Simulated SOFC Interconnect Performance of Crofer 22 APU with ...

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Electrochemical and Solid-State Letters, 11 4 B54-B58 2008
1099-0062/2008/114/B54/5/$23.00 © The Electrochemical Society
Simulated SOFC Interconnect Performance of Crofer 22 APU
with and without Filtered Arc CrAlON Coatings
P. E. Gannon, a,z A. Kayani, b C. V. Ramana, b M. C. Deibert, a R. J. Smith, b and
V. I. Gorokhovsky c, *
a Chemical and Biological Engineering, and b Physics, Montana State University, Bozeman,
Montana 59717-3920, USA
c Arcomac Surface Engineering, Bozeman, Montana 59715, USA
The solid oxide fuel cell SOFC interconnect performance of Crofer 22 APU Crofer 22 APU “High Temperature Alloy” MSDS
No. 8005 June, 2004 ThyssenKrupp VDM 2004 with and without protective large area filtered arc coatings V. I. Gorokhovsky,
R. Bhattacharya, and D. G. Bhat, Surf. Coat. Technol., 140, 822001 from the Cr–Al–O–N elemental system has been investigated
as a function of exposure to simulated SOFC cathode-gas phase conditions at 800°C. Area specific resistance ASR was
measured using a four-point probe method with platinum paste as an electrical contact. The sample surface morphology and
chemical composition were assessed using scanning electron microscope/energy-dispersive spectroscopy and Rutherford backscattering
spectroscopy. Significantly improved surface stability and ASR were observed for coated vs uncoated samples. Changes
in coating composition and/or architecture were also observed to influence the SOFC interconnect relevant performance.
© 2008 The Electrochemical Society. DOI: 10.1149/1.2838038 All rights reserved.
Manuscript submitted October 20, 2007; revised manuscript received December 28, 2007.
Available electronically January 28, 2008.
Planar solid oxide fuel cells SOFCs are increasingly promising
candidates for future energy conversion due to their inherently high
efficiencies and decreased environmentally sensitive emissions. 1 A
typical anode-supported planar SOFC design is diagrammed in Fig.
1. This cell unit is anticipated to produce electric power at
0.5 W/cm 2 at an operating voltage of 0.7 V with a fuel conversion
efficiency of 40%. 2 A 5 kW modular SOFC stack design
with a 100 cm 2 active area at each cell would require 100 individual
cells in electrical series.
SOFC units are separated by hermetic, electrical current collecting,
and stack supporting interconnect components. During operation,
the SOFC interconnect realizes a simultaneous dual atmosphere
wet reducing and oxidizing exposure up to 800°C. The
interconnect/electrode and interconnect/seal interfaces must exhibit
chemical, thermal-mechanical, and electrical stability throughout the
desired SOFC stationary device lifetime of 40,000 h, and endure
several hundred thermal cycles. 2 Previous ceramic SOFC interconnect
materials e.g., LaCrO 3 operated reasonably well at higher
temperatures 1000°C, but suffered from high costs and difficulties
in fabrication. The recent advancement in SOFC materials has
reduced operating temperatures to below 800°C, allowing for a
greater selection of interconnect materials.
High-temperature metallic alloys have received attention for use
as intermediate-temperature 600–800°C SOFC interconnects due
to their higher relative toughness and formability and much lower
costs compared to their ceramic counterparts. Of particular interest
are high Cr content, ferritic stainless steels, which exhibit compatible
thermal expansion coefficients with other SOFC components,
but form electrically resistive thermally grown oxide TGO scales
when exposed to the complex SOFC operating gases. TGO scales
can also introduce adverse chemical and thermal-mechanical incompatibilities
with adjoining SOFC components through deleterious
species volatilization, interdiffusion, and thermal-mechanical
stresses. A thorough investigation of several heat-resistant alloys
concluded that, for improved oxidation resistance and electrical conductivity,
either new alloys will need to be developed or surface
engineering of the existing alloys will be required. 3 Among the candidates
in the former category is Crofer 22 APU, a low-Si ferritic
stainless steel 20–24% Cr, with engineered additions of Mn, Ti,
and La available from ThyssenKrupp VDM. This high-temperature
specialty stainless steel is characterized by the formation of a stable
and electrically conductive Cr–Mn oxide surface layer during SOFC
cathode gas-phase exposure. 4 However, at 800°C, continued TGO
scale growth dominated by an underlying Cr 2 O 3 layer during extended
exposures may create an increased electrical resistance and
other SOFC incompatibilities.
The present work is focused upon further enhancement of Crofer
22 APU via the large-area filter arc deposition LAFAD surface
engineering technique. 5 Various, nanolayered Cr–Al–O–N LAFAD
coatings have been applied to sample coupons of Crofer 22 APU
alloy in an attempt to improve its SOFC-interconnect performance
characteristics. The research is aimed toward understanding the influence
of coating layer characteristics thickness, structure, elemental
and phase composition on long-term stability during SOFC exposure.
The related research in this area is expansive and includes
the deposition of conductive oxide and nitride coatings using a wide
variety of conventional and advanced techniques. 1,6-15
A broader goal of this research is to enable the use of inexpensive
alloys for SOFC interconnects by developing a coating material
system and deposition process to ensure electrical, chemical, and
thermal-mechanical compatibility with other SOFC stack components,
throughout the device’s lifetime. The use of coatings to improve
the oxidation resistance on metal alloys has been known for
many years. The Cr–Al–O–N coating system was selected for this
study due to a previously demonstrated oxidation and wear resistance
at temperatures up to 900°C. 9-11 In this study, nanolayered
structures consisting of alternating AlO/N and CrO/N layers were
investigated the form “O/N” is meant to denote oxygen and
nitrogen-containing solid-state species. It is known that exposing
highly conductive CrN to oxygen at elevated temperatures leads
eventually to the loss of nitrogen and formation first of Cr 2 N, then
Cr 2 O 3 , 16 a semiconductor with sufficiently low resistivity for the
interconnect application at 800°C. 17 However, the volatile Cr species
from the Cr 2 O 3 -based TGO scales electrochemically reduce at
the cathode/electrolyte/gas interface, forming deleterious solid Cr
species. 18 The oxidation of AlN leads to the formation of Al 2 O 3 with
an unacceptably low conductivity for SOFC interconnects, yet alu-
* Electrochemical Society Active Member.
z E-mail: pgannon@coe.montana.edu
Figure 1. Typical anode-supported planar SOFC repeat unit design.

Electrochemical and Solid-State Letters, 11 4 B54-B58 2008
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Table I. Filtered arc Cr(O/N)/Al(O/N) nanolayered coatings.
Sample
ID
Sample description
Coating
thickness
m; Bilayer
thickness
nm
Near surface
composition
atom %
by RBS
B3 CrO/N/AlO/N–3 rpm 1.0 a 30 N, 30 O,
13 Cr, 27 Al
B9 CrO/N/AlO/N–9 rpm 1.0 b 34 N, 20 O,
13 Cr, 33 Al
a Ref. 1.
b Ref. 4.
mina is known to be a good oxidation-resistant diffusion barrier and
may form Cr-retentive and/or blocking phases. Combining the positive
attributes of Cr and Al oxides, nitrides, and oxinitrides in nanolayered
LAFAD coating structures and investigating SOFC
interconnect-relevant behavior is the focus of this study.
Long-term 1000 h testing results from two unique coatings
Table I are presented herein.
These are both 1 m Cr/Al oxinitride nanolayered coatings
with different layer thicknesses. It is expected that the thin
aluminum-containing layers will be sufficiently discontinuous to
have electron conduction pathways, or if continuous, will be sufficiently
thin to have a significant electron transport via defects and/or
tunneling. Of particular interest are the effects of the coatings’
diffusion-barrier properties on the SOFC interconnect-related performance
characteristics. Long-term testing and analyses on other similar
coatings from the Cr–Al–O–N system are ongoing.
The early stages of oxidation behavior of other coatings within the
Cr–Al–O–N system on 440A substrates are reported elsewhere. 19
Experimental
CrO/NAlO/N nanolayered coatings were deposited on
1.6 cm 2 1 mm thick electropolished substrate coupons of Crofer
22 APU by Arcomac Surface Engineering, LLC using the patented
LAFAD technology. 5 LAFAD employs a rectangular plasmaguide
chamber with two rectangular deflecting coils installed on
opposite sides, as shown in Fig. 2. The LAFAD details and design
advantages are provided elsewhere. 5
The substrates were mounted on pedestals distributed about the
outer rim of a rotating carousel in the LAFAD chamber. The substrate
temperature during deposition was about 500°C. The substrates
were first cleaned in an Ar ion plasma at 8 10 −2 Pa for 20
min, followed by 2 min of high-voltage Cr ion etching in Ar 2
10 −2 Pa. Cr and Al ions were then deposited in mixed oxygen/
nitrogen reactive atmospheres at 4 10 −2 Pa with an applied
substrate bias voltage of −50 V at 40 kHz. With both target sources
on and the substrate rotation engaged, the substrates were successively
exposed to Cr, then Al ions, in reactive oxygen/nitrogen atmospheres,
resulting in bilayers of CrO/N/AlO/N. Two separate
LAFAD processes with different substrate rotation speeds yielded
two CrO/N/AlO/N coatings with different bilayer thicknesses
with the same total thickness.
The coatings’ elemental composition was determined by energy
dispersive X-ray spectroscopy EDS and Rutherford backscattering
spectroscopy RBS. Total coating thickness was measured using the
CALO spherical abrasion technique and optical microscopy of the
wear scar with an accuracy of ±0.1 m. The thicknesses of the
coatings’ individual bilayers were estimated using the rotation speed
of the substrate carousel, deposition time, and total coating thickness.
For the coatings considered here Table I, the bilayer thicknesses
were estimated at 3 and 1 nm for 3 and 9 rpm modes,
respectively. The coating adhesion was qualitatively evaluated using
the Mercedes indentation test, using a Rockwell C indenter with
100 N load both coatings presented exhibited an excellent HF1
adhesion. 20 The indentations on the samples also allowed analysis
of the oxidation of highly damaged coating areas.
Oxidation of the sample coupons in Bozeman, MT air was
carried out using a standard bench-top furnace operated with no
control of humidity or air circulation. Measurements of area specific
resistance ASR were made using standard procedures with Pt paste
electrodes 3 on preoxidized samples as a function of time and temperature
for coated and uncoated Crofer 22 APU coupons. A schematic
illustrating the ASR sample measurement apparatus is shown
Figure 2. Schematic drawing of the
LAFAD surface engineering system.

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Electrochemical and Solid-State Letters, 11 4 B54-B58 2008
increasing dc voltage across the sandwich with the electrical current
recorded; when the current density reached 0.5 A/cm 2 , the power
supply was set to the constant current mode and the ASR recorded.
ASR measurements for coated samples extended to 1400 h.
Results
Figure 3. Color online Schematic drawing of the ASR measurement system.
in Fig. 3. Prior to ASR measurements, all coupons were oxidized in
800°C air for 100 h, with a furnace temperature ramp rate of
5°C/min. Subsequent to the coupon preoxidation, the Pt paste was
applied to an 1 cm 2 contact area on two identical samples, and
cured for 30 min at 110°C in air. Two Pt wires were spot welded
local oxide scale/coating removed to the alloys opposite the Pt
paste contact area, and the sample sandwich was assembled. To
ensure sandwich electrical contact, the assembly was placed between
two metal blocks 0.5 kg with alumina spacers. In addition,
one identical “witness” sample for subsequent surface analyses
was placed next to the sandwich. The entire apparatus was then
inserted into the furnace with a type-K thermocouple near the apparatus
and alumina tubes to insulate the Pt wires.
The following sequence of experimental procedures was used.
First, the system was heated to 800°C before applying a small and
The ASR results for the uncoated and coated Crofer 22 APU are
shown in Fig. 4. The oxidation behavior of the uncoated Crofer 22
APU as related to SOFC interconnects has been investigated
extensively. 12,21 The ASR values for the uncoated Crofer 22 APU
typically decreased for 100–300 h, reached a minimum value, and
subsequently increased. This increase was observed to continue during
1800 + hours tests. The TGO scale formed on the Crofer 22
APU is well-characterized by its duplex nature, having a slowforming
conductive Mn–Cr oxide surface layer reported to exhibit
spinel Mn,Cr 3 O 4 crystalline phases with a dominant, underlying
Cr 2 O 3 -based layer. 12,21 A scanning electron microscope SEM
image of the Crofer 22 APU surface after oxidation for 150 h is
shown in Fig. 5a. The crystals are indeed composed of Mn–Cr oxides,
as confirmed by a SEM/EDS elemental map cross section after
a 750 h ASR test Fig. 5b. This further illustrates the duplex nature
of the TGO and provides evidence to explain the observed ASR
behavior. 12 Relative to their uncoated counterpart, a very gradual
decrease in ASR was observed for both coated samples. Sample B3
seemed to reach a minimum and stable ASR value at 1200 h,
whereas the ASR values for sample B9 continued to decrease past
Figure 4. Color online ASR data for
coated and uncoated Crofer 22 APU.
Figure 5. Color online Surface image
a and b cross-section elemental map of
uncoated Crofer 22 APU after a 150 and
b 750 h in 800°C air.

Electrochemical and Solid-State Letters, 11 4 B54-B58 2008
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Figure 6. Surface SEM images of coated Crofer 22 APU before and after 1400 h in 800°C air surface crystallites are Mn-rich oxides.
1400 h. The gaps in the data represent times for thermal cycling
associated with unanticipated power losses and ancillary investigations.
Figure 6 displays SEM surface images of LAFAD coated Crofer
22 APU Table I surfaces before and after 1400 h oxidation.
Both coatings as deposited are predominantly amorphous, as indicated
from the frothlike appearance of their surfaces and the absence
of X-ray diffraction peaks other than from the substrate steel. After
oxidation, Mn-rich oxide crystallites were observed on both coating
surfaces, with a significantly higher concentration on the B9 coating
surface. Away from these surface crystallites, the coating was relatively
unchanged from its preoxidized condition. Mn-rich oxide surface
crystallites were observed to concentrate at the damaged coating
areas. This observation is illustrated in Fig. 7, which shows an
Figure 7. Surface SEM images of indentation
test area on coating B3 before and
after 1400 h in 800°C air.

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Electrochemical and Solid-State Letters, 11 4 B54-B58 2008
indentation mark from adhesion testing on coating B3, before and
after the 1400 h oxidation in 800°C air. Inside the indentation, Mnrich
surface crystallites are abundant, and the composition is similar
to that of uncoated Crofer 22 APU after the same exposure. In the
areas away from the indentation, Mn-rich crystallites are sporadic,
and appear to form preferentially along the roll grinding marks from
the substrate surface finishing process.
Discussion
TGO scale growth is often limited by the outward diffusion of
reactive metal ions from the alloy toward the gas phase, or a dominant
inward diffusion of oxygen ions derived from the atmosphere.
This process is greatly affected by the diffusivity of these species
through the initially established TGO scale. Diffusivity is a function
of the local TGO scale composition and morphology, which is determined
by bulk alloy composition and its thermal exposure history.
The initial ASR decrease observed for uncoated Crofer 22 APU
Fig. 4 was likely a function of the slow-forming, conductive
Mn–Cr oxide surface layer. Upon establishment and development of
the Mn–Cr oxide surface layer, the subsequent ASR increase was
likely the result of a continued growth of the underlying Cr 2 O 3 layer
within the duplex-natured TGO scale. The continued Cr 2 O 3 layer
growth may be limited by the outward diffusion of Cr from the bulk
alloy, inward diffusion of oxygen ions through the Mn–Cr oxide
surface layer from the atmosphere, or some combination thereof.
Whichever the mechanism, in 800°C air, the continued TGO scale
growth on the uncoated Crofer 22 APU will limit its applicability as
an SOFC interconnect material.
The relatively slow ASR decrease observed for both coated Crofer
22 APU samples may indicate the inhibited formation of the
conductive Mn–Cr oxide surface layer observed on the TGO scale
of the uncoated counterpart. Because there was no Mn in the coatings
as deposited, the Mn and perhaps the Cr ionic species transported
through the coating from the Crofer 22 APU substrate to
form the observed oxide crystals at the coating surface Fig. 6. The
Mn-rich oxide surface crystallites were more prevalent on the surface
of the B9 coating vs the B3 coating, perhaps indicating an
enhanced stability in thicker CrO/N/AlO/N bilayers Fig. 6. In
both coated samples, the Mn-rich oxide surface crystals appeared to
concentrate at the damaged areas of the coated Crofer 22 APU, e.g.,
at indentations, and along the roll marks from the rough steel surface
finish Fig. 7. The surface finishing features of the substrate steel
i.e., the roll marks seem to propagate through the coating, which
may serve as enhanced ionic transport pathways, resulting in the
observed Mn–Cr surface crystals concentrating at the damaged areas
of the coating. These ionic transport pathways may also serve as
electronic conduction pathways and, throughout their development,
effectively reduce the ASR of the coating over time Fig. 4. The
electron conduction mechanism through the bulk coating may be a
combination of metallic conduction through regions containing CrN,
mobility of thermally activated charge carriers in the Cr 2 O 3 , or tunneling
through layers of AlN, AlON, or Al 2 O 3 . Because the initial
observed ASR values for the coatings were high, the bulk coating’s
electronic conductivity was unacceptably poor; however, afterward
the ASR was observed to slowly reduce throughout the 1400 h test.
Having a similar or better ASR as the uncoated Crofer 22 APU, the
coated samples exhibited an acceptable conductivity and stability for
the SOFC interconnect application.
The efficacy of further investigation into similar Cr–Al–O–Nbased
coatings is grounded in the increased thermal stability and
decreased ASR observed in LAFAD coated Crofer 22 APU samples.
Future work will concentrate on further enhancement of both specialty
alloys like Crofer 22 APU, and more common hightemperature
ferritic steels, such as AISI 430. In both cases, the surface
finish prior to the coating deposition in its relation to the
surface oxidation behavior will be of interest. In addition, more
accurately simulated SOFC interconnect-related exposures, e.g.,
contact with cathode materials, dual atmospheres, etc., will be explored
to adequately assess the benefit of the LAFAD surface treatments
for the SOFC interconnect application. This will be coupled
with advanced cross-sectional microscopic and spectroscopic analyses
to determine the evolution of these materials during SOFC interconnect
relevant exposures.
Conclusion
Coated and uncoated Crofer 22 APU coupons have been investigated
as a function of exposure to SOFC-cathode gas phase conditions
at 800°C for up to 1400 h. The continued TGO scale growth
observed on the uncoated coupons may indicate long-term incompatibilities
as SOFC interconnects. Nanolayered filtered arc CrO/
N/AlO/N coatings were observed to significantly extend the thermal
stability of Crofer 22 APU. The enhanced improvement is likely
the result of the oxygen diffusion-barrier characteristics of the coatings,
along with an evolving network of conductive Mn-containing
oxides. The filtered arc coating with thicker CrO/N/AlO/N bilayers
3 vs 1 nm exhibited higher diffusion-barrier properties as
judged from the lesser concentration of Mn-rich oxide surface crystallites
after oxidation. Future work will focus on further enhancement
of Crofer 22 APU and similar commercial ferritic steels
through an extension of the basic coating system presented here.
Acknowledgments
We acknowledge the technical assistance of Norm Williams, Lyman
Fellows, and John Getty at Montana State University. Coatings
were skillfully prepared by Duane Jones and Oleg Popov. Portions
of this work were supported through the High-Temperature Electrochemistry
Center HiTEC supported by a DOI and DOE subcontract
from PNNL, no. 3917413060-A.
The Montana State University assisted in meeting the publication costs of
this article.
References
1. S. C. Singhal and K. Kendall, High-Temperature Solid Oxide Fuel Cells: Fundamentals,
Design and Applications, Elsevier Science, Ltd., Oxford 2004.
2. M. Williams, in Proceedings of the 7th International Symposium on Solid Oxide
Fuel Cells (SOFC VII), S. C. Singhal and H. Yokakawa, Editors, p. 3, The Electrochemical
Society Proceedings Series, Pennington, NJ 2003.
3. Z. Yang, K. S. Weil, D. M. Paxton, and J. W. Stevenson, J. Electrochem. Soc., 150,
A1188 2003.
4. Crofer 22 APU “High Temperature Alloy” MSDS No. 8005 June, 2004 Thyssen-
Krupp VDM 2004.
5. V. I. Gorokhovsky, R. Bhattacharya, and D. G. Bhat, Surf. Coat. Technol., 140, 82
2001.
6. S. Elangovan, S. Balagopal, M. Timper, I. Bay, D. Larsen, and J. Hartvigsen, J.
Mater. Eng. Perform., 13, 265 2004.
7. Y. Yoo and M. Dauga, in Proceedings of the 7th International Symposium on Solid
Oxide Fuel Cells (SOFC VII), S. C. Singhal and H. Yokakawa, Editors, p. 837, The
Electrochemical Society Proceedings Series, Pennington, NJ 2001.
8. N. Oishi, T. Namikawa, and Y. Yamazaki, Surf. Coat. Technol., 132, 582000.
9. M. Kawate, A. K. Hashimoto, and T. Suzuki, Surf. Coat. Technol., 165, 163
2003.
10. O. Banakh, P. E. Schmid, R. Sanjines, and F. Levy, Surf. Coat. Technol., 163–164,
57 2003.
11. S. PalDey and S. C. Deevi, Mater. Sci. Eng., A A342, 582003.
12. P. E. Gannon, V. I. Gorokhovsky, M. C. Deibert, R. J. Smith, A. Kayani, P. T.
White, S. Sofie, Z. Yang, D. McCready, S. Visco, et al., Int. J. Hydrogen Energy,
32, 3672 2007
13. C. Johnson, R. Gemmen, J. Poston, C. Scaeffer, N. Orlovskaya, L. Fegely, and C.
Rawn, in S. C. Singhal and J. Mizusaki, Editors, PV 2005–07, p. 1842, The Electrochemical
Society Proceedings Series, Pennington, NJ 2005.
14. T. Armstrong, M. Homel, A. Virkar, PV 2003–07, p. 841, The Electrochemical
Society Proceedings Series, Pennington, NJ 2003.
15. B. Rajendra, K. Nigel, and A. Petric, in S. C. Singhal and J. Mizusaki, Editors, PV
2005-07, p. 1859, The Electrochemical Society Proceedings Series, Pennington, NJ
2005.
16. F. H. Lu, H.-Y. Chen, and C.-H. Hung, J. Vac. Sci. Technol. A, 21, 671 2003.
17. K. Huang, P. Y. Hou, and J. B. Goodenough, Mater. Res. Bull., 36, 812001.
18. S. P. Jiang, Solid State Ionics, 146, 12002.
19. A. Kayani, R. J. Smith, S. Teintze, M. Kopczyk, P. E. Gannon, M. C. Deibert, V. I.
Gorokhovsky, and V. Shutthanandan, Surf. Coat. Technol., 201, 1685 2006.
20. H. Jehn, G. Reiners, and N. Siegel, DIN-Fachbericht(Special Report) 39, Charakterisierung
duenner Schichten (Characterization of Thin Layers), Beuth-Verlag,
Berlin 1993.
21. Z. Yang, J. Hardy, M. Walker, G. Xia, and S. Simner, and J. Stevenson, J. Electrochem.
Soc., 151, A1825 2004.