FU Bingjie ¹, GU Shijia ², ZHAO Zhaoyong ¹, WANG Lianjun ¹, JIANG Wan ^{1, 2}

(1. State Key Laboratory of Advanced Fiber Materials and Fabrics, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China; 2. Center for Functional Materials, Donghua University, Shanghai 201620, China)

Extended abstract:[Background and purposes] Hexagonal boron nitride (h-BN) ceramics exhibit outstanding high-temperature stability, corrosion resistance, electrical insulation and self-lubricating properties, making them highly promising for applications in aerospace, electronics, metallurgy and mechanical manufacturing. However, due to their strong covalent bonding and low self-diffusion coefficient, achieving full densification remains challenging, even under high-temperature or pressure-assisted sintering conditions. A common approach to improve the densification involves the addition of sintering aids, such as B₂O₃. However, the incorporation of sintering additives can degrade high-temperature performance of the ceramics and should therefore be avoided. Cubic boron nitride (c-BN), a polymorphic form of h-BN, has crystal structure and lattice constant similar to those of diamond. Under ambient pressure conditions, c-BN can transform into h-BN. Since c-BN has a higher density than h-BN, this phase transition results in volumetric expansion, which can fill the voids between h-BN platelets. Utilizing phase transformation-assisted sintering, highly dense ceramics with a relative density of up to 97.6% can be obtained. Nevertheless, the mechanical properties of monolithic h-BN ceramics remain relatively low, limiting their practical applications. Incorporating a secondary phase is an effective strategy for enhancing the strength and toughness of ceramic materials. Carbon fibers, known for their low density, high tensile modulus and strength, low thermal expansion coefficient and excellent thermal conductivity, are widely used as one-dimensional reinforcement materials in ceramic, metal and polymer composites. In this study, carbon fibers were employed as the secondary phase to fabricate high-performance composite ceramics, aiming to advance BN ceramic processing technology and broaden its practical applications.[Methods] BN powder (consisting of 80% hexagonal boron nitride and 20% cubic boron nitride) and carbon fibers were used as raw materials. The BN and carbon fiber powders were precisely weighed, mixed, stirred and dried through rotary evaporation. The prepared powder was then sintered using spark plasma sintering (SPS). The heating rate was 100 ℃·min⁻¹ and the temperature was held at 1700 ℃ for 5 min to obtain CF/BN composite ceramics. Phase composition and its evolution during heat treatment were analyzed using X-ray diffraction (XRD). Bulk density of the samples was measured using the Archimedes method. A universal testing machine was used to evaluate flexural properties and stress-strain behavior of the BN ceramics containing 3%, 5%, 10% and 15% carbon fiber with different fiber mesh sizes. A nanoindentation system was employed to measure elastic modulus and microhardness of the composite ceramics. The morphology of the carbon fiber powders and the fracture surfaces of the composite ceramics were examined using scanning electron microscopy (SEM).[Results] As the carbon fiber content was increased, relative density and flexural strength of the composite ceramics initially increased and then decreased. The optimal performance was achieved at a carbon fiber content of 5%, with a flexural strength of 130.5 MPa. Additionally, its fracture strain reached 1.1%, more than twice that of the sample without carbon fiber. However, with increasing carbon fiber content, both Young's modulus and microhardness exhibited a decreasing trend. With 3% carbon fiber, the average Young's modulus reached 30.09 GPa, while the microhardness was 0.43 GPa. As the fiber mesh size increased, relative density and flexural strength of the composite ceramics decreased, with the optimal performance observed at a fiber mesh size of 300. Conversely, Young's modulus and microhardness increased significantly with increasing fiber mesh size, reaching 30.18 GPa and 0.49 GPa, respectively, at a mesh size of 800. Controlling the carbon fiber content within an appropriate range is critical for maximizing the composite material’s mechanical performance. When the carbon fiber content is too low, the reinforcement effect is insufficient, whereas excessive fiber content leads to agglomeration and overlapping, diminishing the strengthening effect. Additionally, fiber size plays a crucial role in mechanical performance. Larger carbon fibers interlock with multiple BN platelets, enhancing mechanical interlocking and stress transfer, thereby improving flexural properties. Conversely, smaller carbon fibers exhibit better dispersion within the matrix, which enhances material uniformity. This uniform dispersion effectively distributes and absorbs externally applied stress, leading to improvements in Young's modulus and hardness.[Conclusions] CF/BN composite ceramics were successfully fabricated at a relatively low sintering temperature of 1700 ℃ using phase transformation-assisted sintering, with carbon fibers as the reinforcing phase. The effects of carbon fiber content and fiber size on microstructure and mechanical properties of the composite ceramics were systematically studied. Comprehensive characterization via XRD, SEM and mechanical testing provided insights into the influence of carbon fiber parameters on phase composition, morphology and mechanical performance of the CF/BN composites, establishing correlations between fiber content, fiber size and the resulting properties. The findings indicate that the optimal composition for CF/BN composite ceramics is 5% carbon fiber with a fiber mesh size of 300, achieving a relative density of 95.6% and a flexural strength of 130.5 MPa. Additionally, the fracture strain reached 1.1%, more than twice that of samples without carbon fiber. The incorporation of carbon fibers significantly enhanced both the strength and toughness of BN ceramics, which holds substantial promise for their practical applications.

Key words: hexagonal boron nitride; carbon fiber; secondary phase reinforcement; flexural strength; phase transformation assisted sintering