Microstructure and thermal cyclic performance of laser-glazed plasma-sprayed ceria–yttria-stabilized zirconia thermal barrier coatings

Abstract

In this study, substrates of Hastelloy-X superalloy coupons were first sprayed with a Ni–22Cr–10Al–1Y bond coat and then with a ceria–yttria-stabilized zirconia (CYSZ) top coat by air plasma spraying (APS). After that, the plasma-sprayed ceria–yttria-stabilized zirconia thermal barrier coatings (TBCs) were glazed using a pulsed CO2 laser. The effects of laser-glazing on the microstructure and cyclic oxidation behavior of the coatings were evaluated. The microstructures of both the as-processed and the tested TBCs were investigated by using optic microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The phases of the coatings were measured with X-ray diffractometry (XRD). An investigation of microstructure revealed that both plasma-sprayed and laser-glazed top coatings consisted of nonequibrium tetragonal (T) phase, cubic (C) phase and minor amount of Ce0.75Zr0.25O2 phase. Minor amount of monoclinic (M) phase found in plasma-sprayed coatings disappeared after laser-glazing. The phase analysis of tested top coat indicated that the mole% of tetragonal (T) phase decreased and that of Ce0.75Zr0.25O2 phase increased after thermal cycling for both sprayed and glazed samples. It was also found that the monoclinic phase in the plasma-sprayed sample disappeared after thermal cycling. Results showed that laser-glazing did not have a substantial effect on reducing the oxidation rate of the bond coat. Cyclic test results showed that the life times of the plasma-sprayed TBCs were enhanced about twofold by laser-glazing for initial spallation life. With regard to 50% spallation life, the laser-glazed samples exhibited about a half lifetime improvement relative to the plasma-sprayed samples. Failure of the TBCs was initiated within the top coat near the bond coat valley region and propagated mainly within the top coat and the out-growth oxide near the top coat–bond coat interface. Segmented cracks produced by laser-glazing improved the strain accommodation ability of top coat and could be identified as the major enhancement mechanism for TBC life extension.