Orientation-Dependent Reinforcing Mechanisms of SiC/Carbon Nanotube Composites: A Reactive Molecular Dynamics...

Carbon nanotubes (CNTs) are effective reinforcing agents in SiC matrices, enhancing their toughness and strength. However, reinforcement is constrained by the complex atomic-scale structural factors of the SiC/CNT composites. Among these, the CNT orientation within the SiC matrix plays a critical role in the performance, though the underlying mechanisms remain unclear. Hence, here, we apply a bond-order-based reactive molecular dynamics simulation method to investigate the mechanical properties and deformation mechanisms of SiC/CNT composites with different CNT orientations. Our research has revealed that CNT orientation affects composites' mechanical properties (including toughness and strength) through two competitive mechanisms: axial stretching and crack bridging. As the angle (θ) between the CNT and tensile direction increases, the axial stretching effect is enhanced, while the crack-bridging effect is weakened. The strength of the SiC/CNT composite is mainly dependent on the axial stretching, hence showing a decreasing trend with θ; whereas the toughness is determined by the competition between the axial stretching and crack-bridging mechanisms. Hence, there is an optimal balance of both mechanisms, resulting in a volcano-type dependence of toughness on θ. Moreover, by accounting for factors such as the SiC crystalline structure, CNT wall number, diameter, aspect ratio, and defects, this study introduces a modified, generalized rule of mixtures. This approach equates the CNT orientation correction factor to the cosine of θ, facilitating the theoretical prediction of the tensile strength in fiber-reinforced ceramic matrix composites.