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

Abstract

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.

Protective coatings for very high temperature reactor applications

“…This typical columnar morphology is said to reduce stress build up within the body of the coating by accommodating stress mismatch between the ceramic layer and the metallic one. The initial thermal conductivity of EBPVD‐PSZ is typically 1 W/m·K but it increases in service up to about 1.5–2 W/m·K 10–12. the TGO .…”

“…They comprise occasional dispersal of γ′ (Ni 3 Al) and α‐Cr phase. Furthermore, some secondary phases like Y 2 O 3 and chromium carbides may coexist 10. EDS analyses indicate an overall composition of approximately 63 at% Ni, 17 at% Al and 20 at% Cr (Y concentration was too low for proper detection) that should correspond to the coexistence of γ, β and γ′ phases, according to the Ni–Cr–Al equilibrium diagram at high temperature (see Fig.…”

The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

“…The as-received powder had a two-phase microstructure. The XRD and EDS results identified the presence of c and b phases; the c phase is fcc and is Co rich with a lattice parameter of 0.358 nm, the b phase is (Co, Ni)Al, an ordered B2, bcc, structure with a lattice parameter of 0.286 nm ( Ref 10,11).…”

Mechanical Properties and Microstructure of VPS and HVOF CoNiCrAlY Coatings

“…The MCrAlY (where M = Co, Ni or combination of both)-type alloys which are used as both stand-alone overlay coatings and thermal barrier coating (TBC) bond coats are among the most important protective coating materials applied to counteract hot corrosion and high temperature oxidation (Ref [1][2][3][4]. Like any other type of protective coating designed for oxidation resistance, MCrAlYs are capable of developing a thermodynamically stable slow growing oxide layer (thermally grown oxide, TGO).…”