Thermal Barrier Coatings (TBCs) are deposited on jet turbine blades to allow engines to run at higher, more fuel efficient, operating temperatures. However, these TBCs are vulnerable to molten silicate penetration, which occurs when siliceous debris (e.g. sand, volcanic ash, runway debris) is ingested by the turbines and subsequently melts, infiltrating the porous TBCs and causing cracks and delamination. Therefore, there is much interest in the development of TBCs that are phase stable at higher temperatures and are resistant to molten silicate infiltration. Rather than investigating the thermal and structural properties of a single TBC, we investigated the use of an overlayer TBC and its effectiveness in preventing a primary TBC from molten silicate penetration. We evaluated the interdiffusion behavior of three separate overlayer TBC compositions with a chosen primary TBC, and then tested each overlayer TBC with two molten silicate compositions, to examine the nature of those interactions.