Journal of Materials Science & Technology  2019 , 35 (12): 2767-2771 https://doi.org/10.1016/j.jmst.2019.05.069

Orginal Article

Preparation and interface modification of Si3N4f/SiO2 composites

Yubo Houa,1, Xuejin Yanga,1, Bin Lib*, Duan Lia*, Shitao Gaoa, Zhongshuai Wua

a.Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha, 410073, China
b.School of Materials, Sun Yat-sen University, Guangzhou, 510275, China

Corresponding authors:   *Corresponding authors.E-mail addresses: libin75@mail.sysu.edu.cn (B. Li)duanli2016@163.com(D. Li).
1 Co-first authors. These authors contributed equally to this work.
duanli2016@163.com(D. Li).
1 Co-first authors. These authors contributed equally to this work.
duanli2016@163.com(D. Li).
1 Co-first authors. These authors contributed equally to this work.

Received: 2019-01-25

Revised:  2019-03-31

Accepted:  2019-05-15

Online:  2019-12-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

In order to modify the interface, SiON coating was introduced on the surface of silicon nitride fiber by perhydropolysilazane conversion method. Si3N4f/SiO2 and Si3N4f/SiONc/SiO2 composites were prepared by sol-gel method to explore the influence of SiON coating on the mechanical properties of composites. The results show that with the protection of SiON coating, Si3N4 fiber enjoys a strength increase of up to 24.1% and Si3N4f/SiONc/SiO2 composites have a tensile strength of 170.5 MPa and a modulus of 26.9 GPa, respectively. After 1000 °C annealing in air for 1 h, Si3N4f/SiONc/SiO2 composites retain 65.0% of their original strength and show a better toughness than Si3N4f/SiO2 composites. The improvement of mechanical properties is attributing to the healing effect of SiON coating as well as its intermediate coefficient of thermal expansion between Si3N4 fiber and SiO2 matrix.

Keywords: Si3N4 fiber ; Perhydropolysilazane ; SiON coating ; Interface ; Mechanical properties

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Yubo Hou, Xuejin Yang, Bin Li, Duan Li, Shitao Gao, Zhongshuai Wu. Preparation and interface modification of Si3N4f/SiO2 composites[J]. Journal of Materials Science & Technology, 2019, 35(12): 2767-2771 https://doi.org/10.1016/j.jmst.2019.05.069

1. Introduction

Silicon nitride fiber (Si3N4 fiber) has been recognized as one of the most optimal reinforcements for wave-transparent composites due to its excellent thermal stability, high specific strength and modulus, good high-temperature mechanical properties as well as decent dielectric behavior [1,2]. Compared with some oxide fibers, like silica fiber (SiO2 fiber), the Si3N4 fiber displays excellent thermal stability and has no crystallization problem at high temperatures, which enables its application in high-speed aircrafts and reentry vehicles [[3], [4], [5], [6]].

As well-known, fiber-matrix interface is critical to the mechanical properties of the continuous fiber reinforced ceramic matrix composites (FRCMCs) [7]. One effective approach to optimize the interfacial structure of composites is fabricating a layer of interfacial coating on the fiber. The function of such coating mainly includes: i) to prevent the interfacial reaction between the fiber and matrix; ii) to reduce the residual thermal stress caused by the thermal mismatch; and iii) to heal the structural defects of fibers. Recently, many researchers have been devoted to the interfacial coating of Si3N4 based composites. K. Sato et al. prepared C-B-Si coatings on Si3N4 fiber by chemical vapor deposition method and fabricated Si-N-C matrix composites [8,9]. The oxidation resistance of composites was significantly improved. However, the presence of carbon is not acceptable for wave-transparent materials. K.W. Nam et al. studied the effect of SiO2 coating on crack healing and bending strength of Si3N4 ceramics [10]. The results show that SiO2 particles can promote crack healing. X. Hu et al. adopted silica sol as precursor and prepared silica coating on Si3N4 fiber by dip coating method. The tensile strength of Si3N4 fiber was increased by 125% [11]. Moreover, perhydropolysilazane (PHPS) has been also employed to the preparation of SiOx or SiON based coating [[12], [13], [14]]. Compared with other methods, the PHPS conversion method is free from water. So, it enables application to the materials which are sensible to water. More importantly, the conversion temperature is relatively low. Ceramic fiber will suffer minor damage throughout the fabrication process. Nevertheless, there is few reports on the Si3N4 fiber coatings prepared by PHPS conversion method.

In this work, SiON coating was formed on Si3N4 fiber by the PHPS conversion method. The Si3N4 fiber bundle reinforced silica matrix composites with (Si3N4f/SiONc/SiO2) and that without the SiON coating (Si3N4f/SiO2) were fabricated through sol-gel method. SiO2f/SiO2 composites were also prepared by the same way for comparison. The effect of SiON coating on the mechanical properties was explored by comparing the microstructure, composition and tensile strength of the fiber and composites.

2. Experiment

2.1. Raw materials

The Si3N4 fiber with carbon content below 1 wt% and oxygen content over 3 wt% was synthesized by the National University of Defense Technology (NUDT). It possesses a diameter of ˜12 μm, a tensile strength of over 1.2 GPa and a Young’s modulus of ˜140 GPa. SiO2 fiber having a diameter of 6-8 μm, a tensile strength ˜1.7 GPa and a Young’s modulus of ˜78 GPa was purchased from Feilihua Quartz Glass Co. Ltd., China. The PHPS was selected as precursor of SiON coating, while silica sol for SiO2 matrix. They were provided by Iota Silicone Oil Co. Ltd., China and Xinyu Chemicals Co. Ltd., China, respectively.

2.2. Experimental details

To fabricate the SiON coating, PHPS solution was firstly prepared by mixing PHPS and butylether with a certain ratio of 5 wt%. Then Si3N4 and SiO2 fiber were soaked in the PHPS solution for 1 h. After that, the SiON coating could be formed by 100 °C heat treatment for several hours.

To fabricate fiber bundle reinforced silica matrix composites, Si3N4 and SiO2 fiber bundles were firstly infiltrated by silica sol in vacuum. Then the bundles filled with silica sol were dried at 100 °C in air. Finally, the dried composites were sintered at 800 °C in N2 under atmosphere pressure.

2.3. Characterization

The phase composition of the fiber and composites was characterized by X-ray diffractometer (XRD, D8 Advance, Bruker/Axc Corp., Germany) using monochromatic Cu Kα radiation (2θ = 10°-80°). The chemical construction was examined by X-ray photoelectron spectroscope (XPS, Thermo ESCALAB 250, Thermo Fisher Scientific Corp., USA) with Al Ka excitation and Fourier transform infrared spectroscopy (FT-IR, Avatar 360, Nicolet Instrument Corp., Wisconsin, USA). Surface and cross-section morphologies of the fiber were observed by scanning electron microscopy (SEM, HITACHI S-4800, Japan).

Single filament tensile test of the fiber was conducted on Instron-type test machine (Micro-350, Testometrix) with a gauge length of 25 mm and a crosshead speed of 1 mm/min. The average strength was obtained by testing at least 24 samples. The tensile strength and modulus of composites were measured by the same way with a gauge length of 150 mm and a crosshead speed of 0.5 mm/min. The average value was obtained by measuring at least 6 samples.

3. Results and discussion

Fig. 1 shows the XRD patterns of SiO2 fiber and Si3N4 fiber after 1 h annealing in air at different temperatures. For the as-received SiO2 fiber, there are only broad peaks existed in the XRD spectrum, showing an amorphous state. For the 800 °C treated SiO2 fiber, there is a trend of crystallization with the intensity of the peak at around 23° becoming stronger. When it comes to the 1200 °C annealed SiO2 fiber, there appears a sharp but not too strong characteristic peak at 23°, which proves the generation of cristobalite. As for the Si3N4 fiber, the characteristic peak of cristobalite is not evident until being heat-treated at 1300 °C. Considering that the crystallization generally results in a strength decrease of fiber, it is reasonable to consider that Si3N4 fiber has better thermal stability than SiO2 fiber.

Fig. 1.   XRD patterns of (a) SiO2 fiber and (b) Si3N4 fiber at different temperatures.

Surface morphologies of the SiO2 and Si3N4 fiber after annealing at different temperatures are illustrated in Fig. 2. It can be seen from Fig. 2(a) and (d) that both the as-received SiO2 fiber and Si3N4 fiber exhibit a relatively smooth surface. With the increase of annealing temperature, several small cores appear on the surface of the 800 °C treated SiO2 fiber (Fig. 2b), while apparent microcracks and wrinkles can be observed at 1200 °C treated SiO2 fiber (Fig. 2c), which is caused by the generation of cristobalite phase. As to the Si3N4 fiber, it maintains a smooth and fine structure even at 1200 °C, despite of several white particles exhibiting on the surface of the 1200 °C annealed fiber. From this point of view, Si3N4 fiber displays good oxidation resistance and is more thermostable than SiO2 fiber, which is in good agreement with the XRD results.

Fig. 2.   Surface morphologies of SiO2 fiber (a) as-received, (b) after annealing at 800 °C, (c) after annealing at 1200 °C, and Si3N4 fiber (d) as-received, (e) after annealing at 1000 °C, (f) after annealing at 1200 °C.

To improve the mechanical properties of Si3N4 fiber, the SiON coating was formed by PHPS conversion method. The tensile strength at different temperatures of the coated and uncoated Si3N4 fiber is presented in Fig. 3. As can be seen, when the annealing temperature is below 1100 °C, the coated Si3N4 fiber possesses a higher tensile strength than the uncoated one. This is attributed to the healing effect of SiON coating. However, when the temperature increases to 1200 °C and 1300 °C, the tensile strength of the coated fiber is slightly lower than that of the uncoated one. The possible reason might be the crystallization of SiON coating leads to the generation of microcracks, and thus make the healing effect totally lost. Meanwhile, the diameter of the coated fiber is greater than the uncoated fiber, thus the measured tensile strength is not the actual strength of silicon nitride fiber.

Fig. 3.   Tensile strength of the coated and uncoated Si3N4 fiber after annealing at different temperatures for 1 h.

Fig. 4 compares the FT-IR spectra of Si3N4f/SiO2 and SiO2f/SiO2 composites. The characteristic peaks at 1100, 800 and 470 cm-1 belong to SiO2, which can be discovered in both two composites. Besides, no distinct difference between Si3N4f/SiO2 and SiO2f/SiO2 composites can be found, except that a peak at 930 cm-1 appears in the spectrum of Si3N4f/SiO2 composite. It is assigned to Si-N-Si bond of Si3N4 fiber.

Fig. 4.   FT-IR spectra of Si3N4f/SiO2 and SiO2f/SiO2 composites.

To have a further understanding on the chemical construction of Si3N4f/SiO2 composites, XPS was employed and the results are shown in Fig. 5. It can be seen that the overall spectrum of Si3N4f/SiO2 composites reveals the presense of O1s, N1s, C1s, Si2s, and Si2p. C component is assigned to the standard calibration for the XPS analysis. The peaks at 103.5 eV in Fig. 5(b) and 533 eV in Fig. 5(c) are attributed to the SiO2 matrix. For the Si2p and N1s core level lines in Fig. 5(b) and (d), the single peaks at 102 eV and 397.9 eV can be assigned to Si_N bond and N_Si bond, respectively.

Fig. 5.   XPS spectra of Si3N4f/SiO2 composites: (a) overall spectrum, (b) Si2p, (c) O1s, (d) N1s.

Fig. 6 plots the load-displacement curves of both the Si3N4f/SiO2, Si3N4f/SiONc/SiO2 and SiO2f/SiO2 composites. The detailed mechanical properties at different temperatures of the above composites are listed in Table 1. In general, all the composites encounter a strength degradation as the annealing temperature increases. For the Si3N4f/SiO2 composites, the tensile strength and modulus at 1000 °C are 95.0 MPa (64.5% retention) and 16.4 GPa (62.1% retention), respectively. At 1200 °C, the values decrease to 57.6 MPa (39.1% retention) and 8.2 GPa (31.1% retention). It can be seen from Fig. 6(a) that the as-produced Si3N4f/SiO2 composite displays a progressive fracture mode. It is reported that the failure of unidirectional fiber reinforced composites starts with the rupture of the Si3N4 fiber, followed by which is the de-bonding of the interface and the fiber pull-out [15]. Progressive fracture in turn proves the reinforcing and strengthening function of Si3N4 fiber. When it comes to the performance at 1000 and 1200 °C, Si3N4f/SiO2 composites, however, shows a brittle fracture mode, as evidenced by the plunge of load at the failure stage. According to the Griffith’s theory, the reduction of tensile strength of composites is partly caused by the increase of matrix micro-cracks with the rise of temperature. However, it is believed in this work that the degradation of Si3N4 fiber might be the main reason for the decline of the strength, because the tensile modulus of composites dramatically declines after high-temperature heat treatment.

Fig. 6.   Load-displacement curves of (a) Si3N4f/SiO2 and (b) Si3N4f/SiONc/SiO2 composites annealed at different temperatures for 1 h.

Table 1   Mechanical properties of Si3N4f/SiO2, Si3N4f/SiONc/SiO2 and SiO2f/SiO2 composites annealed at different temperatures for 1 h.

CompositesAnneal temperature (°C)Tensile strength (MPa)Retention of strength (%)Tensilemodulus (GPa)Retention of modulus (%)
Si3N4f/SiO2As-produced147.210027.6100
1000 °C95.064.516.462.1
1200 °C57.639.18.231.1
Si3N4/SiONc/SiO2As-produced170.510026.9100
1000 °C111.965.013.750.5
1200 °C46.027.02.5214.8
SiO2f/SiO2As-produced162.410019.1100
1000 °C83.551.49.951.7
1200 °C28.517.63.417.9

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In comparison, Si3N4f/SiONc/SiO2 composites display a better mechanical performance especially at 1000 °C. The tensile strength reaches up to 170.5 MPa at room temperature and maintains 111.9 MPa at 1000 °C, and more importantly the fracture mode of Si3N4f/SiONc/SiO2 composites is a progressive one even at 1000 °C (Fig. 6b). This improvement of mechanical properties is not only owing to the healing effect of the SiON coating on the fiber, but also due to the intermediate coefficient of thermal expansion of SiON coating staying between SiO2 matrix and Si3N4 fiber. However, the tensile strength at 1200 °C of Si3N4f/SiONc/SiO2 is relatively lower than that of Si3N4f/SiO2. This reduction is mainly caused by degradation of the coated fiber. Nevertheless, in comparison with SiO2f/SiO2 composites whose strength retention is only 28.5% at 1200 °C, the high-temperature performance of Si3N4 fiber reinforced silica matrix composites is much remarkable.

4. Conclusions

(1)SiO2 fiber shows a tendency of crystallization even at 800 °C and seriously degrades at 1200 °C, while Si3N4 fiber encounters little structural change at 1200 °C and will not crystallize until at 1300 °C, showing much better high-temperature performance.

(2)Owing to the surface healing effect of SiON coating, the tensile strength of the SiON coated Si3N4 fiber can be increased by 24.1%, and the tensile strength and modulus of Si3N4/SiONc/SiO2 composite reach up to 170.5 MPa and 26.9 GPa at room temperature.

(3)After annealing at 1000 °C, Si3N4/SiONc/SiO2 composite maintains strength retention of 65.0%, and shows a progressive fracture mode. The improvement of mechanical properties is attributed to the healing effect of the SiON coating as well as its intermediate coefficient of thermal expansion between SiO2 matrix and Si3N4 fiber.

(4)Si3N4 fiber reinforced silica matrix composite possesses better high-temperature mechanical properties than SiO2f/SiO2 composite, which is one of the most promising wave-transparent materials for the high-temperature application.

Acknowledgments

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51702361) and the Natural Science Foundation of Hunan Province (Grant No. 2017JJ3353).


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