FATIGUE OF THICK THERMAL BARRIER COATINGS

Thick thermal barrier coatings (TTBCs) of Plasma-sprayed 8% Y2O3-ZrO2 were fatigued in compression as part of a reliability and durability s tudy to evaluate their potential use in high-performance diesel engine s. Test specimens were designed to test the bulk ceramic uniaxially, i ndependent of the substrate. A test machine was designed to alleviate the mechanical gripping and alignment difficulties associated with cyc lically stressing brittle ceramics in compression. Higher fatigue limi ts, 375 vs 200 MPa, were observed at 800-degrees-C than at room temper ature. Specimens tested at room temperature after high-temperature com pressive cycling also had higher fatigue limits, indicating that the s trengthening was permanent. At temperatures of 800-degrees-C, the coat ings showed evidence of low-temperature, pressure-induced sintering. T he extent to which sintering occurred was determined by studying the c hange in the elastic modulus as a result of the application of varying temperatures and static stresses.