CMC-Processing from Powders by Hot-Pressing

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1 Advances in Science and Technology Online: ISSN: , Vol. 50, pp doi: / Trans Tech Publications, Switzerland CMC-Processing from Powders by Hot-Pressing S.M. Dong 1, a, Y.S. Ding 2,b, Z. Wang 3,c, Q. Zhou 4,d, X.Y. Zhang 5,e and D.L. Jiang 6,f and A. Kohyama 7,g 1-6 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai , China 7 Institute of Advanced Energy, Kyoto University, Kyoto , Japan a smdong@mail.sic.ac.cn, b ysding@mail.sic.ac.cn, c jeff@mail.sic.ac.cn, d zhouqing@mail.sic.ac.cn, e xyzhang@mail.sic.ac.cn, f dljiang@sunm.shcnc.ac.cn, g kohyama@iae.kyoto-u.ac.jp Keywords: CMC, Hot pressing, microstructural evolution, mechanical behavior Abstract. Hot pressing is an effective way to densify powder compacts, especially during the inclusion of a second phase, such as particles, whiskers or fibers. In the present study, SiC/SiC and C/SiC composites were prepared by hot pressing via liquid phase sintering. Nano-SiC powder was used for matrix formation with sintering additives. The effects of preparation conditions such as sintering temperature, pressure and matrix composition, on the microstructural evolution and mechanical behaviors were discussed. Using micro-sic powder and nano-sic powder for matrix formation, the interaction between fiber and matrix was characterized. Because the matrix compositions can be easily modified in the CMC-processing from powders by hot pressing, the SiC-BN matrix was also studied in the present experiment. The inclusion of BN can either improve the machinability or provide oxidation resistance to the composite. BN was derived through an in-situ reaction between boron acid and urea by hot-pressing. Boric acid and urea were solved into the ethanol and mixed with nano-sic particles, and then infiltrated into the fiber bundles. Correlations among microstructures, properties and compositions will be discussed. Introduction The development of advanced SiC based fibers with a well-crystallized microstructure and a near-stoichiometric composition such as Hi-Nicalon type S and Tyranno SA [1, 2], is beneficial to fabrication SiC/SiC composites under even harsh conditions. The new-formed Si-Al-C Tyranno SA fiber has high tensile strength and modulus and shows no degradation in strength or change in composition on heating to 1900ºC in an inert atmosphere and in air at 1000ºC [1]. These properties provide for extended application areas and potential high performance for the composites. Hot pressing is effective to densify powder compacts. Since earlier developed SiC based fibers such as Nicalon fiber are thermodynamically unstable because of the high oxygen content [3, 4], previous work was mainly concentrated on low-temperature fabrication techniques such as chemical vapor infiltration (CVI), polymer impregnation and pyrolysis (PIP) and reaction sintering (RS) to prepare high performance SiC/SiC composites. Recent years, the development of NITE (Nano-powder Infiltration and Transient Eutectoid) process has provided an effective way to fabricate high performance SiC/SiC composite by hot pressing [5,6]. Further research is still necessary for better understanding the influences of processing on microstructure and properties of the composites. As a promising structural material, C/SiC composite has the potential for use at or above 1500ºC. However, due to the difference of thermal expansion coefficient between the fiber and the matrix, there are some micro-cracks existing in the composite, which represent an easy path for the diffusion of oxygen towards the carbon reinforcements. Consequently, carbon reinforcements are readily oxidized in an oxidizing atmosphere at temperature higher than 450ºC, which significantly reduce the mechanical properties and the lifetime of the composite. In recent years, a lot of work has been done to improve the oxidation resistance of C/SiC composites. In most cases, boron nitride or other All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/09/16,16:17:47)

2 76 Advanced Inorganic Fibrous Composites V boron-bearing materials are selected to achieve the purpose by forming B 2 O 3 fluid when used in oxidizing atmosphere [7, 8, 9]. The purpose of the present work is to prepare SiC/SiC, C/SiC, C/SiC-BN composites by hot pressing, and compare the interaction between fiber and matrix at various sintering conditions. Nano-SiC, micro-sic and BN with sintering additives were used for matrix formation, Tyranno SA fiber and C fiber were used as reinforcement. Experimental Procedure Pyrolytic Carbon (PyC) coated Tyranno SA fibers (Ube Industries Ltd., Japan) were used as reinforcement for SiC/SiC composite fabrication. The thickness of fiber coating was about 0.8 μm. High module carbon fibers (M40JB, Toray international Inc, Japan) were used for C/SiC composites fabrication. High purity β-sic (>99%) micrometer powder (Ibiden Co., Ltd., Japan) with an average particle size of 0.3 μm and β-sic nano-power with an average particle size of 30 nm (Marketech International Inc., America) were used for matrix formation for preparing SiC/SiC composite. Another kind of β-sic nano-powder with an average particle size of 60nm (Kiln Nanometer Technology Development Co. Ltd, China) was used for preparation of C/SiC(BN) composites. Al 2 O 3 (>99.9%) and Y 2 O 3 (>99.9%) were used as sintering aids to lower the sintering temperature. Both SiC/SiC and C/SiC composites were prepared by the method of slurry impregnation, pre-pyrolysis and hot pressing. The SiC slurry was prepared by mixing SiC powder with polycarbonsilane (PCS) and sintering additives in hexane. Boric acid and urea containing slurry was prepared using ethanol as solvent, and mixed with nano-sic particles. Part of the experimental procedures has been described elsewhere [5, 10]. The hot pressed SiC/SiC samples were cut and ground into 3mm 1.6mm 30 mm in size for tensile test in an INSTRON 5581 test machine. The C/SiC(BN) composites were cut and ground into 4mm 2.4mm 40mm rectangles for three-point-bending test. Density of each sample was measured by Archimedes s method. Both the polished cross-section and the fracture surface after tensile or bending test were observed by scanning electron microscopy (SEM). Results and Discussion The hot pressing SiC/SiC composites using nano-sic and micro-sic powders. As reported in literature [11], monolithic SiC could be well sintered via liquid phase sintering at a relatively lower temperature. In the present study, matrix (monolithic SiC) densification was firstly modified using both SiC nano-powder and micro-powder. The selection of high active SiC nano-powder is to decrease the sintering temperature. While the application of SiC micro-powder is expected to improve the fiber intra-bundle infiltration efficiency. Fig.1 shows the variation of the density of monolithic SiC at different temperature under an identical pressure. Using SiC nano-powder, higher density was obtained over 1750ºC under 15MPa. Increasing temperature to 1780ºC, about 99% of the theoretical density could be reached. When micro-sic was applied, high density of monolithic SiC could only be obtained at 1800ºC. Since nano-sic promote the densification at a relatively lower temperature, the related Density (g/cm 3 ) Nano-SiC Micro-SiC Temperatur ( o C) Fig.1 Effect of sintering temperature on the densification of monolithic SiC under an identical pressure of 15MPa

3 Advances in Science and Technology Vol composite (N-SiC/SiC) would also achieve its higher density at 1780ºC, 20MPa, as listed in table 1. Increasing temperature and/or pressure (such as 1800ºC, 20MPa or 1830ºC, 15MPa) were beneficial to the promotion of densification process, but in a limited level. While for using micro-sic, highly densified composite (M-SiC/SiC) could only be obtained at relatively higher temperature (1800ºC), indicating that the densification of this composite was much dependent on sintering temperature, as listed in table 2. Table 1 Effects of sintering conditions on the properties of the composites N-SiC/SiC Sintering conditions [ºC/MPa] 1780/ / / / /15 Density [g/cm 3 ] Tensile strength [MPa] 335.3± ± ± ± ±35.2 Elastic modulus [GPa] 275.2± ± ± ± ±14.7 Table 2 Effects of sintering conditions on the properties of the composites M-SiC/SiC Sintering conditions [ºC/MPa] 1780/ / / /20 Density [g/cm 3 ] Tensile strength [MPa] 274.7± ± ± ±8.9 Elastic modulus [GPa] 258.0± ± ± ±9.8 Microstructural development. SEM observations on the polished cross-section indicated that clear inter-bundle matrix rich layers were formed by the presently adopted fabrication process. The densification of the matrix layers was similar to that of the monolithic SiC, and no big pores was identified in those areas either using nano-sic or micro-sic for matrix formation. Pores mainly distributed in intra-bundle regions. For the well densified N-SiC/SiC such as the composite sintered at 1780ºC, 20MPa, micropores still widely distributed in the intra-bundle region as demonstrated in Fig.2 (a). In intra-bundle regions, well-densified matrix could be observed as shown in Fig.2 (b). Increasing temperature to 1830ºC under 15MPa, only a slight increase in density of the Fig.2 N-SiC/SiC composite sintered at 1780ºC under 20MPa Fig.3 M-SiC/SiC composite sintered at 1800ºC under 20MPa composite was identified. In M-SiC/SiC, matrix was more widely distributed in the intra-bundle surrounding the fibers, indicating the feasibility of intra-bundle infiltration by micro-sic. Compared with the composites sintered at different temperature under the identical pressure of 15MPa, less sintered matrix (especially in intra-bundle areas) was identified when temperature was lower (1780ºC). Higher sintering temperature or pressure was beneficial to densification, but

4 78 Advanced Inorganic Fibrous Composites V simultaneously it might enhance the interaction between fiber and matrix to affect the mechanical properties. Fig.3 indicates the composite sintered at 1800ºC under 20 MPa. Although no obvious pores existed in the intrabundle area, fiber deformation was clearly observed. Mechanical properties and fracture behavior. Table 1 lists some properties of the composites using nano-sic for matrix formation. At higher sintering temperature (such as 1830ºC, 15MPa) or higher pressure (such as 1800ºC, 20MPa), only a little increase in density was found. In both cases, elastic modulus increased, while tensile strength decreased. For the composites using micro-sic for matrix formation, the elevated sintering temperature (1800ºC) under 15 MPa could only increase the elastic modulus compared with the composites sinter at 1780ºC under the identical pressure, as listed in table 2. When pressure increased to 20MPa at 1780ºC, higher tensile strength was obtained. However, the composite sintered at 1800ºC under 20 MPa had a lower strength although its density was high. Stress-strain curves from tensile test with unloading-reloading cycles of M-SiC/SiC composites only demonstrate two portions: a linear region followed by a curved domain, except the composite sintered at 1800ºC under 20 MPa, which indicates only a linear region and no curved domain, as shown in Fig.4. The composite with higher density which was sintered at 1800ºC, 15MPa shows the very narrow hysteresis loops and very narrow curved domain. Relatively wide curved domain revealed in the composites sintered at lower temperature of 1780ºC. No second linear region could be discriminated. SEM observation on the fracture surface after tensile test indicates that in composites N-SiC/SiC, lowly densified composite (sintered at 1780ºC, 15MPa) accompanied by a longer fiber pull-out. While the composite with higher density (sintered at 1780ºC, 20MPa, Fig.4 Tensile stress-strain curves with unloading-reloading cycles for the composites M-SiC/SiC sintered at different conditions: (a) 1780ºC /15MPa, (b) 1780ºC /20MPa, (c) 1800ºC/15MPa, (d) 1800ºC/20MPa Fig.5 SEM micrographs of the composite N-SiC/SiC sintered at 1780ºC under 20MPa. Fig.6 SEM micrographs of the composite M-SiC/SiC sintered at 1780ºC under 20MPa 1800ºC, 20MPa or 1830ºC, 15MPa) exhibited a relatively short fiber pull-out. Typical micrographs are demonstrated in Fig.5. Even for the densely sintered composite, some longer pull-out fibers still could be observed in the intra-bundle areas, as shown by arrows in Fig.5 (a). The fibers that were well

5 Advances in Science and Technology Vol surrounded by matrix mainly exhibited the short fiber pull-out (Fig.5 (b)). In composite M-SiC/SiC, short fiber pull-out predominated the fracture surface for all the composites, as evidenced in Fig.6, especially in the composite with higher density, which was sintered at 1800ºC, 15MPa. The lower temperature (1780ºC, 15MPa) sintered composite had a lowly sintered matrix, especially in intra-bundle areas. Even in this composite, fiber pull-out was still limited. To further understanding the effect of particle size on the fiber/matrix interaction, the lowly sintered N-SiC/SiC and M-SiC/SiC (sintered at 1780ºC, 15MPa) were selected for comparison, as shown in Fig.7. It can be clearly distinguished that the surface of the interphase (PyC) was relatively smooth in N-SiC/SiC, indicating that the interaction between fiber and matrix was relatively relieved by using nano-sic for matrix formation. In M-SiC/SiC, very rough surface of the interphase (PyC) was formed as indicated in Fig.7 (b), indicating that the interaction between fiber and matrix was relatively strong. This effect would be further enhanced when temperature and/or pressure increased. Under the similar sintering conditions, debonding between fibers and matrix in M-SiC/SiC became more difficult than in N-SiC/SiC, providing the different fracture behaviors to the composites. The hot pressing C/SiC composite using nano-sic powder. SEM observation on the polished cross section for the composites fabricated at 1820ºC and 1850ºC under 20MPa was shown in Fig.8 and Fig.9, respectively. When the sintering temperature increased, both the number and the size of the pores decreased. However, many small pores still distributed in the intrabundle areas for both of the composites. The increased sintering temperature also enhanced the fiber/matrix interaction. In Fig.8 (b), clear fiber/matrix interface revealed, while in Fig.9 (b) the formation of a reaction layer could be observed, indicating that the reaction between fiber and matrix occurred during sintering. Further analysis of the reaction layer indicated that O, Al, Y and Si elements existed on the fiber surface. It means that those elements diffused from the matrix to the fiber surface and form the reaction layer. The physical and mechanical properties of C/SiC composites under different sintering conditions are listed in Table 3. Porosity of the composites decreased with the increase of Fig.7 Fracture surface of the composites sintered at 1780ºC under 15MPa (a) N-SiC/SiC, (b) M-SiC/SiC Fig.8 SEM observations on the polished cross section for the composite sintered at 1820ºC under 20MPa Fig.9 SEM observations on the polished cross section for the composite sintered at 1850ºC under 20MPa

6 80 Advanced Inorganic Fibrous Composites V sintering temperature. Simultaneously, bending strength and fracture toughness gradually increased. Highest values were obtained at the temperature of 1850ºC. Although further increase the sintering temperature would lower the porosity of the composite, but the mechanical properties were depressed. Table3. Effects of experimental conditions on the properties of the composites Sintering condition 1800/ / / /20 [ºC/MPa] Density [g/cm 3 ] 2.08± ± ± ±0.01 Bending stress [MPa] 294.8± ± ± ±35.7 Fracture toughness [MPa m 1/2 ] 9.2± ± ± ±0.9 Fig.10 Fracture surface of the composites sintered at (a) 1820ºC /20MPa, (b) 1850ºC /20MPa and (c) 1880ºC /20MPa At the sintering temperature of 1820ºC and 1850ºC, fiber pull-out was obviously observed and relatively long fiber pulled out dominated the fracture surface. When the temperature increased to 1880ºC, only short fibers were pulled out. Fracture surfaces of the composites prepared at the sintering temperature from 1820ºC to 1880ºC under an identical pressure of 20MPa are shown in Fig.10. When sintering temperature was lower (1820ºC), the fiber surface kept smooth. As the sintering temperature increased, fiber surface became rough. These phenomena correspond to the interaction between fiber and matrix during sintering at different conditions. Moderate fiber/matrix interaction would increase the bonding strength between fiber and matrix. As a result, the mechanical properties were improved. However, excess fiber/matrix interaction would either degrade the fibers or form the too strong fiber/matrix interface. These might finally destroy the mechanical behaviors of the composite. The hot pressing C/SiC-BN composite. In this process, silicon carbide nano-powder, boron acid, urea and sintering aids were mixed with the binder in ethanol to form the slurry. The use of Intensity (a.u.) Bending stress (MPa) Theta (degree) Fig.11 XRD patterns of C f /SiC-BN h-bn β SiC C fiber Displacement (mm) Fig.12 Stress-displacement curve from bending test

7 Advances in Science and Technology Vol binder was to improve the wetting ability between fiber and slurry. BN was in-situ formed through the precursors. The infiltrated sheets were firstly heat-treated at 900ºC in a N 2 atmosphere, and then hot pressed at 1800ºC under 20MPa. Volume fraction of BN was controlled at 30%. Fig.11 shows the XRD patterns of the composite, which was taken in the direction that perpendicular to the carbon fibers. In this figure, BN peak at about 26 degrees which is corresponding to the (002) is clearly seen. Typical stress-displacement curve derived from the three-point-bending test is shown in Fig.12. A non-linear fracture behavior was revealed. The pulled out fibers could also be observed, as indicated in Fig.13. However, the ultimate bending strength was still low and the delamination occurred during the fracture. Further characterization was conducted through the observation on the polished cross-section, as shown in Fig.14. Carbon fibers were not well-dispersed. Too much matrix distributed in the inter-bundle regions while too little matrix distributed in the intra-bundle regions. Moreover, the less matrix formation in the fiber bundles resulted in a higher porosity, and therefore the fiber/matrix bonding was Fig.13 Fracture surface of the composite after bending test Fig.14 SEM micrographs of the polished cross-section for the as prepared composite weakened. When the composite was subjected to a load, sometimes the whole fiber bundle was pulled out at the same time, as a result, parts of the fibers do not contribute to strengthen the composite. Anyway, optimization of the present adopted experiment is necessary. This will be the future discussion. Summary Unidirectional SiC/SiC, C/SiC and C/SiC-BN composites were successfully fabricated by hot pressing via liquid phase sintering, and the densification was strongly affected by SiC powders and the sintering conditions. Using nano-sic for matrix formation, densely sintered composites could be obtained either at 1780ºC under 20MPa or 1800ºC under 15MPa. The composites sintered at these conditions had the improved mechanical properties. Further increase either temperature or pressure would enhance the interaction between fiber and matrix. However, tensile strength was simultaneously lowered due to the potential degradation of fiber reinforcement. Well-densified composite using micro-sic for matrix formation required higher sintering temperature. Since the intra-bundle matrix formation was much easier than that of using nano-sic, and the interaction between fiber and matrix was stronger in M-SiC/SiC, a strong fiber/matrix interface formed and therefore limited the fiber pull-out during fracture, especially for the higher temperature (1800ºC) sintered composite. The densification process and the mechanical properties of C/SiC composites were highly dependent on the sintering temperature. At low sintering temperature, the properties of the composite

8 82 Advanced Inorganic Fibrous Composites V were relatively low. With the increase of sintering temperature, density of the composites gradually increased. Simultaneously, the interaction between fiber and matrix was enhanced. However, mechanical properties of the composite dropped significantly when sintering temperature increased to 1880ºC. This might be ascribed to the degradation of the fiber and the over strengthened fiber/matrix bonding, leading to the weakened ability for crack deflection along the fiber/matrix interfaces. BN was successfully introduced into the matrix to form C/SiC-BN composite through the in-situ reaction between boron acid and urea. However, the fibers were not homogenously dispersed in the matrix. Less formed intra-bundle matrix was obviously observed. Continuous work should be concentrated on the improvement of the wettability of the slurry to the fibers and the optimization of sintering conditions. Acknowledgements This work was supported by the National Natural Science Foundation Program of China under Grant No and the Key Project of Science and Technology of Shanghai, China, under Grant No. 04DZ The authors would like to thank Professor R. Naslain for his valuable suggestions. References [1] T. Ishikawa, Y. Kohtoku, K. Kumagawa, T. Yamamura and T. Nagasawa: Nature 391 (1998), 773 [2] S. M. Dong, G. Chollon, C. Labrugere, M. Lahaye, A. Guette, R. Naslain, and D. L. Jiang: J. Mater. Sci. 36 (2001), 2371 [3] L. Porte and A. Sartre: J. Mater Sci. 24, 271 (1989) [4] C. Laffon, A. M. Flank, P. Lagarde, M. Laridjani, R. Hagege, P. Olry, J. Cotteret, J. Dixmier, J. L. Miquel, H. Hammel, and A. P. Legrand: J. Mater. Sci. 24 (1989), 1503 [5] S. M. Dong, Y. Katoh, and A. Kohyama: J. Am. Ceram. Soc. 86 (2003), 26 [6] Y. Katoh, S. M. Dong, and A. Kohyama: Ceram. Trans. 144 (2002), 77 [7] R.Naslain, A.Guette, F. Rebillat, R. Pailler, F. Langlais and X. Bourrat: J. Solid State Chem. 177 (2004), 449 [8] S. Labruquere, H. Blanchard, R. Paillar, and R. Naslain: J. Eur. Ceram. Soc. 22 (2002), 1011 [9] F. Lamouroux, S. Bertrand, R. Pailler, R. Naslain, and M. Cataldi: Compos. Sci. Technol. 59 (1999), 1073 [10] Y. S. Ding, S. M. Dong, Q. Zhou, Z. R. Huang and D. L. Jiang: J. Am. Ceram. Soc. 89 (2006), 1447 [11] T. Radsick, B. Saruhan, and H. Schneider: J. Eur. Ceram. Soc. 20 (2000), 545