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Formation of metastable phases in cold-sprayed soft metallic deposit
Surface & Coatings Technology 198 (2005) 469 – 473 www.elsevier.com/locate/surfcoat
Formation of metastable phases in cold-sprayed soft metallic deposit Chang-Jiu Li*, Wen-Ya Li, Yu-Yue Wang State Key Laboratory for Mechanical Behavior of Materials, Welding Research Institute, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049 PR China Available online 24 November 2004
Abstract Zinc powder was used to deposit coating by cold spraying using nitrogen as driving gas. The microstructure of the deposited coating was characterized by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that the cold-sprayed Zn coating presented a dense microstructure. The nanograins formed at the interface area between deposited particles, while the size of grains in the bulk inside of the particles in the coating was comparable to that in the starting powder. The interface region of the oriented lamellar microstructure was observed. The refinement of grains to the nanocrystallines at the particle–particle interfaces is achieved through the recrystallization of elongated grains resulting from intensive deformation. The lamellar microstructure is formed by the successive deformation of deposited fine particulates resulting from the jetting of impact-induced melt. An evolution model of the interface microstructure in cold spraying is proposed. D 2004 Elsevier B.V. All rights reserved. Keywords: Cold spraying; Zinc coating; Microstructure; Nanocrystalline
1. Introduction Cold spraying, also termed cold gas dynamic spray, has emerged upon the horizon of thermal spray technology in the past few years. Typical feature of this process in comparison with the conventional thermal spray techniques is that a coating is formed by completely solid particles of low temperature and high velocity on impact [1]. It can be employed to produce high-quality metallic coatings without any noticeable oxidation. The low-temperature characteristic also makes it possible to deposit coatings without deteriorating the microstructure of the starting feedstocks such as nanostructured feedstocks [2–4]. In order to deposit coating through solid particle, the deformation intensive enough to result in the unstable flow of materials at impact region is necessary [5]. Such intensive deformation results in highly localized pressure at contact interface upon impact. As a result, the intimate contact of fresh metals takes place through break-up and extrusion of * Corresponding author. Tel.: +86 29 82660970; fax: +86 29 83237910. E-mail address: [email protected] (C.-J. Li). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.063
the thin oxide film on metal particle and/or substrate surfaces, which permits the formation of bonding between deposited particles [6]. On the other hand, the intensive deformation of particles and/or substrate will result in high strain and temperature rise at the localized contact area as well. It is well known that highly localized pressure caused by solid particle impact can lead to the refinement of microstructure. The short peening process as a typical example has been widely used to modify the surface microstructure [7]. Borchers et al. [8] reported the grain refinement to tens of nanometers in Cu particle boundaries of cold-sprayed Cu coating. They suggested that the refinement resulted from the recystallization of the elongated grains. Moreover, metallic jetting may occur under impact conditions in cold spraying as that observed in explosive welding process [5,6]. It was also argued that the melting at the contact area upon impact of solid particle may occur [9–11]. All those phenomena involved in particle impact in cold spraying process would modify the microstructure of the cold-sprayed coating, especially the microstructure near interfacial area. Moreover, the evolution of interfacial microstructure will influence the particle cohesion in cold-sprayed coating. However, few
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C.-J. Li et al. / Surface & Coatings Technology 198 (2005) 469–473
papers were involved in the study of interfacial microstructure evolution in cold spraying. In this study, Zn coating was deposited by cold spraying using nitrogen as driving gas at different temperatures. The evolution of coating microstructure with the emphasis on the interfacial microstructure was examined.
2. Materials and experimental procedure The commercially available Zn powder was used as feedstock. It had a spherical morphology with the size ranging from 5 to 25 Am. The examination of the etched cross-section of powder particle indicated that the grain size of the original powder was in micrometer order. Stainless steel was used as a substrate and sandblasted prior to spraying. A home-installed cold spraying system developed in Xi’an Jiaotong University was employed to deposit coating. The setup of this system has been described in detail elsewhere [12]. The nozzle of the gun had a converging– diverging cross-section with a throat diameter of 2 mm. Nitrogen was used as an accelerating gas. It was operated at a chamber pressure of 2 MPa and different preheating temperatures. Nitrogen was also used as the powder carrier gas. The standoff distance from the nozzle exit to substrate surface was 20 mm. During deposition, the spray gun was manipulated by a robot and traversed at a speed of 80 mm/s relative to the substrate. The microstructure of Zn coating was characterized by optical microscopy (OM; Reichert-Jung, MeF3A), scanning electron microscopy (SEM; Hitachi, S2700), and transmission electron microscopy (TEM; JEOL, JEM-200CX). The SAED analysis was used to examine the microstructural features near the particle–particle interfaces.
3. Results and discussion 3.1. General deformation behavior Fig. 1 shows the typical microstructure of Zn coating deposited at the gas temperature of 410 8C. The coating
Fig. 1. Typical OM microstructure of Zn coating deposited at the gas temperature of 410 8C.
Fig. 2. TEM microstructure of Zn coating deposited at the gas temperature of 320 8C indicating the stacking of dislocation.
presented a dense microstructure. It can be observed that most particles experienced certain deformation. The substantially deformed particles to a pancake shape evidently indicated their deformation and flow directions with regard to impact direction of the particle, as marked by arrows in Fig. 1. On the other hand, the observation of microstructure of the coating on an etched cross-section revealed that no significant difference in the grain size in bulk coating from that in the starting powder was observed. This was due to the fact that the intensive deformation of particle on impact concentrated mainly on the interfaces of particles. 3.2. Microstructure features of interfacial area between particles Fig. 2 shows a TEM microstructure of Zn coating deposited at the gas temperature of 320 8C. The stacking of the dislocation was clearly observed. This indicated the intensive deformation of the particles in deposition process. Fig. 3 shows a typical TEM microstructure near particle interface in the coating deposited at the gas temperature of 410 8C. The large grains of a size in micrometers can be clearly observed in deposited particles across the particle boundary. The interface area shown in Fig. 3a marked by arrows was comparably small in width. On the other hand, the detailed examination showed that the interface areas marked with A and B at both sides of the narrow interface had a size up to several hundreds of nanometers. The examination of microstructure in those regions revealed that the fine grains were evolved and near equiaxed grains were present as shown in Fig. 3b. It can be seen that the grain size of those fine grains were about 100 nm or less. Therefore, it is clear that interface between particles consisted of two distinguishable regions by relative impact position of particle. The region under direct impact of high-velocity particle experienced an intensive deformation and subsequent lateral flow. As a result, the highly localized pressure forced the yielded materials near the interface to flow outwards. The high-temperature rise resulting from the adiabatic shear enhances the instability of plastic flow. The limited size of the interface shown in Fig. 3a suggested that
C.-J. Li et al. / Surface & Coatings Technology 198 (2005) 469–473
Fig. 3. TEM microstructure near particle interface in the coating deposited at the gas temperature of 410 8C (a) and the detailed microstructure of area marked A in panel a (b).
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Fig. 4. A TEM microstructure of the interface of Zn coating deposited at the gas temperature of 410 8C (a) and high magnification of area marked C in panel a indicating an oriented lamellar structure (b).
most of interfacial materials having experienced intensive deformation were extruded from the direct impact region. Therefore, only a narrow interface region with a width up to about 100 nm was formed with the original large grains sandwiched the interface. On the other hand, at the region apart from direct impact, the interface area consisted of materials extruded from direct impact region and was wide in several hundreds of nanometers. This possibly resulted from the decrease of contact pressure on impact radially. Fig. 4 shows another typical examination result of the interface microstructure of Zn coating deposited at the gas temperature of 410 8C. The narrow interface region was at the right side of micrograph marked by an arrow as shown in Fig. 4a. In the interface area of large size up to a width of several hundreds nanometers, an oriented lamellar structure was observed (Fig. 4b). The orientation of those lamellae, which were several tens of nanometers in thickness and several hundreds nanometers in length, was along the interface direction as marked by arrows in Fig. 4b. Such orientation was reasonable by taking into account the deformation of the particles on impact. 3.3. Evolution of microstructure at interfaces It has been argued that metal jetting occurs under high velocity impact of solid metal particle in cold spraying [5,6]. The previous numerical study suggested that melting of particle at interface on impact occurs as soft metallic materials are deposited by cold spraying [13]. In this case,
Fig. 5. Typical SEM morphology of coating surface deposited at the gas temperature of 320 8C (a) and high magnification of area marked D in panel a (b).
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Fig. 6. TEM microstructure of the selected area in the coating deposited at the gas temperature of 410 8C and the SAED pattern indicating the presence of amorphous phase.
a liquid metal jetting rather than viscoplastic jetting may occur. Fig. 5 shows the typical SEM morphology of coating surface deposited at a gas temperature of 320 8C. It was clearly observed that the Zn particles have experienced intensively plastic deformation during deposition. Some particles penetrated obviously into the surface of the coating. Moreover, some large craters were also observed, which were formed possibly owing to the rebound-off of large Zn particles of low velocity. Another important fact observed clearly was that many small particulates of several hundreds of nanometers were present on the hillside of stacked particles in rough coating
surface as shown in Fig. 5b. Moreover, these small particles have a spherical morphology. In comparison with the particle size of the starting powders, it was clear that those small spherical particles were formed during the deposition process. With regarding the possibility of the melting during impact of Zn particle under cold spray condition, it is reasonable to consider that those spherical particles resulted from the jetting of the melted particle materials. The liquid metal jetting along the interface direction encountered the curved particle surface to cause the splashing, and subsequently lead to the formation of small spherical particles. As the following particles impact successively on the top of those particles, the tamping effect in cold spraying [12] will cause the successive deformation of those particles along the interface direction. In comparison of the elongated particle size in Fig. 4b with those in Fig. 5b, it is reasonable to consider that the oriented lamellae is formed through the successive deformation of small spherical particles deposited by liquid metal jetting. Taking the small size of liquid particles into consideration, rapid cooling inherent to small melt particles may lead to the formation of amorphous phase in the deposited coating. Fig. 6 shows a typical TEM microstructure of the selected area in the coating deposited at the gas temperature of 410 8C. The SAED analysis clearly suggested the formation of a trace of amorphous phase in the coating. Therefore, the present study clearly found that the fine microstructure with nanograins and even some amorphous phase were evolved near the interface region of Zn particles
Fig. 7. Schematic diagram of the evolution of the particle interface microstructure in cold-sprayed coating by recrystallization refinement mechanism (a–c) and tamping of melting-induced particulates to lamellae (d–f). Panels a–c show formation of elongated grains resulting from viscoplastic deformation (a), further elongation of grains by the tamping of following particle (b), and recrystallization (c). Panels d–f illustrate formation of melt jetting with splashed particulates (d), tamping by the following particle (e), and successive deformation of the fine particulates by tamping effect to lamellae (f).
C.-J. Li et al. / Surface & Coatings Technology 198 (2005) 469–473
in the coating deposited by cold spraying. Because those microstructures are quasi-stable, the recrystallization may occur to lead to the refinement of grains [8]. For Zn, the deformed grains may recrystallize at a room temperature. The recrystallization will lead to a further refinement of grains, especially those heavily elongated ones under highly localized impact strain. According to the results found in this study, a model for the evolution of interface microstructure in cold spraying of soft metal coating can be proposed. Fig. 7 shows schematically two mechanisms for the evolution of nanograins at the particle–particle interfaces. As shown in Fig. 7a–c, the particle of a sufficiently high-velocity impacts on coating surface, and deforms intensively at the contact interface, which results in the metal jetting by the adiabatic shear instability. The grains near the interface are elongated accompanying the further deformation. Through the successive impacts of particles resulting in tamping effect, the elongation of interface grains further occurs. Moreover, the dynamic refinement of the deformed grains occurs through the recrystallization as reported by Borchers et al. [8]. Another mechanism is concerned with the liquid metal jetting by impact-induced melting. As shown in Fig. 7d–f, a particle impacting on coating surface results in the melt jetting owing to the melting of interface region caused by temperature rise. When melt jetting along the interface direction encounters the surface of adjacent particles, the splashing of liquid occurs to lead to the deposition of fine particulates on the curved surface of the deformed particles. The rapid solidification of the fine particulates forms the fine grains and even amorphous structure. As following particles impact on, these fine particulates are tamped to flatten along the direction of the interface. As a result, the oriented lamellar structure is formed.
4. Conclusions The dense Zn coating was deposited through the intensive deformation in cold spraying. The nanograins formed near the interfaces between deposited particles, while the size of grains in the bulk inside of the particles in the coating was comparable to that in the feedstock. The width of the interface of fine grains at direct particle impact region between particles was evidently different from that near the periphery of deposited particle. A narrow interface was present around the direct impact position, while the
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relative wide interface up to several hundreds of nanometers was present near the periphery of deposited particle. Moreover, the oriented lamellar microstructure at the interface region was observed. The amorphous phase was also evidently present in the interface region. It can be considered that fine nanograins were created by the recrystallization of elongated grains resulting from intensive plastic deformation. The lamellar microstructure was formed by the successive deformation of deposited fine particulates resulting from the jetting of impact-induced melt. The recrystallization of the metastable microstructure at the interface might improve the cohesion between particles in the coating.
Acknowledgments The present project is supported by the National Natural Science Foundation of China (NSFC; Contract No. 50171052) and the Doctoral Foundation of Xi’an Jiaotong University.
References [1] A. Papyrin, Adv. Mater. Process. 159 (9) (2001) 49. [2] R.S. Lima, J. Karthikeyan, C.M. Kay, J. Lindemann, C.C. Berndt, Thin Solid Films 416 (1–2) (2002) 129. [3] Chang-Jiu Li, Guan-Jun Yang, Xin-Chun Huang, Wen-Ya Li, A. Ohmori, Thermal Spray Solutions: Advances in Technology and Application, DVS, Dqsseldorf, 2004, p. 315. [4] Chang-Jiu Li, Wen-Ya Li, Wen-Hua Ma, H. Fukanuma, Thermal Spray Solutions: Advances in Technology and Application, DVS, Dqsseldorf, 2004, p. 371. [5] H. Assadi, F. G7rtner, T. Stoltenhoff, H. Kreye, Acta Mater. 51 (15) (2003) 4379. [6] R.C. Dykhuizen, M.F. Smith, D.L. Gilmore, R.A. Neiser, et al., J. Therm. Spray Technol. 8 (4) (1999) 559. [7] N.R. Tao, M.L. Sui, J. Lu, K. Lu, Nanostruct. Mater. 11 (4) (1999) 433. [8] C. Borchers, F. G7rtner, T. Stoltenhoff, H. Assadi, H. Kreye, J. Appl. Phys. 93 (12) (2003) 10064. [9] M. Grujicic, J.R. Saylor, D.E. Beasley, W.S. DeRosset, D. Helfritch, Appl. Surf. Sci. 219 (3–4) (2003) 211. [10] A.P. Alkhimov, A.I. Gulidov, V.F. Kosarev, N.I. Nesterovich, J. Appl. Mech. Tech. Phys. 41 (1) (2000) 204. [11] D. Zhang, P.H. Shipway, D.G. McCartney, in: C. Moreau, B. Marple (Eds.), Thermal Spray 2003: Advancing the Science & Applying the Technology, ASM International, Materials Park, Ohio, USA, 2003, p. 45. [12] Chang-Jiu Li, Wen-Ya Li, Surf. Coat. Technol. 167 (2003) 278. [13] Chang-Jiu Li, Wen-Ya Li, H. Fukanuma, Thermal Spray Solutions: Advances in Technology and Application, DVS, Dqsseldorf, 2004, p. 335.
Formation of metastable phases in cold-sprayed soft metallic deposit Chang-Jiu Li*, Wen-Ya Li, Yu-Yue Wang State Key Laboratory for Mechanical Behavior of Materials, Welding Research Institute, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049 PR China Available online 24 November 2004
Abstract Zinc powder was used to deposit coating by cold spraying using nitrogen as driving gas. The microstructure of the deposited coating was characterized by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that the cold-sprayed Zn coating presented a dense microstructure. The nanograins formed at the interface area between deposited particles, while the size of grains in the bulk inside of the particles in the coating was comparable to that in the starting powder. The interface region of the oriented lamellar microstructure was observed. The refinement of grains to the nanocrystallines at the particle–particle interfaces is achieved through the recrystallization of elongated grains resulting from intensive deformation. The lamellar microstructure is formed by the successive deformation of deposited fine particulates resulting from the jetting of impact-induced melt. An evolution model of the interface microstructure in cold spraying is proposed. D 2004 Elsevier B.V. All rights reserved. Keywords: Cold spraying; Zinc coating; Microstructure; Nanocrystalline
1. Introduction Cold spraying, also termed cold gas dynamic spray, has emerged upon the horizon of thermal spray technology in the past few years. Typical feature of this process in comparison with the conventional thermal spray techniques is that a coating is formed by completely solid particles of low temperature and high velocity on impact [1]. It can be employed to produce high-quality metallic coatings without any noticeable oxidation. The low-temperature characteristic also makes it possible to deposit coatings without deteriorating the microstructure of the starting feedstocks such as nanostructured feedstocks [2–4]. In order to deposit coating through solid particle, the deformation intensive enough to result in the unstable flow of materials at impact region is necessary [5]. Such intensive deformation results in highly localized pressure at contact interface upon impact. As a result, the intimate contact of fresh metals takes place through break-up and extrusion of * Corresponding author. Tel.: +86 29 82660970; fax: +86 29 83237910. E-mail address: [email protected] (C.-J. Li). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.063
the thin oxide film on metal particle and/or substrate surfaces, which permits the formation of bonding between deposited particles [6]. On the other hand, the intensive deformation of particles and/or substrate will result in high strain and temperature rise at the localized contact area as well. It is well known that highly localized pressure caused by solid particle impact can lead to the refinement of microstructure. The short peening process as a typical example has been widely used to modify the surface microstructure [7]. Borchers et al. [8] reported the grain refinement to tens of nanometers in Cu particle boundaries of cold-sprayed Cu coating. They suggested that the refinement resulted from the recystallization of the elongated grains. Moreover, metallic jetting may occur under impact conditions in cold spraying as that observed in explosive welding process [5,6]. It was also argued that the melting at the contact area upon impact of solid particle may occur [9–11]. All those phenomena involved in particle impact in cold spraying process would modify the microstructure of the cold-sprayed coating, especially the microstructure near interfacial area. Moreover, the evolution of interfacial microstructure will influence the particle cohesion in cold-sprayed coating. However, few
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C.-J. Li et al. / Surface & Coatings Technology 198 (2005) 469–473
papers were involved in the study of interfacial microstructure evolution in cold spraying. In this study, Zn coating was deposited by cold spraying using nitrogen as driving gas at different temperatures. The evolution of coating microstructure with the emphasis on the interfacial microstructure was examined.
2. Materials and experimental procedure The commercially available Zn powder was used as feedstock. It had a spherical morphology with the size ranging from 5 to 25 Am. The examination of the etched cross-section of powder particle indicated that the grain size of the original powder was in micrometer order. Stainless steel was used as a substrate and sandblasted prior to spraying. A home-installed cold spraying system developed in Xi’an Jiaotong University was employed to deposit coating. The setup of this system has been described in detail elsewhere [12]. The nozzle of the gun had a converging– diverging cross-section with a throat diameter of 2 mm. Nitrogen was used as an accelerating gas. It was operated at a chamber pressure of 2 MPa and different preheating temperatures. Nitrogen was also used as the powder carrier gas. The standoff distance from the nozzle exit to substrate surface was 20 mm. During deposition, the spray gun was manipulated by a robot and traversed at a speed of 80 mm/s relative to the substrate. The microstructure of Zn coating was characterized by optical microscopy (OM; Reichert-Jung, MeF3A), scanning electron microscopy (SEM; Hitachi, S2700), and transmission electron microscopy (TEM; JEOL, JEM-200CX). The SAED analysis was used to examine the microstructural features near the particle–particle interfaces.
3. Results and discussion 3.1. General deformation behavior Fig. 1 shows the typical microstructure of Zn coating deposited at the gas temperature of 410 8C. The coating
Fig. 1. Typical OM microstructure of Zn coating deposited at the gas temperature of 410 8C.
Fig. 2. TEM microstructure of Zn coating deposited at the gas temperature of 320 8C indicating the stacking of dislocation.
presented a dense microstructure. It can be observed that most particles experienced certain deformation. The substantially deformed particles to a pancake shape evidently indicated their deformation and flow directions with regard to impact direction of the particle, as marked by arrows in Fig. 1. On the other hand, the observation of microstructure of the coating on an etched cross-section revealed that no significant difference in the grain size in bulk coating from that in the starting powder was observed. This was due to the fact that the intensive deformation of particle on impact concentrated mainly on the interfaces of particles. 3.2. Microstructure features of interfacial area between particles Fig. 2 shows a TEM microstructure of Zn coating deposited at the gas temperature of 320 8C. The stacking of the dislocation was clearly observed. This indicated the intensive deformation of the particles in deposition process. Fig. 3 shows a typical TEM microstructure near particle interface in the coating deposited at the gas temperature of 410 8C. The large grains of a size in micrometers can be clearly observed in deposited particles across the particle boundary. The interface area shown in Fig. 3a marked by arrows was comparably small in width. On the other hand, the detailed examination showed that the interface areas marked with A and B at both sides of the narrow interface had a size up to several hundreds of nanometers. The examination of microstructure in those regions revealed that the fine grains were evolved and near equiaxed grains were present as shown in Fig. 3b. It can be seen that the grain size of those fine grains were about 100 nm or less. Therefore, it is clear that interface between particles consisted of two distinguishable regions by relative impact position of particle. The region under direct impact of high-velocity particle experienced an intensive deformation and subsequent lateral flow. As a result, the highly localized pressure forced the yielded materials near the interface to flow outwards. The high-temperature rise resulting from the adiabatic shear enhances the instability of plastic flow. The limited size of the interface shown in Fig. 3a suggested that
C.-J. Li et al. / Surface & Coatings Technology 198 (2005) 469–473
Fig. 3. TEM microstructure near particle interface in the coating deposited at the gas temperature of 410 8C (a) and the detailed microstructure of area marked A in panel a (b).
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Fig. 4. A TEM microstructure of the interface of Zn coating deposited at the gas temperature of 410 8C (a) and high magnification of area marked C in panel a indicating an oriented lamellar structure (b).
most of interfacial materials having experienced intensive deformation were extruded from the direct impact region. Therefore, only a narrow interface region with a width up to about 100 nm was formed with the original large grains sandwiched the interface. On the other hand, at the region apart from direct impact, the interface area consisted of materials extruded from direct impact region and was wide in several hundreds of nanometers. This possibly resulted from the decrease of contact pressure on impact radially. Fig. 4 shows another typical examination result of the interface microstructure of Zn coating deposited at the gas temperature of 410 8C. The narrow interface region was at the right side of micrograph marked by an arrow as shown in Fig. 4a. In the interface area of large size up to a width of several hundreds nanometers, an oriented lamellar structure was observed (Fig. 4b). The orientation of those lamellae, which were several tens of nanometers in thickness and several hundreds nanometers in length, was along the interface direction as marked by arrows in Fig. 4b. Such orientation was reasonable by taking into account the deformation of the particles on impact. 3.3. Evolution of microstructure at interfaces It has been argued that metal jetting occurs under high velocity impact of solid metal particle in cold spraying [5,6]. The previous numerical study suggested that melting of particle at interface on impact occurs as soft metallic materials are deposited by cold spraying [13]. In this case,
Fig. 5. Typical SEM morphology of coating surface deposited at the gas temperature of 320 8C (a) and high magnification of area marked D in panel a (b).
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Fig. 6. TEM microstructure of the selected area in the coating deposited at the gas temperature of 410 8C and the SAED pattern indicating the presence of amorphous phase.
a liquid metal jetting rather than viscoplastic jetting may occur. Fig. 5 shows the typical SEM morphology of coating surface deposited at a gas temperature of 320 8C. It was clearly observed that the Zn particles have experienced intensively plastic deformation during deposition. Some particles penetrated obviously into the surface of the coating. Moreover, some large craters were also observed, which were formed possibly owing to the rebound-off of large Zn particles of low velocity. Another important fact observed clearly was that many small particulates of several hundreds of nanometers were present on the hillside of stacked particles in rough coating
surface as shown in Fig. 5b. Moreover, these small particles have a spherical morphology. In comparison with the particle size of the starting powders, it was clear that those small spherical particles were formed during the deposition process. With regarding the possibility of the melting during impact of Zn particle under cold spray condition, it is reasonable to consider that those spherical particles resulted from the jetting of the melted particle materials. The liquid metal jetting along the interface direction encountered the curved particle surface to cause the splashing, and subsequently lead to the formation of small spherical particles. As the following particles impact successively on the top of those particles, the tamping effect in cold spraying [12] will cause the successive deformation of those particles along the interface direction. In comparison of the elongated particle size in Fig. 4b with those in Fig. 5b, it is reasonable to consider that the oriented lamellae is formed through the successive deformation of small spherical particles deposited by liquid metal jetting. Taking the small size of liquid particles into consideration, rapid cooling inherent to small melt particles may lead to the formation of amorphous phase in the deposited coating. Fig. 6 shows a typical TEM microstructure of the selected area in the coating deposited at the gas temperature of 410 8C. The SAED analysis clearly suggested the formation of a trace of amorphous phase in the coating. Therefore, the present study clearly found that the fine microstructure with nanograins and even some amorphous phase were evolved near the interface region of Zn particles
Fig. 7. Schematic diagram of the evolution of the particle interface microstructure in cold-sprayed coating by recrystallization refinement mechanism (a–c) and tamping of melting-induced particulates to lamellae (d–f). Panels a–c show formation of elongated grains resulting from viscoplastic deformation (a), further elongation of grains by the tamping of following particle (b), and recrystallization (c). Panels d–f illustrate formation of melt jetting with splashed particulates (d), tamping by the following particle (e), and successive deformation of the fine particulates by tamping effect to lamellae (f).
C.-J. Li et al. / Surface & Coatings Technology 198 (2005) 469–473
in the coating deposited by cold spraying. Because those microstructures are quasi-stable, the recrystallization may occur to lead to the refinement of grains [8]. For Zn, the deformed grains may recrystallize at a room temperature. The recrystallization will lead to a further refinement of grains, especially those heavily elongated ones under highly localized impact strain. According to the results found in this study, a model for the evolution of interface microstructure in cold spraying of soft metal coating can be proposed. Fig. 7 shows schematically two mechanisms for the evolution of nanograins at the particle–particle interfaces. As shown in Fig. 7a–c, the particle of a sufficiently high-velocity impacts on coating surface, and deforms intensively at the contact interface, which results in the metal jetting by the adiabatic shear instability. The grains near the interface are elongated accompanying the further deformation. Through the successive impacts of particles resulting in tamping effect, the elongation of interface grains further occurs. Moreover, the dynamic refinement of the deformed grains occurs through the recrystallization as reported by Borchers et al. [8]. Another mechanism is concerned with the liquid metal jetting by impact-induced melting. As shown in Fig. 7d–f, a particle impacting on coating surface results in the melt jetting owing to the melting of interface region caused by temperature rise. When melt jetting along the interface direction encounters the surface of adjacent particles, the splashing of liquid occurs to lead to the deposition of fine particulates on the curved surface of the deformed particles. The rapid solidification of the fine particulates forms the fine grains and even amorphous structure. As following particles impact on, these fine particulates are tamped to flatten along the direction of the interface. As a result, the oriented lamellar structure is formed.
4. Conclusions The dense Zn coating was deposited through the intensive deformation in cold spraying. The nanograins formed near the interfaces between deposited particles, while the size of grains in the bulk inside of the particles in the coating was comparable to that in the feedstock. The width of the interface of fine grains at direct particle impact region between particles was evidently different from that near the periphery of deposited particle. A narrow interface was present around the direct impact position, while the
473
relative wide interface up to several hundreds of nanometers was present near the periphery of deposited particle. Moreover, the oriented lamellar microstructure at the interface region was observed. The amorphous phase was also evidently present in the interface region. It can be considered that fine nanograins were created by the recrystallization of elongated grains resulting from intensive plastic deformation. The lamellar microstructure was formed by the successive deformation of deposited fine particulates resulting from the jetting of impact-induced melt. The recrystallization of the metastable microstructure at the interface might improve the cohesion between particles in the coating.
Acknowledgments The present project is supported by the National Natural Science Foundation of China (NSFC; Contract No. 50171052) and the Doctoral Foundation of Xi’an Jiaotong University.
References [1] A. Papyrin, Adv. Mater. Process. 159 (9) (2001) 49. [2] R.S. Lima, J. Karthikeyan, C.M. Kay, J. Lindemann, C.C. Berndt, Thin Solid Films 416 (1–2) (2002) 129. [3] Chang-Jiu Li, Guan-Jun Yang, Xin-Chun Huang, Wen-Ya Li, A. Ohmori, Thermal Spray Solutions: Advances in Technology and Application, DVS, Dqsseldorf, 2004, p. 315. [4] Chang-Jiu Li, Wen-Ya Li, Wen-Hua Ma, H. Fukanuma, Thermal Spray Solutions: Advances in Technology and Application, DVS, Dqsseldorf, 2004, p. 371. [5] H. Assadi, F. G7rtner, T. Stoltenhoff, H. Kreye, Acta Mater. 51 (15) (2003) 4379. [6] R.C. Dykhuizen, M.F. Smith, D.L. Gilmore, R.A. Neiser, et al., J. Therm. Spray Technol. 8 (4) (1999) 559. [7] N.R. Tao, M.L. Sui, J. Lu, K. Lu, Nanostruct. Mater. 11 (4) (1999) 433. [8] C. Borchers, F. G7rtner, T. Stoltenhoff, H. Assadi, H. Kreye, J. Appl. Phys. 93 (12) (2003) 10064. [9] M. Grujicic, J.R. Saylor, D.E. Beasley, W.S. DeRosset, D. Helfritch, Appl. Surf. Sci. 219 (3–4) (2003) 211. [10] A.P. Alkhimov, A.I. Gulidov, V.F. Kosarev, N.I. Nesterovich, J. Appl. Mech. Tech. Phys. 41 (1) (2000) 204. [11] D. Zhang, P.H. Shipway, D.G. McCartney, in: C. Moreau, B. Marple (Eds.), Thermal Spray 2003: Advancing the Science & Applying the Technology, ASM International, Materials Park, Ohio, USA, 2003, p. 45. [12] Chang-Jiu Li, Wen-Ya Li, Surf. Coat. Technol. 167 (2003) 278. [13] Chang-Jiu Li, Wen-Ya Li, H. Fukanuma, Thermal Spray Solutions: Advances in Technology and Application, DVS, Dqsseldorf, 2004, p. 335.