Effects of outer coating thickness on the failure strength of a SiC-based nuclear-fuel cladding up to 1200°C

Continuous SiC fibre reinforced SiC matrix composites (SiC f -SiC m CMC) with monolithic SiC coatings are promising candidate accident-tolerant fuel cladding materials for light-water-reactors. Thick monolithic SiC coatings can increase failure strength and improve hermeticity, but may also suppress toughening mechanisms in the cladding system. Here, 2D fibre braided SiC f -SiC m cladding with thick inner and outer SiC coatings were examined, with outer-coating-thickness ranging from ∼240 to ∼440 μm. C-ring compression tests were performed up to 1200°C in Ar, combined with in-situ synchrotron X-ray micro-computed tomography and digital-volume-correlation (DVC) to quantify 3D strain distributions. All specimens exhibited brittle fracture initiated in the outer coating, consistent with localized tensile hoop strain measured by DVC. No obvious crack-deflection toughening at the coating/CMC interfaces was observed before failure. Within the limited six-specimen dataset, apparent nominal hoop strength and DVC-derived localized hoop strain showed a positive association with outer-coating-thickness. Because coating-thickness and testing temperature were not varied independently, this association is an indicative trend rather than a design allowable or general thickness–strength law. Newman–Raju calculations semi-quantitatively rationalized the thickness-associated trend in terms of surface-flaw stress intensity. These results highlight the designing need to balance coating thickness, CMC-layer fraction and interfacial strength.