Fabrication and High-Temperature Degradation Behavior of M8 CVI 2D SiC/SiC Countersunk Bolts

Continuous fiber-reinforced silicon carbide ceramic matrix composite fasteners exhibit exceptional high-temperature mechanical properties and have attracted considerable attention in the aerospace sector. In this study, M8 2D SiC/SiC countersunk-head bolts were fabricated via chemical vapor infiltration (CVI), followed by a whisker modification process for toughening. The tensile and double-shear mechanical properties of the bolts were systematically investigated at room temperature, 300℃, 600℃ and 1000℃. The failure mechanisms and high-temperature performance degradation mechanisms were analyzed based on finite element simulations and microstructural phase characterizations. Microstructural observations of the as-prepared bolts revealed that the porosity predominantly originated from weaving-induced voids, and that the thread teeth exhibited chipping defects caused by variations in cutting forces during machining. Based on experimental and finite element simulation results, the predominant failure mechanism of the M8 2D SiC/SiC bolts under tension was stud fracture. The average tensile strength was 282.58 MPa at room temperature and decreased to 208.4 MPa at 1000℃, corresponding to a strength degradation rate of 26.25%. Under double-shear loading, the primary failure mode was bolt fracture into three segments, and within the tested temperature range, the average double-shear strength did not exhibit a significant temperature dependence. Below 1000℃, oxidation of the SiC matrix and SiC fibers was relatively limited; however, oxidation of the BN interphase occurred, leading to interfacial strengthening and aggravated thermal expansion mismatch, which promoted fiber debonding. It is therefore inferred that oxidative damage to the BN interphase constitutes the dominant factor governing the tensile strength degradation of the M8 2D SiC/SiC bolts.