Characterization of the alumina scale formed on a commercial MCrAlYHfSi coating

A commercial NiCoCrAlYHfSi coating deposited on a Ni-base superalloy substrate was characterized before and after high temperature oxidation. The combination of Y, Hf and Si additions is reported to improve coating performance. Advanced characterization techniques including scanning-transmission electron microscopy were used to study the segregation behavior of Y and Hf ions to the alumina grain boundaries after 200 h at 1050 °C and 100 and 200 h exposures at 1100 °C. After both exposure times, two distinct oxide layers were observed. The outer transient layer included many Y- and Hf-rich oxide particles. The inner layer consisted of columnar α-Al2O3 grains normal to the surface of the coating. Segregation of Y and Hf ions was found on the alumina grain boundaries as has been observed in model alloys with similar compositions. Isothermal exposures for up to 200 h at 1050° and 1100 °C caused a minimal increase in surface roughness. However, 200 1-h cycles at 1100 °C resulted in a more significant increase in surface roughness.