Tensile properties of a non-line-of-sight processed β-γ-γ′ MCrAlY coating at high temperature

The tensile properties of a NiCoCrAlYTa coating manufactured by the Tribomet® process were examined in a wide range of temperatures representative to turbine in-service conditions. Thin and bulky freestanding coating specimens (FSCS) were tested and compared in order to evaluate the mechanical strength and ductility of this material. Comparable results were found for both the specimen geometries, i.e. a brittle-to-ductile transition temperature (BDTT) between 650 and 700°C, a significant increase of the ductility (up to 5.5% at 800°C) and a drop of the tensile strength above the BDTT. Thin FSCS demonstrated slightly lower yield strength and ultimate tensile stress compared to bulky FSCS. Fractographic examinations aimed to suggest that this difference in mechanical properties was partly attributed to a lesser cohesion and brittle behavior of a β-rich layer found in the extreme surface of the overlay coating due to the deposition technique. This poor local mechanical strength is believed to be detrimental for the integrity of thermal barrier coatings (TBC) systems and attentions in the design of such functionally graded materials have to be paid to prevent early in-service damages