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Characterizations of cold-sprayed Nickel–Alumina composite coating with relatively large Nickel-coated Alumina powder
Surface & Coatings Technology 202 (2008) 4855–4860
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Characterizations of cold-sprayed Nickel–Alumina composite coating with relatively large Nickel-coated Alumina powder Wen-Ya Li a,⁎, Chao Zhang b, Hanlin Liao b, Jinglong Li a, Christian Coddet b a b
Shaanxi Key Laboratory of Friction Welding Technologies, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, PR China LERMPS, Université de Technologie de Belfort-Montbéliard, Site de Sévenans, 90010 Belfort Cedex, France
A R T I C L E
I N F O
Article history: Received 11 January 2008 Accepted in revised form 17 April 2008 Available online 26 April 2008 Keywords: Cold spraying Nickel–Alumina composite coating Microstructure Microhardness
A B S T R A C T Previous studies have shown that the fabrication of metal matrix composites (MMCs) by cold spraying is effective and promising. When light materials, such as SiC and Al2O3, were used as reinforcements, it was diffcuclt to obtain a high volume fraction of hard phase in the composite just through the simple powder mixture. Therefore, in this study, a Ni-coated Al2O3 powder, which was produced through hydrothermal hydrogen reduction method, was employed aiming at increasing the volume fraction of ceramic particles in the deposited composite coating. It was found that a dense Ni–Al2O3 composite coating could be deposited with the Ni-coated Al2O3 powder under the present spray conditions. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al2O3 in the composite was about 29 ± 6 vol.%, which is less than that in the feedstock (nominal: 40–45 vol.%) due to the rebound of some Al2O3 particulates upon kinetic impacting. The microhardness of the composite coating was about 173 ± 33Hv0.2. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The fabrication of metal matrix composites (MMCs) is of great significance in applications owing to their excellent combination of higher specific strength and remarkably improved wear resistance over their base alloys [1]. The particle-reinforced MMCs are among the most widely used composite materials, which can be produced through a number of routes including melt processing and powder metallurgy, such as casting, sintering, hot pressing [1–4] and thermal spraying (TS) [5–7]. Recently, cold spraying technique has been widely investigated owing to its high deposition efficiency and volume production of deposits or parts. In this process, the deposition of particles takes place through their intensive plastic deformation upon impact in a solid state at a temperature well below the melting point of spray material. Consequently, the deleterious effects of oxidation, phase transformation, decomposition, grain growth and other problems inherent to conventional TS techniques can be minimized or eliminated [8]. A variety of coatings have been prepared by cold spraying, including metals and alloys [8–12], composites (metal– metal [9,13–19] and metal–ceramics [20–35]), even cermets [19,20,21] and nanostructured materials [11,19,20,34–36]. Although cold spraying demonstrated the great capability to produce MMCs, a large
⁎ Corresponding author. Tel.: +86 29 88460673; fax: +86 29 88491426. E-mail address: [email protected] (W.-Y. Li). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.076
amount of research work is still required from both theory and application aspects. The previous studies [9,14–18,22–33] have shown that a dense MMC coating could be deposited by cold spraying using a simple mechanically mixed powder blend with hard particles uniformly dispersed in a matrix. As noticed in the fabrication of MMCs by conventional routes [1–4], besides the nature of hard particle, particle size, volume fraction and distribution, interface conditions between the soft and hard particle will significantly influence the performance of MMCs. Our previous results [25] showed that the volume fraction of TiN particles in the Al/TiN composite was practically identical to that in the original mechanically mixed Al/TiN blend. The best fit of data yielded a slope of about 1 with the initial mixtures of up to about 59 vol.% TiN particles [25]. However, the studies reported by other researchers [22,23,26–33] indicated that the relative deposition efficiency of ceramic particles to metal materials was much lower. For example, Eesley et al. [23] reported a percentage of 40% of the SiC particles present in the initial powder was retained in the coldsprayed Al/SiC composite coating. That means most of the SiC particles rebounded during deposition. Through a detailed comparison of the spray conditions in these studies, it was considered that besides gas conditions which determine the particle velocity, there are mainly two factors influencing the relative deposition efficiency of hard particles to metal materials. The first factor is the tuning of size distributions of the hard and matrix particles. A comparable size range will benefit the co-deposition [25]. The second is the density of hard particles, which determines their kinetic impacting and embedding processes during
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Fig. 1. SEM morphologies of (a) the used Ni-coated Al2O3 powder, (b) a typical particle and (c) high magnification of (b). (d) OM micrograph of the cross-section of a particle. (e) Size distribution of the used Ni-coated Al2O3 powder.
deposition [25]. Therefore, when diamond, SiC, AlN [22,23,30,32,33], Al2O3 [26,31] and B4C [27] of relatively low density were used as reinforcing particles, a low volume fraction of hard particles in the cold-sprayed composite was obtained. The previous works [13,19–21,34–36] showed that an effective way to increase the volume fraction of hard particles in the composite is to prepare the feedstock powder with a higher volume fraction of hard particles. The ball-milling of the powder mixtures was usually adopted, followed by the agglomerating or sintering of the milled powders to obtain the feedstocks suitable for cold spraying [13,20,34–36]. Another great advantage of ball-milling is that nanocrystalline phases can be otained [20,34–36]. According to the literature [37–39], there is another effective way to obtain the composite powder with the controllable volume fraction of reinforcement, which is the hydrothermal hydrogen reduction method. This method is commonly used to prepare the Ni-coated composite materials, such as graphite [37], diamond [38] and Al2O3 [39]. Therefore, in this study, a Ni-coated Al2O3 compostie powder was used aiming at increasing and controlling the volume fraction of ceramic particles in the
Fig. 2. XRD results of the used Ni-coated Al2O3 powder and the cold-sprayed Ni–Al2O3 composite coating.
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feedstock with a nominal Al2O3 volume fraction of 40–45 vol.%. This kind of powder is usually applied by plasma spraying for anti-wear, resistant to thermal shock and corrosion applications at intermediate temperatures, i.e., not more than 800 °C [39]. It can also be used as the bond coat or gradient layer for thermal barrier coating (TBC) applications [39]. Mild steel plates were used as substrates and they were sandblasted using Alumina prior to spraying. A cold spray system with a commercial cold spray gun (CGT GmbH, Germany) was employed for coating deposition. An optimized nozzle design was adopted, which had an expansion ratio of about 4.9 and a divergent section length of 170 mm. High-pressure compressed air was used as the accelerating gas and argon was used as powder carrier gas. The spraying was conducted with an air inlet pressure of 2.7 MPa and temperature of about 495 °C. The pressure of argon was about 3 MPa. The flow rate of argon was about 40 NL/min and the powder feed rate was estimated to be about 15–20 g/min. The standoff distance from the nozzle exit to the substrate surface was 30 mm. The traverse speed of spray gun was 160 mm/s. The powder size distribution was characterized by a laser diffraction sizer (MASTERSIZER 2000, Malvern Instruments Ltd., UK). The powder and coating microstructures were examined by optical microscope (OM) (Nikon, Japan), SEM (JSM5800LV, JEOL, Japan). X-ray diffraction (XRD) (D8 Advance, Brucker AXS, Germany) analysis was carried out with Cu Kα radiation at a scanning speed of 5°/min. The volume fraction of Al2O3 in the composite coating was estimated through analyzing the SEM micrographs with Scion Image software (NIH, USA). The coating microhardness was tested by a Vickers hardness indenter (Leitz, Germany) with a load of 200 g for 15 s. More than 15 values were randomly tested and averaged to evaluate the coating hardness. The cross-sectional fracture surface of the coating obtained by bending the substrate to peel the coating was also examined by SEM. 3. Results and discussion 3.1. Powder characterization
Fig. 3. OM micrographs of the cold-sprayed Ni–Al2O3 composite coating.
deposited composite. The deposition characteristics of Ni–Al2O3 composite coating by cold spraying were investigated in this article. 2. Experimental procedures Ni-coated Al2O3 powder (KF-45, −80 + 300 mesh, Beijing General Research Institute of Mining and Metallurgy, China) was used as
Fig. 1(a–d) show the morphologies of the used Ni-coated Al2O3 powder. It is found that the powder presents a near-spherical shape in an agglomerated state, which can be seen clearly from Fig. 1(b). This is the typical morphology for the powders made by hydrogen reduction process [37,38]. When observed at higher magnification (Fig. 1(b,c)), it is found that the fine Al2O3 particulates are clad by Ni particulates. This can also be observed from the cross-section of a particle as shown in Fig. 1(d). The Ni thickness is not uniform as shown in Fig. 1(d), which is inherent to the processing technique. Nevertheless, the Ni thickness is estimated to be 10–20 μm. It should be noticed that most of Al2O3 particulates have been lost during polishing, which may be due to the weak bond of Al2O3 particulates in a particle. The measurement of particle size distribution as shown
Fig. 4. SEM micrographs of the cold-sprayed Ni–Al2O3 composite coating. (a) With backscattered electrons and (b) with secondary electrons.
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Table 1 EDS result of the black zone marked by the white arrow in Fig. 4(a) Element
O
Al
Ni
Total
wt.%
41.59
40.67
17.74
100.00
in Fig. 1(e) indicates an average particle size of about 85 μm. From XRD result of powder as shown in Fig. 2, it clearly showed the Ni and α-Al2O3 phases in the Ni-coated Al2O3 powder. The relatively low intensities of Al2O3 peak is because the Al2O3 particulates are almost covered by the Ni particulates. 3.2. Coating microstructure After 5 spray-pass, a very thick (about 1 mm) Ni–Al2O3 composite coating was deposited. Fig. 3(a) shows the OM micrograph of the cross-section surface of the as-sprayed Ni-Al2O3 composite coating. A relatively dense coating well bonded with the steel substrate was observed (See Fig. 3(c)) under the present spray conditions. However, there are some large pores present in the coating. When observed at higher magnification as shown in Fig. 3(b and c), it is believed that these large pores are attributed to the peeling off of the fine Al2O3 particulates during the preparation of the OM sample. A typical deformed particle presented at the top of coating as marked by the white arrow in Fig. 3(b) proves this assertion. In addition, there are also some relatively small pores presented at the interfaces of the deposited particles in the coating. This could be attributed to the fact that the deformation of large particles is not enough to fill up the gaps. Furthermore, some interlamellar cracks are observed at the top layer of coating as marked by black arrows in Fig. 3(b). These interlamellar cracks show a lack of bonding between adjacent deposited layers [11]. The estimation of the porosity based on the image analysis is about 1.1±0.4%. This porosity level is comparable to that of pure Ni coating (about 0.9%) deposited under air pressure of 3.0 MPa and temperature of 585 °C with Ni powder (Medipure 18 99.0%, 5–44 μm) [42]. It should be pointed out
that the fine Ni particles used in the previous study [42] yield much higher velocities than the large composite particles used in this study besides the better gas conditions than those used in this study (2.7 MPa, 495 °C). These results may be attributed to the co-deposition effect of metal and ceramic particles [25]. On the other hand, most of the fine Al2O3 particulates embedded into the Ni matrix and well distributed in the coating as shown in Fig. 4. Table 1 gives a typical EDS result of the black zone indicated by the white arrow in Fig. 4(a), which proved to be the Al2O3 particulate. Further observation with SEM demonstrated that most of the Al2O3 particulates adhered well to the Ni matrix. The observation of the large pores with SEM in Fig. 4(b) also proved the separation of the inner weak-bonded Al2O3 particulates. The estimated volume fraction of Al2O3 in the composite was about 29 ± 6 vol.%, which is less than that in the feedstock (40–45 vol.%). It could be considered that the rebound of part of the Al2O3 particulates upon impact is attributed to the decreasing Al2O3 volume fraction as schematically shown in Fig. 5. During the kinetic impacting process of a Ni-coated Al2O3 composite particle, some cracks will occur at certain position within the Ni layer and result in the rebound of some Al2O3 particulates and the embedding of other Al2O3 particulates. This point can be proved by the fact that there exists obvious Al2O3 dust around the substrate and in the air during spraying. At the same time, some Al2O3 particulates will be embedded into the Ni matrix because of their kinetic energy as observed in the micrographs. The previous simulation results also predicted the embedding of the oxide films in particle surfaces [40], which is similar to the process in this study. The composite coating had the same phase structures as the feedstock, but the peaks of Al2O3 are very weak, which was confirmed by the result in Fig. 2. This could explain that the clad Ni matrix shields the underlying Al2O3 particulates during the XRD analysis. Moreover, it is noticed that there is a slight shift in Ni peaks in the XRD pattern, which is about 0.3°. This may be caused by the distortion of lattice resulting from the extensive plastic deformation of coating surface. This phenomenon was also found for the ball-milled powders in the previous study [36].
Fig. 5. Schematic diagram of the impacting process of a Ni-coated Al2O3 composite particle resulting in the rebound and embedding of Al2O3 particulates.
Fig. 6. SEM surface morphologies of the as-sprayed Ni–Al2O3 composite coating. (b) High magnification of (a).
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Fig. 7. SEM fracture morphologies of the Ni–Al2O3 composite coating obtained through bending the substrate to peel the coating. (b) High magnification of (a).
Fig. 6 shows the typical surface morphologies of the as-sprayed Ni– Al2O3 composite coating. It is clearly seen that, besides some large pores between the deposited particles, small plastic pits on the coating surface were also observed, which is different from the observation of other cold-sprayed coatings with the conventional solid powders [12]. The escape of some Al2O3 particulates could be caused by the scratching or impacting of fine Al2O3 particulates during coating deposition. On the other hand, the measurement of the adhesive strength (according to ASTM C633-01 standard) of the composite coating yielded a value of 36 ± 5 MPa, which is higher than that of pure Ni coating (25 ± 3 MPa [42]). This result is consistent with that of the pure Al5356 coating and its composite coatings [25]. As explained in [25], the peening effect of Alumina particulates could enhance the adhesion of composite coating. Fig. 7 illustrates the typical fractograph of the Ni–Al2O3 composite coating are obtained through bending the substrate to peel the coating. It could be found that the failure occurs mainly between the deposited Al2O3 particulates, where they are connected but weak-bonded. In addition, the fracture is also generated at the interfaces between Ni and Ni. Furthermore, the embedded Al2O3 into Ni matrix can also be observed from the fracture surface. 3.3. Coating microhardness The measurement of coating hardness yields the value of 173.1 ± 33.3 Hv0.2. This value is significantly higher than that of the annealed pure Ni bulk (75Hv [41]). Our previous study showed that the microhardness of pure Ni coating was about 131 ± 8Hv0.2. Therefore, it could be considered that, besides the strain-hardening effect caused by the Ni particles during cold spraying, the strengthening effect of Al2O3 particulates in the composite coating also improved the hardness of coating, which is similar to other cold-sprayed MMCs [25]. However, this strengthening effect is not as big as expected according to the theory of composite. This may be attributed to the weak-bonded interfaces in the coating. Beneteau et al. [28] and Maev et al. [29] reported the deposition of Ni–35 vol.% TiC [28], Ni–10 vol.% SiC and Ni–25 vol.% SiC [29] powders by cold spraying using the low pressure cold spray system. They obtained a coating hardness of 220Hv0.1 [28], 280Hv0.1 and 330Hv0.1 [29], respectively. Although the result in [28] is comparable to the results in this study, the reason for the higher value obtained in [29] is still not well understood. 4. Conclusions A Ni-coated Al2O3 powder produced through hydrothermal hydrogen reduction method was used for cold spraying. The microstructure of cold-sprayed Ni–Al2O3 composite coating was investigated for the potential anti-wear applications at intermediate temperatures. It was found that a dense Ni–Al2O3 composite coating
could be deposited with the Ni-coated Al2O3 powder. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al2O3 in the composite was about 29 ± 6 vol.%, which was less than that in the feedstock (40–45 vol.%) due to the rebound of some Al2O3 particulates upon kinetic impact. The embedding of some Al2O3 particulates into the Ni matrix and the good distribution of these fine Al2O3 particulates increased the microhardness of the composite coating beyond the strain-hardening effect, which was about 173 ± 33Hv0.2. Acknowledgements The authors would like to thank Mr. Hongren WANG and Mr. Xiangbo LI (State Key Laboratory For Marine Corrosion and Protection, Luoyang Ship Material Research Institute (LSMRI), Qingdao, China) for their help in the XRD analysis. References [1] A.M. Davidson, D. Regener, Compos. Sci. Technol. 60 (2000) 865. [2] D.P. Mondal, S. Das, Tribol. Int. 39 (2006) 470. [3] Y.L. Shen, J.J. Williams, G. Piotrowski, N. Chawla, Y.L. Guo, Acta Mater. 49 (2001) 3219. [4] A. Slipenyuk, V. Kuprin, Yu. Milman, V. Goncharuk, J. Eckert, Acta Mater. 54 (2006) 157. [5] K. Ghosh, T. Troczynski, A.C.D. Chaklader, in: C.C. Berndt (Ed.), Practical Solutions for Engineering Problems, ASM International, Cincinnati, OH, Oct 7–11, 1996, p. 339. [6] Fr.W. Bach, L. Engl, L.A. Josefiak, in: B.R. Marple, C. Moreau (Eds.), Advancing the Science and Applying the Technology, ASM International, Orlando, FL, May 5–8, 2003, p. 769. [7] S.O. Chwa, D. Klein, H.L. Liao, L. Dembinski, C. Coddet, Surf. Coat. Technol. 200 (2006) 5682. [8] A. Papyrin, Adv. Mater. Process 159 (2001) 49. [9] T. Stoltenhoff, H. Kreye, H.J. Richter, J. Therm. Spray Technol. 11 (2002) 542. [10] C.-J. Li, W.-Y. Li, Surf. Coat. Technol. 167 (2003) 278. [11] L. Ajdelsztajn, B. Jodoin, J.M. Schoenung, Surf. Coat. Technol. 201 (2006) 1166. [12] W.Y. Li, X.P. Guo, C. Verdy, L. Dembinski, H.L. Liao, C. Coddet, Scripta Mater. 55 (2006) 327. [13] H.K. Kang, S.B. Kang, Scripta Mater. 49 (2003) 1169. [14] S. Marx, A. Paul, A. Köhler, G. Hüttl, in: E. Lugscheider (Ed.), Thermal Spray Connects: Explore Its Surfacing Potential! DVS, Düsseldorf, May 2–4, 2005, (Basil, Switzerland). [15] T. Van Steenkiste, J. Therm. Spray Technol 15 (2006) 501. [16] T. Novoselova, P. Fox, R. Morgan, W. O'Neill, Surf. Coat. Technol. 200 (2006) 2775. [17] S. Shin, S.H. Yoon, Y.D. Kim, C.H. Lee, Surf. Coat. Technol. 201 (2006) 3457. [18] H.Y. Lee, S.H. Jung, S.Y. Lee, K.H. Ko, Mater. Sci. Eng. A 433 (2006) 139. [19] H.J. Kim, C.H. Lee, S.Y. Hwang, Mater. Sci. Eng. A 391 (2005) 243. [20] C.-J. Li, G.-J. Yang, C.-X. Li, Y.-Y. Wang, J. Ma, P.-H. Gao, in: B.R. Marple, M.M. Hyland, Y.-C. Lau, C.-J. Li, R.S. Lima, G. Montavon (Eds.), Global Coating Solutions, ASM International, Materials Park, OH, May 14–16, 2007, p. 1129, (Beijing, China). [21] D.E. Wolfe, T.J. Eden, J.K. Potter, P.J. Adam, J. Therm. Spray Technol. 15 (2006) 400. [22] D.T. Morelli, A.A. Elmoursi, T.H. Van Steenkiste, D.W. Gorkiewicz, B. Gillispie, in: B.R. Marple, C. Moreau (Eds.), Advancing the Science and Applying the Technology, ASM International, OH, 2003, p. 85. [23] G.L. Eesley, A. Elmoursi, N. Patel, J. Mater. Res. 18 (2003) 855. [24] T.H. Van Steenkiste, A. Elmoursi, D. Gorkiewicz, B. Gillispie, Surf. Coat. Technol. 194 (2005) 103. [25] W.-Y. Li, G. Zhang, X.P. Guo, H.L. Liao, C. Coddet, Adv. Eng. Mater. 9 (2007) 577.
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Characterizations of cold-sprayed Nickel–Alumina composite coating with relatively large Nickel-coated Alumina powder Wen-Ya Li a,⁎, Chao Zhang b, Hanlin Liao b, Jinglong Li a, Christian Coddet b a b
Shaanxi Key Laboratory of Friction Welding Technologies, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, PR China LERMPS, Université de Technologie de Belfort-Montbéliard, Site de Sévenans, 90010 Belfort Cedex, France
A R T I C L E
I N F O
Article history: Received 11 January 2008 Accepted in revised form 17 April 2008 Available online 26 April 2008 Keywords: Cold spraying Nickel–Alumina composite coating Microstructure Microhardness
A B S T R A C T Previous studies have shown that the fabrication of metal matrix composites (MMCs) by cold spraying is effective and promising. When light materials, such as SiC and Al2O3, were used as reinforcements, it was diffcuclt to obtain a high volume fraction of hard phase in the composite just through the simple powder mixture. Therefore, in this study, a Ni-coated Al2O3 powder, which was produced through hydrothermal hydrogen reduction method, was employed aiming at increasing the volume fraction of ceramic particles in the deposited composite coating. It was found that a dense Ni–Al2O3 composite coating could be deposited with the Ni-coated Al2O3 powder under the present spray conditions. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al2O3 in the composite was about 29 ± 6 vol.%, which is less than that in the feedstock (nominal: 40–45 vol.%) due to the rebound of some Al2O3 particulates upon kinetic impacting. The microhardness of the composite coating was about 173 ± 33Hv0.2. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The fabrication of metal matrix composites (MMCs) is of great significance in applications owing to their excellent combination of higher specific strength and remarkably improved wear resistance over their base alloys [1]. The particle-reinforced MMCs are among the most widely used composite materials, which can be produced through a number of routes including melt processing and powder metallurgy, such as casting, sintering, hot pressing [1–4] and thermal spraying (TS) [5–7]. Recently, cold spraying technique has been widely investigated owing to its high deposition efficiency and volume production of deposits or parts. In this process, the deposition of particles takes place through their intensive plastic deformation upon impact in a solid state at a temperature well below the melting point of spray material. Consequently, the deleterious effects of oxidation, phase transformation, decomposition, grain growth and other problems inherent to conventional TS techniques can be minimized or eliminated [8]. A variety of coatings have been prepared by cold spraying, including metals and alloys [8–12], composites (metal– metal [9,13–19] and metal–ceramics [20–35]), even cermets [19,20,21] and nanostructured materials [11,19,20,34–36]. Although cold spraying demonstrated the great capability to produce MMCs, a large
⁎ Corresponding author. Tel.: +86 29 88460673; fax: +86 29 88491426. E-mail address: [email protected] (W.-Y. Li). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.076
amount of research work is still required from both theory and application aspects. The previous studies [9,14–18,22–33] have shown that a dense MMC coating could be deposited by cold spraying using a simple mechanically mixed powder blend with hard particles uniformly dispersed in a matrix. As noticed in the fabrication of MMCs by conventional routes [1–4], besides the nature of hard particle, particle size, volume fraction and distribution, interface conditions between the soft and hard particle will significantly influence the performance of MMCs. Our previous results [25] showed that the volume fraction of TiN particles in the Al/TiN composite was practically identical to that in the original mechanically mixed Al/TiN blend. The best fit of data yielded a slope of about 1 with the initial mixtures of up to about 59 vol.% TiN particles [25]. However, the studies reported by other researchers [22,23,26–33] indicated that the relative deposition efficiency of ceramic particles to metal materials was much lower. For example, Eesley et al. [23] reported a percentage of 40% of the SiC particles present in the initial powder was retained in the coldsprayed Al/SiC composite coating. That means most of the SiC particles rebounded during deposition. Through a detailed comparison of the spray conditions in these studies, it was considered that besides gas conditions which determine the particle velocity, there are mainly two factors influencing the relative deposition efficiency of hard particles to metal materials. The first factor is the tuning of size distributions of the hard and matrix particles. A comparable size range will benefit the co-deposition [25]. The second is the density of hard particles, which determines their kinetic impacting and embedding processes during
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Fig. 1. SEM morphologies of (a) the used Ni-coated Al2O3 powder, (b) a typical particle and (c) high magnification of (b). (d) OM micrograph of the cross-section of a particle. (e) Size distribution of the used Ni-coated Al2O3 powder.
deposition [25]. Therefore, when diamond, SiC, AlN [22,23,30,32,33], Al2O3 [26,31] and B4C [27] of relatively low density were used as reinforcing particles, a low volume fraction of hard particles in the cold-sprayed composite was obtained. The previous works [13,19–21,34–36] showed that an effective way to increase the volume fraction of hard particles in the composite is to prepare the feedstock powder with a higher volume fraction of hard particles. The ball-milling of the powder mixtures was usually adopted, followed by the agglomerating or sintering of the milled powders to obtain the feedstocks suitable for cold spraying [13,20,34–36]. Another great advantage of ball-milling is that nanocrystalline phases can be otained [20,34–36]. According to the literature [37–39], there is another effective way to obtain the composite powder with the controllable volume fraction of reinforcement, which is the hydrothermal hydrogen reduction method. This method is commonly used to prepare the Ni-coated composite materials, such as graphite [37], diamond [38] and Al2O3 [39]. Therefore, in this study, a Ni-coated Al2O3 compostie powder was used aiming at increasing and controlling the volume fraction of ceramic particles in the
Fig. 2. XRD results of the used Ni-coated Al2O3 powder and the cold-sprayed Ni–Al2O3 composite coating.
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feedstock with a nominal Al2O3 volume fraction of 40–45 vol.%. This kind of powder is usually applied by plasma spraying for anti-wear, resistant to thermal shock and corrosion applications at intermediate temperatures, i.e., not more than 800 °C [39]. It can also be used as the bond coat or gradient layer for thermal barrier coating (TBC) applications [39]. Mild steel plates were used as substrates and they were sandblasted using Alumina prior to spraying. A cold spray system with a commercial cold spray gun (CGT GmbH, Germany) was employed for coating deposition. An optimized nozzle design was adopted, which had an expansion ratio of about 4.9 and a divergent section length of 170 mm. High-pressure compressed air was used as the accelerating gas and argon was used as powder carrier gas. The spraying was conducted with an air inlet pressure of 2.7 MPa and temperature of about 495 °C. The pressure of argon was about 3 MPa. The flow rate of argon was about 40 NL/min and the powder feed rate was estimated to be about 15–20 g/min. The standoff distance from the nozzle exit to the substrate surface was 30 mm. The traverse speed of spray gun was 160 mm/s. The powder size distribution was characterized by a laser diffraction sizer (MASTERSIZER 2000, Malvern Instruments Ltd., UK). The powder and coating microstructures were examined by optical microscope (OM) (Nikon, Japan), SEM (JSM5800LV, JEOL, Japan). X-ray diffraction (XRD) (D8 Advance, Brucker AXS, Germany) analysis was carried out with Cu Kα radiation at a scanning speed of 5°/min. The volume fraction of Al2O3 in the composite coating was estimated through analyzing the SEM micrographs with Scion Image software (NIH, USA). The coating microhardness was tested by a Vickers hardness indenter (Leitz, Germany) with a load of 200 g for 15 s. More than 15 values were randomly tested and averaged to evaluate the coating hardness. The cross-sectional fracture surface of the coating obtained by bending the substrate to peel the coating was also examined by SEM. 3. Results and discussion 3.1. Powder characterization
Fig. 3. OM micrographs of the cold-sprayed Ni–Al2O3 composite coating.
deposited composite. The deposition characteristics of Ni–Al2O3 composite coating by cold spraying were investigated in this article. 2. Experimental procedures Ni-coated Al2O3 powder (KF-45, −80 + 300 mesh, Beijing General Research Institute of Mining and Metallurgy, China) was used as
Fig. 1(a–d) show the morphologies of the used Ni-coated Al2O3 powder. It is found that the powder presents a near-spherical shape in an agglomerated state, which can be seen clearly from Fig. 1(b). This is the typical morphology for the powders made by hydrogen reduction process [37,38]. When observed at higher magnification (Fig. 1(b,c)), it is found that the fine Al2O3 particulates are clad by Ni particulates. This can also be observed from the cross-section of a particle as shown in Fig. 1(d). The Ni thickness is not uniform as shown in Fig. 1(d), which is inherent to the processing technique. Nevertheless, the Ni thickness is estimated to be 10–20 μm. It should be noticed that most of Al2O3 particulates have been lost during polishing, which may be due to the weak bond of Al2O3 particulates in a particle. The measurement of particle size distribution as shown
Fig. 4. SEM micrographs of the cold-sprayed Ni–Al2O3 composite coating. (a) With backscattered electrons and (b) with secondary electrons.
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Table 1 EDS result of the black zone marked by the white arrow in Fig. 4(a) Element
O
Al
Ni
Total
wt.%
41.59
40.67
17.74
100.00
in Fig. 1(e) indicates an average particle size of about 85 μm. From XRD result of powder as shown in Fig. 2, it clearly showed the Ni and α-Al2O3 phases in the Ni-coated Al2O3 powder. The relatively low intensities of Al2O3 peak is because the Al2O3 particulates are almost covered by the Ni particulates. 3.2. Coating microstructure After 5 spray-pass, a very thick (about 1 mm) Ni–Al2O3 composite coating was deposited. Fig. 3(a) shows the OM micrograph of the cross-section surface of the as-sprayed Ni-Al2O3 composite coating. A relatively dense coating well bonded with the steel substrate was observed (See Fig. 3(c)) under the present spray conditions. However, there are some large pores present in the coating. When observed at higher magnification as shown in Fig. 3(b and c), it is believed that these large pores are attributed to the peeling off of the fine Al2O3 particulates during the preparation of the OM sample. A typical deformed particle presented at the top of coating as marked by the white arrow in Fig. 3(b) proves this assertion. In addition, there are also some relatively small pores presented at the interfaces of the deposited particles in the coating. This could be attributed to the fact that the deformation of large particles is not enough to fill up the gaps. Furthermore, some interlamellar cracks are observed at the top layer of coating as marked by black arrows in Fig. 3(b). These interlamellar cracks show a lack of bonding between adjacent deposited layers [11]. The estimation of the porosity based on the image analysis is about 1.1±0.4%. This porosity level is comparable to that of pure Ni coating (about 0.9%) deposited under air pressure of 3.0 MPa and temperature of 585 °C with Ni powder (Medipure 18 99.0%, 5–44 μm) [42]. It should be pointed out
that the fine Ni particles used in the previous study [42] yield much higher velocities than the large composite particles used in this study besides the better gas conditions than those used in this study (2.7 MPa, 495 °C). These results may be attributed to the co-deposition effect of metal and ceramic particles [25]. On the other hand, most of the fine Al2O3 particulates embedded into the Ni matrix and well distributed in the coating as shown in Fig. 4. Table 1 gives a typical EDS result of the black zone indicated by the white arrow in Fig. 4(a), which proved to be the Al2O3 particulate. Further observation with SEM demonstrated that most of the Al2O3 particulates adhered well to the Ni matrix. The observation of the large pores with SEM in Fig. 4(b) also proved the separation of the inner weak-bonded Al2O3 particulates. The estimated volume fraction of Al2O3 in the composite was about 29 ± 6 vol.%, which is less than that in the feedstock (40–45 vol.%). It could be considered that the rebound of part of the Al2O3 particulates upon impact is attributed to the decreasing Al2O3 volume fraction as schematically shown in Fig. 5. During the kinetic impacting process of a Ni-coated Al2O3 composite particle, some cracks will occur at certain position within the Ni layer and result in the rebound of some Al2O3 particulates and the embedding of other Al2O3 particulates. This point can be proved by the fact that there exists obvious Al2O3 dust around the substrate and in the air during spraying. At the same time, some Al2O3 particulates will be embedded into the Ni matrix because of their kinetic energy as observed in the micrographs. The previous simulation results also predicted the embedding of the oxide films in particle surfaces [40], which is similar to the process in this study. The composite coating had the same phase structures as the feedstock, but the peaks of Al2O3 are very weak, which was confirmed by the result in Fig. 2. This could explain that the clad Ni matrix shields the underlying Al2O3 particulates during the XRD analysis. Moreover, it is noticed that there is a slight shift in Ni peaks in the XRD pattern, which is about 0.3°. This may be caused by the distortion of lattice resulting from the extensive plastic deformation of coating surface. This phenomenon was also found for the ball-milled powders in the previous study [36].
Fig. 5. Schematic diagram of the impacting process of a Ni-coated Al2O3 composite particle resulting in the rebound and embedding of Al2O3 particulates.
Fig. 6. SEM surface morphologies of the as-sprayed Ni–Al2O3 composite coating. (b) High magnification of (a).
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Fig. 7. SEM fracture morphologies of the Ni–Al2O3 composite coating obtained through bending the substrate to peel the coating. (b) High magnification of (a).
Fig. 6 shows the typical surface morphologies of the as-sprayed Ni– Al2O3 composite coating. It is clearly seen that, besides some large pores between the deposited particles, small plastic pits on the coating surface were also observed, which is different from the observation of other cold-sprayed coatings with the conventional solid powders [12]. The escape of some Al2O3 particulates could be caused by the scratching or impacting of fine Al2O3 particulates during coating deposition. On the other hand, the measurement of the adhesive strength (according to ASTM C633-01 standard) of the composite coating yielded a value of 36 ± 5 MPa, which is higher than that of pure Ni coating (25 ± 3 MPa [42]). This result is consistent with that of the pure Al5356 coating and its composite coatings [25]. As explained in [25], the peening effect of Alumina particulates could enhance the adhesion of composite coating. Fig. 7 illustrates the typical fractograph of the Ni–Al2O3 composite coating are obtained through bending the substrate to peel the coating. It could be found that the failure occurs mainly between the deposited Al2O3 particulates, where they are connected but weak-bonded. In addition, the fracture is also generated at the interfaces between Ni and Ni. Furthermore, the embedded Al2O3 into Ni matrix can also be observed from the fracture surface. 3.3. Coating microhardness The measurement of coating hardness yields the value of 173.1 ± 33.3 Hv0.2. This value is significantly higher than that of the annealed pure Ni bulk (75Hv [41]). Our previous study showed that the microhardness of pure Ni coating was about 131 ± 8Hv0.2. Therefore, it could be considered that, besides the strain-hardening effect caused by the Ni particles during cold spraying, the strengthening effect of Al2O3 particulates in the composite coating also improved the hardness of coating, which is similar to other cold-sprayed MMCs [25]. However, this strengthening effect is not as big as expected according to the theory of composite. This may be attributed to the weak-bonded interfaces in the coating. Beneteau et al. [28] and Maev et al. [29] reported the deposition of Ni–35 vol.% TiC [28], Ni–10 vol.% SiC and Ni–25 vol.% SiC [29] powders by cold spraying using the low pressure cold spray system. They obtained a coating hardness of 220Hv0.1 [28], 280Hv0.1 and 330Hv0.1 [29], respectively. Although the result in [28] is comparable to the results in this study, the reason for the higher value obtained in [29] is still not well understood. 4. Conclusions A Ni-coated Al2O3 powder produced through hydrothermal hydrogen reduction method was used for cold spraying. The microstructure of cold-sprayed Ni–Al2O3 composite coating was investigated for the potential anti-wear applications at intermediate temperatures. It was found that a dense Ni–Al2O3 composite coating
could be deposited with the Ni-coated Al2O3 powder. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al2O3 in the composite was about 29 ± 6 vol.%, which was less than that in the feedstock (40–45 vol.%) due to the rebound of some Al2O3 particulates upon kinetic impact. The embedding of some Al2O3 particulates into the Ni matrix and the good distribution of these fine Al2O3 particulates increased the microhardness of the composite coating beyond the strain-hardening effect, which was about 173 ± 33Hv0.2. Acknowledgements The authors would like to thank Mr. Hongren WANG and Mr. Xiangbo LI (State Key Laboratory For Marine Corrosion and Protection, Luoyang Ship Material Research Institute (LSMRI), Qingdao, China) for their help in the XRD analysis. References [1] A.M. Davidson, D. Regener, Compos. Sci. Technol. 60 (2000) 865. [2] D.P. Mondal, S. Das, Tribol. Int. 39 (2006) 470. [3] Y.L. Shen, J.J. Williams, G. Piotrowski, N. Chawla, Y.L. Guo, Acta Mater. 49 (2001) 3219. [4] A. Slipenyuk, V. Kuprin, Yu. Milman, V. Goncharuk, J. Eckert, Acta Mater. 54 (2006) 157. [5] K. Ghosh, T. Troczynski, A.C.D. Chaklader, in: C.C. Berndt (Ed.), Practical Solutions for Engineering Problems, ASM International, Cincinnati, OH, Oct 7–11, 1996, p. 339. [6] Fr.W. Bach, L. Engl, L.A. Josefiak, in: B.R. Marple, C. Moreau (Eds.), Advancing the Science and Applying the Technology, ASM International, Orlando, FL, May 5–8, 2003, p. 769. [7] S.O. Chwa, D. Klein, H.L. Liao, L. Dembinski, C. Coddet, Surf. Coat. Technol. 200 (2006) 5682. [8] A. Papyrin, Adv. Mater. Process 159 (2001) 49. [9] T. Stoltenhoff, H. Kreye, H.J. Richter, J. Therm. Spray Technol. 11 (2002) 542. [10] C.-J. Li, W.-Y. Li, Surf. Coat. Technol. 167 (2003) 278. [11] L. Ajdelsztajn, B. Jodoin, J.M. Schoenung, Surf. Coat. Technol. 201 (2006) 1166. [12] W.Y. Li, X.P. Guo, C. Verdy, L. Dembinski, H.L. Liao, C. Coddet, Scripta Mater. 55 (2006) 327. [13] H.K. Kang, S.B. Kang, Scripta Mater. 49 (2003) 1169. [14] S. Marx, A. Paul, A. Köhler, G. Hüttl, in: E. Lugscheider (Ed.), Thermal Spray Connects: Explore Its Surfacing Potential! DVS, Düsseldorf, May 2–4, 2005, (Basil, Switzerland). [15] T. Van Steenkiste, J. Therm. Spray Technol 15 (2006) 501. [16] T. Novoselova, P. Fox, R. Morgan, W. O'Neill, Surf. Coat. Technol. 200 (2006) 2775. [17] S. Shin, S.H. Yoon, Y.D. Kim, C.H. Lee, Surf. Coat. Technol. 201 (2006) 3457. [18] H.Y. Lee, S.H. Jung, S.Y. Lee, K.H. Ko, Mater. Sci. Eng. A 433 (2006) 139. [19] H.J. Kim, C.H. Lee, S.Y. Hwang, Mater. Sci. Eng. A 391 (2005) 243. [20] C.-J. Li, G.-J. Yang, C.-X. Li, Y.-Y. Wang, J. Ma, P.-H. Gao, in: B.R. Marple, M.M. Hyland, Y.-C. Lau, C.-J. Li, R.S. Lima, G. Montavon (Eds.), Global Coating Solutions, ASM International, Materials Park, OH, May 14–16, 2007, p. 1129, (Beijing, China). [21] D.E. Wolfe, T.J. Eden, J.K. Potter, P.J. Adam, J. Therm. Spray Technol. 15 (2006) 400. [22] D.T. Morelli, A.A. Elmoursi, T.H. Van Steenkiste, D.W. Gorkiewicz, B. Gillispie, in: B.R. Marple, C. Moreau (Eds.), Advancing the Science and Applying the Technology, ASM International, OH, 2003, p. 85. [23] G.L. Eesley, A. Elmoursi, N. Patel, J. Mater. Res. 18 (2003) 855. [24] T.H. Van Steenkiste, A. Elmoursi, D. Gorkiewicz, B. Gillispie, Surf. Coat. Technol. 194 (2005) 103. [25] W.-Y. Li, G. Zhang, X.P. Guo, H.L. Liao, C. Coddet, Adv. Eng. Mater. 9 (2007) 577.
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