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Mixed zone formation

This study was concentrated on the oxidation behaviour of NiCrAlY bond coats, of a particular interest being the formation of a mixed zone thermally grown oxide (TGO), which at the time has been reported only in single-phase â-(Ni,Pt)Al alloys. These mixed Al2O3-ZrO2 microstructures are peculiar to EB-PVD systems. Most of the study was performed on polished samples, focusing on the effects of the EB-PVD deposition environment on TGO evolution. Samples with the more typical grit-blasted surface were run in parallel to identify similarities and difference in behaviour.

The research emphasized characterization by transmission electron microscopy (TEM), which provide information on a scale often absent in many of the studies reported in the literature. The implication, confirmed by comparing samples studied in this work, is that a suitable preoxidation treatment should preclude the incorporation of ZrO2 within the TGO. This study showed that the evolution of alumna scales during TBC deposition on multiphase MCrAlY alloys exhibits the same divergent behaviour between the major â and ã constituents reported in the literature for oxidation of non-coated surfaces. Absent the TBC the differences are primarily confined to the outer TGO surface layer, which is most likely the result of transient oxidation on heating.

The initial oxidation morphologies are quite sensitive to pO2, especially on â that forms a much thicker outer oxide than ã in O2 at ambient pressure, but much thinner in the deposition chamber environment. The mechanistic origin appears to be the rapid growth of è-Al2O3 by outward Al diffusion at pO2 ~105 Pa, and its suppression at pO2 ~10-2 Pa, but the fundamental reasons for this behaviour are not understood. The outer oxide on ã also changes from Ni(Al,Cr)2O4 to (Al,Cr)2O3, but that does not seem to change significantly its thickness. An uderlaying columnar layer of á-Al2O3 was noted in all non-coated substrates, even after relatively short oxidation treatments. The outer oxide, however, can continue to grow concurrently with á as long as the latter is sufficiently thin to allow the requisite outward caution transport.

Oxidation under a depositing TBC regenerates the divergent behaviour observed at the higher pO2, primarily by reestablishing the growth of è-Al2O3 on the â regions. This is believed to result from the dissolution of YSZ in the metastable oxide, delaying its transformation to á. When the transformation finally occurs, the dissolved ZrO2 and Y2O3 precipitate within the á-Al2O3, giving rise to a fine-grain "mixed zone" conceptually similar to those reported for Pt-modified NiAl bond coats. The overall thickness of the TGO and its constituent layers is in generally smaller under low pO2, suggesting that the growth rate of è during TBC deposition is significantly lower that that on oxidation in ambient pressure O2.

Reported by

Max-Planck-Institut fur Metallforschung
Heisenbergstr. 3
D70569 Stuttgart
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