Aluminization of copper for oxidation protection

Surface and Coating.s Technology. 52 (1992) 135--I 39

135

Aluminization of copper for oxidation protection* K. T. Chiang, K. J. Kallenborn and J. L. Yuen Rockwell International Corporation. Rocketdvne Dicision, 6633 Canoga Arenue, Canoga Park, CA 91303 (USA) (Received April 22, 199!; accepted in final form October 3. 1991)

Abstract Copper alloys are of interest for high strength and high thermal conductivity applications at elevated temperatures. However, their usage at high temperatures is usually limited by severe oxidation attack. To improve oxidation resistance, aluminum was diffused into oxygen-free high conductivity (OFHC) copper by a pack cementation process. Subsequent oxidation of the coated alloy at both 649 and 815 C showed a decrease in the oxidation rate by a factor of more than two orders of magnitude. While the untreated copper oxidized to form Cu,O and CuO, the coated material formed an AI,0 3 scale. At 649 ~C,the coating was protective for extended testing. At the higher temperature of 815 -C, coating breakdown was observed after 40—50 h. The coating phases formed by the aluminization process and their relationship to the development and breakdown ofthe protective Al,03 scale are discussed.

1. Introduction Copper-base alloys are of interest for high strength, high thermal conductivity applications at elevated temperatures. However, their usage at high temperatures is usually limited by severe oxidation attack. Oxidation of copper is controlled predominantly by the outward diffusion of metal ions, and the reaction kinetics obey a parabolic rate law [I]. The oxides that developed on the copper surface, namely Cu2O with a thin CuO outer layer, are not protective and are subject to severe spalling during thermal cycling. A surface coating that improves the oxidation resistance of the base metal is required at high temperatures to prevent material degradation. Among the various coating processes to improve oxidation resistance, pack cementation is relatively simplc and suitable for coating a substrate with complex geometry. In particular, pack aluminizing has been widely used to protect gas turbine components [2—4] and alloy steels [5, 6]. In this process, the component is embedded in a powder mixture containing aluminum, inert A1203 filler and ammonium halide activators. The aluminum is diffusionally introduced into the component surface using the halide species to transport aluminum to the component surface. The mechanisms of aluminum diffusion are well documented in the literature for nickelbase superalloys [2, 3, 7]. There the coating obtained consists of an NiAl intermetallic compound layer which forms an A1201 scale on its surface during operation of superalloy components [8]. The purpose of the present *paper presented at the 18th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, April 22—26, 1991.

study was to apply an aluminized coating on copper by pack cementation techniques. The coating phases formed by the aluminization process were characterized. The oxidation behavior of the aluminized copper was exammed after air exposure at 649 and 815 ~C.

2. Experimental details Oxygen-free high conductivity (OFHC) copper material was cut into 10 mm x 15 mm x 2 mm specimens. The specimen surfaces were polished up to 600 grit SiC paper and ultrasonically cleaned. The pack components, 5 wt.% Al powder, 1.5 wt.% NH4C1 and balance Al203 powder, were weighed and mixed. Specimens were enclosed in A1203 crucibles filled with the powder mixture. The pack-aluminizing process was carried out at 800 ~Cfor 5 h under argon. A combination of X-ray diffraction (XRD), metallography and electron probe microanalysis (EPMA) was used for phase identification of the coating layers. An XRD spectrum was first taken on the specimen surface after the aluminization process; the specimen was then polished to remove a layer of material and an additional X-ray spectrum was taken. This process was repeated until all aluminum diffusion layers were removed. The coating morphology studied by optical metallography and scanning was electron microscopy (SEM) with energy-dispersive X-ray analysis (EDXA). The aluminum diffusion profile was analyzed by EPMA. After aluminization, all faces of the specimen were lightly polished, removing approximately 45 .tm from

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IS the aluminum-containing solid phase. According to the Cu Al binary alloy phase diagram [9]. the solubility of aluminum in (Cu) is about ~ wI/U at 800 C. Beneath the two-phase region ((‘Li)

performed in air at both 649 and 815 C with mass gains being measured using a continuous recording microbalance. The surfaces and cross-sections of the oxidized

solution

specimens were examined using Auger electron microscopy. optical metallography and SEM techniques.

toward the unreacted copper matrix, a region containing single-phase (Cu) was detected. The diffraction pattern of (CLI) was distinguishable from those of pure copper in that all the Bragg reflections were shifted to smaller angles, indicating a larger ii spacing than normal. l-igLire 2 shows a cross—sectioned copper specimen after the aluminizing treatment. Three different layers are visible. The oLitermost layer is a two-phase region. ahoLit 75 om thick. The sLirface of this layer is irregLilar and contains entrapped Al,O~ particles. The second layer also contains aluminum and has ahoLit the same thickness as the oLiter layer. The innermost layer is the unreactcd copper sLibstrate. Microprohe line scans of

3. Results and discussion 3.1. (ooiiig microxlrticture and conipo.sition Figure I shows the XRD pattern of the aluminized copper. It was found that the copper surface after aluminization contained many A1201 particles from the pack (Fig. 1(a)). The A1703 particles can be easily removed by a light polish of the specimen’s sLirface (Fig. 1(b)). Two solid phases were identified: (Cu)* and

aluminum and copper show an enrichment in aluminum I

and depletion of copper in the coating layer. The analysis shows a gradient in the concentration of aluminum from the sLirface to the interior ) Fie. 3). The observation that the outermost coating layer contains entrapped particles from the pack indicated that outward diffusion of copper

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Fig. 2. ( ross-section of the alumini,ed copper et~httit, 2))~ [NH ] .S,() ). (a) Scanning electron niicrograph shovvintz the iao— — — a tered coattng microsiruct ure. The outer las-er is I bicker near i he . . -. edge as the resuli of enhanced diffusion in this location. b) Magntlied hess of the coating Ia~ers,

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Aluminization of Cu for oxidation protection

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Fig. 4. Auger depth profile from an aluminized copper specimen oxidized in air for 50 h at 649 ~C. The carbon on the surface was absorbed from the atmosphere.

3.2. Oxidation behavior Table I is a summary of mass change measurements of the untreated and aluminized samples at 649 and 815 The oxidation rate of the aluminized sample at 649 °Cwas extremely slow. The mass gain of the aluminized sample was approximately 340 times less than that of the untreated sample. The EDXA from the top side of the sample shows that the scale was aluminum rich. The oxide film was further examined using Auger electron microscopy. The Auger depth profile from the aluminized copper, after exposure in air at 649 °Cfor

A12O3 scale was formed with a small amount of coppercontaining transient oxides. The thickness of AI2O3 film was approximately 0.1 ~tm. Also, no aluminum depletion zone was observed. The EDXA showed the aluminum concentration immediately beneath the Al203 scale was 8 wt.%. For binary Cu—Al alloy, it has been established that the aluminum concentration in the range of 4—8 wt.% conferred considerable oxidation resistance [10]. In the case of oxidation of Cu—Al alloys, the protection was given by the formation of a thin A12O3 or cupric aluminate scale on the alloy surface [II]. In

50 h, is shown in Fig. 4. It was evident that a continuous

the present investigation, an aluminum-rich surface layer

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form sLirface oxides [12].Alter oxidation for 40 50 h at I S C. the aluminum concentration in the coating was insufficient to reform a protective ALO~ scale. I,ocal disruption of Al~() 1scale may be caLised by mechanical inst ihilit~ of thu oxidus

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4. Summary and conclusions -

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) It was demonstrated that improvement in oxidation resistance is achieved by alLiminizing OFHC copper ) I

was prodLiced by a pack cementation technique. The fact that iio alLiminum depletion zone was observed sLiggests that oLitward dilTLlsion of alLiminum to form Al205 was immediately replenished to the bLilk conceii— tration of aluminum in the coating phase by diffusion. In addition. interdillusion between the alLiminLim— enriched coating layer and the copper substrate ~vas sufficiently slow at 64k) C anLi a large reservoir of aluminum was maintained. In contrast, au untreated copper sample after identical oxidation exposure condilions formed a thick CLi,O layer about ISO pm thick. A thin layer of CuO with whiskers formed at the scale gas interface. At 815 C. the aluminized sample again had a mLlch slower rate than the untreated sample. but the sample oxidized with a continuously increasing rate as shown in Fig. 5. The microstructure of an aluminized sample after oxidation at 815 C for 70 h in air is shown in Fig. 6. In most areas, a continuous A1201 scale approximately I pm thick was formed. The aluminum concen— tration in the alloy immediately beneath the A1,O scale, — — . however, was reduced from tile 8 wt.(’o at the start of oxidation to approximately 2 wt.%. Localized attack in the form of pitting with pronoLinced overlaying copper—

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h a pack cementation tech niqtie. (2) The coating consisted of an oLiter t~so—phase region approximately 75 pm thick with an inner aluminum dill’Lision zone of approximately 75 pm. The outer coat— ing layer consisted of ((‘u) and ;‘ —(‘u~1Al3phases. (3) At 649 C. the coating was protective for an extended time as the resLilt of continLioLis Al,O~ scale formation. (4) At a higher temperature of S IS C. coating break— down was observed after 40 50 h as tile restilt of alumi— nLinl depletion in the coating.

..~cknowledgment ‘fhe authors ~ratefullv acknowledge the contributions of J. B. Lumsden and D. A. Hardwiek of Rockwell Science Center. and P. Y. HOU of Lawrence Berkeley Laboratories.

References -

-

-

-

I (). K uhascliess ski and B. l - Flopk ills, O,vulat ion IUrii’~ 2nd edt! . ttuitervvorths t.ondon. 1962

I!

.‘ilctiilo

wid

2 U. Vs. Howard. H. H. Boone and (7 6. (‘iigginv, ‘l’rittv -1.5 ~I 6(1 10671 226.

K. T. C/ticing et a!.

/

A !untinization

0

I Cu

for oxidation protection

139

3 U. W. Goward and D. H. Boone, Oxid. Met., 3 (1971) 475. 4 S. J. Grisaffe. in C. T. Sims and W. C. Hagel teds.), The Supera!!ov.s, Wiley, New York. 1972, p. 341.

7 S. R. Levine and R. M. Caves, J. Electrocheni. Soc., /21 (1974) 1051. 8 F. S. Pettit, Tran,’i Metall. Soc. A/ME, 239 (1967) 1296.

5 W. A. McGill and M. J. Weinbaum, Met. Prog., /15 )February 1979) 26. Y. Mitani, in S.C. Sunghal (ed), High Temperature Protective Coatings, Metallurgical Society of AIME. Warrcndalc, PA, 1983.

9 T. B. Massalski (ed), Binary Alloy Phase Diagrams, American Society for Metals, Metals Park, OH, 1986, p. 103. 10 M. D. Sanderson and J. C. Scully, Oxid. Met.. 3 (1971) 59. II J. C. Blade and A. Preece. J. Inst. Metals, 88 (1959) 427.

p.251.

12 D. P. Whittle, Acta Metall.. 17(1969)1247.

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