Nondestructive ultrasonic characterization of multilayer coatings

Acoustic Microscopy is used to study the structure and properties of polymer coatings. In a multi-layer coating system, the reflection of an ultrasonic signal takes place at each interface. For thin coatings, the reflected signals from different interfaces superimpose and appear as a single reflected signal. The amplitude and the fine structure of the reflected signal depend on the material properties of the sample. To separate the reflected signal from different interfaces of the multi-layer coatings, the pulse length has to be shorter than the time of flight of the ultrasonic pulse through each layer. However, usually ultrasonic pulses are longer. The approach used here is to model the acoustic signal for different interfaces and compare the model signal with the signal recorded from the degraded coating. Due to thermal and environmental effect, the properties like acoustic impedance, density and thickness of the polymer coatings will change with time. This results in minor modifications of the shape of the reflected signal from a degraded coating. By using a calibrated scale for different kinds of coating properties, coating characteristics can be determined. This paper will discusses the application of the above method to characterize the degradation of aircraft coatings.