Effects of microstructure defects on the internal friction of C/SiC composites
Introduction
Due to their unique properties such as low density, outstanding fatigue resistance and good mechanical properties, especially at high temperatures, carbon fiber reinforced silicon carbide (C/SiC) composites are the great potential materials in the aeronautical, astronautical and energy application fields [1], [2], [3]. In those fields, a considerable part of C/SiC composites’ components such as brake discs and clutch systems, usually suffer high-speed rotating and bear a dynamic load. In this case, the dynamic mechanical properties of C/SiC composites become the main factor controlling the performance of materials. The internal friction or damping capacity, as an important parameter for dynamic properties, mainly refers to the energy dissipation through reversible movement or an irreversible thermoelastic movement in materials under mechanical vibration [4]. The internal friction results from the irreversible transformation (viscoelastic deformation) in materials and is mainly controlled by the viscoelastic behavior. Clearly, the internal friction has great relationship with the constitutions and microstructure of materials and is very sensitive to the microstructural changes, which provides the theoretical basis for judging the microstructure changes by analyzing the internal friction [5], [6], [7]. Compared to lots of studies about the static mechanical properties of C/SiC, the researches about the dynamic mechanical properties are relatively lacking. Therefore, for deeply understanding the performance and the microstructure involution of materials in service, it is essential to investigate the internal friction behavior of C/SiC composites.
Many studies have been done to explore the relationship between the internal friction and microstructure in composites and many internal friction mechanisms have been established [8], [9], [10], [11]. For C/C composites, the internal friction decreased with an increase in bulk density and increased with an increase in the fiber volume fraction and the porosity [12], [13]. After fatigue tests, the internal friction and porosity increased with the progressive damage, the internal friction was an effective and sensitive method for characterizing structural changes indirectly and assessing the internal damage of C/C composites [14]. By comparison the composites reinforced by two different fibers, the effect of fiber type on the internal friction of composites was studies. Because of the fewer microstructural defects in SiC fiber reinforced composites, the composites possessed a lower internal friction and a higher dynamic modulus than carbon fiber reinforced composites [15]. Expect the fiber type, the fiber directionality in CMC preforms also could affect the internal friction and storage modulus of SiC/SiC. The 3D 4-directional composites showed larger internal friction than their 3D 5-directional counterparts, because there were more damage and microstructural defects, such as pores and micro-cracks in the former. The microstructural defects were activated to relax the stress concentration and resulted in the dissipation of elastic energy. That is, the elastic energy was dissipated, so the internal friction level of 3D 5-directional composites was lower than that of 3D 4-directional composites [16].
The interphase layer between fiber and matrix is very important for load transfer and adjusting the bonding strength. The studies about the interphase in 3D braided SiCf/SiC composites had shown that the composites with PyC and SiC interphase presented the lower internal friction than those of composites without an interlayer. Because of the existence of interface, the internal friction in SiCf/SiC composites with interphase became more sensitive to temperature, frequency and amplitude [17]. In Sato's studies, the internal friction of 2D SiCf/SiC composite fabricated by CVI process was found to be largely affected not by the property of the fiber but the microstructure of the matrix. And the internal friction increased with crystallization [18]. In Zhang's studies, the damping capacity of 2D and 3D C/SiC composites decreased after heat treatment at 1500 °C, which is mainly attributed to the microstructure changes in the SiC matrix and interphase bonding [19]. For 2D C/SiC composites with PyC interphase, the heat treatment introduced damage in PyC interphase and improved internal friction. Meanwhile, the heat treatment makes the internal friction of 2D C/SiC composites more insensitive to temperature and frequency, and more sensitive to amplitude [20].
Because of the limitation of the preparation progress, the defects such as pores, cracks and lamination inevitably exist in C/SiC composites. Moreover, in service, various environmental factors, such as O2 and H2O would erode C/SiC composites, introduced defects into composites and changed the microstructure of materials [21], [22]. The defects, especially the interface damage and pore in C/SiC had great effects on the internal friction of C/SiC composites. However, the studies about the effects of the microstructural damage on the internal friction of C/SiC composites have rarely been clarified, and deep investigation is needed to expose the internal friction mechanism in C/SiC with defects. In this paper, the internal friction behaviors of C/SiC composites with different damage were investigated and the internal friction mechanisms were also discussed in detail. Meanwhile, by analyzing the internal friction of C/SiC composites with microstructure defects, it can get some useful information about the relationship between internal friction and microstructure evolution.
Section snippets
Materials preparation
C/SiC composites prepared by the chemical vapor infiltration (CVI) process were used in this study. T300 woven carbon fabrics were laminated to prepare two dimensional (2D) fiber preforms, and the volume fraction of carbon fibers was designed at about 40%. Pyrolytic carbon (PyC) was firstly deposited on the carbon preforms by chemical vapor deposition (CVD) at 900–1000 °C in a reduced pressure of 5 kPa. Methyltrichlorosilane (MTS, CH3-SiCl3) was used as the SiC precursor for depositing SiC
Microstructure
After corrosion at 700 °C, the main microstructure changes in C/SiC composites was the carbon phase oxidation and the increasing porosity. The porosity, weight loss and the oxygen content of C/SiC samples were listed in Table 1. The porosity and the weight loss were illustrated in Fig. 1. The porosity of samples increased evidently from 7.73% to 45.11% with corrosion time prolonged from 0 h to 10 h. There was little oxide formed in SiC matrix because of the relative low temperatures. So, in the
Conclusion
The present study clarified the effect of microstructure defects on the internal friction of C/SiC composites under multi-frequency and multi-amplitude modes, respectively. The microstructure defects introduced by corrosion improved the internal friction of C/SiC, and the rise rates increased with the damage degree in C/SiC samples. There were two mechanisms affected the internal friction of C/SiC with microstructure defects, the interface and pore of internal friction. For the C/SiC samples
Acknowledgements
The authors acknowledge the financial support of China Postdoctoral Science Foundation (No. 2018M643816XB).
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