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Fatigue life prediction of a plasma sprayed thermal barrier coating system
Linköping University, Department of Mechanical Engineering. Linköping University, The Institute of Technology.
Linköping University, Department of Mechanical Engineering. Linköping University, The Institute of Technology.
(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 [en]
Thermal barrier coating, delamination, fatigue, fracture mechanics, modelling, crack growth
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-24277Local ID: 3886OAI: oai:DiVA.org:liu-24277DiVA: diva2:244594
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-01-31
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
2. Failure of thermal barrier coatings under thermal and mechanical fatigue loading: microstructural observations and modelling aspects
Open this publication in new window or tab >>Failure of thermal barrier coatings under thermal and mechanical fatigue loading: microstructural observations and modelling aspects
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Industrial and air-borne gas turbine hot components suffer from creep, oxidation, corrosion and microstructural degradation if not shielded from the hot and aggressive combustion gases. Two major strategies commercially available are adopted; film cooling by pressurised air and application of protective coatings. Protective coatings form a slow-growing oxide that protects from oxidation and corrosion. By application of a thermal insulator, a thermal barrier coating, the material will be protected from high temperature through good insulation properties of the coating system.

If thermal barrier coatings are to be used in situations where capabilities and possibilities for inspections are limited, better knowledge of the fatigue properties of the coatings is also needed. Therefore development of a reliable fatigue life model is needed. The present work aims at serving as a basis from which a general physically founded thermal barrier coating life model can be formulated. The effects of exposure to high temperatures and mechanical loads on thermal barrier coatings under service like conditions have been investigated in the present thesis. Emphasis is put on the coupling between materials science and solid mechanics approaches in order to establish a better knowledge concerning degradation mechanisms and fatigue life issues than what is common if only one discipline is explored.

Investigations of material exposed to isothermal oxidation and thermal cyclic fatigue were performed on plasma-sprayed systems with NiCoCrAIY or NiCrAIY bond coats and yttria partially stabilised zirconia top coats. It has been shown that the thermally grown oxide that will form upon high temperature exposure influences the failure behaviour. If the oxide is composed mainly of alumina, the fatigue properties are good since the adhesion between the ceramic top coat and the metallic bond coat is good. This is also shown in a comparison between different plasma sprayed thermal barrier coating systems. If the oxide formed is based on alurnina and spinel is avoided the fatigue properties benefit from a relatively flat interface where out-of plane stresses are low in comparison to a rough interface between top- and bond coat. These findings indicate that the bonding in air-plasma sprayed systems is dependent on so called chemical bonding if the thermally grown oxide is not voluminous with high growth stresses.

It is possible to establish a fatigue life model for thermal barrier coatings. This has been shown with a model based on a modified Paris law formulation. The formulation needs to be modified with regards to mode rnixity of growth. Results achieved in the present project show that it is possible to extract crack growth data for interfacial crack growth. However a combination of mechanical testing and finite element modelling is needed since the load situation in critical areas cannot be measured. Crack growth results are presented and crack growth data are compared to predictions with good agreement.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2004. 73 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 898
Keyword
thermal barrier coating, TBC, delamination, crack initiation, crack propagation, crack growth, oxidation, alumina, spinel, MCrAIY, diffusion, fatigue, modelling, modeling, degradation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-22793 (URN)2126 (Local ID)91-852-9540-X (ISBN)2126 (Archive number)2126 (OAI)
Public defence
2004-10-29, Sal C3, Hus C, Linköpings Universitet, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-01-31

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Brodin, HåkanJinnestrand, Magnus

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