Investigation of an as-sprayed NiCoCrAIY overlay coating - microstructure and evolution of the coating

A low pressure plasma sprayed (LPPS) NiCoCrAlY aircraft turbine blade overlay coating was investigated in the as-sprayed condition by X-ray diffraction (XRD) and analytical transmission electron microscopic (TEM) techniques. gamma-Ni, beta-NiAl and gamma'-Ni3Al phases were identified by XRD showing predominant gamma phase at the expense of P phase if compared to the fully processed state with gamma and beta at about equal portions. Besides grains of gamma-Ni and beta-NiAl, amorphous metallic grains and Cr-rich oxides were found by TEM. Close to the surface to the atmosphere gamma'-Ni3Al phase was localized in composite grains with an off-plane oriented gamma-gamma' fibrous eutectic structure. They were neighbored by amorphous metallic grains. The evolution of these structures is discussed within the scope of crystal growth behavior. The dominant occurrence of face-centered cubic at the expense of body-centered phases observed in as-sprayed coatings is attributed to their higher growth rates on quenching. Also benefiting from rapid growth h the fibrous eutectic grains will form. hereby relying on essentially binary phase compositions of highly extended solubility ranges. Accumulation of alloy constituents like Cr and Ti by segregation to the growth front is considered to pave the way for the evolution of amorphous grains at the coating surface on top of gamma-gamma' grains. The potentials of the microstructures in service are addressed.