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Applied Surface Science 257 (2011) 5972–5976

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

ffect of vacuum heat treatment on tensile strength and fracture performance ofold-sprayed Cu-4Cr-2Nb coatings

in Yua,b,c,∗, Wen-Ya Lia,b, Chao Zhangd, Hanlin Liaoc

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, PR ChinaShaanxi Key Laboratory of Friction Welding Technologies, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, PR ChinaLERMPS, Université de Technologie de Belfort-Montbéliard, Site de Sévenans, 90010 Belfort Cedex, FranceService de Science des Matériaux, Faculté Polytechnique, Université de Mons, Rue de l’Epargne 56, B-7000 Mons, Belgium

r t i c l e i n f o

rticle history:eceived 4 November 2010eceived in revised form 11 January 2011ccepted 19 January 2011

a b s t r a c t

The previous study [1] indicated that dense thick Cu-4Cr-2Nb coatings could be formed by cold spraying,and the post-spray heat treatment could significantly influence the microstructure and microhardness ofthe as-sprayed Cu-4Cr-2Nb coatings. In this study, the tensile strength and fracture performance of theCu-4Cr-2Nb coatings after annealing were investigated. The vacuum heat treatment was conducted under

vailable online 12 February 2011

eywords:old sprayingu-4Cr-2Nb coatingsacuum heat treatment

10−2 Pa at 850 ◦C for 4 h. Results showed that the heat treatment had a great contribution to the healing-upof the incompleteness of the interfaces between the deposited particles. In addition, the coating micro-hardness decreased from 156.8 ± 4.6 Hv0.2 for the as-sprayed coatings to 101.7 ± 4.5 Hv0.2 for the annealedones. The mean tensile strength of the annealed coatings was approximately 294.1 ± 36.1 MPa comparedto that of 45.0 ± 10.5 MPa for the as-sprayed ones, which results from the partially metallurgically bonded

ited p

ensile strengthracture performance

zones between the depos

. Introduction

The Cu-4Cr-2Nb alloy effectively combines good thermal con-uctivity with excellent mechanical properties, which representsn attractive alternative to other high temperature Cu based alloys2,3]. The traditional fabrication processes for these Cu alloysncluding casting, direct-extruding, hot isostatic pressing and evenacuum plasma spraying are usually employed [2,4,5]. However, its expected to cost-effectively fabricate these Cu alloys for appli-ations. Cold spraying, as an emerging coating technique, has beenidely applied to produce many metallic coatings and composites,

nd even for spray forming of parts [6–9]. A literature survey showshat Cu is the mostly used cold spray material [10–13]. Studies alsohowed that cold-sprayed Cu coatings possess excellent electricalnd thermal conductivities and have been used in rocket engines14].

Recent study on annealed cold-sprayed Cu coatings revealed

hat the homogenization of coating microstructure and the signif-cant improvement of interface bonding were achieved [15–17].herefore, it is expected to cost-effectively produce the Cu alloyoatings by combining cold spraying with post-spray heat treat-

∗ Corresponding author at: LERMPS-UTBM, Site de Sévenans, 90010 Belfort Cedex,rance. Tel.: +33 3 84583736; fax: +33 3 84583286.

E-mail addresses: Min.Yu@utbm.fr (M. Yu), liwy@nwpu.edu.cn (W.-Y. Li).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.01.076

articles inducing by the heat treatment process.© 2011 Elsevier B.V. All rights reserved.

ment compared to the most commonly used vacuum plasmaspraying in applications. The previous study [1] showed thatthe post-spray heat treatment could significantly improve themicrostructure and microhardness of cold-sprayed Cu-4Cr-2Nbcoatings. In addition, our recent study showed that the vacuumdegree during the heat treatment had effects on the microstruc-ture and tensile properties of cold-sprayed Ti coatings [18]. In thepresent study, the tensile strength and fracture performance ofcold-sprayed Cu-4Cr-2Nb alloy coatings after high-vacuum anneal-ing were investigated. The results of this work were also comparedto that of the previous work [1] for a better understanding of thecoating mechanical performance.

2. Experimental procedures

A commercially available cold spray gun (Kinetics 3000, CGTGmbH, Germany) with a MOC type nozzle (expansion ratio: 5.6)was used for coating deposition. The high-pressure compressed airwas used as the driving gas operating at the pressure of 3.0 MPaand temperature of 415 ◦C in the pre-chamber. The standoff dis-tance from nozzle exit to substrate surface was 50 mm. During

spraying, the substrate was mounted on a cylindrical holder rotat-ing at 120 rpm and the spray gun traversed vertically with a speedof 5 mm/s. Gas-atomized Cu-4at.%Cr-2at.%Nb powders (2–25 �m)were produced by LERMPS (UTBM, France). The morphology of thepowders is shown in Fig. 1. Cold-rolled Cu plates were used as sub-

M. Yu et al. / Applied Surface Science 257 (2011) 5972–5976 5973

st

uactqepg

as(5te

Fig. 1. Morphology of the used Cu-4Cr-2Nb powder.

trates and sandblasted using alumina grits (about 147 �m) prioro spraying.

In this study, the vacuum heat treatment was conducted in a vac-um furnace, which consists of heating to 850 ◦C in about 15 minnd holding the temperature for 4 h under 10−2 Pa, followed by theooling with the furnace in about 45 min, compared to the condi-ion of 850 ◦C for 2 h under 1300 Pa in the previous study [1]. For theuantitative analysis of mechanical properties of the coatings, sev-ral micro-flat tensile specimens were prepared using the methodroposed by Gärtner et al. [16]. The dimensions of the specimen areiven in Fig. 2.

The polished cross-section of the coatings was etched with anqueous solution of 5 g FeCl3 + 10 ml HCl + 100 ml H2O. The cross-

ectional microstructure was observed using an optical microscopeOM, Nikon, Japan) and a scanning electron microscope (SEM, JSM-800LV, Japan). The microhardness and mechanical properties ofhe coatings were measured at room temperature using a Vick-rs harness tester (Struers Duramin-A300, Germany) with a load of

Fig. 3. SEM micrographs of the as-sprayed Cu-4Cr-2Nb coating (a), the annealed coatin

Fig. 2. Dimensions of the micro-flat tensile specimen (unit: mm).

200 g for 15 s and a Shimadzu tensile tester (Shimadzu AG-X, Japan),respectively. More than 15 values were randomly tested and aver-aged to evaluate the coating hardness and four samples were fortensile strength. The fracture morphologies of the coatings aftertensile tests were also examined by SEM.

3. Results and discussion

3.1. Effect of annealing on coating microstructure

For better observing the Cr2Nb precipitates, the polished cross-sections of the as-sprayed and annealed coatings were etched. Theresultant micrographs of the as-sprayed and annealed Cu-4Cr-2Nbcoatings are shown in Fig. 3. From Fig. 3a, it could be found thatthe Cr2Nb precipitates (white particles as marked by arrows inFig. 3a) are dispersed in the matrix and the interfaces between thedeposited particles are clearly revealed even if the particles haveexperienced intensive deformation to get the elongated shapes[1]. Fig. 3b and c show the microstructure of the annealed Cu-

◦

4Cr-2Nb coatings. As the heat treatment was conducted at 850 Cfor 2 h under 1300 Pa, the obtained microstructure as shown inFig. 3b reveals that the interfaces tend to disappear, while at 850 ◦Cfor 4 h under 10−2 Pa, these interfaces have almost disappearedexcept some relatively large near-spherical pores, This on one hand

gs at 850 ◦C for 2 h under 1300 Pa (b) and for 4 h under 10−2Pa (c) after etching.

5 e Science 257 (2011) 5972–5976

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Fig. 5. OM micrograph of a representative indentation.

Table 1Tensile test data of the annealed coatings.

Sample no. Ultimate strength (MPa) Elongation to failure (%)

1 248.0 2.9

to failure of about 248 MPa and 2.9%, 319 MPa and 1.6%, respec-tively. The different fracture behaviors between samples 1 (1#)and 4 (4#) may be related to the microstructures of the sampleswhich have different defects and defect distributions (especially

974 M. Yu et al. / Applied Surfac

ay suggest the recrystallization had occurred and a metallurgicalonding between the deposited particles was formed as reportedor cold-sprayed Cu coatings [15,17]. On the other hand, these rel-tively large micro-pores could result from the coalescence of thencompleted interfaces in the as-sprayed coatings through the atomiffusion during the heat treatment with relatively high tempera-ure and long time, such as 850 ◦C for 4 h in the present experiment.his phenomenon has also been found in the annealed Ti coatings19]. In conclusion, the heat treatment has a contribution to theealing-up of the incomplete interfaces between the deposited par-icles. Therefore, the improved mechanical properties are expected.

.2. Effect of annealing on coating microhardness

Fig. 4 shows the microhardness of the as-sprayed andnnealed coatings. The hardness of the as-sprayed coatings is56.8 ± 4.6 Hv0.2. A considerable decrease in hardness after thereatment at 850 ◦C for 2 h under 1300 Pa (110.1 ± 13.8 Hv0.2) coulde explained by the grain size growth of Cr2Nb phase as well ashe softening of the matrix induced by the heat treatment process1]. However, the hardness of the coatings annealed at 850 ◦C forh under 10−2 Pa presents a lower value, 101.7 ± 4.5 Hv0.2, whichay result from longer heat treatment time and correspondingly

igher softening of the matrix. Nevertheless, these hardness val-es are much higher than the annealed Cu coatings (e.g., 76.3 Hv0.2nd 46 Hv0.2 at 650 ◦C for 1 h and 12 h, respectively [17]). Fig. 5hows a typical indentation after the hardness test and suggestsgood adhesion and ductile behavior characterized by the regu-

ar shape and no cracks in the indentation of the annealed coating.he dispersed Cr2Nb particulates contribute to the relatively highardness. For a better understanding of this, the tensile strengthest was carried out and will be discussed in the next section.

.3. Effect of annealing on coating tensile strength

A mean ultimate tensile strength value of about 45.0 ± 10.5 MPaas obtained for the as-sprayed coatings. The elongations to fail-re of all the as-sprayed coatings do not exceed 0.2%, which reflectshe brittleness of as-sprayed coatings. The data of the annealedoatings are shown in Table 1. After annealing at 850 ◦C for 4 h

−2

nder 10 Pa, the mean ultimate strength tremendously increaseso 294.1 ± 36.1 MPa, compared to about 400 MPa for the strength ofhe bulk cast Cu-4Cr-2Nb alloy [20]. Therefore, it is possible to cost-ffectively produce the Cu alloy coatings owing to the equivalentechanical properties by combining cold spraying with post-spray

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Annealed at 10-2 Pa for 4h

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ig. 4. Effect of annealing on the microhardness of cold-sprayed Cu-4Cr-2Nb coat-ng.

2 283.1 4.83 326.4 5.54 319.1 1.6

heat treatment. In addition, notwithstanding the elongation to fail-ure of the annealed coatings was increased, it is still very low (seeTable 1). Representative stress–strain plots of the annealed coatingsand the photos of the fractured samples are shown in Fig. 6. Sam-ples 1 (1#) and 4 (4#) yield the ultimate strength and elongation

Fig. 6. Stress–strain curve of the Cu-4Cr-2Nb coatings annealed at 850 ◦C for 4 hunder 10−2 Pa (a) and the fractured 4# specimen (b).

M. Yu et al. / Applied Surface Science 257 (2011) 5972–5976 5975

annea

tist[it

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Fig. 7. SEM fracture morphologies of the as-sprayed (a and b) and

he pores). These defects would induce the fracture taking placen advance. Therefore, this discrepancy is rational. In addition, themall elongation to failure of the annealed coating indicates a brit-le behavior. Compared to the annealed cold-sprayed Cu coatings17], the ultimate strength is higher in this study, but the elongations lower. Future work may focus on the improvement of the coatingoughness.

In order to obtain the detailed information on the micro-echanisms of deformation of the as-sprayed and annealed

oatings, the fracture surfaces of the typical tensile samples werenalyzed and are presented in Fig. 7. A locally smooth fracture sur-ace for the as-sprayed coating can be seen in Fig. 7a. The crackropagation occurs along the interfaces of the deposited particlesather than through the deformed particles as shown in Fig. 7b. Itan be rationalized that the mechanical properties of the as-sprayedoatings are determined by the mechanical interlocking betweenhe deformed particles. Therefore, the tensile strength of the as-prayed coatings is relatively low, and the fracture is the brittleype. After annealing, however, the fracture morphology as shownn Fig. 7c and d appears a different feature. Many tearing zones

arked by the white arrows in Fig. 7c can be clearly seen, espe-ially observed at a high magnification (Fig. 7d). Typical dimplesre observed, which indicates the metallurgical bonding betweenhe deposited particles and suggests the local ductile fracture. Inddition, some pores present in the metallurgically bonded regions

mply the interfaces of multi-particles in the as-sprayed coating ashown in Fig. 3c. Therefore, the tensile strength of the annealedoating is about six times as that of the as-sprayed coatings due tohe existence of these metallurgically bonded regions induced byhe heat treatment process.

led (c and d) Cu-4Cr-2Nb coatings at 850 ◦C for 4 h under 10−2 Pa.

4. Conclusions

Based on the results obtained in this study, it can be concludedthat vacuum heat treatment contributed to the healing-up of theinterface incompleteness of cold-sprayed Cu-4Cr-2Nb coatings andconsequently improved coating mechanical properties. The coatingmicrohardness decreased from 156.8 ± 4.6 Hv0.2 for the as-sprayedcoatings to 101.7 ± 4.5 Hv0.2 for the annealed ones. The tensilestrength increased from about 45.0 ± 10.5 MPa for the as-sprayedcoatings to 294.1 ± 36.1 MPa for the annealed ones due to the met-allurgically bonded zones between the deposited particles existingin the annealed coatings.

Acknowledgements

This work was partially supported by the Ao-Xiang Star Projectof NPU (Northwestern Polytechnical University), the Program forNew Century Excellent Talents in University by the Ministry ofEducation of China (NCET-08-0463), the Research Fund of the StateKey Laboratory of Solidification Processing (NPU) (grant no. 69-QP-2011) and the 111 Project (B08040).

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