Evaluation of Environmental Barrier Coatings by High-temperature Optical Microscopic Observation and Strain Measurement Technique

• Published in: Journal of Japan Thermal Spray Society Vol. 60; no. 2; pp. 111 - 117
• Main Authors: KAKISAWA, Hideki, INOUE, Ryo, ARAI, Yutaro
• Format: Journal Article
• Language: Japanese
• Published: Japan Thermal Spray Society 2023
• Subjects: coefficient of thermal expansion; cracking; environmental barrier coating; High temperature observation; strain mapping
• ISSN: 0916-6076; 2186-1080
• DOI: 10.11330/jtss.60.111

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.