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Tribological behavior of cold-sprayed nanocrystalline and conventional copper coatings
Applied Surface Science 258 (2012) 7490–7496
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Tribological behavior of cold-sprayed nanocrystalline and conventional copper coatings Jingchun Liu a , Xianglin Zhou a , Xiong Zheng a , Hua Cui b , Jishan Zhang a,∗ a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 5 March 2012 Accepted 11 April 2012 Available online 21 April 2012 Keywords: Coating Cold spray Wear property Microstruture
a b s t r a c t Both the cryomilled Cu powder and the gas-atomized Cu powder were sprayed onto aluminum substrate using the cold spray process. This study focused on the formation and tribological behavior of the nanocrystalline (NC) Cu coating in comparison to its coarse-grained (CG) Cu counterpart. The results showed that the as-sprayed deposit presented a dense microstructure. The mean grain size of the NC Cu coating was about 30 nm. Investigations on the worn surface of the NC coating revealed that the plastic deformation with grooves and some debris were prominent with no visible cracking. Nanocrystalline Cu coating showed a good wear resistance with a low friction coefficient. The enhancement of the wear properties of the NC Cu was attributed to the grain refinement and the superior hardness. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their extraordinary properties, such as mechanical/physical properties and electrochemical properties, nanocrystalline materials have been drawing attention in the engineering field [1–3]. Nanocrystalline materials have exhibited very high strength and hardness compared with conventional polycrystalline as a result of the considerable reduction of grain size and their significant volume fraction of grain boundaries [1,2]. These exceptional properties suggest various potential structural applications, and research interest in the NC materials has been growing rapidly for the past several years [3,4]. Although there are numerous studies on the mechanical behavior of nanocrystalline metals through standard hardness, compression or tension testing, studies on the mechanisms of friction and wear are relatively limited in the nanocrystalline range, perhaps owing to the difficulty in producing bulk samples suitable for wear tests [5–7]. Cold spray is expected to be a good means to overcome the difficulty of fabricating bulk nanocrystalline materials and to improve the properties (including tribological properties) of conventional materials [8–13]. In this study, the NC Cu and CG Cu were sprayed using the cold spray process. In present study, a nanocrystalline and a coarse grain copper coatings on AA6061 substrate were achieved by means of cold spray. The objective of this study is to compare the wear
∗ Corresponding author. Tel.: +86 10 62334717; fax: +86 10 62332508. E-mail address: [email protected] (J.S. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.070
characteristics of a cold-sprayed nanocrystalline Cu coating with a cold-sprayed conventional Cu coating with coarse grains, and to understand the nature of the difference of the friction and wear behavior with different microstructure. 2. Experimental procedure Both the cryomilled powder and the gas-atomized powder were sprayed onto polished 6061 aluminum substrate using the homemade cold spray system. Nitrogen gas at the temperature of about 300 ◦ C was used as the driving and carrier gas. While producing the nanocrystalline Cu coating, the accelerating gas stagnation pressure was set at 2.0 MPa and the powder carrier gas was kept 0.2 MPa higher than that of the accelerating gas in order to facilitate the injection of the powder into the jet. The substrate was moved at the speed of 5 mm/s relative to the rest gun, under the nozzle jet, to overlay the thickness of the coating in one pass. And the stand-off distance is 10 mm. During the preparation of the conventional Cu coating, the accelerating gas stagnation pressure was set at 1.5 MPa. The substrate was manipulated at the speed of 10 mm/s relative to the rest gun, and the stand-off distance is 20 mm. The microstructure of the coatings were characterized by using scanning transmission electron microscope (STEM) on a Philips/FEI Tecnai F20 operated at an accelerating voltage of 200 kV. The samples for TEM were in the form of a 3 mm disc, which were punched from the coating less than 50 m thick prepared by grinding on a 2000 grit abrasive paper. The final thinning was carried out by ionbeam thinning at a current of 0.5 mA and an inclination angle of 12◦ (Model 691 PIPS, Gatan, Inc., CA, USA).
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Fig. 1. A cross-sectional observation of the cold sprayed nanocrystalline Cu coating (a) and conventional Cu coating (b).
Sliding wear tests of the cold sprayed nanocrystalline Cu were performed on an Optimol SRVII oscillating friction and wear tester in a ball-on-disc contact configuration under dry condition at room temperature (20–25 ◦ C) in air with a relative humidity of 40%. Discs were cut from the sprayed specimens to a dimension of Ø24 mm × 7.88 mm. The balls of 10 mm in diameter were made of Gr Cr15. The friction and wear tests were carried out at an oscillating stroke of 1 mm, normal loads of 5–30 N, a frequency of 50 Hz, and a duration of 5 min. The wear tests of the cold sprayed conventional Cu coatings were also conducted under the same conditions as a comparison. The friction coefficient values reported in this paper, which were continuously recorded, are considered normal values
that represent the predominant behavior during the majority of each test. After each test, the wear volume loss was determined from the wear track profiles obtained by a non-contact 3D (3-dimensional) surface profiler (MicroMAXTM, ADE Phase Shift, AZ, US), which clearly determined the shape and depth of the wear track, and thus the wear volume. The morphologies of the worn surface at different wear conditions were investigated by using ZEISS SUPRA55 field emission scanning electron microscope (SEM). Energy dispersion spectroscope (EDS) was used to analyze the composition of the worn surface.
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3. Results and discussion 3.1. Microstructure and microhardness The typical cross-sectional microstructure of nanostructured Cu coating and conventional Cu coating analyzed via EPMA were seen in Fig. 1(a and b), respectively. Both of the coatings showed excellent interface with the substrate. Cu particles experienced severe deformation resulting in a relatively low porosity in the two kinds of coatings with irregular surface were observed. The thickness of the nanocrystalline coating was about 100–150 m and the thickness of the conventional coating was about 300 m. The microstructure clearly showed that dense nanocrystalline and conventional Cu coatings were deposited via cold spray. It could be considered that the high velocity impact of the particles led to densification [12]. The densification would pack the microscopic particles, eliminating the voids or dark gaps between them. The TEM image in Fig. 2 showed that the microstructure of the cryomilled powders was retained after the cold spray process. The coating consisted of grains of irregular shape and random orientation, as shown in Fig. 2(a). The corresponding SAED pattern in Fig. 2(b) showed fairly uniform rings, indicating a continuous and wide distribution of misorientations among the nanograins. Closely examining the phases computed based on the d-spacing from the ring pattern also shown the rings corresponding to Cu2 O along with those of copper in Fig. 2(b). Oxygen was derived from a combination of the oxide layer on the starting powder and reformation of a new layer on the particles after milling, despite efforts to minimize exposure during handling of the powder [4]. The grain size distribution measurements from the TEM bright field micrographs were presented in Fig. 2(c) and showed an average grain size of 30.4 nm. The microstructure composed mainly of small nanometer grains that were 5–45 nm in size, as shown in Fig. 2(a). Other investigators [9–12] also reported similar results on nanocrystalline Ni, Al1100 alloy, Al2618 alloy and Fe–Si alloy coatings. It is clear that the nanostructured phase of the powders was successfully transferred to the coating via cold spray. Moreover, the present study suggests that cryomilling process combined with the cold spray technique are a viable means of producing nanocrystalline coatings for wear test. In the conventional Cu coatings, a large number of deformation bands could be observed, as shown in Fig. 3(a). This was attributed to the peening effect by the Cu particles during high-velocity impact on to the substrate. A close examination of the corresponding SAED pattern in Fig. 3(b) showed no oxide in the coating. The microhardness tests on the NC Cu coating showed that a hardness value of 3.02 GPa was obtained, which was nearly two times as high as that of the CG Cu coating (1.7 GPa). The hardness values are comparable to those of the coating formed by surface mechanical attrition treatment (SMAT) as reported by Zhang et al. [14,15]. On one hand, the increase in hardness with the refinement of the grain structure is well document based on the experimental and theoretical results, or termed as Hall–Petch effect [16–20]. On the other hand, dispersed oxide strengthening may also have a little effect on the microhardness [12]. According to Sudharshan Phani et al. [21], the hardness of cold sprayed Cu (HCu ) can be obtained by linear superposition of various strengthening mechanisms to give: HCu = Ho,Cu + Hg + Hcw
(1)
In Eq. (1), Ho,Cu is the intrinsic hardness of CG Cu and should usually include the contributions of dislocation strengthening caused by dislocations present in Cu and solid solution hardening effects caused by the oxygen present. Hg is the strengthening owing to grain boundaries and the grain boundary effect for hardness can be described using the well-known Hall–Petch relationship. Finally, Hcw is the increase in hardness of the coating owing to cold work
Fig. 2. A bright-field TEM image (a) and a corresponding SAED pattern (b) and grain size distribution (c) of the cold sprayed nanocrystalline Cu coating.
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Fig. 4. Variation of the friction coefficient of the cold sprayed nanocrystalline Cu and the conventional Cu coatings with the applied load.
The friction coefficient is determined by the real contact area, the contact state and the lubricant role of debris [14]. For the case of a material undergoing plastic leading to the deformation at low sliding speeds, the variations of flash temperature with load can be described by [14,22]:
Tf = v
Fig. 3. A bright-field TEM image (a) and a corresponding SAED pattern (b) of the cold sprayed conventional Cu coating.
induced by the cold spray process. In the case of cold sprayed NC Cu coating, the additional strengthening is due to the grain refinement strengthening effect, the cold work induced by the cold spray process and the dispersed nano-sized Cu2 O particles in the NC Cu matrix. Otherwise, there is no Cu2 O particles strengthening and the grain refinement strengthening is only marginal in the cold sprayed CG Cu coating. In the case of cold sprayed CG Cu, the contribution of Hcw to the hardness is nearly 50% while the contribution of Hg is marginal (∼10%) [21]. However, the contribution of Hcw to overall hardness in the case of cold sprayed NC Cu is only marginal. This is essentially due to the much higher hardness of NC Cu compared to CG Cu. Therefore, the grain refinement strengthening is the most dominant strengthening mechanism in the NC Cu coating. The higher hardness may indicate that the NC Cu coating would have good wear resistance.
FN y 8k
(2)
where is the friction coefficient, the sliding speed, FN the normal load on the pin, y the compression yield stress of the coating, and k the thermal conductivity of the coating. It is expected that friction contact temperature increase with normal load, leading to the softening of the material and the decrease of subsurface shear strength [14,23]. At the same time, the decrease of friction coefficient with load may also originate from slow increase of actual contact area of the friction couples. Otherwise, the oxidation of the coating becomes available while the contact temperature increases. The variation of wear volume with applied load for NC and the CG Cu coatings is shown in Fig. 5. As expected, wear volumes of both coatings increased with the increase in the applied load. An obvious difference was found between the NC and CG Cu coatings. For the NC Cu coating, wear volume was always lower than that of the CG Cu coating for an applied load range from 5 N to 30 N,
3.2. Friction and tribological behavior Fig. 4 shows the variation of the steady-state friction coefficient with applied load for the NC and CG Cu coatings. For both coatings, the friction coefficient decreases with an increase of the load and tends to a saturation value (∼0.5) when load exceeds about 20 N. It can be seen that the friction coefficient the NC Cu coating is obviously lower than that of the CG Cu coating when the load is below 20 N. When the applied load exceeded 20 N, the steady-state friction coefficient of the NC Cu (about 0.50 under the load of 20 N and 0.48 under the load of 30 N) was close to that of the CG Cu (0.51).
Fig. 5. Variation of wear volume with the applied load for the cold sprayed nanocrystalline Cu and the cold sprayed conventional Cu samples.
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Fig. 6. Three-dimensional reconstructions by white-light interferometry on the NC Cu coating under the load of 10 N (a), 30 N (b), and the CG Cu coating under the load of 10 N (c), 30 N (d).
which showed that the better wear resistance of NC Cu coating as compared to their conventional counterparts, CG Cu. Both friction and wear are simultaneously the results of the same tribological contact process that takes place between two sliding surfaces. It is well known that the low friction corresponds to low wear. The way in which the removal of material from the surface takes place
is described by several wear mechanisms. It is very common that in a real contact, more than one wear mechanism is acting at the same time. So, it becomes quite pertinent to explain the wear phenomenon based on worn surface morphology. Further, quite similar results for the wear resistance and friction coefficient are obtained in other literatures [6,14,15,24].
Fig. 7. Surface morphologies of wear scars for the NC Cu coating under the load of 10 N (a), 30 N (b), and the CG Cu coating under the load of 10 N (c), 30 N (d).
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The enhancement of hardness due to grain size reduction following the well-known Hall–Petch relationship [16–20], and then to result in the improvement in wear resistance according to Archard’s law of wear [25], i.e., W =k
L×S H
(3)
which gives the relationship between wear volume W, applied load L, sliding distance S, hardness H of the softer materials in contact and k is the wear coefficient. The experimental results given above indicated that the improvement of wear resistance of the NC Cu was associated with the increase of hardness, which was attributed to grain refinement. It has also been reported that the nanocrystalline metal and alloy may have lower friction coefficient than coarse-grained metal and alloy when the grain size was small enough [14,26], which may be attributed to high hardness and other reasons. It is clearly that the reduction of friction and wear of NC Cu coating under dry friction condition was attributed to the high hardness and load bearing. 3.3. Worn surfaces and wear mechanism The difference in the wear behavior between the NC and the CG Cu samples can be understood by observation of the worn surface morphologies. Fig. 6 shows SEM micrographs of the worn surfaces of the NC Cu and the CG Cu samples under loads of 10 N and 30 N. All the worn surfaces is found to be elliptical with large dimensions along the sliding direction. For both the CG and the NC Cu samples, at the low load of 10 N, the worn surfaces (Fig. 7) exhibited similar morphologies with many grooves. The dominant wear loss was caused by ploughing. As expected, the morphology of the worn surface indicated that NC Cu coating experienced plastic deformation with grooves and some debris and no visible cracking or fracture after the wear test, as presented in Fig. 7(a and b). Otherwise, the worn surface of the CG Cu exhibited deep grooves and big scars, which caused the rough surface and increased the friction coefficient and weight loss, as shown in Fig. 7(c and d). The difference in the worn surface morphologies between the two Cu samples was that the plastic deformation on the CG Cu sample surface was more intensive than that on the NC Cu sample surface. Several investigations of the friction and wear behavior of NC Cu showed different tribological characteristics [6,14,15,24]. However, most of researchers observed similar plastic deformation patterns on the worn surface due to the friction heating. The EDS analysis of the worn track of the NC Cu coating and its conventional counterparts under the load of 10 N and 30 N were presented in Fig. 8. Obviously, the O content of the worn surface of the NC Cu coating in Fig. 8(a and b) is higher than that of the CG Cu coating in Fig. 8(c and d), which may attribute to the process of the powder preparation and the sliding wear process. However, quantitative analysis for O is not accurate by SEM–EDS, it is only considered as a change trend. After cryomilling, the handling of the powders may result in oxidation of the Cu powder. For sliding in air, the NC Cu coating becomes more susceptible to oxidation compared to the CG Cu coating while the contact temperature increases. The discrepancy of the two kinds of coatings is attributed to the more defects the NC coating has. The defects of the NC coating is mainly attributed to the cryomilling process. It is common to produce oxides which are then available to mix mechanically with unoxidized metal to form a mechanical mixed surface layer [14,27,28]. It is indicated that the damage is characterized by delamination of the mechanical mixed layer as can be seen from surface debris in Fig. 7(b). During sliding wear test, the surface and subsurface of the sample are subjected to alternate tensile stress and compression stress [14], and this is in fact a low
Fig. 8. The EDS analysis of the worn surface of the NC Cu coating under the load of 10 N (a), 30 N (b), and the CG Cu coating under the load of 10 N (c), 30 N (d).
frequency fatigue process. The reciprocating sliding action caused the repetitive work-hardening with increased the load. Severe plastic deformation and shear strains in the worn surface giving rise to the formation of delamination cracks which propagate subsequently to cause spalling of the material in the coating [14]. The sharp increase of wear volume, as presented in Fig. 5, may result from the change of the wear mechanism from local damage to delamination of the mechanical mixed layer under the load of 30 N. For the CG Cu coating, with the increase of the load, some debris induced by severe deformation and work-hardening is present on the worn surface, as shown in Fig. 7(c and d). For a nanocrystalline metal, it is easy to form a mixed surface layer because of the presence of more grain boundaries which act as nucleation sites for the oxides and diffusion paths of oxygen during the mixing process caused by sliding. Otherwise, it cannot form a continuous and effective mixed surface layer for the CG Cu coating. Grain refinement not only increases the hardness of the NC Cu coating, but also improves the stability of a mixed surface layer for the NC Cu coating. Both effects can enhance the wear resistance of the NC Cu coating in present study. Otherwise, according to Eq. (1), the grain refinement strengthening is the most dominant strengthening mechanism in the NC Cu coating. The enhancement of the wear behavior concurrent with higher hardness of the NC Cu coating is mainly attributed to the grain refinement strengthening. 4. Conclusions When nitrogen was used as the process gas, nanocrystalline Cu coating and coarse grain Cu coating could be formed via the cold spray process. The microhardness of the NC Cu coating was nearly two times that of CG Cu coating. Nanocrystalline Cu coating showed a good wear resistance with a low friction coefficient.
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The enhancement of the wear properties of the NC Cu was associated with the high hardness, the small work-hardening rate of the nanocrystalline structure, and easily being oxidized of wear debris, which was attributed to grain refinement. The worn surface of the NC Cu mainly revealed that the plastic deformation with grooves and some debris were observable with no visible cracking or fracture. It seemed to be concluded that the cold spray process was suitable for the deposition of NC Cu coating, a potential material for application in bearing parts. Acknowledgements The authors wish to acknowledge the financial support provided by the National Natural Science Foundation of China under grants No. 50874009 and No. 50871019. References [1] H. Gleiter, Progress in Materials Science 33 (1989) 223–315. [2] M.A. Meyers, A. Mishra, D.J. Benson, Progress in Materials Science 51 (2006) 427–556. [3] E.J. Lavernia, B.Q. Han, J.M. Schoenung, Materials Science and Engineering A 493 (2008) 207–214. [4] D.B. Witkin, E.J. Lavernia, Progress in Materials Science 51 (2006) 1–60. [5] M. Dao, L. Lu, R.J. Asaro, J.T.M. De Hosson, E. Ma, Acta Materialia 55 (2007) 4041–4065. [6] Z. Han, L. Lu, K. Lu, Tribology Letters 21 (2006) 47–52.
[7] S. Bellemare, M. Dao, S. Suresh, International Journal of Solids and Structures 44 (2007) 1970–1989. [8] R.S. Lima, J. Karthikeyan, C.M. Kay, J. Lindemann, C.C. Berndt, Thin Solid Films 416 (2002) 129–135. [9] L. Ajdelsztajn, B. Jodoin, J.M. Schoenung, Surface and Coatings Technology 201 (2006) 1166–1172. [10] L. Ajdelsztajn, B. Jodoin, G.E. Kim, J.M. Schoenung, Metallurgical and Materials Transactions A 36 (2005) 657–666. ˇ B. Jodoin, E.J. Lavernia, Journal of Thermal Spray Tech[11] L. Ajdelsztajn, A. Zúniga, nology 15 (2006) 184–190. [12] W.Y. Li, C.J. Li, Applied Surface Science 256 (2010) 2193–2198. [13] M. Yandouzi, A.J. Bottger, R.W.A. Hendrikx, M. Brochu, P. Richer, A. Charest, B. Jodoin, Surface and Coatings Technology 205 (2010) 2234–2246. [14] Y.S. Zhang, Z. Han, K. Wang, K. Lu, Wear 260 (2006) 942–948. [15] Y.S. Zhang, Z. Han, K. Lu, Wear 265 (2008) 396–401. [16] T.G. Nieh, J. Wadsworth, Scripta Metallurgica et Materialia 25 (1991) 955–958. [17] T.G. Nieh, J.G. Wang, Intermetallics 13 (2005) 377–385. [18] V. Bata, E.V. Pereloma, Acta Materialia 52 (2004) 657–665. [19] K.E. Aifantis, A.A. Konstantinidis, Materials Science and Engineering B 163 (2009) 139–144. [20] T. Shanmugasundaram, M. Heilmaier, B.S. Murty, V. Subramanya Sarma, Materials Science and Engineering A 527 (2010) 7821–7825. [21] P. Sudharshan Phani, V. Vishnukanthan, G. Sundararajan, Acta Materialia 55 (2007) 4741–4751. [22] J.F. Archard, Wear 2 (1959) 438–455. [23] G. Straffelini, M. Pellizzari, A. Molinari, Wear 256 (2004) 754–763. [24] Y.S. Zhang, K. Wang, Z. Han, G. Liu, Wear 262 (2007) 1463–1470. [25] J.F. Archard, Journal of Applied Physics 24 (1953) 981–988. [26] R. Mishra, B. Basu, R. Balasubramaniam, Materials Science and Engineering A 373 (2004) 370–373. [27] D.A. Rigney, Wear 245 (2000) 1–9. [28] X.Y. Li, K.N. Tandon, Wear 245 (2000) 148–161.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Tribological behavior of cold-sprayed nanocrystalline and conventional copper coatings Jingchun Liu a , Xianglin Zhou a , Xiong Zheng a , Hua Cui b , Jishan Zhang a,∗ a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 5 March 2012 Accepted 11 April 2012 Available online 21 April 2012 Keywords: Coating Cold spray Wear property Microstruture
a b s t r a c t Both the cryomilled Cu powder and the gas-atomized Cu powder were sprayed onto aluminum substrate using the cold spray process. This study focused on the formation and tribological behavior of the nanocrystalline (NC) Cu coating in comparison to its coarse-grained (CG) Cu counterpart. The results showed that the as-sprayed deposit presented a dense microstructure. The mean grain size of the NC Cu coating was about 30 nm. Investigations on the worn surface of the NC coating revealed that the plastic deformation with grooves and some debris were prominent with no visible cracking. Nanocrystalline Cu coating showed a good wear resistance with a low friction coefficient. The enhancement of the wear properties of the NC Cu was attributed to the grain refinement and the superior hardness. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their extraordinary properties, such as mechanical/physical properties and electrochemical properties, nanocrystalline materials have been drawing attention in the engineering field [1–3]. Nanocrystalline materials have exhibited very high strength and hardness compared with conventional polycrystalline as a result of the considerable reduction of grain size and their significant volume fraction of grain boundaries [1,2]. These exceptional properties suggest various potential structural applications, and research interest in the NC materials has been growing rapidly for the past several years [3,4]. Although there are numerous studies on the mechanical behavior of nanocrystalline metals through standard hardness, compression or tension testing, studies on the mechanisms of friction and wear are relatively limited in the nanocrystalline range, perhaps owing to the difficulty in producing bulk samples suitable for wear tests [5–7]. Cold spray is expected to be a good means to overcome the difficulty of fabricating bulk nanocrystalline materials and to improve the properties (including tribological properties) of conventional materials [8–13]. In this study, the NC Cu and CG Cu were sprayed using the cold spray process. In present study, a nanocrystalline and a coarse grain copper coatings on AA6061 substrate were achieved by means of cold spray. The objective of this study is to compare the wear
∗ Corresponding author. Tel.: +86 10 62334717; fax: +86 10 62332508. E-mail address: [email protected] (J.S. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.070
characteristics of a cold-sprayed nanocrystalline Cu coating with a cold-sprayed conventional Cu coating with coarse grains, and to understand the nature of the difference of the friction and wear behavior with different microstructure. 2. Experimental procedure Both the cryomilled powder and the gas-atomized powder were sprayed onto polished 6061 aluminum substrate using the homemade cold spray system. Nitrogen gas at the temperature of about 300 ◦ C was used as the driving and carrier gas. While producing the nanocrystalline Cu coating, the accelerating gas stagnation pressure was set at 2.0 MPa and the powder carrier gas was kept 0.2 MPa higher than that of the accelerating gas in order to facilitate the injection of the powder into the jet. The substrate was moved at the speed of 5 mm/s relative to the rest gun, under the nozzle jet, to overlay the thickness of the coating in one pass. And the stand-off distance is 10 mm. During the preparation of the conventional Cu coating, the accelerating gas stagnation pressure was set at 1.5 MPa. The substrate was manipulated at the speed of 10 mm/s relative to the rest gun, and the stand-off distance is 20 mm. The microstructure of the coatings were characterized by using scanning transmission electron microscope (STEM) on a Philips/FEI Tecnai F20 operated at an accelerating voltage of 200 kV. The samples for TEM were in the form of a 3 mm disc, which were punched from the coating less than 50 m thick prepared by grinding on a 2000 grit abrasive paper. The final thinning was carried out by ionbeam thinning at a current of 0.5 mA and an inclination angle of 12◦ (Model 691 PIPS, Gatan, Inc., CA, USA).
J. Liu et al. / Applied Surface Science 258 (2012) 7490–7496
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Fig. 1. A cross-sectional observation of the cold sprayed nanocrystalline Cu coating (a) and conventional Cu coating (b).
Sliding wear tests of the cold sprayed nanocrystalline Cu were performed on an Optimol SRVII oscillating friction and wear tester in a ball-on-disc contact configuration under dry condition at room temperature (20–25 ◦ C) in air with a relative humidity of 40%. Discs were cut from the sprayed specimens to a dimension of Ø24 mm × 7.88 mm. The balls of 10 mm in diameter were made of Gr Cr15. The friction and wear tests were carried out at an oscillating stroke of 1 mm, normal loads of 5–30 N, a frequency of 50 Hz, and a duration of 5 min. The wear tests of the cold sprayed conventional Cu coatings were also conducted under the same conditions as a comparison. The friction coefficient values reported in this paper, which were continuously recorded, are considered normal values
that represent the predominant behavior during the majority of each test. After each test, the wear volume loss was determined from the wear track profiles obtained by a non-contact 3D (3-dimensional) surface profiler (MicroMAXTM, ADE Phase Shift, AZ, US), which clearly determined the shape and depth of the wear track, and thus the wear volume. The morphologies of the worn surface at different wear conditions were investigated by using ZEISS SUPRA55 field emission scanning electron microscope (SEM). Energy dispersion spectroscope (EDS) was used to analyze the composition of the worn surface.
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3. Results and discussion 3.1. Microstructure and microhardness The typical cross-sectional microstructure of nanostructured Cu coating and conventional Cu coating analyzed via EPMA were seen in Fig. 1(a and b), respectively. Both of the coatings showed excellent interface with the substrate. Cu particles experienced severe deformation resulting in a relatively low porosity in the two kinds of coatings with irregular surface were observed. The thickness of the nanocrystalline coating was about 100–150 m and the thickness of the conventional coating was about 300 m. The microstructure clearly showed that dense nanocrystalline and conventional Cu coatings were deposited via cold spray. It could be considered that the high velocity impact of the particles led to densification [12]. The densification would pack the microscopic particles, eliminating the voids or dark gaps between them. The TEM image in Fig. 2 showed that the microstructure of the cryomilled powders was retained after the cold spray process. The coating consisted of grains of irregular shape and random orientation, as shown in Fig. 2(a). The corresponding SAED pattern in Fig. 2(b) showed fairly uniform rings, indicating a continuous and wide distribution of misorientations among the nanograins. Closely examining the phases computed based on the d-spacing from the ring pattern also shown the rings corresponding to Cu2 O along with those of copper in Fig. 2(b). Oxygen was derived from a combination of the oxide layer on the starting powder and reformation of a new layer on the particles after milling, despite efforts to minimize exposure during handling of the powder [4]. The grain size distribution measurements from the TEM bright field micrographs were presented in Fig. 2(c) and showed an average grain size of 30.4 nm. The microstructure composed mainly of small nanometer grains that were 5–45 nm in size, as shown in Fig. 2(a). Other investigators [9–12] also reported similar results on nanocrystalline Ni, Al1100 alloy, Al2618 alloy and Fe–Si alloy coatings. It is clear that the nanostructured phase of the powders was successfully transferred to the coating via cold spray. Moreover, the present study suggests that cryomilling process combined with the cold spray technique are a viable means of producing nanocrystalline coatings for wear test. In the conventional Cu coatings, a large number of deformation bands could be observed, as shown in Fig. 3(a). This was attributed to the peening effect by the Cu particles during high-velocity impact on to the substrate. A close examination of the corresponding SAED pattern in Fig. 3(b) showed no oxide in the coating. The microhardness tests on the NC Cu coating showed that a hardness value of 3.02 GPa was obtained, which was nearly two times as high as that of the CG Cu coating (1.7 GPa). The hardness values are comparable to those of the coating formed by surface mechanical attrition treatment (SMAT) as reported by Zhang et al. [14,15]. On one hand, the increase in hardness with the refinement of the grain structure is well document based on the experimental and theoretical results, or termed as Hall–Petch effect [16–20]. On the other hand, dispersed oxide strengthening may also have a little effect on the microhardness [12]. According to Sudharshan Phani et al. [21], the hardness of cold sprayed Cu (HCu ) can be obtained by linear superposition of various strengthening mechanisms to give: HCu = Ho,Cu + Hg + Hcw
(1)
In Eq. (1), Ho,Cu is the intrinsic hardness of CG Cu and should usually include the contributions of dislocation strengthening caused by dislocations present in Cu and solid solution hardening effects caused by the oxygen present. Hg is the strengthening owing to grain boundaries and the grain boundary effect for hardness can be described using the well-known Hall–Petch relationship. Finally, Hcw is the increase in hardness of the coating owing to cold work
Fig. 2. A bright-field TEM image (a) and a corresponding SAED pattern (b) and grain size distribution (c) of the cold sprayed nanocrystalline Cu coating.
J. Liu et al. / Applied Surface Science 258 (2012) 7490–7496
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Fig. 4. Variation of the friction coefficient of the cold sprayed nanocrystalline Cu and the conventional Cu coatings with the applied load.
The friction coefficient is determined by the real contact area, the contact state and the lubricant role of debris [14]. For the case of a material undergoing plastic leading to the deformation at low sliding speeds, the variations of flash temperature with load can be described by [14,22]:
Tf = v
Fig. 3. A bright-field TEM image (a) and a corresponding SAED pattern (b) of the cold sprayed conventional Cu coating.
induced by the cold spray process. In the case of cold sprayed NC Cu coating, the additional strengthening is due to the grain refinement strengthening effect, the cold work induced by the cold spray process and the dispersed nano-sized Cu2 O particles in the NC Cu matrix. Otherwise, there is no Cu2 O particles strengthening and the grain refinement strengthening is only marginal in the cold sprayed CG Cu coating. In the case of cold sprayed CG Cu, the contribution of Hcw to the hardness is nearly 50% while the contribution of Hg is marginal (∼10%) [21]. However, the contribution of Hcw to overall hardness in the case of cold sprayed NC Cu is only marginal. This is essentially due to the much higher hardness of NC Cu compared to CG Cu. Therefore, the grain refinement strengthening is the most dominant strengthening mechanism in the NC Cu coating. The higher hardness may indicate that the NC Cu coating would have good wear resistance.
FN y 8k
(2)
where is the friction coefficient, the sliding speed, FN the normal load on the pin, y the compression yield stress of the coating, and k the thermal conductivity of the coating. It is expected that friction contact temperature increase with normal load, leading to the softening of the material and the decrease of subsurface shear strength [14,23]. At the same time, the decrease of friction coefficient with load may also originate from slow increase of actual contact area of the friction couples. Otherwise, the oxidation of the coating becomes available while the contact temperature increases. The variation of wear volume with applied load for NC and the CG Cu coatings is shown in Fig. 5. As expected, wear volumes of both coatings increased with the increase in the applied load. An obvious difference was found between the NC and CG Cu coatings. For the NC Cu coating, wear volume was always lower than that of the CG Cu coating for an applied load range from 5 N to 30 N,
3.2. Friction and tribological behavior Fig. 4 shows the variation of the steady-state friction coefficient with applied load for the NC and CG Cu coatings. For both coatings, the friction coefficient decreases with an increase of the load and tends to a saturation value (∼0.5) when load exceeds about 20 N. It can be seen that the friction coefficient the NC Cu coating is obviously lower than that of the CG Cu coating when the load is below 20 N. When the applied load exceeded 20 N, the steady-state friction coefficient of the NC Cu (about 0.50 under the load of 20 N and 0.48 under the load of 30 N) was close to that of the CG Cu (0.51).
Fig. 5. Variation of wear volume with the applied load for the cold sprayed nanocrystalline Cu and the cold sprayed conventional Cu samples.
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Fig. 6. Three-dimensional reconstructions by white-light interferometry on the NC Cu coating under the load of 10 N (a), 30 N (b), and the CG Cu coating under the load of 10 N (c), 30 N (d).
which showed that the better wear resistance of NC Cu coating as compared to their conventional counterparts, CG Cu. Both friction and wear are simultaneously the results of the same tribological contact process that takes place between two sliding surfaces. It is well known that the low friction corresponds to low wear. The way in which the removal of material from the surface takes place
is described by several wear mechanisms. It is very common that in a real contact, more than one wear mechanism is acting at the same time. So, it becomes quite pertinent to explain the wear phenomenon based on worn surface morphology. Further, quite similar results for the wear resistance and friction coefficient are obtained in other literatures [6,14,15,24].
Fig. 7. Surface morphologies of wear scars for the NC Cu coating under the load of 10 N (a), 30 N (b), and the CG Cu coating under the load of 10 N (c), 30 N (d).
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The enhancement of hardness due to grain size reduction following the well-known Hall–Petch relationship [16–20], and then to result in the improvement in wear resistance according to Archard’s law of wear [25], i.e., W =k
L×S H
(3)
which gives the relationship between wear volume W, applied load L, sliding distance S, hardness H of the softer materials in contact and k is the wear coefficient. The experimental results given above indicated that the improvement of wear resistance of the NC Cu was associated with the increase of hardness, which was attributed to grain refinement. It has also been reported that the nanocrystalline metal and alloy may have lower friction coefficient than coarse-grained metal and alloy when the grain size was small enough [14,26], which may be attributed to high hardness and other reasons. It is clearly that the reduction of friction and wear of NC Cu coating under dry friction condition was attributed to the high hardness and load bearing. 3.3. Worn surfaces and wear mechanism The difference in the wear behavior between the NC and the CG Cu samples can be understood by observation of the worn surface morphologies. Fig. 6 shows SEM micrographs of the worn surfaces of the NC Cu and the CG Cu samples under loads of 10 N and 30 N. All the worn surfaces is found to be elliptical with large dimensions along the sliding direction. For both the CG and the NC Cu samples, at the low load of 10 N, the worn surfaces (Fig. 7) exhibited similar morphologies with many grooves. The dominant wear loss was caused by ploughing. As expected, the morphology of the worn surface indicated that NC Cu coating experienced plastic deformation with grooves and some debris and no visible cracking or fracture after the wear test, as presented in Fig. 7(a and b). Otherwise, the worn surface of the CG Cu exhibited deep grooves and big scars, which caused the rough surface and increased the friction coefficient and weight loss, as shown in Fig. 7(c and d). The difference in the worn surface morphologies between the two Cu samples was that the plastic deformation on the CG Cu sample surface was more intensive than that on the NC Cu sample surface. Several investigations of the friction and wear behavior of NC Cu showed different tribological characteristics [6,14,15,24]. However, most of researchers observed similar plastic deformation patterns on the worn surface due to the friction heating. The EDS analysis of the worn track of the NC Cu coating and its conventional counterparts under the load of 10 N and 30 N were presented in Fig. 8. Obviously, the O content of the worn surface of the NC Cu coating in Fig. 8(a and b) is higher than that of the CG Cu coating in Fig. 8(c and d), which may attribute to the process of the powder preparation and the sliding wear process. However, quantitative analysis for O is not accurate by SEM–EDS, it is only considered as a change trend. After cryomilling, the handling of the powders may result in oxidation of the Cu powder. For sliding in air, the NC Cu coating becomes more susceptible to oxidation compared to the CG Cu coating while the contact temperature increases. The discrepancy of the two kinds of coatings is attributed to the more defects the NC coating has. The defects of the NC coating is mainly attributed to the cryomilling process. It is common to produce oxides which are then available to mix mechanically with unoxidized metal to form a mechanical mixed surface layer [14,27,28]. It is indicated that the damage is characterized by delamination of the mechanical mixed layer as can be seen from surface debris in Fig. 7(b). During sliding wear test, the surface and subsurface of the sample are subjected to alternate tensile stress and compression stress [14], and this is in fact a low
Fig. 8. The EDS analysis of the worn surface of the NC Cu coating under the load of 10 N (a), 30 N (b), and the CG Cu coating under the load of 10 N (c), 30 N (d).
frequency fatigue process. The reciprocating sliding action caused the repetitive work-hardening with increased the load. Severe plastic deformation and shear strains in the worn surface giving rise to the formation of delamination cracks which propagate subsequently to cause spalling of the material in the coating [14]. The sharp increase of wear volume, as presented in Fig. 5, may result from the change of the wear mechanism from local damage to delamination of the mechanical mixed layer under the load of 30 N. For the CG Cu coating, with the increase of the load, some debris induced by severe deformation and work-hardening is present on the worn surface, as shown in Fig. 7(c and d). For a nanocrystalline metal, it is easy to form a mixed surface layer because of the presence of more grain boundaries which act as nucleation sites for the oxides and diffusion paths of oxygen during the mixing process caused by sliding. Otherwise, it cannot form a continuous and effective mixed surface layer for the CG Cu coating. Grain refinement not only increases the hardness of the NC Cu coating, but also improves the stability of a mixed surface layer for the NC Cu coating. Both effects can enhance the wear resistance of the NC Cu coating in present study. Otherwise, according to Eq. (1), the grain refinement strengthening is the most dominant strengthening mechanism in the NC Cu coating. The enhancement of the wear behavior concurrent with higher hardness of the NC Cu coating is mainly attributed to the grain refinement strengthening. 4. Conclusions When nitrogen was used as the process gas, nanocrystalline Cu coating and coarse grain Cu coating could be formed via the cold spray process. The microhardness of the NC Cu coating was nearly two times that of CG Cu coating. Nanocrystalline Cu coating showed a good wear resistance with a low friction coefficient.
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The enhancement of the wear properties of the NC Cu was associated with the high hardness, the small work-hardening rate of the nanocrystalline structure, and easily being oxidized of wear debris, which was attributed to grain refinement. The worn surface of the NC Cu mainly revealed that the plastic deformation with grooves and some debris were observable with no visible cracking or fracture. It seemed to be concluded that the cold spray process was suitable for the deposition of NC Cu coating, a potential material for application in bearing parts. Acknowledgements The authors wish to acknowledge the financial support provided by the National Natural Science Foundation of China under grants No. 50874009 and No. 50871019. References [1] H. Gleiter, Progress in Materials Science 33 (1989) 223–315. [2] M.A. Meyers, A. Mishra, D.J. Benson, Progress in Materials Science 51 (2006) 427–556. [3] E.J. Lavernia, B.Q. Han, J.M. Schoenung, Materials Science and Engineering A 493 (2008) 207–214. [4] D.B. Witkin, E.J. Lavernia, Progress in Materials Science 51 (2006) 1–60. [5] M. Dao, L. Lu, R.J. Asaro, J.T.M. De Hosson, E. Ma, Acta Materialia 55 (2007) 4041–4065. [6] Z. Han, L. Lu, K. Lu, Tribology Letters 21 (2006) 47–52.
[7] S. Bellemare, M. Dao, S. Suresh, International Journal of Solids and Structures 44 (2007) 1970–1989. [8] R.S. Lima, J. Karthikeyan, C.M. Kay, J. Lindemann, C.C. Berndt, Thin Solid Films 416 (2002) 129–135. [9] L. Ajdelsztajn, B. Jodoin, J.M. Schoenung, Surface and Coatings Technology 201 (2006) 1166–1172. [10] L. Ajdelsztajn, B. Jodoin, G.E. Kim, J.M. Schoenung, Metallurgical and Materials Transactions A 36 (2005) 657–666. ˇ B. Jodoin, E.J. Lavernia, Journal of Thermal Spray Tech[11] L. Ajdelsztajn, A. Zúniga, nology 15 (2006) 184–190. [12] W.Y. Li, C.J. Li, Applied Surface Science 256 (2010) 2193–2198. [13] M. Yandouzi, A.J. Bottger, R.W.A. Hendrikx, M. Brochu, P. Richer, A. Charest, B. Jodoin, Surface and Coatings Technology 205 (2010) 2234–2246. [14] Y.S. Zhang, Z. Han, K. Wang, K. Lu, Wear 260 (2006) 942–948. [15] Y.S. Zhang, Z. Han, K. Lu, Wear 265 (2008) 396–401. [16] T.G. Nieh, J. Wadsworth, Scripta Metallurgica et Materialia 25 (1991) 955–958. [17] T.G. Nieh, J.G. Wang, Intermetallics 13 (2005) 377–385. [18] V. Bata, E.V. Pereloma, Acta Materialia 52 (2004) 657–665. [19] K.E. Aifantis, A.A. Konstantinidis, Materials Science and Engineering B 163 (2009) 139–144. [20] T. Shanmugasundaram, M. Heilmaier, B.S. Murty, V. Subramanya Sarma, Materials Science and Engineering A 527 (2010) 7821–7825. [21] P. Sudharshan Phani, V. Vishnukanthan, G. Sundararajan, Acta Materialia 55 (2007) 4741–4751. [22] J.F. Archard, Wear 2 (1959) 438–455. [23] G. Straffelini, M. Pellizzari, A. Molinari, Wear 256 (2004) 754–763. [24] Y.S. Zhang, K. Wang, Z. Han, G. Liu, Wear 262 (2007) 1463–1470. [25] J.F. Archard, Journal of Applied Physics 24 (1953) 981–988. [26] R. Mishra, B. Basu, R. Balasubramaniam, Materials Science and Engineering A 373 (2004) 370–373. [27] D.A. Rigney, Wear 245 (2000) 1–9. [28] X.Y. Li, K.N. Tandon, Wear 245 (2000) 148–161.