A study of the apparent detrimental effect of a surface applied HfO2 coating on the oxidation behavior of alloys

This paper reports on additions of reactive elements, such as Hf, Ce, Y, or Zr, which have a number of beneficial effects on the oxidation behavior of Cr2O3 and Al2O3 forming alloys at high temperatures. These reactive elements have been found effective as minor alloying additions, as oxide dispersions in the alloy, as implanted ions in the alloy's surface, or as thin surface coatings. In general, one element which is effective when present in the alloy is also effective when applied on the surface. However, there have been a number of exceptions. Our earlier work, which studied the effect of several reactive element oxide coatings on Ni-Cr and Co-Cr alloys indicated that while surface oxides of Ce, La, and Y were as effective as their alloying additions, coatings of ZrO2 and CaO had little effect, and the HfO2 coating even caused breakaway oxidation