Correlation between microstructure, phase composition and oxidation behavior of CoNiCrAlY/nano-Al2O3 coatings using satellited feedstock deposited by LPPS process

Abstract

In this study, satellited CoNiCrAlY/2, 4 wt% nano-Al2O3, and conventional CoNiCrAlY feedstocks were deposited through the low-pressure plasma spraying (LPPS) process on Inconel738 substrate. The oxidation test was done at 1050 °C for 10 min and 5, 50, 100, 150, and 200 h. The microstructure and phase composition of the powders and coatings were characterized by the FESEM and XRD, respectively. The results showed that the CoNiCrAlY and Al2O3 powders comprised γ-CoNiCr and β-(Co,Ni)Al, and α-Al2O3, respectively. In contrast, the as-sprayed coatings included supersaturated γ-CoNiCr and meta-stable γ-Al2O3 phases. With the increase in the percentage of nanoparticles (from 2 to 4 wt%), the amount of porosity (from 2 to 3.76 vol%), and the roughness of the coating (from 6.1 to 7.5 μm) increased. With oxidation time, dissolved β-(Co,Ni)Al was recovered and then gradually consumed to create a thermally grown oxide (TGO) layer. In the composite coatings, the transformation of γ-Al2O3 to stable α-Al2O3 occurred, creating defects across the TGO layer. Therefore, with the addition of Al2O3 nanoparticles, the oxidation rate was continuously increased. The oxidation rate constant (Kp × 10−13) of conventional and composite coatings (2, 4 %) was measured as 3.89, 4.35 and 5.08 cm2.s−1, respectively.