Abstract
Infiltration of molten calcium–magnesium–alumina–silicates (CMAS) through thermal barrier coatings (TBCs) causes structural degradation of TBC layers. The infiltration kinetics can be altered by careful tailoring of the electron beam physical vapor deposition (EB-PVD) microstructure such as feather arm lengths and inter-columnar gaps, etc. Morphology of the feathery columns and their inherent porosities directly influences the infiltration kinetics of molten CMAS. To understand the influence of columnar morphology on the kinetics of the CAMS flow, a finite element based parametric model was developed for describing a variety of EB-PVD top coat microstructures. A detailed numerical study was performed considering fluid-solid interactions (FSI) between the CMAS and TBC top coat (TC). The CMAS flow characteristics through these microstructures were assessed quantitatively and qualitatively. Finally, correlations between the morphological parameters and CMAS flow kinetics were established. It was shown that the rate of longitudinal and lateral infiltration could be minimized by reducing the gap between columns and increasing the length of the feather arms. The results also show that the microstructures with long feather arms having a lower lateral inclination decrease the CMAS infiltration rate, therefore, reduce the CMAS infiltration depth. The analyses allow the identification of key morphological features that are important for mitigating the CMAS infiltration.
Highlights
Hot section engine components of gas turbines are insulated with thermal barrier coatings (TBCs) to protect them against excessive heat
The TBC layers fabricated by electron beam physical vapor deposition (EB-PVD) show superior structural stability and long-life compared to the TBCs produced by the atmospheric plasma spray (APS) method due to their feathery columnar microstructure [1,2,3,4,5,6]
CMAS considering the morphological variation of EB-PVD TBCs
Summary
Hot section engine components of gas turbines are insulated with thermal barrier coatings (TBCs) to protect them against excessive heat. State-of-the art standard 7YSZ (7 wt.% yttria stabilized zirconia) coatings are vulnerable against environmental attacks, i.e., sand, volcanic ash, etc., that comes from dusty air intake during engine operation. These silicate-based particles melt under high engine temperature forming calcium–magnesium–alumina–silicates (CMAS), deposit on coated parts, and infiltrates through the porous channels of the TBC layers [7,8]. The contamination of CMAS eventually degrades the structural properties of TBCs, causing spallation failure under thermal cycles [9,10,11,12,13]. Several methodologies were suggested to enforce the mitigation mechanisms against CMAS, which fall under two broad categories
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