Thermal cycle test of functionally graded and composite environmental barrier coatings in the steam environment

Abstract

Protecting hot section parts of gas turbine engines made of SiC/SiC CMCs from atmospheric corrosion has become challenging. Three-layer environmental barrier coatings are recommended to eliminate this problem. However, thermal expansion incompatibilities between layers limit the lifespan of coatings. In this study, environmental barrier coatings were produced with 4 different functionally graded and composite designs to tolerate thermal expansion incompatibilities. For this purpose, 50-50 wt% YbSi-mullite and mullite-Si layers were produced between the layers in three-layer environmental barrier coatings for composite design. In functionally graded designs, 25 wt% graded layers were deposited between Si-mullite, mullite-YbSi, and Si-mullite-YbSi layers. The coatings were subjected to thermal cycle tests above 1450 ± 50 °C in a water vapor environment. Before and after the tests, the coatings were subjected to structural characterizations such as scanning electron microscopy and X-ray diffraction. An evaluation of the damage mechanism of the coating was carried out. According to thermal cycle tests, EBC produced with 50 wt% mullite+Si and 50 wt% mullite+YbSi composite interlayer design exhibited the longest thermal cycle life. The shortest thermal cycle life was observed in EBC produced by functionally grading the mullite+Si layer. The thermal cycle life of EBCs produced with 50 wt% mullite+Si and 50 wt% mullite+YbSi composite interlayers and trilayer functionally graded designs was longer than that of traditional trilayer EBCs. These new designs helped reduce stress accumulation resulting from thermal expansion mismatch between layers, thereby extending the thermal cycle life of the coatings. The study also observed that phase transformations in the YbSi layer, along with water vapor corrosion, were the primary factors contributing to crack formation during the thermal cycle tests.