Parameters influencing the oxidation behavior of cryomilled CoNiCrAlY

A nanostructured CoNiCrAlY coating was obtained by thermally spraying cryomilled feedstock powder using the HVOF process. The resulting coating is intended for use in a harsh oxidizing environment. The oxidation behavior of the nanostructured powder and coating was compared to their conventional counterparts by performing static oxidation tests at 900, 925, 950 and 1050 °C for 72 h and at 1000 °C for durations ranging from 1 to 96 h. It has been found that oxidation of the conventional powder yielded a duplex oxide scale composed of an inner Al2O3 with an outer layer composed of Cr2O3, NiAl2O4 and other spinels. Interestingly, oxidation of the as-sprayed nanostructured coating yielded a similar duplex oxide scale while oxidation of the ground coating resulted in a single Al2O3 layer. This single alumina layer formed after surface treatment has substantially lower activation energy than the oxide formed on untreated samples showing nearly spontaneous oxidation of aluminum and exhibits notably slower kinetics. These findings indicated that surface oxide seeds act as nucleation sites for the growth of the mixed oxide layer. Also, it was determined that nanostructuring of the coating promotes the nucleation and growth of a single and adherent α-Al2O3 layer by increasing both the number of diffusion channels and the aluminum concentration at grain boundaries.