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

✍ Scribed by K. Fritscher; Y.-T. Lee

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: German
• Weight: 325 KB
• Volume: 56
• Category: Article
• ISSN: 0947-5117
• DOI: 10.1002/maco.200403808

✦ Synopsis

Investigation of an as-sprayed NiCoCrAlY overlay coating -microstructure and evolution of the coating K. Fritscher* and Y.-T. Lee 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. c-Ni, b-NiAl and c 0 -Ni 3 Al phases were identified by XRD showing predominant c phase at the expense of b phase if compared to the fully processed state with c and b at about equal portions. Besides grains of c-Ni and b-NiAl, amorphous metallic grains and Cr-rich oxides were found by TEM. Close to the surface to the atmosphere c 0 -Ni 3 Al phase was localized in composite grains with an off-plane oriented c-c 0 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 assprayed coatings is attributed to their higher growth rates on quenching. Also benefiting from rapid growth the c-c 0 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 c-c 0 grains. The potentials of the microstructures in service are addressed.