Non Destructive Evaluation of Urethane-Epoxy Coating Systems Using the Scanning Kelvin Probe Technique

Tuesday, 3 October 2017
Prince George's Exhibit Hall D/E (Gaylord National Resort and Convention Center)
D. Borth and D. C. Hansen (University of Dayton Research Institute)
High performance aircraft are subjected to harsh environmental factors during operation including impacts with water and other particles, as well as high temperature and humidity. Rain erosion coatings (REC), made from polyurethane (PU), are used to protect the metal and other coatings from these damaging elements by absorbing the force of impacts and providing a barrier against corrosive substances. Over time these coatings break down and may fail in flight by delamination. Failure of a coating leads to unplanned downtime and expensive refinishing procedures. Coatings that are exposed to high temperature and humidity are particularly susceptible to degradation, leading to the assumption that the cause is hydrolysis or reversion. Hydrolysis affects the esters linkages whereas reversion targets the urethane bonds. These types of degradation lead to a breakdown of the polymer network structure and reduce both the tensile strength and melting point of the coating, increasing the likelihood of failure.

The dynamic nature of weather makes predicting when the PU REC will fail is extremely difficult when using traditional lifecycle estimates, since aircraft are exposed to different environmental conditions at different geographical locations or areas of operations. Currently, no method exists for accurately predicting when a particular aircrafts coating will fail. Therefore, there is a need for a reliable non-destructive method to detect the changes in the surface coating that indicate when it is close to failure or has begun to fail.

It is hypothesized that the Scanning Kelvin Probe (SKP) can be used to detect changes in the work function of the REC’s associated with degradation of the polymer structure. If successful, it will serve as the justification for developing a portable scanning unit that can be deployed by maintenance crews to determine when a coating needs to be reapplied. To test this, 4X6 inch 2024 aluminum alloy test panels were exposed in an autoclave to high heat and humidity (121˚C and 100% RH) for a variety of times to simulate the degradation process. They were then scanned with the SKP to characterize the degradation as a function of exposure time. Initial data shows that there is a change in work function with increasing exposure as well as localized areas of higher work function. Raman spectroscopic analysis indicates changes in coating polymer chemistry consistent with hydrolysis of the ester bonds. This data correlates with the changes in work function seen by the Kelvin probe, showing that the SKP can non-destructively detect polymer degradation.