The mechanisms of controlling FCT life and failure mode of Ni-based EBPVD YSZ thermal barrier coatings via the effects of both substrate elements and reactive element doping

Abstract

Electron-beam physical vapor deposition (EBPVD) of NiCoCrAlY- and Hf-modified bond coats on (1) selected polycrystalline, directionally solidified, (2) single crystalline substrate alloys and (3) an uncoated NiCrAl bond-coat surrogate substrate, all of them covered with standard EBPVD YSZ topcoats were subjected to cyclic furnace testing (FCT) at 1100 °C. The lifetime and spallation failure upon FCT were evaluated. A typical mixed layer zone (MZ) of alumina and zirconia has formed during topcoat processing above the thermally growing oxide layer. The MZ was investigated by energy-dispersive X-ray spectroscopy after intermediate lifetimes and at the end of life. Chemical composition of the MZ and lifespan data were related to each other thus accounting for rate-determining reactions which could be assigned to either cation- or anion-controlled transport mechanisms. These provide a new approach to address FCT life and failure mode of even complex TBC systems containing reactive elements (e.g. Y and Hf). The cation-controlled processes are accelerated according to their concentration by tetravalent elements of the substrates, while the anion-controlled processes are unaffected by this and only adopt a cation-dominated mode when alloying elements of a low valence (e.g. Ti+) reach a supercritical concentration.

The Reactive Element Effect

“…The dependence of the service life of TBC samples was investigated with regard to the chemistry of the TGO formed on the Ni-base substrates. [12] The standard NiCoCrAlY bond coat composition of the experimental campaign is Ni-21Co-19Cr-12Al-0.15Y in wt pct. The EBPVD process runs in a high vacuum atmosphere.…”

“…7-Y + Zr content (at pct) in the mixed zone vs time at 1100 °C during FCT of Hf-holding TBC samples and Hf-modified bond-coated samples showing a logarithmic rate of Y + Zr reduction with time. The half-life value of 1200 h is calculated from the downtilt; it allows to estimate the RE content at FCT failure.Reprinted from Ref [12]. under the terms of the CC BY license.…”