A cyclic furnace study was conducted on thermal barrier systems to evaluate the effects of yttrium, chromium, and aluminum in nickel-base alloy bond coatings and the effect of bond coating thickness on yttria-stabilized zirconia thermal barrier coating life. Without yttrium in the bond coatings, the zirconia coatings failed very rapidly. Increasing chromium and aluminum in the Ni-Cr-Al-Y bond coatings increased total coating life. This effect was not as great as that due to yttrium. Increased bond coat thickness was also found to increase life

Stecura, S.

English

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NASA Technical Memorandum 81485
(NASA-TM-81485) EFFECTS GE YTTRIUM,
	 N60-22464
ALUMI19M AND CBROMIUM CGNCFhTRATIONS INI BOND
COATINGS CN ?HE PERFOhMANCE CP
ZIRCONIA-YSTBIA THERMAL BABRIERS (NASA)
	 Unclas
12 p HC AC2/MF A01
	 CSCL 11E G3/26 17998
EFFECTS OF YTTRIUM, ALUMINUM AND
CHROMIUM CONCENTRATIONS IN BOND
COATINGS ON THE PERFORMANCE OF
ZIRCONIA-YTTRIA THERMAL BARRIERS
Stephan Stecura
Lewis Research Center
Cleveland, Ohio
Prepared for the
International Conference on Metallurgical Coatings
sponsored by the American Vac,ium Society
San Diego, California, April 21-25, 1980
EFFECTS OF YTTRIUM, ALUMINUM AND CHROMIUM CONCENTRATIONS IN
BONO COATINGS ON THE PERFORMANCE OF ZIRCONIA-YTTRIA THERMAL BARRIERS
by Stephan Stecura
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
A cyclic furnace study was conducted on thermal barrier systems to eval-
uate the effects of yttrium, chromium, and aluminum in nickel-base alloy bond
coatings and the effect of bond coating thickness on yttria-stabilized zir-
conic thermal barrier coating life. Without yttrium in the bond coatings, the
zirconia coatings failed very rapidly. Increasing chromium and aluminum in
the Ni-Cr-A1-Y bond coatings increased total coating life. This effect was
not as great as that due to yttrium. Increased bond coat thickness was also
found to increase life.
INTRODUCTION
Significant progress has been made in improving the performance and add-
herence of two-layer thermal barrier systems (TBS) on metallic substratesl-4.
The reported Lewis Research Center data indicate that the performance of the
NiCrAlY/Zr0 2
-Y2O 3 duplex system is best at certain specific NiCrAI-Y bond coat-
ing and Zr022-Y203 (TBC) compositions. One of the better systems, Ni-11.0
Cr-5.4A1-0.35Y/Zr0--7.9Y^0 3*, has withstood 2000 1-hour cycles in a Mach 1.0
burner rig at a 14 250 0 E surface temperature.
This study was conducted to examine the effects of yttrium, chromium, and
aluminum in the bond coating and the effect of bond coating thickness on the
life of Zr02-Y203 . The evaluations were done in a cyclic furnace.
EXPERIMENTAL
The compositions of the bond coatings and Zr0L-Y 0 3 are given in tables
1-V. Substrates were cast nickel-base alloys, B-1900*Hf and MAR-M-200*Hf.
Flat specimens with all corners and edges rounded were used. The plasma spray
depositions of the bond coatings and ZrU 2
-Y20 3 was done in the same manner as
described in references 1 and 4. Coating thicknesses reported in the tables
are maximums. Bond coating thicknesses on the specimens used in the evalua-
tion of the effect of bond coating thickness on life were measured with a mi-
crometer. These measurements showed that thickness could vary by * 0.004 cm.
Evaluations were done in a furnace cycled between eitker 990 0 and 280 0 C or
1095 0 and 280 00. The cycle consisted of a 6 minute heat - lip, 60 minutes at
* All compositions in weight percent.
i
temperature, and 60 minutes of cooling to about 280 * C. Specimens were in-
spected every 12 cycles until failure, denoted by a visible crack, occurred in
the TBC. Cyclic furnace tests appear to be reliable for comparison purposes.
From prior investigations, it was found that the TBS having the longest life
in the cyclic furnace tests also had the longest life in cyclic natural gas-
oxygen torch rig tests as well as in cyclic Mach 1.0 burner rig testsl,2,4.
RESULTS AND DISCUSSION
Effect of yttrium-free Ni-Cr, Ni-A1 and NiCrAI bond coatings - The data in
table 1 indicate that the use of an yttrium-free bond coating (8.9 micrometers
rms) slightly improves the life of the yttria-stabilized zirconia TBC compared
to the case with no bond coating, where cleaned substrate surfaces were rela-
tively smooth (2.5 micrometers rms). if mechanical forces, due to interlock-
ing, play the major role in determining TBS life, then the presence of a bond
coating should result in a significant increase in life and the three bond
coatings in table 1 should have similar lives. This is not the case. The
greatest changes result from changes in bond coating composition. However
considerable roughness is required to achieve adherence of the oxide layer;.
Thus, chemi.al-electrostatic bonding and mechanical forces responsible for ad-
herence can act through a larger area. Failure of ZrU2-Y 03 applied directly
B-1900*Hf occurred by separation of the TBC from the subs rate. Many small
cracks in the Zr02-7.8Y 20 3
 and Zr02-11.5Y203 TBCs oriented either parallel or
acute angles to the substrate were observed (Fig. 1).
Effect of ttrium on NiCr and NiAI bond coatings - The data in tables 11 and
TIT Show	 at	 e addition o y riumhads very signifiant beneficial effect
on TBS life. Additions of 0.52Y improved life many times while 1.52Y addi-
tions were only slightly beneficial. Substitution of chromium for aluminum
does not affect the life of TBCs as much as does the addition of yttrium.
Metallographic studies of failed ZrU2-7.8Y 203 coated specimens having yttri-
um-free bond coatings revealed failure at the substrate-bond coating inte^face
and complete bond oxidation at 990 6 C. The Ni-19.8Cr-0.53Y bond coating after
longer exposure at 990 ` C was less extensively oxidized : However, the Ni-19.8
Cr-0.53Y bond coating underwent more oxidation at 1095 C (Fig. 2a) than did
the Ni-19.3AI-0.52Y bond coating (Fig. 2b). With yttrium in the bond coatings
failure typically initiated through formation of cracks within the TBC near
the bond coating, as shown in figure 2, and extension of these internal cracks
to the surface. The oxide stringers formed within the as-deposited Ni-19.3
Al -0.52Y bond coating were identified to be principally aluminum oxides.
These stringers grew during testing. Electron microprobe data also suggested
that yttrium in the bond coating is diffusing toward the bond coating-TBC in-
terface.
Effect of bond coating Cr and Al content - The data in tale 1V show that Cr
and Al alsoave a significant ertecFon TBS life. At 990 C, all but the
Zr42_17.4Y2033 TBCs withstood 1500 1-hour cycles. At 1095 0 C, the best bond coat-
ing was Ni-25.7Cr-5.6A1-0.3?.Y, which when coupled with Zr02_7.8Y20 3
 gave more
than twice the life of a Ni-16.4Cr-5.1A1-0.15Y/ZrO 2
-7.8Y 20 3 system.
Figure 3 shows Ni-25.7Cr-5.6A1-0.32Y before testing. It is representative
of other bond coatings sprayed in air in that it contains oxide particles and
on
at
stringers at particle boundaries, is quite rough and convoluted, and is not of
uniform thickness. Figure 4 shows the degree of oxidation of the bong coating
in the Ni-25.7Cr-5.6A1-0.32Y/1r0 2-7.8Y203 system after testing as 990 and 1095°C.
Metallography indicated that oxidation resistance decreased from Ni-25.7Cr-5.6
A1-0.32Y, to Ni-19.9Cr-19.2A1-0.33Y, to Ni-16.6Cr-10.6A1-0.33Y as did the life
of the TBS. Failure of the oxides with these bond coatings originated within
the TBC near the bond coating-TBC interface.
Effect of bond coating thickness - The data in table V show that the thicker
the bond coating, the longer the TBS life. Uniformity of bond coating thick-
ness is probably also important because a thin spot could lead to a local,
premature failure. The data indicate that manually applied bond coatings
should be at least 0.015 to 0.020 cm thick. The bond coating thickness values
reported in table V indicate the ranges measured with a micrometer before the
oxide was applied. Thinner areas were observed by metallographic examina-
tions. Photomicrographs of the NiCrAI-0.32Y/LrO2-1.dY203 TBS showed that the
0.003 to 0.007 cm thick bond coating, which failed after 1419 1-hour cycles at
990 `C, was nearly completely oxidized. Metallographic examinations of speci-
mens tested at 990'C showed that as bond coating thickness increased, the
thickness of the unoxidized bond coating next to the substrate also in-
creased. Similar trends were noted after 1095°C tests.
The effect of bond coating thickness on life is even more strikingly shown
by results obtained at 1095°C with 0.020 cm thick Ni-25.7Cr-5.6A1-0.32Y bond
coatings coupled with 1.8Y 0 3- and 6.1Y 20 3-stabilized zirconia coatings. The
former withstood 635 and 696 cycles and the latter withstood 668 and 681
1-hour cycles. This is about a 2.5-fold increase in life resulting from an
increase in bond coating thickness from about 0.011 to about 0.020 cm. Bond
coating thickness did not affect the failure mechanism.
SUMMARY OF RESULTS AND CONCLUSIONS
Bond coating resistance to oxidation has a significant effect on the ad-
herence and life of the TBS. The presence of yttrium in the bond coating is
very critical. Without it, the TBS fails rapidly at the substrate-bond coat-
ing interface. With yttrium, failure occurs within the oxide coating near the
bond coating-oxide coating interface. Yttrium, aluminum, and/or chromium in
the bond coating critically affect TBS life. The best bond coating was
Ni-25.7Cr-5.6A10.32Y which was about 2 times better than Ni-16.4Cr-5.1
N1-0.15Y. Coating life improvements of about 1.5 to 2.0 times were obtained
with bond coatings exceeding 0.010-0.012 em.
REFERENCES
1. S. Stecura, NASA TM X-3425, (1976).
2. S. Stecura, Am. Ceram. Soc. Bull. 56, 1082, (1911).
3. S. Stecura and C. H. Liebert, U.S. Patent No. 4,055,705, (1977).
4. S. Stecura, NASA TM X-78976, (1978).
5. R. C. Tucker, T. A. Taylor, and M. H. Weatherly, Third Conference on Gas
Turbine Materials in a Marine Environment, (Bath Univ., England, 1976),
ession VII, Paper 2.
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TABLE V. - EFFECT OF BOND COATING TMCKNBU ON YTTRIA-11TABILIZED
ZIRCONIA THERMAL BARRIER COATING LIFE
(Cyclic furnace tot results)
Bond coating Thermal barrier coating Average number
of cycles toComposition. Thickness Com poritinm. Thl.kturse. failure for two
n-t% ranwe. wt% cm specimens
cut
9900 - 280o C
NI-16.4Cr-5,eAi-0.33Y 0,003-0.007 ZrO2T.AY203 0.026 1419
-0.32Y .005- .006 -7.$Y203 .028 1529
-0.32Y .012-	 .018 -7,6Y303 .032 '1610
-'1,32Y .018- ,026 -7,$Y203 .032 "1610
-0.32Y .026- .032 -7.8Y203 ,030 '1629
Ni-16.4Cr-5.8A1-0.32Y .002- .007 Zr0_ 11.8Y203 ,036
-0.32Y .004-	 .010
-11.8Y203 ,034 7 t
-0.32Y .009- .015 -11.8Y203 .039 1294
-0.32Y .018-	 ,027 -11.8Y203 .038 1119
-0.32Y .020-	 .027
-11.8Y203 ,039 1132
Nt-16,$Cr-5.b Al-0.62Y .005-	 .009 Zr01 7.8Y20s .036 676
-0.62Y .007-	 .010 -7,NY203 .040 726
-0,62Y Oli-	 .C1$ -7,$Y203 .948 87-
1095'm - 2N00 C
Ni-16,4Cr-5.8A1-0.32Y 0,005-0,00$ Zr02 T,NY 203 0.033 46
-0.32Y .007 -	 ,010
-7.sY,,03 .036 60
-0.32Y .013-	 .019 -7. el .038 104
Ni-16,4Cr-5,8A1-0.32Y 004- ,007 ZrO: 11.8Y,,0 3 033 23
-0.32Y .007- ,010
-ll.8Y203 ,036 35
-0.32Y 011-	 .016 -11.8Y209 .038
'No failure,
Figure 1. -Light optical p hotomicrograph of edge of B-1900 + Hf specimen
coated with Zr02 -7. 8y ?0 3 after testing for 85 cycles at NOO C.
OF 
►GI^NV 
PA
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ond LUdtl I	 -no;	 -L-* —, -t %—%
I	 Depletion zone
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?00 + of %pet Irlun tMe.l with ',I I Q . iAl 0. 5:) .ind ,'rO, i. BY ?03dtter 198 1-hour i^ile5 at IOQ5')
Figure 3. -Light optical p hotomicrograph of flat surface of 8-1900 + Nf speci-
men coated with Ni-25. 7Cr-5.6A1-0. VY and Zr0? 1 8Y ?U 3
 after plasma
spraying with no cyclic testing.
VNIGINAL PAI R 1S
OF POOR QUAI"
14.1
	 Bond coatinq
Depletion zone
A
Substrati,
Ar. . Lei
(a)
vevirtlull lulir
A	 ".10	 A%
Vk
Suh%tralp	 0.? rn M
- — I
I b)
Figure 4. - Fight optical photomicrograph of B- POO + lif specimen coated with
Ni-25. 7Cr-5. 6AI-O. 32Y and Zr0 ? -7. " ?0 3:(a , flat surface of specimen after
1500 1-hour cycles and no failure at 990 0 C, and Ih' flat surface of specimen
after 249 1-hour cycles and failure at 10950 C.


Effects of yttrium, aluminum and chromium concentrations in bond coatings on the performance of zirconia-yttria thermal barriers

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