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Functional Performance of Gadolinium Zirconate/Yttria Stabilized Zirconia Multi-Layered Thermal Barrier Coatings
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)ORCID iD: 0000-0003-2475-9284
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Yttria stabilized zirconia (YSZ) is the state-of-the-art ceramic top coat material used for thermal barrier coating (TBC) applications. Demand for higher efficiency in gas turbine engines has led to a continuous increase in the gas in let temperature. However, this increase in temperature has pushed YSZ to its upper limit. Above1200 °C, issues such as poor phase stability, high sintering rate, and susceptibility to CMAS (Calcium Magnesium Alumino Silicates) degradation limit the durability of YSZ based TBCs. Among the new top coat materials suggested for high temperature TBC applications, gadolinium zirconate (GZ) is an interesting alternative to YSZ since it has shown attractive properties which include a better resistance to CMAS attack. However, GZ has poor thermo-chemical compatibility with the thermally grown oxide (alumina), leading to poor thermalcyclic fatigue performance of single layered GZ TBCs. Therefore, a multi-layered GZ/YSZ based TBC design seems promising. This work presents a new approach of depositing multi-layered (double and triple layered) GZ/YSZ TBCs using the recently developed suspension plasma spray(SPS) process. SPS was employed in this work because of its capability to mimic the electron beam physical vapour deposition (EB-PVD) process in terms of producing columnar microstructured TBCs. Single layer YSZ TBCs were also deposited by SPS process and used as a reference for comparing the functional performance of multi-layered GZ based TBCs. The primary aim of this work was to improve the durability of GZ based multilayered TBCs at high temperatures. Durability tests were performed in the temperature range 1100 °C - 1400 °C under different thermal cyclic test conditions (with and without thermal gradient). The results indicate that multilayered GZ based TBCs improve durability compared to the single layer YSZTBCs at all the test temperatures. Failure analysis of the multi-layered GZ/YSZTBCs revealed spallation within the GZ layer close to GZ/YSZ interface and the reason was believed to be the inferior fracture toughness of GZ. In order to improve the fracture toughness in the region of failure, a composite approach comprising multi-layered GZ+YSZ based TBC was considered. It was shown that the composite GZ+YSZ based TBCs did not improve the thermal cyclic lifetime, although improvement in fracture toughness was observed. As a further extension of this work, the influence of YSZ layer thickness on the durability of GZ/YSZTBCs was investigated. It was shown that an increase in YSZ layer thickness in the GZ/YSZ TBC led to poor durability. Additionally, the other important performance criteria for TBCs, i.e. thermal conductivity, was measured experimentally and compared with the single layer YSZ TBC. It was shown that the GZ based TBCs had lower thermal conductivity than YSZ. The second aim was to investigate and compare the erosion performance of multi-layered GZ based TBCs and single layered YSZ TBCs. In the erosion testconducted at room temperature, the GZ based TBCs showed lower erosion resistance compared to the single layer YSZ TBC. The main reason for this difference was attributed to the inferior fracture toughness of GZ. In case of the composite multi-layered GZ+YSZ based TBC, an improvement in erosion resistance was observed compared to the multi-layered GZ based TBC. Based on the results obtained, this work has demonstrated that SPS is a promising processing technique to produce columnar microstructured TBCs irrespective of the composition (GZ, YSZ, GZ+YSZ). It was also shown that GZ/YSZ multilayered TBCs are promising for high temperature TBC applications due to theirl ow thermal conductivity and high thermal cyclic fatigue lifetime. However, low erosion resistance for certain applications might be an issue for the GZ basedTBCs.

Place, publisher, year, edition, pages
Trollhättan: University West , 2018. , p. 82
Series
PhD Thesis: University West ; 18
Keywords [en]
Composite; Erosion; Gadolinium Zirconate; Suspension Plasma Spray; Thermal Barrier Coatings; Thermal Cyclic Test; Thermal Conductivity; Yttria Stabilized Zirconia.
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-12301ISBN: 978-91-87531-86-6 (print)ISBN: 978-91-87531-85-9 (electronic)OAI: oai:DiVA.org:hv-12301DiVA, id: diva2:1206045
Public defence
2018-06-05, F104, Trollhättan, 10:15 (English)
Opponent
Supervisors
Available from: 2018-05-16 Created: 2018-05-15 Last updated: 2018-05-16Bibliographically approved
List of papers
1. Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray
Open this publication in new window or tab >>Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray
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2017 (English)In: Journal of thermal spray technology (Print), ISSN 1059-9630, E-ISSN 1544-1016, Vol. 26, no 1-2, p. 108-115Article in journal (Refereed) Published
Abstract [en]

7-8 wt.% Yttria-stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used by the gas turbines industry due to its excellent thermal and thermo-mechanical properties up to 1200 °C. The need for improvement in gas turbine efficiency has led to an increase in the turbine inlet gas temperature. However, above 1200 °C, YSZ has issues such as poor sintering resistance, poor phase stability and susceptibility to calcium magnesium alumino silicates (CMAS) degradation. Gadolinium zirconate (GZ) is considered as one of the promising top coat candidates for TBC applications at high temperatures (>1200 °C) due to its low thermal conductivity, good sintering resistance and CMAS attack resistance. Single-layer 8YSZ, double-layer GZ/YSZ and triple-layer GZdense/GZ/YSZ TBCs were deposited by suspension plasma spray (SPS) process. Microstructural analysis was carried out by scanning electron microscopy (SEM). A columnar microstructure was observed in the single-, double- and triple-layer TBCs. Phase analysis of the as-sprayed TBCs was carried out using XRD (x-ray diffraction) where a tetragonal prime phase of zirconia in the single-layer YSZ TBC and a cubic defect fluorite phase of GZ in the double and triple-layer TBCs was observed. Porosity measurements of the as-sprayed TBCs were made by water intrusion method and image analysis method. The as-sprayed GZ-based multi-layered TBCs were subjected to erosion test at room temperature, and their erosion resistance was compared with single-layer 8YSZ. It was shown that the erosion resistance of 8YSZ single-layer TBC was higher than GZ-based multi-layered TBCs. Among the multi-layered TBCs, triple-layer TBC was slightly better than double layer in terms of erosion resistance. The eroded TBCs were cold-mounted and analyzed by SEM.

Keywords
columnar microstructureerosion, test gadolinium, zirconatemulti-layered thermal barrier, coating, suspension plasma spray, yttria-stabilized zirconia
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-10498 (URN)10.1007/s11666-016-0479-4 (DOI)000392060300011 ()2-s2.0-85001975445 (Scopus ID)
Note

This article is an invited paper selected from presentations at the 2016 International Thermal Spray Conference, held May 10-12, 2016, in Shanghai, P. R. China, and has been expanded from the original presentation.

Available from: 2017-01-09 Created: 2017-01-09 Last updated: 2018-06-18Bibliographically approved
2. Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray
Open this publication in new window or tab >>Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray
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2017 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 318, p. 208-216Article in journal (Refereed) Published
Abstract [en]

7-8 wt% yttria stabilized zirconia (YSZ) is the standard ceramic top coat material used in gasturbines to insulate the underlying metallic substrate. However, at higher temperatures(>1200 °C), phase stability and sintering becomes an issue for YSZ. At these temperatures,YSZ is also susceptible to CMAS (calcium magnesium alumino silicates) infiltration. New ceramic materials such as pyrochlores have thus been proposed due to their excellent properties such as lower thermal conductivity and better CMAS attack resistance compared to YSZ. However, pyrochlores have inferior thermo mechanical properties compared to YSZ.Therefore, double-layered TBCs with YSZ as the intermediate layer and pyrochlore as the top ceramic layer have been proposed. In this study, double layer TBC comprising gadoliniumzirconate (GZ)/YSZ and triple layer TBC (GZdense/GZ/YSZ) comprising relatively denser GZtop layer on GZ/YSZ were deposited by suspension plasma spray. Also, single layer 8YSZ TBC was suspension plasma sprayed to compare its functional performance with the multilayered TBCs. Cross sections and top surface morphology of as sprayed TBCs were analyzed by scanning electron microscopy (SEM). XRD analysis was done to identify phases formed in the top surface of as sprayed TBCs. Porosity measurements were made using water intrusionand image analysis methods. Thermal diffusivity of the as sprayed TBCs was measured using laser flash analysis and thermal conductivity of the TBCs was calculated. The multi-layered GZ/YSZ TBCs were shown to have lower thermal conductivity than the single layer YSZ. Theas sprayed TBCs were also subjected to thermal cyclic testing at 1300 ºC. The double and triple layer TBCs had a longer thermal cyclic life compared to YSZ. The failed samples were cold mounted and analyzed by SEM.

Keywords
Columnar microstructure, Gadolinium zirconate, Multi-layered thermal barrier coating, Suspension plasma spray, Thermal conductivity, Thermal cyclic test, Yttria stabilized zirconia
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9855 (URN)10.1016/j.surfcoat.2016.12.062 (DOI)000402356100024 ()2-s2.0-85009266708 (Scopus ID)
Note

Ingår i licuppsats

Available from: 2016-09-05 Created: 2016-09-05 Last updated: 2018-05-15Bibliographically approved
3. Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray
Open this publication in new window or tab >>Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray
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2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 283, p. 329-336Article in journal (Refereed) Published
Abstract [en]

Rare earth zirconates have lower thermal conductivity, better phase stability, improved sintering resistance and CMAS (calcium magnesium alumino silicates) infiltration resistance than yttria stabilized zirconia (YSZ) at temperatures above 1200 °C. However, their lower fracture toughness and lower coefficient of thermal expansion (CTE) compared to YSZ lead to premature coating failure. In order to overcome these drawbacks at higher temperatures, a multilayered coating approach is attempted in this study and compared with the single layer YSZ. Suspension plasma spray of single layer YSZ, single layer gadolinium zirconate (GZ) and double layer GZ/YSZ was carried out. Additionally, a triple layer coating system, with denser gadolinium zirconate on top of the GZ/YSZ system was sprayed to impart an added functionality of sealing the TBC from CMAS infiltration. Microstructural analysis was done using scanning electron microscopy and optical microscopy. Columnar microstructure with vertical cracks was observed. XRD analysis was used to identify phases formed in the as sprayed TBC samples. Porosity measurements were done using water impregnation method. Thermal diffusivity of single and multi-layered coatings was obtained by laser flash analysis and thermal conductivity of the coating systems was determined. It was found that the thermal conductivity of single layer gadolinium zirconate was lower than YSZ and that the thermal conductivity of multilayered systems were between their respective single layers. The single (YSZ), double (GZ/YSZ) and triple (GZ dense/GZ/YSZ) layer TBCs were subjected to thermal cyclic fatigue (TCF) test at 1100 °C and 1200 °C. It was observed that the single layer YSZ had lowest TCF life whereas the triple layer TBC had highest TCF life irrespective of test temperature.

Place, publisher, year, edition, pages
Lausanne: Elsevier Sequoia, 2015
Keywords
Gadolinium zirconate; Multilayered thermal barrier coating; Suspension plasma spray; Thermal conductivity; Thermal cyclic fatigue; Thermal diffusivity; Yttria stabilized zirconia
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-8779 (URN)10.1016/j.surfcoat.2015.11.009 (DOI)2-s2.0-84949467152 (Scopus ID)
External cooperation:
Available from: 2015-12-09 Created: 2015-12-09 Last updated: 2018-05-15Bibliographically approved
4. Failure analysis of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings subjected to thermal cyclic fatigue
Open this publication in new window or tab >>Failure analysis of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings subjected to thermal cyclic fatigue
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2016 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 689, p. 1011-1019Article in journal (Refereed) Published
Abstract [en]

8 wt.% yttria stabilized zirconia (8YSZ) is the standard ceramic top coat material used in thermal barrier coatings (TBCs) due to its excellent thermo-physical and thermo-mechanical properties. However, above 1200 °C, YSZ has issues such as susceptibility to CMAS (Calcium Magnesium Alumino Silicates) attack and enhanced sintering which could lead to catastrophic failure of the TBC. Pyrochlores of rare earth zirconate composition such as gadolinium zirconate have shown to be resistant to CMAS attack and at the same time possess several other attractive properties. However, poor thermal cycling life of single layer gadolinium zirconate (GZ) TBC compared to single layer YSZ has been reported. Therefore, a double layered GZ/YSZ TBC with YSZ as the intermediate coating and GZ as the top coat and a single layer 8YSZ were deposited by the axial suspension plasma spray process. Additionally, a triple layer TBC (GZdense/GZ/YSZ) comprising of denser GZ coating on top of GZ/YSZ TBC was deposited. SEM analysis revealed a columnar microstructure in the single, double and triple layer TBCs. XRD analysis confirmed the presence of tetragonal prime and defect fluorite phases in the top surface of YSZ and GZ based as sprayed TBCs respectively. The single layer YSZ and GZ/YSZ multi-layered TBCs were subjected to thermal cyclic fatigue (TCF) testing at 1100 °C and 1200 °C. The triple layer TBC showed a higher thermal cyclic life at both the temperatures compared to the single and double layer TBCs. The failed TBCs at 1100 °C were analyzed by SEM/EDS and image analysis. It was found that the failure modes in single layer YSZ and GZ based TBCs were different.

Keywords
Gadolinium zirconate, yttria stabilized zirconia, axial suspension plasma spray, multi-layered thermal barrier coating, columnar microstructure, thermal cyclic fatigue
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-9811 (URN)10.1016/j.jallcom.2016.07.333 (DOI)000384427200129 ()2-s2.0-84982239051 (Scopus ID)
Available from: 2016-08-26 Created: 2016-08-26 Last updated: 2018-06-18Bibliographically approved

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