Fiber–Filler Size Matching Enables Toughening of Fine-Grained Isostatic Graphite

Graphite, like many ceramics, exhibits brittle fracture behavior. In high-temperature gas-cooled reactors (HTRs), this brittleness makes fracture toughness a key determinant of the structural reliability and service life of nuclear graphite. Although extensive studies have clarified the fracture behavior of commercial nuclear graphites, strategies that regulate crack paths and improve crack resistance through structural design are still lacking, which limits improvements in reactor safety. In brittle ceramics, whisker toughening has been demonstrated to be highly effective. Inspired by this concept, we develop a fine-grained isostatically molded graphite reinforced with short-cut carbon fibers (SCF), and systematically evaluate the effects of fiber content and fiber–filler size matching on fracture toughness, flexural strength, and thermal conductivity. The results show that the introduction of fibers effectively improves the physical properties of graphite. In graphite prepared with 3.5 μm fillers, an 8 wt.% fiber addition increased the fracture toughness by 36.5% to 1.01 MPa·m 1/2 and the flexural strength to 46.62 MPa, which are 16.1% and 18.9% higher than those of IG-11, respectively, while maintaining a thermal conductivity comparable to IG-11. Fractographic analysis and R-curve analysis reveal that the toughening behavior is governed by the size relationship between fibers and fillers. When the fiber length exceeds the filler size, crack bridging, fiber pull-out, and crack deflection dominate, leading to increases in both the initiation strain energy release rate ( G init ) and the critical strain energy release rate ( G Ic ), thereby enhancing the fracture toughness of graphite. By contrast, when the fiber and filler sizes are comparable, fibers tend to split axially, resulting in reduced toughness. These findings not only demonstrate the feasibility of fiber-toughened graphite, but also establish fiber–filler size matching as a design principle for toughening, providing an effective pathway to develop safer and longer-lived nuclear graphite and a broadly applicable toughening strategy for other brittle carbon-based materials.