High-temperature wave-transparent composites are essential components for radomes and antenna windows in hypersonic vehicles, enabling stable communication under extreme flight conditions. Aerodynamic heating during high-speed flight presents significant challenges to both the thermal and structural integrity of these materials. To perform reliably, they must simultaneously satisfy multiple stringent requirements: low dielectric constant and dielectric loss tangent, high mechanical strength, and excellent thermal stability.
Among the various candidates, Si 3 N 4 fibers offer distinct advantages over other reinforcements. Compared with SiO 2 fibers, Si 3 N 4 fibers exhibit higher crystallization temperatures and better strength retention at elevated temperatures; relative to BN fibers, they provide superior mechanical properties and markedly improved high-temperature stability in air. However, Si 3 N 4 itself has a relatively high dielectric constant, necessitating the introduction of low-dielectric-constant materials such as SiO 2 and BN as matrix components to reduce overall dielectric parameters.
A team led by Professor Bin Li from Sun Yat-sen University designed and fabricated three types of composites: Si 3 N 4f /BN (fabricated via precursor infiltration and pyrolysis), Si 3 N 4f /SiO 2 (via sol-gel), and Si 3 N 4f /SiO 2 -BN (via a sequential combination of sol-gel and PIP). Their goal was to systematically uncover how matrix composition influences interfacial structure and how this, in turn, determines the overall mechanical and dielectric performance.
The team published their work in Journal of Advanced Ceramics on August 1 0 , 2026.
“The fiber-matrix interface serves as a mechanical fuse and plays a pivotal role in determining composite mechanical behavior,” said Professor Li. “Optimal interfacial shear strength enables effective load transfer between fibers and matrix, but when the bonding becomes too strong, it suppresses interfacial debonding and fiber pull-out, ultimately leading to brittle fracture.”
The TEM observations revealed striking differences in interfacial diffusion region thickness among the three composites: approximately 9 nm for Si 3 N 4f /SiO 2 , 38 nm for Si 3 N 4f /BN, and 53 nm for Si 3 N 4f /SiO 2 -BN. X-ray photoelectron spectroscopy analysis further showed that in Si 3 N 4f /SiO 2 -BN, the BN precursor (B 3 N 3 H 6 ) reacts with residual silanol groups (Si-OH) in the SiO 2 matrix, shifting the B–O peak from 192.81 eV to 192.98 eV and the Si–O peak from 103.88 eV to 103.69 eV. This reaction enhances chemical compatibility between fiber and matrix while introducing defects into the silica network that facilitate elemental interdiffusion. Fiber push-in tests quantified the interfacial shear strength: Si 3 N 4f /SiO 2 -BN exhibited a remarkably high value of 443.68 ± 20.12 MPa—1.86 times that of Si 3 N 4f /BN (238.15 ± 28.21 MPa) and 2.35 times that of Si 3 N 4f /SiO 2 (188.80 ± 14.02 MPa). The interfacial shear strength showed a clear positive correlation with the thickness of the diffusion region.
Three-point bending tests revealed that Si 3 N 4f /BN exhibited the highest flexural strength, followed by Si 3 N 4f /SiO 2 , while Si 3 N 4f /SiO 2 -BN showed the lowest. Digital image correlation full-field strain analysis revealed starkly different fracture modes: Si 3 N 4f /BN exhibited strain concentration on the tensile surface with crack propagation along fiber bundle directions; Si 3 N 4f /SiO 2 showed more distributed strain with extensive single-fiber pull-out indicative of excellent damage tolerance; and Si 3 N 4f /SiO 2 -BN displayed the lowest strain levels with cracks rapidly penetrating through the specimen–a classic brittle fracture signature.
“The excessively strong interfacial bonding in Si 3 N 4f /SiO 2 -BN, together with the thermal residual stress arising from the thermal expansion mismatch between the Si 3 N 4 fiber and the matrix, degrades both flexural and compressive strengths,” explained Professor Li. “Moreover, this excessive bonding restricts interfacial debonding and fiber pull-out, leading to a brittle fracture mode.” SEM images of fracture surfaces confirmed these observations: Si 3 N 4f /BN showed primarily fiber bundle pull-out; Si 3 N 4f /SiO 2 exhibited extensive long single-fiber pull-out; and Si 3 N 4f /SiO 2 -BN displayed smooth fracture surfaces with minimal fiber pull-out.
Despite the significant differences in mechanical performance, all three composites demonstrated excellent dielectric properties. From room temperature to 1100°C, they maintained low dielectric constants (<4.5) and low dielectric loss tangents (<0.005) across the 7–18 GHz frequency band. The dielectric constant temperature coefficients were all on the order of 10 -5 °C -1 –7.9×10 -5 for Si 3 N 4f /BN, 6.3×10 -5 for Si 3 N 4f /SiO 2 , and 5.4×10 -5 for Si 3 N 4f /SiO 2 -BN. Notably, while the fiber-matrix interface significantly influenced mechanical properties, its effect on dielectric performance was minimal.
“The use of SiO 2 and BN matrices effectively reduces both the dielectric constant and dielectric loss tangent of Si 3 N 4 fiber-reinforced ceramic composites,” Professor Li added. “These findings provide valuable insights into the design of high-temperature wave-transparent composites operated in extreme environments.”
Other contributors include Yingpeng Zhang (first author, Ph.D. candidate at Sun Yat-sen University)
About Author
Bin Li is a Professor and Vice Dean of the School of Materials at Sun Yat-sen University. His research focuses on aerospace composites and intelligent sensing materials. He pioneered the systems of polymer-derived nitride wave-transparent composites and rare-earth element compound-modified wave-transparent composites.
Zhilin Tian is an Associate Professor and Doctoral Supervisor at the School of Materials, Sun Yat-sen University. His research focuses on the design, fabrication, evaluation, and application of ceramic materials for extreme environments, including structural-functional integrated ceramics for aerospace applications, high-entropy ceramics, power electronic packaging materials, and additive manufacturing of ceramics.
Funding
This work was funded by the National Natural Science Foundation of China (Grant no. 52202078), the Leading Talent Project of the National Special Support Program (2022WRLJ003), Guangdong Basic and Applied Basic Research Foundation for Distinguished Young Scholars (Grant no. 2021B1515020083), Guangdong Basic and Applied Basic Research Foundation (Grant no. 2022A1515012201).
DOI LINK : 10.26599/JAC.2026.9221338
About Journal of Advanced Ceramics
Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen . JAC’s 2025 IF is 14, ranking in Top 1 (1/34, Q1) among all journals in “Materials Science, Ceramics” category, and its 2025 CiteScore is 24.6 (6/133) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
Journal of Advanced Ceramics
Revealing the influence of composition and interface on the mechanical and dielectric properties of Si3N4 fiber-reinforced ceramic composites
10-Aug-2026