In the present study, isothermal oxidation behavior of nanostructured compositionally graded layer (CGL) thermal barrier coatings (TBC) has been investigated. Moreover, the property of the produced coating was compared with that of a micro CGL, as well as a conventional bilayer TBC. For this purpose, nanostructured and micro Y2O3-stabilized ZrO2 (YSZ) were used as ceramic-powder feedstocks and three coatings were deposited by air plasma spray (APS) procedure on IN-738LC superalloy. The first TBC consisted of 100 wt% NiCrAlY as the bond coat, NiCrAlY and nanostructured YSZ in the weight ratio of 50:50 as the graded layer, and 100 wt% YSZ as the top coat. In the second coating a micro YSZ feedstock, instead of the nanostructured one, was used. The third coating was a conventional bilayer coating consisted of a bond coat and top coat. Isothermal oxidation was carried out at 1100 °C in an atmospheric furnace for 10, 50, 100 and 150 h to investigate the kinetic of growth of thermally grown oxide (TGO). Microstructure evaluation, elemental and phase analysis, and hardness test were performed using field emission scanning electron microscopy, X-ray diffraction and Vickers hardness test. The results showed that nanostructured compositionally graded layer TBC has the lowest TGO growth rate compared with the other coatings. The hardness values of this kind of coating increased rapidly after 10 h exposure at 1100 °C, and then increased slightly to a certain constant value. Nanostructured coating showed different sintering effect due to its bimodal structure. Upon isothermally oxidation, XRD peaks were shifted slightly and tetragonal phase with a higher c/a ratio was appeared.

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