Sulfidation behavior of a thermal barrier coating consisted of CoNiCrAlY and YSZ (ZrO
2-8mol%Y
2O
3) coating, prepared by low pressure and air plasma sprayings, respectively, on a nickel based heat resistant alloy was investigated in a H
2S-H
2 atmosphere with a 10
-0.65Pa sulfur partial pressure at 1173K for 43.2ks. The corrosion product with a network structure was identified by X-ray diffraction and electron probe microanalyses to be Zr
3Y
4S
3.5O
9.4, which was formed from the Zr
3Y
4O
12 by replacement of oxygen by sulfur, having the elongated c-axis and the shrunken a-axis in contrast to those of the Zr
3Y
4O
12. Sulfur and/or hydrogen sulfide gases were assumed to diffuse through flaws such as cracks and voids across the YSZ coating and reacted with Cr to form Cr-sulfide surrounding the aluminum-rich oxide layer at the YSZ/CoNiCrAlY interface.
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