Evolution of mixed oxide and failure mechanisms of high-temperature-resistant and radar/infrared-compatible stealth coatings under isothermal exposure

Stealth coatings encounter severe spallation failures in high-temperature service environments. Investigating the failure mechanisms of stealth coatings under isothermal exposure is critical to identify the weak links in coating systems and guide the development of novel high-temperature-resistant designs. This study investigated a high-temperature-resistant and radar/infrared-compatible stealth coating (HRISC) system, comprising: a bonding layer (BL), a thermal barrier ceramic layer (TBL1), a dielectric layer (DL), a resistive periodic pattern, a protective layer, and a capacitive frequency selective surface. Isothermal tests were conducted at 1150 °C to analyze the microscopic failure behavior and macroscopic failure morphology of the HRISC. The results indicated that the HRISC failed through the delamination along the DL-TBL1 interface and the spallation along the TBL1-TGO interface. Throughout the HRISC isothermal lifetime, the DL underwent sintering. Microcracks healing reduced porosity, resulting the increased Young's modulus and hardness of the DL. This likely reduced the DL strain tolerance capacity, thereby causing the HRISC delamination along the DL-TBL1 interface. Moreover, during the early stage of the isothermal lifetime, the thermally grown oxide (TGO) already contained continuous α-Al₂O₃ and limited locally grown mixed oxide (MO). To quantify the localized growth features of the MO, this study proposed a coefficient of MO non-uniform growth. Analysis revealed that during the mid-to-late stages, the locally grown MO transformed into layered MO. The increased coverage ratio of the loose and porous MO continuously weakened the bonding strength of the TBL1-TGO interface. Simultaneously, the locally grown MO and the layered MO synergistically generated large stresses within the TBL1 near the TBL1-TGO interface, which could not be relieved by high-temperature creep, causing the HRISC spallation failure along the TBL1-TGO interface.