Application of small punch creep testing to a thermally sprayed CoNiCrAlY bond coat

High velocity oxy-fuel thermal spraying was used to prepare free-standing CoNiCrAlY (Co-31.7% Ni-20.8% Cr-8.1% Al-0.5% Y (wt%)) bond coat alloy samples approximately 0.5 mm thick. Creep tests were conducted at 750 °C on these samples using a small punch (SP) creep test method. The samples were characterised before and after creep testing using scanning electron microscopy with electron backscatter diffraction (EBSD). EBSD revealed a two phase fcc γ-Ni and bcc B2 β-NiAl microstructure with grain sizes ~1-2 μm for both phases, which did not change significantly following testing. The constant temperature SP test data were characterised by a minimum creep strain rate, ε̇min, and a total time to failure, tf, at different applied stresses. The data are fitted to conventional power law equations with a stress exponent for creep close to 8 in the Norton power law and between 7 and 10 in the Monkman-Grant creep rupture law. Creep rupture was predominantly due to creep cavitation voids nucleating at both the γ-β interphase boundaries and the γ-γ grain boundaries leading to final failure by void linkage. However, rupture life was influenced by the quantity of oxide entrained in the coating during the spray deposition process.