Effects of irradiation on interfacial strength and microstructure of double-layer mullite and alumina coating on SiC

• The irradiation effects on the interfacial strength of mullite and alumina double-layer coatings on SiC were investigated. • A transition layer, approximately 2 µm thick, formed at the SiC/mullite interface during the coating process. • The interfacial strength was evaluated following Si-ion irradiation using the microscale double-notch shear testing technique. • The strength of both the SiC/mullite and alumina/mullite interfaces exhibited no significant degradation following irradiation. • Alterations in Al-O bonding within the transition layer may contribute to the preservation of interfacial strength after irradiation. Silicon carbide (SiC) ceramics hold great potential for use in nuclear-reactor components due to characteristics such as high-temperature strength and low activation. Despite their resistance against corrosion in harsh environments, they suffer elevated corrosion rates under particle irradiation. Thin anti-corrosion coatings, such as mullite bond layers and alumina top layers, are essential to enhance the irradiation stability of SiC. The objective of this study was to evaluate the irradiation stability of a double-layer coating developed to enhance the corrosion resistance of SiC in nuclear applications. The coating, which comprised a mullite bond layer and an alumina top layer, was applied to a SiC substrate using laser chemical vapor deposition. Irradiation experiments were conducted at 300 °C with 5.1-MeV Si ions up to 10 displacements per atom. To assess the interfacial strength, a novel testing method, called the double-notch shear compression testing method, was developed based on ASTM standards and was implemented using a nanoindenter. This approach enabled precise measurements of the mechanical integrity at the interfaces of the coating under irradiation. The results showed an increase in the interfacial strength at the SiC/mullite and alumina/mullite interfaces with irradiation. Microstructural analysis of the fracture surface through scanning electron microscopy–energy-dispersive X-ray spectroscopy revealed that cracks propagated within the mullite layer, indicating the presence of mullite on the fracture surface. Transmission electron microscopy (TEM) images indicated that a 2-µm-thick transition layer existed at the SiC/mullite interface but not at the alumina/mullite interface. The TEM–electron energy loss spectroscopy suggested that the Al-O bonding structures in the transition layer were changed from tetrahedral (AlO 4 ) to octahedral (AlO 6 ) through irradiation, and this structural transition may directly affect the strength.