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Behaviour of a Thermal Barrier Coating during High Temperature Oxidation
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 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 [en]
APS, thermal barrier coating, oxidation, growth rate, NiCoCrAlY, geometry
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-102045OAI: oai:DiVA.org:liu-102045DiVA: diva2:667991
Available from: 2013-11-28 Created: 2013-11-28 Last updated: 2013-11-28
In thesis
1. Aspects of fatigue life in thermal barrier coatings
Open this publication in new window or tab >>Aspects of fatigue life in thermal barrier coatings
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:nbn:se:liu:diva-30049 (URN)15509 (Local ID)91-7373-085-8 (ISBN)15509 (Archive number)15509 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28

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Brodin, HåkanEricsson, Torsten

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