The influence of yttrium on oxide scale growth and adherence

Alloys and coatings for high-temperature service are designed to form selectively chromia scales, alumina scales, or, to a limited extent, silica scales upon exposure to the environment. For such oxide scales to be protective, they should be both slow growing and adherent. It turns out that the addition of yttrium to such alloys can often impart both characteristics to the oxide scale. However, the actual operating mechanisms continue to be a matter of controversy among researchers in the area of oxidation. In the present study, the growth and adherence of alumina and chromia scales on alloys containing yttrium, either as an oxide dispersion or as an intermetallic phase, have been investigated in conjunction with detailed oxide scale characterization using the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS). The results of the study are used for critical assessment of the proposed mechanisms, especially the more recent ones, and to suggest some new mechanisms for adherence.