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Initiation and growth of delamination cracks in vacuum plasma sprayed thermal barrier coatings
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2004 (English)In: Surface Engineering 2004: Proceedings of the 3rd International Surface Engineering Congress (ASM International), 2004, 284-290 p.Conference paper, Published paper (Refereed)
Abstract [en]

Thermal barrier coatings are widely used in air-borne and land-based gas turbines. In these applications they serve as thermal insulators in hot components (burner cans/liners, turbine vanes and turbine blades). The present paper is aimed to describe the failure mechanism of a vacuum plasma sprayed thin thermal barrier coating system. Also the coating degradation mechanism (delamination followed by spallation) in terms of interfacial crack growth data is investigated and presented. In the present paper a 200 µm thick NiCrAlY bond coat was chosen together with a 350µm thick 7wt% yttria stabilised zirconia top coat. The coating system was exposed to thermal cyclic fatigue with a thermal cycle ranging from 100 °C to 1100 °C and a cycle time of 70 minutes. After thermal cyclic testing all specimen were subjected to standard sample preparation routines and inspected by optical microscopy and scanning electron microscopy. In order to describe interfacial crack growth an interface damage measure is used. For the present coating system under current thermal load conditions a mainly black interface fracture is discovered. The top coat exhibit microcrack formation after thermal fatigue, but these cracks do not contribute to the final fracture appearance. Crack growth rates are compared to local stress intensity levels at the top coat / bond coat interface. From crack length measurements are crack growth data da/dN = f(ΔKeff) calculated.

Place, publisher, year, edition, pages
2004. 284-290 p.
Keyword [en]
thermal barrier coating, crack growth, delamination, interface, NiCrAlY
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-22779Local ID: 2110ISBN: 0871708191 (print)ISBN: 978-0871708199 OAI: oai:DiVA.org:liu-22779DiVA: diva2:243092
Conference
3rd International Surface Engineering Conference, Orlando FL, August 2-4 2004
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-01-31
In thesis
1. 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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