Non-Destructive Detection of Coating Delamination Using Reflected Time-Domain Terahertz Waveforms

Delamination of anti-corrosion coatings from substrates compromises structural integrity by introducing thin gaps that allow ingress of corrosive agents like water. This study investigates the capability of terahertz pulsed imaging to detect air gaps thinner than a THz system's minimum resolvable thickness by measuring apparent thickness changes in overlying layers. The minimum resolvable thickness in terahertz pulsed imaging, referred to as the axial resolution, is determined by the system’s bandwidth. To test this approach, THz measurements were conducted on a 2 mm thick quartz window positioned above a metal substrate with an intervening 19 µm air gap. An increase in the apparent quartz window thickness was observed, consistent with theoretical expectations, despite the air gap remaining undetectable as a distinct reflection. This analysis was extended to multi-layered paint systems consisting of top, mid, and base coatings on steel plates. Cyclic ageing induced a delamination layer between the metal substrate and base coating, measured to be 10 µm by scanning electron microscopy. Terahertz pulsed imaging analysis showed a corresponding 9.9-13 µm increase in the apparent base coating thickness, consistent with the quartz window results. These findings demonstrate that terahertz pulsed imaging offers a non-destructive method for identifying sub-resolution air gaps, enabling early-stage detection of delamination in protective coatings.