Plasma sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten Ca–Mg–Al–silicate glass

Surface & Coatings Technology 206 (2012) 3911–3916

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Plasma sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten Ca–Mg–Al–silicate glass Julie M. Drexler a, Chun-Hu Chen a, Andrew D. Gledhill a, Kentaro Shinoda b, Sanjay Sampath b, Nitin P. Padture c,⁎ a b c

Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794, USA School of Engineering, Brown University, Providence, RI 02912, USA

a r t i c l e

i n f o

Article history: Received 25 February 2012 Accepted in revised form 16 March 2012 Available online 27 March 2012 Keywords: Thermal barrier coatings Ceramics Gadolinium zirconate Silicate glass Crystallization

a b s t r a c t Air plasma sprayed (APS) Gd2Zr2O7 thermal barrier coatings (TBCs) are found to be highly effective in resisting high-temperature (1200 °C) penetration of molten Ca–Mg–Al–silicate (CMAS) glass deposit for prolonged durations (up to 1 week). In contrast, conventional APS 7YSZ TBCs are found to be fully penetrated by the molten CMAS glass under the same testing conditions. This resistance is attributed to the formation of a sealing layer made of crystalline Ca–apatite phase (based on Ca2Gd8(SiO4)6O2) as a result of the hightemperature chemical interactions between the APS Gd2Zr2O7 TBC and the CMAS glass. The resistance to penetration of molten silicate deposits offered by Gd2Zr2O7 composition TBCs is relatively insensitive to both the type of molten silicate deposits (CMAS sand, volcanic ash, coal fly ash) and the TBC microstructure (APS, EB-PVD). © 2012 Elsevier B.V. All rights reserved.

1. Introduction Thermal barrier coatings (TBCs) made of ZrO2 ceramics stabilized by Y2O3 (YSZ) are being widely used in gas-turbine engines to insulate and protect hot-section metal components (see e.g. reviews [1–3]). TBCs (100 μm to 1 mm thickness) have highly defective, porous microstructures, which impart them with the desirable properties of low thermal conductivity and high compliance (or straintolerance). There are two main types of commercial TBCs: those deposited by electron-beam physical vapor deposition (EB-PVD) with columnar grains normal to the coating/substrate interface and intercolumnar porosity, and others deposited by the low-cost process of air plasma spray (APS) which have porosity and cracks generally running parallel to the interface [1–3]. Both types of TBCs are enabling higher engine operating temperatures, which translate to higher power and efficiency. However, these higher operating temperatures are now engendering new materials issues in aircraft engines. Specifically, fine sand particles ingested by the engine deposit on the hotter TBC surfaces as molten Ca–Mg–Al–silicate (CMAS) glass, which penetrates 7YSZ TBCs resulting in loss of strain tolerance and premature failure of the TBCs [4–17]. Since airborne sand particles are ubiquitous, and there is an increasing demand for higher

⁎ Corresponding author. Tel.: + 1 401 863 2859; fax: + 1 401 863 9025. E-mail address: [email protected] (N.P. Padture). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.03.051

engine-operating temperatures, the CMAS-attack of 7YSZ TBCs, both APS and EB-PVD, is becoming a critical issue in the development of next-generation gas-turbine engines. To that end, TBCs of alternate compositions are being explored for mitigating attack by molten CMAS sand [10,13,15,17–21]. Majority of the published research in this area has been on: (i) APS TBCs of 7YSZ containing 20 mol% Al2O3 and 5 mol% TiO2 (7YSZ + Al + Ti) in the form of a solid solution [10,15,17,22] and (ii) EB-PVD TBCs of Gd2Zr2O7 composition [13]. While both EB-PVD and APS TBCs of Gd2Zr2O7 composition have been claimed to mitigate attack by molten CMAS sand in patents [18,20,21], detailed research results only for EB-PVD Gd2Zr2O7 TBCs have been published in the open literature [13]. In the EB-PVD Gd2Zr2O7 TBCs it has been shown, with the aid of a detailed microstructure-based explanation, that the formation of apatitetype phases results in the arrest of the penetrating molten CMAS glass front [13]. However, what has not been shown in the open literature is if similar mitigation mechanisms are operative in Gd2Zr2O7 TBCs made by the APS method that have microstructures vastly different from those of EB-PVD Gd2Zr2O7 TBCs. Recently, we have demonstrated that APS Gd2Zr2O7 TBCs are highly effective in resisting high-temperature attack by both Eyjafjallajökull volcanic ash [22], and lignite coal fly ash in the context of syngas-fired land-based gas-turbines engines for electricity generation [23]. Thus, the objective of this research is to investigate and report on the possible influence of TBCs microstructure on the effectiveness of APS Gd2Zr2O7 TBCs in mitigating attack by molten CMAS sand.

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Table 1 Chemical composition (wt%) of the CMAS sand used in this study [10]. Compositions of Eyjafjallajökull volcano ash [22] and lignite coal fly ash [23] are included for comparison. Trace constituents (b 0.5%) are not reported. wt.%

SiO2

CaO

FeO

Al2O3

Cr2O3

MgO

SO3

TiO2

SrO

MnO

K2O

Na2O

CMAS sand Volcano ash Fly ash

49.7 58.0 29.7

35.3 5.5 25.4

2.4 9.8 14.8

6.7 14.9 14.7

– – 5.1

3.3 2.3 3.6

– – 1.8

– 1.8 1.1

– – 1.0

– – 0.9

1.6 1.8 0.8

1.0 5.0 0.6

2. Experimental procedures All TBCs were deposited on Ni-based superalloy substrate (Grade 214, Haynes International, Kokomo, IN) “buttons” (25.4 mm diameter, 3.2 mm thickness), which is an oxidation-resistant, wrought alloy supplied in the solution-treated form. One circular surface of each “button” substrate was roughened by grit-blasting prior to coating deposition (a bond-coat was not used here). TBCs (thickness ~ 200 μm) of Gd2Zr2O7 composition were deposited by the APS method using an atmospheric direct-current plasma torch with an 8-mm diameter nozzle and a swirl flow gas distribution ring (Model F4-MB, Sulzer Metco Inc., Westbury, NY), as described elsewhere [22,23]. Commercially available granulated powders of Gd2Zr2O7 (TransTech, Adamstown, MD) were used as feedstock. The APS deposition parameters were determined through an optimization process [24] to produce high quality TBCs of similar microstructures. Reference APS 7YSZ TBCs deposited on the same substrate from previous studies were used for comparison purposes [22,23]. 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 [10]. Simulated CMAS glass frit of predetermined composition (Table 1) was prepared using procedures given elsewhere [10]. Note that this CMAS composition is similar to the one used in other studies [8,9], and it is typical of sand deposits in engines [8]. For comparison, compositions of Eyjafjallajökull volcano ash [22] and lignite coal fly ash [23] are also included in Table 1. CMAS/TBC interaction experiments were performed using the same procedure employed in the previous studies [10,17]. Briefly, a predetermined amount of the CMAS frit paste was used to cover uniformly the top surface of as-sprayed APS Gd2Zr2O7 and APS 7YSZ 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 (1 day) or 168 h (1 week) using a box furnace (Thermolyne, Dubuque, IA). As-deposited TBCs and CMAS-interacted TBCs were cross-sectioned and polished to a 1-μm finish using routine metallographic techniques. The cross-sections were then observed in a scanning electron microscope (SEM; Sirion, FEI Company, Hillsboro, OR), equipped with an energy dispersive spectrometer (EDS; EDAX, Mahwah, NJ) capable of determining elemental distributions in the microstructure. Porosities of the as-deposited TBCs were estimated from the SEM micrographs in conjunction with image analysis. Transmission electron microscopy (TEM) specimens from specific locations within the CMAS-interacted TBC cross-sections were extracted using focused ion beam (FIB; Helios 600, FEI, Hillsboro, OR) and in situ lift-off. These specimens were observed in a TEM (CM200, Philips, The Netherlands) operated at 200 kV. The TEM is equipped with EDS for elemental analysis. Indexing of the selected area electron diffraction patterns (SAEDPs) and phase identification was performed using standard procedures. 3. Results Fig. 1A and B are cross-sectional SEM images showing the microstructures of as-sprayed APS Gd2Zr2O7 TBC and APS 7YSZ TBC from

previous studies [22,23], respectively. The porosities in both TBCs are estimated at ~ 20%, and their typical APS microstructures are nominally the same. Fig. 2 shows data from a previous study [22] on high temperature (1200 °C; 24 h) interactions between molten CMAS glass and APS 7YSZ TBC. Fig. 2A is a cross-sectional SEM micrograph of APS 7YSZ TBC with CMAS on top that has been heat-treated at 1200 °C for 24 h, showing the complete penetration of the TBC. The dashed red line indicates the original top surface of the TBC. Fig. 2B, C, and D are corresponding Zr, Ca, and Si EDS elemental maps, respectively, showing clear evidence of Ca and Si (i.e. CMAS) reaching the bottom of the TBC, a penetration depth of ~200 μm. The general mechanisms involved in the CMAS attack of APS 7YSZ TBCs are described elsewhere [10,17,22,23]. In contrast, APS Gd2Zr2O7 TBCs subjected to the same CMAS interaction test (1200 °C; 24 h) show remarkable resistance to CMAS penetration in Fig. 3. The cross-sectional SEM micrograph of APS Gd2Zr2O7 TBC is shown in Fig. 3A, where the dashed red line indicates the original top surface of the TBC. Fig. 3B, C, and D are corresponding Zr, Ca, and Si EDS elemental maps, respectively, showing clear evidence of arrest of CMAS at a penetration depth of ~20 μm (~ 10% penetration). Fig. 4A–C are corresponding higher magnification images/ maps of the area denoted by the red box in Fig. 3A, showing the abrupt arrest of the CMAS front. Fig. 5A–D show the same dramatic CMAS-front arrest in APS Gd2Zr2O7 TBCs after prolonged CMAS

A

Substrate

100 µm

B

100 µm

Substrate

Fig. 1. Cross-sectional SEM micrographs of as-sprayed APS TBCs on Ni-based superalloy substrates: (A) 7YSZ and (B) Gd2Zr2O7 (adapted from [23]).

J.M. Drexler et al. / Surface & Coatings Technology 206 (2012) 3911–3916

A

3913

B

100 µm

Zr Map

C

D

Ca Map

Si Map

Fig. 2. (A) Cross-sectional SEM micrograph of APS 7YSZ 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 (adapted from [17]).

interaction of 168 h (1 week) at 1200 °C. (Note that the CMAS layer on top became detached during metallographic sample preparation.) Fig. 6A is a bright-field TEM image of a representative specimen extracted from the CMAS arrest region denoted by the white box in Fig. 3 (1200 °C; 24 h). This image shows a combination of rounded grains (denoted by B and D) and intergranular phase (denoted by C). Indexing of the selected area electron diffraction patterns (SAEDPs) (Fig. 6B) from bottom region B reveals the presence of the Gd2Zr2O7 phase, which is the unreacted TBC. The region C consists primarily of CMAS glass, as evinced by the amorphous halo in the SAEDP from that region (Fig. 6C). SAEDP in Fig. 6D from the arrest region (D) indicates the presence of a Ca–apatite phase, specifically one based on Ca2Gd8(SiO4)6O2. However, the presence of small

A

amounts of other crystalline phases in the CMAS arrest region cannot be ruled out. CMAS glass could not be detected in areas below the arrest region (not shown here). 4. Discussion The results presented here show clearly that the APS Gd2Zr2O7 TBCs are highly effective in resisting molten CMAS penetration at high temperatures (1200 °C) and for prolonged durations (upto 1 week). It is interesting to note that very similar results have been observed in studies involving interactions between APS Gd2Zr2O7 TBCs and other molten deposits of vastly different chemical compositions (Table 1): Eyjafjallajökull volcano ash [22] and lignite coal fly

B

100 µm

Zr Map

C

D

Ca Map

Si Map

Fig. 3. (A) Cross-sectional SEM micrograph of APS Gd2Zr2O7 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.

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B

A

20 µm

C

Zr Map

D

Ca Map

Si Map

Fig. 4. (A) Higher magnification cross-sectional SEM micrograph of APS Gd2Zr2O7 TBC that has interacted with CMAS glass (1200 °C, 24 h) from the region denoted by the red box in Fig. 3A, and corresponding elemental maps: (B) Zr, (C) Ca, and (D) Si.

A

B

100 µm Si Map

C

Gd Map Fig. 5. (A) Cross-sectional SEM micrograph of APS Gd2Zr2O7 TBC that has interacted with CMAS glass (1200 °C, 168 h or 1 week), and corresponding elemental maps: (B) Zr, (C) Ca, and (D) Si. The horizontal dashed line denotes top surface of the original TBC. (The CMAS layer on top became detached during metallographic sample preparation.)

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A

B

3915

SiO2

622 622

Volcano Ash

040 CMAS Sand

B=[103]

Lignite Fly Ash

CaSi

Ca3Si2

C

D

C

CaAl2Si2

Ca2Si Ca3Si

Al3Si2

Ca2Al2Si

CaO

Ca3Al2

Ca6Al7

CaAl2

CaAl4

CaAl12 AlO

1.5

Fig. 7. SiO2―CaO―AlO1.5 ternary diagram (mol%) showing compositions of the three deposits in terms of the three major components. (Oxygen is omitted in the binary and ternary compounds labels.)

D

103 111

B 2 µm

212

321

B=[341]

Fig. 6. (A) Bright-field cross-sectional TEM micrograph micrograph of APS Gd2Zr2O7 TBC that has interacted with CMAS glass (1200 °C, 24 h) from the region denoted by the white box in Fig. 3A. Indexed SAEDPs (white circle denotes transmitted beam; B denotes the zone axis): (B) from region B in Fig. 6A showing the presence of Gd2Zr2O7, (C) from region C in Fig. 6A showing the presence of CMAS glass, and (D) from region D in Fig. 6A showing the presence of Ca–apatite based on Ca2Gd8(SiO4)6O2.

ash [23] (for ready comparison, the compositions of these deposits in terms of only the three major components are plotted on a SiO2–CaO– AlO1.5 ternary diagram in Fig. 7). In those studies, Ca2Gd8(SiO4)6O2 apatite was also found to be the primary crystalline phase present in the arrest regions [22,23]. The general principles of the moltenglass arrest mechanisms in those studies are as follows. The Gd2Zr2O7 grains in the TBCs are wetted by the molten glass in contact with them and react with the glass, resulting in the incorporation of Gd in the glass locally. This triggers the crystallization of the Gd-rich glass into Ca2Gd8(SiO4)6O2 apatite, essentially providing a sealing layer that prevents further penetration of the molten glass above that layer. While small amounts of other phases do crystallize depending on the glass composition, it appears that the Ca2Gd8(SiO4)6O2 apatite phase plays the most important role in the arrest of the molten-glass front [22,23]. It appears that similar mechanisms are operative in the molten CMAS glass case studied here, despite its composition being quite different from the two ashes (Table 1 and Fig. 7). Since the crystallization of Ca2Gd8(SiO4)6O2 apatite needs just Ca-containing silicate glass enriched in Gd content locally, it is not surprising that the resistance offered by APS Gd2Zr2O7 TBCs to molten-glass penetration is relatively insensitive to the composition of the three molten deposits considered: CMAS sand, volcanic ash, and coal fly ash. Also, Ca2Gd8(SiO4)6O2 apatite is in equilibrium with the molten CMAS glass [13], and as such it continues to provide protection against glass penetration into the TBCs for a prolonged duration (1 week) at 1200 °C (Fig. 5).

Similar resistance to molten CMAS glass penetration has been observed in EB-PVD Gd2Zr2O7 TBCs [13], despite the vastly different microstructures of the EB-PVD and the APS TBCs. A detailed explanation for this resistance based on both the unique EB-PVD microstructures (vertical columnar grains, gaps between columns, column tips crystallography, feathery porosity, etc.) and the high-temperature chemical interactions has been provided by Krämer et al. [13]. However, the results presented here show that it is the high-temperature chemical interactions between the molten CMAS glass and Gd2Zr2O7 that dominate the observed effects, and the exact nature of the TBC microstructure does not appear to play a major role. Thus, it is argued that EB-PVD Gd2Zr2O7 TBCs are also likely to be resistant to penetration by other molten silicate deposits, such as volcano ash and coal fly ash. While Gd2Zr2O7 TBCs have significantly lower thermal conductivities relative to 7YSZ [3,25–29], they are more brittle and less erosionresistant reltaive to 7YSZ TBCs. Also, relative to 7YSZ, Gd2Zr2O7 is not as stable in contact with the Al2O3 thermally grown oxide (TGO) that forms between the ceramic top-coat and the metallic bond-coat at high temperatures [30]. However, the resistance to penetration of molten silicate deposits offered by Gd2Zr2O7 composition TBCs that is relatively insensitive to both the chemical composition of the silicate glass and the TBC microstructure makes these TBCs rather robust. 5. Conclusions APS Gd2Zr2O7 TBCs are found to be highly effective in resisting penetration of molten CMAS glass for prolonged durations. In contrast, conventional APS 7YSZ TBCs are found to be fully penetrated by the molten CMAS glass under the same testing conditions. This resistance is attributed to the formation of a sealing layer made of crystalline Ca2Gd8(SiO4)6O2 apatite phase as a result of the hightemperature chemical interactions between the APS Gd2Zr2O7 TBC and the CMAS glass. The resistance to penetration of molten silicate deposits offered by Gd2Zr2O7 composition TBCs is relatively insensitive to both the chemical composition of the silicate glass (CMAS sand, volcanic ash, coal fly ash) and the TBC microstructure (APS, EB-PVD). Acknowledgements The authors thank Mr. L. Flower (Haynes), Dr. R. Kowalik (NavAir), and Dr. B. Nagaraj (GE Aviation) for their help with this project. This research was supported by the Office of Naval Research (award nos.

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