Phase stability and thermal behavior of single layered PSZ and bi-layered PSZ/Gd2Zr2O7 on bond coated Inconel-718 substrate

Abstract

In the present study, structural properties of thermally treated thermal barrier coatings made of plasma sprayed NiCrAlY bond coat with single layered yttria (7 wt%) partially stabilized zirconia (PSZ) and bi-layered PSZ/gadolinium zirconate (GZ) (PSZ/GZ) as topcoat prepared by electron beam physical vapor deposition have been investigated. X-ray diffraction was used to understand the phase distribution, crystallite size, strain and preferred orientation in the as-deposited as well as thermally treated topcoat materials. SEM examination of the cross-sectional microstructure of the bond coat showed lamellar microstructure, while both the topcoats exhibited feathery columnar microstructure associated with fine pores. The single layered PSZ and bi-layered PSZ/GZ coatings were isothermally oxidized at 1273 K, 100 h to investigate the microstructure and crack distribution in these coatings. Mechanical properties of the as-deposited and isothermally oxidized single layered PSZ and bi-layered PSZ/GZ coatings were evaluated. The phase stability and lifetime of the TBCs were studied for the samples subjected to isothermal and cyclic oxidation tests. Results revealed that the coatings were thermally stable with no phase transformation and no spallation on both PSZ and PSZ/GZ coating subjected thermal cycling at 1273 K, 200 cycles. However, the bi-layered PSZ/GZ coatings performed better than single layered PSZ coatings subjected to thermal cyclic at 1473 K, 100 cycles. The reduced hardness obtained after thermal cycling of the PSZ/GZ suggests that the latter coatings are better than the single layered PSZ or GZ coatings as top coat material.