Companies that produce aerospace and power generation parts such as industrial gas turbine (IGT) engine components, hot gas path blades, and nozzle guide vanes apply a protective coating to turbine blades, enabling them to withstand the high temperatures they are exposed to while in operation. The thickness and microstructure of the coating must be evaluated during the application and throughout the blade’s lifespan to ensure its fitness for service. Aerospace and power generation companies can accomplish this analysis through destructive testing (DT) or nondestructive testing (NDT) techniques.
One DT method involves using electrical discharge machining (EDM) techniques to cut up blade sections for analysis with microscopy, providing a detailed look at the size and structure of coating layers. However, each blade can cost more than $1,000 to produce, and the typical requirement is to check one in 20 blades, making this method expensive, time-consuming, impractical for large volumes, and restricted to local analysis. A common NDT technique for this measurement is eddy current testing which uses electromagnetic induction to measure coating thickness by detecting changes in electrical conductivity caused by the presence of the coating. However, this approach is limited by the magnetic properties from which the turbine blades are made.