Tailoring the geometry of silicon nitride nanofillers to simultaneously strengthen and toughen carbon/carbon composites

Incorporating one-dimensional (1D) nanofillers into carbon/carbon composites (C/Cs) can increase their mechanical strength. However, this method is ineffective in improving the fracture toughness of C/Cs because cracks can easily pass over the 1D nanofillers, resulting in a flat fracture surface. Herein, we propose a new technique to significantly improve both the strength and toughness of C/Cs by incorporating 2D ribbon-shaped nanofillers. Silicon nitride nanoribbons (SiNNRs) are assembled into a film-like structure and inserted into each layer of a carbon fiber laminate, which is then densified with a pyrocarbon matrix. Due to the large interfacial contact area between the nanoribbon and matrix, ribbon-shaped SiNNRs can efficiently arrest and deflect cracks in both two and three dimensions, exhibiting a higher reinforcement efficiency than that of slender silicon nitride nanowires (SiNNWs). Mechanical tests show that the percentage increases in fracture toughness, flexural strength, and compressive strength of C/Cs produced by SiNNRs are approximately 9.8, 1.8, and 1.25 times higher than those produced by SiNNWs at the same volume fraction of nanofillers, respectively. This study suggests that 2D ribbon-shaped nanofillers are more effective than 1D fibrous nanofillers in enhancing the strength and toughness of C/Cs.