Accession Number : ADA326221

Title :   Influence of High Cycle Thermal Loads on Thermal Fatigue Behavior of Thick Thermal Barrier Coatings.

Descriptive Note : Technical paper,

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

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

PDF Url : ADA326221

Report Date : MAY 1997

Pagination or Media Count : 51

Abstract : Thick thermal barrier coating systems in a diesel engine experience severe thermal low cycle fatigue (LCF) and high cycle fatigue (HCF) during engine operation. In the present study, the mechanisms of fatigue crack initiation and propagation, as well as of coating failure, under thermal loads which simulate engine conditions, are investigated using a high power CO2 laser. In general, surface vertical cracks initiate early and grow continuously under LCF and HCF cyclic 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, which induce tensile stresses in the coating after cooling. Experiments show that the HCF cycles are very damaging to the coating systems. The combined LCF and HCF tests produced more severe coating surface cracking, microspallation and accelerated crack growth, as compared to the pure LCF test. It is suggested that the HCF component cannot only accelerate the surface crack initiation, but also interact with the LCF by contributing to the crack growth at high temperatures. The increased LCF stress intensity at the crack tip due to the HCF component enhances the subsequent LCF crack growth. Conversely, since a faster HCF crack growth rate will be expected with lower effective compressive stresses in the coating, the LCF cycles also facilitate the HCF crack growth at high temperatures by stress relaxation process. A surface wedging model has been proposed to account for the HCF crack growth in the coating system. This mechanism predicts that HCF damage effect increases with increasing temperature swing, the thermal expansion coefficient and the elastic modulus of the ceramic coating, as well as the HCF interacting depth. A good agreement has been found between the analysis and experimental evidence.

Descriptors :   *DIESEL ENGINES, *CRACK PROPAGATION, *THERMAL FATIGUE, *BARRIER COATINGS, STRESS ANALYSIS, HIGH TEMPERATURE, TENSILE STRESS, CRACKING(FRACTURING), MODULUS OF ELASTICITY, FRACTURE(MECHANICS), CERAMIC MATERIALS, THERMAL EXPANSION, FAILURE(MECHANICS), ACCELERATED TESTING, THERMAL STRESSES, CARBON DIOXIDE LASERS, CYCLIC TESTS, SINTERING, CREEP STRENGTH, STRESS RELAXATION.

Subject Categories : Mechanics
      Reciprocating and Rotating Engines
      Coatings, Colorants and Finishes

Distribution Statement : APPROVED FOR PUBLIC RELEASE