Novel capacitance sensor design for measuring coating debonding: the effects of thermal-hygroscopic cycling

The effects of combined thermal and hygroscopic cycling on the adhesion performance of an epoxy coating were measured using a novel electrode sensor. The sensor is uniquely designed, consisting of a series of independent interdigitated electrode traces which are arranged parallel to the sensor edges. Coupled with single-frequency capacitance measurements, the sensor detects changes in capacitance in the adhered coating–sensor interfacial region as a function of the distance from the edge of the sensor, x. Recently, this sensor was utilized by O'Brien and co-workers to measure interfacial diffusion and the concentration profile of fluid in an adhesive joint (Int. J. Adhesion Adhesives 23, 335–338 (2003)). In the present work, large capacitance changes due to debonding and displacement of the coating by fluids at the sensor surface were used to monitor coating delamination. The apparent debond growth rate and number of cycles until failure were determined as a function of coating thickness, fluid environment and sensor surface chemistry. The results show that the coating becomes more durable as the thickness is reduced; and also that thermal and hygroscopic cycling of coatings produces different results than conventional continuous adhesion tests. This study suggests that this novel sensor or a similar design is applicable for the study of adhesion loss and interfacial diffusion processes, and could be extended to other coatings or adhesives in a variety of environments. General trends about coating durability are also discussed.