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Evolution of thermal conductivity of dysprosia stabilised thermal barrier coating systems during heat treatment
University West, Department of Engineering Science. (Thermal Spray, PTW)ORCID iD: 0000-0003-0209-1332
University of Manchester.
2012 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, ISSN 0257-8972, Vol. 209, 38-43 p.Article in journal (Refereed) Published
Abstract [en]

Dysprosia stabilised zirconia coatings offer a potential reduction in thermal heat transfer for thermal barrier coating systems with the added benefit of being producible with existing equipment and spray knowledge. However, there is little information on the long term performance of such systems relative to the standard coatings. While a low thermal conductivity is important for a gas turbine; sintering resistance is important to maintain properties over the lifetime of a component.

In this study, four dysprosia stabilised zirconia coatings are compared with a standard yttria stabilised coating in present industrial use.

Samples were exposed to isothermal furnace conditions at 1150 °C from 5 to 200 hours to observe the sintering resistance of the coating systems. Tests carried out include microstructural analysis with SEM, thermal conductivity measurements using laser flash analysis and porosity changes monitored using image analysis.

Place, publisher, year, edition, pages
2012. Vol. 209, 38-43 p.
Keyword [en]
Thermal barrier coating; Atmospheric plasma spraying; Thermal conductivity; Sintering
National Category
Materials Engineering
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-4692DOI: 10.1016/j.surfcoat.2012.08.018ISI: 000310656200006Scopus ID: 2-s2.0-84866883350OAI: oai:DiVA.org:hv-4692DiVA: diva2:557114
Available from: 2012-09-27 Created: 2012-09-27 Last updated: 2015-06-24Bibliographically approved
In thesis
1. Design of Thermal Barrier Coating Systems
Open this publication in new window or tab >>Design of Thermal Barrier Coating Systems
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Thermal barrier coatings (TBC’s) are used to provide both thermal insulation and oxidation protection to high temperature components within gas turbines. The development of turbines for power generation and aviation has led to designs where the operation conditions exceed the upper limits of most conventional engineering materials. As a result there has been a drive to improve thermal barrier coatings to allow the turbine to operate hotter for longer.

The focus of this study has been the development of a new generation of TBC system for industrial implementation. The goal for these new coatings was to achieve lower conductivity and longer lifetime than those coatings used today. The route taken to achieve these goals has been twofold. Firstly an alternative stabiliser has been chosen for the zirconium oxide system in the form of dysprosia. Secondly, Control of the powder morphology and spray parameters has been used to generate coating microstructures with favourable levels of porosity.

Samples have been heavily characterised using the laser flash technique for evaluation of thermal properties. Measurements were performed at room temperature and at intervals up to 1200°C. Samples have also been tested in their as produced state and after heat treatments of up to 200 hours.

Lifetime evaluation has been performed using the thermo-cyclic fatigue test to expose coating systems to successive cycles of heating and cooling combined with oxidation of the underlying metallic coating.

Microstructures have been prepared and analysed using SEM. An image analysis routine has been used to attempt to quantify changes in microstructure features between coating types or coating exposure times and to relate those changes to changes in thermal properties

Results show that dysprosia as an alternative dopant gives a reduction in thermal conductivity. While small at room temperature and in the as produced state; the influence becomes more pronounced at high temperatures and with thermal exposure time. Overall, the greatest sustained influence on thermal conductivity has been from creating coatings with high levels of porosity.

In relation to lifetime, the target of double the thermo-cyclic fatigue life was achieved using a coating with engineered porosity. Introducing a polymer to the spray powder helps to generate large globular pores within the coating together with a large number of delaminations. Such a structure has shown to be highly resistant to TCF testing.

Place, publisher, year, edition, pages
Gothenburg: Chalmers University of Technolgy, 2012. 75 p.
Keyword
Thermal Barrier Coating, Atmopsheric Plasma Spraying, Thermal Conductivity, Thermo-cyclic fatigue
National Category
Materials Engineering
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-4898 (URN)
Presentation
2012-06-08, C118, University West, Gustava Melins Gata 2, Trollhättan, 14:27 (English)
Opponent
Supervisors
Available from: 2012-12-20 Created: 2012-12-17 Last updated: 2015-10-02Bibliographically approved
2. Design of Thermal Barrier Coating Systems
Open this publication in new window or tab >>Design of Thermal Barrier Coating Systems
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thermal barrier coatings (TBC’s) are used to provide both thermal insulation and oxidation protection to high temperature components within gas turbines. The development of turbines for power generation and aviation has led to designs where the operation conditions exceed the upper limits of most conventional engineering materials. As a result there has been a drive to improve thermal barrier coatings to allow the turbine to operate at higher temperatures for longer.

The focus of this thesis has been to design thermal barrier coatings with lower conductivity and longer lifetime than those coatings used in industry today. The work has been divided between the development of new generation air plasma spray (APS) TBC coatings for industrial gas turbines and the development of suspension plasma spray (SPS) TBC systems.

The route taken to achieve these goals with APS TBC’s has been twofold. Firstly an alternative stabiliser has been chosen for the zirconium oxide system in the form of dysprosia. Secondly, control of the powder morphology and spray parameters has been used to generate coating microstructures with favourable levels of porosity.

In terms of development of SPS TBC systems, these coatings are relatively new with many of the critical coating parameters not yet known. The focus of the work has therefore been to characterise their lifetime and thermal properties when produced in a complete TBC system.

Results demonstrate that dysprosia as an alternative stabiliser gives a reduction in thermal conductivity. While small at room temperature and in the as produced state; the influence becomes more pronounced at high temperatures and with longer thermal exposure time. The trade-off for this lowered thermal conductivity may be in the loss of high temperature stability. Overall, the greatest sustained influence on thermal conductivity has been from creating coatings with high levelsof porosity.

In relation to lifetime, double the thermo-cyclic fatigue (TCF) life relative to the industrial standard was achieved using a coating with engineered porosity. Introducing a polymer to the spray powder helps to generate large globular pores within the coating together with a large number of delaminations. Such a structure was shown to be highly resistant to TCF testing.

SPS TBC’s were shown to have much greater performance relative to their APS counterparts in thermal shock life, TCF life and thermal conductivity. Columnar SPS coatings are a prospective alternative for strain tolerant coatings in gas turbine engines.

Place, publisher, year, edition, pages
Trollhättan: University West, 2014. 109 p.
Series
PhD Thesis: University West, 3
Keyword
Industrial Gas Turbine; Aero Turbine; Thermal Barrier Coating; Atmospheric Plasma Spraying; Suspension Plasma Spraying; Thermal Conductivity; Thermo-cyclic fatigue; Thermal Shock
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-5931 (URN)978-91-977943-9-8 (ISBN)978-91-977943-8-1 (ISBN)
Public defence
2014-02-28, F127, University West, Gustava Melins Gata 2, Trollhättan, 09:53 (English)
Opponent
Supervisors
Available from: 2014-02-19 Created: 2014-02-19 Last updated: 2015-06-24Bibliographically approved

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Publisher's full textScopushttp://www.sciencedirect.com/science/article/pii/S0257897212007931?v=s5

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