Aircraft engine components are protected by a 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 grows in between these two during YSZ deposition, which then thickens during operation, ultimately leading to failure of the coating. In order to extend coating lifetime, it is critical to understand failure mechanisms and design circumvention strategies. Here first principles quantum mechanics (periodic density functional theory) calculations are used to test hypotheses about impurities that harm TBCs and transition metal (TM) dopants that extend TBC lifetime. Given that alumina thickening correlates with TBC failure, hindering alumina growth is critical. Some TMs such as Pt, Hf, and Y improve TBC stability, though their roles are not known precisely. Some TMs segregate to alumina grain boundaries and alumina growth may be inhibited by their presence; we explore this hypothesis here.