FATIGUE BEHAVIOR OF A PLASMA-SPRAYED 8-PERCENT-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 operati ng temperatures of about 800 degrees C. In this environment, a coating will experience thermomechanical cycling due to differences in elasti c and thermal expansion properties between the coating and the substra te. The inelastic constitutive behavior of the coating material result s 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 tem perature (800 degrees C) to simulate the loading environment to which the coating materials will be exposed during service. At high temperat ure, the compressive fatigue strength of the coating material increase d by nearly 100%. Fatigue tests in tension and combined tension/compre ssion were conducted at room temperature to evaluate the effect of mea n stress. It was observed that a varying mean stress had no significan t impact on the fatigue lives of the coating material and the fatigue life was controlled by the maximum tensile stress of the cycle. Result s from fatigue tests and SEM observations indicated that the damage ac cumulated during the tensile and the compressive portions of the fatig ue cycle were independent of each other.