DEVELOPMENT OF LaAlO3-BASED THERMAL BARRIER COATINGS BY SOLUTION PRECURSOR THERMAL SPRAY E. Marathoniti1, N. Vourdas1, I. Georgiopoulos2, O-D. Trusca3, I. Trusca3 C. Andreouli1, V.N. Stathopoulos1* 1 School of Technological Applications, Technological Educational Institute of Sterea Ellada, 34400 Psahna, Chalkida Evia Greece 2 MIRTEC S.A, 72nd km Athens- Lamia National Road, 34100, Chalkida, Greece 3 Plasma Jet s.r.l, Magurele-Ilfov, 077125, Romania * Prof. V.N. Stathopoulos,
[email protected] ABSTRACT Improvements in turbine engine efficiency are strongly connected to the performance of thermal barrier coatings (TBC) operating at temperatures over 1200 oC, where the state-ofthe-art topcoat material, i.e. Yttria Stabilized Zirconia (YSZ), exhibits fatal phase transformations. Various materials and structures are evaluated as YSZ-replacements, that will enable turbine engine operation at this ranges. In this direction particular interest have been gained by perovskites. In this study we evaluate simple structured LaAlO3 (LA) in TBC applications, by means of solution precursor thermal spraying (SPTS). LA solution precursors were synthesized and characterized, towards their flow-related characteristics, and capability to deliver stoichiometric perovskite structures. The concentration of the solution was set to demonstrate a viscosity of ca. 3 cP, while XRD and ATR-IR confirmed the formation of pure perovskite. SPTS deposition of LA was performed and process parameters were tuned towards final coating performance optimization.
1. INTRODUCTION Thermal Barrier Coatings (TBCs) are used to protect and insulate the metallic gas turbine engine component from the hot gas stream, against high temperature corrosion, and subsequent damage. Improvement in this field will facilitate higher combustion temperatures and thus improved engine efficiency not only in power generation but also in aerospace and marine propulsion (1-3). In general a typical TBC consists of two deposited layers, the bondcoat (BC) and the topcoat (TC) and one that is evolved during processing and operation, the Thermal Grown Oxide (TGO). The state of the art TC material is the Y2O3-stabilized zirconia (7% wt Y2O3 in ZrO2, YSZ). This particular Y2O3 percentage was selected based on thermal cycling measurements (4). Its comparable higher thermal cycling performance is leveraged by a combination of suitable 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 (5, 6). YSZ performance is affected by a variety of interconnected parameters, such as the compatibility with the BC material, the BC/TC interface state, the microstructure, the deposition method etc. Usually Atmospheric Plasma Spray (APS) or Electron Beam Physical Vapor Deposition (EB-PVD) is employed to deliver the TC layer. In general APS coatings demonstrate low thermal conductivity and particular lamellar microstructure characteristics, i.e. ‘horizontal’ cracks parallel to the substrate induced by rapid solidification. EB-PVD coatings exhibit a column-like 1-195
microstructure perpendicular to the surface with better-off mechanical properties, but also higher thermal conductivity (7). More recently Solution Precursor Thermal Spraying (SPTS) has been utilized in this field, providing another means to improve the thermal YSZ cycling performance (8). However YSZ operation temperature is limited at ~1200 oC, above which the phase transformation to monoclinic phase, becomes favourable. This transformation introduce fatal volume changes that degrade the integrity upon thermal cycling, thus inseminating the necessity for studies of novel TC materials operating at temperatures above 1200 oC (9-11). Materials exhibiting perovskite structure in particular have been attracted much attention as YSZ replacements mainly due to their high melting point, high TEC and relatively low thermal conductivity. The drawback of materials exhibiting simple perovskite structure is mainly their inferior fracture-related mechanical properties. SrZrO3 is one of the few exceptions with comparable to YSZ fracture toughness and has been studied as single layer TC (12, 13) or as a double layer with YSZ (14), with or without Gd2O3 or Yb2O3 doping to lower the thermal conductivity and improve phase stability. Other simple perovskite structured zirconates such as BaZrO3 (12) and CaZrO3 (15) and LaYbO3 (16) have been also evaluated, but their poor thermochemical stability and low melting point, respectively, prevail their application as YSZ replacements. MgZrO3 has been used as a buffer layer between YSZ and BC to enhance the damage resistance of the TBC (17). Simple perovskite structured aluminates have attracted less attention. An important aspect of perovskites, however, is that they may accommodate substitution in both A- and B-site, thus allowing their properties to be tailored towards specific requirements. In this direction, and based on structures previously synthesized for various applications, complex substituted structures have been studied as YSZ replacements; Ba(Mg1/3Ta2/3)O3 (BMT) with TEC=10.9 10-6 K-1 and k=2.71 W/mK and La(Al1/4Mg1/2Ta1/4)O3 (LAMT) with TEC=9.7 10-6 K-1 and k=1.82W/mK (18) (TEC values within the range of 30-1000 oC, and k at 1000 oC) have been synthesized and evaluated as one layer TC or as double layer TC on YSZ (11). Even though their fracture toughness is lower compared to YSZ their performance at operating temperatures of 1400 oC is characterized as encouraging. However the presence of Mg introduces implications upon spraying (19) due to relatively high vapor pressure of MgO (order of 10-4 Torr at 1300 oC), compared to Ta2O5 (order of 10-4 Torr at 1920 oC), to Al2O3 (order of 10-4 Torr at 1550 oC) and to La2O3 (10-4 Torr at 1400 oC). Even though LaAlO3 is claimed as a potential YSZ replacement in various patents, (20, 21), no pertaining study has been published so far to our knowledge, contrary to the respective substituted Lanthanum aluminates exhibiting the hexaaluminate structure (22, 23). Even in hexaaluminates, as well as in Lanthanum zirconates (24) etc, LaAlO3 gradually evolves with thermal aging (23), 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 (25). Therefore its evaluation as material incorporated in the TC of a TBC, is of crucial importance not only as standalone TC layer, but also for various La-containing YSZ replacements. Simple LaAlO3 structure demonstrate poor mechanical properties (26), has a relatively low melting point (ca. 2100 oC), and exhibits a phase transition from the rhombohedral (R3c) to the ideal cubic (Pm3m) at ca. 810-840 oC, accompanied with a TEC discontinuity. The extent of this discontinuity is however debatable; from 1-196
prohibitively large (27, 28) to undistinguishable (29). On the other LaAlO3 is also susceptible for doping in both A- and B- site (27, 29-31), thus enabling the formation of stabilized structures up to more than 1600 oC, with lower the thermal conductivity (32), and improved the mechanical properties (33). In this study we explore the application of LaAlO3 by SPTS as a TC layer. Results from this study will be correlated to the substituted structures of LaAlO3, that are currently synthesized. Here we present the synthesis route for the LaAlO3 solution precursor, and we characterize the powder and respective pellet obtained by this precursor. The development and characterization of the respective coatings delivered from SPTS is also presented. 2. EXPERIMENTAL 2.1. Synthesis of LaAlO3 solution precursor and powder LaAlO3 powder was synthesized based on a citrate-precursor technique described in detail elsewhere (34). La(NO3)3.6H2O and Al(NO3)3.9H2O from Sigma Aldrich were used as La and Al sources. An 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 feed for SPTS. In order to test the capability of this precursor to deliver pure perovskite structure, NH4OH up to pH values between 9-10 was added and heated to 90 oC, while stirring. The formed gel was then charred into a powder after firing 350 oC for 1 h. This powder was finally annealed at 1100 oC for 2 h to produce the LaAlO3. For characterization purposes LaAlO3 powder was uniaxially pressed into pellets. Previously LaAlO3 powder size was reduced by wet ball milling using zirconia grinding balls. 2.2. SPTS deposition Deposition of LaAlO3 perovskite-type oxide has been performed using SPTS. A Praxair SG-100 atmospheric plasma spraying gun, attached to a 6-axis Motoman robotic arm, was used. Coatings were deposited on sand blasted Nimonic 90 and SL316 stainless steel substrates (15 mm x 15 mm x 3 mm). During spraying, the substrates were adequately cooled. Extensive cooling is necessary since the SPTS requires multiple passes of the spraying gun over the substrate, to deliver the desirable coating thickness. If no cooling is used the substrate temperature will increase and subsequent thermal stresses would be introduced. The substrate temperature, measured using infrared pyrometers, was kept lower than 100 °C. The solution (Solution B) was fed to the plasma gun through a BETE–XAPR atomizer using peristaltic and dosing pumps and Ar as atomizing gas. The particular nozzle may accommodate low viscosity liquid feeds. 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. 2.3. Characterization Phase formation was confirmed using a Siemens D600 XRD system employing CuKa radiation. The particle size was checked by laser particle size analysis using a Malvern 2000 Mastersizer. Viscosity of the solution precursor was measured using 1-197
the Brookfield, DV-II-Pro viscometer at temperatures of 21.0±0.5 oC. Microstructure was investigated using scanning electron microscopy (SEM, JEOL 6300), combined with EDS analysis for elemental analysis. Micrographs of polished cross-sections were studied using routine metallographic techniques. Apparent porosity was measured utilizing the Archimedes method and following the respective standard method (35). Bulk density of pellets has been calculated as the mass of the dry pellet over the apparent volume of the pellet, based on the geometrical characteristics of the pellet. Mechanical strength characterization was performed using a Shimadzu HMV2 microhardness tester with a Vickers indenter. 3. RESULTS AND DISCUSSION 3.1. Solution precursor characterization The properties of the solution precursor is of crucial importance for the final outcome of the SPTS. The molar ratio of the La and Al sources as well as of the citric acid, was kept constant in all cases, following previous studies (34). The solution concentration however is subject to optimization. The upper concentration limit is dictated by the maximum solubility of the Al(NO3)3.9H2O, which is ca. 69 g/100 ml H2O, and corresponds to 1.84 M. High concentrated solutions (below the upper limit) are expected to increase the yield since its La and Al load is higher. On the other hand as the concentration of the solution is lowered, both the density and the viscosity of the solution is decreased, which is generally beneficial for the SPTS process, since the solution precursor flow becomes more convenient. To our knowledge no strict viscosity and density specifications have been set up to now. However their values will dictate the selection of the appropriate nozzle type. In any case the characterization of the solution precursor and control is valuable information for the SPTS process. In Fig. 1 we present experimental data of the solution precursor viscosity and density vs. the concentration of Al3+. In all cases the molar ratio of Al : La : Citric acid was equal to 1 : 1 : 4.5. 40
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Fig. 1. Viscosity and mass density of the solution precursor against concentration of Al3+. In all cases the molar ratio of Al : La : Citric Acid was kept constant and equal to 1:1:4.5. 3.2. Powder characterization
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In Fig. 2(b) the particle size distribution of the LaAlO3 powder after 60 h of ball milling is shown. The powder exhibits a D(0.5) and D(0.9) equal to 10.2 μm and 26.2 μm respectively. 20
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Fig. 3. XRD pattern of (a) LaAlO3 powder made after firing the solution precursor at 350 oC and then annealed at 1100 oC (as synthesized), (b) LaAlO3 pellet from powder as in (a) and then sintered at 1200 oC for 2h, (c) LaAlO3 pellet from powder as in (a) and then annealed at 1500 oC for 2h. The ATR-IR spectrum of the powder as synthesized is presented in Fig. 4. The dominating band is asymmetric and located at ca. 650 cm-1. Based on previous studies regarding the optical properties of LaAlO3 (36), this is the result from of two primary peaks at 692 cm-1 and 652 cm-1. A smaller band at ca. 556 cm-1 is also recorded. These, namely, three bands, correspond to the formation of AlO6 octahedra (34, 37). One small band at ca. 495 cm-1 is also expected (36), whereas we record another at 406 cm-1. The deconvolution of the spectrum is also shown in Fig. 4. Four Lorentz oscillators were used and fitted to the measured spectrum. The fitted parameters, i.e. position (xc), width (w) and intensity (A) are also presented in Fig. 4. The positions of the 1-199
Transmittance (a.u.)
Lorentz oscillators, as well as the comparably larger width of the oscillator at 691 cm-1, are in accordance to the pertaining the tabulated values (36).
Measurement Primary bands Sum of primary bands
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Value 690.85 76.16 4531.39 649.57 33.16 2791.77 551.37 47.49 1236.71 407.94 42.20 2250.47
Error 1.07 3.08 269.92 0.25 1.06 134.74 0.69 2.72 73.75 0.64 2.04 120.52
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Fig. 4. (left) Deconvolution of the ATR-IR spectra of the LaAlO3 powder after firing the solution precursor at 350 oC and then annealed at 1100 oC (as synthesized). (right) 3.3. Pellet characterization Pellets were formed by means of uniaxial pressing the annealed at 1100 oC powder under 74 MPa, using identical conditions. In all cases the green pellet diameter was 13.00 mm, the height was 3.5 mm and the bulk density was 3.05 g/cm3. The green bodies were then sintered at various temperatures for 2 h each. Table 1 summarizes the measured and calculated properties of the sintered pellets. The XRD patterns of the sintered pellets at 1200 oC and 1500 oC are presented in Fig. 3(b) and Fig. 3(c) respectively, for comparison to the respective pattern of the as synthesized powder. No changes are recorded compared to the starting powder, thus indicating a phase stability of the LaAlO3 structures at these temperatures for the time studied. Even though the LaAlO3 structure is stable within these sintering conditions, a substantial sintering is recorded. Open porosity is reduced from 47.5% to virtually zero, and bulk density is decreased from 3.21 g/cm3 to 5.81 g/cm3 as the sintering temperature increases from 1200 oC to 1500 oC. Single crystal R3c LaAlO3 specimens exhibit density of 6.51 g/cm3. Table 1. Effect of the sintering temperature on the pellet diameter, height, bulk density and porosity. Sintering temperature (oC) 1200 1300 1400 1500
Pellet diameter after sintering (mm) 12.75 11.50 10.50 10.25
Height of pellet after sintering (mm) 3.45 3.20 3.00 2.95
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Bulk density (g/cm3) 3.21 4.26 5.47 5.81
Open porosity (%) 47.5 34.4 14.7 1.8
In Fig. 5 the ATR-IR spectra of powders from the sintered pellets are presented. In both cases the main convoluted band at ca. 650 cm-1 is preserved compared to the annealed powder given in Fig. 4. For the sample sintered at 1500 oC the bands at lower wavenumbers are substantially lowered.
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Fig. 5. ATR-IR spectra LaAlO3 powder from pellets sintered at 1200 oC and 1500 oC. 3.4. SPTS coatings characterization In Table 2 we provide the process parameters of the four (A-D) SPTS experiments performed. The effect of solution precursor feed rate (A-B) was studied for two values, i.e. 21.80 and 10.00 ml/min. The respective study for the effect of plasma power (B-C) was conducted for 56.0 and 50.0 kW, while two values of atomization gas flow rate were studied (C-D); i.e. 12.0 and 15.0 slpm. SEM images of the SPTS coatings, for each process conditions are presented in Fig. 6. A decrease of the feed rate from 21.8 to 10 ml/min (A-B) results in better coating quality, regarding microstructure, and higher deposition rate from 7 to 20 um/pass respectively. Under high feed rate, i.e. 21.8 um/pass, the coating consists of unmelted particles with the simultaneuous presence of pores of less than 5 microns distributed within the coating as seen in Fig. 6(a). Decrease of the feed rate to 10 ml/min (B) results in enhanced microstrustural characteristics with less unmelted particles, areas of well melted material and fewer as well as smaller pores in the LaAlO3 coating (Fig. 6(b)). This is attributed to the fact that with low solution feed rates the thermal energy of the flame is enough for solvent evaporation, LaAlO3 phase formation and solid particle melting prior to solidification on the substrate. Table 2. Deposition conditions and experimental results for the SPTS coating.
Feed rate (ml/min) Plasma Power (kW) Atomization gas flow rate (slpm)
A 21.80 56.0 12.0
Process B C 10.00 10.00 56.0 50.0 12.0 12.0
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Plasma power increase (C-B) results in higher deposition rate from 5 to 20 um/pass as well as process yield from 35% to 45%. In this case also the microstructural characteristics of the developed coatings using lower plasma power, Fig. 6(c), are significantly inferior compared to LaAlO3 deposits with increased plasma power, Fig. 6(b). B
A
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Fig. 6. SEM images of the coatings delivered by SPTS using respective process condition described in Table 2. Increase of the flow rate results in higher deposition rate 7 instead of 5 μm/pass and higher process yield 60% instead of 35%. The presence of larger unmelted particles as well as pores size is observed with increasing the atomization flow rate to 15 slpm. The XRD pattern of the SPTS coatings delivered using the four processes described in Table 2 is given in Fig. 7. In all cases the dominating structure is the respective LaAlO3 structure (R3c). This is a promising result confirming the advantage of the SPTS as a single step procedure towards coatings deposition integrating various physical/chemical processes i.e. chemical reaction, solid state phenomena, crystal phase formation etc.
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Fig. 7. XRD pattern of SPTS coated layers for each process as described in Table 2. 4. CONCLUSIONS We evaluated the application of simple perovskite structured LaAlO3, as a YSZ replacement in TBS. LaAlO3 solution precursor was synthesized and deposited by means of SPTS. The rheology-related characteristics of the solution precursor were studied. The concentration of the solution precursor was set to exhibit a low viscosity of ca. 3 cP. The solution was fired and annealed, to confirm its capability to deliver pure perovskite structures, by means of XRD and ATR-IR. Sintering studies from 1200 to 1500 oC revealed a significant densification from 3.21 to 5.81 g/cm3. SPTS process parameters were tuned towards optimization of the coating performance. 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 1. 2. 3. 4.
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