The ongoing development of environmental barrier coatings (EBCs) offers the prospect to implement the full potential of silicon-based ceramics for high temperature structural applications. The current state-of-the-art EBC system comprises a Si bond coat, a mullite (3Al 2 O 3 ·2SiO 2 ) interlayer and a (1-x)BaO·xSrO·Al 2 O 3 ·2SiO 2 , 0 ≤ x ≤ 1 (BSAS) crack-resistant and water vapour attack resistant top coat. In this study, the influence of water vapour corrosion on the structural and mechanical properties of plasma-sprayed Si/Mullite/BSAS architectures was assessed by furnace thermal cycle testing (e.g., 100 cycles, 2h/cycle at 1300°C). Commercially available mullite and BSAS powders were used to produce crystalline coatings by air plasma spraying. Fully crystalline mullite and celsian BSAS coatings were engineered under controlled conditions on silicon coated, sintered α-SiC Hexoloy substrates. The overall performance at high-temperature of these functionally graded EBCs is discussed and correlated to their microstructural characteristics.

Optical coherence tomography (OCT) is evaluated as a promising technique for microstructure characterization of plasma-sprayed ceramic coatings. OCT combines the principles of low coherence interferometry and optical heterodyne detection to obtain both a high sensitivity to weakly backscattered light and a high axial resolution. It can be used to accurately locate interfaces where the refractive index changes abruptly within translucent materials. In the present work, OCT cross-sectional images of thin yttria-stabilized zirconia (YSZ) coatings are considered. The interferograms forming the images are analyzed individually to successfully gather information related to light penetration depth inside coatings. The interferogram analysis allows the evaluation of the refractive index of the YSZ non-transformable tetragonal phase.