Thickness measurement of polychlorotrifluoroethylene coating over metallic seal using terahertz time-domain spectroscopy

Polychlorotrifluoroethylene is used as a coating material over metallic seals in low-temperature applications to arrest fluid leakage from the impeller side in turbopumps. Typically, polychlorotrifluoroethylene coating is applied on V-type seals, with a thickness ranging from 80 to 130 mu m by spraying an emulsion over the substrate followed by heat treatment. An attempt has been made to measure the polychlorotrifluoroethylene coating thickness over V-type seals using terahertz time-domain spectroscopy in reflection geometry, a noncontact, non-invasive NDT method. When the terahertz pulse from a transmitter photo-conductive antenna is incident on the V-type seal, it penetrates through the polychlorotrifluoroethylene coating. It gets reflected from the coating/base coat interface. Here, the reflected echoes from the air-to-polychlorotrifluoroethylene coating interface and polychlorotrifluoroethylene coating to the basecoat interface get overlapped in the time domain as the polychlorotrifluoroethylene coating layer is very thin. The sparse deconvolution technique separates the individual reflected signals and obtains the time delay signals from various interfaces. From the estimation of time delay values, the thickness of the coating has been computed using the refractive index value extracted using terahertz time-domain spectroscopy in transmission mode before the reflection measurements. The obtained thickness values are in close agreement with the coating thickness measured using optical microscopy.