Effects of water vapor on high temperature oxidation of cryomilled NiCoCrAlY coatings in air and low-SO2 environments

Nanostructured NiCoCrAlY coatings were obtained through cryomilling and subsequent HVOF-spraying of the nanostructured powder onto copper substrates, followed by leaching of the Cu to obtain freestanding coatings. Oxidation tests in both air and an environment bearing 0.5% SO2 with water vapor levels varying from 0 to 8% were conducted for 24 and 48 h at 1000 °C. It has been found that the oxides formed under all conditions are alumina followed by a spinel phase consisting primarily of cobalt aluminum oxide. In both air and the SO2 environment an increase in total mass gain was seen up to 4% water vapor, followed by a decrease at 6 and 8%. This behavior is attributed to an extension of the transient oxidation stage, resulting first in a variation in scale thickness through increased spinel growth and second in a rise in internal oxidation caused by increased oxygen transport through the scale in the presence of water vapor. It would appear that above 4% water vapor the thickened spinel is able to compensate for alumina loss and create a diffusion barrier to internal oxidation, once again reducing mass gain.