Substrate and bond coat compositions: factors affecting alumina scale adhesion

Thermally grown oxide scales that form beneath ZrO₂ top coats play an important role in determining the performance of thermal barrier coatings. Numerous factors, including the composition of both the alloy substrate and the bond coat, affect adhesion of the α-Al₂O₃ scale. Three areas of focus in the formation of an 'ideal', adherent scale encompass: (1) migration of Al and other elements in the metal substrate, (2) segregation of elements to the metal-scale interface and the scale grain boundaries, and (3) generation of stresses in the scale. Examples of the effects of reactive elements, Pt, indigenous S, and reaction temperature on scale adhesion are discussed.