Aircraft engine components are protected by a multilayer, multi-component
thermal barrier coating (TBC), consisting of a NiAl-based bond coat alloy
layer onto which yttria-stabilized zirconia (YSZ) is deposited. A layer
of alumina (or "thermally grown oxide," TGO) grows in between
these two materials during YSZ deposition, which then subsequently thickens
during engine use. Current TBCs fail after ~16,000 hrs of thermal
cycling. In order to extend engine service lifetime, it is critical
to understand mechanisms of failure and then to design circumvention strategies.
We present results of first principles quantum mechanics - specifically
periodic density functional theory (DFT) - calculations that are used to
test hypotheses about impurities that harm TBCs and transition metal (TM)
dopants that extend TBC lifetime, including the role of Pt and early TMs
in NiAl-based bond coat alloys. For example, it has been shown experimentally
that TGO thickening is correlated with TBC failure. Thus, hindering
alumina growth is thought to be critical to prolonging TBC lifetimes. Empirically,
it is known that TMs such as Pt, Hf, and Y improve the stability of the
TBC, though their mechanism of action is not well characterized. Some
of these TMs (e.g., Y) segregate to grain boundaries in the TGO, and it
has been suggested that alumina growth may be inhibited by their presence.
As it is thought that growth of the alumina layer is controlled by Al and
O diffusion at grain boundaries, we explore the structure and energy landscape
at an alumina Σ11 
|| 
tilt
grain boundary for Al, O, early TMs, and rare earth elements. We also consider
how Pt affects the thermodynamic stability of defects and high temperature
diffusion kinetics in the NiAl bond coat, in order to elucidate the role
of Pt in stabilizing TBCs. Implications for growth of the alumina layer
and the fate of TBCs will be discussed.