The NbSi2/Nb/γ-TiAl and NbSi2/Nb functionally graded materials (FGMs) were prepared and their tolerances tested by exposing them to temperatures from 1323 K to 1523 K under vacuum and in air. Oxygen resistivity was estimated from metallographic investigations. The FGM lifetime was estimated by using a diffusion equation that considers the disappearance of the NbSi2 and Nb interlayer. These occurred during NbSi2 oxidation and Si diffusion from NbSi2 to Nb and interdiffusion between Nb and γ-TiAl. The results were validated by diffusion equations. The results indicate that a coating used at 1323 K for 380 h requires a NbSi2 + Nb coating thickness of more than 77 μm.