Shear properties at the PyC/SiC interface of a TRISO-coating

The failure behavior of TRISO-coated fuel particles depends significantly on the shear strength at the interface between the inner pyrolytic carbon (PyC) and silicon carbide (SiC) coatings. In this study, a micro-indentation fiber push-out test was applied to measure the interfacial shear properties of a model TRISO-coated tube. Of particular emphasis is that this study developed a non-linear shear-lag model for a transversely isotropic composite material due to insufficiency in the existing isotropic models as applied to layered TRISO-coating systems. In the model, the effects of thermal residual stresses and the roughness-induced clamping stress were identified as particularly important. The rigorous model proposed in this study provides more reasonable data on two important interfacial parameters: the interfacial debond shear strength and the interfacial friction stress. The modified model coupled with experiments yields an interfacial debond shear strength of 240 ± 40 MPa. This high interfacial strength, though slightly lower than that obtained by the existing isotropic model (∼280 MPa), allows significant loads to be transferred between inner PyC and SiC in application. Additionally, an interfacial friction stress of 120 ± 30 MPa was determined. This high friction stress is attributed primarily to the roughness at the cracked interface rather than clamping effects due to differing coefficients of thermal expansion.