The cross-sectional morphologies of the SiCf/SiC composites indicate that both the fiber and the interphase remain intact (Fig. 1). Besides, the SiC matrix is almost distributed homogeneously and densely between the fibers. These structure features are quite different from the SiCf/SiC composites derived from polymer infiltration and pyrolysis (PIP) [23,24] and chemical vapor infiltration (CVI) [25,26], which show rather porous structures due to the restricted mass transfer during the fabrication process. The high temperature/pressure of NITE process adopted in present study contributes to the highly dense structure, but can also lead to high residual stress in the composites. It should be reminded here that the BN interphase is unobserved in the XRD patterns (Fig. 1d), which is mainly attributed to its low volume content (~3.3 vol%). In addition, the SiC phase exhibits two sets of crystal parameters that have subtle difference, which may correspond to the SiC fiber and the SiC matrix respectively. The energy dispersive spectra (Fig. 2) of the composites confirm that the BN interphase is mainly composed of B and N elements. The interphase almost remains unreacted with the fiber and the matrix. This makes it feasible to measure residual stress in the interphase using Raman spectroscopy.
Relaxation of residual stress is usually realized via annealing process, during which the material is heated at temperature lower than the preparation temperature. With the aid of thermal excitation, the crystallographic microstructures (grain size, grain boundary, grain orientation, etc.) can achieve significant transformation through atom diffusion or defect annihilation. Accompanying with the microstructure transformation, residual stress also experiences remarkable relaxation. Here, residual stress variation of the SiCf/SiC composites was in-situ investigated by a high-temperature Raman Spectrometer, and the Raman spectra at RT~1400℃ are shown in Fig. 3.
As the composites are composed of multiple constituents (matrix, interphase and fiber), the Raman spectra of the constituents were collected separately (Fig. 3a). The Raman spectra of the SiC matrix contain two main peaks in the wavenumber range of 500~1200 cm-1: transverse optical peak at ~796 cm-1 and longitudinal optical peak at ~973 cm-1 (Fig. 3b) [27,28]. The Raman spectra of the BN interphase show a sharp and strong peak at ~1365 cm-1, which corresponds to the in-plane ring vibration (Fig. 3c) [29,30]. As the resolution of Raman spectroscopy is ~1 μm (larger than the thickness of the BN interphase), other peaks ascribed to the fiber and the matrix are also present in the Raman spectra of the BN interphase. The typical Raman spectra of the SiC fiber present three peaks: one sharp peak (~830 cm-1) corresponding to SiC microcrystalline and two broad peaks (1330~1360 cm-1 and 1585~1600 cm-1) corresponding to graphite phase surrounding the SiC microcrystalline (Fig. 3d) [31]. The graphite peak at larger wavenumber is called G peak, which is resulted from the vibration of carbon network plane, and has proved to be highly sensitive to the residual stress of SiC fiber [32,33]. With the temperature rising from room temperature (RT) to 1400℃, slight peak shift occurs as indicated by the dashed lines in the spectra, indicating that the residual stress of the composites experiences significant variation.
To calculate the residual stress, the peak shift (dλ) should be measured as precisely as possible. In general, dλ can be expressed as the difference between the wavenumbers corresponding to the peak summits (dλ = λ’ - λ). This may cause large measurement error when the peak shape is not symmetrical. Considering the irregular peak shape, barycenter method is used here to determine the dλ value [34,35]. The calculation of dλ based on barycenter method can be schematically shown in Fig. 4. Firstly, Raman peak intensity is normalized by the formula below: (see Equation 1 in the Supplementary Files)
Where In and Ii represent the normalized and the initial intensities of the Raman spectra, respectively. Imin and Imax are the minimum and the maximum intensities of the Raman spectra, respectively. Secondly, the baseline intensity (I0) of the Raman spectra is defined as: (see Equation 2 in the Supplementary Files)
Import the data of In and I0 into Origin software and then a normalized peak and a horizontal baseline can be obtained, which generates two intersections (λ1 and λ2). Then the barycenter position (λb) is calculated according to the formula as below: (see Equation 3 in the Supplementary Files)
Then the peak position shift (dλ) and the residual stress (σr) can be obtained as follows: (see Equations 4 and 5 in the Supplementary Files)
Where and are the barycenter positions of the Raman peaks at stressed and stress-free states, respectively. C is conversion factor of Raman shift to residual stress. Characteristic peaks at 796 cm-1, 1365 cm-1 and 1585 cm-1 are chosen as the calculation references for the matrix, interphase and fiber, respectively. The corresponding C values are listed in Table 1.
Table 1 Characteristic peak positions at stress-free states and conversion factors of peak shift to stress
Constituent
|
Characteristic peak position at stress-free state (cm-1)
|
Conversion factor C
(GPa∙cm-1)
|
SiC matrix
|
796
|
-0.2833 [28]
|
BN interphase
|
1365
|
-0.2950 [36]
|
SiC fiber
|
1585
|
-0.1038 [31]
|
The peak shift (dλ) and residual stress (σr) at RT~1400℃ are shown in Fig. 5. Both the matrix and the fiber show positive dλ while the interphase shows negative dλ in the temperature range. This indicates the matrix and the fiber bear compressive stress while the interphase bears tensile stress. With the temperature rising from RT to 1400℃, dλ of the matrix and the fiber decreases from 4.56 cm-1 to 2.20 cm-1 and from 8.07 cm-1 to 5.30 cm-1, respectively, while that for the interphase increases from -0.55 cm-1 to -0.35 cm-1. Correspondingly, the residual stresses of the matrix and the fiber decrease from 1.29 GPa to 0.62 GPa and from 0.84 GPa to 0.55 GPa in compression, while that for the interphase decreases from 0.16 GPa to 0.10 GPa in tension. The thermal expansion coefficient of the interphase is ~2.7×10-6 ℃-1, which is much lower than the fiber (~5.0×10-6 ℃-1) and the matrix (~4.6×10-6 ℃-1). This makes the interphase in tension while both the fiber and the matrix in compression due to the mismatch of the thermal expansion coefficients. The different residual stress states of the constituents can be further confirmed by the mechanical properties of the composites, as residual stress can significantly influence the mechanical behavior by changing the stress distribution.
The mechanical response of SiCf/SiC composites can be roughly divided into three stages. Firstly, the SiC matrix cracks when the applied stress reaches to the critical value beyond the matrix can endure. With the increase of the applied stress, the cracks in the matrix can propagate to the interphase and lead to fiber/matrix debonding. Finally, the fiber pulls out and fractures as the applied stress rises to the fracture strength. Based on the calculation results (Fig. 5b), the matrix of the composites bears residual compressive stress, which can suppress the formation of cracks in the matrix. Meanwhile, the interphase bears residual tensile stress, which can accelerate crack propagation to the interphase and thus facilitate fiber/matrix debonding and fiber pulling-out. With the increase of temperature, fiber/matrix debonding and fiber pulling-out can be restricted due to the decreasing residual tensile stress. This leads to different fracture modes in terms of “easy” and “hard” fiber pulling-out (Fig. 6). However, the mechanical behavior of the composites should be much more complicated considering the synergistic effect of the residual stresses in the matrix and the fiber.
In order to investigate the effects of residual stress on the mechanical properties, tensile mechanical tests were performed on the as-received and heat-treated composites. The composites were heat-treated at 1000℃ and 1400℃ for 10h, respectively. Higher temperature heat treatment contributes to lower residual stress in the composites according to Fig. 5b. The tensile stress-strain curves of the composites are shown in Fig. 7 and the mechanical properties are summarized in Table 2. The tensile strength almost remains unchanged regardless of the heat-treatment, while a slight decrease can be seen in the tensile strain of heat-treated composites compared with the as-received ones. The unchanged tensile strength indicates that the decreasing residual stress has little effect on the mechanical strength. The decrease of tensile strain should be attributed to the restricted fiber/matrix debonding and fiber pulling-out caused by the lower residual tensile stress in the interphase.
Table 2 Mechanical properties of as-received and heat-treated SiCf/SiC composites
Sample
|
Tensile strain (%)
|
Tensile strength (MPa)
|
As-received SiCf/SiC
|
0.57±0.07
|
236±15
|
1000℃/10h
|
0.52±0.04
|
251±13
|
1400℃/10h
|
0.48±0.03
|
247±17
|
The tensile fracture surfaces of the composites (Fig. 8) confirm that the composites show different fracture modes due to the variation of residual stress. The fiber pull-outs in the heat-treated composites are a little shorter than those in the as-received samples. This is resulted from lower residual tensile stress in the interphase which restricts fiber pulling-out. Furthermore, the fractured fiber surface of the heat-treated composites is rougher than that of the as-received composites. As the residual tensile stress in the composites heat-treated at 1400℃ is much smaller than the other composites, interphase debonding and fiber pulling-out are suppressed violently. This leads to rougher fiber surface and shorter fiber pull-outs. The effects of residual stress on the mechanical properties should be very intricate considering the anisotropic structures and multiple constituents of the composite. Furthermore, the residual stress is strongly dependent on preparation methods and post-treatment conditions. Therefore, the overall understanding of residual stress in the composites still needs more researches. Nevertheless, this study promotes the research by revelation of residual stress variation in SiCf/SiC composites and its effects on the mechanical behavior of the composites.