XUE Longbo, CHEN Jianjun

(School of Materials Science & Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, China)

Extended abstract:

[Background and purposes] Carbon fiber-reinforced silicon carbide (C_f/SiC) composites find extensive applications as advanced thermal structural and protection materials in the aerospace industry. They possess exceptional high-temperature mechanical properties, low density and high thermal shock resistance. However, a single SiC matrix shows inherent limitations under increasingly extreme environments. Incorporating secondary phases like boron carbide (B₄C) modifies the matrix effectively. B4C offers high hardness and forms a protective borosilicate glass layer at high temperatures, which blocks oxygen diffusion. Therefore, C_f/SiC-B₄C composites display broad application prospects. Researchers widely use reactive melt infiltration (RMI) to fabricate these composites, because it achieves rapid densification and near-net shape manufacturing. Nevertheless, the high-temperature molten silicon severely damages the carbon fibers during infiltration. Furthermore, incomplete reactions leave abundant brittle residual silicon in the matrix, which significantly degrades mechanical properties of the final composites. To overcome these critical shortcomings, a cost-effective and continuous pyrolytic carbon/boron nitride (PyC/BN) bilayer interphase were designed to protect the carbon fibers. Additionally, this study was aimed to optimize pore structure of the preforms by adjusting the molding pressure. By combining polymer infiltration and pyrolysis (PIP) with RMI, this methodology was intended to maximize carbon source introduction, minimize residual silicon and ultimately enhance the mechanical and fracture behaviors of the C_f/SiC-B₄C composites.

[Methods] The surface was treated by depositing a PyC/BN bilayer interphase onto polyacrylonitrile (PAN)-based 2D carbon fiber cloths. Initially, chemical vapor deposition (CVD) was used to synthesize the PyC inner layer at 1000 ℃, using methane as the carbon source. Subsequently, a combined dip-coating and in-situ nitridation method were employed to develope the BN outer layer. The PyC-coated fibers were immersed into an ethanol solution containing B₄C powder and urea, followed by nitridation in a tube furnace at 1400 ℃ in nitrogen. For the composite fabrication, a planetary ball mill was employed to mix SiC powder, B₄C powder, phenolic resin and graphite powder into a homogeneous slurry. The manufacturing process was to coat the interphase-modified fibers with this slurry and stacked them. A hot-pressing machine was used to cure the stacked preforms at 150 ℃ and four molding pressures, including 2 MPa, 4 MPa, 6 MPa and 8 MPa. After pyrolysis at 900 ℃, the PIP process was cyclized to infiltrate the porous preforms with a phenolic resin solution, until they showed no obvious weight gain. Finally, the RMI process densified the preforms at 1450 ℃ for 1 h using molten silicon. The element composition of the interface layer was measured by using XPS. An X-ray diffractometer was used to identify the phase composition. The Archimedes method was adopted to measure the bulk density and porosity. A universal testing machine was used to test the flexural strength and fracture toughness. Finally, a scanning electron microscope was used to characterize the microstructures and fracture morphologies.

[Results] The CVD combined with the in-situ nitridation method can be used to successfully form a uniform PyC/BN bilayer interphase with a thickness of 232.8 nm. During nitridation, urea decomposes to generate ammonia, which reacts spontaneously with B₄C at high temperatures to form a distinct granular BN outer layer. Regarding the preform architecture, the molding pressure significantly dictates the initial pore structure. Low pressure (2 MPa) leaves large inter-laminar voids, whereas excessive pressure (8 MPa) closes the pore channels prematurely. A molding pressure of 6 MPa creates an optimal highly interconnected porous network. Consequently, the 6 MPa preform exhibits the highest PIP densification efficiency, reaching a bulk density of 1.73 g·cm⁻³ and a low porosity of 12.9% after five PIP cycles. After the final RMI treatment, the 6 MPa C_f/SiC-B₄C composite displayed the highest overall densification, exhibiting a bulk density of 2.24 g·cm⁻³ and a porosity of only 7.3%. X-ray diffraction confirms that the matrix primarily consists of SiC with minor amounts of B₄C, C and residual Si. Notably, the 6 MPa sample displays the lowest residual silicon peak intensity. This indicates that its optimal pore structure facilitates the thorough reaction between molten silicon and the well-distributed PIP-derived carbon skeleton. The composites fabricated at 6 MPa possess the most outstanding mechanical properties. The flexural strength reaches 234 MPa, while the fracture toughness is 7.5 MPa·m^1/2. Compared with the composites molded at 2 MPa, the two values increased by 61.4% and 74.4%, respectively. All samples exhibit pseudo-plastic fracture behaviors. The PyC/BN dual interphase effectively shields the carbon fibers from erosive molten silicon. Moreover, an ideal weak-interface effect was present, which induced massive interfacial debonding and extensive fiber pull-out during fracture. Conversely, the composites molded at 8 MPa show degraded mechanical performance, because the blocked pore channels restrict precursor infiltration, while excessive pressure induces microscopic damage to the carbon fibers.

[Conclusions] In this study, a dual-layer PyC/BN interphase was successfully fabricated on carbon fibers via a combined chemical vapor deposition (CVD) and dip-coating/in-situ nitridation approach. Subsequently, C_f/SiC-B₄C composites were manufactured using hot-press molding combined with a PIP-RMI process. The PyC/BN dual interphase played a crucial synergistic role in resisting melt infiltration damage and enhancing toughness. Specifically, the in-situ synthesized BN outer layer acted as a thermophysical barrier to effectively shield the carbon fibers from liquid silicon erosion during the RMI stage, while the inner PyC layer provided an appropriate interfacial bonding strength to dissipate fracture energy through interfacial debonding and fiber pull-out, thereby effectively averting brittle failure. Furthermore, the molding pressure significantly regulated the pore characteristics of the C_f/SiC-B₄C preforms. At an optimal pressure of 6 MPa, the preform exhibited highly interconnected pore channels without distinct delamination, facilitating the infiltration of liquid precursors. After 5 PIP cycles, this optimized preform exhibited the highest bulk density (1.73 g·cm⁻³) and the lowest porosity (12.9%), providing an adequate carbon source for the subsequent RMI process. Conversely, an excessive pressure (8 MPa) led to severe pore closure, which hindered infiltration. Consequently, the C_f/SiC-B₄C composite prepared at 6 MPa exhibited the minimum residual silicon content, a dense uniform matrix and a typical pseudo-plastic fracture behavior, achieving optimal mechanical properties with a flexural strength of 234 MPa and a fracture toughness of 7.5 MPa·m^1/2. These values represent substantial improvements of 61.4% and 74.4%, respectively, as compared with the composite processed at 2 MPa.

Key words: C_f/SiC-B₄C composites; PyC/BN bilayer interphase; PIP-RMI combined process; molding pressure; mechanical properties