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Investigation by 3D FE simulations of delamination crack initiation in TBC caused by alumina growth
Linköping University, Department of Mechanical Engineering. Linköping University, The Institute of Technology.
Linköping University, Department of Mechanical Engineering. Linköping University, The Institute of Technology.
2001 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 135, no 2-3, 188-195 p.Article in journal (Refereed) Published
Abstract [en]

In gas turbines, thermal barrier coatings (TBCs) applied by air plasma spraying are widely used to reduce the temperature in hot components. The TBC allows higher gas temperature and/or reduces the need for internal cooling in the hot components, thus increasing the efficiency of the gas turbine. Spallation is a common failure mechanism of TBC and occurs after a critical number of thermal cycles, when the alumina layer has grown to a critical thickness. The influence of the growing alumina layer and the top/bond-coat interface roughness in the TBC has been investigated. The primary goal was to identify failure mechanisms that can be incorporated into a life model of the TBC, and to increase the understanding of the delamination process in the TBC. A new formulation of alumina growth is proposed, in which the swelling strains caused by the volumetric increase during alumina growth depends on the stress state. The alumina growth model is used in 3D FE thermal cycling simulations of a TBC in which the thermal cycle time is long enough to characterize a typical cycle of a gas turbine. From the simulations, the growing alumina layer is observed to be one failure mechanism of the TBC. Without an alumina layer in the model, high delamination stress is observed at room temperature, above ridges of the top/bond-coat interface in the top coat. When the alumina is growing, the point of maximum delamination stress is moved towards the valleys. When the thickness of the alumina layer has grown to approximately 8–10 μm, positive delamination stress is found above the valleys in the top coat. The movement of the positive delamination stress region can explain why a delamination crack develops, which will cause spallation of the TBC during shutdown to room temperature.

Place, publisher, year, edition, pages
2001. Vol. 135, no 2-3, 188-195 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-47480DOI: 10.1016/S0257-8972(00)01084-7OAI: oai:DiVA.org:liu-47480DiVA: diva2:268376
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2013-01-23
In thesis
1. Delamination in APS applied thermal barrier coatings: life modelling
Open this publication in new window or tab >>Delamination in APS applied thermal barrier coatings: life modelling
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thermal barrier coatings, TBCs, are used in gas turbines as a thermal shield resulting in lower temperature in coated components. The decrease of temperature allows higher gas temperatures in the turbine, which increase the efficiency. The bimaterial construction with an outer ceramic layer applied onto a metallic material give rice to problems during thermal cycling. Thermal induced stresses will gradually break down the coating. The ceramic layer will delaminate from the substrate, resulting in spallation, and the component will break down due to overheating.

The delamination process is investigated in this thesis by finite element simulations. The growth of an internal alumina layer in the top/bond coat interface is investigated by 3D finite element simulations which show that the local stress state change in such way that the alumina growth help nucleation and growth of small delamination cracks. Finite element simulations, in which t he energy release rate and stress intensity factors are calculated, investigate the growth of small delamination cracks in or close to the top/bond coat interface. Experiments show that these cracks grow parallel to or in the sinusoidal top/bond coat interface and the results of the simulations show that the mode mixity changes as the delamination cracks grow.

A new delamination life model is proposed which is based on results of the fracture mechanical simulations and experimental observations. The model predicts the growth of small cracks in the TBC before they form a large delamination crack. The model is based on a modified Paris law where a mode mixity dependence on the crack growth rate is included, meaning lower crack growth rate in mode 2 load compared with mode 1. Parameters of the model are obtained by optimisation of the model against experimental data, describing the delamination damage evolution in the TBC. The data are obtained from interrupted thermal cycling tests and the prediction of the model corresponds well with these data.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2004. 20 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 902
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-24067 (URN)3627 (Local ID)91-85295-55-8 (ISBN)3627 (Archive number)3627 (OAI)
Public defence
2004-10-27, Sal C3, Fysikhuset, Linköpings Universitet, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-01-23

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Jinnestrand, MagnusSjöström, Sören

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