Accession Number : ADA327220

Title :   Thermal Fatigue Testing of ZrO2-Y2O3 Thermal Barrier Coating Systems Using a High Power CO2 Laser.

Descriptive Note : Technical memo.,

Corporate Author : NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND OH LEWIS RESEARCH CEN TER

Personal Author(s) : Zhu, Dongming ; Miller, Robert A.

PDF Url : ADA327220

Report Date : JUN 1997

Pagination or Media Count : 30

Abstract : In the present study, the mechanisms of fatigue crack initiation and propagation, and of coating failure, under thermal loads that simulate diesel engine conditions, are investigated. The surface cracks initiate early and grow continuously under thermal low cycle fatigue (LCF) and high cycle fatigue (HCF) stresses. It is found that, in the absence of interfacial oxidation, the failure associated with LCF is closely related to coating sintering and creep at high temperatures. Significant LCF and HCF interactions have been observed in the thermal fatigue tests. The fatigue crack growth rate in the ceramic coating strongly depends on the characteristic HCF cycle number, N* sub HCF, which is defined as the number of HCF cycles per LCF cycle. The crack growth rate is increased from 0.36 micrometer/LCF cycle for a pure LCF test to 2.8 micrometers/LCF cycle for a combined LCF and HCF test at NH* sub HCF about 20,000. A surface wedging model has been proposed to account for the HCF crack growth in the coating systems. This mechanism predicts that HCF damage effect increases with increasing surface temperature swing, the thermal expansion coefficient and the elastic modulus of the ceramic coating, as well as with the HCF interacting depth. A good agreement has been found between the analysis and experimental evidence.

Descriptors :   *CERAMIC MATERIALS, *COATINGS, *THERMAL FATIGUE, *CARBON DIOXIDE LASERS, *YTTRIUM OXIDES, *ZIRCONIUM OXIDES, *BARRIER COATINGS, TEST AND EVALUATION, STRESSES, THERMAL PROPERTIES, HIGH POWER, SIMULATION, DIESEL ENGINES, DAMAGE, INTERACTIONS, INTERFACES, HIGH TEMPERATURE, LOADS(FORCES), CRACKING(FRACTURING), MODULUS OF ELASTICITY, FAILURE, RATES, DEPTH, CYCLES, CRACK PROPAGATION, OXIDATION, COEFFICIENTS, THERMAL EXPANSION, CREEP, SURFACE TEMPERATURE, FATIGUE TESTS(MECHANICS), SINTERING.

Subject Categories : Inorganic Chemistry
      Lasers and Masers
      Coatings, Colorants and Finishes
      Mechanics

Distribution Statement : APPROVED FOR PUBLIC RELEASE