ELSEVIER Surface and Coatings Technology 76-77 (1995) 14-19

SURFACE&COI11'IiS

HC6KOLOGY

Blanching resistant Cu-Cr coating by vacuum plasma spray

K.T. Chiang, P.D. Krotz, lL. YuenRockwell International Corporation, Rocketdyne Division, 6633 Canoga Avenue, Mail CodeIB-17, CanogaPark, CA 91303, USA

Abstract

Copper alloy rocket engine combustion chamber linings have been found to deteriorate when exposed to cyclic reducingoxidizing (redox) environments, which are a consequence of the combustion process. The deterioration, known as blanching, canbe characterized by increased roughness and burn-through sites in the wall of the combustion chamber lining and can seriouslyreduce the operational lifetime of the combustion chamber. A Cu-30 vol.%Cr coating produced by vacuum plasma spraying waseffective in protecting the copper alloy substrate against blanching. The coating propertieswerecharacterized after cyclic oxidationexposure to 650°C in air followed by high pressure hydrogen charging. When exposed to an oxidizing environment at hightemperatures, the coating formed a protective chromia scale that was substantially unreduced by high pressure hydrogen.

Keywords: Blanching; Cu-Cr coating; Vacuum plasma spray; Chromia scale

1. Introduction

A deterioration mechanism of rocket engine combus­tion chamber linings arises from a cyclic reducing­oxidizing (redox) environment, which is a consequenceof the combustion process. In general, these combustionchamber liners are fabricated from a copper alloy forg­ing. The deterioration, known as blanching, can becharacterized by increased roughness and burn-throughsites in the wall of the combustion chamber lining. Thiscan seriously reduce the operational lifetime of thecombustion chamber [1,2]. For a rocket engine thatburns a mixture of liquid oxygen and liquid hydrogen,blanching is a self-aggravating process. When the copperalloy is exposed in an oxidizing environment at hightemperatures, copper oxides form. Later, when exposedto a reducing environment, these copper oxides arereduced. The redox phenomenon causes alloy surfacerecession and degrades materials properties of the wall.This, in turn, can result in localized hot spots and furtheralloy oxidation.

One approach to a protective coating for the copper­based alloys uses chromia scales, similar to those formedon M-Cr alloys (where M is Ni, Co, or Fe) that impartoxidation resistance when formed on the alloy. Onerequirement of such coatings is that the chromiumconcentration at the surface must be high enough toform a continuous CrZ03 scale that must be stable inthe service environment. In previous studies, the forma­tion of a protective CrZ03 scale was demonstrated on

Elsevier Science S.A.SSDl 0257-8972 (95) 02572-3

Cu-30 vol.%Cr coatings produced by co-evaporation ofCu and Cr either from dual sources [3] or from a singleCu-Cr composite source [4]. This investigation usedan alternate coating technique, vacuum plasma spraying(VPS), to deposit a prealloyed Cu-30 vol.%Cr powderonto a Cu-15 vol.%Nb substrate. The coating micro­structures were characterized. The oxidation behaviorand the effect of hydrogen charging on stability of theoxides were evaluated at 650°C.

VPS has been widely used for depositing variousmetallic coatings because of its high deposition rate, andits attractive coating properties such as high coatingdensity, low porosity, and reduced oxide levels [5]. Theprocess has also been used for fabrication of net-shapecopper alloy components for rocket engine applica­tions [6,7].

2. Experimental Details

Cu-15 vol.%Nb specimens 18 mm by 12 mm by 1 mmin size were cut from a rolled sheet. The microstructureof the Cu-15 vol.%Nb specimen consisted of fine,ribbon-shaped Nb filaments in a Cu matrix. Details ofthe fabrication method and the initial microstructure ofthe specimens were described elsewhere [8]. The speci­mens were polished using 600 grit SiC abrasive paper,grit blasted, and ultrasonically cleaned in an acetonebath and an alcohol bath before coating deposition.

Cu-30 vol.%Cr powder was produced from the highpressure gas atomization system [9,10] at the Ames

K.T . Chiang et al.jSurfa ce and Coatings Technology 76-77 ( 1995) 14-1 9 15

3. Results and discussion

Table 1Vacuum plasma spraying parameters for the Cu-Cr coa ting deposition

(a)

(b)

two-phase microstructure has been generated in thegas-atomized Cu-Nb powder [1 2].

Fig. 2(a) shows the surface morphology of theas-sprayed Cu- 30 vol.% Cr coating. The coating surfaceexhibited a "splat " structur e typical of the pla sma sprayprocess. During VPS dep osition, the Cu-Cr powderswere heated by the plasm a gas to a molten state andpropelled toward the Cu- 15 vol.%Nb substrate at highvelocity. The molten-powder particles were flattened onimpact, solidified, and integrated into the coating. TheXRD of the as-deposited coating surface revealed thatonly f.c.c. Cu and b.c.c. Cr phases were present. Thecomposition of the coating was analyzed using quantita­tive energy-dispersive X-ray (EDX) analysis. The averagemeasured Cr concentration was 25.9 wt.% (30.2 vol.%),which was within I vol.% of the nominal target composi-

Fig. l. (a) Scanning electr on micrograph of gas-atomizedCu-30 vol.%Cr powder. (b) Cross-section of the Cu-Cr powder.

75 Ar25 He

140044304530

635

Paramet ers

Plasma gases (%)

Description

Current (A)Voltage (V)Powder feed rate (g min - 1)

Chamber pressure (Torr)Spray distance (em)Traverse speed (ern min -1)

3.1. Coating microstructure and composition

Fig. 1(a) shows the surface morphology of the gas­atomized Cu-30 vol.%Cr powder. The powder was veryspherical in shape with a very few small satellite powderparticles attached to the large-sized powder. Excessiveagglomeration of powder particles was not observed,which resulted in excellent flow during plasma spray.Fig . l (b) shows a cross-section of the powder. Themicro structure of the powder consisted of finely dis­persed Cr particles in an essent ially pure matrix of Cu.Each individual Cr particle was less than 1 11m in size.Note that the refined distribution of Cu and Cr exist­ed not only in the larger-sized powder particles withdiameters of 10-44 11m, but also existed on all thesmaller-sized (less than 10 11m) part icles. A similar refined

Laboratories (Ames, IA). The gas-atomized powderswere sieved to - 325 mesh (below 44 11m) for plasmaspraying. The powders were handled under an argonenvironment during all stages of the powder processingto prevent oxidation of the powder. Coating depositionwas conducted using a 120 kW d.c. plasma spray system.The deposition parameters are summarized in Table 1.A water- cooled fixture was used to hold the specimensso that the substrate temperature was maintained below450 °C during coating deposition.

Isothermal oxidation tests were conducted in slowlyflowing air at 650 °C with mass gains being measuredusing a continuously recording microbalance. Thecoated specimens were also exposed to repetitive thermalcycles between room temperature and 650 °C in air.Each cycle lasted 30 min at 650 °C followed by 30 mincooling. To evaluate the effect of hydrogen environmenton oxide scale morphology, the oxidized specimens werethermall y charged with 34.5 MPa pressure hydrogen for30 min at 650 °C, which simulated a reducing environ­ment in a combustion chamber [11]. The surfaces andcross-sections of the specimens were examined usingconventiona l op tical metallogra phy, X-ray diffraction(XRD), and scanning electron micro scopy (SEM)techniques.

16 K T. Chiang et al./Surfaceand Coatings Technology 76-77 (1995) 14-19

(a)

~ JI~~

I I

Cu (fcc)

Cr (bee)

100(%)

64

C 36"iiic:

~16

4.0

0.040

Cu30vol.%Cr I II

I(b)

50II

60 70 80I

90II

100 (°2 theta)I

Fig. 2. Surface morphology and XRD analysis of VPS Cu-30 vol.%Cr coating: (a) SEM micrograph; (b) XRD pattern of the coating surface.

tion. The composition analysis was obtained from theaverage of three area scans at low magnifications.

Fig. 3(a) shows a cross-section of the Cu-30 vol.%Crcoating created by VPS. Using the backscatteredelectron image mode associated with SEM, three regionsof different atomic contrasts are visible. The EDX analy­ses were performed in these regions and the results areshown in Figs. 3(b), 3(c), and 3(d). The light gray region(point A) was primarily a matrix of Cu while the darkgray phases (point B) were the finely dispersed Crparticles. Both the Cu and the Cr phases were presentwithin a single lamella structure, indicating they werederived from a large-sized molten particle. The EDXanalysis of the intermediate gray region (point C), con­versely, revealed that the region was Cu rich but con­tained substantial amount of Cr. Quantitative EDX

analysis of the region showed that the Cr content was25.3 wt.% (29.5 vol.%), which was close to the nominalcoating composition. This region was, therefore, anintimate mixture of Cu and Cr phases that might bederived from the smaller-sized molten powder particles.The analysis showed that the distribution of Cu and Crphases in the original gas-atomized powder was retainedor even further refined in the plasma-sprayed coating.

3.2. Oxidation behavior

The oxidation kinetics of the VPS Cu-Cr coatedCu-15 vol.%Nb in air at 650°C is shown in Fig. 4. Theoxidation rate of the uncoated Cu-I5 vol.%Nb is alsopresented for comparison. The oxidation rate is seen tobe markedly reduced with the coating. For the uncoated

K. T. Chiang et al.tSurface and Coatings Technology 76-77 (1995) 14-19 17

Cul,« CuK ex

(c)

(a)

cu.«

2.00

CrK ex

4.00 6.00Energy, kev

8.00

(b)

(d)

2.00

2.00

4.00 6.00Energy, kev

CrK ex

4.00 6.00Energy, kev

8.00

CuK ex

8.00

Fig. 3. Cross-section of the VPS Cu-30 vol.%Cr coating: (a) SEM backscattered electron image; (b) EDX spectrum of the point A; (c) EDXspectrum of point B; (d) EDX of spectrum of point C.

Fig. 4. Oxidation rate of Cu-30vol.%Cr coated Cu-15vol.%Nb inair at 650°C.

Cu-15 vol.%Nb, the oxidation kinetics was controlledby outward diffusion of Cu ions to form external Cuoxides [13]. The Cu oxides that formed were notprotective and spalled severely during cooling to room

•••• uncoated..

••••• VPS Cu-30vol. '¥oCr coaledt~:JEI '

[;] a I!I

5

4N

E~.§. 3c:'0;<!lE.~ 23:

oo 10 20 30

Time (h)40 50

temperature. The VPS Cu-Cr coated specimen exhibiteda relatively high oxidation rate during an initial transientperiod of approximately 2 h, but reduced to a muchslower rate for longer times. The steady state oxidationkinetics followed a parabolic rate law. The parabolicrate constant after the transient period of 2 h wasanalyzed by regression analysis to be4.3 x 10- 3 mg" em-4 h -1. This rate was consistent withthe formation of a continuous CrZ0 3 scale on theCu-30 vol.%Cr coating produced by vaporco-evaporation of Cu and Cr [3,4],

The oxide scale that formed on the VPSCu-30 vol.%Cr coating, and the corresponding X-raymaps of Cu, Cr, and oxygen are presented in Fig. 5. Thespecimen was oxidized isothermally for 40 h in air at650°C. The Cu and oxygen X-ray maps (Figs. 5(b) and5(d)) and XRD indicated that the external oxide layerwas CuO. Underneath the external scale, a continuousCrZ0 3 scale was formed (see Cr and oxygen X-ray mapsin Figs. 5(c)and 5(d)). The CuO scale was formed duringthe transient stage and contributed to the more rapidinitial oxidation rate. As the continuous CrZ0 3 scale

18 K T. Chiang et al./Surface and Coatings Technology 76-77 (1995) 14-19

(c)

(b)

(d)

Fig. 5. Microstructure of VPS Cu-30 vol."ioCr coating and its scale after 40 h of isothermal exposure in air at 650°C: (a) SEM backscatteredelectron image; (b) copper X-ray map; (c) chromium X-ray map; (d) oxygen X-ray map.

developed, it cut off the growth of transient oxide andbecame a barrier for inward oxygen penetration. Theinner part of the coating was free of oxygen, and theCu-Nb substrate was protected from oxidationdegradation.

Thermal cycle tests were also performed on Cu-Crcoated Cu-Nb specimens for up to 100 cycles. Nocoating cracking or spallation was observed. The speci­mens exhibited a slow, positive mass gain indicating therate was controlled by slow growth of CrZ03 scale. Thetotal mass gain after 100 cycles was 0.8 mg cm-z, whichwas in good agreement with the mass change fromisothermal oxidation.

A series of specimens after various times during cyclicoxidation were charged with 34.5MPa hydrogen toevaluate the effect of hydrogen exposure to oxide sta­bility. XRD of the specimen showed that the CuOsurface oxide was reduced to pure Cu, but the CrZ0 3

scale was not affected. Fig. 6 shows the typical scalemorphology after hydrogen charging. While the outerCuO layer was reduced to discrete Cu particles, theinner layer of CrZ03 was adherent and was not reduced.

The Cr20 3 scale and the coating separated the Cu alloysubstrate from the oxidizing-reducing environment andprevented oxidation attack of the copper substrate.

4. Summary and conclusions

(1) A Cu-30 vol.%Cr coating that protects the copperalloy substrate against redox degradation (blanching)was demonstrated on a Cu-15 vol.%Nb substrate. Thecoating was produced by VPS of gas-atomized Cu-Crpowder.

(2) The microstructure of the VPS Cu-30 vol.%Crcoating consisted of an intimate mixture of f.c.c. Cu andb.c.c. Cr phases. The Cr constituent phase was presentin the gas-atomized powder as finely dispersed particlesin a Cu-rich matrix.

(3) The coating formed a continuous, protectiveCrZ0 3 scale in air at 650°C. On exposure to 34.5 MPahydrogen at 650°C, the CrZ0 3 scale was stable and wasnot reduced by high pressure hydrogen.

K. T. Chianget al.ISurface and Coatings Technology 76-77 (1995) 14-19 19

(a)

Fig. 6. Microstructure of Cu-30 vol.%Cr coated Cu-15vol. %Nb oxi­dized cyclically at 650 DC for 80 cycles of 0.5 h each, followed by30 min exposure to 34.5MPa hydrogen at 650°C: (a) SEM image;(b) higher magnification view of the coating surface.

Acknowledgements

This work was supported by the US Air ForceContract F33657-91-C-2012.The technical contributionsof D.R. Dietrich and P.L. Sterling of the RocketdyneAnalytical Laboratory are gratefully acknowledged.

References

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[2] D. Morgan, l Franklin, A. Kobayashi and T. Nguyentat,Investigation of copper alloy combustion chamber degradationby blanching, in AdvancedEarth-to-Orbit Propulsion TechnologyConf. 1988, Vol.2, p. 506.

[3] K.T. Chiang and lL. Yuen, Surf. Coat. Technol., 61 (1993) 20.[4] K.T. Chiang and lP. Ampaya, Oxidation Kinetics of

Cu-30vol.%Cr Coating, Surf. Coat. Technol., in press.[5] H. Herman, Sci. Am., (September 1988) 112.[6] R Holmes, D. Burns and T. McKechnie, Vacuum plasma spray

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[7] T. McKechnie, Y. Liaw, F. Zimmerman and R Poorman,Metallurgy and properties of plasma spray formed materials, inThermal Spray: International Advances in Coatings Technology,ASM International, Materials Park, OH, 1992,p. 837.

[8] P.D. Krotz, lA. Fint, lL. Yuen and N.E. Paton, Mater. Sci.Eng., A149 (1992) 225.

[9] US Patent 4,619,845.[10] I.E. Anderson and RR Rath, in S.K. Das, RH. Kear and C.M.

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[11] L.G. Fritzemeier, Rl Walter, A.P. Meisels and R.P. Jewett,Hydrogen embrittlement research: a Rocketdyne overview, inN.R Moody and A.W. Thompson (eds.), Hydrogen Effects onMaterial Behavior, AIME, Warrendale, PA, 1990, p. 941.

[12] K.L. Zeik, D.A. Koss, I.E. Anderson and P.R. Howell, Metall.Trans. A, 23 (1992) 2159.

[13] K.T. Chiang, KJ. Kallenborn, lL. Yuen and N.E. Paton, Mater.Sci. Eng., A156 (1992) 85.