Fabrication of SiC ceramic architectures using stereolithography combined with precursor infiltration and pyrolysis

Fabrication of SiC ceramic architectures using stereolithography combined with precursor infiltration and pyrolysis

Ceramics International 45 (2019) 14006–14014 Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/loc...

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Ceramics International 45 (2019) 14006–14014

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Fabrication of SiC ceramic architectures using stereolithography combined with precursor infiltration and pyrolysis

T

Rujie Hea,b,c,∗, Guojiao Dinga, Keqiang Zhanga, Ying Lia,b,∗∗, Daining Fanga,b a

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing, 100081, China Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and Structures, Beijing Institute of Technology, Beijing, 100081, China c State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Silicon carbide Stereolithography Additive manufacturing Precursor infiltration and pyrolysis

Stereolithography based additive manufacturing provides a new route to produce ceramic architectures with complex geometries. In this study, 3D structured SiC ceramic architectures were fabricated by stereolithography based additive manufacturing combined with precursor infiltration and pyrolysis (PIP). Firstly, photosensitive SiC slurry was prepared. Then, stereolithography was conducted to fabricate complex-shaped green SiC parts. Polymer burn-out was subsequently performed, and porous SiC preforms were produced. After that, precursor infiltration and pyrolysis was used to improve the density and strength. Finally, 3D-structured SiC ceramic architectures with high accuracy and quality were obtained. It is believed that this study can give some fundamental understanding for the additive manufacturing of SiC ceramic structures.

1. Introduction Silicon carbide (SiC) ceramic has attracted much attention due to its unique combination of mechanical, chemical, and thermal properties [1–3], finding its wide applications in aerospace [4], nuclear [5] and transportation areas [6]. Recently, various traditional shaping techniques, such as dry pressing [7], cold isostatic pressing [8], slip casting [9], injection molding [10], tape casting [11] and gel casting [12], have been developed to produce SiC ceramic with improved density, superior uniformity, and high performance reliability. However, these traditional forming techniques usually have difficulties in fabricating complex-shaped ceramic parts (internal holes, sharp corners, etc.) and components with high accuracy [13,14]. Besides, these above-mentioned processing techniques are also very time-consuming and highcost. Therefore, it is deemed necessary to develop a more effective and economical approach to form complex-shaped SiC ceramic. Fortunately, additive manufacturing (AM) techniques, based on a layer-by-layer building process, are rapidly gaining interest for manufacturing 3D-structured ceramic architectures without molds, and offer good alternative [15–19]. Many additive manufacturing techniques have been reported, such as 3D printing (3DP) [20,21], selective laser sintering (SLS) [22,23], direct ink writing (DIW) [24,25] and stereolithography [26–29]. Many kinds of ceramics have been prepared using these additive manufacturing techniques. However, the additive ∗

manufacturing of SiC ceramic has rarely been studied. Travizky et al. [30] prepared SiSiC composites from starch-cellulose powder by using 3D printing combined with liquid silicon infiltration (LSI). Compton et al. [31] fabricated SiC/C-filled ceramics by using 3D printing. Larson et al. [32] prepared 3D SiC structures via direct ink writing of borosiloxane-colloidal SiC ink blends. Tu [33] obtained SiC reticulated porous ceramics by 3D printing, gelcasting and liquid drying, and Chen et al. [34] prepared complicated silicon carbide ceramic components using combined 3D printing with gelcasting. However, 3D printing and direct ink writing always limit to their low manufacturing accuracy, which make them not suitable for fabricating engineering components with high accuracy. Jin et al. [35] prepared SiC ceramics by using selective lase sintering combined with cold isostatic pressing and polymer infiltration pyrolysis (PIP). Liu et al. [36] used laser additive manufacturing to fabricate homogeneous complicated shape SiC ceramic parts. Selective lase sintering usually needs high heat power, and inevitable high thermal gradient during processing usually induces undesired residual thermal stress and cracks. Among these reported additive manufacturing techniques, stereolithography, a more effective UV light-cured technology, exhibits a great potential for the fabrication of complex-shaped ceramic parts with high accuracy and quality [37,38]. Stereolithography is a processing method based on the photopolymerization of a photosensitive ceramic slurry. 3D model is imported with the processing parameters, including x-y resolution, layer

Corresponding author. Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing, 100081, China. Corresponding author. Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing, 100081, China. E-mail addresses: [email protected] (R. He), [email protected] (Y. Li).

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https://doi.org/10.1016/j.ceramint.2019.04.100 Received 25 March 2019; Received in revised form 8 April 2019; Accepted 11 April 2019 Available online 13 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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thickness and exposure time. Each individual layer is cured by a UV light selective scanning on the slurry. After the first layer is cured, the supporting platform is moved up, and the ceramic slurry is recoated with a blade. Then, the second layer is cured analogously. These steps are repeated until the whole green part is eventually produced. Al2O3 [27,29], ZrO2 [39-40], fused silica [41], hydroxyapatite [42,43] and other oxide dense ceramics [44–47] have been widely reported using this method in the past years. However, to the best of our knowledge, the stereolithography of SiC ceramic has not been reported so far. Besides, the SiC ceramic always exhibits a low relative density and low strength through additive manufacturing, therefore, it is essential to find a way to improve both the density and strength. In the past, both liquid silicon infiltration (LSI) [30] and precursor infiltration and pyrolysis (PIP) [35] have been reported to improve the density and strength of SiC ceramic. Especially, precursor infiltration and pyrolysis is the most using method to prepare large-scale complex-shaped SiC ceramic at low temperature and with high performance and low cost [48–51]. Jin et al. [35] used this method to improve the density and strength of selective lase sintering (SLS) prepared SiC ceramic successfully. It is thus believed that precursor infiltration and pyrolysis can also be used for improving the property of the stereolithography prepared SiC ceramic. Thus, the objective of this study was to validate the feasibility of manufacturing SiC ceramic through stereolithography based additive manufacturing. In this paper, SiC ceramic was fabricated using stereolithography combined with precursor infiltration and pyrolysis technique. Photosensitive SiC slurries with solid loading ranging 30–40 vol % were produced, and green SiC parts were fabricated using stereolithography. After that, polymer burn-out and precursor infiltration and pyrolysis were conducted. Finally, 3D structured SiC ceramic architectures were obtained. The purpose of this study is to demonstrate the feasibility of this novel process for the manufacturing of SiC ceramic, how to further achieve full dense SiC ceramic is not the focus of this study and has to be further investigated in more detail in our future work. 2. Experimental section 2.1. Raw materials Commercial SiC powders (purity > 99.5%; Beijing Zhongjinyan Co. Ltd., China) were used as raw material. Fig. 1 shows the particle size distribution and morphology of the raw SiC particles. The average particle size was ∼1.1 μm, and the particles were nearly in rod-shape. For stereolithography, 1,6-Hexanediol diacrylate (HDDA, Sinopharm Chemical Reagent Co., Ltd., China) and triethyleneglycol divinyl ether (DVE-3, Sinopharm Chemical Reagent Co., Ltd., China) were used as resin monomers. The volume ratio of HDDA to DVE-3 was chosen as 1:1. Free radical photoinitiator Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, Sinopharm Chemical Reagent

Fig. 2. Fabrication procedure of SiC ceramic using stereolithography combined with precursor infiltration and pyrolysis.

Co., Ltd., China) and cationic photoinitiator (4-Methylphenyl)[4-(2methylpropyl)phenyl] iodonium hexafluorophosphate (250, Sinopharm Chemical Reagent Co., Ltd., China) were used to trigger polymerization. KOS110 (Guangzhou Kang'oushuang Trade Co., Ltd., China) was used as dispersant for the dispersion of SiC particles. The doping content of TPO, 250 and KOS110 was set as 1, 1 and 5 wt% based on the slurry weight, respectively. For precursor infiltration and pyrolysis, polycarbosilane (PCS, Suzhou Saifei Group Co., Ltd., China) were used as pre-ceramic polymer, and Di-Vinyl-Benzene (DVB, Sinopharm Chemical Reagent Co., Ltd., China) as solvent. 2.2. Stereolithography SiC ceramic was fabricated using stereolithography based additive manufacturing combined with precursor infiltration and pyrolysis, as shown in Fig. 2. Firstly, SiC particles and HDDA-DVE3 monomers were weighed according to different solid loading, and then ball-milled for 6 h with zirconia ball media in a planetary mill (QM-3SP2, Nanjing

Fig. 1. (a) Particle size distribution and (b) morphology of raw SiC particles. 14007

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2.5. Characterization

Fig. 3. Schematic diagram of the stereolithography equipment: 1. Working stage; 2. blade; 3. SiC slurry; 4. Slurry tank, 5. UV light source; 6. Mirror; 7. Lifting frame; 8. SiC green body; 9. SiC layer; 10. Glass window. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Viscosity of the SiC slurry was determined using a rotational viscometer (NDJ-1B, Shanghai Pingxuan Instrument Co. Ltd., China). To access the curing ability of different ceramics under the UV light, the curing thickness was measured by using a digital micrometer (211-101, Anyi Instrument Co., Ltd., China) after erasing the uncured slurry carefully. Binder burnout behavior of the green body was determined using a thermogravimetric and differential thermal analysis (TG-DTA, Netzsch STA 449C, Germany) in N2 at a heating rate of 2 °C/min up to 1000 °C. Relative density after polymer burn-out, as well as relative density after precursor infiltration and pyrolysis were measured by Archimedes’ method in deionized water. Crystalline phase of the ceramic after pyrolysis at different temperature was characterized using a X-ray diffraction (XRD, Holland, CuKα = 1.5418 Å). Strength was measured via three-point bending tests on a universal mechanical testing machine (Instron Legend 2367 testing system, USA), using a loading span of 30 mm with a crosshead speed of 0.5 mm/min at room temperature. A minimum number of 5 specimens were tested to obtain average value. Microstructure was observed using a scanning electron microscopy (SEM, ZEISS EVO®18, Germany). 3. Results and discussion 3.1. Stereolithography

University Instrument Plant, China) at 400 rpm. After that, photoinitiators (TPD and 250) and dispersant (KOS110) were added to the slurry, and continued milling for 2 h. Finally, dispersed homogeneous photosensitive SiC slurry with different solid loading was obtained. Stereolithography was then conducted on the as-prepared photosensitive SiC slurry using a digital light processing stereolithography equipment (AutoCera, Beijing 10dim Tech., Co., Ltd., China), as shown in Fig. 3. Firstly, a 3D STL model was drawn using Solidworks and imported into the equipment, and the thickness for each slice layer was set as 50 μm. Then, the SiC slurry was poured into the tank, and homogeneous coated on a glass sheet using a blade. After that, the slurry was exposed to a UV light (Wavelength: 405 nm; Intensity: 7500 μw/cm2; Exposure time: 90 s), and cross-linked to form a single layer. Then, the working stage was moved upwards and the slurry was recoated on the sheet, continued to the solidification of next layer. After such cycles, green SiC ceramic part was obtained and removed from the working stage with caution.

2.3. Polymer burn-out

Stereolithography is based on the photopolymerization of photosensitive ceramic slurry. Therefore a photosensitive ceramic slurry with high solid loading and low viscosity is very important for the following additive manufacturing as well as pyrolysis and sintering. The approaches for obtaining photosensitive ceramic slurries have been widely reported in the past years. In our previous work [51], a Al2O3 ceramic slurry with ultra high solid loading (60 vol%) and low viscosity was successfully prepared. How to achieve an optimal SiC slurry was not the main focus of this study, and would be discussed in our other paper. In this study, SiC ceramic particles and HDDA-DVE3 monomers were ball-milled to produce photosensitive SiC slurries with solid loading of 30, 35 and 40 vol%. Photoinitiators (TPD and 250) and dispersant (KOS110) were added to the slurry to further produce homogeneous SiC slurries. Fig. 4 shows the viscosity of these SiC slurries. The viscosity of the SiC slurries with 30, 35 and 40 vol% was 951, 1250 and 1737 mPa s at a shear rate of 60 s−1, respectively. With the increasing of solid loading, the viscosity increased correspondingly. Certainly, solid loading could be further improved, however, this study rather aimed to propose a method to produce SiC ceramic structures than their property, therefore the effects of solid loading were not

After stereolithography, the green SiC body was pyrolyzed at 800 °C in N2 atmosphere using a tube-furnace (SK-G03123K, Tianjin Zhonghuan Lab Furnace Co., Ltd, China) to burn out the polymers among the green body. The sample was firstly heated to 800 °C at a heating rate of 0.5 °C/min, then soaked at 800 °C for 2 h, and finally cooled to room temperature naturally.

2.4. Precursor infiltration and pyrolysis In order to improve the density and strength, precursor infiltration and pyrolysis was conducted. Firstly, PCS-DVB precursor solution was prepared by mixing PCS and DVB with a ratio of 1:0.5 (wt./wt.). Then, the porous SiC preform after polymer burn-out was infiltrated into the solution under vacuum and cured at 60 °C for 1 h. After that, the sample was pyrolyzed at 1200 °C for 1 h in a graphite furnace (ZT-70-23Y, Shanghai Chenxin Furnace Co., Ltd., China) under an inert atmosphere. The precursor infiltration and pyrolysis process was conducted 8 cycles. Fig. 4. Viscosity of the photosensitive SiC slurries with different solid loading. 14008

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Fig. 5. Curing thickness of the photosensitive SiC slurries with different solid loading.

further investigated in detailed. For stereolithography, the viscosity of ceramic slurry should be lower than 5–10 Pa s as reported in our previous paper [52]. Therefore, these as-prepared SiC slurries were suitable for subsequent stereolithography based additive manufacturing. Owing to the high light absorbance value of SiC ceramic, the stereolithography of SiC is known to be very different. All the SiC slurries with 30, 35 and 40 vol% solid loading were exposure to the UV light with a wavelength of 405 nm for 90s. The curing thickness of the SiC slurries were measured by using a digital micrometer after erasing the uncured slurry carefully, as shown in Fig. 5. The curing thickness of the SiC slurries with 30, 35 and 40 vol% solid loading were about 75, 66 and 48 μm, respectively. It was found that the curing ability of the SiC slurry became much worse with the increasing of the solid loading. The reason for this phenomenon was that the reflection of the slurry became larger when the solid loading got larger, and thus the polymerization became more difficult. According to the ability of the 3D printer equipment in this study, each layer of the stereolithography was set as 50 μm in thickness, therefore, the curing thickness for 40 vol% SiC slurry did not reach up to this standard. Thus, the SiC slurry with 40 vol % would become not suitable for stereolithography based on this equipment. Using a new stereolithography equipment with higher light penetration ability and thicker layer, or finding some approaches to improve the curing ability of high solid loading slurry, will be discussed in our future work. Based on the as-obtained SiC slurries, stereolithography based additive manufacturing was conducted. In this work, both rod-like SiC samples for strength testing and 3D structured SiC ceramic architectures (pyramid and hollowed basket structures) were produced. Fig. 6 presents the stereolithography of two typical 3D structured SiC ceramic architectures: pyramid structure and hollowed basket structure, from 35 vol% solid loading slurry. Firstly, 3D STL models were drawn using Solidworks, the thickness for each slice layer was set as 50 μm. and then imported into the digital light processing based stereolithography equipment (Fig. 6a). Then, photosensitive SiC slurry was poured into the tank, and homogeneous coated on a glass sheet using a blade. After that, the slurry was exposed to a UV light to form a single layer, during the process the slurry was cross-linked (Fig. 6b–i, iii). The working stage was subsequently moved upwards and the slurry was recoated on the sheet, continued to the solidification of next layer (Fig. 6b–ii, iv). After such cycles, green SiC ceramic structures were obtained (Fig. 6c). It was found that the green 3D structured SiC ceramic architectures exhibited high accuracy and high quality. No obvious burrs, pores, cracks and breaks were detected. The stereolithography showed a great possibility for preparing 3D SiC ceramic

structures. Different solid loading induces different microstructure and mechanical property. Fig. 7 gives the relative green density and green strength of the green SiC ceramics produced by stereolithography. Owing to the bonding effects of resin networks, green SiC ceramics maintained high relative density. The relative green density of the green SiC ceramics made from 30, 35, 40 vol% slurries were 94.9, 94.3 and 91.0%, respectively. The ceramic was fabricated layer by layer, thus some few bubbles were difficult to eliminate during the process, resulting in about 5–10% pores among the ceramic bodies. Besides, the relative green density decreased with the increasing of solid loading, mainly because the viscosity increased with the increasing of solid loading. High viscosity induced less flowability during processing, and thus more bubbles locating along the layer boundary. Fig. 8 abc presents the microstructure of the green SiC ceramics made from 30, 35, 40 vol% slurries, respectively. Some visible pores were detected, and it was found that the pores became more with the increasing of solid loading, which was in agreement with the density results. From Fig. 7, it was also found that the green strength of the green SiC ceramics made from 30, 35, 40 vol% slurries were 28.7, 26 and 24 MPa, respectively. Actually, green strength mainly resulted from the bonding strength of the resin networks. According to previous report [51], the green strength of these green bodies were in the same level of resin cured by a UV light [53]. Besides, the green strength were not so high and exhibited a decreasing trend with the increasing of solid loading, due to the pores became more and more, which was also in good agreement with the density results and the microstructural observation. 3.2. Polymer burn-out In order to get SiC ceramic, polymers among the green body, including resin, dispersant, must be burnt out. TG-DTA analysis was used to evaluate the polymer burn-out behavior. Although the green SiC ceramics were prepared from different solid loading slurries, the composition among these green bodies were almost the same. It was believed that all these green SiC ceramics made from different solid loading slurries maintained a similar polymer burn-out behavior. Therefore, the green SiC ceramic made from 35 vol% solid loading slurry was taken for TG-DTA analysis. Fig. 9 shows the TG-DTA testing curves. The vaporization of the absorbed water (maybe absorption in air after printing) caused obvious exothermic DTA peak at 38 °C. Furthermore, the polymer additives were burnt out at temperatures ranging from 200 to 600 °C, resulting in obvious weight loss. The polymer binders among the green body could be totally burnt out at temperatures above 600 °C. Therefore, the polymer burn-out of the green SiC ceramic was conducted at 800 °C. After pyrolysis at 800 °C, polymers among the green SiC ceramic bodies were totally burnt out, and large pores were residual existed among the bodies where the polymers were, as shown in Fig. 8. Fig. 8 def presents the microstructure of the porous SiC ceramic preforms after polymer burn-out. It was found that large pores existed among these porous SiC ceramic preforms after polymer burn-out. SiC particles dispersed separately, and nearly no obvious bonding between the ceramic particles was observed. It was because such a low temperature treatment could only burn out the polymers, had no ability to promote the grain boundary migration and grain boundary diffusion of SiC ceramic particles to achieve densification. Therefore, there were so many pores among the porous ceramic preforms after polymer burnout. Besides, it was further observed that the pores became more with the increasing of solid loading, exhibiting the same trend of the green microstructure observation. Moreover, there was some large aggregates or particles, mainly composed of small SiC particle, were found in the green SiC part after binder burn-out, maybe resulting from the particle migration and particle aggregation during the heating. Detailed reason for this phenomenon is still un-known, and need to be further studied in the next future. Fig. 10 gives the relative density and strength of the

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Fig. 6. The stereolithography of SiC ceramic architectures with pyramid and hollowed basket structures: (a) STL files; (b) stereolithography: (i, iii) curing, (ii, iv) lifting; (c) green SiC structures; (d) final SiC structures. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7. Relative green density and green strength of the green SiC ceramics. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

porous SiC ceramic preforms after polymer burn-out. The relative density of the porous SiC ceramic preforms made from 30, 35, 40 vol% slurries were 32.7, 36.3 and 44.1%, respectively. After polymer burn

out, organic binders left and no obvious densification occurred, therefore the density was low. However, it was also observed that there was a slight increasing compared to the initial solid loading. The strength of the porous SiC ceramic preforms were very low owing to there was no efficient bonding between ceramic particles since the bonding resin networks had been burnt out. The strength of the porous SiC ceramic preforms made from 30, 35, 40 vol% slurries were as low as 15.2, 14.5 and 14.1 MPa, respectively. That was the reason why we must treat the porous SiC ceramic preforms using following precursor infiltration and pyrolysis. One of the key issues for stereolithography is how to achieve ceramic structures with high quality and high accuracy. Thus, the shrinkage behavior of the SiC ceramic parts were also investigated. Fig. 11 shows the linear shrinkage of the porous SiC ceramic preforms after polymer burn-out in Z-direction of the specimen. The linear shrinkage of the porous SiC ceramic preforms made from 30, 35, 40 vol % slurries were as low as 5.383, 4.734 and 4.341%, respectively. The linear shrinkage of the porous SiC ceramic preforms deceased with the increasing of solid loading. The reason was that more ceramic particles existed when the solid loading became larger, therefore the linear shrinkage became smaller correspondingly. Importantly, the processing procedure became easy controllable when we figured out the shrinkage behavior, and it was possible to manufacture 3D-structured ceramic

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Fig. 8. Microstructure of the green SiC ceramics: (a) 30 vol%; (b) 35 vol% and (c) 40 vol%; and microstructure of the porous SiC preforms after polymer burn-out: (d) 30 vol%; (e) 35 vol% and (f) 40 vol%.

Fig. 11. Linear shrinkage of the porous SiC ceramic preforms after polymer burn-out and the final SiC ceramics after precursor infiltration and pyrolysis. Fig. 9. TG-DTA curves of the green SiC ceramic body in N2. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

architectures with high accuracy and high quality, which was the advantage of stereolithography of ceramics than other reported additive manufacturing techniques.

3.3. Precursor infiltration and pyrolysis

Fig. 10. Relative density and strength of the porous SiC ceramic preforms after polymer burn-out.

SiC ceramic after polymer burn-out had low density and low strength. Precursor infiltration and pyrolysis (PIP) is an effective method to improve the density and strength. Polycarbosilane (PCS) preceramic polymer was infiltrated into the porous SiC ceramic preforms, and in-situ pyrolyzed into SiC among the porous preforms, and finally formed a final SiC ceramic body with high density and high strength. As a liquid, the polycarbosilane pre-ceramic was flowed into the pores among the porous SiC ceramic preforms due to the capillary force under vacuum, and then converted to SiC phase after pyrolysis. Usually, the PIP treatment should be repeated for several cycles until no more preceramic could be further immersed into the porous preform and the weight of the body had no change. Fig. 12 shows the weight increasing rates of the SiC ceramics after each PIP cycle. Firstly, the weight of SiC increased progressively after each PIP treatment. This meant PCS preceramic was successfully infiltrated into the body and converted into SiC phase. Secondly, the weight increasing rate of the SiC ceramic made from 30 vol% solid loading slurries was the highest, which was attributed to its porosity was the lowest. Moreover, all of thee SiC ceramics did not achieve full densification even after 8 cycles of PIP treatment.

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Fig. 14. XRD patterns of the pyrolyzed product of PCS and the final SiC ceramics.

Fig. 12. Weight increasing rates of the SiC ceramics after each PIP cycle.

Fig. 13. Relative density and strength of the final SiC ceramics after PIP treatment.

Fig. 13 gives the relative density of the final SiC ceramics after PIP treatment. The relative density of the SiC ceramics made from slurries with 30, 35 and 40 vol% solid loading were 79.2, 84.8 and 82.6%, respectively. The reason for the phenomena of no full densification was that when the PIP was conducted for several cycles, the formed SiC filled the pores among the preform from outside to inside, no more preceramic could be immersed into the inside, and the density would therefore no more increased. Moreover, the SiC ceramic made from 35 vol% solid loading slurry had the highest density, which might be attributed to its fine microstructure and pore structures. The ultimate purpose is to fabricate a SiC ceramic body with full densification. However, it was noted that the aim of this study is to demonstrate the feasibility of manufacture complex shaped SiC ceramic parts by using this novel stereolithography combined with precursor infiltration and pyrolysis. How to further achieve the full densification of the SiC ceramic has not been fully understood yet and has to be further investigated in more detail in our future work. Fig. 14 shows the XRD patterns of the pyrolyzed product of PCS and the final SiC ceramics. It was clearly found that the crystalline phase of the pyrolyzed product of the PCS preceramic was 6HeSiC phase (Card 74–1302). And the crystalline phases of the SiC ceramics made from 30, 35 and 40 vol% solid loading slurries were also 6HeSiC phase (Card 74–1302). These results indicated that the PCS pre-ceramic was successfully converted to SiC. Fig. 13 also gives the strength of the final SiC ceramics after PIP treatment. The SiC ceramics made from 30, 35 and 40 vol% solid loading slurries had the bending strength of 142.1, 204.6 and 184.2 MPa, respectively. The strength was significantly improved after PIP treatment compared with the strength after simple polymer

burn-out. It demonstrated that PIP treatment not only improved the density but also the strength. The SiC ceramic made from 35 vol% solid loading slurry maintained the highest final strength, which was mainly attributed to its highest density. Moreover, Fig. 11 also shows the linear shrinkage of the final SiC ceramics after PIP treatment. The linear shrinkage of the SiC ceramics from 30, 35, 40 vol% slurries were 6.333, 5.334 and 4.537%, respectively. The linear shrinkage of the SiC ceramic bodies deceased with the increasing of solid loading. Compared with the linear shrinkage of the SiC preforms after polymer burn-out, the linear shrinkage of the final SiC ceramics after PIP treatment had obvious improvements, which meant the ceramic particles moved together and the density thus increased correspondingly. By obtaining these precise shrinkage data, the fabrication of SiC ceramic structures with high accuracy and high quality could be easily controlled. Fig. 15 shows the microstructure of the final SiC ceramics after PIP treatment. Firstly, all these SiC ceramics had obvious pores, resulting in the non-densification and low strength (Fig. 15 abc). Secondly, it was observed the microstructure of the SiC ceramic made from 35 vol% solid loading slurry was much denser that the other two ceramics (Fig. 15b), which was in good agreement with the density results shown in Fig. 13. Moreover, high magnification SEM images showed some detailed information (Fig. 15 def). Small grains located between the large SiC particles were the pyrolyzed SiC after PIP treatment. It was found that the in-situ formed SiC particles exhibited smaller particles size and finer microstructure. It was also observed the in-situ formed SiC particles was much denser among the ceramic from 35 vol% solid loading slurry (Fig. 15e) than others. This phenomena was agree with the relative density and strength results well. Therefore, the SiC ceramic made from 35 vol% solid loading had the highest density, the highest strength and the finest microstructure after stereolithography based additive manufacturing, polymer burn-out and precursor infiltration and pyrolysis. Actually, the density, strength and microstructure are closely related to the initial state of the SiC slurry, including the solid loading, viscosity and dispersion behavior. Usually, a ceramic slurry with a simultaneous behavior combined with high solid loading, low viscosity, fine dispersion, will be helpful for obtain a ceramic with high relative density, high strength and fine microstructure. Therefore, the optimization of the SiC slurry is very important and will be investigated in detailed in our next study. Fig. 6d shows the photographs of the as-prepared 3D-structured SiC pyramid and hollowed basket architectures using stereolithography based additive manufacturing, polymer burn-out and precursor infiltration and pyrolysis. The 3D-structured SiC ceramic architectures exhibited high accuracy and high quality. Therefore, it demonstrated that complex-shaped SiC ceramic could be manufactured using the novel stereolithography based additive manufacturing combined with

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Fig. 15. Microstructure of the final SiC ceramics after PIP treatment: (a, d) 30 vol%; (b,e) 35 vol% and (c, f) 40 vol%.

precursor infiltration and pyrolysis technique.

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4. Conclusions

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In this study, SiC ceramic was fabricated using stereolithography based additive manufacturing combined with precursor infiltration and pyrolysis. Main conclusions are listed as follows, (1) Photosensitive SiC slurries were produced by using SiC as raw material, HDDA and DVE-3 as resin monomers, TPO and 250 as photoinitiators, and KOS110 as dispersant. SiC slurries with solid loading of 30, 35 and 40 vol% were obtained, which was suitable for the following stereolithography. (2) 3D-structured green SiC ceramic architectures with high accuracy and high quality were prepared using stereolithography from 35 vol % solid loading slurry. No obvious burrs, pores, cracks and breaks were detected. (3) Polymer burn-out was conducted at 800 °C. After polymer burn-out, the polymers among the SiC ceramic green bodies were totally burnt out, and large pores were residual existed among the bodies. (4) Precursor infiltration and pyrolysis was further conducted to improve the density and strength. After 8 PIP cycles, the final SiC ceramics exhibited high relative density and strength. The as-produced 3D-structured SiC ceramic architectures exhibited high accuracy and high quality. Therefore, it demonstrated that complex-shaped SiC ceramic could be manufactured using the stereolithography based additive manufacturing combined with precursor infiltration and pyrolysis. We believe that this paper can give some thinking for the additive manufacturing of SiC ceramic. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No. 51772028), the Beijing Natural Science Foundation (No. 2182064), the Young Elite Scientist Sponsorship (YESS) Program by CAST (No.2015QNRC001), the Open Research Fund of Key Laboratory of Space Utilization, Chinese Academy of Sciences (No. LSU_ZHBZ_2017_01), State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (No. P2020-005), and the program of China Scholarships Council (No. 201806035020).

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