Comparison of oxidation behavior of Ni-20Cr alloy and Ni-base self-fluxing alloy during air plasma spraying

Oxidation behavior of Ni-20Cr alloy and Ni-base self-fluxing (NiCrSiBC) alloy in atmospheric plasma spraying was studied experimentally. The in-flight particles were collected by quenching into liquid nitrogen. The oxygen contents in the collected particles and the coatings deposited on a substrate were analyzed by the inert gas fusion method. The oxide distribution, morphology and phase composition were analyzed using SEM, EDX, XRD, and AES. The results clearly show that the oxygen content in the NiCrSiBC coating was remarkably lower than that in the Ni20Cr coating by a factor of over 10. The formation of Cr2O3 and its vaporization primarily occurred during the flight of Ni20Cr particles, which dominated the oxidation in the coating. In contrast, little oxygen pickup occurred during flight for the NiCrSiBC alloy particles and a thin surface layer of 5 nm with rich in oxygen was found on the surface of NiCrSiBC splats. The mechanism of protecting NiCrSiBC alloy particles from oxidation is preferential oxidation of C, Si and B and simultaneous vaporization of the formed oxides.