Abstract
A fundamental understanding of failure mechanisms for thermal barrier coatings (TBC) is important for accurate life-time prediction and hence of much interest for industry. Failure (i.e. spallation or cracking) of the TBC usually occurs immediately upon cooling the specimen. However, in some cases spallation of the TBC is observed with a delay of several hours or even days after cooling, when the specimen is at ambient temperature and exposed to laboratory air. Because laboratory air contains water vapour, one hypothesis is that water plays a role in delayed failure of TBCs. This hypothesis is strongly supported by experiments in which the application of liquid water to a pre-oxidized TBC leads to spontaneous spallation/delamination at room temperature. The aim of this work is to study the effect of moisture on TBC systems in more detail. A series of experiments including acoustic emission techniques for in situ detection of cracking within the specimen and nuclear reaction analysis to determine hydrogen concentration depth profiles support the proposed hypothesis. Optical micrographs of APS TBCs isothermally oxidized at 1100°C show increased inward growing oxidation in cauliflower-like structures for specimens oxidized in moist atmospheres.
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