Abstract Automotive coating plays an important role in car industry. They protect the car bodies from various aggressive weathering and mechanical deformations. Although many techniques have been developed for car coating, there is still a demand for non destructive characterization of multilayer coatings. Therefore, research on new measurement methods is still ongoing. In this paper, we investigate thin coating layers on metal substrate using THz time-domain spectroscopy (THz- TDS). We are able to apply existing algorithm but in reflection method using THz-TDS to distinguish the thickness as thin as 20 µm of individual layers. I. INTRODUCTION ODAY, a large percentage of vehicles are painted with at least some layers of protection coating. The coating layers often contain filling materials or pigments to enhance their protecting properties aesthetics. The filling materials and pigments affect the refractive index of the coating materials, which make the thickness determination of individual layers more challenging, especially when non-destructive testing (NDT) is required. THz-TDS has been demonstrated as an effective non- destructive and contact free testing method for multilayer thin films investigation [1 2 3]. In previous publications [1 2 3], samples were studied in transmission mode. However, while working with automotive coating, transmission mode is not suitable due to the metal substrates. As a new coating is applied, there is a need to investigate the properties of the coating layers. Typical automotive coating system includes multiple layers. Each layer has different roles either to enhance adhesion, corrosion protection or preventing mechanical deformation, etc. In this paper, we study a coating system which includes 4 layers. Reflection mode was applied and the results were modeled to extract the desired properties of the sample. II. METHOD The algorithm is based on Fresnel equations which describe the transmission and reflection parts of the THz wave going into the coating layers and bounced back from individual layers’ surfaces. In this model, the THz reflection pulse is considered as an interference of individual electric fields reflected from multiple layers’ surfaces. A reference THz pulse was measured in the air in the same conditions as the measurements done on the sample. Based on this measured reference, the propagation of individual THz electronic field reflected from the interfaces between coating layers was modeled. For example, a THz pulse reflected from an interface between two media is affected by a 180° phase change if the incident media is less condensed than the refracted one, supposing that both are non-dispersive materials. Then the reflected electric field is proportional to the measured reference pulse with the phase shifted by 180° and the intensity is multiplied by a reflectance factor. By super- imposing these reflected waves together, a model of the complete THz reflection pulse was built. Delay times and intensity of reflected waves were varied to fit the model with the experimental results. The refractive indices could be calibrated by using the samples of single layer with known thickness. The coating sample includes 4 layers which were coated one after another when the previous layer was dried. Fig.1 presents schematically the structure of the coating system. The substrate is Zn-coated and phosphated steel which is regularly used in automotive industry. The two middle layers are of Epoxy which is injected with pigment material. Due to the similarity of these two layers and their thin thickness, the reflection from the interface between these two layers is supposed to be weak. Therefore we assume to have 3 layers to be calculated in the model. In addition, incident angle of the THz beam was varied in order to increase the optical path length between layers. Fig.2 shows the measured reflection signals of the THz wave at Dinh T. Nguyen 1 , Katharina Weber 2 , Wegmann Volker 2 , Yves Hernandez 1 1 Multitel ASBL, 7000 Mons, Belgium 2 Fraunhofer-Institut für Produktionstechnik und Automatisierung IPA, 70569 Stuttgart, Germany Non-destructive measurement of thickness and refractive index of multilayer coating on metal substrate T Fig. 2. Reflections of THz wave from the coating layers at different incident angles. Fig. 1. (a) Construction of the coating system.