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Failure mechanisms in APS and SPS thermal barrier coatings during cyclic oxidation and hot corrosion
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Thermal Barrier Coatings (TBCs) are advanced material systems that are being used in the hot sections of gas turbines such as combustor, turbine blades, and vanes. The top ceramic coating in TBCs provides insulation against the hot gases and the intermediate metallic bond coat provides oxidation and corrosion resistance to the underlying turbine components.

Durability of thermal barrier coatings is very important for the overall performance of the gas turbine. TBCs can fail in several different ways and there is a combination of more than one failure mechanism in most situations. One of the most widely used TBC is atmospheric plasma sprayed (APS) yttria stabilized zirconia (YSZ). Both the deposition technique and the TBC material have certain limitations. The main aim of this research is to study new TBC materials and/or new deposition techniques and compare with the conventional YSZ and understand their failure mechanisms during cyclic oxidation and hot corrosion.

Thermal cyclic oxidation of a newly developed high purity nano YSZ thermal barrier coating has been studied. Cross sectional analysis of exposed as well as completely failed samples showed a mixed-type failure caused by crack propagation parallel to the bond coat/top coat interface. The majority of the damage occurred towards the end of the coating life. A finite element model has been developed to study the probability of crack growth along different paths that leads to the final failure.

Hot corrosion mechanism in suspension plasma sprayed two-layer gadolinium zirconate/YSZ, three-layer dense gadolinium zirconate/gadolinium zirconate/YSZ, and a single-layer YSZ has been studied in the presence of sodium sulfate and vanadium pentoxide. The test results showed that gadolinium zirconate coatings were more susceptible to corrosion compared to YSZ coatings despite gadolinium zirconate coatings having lower reactivity with the corrosive salts.

Thermal cycling behavior of a high chromium bond coat has been studied. Cross-sectional analysis showed formation of sandwich type microstructure with chromium rich oxide and alumina as the top and the bottom layers.

Inter-diffusion of minor elements between different MCrAlY coatings – substrate systems has been studied using, diffusion simulation software, DICTRA. The simulation results showed that the diffusion of minor elements in the coatings is dependent on the rate of β phase depletion in the beginning. After the depletion of β phase there was no clear dependence of the coating composition on the diffusion of minor elements.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. , p. 45
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1770
National Category
Manufacturing, Surface and Joining Technology Corrosion Engineering Materials Chemistry Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:liu:diva-134379DOI: 10.3384/lic.diva-134379ISBN: 9789176855942 (print)OAI: oai:DiVA.org:liu-134379DiVA, id: diva2:1072644
Presentation
2017-03-10, A37, A-huset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-02-27Bibliographically approved
List of papers
1. Hot Corrosion Mechanism in Multi-Layer Suspension Plasma Sprayed Gd2Zr2O7 /YSZ Thermal Barrier Coatings in the Presence of V2O5 + Na2SO4
Open this publication in new window or tab >>Hot Corrosion Mechanism in Multi-Layer Suspension Plasma Sprayed Gd2Zr2O7 /YSZ Thermal Barrier Coatings in the Presence of V2O5 + Na2SO4
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2017 (English)In: Journal of thermal spray technology (Print), ISSN 1059-9630, E-ISSN 1544-1016, Vol. 26, no 1, p. 140-149Article in journal (Refereed) Published
Abstract [en]

This study investigates the corrosion resistance of two-layer Gd2Zr2O7/YSZ, three-layer dense Gd2Zr2O7/ Gd2Zr2O7/YSZ, and a reference single-layer YSZ coating with a similar overall top coat thickness of 300-320 µm. All the coatings were manufactured by suspension plasma spraying resulting in a columnar structure except for the dense layer. Corrosion tests were conducted at 900 °C for 8 h using V2O5 and Na2SO4 as corrosive salts at a concentration of approximately 4 mg/cm2. SEM investigations after the corrosion tests show that Gd2Zr2O7-based coatings exhibited lower reactivity with the corrosive salts and the formation of gadolinium vanadate (GdVO4), accompanied by the phase transformation of zirconia was observed. It is believed that the GdVO4 formation between the columns reduced the strain tolerance of the coating and also due to the fact that Gd2Zr2O7 has a lower fracture toughness value made it more susceptible to corrosion-induced damage. Furthermore, the presence of a relatively dense layer of Gd2Zr2O7 on the top did not improve in reducing the corrosion-induced damage. For the reference YSZ coating, the observed corrosion-induced damage was lower probably due to combination of more limited salt penetration, the SPS microstructure and superior fracture toughness of YSZ.

Place, publisher, year, edition, pages
New York: Springer, 2017
Keywords
gadolinium zirconatehot corrosionmulti-layer thermal barrier coatingssuspension plasma sprayingvanadium pentoxide + sodium sulfate
National Category
Corrosion Engineering Manufacturing, Surface and Joining Technology Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-134375 (URN)10.1007/s11666-016-0486-5 (DOI)000392060300014 ()
Note

Funding agencies: Vinnova in Sweden

Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-11-29Bibliographically approved
2. Thermal fatigue failure of thermal barrier coatings with a high-Cr MCrAIY bond coat
Open this publication in new window or tab >>Thermal fatigue failure of thermal barrier coatings with a high-Cr MCrAIY bond coat
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2016 (English)In: Proceedings of the International Thermal Spray Conference (ITSC), 2016, Vol. 324, p. 273-278Conference paper, Published paper (Refereed)
Abstract [en]

Thermal barrier coatings (TBCs) were air-plasma sprayed onto Hastelloy X substrates. The TBCs consisted of a high-Cr MCrAlY (M for Ni and Co) bond coat and a yttria-stabilized zirconia (YSZ) top coat. The TBC samples were thermally cycled between 100 ºC and 1100 ºC with 1 hour dwell time at 1100 ºC. The thermal fatigue failure of the TBCs was investigated via microstructure analyses. The final fatigue failure of the TBCs was caused by the formation of interface-parallel cracks in the YSZ top coat. The formation of the cracks was found to be strongly related to the oxidation behaviour of the MCrAlY bond coat. The development of the oxide layers was therefore studied in detail. A thermokinetic model was also used to deepen the understanding on the elemental diffusion behavior in the materials.

National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials Materials Chemistry Composite Science and Engineering
Identifiers
urn:nbn:se:liu:diva-134377 (URN)
Conference
International Thermal Spray Conference (ITSC), May 10-12, 2016, China
Available from: 2017-02-08 Created: 2017-02-08 Last updated: 2017-02-16Bibliographically approved

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