The importance of thermal contact resistance between layers in heat transfer through two layer, plasma sprayed, thermal barrier coatings applied to turbine vanes was investigated. Results obtained with a system of NiCrAlY bond and yttria stabilized zirconia ceramic show that thermal contact resistance between layers is negligible. These results also verified other studies which showed that thermal contact resistance is negligible for a different coating system of NiCr bond calcia stabilized zirconia ceramic. The zirconia stabilized ceramic thermal conductivity data scatter presented in the literature is ?20 to -10 percent about a curve fit of the data. More accurate predictions of heat transfer and metal wall temperatures are obtained when the thermal conductivity values are used at the ?20 percent level

Gaugler, R. E.

Liebert, C. H.

English

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NASA Technical Memorandum 81483
(NASA-TR-81483) SIG NIFCCANCE OF THERMALCONTACT R ESISTANC2 11 Tito•-LAYER
t . THERRAL-BARRIER-COATED TURBINE VAIES (NASA)i	 1 1 Ip HC A02/KF A01	 CSCL 21Er
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NSO-23310
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.:j	 SIGNIFICANCE OF THERMAL CONTACT
RESISTANCE IN TWO-LAYER, THERMAL-
BARRIER-COATED TURBINE VANES
Curt Ii. Liebert and Raymond E. Gaugler
Lewis Research Center
Cleveland' Ohio
1	 Prepared for the
International Conference on Metallurgical.
 Coatings
sponsored by the American Vacuum Society
_	 San Diego, California, April 21-25, 1980
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SIGNIFICANCE OF THERMAL CONTACT RESIST A NCE IN TWO-LAYER,
THERMAL-BARRIER-COATED TURBINE VANES
by
Curt H. Liebert and Raymond E. Gaugler
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
CD	
A study was conducted to determine if thermal contact resistance
between layers is important in heat transfer through two-layer, plasma-
sprayed thermal barrier coatings applied to turbine vanes at the NASA Lewis
Research Center. Results obtained herein with a system of NiCrAlY bond and
yttria-stabilized zirconia ceramic show that thermal contact resistance
between layers is negligible. These results also verified other studies at
Lewis which showed that thermal  contact resistance is negligible for a
different coating system of NiCr bond and calcia-stabilized zirconia cera-
mic. The zirconia-stabilized ceramic thermal conductivity data scatter
presented in the literature is +20 to -10 percent about a curve fit of the
data. The study herein shows that the designer can more accurately predict
heat transfer and metal wall temperatures when the thermal conductivity
values are used at the +20 percent level...
INTRODUCTION
Ceramic thermal barrier coating (TBC) systems are of growing importance
as a class of materials for aerospace applications primarily because of
their high thermal resistance or low thermal conductivity k property.
These coatings have been studied extensively at the Lewis Research Center
because of their potential for reducing the cooling-air requirements in
turbine blades. The TBC is applied to turbine vanes and blades at Lewis by
plasma spraying, and it consists of a NiCrAlY bond coat covered by a ceramic
layer of yttria-stabilized zirconia. Data [ref. 1] have been obtained for
the thermal conductivity of the individual layers of the materials of the
TBC used at Lewis. However, thermal conductivity data alone may not be suf-
ficient for heat transfer calculations on coated turbine components because
the overall heat resistance may also be sensitive to the thermal contact
resistance (71CR) at the ceramic-bored and bond - metal wall interfaces.
Reference 2 showed that for plas=c-;prayed layers of ceramic and bond
materials, TC.R was significant. Studies of the Battelle Laboratories [ref.
3], however, indicated that TCR was not significant.
A study was conducted at the NASA Lewis Research Center to determine if
TCR between layers is important in heat transfer through two-layer, plasma
sprayed thermal barrier coatings as applied to turbine vanes. Comparisons
were made between calculated and experimental metal wall temperatures. The
calculated metal wall temperatures were determined with a quasi-three-
dimensional heat transfer model [ref. 41 in which published thermal
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conductivity data of metal and ceramic materials were used. The experimen-
tal metal temperatures were obtained at the leading-edge region frcm tests
with two air-cooled turbine vanes in a hot cascade facility. Experimental
data from the uncoated vanes were used as a basis for adjustment of,a
prediction model to give best agreement with uncoated metal temperatures.
This model and the resulting data scatter between predicted and measured
metal temperatures for uncoated vanes was compared with the data scatter
with coated vanes. The comparisons permitted the evaluation of the signifi-
cance of TCR. The gas temperatures and pressures for the tests were 1550 K
and 85 N/cm2 over a range of coolant-to-gas flow ratios of 0.029 to
0.065. The range of **+easured metal temperatures was 655 to 1205 K. The
thickness of the bond coating was 0.013 centimeter, and the thickness of the
ceramic coating was 0.037 to 0.052 centimeter.
This report presents the results of this experimental and analytical
study. Plots of measured versus calculated data are presented showing the
extent of data scatter. Conclusions are made about the importance of TCR.
The significance of the data scatter and of the published ceramic thermal
conductivity values is discussed.
APPARATUS AND PROCEDURE
Hot cascade Facility
The cascade facility was designed for operation at gas temperatures and
pressures to 1600 K and 100 N/cant . The facility [ref. 51 consisted of
five sections: inlet, combustor, transition, test, and exit. The test
section walls were coated with a ceramic to increase their surface tempera-
ture and thus minimize heat radiation loss from the vanes. For the tests
herein, experimental vane metal temperatures were obtained at gas tempera-
tures and pressures of 1550 K and 85 N/can t with coolant flow ratios
through the test vanes ranging from 0.029 to 0.065.
Uncoated and Coated Test Vanes
Experimental leading-edge metal wall temperatures were obtained with
two air-cooled turbine vanes. The vanes were full-size J-75 vanes and were
internally impingement cooled. The midspan of both vanes was instrumented
with Chromel-Alumel thermocouples at the stagnation point and at distances
of 0.39 and 0.49 centimeter from the stagnation point on the pressure and
suction surfaces, respectively (fig. 1). The method of thermocouple
assembly and, installation is described in reference 5. Thermocouple cali-
bration was done after the thermocouple assemblies were installed in the
vane walls.
The procedure used to deposit the ceramic coating [ref. b] onto the
metal wall was to prepare the wall by grit-blasting with pure aluminum oxide
material, plasma-spray on a bond coat of Ni-16Cr-6A1-.6Y, and then plasma-
spray on a cerwmic coating of 12-weight-percent yttria-stabilized zirconia.
Further details on coating procedure are given in reference 7. Coating
thickness was measured by comparing 10-times-size airfoil profiles drawn
with a plotting machine of the uncoated vanes and of the vanes after coat-
ing. Ceramic coating thickness was 0.037 to 0.052 centimeter, band coating
thickness was 0.013 centimeter, and metal thickness was 0.152 centimeter.
S,
The heat transfer effects of the coating were determined by first test-
ing the uncoated vanes and then testing the same vanes after the TBC had
been applied. In these tests the combustion gas temperatures and pressures
were established and then the cooling-air flow rate was varied in a step-
wise manner from test point to test point. Steady-state data were recorded
at each cooling-air flow rate. Twenty-four data points were obtained with
	 Athe uncoated vanes at coolant-to-gas flow ratios of 0.029, 0.037, 0.059, andy.
	
	
0.065; also 24 data points were obtained with the same vanes after coating
at coolant-to-gas flow ratios of 0.036, 0.049, 0.058, and 0.065.
ANALYSIS
Heat Transfer
The calculated metal temperatures were determined with a quasi-three-
dimensional heat transfer model fref. 4) in which radiation was assumed to
be negligible. Because of limitations in the heat transfer model, the metal
bond coating thickness was added to the metal wall thickness, and the ther-
mal conductivity k of the metal wall (MAR-M-302, ref. 8) was used for this
composite of wall and bond materials. These simplifications can be shown to
have a negligible effect on the results.
Parameters and Uncertainties
Gas-side parameters. - The gas temperature and pressures for these
tests are known within +2 percent. The external heat transfer coefficients
are affected not only by gas temperature and pressure, but also by such
factors as qas flow conditions, airfoil curvature, and free-stream turbu-
lence. Unpublished experimental investigations of vane pressure distribu-
tion, gas turbulence, and boundary layer tt ►ickness carried out in the
cascade indicated that the external heat transfer coefficients around the
leading-edge region are known +10 percent.
Coolant-side parameters. = It;e coolant flow temperatures, pressures,
and flows were measured +1 percent. The local coolant-side heat transfer
coefficient accuracy is not well knmm because of difficulties in fabrica-
tion of the insert and associated unknown positions of the insert relative
to the metal wall profile. This large uncertainty necessitates the use of a
statistical control concept on uncoated vanes to establish a semi-empirical
heat transfer model for evaluating the significance of TCR on coated vanes.
This was done by adjusting the coolant-side heat transfer coefficient equa-
tions of reference 4 in order to obtain a modified equation, which, when
used in the computer code in reference 4 (TAM), gave the best correspondence
of experimental versus calculated uncoated vane metal wall temperatures at
coolant-to-gas flow ratios of 0.029 to 0.065.
Ceramic, bond, and metal wall material parameters. The thermal con-
ductivities of the metal wall and bond coating arenonown within +10 percent.
The experimental error of metal wall temperature measurement was +1 per-
cent. Values of k for yttri.a-stabilized zirconia are known within +20
percent to -10 percent of a curve fit of the data given in reference 1.
This uncertainty is due to measurement inaccuracies and variations of cera-
mic material structure with time, such as sintering and densification
changes as k was being measured.: A curve fit of the data presented in
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reference 1 gives the following equations for k as a function of tempera-
ture for this ceramic:
k = 0.483 + 1 x 10-4T, WrW, at temperatures of 370 to 970 K
	
(1)
k = 0.275 + 3.14 x 10-4T, W/mK, at temperatures of 970 to 1320 K
	
(2)
"	 These equations were used to analytically determine if TCR is negligible.
°
	
	 This was done by assuming that TCR was negligible and then varying k
values of the ceramic material throughout the range of these data... The
three levels of k considered were 1.2, 1.0, and 0.9 timss the k-values
calculated with equations (1) and (2).
}	 Statistical Inference
To determine the significance of the data scatter, the analysis presented
in references 9 and 10 was applied to the data points of measured versus cal-
culated metal wall temperatures obtained with the uncoated vanes and with the
coated vanes. The analysis was used at a 95 percent confidence level to
determine if the results and conclusions obtained from the sample data would
still be valid if larger amounts of data had been obtained.
.	 . RESULTS AMID DISCUSSION
Uncoated Vanes
Corres	 ndence of measured wall temperatures. - Figure 1 presents a
` comparision ot uncoated metal wall temperatures at the stagnation point, the
pressure surface, and the suction surface leading edge region for vanes desig-
nated A and B.	 Measurement agreement was within 10 percent and repeatability
was within 2 percent. 	 This agreement and repeatability show that the cascade
is functioning in a reasonably consistent manner. 	 It is not unusual to find
this amount of experimental uncertainty in hot cascades and engines.
Corres	 ndence of calculated and measured wall t 	 ratures. - Data
points o	 ca cu at
	
versus measureduncoated vane metal wall temperatures at
1- the leading-edge locations are plotted in figure 2.
	
The data scatter is
within 10 percent about a line of perfect agreement of calculated and measured
temperatures.	 This scatter demonstrates that the TAM computer program and
the adjustment made to the coolant-side heat transfer coefficient equations
used in this program result in a predictive model that can be used to cor-
relate control-vane heat transfer data over the range of coolant flow ratios
considered herein.	 This spread of +10 percent in predicted versus measured
temperature and the predictive model was used as the basis (control) for
determining the significance of the thermal contact resistance.
Coated Vanes
Data points of calculated versus measured coated vane metal wall tempera-
tures at the same leading-edge locations as for the uncoated vanes are pre-
sented in figures 2 and 3. The results at the 1.2 k level are shown in figure
2. The data scatter is within the +10 percent scatter about the line of
perfect agreement and, because of tFie TBC, the data are lower than the
measured wall temperatures of the uncoated vane. If thermal contact resis-
tance (TAR) were added in the analysis, then values of k which are higher
than the maximum of published values would have to be used, and this would be
unreasonable.
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Figure 3 presents calculated versus measured metal wall temperatures at
the other k levels of 1.0 k and 0.9 k. This figure shows some data which
are displaced below the +10 percent control region obtained with the uncoated
vanes and with the coateU vanes at the 1.2 k level. If MR were included in
the calculation with 0.9 k and 1.0 k, then there would be even greater
displacement and less correspondence of calculated and measured metal wall
temperatures. These trends in figures 2 and 3 support the initial assumption
that 4CR was negligible. This conclusion agrees with calculations presented
in reference 11, which also neglected TCR on a two-layer TBC using'NiCr bond
and 5-weight-percent calcia-stabilized zirconia ceramic. The results also
show that the designer can more accurately predict heat transfer and metal
temperature when k values of yttria-stabilized zirconia are used at
published values which are 20 percent above the curve fit of the data. The
statistical analysis used at the 95 percent confidence level showed that there
is a 95 percent probability that these results would still be valid if larger
amounts of data had been taken.
CONCLUDING REMARKS
Results of comparisons of calculated and measured metal wall temperatures
of uncoated vanes and the same vanes coated with a thermal barrier coating
system of NiCrAlY bond and yttria-stabilized zirconia ceramic showed that
thermal contact resistance between layers is negligible. This result also
verified other results obtained with a different coating system of NiCr bond
and calcia-stabilized zirconia ceramic. The data showed. that the designer can
more accurately predict heat transfer and metal temperatures when equations
presented herein are used at published values which are 20 percent above a
curve fit of the data. Statistical analysis showed that there is a 95 percent
probability that these observations would still be valid if a larger amount of
data had been taken.
s
5
1. L. T. Shiembob, "Development of a Plasma Sprayed Ceramic Gas Path Seal for
High Pressure Turbine Applications," (PWA-5521, Pratt & Whitney
Aircraft: NASA Contract NAS3-19759). NASA CR-135183, 1978.
..
	 2. H. G. Price, Jr., R. L. Schact and R. J. Quentmeyer, "Reliability and
Effective Thermal Conductivity of Three Metallic -Ceramic Composite
Insulating Coatings on Cooled Hydrogen -Oxygen Rockets," NASA TN D-7392,
1973.
3. K. E. Wilkes and J. F. Iagedrost, "Thermophysical Properties of Plasma
Sprayed Coatings," (Battelle Columbus Labs.; NAS3-13329) NASA CR-121144,
1973.
4. R. E. Gaugler, "TACT1, A Computer Program for the Transient Thermal
Analysis of a Cooled Turbine Blade or Vane Equipped with a Coolant
Insert, 1. Users Manual," NASA TP-1271, 1978.
5. H. J. Gladden and C. H. Liebert, "Effects of a Ceramic Coating on Metal
''	 Temperatures of an Air Cooled Turbine Vane," NASA TP-1598, 19...
6. S. Stecura and C. H. Liebert, "Thermal Barrier Coating System,"
U.S. Patent 4,055,705, Oct. 1977.
7. C. H. Liebert, R. E. Jacobs, S. Stecura and C. R. Morse,, "Durability
of Zirconia Thermal -Barrier Ceramic Coatings on Air -Cooled Turbine Blades
in Cyclic Jet Engine Operation," NASA ZM X-3410, 1976.
8. "MAR-M-302. High Temperature, High Strength Nickel Base Alloys,"
(The International Nickel Co., Inc., New York, Rev. ed., 1964), Table
III, p. 9.
9.	 N. L. Johnson and F. C. Leone, Statistics and Experimental Design In
Engineering and the Physical Sciences, (Wiley, New York, 1964).
10.	 M. R. Spiegel, Schaum's Outline of Theory and Problems of Statistics,
'	 (McGraw-Hill, New York, 1961).
_	 11.	 C. H. Liebert and F. S. Stepka, J. Aircr. 14, 487	 ( 1977).
6
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Figure 1. - Comparison of experimental metal wall temperatures at leading
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Figure 2 - Comparison of calculated and measured vane metal temperatures at
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Figure 3. - Comparison of calculated and measured vane metal tem-
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	 (Ceramic thermal con-
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ref.	 1.1
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Significance of thermal contact resistance in two-layer thermal-barrier-coated turbine vanes

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