Short communicationThe control of interfacial bonding state and optimization of mechanical properties of Si3N4f/BN/Si3N4 composites via different synthesis technologies
Introduction
With the development of aircraft technology, the requirements of electronic communication and electronic countermeasure technology in the modern battlefield are becoming stricter. The high-performance radomes materials that need to serve in harsher environments have always been the research hotspot, which possess not only good electromagnetic (EM) wave-transparent performance, but also excellent mechanical properties, high temperature stability, oxidation resistance, etc [[1], [2], [3]]. The wave-transparent properties are mainly based on the intrinsic composition and structure of material systems [[4], [5], [6]], which can be achieved through the preparation process of materials. It is challenging for wave-transparent materials to increase their mechanical properties. Generally, it is necessary to optimize the interface structure and achieve the matching between structure and properties of each component of composites.
Silicon nitride fibers-reinforced silicon nitride (Si3N4f/BN/Si3N4) composites with BN interphase combines the high strength, good wave-transparent properties, excellent chemical stability and high temperature stability of Si3N4 ceramic [[7], [8], [9], [10]], while overcoming its shortcoming of defect sensitivity, which is an excellent candidate of wave-transparent materials [11]. Given the current characteristics of composites, the high load bearing is mainly determined by the fibers [12,13]. At present, the developed Si3N4 fibers have lower strength and modulus compared with silicon carbide (SiC) fibers [[14], [15], [16]]. How to give full play to their mechanical properties is the key to extend the application of Si3N4f/BN/Si3N4 composites. In the composite systems, to achieve good mechanical properties, the intrinsic characteristics of the fibers and matrix need to meet a certain matching relationship, such as modulus matching and thermal physical properties matching, and the composites should possess a correspondingly proper interface bonding state between fibers and matrix at the same time, which is the essential way to develop the effective bearing capacity of fibers [17,18]. With the protection of BN interphase, the damage to fibers during fabricating composites is weak. The characteristics of Si3N4 matrix prepared by different technologies (such as chemical vapor infiltration (CVI), precursor infiltration pyrolysis (PIP), and reactive sintering (RS)) may be different, which will affect the interface bonding state and modulus matching relationship between fibers and matrix, and ultimately lead to difference in the final mechanical properties of composites. At present, few studies on Si3N4f/BN/Si3N4 composites fabricated by CVI and PIP process have been reported. It is necessary to explore the relationship between synthesis technology and microstructure and properties of Si3N4f/BN/Si3N4 composites.
Based on the above analysis, this paper studied the microstructure and mechanical properties of Si3N4f/BN/Si3N4 composites fabricated by two different technologies (CVI and PIP), focusing on the interface state and bonding control, and the modulus matching relationship between fibers and matrix of composites prepared by different technologies. It also reveals the correlation between micro-nano mechanical and macro-mechanical properties of Si3N4f/BN/Si3N4 composites, which offers an effective way to design and optimize composites structure, and improve the performance of Si3N4f/BN/Si3N4 composites.
Section snippets
Preparation of Si3N4f/BN/Si3N4 composites
Continuous Si3N4 fibers with a density of 2.35 g∙cm−3, diameter of around 12.2 μm, and a filament tensile strength of 1.3 ± 0.2 GPa were provided by the Xiamen University in China [19,20]. The Si3N4 fiber fabrics with a three-dimensional (3D) four-directional structure and a fiber volume fraction of 38 % were prepared by Shaanxi textile research institute. Before fabricating Si3N4 matrix, BN interphase with a thickness of about 500 nm was uniformly deposited on the surface of Si3N4 fibers by
Microstructure and mechanical properties of as-fabricated Si3N4f/BN/Si3N4 composites
Fig. 1 shows the basic structural characteristics of Si3N4f/BN/Si3N4 composites. The polished cross-section of Si3N4f/BN/Si3N4 composites is shown in Fig. 1(a and c), from which what can be seen is that the composites fabricated by CVI have many closed pores with different sizes and uneven distribution due to the possible bottleneck effect of CVI process [24], while the composites fabricated by PIP have fewer closed pores and higher compactness in macroscopic structure. The as-fabricated
Conclusion
The Si3N4f/BN/Si3N4 composites were fabricated successfully via CVI and PIP technologies. The low-modulus Si3N4 matrix with micropores prepared by PIP process made great contribution to the improvement of mechanical properties of composites comparing with Si3N4 matrix by CVI process. PIP Si3N4f/BN/Si3N4 composites possessed a weak interfacial bond and proper modulus matching relationship, which exerted the strengthening and toughening effect of fibers effectively by increasing co-bearing
Declaration of Competing Interest
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript. The article is original and unpublished and has not or is not being considered for publication elsewhere. Also it has to be declared that all authors have seen and approved the manuscript.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51632007, 51872229, 51521061), the 111 Project of China (B08040), and National Science and Technology Major Project (Grant: 2017-VⅠ-0007-0077). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for the kind assistance with electron microscopic characterization in this work.
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