Effect of Applied Pressure During Sintering on the Densification and Mechanical Properties of SiCf/SiC Composites Prepared by Electrophoretic Infiltration

SiC-fiber-reinforced SiC matrix ceramic (SiCf/SiC) composites were successfully fabricated by electrophoretic infiltration and sintering at various applied pressures. The effect of applied pressure (i.e., 5, 10, and 20 MPa) was thoroughly examined, and applied pressure appears to influence the densification and mechanical properties of SiCf/SiC composites. The densities of SiCf/SiC composites prepared at applied pressure of 5, 10, and 20 MPa were 2.99, 3.10, and 3.16 g/cm, respectively. All samples showed dense microstructures in their matrix areas; however, many closed pores were found with increasing pressure. Pressure induced densification in the fiber area, and 20 MPa was determined to be the minimum pressure required for adequate densification. The liquid phase in the matrix area was effectively maintained at a high applied pressure, which resulted in densification. However, the liquid phase redistributed to the matrix or near the matrix area at applied pressures of <20 MPa, leading to pores in the fiber areas of these samples. The highest flexural strength of 262 ± 18 MPa for the SiCf/SiC composite was achieved at an applied pressure of 20 MPa. Meanwhile, the flexural strengths of the composites prepared at 5 and 10 MPa were 198 ± 41 and 238 ± 32 MPa,


Introduction
High-temperature applications of SiC-fiber-reinforced SiC matrix ceramic (SiCf/SiC) composites require these composites to be dense because the mechanical properties of these composites improve with density. Moreover, SiC fibers need to maintain their original shapes to prevent damage to the fibers.
Several well-known methods have been used to fabricate SiCf/SiC composites. High-purity SiCf/SiC composites are obtained by chemical vapor infiltration (CVI), which is an ideal method for the fabrication of these composites [1][2][3]. The precursors used in this method are gaseous; therefore, extended processing times are required to achieve porosities of approximately 10-20%. Polymer impregnation and pyrolysis (PIP) is another method for the fabrication of SiCf/SiC composites [4][5][6]. In contrast to CVI, this method uses polymer precursors instead of gases; however, it requires multiple impregnations to obtain the desired density. To utilize the advantages of these two methods, Ortona et al. reported a combined CVI/PIP method [7]. Another approach for the fabrication of SiCf/SiC composites involves slurry infiltration combined with sintering [8][9], and electrophoretic infiltration is one such method [10][11][12][13][14][15][16].
Electrophoretic infiltration was initially used to deposit a coating layer on a substrate [17][18]. Novak et al. investigated electrophoretic infiltration as a possible infiltration method for the production of SiCf/SiC composites [10] and determined that submicron SiC powder sufficiently infiltrates SiC fibers compared with nanosized SiC powder. In another report, Novak et al. discovered that the zeta potential is the most important parameter for electrophoretic infiltration and dense deposits can be obtained from suspensions with high zeta potentials [11]. A method that combined electrophoretic infiltration and PIP was used to prepare high-density SiCf/SiC composites [12], and a SiCf/SiC composite with a relative density of 86.5% was obtained after six PIP cycles. Gil and Yoon reported that SiCf/SiC composites prepared by electrophoretic infiltration exhibited the highest density [14], wherein a relative density of 99.5% was achieved by combined  [14][15][16]; however, to the best of our knowledge, no studies that examine the effect of external pressure during sintering of SiCf/SiC composites prepared by electrophoretic infiltration have been reported.
This study aimed to examine the effect of external pressure on the densification and mechanical properties of a SiCf/SiC composite prepared by electrophoretic infiltration. The density, microstructural, and mechanical property data were analyzed to elucidate the effect of external pressure on the SiCf/SiC composite.

Materials and Methods
Commercial β-SiC powder (>97.5% purity; 4620KE, NanoAmor Inc., USA) with average particle size and specific surface area of 52 nm and 80 m 2 /g, respectively, was used as the matrix phase. The SiC powder particles contain thin SiO2 layers because of the reaction between SiC and O2 in the air [19]. To decrease the sintering temperature and increase the density of SiC, 12 wt% of sintering additives (i.e., Al2O3, Dm = 150 nm, >99.9% purity [Baikowski, Japan] and Y2O3, Dm = 200 nm, >99.9% purity [Acros Organics, USA] in a 60:40 mass ratio) was added to the commercial β-SiC powder. The mixed powder was dispersed in ethanol and subjected to ball milling for 24 h using 5-mm SiC balls to minimize contamination. Prior to ball milling, the mixed-powder dispersion was enhanced by the addition of 3 wt% of Hypermer KD1 as the dispersant. Moreover, the zeta potential of the mixed powder was controlled at >40 mV by setting the pH to 3. The β-SiC slurry containing the sintering additives was used to electrophoretically infiltrate a woven Tyranno SA3 fabric (Ube Industries Ltd., Japan) coated with 400-nmthick pyrolytic carbon (PyC). The distance between fabric and electrode in the dual electrophoretic infiltration system was 20 mm. The applied voltage and electrophoretic infiltration time were 10 V and 30 min, respectively. Ultrasonication (HD 2070, Bandelin, Germany) was applied over the first 20 min to minimize the surface sealing effect. After the infiltrated fabric was dried, 15 layers of fabric were laminated under an applied pressure of 10 MPa at 80 °C. Binder burnout was conducted at 350 °C for 2 h in the air at a heating rate of 0.5 °C/min to remove organic matter from the infiltrated fabric. Sintering was performed in a hot-pressing furnace at 1,750 °C for 1 h under Ar. Three different ap-plied pressures were used (i.e., 5, 10, and 20 MPa) during sintering.
The densities of the SiCf/SiC composites were measured using the Archimedes principle, with distilled water as the medium. Scanning electron microscopy (SEM; S-4800, Hitachi, Japan) was used to analyze the microstructures of the sintered SiCf/SiC composites. Thermal etching was performed at 1,500 °C for 30 min under vacuum after each sample had been polished. The average grain size of each sample (i.e., 100 grains) was estimated from the SEM image after thermal etching and analyzed statistically. Three-point bend testing was performed using an ultimate testing machine (UTM AG-50E, Shimadzu, Japan). Each composite was cut into a 4 mm × 2 mm × 20 mm piece and polished using a 1-µm diamond paste to remove scratches from the sample before bend testing. Figure 1 shows the images of the SiC fabric after electrophoretic infiltration. Figure 1(a) shows that the SiC fabric is fully covered by SiC powder after electrophoretic infiltration for 30 min at an applied voltage of 10 V. SiC powder not only covers the surface of the SiC fabric but also deeply infiltrates the fiber area of the fabric to entirely cover its fibers (Figure 1(b)). These images indicate that electrophoretic infiltration is an effective method for the preparation of SiCf/SiC composites. Figure 2 shows the densities and relative densities of the SiCf/SiC composites sintered at different applied pressures, which indicate that density increases with increasing applied pressure. The average densities of the SiCf/SiC composites prepared at 5, 10, and 20 MPa were determined to be 2.99, 3.10, and 3.16 g/cm 3 , respectively. The standard deviations of the average densities are large at 5 and 10 MPa. By contrast, the standard deviations of the average densities are small at 20 MPa. These results indicate that the SiCf/SiC composites prepared at 5 and 10 MPa contain many pores and have not been completely densified. Given that the theoretical density of SiC is 3.21 g/cm 3 , the relative densities of the SiCf/SiC composites prepared at 5, 10, and 20 MPa are 93.1%, 96.5%, and 98.4%, respectively. Figure 3 shows the SEM images of sintered SiCf/SiC composites prepared at different applied pressures, which indicate that all samples have dense structures in their matrix areas (Figures 3(a), 3(c), and 3(e)); however, pores are visible in the fiber areas, particularly for the composites prepared at 5 MPa (Figure 3(b)) and10 MPa (Figure 3(d)). These results are consistent with the data presented in Figure 2, that is, the SiCf/SiC composites prepared at 5 and 10 MPa are less dense than that   (Figure 3(e)), as well as in its fiber area (Figure 3(f)). Notably, white phases are present in the matrix and fiber areas, particularly for the SiCf/SiC composite prepared at 20 MPa. Energydispersive X-ray spectroscopy (EDS) analysis ( Figure 4) showed that the white phase is composed of Al, Y, Si, O, and C, which correspond to the elements present in the sintering additives (i.e., Al2O3 and Y2O3). The source of Si may be SiO2 on the surface of SiC, which reacts with Al2O3 and Y2O3 and reduces the temperature to induce the formation of the liquid phase. The eutectic temperatures of Al2O3-Y2O3 and Al2O3-Y2O3-SiO2 are 1,760 °C and 1,379 °C [20], respectively; hence, the white phases are deduced to be the liquid phases that form because of the reactions between Al2O3, Y2O3, and SiO2. This observation is consistent with that reported in other studies [16,21,22] in which a liquid phase was detected in the fiber area at an applied pressure of 20 MPa. This liquid phase helps densify SiC through a liquid-phase sintering mechanism. The liquid phase is homogeneously dispersed in the matrix area of the SiCf/SiC composite at an applied pressure of 5 MPa. The liquid phase segregates near the fiber area ( Figure  3(c)) or exists in the fiber area (Figure 3(f)) at high pressures. The absence of a liquid phase in the fiber area of the SiCf/SiC composite prepared at 5 or 10 MPa results in the formation of pores and is the reason why densification is ineffective at these pressures. Moreover, poor densification in the fiber area results in damage to the PyC coating, as shown in Figures 3(b) and 3(d).

Figure 5. SEM Images Showing the Sizes of the Grains in the Matrices of the SiCf/SiC Composites Prepared at (a) 5 MPa, (b) 10 MPa, and (c) 20 MPa
Shimoda et al. reported that densification in the fiber area is essential to protect the PyC coating and the fiber from damage [21], which leads to minimal degradation of the mechanical properties of the SiCf/SiC composite. One notable finding in this study is the lack of infiltrated particles in the fiber areas of the SiCf/SiC composites prepared at applied pressures of 5 and 10 MPa, even though electrophoretic infiltration covered the fabric, as well as the fibers, as shown in Figure 1. This observation is possibly ascribable to particle redistribution re-sulting from the formation of liquid phases in the first stage of sintering, as well as capillary forces, Ostwald ripening, and attraction of particles in the fiber area of the matrix or the vicinity of the matrix. Consequently, the fiber area was poorly densified because of the absence of infiltrated particles composed of SiC and the sintering additives. In contrast, an applied pressure of 20 MPa hinders the redistribution of infiltrated particles to the matrix area, resulting in the densification of the fiber area. Therefore, 20 MPa is the minimum applied pressure required for the fabrication of SiCf/SiC composites prepared by electrophoretic infiltration to ensure the densification of the matrix and fiber areas.
The sizes of the grains in the matrices of the SiCf/SiC composites prepared at various applied pressures are shown in Figure 5. Notably, pressure influences grain size, even though the differences are insignificant. The average grain sizes in the matrix areas of the composites prepared at 5, 10, and 20 MPa are 1,464, 1,354, and 1,282 nm, respectively. Moreover, the matrix of the SiCf/SiC composite prepared at 5 MPa has a dense microstructure with a few pores, as shown in Figure 5(a). The number of pores in the matrix area increases with increasing applied pressure, as shown in Figures 5(b) and 5(c), which is due to the homogeneous distribution of the liquid phase in the matrix area of the SiCf/SiC composite prepared at 5 MPa (as discussed in the previous paragraph). Meanwhile, the other samples exhibited segregated liquid phases. Figure 6 shows the flexural strength of the SiCf/SiC composite as a function of applied pressure. Indeed, the SiCf/SiC composite prepared at an applied pressure of 20 MPa has higher flexural strength than those prepared at 5 and 10 MPa. The flexural strengths of the 5, 10, and 20 MPa samples are 198 ± 41, 238 ± 32, and 262 ± 18 MPa, respectively. The flexural strength of the composite is observed to be influenced by its density. For instance, the dense structure of the SiCf/SiC composite prepared at an applied pressure of 20 MPa reached its maximum flexural strength earlier than those prepared at applied pressures of 5 and 10 MPa. Interestingly, although the SiCf/SiC composite prepared at 20 MPa shows an abrupt decrease in load after fracturing, which indicates the brittle nature of the composite, it still exhibits a short tail extension, which is ascribable to single-fiber pullout, as shown in Figure 7(c). Meanwhile, the SiCf/SiC composites prepared at applied pressures of 5 and 10 MPa show relatively long tail extensions, as shown in Figure 6. The tail-extension mechanism of the composites prepared at 5 and 10 MPa is different from that of the composite prepared at 20 MPa. It seems that composite delamination is the main factor responsible for tail extension, as shown in Figures 7(a) and 7(b). The SiCf/SiC composites prepared at 5 and 10 MPa delaminate because of the presence of pores, which results in low-density fiber areas.

Conclusion
In this study, the effect of pressure on the densification and mechanical properties of SiCf/SiC composites prepared by electrophoretic infiltration was examined. applied pressures of <20 MPa. Poor densification in the fiber areas of the SiCf/SiC composites prepared at applied pressures of 5 and 10 MPa is due to liquid phase redistribution to the matrix or near the matrix area. Contrary, the SiCf/SiC composite prepared at 20 MPa successfully maintained infiltrated particles in its fiber area, which resulted in the formation of dense fibers. This result is supported by the homogenous distribution of the liquid phase in the matrix area of the SiCf/SiC composite prepared at an applied pressure of 5 MPa, with the liquid phase more likely from the fiber area. Hence, a dense microstructure with few pores was detected in the matrix area of the SiCf/SiC composite prepared at an applied pressure of 5 MPa compared to those prepared at applied pressures of 10 and 20 MPa. Therefore, the density of the SiCf/SiC composite increases with increasing pressure. For instance, the densities of the SiCf/SiC composites prepared at applied pressures of 5, 10, and 20 MPa were 2.99, 3.10, and 3.16 g/cm 3 , respectively, and the highest flexural strength of 262 ± 18 MPa was obtained at an applied pressure of 20 MPa. This high flexural strength is certainly due to the excellent density of this composite. In contrast, the flexural strengths of the composites prepared at applied pressures of 5 and 10 MPa were 198 ± 41 and 238 ± 32 MPa, respectively.