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Microstructural development and factors affecting the performance of a reaction-bonded silicon carbide composite
Ceramics International 45 (2019) 17987–17995
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Microstructural development and factors affecting the performance of a reaction-bonded silicon carbide composite
T
Suocheng Songa,∗, Bingheng Lua, Zongqiang Gaob, Chonggao Baoc, Yana Mac a
Collaborative Innovation Center of High-end Manufacturing Equipment, Xi'an Jiaotong University, Xi'an, 710054, China The Second Affiliated Hospital of Xi'an Jiaotong University, Xi'an Jiaotong University, Xi'an, 710004, China c State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC-Based ceramic Carbon density Reaction mechanism Performance control Capillary channel blockage
Due to its near net-shape forming characteristic, reaction-bonded silicon carbide (RBSC) is an ideal sintering material to prepare complicated structural SiC-based ceramics when combined with selective laser sintering (SLS). With this understanding, various factors affecting RBSC composites and the performance control methods were analyzed in this work. By studying the microstructural development and reaction mechanism during the reaction-bonded process, the main factor affecting the performance was found to be the carbon density of SiC/C preform. The residual silicon content decreased with the increase in the carbon density of preform, thereby improving the performance. When the carbon density of preform exceeded the maximum value, some unreacted carbon existed in the sintered body, resulting in the degradation of performance. In order to break the limit of maximum carbon density of the preform, two effective approaches, namely the slow-release carbon source and new-style structure capillary channel were proposed in this work.
1. Introduction The hot section components of aero-engines are usually made of high-temperature alloys [1–4]. With the help of cooling and thermal barrier technologies, the working temperatures of high-temperature alloys are found to be higher than 1150 °C, which are nearly the 80% of the melting points of individual alloys. Therefore, the potential to further increase the service temperature is limited. In this context, advanced structural SiC-based ceramics possessing a number of superior characteristics, such as high temperature resistance, low density, high specific strength, high specific modulus, high oxidization resistance and ablation resistance are considered as one of the most promising candidates to replace alloys as new high-temperature structural materials in aero-engines [4–6]. Besides, SiC-based ceramics have the potential to be used in automobile industry, military applications, nuclear industry and so forth [7–9]. Nevertheless, certain drawbacks, such as forming difficulties, low accuracy, bad machinability, high sintering temperature, high sintering shrinkage and high brittleness, have hindered the application and development of complicated structural SiC-based ceramic components in aero-engines. At present, there is a great tendency to fabricate complicated structural SiC-based ceramics using selective laser sintering (SLS)
technology. Our complicated structural porous SiC-based preforms, prepared using SLS, are shown in Fig. 1. In general, the SiC-based preforms, prepared using SLS, need high temperature sintering. It is usually accompanied by volume shrinkage, due to which, it is unable to achieve the expected precision through traditional sintering methods, including liquid-phase sintering, hot pressed sintering and hot isostatic pressing sintering. Therefore, it is important to choose a suitable sintering method. Reaction-bonded SiC (RBSC) is a process, in which liquid silicon infiltrates the porous SiC/C preform to react with the carbon source in the preform. The reaction-bonded process has a lot of advantages, including near net-shape forming, short sintering time, low cost and low sintering temperature [10–13]. It is an excellent sintering method when combined with SLS. Because of the existence of residual silicon phase in RBSC composites, the overall performance of the RBSC composites becomes low, thus limiting their practical applications. In the past researches about SiC-based ceramics using SLS, a variety of carbon sources, including reactive polymer binder [14,15], high carbon yield resin [16], and carbon fibers [17,18] have been used to prepare complicated structural SiC/C preforms. Unfortunately, the factors affecting the performance were not systematically studied in these studies. In order to solve these problems and simplify the research process,
∗ Corresponding author. Collaborative Innovation Center of High-end Manufacturing Equipment, Xi'an Jiaotong University, 99Yanxiang Road, Xi'an, Shanxi Province, 710054, PR China. E-mail address: [email protected] (S. Song).
https://doi.org/10.1016/j.ceramint.2019.06.017 Received 12 April 2019; Received in revised form 28 May 2019; Accepted 3 June 2019 Available online 04 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. Preparation process of the complicated structural SiC-based preforms: (a) preparation process; (b) complicated structural parts.
SiC/C preforms were prepared through pressure forming to replace SLS and carbon black was chosen as the carbon source. Especially, a socalled two-step blending process was employed to blend raw materials. In this regard, the reaction mechanism, the causes leading to the infiltration blockage of liquid silicon and the control method of RBSC composites’ performance were studied. According to the analysis of every step of RBSC process, the sintering process was elucidated. During the RBSC process, the dissolution-precipitation process led to the formation of continuous SiC ceramic skeleton and the core-rim structure that consisted of two kinds of SiC with different crystal structures. Besides, the factors affecting the phase composition of RBSC composites were also investigated. The results indicated that the main factor affecting the residual silicon content and performance of RBSC composites was the carbon density of SiC/C preform. The residual silicon content reduced, which enhanced the performance of RBSC composites, as the carbon density increased appropriately. The liquid silicon could not infiltrate the preform when the carbon density was too high, which led to the existence of unreacted carbon and a sharp decline in the overall performance. The carbon density of preform had a peak value. Before fabricating the porous SiC/C preform using SLS or other forming methods, it is necessary to conduct a series of tests to confirm the peak value of carbon density so as to obtain the optimum ratio of raw materials. Interesting enough, after analyzing the factors leading to the liquid silicon infiltration blockage, two different approaches, namely the slow-release carbon source and new-style structure capillary channel were put forward to promote the liquid silicon infiltration and break the limit of maximum carbon density. Through these two approaches, the residual silicon content could be reduced sharply, while the performance could be enhanced. This research provided a new perspective for developing the preparation technology for complicated structural SiC-based ceramics. 2. Experimental In this study, starting raw materials consisted of commercial grade SiC powder (alpha, D0.5 = 14 μm, purity of 98.5%, Fengye Metal Material Co., Ltd., China), carbon black (N330, purity > 99.8%),
phenolic resin (25% wt.% of carbon yield, Xiangtong Chemical Engineering Co., Ltd., China) and coarse silicon particles (D0.5 = 4 mm, purity > 97.5%). The preforms were prepared using compression moulding forming, and then, infiltrated with liquid silicon for 30 min at 1600 °C. The carbon density of the preform was controlled by adjusting the carbon content, added to the system, and the moulding pressure. The RBSC composites with the dimensions of 25 mm × 3 mm × 4 mm were made for the three-point bending and density tests, respectively. All the samples were hand-polished using 0.5 μm diamond paste. Archimedes principle was employed to measure the bulk densities of the samples. Three-dimensional laser topography instrument (Keyence VK9700K, Japan) and scanning electron microscope (SEM; Tescan VEGA 2 XMU, Czech Republic) with an energy dispersive spectrometer (EDS) were employed to observe the microstructures of samples. X-ray diffractometer (XRD; Shimadzu XRD-7000, Japan) was used to acquire the phase composition of samples. 3. Results and discussion 3.1. Microstructures 3.1.1. Microstructure of the SiC/C preform The microstructure and performance of RBSC composites are determined based upon uniformity, material composition and bulk density of SiC/C preform. Therefore, it is important to prepare the preform. By means of successive wet ball-milling and dry ball-milling, the raw materials could be perfectly mixed with a uniform distribution of granularity. We called this process the two-step blending process. During the wet ball-milling process (200 r/min), the alcohol left a good dispersive effect on the tiny mixed powder. However, when the mixed slurry was heated and stirred, the alcohol evaporated, causing the raw materials reunite, thus increasing the granularity. This process was the main reason that the microstructure of sintered body was nonuniform. Fig. 2(a) shows the schematic of the particle aggregation after wet ballmilling process. The carbon black (black dots) and original SiC (red circles) were agglomerated. In order to obtain uniformly mixed particles, dry ball-milling (150 r/min) was used to break the agglomeration
Fig. 2. Schematic of the raw mixed particles: (a) particle aggregation after wet ball-milling process; (b) uniformly dispersed mixed particles after the two-step blending process. 17988
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Fig. 3. Morphology of the SiC/C preform: (a) after drying; (b) element area profile in (a); (c) SiC/C preform after high-temperature degreasing and (d) element area profile in (c).
of materials. Fig. 2(b) shows the schematic of mixed particles after the two-step blending process. Finally, the uniformly mixed particles with relatively good fluidity were obtained after the two-step blending and sieving process. The morphology of SiC/C preform after drying is shown in Fig. 3(a). The light gray particles are the original SiC, while the other region is the mixture of carbon black and phenolic resin. By using the pre-processed mixed powders, the original SiC particles are uniformly dispersed in the preform, while the carbon black and phenolic resin fill in the intergranular pores, which is the pre-condition for the homogeneity of structure. Fig. 3(b) shows the element area profile for the results presented in Fig. 3(a). After pyrolysis treatment, the morphology of SiC/C preform is shown in Fig. 3(c). Fig. 3(d) shows the element area profile for the results shown in Fig. 3(c). The morphology undergoes almost no change and all the phenolic resin is converted into fluffy carbon. It can be seen from these results that the phenolic resin used as bonding agent not only provides carbon source for the reaction-bonded process, but also ensures that the preform possesses a certain strength and stability. The function of phenolic resin is similar to that of vitrified bond during the ceramic additive manufacturing process. Both of them are converted into carbon source and react with silicon to form SiC, which avoids introducing impurities into the composites. 3.1.2. Microstructure of the RBSC composites The SiC/C preform (73 wt% SiC, 15 wt% carbon black and 12 wt% phenolic resin) was infiltrated with liquid silicon. Considering the surface morphology shown in Fig. 4(a), it can be seen that the material is composed of two different phases. The gray phase connects together and forms a continuous structure. The pores among the gray phase are filled with a white phase. The phase composition is identified using the XRD diffraction pattern shown in Fig. 4(b). The XRD pattern shows that the sintered body is composed of three phases, namely the α-SiC, the βSiC and the silicon itself. Among them, α-SiC is the original SiC, while β-SiC is the newly-formed SiC, which came into existence through C–Si reaction. When the back-scattered electron detector shown in Fig. 4(c)
is analyzed, it is found that the material is composed of two different phases. Combined with the EDS analysis of Points 1 and 2 (as presented in Table 1), the white phase comes out to be silicon, while the gray phase turns out to be SiC. These silicon islands are the major cause of the degradation of performance of RBSC composites [19]. It is worth noticing that the α-SiC and β-SiC cannot be identified due to identical chemical compositions. It is interesting to note that the use of a secondary electron detector reveals a core-rim structure, as shown in Fig. 4(d). According to the EDS analysis of Points 3 and 4 (presented in Table 1) and the morphology and size of the original SiC particle, the bright white particles turn out to be the original SiC, while the thin layer around them corresponds to the newly-formed β-SiC. The other dark gray region is the silicon (Point 5 presented in Table 1). The newly-formed SiC grew onto the original SiC particle to form the corerim structure during the reaction-bonded process. In order to clearly observe the microstructure, we used a mixed acid (hydrofluoric acid and nitric acid in the volumetric ratio of 1:7, respectively) to remove residual silicon (see Reaction Equation (1)). 3Si+4HNO3+12HF→4NO↑+3SiF4↑+8H2O
(1)
The microstructure of RBSC composites after etching is shown in Fig. 5(a). The newly-formed SiC bonded the original SiC. Fig. 5(b) shows the three-dimensional topography presented in Fig. 5(a). Two kinds of SiC phases were found to form the continuous ceramic skeleton. In order to explore the reaction-bonded process, the fracture in sintered body was etched to remove the residual silicon. During the whole process, it is of great significance to analyze the microstructural development. The morphologies of original SiC particles are shown in Fig. 6(a). The newly-formed SiC gradually grew on the original SiC. The growth models can be divided into two categories, namely the refined newly-formed SiC grains growing around the original SiC boundary (see Fig. 6(b)) and the compact connection layer bonding those original SiC particles (see Fig. 6(c)). Fig. 6(d) shows the newly-formed SiC among the original SiC particles and the two growth models mentioned above.
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Fig. 4. Microstructures and XRD diffraction pattern of RBSC composites: (a) optical photograph; (b) XRD diffraction pattern; (c) SEM photograph obtained using the back-scattered electron detector and (d) SEM photograph obtained using the secondary electron detector. Table 1 EDS analysis results, as shown in Fig. 4(c) and (d). Point
1 2 3 4 5
Element (at.%) C
Si
48.12 0 47.37 45.29 0
51.88 100 52.63 54.71 100
According to the above-mentioned analysis, the newly-formed SiC exists in two different forms, namely the compact SiC connection layer and the refined newly-formed SiC grains. This result agrees with the one reported in a previous work [20]. 3.1.3. Reaction-bonded mechanisms Ness and Page [21] argued that the dissolution of carbon source in liquid silicon is an exothermic process and that the carbon source might exist in three forms, namely C, C–Si and CSi4. Furthermore, with the increase in temperature, the solubility of carbon in liquid silicon
increased, which promoted the diffusion of carbon from higher temperature region to cooler original SiC. The carbon diffused along the carbon concentration gradient until a supersaturation occurred in the locally cooler sites. Then, the newly-formed SiC precipitated on the surface of original SiC particles. Fig. 7 shows the schematic of the reaction-bonded process. Fig. 7(a) shows the structure of porous SiC/C preform. Fig. 7(b) reveals that the liquid silicon enters the preform, after which, the RBSC process begins. As the reaction progresses, the newly-formed SiC grows on the newly-formed SiC particles, which leads to the occlusion of some capillary channels. Ultimately, the entire carbon source is converted to SiC, which precipitates on the surface of original SiC particles. The liquid silicon infiltrates the residual pores and forms the island of residual silicon. This silicon island is detrimental to the performance of RBSC composites (see Fig. 7(c). In the later cooling process, carbon in liquid silicon becomes supersaturated and the viscosity of C–Si solution increases with the decrease in temperature. Some nanometer-sized newly-formed SiC grains are generated in situ, which are in accordance with the microstructures shown in Fig. 6(b), (c) and 6(d).
Fig. 5. Microstructure of RBSC composites after etching: (a) surface appearance; (b) three-dimensional topography. 17990
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Fig. 6. Microstructural development during the RBSC process: (a) original SiC particle; (b) refined newly-formed SiC; (c) compact connection layer and (d) two growth models of the newly-formed SiC.
3.2. Optimization of phase composition of RBSC composites 3.2.1. Control method According to the XRD pattern and microstructural observations, the RBSC composites consist of three phases (original α-SiC, newly-formed β-SiC and silicon). The density of silicon (2.33 g/cm3) is larger than the density of SiC (3.21 g/cm3). The residual silicon decreases with the increase in the density of RBSC composites. The silicon phase is detrimental to the performance of RBSC composites, which results from inferior flexural strength (≤100 MPa) and low melting point (ca. 1410 °C) [22,23]. It is essential to decrease the dimension and content of residual silicon [20]. RBSC is a near net-shape forming technique for ceramics, which has provided an effective approach for resolving the difficulty of
manufacturing and processing complicated structural ceramics. The entire carbon source is converted to SiC, which is accompanied by volume expansion. The volume expansion times (T) can be acquired using Equation (2).
T = MSiC DCarbon /MC DSiC
(2)
where MSiC is the relative molecular mass of SiC, MC is the relative atomic mass of C, DCarbon (g/cm3) is the theoretical density of C and DSiC (g/cm3) is the theoretical density of SiC. Based upon the reaction-bonded process, the newly-formed SiC fills some pores and the residual silicon fills the rest. In order to decrease the residual silicon content, the compression moulding pressure was adjusted to control the density of preform, thereby decreasing the porosity. Fig. 8 shows the variation in preform density with the
Fig. 7. Schematic of the RBSC process: (a) structure of the porous SiC/C preform; (b) liquid silicon entering the preform and (c) densification of the RBSC composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 17991
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Fig. 9. Variations in DRBSC, bulk density and flexural strength for different carbon densities of the preforms.
Fig. 8. Variation in preform density with the compression moulding pressure.
compression moulding pressure. When the pressure increases from 120 MPa to 180 MPa, the bulk density of preform obviously improves. It is worth noticing that the preform densities decrease with the further increase in pressure. When the pressure is too high, the original particles are in close contact with each other and the air among these original particles cannot be discharged. The preform springs back during the demoulding process, thereby decreasing the bulk density of preform. It is observed that the entire carbon source is converted to SiC. In other words, all carbon element in the preform exists in the form of SiC. Therefore, the carbon density of preform helps in calculating the value of theoretical density of RBSC composite. Ultimately, the carbon density of preform is adjusted to control the value of volume fraction of residual silicon. The carbon density of preform (DC) can be calculated using Equation (3).
DC = mpreform ωSiC MC /MSiC + mpreform ωmixedC /Vpreform
(3)
where mpreform (g) is the preform mass, ωSiC is the mass fraction of SiC, ωmixedC is the mass fraction of mixed carbon, and Vpreform (cm3) is the volume of preform. According to the principle of mass conservation, the near net-shape forming characteristic and the phase composition of RBSC composites, the relationship between DC and the theoretical density of RBSC composite (DRBSC) is given by Equation (4).
DRBSC = DC MSiC /MC + DSi (1 − DC MSiC /MC DSiC) = 2. 33 + 0. 92DC (4) where Dsi (g/cm3) is the theoretical density of silicon. It can be seen that DC is the major factor contributing to the residual silicon content and DRBSC. It can also be used to evaluate the preform quality. Furthermore, the volume fraction of residual silicon (φSi) can be calculated using Equation (5).
ϕSi = 3. 21 − DRBSC /0. 88
(5)
The residual silicon content decreases with the increase in the value of DRBSC. It is worth mentioning that DRBSC is an important factor affecting the performance of ceramics. 3.2.2. Maximum carbon density of the preform In some cases, some unreacted carbon, called the residual carbon, is found in the RBSC composites. The existence of residual carbon in the internal zone of RBSC composites greatly reduces their performance. Popper [24] reported that carbon source was converted to SiC along with some volumetric expansion, which was detrimental to the infiltration of liquid silicon (the skin effect phenomenon). According to the reaction-bonded process and computed results, it is a very effective way to improve the value of DC so as to reduce the residual silicon
content in the RBSC composites. However, some unreacted carbon will exist in the RBSC composites with the increase in the value of DC. It is important to find the maximum value of DC. In this case, the residual silicon content reaches the minimum level and all the carbon is converted to SiC. For this reason, some porous SiC/C preforms with a series of carbon densities were prepared to find the maximum value of DC. Fig. 9 shows the variations in DRBSC, bulk density, and flexural strength for different carbon densities of the preforms. As can be seen, the bulk densities of RBSC composites increase with the increase in value of DC. When the value of DC is less than 0.84 g/cm3, the bulk densities of RBSC composites are in good agreement with the theoretical densities. As the value of DC increases further, the bulk densities of RBSC composites decrease. This is mainly because the excess carbon source was converted to plenty of newly-formed SiC, which blocked the capillary channel and led to the difficulty of liquid silicon infiltration. The pores and unreacted carbon existed in the RBSC composites, thereby decreasing the bulk densities of RBSC composites. The maximum value of DC is 0.84 g/cm3. In other words, the value of DC should be controlled to obtain the optimal raw material ratio before the reaction-bonded process. The value of DC is a useful performance parameter for evaluating the performance of RBSC composites. Porous SiC ceramics have many advantages, including good thermal-shock stability, high mechanical properties and chemical stability. It is widely used for metal filtration, biological carrier and lightweight heat-resistant components. It is interesting to note that porous SiC ceramic can be prepared by removing the residual silicon in RBSC composites under the conditions of high temperature and high vacuum. In Liu's research [25], porous α-SiC ceramics were prepared by recrystallization method using silicon as the template. The porosity of SiC ceramics was regulated by controlling the amount and size of silicon particle and sintering temperature. The silicon phase was carried off into the external environment in a gaseous form at high temperatures. Afterwards, the original SiC and the newly-formed SiC connected together through recrystallization. Considering the controlling of silicon content by adjusting the value of DC, porous SiC ceramics with different porosities can be prepared using the above-mentioned method. In addition, the liquid silicon infiltrates the capillary channel and a through-hole is formed after the removal of residual silicon. When the value of DC is 0.84 g/cm3, the volume fraction of residual silicon is 12.5%. In general, the porous SiC ceramics with different porosities (≥12.5%) can be prepared by adjusting the value of DC.
3.2.3. Process of capillary channel blockage Fig. 10(a) shows the fracture morphology of RBSC composites with the corresponding carbon density of 0.85 g/cm3. As can be seen, the
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Fig. 10. Fracture morphology of RBSC composites with excessive carbon density: (a) porous fracture appearance with the corresponding carbon density of 0.85 g/cm3; (b) magnification of the red rectangle shown in (a); (c) unreacted fracture appearance with the corresponding carbon density of 0.86 g/cm3 and (d) magnification of the red rectangle shown in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
structure close to the outer edge is compact, while that in the center is porous. The porous area is small, and the carbon source reacted completely. Fig. 10(b) shows the detailed view of red rectangle shown in Fig. 10(a). The liquid silicon is unable to infiltrate the center due to capillary channel blockage, thereby forming a porous structure. Fig. 10(c) shows the fracture morphology of RBSC composites with the corresponding carbon density of 0.86 g/cm3. The thickness of the outer compact edge is about 0.3 mm and a lot of raw materials exist in the center. Fig. 10(d) shows the detailed view of red rectangle shown in Fig. 10(c). The outer thin edge is compact and its structure is in agreement with the traditional RBSC composites. The structure in the center region agrees with that of the SiC/C preform. Combined with the reaction mechanism, the process of capillary channel blockage is summarized in Fig. 11. The complexity was reduced, and the formation, narrowing and blockage of capillary channel during the reaction-bonded process were described (see Fig. 11(a), (b) and 11(c)). In Fig. 11(a), the pores among those original SiC particles formed the capillary channels. With the increase in temperature, the solubility of carbon in liquid silicon increased, which promoted the diffusion of carbon from higher temperature region to cooler original SiC. The carbon diffused along the carbon concentration gradient until a supersaturation occurred in the locally cooler sites. Then, the newlyformed SiC precipitated on the surface of original SiC particles. The above process caused the capillary channel to become narrower (see Fig. 11(b)). As the reaction progressed, the capillary channel closed (see Fig. 11(c)) and liquid silicon could not further infiltrate the preform.
3.3. Assumption of two kinds of approaches to break the limit of maximum carbon density of the preform Considering the process of capillary channel blockage, two kinds of approaches are put forward to solve the blockage, thereby breaking the limit of maximum value of DC and improving the performance of RBSC composites.
3.3.1. Utilizing the slow-release carbon source The nanometer-sized carbon black is amorphous and has high specific surface area. Therefore, it has high activity and its siliconization reaction is fast. When the value of DC is too high, the capillary channel is prone to be blocked. One solution to this problem is to use the carbon with low activity as the source. This carbon source, herein called the slow-release carbon source, exhibits large size (low specific surface area) and high graphitization. During the liquid silicon infiltration, the carbon with low activity is gradually released into the liquid silicon. This process prolongs the siliconization reaction, thereby postponing the capillary channel blockage. If so, it has plenty of time for liquid silicon to infiltrate the porous SiC/C preform. The unreacted residual carbon is usually eliminated through the follow-up heat treatment. The unreacted residual carbon reacts with the residual silicon through diffusion. The size of slow-release carbon source should not be too large. The addition of oversized slow-release carbon source results in an uneven structure and some unreacted residual carbon in the system. As long as the amount of selective slow-release carbon source is reasonable, the maximum value of DC can be improved further. More
Fig. 11. Schematic of the process of capillary channel blockage: (a) original SiC particles forms the capillary channel; (b) capillary channel becomes narrower and (c) capillary channel closes. 17993
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Fig. 12. Schematic of the influence of slowrelease carbon on the reaction-bonded process.
Fig. 13. Schematic of fabricating the new-style structure capillary channel.
newly-formed SiC is produced and the residual silicon content decreases further. In this method, a single species of carbon can be used as the slow-release carbon source, whereas the reaction-bonded time becomes long. Mixed carbon, which is composed of two or more kinds of carbons with different activities, can also be used in the reactionbonded process. The carbon with high activity will preferentially react with liquid silicon. The refined carbon particle with high activity fills the available pores, while the large carbon particle with low activity prolongs the siliconization reaction, thereby postponing the capillary channel blockage. Compared with a single species of carbon, mixed carbon can adjust the siliconization reaction time and decrease the residual silicon content to some degree. Fig. 12 shows the influence of slow-release carbon on the reaction-bonded process. The black circles with different particle sizes represent the mixed slow-release carbon, which is evenly distributed among the original SiC particles. When the reaction-bonded process begins, the mixed slow-release carbon releases the carbon element into the liquid silicon in a controlled fashion, thereby solving the problem of capillary channel blockage to some extent.
3.3.2. Fabricating a new-style structure capillary channel The capillary channel built by the original SiC particles is evenly distributed in the whole of preform. The capillary channel becomes narrower and narrower as the reaction progresses. It is a good choice to fabricate a capillary channel with a new-style structure to solve this problem. The characteristics of the capillary channel are as follows. (1) The new-style structure capillary channel possesses stable physical and chemical properties, which ensure an almost unchanged width and structure during the reaction process. The new-style structure capillary channel, whose surface cannot be coated by the newlyformed SiC phase, can be on active service until the end of reaction. (2) The new particles, forming the new-style structure capillary channel, bring no negative impact on the RBSC composites, whereas the width of capillary channel is tunable. Considering this, a multimodal particle size distribution can be used to adjust the width of capillary channel.
mentioned conditions. In a previous research [20], boron carbide was used as the reinforced particles, which formed the capillary channel with new-style structure. The content and size of residual silicon decreased sharply, thereby effectively improving the performance of RBSC composites. More importantly, the production of a large number of nanometer-sized newly-formed SiC particles effectively enhanced the performance of RBSC composites. In the same way, other suitable particles can be chosen to replace boron carbide. Fig. 13 shows the schematic of fabricating the new-style structure capillary channel, which is evenly distributed in the whole of preform. Its stable structure ensures adequate infiltration of liquid silicon and can effectively solve the problem of capillary channel blockage.
4. Conclusions In order to control the performance of RBSC composites, prepared using additive manufacturing (AM) or other forming methods, the reaction-bonded process and the microstructural development of RBSC composites were studied in this work. The key factor affecting the performance and residual silicon content of RBSC composites was found to be the carbon density of SiC/C preform. The reason of the capillary channel blockage was the excessive carbon density. In this research, the maximum value of carbon density of preform was 0.84 g/cm3, while the corresponding bulk density was 3.10 g/cm3. The volume fraction of residual silicon was 12.5% and the flexural strength was 349 MPa. The parameters of raw materials (original SiC particle shape and size, carbon particle size and activity) determined the maximum value of carbon density. The maximum value of carbon density could be determined by analyzing the data for different carbon densities. The performance of RBSC composites could be reasonably predicted and controlled by regulating and measuring the carbon density of preform. Furthermore, two kinds of approaches were put forward to solve the problem of capillary channel blockage: (i) utilizing the slow-release carbon source and (ii) fabricating a new-style structure capillary channel. This research lays an elementary foundation for manufacturing SiC-based ceramics.
The value of DC can be improved suitably based on the above17994
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Acknowledgement This work was supported by the Basic Scientific Research of Xi’an Jiaotong University (No. xjh012019007), the Natural Science Foundation of China (No. 51372194) and the Social Development Fund of Shaanxi Province (No. 2015KTCQ03-01). References [1] Y.W. Kim, S.L. Kim, Advances in gammalloy materials–processes–application technology: successes, dilemmas, and future, J. Occup. Med. 70 (2018) 553–560. [2] M. Perrut, P. Caron, M. Thomas, A. Couret, High temperature materials for aerospace applications: Ni-based superalloys and γ -TiAl alloys, C. R. Phys. 19 (2018) 657–671. [3] T. Tetsui, T. Kobayashi, H. Harada, Achieving high strength and low cost for hotforged TiAl based alloy containing beta phase, Mat. Sci. Eng. A Struct. 552 (2012) 345–352. [4] S.V. Raj, Development and characterization of hot-pressed matrices for engineered ceramic matrix composites (E-CMCs), Ceram. Int. 45 (2019) 3608–3619. [5] C. Xu, C. Xu, F. Han, F. Zhang, W. Wei, Z. Zhong, W. Xing, Fabrication of high performance macroporous tubular silicon carbide gas filters by extrusion method, Ceram. Int. 44 (2018) 17792–17799. [6] Q. Tian, N. Wu, B. Wang, Y. Wang, Fabrication of hollow SiC ultrafine fibers by single-nozzle electrospinning for high-temperature thermal insulation application, Mater. Lett. 239 (2019) 109–112. [7] I.G. Crouch, M. Kesharaju, R. Nagarajah, Characterisation, significance and detection of manufacturing defects in reaction sintered silicon carbide armour materials, Ceram. Int. 41 (2015) 11581–11591. [8] A. Tasdemirci, G. Tunusoglu, M. Güden, The effect of the interlayer on the ballistic performance of ceramic/composite armors: experimental and numerical study, Int. J. Impact Eng. 44 (2012) 1–9. [9] J.M. Fernández, A. Munoz, A.R.D. López, F.M.V. Feria, A. Dominguez-Rodriguez, M. Singh, Microstructure-mechanical properties correlation in siliconized silicon carbide ceramics, Acta Mater. 51 (2003) 3259–3275. [10] Y.Y. Zhang, C.Y. Hsu, S. Aubuchon, P. Karandikar, C.Y. Ni, Microstructural and thermal property evolution of reaction bonded silicon carbide (RBSC), J. Alloy, Compd 764 (2018) 107–111.
[11] S. Meyers, L. De Leersnijder, J. Vleugels, J.P. Kruth, Direct laser sintering of reaction bonded silicon carbide with low residual silicon content, J. Eur. Ceram. Int. 38 (2018) 3709–3717. [12] B.L. Wing, J.W. Halloran, Relaxation of residual microstress in reaction bonded silicon carbide, Ceram. Int. 44 (2018) 11745–11750. [13] N. Song, H. Liu, J. Fang, Fabrication and mechanical properties of multi-walled carbon nanotube reinforced reaction bonded silicon carbide composites, Ceram. Int. 42 (1) (2016) 351–356. [14] R.S. Evans, D.L. Bourell, J.J. Beaman, M.I. Campbell, Rapid manufacturing of silicon carbide composites, Rapid Prototyp. J. 11 (1) (2005) 37–40. [15] B. Stevinson, D.L. Bourell, J.J. Beaman, Over-infiltration mechanisms in selective laser sintered Si/SiC preforms, Rapid Prototyp. J. 14 (3) (2008) 149–154. [16] X. Tian, D. Li, Rapid manufacture of net-shape SiC components, Int. J. Adv. Manuf. Technol. 46 (5) (2010) 579–587. [17] W. Zhu, C. Yan, Y. Shi, S. Wen, J. Liu, Q. Wei, Y. Shi, A novel method based on selective laser sintering for preparing high-performance carbon fibres/polyamide12/epoxy ternary composites, Sci. Rep. 6 (2016) 33780. [18] W. Zhu, H. Fu, Z. Xu, R. Liu, P. Jiang, X. Shao, Y. Shi, C. Yan, Fabrication and characterization of carbon fiber reinforced SiC ceramic matrix composites based on 3D printing technology, J. Eur. Ceram. Int. 38 (2018) 4604–4613. [19] U. Paik, H.C. Park, S.C. Choi, C.G. Ha, J.W. Kim, Y.G. Jung, Effect of particle dispersion on microstructure and strength of reaction-bonded silicon carbide, Mat. Sci. Eng. A Struct. 334 (2002) 267–274. [20] S. Song, C. Bao, Y. Ma, K. Wang, Fabrication and characterization of a new-style structure capillary channel in reaction bonded silicon carbide composites, J. Eur. Ceram. Int. 37 (2017) 2569–2574. [21] J.N. Ness, T.F. Page, Microstructural evolution in reaction-bonded silicon carbide, J. Mater. Sci. 21 (1986) 1377–1397. [22] O.P. Chakrabarti, S. Ghosh, J. Mukerji, Influence of grain size, free silicon content and temperature on the strength and toughness of reaction-bonded silicon carbide, Ceram. Int. 20 (5) (1994) 283–286. [23] S. Hayun, A. Weizmann, M.P. Dariel, N. Frage, The effect of particle size distribution on the microstructure and the mechanical properties of boron carbide-based reaction-bonded composites, Int. J. Appl. Ceram. Technol. 6 (4) (2009) 492–500. [24] P. Popper, Special Ceramics V[M], (1970), p. 99. [25] R. Liu, G. Liu, J. Yang, H. Jin, Y. Lu, W. Gu, Effect of removal of silicon on preparation of porous SiC ceramics following reaction bonding and recrystallization, Mat. Sci. Eng. A Struct. 552 (2012) 9–14.
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Microstructural development and factors affecting the performance of a reaction-bonded silicon carbide composite
T
Suocheng Songa,∗, Bingheng Lua, Zongqiang Gaob, Chonggao Baoc, Yana Mac a
Collaborative Innovation Center of High-end Manufacturing Equipment, Xi'an Jiaotong University, Xi'an, 710054, China The Second Affiliated Hospital of Xi'an Jiaotong University, Xi'an Jiaotong University, Xi'an, 710004, China c State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: SiC-Based ceramic Carbon density Reaction mechanism Performance control Capillary channel blockage
Due to its near net-shape forming characteristic, reaction-bonded silicon carbide (RBSC) is an ideal sintering material to prepare complicated structural SiC-based ceramics when combined with selective laser sintering (SLS). With this understanding, various factors affecting RBSC composites and the performance control methods were analyzed in this work. By studying the microstructural development and reaction mechanism during the reaction-bonded process, the main factor affecting the performance was found to be the carbon density of SiC/C preform. The residual silicon content decreased with the increase in the carbon density of preform, thereby improving the performance. When the carbon density of preform exceeded the maximum value, some unreacted carbon existed in the sintered body, resulting in the degradation of performance. In order to break the limit of maximum carbon density of the preform, two effective approaches, namely the slow-release carbon source and new-style structure capillary channel were proposed in this work.
1. Introduction The hot section components of aero-engines are usually made of high-temperature alloys [1–4]. With the help of cooling and thermal barrier technologies, the working temperatures of high-temperature alloys are found to be higher than 1150 °C, which are nearly the 80% of the melting points of individual alloys. Therefore, the potential to further increase the service temperature is limited. In this context, advanced structural SiC-based ceramics possessing a number of superior characteristics, such as high temperature resistance, low density, high specific strength, high specific modulus, high oxidization resistance and ablation resistance are considered as one of the most promising candidates to replace alloys as new high-temperature structural materials in aero-engines [4–6]. Besides, SiC-based ceramics have the potential to be used in automobile industry, military applications, nuclear industry and so forth [7–9]. Nevertheless, certain drawbacks, such as forming difficulties, low accuracy, bad machinability, high sintering temperature, high sintering shrinkage and high brittleness, have hindered the application and development of complicated structural SiC-based ceramic components in aero-engines. At present, there is a great tendency to fabricate complicated structural SiC-based ceramics using selective laser sintering (SLS)
technology. Our complicated structural porous SiC-based preforms, prepared using SLS, are shown in Fig. 1. In general, the SiC-based preforms, prepared using SLS, need high temperature sintering. It is usually accompanied by volume shrinkage, due to which, it is unable to achieve the expected precision through traditional sintering methods, including liquid-phase sintering, hot pressed sintering and hot isostatic pressing sintering. Therefore, it is important to choose a suitable sintering method. Reaction-bonded SiC (RBSC) is a process, in which liquid silicon infiltrates the porous SiC/C preform to react with the carbon source in the preform. The reaction-bonded process has a lot of advantages, including near net-shape forming, short sintering time, low cost and low sintering temperature [10–13]. It is an excellent sintering method when combined with SLS. Because of the existence of residual silicon phase in RBSC composites, the overall performance of the RBSC composites becomes low, thus limiting their practical applications. In the past researches about SiC-based ceramics using SLS, a variety of carbon sources, including reactive polymer binder [14,15], high carbon yield resin [16], and carbon fibers [17,18] have been used to prepare complicated structural SiC/C preforms. Unfortunately, the factors affecting the performance were not systematically studied in these studies. In order to solve these problems and simplify the research process,
∗ Corresponding author. Collaborative Innovation Center of High-end Manufacturing Equipment, Xi'an Jiaotong University, 99Yanxiang Road, Xi'an, Shanxi Province, 710054, PR China. E-mail address: [email protected] (S. Song).
https://doi.org/10.1016/j.ceramint.2019.06.017 Received 12 April 2019; Received in revised form 28 May 2019; Accepted 3 June 2019 Available online 04 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. Preparation process of the complicated structural SiC-based preforms: (a) preparation process; (b) complicated structural parts.
SiC/C preforms were prepared through pressure forming to replace SLS and carbon black was chosen as the carbon source. Especially, a socalled two-step blending process was employed to blend raw materials. In this regard, the reaction mechanism, the causes leading to the infiltration blockage of liquid silicon and the control method of RBSC composites’ performance were studied. According to the analysis of every step of RBSC process, the sintering process was elucidated. During the RBSC process, the dissolution-precipitation process led to the formation of continuous SiC ceramic skeleton and the core-rim structure that consisted of two kinds of SiC with different crystal structures. Besides, the factors affecting the phase composition of RBSC composites were also investigated. The results indicated that the main factor affecting the residual silicon content and performance of RBSC composites was the carbon density of SiC/C preform. The residual silicon content reduced, which enhanced the performance of RBSC composites, as the carbon density increased appropriately. The liquid silicon could not infiltrate the preform when the carbon density was too high, which led to the existence of unreacted carbon and a sharp decline in the overall performance. The carbon density of preform had a peak value. Before fabricating the porous SiC/C preform using SLS or other forming methods, it is necessary to conduct a series of tests to confirm the peak value of carbon density so as to obtain the optimum ratio of raw materials. Interesting enough, after analyzing the factors leading to the liquid silicon infiltration blockage, two different approaches, namely the slow-release carbon source and new-style structure capillary channel were put forward to promote the liquid silicon infiltration and break the limit of maximum carbon density. Through these two approaches, the residual silicon content could be reduced sharply, while the performance could be enhanced. This research provided a new perspective for developing the preparation technology for complicated structural SiC-based ceramics. 2. Experimental In this study, starting raw materials consisted of commercial grade SiC powder (alpha, D0.5 = 14 μm, purity of 98.5%, Fengye Metal Material Co., Ltd., China), carbon black (N330, purity > 99.8%),
phenolic resin (25% wt.% of carbon yield, Xiangtong Chemical Engineering Co., Ltd., China) and coarse silicon particles (D0.5 = 4 mm, purity > 97.5%). The preforms were prepared using compression moulding forming, and then, infiltrated with liquid silicon for 30 min at 1600 °C. The carbon density of the preform was controlled by adjusting the carbon content, added to the system, and the moulding pressure. The RBSC composites with the dimensions of 25 mm × 3 mm × 4 mm were made for the three-point bending and density tests, respectively. All the samples were hand-polished using 0.5 μm diamond paste. Archimedes principle was employed to measure the bulk densities of the samples. Three-dimensional laser topography instrument (Keyence VK9700K, Japan) and scanning electron microscope (SEM; Tescan VEGA 2 XMU, Czech Republic) with an energy dispersive spectrometer (EDS) were employed to observe the microstructures of samples. X-ray diffractometer (XRD; Shimadzu XRD-7000, Japan) was used to acquire the phase composition of samples. 3. Results and discussion 3.1. Microstructures 3.1.1. Microstructure of the SiC/C preform The microstructure and performance of RBSC composites are determined based upon uniformity, material composition and bulk density of SiC/C preform. Therefore, it is important to prepare the preform. By means of successive wet ball-milling and dry ball-milling, the raw materials could be perfectly mixed with a uniform distribution of granularity. We called this process the two-step blending process. During the wet ball-milling process (200 r/min), the alcohol left a good dispersive effect on the tiny mixed powder. However, when the mixed slurry was heated and stirred, the alcohol evaporated, causing the raw materials reunite, thus increasing the granularity. This process was the main reason that the microstructure of sintered body was nonuniform. Fig. 2(a) shows the schematic of the particle aggregation after wet ballmilling process. The carbon black (black dots) and original SiC (red circles) were agglomerated. In order to obtain uniformly mixed particles, dry ball-milling (150 r/min) was used to break the agglomeration
Fig. 2. Schematic of the raw mixed particles: (a) particle aggregation after wet ball-milling process; (b) uniformly dispersed mixed particles after the two-step blending process. 17988
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Fig. 3. Morphology of the SiC/C preform: (a) after drying; (b) element area profile in (a); (c) SiC/C preform after high-temperature degreasing and (d) element area profile in (c).
of materials. Fig. 2(b) shows the schematic of mixed particles after the two-step blending process. Finally, the uniformly mixed particles with relatively good fluidity were obtained after the two-step blending and sieving process. The morphology of SiC/C preform after drying is shown in Fig. 3(a). The light gray particles are the original SiC, while the other region is the mixture of carbon black and phenolic resin. By using the pre-processed mixed powders, the original SiC particles are uniformly dispersed in the preform, while the carbon black and phenolic resin fill in the intergranular pores, which is the pre-condition for the homogeneity of structure. Fig. 3(b) shows the element area profile for the results presented in Fig. 3(a). After pyrolysis treatment, the morphology of SiC/C preform is shown in Fig. 3(c). Fig. 3(d) shows the element area profile for the results shown in Fig. 3(c). The morphology undergoes almost no change and all the phenolic resin is converted into fluffy carbon. It can be seen from these results that the phenolic resin used as bonding agent not only provides carbon source for the reaction-bonded process, but also ensures that the preform possesses a certain strength and stability. The function of phenolic resin is similar to that of vitrified bond during the ceramic additive manufacturing process. Both of them are converted into carbon source and react with silicon to form SiC, which avoids introducing impurities into the composites. 3.1.2. Microstructure of the RBSC composites The SiC/C preform (73 wt% SiC, 15 wt% carbon black and 12 wt% phenolic resin) was infiltrated with liquid silicon. Considering the surface morphology shown in Fig. 4(a), it can be seen that the material is composed of two different phases. The gray phase connects together and forms a continuous structure. The pores among the gray phase are filled with a white phase. The phase composition is identified using the XRD diffraction pattern shown in Fig. 4(b). The XRD pattern shows that the sintered body is composed of three phases, namely the α-SiC, the βSiC and the silicon itself. Among them, α-SiC is the original SiC, while β-SiC is the newly-formed SiC, which came into existence through C–Si reaction. When the back-scattered electron detector shown in Fig. 4(c)
is analyzed, it is found that the material is composed of two different phases. Combined with the EDS analysis of Points 1 and 2 (as presented in Table 1), the white phase comes out to be silicon, while the gray phase turns out to be SiC. These silicon islands are the major cause of the degradation of performance of RBSC composites [19]. It is worth noticing that the α-SiC and β-SiC cannot be identified due to identical chemical compositions. It is interesting to note that the use of a secondary electron detector reveals a core-rim structure, as shown in Fig. 4(d). According to the EDS analysis of Points 3 and 4 (presented in Table 1) and the morphology and size of the original SiC particle, the bright white particles turn out to be the original SiC, while the thin layer around them corresponds to the newly-formed β-SiC. The other dark gray region is the silicon (Point 5 presented in Table 1). The newly-formed SiC grew onto the original SiC particle to form the corerim structure during the reaction-bonded process. In order to clearly observe the microstructure, we used a mixed acid (hydrofluoric acid and nitric acid in the volumetric ratio of 1:7, respectively) to remove residual silicon (see Reaction Equation (1)). 3Si+4HNO3+12HF→4NO↑+3SiF4↑+8H2O
(1)
The microstructure of RBSC composites after etching is shown in Fig. 5(a). The newly-formed SiC bonded the original SiC. Fig. 5(b) shows the three-dimensional topography presented in Fig. 5(a). Two kinds of SiC phases were found to form the continuous ceramic skeleton. In order to explore the reaction-bonded process, the fracture in sintered body was etched to remove the residual silicon. During the whole process, it is of great significance to analyze the microstructural development. The morphologies of original SiC particles are shown in Fig. 6(a). The newly-formed SiC gradually grew on the original SiC. The growth models can be divided into two categories, namely the refined newly-formed SiC grains growing around the original SiC boundary (see Fig. 6(b)) and the compact connection layer bonding those original SiC particles (see Fig. 6(c)). Fig. 6(d) shows the newly-formed SiC among the original SiC particles and the two growth models mentioned above.
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Fig. 4. Microstructures and XRD diffraction pattern of RBSC composites: (a) optical photograph; (b) XRD diffraction pattern; (c) SEM photograph obtained using the back-scattered electron detector and (d) SEM photograph obtained using the secondary electron detector. Table 1 EDS analysis results, as shown in Fig. 4(c) and (d). Point
1 2 3 4 5
Element (at.%) C
Si
48.12 0 47.37 45.29 0
51.88 100 52.63 54.71 100
According to the above-mentioned analysis, the newly-formed SiC exists in two different forms, namely the compact SiC connection layer and the refined newly-formed SiC grains. This result agrees with the one reported in a previous work [20]. 3.1.3. Reaction-bonded mechanisms Ness and Page [21] argued that the dissolution of carbon source in liquid silicon is an exothermic process and that the carbon source might exist in three forms, namely C, C–Si and CSi4. Furthermore, with the increase in temperature, the solubility of carbon in liquid silicon
increased, which promoted the diffusion of carbon from higher temperature region to cooler original SiC. The carbon diffused along the carbon concentration gradient until a supersaturation occurred in the locally cooler sites. Then, the newly-formed SiC precipitated on the surface of original SiC particles. Fig. 7 shows the schematic of the reaction-bonded process. Fig. 7(a) shows the structure of porous SiC/C preform. Fig. 7(b) reveals that the liquid silicon enters the preform, after which, the RBSC process begins. As the reaction progresses, the newly-formed SiC grows on the newly-formed SiC particles, which leads to the occlusion of some capillary channels. Ultimately, the entire carbon source is converted to SiC, which precipitates on the surface of original SiC particles. The liquid silicon infiltrates the residual pores and forms the island of residual silicon. This silicon island is detrimental to the performance of RBSC composites (see Fig. 7(c). In the later cooling process, carbon in liquid silicon becomes supersaturated and the viscosity of C–Si solution increases with the decrease in temperature. Some nanometer-sized newly-formed SiC grains are generated in situ, which are in accordance with the microstructures shown in Fig. 6(b), (c) and 6(d).
Fig. 5. Microstructure of RBSC composites after etching: (a) surface appearance; (b) three-dimensional topography. 17990
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Fig. 6. Microstructural development during the RBSC process: (a) original SiC particle; (b) refined newly-formed SiC; (c) compact connection layer and (d) two growth models of the newly-formed SiC.
3.2. Optimization of phase composition of RBSC composites 3.2.1. Control method According to the XRD pattern and microstructural observations, the RBSC composites consist of three phases (original α-SiC, newly-formed β-SiC and silicon). The density of silicon (2.33 g/cm3) is larger than the density of SiC (3.21 g/cm3). The residual silicon decreases with the increase in the density of RBSC composites. The silicon phase is detrimental to the performance of RBSC composites, which results from inferior flexural strength (≤100 MPa) and low melting point (ca. 1410 °C) [22,23]. It is essential to decrease the dimension and content of residual silicon [20]. RBSC is a near net-shape forming technique for ceramics, which has provided an effective approach for resolving the difficulty of
manufacturing and processing complicated structural ceramics. The entire carbon source is converted to SiC, which is accompanied by volume expansion. The volume expansion times (T) can be acquired using Equation (2).
T = MSiC DCarbon /MC DSiC
(2)
where MSiC is the relative molecular mass of SiC, MC is the relative atomic mass of C, DCarbon (g/cm3) is the theoretical density of C and DSiC (g/cm3) is the theoretical density of SiC. Based upon the reaction-bonded process, the newly-formed SiC fills some pores and the residual silicon fills the rest. In order to decrease the residual silicon content, the compression moulding pressure was adjusted to control the density of preform, thereby decreasing the porosity. Fig. 8 shows the variation in preform density with the
Fig. 7. Schematic of the RBSC process: (a) structure of the porous SiC/C preform; (b) liquid silicon entering the preform and (c) densification of the RBSC composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 17991
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Fig. 9. Variations in DRBSC, bulk density and flexural strength for different carbon densities of the preforms.
Fig. 8. Variation in preform density with the compression moulding pressure.
compression moulding pressure. When the pressure increases from 120 MPa to 180 MPa, the bulk density of preform obviously improves. It is worth noticing that the preform densities decrease with the further increase in pressure. When the pressure is too high, the original particles are in close contact with each other and the air among these original particles cannot be discharged. The preform springs back during the demoulding process, thereby decreasing the bulk density of preform. It is observed that the entire carbon source is converted to SiC. In other words, all carbon element in the preform exists in the form of SiC. Therefore, the carbon density of preform helps in calculating the value of theoretical density of RBSC composite. Ultimately, the carbon density of preform is adjusted to control the value of volume fraction of residual silicon. The carbon density of preform (DC) can be calculated using Equation (3).
DC = mpreform ωSiC MC /MSiC + mpreform ωmixedC /Vpreform
(3)
where mpreform (g) is the preform mass, ωSiC is the mass fraction of SiC, ωmixedC is the mass fraction of mixed carbon, and Vpreform (cm3) is the volume of preform. According to the principle of mass conservation, the near net-shape forming characteristic and the phase composition of RBSC composites, the relationship between DC and the theoretical density of RBSC composite (DRBSC) is given by Equation (4).
DRBSC = DC MSiC /MC + DSi (1 − DC MSiC /MC DSiC) = 2. 33 + 0. 92DC (4) where Dsi (g/cm3) is the theoretical density of silicon. It can be seen that DC is the major factor contributing to the residual silicon content and DRBSC. It can also be used to evaluate the preform quality. Furthermore, the volume fraction of residual silicon (φSi) can be calculated using Equation (5).
ϕSi = 3. 21 − DRBSC /0. 88
(5)
The residual silicon content decreases with the increase in the value of DRBSC. It is worth mentioning that DRBSC is an important factor affecting the performance of ceramics. 3.2.2. Maximum carbon density of the preform In some cases, some unreacted carbon, called the residual carbon, is found in the RBSC composites. The existence of residual carbon in the internal zone of RBSC composites greatly reduces their performance. Popper [24] reported that carbon source was converted to SiC along with some volumetric expansion, which was detrimental to the infiltration of liquid silicon (the skin effect phenomenon). According to the reaction-bonded process and computed results, it is a very effective way to improve the value of DC so as to reduce the residual silicon
content in the RBSC composites. However, some unreacted carbon will exist in the RBSC composites with the increase in the value of DC. It is important to find the maximum value of DC. In this case, the residual silicon content reaches the minimum level and all the carbon is converted to SiC. For this reason, some porous SiC/C preforms with a series of carbon densities were prepared to find the maximum value of DC. Fig. 9 shows the variations in DRBSC, bulk density, and flexural strength for different carbon densities of the preforms. As can be seen, the bulk densities of RBSC composites increase with the increase in value of DC. When the value of DC is less than 0.84 g/cm3, the bulk densities of RBSC composites are in good agreement with the theoretical densities. As the value of DC increases further, the bulk densities of RBSC composites decrease. This is mainly because the excess carbon source was converted to plenty of newly-formed SiC, which blocked the capillary channel and led to the difficulty of liquid silicon infiltration. The pores and unreacted carbon existed in the RBSC composites, thereby decreasing the bulk densities of RBSC composites. The maximum value of DC is 0.84 g/cm3. In other words, the value of DC should be controlled to obtain the optimal raw material ratio before the reaction-bonded process. The value of DC is a useful performance parameter for evaluating the performance of RBSC composites. Porous SiC ceramics have many advantages, including good thermal-shock stability, high mechanical properties and chemical stability. It is widely used for metal filtration, biological carrier and lightweight heat-resistant components. It is interesting to note that porous SiC ceramic can be prepared by removing the residual silicon in RBSC composites under the conditions of high temperature and high vacuum. In Liu's research [25], porous α-SiC ceramics were prepared by recrystallization method using silicon as the template. The porosity of SiC ceramics was regulated by controlling the amount and size of silicon particle and sintering temperature. The silicon phase was carried off into the external environment in a gaseous form at high temperatures. Afterwards, the original SiC and the newly-formed SiC connected together through recrystallization. Considering the controlling of silicon content by adjusting the value of DC, porous SiC ceramics with different porosities can be prepared using the above-mentioned method. In addition, the liquid silicon infiltrates the capillary channel and a through-hole is formed after the removal of residual silicon. When the value of DC is 0.84 g/cm3, the volume fraction of residual silicon is 12.5%. In general, the porous SiC ceramics with different porosities (≥12.5%) can be prepared by adjusting the value of DC.
3.2.3. Process of capillary channel blockage Fig. 10(a) shows the fracture morphology of RBSC composites with the corresponding carbon density of 0.85 g/cm3. As can be seen, the
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Fig. 10. Fracture morphology of RBSC composites with excessive carbon density: (a) porous fracture appearance with the corresponding carbon density of 0.85 g/cm3; (b) magnification of the red rectangle shown in (a); (c) unreacted fracture appearance with the corresponding carbon density of 0.86 g/cm3 and (d) magnification of the red rectangle shown in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
structure close to the outer edge is compact, while that in the center is porous. The porous area is small, and the carbon source reacted completely. Fig. 10(b) shows the detailed view of red rectangle shown in Fig. 10(a). The liquid silicon is unable to infiltrate the center due to capillary channel blockage, thereby forming a porous structure. Fig. 10(c) shows the fracture morphology of RBSC composites with the corresponding carbon density of 0.86 g/cm3. The thickness of the outer compact edge is about 0.3 mm and a lot of raw materials exist in the center. Fig. 10(d) shows the detailed view of red rectangle shown in Fig. 10(c). The outer thin edge is compact and its structure is in agreement with the traditional RBSC composites. The structure in the center region agrees with that of the SiC/C preform. Combined with the reaction mechanism, the process of capillary channel blockage is summarized in Fig. 11. The complexity was reduced, and the formation, narrowing and blockage of capillary channel during the reaction-bonded process were described (see Fig. 11(a), (b) and 11(c)). In Fig. 11(a), the pores among those original SiC particles formed the capillary channels. With the increase in temperature, the solubility of carbon in liquid silicon increased, which promoted the diffusion of carbon from higher temperature region to cooler original SiC. The carbon diffused along the carbon concentration gradient until a supersaturation occurred in the locally cooler sites. Then, the newlyformed SiC precipitated on the surface of original SiC particles. The above process caused the capillary channel to become narrower (see Fig. 11(b)). As the reaction progressed, the capillary channel closed (see Fig. 11(c)) and liquid silicon could not further infiltrate the preform.
3.3. Assumption of two kinds of approaches to break the limit of maximum carbon density of the preform Considering the process of capillary channel blockage, two kinds of approaches are put forward to solve the blockage, thereby breaking the limit of maximum value of DC and improving the performance of RBSC composites.
3.3.1. Utilizing the slow-release carbon source The nanometer-sized carbon black is amorphous and has high specific surface area. Therefore, it has high activity and its siliconization reaction is fast. When the value of DC is too high, the capillary channel is prone to be blocked. One solution to this problem is to use the carbon with low activity as the source. This carbon source, herein called the slow-release carbon source, exhibits large size (low specific surface area) and high graphitization. During the liquid silicon infiltration, the carbon with low activity is gradually released into the liquid silicon. This process prolongs the siliconization reaction, thereby postponing the capillary channel blockage. If so, it has plenty of time for liquid silicon to infiltrate the porous SiC/C preform. The unreacted residual carbon is usually eliminated through the follow-up heat treatment. The unreacted residual carbon reacts with the residual silicon through diffusion. The size of slow-release carbon source should not be too large. The addition of oversized slow-release carbon source results in an uneven structure and some unreacted residual carbon in the system. As long as the amount of selective slow-release carbon source is reasonable, the maximum value of DC can be improved further. More
Fig. 11. Schematic of the process of capillary channel blockage: (a) original SiC particles forms the capillary channel; (b) capillary channel becomes narrower and (c) capillary channel closes. 17993
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Fig. 12. Schematic of the influence of slowrelease carbon on the reaction-bonded process.
Fig. 13. Schematic of fabricating the new-style structure capillary channel.
newly-formed SiC is produced and the residual silicon content decreases further. In this method, a single species of carbon can be used as the slow-release carbon source, whereas the reaction-bonded time becomes long. Mixed carbon, which is composed of two or more kinds of carbons with different activities, can also be used in the reactionbonded process. The carbon with high activity will preferentially react with liquid silicon. The refined carbon particle with high activity fills the available pores, while the large carbon particle with low activity prolongs the siliconization reaction, thereby postponing the capillary channel blockage. Compared with a single species of carbon, mixed carbon can adjust the siliconization reaction time and decrease the residual silicon content to some degree. Fig. 12 shows the influence of slow-release carbon on the reaction-bonded process. The black circles with different particle sizes represent the mixed slow-release carbon, which is evenly distributed among the original SiC particles. When the reaction-bonded process begins, the mixed slow-release carbon releases the carbon element into the liquid silicon in a controlled fashion, thereby solving the problem of capillary channel blockage to some extent.
3.3.2. Fabricating a new-style structure capillary channel The capillary channel built by the original SiC particles is evenly distributed in the whole of preform. The capillary channel becomes narrower and narrower as the reaction progresses. It is a good choice to fabricate a capillary channel with a new-style structure to solve this problem. The characteristics of the capillary channel are as follows. (1) The new-style structure capillary channel possesses stable physical and chemical properties, which ensure an almost unchanged width and structure during the reaction process. The new-style structure capillary channel, whose surface cannot be coated by the newlyformed SiC phase, can be on active service until the end of reaction. (2) The new particles, forming the new-style structure capillary channel, bring no negative impact on the RBSC composites, whereas the width of capillary channel is tunable. Considering this, a multimodal particle size distribution can be used to adjust the width of capillary channel.
mentioned conditions. In a previous research [20], boron carbide was used as the reinforced particles, which formed the capillary channel with new-style structure. The content and size of residual silicon decreased sharply, thereby effectively improving the performance of RBSC composites. More importantly, the production of a large number of nanometer-sized newly-formed SiC particles effectively enhanced the performance of RBSC composites. In the same way, other suitable particles can be chosen to replace boron carbide. Fig. 13 shows the schematic of fabricating the new-style structure capillary channel, which is evenly distributed in the whole of preform. Its stable structure ensures adequate infiltration of liquid silicon and can effectively solve the problem of capillary channel blockage.
4. Conclusions In order to control the performance of RBSC composites, prepared using additive manufacturing (AM) or other forming methods, the reaction-bonded process and the microstructural development of RBSC composites were studied in this work. The key factor affecting the performance and residual silicon content of RBSC composites was found to be the carbon density of SiC/C preform. The reason of the capillary channel blockage was the excessive carbon density. In this research, the maximum value of carbon density of preform was 0.84 g/cm3, while the corresponding bulk density was 3.10 g/cm3. The volume fraction of residual silicon was 12.5% and the flexural strength was 349 MPa. The parameters of raw materials (original SiC particle shape and size, carbon particle size and activity) determined the maximum value of carbon density. The maximum value of carbon density could be determined by analyzing the data for different carbon densities. The performance of RBSC composites could be reasonably predicted and controlled by regulating and measuring the carbon density of preform. Furthermore, two kinds of approaches were put forward to solve the problem of capillary channel blockage: (i) utilizing the slow-release carbon source and (ii) fabricating a new-style structure capillary channel. This research lays an elementary foundation for manufacturing SiC-based ceramics.
The value of DC can be improved suitably based on the above17994
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Acknowledgement This work was supported by the Basic Scientific Research of Xi’an Jiaotong University (No. xjh012019007), the Natural Science Foundation of China (No. 51372194) and the Social Development Fund of Shaanxi Province (No. 2015KTCQ03-01). References [1] Y.W. Kim, S.L. Kim, Advances in gammalloy materials–processes–application technology: successes, dilemmas, and future, J. Occup. Med. 70 (2018) 553–560. [2] M. Perrut, P. Caron, M. Thomas, A. Couret, High temperature materials for aerospace applications: Ni-based superalloys and γ -TiAl alloys, C. R. Phys. 19 (2018) 657–671. [3] T. Tetsui, T. Kobayashi, H. Harada, Achieving high strength and low cost for hotforged TiAl based alloy containing beta phase, Mat. Sci. Eng. A Struct. 552 (2012) 345–352. [4] S.V. Raj, Development and characterization of hot-pressed matrices for engineered ceramic matrix composites (E-CMCs), Ceram. Int. 45 (2019) 3608–3619. [5] C. Xu, C. Xu, F. Han, F. Zhang, W. Wei, Z. Zhong, W. Xing, Fabrication of high performance macroporous tubular silicon carbide gas filters by extrusion method, Ceram. Int. 44 (2018) 17792–17799. [6] Q. Tian, N. Wu, B. Wang, Y. Wang, Fabrication of hollow SiC ultrafine fibers by single-nozzle electrospinning for high-temperature thermal insulation application, Mater. Lett. 239 (2019) 109–112. [7] I.G. Crouch, M. Kesharaju, R. Nagarajah, Characterisation, significance and detection of manufacturing defects in reaction sintered silicon carbide armour materials, Ceram. Int. 41 (2015) 11581–11591. [8] A. Tasdemirci, G. Tunusoglu, M. Güden, The effect of the interlayer on the ballistic performance of ceramic/composite armors: experimental and numerical study, Int. J. Impact Eng. 44 (2012) 1–9. [9] J.M. Fernández, A. Munoz, A.R.D. López, F.M.V. Feria, A. Dominguez-Rodriguez, M. Singh, Microstructure-mechanical properties correlation in siliconized silicon carbide ceramics, Acta Mater. 51 (2003) 3259–3275. [10] Y.Y. Zhang, C.Y. Hsu, S. Aubuchon, P. Karandikar, C.Y. Ni, Microstructural and thermal property evolution of reaction bonded silicon carbide (RBSC), J. Alloy, Compd 764 (2018) 107–111.
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