Volume 6 Preprint 3
Protection of Advanced Copper Alloys with Lean Cu-Cr Coatings
Keywords: Cu-Cr, coatings, oxidation, blanching, protection, Cr2O3
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Volume 6 Paper H072
Protection of Advanced Copper Alloys with Lean Cu-Cr
QSS Group, MS 106-1, NASA Glenn Research Center, Cleveland OH 44135
Phone: 216/433-6463; e-mail: firstname.lastname@example.org
Advanced copper alloys are used as liners of rocket thrusters and nozzle
ramps to ensure dissipation of the high thermal load generated during
launch, and Cr-lean coatings are preferred for the protection of these
liners from the aggressive ambient. It is shown that adequate protection
can be achieved with thin Cu-Cr coatings containing as little as 17 % Cr.
Key words: Cu-Cr, coatings, oxidation, blanching, protection, Cr2O3
The excellent thermal conductivity of copper alloys makes them preferred
liner materials for rocket engine combustors and nozzle ramps, where
dissipation of a huge thermal load is important. Pure Cu has a thermal
conductivity of 300 W/mK (at 20oC), the highest for any material; and
comparable values for the advanced Cu alloys being considered for liner
application (CuCrNb, CuAgZr, and ODS Cu-Al2O3) are 300-360 W/mK ,
representing a low-to-moderate 6-23% drop from Cu. However, the poor
resistance of Cu and its alloys to oxidative degradation needs to be
overcome. One solution is the development of special copper alloys with
better oxidation resistance, a good example being Cu-8Cr-4Nb [1,2].
Another is the use of an environmental-barrier coating, provided the
coating material does not seriously impair thermal conductivity (and
ductility) of the liner alloy.
Cu-Cr coatings are favored because they can provide a protective Cr2O3
scale upon oxidation, while maintaining good chemical match as well as
acceptable thermal match with a Cu-alloy substrate. Ken Chiang and his
co-workers have demonstrated the formation of a continuous Cr2O3 sub-
layer from Cu-30vol.%Cr coatings on Cu alloys [4-6]. NiCrAly is another
candidate coating material. Though its intrinsic thermal conductivity is
lower than that of Cu-Cr compositions, it enables liner service at higher
wall temperatures, while its preferential oxidation to Al2O3 (when its Al
content exceeds ~5%) yields an even more protective oxide scale.
Our aim was to develop Cu-Cr coatings to protect Cu alloys via adequate
formation of Cr2O3, while keeping Cr content low enough not to impair
ductility or thermal conductivity of the system. A rough, rule-of-mixtures
estimation shows that thermal conductivity of a Cu-30%Cr composition
should be ~240 W/mK, only 60% of the value for pure Cu, and 20-35%
below the values given above for candidate liner alloys. It is important to
reduce this deficit of thermal conductivity – by reducing its Cr content
while preserving its ability to provide Cr2O3 cover.
A straight-forward way to do it is to make the coating fine-textured and
homogeneous. The coatings studied by Chiang et al. were rather coarse
aggregates of Cu and Cr particles. Of course, oxidation would yield Cr2O3
islands which conform to the same texture but coalesce eventually into a
continuous Cr2O3 layer. That is perhaps why they needed 30% and higher
Cr levels to achieve protection. Advanced coating techniques now enable
the routine deposition of coatings with fine microstructures. Accordingly,
one goal of this work was to demonstrate that Cr-lean coatings can be
put down on Cu alloys from powder, using plasma spray or cold spray.
The coating compositions used in our study were Cu-8.5Cr, Cu-17.1Cr,
Cu-21.3Cr, and Cu-25.6 Cr (Cr content in wt %). They were deposited
from corresponding powders by low-pressure-plasma spray (LPPS) or by
a variant of cold-spray called “kinetic metallization” (KM). The powders, -
635 mesh (<15µm average particle size) were made by Crucible Research
of Pittsburgh, PA, with O2 contents ranging from 425 ppm by wt. (wppm)
for Cu-8.5Cr to 650 wppm for Cu-25.6Cr. KM deposition was by Inovati
of Santa Barbara, CA, and LPPS was done in-house. The substrates were
polished coupons (19mm in diameter, 1.0mm thick). Consolidation of the
coatings was done by post-anneal or by HIPing (100 MPa) in Ar at 930oC.
Static oxidation test was done in a thermo-gravimetric analyzer (TGA) in
2.2 vol% O2 (bal Ar), while cyclic oxidation was by TGA in air. This choice
of low-p(O2) ambient was based on a typical thrust-cell environment .
Rods of the same Cu-Cr compositions made by KM were cut into mini-
disks 5-7 mm in diameter for oxidation-reduction studies, polished to a
mirror finish, and subjected to in-situ cycles of oxidation-reduction (air-
to-H2/Ar). Details of this novel test are given in a companion paper .
Results and Discussion
Results presented here for coatings on Cu-8Cr-4Nb were similar to those
obtained with corresponding coating on Cu-3Ag-0.5Zr (the current Space
Shuttle Main Engine liner). So, substrate details did not affect the results.
LPPS coatings were brownish due to oxygen pick-up, and turned black
upon anneal as the oxygen was incorporated into oxide. In contrast, KM
coatings retained their color and luster through deposition and anneal,
indicating they were significantly oxygen-free. Accordingly, the rest of
this paper considers only the KM coatings. (LPPS coatings were mentioned
just to highlight an advantage of cold deposition for Cu alloys.)
Fig. 1 shows a KM-coated sample after 10hr oxidation at 650oC. In (A) a
Cu-25.6Cr coating with an oxide skin (to the right) lies on the featureless
substrate (to the left). Details of the coating appear in (B): dark Cr lumps
within the lighter Cu matrix; (C) and (D) are corresponding EDS spectra.
Fig. 2 presents histograms of the static oxidation weight gains in 2.2% O2
at various temperatures, and a kinetic plot comparing 650oC weight gain
rates for the bare substrate and a coated substrate.
Fig. 1, SEM section of Cu8Cr4Nb, left in (A), coated with Cu-25.6%Cr
and oxidized 10h at 650oC. Higher magnification (B) reveals coating
texture; EDS spectra (C), (D) identify Cu (light) and Cr (dark) phases
Sp.Wt. Gain (mg/cm )
wt.% Cr in Cu-Cr Coating (on Cu8Cr4Nb)
SpWt Gain (mg/sq-cm)
Oxidation Time (H)
Fig. 2: (a) Static oxidation weight gains (at top), and (b) kinetics
(bottom) at 650oC, for Cu8Cr4Nb with and without Cu-Cr coatings
The histograms illustrate two key facts. The first is that coating reduces
oxidation weight gain, by up to a factor of 4 compared with the uncoated
alloy; indeed, the kinetic plots below the histograms illustrate the large
differences in oxidation rates (i.e. in slopes) between the bare and coated
alloy. The second is that the greater the Cr content of the coating, the
more effective it was in reducing weight gain. A third fact, perhaps more
interesting than the first two, is evident in the kinetic plot: For the coated
samples, up to 80% of the net weight gain was registered in the first ~1.5
hours, when parabolic kinetics associated with protective Cr2O3 had not
set in. Fig. 1(b) also shows that after 1 hr. the coated sample has gained
30% more weight than the uncoated substrate: Oxidation weight gains in
the pre-parabolic stage were consistently higher for the coated samples
than for the bare alloy, a result that is contra-intuitive.
This seeming anomaly is explained by a point made earlier. The Cr2O3
layer starts as discrete nuclei, hence a coarse distribution of Cr will result
in a coarse distribution of nuclei. Only when these nuclei coalesce into a
continuous Cr2O3 layer does it become a protective diffusion barrier. Up
to that point, oxidation of Cu in the coating proceeds unabated. Hence,
the early-stage oxidation produces by far the most weight gain because
it is strongly influenced by coating texture. While post-deposition heat
treatment may consolidate the coating in the sense of improving particle
adhesion and cohesion, it is unlikely to alter the texture. Coating texture
is ultimately determined by the size of particle aggregates or “splats” that
strike the substrate during deposition. This size factor may be improved
by going to finer powders; however, the handling and delivery of very fine
powders are not easy tasks. Apart from the coating texture, there may be
inhomogeneity in the Cu-Cr distribution carried over from the powder;
this is evident in Fig. 1. These inhomogeneities in coating textural and
compositional determine when a continuous Cr2O3 layer begins to grow
and oxidation protection becomes established.
Fig. 2(a) shows also that above ~8%Cr and at temperatures below 750oC,
coating composition did not significantly affect sample weight gain. The
reason for this is evident in Figs. 3 and 4. Fig. 3 is from Cu-8.5Cr coating
on a sample after oxidation at 650oC. (Inset is a low-magnification image
that includes the alloy substrate as well.) This figure shows a filamentous
early network of Cr2O3 just above the alloy substrate, separated from the
copper oxide layers by a zone of friable material. (By analogy to similar
features found in oxidized Cu-8Cr-4Nb [1,2], this zone is though to be
highly porous Cu2O.) Fig. 4 shows the same alloy coated with Cu-17.1Cr
(a), and Cu-25.6Cr (b), and oxidized at 650oC. A robust and continuous
Cr2O3 layer is evident in both images. Thus, from the standpoint of Cr2O3
protection, there is no obvious advantage in Cr contents above ~17%. The
question of whether the Cr reservoir in a Cu-17.1Cr coating is enough to
sustain Cr2O3 growth is really a matter of the service life desired. For a
space launch thruster, 10 hrs. of cumulative service (which represents
hundreds of missions) is quite a long life.
Fig. 3, SEM section of Cu8Cr4Nb coated with Cu-8.5%Cr, oxidized
10h at 650oC, showing nascent Cr2O3 layer beneath copper oxides
Fig. 4, SEM section of Cu-8Cr-4Nb coated with (a) Cu-17.1Cr and
(b) Cu-25.6Cr and oxidized 10h at 650oC. Note Cr2O3 networks.
Oxide films on these coatings and on the Cu-Cr-Nb substrate tended to
spall off upon cooling; hence, resistance to cyclic oxidation is considered
an important metric of performance, especially if a substantial thickness
of oxide can build up during service. Cylic oxidation results for Cu17.1Cr and Cu-21.3Cr coatings on Cu8Cr4Nb are illustrated in Fig. 5.
Normalized Sample Wt.
Cycle # (15-Minute Cycles)
Fig. 5, Cyclic-oxidation results for Cu-17.1Cr and Cu-25.5Cr:
Fig. 5(a) is a chart of weight gains at 750oC; the numbers in parentheses
within the legend refer to repeat experiments. The Cu-21.3%Cr coating
did not show any weight loss at 750oC; hence there was no necessity to
explore higher-Cr coatings. The Cu-17.1Cr coating began losing weight
after ~3.5h at 750oC; after 10h it was down to 90% of its starting weight.
At 650oC and below, none of the coatings registered any weight loss. In a
separate experiment  the substrate alloy, Cu-8Cr-4Nb, was found to
have lost 15% of its initial weight after similar oxidation cycles at 650oC;
and other Cu alloys were found to have lost significantly more weight.
Hence, the effectiveness of these Cu-Cr coatings in protecting against
cyclic-oxidation degradation is obvious.
Fig. 5(b) shows the surface appearance of the samples after 40 oxidation
cycles at various temperatures. The only visible degradation appeared on
the Cu-17.1Cr coating after exposure at 750oC: The oxide cover became
breached around the edge and center hole, no doubt causing weight loss
to begin in the 4th hour (Fig. 5a). Since temperatures at the wall are not
expected to exceed 700oC in service, the 750oC results may be seen as
approximating a worst-case cyclic-oxidation scenario.
This aspect of the study was intended to determine how Cu-Cr coatings
might fare in an environment in which Cu alloys are usually degraded by
“blanching” (which is elaborated elsewhere [1,2] and in the companion
paper ). Behavior in a cyclic oxidation-reduction test was considered
an indicator of blanching resistance. The controls in this study were the
alloys 3Ag-0.5Zr and Cu-8Cr-4Nb. The former exhibits no net change of
weight in oxidation-reduction cycling and emerges with deep surface pits
and fissures similar to features seen on this alloy after blanching attack in
service; the latter registers a monotonic weight gain due to the growth of
a protective oxide cover that resists reduction .
Fig. 6 summarizes the results of oxidation-reduction cycling for Cu-Cr.
Fig. 6(a) plots weight change versus time for the three Cu-Cr coatings in
the legend; all three behaved very similarly. Fig 6(b) is a high-resolution
segment of the Cu-21.3Cr plot, showing individual weight gain-and-loss
(oxidation-reduction) cycles; 6(c) is a plot of weight gain versus square
root of time, for the same Cu-21.3Cr, indicating overall parabolic weight
gain, as may be expected for Cr2O3 protection. Fig. 6(d) is an SEM image
of the Cu-21.3Cr sample at the end of cycling, showing a covering mat of
oxide which was identified by EDS as Cr2O3. Cu-17.1Cr and Cu-25.6Cr
gave the same results: The Cu2O/CuO that formed during oxidation was
completely removed by reduction in each cycle, leaving only Cr2O3.
Fig. 6, Charts and image showing that oxidation-reduction cycling
caused parabolic growth of Cr2O3 (only) on Cu-Cr coatings at 800oC
Note, however, that this result was obtained at 800oC. It is not known
what would happen at lower temperatures (say, 600oC), where the growth
of Cr2O3 is quite sluggish, while the growth of more easily reduced
copper oxides is comparably brisk. This point is under investigation.
Summary and Conclusion:
To assess their effectiveness as environmental barriers for Cu alloys, CuCr coatings were tested for resistance to three categories of oxidationrelated degradation: (1) static oxidation in 2.2% O2, (2) cyclic oxidation in
air, and (3) oxidation-reduction cycling (in air and 5%H2 environments).
Cu-17.1Cr was found to be borderline adequate, whereas Cu-21.3Cr and
Cu-25.5Cr were superior. But since Cu-21.3Cr and Cu-25.5Cr showed
about the same level of resistance to degradation in all categories, there
would be no need to go above 21.3%Cr content, in view of the attendant
deficit in coating ductility and thermal conductivity. Further improvement
may come from refining the coating texture and also homogenizing its
composition. Using finer (<10-micron) powders and co-deposition of
elemental Cu and Cr rather than pre-blended Cu-Cr powders are two
ways to go. These approaches will be explored in future work.
1. L. Ogbuji, “The General Isothermal Oxidation Behavior of Cu-8Cr-4Nb”
(submitted to Materials at High Temperatures)
2. L. Ogbuji and D. Humphrey, “Comparison of the Oxidation Rates of
Some New Copper Alloys” (submitted to Oxidation of Metals)
3. L. Ogbuji and D.L. Humphrey, “Oxidation Behavior of Cu Alloy
Candidates for Rocket Engine Applications”, poster paper submitted at
the National Space & Missile Materials Conf., Colorado Springs, USA,
June 24-28, 2002
4. K.T. Chiang and J.L. Yuen, “Oxidation Studies of Cu-Cr-Coated Cu-Nb
Microcomposite”, Surf. & Coat. Tech., 61 (1993) 20-24
5. K.T. Chiang and J.P. Ampaya, “Oxidation Kinetics of Cu-30vol.%Cr
Coating”, Surf. & Coat. Tech., 78 (1996) 243-247
6. K.T. Chiang, P.D. Krotz, and J.L. Yuen, “Blanching Resistant Cu-Cr
Coating by Vacuum Plasma Spray”, Surf. & Coat. Tech., 76-77 (1995)
7. L.U. Ogbuji, “Oxidation-Reduction Resistance of Advanced Copper
Alloys”, pp… of these proceedings (Paper H071)
8. D.B. Morgan and A.C. Kobayashi, NASA CR-184345 of August 1989