Effects of pre-cracking on small punch creep testing of a vacuum plasma-sprayed CoNiCrAlY coating

Abstract

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 ℃. 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 γ-Ni matrix and a bcc β-NiAl secondary phase was found to exist. A large number of partially melted powder particles, with a semi-continuous β 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 β 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.