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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 937–943

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nnealing effects on the intermetallic compound formationf cold sprayed Ni, Al coatings

. Lee, S. Lee, K. Ko ∗

epartment of Materials Science and Engineering, Ajou University, Suwon 443-749, Republic of Korea

r t i c l e i n f o

rticle history:

eceived 6 February 2007

eceived in revised form

5 February 2008

ccepted 2 March 2008

a b s t r a c t

The annealing of Ni and Al coatings under various conditions on substrates fabricated by a

cold gas dynamic spray process (CDSP) were investigated. The powder particles were accel-

erated through a standard De Laval-type nozzle with air used as the main carrying gas.

The coatings were annealed at 450–550 ◦C in either argon or air atmospheres for 4 h. In

the case of Ni coatings during annealing both in argon and air atmospheres, intermetallic

compound layers such as Al3Ni and Al3Ni2 were observed at the interfaces between the Ni

coating and Al substrate. Also, the intermetallic layer formation of Al3Ni and Al3Ni2 at the

eywords:

nnealing

ntermetallic compounds

old spray

interfaces depended on the solid-state diffusion and the annealing temperature. The inter-

metallic compound AlNi was obtained at the interface of Al coating on a Ni substrate by

low-temperature annealing under the melting temperature.

© 2008 Elsevier B.V. All rights reserved.

oatings

. Introduction

ntermetallic compounds are promising for several high-emperature applications as structural or non-structural

aterials (heat resistance, corrosion resistance, electronicevices, magnets, and superconductors). Aluminide com-ounds have been studied extensively for structural appli-ations, heat resistance and corrosion resistance because ofheir low density, high-melting points, high-thermal conduc-ivity and high corrosion and oxidation resistance at highemperatures (Khor et al., 1995; Sierra and Vazquez, 2006;uarte et al., 2006). Aluminide coatings can be obtained by dif-

erent processes: plasma spray (Khor et al., 1995; Mishra et al.,

005), high-velocity oxy-fuel (HVOF) (Gang et al., 2005), wire-rc (Xua et al., 2004), and flame spray. However, such coatingsre easily deteriorated because the melted and semi-meltedarticles are impacted onto the substrate. In another pro-

∗ Corresponding author at: Department of Materials Science and Enginee43-749, Republic of Korea. Tel.: +82 31 219 2534; fax: +82 31 219 2467.

E-mail addresses: khko@ajou.ac.kr, materialist@empal.com (K. Ko).924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2008.03.001

cessing method, the surface mechanical attrition treatmenttechnique has been developed for obtaining nanostructuredsurface layers (Romankov et al., 2006; Wang et al., 2003). Inprevious papers, the surface layer was subjected to repeatedball collisions and, as a result, the coarse grained structureat the surface became refined on the nanometer scale. It hasbeen demonstrated that ultrafine grains could accelerate dif-fusion (Gao et al., 2001; Liu et al., 1998) and chemical reactionson the surface (Bhushan and Li, 2003). So, mechanical alloyingwith high-energy ball milling can produce ultra-fine powdersand a large quantity of defects, enhancing atomic diffusionover a short distance (Suryanarayana, 2001). In this paper, thecold gas dynamic spray process (CGDS or simply cold spray)

ring, Ajou University, San5, Wonchun-dong, Yeongtong-gu, Suwon

was chosen as a coating process (Lee et al., 2004, 2005). Whenmetallic powders are impinged onto a substrate, the con-version of kinetic energy makes it possible to proceed withmechanical deformation of the particles resulting in a large

n g t

938 j o u r n a l o f m a t e r i a l s p r o c e s s i

quantity of defects similar to mechanical alloying. However,in the case of the Al–Ni reaction, the intermetallic compoundsneed to be formed by post-annealing. Thus, the intermetalliccompound formation in the cold-sprayed Al and Ni coatings bypost-annealing of solid-state diffusion needs to be consideredas an option for aluminide coating formation. In this study,coatings were produced using the cold spray process, and themicrostructure and reaction of coatings at the interfaces wereinvestigated.

2. Experimental

A home-made cold spray system which consisted of a nozzle,a heater, and a compressed gas supply unit was used in theseexperiments. The particles were accelerated through a stan-dard De Laval-type nozzle with a circular exit cross-section

(diameter of 7 mm and throat diameter of 1 mm). Instead ofHe gas, which is typically utilized, air was used as the maingas and particle carrying gas. The pressure prior to enteringthe gas heater was fixed at 1.4 MPa and the temperature of

Fig. 1 – SEM images of as-purchased Al and Ni used in the experXRD patterns.

e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 937–943

the gas through the nozzle was 280 ◦C. The standoff distanceto the substrate was 10 mm. Fig. 1 shows the SEM images ofthe raw material powders of Al and Ni. As-purchased pow-ders with 0.3 wt.% oxygen content and purities of 99.9% wereused in these experiments. Aluminum powders were sievedto −200 mesh (−77 �m) and had an irregular shape (Fig. 1(a))while Ni powders were 3 �m in diameter and exhibited aggre-gation (Fig. 1(b)). The as-purchased powders analyzed by XRDare shown in Fig. 1(c). As shown, only Al and Ni peaks weredetected in the powders. The Ni was coated on a smooth Alsubstrate without sand-blasting and an Al coating was alsoapplied to a smooth Ni substrate without sand-blasting. Theas-coated samples were annealed between 450 ◦C and 550 ◦Ceither in an argon or air atmosphere for 4 h in order to formthe intermetallic compounds by solid-state diffusion at theinterface between the coating and the substrate. Repeats ofsample preparation were carried out with the same coat-

ing and annealing parameters and the reproducibility of thecoating preparation was confirmed. The microstructure andcomposition of coatings were measured by ESEM with EDS(Philips, XL 30 ESEM-FEG) and an optical microscope (OM).

iments, (a) Al powders (X300) (b) Ni powders (X3k), and (c)

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. Results and discussion

he OM and BSE (backscattered electron) images of as-coatedi and Al coatings are shown in Fig. 2. It was found that the

nterface between a Ni coating and an Al substrate was roughecause the Ni particles accelerated by high pressure had an

mpact on the Al substrate and the craters were formed byard Ni particles (Fig. 2(a) and (c)). On the contrary, the Al par-icles are softer and did not create craters at the interface whenprayed onto a Ni substrate (Fig. 2(b) and (d)). Further, there iso interaction between as-coated coatings and substrates andure Al and Ni were detected based on the EDS results (Fig. 2(c)nd (d)).

Fig. 3 shows the OM images of Ni and Al coatings on sub-trates annealed at 450 ◦C in argon and air atmospheres. Theeaction was observed at the interfaces at 450 ◦C in both argonnd air by solid-state diffusion. The fact that only a singleolor (light gray) of reaction product exists at the interfacesfter annealing in both argon and air was confirmed for thei on Al substrate samples (Fig. 3(a) and (b)). In the case of Al

oatings on Ni substrates, it was observed that the Al coatingide was light gray while dark gray layers on the Ni sub-trate side were formed (Fig. 3(c) and (d)). With an increasedost-annealing temperature of 500 ◦C in both argon and air

ig. 2 – Polished cross-section images (OM, BSE) of as-coated coai coating (BSE), and (d) Al coating (BSE).

h n o l o g y 2 0 9 ( 2 0 0 9 ) 937–943 939

atmospheres, the light gray layers on the Al side and darkgray layers on Ni side were found in the Ni coating on anAl substrate (Fig. 4(a) and (b)). However, it was found thatthe interlayers between the Al coating and Ni substrate werethicker and had a dark gray color while the light gray layerdisappeared in both argon and air (Fig. 4(c) and (d)). Afterpost-annealing at 550 ◦C, the reaction layers became thicker,up to 25 �m, and only a single dark gray layer was formed inthe Ni coating (Fig. 5(a) and (b)). It seemed, however, that thecross-section images of Al coating on Ni substrate annealedin argon and air atmospheres were different (Fig. 5(c) and (d)).The reaction layer created by solid-state diffusion in an argonatmosphere was dark gray but the thickness of whole reactionlayer remained unchanged in spite of an increased anneal-ing temperature. However, the interface with a dark gray coloron the bulk Ni side became thicker and the light gray layerwas still observed on the Al side when the Al coating wasannealed in an air environment. From these results, it waslikely that the diffusion of coated Ni and Al particles withsevere plastic deformation was very fast because of the largeamount of defects in the Al and Ni due to the accelerating inci-

dent particles’ impact. So, it is possible that the intermetallicformation of cold-sprayed coatings occurred under low tem-perature annealing in argon even after post-annealing in an airenvironment.

tings, (a) Ni coating (OM, X500), (b) Al coating (OM, X500), (c)

940 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 937–943

Fig. 3 – Polished cross-section images (OM) of coatings on substrates after annealing at 450 ◦C (X500) for (a) Ni on Al in air,(b) Ni on Al in argon, (c) Al on Ni in air, and (d) Al on Ni in argon. The arrows indicate the reactions.

Fig. 4 – Polished cross-section images (OM) of coatings on substrates after annealing at 500 ◦C (X500) for (a) Ni on Al inargon, (b) Ni on Al in air, (c) Al on Ni in argon, and (d) Al on Ni in air. The arrows indicate the reactions.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 937–943 941

Fig. 5 – Polished cross-section images (OM) of coatings on substrates after annealing at 550 ◦C (X500) for (a) Ni on Al inargon, (b) Ni on Al in air, (c) Al on Ni in argon, and (d) Al on Ni in air. The arrows indicate the reactions.

Fig. 6 – The backscattered electron (BSE) images and the composition of coatings after annealing at 450 ◦C using FESEM withEDS for (a) Ni on Al in air and (b) Al on Ni in air.

942 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 937–943

Fig. 7 – The backscattered electron (BSE) images and the composition of coatings after annealing at 500 ◦C using FESEM withEDS for (a) Ni on Al in air and (b) Al on Ni in air.

Fig. 8 – The backscattered electron (BSE) images and the composition of coatings after annealing at 550 ◦C using FESEM withEDS for (a) Ni on Al in air, (b) Al on Ni in air, and (c) Al on Ni in argon.

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Suryanarayana, C., 2001. Prog. Mater. Sci. 46, 1–184.Wang, Z.B., Tao, N.R., Tong, W.P., Lu, J., Lu, K., 2003. Acta Mater. 51,

4319–4329.Xua, B., Zhu, Z., Ma, S., Zhang, W., Liu, W., 2004. Wear 257,

j o u r n a l o f m a t e r i a l s p r o c e s s i n g

In order to clearly determine the reactants’ compositiont the interfaces between coatings and substrates, the inter-ayer composition for various annealing temperatures in argonnd air atmospheres were measured by EDS. Fig. 6 shows EDSnd backscattered images of the interfaces at an annealingemperature of 450 ◦C. Comparing the composition of coat-ngs with the OM data (Fig. 3(a) and (b)), it was confirmed thathe intermetallic compound Al3Ni was formed at the inter-ace of Ni and Al substrate when annealing at 450 ◦C in airFig. 6(a)). It was also found that the Al3Ni2 (dark gray in OM,ig. 3(c) and (d)) was generated in the Ni substrate side due tohe higher availability of Ni, while Al3Ni (light gray in OM) wasormed on the Al coating side due to the higher availabilityf Al when the Al coating was annealed at 450 ◦C (Fig. 6(b)).ore Ni particles reacted with bulk Al in air annealing at

00 ◦C, and thus Al3Ni2 phases were formed at the interfaceFig. 7(a)). A very thick single layer of Al3Ni2 between the Aloating and Ni substrate was confirmed at the same anneal-ng conditions (Fig. 7(b)). The EDS and backscattered imagesf the interfaces prepared at an annealing temperature of50 ◦C are shown in Fig. 8. The intermetallic compound Al3Ni2dark gray in OM) at the interface of coated Ni/Al substrateas confirmed with air annealing (Fig. 8(a)). In the case of the

ir-annealed Al coating on a Ni substrate, the intermetallicompound Al3Ni still existed and the Al3Ni2 phase interlayerecame thicker (Fig. 8(b)). The interlayer of the argon-annealedl coating on a Ni substrate did not change but the phase

ransformation of intermetallic compounds as relatively high-emperature reaction phases (AlNi) occurred at the interfaceFig. 8(c)). Based on these data, the diffusion of the coated Alnto the Ni bulk was not thought to result in the formationf a thick layer of Al3Ni2 (low-temperature reaction phase, Al-ich phase). Rather, it resulted in the transformation of thentermetallic compound (high-temperature reaction phase) athe interface between coated Al layer and Ni substrate. There-ore, this study clearly confirmed that the diffusion of severelylastic-deformed Al and Ni (coatings)/tamped Al and Ni (sub-trates) was very fast due to the quantity of defects and theact that intermetallic compounds could be formed at inter-aces between the coatings and substrates by post-annealingt relatively low temperatures even in an air atmosphere.

. Conclusion

n conclusion, the intermetallic compounds Al3Ni and Al3Ni2ere successfully fabricated in the coatings by annealing in

h n o l o g y 2 0 9 ( 2 0 0 9 ) 937–943 943

both argon and air atmospheres. There is a good possibilitythat cold spray coatings with intermetallic compounds canbe successfully produced by solid-state diffusion. In addition,the formation of intermetallic compounds can occur at lowannealing temperatures because of the high velocity impact ofparticles/surfaces in the incident stream of spraying. This canresult in high strains and defects. Therefore, the high velocityimpact coatings by cold spray could have an influence on lowtemperature intermetallic compound formation in the coat-ings due to annealing effects.

Acknowledgment

This research was supported by a grant from the Center forAdvanced Materials Processing of the 21st Century FrontierR&D Program funded by the Ministry of Science and Technol-ogy, Republic of Korea.

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Gang, J., Elkedimc, O., Grosdidiera, T., 2005. Surf. Coat. Technol.190, 406–416.

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Process. Technol. 48, 413–419.Lee, H., Yu, Y., Lee, Y., Hong, Y., Ko, K., 2004. J. Thermal Spray

Technol. 13 (2), 184–189.Lee, H., Yu, Y., Lee, Y., Hong, Y., Ko, K., 2005. J. Thermal Spray

Technol. 14 (2), 183–186.Liu, Z., Gao, W., Dahm, K., Wang, F., 1998. Acta Mater. 46,

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