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 study to evaluate their potential use in high-performance diesel engines. Test specimens were designed to test the bulk ceramic uniaxially, independent of the substrate. A test machine was designed to alleviate the mechanical gripping and alignment difficulties associated with cyclically stressing brittle ceramics in compression. Higher fatigue limits, 375 vs 200 MPa, were observed at 800-degrees-C than at room temperature. Specimens tested at room temperature after high-temperature compressive cycling also had higher fatigue limits, indicating that the strengthening was permanent. At temperatures of 800-degrees-C, the coatings showed evidence of low-temperature, pressure-induced sintering. The extent to which sintering occurred was determined by studying the change in the elastic modulus as a result of the application of varying temperatures and static stresses.