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Failure of thermal barrier coatings under thermal and mechanical fatigue loading: microstructural observations and modelling aspects
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
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 [en]
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: urn:nbn:se:liu:diva-22793Local ID: 2126ISBN: 91-852-9540-X (print)OAI: oai:DiVA.org:liu-22793DiVA: diva2:243106
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
List of papers
1. Influence of high temperature exposure on thermal barrier coating behaviour
Open this publication in new window or tab >>Influence of high temperature exposure on thermal barrier coating behaviour
(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
APS, thermal barrier coating, oxidation, growth rate, NiCoCrAlY, geometry
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-88205 (URN)
Available from: 2013-01-31 Created: 2013-01-31 Last updated: 2013-01-31
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. The influence of oxidation on mechanical and fracture behaviour of an air plasma-sprayed NiCoCrAlY bondcoat
Open this publication in new window or tab >>The influence of oxidation on mechanical and fracture behaviour of an air plasma-sprayed NiCoCrAlY bondcoat
2004 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 187, no 1, 113-121 p.Article in journal (Refereed) Published
Abstract [en]

The influence of isothermal oxidation on room-temperature mechanical and fracture behaviour of an air plasma-sprayed Ni-23Co-17Cr-12Al-0.5Y bondcoat was investigated by the miniaturised disc bending test (MDBT) technique. Disc specimens were extracted from the bondcoat region of both as-received and oxidised thermal barrier coating (1000 °C, 1000 h). Microstructure analysis revealed that the non-oxidised bondcoat consisted mainly of γ-phase (Ni-structure) and β-NiAl. After 500 h of oxidation no NiAl remained in the bondcoat, an effect due to internal as well as external oxidation of Al. The former resulted in the formation of an extensive oxide network and the latter in the formation of an oxide scale between the topcoat and the bondcoat. The crack propagation behaviour of the bondcoat, both in non-oxidised and oxidised condition can be characterised as intergranular with stable growth. The crack propagation resistance is substantial due to the lamellar grain (splat) orientation and the extensive intergranular oxide network, acting as crack deflection and crack branching mechanisms. As an effect of oxidation, crack propagation resistance of the bondcoat increases but the strain to crack initiation decreases.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-22777 (URN)10.1016/j.surfcoat.2003.12.021 (DOI)2108 (Local ID)2108 (Archive number)2108 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2017-12-13
4. Bond coat influence on thermal fatigue behaviour of thermal barrier coatings
Open this publication in new window or tab >>Bond coat influence on thermal fatigue behaviour of thermal barrier coatings
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In the present study the influence of bond coat composition and coating process on thermal barrier coating (TBC) life has been evaluated. Six different coatings have been subjected to thermal cycling between 100 and 1100°C. After this the different systems have been characterised by light microscopy and SEM-EDS. Various commercial bond coats have been applied on one substrate material (Haynes alloy 230). The total fatigue life of the different TBC systems varies with 30% what is believed to be influenced by diffusion and oxidation phenomena in the bond coat. It is found that when the aluminium concentration is decreased phases other than alumina form at the top coat / bond coat interface. Oxides formed during later stages of the thermal cyclic test are rich in nickel, cobalt and chromium and the results can be interpreted as formation of nickel-, chromium- or spinel oxides. The reason for TBC failure is coupled to aluminium depletion, which here is believed to be due to inward diffusion and formation of thermally grown oxides (TGO) at the ceramic top coat (TC) metallic bond coat (BC) interface as well as growth of internal oxides in the bond coat.

Keyword
thermal barrier coating, NiCoCrAlY, CoNiCrAlY, alumina, thermal fatigue, oxide growth, interdiffusion
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-88206 (URN)
Available from: 2013-01-31 Created: 2013-01-31 Last updated: 2013-01-31
5. 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
6. Initiation and growth of delamination cracks in vacuum plasma sprayed thermal barrier coatings
Open this publication in new window or tab >>Initiation and growth of delamination cracks in vacuum plasma sprayed thermal barrier coatings
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.

Keyword
thermal barrier coating, crack growth, delamination, interface, NiCrAlY
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-22779 (URN)2110 (Local ID)0871708191 (ISBN)978-0871708199 (ISBN)2110 (Archive number)2110 (OAI)
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

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