Terahertz characterisation of graphene content in silver-graphene composite coatings

Accurate non-destructive testing of graphene content and equivalent permittivity in graphene-reinforced silver composite coatings represents a critical challenge for performance evaluation. Terahertz time-domain spectroscopy is sensitive to dielectric parameters; however, accurate measurement of coating thickness is difficult, and the characteristics of THz signals are influenced by several coupling factors such as refractive index, surface roughness, and dispersion effects, limiting the precision of traditional methods that rely on calibrations or thickness priors. This study developed an analytical model for the THz wave-coating interaction that comprehensively considers interface scattering and dispersion effects, revealing a negative correlation between graphene content and the amplitude of THz echo signals. Utilising a teaching-learning-based optimisation algorithm, the method improves the fitness function by focusing on the amplitude of the main echo peak, enabling refractive index inversion without prior knowledge. Systematic validation with graphene-doped gradient samples shows that the inverted results align well with predictions from effective medium theory, with a mean error below 2% and a fluctuation error below 3%. Compared to traditional calibration methods, the proposed approach demonstrates significant advantages in consistency and interference resistance. This study provides a reliable technical pathway for the rapid, non-destructive assessment and process optimisation of silver-graphene composite coatings.