A Review of Active Thermography for Defect Detection in TBCs: Status and Prospects for Multifunctional Coatings

Thermal barrier coatings (TBCs) require nondestructive inspection because cracking and interfacial separation can compromise thermal protection. This review critically examines active infrared thermography for defect detection and characterization in conventional TBCs and evaluates prospects for multifunctional coatings. Excitation approaches are organized into four practical groups: external optical heating, internal mechanical heating, internal electromagnetic heating, and material-enabled or embedded excitation. The reviewed studies address interfacial debonds and delaminations, surface microcracks, top coat thickness and overthickness, ageing-related changes in thermal diffusivity, CMAS-induced failure and substrate cracks beneath the coating. Optical thermography has the most established evidence base, with debonds of about 1–2 mm beneath 0.2–0.5 mm top coats, surface cracks about 10 µm wide and top coat thickness within a few percent reported under laboratory conditions, while mechanical and induction excitation offer complementary sensitivity to contacting cracks, debonds and substrate cracks. Detection and localization must therefore be distinguished from validated sizing, depth estimation and assessment of residual adhesion or remaining life. Direct thermographic validation of multifunctional TBCs remains limited in the reviewed literature, despite demonstrated embedded sensing and functional material integration. Research priorities include validation on representative damage, independent measurements, uncertainty assessment and probability of detection studies.