Failure processes within ceramic coatings at high temperatures
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Plasma sprayed coatings have a complex structure which is produced by the overlaying of many molten or semi-molten particles in the diameter range of 20 to 120 µm. There is a need to characterize the failure behaviour of coatings and this has been carried out by using acoustic emission (AE) methodology.
Coatings of NiCrAIY bond coat with a zirconia-12 wt% yttria overlay were applied to discshaped specimens of U-700 alloy. A waveguide of 1 mm diameter platinum was TIG welded to the specimen and allowed it to be suspended in a tubular furnace. The specimen was thermally cycled to 1150° C and the AE monitored.
One method of examining the AE is from the viewpoint of the accumulative count data. It is also convenient to establish the temperatures for “initial” AE and “significant” AE (i.e., the temperature at which 100 counts is exceeded) so that coatings may be compared. Several other analyses have been carried out with the aim of establishing parameters which are related to the crack size and crack population. These studies have been used to postulate types of cracking mechanisms which may occur in plasma sprayed coatings during thermal cycling.
It is shown that microcracking gave rise to a large amount of AE. However, this coating still survived more thermal cycles than a coating which exhibited macrocracking events. Data of this nature will be presented and the results discussed.
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