Transition of failure mechanisms of 2D-C/SiC under on-axis and off-axis load considering temperature effects

In this study, the tensile behaviors of two-dimensional plain-woven carbon fiber-reinforced silicon carbide composite (2D-C/SiC) prepared by chemical vapor infiltration (CVI) were investigated through on-axis and off-axis tensile experiments at different temperatures. The stress-strain curves exhibited obvious nonlinearity for both loading conditions. As the temperature rose from 25 °C (room temperature) to 900 °C, the tensile strength of on-axis specimens decreased by 13.1 %, from 298.7 MPa to 259.5 MPa. On the contrary, the tensile strength of off-axis specimens increased by 34.5 %, from 146.9 MPa to 197.6 MPa. The macroscopic fracture surfaces exhibited a wedge-shaped morphology for on-axis tension, whereas form an angle of 45° for off-axis tension. Microscopic analysis of the fracture sections using scanning electron microscopy (SEM) indicated that failure in on-axis tension was primarily dominated by fiber fracture, while the failure in off-axis tension was mainly governed by matrix fracture. The matrix effects and shear coupling were the main reasons for the lower off-axis strength compared with that of on-axis condition. The oxidation of fibers and PyC interface was identified as the main factor contributing to the reduction in on-axis tensile strength at elevated temperatures. In contrast, the off-axis tensile strength was less affected by the high-temperature oxidation reaction, and instead, the thermal strengthening effect of the SiC matrix led to a slight increase in strength at elevated temperatures.