Effect of thermal and mechanical loadings on the degradation and failure modes of APS TBCs

Thermal barrier coatings (TBCs) are applied for the thermal protection of hot section components of gas turbines to improve their life-time or to raise the power and thermal efficiency of the gas turbine by increasing the turbine inlet temperature. TBCs limit the lifetime of the coated component either by spallation or by affecting the surface properties of the component and therefore the fatigue resistance. The respective failure is influenced by a series of degradation parameters and processes such as bond coat oxidation, sintering of the ceramic coating, beta-depletion in the bond coat, roughness of the metal/ceramic interface, mismatch of thermal expansion of the coatings and resulting stresses, creep and stress relaxation processes and so forth. These parameters promote the particular damage and failure modes differently and therefore, changes in the complex load scenario during service affect the failure mode and the lifetime of coated components. The present work thus focuses on the correlation between the load scenario or type of loads and the resulting damage, failure modes and lifetime. Moreover, the kinetics of damage evolution and the major mechanisms and parameters, which control the durability of TBCs were investigated. A TBC system investigated consisted of air plasma-sprayed partially stabilized zirconia (ZrO2-7-8wt.%Y2O3) with a NiCoCrAlY bond coat onto a CMSX-4 substrate and was subjected to six types of tests: (i) isothermal furnace tests, (ii) thermal cycling, (iii) cyclic oxidation, (iv) cyclic oxidation with temperature gradient, (v) thermomechanical fatigue, (vi) thermomechanical fatigue with a dwell time at high temperature in order to systematically analyze the influence of the type of the load profile on the damage and failure mode of the TBC composite, as well as on the time and number of cycles to failure. Analysis of the damage and failure modes of thermal barrier coatings was based on the representation of these six test types or load profiles as a combination of four basic load components such as isothermal exposure, thermal cycling, mechanical cycling and temperature gradient. The activation of each of these load components was found to have a strong influence on the time and number of cycles to failure and the damage and failure modes, which were distinguished by the particular crack paths on which spallation of the coating had occurred and which were more or less activated by the respective load components. The corresponding damage mapping has been developed and is presented in this thesis. The deformation mechanisms and corresponding microstructural changes of the ceramic top coat were additionally investigated on free-standing plasma-sprayed TBCs. Compressive deformation experiments revealed high creep rates and therefore high stress relaxation rates for APS-TBCs. Respective microstructural investigations displayed that macroscopic creep deformation comprises crack related as well as bulk related deformation.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2006

Author dc:creator

• Trunova, Olena

Contributors dc:contributor

• Schneider, Jochen M.

• info:eu-repo/classification/ddc/620
• Ingenieurwissenschaften
• Wärmeisolierstoff
• Plasmaspritzen
• Thermomechanische Eigenschaft
• Mechanisches Versagen
• Rissbildung
• TBC
• TMF
• failure modes
• damage evolution
• lifetime

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123202%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61547