Effect of manufacturing related parameters on oxidation properties of MCrAlY-bondcoats

The effect of MCrAlY-bondcoat manufacturing parameters prior to TBC deposition on the bondcoat oxidation behavior and TBC lifetime was studied. The studied material was a NiCoCrAl(Y/Hf) free-standing coating. It was found that variation of oxygen partial pressure during vacuum plasma spraying and the vacuum heat treatment procedure significantly affects the yttrium and hafnium distribution in the coating. In coatings sprayed at high pO2, yttrium and hafnium were mainly tied up into oxide precipitates. This effect is apparently responsible for an early alumina scale spallation and failure of the initially studied TBC system during cyclic oxidation. In contrast, the coating sprayed at low pO2 revealed an overdoping effect, i.e. extensive yttrium and hafnium incorporation into the scale resulting in an accelerated scale growth rate and internal oxidation. It was shown that by variation of the vacuum heat treatment parameters the yttrium and hafnium distribution in the near-surface regions of the low oxygen coating can be modified. The latter result demonstrates the potential of minimizing the negative overdoping effect on the scale growth in the thermal-sprayed MCrAlY coatings with low oxygen and/or high reactive element contents by optimization of the vacuum heat treatment procedure.