Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature attack by glassy deposits

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Acta Materialia 58 (2010) 6835–6844 www.elsevier.com/locate/actamat

Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature attack by glassy deposits Julie M. Drexler a, Kentaro Shinoda b, Angel L. Ortiz c, Dongsheng Li a, Alexander L. Vasiliev d, Andrew D. Gledhill a, Sanjay Sampath b, Nitin P. Padture a,⇑ b

a Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA Center for Thermal Spray Research, Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794, USA c Departamento de Ingenierı´a Meca´nica, Energe´tica y de los Materiales, Universidad de Extremadura, 06071 Badajoz, Spain d Institute of Crystallography, Russian Academy of Sciences, Moscow 119333, Russia

Received 19 July 2010; received in revised form 5 September 2010; accepted 7 September 2010

Abstract Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, at these high operating temperatures, environmentally ingested airborne sand/ash particles melt on the hot TBC surfaces and form calcium–magnesium–aluminosilicate (CMAS) glass deposits. The molten CMAS glass penetrates the TBCs, leading to loss of strain tolerance and TBC failure. Here we demonstrate the use of the commercial manufacturing method of air-plasmaspray (APS) to fabricate CMAS-resistant yttria-stabilized zirconia (YSZ)-based TBCs containing Al and Ti in solid solution. Results from thermal stability studies of these new TBCs and CMAS/TBC interaction experiments are presented, together with a discussion of the CMAS mitigation mechanisms. The ubiquity of airborne sand/ash particles and the ever-increasing demand for higher operating temperatures in future high efficiency/performance gas-turbine engines will necessitate CMAS resistance in all hot-section components of those engines. In this context the versatility, ease of processing, and low cost offered by the APS method has broad implications for the design and fabrication of next-generation CMAS-resistant TBCs for future engines. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings; Zirconia; Anorthite; Glass; Crystallization

1. Introduction Thermal barrier coatings (TBCs) made of ZrO2 ceramics stabilized by Y2O3 (YSZ) are being used widely in gas-turbine engines to insulate and protect hot-section metal components (see e.g. Refs. [1–7]). The higher engine-operating temperatures enabled by TBCs in aircraft engines are now engendering new materials issues. Specifically, fine sand/ash particles ingested by the engine during operation deposit on the hotter TBC surfaces as molten calcium– magnesium–aluminosilicate (CMAS) glass, which penetrates the TBCs, resulting in loss of strain tolerance and premature failure of the TBCs [8–21]. Since airborne ⇑ Corresponding author.

E-mail address: [email protected] (N.P. Padture).

sand/ash particles are ubiquitous, and there is an increasing demand for higher and higher engine-operating temperatures, the CMAS attack of TBCs (and also environmental barrier coatings or EBCs [15]) is becoming a critical issue in the development of next-generation gas-turbine engines. To that end, some approaches for mitigating CMAS attack on YSZ TBCs have been reported in the literature [18,22–24]. Recently, a new approach for mitigating CMAS attack has been demonstrated by some of the present coauthors [14], where up to 20 mol.% Al2O3 and 5 mol.% TiO2 in the form of a solid solution have been incorporated into YSZ TBCs (referred to as YSZ+20Al+5Ti). The solution-precursor plasma spray (SPPS) process [25–30], which can produce coatings of metastable ceramics with extended solid-solubility [31,32], is used to deposit such TBCs with engineered chemistries. Here the YSZ+20Al+5Ti SPPS

1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2010.09.013

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Table 1 Chemical composition of the powder used to produce YSZ+20Al+5Ti APS TBCs. Composition

ZrO2

Y2O3

Al2O3

TiO2

mol.% wt.%

71.4 62.4

3.6 5.8

20.0 28.9

5.0 2.9

TBC serves as a reservoir of Al and Ti solutes, which are incorporated into the molten CMAS glass that is in contact with the TBC. An accumulation of Al concentration in the CMAS glass, as it penetrates the TBC, shifts the glass composition from the difficult-to-crystallize pseudowollastonite (CaSiO3) field to the relatively easy-to-crystallize anorthite (CaAl2Si2O8) field. The incorporation of Ti in the glass promotes crystallization of the CMAS glass by serving as a nucleating agent. This combined effect results in the near-complete crystallization of the leading edge of the penetrating CMAS front into crystalline anorthite, arresting that front [14]. Note that it is important to have the Al2O3 in solid solution or as small precipitates in these YSZ-based TBCs to avoid local coefficient of thermal expansion (CTE) mismatch stresses associated with multiphase TBCs. In related experiments, TBCs of the conventional composition of 93 wt.% ZrO2–7 wt.% Y2O3 (7YSZ), regardless of whether they were made using the air-plasma-spray (APS) method or the SPPS method, were found to be penetrated completely by CMAS under the same conditions, resulting in the total destruction of the TBCs [14]. More recently, YSZ+20Al+5Ti SPPS TBCs were also found to be CMAS-resistant and durable under tests simulating actual engine conditions, where a cyclic thermal gradient was applied across the TBC thickness with simultaneous deposition of CMAS on the hot surface of the TBC [20]. The objective of this study is to explore the possibility of depositing solid-solution YSZ+20Al+5Ti TBCs using the conventional method of APS and to investigate their thermal stability and CMAS-resistance properties. The motivation for this being the fact that the widely used APS method would be more amenable to commercial manufacturing of the new CMAS-resistant TBCs, relative to the developmental SPPS method. 2. Experimental procedure Single-phase solid-solution powders of the composition listed in Table 1 were synthesized using a chemical method, and were sprayed dried using a laboratory-scale spray

dryer (Model B-290, Buchi, Flawil, Switzerland). This composition is the same as the composition of the YSZ+20Al+5Ti SPPS TBCs used in previous studies [14,20]. That composition was based on striking a balance between the maximum amount of Al solute that can be practically incorporated in the YSZ TBCs, and the minimum amount of Al needed to shift the composition of the penetrating CMAS to the anorthite field [14,20]. The spray-dried powders were air-plasma-sprayed onto Ni-based superalloy (Haynes 214, Haynes International, Kokomo, IN) “button” substrates (25 mm diameter, 6 mm thickness; grit-blasted) to create the APS TBCs (referred to as YSZ+20Al+5Ti APS; 125 lm thickness). An atmospheric DC plasma torch with an 8-mm diameter nozzle and a swirl flow gas distribution ring (F4-MB, Sulzer Metco Inc., Westbury, NY, USA) was used for APS deposition. The substrate was preheated to 400 °C prior to the APS deposition. The APS processing parameters are listed in Table 2. These parameters were developed based on extensive optimization studies using the concept of process maps to tailor both thermal conductivity and compliance of TBCs [33–35]. The YSZ+20Al+5Ti TBCs were air-plasma-sprayed using optimized conditions of this process map with average particle temperatures and velocities of 2700 °C and 239 m s1, respectively, which are similar to those used to deposit conventional 7YSZ TBCs [33– 35]. Reference TBCs of conventional 7YSZ composition were also APS deposited (200 lm thickness) onto separate Haynes 214 “button” substrates. The as-sprayed YSZ+20Al+5Ti APS TBC specimens were heat-treated isothermally at 1200 °C or 1300 °C for 24 h or 72 h in air using a box furnace (Thermolyne, Dubuque, IA) to study their thermal stability. The use of simulated CMAS glass frit, of same composition as the actual sand deposits found in engines, was determined to be the best choice for conducting the CMAS/TBC interaction experiments in a controlled and reproducible fashion [14]. Other choices include the use of actual sand particles from the field or the use of a mixture of oxides (same composition as the sand) [13]. Unfortunately, actual sand from the field is not available in needed quantity, and uncertainties in the oxides-mixture case are expected in the melting behavior and the homogeneity of the resulting CMAS glass that forms in contact with the TBC. Thus, simulated CMAS glass frit of predetermined composition (Table 3) was prepared using procedures given elsewhere [14]. Note that this CMAS composition is very similar to that used in other studies [12,13] and is typical of sand deposits in engines.

Table 2 APS processing parameters for the deposition of YSZ+20Al+5Ti APS TBCs. Gas flow rates (std. l min1) Primary Ar

Secondary H2

Carrier Ar

47.5

6.0

4.0

Current (A)

Power (kW)

Spray distance (mm)

Raster speed (mm s1)

Powder rate (g min1)

Number of passes

550

34

100

500

7

24

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Table 3 Composition of the sand used to prepare the simulated CMAS glass. Composition

SiO2

CaO

MgO

Al2O3

Na2O

K2O

Fe3O4

mol.% wt.%

50.0 49.6

38.0 35.2

5.0 3.3

4.0 6.7

1.0 1.0

1.0 1.6

1.0 2.6

CMAS/TBC interaction experiments were performed using the same procedure employed in the previous study [14]. Briefly, a predetermined amount of the CMAS-frit paste was used to cover uniformly the top surface of assprayed YSZ+20Al+5Ti APS and 7YSZ APS TBCs, such that a constant CMAS concentration per unit TBC area of 35 mg cm2 was maintained in all the experiments. These specimens were laid flat on an alumina plate, with the CMAS-coated TBC top surface facing up, and they were then heat-treated at 1200 °C in air for 24 h using a box furnace (Thermolyne, Dubuque, IA). X-ray diffraction (XRD) patterns of as-sprayed and heat-treated (without CMAS) YSZ+20Al+5Ti APS TBCs were obtained using a powder diffractometer (Ultimate III, Rigaku, Woodlands, TX). Detailed Rietveld analysis of these XRD patterns was performed to determine the phases present in the as-sprayed TBCs, and to understand the phase evolution with heat-treatment. Standard protocol of refinement was used to perform the Rietveld analyses [36,37], employing FullProf integrated into the WinPLOTR software package [38]. The phase composition was calculated from the ultimate Rietveld scale factors of the phases present [39]. The average size of the precipitates was determined from XRD peak broadening using the Scherrer method [40]. As-sprayed TBC specimens and CMAS-interacted TBC specimens were cut in half to produce cross-sections. The cross-sections were polished to a 1 lm finish using routine metallographic techniques and observed in a scanning electron microscope (SEM; Electroscan, Philips Electron Optics, The Netherlands) equipped with an energy dispersive spectrometer (EDS; EDAX, Mahwah, NJ). Porosity of the as-sprayed YSZ+20Al+5Ti APS TBCs was estimated from the SEM micrographs in conjunction with detailed image analysis. Transmission electron microscopy (TEM) specimens of as-sprayed TBCs, heat-treated TBCs (without CMAS), and CMAS-interacted TBCs were prepared using focused ion beam (FIB; Helios 600, FEI, Hillsboro, OR). These specimens were observed in a CM200 TEM (CM200, Philips Electron Optics, The Netherlands; 200 kV) or a Tecnai TEM (Tecnai F20, FEI, Hillsboro, OR; 200 kV) or a Titan TEM (Titan 80–300, FEI, Hillsboro, OR; 300 kV), all equipped with an EDS. 3. Results Fig. 1A shows a cross-sectional SEM micrograph of the as-sprayed YSZ+20Al+5Ti APS TBC. The microstructure appears uniform throughout the thickness, with an esti-

Fig. 1. Cross-sectional SEM micrographs of as-sprayed APS TBCs onto a superalloy substrates: (A) YSZ+20Al+5Ti and (B) 7YSZ.

mated porosity of 21%. This microstructure is very similar to that of the reference 7YSZ APS TBC shown in Fig. 1B. Fig. 2A shows a bright-field TEM micrograph of the as-sprayed YSZ+20Al+5Ti APS TBC. The microstructure appears to comprise of ZrO2, with no evidence of any other crystalline phases (such as Al2O3, TiO2, or Y2O3). The ZrO2 appears to be tetragonal (t) but this could not be confirmed conclusively using selected area electron diffraction patterns (SAEDPs). EDS reveals the presence of Al, Ti and Y, which appear to be in solid solution in the ZrO2. However, the EDS data show that there are Al-rich (Fig. 2B) and Al-lean (Fig. 2C) regions within the YSZ+20Al+5Ti APS TBC. Fig. 3 shows XRD data (black points) from as-sprayed YSZ+20Al+5Ti APS TBC. Besides t-ZrO2 phases, there is no measurable evidence of any other crystalline phases (such as Al2O3, TiO2, or Y2O3) in the XRD data. Rietveld analysis of the XRD pattern was performed assuming the presence of three t-ZrO2 phases, one Al-rich and two Allean, based on the TEM/EDS results. As can be observed, this analysis shows an excellent fit (Fig. 3 top; red curve). Considering only two t-ZrO2 phases, one Al-rich and the other Al-lean, the Rietveld analysis of the XRD pattern results in a poor fit (Fig. 3 bottom; red curve). The phase composition and lattice parameters from the analysis using three t-ZrO2 phases are presented in Table 4. The Al-rich tZrO2 phase has low tetragonality, and it accounts for 39 wt.% of the phase content. The two Al-lean t-ZrO2 phases (Al-lean-1 and Al-lean-2) are so designated because of their higher tetragonality (>1.000), and they account for

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Fig. 3. XRD data (black points) of the as-sprayed YSZ+20Al+APS TBC, with Rietveld analyses best fits (red curves) for two-phases model (bottom) and three-phases model (top). Positions of a-Al2O3 Bragg reflections are shown on the top (black vertical bars) as reference. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (A) Bright-field TEM micrograph of the air-plasma-sprayed (APS) YSZ+20Al+5Ti TBC. EDS spectra from (B) Al-rich and (C) Al-lean (right) regions; vertical red lines denote energies for Al and Zr elemental lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

61 wt.% of the phase content. Thus, the combination of TEM/EDS and XRD results confirms that the APS method can be used to deposit tetragonal YSZ TBCs with 20 mol.% Al2O3 and 5 mol.% TiO2 in complete solid solution. Fig. 4 compares XRD data (black points) from assprayed and heat-treated (without CMAS) YSZ+20Al+5Ti APS TBCs. The red curves are corresponding Rietveld analysis best fits, with the corresponding composition and lattice parameter results presented in Table 4. All the heat-treated YSZ+20Al+5Ti APS TBCs are found to contain one Al-lean t-ZrO2 phase and aAl2O3, in addition to minor amounts of TiO2 and Y2O3 phases. The t-ZrO2 phase in the 1200 °C 24 h heat-treated TBC shows higher tetragonality (1.015), which is the result of Al-depletion by diffusion [41] and concomitant precipitation of a-Al2O3 (16 wt.%). Fig. 5A is a bright-field TEM micrograph confirming the presence of up to 300 nm sized a-Al2O3 precipitates. Analysis of XRD peak broadening yielded an average a-Al2O3 precipitate size of 200 nm. Longer heat-treatment at 1200 °C (72 h) does

not result in much change in the phase composition, the lattice parameters, and the precipitate size. By comparing Al2O3 concentrations in Tables 1 and 4, it appears that little more than half of the Al2O3 in solid solution in the assprayed YSZ+20Al+5Ti APS TBC precipitates out as aAl2O3. Higher temperature heat-treatments (1300 °C) result in even higher tetragonality in the t-ZrO2 phase, and is consistent with the higher concentrations of precipitated a-Al2O3 (Table 4). The heat-treatment of YSZ+20Al+5Ti APS TBC at 1300 °C for 72 h results in about a third of the added Al2O3 remaining in solid solution in t-ZrO2. Once again, the bright-field TEM micrograph in Fig. 5B confirms the presence of a-Al2O3 precipitates (300 nm) in that heat-treated TBC. Also, XRD peak broadening analysis showed the same average a-Al2O3 precipitate size as the 1200 °C (24 h) case: 200 nm. These thermal stability studies show that a significant amount of Al2O3 remains in solid solution in the YSZ+20Al+5Ti despite prolonged high-temperature heat-treatments. Also, the size of the a-Al2O3 second phase that does precipitate out appears to be too small to have an adverse effect on the CTE mismatch issue mentioned earlier, as discussed in Section 4. Figs. 6–8 show results from the TBC/CMAS interaction studies. As shown in the previous study, 7YSZ APS TBCs are completely penetrated by CMAS at 1121 °C (24 h) and totally destroyed [14]. In the present study, Fig. 6 shows a similar result for a higher temperature of 1200 °C (24 h) using the same CMAS glass. Fig. 6A is a cross-sectional SEM micrograph of 7YSZ APS TBC with CMAS on top that has been heat-treated at 1200 °C for 24 h, showing destruction of the TBC. The dashed red line shows the original top surface of the TBC. Fig. 6B–D shows corresponding Zr, Ca, and Si EDS elemental maps, respectively,

J.M. Drexler et al. / Acta Materialia 58 (2010) 6835–6844 Table 4 Phase composition of the YSZ+20Al+5Ti APS TBCs from XRD Rietveld analysis.

6839

Heat-treatment

Phases present

wt.%

˚) Lattice parameters (A

pffiffiffi Tetragonality c=ða 2Þ

Remarks

As-sprayed

t-ZrO2 t-ZrO2 t-ZrO2

39 41 20

a = 3.6154, c = 5.1086 a = 3.5665, c = 5.0466 a = 3.5025, c = 4.9560

0.9992 1.0006 1.0006

Al-rich Al-lean-1 Al-lean-2

1200 °C, 24 h

t-ZrO2 a-Al2O3 Other

81 16 3

a = 3.6071, c = 5.1762 – –

1.0147 – –

Al-lean – Y2O3, TiO2

1200 °C, 72 h

t-ZrO2 a-Al2O3 Other

79 18 3

a = 3.6067, c = 5.1756 – –

1.0147 – –

Al-lean – Y2O3, TiO2

1300 °C, 24 h

t-ZrO2 a-Al2O3 Other

76 18 6

a = 3.5985, c = 5.1710 – –

1.0161 – –

Al-lean – Y2O3, TiO2

1300 °C, 72 h

t-ZrO2 a-Al2O3 Other

74 20 6

a = 3.5975, c = 5.1732 – –

1.0168 – –

Al-Lean – Y2O3, TiO2

Fig. 4. XRD data (black points) of the as-sprayed and heat-treated YSZ+20Al+5Ti APS TBCs, with Rietveld analyses best fits (red curves). Positions of a-Al2O3 Bragg reflections are shown on the top (black vertical bars) as reference. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

showing clear evidence of Ca and Si reaching the bottom of the TBC, a penetration depth of 200 lm. In contrast, the YSZ+20Al+5Ti APS TBC is found to be highly resistant to CMAS penetration under the same conditions, as seen in Fig. 7. The cross-sectional SEM micrograph (Fig. 7A) shows that the original TBC is intact and that the CMAS front is arrested (denoted by the arrow). The Ca (Fig. 7C) and the Si (Fig. 7D) EDS elemental maps indicate that only a depth of 40 lm is penetrated by the CMAS, which is about a third of the original YSZ+20Al+5Ti APS TBC thickness. Fig. 8A is a bright-field TEM micrograph of the CMASinteracted YSZ+20Al+5Ti APS TBC from the CMASarrest region (similar to that shown by the arrow in Fig. 7A). This micrograph shows globular, Al-depleted ZrO2 grains, which are similar to the ones observed in

Fig. 5. Bright-field TEM micrographs of YSZ+20Al+5Ti APS TBCs heat-treated at: (A) 1200 °C for 24 h and (B) 1300 °C for 72 h.

the earlier studies [14,20]. Fig. 8B shows an indexed SAEDP from one of the ZrO2 grains indicating tetragonal

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B

A

100 µm

Zr Map

Substrate

C

D

Ca Map

Si Map

Fig. 6. (A) Cross-sectional SEM micrograph of 7YSZ APS TBC that has interacted with CMAS glass (1200 °C, 24 h), and corresponding elemental maps: (B) Zr, (C) Ca, and (D) Si. The horizontal dashed line denotes top surface of the original TBC.

A

50 µm

B

Substrate

C

Zr Map

D

Ca Map

Si Map

Fig. 7. (A) Cross-sectional SEM micrograph of YSZ+20Al+5Ti APS TBC that has interacted with CMAS glass (1200 °C, 24 h), and corresponding elemental maps: (B) Zr, (C) Ca, and (D) Si. The horizontal dashed line denotes top surface of the original TBC. Arrow in (A) denotes arrest of the CMAS front.

phase. Small regions of CMAS glass were also observed, as confirmed by the SAEDP (Fig. 8C) showing amorphous “halo.” The remainder of the matrix was found to be the anorthite (CaAl2Si2O8) phase, as confirmed by SAEDP (Fig. 8D) and EDS (Fig. 8E), with the latter showing almost equal concentrations of Al and Si.

4. Discussion Here we have demonstrated that the conventional method of APS can be used to deposit tetragonal YSZ TBCs with 20 mol.% Al2O3 and 5 mol.% TiO2 in complete solid solution, even when these oxides are expected to have

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Fig. 8. (A) Bright-field TEM micrograph of CMAS-interacted (1200 °C, 24 h) YSZ+20Al+5Ti APS TBC from the CMAS-arrest region (arrow in Fig. 7A). SAEDPs from: (B) ZrO2, (C) CMAS glass, and (D) CaAl2Si2O8 (anorthite) regions. In (B) and (D) “T” indicates transmitted spot, and B indicates zone axis. (E) EDS spectrum from the CaSi2Al2O8 region; vertical red lines denote energies for Al and Si elemental lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

little or no solubility in YSZ [42,43]. This extended solubility is attributed to the atomic-level mixing of the constituent elements in the chemically prepared powder feedstock and the rapid processes involved in the APS method that kinetically suppress any precipitation [44,45]. The presence of three types of t-ZrO2 phases in the as-sprayed YSZ+20Al+5Ti APS TBCs could be due to local heterogeneities in powder compositions and/or in thermal histories during the APS processing. We have also shown that the YSZ+20Al+5Ti APS TBCs have good thermal stability. The precipitation of stable a-Al2O3 upon heat-treatment (without CMAS) of YSZ+20Al+5Ti APS TBCs is the result of diffusion of metastable Al solute out of t-ZrO2. Earlier thermal stability studies on metastable ZrO2-based SPPS coatings [31,32] and APS powders [45], both with Al2O3 in complete solid solution, showed similar a-Al2O3 precipitation. While near-complete Al-depletion and destabilization of the tetragonal ZrO2 to the monoclinic phase were observed in some of those earlier studies [31,32], in the present study complete Al-depletion and destablization of t-ZrO2 is not observed. It appears that the concentration of remaining solutes (Al, Y) is still sufficient to stabilize completely the tetragonal phase of ZrO2. These differences are most likely due to the higher heat-treatment temperatures (1400– 1500 °C) used in those earlier studies [31,32] compared to the present study (1200–1300 °C), resulting in faster precip-

itation kinetics. Note that 1300 °C was used as the highest temperature in the present study because it represents the upper end of typical TBC surface temperatures found in modern gas-turbine engines. Also note that TBCs in actual engines have a thermal gradient across their thickness, resulting in temperatures lower than 1200 °C in the interior of the TBCs, further slowing the a-Al2O3 precipitation kinetics. It appears that the diameter of the a-Al2O3 precipitates observed in Fig. 5 is limited to 300 nm. This size of the second phase a-Al2O3 precipitates is well below the estimated critical inclusion diameter lC of 8.6 lm for spontaneous microcracking, for a situation where inclusions of a lower CTE are embedded in a brittle matrix with a higher CTE. This is calculated using the following simple relation [46]: 2

lC ¼ 11pðK IC =rR Þ

ð1Þ

where KIC is the toughness of the matrix and rR is the CTE mismatch residual stress in the inclusion given by [47]: rR ¼ fDaDT g=fð1 þ mM Þ=2EM þ ð1  2mI Þ=EI g

ð2Þ

Here Da is the CTE mismatch, DT is the temperature change, m is the Poisson’s ratio, and E is the Young’s modulus, with subscripts M and I representing matrix and inclusion, respectively. Assuming a dilute concentration of spherical a-Al2O3 inclusions embedded in a locally dense

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ZrO2 matrix and using the following values [46,47]: aM = 11  106 °C1, aI = 8  106 °C1, EM = 250 GPa, EI = 400 GPa, mM = mI = 0.22, and DT = 1275 °C (1300  25 °C) in Eq. (2), rR is estimated at 1 GPa. Using a nominal toughness of KIC = 0.5 MPa m0.5 for a YSZ APS TBC [48] in Eq. (1), lC is estimated at 8.6 lm. It is important to note that due to the extremely limited equilibrium solubility of Al in ZrO2 [42,43], significant coarsening of the a-Al2O3 precipitates is unlikely to occur during prolonged high-temperature exposure under actual engine conditions, assuring requisite thermal stability. Furthermore, this smallness of the a-Al2O3 precipitates is not only important for preventing microcracking but also for assisting in long-term CMAS-attack resistance, as discussed below. Here we have shown that the YSZ+20Al+5Ti APS TBCs are as effective in resisting CMAS attack as the SPPS TBCs of the same composition from the previous study [14]. Whereas the conventional 7YSZ APS TBCs of thickness 200 lm are completely penetrated and destroyed by the CMAS glass (Fig. 6), the YSZ+20Al+5Ti APS TBCs allow only 40 lm of CMAS penetration (Fig. 7) under the same conditions. These results also reinforce the important role played by Al and Ti in the YSZ+20Al+5Ti APS TBC in mitigating CMAS attack. First, consider CMAS attack of conventional YSZ APS TBCs. While the exact thermo-chemical mechanisms by which this occurs are not entirely clear at this time, there is general consensus that CMAS attack occurs by a combination of four phenomena occurring simultaneously [13,14]: 1. The molten CMAS glass wets YSZ and infiltrates into pores and cracks in the TBCs. At 1200 °C the CMAS glass of composition given in Table 3 has a viscosity of 20 Pa s, as estimated using the Iida model [14,49], allowing easy flow. 2. YSZ grains dissolve in the molten CMAS glass and then reprecipitate as globular grains that are depleted in Y solute. 3. CMAS glass penetrates YSZ grain boundaries, resulting in the energetically favorable dispersion of exfoliated YSZ grains in glass. 4. Possible diffusion of Y from YSZ grains to the CMAS glass occurs, in conjunction with counter-diffusion of Si, Ca, and Mg from the CMAS glass into YSZ grains. As mentioned earlier, in the case of TBCs in actual engines, the TBC temperature decreases in the interior, slowing the CMAS-attack kinetics as the penetration deepens [18]. Now consider the mitigation of CMAS attack in YSZ+20Al+5Ti APS TBCs. The clear evidence for crystalline anorthite in the CMAS-arrest region (Fig. 8) indicates that the CMAS mitigation mechanisms in the YSZ+20Al+5Ti APS TBCs are identical to those in the SPPS TBCs of the same composition in the earlier studies [14,20]. As mentioned in the Section 1, those mechanisms

involve uptake and accumulation of Al and Ti in the penetrating CMAS glass from the TBC, resulting in a shift of the CMAS composition to the anorthite field. Anorthite composition glasses containing Ti as a nucleating agent readily crystallize as anorthite (melting point 1553 °C), resulting in the formation of a crystalline anorthite/zirconia composite at the leading edge of the CMAS front, arresting its further penetration [14,20]. It appears that the Al and Ti uptake occurs during dissolution–reprecipitation of the YSZ+20Al+5Ti solid-solution grains in the CMAS glass and by direct diffusion of Al and Ti to the CMAS glass. Without such uptake in the conventional 7YSZ TBCs that are devoid of Al and Ti, CMAS glass does not crystallize and continues to penetrate into the TBC. Note that in these TBC/CMAS interaction experiments, the thickness of the CMAS layer used (35 mg cm2) is very high, and it corresponds to the final thickness of accumulating CMAS in an actual engine over prolonged service durations [18]. Therefore, while the proof-of-concept CMAS mitigation in YSZ+Al+Ti APS TBCs is demonstrated for only a 24 h duration here, the thermo-chemical integrity of the CMAS-arresting crystalline layer of anorthite/zirconia composite is likely to be sound for prolonged high-temperature exposure. This has been demonstrated in the earlier study [14] in the case of the SPPS TBCs. During the heat-treatment at 1200 °C it has been shown that a-Al2O3 precipitates form. However, by virtue of their small size (300 nm) and their homogeneous distribution they are likely to dissolve in the molten CMAS glass easily, because particle-dissolution times in molten glass typically scale with the square of the particle size [50]. This will contribute to Al uptake by the CMAS glass and assist in CMAS-attack mitigation. These results indicate that, provided the Al- and Ti-bearing second phases in YSZ-based TBCs are small, those TBCs will still resist CMAS attack. In other words, Al and Ti need not be in complete solid solution in CMAS-resistant YSZ TBCs, making the APS processing of these TBCs less demanding. In the earlier study it was shown that, while under isothermal conditions (no thermal gradient) the SPPS TBCs (YSZ+20Al+5Ti) are thermo-chemically resistant to CMAS attack, those TBCs failed thermo-mechanically [14]. This is because partial CMAS penetration is sufficient for causing loss of strain-tolerance, leading to spontaneous thermo-mechanical failure of the CMAS-penetrated TBCs in a single thermal cycle (no thermal gradient). However, the recent study by Drexler et al. [20] has shown that SPPS TBCs (YSZ+20Al+5Ti) tested under thermal gradient conditions with simultaneous CMAS deposition (simulating conditions in actual engines) are both thermo-chemically and thermo-mechanically resistant to CMAS attack. Since the thermo-chemical CMAS mitigation mechanisms in both SPPS and APS TBCs are identical, the new YSZ+20Al+5Ti APS TBCs are also expected to be thermo-mechanically durable in tests involving cyclic thermal gradient conditions with simultaneous CMAS accumulation on the TBC hot surface.

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5. Conclusions We have demonstrated that the commercial manufacturing method of APS can be used to fabricate CMASresistant YSZ-based TBCs containing 20 mol.% Al2O3 and 5 mol.% TiO2 in solid solution. The as-sprayed YSZ+20Al+5Ti APS TBCs comprise t-ZrO2 phases, one Al-rich and two Al-lean. No other crystalline phases (such as Al2O3, TiO2, or Y2O3) were detected in the as-sprayed YSZ+20Al+5Ti APS TBCs. Upon prolonged heat-treatment of these TBCs in air at 1200 °C or 1300 °C, aAl2O3 is found to precipitate. However, the size of those precipitates is limited to 300 nm, which is well below the critical size for CTE-mismatch microcracking. While conventional 7YSZ APS TBCs are fully penetrated and destroyed by molten CMAS glass (at 1200 °C), the YSZ+20Al+5Ti APS TBCs are found to be highly resistant to CMAS attack under the same conditions. The molten CMAS front is found to penetrate only about a third of the YSZ+20Al+5Ti APS TBCs thickness before it is arrested due to the formation of crystalline anorthite at the leading edge. The CMAS-attack mitigation mechanisms are found to be identical to those found in earlier studies involving YSZ+20Al+5Ti TBCs deposited using the SPPS method [14,20]. The versatility, ease of processing, and low cost offered by the APS method has broad implications for the design and fabrication of next-generation CMAS-resistant TBCs for future gas-turbine engines. Acknowledgements

[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

[23]

[24] [25] [26] [27] [28]

The authors thank Mr. L. Flower (Haynes), Dr. R. Kowalik (NavAir) and Dr. B. Nagaraj (GE Aviation), for their help with this project. We also thank Ms. N. Ahlborg, Dr. K.M. Reddy, and Ms. R. Sample of the Ohio State University for experimental assistance. The research at the Ohio State University was supported by a grant from ONR (award no. N00014-08-1-0458) monitored by Dr. D. Shifler. Additional support was provided by the DoE (award no. DE-NT0006552). The research at the University of Extremadura was supported by a grant from the Ministerio de Ciencia y Tecnologı´a (award no. MAT 2007-61609). The research and facilities at Stony Brook University is supported in part by NSF (award no. CMMI-0605704) and via the Consortium for Thermal Spray Technology.

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