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Delamination in APS applied thermal barrier coatings: life modelling
Linköping University, Department of Mechanical Engineering, Solid Mechanics. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-24067Local ID: 3627ISBN: 91-85295-55-8 (print)OAI: oai:DiVA.org:liu-24067DiVA: diva2:244383
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
List of papers
1. Investigation by 3D FE simulations of delamination crack initiation in TBC caused by alumina growth
Open this publication in new window or tab >>Investigation by 3D FE simulations of delamination crack initiation in TBC caused by alumina growth
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.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-47480 (URN)10.1016/S0257-8972(00)01084-7 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
2. Crack initiation and propagation in air plasma sprayed thermal barrier coatings, testing and mathematical modelling of low cycle fatigue behaviour
Open this publication in new window or tab >>Crack initiation and propagation in air plasma sprayed thermal barrier coatings, testing and mathematical modelling of low cycle fatigue behaviour
2004 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 379, no 1-2, 45-57 p.Article in journal (Refereed) Published
Abstract [en]

In the present paper failure mechanisms in air plasma sprayed thermal barrier coatings for land-based gas turbines have been studied. This has been done by finite element simulations and fractographic investigations of low cycle fatigue (LCF) tested material, here chosen as an 350 μm thick partially stabilised zirconia top coat (TC) together with a 150 μm thick Ni-Co-Cr-Al-Y bond coat (BC) on a nickel base substrate (Haynes 230). Both LCF testing, modelling results and fractographic investigations point in the same direction. An increased thickness of the thermally grown oxide (TGO) does decrease the LCF life of a coated structural alloy. Several points of crack initiation were found, in the TGO at the TC/BC interface, at the oxide network within the BC and at oxide inclusions between BC and substrate. During LCF tests the initiated cracks will grow radially into the substrate material. The behaviour is explained by increased TC/BC delamination stresses and changed oxidation behaviour with increased oxidation times.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-22790 (URN)10.1016/j.msea.2003.12.063 (DOI)2123 (Local ID)2123 (Archive number)2123 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2017-12-13
3. Fracture mechanics analysis of an APS applied TBC
Open this publication in new window or tab >>Fracture mechanics analysis of an APS applied TBC
(English)Manuscript (preprint) (Other academic)
Abstract [en]

An air plasma sprayed thermal barrier coating is investigated by fracture-mechanical analysis. The virtual crack extension method is used to calculatethe energy release rate, G, and the displacements of the crack surfaces are used to split the energy release rate into mode 1 and 2 stress intensity factors, K1 and K2 . The results of the investigation are evolutions of G, K1 and K2 as functions of delamination crack length for small delamination cracks.

An approximation of a hot spot is investigated by applying an out-of-phase load at room temperature. The results show that the out-of-phase load results in a more open crack compared with only a thermal load on the TBC system. This can explain why the out-of-phase load is dangerous.

The results show that it might be possible to base a delamination life model on fracture-mechanical data of an interface crack in the top/bond coat interface. If so, it is important that the mode mixity effects on crack growth rate is included in the life model.

Keyword
Thermal barrier coating, Virtual crack extension, Interface crack, Crack initiation, Life modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87826 (URN)
Available from: 2013-01-23 Created: 2013-01-23 Last updated: 2013-01-23
4. Fatigue life prediction of a plasma sprayed thermal barrier coating system
Open this publication in new window or tab >>Fatigue life prediction of a plasma sprayed thermal barrier coating system
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Thermal barrier coatings are commonly used in gas turbines for power generation. One major issue in TBC design is how to determine fatigue life in these material systems. The present paper aims at presenting a model for determination of TBC life based on the behavior of an air plasma sprayed coating system. This is done by analysis of fracture behavior and evaluation of data from thermal fatigue tests. The knowledge regarding fracture behavior in thermal fatigue tests is used as an input to the modelling work. For formulation of the fatigue life model, a Paris law approach has been used. FE calculations are used to obtain energy release rate and stress intensity factors, KI and KII, for a propagating delamination crack in the top/ bond coat interface. As a measure of failure, a delamination damage measure is used. A method for determination of delamination crack growth data is presented.

Keyword
Thermal barrier coating, delamination, fatigue, fracture mechanics, modelling, crack growth
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-24277 (URN)3886 (Local ID)3886 (Archive number)3886 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-01-31

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