Failure processes of a gradient 2D-braided SiC-based nuclear-fuel cladding material at room temperature and 1200°C: an in situ X-ray tomography study

Continuous SiC-fibre-reinforced SiC-matrix composites (SiC f -SiC m CMCs) with SiC coatings are widely considered as candidate accident-tolerant fuel cladding materials for advanced light-water reactors. In this work, a four-layer gradient braided SiC f -SiC m cladding material with a bilayer outer SiC coating was investigated under C-ring compression at room temperature (RT) and 1200°C in argon (Ar) using in situ X-ray micro-computed tomography (XCT). Two specimens were tested at each temperature. The mean maximum nominal hoop stresses were 77.3 ± 2.7 MPa at RT and 80.5 ± 7.3 MPa at 1200°C (mean ± standard deviation), indicating comparable peak nominal stress levels within the limited specimen set. The inner coating sub-layer exhibited a more pronounced columnar-grain morphology approximately normal to the coating surface, whereas the outer sub-layer showed less clearly aligned columnar features. Crack deflection was repeatedly observed near the interface between the two coating sub-layers under C-ring loading. Within the braided CMC layer, XCT-resolvable cracks underwent redirection and bifurcation and were arrested near fibre-bundle interfaces, and no XCT-resolvable through-thickness crack was observed within the investigated volume at either temperature. A four-stage failure-process model is proposed based on the in situ XCT observations. Hermeticity was not evaluated, and the observed crack morphology should not be interpreted as evidence of retained gas tightness or functional cladding integrity.