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Influence of high temperature exposure on thermal barrier coating behaviour
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

An air plasma sprayed thermal barrier coating was investigated in order to clarify links between heat treatment, oxidation and diffusion behaviour. In the study a thin Zirconia (PSZ) layer was used as top coat together with a NiCoCrAIY bond coat. The investigation was focused on differences for three geometries. Thermal barrier coatings on flat, concave and convex surfaces were studied. Isothermal oxidation was performed up to 1000 hrs at 1000°C in order to simulate true working conditions for the interface between ceramic top coat and metallic bond coat. The investigations show presence of Al-rich oxides for shorter times. When the coating system is heat-treated for 1000 hrs a change of oxide composition is obvious and beside AI the oxides contain Ni, Cr and Co. The oxides tend to grow with different rates depending on the macroscopic surface geometry. In the study concave surfaces reveal the highest oxide growth rates and convex the lowest growth rates. At 1000 hrs and 1 000°C the difference between the fastest and the slowest growing oxide layer is 1µm. Some interdiffusion is obvious. Between the superalloy substrate and the bond coat outward diffusion of Ni, W and Cr is present together with inward diffusion of Co and to some extent Al.

Keyword [en]
APS, thermal barrier coating, oxidation, growth rate, NiCoCrAlY, geometry
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-88205OAI: oai:DiVA.org:liu-88205DiVA: diva2:602094
Available from: 2013-01-31 Created: 2013-01-31 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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Brodin, HåkanEricsson, Torsten

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