In-situ SEM mechanical testing of bond coating at extreme temperatures

Abstract text (incl. figure legends and references)

Pt-aluminide (PtAl) bond coats are an integral part of thermal barrier coating (TBC) systems operating in aggressive environments such as gas turbine engines. We explored deformation mechanisms of diffusion bond coats and Ni-based superalloys at elevated and at cryogenic temperatures.

An advanced in situ SEM nanomechanical instrument, Hysitron PI 89 PicoIndenter (Bruker, Minneapolis, USA) with an integrated 1000°C high-temperature stage and active 1000°C tip heating was used to conduct uniaxial compression of micropillar samples. An integrated cryogenic stage was used to conduct pillar compression tests at -130°C.

Micropillars of dimensions 8 μm x 8 μm in cross-section and 25 μm in height were prepared from the outer layer of PtNiAl coating and Ni-base superalloy region by focused ion beam (FIB) machining. Using the displacement-controlled feedback mode of the system, the pillars were compressed to 5-12% strain at a strain rate of 10-3 s-1.

A switch in plasticity mechanism to grain boundary sliding at higher temperature was observed on the surface of the bond coating pillars (fig. 1). The elastic modulus of the bond coating remains nearly constant up to 900°C, whereas yield stress of the coating decreases to ~50% (fig. 2).

Fig.1 Morphology of bond coating pillar after compression at 700°C.

Fig. 2 Stress-strain curves of bond coating at different temperatures