- Nano Express
- Open Access
Sintering Behavior of Spark Plasma Sintered SiC with Si-SiC Composite Nanoparticles Prepared by Thermal DC Plasma Process
© The Author(s). 2017
- Received: 16 July 2017
- Accepted: 10 November 2017
- Published: 25 November 2017
The Si-coated SiC (Si-SiC) composite nanoparticle was prepared by non-transferred arc thermal plasma processing of solid-state synthesized SiC powder and was used as a sintering additive for SiC ceramic formation. Sintered SiC pellet was prepared by spark plasma sintering (SPS) process, and the effect of nano-sized Si-SiC composite particles on the sintering behavior of micron-sized SiC powder was investigated. The mixing ratio of Si-SiC composite nanoparticle to micron-sized SiC was optimized to 10 wt%. Vicker’s hardness and relative density was increased with increasing sintering temperature and holding time. The relative density and Vicker’s hardness was further increased by reaction bonding using additional activated carbon to the mixture of micron-sized SiC and nano-sized Si-SiC. The maximum relative density (97.1%) and Vicker’s hardness (31.4 GPa) were recorded at 1800 °C sintering temperature for 1 min holding time, when 0.2 wt% additional activated carbon was added to the mixture of SiC/Si-SiC.
- Si-SiC nanoparticles
- Plasma processing
- Spark plasma sintering
Silicon carbide (SiC) ceramics have been attracting great attention due to its phenomenal properties, such as high-temperature hardness, wear resistance, low thermal expansion coefficient, high thermal conductivity, strong corrosion resistance, and high stability in aggressive environment, and have been applied for various fields such as turbine blades, diesel engine parts, and aerospace and nuclear reactor materials [1–6]. However, it is difficult to densify the SiC without additives because of the covalent nature of Si–C bonding and low self-diffusion coefficient [7, 8]. The bulk SiC materials are usually prepared either by the solid state sintered silicon carbide (SSS-SiC) or by the liquid phase sintered silicon carbide (LPS-SiC) from the starting SiC crystalline powders [7, 8]. In the case of SSS-SiC, no liquid forming additives, such as boron, aluminum, carbon, or their compounds, have been used for densification of SiC by the reduction of the surface energy of grains and the reaction between silica present on surface and carbon. However, this process requires over 2000 °C temperature for sintering [7, 9, 10]. LPS-SiC is governed by liquid phase formation of metal oxide additive at sintering temperature and this liquid phase act as a mass transport media during SiC sintering [8, 11, 12]. Except magnesia and alumina, yttria and other rare earth oxides are mostly used as sintering additives, and sintering temperature can be decreased down to 1850 °C, depending upon the used combination of sintering additives [11, 12]. However, presence of the amorphous silicate compound at grain boundaries and the triple points cause decrease of hardness and high-temperature creep resistance as compared with the SSS-SiC . But the additives used to enhance processing invariably become a “weak” secondary phase in the final ceramic, which usually lower its mechanical properties at high temperature . This detrimental effect infers that the smallest fraction of additives is desirable. In addition, the effectiveness of the additives greatly depends on the homogeneity of their distribution . There is also another approach for fabrication of bulk SiC, which is called as reaction bonded silicon carbide (RB-SiC). In RB-SiC, the reaction of molten silicon with carbon powder results in a formation of SiC [13–16]. Although this approach requires lower sintering temperature and there is no limitation of product shape and size, low density of the bodies is a disadvantage [17, 18]. However, lowering of sintering temperature is essential to save the energy. In recent time, energy saving becomes the driving force to find other methods suitable for the preparation of bulk SiC ceramics at low temperature.
Recently, nano-sized SiC has been widely investigated to examine their mechanical, physical, and chemical properties that are different from those in bulk forms and often useful [19–21]. For example, nanopowders primarily due to the higher specific surface areas and surface activities can provide the low-temperature sinterability of nano-sized SiC in the consolidation processing and the improvement of mechanical properties by making it possible to reach high densities . Therefore, in present, we have developed a new method to prepare Si-coated SiC (Si-SiC) nanoparticle to apply as a sintering additive by using non-transferred thermal DC plasma processing of solid-state synthesized SiC powder .
In this study, the nano-sized Si-SiC composite particle as a sintering additive was applied for preparing bulk SiC ceramic by spark plasma sintering (SPS) process, and the effect of addition of the nano-sized Si-SiC composite particle on sintering temperature, relative density, and Vicker’s hardness of sintered SiC ceramic was investigated. In addition, to further increase the relative density and hardness of sintered SiC, reaction bonding between free silicon of nano-sized Si-SiC particle and activated carbon which was additionally added was newly introduced to SiC sintering process. The sintering mechanism of the SiC ceramic produced with nano-sized Si-SiC composite additive through SPS process was also discussed on the basis of nano-size effect and reaction bonding effect. This study provides a new promising strategy to be able to prepare the SiC ceramic with high density and hardness at a relatively low sintering temperature.
Micron-sized SiC Powder Preparation
The micron-sized SiC was synthesized by using Si powders with an average particle size of 25 μm (99.9%; Neoplant Co. LTD.) and activated carbon with an average particle size of 32 μm (Sigma-Aldrich). In a typical procedure, 1:1.5 mol ratio of Si and carbon were mixed together by using ball mill for 15 h. The mixed powder was placed in a vertical tube furnace and heated at 1300 °C for 2 h with 10 °C/min heating rate in the presence of argon gas (1 L/min). After completion of the reaction, the obtained powder was grinded in agate mortar for further characterization.
Plasma Processing of Synthesized SiC Powders
Plasma processing was carried out by non-transferred arc thermal plasma reactor as reported in our previous work [21, 23]. The milled SiC powder was fed into the plasma arc through the internal feeding pipeline of 2-mm inner diameter in the plasma torch using a specially designed powder feeder. The powder feeding system consisted of a sample container, a vibrator, and a carrier gas line. Powders were fed by vibrating feeder at 70 V with 1 g/min feeding rate. Typical synthesis experiments were operated at system pressures of 200 Torr, with Ar plasma gas flow rates of 30 L/min, H2 gas flow rates of about 3 L/min, and DC current of 300 A (at 45 V). After plasma ignition, a micron-sized SiC powder was supplied by feeder. The synthesized nanopowders were collected from the reactor wall and bottom of the plasma reactor system. The yield was about 80–85%.
Preparation of Sintered SiC Pellet
Sintered SiC pellet was prepared by SPS process (as shown in Fig. 1). Both SiC materials, i.e., the micron-sized SiC powder synthesized by calcination process and the nano-sized Si-SiC powder obtained from plasma process, were used without additional additives. The mixing content of Si-SiC nanoparticles in micron-sized SiC powder was changed from 5 to 15 wt%.
The mixed powders were put into a graphite die (20 mm in diameter) and sintered with SPS system in vacuum atmosphere (10−2 Torr). The heating rate was fixed at 600 °C/min, and the applied pressure was 80 MPa. The sintering temperature was changed from 1600 to 1800 °C. The holding time at target temperature was varied from 0 to 1 min at 1800 °C. After sintering, the sample surfaces were grounded to remove the graphite layer and then polished by a diamond paste. The density of the sintered specimens was measured by the Archimedes method in deionized water as an immersion medium.
The crystallographic structures of the solid samples were determined using a XRD (D/Max 2005 Rigaku) equipped with graphite monochromatized high-intensity Cu-Kα1 radiation (λ = 1.5405 Å). The XRD patterns were recorded from 20° to 80° (2θ) with a scanning speed of 0.04°/s. Particle size and morphology were investigated by a scanning electron microscope (SEM; JSM-5900, JEOL) and transmission electron microscope (TEM; JEM-2010, JEOL).
Composition of micron-sized SiC and nano-sized Si-SiC composite particle, temperature, and pressure for preparing sintered SiC pellets and their relative densities and Vicker’s hardness
Holding time (min)
Relative density (%)
Vicker’s hardness (GPa)
The changes of relative density and Vicker’s hardness of the sintered SiC according to sintering temperature and holding time at target sintering temperature are also given in Table 1. Relative density and hardness increases with increasing sintering temperature, and highest relative density (87.4%) and hardness (18.6 GPa) were recorded at 1800 °C. The relative density and hardness was further increased to 88.2% and 21.2 GPa, respectively, with increasing holding time from 0 to 1 min at 1800 °C sintering temperature. It suggest that relative density and hardness increases with increasing holding time; unfortunately, the holding time at 1800 °C could not be increased further due to limitation of the SPS system.
Composition of micron-sized SiC, nano-sized Si-SiC composite particle, and activated carbon for preparing sintered SiC pellets, and their relative densities and Vicker’s hardness (sintering temp. 1800 °C, holding time 1 min, pressure 80 MPa)
Relative density (%)
Vicker’s hardness (GPa)
Micron-sized SiC (2–5 μm) powder was synthesized by a solid-state method using Si powder and activated carbon sources. Nano-sized Si-SiC composite powder, having 20–70 nm particle size, was prepared by non-transferred arc thermal plasma process. Sintered SiC pellets were prepared by SPS process using the mixture with different ratio of micron-sized SiC powder and nano-sized Si-SiC composite particle as a sintering additive. At a fixed ratio of micron-sized SiC and nano-sized Si-SiC (90:10), the relative density and Vicker’s hardness increased with increasing sintering temperature and holding time. The maximum relative density (88.2%) and Vicker’s hardness (21.2) were recorded at 1800 °C sintering temperature for 1 min holding time. The relative density and Vicker’s hardness was further increased by addition of extra activated carbon to the mixture of micron-sized SiC and nano-sized Si-SiC. The relative density and Vicker’s hardness were increased to 97.1% and 31.4 GPa, respectively, with the addition of 0.2 wt% of extra activated carbon to the SiC/Si-SiC mixture. It was found that the nano-size effect of Si-SiC composite particle and the exothermic nature of silicon–carbon reaction bonding were responsible for the increase in relative density and hardness. Therefore, it was suggested that the nano-sized Si-SiC composite particle could be a promising additive for sintering of SiC ceramics.
This paper was supported by the (1) BK21 plus program from the Ministry of Education and Human-Resource Development, (2) National Research Foundation grant funded by the Korean Government (MSIP) (BRL No. 2015042417, 2016R1A2B4014090), and (3) Research Base Construction Fund Support Program funded by Chonbuk National University in 2016.
This research work was supported by the funding of National Research Foundation grant funded by the Korean Government (MSIP) and Research Base Construction Fund Support Program of Chonbuk National University in 2016.
Y-TY designed the research work. He has carried out the interpretation of all the data such as the XRD patterns, SEM images, and hardness and density changes of sintered SiC pellets. He also proposed the sintering mechanism from shrinkage displacement change of SiC pellets sintered with nano-sized Si-SiC composite nanoparticles during SPS process. GKN has contributed in observing the morphology and crystal structure of Si-SiC composite nanoparticles and studied on the formation of metallic Si-loaded SiC nanoparticles in DC thermal plasma reactor. Y-BL has prepared the micron-sized SiC and nano-sized Si-SiC composite powders by using calcination furnace and DC plasma reactor. He has also sintered the mixture of those powders by using the SPS (spark plasma sintering) processing and analyzed the surface morphology, relative density, and Vicker’s hardness of the SiC sintered pellets. J-MY has designed the non-transferred DC arc thermal plasma reactor and tuned the operating condition for preparing the nano-sized Si-SiC composite powders. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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