**2. Main requirements for TBCs**

TBCs must have low weight and low thermal conductivity and they should withstand large stress variations due to heating and cooling, as well as thermal shock. They must be chemically compatible with the underlying metal and the thermally grown oxide (TGO) and should operate in an oxidizing environment. TBCs must provide thermal insulation to the underlying superalloy engine parts. They must have strain compliance in order to minimize the thermal expansion mismatch stresses with the superalloy parts. Additionally, they must reflect much of the radiant heat from the hot gas and thereby, preventing it from reaching the superalloy substrate. Further, TBCs must provide thermal protection to the substrate for prolonged service times and thermal cycles without failure [4].
