Thermal expansion behavior of new Co-based alloys and implications for coatings

The thermal expansion behavior of five γ′-(L12) containing Co-based superalloys was measured using push-rod dilatometry. These results were analyzed to determine the mean coefficient of thermal expansion (CTE), the instantaneous CTE, and γ′-solvus temperature of the alloys. The CTE was found to depend strongly on the γ′-solvus temperature. In turbine applications, these alloys will be part of a multilayer system that generates thermal stresses due to the CTE mismatch between the components. To investigate this phenomenon, the CTE data was incorporated into a thermo-mechanical model to calculate the energy release rate (ERR) for the delamination of coatings from these alloys. The ERR of a thermally grown oxide (TGO) formed on the bare and bond coated Co-based alloys was determined as a function of superalloy and bond coat CTE. The ERR of a thermal barrier coating (TBC) was evaluated as a function of temperature, superalloy CTE, and coating thickness. The ERR was found to be about 1.5× greater in Co-based superalloys compared to Ni-based superalloys cooled from T=1100°C to ambient. Strategies for reducing the driving force for delamination are discussed. •The thermal expansion behavior and γ′-solvus was determined for new Co-based γ/γ’ alloys.•The thermal expansion coefficient strongly depends on the γ’-solvus•The driving force for delamination of coatings was calculated using a thermo-mechanical model.•Implications of alloy composition on thermal expansion and the effect of CTE on the durability of coatings are discussed.