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@PHDTHESIS{Bednarz:55875,
      author       = {Bednarz, Piotr},
      title        = {{F}inite {E}lement {S}imulation of {S}tress {E}volution in
                      {T}hermal {B}arrier {C}oating {S}ystems},
      volume       = {60},
      school       = {RWTH Aachen},
      type         = {Dr. (Univ.)},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {PreJuSER-55875},
      isbn         = {978-3-89336-471-8},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Energietechnik / Energy Technology},
      pages        = {121 S.},
      year         = {2007},
      note         = {Record converted from VDB: 12.11.2012; RWTH Aachen, Diss.,
                      2006},
      abstract     = {Gas turbine materials exposed to extreme high temperature
                      require protective coatings. To design reliable components,
                      a better understanding of the coating failure mechanisms is
                      required. Damage in $\textit{Thermal Barrier Coating
                      Systems}$ (TBCs) is related to oxidation of the
                      $\textit{Bond Coat}$, sintering of the ceramic, thermal
                      mismatch of the material constituents, complex shape of the
                      BC/TGO/TBC interface, redistribution of stresses via creep
                      and plastic deformation and crack resistance. In this work,
                      experimental data of thermo-mechanical properties of CMSX-4,
                      MCrAlY (Bond Coat) and APS-TBC (partially stabilized
                      zirconia), were implemented into an FE-model in order to
                      simulate the stress development at the metal/ceramic
                      interface. The FE model reproduced the specimen geometry
                      used in corresponding experiments. It comprises a periodic
                      unit cell representing a slice of the cylindrical specimen,
                      whereas the periodic length of the unit cell equals an
                      idealized wavelength of the rough metal/ceramic interface.
                      Experimental loading conditions in form of thermal cycling
                      with a dwelltime at high temperature and consideration of
                      continuous oxidation were simulated. By a stepwise
                      consideration of various material properties and processes,
                      a reference model was achieved which most realistically
                      simulated the materials behavior. The influences of
                      systematic parameter variations on the stress development
                      and critical sites with respect to possible crack paths were
                      shown. Additionally, crack initiation and propagation at the
                      peak of asperity at BC/TGO interface was calculated. It can
                      be concluded that a realistic modeling of stress development
                      in TBCs requires at least reliable data of i) BC and TGO
                      plasticity, ii) BC and TBC creep, iii) continuous oxidation
                      including in particular lateral oxidation, and iv) critical
                      energy release rate for interfaces (BC/TGO, TGO/TBC) and for
                      each layer. The main results from the performed parametric
                      studies of material property variations suggest that
                      porosity in the TBC should be increased and sintering
                      decreased, in order to prevent or hinder continuous paths of
                      tensile stresses above the valleys in the TBC. It was shown
                      that variations of creep rates in the BC influence marginaly
                      stress values in TBCs. Therefore neither a positive nor a
                      negative influence on the lifetime can be extrapolated. It
                      was shown that higher creep rates in the TBC layer led to a
                      lower stress level. The extreme variations of thermal
                      expansion coefficient (±50\%) help in better understanding
                      of these variations on stress development. The creep of base
                      material only slightly affects stress field development,
                      under pure thermal cycling and can therefore be neglected in
                      this case. As the tensile stresses increase with a
                      relatively high fraction of lateral oxidation not only the
                      out-of-plane oxidation kinetics, but also its lateral
                      component should be low. The modification of amplitude and
                      wavelength of the asperity showed that with increasing
                      roughness a continuous radial tensile path in the TBC and
                      partially in the TGO was formed already after 161 cycles.
                      The variations of wavelength, amplitude and shapes improve
                      the understanding of stress development. The large variety
                      of parametric variations studied by the present work in a
                      highly complex and rather realistic FE model contribute
                      significantly to an enhanced understanding of TBCs. This is
                      supported by the final conclusion, that the set of crucial
                      parameters could be reduced to the time dependent
                      deformation behavior of TBC and TGO, the oxidation kinetics,
                      including lateral oxidation and the shape function of the
                      interface asperity.},
      cin          = {IEF-2},
      ddc          = {620},
      cid          = {I:(DE-Juel1)VDB810},
      pnm          = {Rationelle Energieumwandlung},
      pid          = {G:(DE-Juel1)FUEK402},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/55875},
}