- Email: [email protected]
LaAlO3 as overlayer in conventional thermal barrier coatings
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Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2018) 000–000
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Procedia Structural Structural IntegrityIntegrity Procedia1000(2018) (2016)280–287 000–000
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
1st International Conference of the Greek Society of Experimental Mechanics of Materials
LaAlO3 as overlayer in conventional thermal barrier coatings
XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal
I. Georgiopoulosa, N. Vourdasb, S. Mirzac, C. Andreoulia, V. Stathopoulosb,*
Thermo-mechanical modeling of a high pressure turbine blade of an MIRTEC S.A., Thiva Branch, 72 km of Athens-Lamia National Road, 34100, Chalkida, Greece Technological Education Insttute of Sterea Psachna Campus, 34400, Evia, Greece airplane gasEllada, turbine engine ELEMENT S.A., Hitchin SG4 0TW, UK a
nd
b
c
P. Brandãoa, V. Infanteb, A.M. Deusc* a AbstractDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, b
Portugal
IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, The performance enhancement of Thermal Barrier Coatings (TBC) towards higher service temperatures and longer service life will Portugal havecCeFEMA, a tremendous positive impact on the energy efficiency of turbines for various de applications. In this Pais, work1,a1049-001 study for the Department of Mechanical Engineering, Instituto Superior Técnico, Universidade Lisboa, Av. Rovisco Lisboa, evaluation of LaAlO3 as an overlayer to the YSZ-based TBC isPortugal presented. Overlayers on TBC, are desirable in order to add functionalities, not exhibited by the YSZ, such as CMAS tolerance. However, their effect on the thermal cycling performance of the multi-layered system needs to be examined. NiCrAlY bond-coated nimonic substrates were coated with Yttria Stabilized Zirconia Abstract (YSZ) delivered either by Atmospheric Plasma Spraying (APS) or by Suspension Plasma Spraying (SPS). On top of the YSZ layer, a LaAlO3 overlayer was developed directly from solution feedstock, employing Solution Precursor Plasma Spraying (SPPS). The Duringoftheir operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, structure the LaAlO 3 was confirmed by means of X-ray diffraction. The effect of the LaAlO3 overlayer on the thermal cycling o o especially theFor high blades. Such conditions cause these parts to undergo different types of time-dependent was evaluated. allpressure cases theturbine LaAlO(HPT) 3 improved the performance during thermal cycling at both 1100 C and 1200 C. degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict © 2018creep The Authors. Published by Elsevier Ltd. behaviour of HPT © the 2018 The Authors. Published by blades. Elsevier Flight Ltd. data records (FDR) for a specific aircraft, provided by a commercial aviation This is an openwere access article the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) company, used to under obtain and mechanical data for three different flight cycles. In order to create the 3Dunder model This is an open access article under thermal the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review Peer-review under of theascientific committee the 1st International Conference of the Greek Society Experimental Mechanics needed for theresponsibility FEM analysis, HPT blade scanned, and its chemical composition andofmaterial properties were st scrapofwas International Conference of the Greek Society of Experimental Mechanics of Materials responsibility of the scientific committee of the 1 of obtained. Materials. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The Keywords: Thermal barrier coatings; LaAlO3; thermal cycling; plasma spraying overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
1. ©Introduction 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016.
Thermal Barrier Coatings (TBCs) are used to insulate and protect the metallic gas turbine engine components Keywords: Blade; Creep; Finite Element Method; and 3D Model; Simulation. from the hotHigh gasPressure stream,Turbine against high temperature corrosion, subsequent damage. Performance improvement and
* Corresponding author. Tel.: +30 22280 99688 E-mail address: [email protected] Received: May 03, 2018; Received in revised form: July 12, 2018; Accepted: July 20, 2018
2452-3216 © 2018 The Authors. Published by Elsevier Ltd.
* Corresponding author. Tel.: under +351 218419991. This is an open access article the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under E-mail address: [email protected] responsibility of the scientific committee of the 1 st International Conference of the Greek Society of Experimental Mechanics of Materials 2452-3216 © 2016 The Authors. Published by Elsevier B.V. 2452-3216 2018 The Authors. Published by Elsevier Ltd. Peer-review underarticle responsibility of the Scientific Committee of PCF 2016. This is an open access under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 10.1016/j.prostr.2018.09.039
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increased thermal stability will facilitate higher combustion temperatures and thus improved engine efficiency, in numerous fields including power generation, aerospace, marine propulsion etc (Padture et al. (2002); Clarke et al. (2012); Xu & Guo (2011)). The pertinent market is growing fast and is expected over the next 10 years reaching nearly 228,000 aviation gas turbine engines valued in $1.232 trillion through 2030 and 5480 power generation engines worthing $105.3 billion (Bakan & Vaßen (2017)). In general, a typical TBC consists of two deposited layers, the bond coat (BC) and the top coat (TC). The state-of-the art BC consists of (Ni, Co) CrAlY or aluminides of Pt and Ni and the TC consists of Yttria-stabilized Zirconia (YSZ). The YSZ better-off performance is leveraged by a combination of convenient properties, i.e., high melting point, 2680 °C, high thermal expansion coefficient (TEC) 11.5 10-6 K-1 at 1273 K, low thermal conductivity (k) 2.12 W/mK at 1273 K and high fracture toughness 1-2 MPa m1/2 (Fu et al. (2011); Subramanian et al. (2002)). However, YSZ exhibits accelerated sintering above 1200 oC while its insufficient phase stability after long-term exposure in these high temperatures reduces the lifetime of the TC (Gadow & Lishka (2002)). Recently lanthanum aluminates have been used due to their refractory properties (high melting point, coefficient of thermal expansion > 8.5x10-6 K-1 and thermal conductivity < 2.2 W/mK), towards increasing the performance of TBC systems. Even though LaAlO3 is claimed as a potential YSZ replacement in various patents, (Fu et al. (2011); Subramanian et al (2002)) the focus is more for the substituted lanthanum aluminates exhibiting the hexaaluminate structure (Gadow & Lishka (2002); Chen et al. (2011); Ovaneysyan et al. (2014); Yeganan et al. (2015)). Even in hexaaluminates, as well as in lanthanum zirconates (Cao et al. (2001); Wang & Xiao (2014) etc), LaAlO3 gradually evolves with thermal aging (Chen et al. (2011)), due to the reaction of La with the Al from the evolved TGO. More interestingly La oxide seems to stabilize the sintering and hence the creep resistance of Al oxide (Schaper et al. (1983)). Its evaluation as material in multi-layered TC structures in conventional TBC systems as well as in nanocomposites has been reported recently, (Stathopoulos et al. (2016); Georgiopoulos et al. (2014); Vourdas et al. (2018)). Moreover, use of the Suspension or Solution Precursor Plasma Spray techniques (SPS & SPPS, respectively) provides the advantage of finer sized droplets deposition resulting in different microstructures, compared to conventional Atmospheric Plasma Spraying (APS), exhibiting a high cumulative porosity mainly consisting of submicrometer range pores and/or high segmentation crack density with excellent thermal cycling performance. TBC’s performance (lifetime and failure) depends on several parameters. During service, they undergo thermal cycling loading due to the cycling variation of the environmental temperature and the different thermal expansion coefficients of the different TBC layers. Stress variations induced by thermal shock loading generated during start up and shut down of the turbine engine result in coating system failure with the highest stresses and consequent failure occurring at the TC-BC interface. This work aims to develop, optimize and study the properties and the performance of innovative TBC systems involving LaAlO3 (LA) as an overlayer. More specifically, experimental results for the evaluation of LA as overlayer in double-layered TC structures of YSZ-based TBCs using different thermal spraying techniques (SPS, SPPS and APS) for the deposition of the different layers are presented. Consequently, different multi-layered structures in terms of materials and deposition techniques have been developed, evaluated and compared. LA overlayer is expected to provide high strain tolerance and low thermal conductivity in the TBC systems. Macroscopic together with microstructural analysis of thermal cycling and thermal shock experiments in temperatures up to 1200 oC reveal interesting results for the perovskite overlayer. 2. Experimental protocol 2.1. Materials synthesis and suspension optimization The multi-layered TBC structures investigated in this study consisted of different materials. Commercial NiCrAlY (H. C. Starck Amperit 413) was used as BC deposited using APS, commercial YSZ (H. C. Starck Amperit 827) as TC deposited with APS technique, commercial YSZ by Unitec Materials (after water based suspension optimization) as TC deposited using the SPS technique and nitrate precursor LaAlO 3 solution as overlayer deposited using the SPPS technique. Particle size distribution of raw powders and suspensions were measured using a Malvern 2000 laser particle sizer. LaAlO3 (perovskite) powder was synthesized based on a citrate-precursor technique described in detail elsewhere (Vourdas et al. (2018)). La(NO3)3.6H2O and Al(NO3)3.9H2O (Sigma Aldrich) were used as La and Al sources. An
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3
aqueous solution of 0.407 M in La(NO3)3.6H2O and 0.407 M in Al(NO3)3.6H2O was formed (Solution A). In solution A citric acid was added to a molar ratio of La:Al:Citric acid = 1:1:4.5 (Solution B). Solution B was used as a feedstock material in Solution Precursor Plasma Spraying. Also development and characterization of stable YSZ water-based suspension inks suitable to be used in SPS using 7 wt% yttria stabilized zirconia fine powder (7YSZ) has been performed. The 7YSZ particles were dispersed in the water using 1.5 %wt dispersant. Suspension with 15%wt solids after several hours ball milling and pH adjustment was produced fulfilling necessary requirements such as long-term stability, re-dispersability, low viscosity (6-14 cP) and small particle size (D50 = 0.31 μm) as shown in Fig. 1.
Fig. 1. Particle size distribution of the 7YSZ stable suspensions after ball milling using different dispersants after stabilization.
2.2. TBC systems development A Praxair SG-100 atmospheric plasma spraying gun was used for the different layers deposition. Coatings were deposited on rectangular sand blasted nimonic 901 substrates (20x25x2 mm3, 20x38x2 mm3). During spraying, the substrates were adequately cooled. If no cooling is used the substrate temperature will increase and subsequent thermal stresses will be introduced. The substrate temperature, measured using infrared pyrometers, was kept lower than 100°C. The solutions/suspensions were fed to the plasma gun through an atomizer. 7YSZ coatings using SPS were developed using plasma power in the range of 27-33 kW while the atomization gas flow rate ranged from 15 to 20 slpm. A spraying distance of 7.5 cm was used during deposition. Deposition of LaAlO3 perovskite-type oxide has been performed using the SPPS technique. Ar/He mixtures as plasma gases with plasma power ranging from 50 kW to 57 kW, solution feed rates of 4.50-21.8 ml/min and atomization gas flow rates 12-19 slpm were tested. In Table 1 the different TBC systems compared in this study are shown. Table 1. Overview of the TBC systems prepared and compared. TBC System
Substrate
Bond Coat
Top Coat
Overlayer
1
Nimonic
NiCrAlY(APS)
YSZ(SPS)
-
2
Nimonic
NiCrAlY(APS)
YSZ(SPS)
LA(SPPS)
2.3. TBC systems characterization Phase composition of powders and deposited coatings was evaluated using a Siemens D500 X-Ray Diffractometer (Cu, Ka). Microstructural analysis of the developed coatings was performed using a Jeol 6300 Scanning Electron
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Microscope. Coatings porosity was estimated using a Leica microscope and Image Analysis software. Vickers microhardness measurements on coatings was performed using a Shimadzu micro hardness tester. Thermal cyclic testing (Fig. 2) has been performed on rectangular, one-side coated samples under the following conditions: Short Cycle: Heat up to 1100 oC or 1200 °C in 16 min; hold at temperature for 1 hour; cool down to room temperature (18 °C) in approximately 50 min. Long Cycle: Heat up to 1100 oC or 1200 °C in 16 min; hold temperature for 25 hours; cool down to room temperature (18 °C) in approximately 50 min. In both cases tests were continued until 10-20% spallation of the TBC area was observed. Total duration (number of cycles) under thermal cycling at 1100 oC and 1200 oC were studied.
Fig. 2. Temperature profile of the thermal cycling testing at 1200 oC (left) short cycle and (right) long cycle.
3. Results and discussion 3.1. Top coat layers deposition parameters optimization 7YSZ layer using SPS: The obtained coating thickness was in the range of 25-50 μm. Its porosity ranged from 9 to 18% while its microhardness HV100 from 300 to 772. In Fig. 3a &b the microstructure of the optimum coating is shown. Its thickness is 38 μm and from the polished cross section (Fig. 3b) a continuous, homogeneous, microcrack free, without any evidence of delamination coating is revealed. Its microhardness HV100 is 633 and has a porosity of 9% (Fig. 3c).
a
b
c
Fig. 3. 7YSZ SPS coating (a, b) SEM micrograph of polished cross-section and (c) porosity measurement of this coating through image analysis.
LaAlO3 layer using SPPS: The effect of critical plasma spray parameters on the quality of the developed coatings has been investigated. In detail, the effect of the solution feed rate on the coating microstructure (Fig. 4a, b) showed that decrease of the feed rate results in higher deposition rate from 7 to 20 μm/pass and increase of the process yield.
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Plasma power: 56 kW, Feed rate: 21.8 ml/min, Atomization gas flow rate: 12 slpm
Plasma power: 56 kW, Feed rate: 10 ml/min, Atomization gas flow rate: 12 slpm
Plasma power: 50 kW, Feed rate: 10 ml/min, Atomization gas flow rate: 12 slpm
5
Plasma power: 50 kW, Feed rate: 10 ml/min, Atomization gas flow rate: 15 slpm
Fig. 4. SEM cross sections of LaAlO3 SPPS deposited coatings with different sets of plasma spray parameters.
Furthermore the applied plasma power effect showed that power increase results in higher quality microstructure (Fig. 3b, c) as well as deposition rate from 5 to 20 μm/pass and process yield from 35 to 45%. Investigation of the effect of the atomization gas flow rate on the deposited LaAlO3 coatings revealed that increase of the flow rate results in a maximum deposition rate of 7 μm/pass and maximum process yield 60% at the flow rate value of 15 slpm. By the X ray diffraction results the phase composition of the deposited coatings under the selected sets of plasma spraying parameters investigated, revealed the formation of LaAlO3 phase as the predominant crystal phase (Stathopoulos et al. (2018)). The coating shows only traces of La as a secondary phase (Fig. 5). This is a very important result since it confirms the advantage of the SPPS coating deposition as a single step technique towards the formation of LaAlO3 oxide without the need of post annealing. In the minimum time of the liquid feedstock precursor material flight within the high temperature plasma flame all required chemical synthesis steps occurs. Thus the citrate-precursor La – Al compounds form a high purity LaAlO3 oxide as a deposited coating.
Fig. 5. XRD patterns of the LaAlO3 coatings deposited using SPPS technique under the sets of parameters as described in Fig. 4.
3.2. Multi-layered TBC development and characterization Thermal spray deposition parameters optimization of the above mentioned investigated materials as two-layered (top coat-bond coat) systems, was followed by the deposition and characterization of three layered systems (overcoat- top coat-bond coat) as shown in Table 1 and Fig. 6.
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Nimonic/NiCrAlY(APS)/ YSZ(SPS)
285
Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS)
Fig. 6. Photos of the ceramic coating surface structure before thermal cycling testing.
For the Nimonic/NiCrAlY(APS)/YSZ(SPS) samples spallation occurred after 3 long cycles at 1200°C (Fig. 7), while no spallation occurred after 14 long cycles (Fig. 8), as well as after 57 short cycles at 1100°C (Fig. 9). For the Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS), that is YSZ (SPS) top coated with LaAlO3 over layer using SPPS method, spallation at 1200 °C occurred after 4 long cycles and after 40 short cycles, as shown in Fig. 10 and Fig. 11, respectively. At 1100 °C no spallation occurred after 4 long and 57 short cycles, as shown in Fig. 12 and Fig. 13, respectively. Nimonic/NiCrAlY(APS)/YSZ(SPS)
Fig. 7. Thermal cycling at 1200°C, after 3 long cycles.
Fig. 8. Thermal cycling at 1100°C, after 14 long cycles.
Fig. 9. Thermal cycling at 1100°C, after 57 short cycles.
Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS)
Fig. 10. Thermal cycling at 1200°C, after 4 long cycles.
Fig. 11. Thermal cycling at 1200°C, after 40 short cycles.
Fig. 12. Thermal cycling at 1100°C, after 14 long cycles.
Fig. 13. Thermal cycling at 1100°C, after 57 short cycles.
In Table 2 below the results of the thermal cycling performance of the two TBC systems studied herein are exhibited.
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Table 2. Overview of the thermal cycling performance of the TBC tested. Nr of long cycles @1200oC
Nr. of short cycles @1200oC
Nr of long cycles @1100oC
Nr. of short cycles @1100oC
Nimonic/NiCrAlY(APS)/YSZ(SPS)
3 (spallation)
36 (spallation)
14 (no spallation)
57 (no spallation)
Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS)
4 (spallation)
40 (spallation)
14 (no spallation)
57 (no spallation)
System
4. Conclusions LaAlO3 was used as over layer in multi-layered TBC systems. The additional thermal sealing provided by LaAlO3 along with its lower fracture toughness improved the TBC performance during thermal cycling. SPS and SPPS thermal spray techniques proved effective for the deposition of high quality LaAlO3 perovskite oxide coatings using liquid feedstocks. Use of precursor solutions and suspension materials provide the advantage of using nano-sized materials to develop coatings with unique microstructure, low porosity levels and microcrack free coatings. Furthermore, LaAlO3 perovskite synthesis by a chemical reaction occurring during the liquid precursor materials flight within the plasma flame and subsequent coating deposition in a single step procedure is verified by using the SPPS technique. The effect of the deposition of LaAlO3 over the YSZ was assessed, on the thermal cycling performance at both 1100 oC and 1200 oC. At 1100 oC the LaAlO3 over layer does not affect the thermal cycling performance up to 14 long (ca. 350 h at 1100 oC) as well as after 57 short (ca. 57 h at 1100 oC) cycles. At 1200 °C the LaAlO3 over-layered TBC performed slightly better compared to the not over-layered samples, indicating a satisfactory potential of LaAlO3 perovskite over layers, worthing further investigation in the near future. Acknowledgements Financial support by «THEBARCODE - Development of multifunctional Thermal Barrier Coatings and modeling tools for high temperature power generation with improved efficiency» FP7-NMP-2012-SMALL-6, Collaborative project. References Bakan, E., Vaßen, R., 2017. Ceramic top coats of plasma-sprayed thermal barrier coatings: materials, processes, and properties. Journal of Thermal Spray Technology 26, 992-1010. Cao, X.Q., Vassen, R., Jungen, W., Schwartz, S., Tietz, F., Stöver, D., 2001. Thermal stability of lanthanum zirconate plasma-sprayed coating. Journal of the American Ceramic Society 84, 2086-2090. Chen, X., Zhao, Y., Huang, W., Ma, H., Zou, B., Wang, Y., Cao, X., 2011. Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating. Journal of the European Ceramic Society 31, 2285-2294. Clarke, D.R., Oechsner, M., Padture, N.P., 2012. Thermal-barrier coatings for more efficient gas-turbine engines. MRS Bulletin 37, 891-898. Fu, M., Darolia, R., Gorman, M., Nagaraj, B.A., 2011. Thermal barrier coating systems including a rare earth aluminate layer for improved resistance to CMAS infiltration and coated articles, US8062759 B2, US 11/964,953. Gadow, R., Lischka, M., 2002. Lanthanum hexaaluminate - novel thermal barrier coatings for gas turbine applications - materials and process development. Surface and Coatings Technology 151-152, 392-399. Georgiopoulos, I., Marathoniti, E., Vourdas, N., Andreouli, K., Stathopoulos, V., 2014. Comparative study on liquid plasma sprayed lanthanum aluminate oxide coatings using different feedstock materials for potential TBC application, 25th Advanced Aerospace Materials and Processes (AeroMat) Conference and Exposition, ASM, Orlando, Florida, USA. Marathoniti, E., Vourdas, N., Georgiopoulos, I., Trusca, O.D, Trusca, I., Andreouli, C., Stathopoulos, V.N., 2014. Development of LaAlO3-based thermal barrier coatings by solution precursor thermal spray, International Conference on Material Technologies and Modeling-MMT 2014 Ariel, Israel, pp. 195-205. Ovanesyan, K.L., Kuzanyan, A.S., Badalyan, G.R., Yeganyan, A.V., Sargsyan, R.V., Kuzanyan, V.S., Petrosyan, A.G., Stathopoulos, V., 2014. Preparation and investigation of rare earth magnesium hexaaluminate solid solutions. Journal of Contemporary Physics (Armenian Academy of Sciences) 49, 220-227. Padture, N.P., Gell, M., Jordan, E.H., 2002. Thermal barrier coatings for gas-turbine engine applications. Science 296, 280-284. Schaper, H., Doesburg, E.B.M., Van Reijen, L.L., 1983. The influence of lanthanum oxide on the thermal stability of gamma alumina catalyst supports. Applied Catalysis 7, 211-220.
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Stathopoulos, V., Kuznetsova, T., Lapina, O., Khabibulin, D., Pandis P.K., Krieger, T., Chesalov, Y., Gulyalev, R., Krivensov, V., Larina, T., Sadykov, V., 2018. Evolution of bulk and surface structures in stoichiometric LaAlO3 mixed oxide prepared by using starch as template. Materials Chemistry and Physics 207, 423-434. Stathopoulos, V., Sadykov, V., Pavlova, S., Bespalko, Y., Fedorova, Y., Bobrova, L., Salanov, A., Ishchenko, A., Stoyanovsky, V., Larina, T., Ulianitsky, V., Vinokurov, Z., Kriventsov, V., 2016. Design of functionally graded multilayer thermal barrier coatings for gas turbine application. Surface and Coatings Technology 295, 20-28. Subramanian, R., Sabol, S.M., Goedjen, J.G., Sloan, K.M., Vance, S.J., 2002. Thermal barrier coatings for turbine components, US6365281 B1, US 09/405,498). Vourdas, N., Marathoniti, E., Pandis, P., Argirusis, C., Sourkouni, G., Legros, C., Mirza, S., Stathopoulos, V., 2018. Evaluation of LaAlO3 as top coat material for thermal barrier coatings. Transactions of Nonferrous Metals Society of China 28, 1582-1592. Wang, Y., Xiao, P., 2014. The phase stability and toughening effect of 3Y-TZP dispersed in the lanthanum zirconate ceramics. Materials Science and Engineering A 604, 34-39. Xu, H., Guo, H., 2011. Thermal barrier coatings. Elsevier Science. Yeganyan, A.V., Kuzanyan, A.S., Stathopoulos, V.S., 2015. Estimation of thermal expansion coefficient of solid crystalline dielectrics at high temperatures. Journal of Contemporary Physics (Armenian Academy of Sciences) 50, 79-83.
ScienceDirect
Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
Procedia Structural Structural IntegrityIntegrity Procedia1000(2018) (2016)280–287 000–000
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
1st International Conference of the Greek Society of Experimental Mechanics of Materials
LaAlO3 as overlayer in conventional thermal barrier coatings
XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal
I. Georgiopoulosa, N. Vourdasb, S. Mirzac, C. Andreoulia, V. Stathopoulosb,*
Thermo-mechanical modeling of a high pressure turbine blade of an MIRTEC S.A., Thiva Branch, 72 km of Athens-Lamia National Road, 34100, Chalkida, Greece Technological Education Insttute of Sterea Psachna Campus, 34400, Evia, Greece airplane gasEllada, turbine engine ELEMENT S.A., Hitchin SG4 0TW, UK a
nd
b
c
P. Brandãoa, V. Infanteb, A.M. Deusc* a AbstractDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, b
Portugal
IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, The performance enhancement of Thermal Barrier Coatings (TBC) towards higher service temperatures and longer service life will Portugal havecCeFEMA, a tremendous positive impact on the energy efficiency of turbines for various de applications. In this Pais, work1,a1049-001 study for the Department of Mechanical Engineering, Instituto Superior Técnico, Universidade Lisboa, Av. Rovisco Lisboa, evaluation of LaAlO3 as an overlayer to the YSZ-based TBC isPortugal presented. Overlayers on TBC, are desirable in order to add functionalities, not exhibited by the YSZ, such as CMAS tolerance. However, their effect on the thermal cycling performance of the multi-layered system needs to be examined. NiCrAlY bond-coated nimonic substrates were coated with Yttria Stabilized Zirconia Abstract (YSZ) delivered either by Atmospheric Plasma Spraying (APS) or by Suspension Plasma Spraying (SPS). On top of the YSZ layer, a LaAlO3 overlayer was developed directly from solution feedstock, employing Solution Precursor Plasma Spraying (SPPS). The Duringoftheir operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, structure the LaAlO 3 was confirmed by means of X-ray diffraction. The effect of the LaAlO3 overlayer on the thermal cycling o o especially theFor high blades. Such conditions cause these parts to undergo different types of time-dependent was evaluated. allpressure cases theturbine LaAlO(HPT) 3 improved the performance during thermal cycling at both 1100 C and 1200 C. degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict © 2018creep The Authors. Published by Elsevier Ltd. behaviour of HPT © the 2018 The Authors. Published by blades. Elsevier Flight Ltd. data records (FDR) for a specific aircraft, provided by a commercial aviation This is an openwere access article the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) company, used to under obtain and mechanical data for three different flight cycles. In order to create the 3Dunder model This is an open access article under thermal the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review Peer-review under of theascientific committee the 1st International Conference of the Greek Society Experimental Mechanics needed for theresponsibility FEM analysis, HPT blade scanned, and its chemical composition andofmaterial properties were st scrapofwas International Conference of the Greek Society of Experimental Mechanics of Materials responsibility of the scientific committee of the 1 of obtained. Materials. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The Keywords: Thermal barrier coatings; LaAlO3; thermal cycling; plasma spraying overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
1. ©Introduction 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016.
Thermal Barrier Coatings (TBCs) are used to insulate and protect the metallic gas turbine engine components Keywords: Blade; Creep; Finite Element Method; and 3D Model; Simulation. from the hotHigh gasPressure stream,Turbine against high temperature corrosion, subsequent damage. Performance improvement and
* Corresponding author. Tel.: +30 22280 99688 E-mail address: [email protected] Received: May 03, 2018; Received in revised form: July 12, 2018; Accepted: July 20, 2018
2452-3216 © 2018 The Authors. Published by Elsevier Ltd.
* Corresponding author. Tel.: under +351 218419991. This is an open access article the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under E-mail address: [email protected] responsibility of the scientific committee of the 1 st International Conference of the Greek Society of Experimental Mechanics of Materials 2452-3216 © 2016 The Authors. Published by Elsevier B.V. 2452-3216 2018 The Authors. Published by Elsevier Ltd. Peer-review underarticle responsibility of the Scientific Committee of PCF 2016. This is an open access under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 10.1016/j.prostr.2018.09.039
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increased thermal stability will facilitate higher combustion temperatures and thus improved engine efficiency, in numerous fields including power generation, aerospace, marine propulsion etc (Padture et al. (2002); Clarke et al. (2012); Xu & Guo (2011)). The pertinent market is growing fast and is expected over the next 10 years reaching nearly 228,000 aviation gas turbine engines valued in $1.232 trillion through 2030 and 5480 power generation engines worthing $105.3 billion (Bakan & Vaßen (2017)). In general, a typical TBC consists of two deposited layers, the bond coat (BC) and the top coat (TC). The state-of-the art BC consists of (Ni, Co) CrAlY or aluminides of Pt and Ni and the TC consists of Yttria-stabilized Zirconia (YSZ). The YSZ better-off performance is leveraged by a combination of convenient properties, i.e., high melting point, 2680 °C, high thermal expansion coefficient (TEC) 11.5 10-6 K-1 at 1273 K, low thermal conductivity (k) 2.12 W/mK at 1273 K and high fracture toughness 1-2 MPa m1/2 (Fu et al. (2011); Subramanian et al. (2002)). However, YSZ exhibits accelerated sintering above 1200 oC while its insufficient phase stability after long-term exposure in these high temperatures reduces the lifetime of the TC (Gadow & Lishka (2002)). Recently lanthanum aluminates have been used due to their refractory properties (high melting point, coefficient of thermal expansion > 8.5x10-6 K-1 and thermal conductivity < 2.2 W/mK), towards increasing the performance of TBC systems. Even though LaAlO3 is claimed as a potential YSZ replacement in various patents, (Fu et al. (2011); Subramanian et al (2002)) the focus is more for the substituted lanthanum aluminates exhibiting the hexaaluminate structure (Gadow & Lishka (2002); Chen et al. (2011); Ovaneysyan et al. (2014); Yeganan et al. (2015)). Even in hexaaluminates, as well as in lanthanum zirconates (Cao et al. (2001); Wang & Xiao (2014) etc), LaAlO3 gradually evolves with thermal aging (Chen et al. (2011)), due to the reaction of La with the Al from the evolved TGO. More interestingly La oxide seems to stabilize the sintering and hence the creep resistance of Al oxide (Schaper et al. (1983)). Its evaluation as material in multi-layered TC structures in conventional TBC systems as well as in nanocomposites has been reported recently, (Stathopoulos et al. (2016); Georgiopoulos et al. (2014); Vourdas et al. (2018)). Moreover, use of the Suspension or Solution Precursor Plasma Spray techniques (SPS & SPPS, respectively) provides the advantage of finer sized droplets deposition resulting in different microstructures, compared to conventional Atmospheric Plasma Spraying (APS), exhibiting a high cumulative porosity mainly consisting of submicrometer range pores and/or high segmentation crack density with excellent thermal cycling performance. TBC’s performance (lifetime and failure) depends on several parameters. During service, they undergo thermal cycling loading due to the cycling variation of the environmental temperature and the different thermal expansion coefficients of the different TBC layers. Stress variations induced by thermal shock loading generated during start up and shut down of the turbine engine result in coating system failure with the highest stresses and consequent failure occurring at the TC-BC interface. This work aims to develop, optimize and study the properties and the performance of innovative TBC systems involving LaAlO3 (LA) as an overlayer. More specifically, experimental results for the evaluation of LA as overlayer in double-layered TC structures of YSZ-based TBCs using different thermal spraying techniques (SPS, SPPS and APS) for the deposition of the different layers are presented. Consequently, different multi-layered structures in terms of materials and deposition techniques have been developed, evaluated and compared. LA overlayer is expected to provide high strain tolerance and low thermal conductivity in the TBC systems. Macroscopic together with microstructural analysis of thermal cycling and thermal shock experiments in temperatures up to 1200 oC reveal interesting results for the perovskite overlayer. 2. Experimental protocol 2.1. Materials synthesis and suspension optimization The multi-layered TBC structures investigated in this study consisted of different materials. Commercial NiCrAlY (H. C. Starck Amperit 413) was used as BC deposited using APS, commercial YSZ (H. C. Starck Amperit 827) as TC deposited with APS technique, commercial YSZ by Unitec Materials (after water based suspension optimization) as TC deposited using the SPS technique and nitrate precursor LaAlO 3 solution as overlayer deposited using the SPPS technique. Particle size distribution of raw powders and suspensions were measured using a Malvern 2000 laser particle sizer. LaAlO3 (perovskite) powder was synthesized based on a citrate-precursor technique described in detail elsewhere (Vourdas et al. (2018)). La(NO3)3.6H2O and Al(NO3)3.9H2O (Sigma Aldrich) were used as La and Al sources. An
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aqueous solution of 0.407 M in La(NO3)3.6H2O and 0.407 M in Al(NO3)3.6H2O was formed (Solution A). In solution A citric acid was added to a molar ratio of La:Al:Citric acid = 1:1:4.5 (Solution B). Solution B was used as a feedstock material in Solution Precursor Plasma Spraying. Also development and characterization of stable YSZ water-based suspension inks suitable to be used in SPS using 7 wt% yttria stabilized zirconia fine powder (7YSZ) has been performed. The 7YSZ particles were dispersed in the water using 1.5 %wt dispersant. Suspension with 15%wt solids after several hours ball milling and pH adjustment was produced fulfilling necessary requirements such as long-term stability, re-dispersability, low viscosity (6-14 cP) and small particle size (D50 = 0.31 μm) as shown in Fig. 1.
Fig. 1. Particle size distribution of the 7YSZ stable suspensions after ball milling using different dispersants after stabilization.
2.2. TBC systems development A Praxair SG-100 atmospheric plasma spraying gun was used for the different layers deposition. Coatings were deposited on rectangular sand blasted nimonic 901 substrates (20x25x2 mm3, 20x38x2 mm3). During spraying, the substrates were adequately cooled. If no cooling is used the substrate temperature will increase and subsequent thermal stresses will be introduced. The substrate temperature, measured using infrared pyrometers, was kept lower than 100°C. The solutions/suspensions were fed to the plasma gun through an atomizer. 7YSZ coatings using SPS were developed using plasma power in the range of 27-33 kW while the atomization gas flow rate ranged from 15 to 20 slpm. A spraying distance of 7.5 cm was used during deposition. Deposition of LaAlO3 perovskite-type oxide has been performed using the SPPS technique. Ar/He mixtures as plasma gases with plasma power ranging from 50 kW to 57 kW, solution feed rates of 4.50-21.8 ml/min and atomization gas flow rates 12-19 slpm were tested. In Table 1 the different TBC systems compared in this study are shown. Table 1. Overview of the TBC systems prepared and compared. TBC System
Substrate
Bond Coat
Top Coat
Overlayer
1
Nimonic
NiCrAlY(APS)
YSZ(SPS)
-
2
Nimonic
NiCrAlY(APS)
YSZ(SPS)
LA(SPPS)
2.3. TBC systems characterization Phase composition of powders and deposited coatings was evaluated using a Siemens D500 X-Ray Diffractometer (Cu, Ka). Microstructural analysis of the developed coatings was performed using a Jeol 6300 Scanning Electron
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Microscope. Coatings porosity was estimated using a Leica microscope and Image Analysis software. Vickers microhardness measurements on coatings was performed using a Shimadzu micro hardness tester. Thermal cyclic testing (Fig. 2) has been performed on rectangular, one-side coated samples under the following conditions: Short Cycle: Heat up to 1100 oC or 1200 °C in 16 min; hold at temperature for 1 hour; cool down to room temperature (18 °C) in approximately 50 min. Long Cycle: Heat up to 1100 oC or 1200 °C in 16 min; hold temperature for 25 hours; cool down to room temperature (18 °C) in approximately 50 min. In both cases tests were continued until 10-20% spallation of the TBC area was observed. Total duration (number of cycles) under thermal cycling at 1100 oC and 1200 oC were studied.
Fig. 2. Temperature profile of the thermal cycling testing at 1200 oC (left) short cycle and (right) long cycle.
3. Results and discussion 3.1. Top coat layers deposition parameters optimization 7YSZ layer using SPS: The obtained coating thickness was in the range of 25-50 μm. Its porosity ranged from 9 to 18% while its microhardness HV100 from 300 to 772. In Fig. 3a &b the microstructure of the optimum coating is shown. Its thickness is 38 μm and from the polished cross section (Fig. 3b) a continuous, homogeneous, microcrack free, without any evidence of delamination coating is revealed. Its microhardness HV100 is 633 and has a porosity of 9% (Fig. 3c).
a
b
c
Fig. 3. 7YSZ SPS coating (a, b) SEM micrograph of polished cross-section and (c) porosity measurement of this coating through image analysis.
LaAlO3 layer using SPPS: The effect of critical plasma spray parameters on the quality of the developed coatings has been investigated. In detail, the effect of the solution feed rate on the coating microstructure (Fig. 4a, b) showed that decrease of the feed rate results in higher deposition rate from 7 to 20 μm/pass and increase of the process yield.
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Plasma power: 56 kW, Feed rate: 21.8 ml/min, Atomization gas flow rate: 12 slpm
Plasma power: 56 kW, Feed rate: 10 ml/min, Atomization gas flow rate: 12 slpm
Plasma power: 50 kW, Feed rate: 10 ml/min, Atomization gas flow rate: 12 slpm
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Plasma power: 50 kW, Feed rate: 10 ml/min, Atomization gas flow rate: 15 slpm
Fig. 4. SEM cross sections of LaAlO3 SPPS deposited coatings with different sets of plasma spray parameters.
Furthermore the applied plasma power effect showed that power increase results in higher quality microstructure (Fig. 3b, c) as well as deposition rate from 5 to 20 μm/pass and process yield from 35 to 45%. Investigation of the effect of the atomization gas flow rate on the deposited LaAlO3 coatings revealed that increase of the flow rate results in a maximum deposition rate of 7 μm/pass and maximum process yield 60% at the flow rate value of 15 slpm. By the X ray diffraction results the phase composition of the deposited coatings under the selected sets of plasma spraying parameters investigated, revealed the formation of LaAlO3 phase as the predominant crystal phase (Stathopoulos et al. (2018)). The coating shows only traces of La as a secondary phase (Fig. 5). This is a very important result since it confirms the advantage of the SPPS coating deposition as a single step technique towards the formation of LaAlO3 oxide without the need of post annealing. In the minimum time of the liquid feedstock precursor material flight within the high temperature plasma flame all required chemical synthesis steps occurs. Thus the citrate-precursor La – Al compounds form a high purity LaAlO3 oxide as a deposited coating.
Fig. 5. XRD patterns of the LaAlO3 coatings deposited using SPPS technique under the sets of parameters as described in Fig. 4.
3.2. Multi-layered TBC development and characterization Thermal spray deposition parameters optimization of the above mentioned investigated materials as two-layered (top coat-bond coat) systems, was followed by the deposition and characterization of three layered systems (overcoat- top coat-bond coat) as shown in Table 1 and Fig. 6.
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Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS)
Fig. 6. Photos of the ceramic coating surface structure before thermal cycling testing.
For the Nimonic/NiCrAlY(APS)/YSZ(SPS) samples spallation occurred after 3 long cycles at 1200°C (Fig. 7), while no spallation occurred after 14 long cycles (Fig. 8), as well as after 57 short cycles at 1100°C (Fig. 9). For the Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS), that is YSZ (SPS) top coated with LaAlO3 over layer using SPPS method, spallation at 1200 °C occurred after 4 long cycles and after 40 short cycles, as shown in Fig. 10 and Fig. 11, respectively. At 1100 °C no spallation occurred after 4 long and 57 short cycles, as shown in Fig. 12 and Fig. 13, respectively. Nimonic/NiCrAlY(APS)/YSZ(SPS)
Fig. 7. Thermal cycling at 1200°C, after 3 long cycles.
Fig. 8. Thermal cycling at 1100°C, after 14 long cycles.
Fig. 9. Thermal cycling at 1100°C, after 57 short cycles.
Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS)
Fig. 10. Thermal cycling at 1200°C, after 4 long cycles.
Fig. 11. Thermal cycling at 1200°C, after 40 short cycles.
Fig. 12. Thermal cycling at 1100°C, after 14 long cycles.
Fig. 13. Thermal cycling at 1100°C, after 57 short cycles.
In Table 2 below the results of the thermal cycling performance of the two TBC systems studied herein are exhibited.
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Table 2. Overview of the thermal cycling performance of the TBC tested. Nr of long cycles @1200oC
Nr. of short cycles @1200oC
Nr of long cycles @1100oC
Nr. of short cycles @1100oC
Nimonic/NiCrAlY(APS)/YSZ(SPS)
3 (spallation)
36 (spallation)
14 (no spallation)
57 (no spallation)
Nimonic/NiCrAlY(APS)/YSZ(SPS)/LA(SPPS)
4 (spallation)
40 (spallation)
14 (no spallation)
57 (no spallation)
System
4. Conclusions LaAlO3 was used as over layer in multi-layered TBC systems. The additional thermal sealing provided by LaAlO3 along with its lower fracture toughness improved the TBC performance during thermal cycling. SPS and SPPS thermal spray techniques proved effective for the deposition of high quality LaAlO3 perovskite oxide coatings using liquid feedstocks. Use of precursor solutions and suspension materials provide the advantage of using nano-sized materials to develop coatings with unique microstructure, low porosity levels and microcrack free coatings. Furthermore, LaAlO3 perovskite synthesis by a chemical reaction occurring during the liquid precursor materials flight within the plasma flame and subsequent coating deposition in a single step procedure is verified by using the SPPS technique. The effect of the deposition of LaAlO3 over the YSZ was assessed, on the thermal cycling performance at both 1100 oC and 1200 oC. At 1100 oC the LaAlO3 over layer does not affect the thermal cycling performance up to 14 long (ca. 350 h at 1100 oC) as well as after 57 short (ca. 57 h at 1100 oC) cycles. At 1200 °C the LaAlO3 over-layered TBC performed slightly better compared to the not over-layered samples, indicating a satisfactory potential of LaAlO3 perovskite over layers, worthing further investigation in the near future. Acknowledgements Financial support by «THEBARCODE - Development of multifunctional Thermal Barrier Coatings and modeling tools for high temperature power generation with improved efficiency» FP7-NMP-2012-SMALL-6, Collaborative project. References Bakan, E., Vaßen, R., 2017. Ceramic top coats of plasma-sprayed thermal barrier coatings: materials, processes, and properties. Journal of Thermal Spray Technology 26, 992-1010. Cao, X.Q., Vassen, R., Jungen, W., Schwartz, S., Tietz, F., Stöver, D., 2001. Thermal stability of lanthanum zirconate plasma-sprayed coating. Journal of the American Ceramic Society 84, 2086-2090. Chen, X., Zhao, Y., Huang, W., Ma, H., Zou, B., Wang, Y., Cao, X., 2011. Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating. Journal of the European Ceramic Society 31, 2285-2294. Clarke, D.R., Oechsner, M., Padture, N.P., 2012. Thermal-barrier coatings for more efficient gas-turbine engines. MRS Bulletin 37, 891-898. Fu, M., Darolia, R., Gorman, M., Nagaraj, B.A., 2011. Thermal barrier coating systems including a rare earth aluminate layer for improved resistance to CMAS infiltration and coated articles, US8062759 B2, US 11/964,953. Gadow, R., Lischka, M., 2002. Lanthanum hexaaluminate - novel thermal barrier coatings for gas turbine applications - materials and process development. Surface and Coatings Technology 151-152, 392-399. Georgiopoulos, I., Marathoniti, E., Vourdas, N., Andreouli, K., Stathopoulos, V., 2014. Comparative study on liquid plasma sprayed lanthanum aluminate oxide coatings using different feedstock materials for potential TBC application, 25th Advanced Aerospace Materials and Processes (AeroMat) Conference and Exposition, ASM, Orlando, Florida, USA. Marathoniti, E., Vourdas, N., Georgiopoulos, I., Trusca, O.D, Trusca, I., Andreouli, C., Stathopoulos, V.N., 2014. Development of LaAlO3-based thermal barrier coatings by solution precursor thermal spray, International Conference on Material Technologies and Modeling-MMT 2014 Ariel, Israel, pp. 195-205. Ovanesyan, K.L., Kuzanyan, A.S., Badalyan, G.R., Yeganyan, A.V., Sargsyan, R.V., Kuzanyan, V.S., Petrosyan, A.G., Stathopoulos, V., 2014. Preparation and investigation of rare earth magnesium hexaaluminate solid solutions. Journal of Contemporary Physics (Armenian Academy of Sciences) 49, 220-227. Padture, N.P., Gell, M., Jordan, E.H., 2002. Thermal barrier coatings for gas-turbine engine applications. Science 296, 280-284. Schaper, H., Doesburg, E.B.M., Van Reijen, L.L., 1983. The influence of lanthanum oxide on the thermal stability of gamma alumina catalyst supports. Applied Catalysis 7, 211-220.
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