The Scanning Kelvin Probe As a Non Destructive Evaluation Tool for Monitoring the Degradation of Urethane-Epoxy Coating Systems

Abstract

Performance degradation of urethane and epoxy coatings on high performance aircraft due to chemical reversion, while not visible on the surface, can be the result of changes in physical and chemical composition, leading to premature coating failure. Standard lifetime predictions fail to estimate the functional lifetime of urethane coatings on operational aircraft due to the wide variety of environmental factors and combinations of factors that trigger degradation at different rates. To better understand the role that environmental conditions play in the overall lifetime performance of these coatings, a non-destructive technique is necessary to quantitatively assess the degree of degradation/reversion that can occur. The scanning Kelvin probe (SKP) has demonstrated itself to be sensitive enough to detect changes in the alkyl chain lengths of polymers and their terminal groups as well as determine the interfacial diffusion of water through epoxy coatings as a function of the chemical structure and functional groups present within the coating. It has also demonstrated its capability to detect corrosion under coatings at the coating-metal interface. These capabilities make the use of the scanning Kelvin probe technique a compelling approach to detecting and characterizing the degradation behavior of multi-layer urethane and epoxy coating systems as well as any corrosion occurring at the coating-metal substrate interface. The objective of this effort is to develop a non-destructive evaluation technique for measurement and analysis of a multi-layer polyurethane/epoxy coating system subjected to elevated temperature and relative humidity (100 C, 100%RH) degradation conditions. Measurements of changes in the work function of non-exposed and exposed rain erosion coating (REC) system for various times at the elevated conditions have been made to correlate the degree of degradation within the coating system as determined by Raman/FTIR spectroscopy.