Fatigue behavior of a plasma-sprayed 8%Y2O3-ZrO2 thermal barrier coating

Thick thermal barrier coatings with thicknesses on the order of a few millimeters are being developed for use in diesel engines with operating temperatures of about 800 degrees C. In this environment, a coating will experience thermomechanical cycling due to differences in elastic and thermal expansion properties between the coating and the substrate. The inelastic constitutive behavior of the coating material results in both compressive and tensile stresses. To observe the effects of such stresses, specimens of plasma-sprayed 8%Y2O3-ZrO2 were fabricated to allow testing of the coating material independent of the substrate. Cyclic compression fatigue tests were conducted at room and high temperature (800 degrees C) to simulate the loading environment to which the coating materials will be exposed during service. At high temperature, the compressive fatigue strength of the coating material increased by nearly 100%. Fatigue tests in tension and combined tension/compression were conducted at room temperature to evaluate the effect of mean stress. It was observed that a varying mean stress had no significant impact on the fatigue lives of the coating material and the fatigue life was controlled by the maximum tensile stress of the cycle. Results from fatigue tests and SEM observations indicated that the damage accumulated during the tensile and the compressive portions of the fatigue cycle were independent of each other.