Investigation of an as-sprayed NiCoCrAlY overlay coating.: A thermoanalytical approach

A low pressure plasma sprayed (LPPS) NiCoCrAlY aircraft turbine blade overlay coating was investigated by differential thermal analysis (DTA). A pronounced irreversible endothermic reaction peak was observed at 975°C for the as-sprayed coating. No equivalent reaction is observed for the LPPS coating in the annealed state and in Ni–Co–Cr–Al (Ni:Co = 2:1) compositions after slow furnace cool-down as well. The reaction scheme for the partial ternary AlNi–Cr–Ni system was taken as a reference standard for understanding the irreversible reaction, and was reexamined where necessary. The four-phase solid state reaction in the ternary system at 995 °C, according to γ-Ni + β-NiAl → γ′-Ni3Al + α-Cr, is compared with the occurrence of the reaction observed at 975 °C on the as-sprayed NiCoCrAlY coating material. It runs most likely according to γ-Ni + γ′-Ni3Al → α-Cr + β-NiAl. The reaction at 975 °C is discussed in terms of metastable phase formation sequences.