This paper presents the results of small punch creep testing (SPCT) of a vacuum plasma-sprayed (VPS) CoNiCrAlY coating (Co-31.7%, Ni-20.8%, Cr-8.1%, Al-0.5%, Y, all in wt%) carried out at 750 C. Coating cross-sections, after initial heat treatment, after sample clamping in the test rig prior to testing and after SPCT, were examined using scanning electron microscopy (SEM). A dual phase structure consisting of a fcc-Ni matrix and a bcc-NiAl secondary phase was found to exist. A large number of partially melted powder particles, with a semi-continuous layer surrounding most of the partially melted particles, were found in the coating. Pre-cracking was observed to occur around the partially melted particles after sample clamping was applied in the test rig prior to testing. This was due to the inherent brittleness of the layer around those particles. Very short failure times were obtained from the SPCT of the specific VPS CoNiCrAlY coatings, due to the pre-existing cracks formed during sample clamping.
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, all in wt%) coatings of an approximate thickness of 0.5 mm. Small punch tests under multi-step loading conditions were performed between room temperature and 600 C on these samples to evaluate the ductile-to-brittle transition temperature. The microstructure of the coatings was characterised using a scanning electron microscope with energydispersive X-ray analysis. A two-phase structure consisting of fcc g-Ni and bcc b-NiAl was found to exist. The displacements obtained from small punch multi-step loading tests at each load increment were relatively small and similar at temperatures below 500 C but a significant increase in displacement was noted at 600 C. Fractographic investigation showed that the main fracture mode was dominated by extensive g matrix tearing at elevated temperatures. A distinct stress and strain behaviour was found at 600 C, indicating that the ductile-to-brittle transition temperature of this CoNiCrAlY coating occurred between 500 C and 600 C.
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