Enigmatic Moisture Effects on Al₂O₃ Scale and TBC Adhesion

Abstract

Abstract. Alumina scale adhesion on high temperature alloys is known to be affected primarily by sulfur segregation and reactive element additions. However adherent scales can become partially compromised by excessive strain energy and cyclic cracking. With time, exposure of such scales to moisture can lead to spontaneous interfacial decohesion, occurring while the samples are maintained at ambient conditions. Examples of this Moisture-Induced Delayed Spallation (MIDS) are presented for NiCrAl and single crystal superalloys, becoming more severe with sulfur level and cyclic exposure conditions. Similarly, delayed failure or Desk Top Spallation (DTS) results are reviewed for TBC's, culminating in the water drop failure test. Both phenomena are discussed in terms of moisture effects on bulk alumina and bulk aluminides. A mechanism is proposed based on hydrogen embrittlement and is supported by a cathodic hydrogen charging experiment. Hydroxylization of aluminum from the alloy interface appears to be the relevant basic reaction.

High-Temperature Oxidation Resistance of Al2O3- and Cr2O3-Forming Heat-Resisting Alloys with Noble Metals and Rare Earths

“…Oxide spallation increased by the formation of aluminum hydroxide (34)(35) and by the weakness of the scale/alloy interface (32)(33). In the present study, oxide spallation occurred during cooling.…”

Moisture-Induced TBC Spallation on Turbine Blade Samples

“…However, we have discussed the general elements of DTS in the context of MIDS [11]. That is to say, hydrogen "embrittlement" had been proposed as the mechanism fundamental to both.…”

Compiled furnace cyclic lives of EB-PVD thermal barrier coatings

“…No distinction has been made regarding cooling temperature, which is very important, although most attain 30-80°C each cycle. It is well known that lower cooling temperatures, even near room temperature, and exposure to moisture, including ambient humidity, can accelerate scale spallation and TBC failure [14][15][16][17][18][19][20][21]. Typically a similar 2.54 cm dia.…”

“…These factors typically include ratcheting/rumpling/interdiffusion effects that occur preferentially on the coated superalloys, but need not be discussed further for the purpose of this compilation. Other factors, also not discussed at length here, but deemed to be relevant for TBC life, would be the sulfur and reactive element contents of the system, ambient moisture content, and temperature achieved on cooling [18][19][20][51][52][53][54].…”