-SiC grains are precipitated in amorphous SiC matrix. Hence, 3D SiCw/SiC lattices are composed of submicron b-SiCw and nano b-SiC particles dispersed in amorphous SiCxOy matrix. 3.5. Mechanical properties Fig. 10 shows the bending and compression strength of 3D SiCw/ SiC lattices, and the fracture morphology of a broken filaments. The
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Fig. 6. Photos of the printed and pyrolyzed 3D SiCw/PCS lattices at 1200 C. The complex structure are maintained after the heat-treatment. SiCw/PCS-based suspension with mass loading of 62.3 vol% is used. The printing height and speed are 0.15 mm and 14 mm/s, respectively. The diameter of nozzle is 160 mm. The gas pressure is 65 psi.
bending and compression strength of 3D-lattices are 33.2 ± 5.2 MPa and 30.6 ± 4.3 MPa, respectively. Two set of load-displacement curves in Fig. 10 (a) refer to the same samples by DIW of
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suspensions with solid loading of 62.3 vol%. The fracture of filaments can be evaluated by the final strength of lattices divided by the effective fracture area ratio of fractured filaments. Based on Fig. 9, the bending strength of a single filament was evaluated to be around 105 MPa. During the three point bending test, only the filaments across over the span pillars are broken up, as shown in Fig. 9(d). Fig. 10(b) shows the fracture morphology of the pyrolyzed filaments. Whiskers are pulled out during the fracture and lead to satisfactory strength. Therefore, much more energy is consumed during the bending test, resulting in satisfactory strength. During the compression test, the non-linear stage occurs indicating the supporting strength from the whiskers. This phenomena is usually known as pseudo-ductility, which is also reported in fibrereinforced composites [24,37]. General, the curve of bending strength exhibits the feature of brittle failure. Only a limited amount of whiskers are pulled out, resulting in limited strength of the 3D SiC. On the contrary, for the compression test, the effect of whiskers works, resulting in graceful failure of the curve. According to the morphology in Fig. 9(a), the porosity of 3D SiCw/SiC lattices is around 62.3%. The mechanical properties of 3D SiCw/SiC lattices are compared with SiC-based composites in other works, as shown in Table 2. When using the same method of DIW, strength of the SiCw/SiC lattices in this work is much higher than that of Cf/SiC with a higher porosity of 75%. The Cf/SiC composite via DIW and PIP show a much higher strength over 100 MPa, owing to a much lower porosity of 25.9e34.8%. For composite with the similar porosity of around 60%, mechanical properties of SiCw/SiC lattices here show advantage over those of the SiC-based composites prepared via foam replication or powder metallurgy. Nevertheless, carbon fibre or whisker-reinforced SiC by freeze casting show higher strength but a slightly lower compression strength. Therefore, the prepared 3D SiCw-SiC porous ceramics by DIW show potential application, such as catalysis supports, filters, or thermal insulators, which require porous structures able to combine structural stability with high surface area.
Fig. 7. SEM images of SiCw/SiC lattices under different printing height or using suspensions with different solid loading. The diameter of nozzle is 160 mm and the printing speed is 14 mm/s (a-c, f) Lattices using SiCw/PCS-based suspensions with the solid loading of 62.3 vol%. The printing height of (aec) and (f) is 0.60 mm, 0.50 mm, 0.40 mm and 0.30 mm, respectively. (def) Lattices using SiCW/PCS-based suspensions with solid loading of 57.7 vol%, 59.9 vol% and 62.3 vol%. Low viscosity of the suspensions leads to the deformation of filaments after deposition. (hej) Enlarged views of the red squares in (def). Suspensions with increased viscosity are beneficial for obtaining filaments with aligned whiskers. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 8. Schematic illustration of the morphology of extruded filaments with different printing height. Higher printing height results in the larger circle path of filaments.
Fig. 9. Microstructure and composition of SiCw/SiC lattices using SiCw/PCS-based suspensions with the solid loading of 62.3 vol%. The diameter of nozzle is 160 mm and the printing height and speed is 0.3 mm and 0.14 mm/s, respectively. The layer distance is kept at 0.15 mm. (aec) Surface morphology with different magnifications. (def) Fracture morphology with different magnifications; inset of (e1) is the EDS result. (h) TEM views of SiCw and SiC matrix. (i, j) High resolution TEM of SiCw and PCS-derived SiC.
4. Conclusions 3D SiC ceramics containing aligned SiC whiskers were fabricated via the direct ink writing of SiCw/PCS-based suspensions. Printability was evaluated by exploring the morphology evolution and dimensions of filaments concerning different printing height and speed. Microstructure and mechanical properties of SiCw/SiCbased lattices were discussed. Conclusions was summarized as follows: (1) With the adjustment of print height and speed, four-types morphology of filaments were obtained, which were
accumulation, coiling, meandering and straight, respectively. SiCw/PCS/N-hexane suspensions acted like the pure polymer suspensions, exhibiting the expanding feature after the extrusion from the nozzle. (2) Higher printing height changed the morphology of 3D SiCw/ SiC lattices from straight to meandering, and then coiling. Low solid loading of SiCw/PCS-based suspension caused the deformation of the filaments, resulting in the weak direction of SiCw. 3D lattices with straight filaments and good retention of shape could be achieved only when applying a suitable print parameters for SiCw/PCS suspensions with proper viscosity.
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Fig. 10. (a) Bending and compressive strength curves of 3D SiCw/SiC lattices with the solid loading of 62.3 vol%. The printing speed and height are 14 mm/s and 0.3 mm, respectively. The typical morphology of tested samples can be referred to images of Fig. 8. (b) Fracture morphology of a filament, in which the whisker pullout is confirmed.
Table 2 Comparision of properties of SiC-based composites using different methods. Composition Starting materials
Methods
Porosity (%) Bending strength (MPa) Compression strength (MPa) Ref.
SiC SiC Si3N4/SiC Cf/SiC Cf/SiOC Cf/SiC
Foam replication Powder metallurgy Freeze casting Freeze casting DIW DIW þ PIP (5 times)
63.7 57 43 58 75 25.9e34.8
9.74 13 80.5 63 ± 21 3.8 ± 1.2 (84e123)±10
19.8 ± 0.7 26 e 20 ± 11 4 e
[38] [39] [40] [41] [23] [24]
DIW
62
33.2 ± 5.2
30.6 ± 4.3
This work
SiCw/SiC
SiC @ silicone resin powders Nano b-SiC powders ɑ-SiC powders, Al2O3, Y2O3 Carbon fibre, ɑ-SiC powders poly(methyl-silsesquioxane), carbon fibre, SiC powders Short carbon fibre, ɑ-SiC powders Polycarbosilane, b-SiC whiskers
(3) For the 3D SiCw/SiC lattices with the porosity of 62%, the bending and compression strength were 33.2 ± 5.2 MPa and 30.6 ± 4.3 MPa, respectively. The satisfactory properties of 3D SiCw/SiC lattices were contributed by the pullout or broken of the highly aligned whiskers.
[9]
[10] [11]
Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51672311 and 51704335), Science and Technology Project of Hunan Province, China (No. 2016WK2022) and supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. This research was also supported by the Open Sharing Fund for the Large-scale Instruments and Equipments of Central South University. References [1] A. Zocca, P. Colombo, C.M. Gomes, J. Gunster, Additive manufacturing of ceramics: issues, potentialities, and opportunities, J. Am. Ceram. Soc. 98 (2015) 1983e2001. [2] J.W. Halloran, Ceramic stereolithography: additive manufacturing for ceramics by photopolymerization, Annu. Rev. Mater. Res. 46 (2016) 19e40. [3] J. Wilkes, Y.C. Hagedorn, W. Meiners, K. Wissenbach, Additive manufacturing of ZrO2-Al2O3 ceramic components by selective laser melting, Rapid Prototyp. J. 19 (2013) 51e57. [4] K. Shahzad, J. Deckers, Z.Y. Zhang, J.P. Kruth, J. Vleugels, Additive manufacturing of zirconia parts by indirect selective laser sintering, J. Eur. Ceram. Soc. 34 (2014) 87e95. [5] B.Y. Nan, X.W. Yin, L.T. Zhang, L.F. Cheng, Three-dimensional printing of Ti3SiC2-based ceramics, J. Am. Ceram. Soc. 94 (2011) 969e972. [6] H.Y. Xing, B. Zou, S.S. Li, X.S. Fu, Study on surface quality, precision and mechanical properties of 3D printed ZrO2 ceramic components by laser scanning stereolithography, Ceram. Int. 43 (2017) 16340e16347. [7] J.A. Lewis, Direct ink writing of 3D functional materials, Adv. Funct. Mater. 16 (2010) 2193e2204. [8] C. Zhu, T.Y. Han, E.B. Duoss, A.M. Golobic, J.D. Kuntz, C.M. Spadaccini,
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