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Aspects of fatigue life in thermal barrier coatings
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2001 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Thermal barrier coatings (TBC) are applied on hot components in airborne and land-based gas turbines when higher turbine inlet temperature, meaning better thermal efficiency, is desired. The TBC is mainly applied to protect underlying material from high temperatures, but also serves as a protection from the aggressive corrosive environment.

Plasma sprayed coatings are often duplex TBC's with an outer ceramic top coat (TC) made from partially stabilised zirconia - ZrO2 + 6-8% Y2O3. Below the top coat there is a metallic bond coat (BC). The BC is normally a MCrAlX coating (M=Ni, Co, Fe ... and X=Y, Hf, Si ... ). In gas turbine components exposed to elevated temperatures nickel-based superalloys are commonly adopted as load carrying components. In the investigations performed here a commercial wrought Ni-base alloy Haynes 230 has been used as substrate for the TBC. As BC a NiCoCrAlY serves as a reference material and in all cases 7% yttria PS zirconia has been used. Phase development and failure mechanisms in APS TBC during service-like conditions have been evaluated in the present study. This is done by combinations of thermal cycling and low cycle fatigue tests. The aim is to achieve better knowledge regarding how, when and why thermal ban'ier coatings fail. As a fmal outcome of the project a model capable of predicting fatigue life of a given component will help engineers and designers of land based gas turbines for power generation to better optimise TBC's.

In the investigations it is seen that TBC life is strongly influenced by oxidation of the BC and interdiffusion between BC and the substrate. The bond coat is known to oxidise with time at high temperature. The initial oxide found during testing is alumina. With increased time at high temperature Al is depleted from the bond coat due to interdiffusion and oxidation. Oxides others than alumina start to form when the Al content is reduced below a critical limit. It is here believed that spinel appears when the Al content is lowered below 2w/o in the bond coat. Here it was shown that a faster growing oxide, rich in Ni, Cr and Co forms at the interface. Al depletion is also linked to BC phases. Initially the bond coat is a γ/ß-material possibly with very fine dispersed γ'. Simultaneously with Al-depletion the ß-phase is found to disappear. This occurs simultaneously with the formation of spinel. However, oxidation is not only a disadvantage. Low cycle fatigue tests reveal that oxide streaks within the bond coat will slow down crack growth due to crack deflection and crack branching. Therefore benefit of or damage from oxide growth on crack initiation and propagation is dependent on crack mode, spalling of the ceramic TC or growth of "classic" cracks perpendicular to the surface.

From the observations conclusions are drawn regarding fatigue behaviour ofTBC systems. The basic idea is that all cracks leading to failure initiate in the thermally grown oxide (TGO). Following the initiation, they can, however, grow to form either delamination cracks leading to top coat spallation or cracks transverse to the surface leading to component failure.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2001. , 50 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 898
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-30049Local ID: 15509ISBN: 91-7373-085-8 (print)OAI: oai:DiVA.org:liu-30049DiVA: diva2:250870
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28
List of papers
1. Behaviour of a Thermal Barrier Coating during High Temperature Oxidation
Open this publication in new window or tab >>Behaviour of a Thermal Barrier Coating during High Temperature Oxidation
(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 NiCoCrAlY 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 Al the oxides contain Ni, Cr and Co. The oxides tend to grow with different rates depending on the macroscopic surface geometry. In the study convex surfaces reveal the highest oxide growth rates and concave the lowest growth rates. At 1000 hrs and 1000°C the difference between the fastest and the slowest growing oxide layer is 1 μm. 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-102045 (URN)
Available from: 2013-11-28 Created: 2013-11-28 Last updated: 2013-11-28
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. Bond Coat Influence on TBC Life
Open this publication in new window or tab >>Bond Coat Influence on TBC Life
(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 I 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, NiCoCrAIY, CoNiCrAIY, alumina, thermal fatigue, oxide growth, interdiffusion
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-102046 (URN)
Available from: 2013-11-28 Created: 2013-11-28 Last updated: 2013-11-28

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