Structure of and ion segregation to an alumina grain boundary: Implications for growth and creep
- PDF / 805,215 Bytes
- 15 Pages / 612 x 792 pts (letter) Page_size
- 69 Downloads / 214 Views
Using periodic density-functional theory (DFT), we investigated the structure and cohesive properties of the ␣-alumina ⌺11 tilt grain boundary, with and without segregated elements, as a model for the thermally grown oxide in jet engine thermal barrier coatings. We identified a new low-energy structure different from what was proposed previously based on electron microscopy and classical potential simulations. We explored the structure and energy landscape at the grain boundary for segregated Al, O, and early transition metals (TMs) Y and Hf. We predict that the TMs preferentially adsorb at the same sites as Al, while some adsites favored by O remain unblocked by TMs. All segregated atoms have a limited effect on grain boundary adhesion, suggesting that adhesion energies alone cannot be used for predictions of creep inhibition. These findings provide some new insights into how TM dopants affect alumina growth and creep kinetics.
I. INTRODUCTION
Jet turbine engines operate at temperatures higher than the melting point of the Ni-based superalloy used for many of the engine parts. This feat is accomplished via the use of a thermal barrier coating (TBC), which acts as a heat shield to reduce the effective temperature experienced by the underlying alloy. A typical TBC consists of three layers: (i) a NiAl-based bond coat alloy deposited on the Ni superalloy substrate to improve TBC adhesion and provide an abundant source of Al, (ii) a yttriastabilized zirconia (YSZ) topcoat for thermal protection, and (iii) a thin layer of a thermally grown oxide (TGO) in between, the purpose of which is to protect the superalloy from oxidative corrosion, since oxygen readily diffuses through the YSZ layer. Alumina (Al2O3) is the optimum TGO, as it has the lowest oxygen mobility of all oxide ceramics up to very high temperatures, thereby providing corrosion protection, while having a relatively slow oxide growth rate.1,2 This slow growth rate is critical, since thermal cycling produces stresses due to the coefficient of thermal expansion mismatch between alumina and the metal alloy. Once the TGO grows past a critical thickness (typically
Data Loading...
