Fig 21 Thermal Shock Test During Cooling Cycle

Bend specimens are loaded into a rotating carousel and placed directly into a very hot combustion flame for preset amounts of time to bring the ceramic surface up to temperature. They are alternately removed from the flame into an air jet to thermally shock the ceramic surface in a cycle similar to what an engine component would experience in a shutdown. Not quite clear in these visuals is that the backside of the coupon is air-jet cooled to maintain a desired temperature gradient from the top surface of the ceramic to the interior of the base material. This, of course, is more of a screening test than any type of quantitative testing but it will give us a ranking of the coatings as far as the porosity levels and of the processing. It will also give us some idea of the stability of the two types of yttria stabilized zirconia coatings.

The next phase of the work, once we have our baselines established, is to grade the coating. The objective of grading is to reduce the thermal stresses induced in the thermal barrier coating by the differences in thermal coefficient of expansion between the bond coat and the ceramic. We plan to accomplish this by mixing the bond coat and the ceramic with the idea of decreasing the tensile stresses developed in the coatings (Fig. 22).


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