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Standardized Test Methodologies for Specialty Coating Durability
Dr. Karen Schultz | Boeing
Replacing exterior coating systems that no longer meet their performance requirements generates a significant amount of environmentally hazardous materials. Low observable (LO) coating systems, in particular, are proving to be much less durable in service than is predicted by current accelerated test methods. The more frequent replacement of these coatings is a cost and waste management issue for Department of Defense (DoD) depot and field operations. The objective of this project was to develop an understanding of multilayer coating system degradation and failure, and to develop an accelerated test protocol and analysis methodology to accurately replicate failure modes observed, that can be used to better predict performance of current and future coating systems in service environments.
Current accelerated testing methods for multilayer coating systems do not accurately simulate damage modes that are relevant to field failures and do not generate data that can be used to accurately predict coating performance in service. To overcome this issue, both in situ monitoring of individual coating layers and ex situ measurements of physical and chemical changes were used to characterize moisture ingress in the individual coating system components and to determine the causes and damage modes of failure. Using these data, along with modeling to understand transport mechanisms and property change rates, temperature and humidity cycles were designed to control moisture gradients across the coating system. Test coupons and fixtures were designed so that mechanical stresses could be applied to the coating system in the exposure environments to evaluate the combined effects of mechanical stresses and environmental exposure (temperature, humidity, etc.).
The combined effects test method developed during this project introduces relevant environmental stressors of cyclic temperature and humidity and, with the application of vibratory or dynamic strain, produces coating cracking with features similar to that observed in service over structural discontinuities. Testing indicates that for this multilayer system of surrogate LO coatings, crack initiation likely begins in the primer layer, definitely starts below the surface, and propagates to and through the topcoat as cracks grow and coalesce.
The ability of this test method to detect subsurface cracking in underlying conductive coating layers, as well as the observation that the impedance of even a cracked conductive coating decreases when applied strain is reduced and the coating is allowed to come back into contact, has important implications for multilayer coating system selection, maintenance, and repair. In the future, this test method, with environmental and mechanical cycles adjusted for the properties of the coatings of interest, could be used to identify weak layers, failure initiation points, and ultimately to select coatings that perform better together in service environments.