Influence of Si and SiC Coating on the Microstructures and Mechanical Properties of C/C Bolts

High-performance bolts made of carbon/carbon (C/C) composites are necessary for connecting thermally-insulating structural components of aerospace vehicles. To enhance the mechanical properties of the C/C bolt, a new silicon infiltration-modified C/C (C/C-SiC) bolt was developed via vapor silicon infiltration. The effects of silicon infiltration on microstructure and mechanical properties were systematically studied. Findings reveal that dense and uniform SiC-Si coating has been formed after silicon infiltration of the C/C bolt, strongly bonding with the C matrix. Under tensile stress, the C/C-SiC bolt undergoes a tensile failure of studs, while the C/C bolt is subject to the pull-out failure of threads. The breaking strength of the former (55.16 MPa) is 26.83% higher than the failure strength of the latter (43.49 MPa). Under double-sided shear stress, both the crushing of threads and the shear failure of studs occur within two bolts. As a result, the shear strength of the former (54.73 MPa) exceeds that of the latter (43.88 MPa) by 24.73%. According to CT and SEM analysis, matrix fracture, fiber debonding, and fiber bridging are the main failure modes. Therefore, a mixed coating formed by silicon infiltration can effectively transfer loads from coating to carbon matrix and carbon fiber, thereby enhancing the load-bearing capacity of C/C bolts.