Surface Degradation of Ductile Metals in Elevated
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Wear, 111 (1986) 173 - 186 173
SURFACE DEGRADATION OF DUCTILE METALS IN ELEVATED
TEMPERATURE GAS-PARTICLE STREAMS
ALAN LEVY and YONG-FA MAN
Materi als and Molecular Research Di vision, L awrence Berkeley Laboratory, Berkeley,
CA 94720 (U.S.A.)
(Received February 15, 1985; revised November 5, 1985; accepted December 20,1985)
Summary
The mechanisms and rates of erosion and combined erosion-corrosion
of SCr-1Mo steel (where the composition is in approximate weight per cent)
and type 310 stainless steel at elevated temperatures were investigated to
understand better the behavior of piping steels in fluidized bed combustor
environments. Tests were performed in a partially inert gas atmosphere to
study erosion behavior and in an air atmosphere to study combined erosion-
corrosion behavior. I t was determined that the erosion rate remained con-
stant or decreased with increasing temperature in nitrogen unti l a temper-ature was reached at which the tensile strength started to decrease more
rapidly with increasing test temperature. Above this temperature the erosion
rate increased rapidly with temperature.
In an erosion-corrosion environment, corrosion was the. dominant
mechanism at all test conditions. At higher temperatures and velocities the
material loss mechanism changed from low loss rate chipping of the scale to
high loss rate periodic spalling. The continuous scale formed on SCr-1Mo
steel in air appeared to protect the metal surface, decreasing its loss rate in
(Y= 30” tests compared with that of type 310 stainless steel tested in the
same conditions in nitrogen where a continuous scale did not form.
1. Introduction
The surface degradation of metals that occurs in aggressive environ-
ments containing both corrosive and erosive media has been an important
design consideration in the construction of equipment for several different
industries. The loss of sound structural metal by erosion-corrosion can be
experienced in such diverse equipment as gas turbines [l 1 and fluidizedbed combustors [ 41. There are major differences in the operating environ-
ments of key components in the two equipment examples referred to [5,6].
However, both the turbine blades in the gas turbine and the heat exchanger
0043-1648/86/$3.50 @ Elsevier Sequoia/Printed in The Netherlands
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tubes in the fluidized bed combustor can lose sound metal by a combined
erosion-corrosion mechanism in an elevated temperature env~onment,
It is the purpose of this paper to describe the material loss mechanisms
which have been observed to occur in structural metal alloys tested in condi-tions which lie between those of the bed of a fluidized bed combustor and
the turbine stage of a gas turbine. The test conditions were selected for
several reasons, major among them being the need to use rates that were
neither too fast nor too slow so that the behavior could be reasonably well
observed in a laboratory test. It is felt that the results of the investigations
reported herein are more applicable to the fluidized bed combustor case
than they are to the gas turbine case. However, it is possible that the types
of metal and scale surface morphology described have been observed on gas
turbine components and might be of use in helping to understand their
erosion-co~osion degradation.
The tests performed in this investigation were carried out in sulfur-free
gases because of limitations of the test equipment to contain toxic gases.
However, the oxide scale morphologies that were developed on the test
surfaces of the metals were very similar to the sulfide scales observed in
multi~omponent gas corrosion tests [ 71. Since the corrosion mechanism was
dominant over the erosion mechanism in ah the erosion-corrosion tests
reported herein, it is felt that the observations made have at least some
applicability to erosion-corrosion in oxygen- and sulfur-bearing gas atmo-
spheres.
2. Test conditions
The experiments were carried out in the elevated temperature erosion
tester described in the previous paper [ 81. It can use a variety of erodents
and air, argon or nitrogen carrier gases. Temperatures T from 20 to 900 “C
are achievable with a ~mpera~re variation not exceeding &15 “C over the
test range. The tests were performed at three different impingement angIes:
(II= zoo, o!= 30” and CY 90”. Angular Sic or rounded agglomerates of A1,03erodent particles with mean diameters of 100 - 250 pm were used [2, 33.
Particle velocities v from 10 to 70 m s-i were used at a solids loading of 2.5
g min-‘. Particle velocities were established by setting a pressure drop across
the nozzle using a metering system that was connected to a shop air supply.
The air or nitrogen pressures for a desired velocity were determined using the
computer calculation developed in ref. 9 to take elevated temperatures into
account. Undried nitrogen was used as the carrier for the erodent particles
in the type 310 stainless steel test to prevent corrosion from occurring
during the test.
Test durations t of 30 min and 5 h were used, depending on the nature
of the test. Both times were sufficient to produce steady state degradation
rates, The impact of small erodent particles on the test surface during the
corrosion process modified the effects of elemental diffusion through the
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175
scale. This overcame the parabolic reaction rates which make the degradation
process much more time dependent in straight corrosion testing.
The data reported and analyzed in this paper are a part of a compre-
hensive program to investigate the elevated temperature behavior of a
number of alloys [ 8, lo]. The steels tested.in the total program were com-
mercial alloys commonly used in steam boiler and chemical process plant
components. Their designations are as follows: 1018; 2iCr-1Mo; 5Cr-+Mo;
SCr-1Mo; type 410 stainless steel; type 304 stainless steel; type 310 stainless
steel; 17-4PH. Their compositions are listed in ref. 10.
The two alloys reported on herein, type 310 stainless steel and 9Cr-
1Mo steel, behaved in a manner that was representative of all the alloys
investigated. The type 310 stainless steel was selected because it forms a
protective Cr,03 scale at the selected test conditions. The SCr-1Mo steelwas selected because its lower chromium content is marginal for forming a
protective scale at the test conditions. The erosion of the other alloys is
reported in ref. 10. They are comparatively simple alloys of iron having their
major variable element, chromium, in the range from 0 to 25 wt.%. This
range of chromium contents was selected because the oxide scales that form
on the metals in straight elevated temperature corrosion tests provide from
none to a fully protective Cr,Os scale.
The size of the specimens tested in the nozzle tester was 17.5 mm X
17.5 mm X 2 mm. The degradation rates of the test specimens were deter-
mined either by mass loss or thickness loss. To prevent oxidation of the testsurface prior to the tests in the elevated temperature erosion tester, undried
nitrogen was passed through the erosion tester until the specimen reached
the test temperature. After the test the specimen was quickly removed from
the furnace section of the tester and placed under a protective flow of
nitrogen until it had cooled to approximately 300 “C to prevent further
oxidation. Some spalling of the scale on the test surface occurred during
cooling. Optical and scanning electron microscopes were used to observe the
specimens’ surfaces and cross sections. Energy-dispersive X-ray analysis and
X-ray diffraction were used to determine the composition of the scales.
3. Results and discussion
3.1. Effect of t emperat ure on t he st rai ght erosi on of t ype 310 stai nl ess st eel
Figure 1 shows the erosion rate as a function of test temperature of
the highest chromium content most-corrosion-resistant steel tested, type
310 stainless steel [lo]. Each data point was obtained from a separately
tested specimen. The alloy was tested in a nitrogen carrier gas to prevent
corrosion from occurring. Four different test series were carried out to
determine the reproducibility of the data generated in the elevated tem-
perature erosion tester. At Q = 30” the erosion rate did not change as the
temperature was increased until 400 “C was reached. Above this temper-
ature the erosion rate increased with higher test temperatures at an increas-
ing rate.
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'.'O 200 400 600 800 1000
Temperature PC)
Fig. 1. Erosion rate of type 310 stainless steel us. test temperature (20 pm Sic; u = 30m s-l) at (Y= 30” (A, 0, 0, 0) and (Y= 90” (A, m, 0, 4): A, A, run 1; 0, ., run 2; 0, 0, run 3;
0, +, run 4.
The shape of the curve for the CY 90” tests is somewhat different from
the cr = 30” curve. The erosion rate decreased from ambient temperature to
400 “C and then increased with temperature at an increasing slope. All the
alloys listed above showed this type of behavior with the decrease in erosion
rate at the lower elevated temperatures varying from essentially 0% to 60%less than the rate at room temperature.
The temperature at which the alloy steels started to undergo an increas-
ing erosion rate with test temperature correlated well with the temperature
at which their short-time tensile properties started to decrease at an increas-
ing rate. The decrease in erosion rate with test temperature at the lower
temperature showed the same trend as increases in the impact strength of
the alloys as the test temperature was increased above ambient temperature.
The reasons for these correlations are not known.
The effect of particle velocity on the erosion rate of type 310 stainless
steel tested at 800 “C is shown in Fig. 2. The velocity exponent of 1.23 isapproximately one-half of that reported for ductile metals at room temper-
ature [ll] in the range of velocities used in these tests. This indicates that
the relationship between the kinetic energy of the impacting particles andthe amount of material removed from the eroded surface that has been
observed in room temperature tests [ll] and modeled extensively is mod-
ified at elevated temperatures.
The erosion rate for the type 310 stainless steel in Figs. 1 and 2 for the
same test condition, particle velocity u = 30 m s-l at 800 “C, differs becauseof the impingement angle used. The erosion rate of 0.25 X lo-” g g-l in
Fig. 2 is greater than the value of 0.13 X 10m4g g-l in Fig. 1 because the data
in Fig. 2 were obtained at (x = 20”. This angle was nearer the peak rate im-pingement angle for type 310 stainless steel than was the (II= 30” angIe used
in the tests plotted in Fig. 1.
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I
40 80 100
Velocity, m/s
Fig. 2. Erosion rate of type 310 stainless steel us. particle
T = 800 “C; cz = 20”).
velocity (240 Sic; nitrogen;
The appearance of the eroded surface of the type 310 stainless steel at
several test temperatures and two impingement angles is shown in F ig. 3. The
surfaces are filled with platelets and shallow craters which are representative
of the platelet mechanism of erosion [X2]. It can be seen that the erodedsurface texture was the same at all temperatures and both impingement
angles even though the erosion rates were significantly different. The dif-
ferences in the erosion rates are due to the size of the platelets that are
formed and knocked off the surface 1131. They are much larger at higher
particle velocities. The same eroded surface texture has been observed in
tests carried out at the highest possible velocity in the erosion tester, 130 m
s-i [13]. Whether this mechanism of erosion also occurs at the 300 m s-’
velocities that are common on turbine blade surfaces is not known.
The absence of any corrosion product can be seen on all the surfaces
pictured except the ~1s 30*, T = ‘775 “C test micrograph. The small nodules
that can be seen on the surface in this test condition are the beginning of the
formation of oxide scale. The scale formed on the type 310 stainless steel
was not continuous, even at the top test temperature, 900 “C. Once condi-
tions are right for a continuous corrosion scale to form, the nature of the
combined erosion-corrosion process enhances the growth of the scale and
corrosion becomes the dominant mechanism [ 141.
3.2. Combined erosion-corrosion of SCr-1iWo steel scale morphologyThe ability of impacting solid particles to promote the growth of the
scale is seen in Fig. 4. SCr-1Mo steel was selected for this series of tests
because it had a marginal chromium content for producing a continuous
protective Cr20s scale under ideal corrosion conditions but not enough
chromium to produce such a scale under all corrosion conditions. Thus the
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(a) (b)
Fig. 3. Micrographs of an eroded type 310 stainless steel surface at various test temper-atures (nozzle tester; erosion; 240 pm Sic; u = 30 m s-l; t =: 30 min).
Figure 01 deg) T WI
3(a) 30 775
3& f 90 710
3(c) 30 397
3(d) 30 25
3(e) 90 25
(d)
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(clFig. 4. Scale morphology on QCr-1Mo steel at (a), (b) 750 “C and (c) 900 “C (nozzle
tester; 130 firn A&03; air; U = 70 m s-l; t = 30 min; or = 90”) showing (a), (c) dynamic
corrosion and (b) erosion-corrosion.
effect of erosion-corrosion could be observed more readily. Figure 4(a)
shows the surface of a SCr-1Mo steel specimen that was exposed to dynamic
corrosion in a u =70 m s-l air blast without any erodent particles in it at
750 “C. Tbe smooth thin scale that formed is typical protective Cr,Os that
forms on chromium-containing steels in oxidizing atmospheres.
When 130 pm Al,Os particles were added to the air blast, the Fe203
scale shown in’ Fig. 4(b) resulted. This scaIe is several microns thick and has
a segmented domain type of microstructure [15]. The micrograph in Fig.
4(c) shows a similar segmented domain type of scale morphology to that
shown in Fig. 4(b). However, this scale occurred as the result of a dynamic
corrosion test without particles in the gas at a test temperature of 900 “C.
Thus the impact of erodent particles on the corroding surface at 750 “C
produced a scale which occurred in a dynamic corrosion test at a temper-
ature 150 “C higher.The morphology of the scale formed on the surfaces of steels has a
strong relation to the metal loss rates which occur in erosion-co~osion
environments [ 8,15,16]. The effect of the test temperature on the scale
morphology in erosion-corrosion tests carried out at a particle velocity u of
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(a)
(c)
Fig. 5. Effect of test temperature on the scale morphology of SC&1Mo steel at L’= 70 m
s-l (nozzle tester; erosion-corrosion; 130 pm Al,Os; air; t = 30 min; Cx= 90”): (a) T =
750 “C; (b) T = 850 “C; (c) T = 90 0 C.
70 m s-l is shown in Fig. 5. At 750 “C the scale was segmented into domains
as was shown in Fig. 4. At 850 “C the scale has been condensed and con-
solidated somewhat by what is speculated to be a hot isostatic pressing type
of action from the impacting particles. This consolidation is more pro-
nounced in the scale that formed at 900 “C (Fig. 5(c)).The great difference in the scale morphology as the result of particle
impacts can be seen by comparing the appearance of dynamic corrosion scale
formed on the SCr-1Mo steel at 900 “C in Fig. 4 with F ig. 5(c). The scale
formed on the erosion-corrosion specimen at 900 “C is essentially contin-
uous, having no sharply defined segments as occurred on the 900 “C speci-
men in the dynamic corrosion test or the 750 “C specimen in the erosion-
corrosion test (Fig. 5(a)).
The same type of transition of the scale morphology from segmented
domains to a consolidated densified continuous scale was also observed to
occur as a function of increasing velocity at the higher temperature. Figure 6
shows a sequence of scales that formed on SCr-1Mo steel as the particle
impact velocity was increased from 10 to 70 m s-l in 850 “C tests. At the
10 m s-r velocity the force of the impacting particles was not sufficient to
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181
(a) (b)
(c) (d)
Fig. 6. Effect of particle velocity on the scale morphology of SCr-1Mo steel in 5 h testsat (Y= 90” (nozzle tester; erosion-corrosion; 130 pm Al203; air; T= 850 “C; primaryzone): (a) u = 10 m s-l; (b) u = 30 m s-l; (c) u = 45 m s-l; (d) IJ = 70 m s-l.
consolidate the scale and the segmented domain type of morphology re-
sulted. At a particle velocity of 30 m s-i a transition in the morphology can
be seen. The scale still has segments, but they appear to be more densified
with smoother surfaces. In the 45 m s-l test the distinctly separated domainshave essentially disappeared and in the 70 m s-l test there is no evidence that
segmented domains remain. The differences in the morphology discussed
can be clearly seen in the original glossy micrographs, but in the printed
photographs these important differences are much harder to see.
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3.3. Metal loss rates
The effect of the differences in the scale morphology in erosion-corro-
sion tests as a function of velocity and temperature on metal loss rates is
seen in Fig. 7 and Fig. 8 respectively. Figure 7 plots the metal thickness lossas a function of velocity at two impingement angles: Q!= 90’ and (x = 30”.
The (Y= 90” curve is a classic S-shaped transition curve, indicating that a
marked difference occurred in the erosion-corrosion mechanism in the
velocity region around v = 30 m s-l. Below the transition velocity, the
scale was eroded by a comparatively slow mechanism of chipping of small
pieces of scale [ 171. Above the transition velocity, the scale was removed
by a much faster mechanism, the periodic spalling of relatively large pieces
of scale [ 81. The scale loss rates on the corrosion-dominated surface trans-
lated into the underlying metal loss rates that were measured by an optical
micrometer on a specimen cross section, The curve for the CY 30” tests
will be discussed later.
It is thought that the change in thickness loss rate of the SCr-1Mo steel
at the higher particle velocities in the Q!= 90” tests is due to the change in
the manner in which the scale is removed rather than because of a change
from primarily corrosion to a synergistic combined erosion-corrosion
mechanism. Corrosion was observed as the dominant mechanism on all
surfaces of the specimens in all the tests at all velocities. The sharp increase
IO 20 so 40 so 80 70 80
Vek@ty of Pdcle m/s
I
I
760 800 860 900
Temperature 'C
Fig. 7. Effect of particle velocity on the metal thickness loss of SCr-1Mo steel in TE850 “C tests at a! = 30” (A) and a! = 90” (0) (130 pm Al,O,; air; t = 5 h).
Fig. 8. Effect of test temperature on the metal thickness loss of $Cr-1Mo steel in u = 70ms-1testsat&=900(130~Alz0~;air;t=5h).
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in metal loss could not be the result of a sudden increase in erosivity of the
particles between 30 and 40 m s-i because the erosivity is a function of the
kinetic energy which increases uniformly as a function of the velocity (see
Fig. 2). The initiation of loss of larger pieces of scale by spalling at u = 30 ms-l was observed and this is thought to account for the increased metal loss.
The difference between the scale loss mechanism at the lower andhigher velocities is postulated to be due to the difference between the scale
morphologies. As seen in Fig. 6, at about u = 30 m s-l the force of the im-
pacting particles begins to densify and consolidate the segmented domainsof scale which occurred at the lower velocities. In its more continuous form
at the higher velocities, the scale can develop sufficiently high internal stresslevels to cause periodic spalling. At the lower velocities the scale’s separate
domains prevent these high stress levels from occurring and scale removalcan only occur by the chipping mechanism.The behavior is thought to be similar to that of plasma-sprayed thermal
barrier ceramic coatings on gas turbine components. They are purposelymicrocracked during application to reduce thermal fatigue failures by coat-
ing spallation [ 18,191. The large number of subcritical microcracks reducethe elastic modulus and, therefore, minimize the stresses that can develop
in the coating layer for a given strain level. This results in a strain accom-modation that reduces the spalling tendency of the coatings.
3.4. Protective scaleOn the right-hand ordinate in Fig. 7 the approximate mass loss of the
specimen was plotted. It was calculated from the eroded area, the metal thick-ness loss and the metal density. It is approximate because the contour of the
eroded area varies somewhat. Comparing the erosion rates for the type 310
stainless steel in Fig. 1 at 800 - 850 “C with the rates plotted in Fig. 7 for the
SCr-1Mo steel shows that the rates based on mass loss were the same at(Y= 90” and 3.0 X lop6 g g-l. At CY 30” the type 310 stainless steel had an
erosion rate that was five times that of the SCr-1Mo steel.There are several factors regarding the erosion behavior of the two
steels that indicate that the corrosion scale that formed on the SCr-1Mosteel in the 850 “C test could have provided some protection to the basemetal in the small-angle (a = 30”) tests.
(1) The type 310 stainless steel was tested in a nitrogen gas atmosphereand did not form a continuous corrosion scale.
(2) The general erosion behavior of brittle material such as the com-paratively thick continuous scale which formed on the SCr-1Mo steel is toundergo their highest erosion rate at a! = 90” and, by comparison, to be muchmore erosion resistant at o = 30”.
(3) In other investigations [ 201 it has been observed that austeniticstainless steels are more erosion resistant than ferritic steels.
(4) The tensile strength of the type 310 stainless steel at 850 “C is17.5 kgf mmP2 (25000 lbf in-‘) while that of the SCr-1Mo steel is 7 kgfmmm2 (10 000 Ibf inM2).
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Erosion rates have been observed to increase as tensile strength de-
crew% i n elevated temperature tests [ 12 1. I tems (3) and (4) indicate that
the type 310 stainless steel should have had a lower erosion rate than the
SC%-1Mo steel.All these factors can be used to explain the higher erosion rate for
the type 310 stainless steel than for the SCr-1Mo steel when tested at
850 “C, u = 30 m s-’ and LY= 30”. I t appears that the decrease in the metal
loss rate of the SCr-1Mo steel was the result of the formation of a protective
scale on the surface of the steel at a formation rate that was enhanced by
the impacting erodent particles [16]. The equal erosion rate for the two
steels tested at LY= 90” indicates that the scale on the SCr-lM0 steel is
eroding at a faster rate because it is a brittle material and did not provide
as much protection to the metal as it did in the Q!= 30” tests. If the scale
on the SC+-1Mo steel was not providing some protection, the generally
higher corrosion resistance of austenitic stainless steels compared with
ferritic steels and the higher elevated temperature strength of the type 310
stainless steel at the 850 “C test temperature compared with that of 9Cr-
1Mo steel should have made the SC%--1Mo steel erode at a faster rate.
3.5. Erosi on-corrosion at CY 30”
The morphology of the scale on the SCr-1Mo steel as well as its thick-
ness were considerably different at an impingement angle (Y of 30” than ata = 90” [16]. Segmented domains of scale with different morphologies were
presented at all velocities up to u = 70 m s-i at a = 30”. The scales at the
higher velocities were observed to be relatively thin. Almost no spalled
regions were found on the specimens after the a! = 30” tests while major
spalling was observed on the LX=90” test specimens. The resulting metal
thickness loss curve for the 01 = 30” tests is shown in F ig. 7, I ts shape indi-
cates that metal loss is occurring only as the result of the slower scale-
chipping mechanism at all velocities.
3.6. Effect of t est t e~~er~t ~re
The effect of the test temperature on the metal thickness loss is plotted
in Fig. 8. The three available data points are connected by straight lines to
get an idea of the basic shape of the curve to compare with the particle
velocity effect curve. The curve has a steep slope between 750 and 850 “C
and a smaller slope between 850 and 900 “C. The shape of the curve together
with micrographs of the morphology of the scale at each temperature (Fig.
5) indicate that the curve in Fig. 8 is the transition portion of the same type
of S-shaped curve shown for the cy= 90” tests in Fig. 7. As in the effect of
velocity on scale morphology, the effect of the test temperature at the
higher velocities on consolidating the scale is thought to be responsible for
the change in the metal loss rates as the test temperature was increased.
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4. Conclusions
4.1. Elevated temperature erosion
(1) An increase in the erosion rate of steels as a function of temperature
occurs at the temperature at which the tensile strength uersus temperature
curve of the steel increases its downward slope.
(2) The initial decrease in the erosion rate of some steels as a function
of temperature at lower elevated temperatures appears to be related to
increasing impact strength.
(3) The velocity exponent for the erosion of type 310 stainless steel at
800 “C is one-half of its exponent at 25 “C.
(4) At all test temperatures and velocities, erosion occurred by the
platelet mechanism of erosion in the absence of a continuous scale layer onthe metal surface.
4.2. Elevated temperature erosion-corrosion
(5) Corrosion was the dominant mechanism at all test conditions,
producing a scale layer on the metal surface.
(6) Impacting erodent particles on a surface in an oxidizing atmosphere
effectively increase its scale formation temperature by about 150 “C.
(7) At higher test temperatures and velocities the scale is consolidated
and densified by the impacting erodent particles.
(8) When the scale is consolidated, the erosion-corrosion loss mecha-nism of the scale changes from low loss rate chipping to high loss rate peri-
odic spa&g.
(9) Different erosion-corrosion mechanisms occur at (Y= 30” and (Y=
90” at the higher temperatures and velocities.
(10) The scale on the SCr-1Mo steel appears to be protective in the
(Y= 30” test at 850 “C compared with the base metal erosion of type 310
stainless steel under the same test conditions.
Acknowledgment
This research was sponsored by the U.S. Department of Energy under
DOE/FEAA 15 10 10 0, Advanced Research and Technical Development,
Fossil Energy Materials Program, Work Breakdown Structure Element
LBL-3.5, and under Contract DE-AC03-76SF00098.
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I . E. Sumner and D. Ruckle, Development of improved durability plasma sprayedceramic coatings for gas turbine engines, AZ AA Paper 80-l 193, 1980 (AmericanInstitute for Astronautics and Aeronautics).
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