Reaction, transformation and delamination of samarium zirconate thermal barrier coatings

The rare earth zirconates have attracted interest for thermal barrier coatings (TBCs) because they have very low intrinsic thermal conductivities, are stable above 1200°C and are more resistant to sintering than yttria-stabilized zirconia (YSZ). Samarium zirconate (SZO) has the lowest thermal conductivity of the rare earth zirconates and its pyrochore structure is stable to 2200°C but little is known about its response to thermal cycling. Here, columnar morphology SZO coatings have been deposited on bond coated superalloy substrates using a directed vapor deposition method that facilitated the incorporation of pore volume fractions of 25 to 45%. The as-deposited coatings had a fluorite structure which transformed to the pyrochlore phase upon thermal cycling between 100 and 1100°C. This cycling eventually led to delamination of the coatings, with failure occurring at the interface between the TGO and a “mixed zone” that formed between the thermally grown alumina oxide (TGO) and the SZO. While the delamination lifetime increased with coating porosity (reduction in coating modulus), it was significantly less than that of similar YSZ coatings applied to the same substrates. The reduced life resulted from a reaction between the rare earth zirconate and the alumina-rich bond coat TGO, leading to the formation of a mixed zone consisting of SZO and SmAlO3. Thermal strain energy calculations show that the delamination driving force increases with TGO and mixed layer thicknesses and with coating modulus. The placement of a 10μm thick YSZ layer between the TGO and SZO layers eliminated the mixed zone and restored the thermal cyclic life to that of YSZ structures. ► The Sm2Zr2O7 coatings transformed from fluorite to pyrochlore upon thermal cycling. ► The coating delamination occurred at the alumina-TGO/mixed zone interface. ► A chemical reaction between SZO and alumina led to the formation of SmAlO3. ► A YSZ layer between TGO and SZO retarded the migration of samaria to the TGO. ► It inhibited the chemical reaction and increased the lifetime of bi-layer coatings.