2023 Volume 60 Issue 2 Pages 111-117
Applications of a unique high temperature optical microscopic observation system to the evaluation of plasma sprayed environmental barrier coatings (EBCs) for SiC/SiC composites in jet engines are introduced. Concept of the developed system using a light source with shorter wave length than that of thermal radiation and a band-pass filter cutting off the radiation light is illustrated. The system is used for the in-situ observation of an amorphous mullite environmental barrier coating while it is heated from 25℃ to 1400℃. The system detects the critical moment of mud-cracking which occurs at 1000℃ by the shrinkage due crystallization. Digital image correlation (DIC) provides strain mapping which indicates that large principal strain is induced along the mud-cracks, showing the cracking moment more clearly. Another application of the system is the measurement of coefficient of thermal expansion (CTE) in the in-plane direction of coatings. Thermal strain is measured from a series of images of a self-standing EBC at various temperatures using DIC and then converted to the CTE. For cases in which a self-standing coating cannot be obtained, a measurement method for a coating adhered to a substrate is also proposed.
