Key Takeaways & Executive Findings
- •• • The composite with 40 wt% hBN content achieves a flexural strength of 183.4 MPa and fracture toughness of 2.06 MPa m1/2, representing 3.6-fold and 4-fold improvements over pure fused SiO2 (52.2 MPa and 0.58 MPa m1/2), enabling structural use in high-stress aerospace components. • • Dielectric constant remains low at 3.58–3.69 and dielectric loss below 0.0087 at 1 MHz, preserving signal integrity for radome and electronic communication applications despite mechanical reinforcement. • • The phase-transition-assisted synthesis via SPS yields randomly oriented hBN nanoplates that force crack propagation along grain boundaries, enhancing toughness without sacrificing dielectric performance—a critical advantage over conventional reinforcements like Si3N4 or AlN that raise dielectric loss. • • The process achieves these properties while maintaining relative density and Young's modulus (values not explicitly stated but implied by Figure 3), indicating a scalable route for manufacturing high-performance SiO2-based ceramics.
Abstract
Fused silica (SiO2) exhibits exceptional thermal stability and dielectric properties, making it an attractive material for aerospace and military applications. However, its relatively poor mechanical performance has limited its widespread practical utilization. This study proposed an innovative approach to fabricate SiO2-hexagonal boron nitride (hBN) composite ceramics via spark plasma sintering (SPS), leveraging the high-temperature phase transformation of cubic boron nitride (cBN) to introduce randomly oriented hBN as a reinforcing phase within the SiO2 matrix. The randomly oriented hBN nanoplates allow cracks to propagate along stronger grain boundaries, rather than along weaker interlayers of hBN, significantly improving the overall strength and fracture toughness of the composite. The maximum flexural strength and fracture toughness achieved are 183.4 MPa and 2.06 MPa m1/2 respectively, which are 3.6 times and 4 times that of fused SiO2. Concurrently, the composites exhibit low dielectric constants (ε = 3.58–3.69) and dielectric losses (tan δ < 0.0087) at 1 MHz. This work successfully enhanced the mechanical performance of fused SiO2 while preserving its excellent dielectric characteristics, opening new possibilities for its potential applications in advanced structural and functional fields.
1. Introduction
Fused silica (SiO2) is a critical ceramic material distinguished by its low thermal expansion coefficient, exceptional thermal stability, superior thermal shock resistance, and outstanding dielectric properties. These attributes render it an ideal candidate for advanced thermal protection materials in aerospace and military missile applications. However, compared with its excellent thermal and electrical performance, the flexural strength (52.2 MPa) and fracture toughness (0.58 MPa m1/2) of fused SiO2 are relatively low, making it difficult to meet the requirements of high stress application scenarios. Numerous studies have shown that adding reinforcing phases for composite is a feasible strategy to improve the mechanical properties of fused SiO2. For instance, adding 5 vol% Si3N4 particles into fused SiO2 matrix can increase its flexural strength and fracture toughness to 95.4 MPa and 1.22 MPa m1/2, respectively. On this basis, further adding 10 vol% carbon fibers will increase its fracture toughness to 2.40 MPa m1/2, with the flexural strength slightly decreasing. Similarly, introducing 30 vol% aluminum nitride (AlN) particles into SiO2 matrix can significantly improve its mechanical properties, achieving a flexural strength of 200 MPa and a fracture toughness of 2.96 MPa m1/2. However, it is worth noting that the dielectric constant and dielectric loss of these reinforcing phases are obviously higher than those of SiO2, which often leads to an increase in dielectric constant and dielectric loss of these composite ceramics with the addition of reinforcing phases, not conducive to some applications in the field of electronic communication.
Hexagonal boron nitride (hBN) represents an ideal reinforcement phase for fused SiO2 composite ceramics, owing to its low dielectric constant, excellent thermal stability, and high thermal conductivity. Recent investigations have explored various BN phases used to enhance fused SiO2, such as BN microparticles, BN nanosheets, BN whiskers, BN fibers, and BN nanotubes. These studies demonstrate that the BN-reinforced SiO2 composite not only exhibits significantly enhanced mechanical properties, but also maintains low dielectric constant and loss. However, conventional methods of directly adding hBN often lead to agglomeration and poor interfacial bonding, limiting the improvement. This study introduces a novel strategy: using metastable cBN as a precursor that transforms to randomly oriented hBN during spark plasma sintering, ensuring uniform dispersion and strong interfacial bonding, thereby overcoming the bottleneck of previous approaches.
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Xiaoyu Wang, Yang Zhang, Zewen Zhuge, Langlang Huo, Yuqing Chang, Zitai Liang, Lei Sun, Bing Liu, Baozhong Li, Mengdong Ma, Julong He, Yingju Wu, Zhisheng Zhao, Yongjun Tian (2026). Phase-Transition Assisted Synthesis of High-Strength, Low-Dielectric Fused Silica/hBN Composite Ceramics. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3686-6
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Frequently Asked Questions
What is the maximum flexural strength and fracture toughness achieved in the SiO2/hBN composite, and how do they compare to pure fused silica?
The composite with 40 wt% hBN content achieves a flexural strength of 183.4 MPa and fracture toughness of 2.06 MPa m1/2, which are 3.6 times and 4 times that of pure fused silica (52.2 MPa and 0.58 MPa m1/2), respectively.
How does the phase transformation of cBN to hBN during sintering contribute to the mechanical enhancement?
The phase transformation of cBN to hBN during spark plasma sintering produces randomly oriented hBN nanoplates within the SiO2 matrix. This random orientation forces cracks to propagate along grain boundaries rather than along the weak interlayers of hBN, leading to significant improvements in strength and fracture toughness.
What are the dielectric properties of the composite at 1 MHz, and how do they compare to pure fused silica?
The composite exhibits a dielectric constant (ε) in the range of 3.58–3.69 and dielectric loss (tan δ) below 0.0087 at 1 MHz. These values are comparable to pure fused silica, indicating that the mechanical reinforcement does not compromise the dielectric performance.
What is the significance of using cBN as a precursor instead of directly adding hBN?
Directly adding hBN often leads to agglomeration and poor interfacial bonding, limiting mechanical improvement. Using cBN as a precursor allows for in-situ formation of hBN during sintering, ensuring uniform dispersion and strong interfacial bonding, which is critical for achieving the observed mechanical enhancements.
What are the potential applications of this composite material?
The combination of high mechanical strength and low dielectric constant/loss makes this composite suitable for advanced structural and functional applications, particularly in aerospace and military missile components where both mechanical integrity and electromagnetic transparency are required, such as radomes and thermal protection systems.
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