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SiC composites
Materials Science and Engineering A 473 (2008) 254–258
Effect of SiC coating and heat treatment on damping behavior of C/SiC composites Qing Zhang, Laifei Cheng ∗ , Wei Wang, Litong Zhang, Yongdong Xu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an 710072, China Received 30 October 2006; received in revised form 19 March 2007; accepted 19 June 2007
Abstract Three groups of 2D and 3D C/SiC composites were fabricated by chemical vapor infiltration (CVI) process: the first group was as received, the second group was treated at 1500 ◦ C in vacuum atmosphere for 2 h, and the third group was deposited with a chemical-vapor-deposited (CVD) SiC coating. Damping properties of these composites were measured by dynamical mechanical analyzer (DMA) at different frequencies from room temperature to 400 ◦ C in air atmosphere. The results show that SiC coating and heat treatment decrease damping capacity of C/SiC composites and the damping peak disappears or decreases in the testing temperature range. The effect of CVD SiC coating on damping behavior of 2D and 3D C/SiC composites is mainly related to the change of porosity and is independent of fiber preform architecture. However, the effect of heat treatment on damping behavior of 2D and 3D C/SiC composites is mainly attributed to the change in the SiC matrix and interphase bonding, and it is dependent on fiber preform architecture. Both of CVD SiC coating and heat treatment studied in this paper have no influence on relationship between damping behavior of C/SiC composites and frequency. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon fiber composites; Chemical vapor infiltration; Damping properties
1. Introduction Carbon fiber reinforced silicon carbide (C/SiC) composites are considered to be the most potential candidates in the fields of aeronautics and astronautics due to low density and good performance at high temperatures [1,2]. They are widely used to fabricate items such as nozzles, tubes, plates, and shells. It is well recognized that the performance of these applications is influenced by the change of microstructure of the materials. The damping capacity, Q−1 , is an important means not only for evaluating the performance of materials in vibration environment, but also for investigating the evolution of microstructure [3–7], for instance, diffusion of elements, defects and cracks of matrix materials, debonding and sliding at interphase and so on. However, there were few reports on the damping properties of ceramic matrix composites, especially C/SiC composite materials. CVD SiC coating and heat treatment (HT) are important technics to improve the properties of the materials, and they
∗
Corresponding author. Tel.: +86 29 8849 4616; fax: +86 29 8849 4620. E-mail address: [email protected] (L. Cheng).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.06.049
have certain effects on microstructure of C/SiC composites. It is the purpose of this paper to investigate the effects of CVD SiC coating and heat treatment on damping behavior of C/SiC composites. The damping mechanisms related to the evolution of microstructure have been discussed, too. 2. Experimental 2.1. Materials procedure Two kinds of fiber preforms were prepared: two-dimensional (2D) by laminating 1K T-300 woven carbon fabrics and threedimensional (3D) by braiding 3K T-300 carbon fibers in four-step method. The volume of fibers was controlled in the range from 40 to 45%. The preforms were infiltrated with both pyrolysis carbon (PyC) as interphase and SiC as matrix to fabricate C/SiC composites by low-pressure chemical vapor infiltration (LPCVI) method using butane and methyltrichlorosilane (MTS) [8]. The infiltration conditions of PyC interlayer were as follows: temperature 960 ◦ C, pressure 5 KPa, time 20 h, Ar flow 200 ml min−1 , butane flow 15 ml min−1 . The infiltration conditions of SiC matrix were as follows: temperature 1000 ◦ C, pressure 5 KPa, time 120 h, H2 flow 350 ml min−1 ,
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Ar flow 350 ml min−1 , and the molar ratio of H2 and MTS 10. The porosity of the composites was about 16%. Specimens were machined from the fabricated composite in the size of 3 mm × 12 mm × 50 mm without further deposition of SiC. Then, the specimens were divided into three groups: the first group was just as received, and the second group was treated at 1500 ◦ C in vacuum atmosphere for 2 h, and the third group was deposited with a 20 m-thick CVD SiC coating under the same conditions as SiC matrix. 2.2. Measurement procedure Dynamical mechanical analyzer made by TA Company of USA was employed for measurements of the dynamic storage modulus (E ) and loss modulus (E ) with increasing temperature and forced vibrations by three-point bending method. The damping capacity, Q−1 , can be expressed as: Q−1 = E /E . All of measurements were carried out in air atmosphere from room temperature to 400 ◦ C, the vibration amplitude being 2 m and the testing frequency 1, 2, 5, and 10 Hz, respectively. 3. Results and discussion 3.1. Damping behavior of 2D and 3D C/SiC composites The dependence of damping capacity Q−1 on temperature and frequency for the as-received 2D and 3D C/SiC composites is shown in Fig. 1. It is clearly shown that the damping capacity changed as a function of the temperature for both of C/SiC composites. The damping capacity increased gradually with increasing temperature and then decreased after damping peak appeared in the temperature range of 250–300 ◦ C. Compared with 2D C/SiC composites, damping peaks for 3D C/SiC composites were lower at equal frequency. It resulted from interlayer friction proper to 2D C/SiC composites under an alternate stress due to its laminated structure. This could be confirmed by delamination of tested 2D C/SiC composites.
Fig. 2. Debonding occurred at the boundary between PyC interphase and SiC matrix.
Cooling from manufacture temperature to room temperature, kinds of defects, such as dislocation [9] and microcracks in SiC matrix, distortion in PyC interphase, debonding between interphase and matrix and so on, would be produced in the composites because of thermal stress induced by thermal mismatch of three different components. After the forced vibration was applied, the dislocation vibrated around its equilibrium position and the mobility of dislocation gradually increased with the temperature increasing, then the damping capacity grew before 250 ◦ C. The damping peak appeared in the temperature range of 250–300 ◦ C induced by interfacial friction at the debonding region between interphase and matrix (Fig. 2) and possibly microcracks expanding in SiC matrix [10]. As the temperature increasing, the damping capacity decreased due to the decrement of interstices in the composites caused by thermal expansion and residual stress releasing gradually. The oxidation effect was not taken into account because no oxidation was observed after the composites were tested. As Parrini and Schaller [11] reported, the damping capacity Q−1 is in inverse proportion to the vibration frequency (f): Q−1 =
const (dT/dt) f
(1)
where dT/dt is the rate of temperature change. For C/SiC composite studied in this work, damping capacity and peak values decreased gradually with the increasing testing frequency and the damping peak shifted to the lower temperatures. The similar results were reported in Refs. [12,13]. 3.2. The effect of SiC coating on damping behavior of 2D and 3D C/SiC composites
Fig. 1. Temperature dependence of damping capacity for the as-received 2D and 3D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 3D (2D) C/SiC composites.
The dependence of damping capacity on temperature and frequency for 2D C/SiC composites with and without CVD SiC coating is shown in Fig. 3. It can be seen that CVD SiC coating decreases damping capacity for 2D C/SiC composites and makes damping peak disappear. The damping capacity of
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Fig. 3. Variations of damping capacity with temperature and frequency for 2D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 2D C/SiC composites without (with) SiC coating.
SiC-coated 2D C/SiC composites increases monotonously in the whole testing temperature range. Fig. 4 illustrates the dependence of damping capacity on temperature and frequency for 3D C/SiC composites with and without CVD SiC coating. Similarly, CVD SiC coating decreases damping capacity for 3D C/SiC composites and makes damping peak disappear. The damping capacity for SiC-coated 3D C/SiC composites changes in a similar way like that of SiCcoated 2D C/SiC composites, i.e. increasing monotonously in the whole testing temperature range. With the testing frequency increasing, the damping capacity for all of 2D and 3D C/SiC composites decreases gradually. The data indicate that for 2D and 3D C/SiC composites, CVD SiC coating decreases damping capacity and makes damping peak disappear. It is considered that the influence of CVD SiC
Fig. 4. Variations of damping capacity with temperature and frequency for 3D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 3D C/SiC composites without (with) SiC coating.
coating on damping capacity of C/SiC composites is mainly related to the change of porosity.1 It can be confirmed by the decreasing porosity from 16% before coated to 13% after coated. There were plenty of open pores at the surface of specimens before a SiC coating deposited, as shown in Fig. 5(a). During the depositing process, these open pores were filled with SiC grains and then the coating was formed, as shown in Fig. 5(b). Therefore, the porosity would decrease after the composites were deposited with a SiC coating. This implies that pores in C/SiC composites play an important role in their damping capacity. As Wang et al. [6] researched, porosity damping of carbon-fiberreinforced porous composites is approximately proportional to porosity. The elastic energy is dissipated through the dilatational and distortional energy of pores. Therefore, the damping capacity decreases with decreasing porosity. Furthermore, CVD SiC coating has no influence on the relationship between damping behavior and frequency, and the effect of CVD SiC coating on damping behavior of C/SiC composites is independent of fiber preform architecture. 3.3. The effect of heat treatment on damping behavior of 2D and 3D C/SiC composites Fig. 6 shows the variations of damping capacity with temperature and frequency for 2D C/SiC composites with and without heat treatment. It can be seen that heat treatment decreases damping capacity and peak values for 2D C/SiC composites and changes the shift of damping peak in direction. As the testing frequency increasing, damping capacity and peak value for as-received 2D C/SiC composites decreases gradually accompanied with a shift of damping peak towards the lower temperatures, while for heat-treated 2D C/SiC composites the temperature of damping peak shifts to the higher temperatures. With the testing frequency increasing, damping capacity for both of the 2D C/SiC composites decreases gradually. Fig. 7 shows the dependence of damping capacity on temperature and frequency for 3D C/SiC composites with and without heat treatment. It is similar to 2D C/SiC composites that heat treatment decreases damping capacity and peak values for 3D C/SiC composites except at 1 Hz, while it does not change the shift of damping peak in direction. Damping capacity and peak value for both 3D C/SiC composites decreases gradually accompanied with a shift of damping peak towards the lower temperatures with the testing frequency increasing. Damping peak for heat-treated 3D C/SiC composites at 1 Hz slightly increases. The data indicate that heat treatment generally decreases damping capacity for 2D and 3D C/SiC composites. It can be attributed to the change in SiC matrix. Tiny SiC grains were generated in CVI process with great internal stress and many of structural defects, such as dislocation. The relaxation of internal stress and the mobility of dislocation became the source of
1
The porosity in this paper means open porosity, which was measured based on the Archimedes method. The closed porosity was not taken into account because it never changed after the specimens were coated.
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Fig. 5. The appearance of C/SiC composites: (a) before coated and (b) after coated.
Fig. 6. Variations of damping capacity with temperature and frequency for 2D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 2D C/SiC composites untreated (treated) SiC coating.
Fig. 7. Variations of damping capacity with temperature and frequency for 3D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 3D C/SiC composites untreated (treated) SiC coating.
Fig. 8. SEM photographs of polished 3DC/SiC composites: (a) without heat treatment and (b) with heat treatment.
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energy dissipation in vibration environment, and the composites showed high damping capacity. However, the stabilizing heat treatment at 1500 ◦ C results in the release of internal stress, the annealing of defects and grain growth [14]. Then, the damping capacity decreases due to the source of energy dissipation decreasing. On the other hand, heat treatment can decrease interphase bonding strength and enlarge the debonding of fiber and matrix [15,16]. For the as-received C/SiC composites, textured graphitoid areas were observed by Li [17] at the edge of PyC interphase near by the fibers, and the other part was in turbostratic stacking structure. During the processing of heat treatment, sliding occurred in the textured graphitoid areas to release the thermal stresses induced by the thermal mismatch of fibers and PyC, and then interphase bonding strength was decreased [18]. Moreover, as shown in Fig. 8, the debonding areas were enlarged after heat treatment because of the thermal mismatch of fiber and matrix. Therefore, friction in interphase area is enhanced and can dissipate much energy in vibration, which is considered as the reason why damping peak for heat-treated 3D C/SiC composites at 1 Hz slightly increases. Moreover, heat treatment has different effects on the shift of damping peak in direction for 2D and 3D C/SiC composites, which is conceived that it is related to different fiber preform architectures. Furthermore, heat treatment has no influence on the relationship between damping behavior and frequency. 4. Conclusions The effects of CVD SiC coating and heat treatment on damping behavior of 2D and 3D C/SiC composites and the related mechanisms have been investigated. Damping capacity of SiCcoated and heat-treated C/SiC composites are mostly lower than that of as-received C/SiC composites. The reduction in the damping capacity of 2D and 3D SiC-coated C/SiC composites mainly results from the decrease of porosity. However, the decrease of the damping capacity of 2D and 3D heat-treated C/SiC composites is mainly attributed to the change of microstructure in the SiC matrix and interphase bonding. Heat treatment has different effects on the shift of damping peak in direction for 2D and 3D C/SiC composites, which is related to different fiber
preform architectures. Both of CVD SiC coating and heat treatment have no influence on relationship between damping behavior of C/SiC composites and frequency. Acknowledgements The authors acknowledge the financial support of Natural Science Foundation of China (Contract No. 90405015), National Young Elitists Foundation (Contract No. 50425208), Program for Changjiang Scholars and Innovative Research Team in University. References [1] J.F. Jamet, P. Lamicq, in: R. Naslain, J. Lamon, D. Doumeingts (Eds.), High Temperature Ceramic Matrix Composites I, HT-CMC I, Woodhead Publishing Limited, Cambridge, England, 1993, pp. 735–742. [2] S. Jacques, A. Guette, F. Langlais, R. Naslain, S. GouJard, J. Eur. Ceram. Soc. 17 (1997) 1083–1092. [3] K. Nishiyama, M. Yamanaka, M. Omori, S. Umekawa, J. Mater. Sci. Lett. 9 (1990) 526–528. [4] R. Chandra, S.P. Singh, K. Gupta, Compos. Struct. 46 (1999) 41–51. [5] W.Z. Li, P.Q. Zhang, J.H. Ruan, J. China Univ. Sci. Technol. 4 (2000) 393–400. [6] C. Wang, Z.G. Zhu, X.H. Hou, H.J. Li, Carbon 38 (2000) 1821–1824. [7] S. Sato, H. Serizawa, T. Noda, A. Kohyama, J. Alloys Compd. 355 (2003) 142–147. [8] L.F. Cheng, Y.D. Xu, Q. Zhang, L.T. Zhang, Carbon 41 (2003) 707–711. [9] X.Y. Yang, Ph.D. Thesis, Institute of Metal Research, Chinese Academy of Science, Shenyang, China, 2000. [10] V. Birman, L.W. Byrd, Int. J. Solids Struct. 40 (2003) 4239–4256. [11] L. Parrini, R. Schaller, Scripta Metall. Mater. 28 (1993) 763–767. [12] X. Zhang, R. Wu, X. Li, Z.X. Guo, Sci. China E 32 (2002) 14–19 (in Chinese). [13] L. Parrini, R. Schaller, Acta Mater. 44 (1996) 4881–4888. [14] Y.D. Xu, L.T. Zhang, L.F. Cheng, Acta Aeronaut. Astronaut. Sin. 18 (1997) 123–126. [15] H. Kodama, H. Sakamoto, T. Miyoshi, J. Am. Ceram. Soc. 72 (1989) 551–558. [16] F. Lamouroux, G. Camus, J. Thebault, J. Am. Ceram. Soc. 77 (1994) 2049–2057. [17] J.Z. Li, Ph.D. Thesis, Northwestern Polytechnical University, Xian, China, 2007. [18] Y.J. Ye, M.S. Thesis, Northwestern Polytechnical University, Xian, China, 2003.
Effect of SiC coating and heat treatment on damping behavior of C/SiC composites Qing Zhang, Laifei Cheng ∗ , Wei Wang, Litong Zhang, Yongdong Xu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an 710072, China Received 30 October 2006; received in revised form 19 March 2007; accepted 19 June 2007
Abstract Three groups of 2D and 3D C/SiC composites were fabricated by chemical vapor infiltration (CVI) process: the first group was as received, the second group was treated at 1500 ◦ C in vacuum atmosphere for 2 h, and the third group was deposited with a chemical-vapor-deposited (CVD) SiC coating. Damping properties of these composites were measured by dynamical mechanical analyzer (DMA) at different frequencies from room temperature to 400 ◦ C in air atmosphere. The results show that SiC coating and heat treatment decrease damping capacity of C/SiC composites and the damping peak disappears or decreases in the testing temperature range. The effect of CVD SiC coating on damping behavior of 2D and 3D C/SiC composites is mainly related to the change of porosity and is independent of fiber preform architecture. However, the effect of heat treatment on damping behavior of 2D and 3D C/SiC composites is mainly attributed to the change in the SiC matrix and interphase bonding, and it is dependent on fiber preform architecture. Both of CVD SiC coating and heat treatment studied in this paper have no influence on relationship between damping behavior of C/SiC composites and frequency. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon fiber composites; Chemical vapor infiltration; Damping properties
1. Introduction Carbon fiber reinforced silicon carbide (C/SiC) composites are considered to be the most potential candidates in the fields of aeronautics and astronautics due to low density and good performance at high temperatures [1,2]. They are widely used to fabricate items such as nozzles, tubes, plates, and shells. It is well recognized that the performance of these applications is influenced by the change of microstructure of the materials. The damping capacity, Q−1 , is an important means not only for evaluating the performance of materials in vibration environment, but also for investigating the evolution of microstructure [3–7], for instance, diffusion of elements, defects and cracks of matrix materials, debonding and sliding at interphase and so on. However, there were few reports on the damping properties of ceramic matrix composites, especially C/SiC composite materials. CVD SiC coating and heat treatment (HT) are important technics to improve the properties of the materials, and they
∗
Corresponding author. Tel.: +86 29 8849 4616; fax: +86 29 8849 4620. E-mail address: [email protected] (L. Cheng).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.06.049
have certain effects on microstructure of C/SiC composites. It is the purpose of this paper to investigate the effects of CVD SiC coating and heat treatment on damping behavior of C/SiC composites. The damping mechanisms related to the evolution of microstructure have been discussed, too. 2. Experimental 2.1. Materials procedure Two kinds of fiber preforms were prepared: two-dimensional (2D) by laminating 1K T-300 woven carbon fabrics and threedimensional (3D) by braiding 3K T-300 carbon fibers in four-step method. The volume of fibers was controlled in the range from 40 to 45%. The preforms were infiltrated with both pyrolysis carbon (PyC) as interphase and SiC as matrix to fabricate C/SiC composites by low-pressure chemical vapor infiltration (LPCVI) method using butane and methyltrichlorosilane (MTS) [8]. The infiltration conditions of PyC interlayer were as follows: temperature 960 ◦ C, pressure 5 KPa, time 20 h, Ar flow 200 ml min−1 , butane flow 15 ml min−1 . The infiltration conditions of SiC matrix were as follows: temperature 1000 ◦ C, pressure 5 KPa, time 120 h, H2 flow 350 ml min−1 ,
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Ar flow 350 ml min−1 , and the molar ratio of H2 and MTS 10. The porosity of the composites was about 16%. Specimens were machined from the fabricated composite in the size of 3 mm × 12 mm × 50 mm without further deposition of SiC. Then, the specimens were divided into three groups: the first group was just as received, and the second group was treated at 1500 ◦ C in vacuum atmosphere for 2 h, and the third group was deposited with a 20 m-thick CVD SiC coating under the same conditions as SiC matrix. 2.2. Measurement procedure Dynamical mechanical analyzer made by TA Company of USA was employed for measurements of the dynamic storage modulus (E ) and loss modulus (E ) with increasing temperature and forced vibrations by three-point bending method. The damping capacity, Q−1 , can be expressed as: Q−1 = E /E . All of measurements were carried out in air atmosphere from room temperature to 400 ◦ C, the vibration amplitude being 2 m and the testing frequency 1, 2, 5, and 10 Hz, respectively. 3. Results and discussion 3.1. Damping behavior of 2D and 3D C/SiC composites The dependence of damping capacity Q−1 on temperature and frequency for the as-received 2D and 3D C/SiC composites is shown in Fig. 1. It is clearly shown that the damping capacity changed as a function of the temperature for both of C/SiC composites. The damping capacity increased gradually with increasing temperature and then decreased after damping peak appeared in the temperature range of 250–300 ◦ C. Compared with 2D C/SiC composites, damping peaks for 3D C/SiC composites were lower at equal frequency. It resulted from interlayer friction proper to 2D C/SiC composites under an alternate stress due to its laminated structure. This could be confirmed by delamination of tested 2D C/SiC composites.
Fig. 2. Debonding occurred at the boundary between PyC interphase and SiC matrix.
Cooling from manufacture temperature to room temperature, kinds of defects, such as dislocation [9] and microcracks in SiC matrix, distortion in PyC interphase, debonding between interphase and matrix and so on, would be produced in the composites because of thermal stress induced by thermal mismatch of three different components. After the forced vibration was applied, the dislocation vibrated around its equilibrium position and the mobility of dislocation gradually increased with the temperature increasing, then the damping capacity grew before 250 ◦ C. The damping peak appeared in the temperature range of 250–300 ◦ C induced by interfacial friction at the debonding region between interphase and matrix (Fig. 2) and possibly microcracks expanding in SiC matrix [10]. As the temperature increasing, the damping capacity decreased due to the decrement of interstices in the composites caused by thermal expansion and residual stress releasing gradually. The oxidation effect was not taken into account because no oxidation was observed after the composites were tested. As Parrini and Schaller [11] reported, the damping capacity Q−1 is in inverse proportion to the vibration frequency (f): Q−1 =
const (dT/dt) f
(1)
where dT/dt is the rate of temperature change. For C/SiC composite studied in this work, damping capacity and peak values decreased gradually with the increasing testing frequency and the damping peak shifted to the lower temperatures. The similar results were reported in Refs. [12,13]. 3.2. The effect of SiC coating on damping behavior of 2D and 3D C/SiC composites
Fig. 1. Temperature dependence of damping capacity for the as-received 2D and 3D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 3D (2D) C/SiC composites.
The dependence of damping capacity on temperature and frequency for 2D C/SiC composites with and without CVD SiC coating is shown in Fig. 3. It can be seen that CVD SiC coating decreases damping capacity for 2D C/SiC composites and makes damping peak disappear. The damping capacity of
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Fig. 3. Variations of damping capacity with temperature and frequency for 2D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 2D C/SiC composites without (with) SiC coating.
SiC-coated 2D C/SiC composites increases monotonously in the whole testing temperature range. Fig. 4 illustrates the dependence of damping capacity on temperature and frequency for 3D C/SiC composites with and without CVD SiC coating. Similarly, CVD SiC coating decreases damping capacity for 3D C/SiC composites and makes damping peak disappear. The damping capacity for SiC-coated 3D C/SiC composites changes in a similar way like that of SiCcoated 2D C/SiC composites, i.e. increasing monotonously in the whole testing temperature range. With the testing frequency increasing, the damping capacity for all of 2D and 3D C/SiC composites decreases gradually. The data indicate that for 2D and 3D C/SiC composites, CVD SiC coating decreases damping capacity and makes damping peak disappear. It is considered that the influence of CVD SiC
Fig. 4. Variations of damping capacity with temperature and frequency for 3D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 3D C/SiC composites without (with) SiC coating.
coating on damping capacity of C/SiC composites is mainly related to the change of porosity.1 It can be confirmed by the decreasing porosity from 16% before coated to 13% after coated. There were plenty of open pores at the surface of specimens before a SiC coating deposited, as shown in Fig. 5(a). During the depositing process, these open pores were filled with SiC grains and then the coating was formed, as shown in Fig. 5(b). Therefore, the porosity would decrease after the composites were deposited with a SiC coating. This implies that pores in C/SiC composites play an important role in their damping capacity. As Wang et al. [6] researched, porosity damping of carbon-fiberreinforced porous composites is approximately proportional to porosity. The elastic energy is dissipated through the dilatational and distortional energy of pores. Therefore, the damping capacity decreases with decreasing porosity. Furthermore, CVD SiC coating has no influence on the relationship between damping behavior and frequency, and the effect of CVD SiC coating on damping behavior of C/SiC composites is independent of fiber preform architecture. 3.3. The effect of heat treatment on damping behavior of 2D and 3D C/SiC composites Fig. 6 shows the variations of damping capacity with temperature and frequency for 2D C/SiC composites with and without heat treatment. It can be seen that heat treatment decreases damping capacity and peak values for 2D C/SiC composites and changes the shift of damping peak in direction. As the testing frequency increasing, damping capacity and peak value for as-received 2D C/SiC composites decreases gradually accompanied with a shift of damping peak towards the lower temperatures, while for heat-treated 2D C/SiC composites the temperature of damping peak shifts to the higher temperatures. With the testing frequency increasing, damping capacity for both of the 2D C/SiC composites decreases gradually. Fig. 7 shows the dependence of damping capacity on temperature and frequency for 3D C/SiC composites with and without heat treatment. It is similar to 2D C/SiC composites that heat treatment decreases damping capacity and peak values for 3D C/SiC composites except at 1 Hz, while it does not change the shift of damping peak in direction. Damping capacity and peak value for both 3D C/SiC composites decreases gradually accompanied with a shift of damping peak towards the lower temperatures with the testing frequency increasing. Damping peak for heat-treated 3D C/SiC composites at 1 Hz slightly increases. The data indicate that heat treatment generally decreases damping capacity for 2D and 3D C/SiC composites. It can be attributed to the change in SiC matrix. Tiny SiC grains were generated in CVI process with great internal stress and many of structural defects, such as dislocation. The relaxation of internal stress and the mobility of dislocation became the source of
1
The porosity in this paper means open porosity, which was measured based on the Archimedes method. The closed porosity was not taken into account because it never changed after the specimens were coated.
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Fig. 5. The appearance of C/SiC composites: (a) before coated and (b) after coated.
Fig. 6. Variations of damping capacity with temperature and frequency for 2D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 2D C/SiC composites untreated (treated) SiC coating.
Fig. 7. Variations of damping capacity with temperature and frequency for 3D C/SiC composites: (rhombus) 1 Hz, (square) 2 Hz, (triangle) 5 Hz, and (circle) 10 Hz. Open (closed) symbols indicate 3D C/SiC composites untreated (treated) SiC coating.
Fig. 8. SEM photographs of polished 3DC/SiC composites: (a) without heat treatment and (b) with heat treatment.
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energy dissipation in vibration environment, and the composites showed high damping capacity. However, the stabilizing heat treatment at 1500 ◦ C results in the release of internal stress, the annealing of defects and grain growth [14]. Then, the damping capacity decreases due to the source of energy dissipation decreasing. On the other hand, heat treatment can decrease interphase bonding strength and enlarge the debonding of fiber and matrix [15,16]. For the as-received C/SiC composites, textured graphitoid areas were observed by Li [17] at the edge of PyC interphase near by the fibers, and the other part was in turbostratic stacking structure. During the processing of heat treatment, sliding occurred in the textured graphitoid areas to release the thermal stresses induced by the thermal mismatch of fibers and PyC, and then interphase bonding strength was decreased [18]. Moreover, as shown in Fig. 8, the debonding areas were enlarged after heat treatment because of the thermal mismatch of fiber and matrix. Therefore, friction in interphase area is enhanced and can dissipate much energy in vibration, which is considered as the reason why damping peak for heat-treated 3D C/SiC composites at 1 Hz slightly increases. Moreover, heat treatment has different effects on the shift of damping peak in direction for 2D and 3D C/SiC composites, which is conceived that it is related to different fiber preform architectures. Furthermore, heat treatment has no influence on the relationship between damping behavior and frequency. 4. Conclusions The effects of CVD SiC coating and heat treatment on damping behavior of 2D and 3D C/SiC composites and the related mechanisms have been investigated. Damping capacity of SiCcoated and heat-treated C/SiC composites are mostly lower than that of as-received C/SiC composites. The reduction in the damping capacity of 2D and 3D SiC-coated C/SiC composites mainly results from the decrease of porosity. However, the decrease of the damping capacity of 2D and 3D heat-treated C/SiC composites is mainly attributed to the change of microstructure in the SiC matrix and interphase bonding. Heat treatment has different effects on the shift of damping peak in direction for 2D and 3D C/SiC composites, which is related to different fiber
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