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Investigation of modification of Cu-Ni-graphite composite by silver
Materials Chemistry and Physics 239 (2020) 121990
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Investigation of modification of Cu-Ni-graphite composite by silver Yiran Wang a, b, *, Yimin Gao a, Jun Takahashi b, Yi Wan b, Mengting Li a, Bing Xiao b, Yunqian Zhang b, Xiangdong He b a b
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, PR China Department of Systems Innovation, School of Engineering, The University of Tokyo, Tokyo, Japan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Silver phase in graphite exists as silver nanowires and nanoparticles, and as silver precipitate phase in the matrix. � Silver nanowires and α-Cu phase are generated at the interface. � Silver efficiently improve the mechani cal properties.
A R T I C L E I N F O
A B S T R A C T
Keywords: Silver Surface treatment Interfacial modification Composite
Surface treatment of graphite by silver is applied to modify microstructure and mechanical properties of Cu-Nigraphite composites. In this study, flake graphite is used to create silver-coated graphite by Tollens’ reagent, and the silver-modified Cu-Ni-graphite composite is prepared by powder metallurgy. The microstructure character ization and modification mechanism of the composite are the emphases of this investigation. The results showed that the silver coating is composed of a pure silver phase with a rough and granular surface and the thickness of the layer is about 1.52 μm. The composite consists of graphite, an α-Cu phase and a silver phase. The silver phase in graphite exists as silver nanowires and nanoparticles, and a silver precipitate phase is generated in the matrix. At the interface of the composite, silver nanowires and α-Cu phase is generated in the frontier of the matrix and the graphite. These silver nanowires and the α-Cu phase that joint the interface can efficiently enhance the interface strength and dramatically improves mechanical properties. Through the silver modification process of the composite, hardness increases by approximately 49.9%, the improvement of relative density is increased by approximately 2.1%. Additionally, the flexural strength increases by approximately 187.8% compared with the composites without modification.
1. Introduction In China, high-speed railways are in the age of hypered development.
Switch slide baseplates (SSBs), one of the most significant railway components, urgently require new materials to increase the efficiency of railway capacity [1]. To date, Q235 steel, a type of low carbon steel,
* Corresponding author. State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, PR China. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.matchemphys.2019.121990 Received 26 April 2019; Received in revised form 21 July 2019; Accepted 7 August 2019 Available online 13 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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coated with chromium has been used for SSBs, and grease lubrication has typically been used to decrease the friction coefficient of the SSBs [2]. In outdoor operation conditions, rain fall, or contamination from sources such as ash and sand grit could easily make the lubrication decay. Therefore, self-lubricating materials are of interest to many re searchers. In recent years, many studies have focused on the use of self-lubricating materials for SSBs to overcome the shortcomings of current SSB materials [3–5]. Solid self-lubricating materials have a great potential to be considered for use in SSBs. Copper matrix composites have the advantages of excellent anticorrosion in the outdoors and unique tribology properties when sliding against steel compared with other self-lubricating composites. Until now, copper matrix composites have been widely used in bearings and others fields [6–11]. In our previous study [12–16], the tribology and mechanical properties of a Cu-Ni-graphite composite were tested under SSB operation conditions. The results show that due to the weak bonding strength at the interface of the graphite reinforced copper matrix composites, graphite as a lubricating phase severely reduces the mechanical properties in the composites. Due to debonding and pulling out of the composites, the graphite loses its capacity under service conditions. Ni plays a significant role in solution strengthening with the copper matrix in the Cu-Ni-graphite composite and enhance the corro sion properties in outdoor operation conditions. However, the Ni element in the matrix cannot effectively improve the weak interface bonding. Surface modification of graphite is one of the best ways to improve the metal and graphite interface in metal matrix composites. Many studies have confirmed that metal coated graphite is the most effective application for surface modification [17–23,33]. The electroless plating technology for copper and nickel metals has been used for many years, and therefore, many studies on copper- or nickel-coated graphite rein forced copper matrix composites have been published [24–29]. In this case, the modifying the Cu-Ni-graphite composites with nickel [14] and copper [16] has been attempted to increase both the mechanical and tribological properties. These metals enhance the mutual diffusion of copper and carbon atoms and increase the interface strength accord ingly. The mechanical properties of Cu-Ni-graphite composites are effectively improved; however, the tribological properties show limited increases. To achieve more improved properties, new metals are tested for Cu-Ni-graphite composites. In addition to nickel and copper, other functional metals have been considered to modify the surface of graphite. Silver, which is a soft metal, can play an all-important function as not simply a reinforced phase but also a lubrication phase. Compared with nickel and copper, the silver phase can decrease the friction coefficient when sliding against steel [30]. Up to now, few studies have focused on silver-coated graphite modified copper matrix graphite composites, and few researchers have studied the modification mechanism of silver in composites [31,32]. Therefore, it is worth studying the modification of Cu-Ni-graphite composites by silver and the associated strength mechanism. This study investigates the way in which a silver covering on graphite for modifies the microstructure and mechanical properties of Cu-Nigraphite composites. Silver-coated graphite was prepared with Toll ens’ reagent, and then a silver-modified Cu-Ni-graphite composite was constructed from silver-coated graphite, copper and nickel powders by powder metallurgy. The microstructure characterizations of the silvercoated graphite and silver-modified Cu-Ni-graphite composite were studied. Then, the interfacial morphologies and modification mecha nisms of the silver-modified Cu-Ni-graphite composite were investi gated. The mechanical properties of the silver-modified Cu-Ni-graphite composite with different contents were also discussed.
2. Experiments 2.1. Materials and preparation Flake graphite with irregular shapes was selected as a raw material. The microstructure of the flake graphite, which is shown in Fig. 1 (a) & (b), reveals a mean size of 45 μm and a thickness of 5 μm. The crosssection morphology is given in Fig. 1 (c), and it can be seen that flake graphite has a lamellar structure inside the particle. The XRD pattern in Fig. 1 (d) also shows a close-packed hexagonal structure and a layer substructure in the (002) crystal plane. In this study, silver-coated graphite was prepared with Tollens’ re agent. The specific constituents of Tollens’ reagent are reported in Table 1, and the principle of Tollens’ reaction can be explained by equations (1)–(3). AgNO3þ NH3⋅H2O →AgOH ↓ þNH4NO3
(1)
AgOH þ NH3⋅H2O→Ag(NH3)2 OH þ 2H2O
(2)
4Ag[(NH3)2]OH þ HCHO →4Ag þ 2H2O þ 6NH3 þ (NH4)2CO3
(3)
The preparation procedures can be divided into five steps, and the schematic diagram is illustrated in Fig. 2. First, flake graphite was pre cleaned for 30 min. The precleaning was conducted with a 20% NaOH aqueous solution to wash off the impurities attached to the surface of the graphite. Then, graphite was sensitized in a 15 ml/L HCl & 10 g/L SnCl2 aqueous solution. The sensitization procedure generated an Sn(OH)Cl film on the graphite. The activation procedure was conducted in a 15 ml/L HCl & 0.3 g/L PbCl2 aqueous solution to deposit a (Pb, Sn2þ) film on the surface of the graphite. Finally, a reduction procedure was carried out in a 40 g/L NaH2PO2⋅H2O aqueous solution. In the reduction, the activated Pb coating was attached to the surface of the flake graphite. Through the above procedures, the silver could be deposited more easily because graphite has an inactive surface onto which metal can hardly be deposited. The precleaning, sensitization, activation and reduction procedures could activate the surface of the graphite by attaching active ions (Pb). The last procedure was chemical plating, and the flake graphite with activated Pb was added into a Tollens’ reagent for plating. The graphite powder was plated at 323 K in an aqueous solution consisting of 36 g/L of AgNO3, 36 g/L of NH3⋅H2O, 24 ml/L of HCHO, and 95 ml/L of CH₃CH₂OH. The activated Pb surface facilitated deposition of the silver ion coating. After 1 h of the chemical plating process, the graphite was successfully coated with silver. The silver-modified Cu-Ni-graphite composite was prepared from silver-coated graphite, copper and nickel powders. Information about the raw materials and the composition of the silver-modified Cu-Nigraphite composites with different graphite contents is listed in Table 2. In the experiment, the 36 g/L of AgNO3 in the Tollens’ reagent and the 1-L aqueous solution reacted to yield approximately 17.8 g of silver. Moreover, the 1-L aqueous solution could plate 20 g of graphite, and the plating efficiency was approximately 30%. Therefore, 20 g of graphite could produce 5.34 g of silver, and the mass fraction of the silver in the silver-coated graphite was 21.1 wt.%. As a conclusion, the silver con tents introduced in the Cu-Ni-graphite composites with graphite con tents of 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%, 5 wt.% and 6 wt.% were approximately 0.2111wt.%, 0.42 wt.%, 0.63 wt.%, 0.84 wt.%, 1.06 wt. % and 1.27 wt.%, respectively. The silver modified Cu-Ni-graphite composite was prepared by a powder metallurgy process. The process consisted of ball milling, com pacting, sintering, recompacting and resintering. The specific prepara tion parameters are reported in Table 3. Finally, specimens with diameter and thickness of 44 mm and 6 mm, respectively, were prepared.
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Fig. 1. Raw flack graphite. (a) Low magnification morphology. (b) High magnification. (c) Cross-section morphology. (d) XRD pattern.
spectrum. An X-ray diffraction detector (XRD) scanned the samples with an the angular 2θ range from 20� to 90� and a step size of 0.02. Highresolution transmission electron microscopy (TEM) equipped with a top-entry goniometer which has a point-to-point resolution of 0.17 nm at 400 kV, was used. Thin foil specimens were prepared by punching 3 mm discs from wafers cut from large blocks and, mechanically grinding by hand to 50 μm. The specimens were twin-jet electropolished to electron transparency at 50 V in a mixture of 5% perchloric acid, 20% glycerol and 75% methanol, with the solution being cooled to 253 K with liquid nitrogen, following the standard techniques of thin foil used for TEM. The powder specimens were directly observed for TEM by
Table 1 Composition of Tollens’ reagent. Solution
AgNO3
NH3⋅H2O
HCHO
CH₃CH₂OH
Content
36 g/L
36 g/L
24 ml/L
95 ml/L
2.2. Microstructure characterization and mechanical properties The microstructure characterizations of the silver-coated graphite and the silver-modified Cu-Ni-graphite composites were investigated by a scanning electronic microscope (SEM), including the energy dispersive
Fig. 2. Schematic of preparation of silver-coated graphite. 3
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3. Results
Table 2 Composition of silver modified Cu-Ni-graphite composite. Element
Content/wt.%
Silver-coated graphite Ni Cu
1–6 8 Bal.
3.1. Microstructure characterization of silver-coated graphite Fig. 3 exhibits the morphologies of the silver-coated graphite with rough and granular surfaces. EDS tests were also conducted to examine the composition of the silver-coated graphite in Fig. 3 (a). The results of the EDS test are given in Table 4, and it can be observed that the coating is composed of the purest silver. The composition of the entire silver coating is uniform, and no other impurities could be detected in the coating. As shown in Fig. 3 (b), the coating is composed of silver nanoparticles with an intermediate size of 200 nm in the high magni fication range. The compact silver coating has a low porosity and is organized by the deposition process of silver atoms during chemical plating. From the TEM morphologies of the silver coating in Fig. 3 (c) & (d), it can been seen that the silver atoms deposited on the surface of the graphite rapidly crystallized, and afterward, the silver atoms grew at the preferred orientation of (200). In agreement with the deposition process, the silver nanoparticles were generated and bound to the thick silver coating. The XRD results given in Fig. 4 (a) also support the phases of silver coated graphite. It can be inferred that silver-coated graphite consists of a silver phase and a graphite phase. Moreover, no reaction was gener ated between the silver and the carbon, and no new phase was formed. Base on the cross-section microstructure shown in Fig. 4 (b), it can be easily summarized that the thickness of the coating is approximately
Table 3 Preparation parameters of silver modified Cu-Ni-graphite composite. Step
Process
Parameter
1 2 3 4 5
Ball-milling Compacting Sintering Re-compacting Re-sintering
15 h 600 MPa 1123 K þ 1 h 600 MPa 1073 K þ 0.5 h
being loaded in a copper grid. The Archimedes’ principle was measured to determine the relative density of silver modified Cu-Ni-graphite composites with different graphite contents. Hardness experiments were tested by Vickers hard ness meters by subjecting samples to 5 N loads for 15 s of dwell time. The flexural strength was examined by conducting three-point tests on a universal testing machine with specimens that had been cut into 30 mm � 6 mm � 6 mm bars. The results were calculated according to the following equation (4)
σf ¼
3PL 2bd2
(4)
Table 4 EDS results of silver coated graphite.
where P is the maximum load at fracture, L is the span length, b is the width of the samples, and d is the thickness of the samples. Thus, the microstructure of the fractures was observed by SEM.
Point 1 2 3 4
C
Ag
wt.%
at.%
wt.%
at.%
13.6 9.9 11.4 7.8
54.0 52.7 53.1 52.4
86.4 90.1 88.6 92.2
46.0 47.3 46.9 47.6
Fig. 3. Silver-coated graphite (I). (a) Low magnification morphology. (b) High magnification. (c) TEM morphology. (d) High-resolution TEM at (111) plane. 4
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Fig. 4. Silver-coated graphite (II). (a) XRD pattern. (b) Cross-section morphology. (c) Interface morphology. (d) Interface EDS results.
1.52 μm, and silver nanoparticles were deposited onto the surface of the graphite, exhibiting densification. From the EDS test shown in Fig. 4 (c) & (d), it can be found that the diffused intensity of silver is reduced as the diffused distance increases, and the distribution of silver in the inner graphite is relatively weak. Additionally, the carbon element diffused into the silver coating, and its diffusion intensity decreases as the diffused distance increases.
matrix, plenty of silver atoms were dispersed and dissolved in the ma trix. Additionally, copper and nickel element have no visible presence in the graphite zone, and a minimal amount of carbon is found in the matrix. Therefore, the silver coating on the graphite restrained the mutual diffusion of the matrix and graphite during the sintering process. As shown in Fig. 7, the microstructure characterization of the silvermodified Cu-Ni-graphite composite was further analyzed by TEM research. It can be deduced that the silver atoms formed not only silver nanoparticles but also silver nanowires in the graphite in Fig. 7 (a) & (c). These silver nanowires are as a kind of substructure that exists in the graphite phase. From the FFT image in Fig. 7 (b), the interplanar spacing of silver nanoparticle was d(111) ¼ 0.2418 nm, which is a slightly larger than the average value (d(111) ¼ 0.2359 nm). It can be asserted that the carbon atoms dissolved into the silver crystal lattice and resulted in lattice distortion. Furthermore, through observing the nanostructure in the matrix in Fig. 7 (d), it is indicated that silver atoms dissolved into the matrix (α-Cu) and made the interplanar spacing of α-Cu increase from d(111) ¼ 0.2086 nm to d(111) ¼ 0.2218 nm. Due to the extremely low solid solubility in the α-Cu phase, silver atoms finally dissolved out of the matrix and generate the precipitated phase. From Fig. 8 (a), the XRD pattern results of the silver-modified Cu-Ni-graphite composite also prove that the composite consisted of three phases: α-Cu, graphite and the silver phase.
3.2. Microstructure of silver-modified Cu-Ni-graphite composite Fig. 5 expresses the microstructure of the silver-modified Cu-Nigraphite composite. The black phase is graphite, and the gray zone is α-Cu as the matrix (Cu-Ni solid solution). Because it is affected by the compacting process, the graphite has two different morphologies in the composite. In Fig. 5 (a), (c), and (e), the graphite exhibited an irregular shape in the microstructure perpendicular to the compacting direction, whereas the graphite expressed a linear shape in the microstructure parallel to the compacting direction shown in Fig. 5 (b), (d), and (f). As the graphite content increases, the volume fraction of the graphite distributed in the composite also increases. According to our previous work, the graphite showed a continuous net distribution when the graphite content reached 6 wt.% in the unmodified Cu-Ni-graphite composite. However, the graphite shows a dispersed and uniform dis tribution in the composite utilizing the silver modification process. Further analysis of the microstructure of the silver-modified Cu-Nigraphite composite at high magnification and with the EDS test is shown in Fig. 6. From the high magnification microstructure in Fig. 6 (a), it can be seen that the silver phase existed and dispersed in both the matrix and the graphite. It is notable that the silver element was generated as silver nanoparticles in the graphite phase shown in Fig. 6 (b) and formed the precipitated phase in both the matrix and the interface expressed in Fig. 6 (c) & (d). The EDS test results given in Fig. 6 (e) ~ (f) imply that the amount of silver distributed in the matrix is more significant than the amount in the graphite. In addition to the precipitated phase in the
3.3. Interfacial characterization of silver-modified Cu-Ni-graphite composite Fig. 8 (b) express the interface of the silver-modified Cu-Ni-graphite composite, The black zone on the left is the graphite phase, the gray zone on the right is the matrix (α-Cu), and the white district is the silver phase. In Fig. 8 (b), EDS tests at three different regions were conducted to determine the element distribution and atom diffusion at the inter face. In terms of EDS line 1, the silver precipitated phase was located in the interface and joints to the graphite and matrix. From the results of 5
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Fig. 5. Silver-modified Cu-Ni-graphite composite(I). (a) 2 wt.% Top surface. (b) 2 wt.% Cross-section. (c) 4 wt.% Top surface. (d) 4 wt.%Cross-section. (e) 6 wt.% Top surface. (f) 6 wt% Cross-section.
Fig. 6. Silver-modified Cu-Ni-graphite composite (II). (a) High magnification morphology. (b) Silver nanoparticle. (c) Silver precipitated phase. (d) Silver precip itated phase at the interface. (e) EDS results of the silver element. (f) Carbon element. (g) Copper element. (h) Nickel element.
EDS line 1 in Fig. 8 (c), mutual diffusion with a width of 1.32 μm was generated at the interface. The diffusion layer of copper was approxi mately 1.35 μm as well as the carbon was approximately 1.06 μm. On
part of EDS line 2, there was no silver phase at the interface as shown in Fig. 8 (d). Only a small amount of mutual diffusion was generated at the interface. The width of mutual diffusion was the only 0.66 μm, and the 6
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Materials Chemistry and Physics 239 (2020) 121990
Fig. 7. TEM morphology. (a) Silver nanowires/nanoparticles in the graphite. (b) Silver nanoparticles. (c) Silver nanowires. (d) Silver precipitated phase.
Fig. 8. Interface microstructure. (a) XRD pattern. (b) Interface morphology. (c) EDS line 1. (d) EDS line 2. (e) EDS line 3. (f) EDS of unmodified Cu-Nigraphite composite.
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diffusion layer of copper and carbon was approximately 0.92 μm and 1.04 μm, respectively. For EDS line 3, there was a silver nanoparticle in the graphite near the interface. The result in Fig. 8 (e) shows that the mutual diffusion in line 3 is the same as that of line 2, and the silver nanoparticle near the interface could hardly affect the diffusion of the graphite and the matrix. For comparison, the EDS result of the interface of the graphite/Cu composites without coating is shown in Fig. 8 (f), and the diffusion layer of carbon was only approximately 1.856 μm, and copper could hardly generate diffusion because the mutual solubility of Cu and C is low in combination with the absence of an intermediate compound, resulting in weak interface mechanical bonding between the matrix and the graphite. The TEM results shown in Fig. 9 further demonstrate the micro structure of the interface of the graphite and matrix. It is noticed that the graphite phase and the matrix are joined by an intermediate phase in Fig. 9 (a) according to the lattice structure of the high magnification image in Fig. 9 (b). The results show plenty of silver nanowires existed at the interface of the composite and play a significant role in the suture interface. It is further supported by the fact that these silver nanowires were generated from graphite and grew towards the matrix. During the sintering process, the silver atoms diffused from the silver coating to the matrix and the graphite. The silver atoms generated a precipitated phase in the matrix and the silver nanowires and nanoparticles formed in the graphite. The silver did not react with carbon and had no solid solubility in the graphite. In this case, the atoms diffused and aggregated on the surface of the graphite. The silver had the preferred orientation growth at the (111) plane and was elongated unidirectionally. The elongated silver atoms, which were incompatible with graphite, crystallized and grew to the silver nanowires. Therefore, the silver coating transformed to the many silver nanowires on the surface of the graphite through sintering. As surgical sutures, these silver nanowires formed metallurgical bonding between the graphite and the matrix. Therefore, they enhanced the interfacial bonding and transferred the mechanical bonding to metallurgical bonding. In addition, it is worth noting that a Cu-Ag solid solution (another kind of α-Cu) emerged at the interface. This Cu-Ag phase formed by the mutual diffusion of silver and copper in the fron tier of the matrix. Consequently, silver nanowires and Cu-Ag phase at the interface reinforced the bonding of graphite and the matrix and increased the interfacial strength. In Fig. 9 (c), a Cu-Ni solid solution (α-Cu) was observed mixing with the silver nanowires in the frontier of
the graphite phase. The Cu-Ni solid solution dispersed with the silver nanowires also enhanced the bonding strength at the interface. Never theless, there were still some pores at the interface found in Fig. 9 (d). 3.4. Mechanical properties of silver-modified Cu-Ni-graphite composite The hardness values of the silver-modified Cu-Ni-graphite composite with different graphite contents are illustrated in Fig. 10 (a). It can be concluded that the hardness of the Cu-Ni-graphite composite increased by 49.9% through the silver modification process. This result can be explained by the generation of a silver precipitate phase and the strength of interfacial bonding. The relative density of the Cu-Ni-graphite com posite was enhanced by 2.1% by utilizing the silver modification process in Fig. 10 (b). This result implies that the number of pores at the inter face decreases dramatically through the joint operation of the silver nanowire and the α-Cu phase. As shown in Fig. 10 (c), the flexural strength increased by approxi mately 187.8%. This phenomenon can be attributed to the improvement of the interface and the precipitation strengthening in the matrix. From Fig. 10 (d), it can be inferred that dimples exist in the matrix an no spelling pits from graphite emerging were founded. Fig. 10 (e) illustrates that the fracture of the graphite is a transgranular fracture. Moreover, it can be seen that plenty of silver nanoparticles existed in the graphite, which can resist the rupture efficiently. When the cracks spread along with the interface shown in Fig. 10 (f), some regions didn’t generate fracture at the interface for efficient bonding. The crack is deflected at 45� and propagated at another crack resources. It can be summarized that the work of rupture at the interface increases through the strength of interfacial bonding. In our previous study, the fracture mechanism of the Cu-Ni-graphite composite without modification was dimples and an intergranular fracture. Consequently, the matrix, graphite and interface are strengthened by the silver element with different phases. Further more, the flexural strength is significantly increased, and the fracture mechanism is changed from an intergranular to a transgranular fracture. 4. Discussion 4.1. Modification process by silver According to the previous results, the modification process of the CuNi-graphite composite by silver can be concluded as illustrated in
Fig. 9. Interface. TEM morphology. (a) Low magnification. (b) High magnification. (c) Silver nanowire. (d) Pores at the interface. 8
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Materials Chemistry and Physics 239 (2020) 121990
Fig. 10. Mechanical properties (a) Vickers hardness. (b) Relative porosity. (c) Flexural strength. (d) Fracture morphology. (e) Fracture of the graphite. (f) Fracture of the interface.
Fig. 11. The entire process can be divided into four steps. As shown in Fig. 11 (a), in the initial stage, the graphite is plated by the silver coating before sintering. Then, after the ball milling and compacting process, the silver coated graphite copper powder, and nickel powder are sintered at 1123 K in Fig. 11 (b). During the sintering process, the silver atoms diffuse to the graphite and the copper powder separately. The silver atoms dissolve into the copper crystal lattice and form the solid solution in the matrix. At the same time, silver atoms diffuse and crystallize as nanowires in the graphite. With the further sintering process, silver atoms saturate and gradually dissolve out of the matrix. As a result, the silver precipitate phase is generated in the matrix. With further diffusion in the graphite, more silver atoms crystallize and grow to silver nano wires. These increasing nanowires form wet nanostructure in the graphite, and the initial nanowires grow to silver nanoparticles, which are shown in Fig. 11 (c). With the development of the sintering process, in the final stage illustrated Fig. 11 (d), plenty of silver nanowires diffuse from the graphite to the interface and join to the interface of the graphite and the matrix. The copper and nickel atoms also diffuse along the silver nanowires and form Cu-Ni and Cu-Ag solid solutions (α-Cu phase), which disperse in the frontier of the graphite and the matrix.
main strengthening mechanisms for the matrix of the silver-modified Cu-Ni-graphite composite. Silver atoms diffuse into the matrix to form the Cu-Ni and Cu-Ag solid solutions (α-Cu phase), and therefore, solid solution strengthening can strengthen the matrix. In addition, due to the low solid solubility, the silver precipitate phase is largely generated in the matrix and the interface. According to the Orowan mechanism, the dispersed precipitate phase can effectively hinder dislocations of the α-Cu phase and strengthen the matrix by second-phase reinforcement. In this study, the amount of sliver atoms forming the precipitate phase is much greater than the amount of silver diffusing into the matrix. Therefore, the effect of precipitation strengthening is greater than that of solid solution strengthening. The graphite phase is also strengthened by the formation of silver nanowires and nanoparticles. The flake graphite powder shows a low strength and brittle properties, whereas both ductility and toughness are increased by the surface modification of the silver coating. The facture of the graphite shows a transgranular fracture instead of spalling off. The silver nanowires and the α-Cu phase jointing the interface can efficiently strengthen the interface bonding and largely improve the mechanical properties. Metallurgical bonding is formed at the interface through the suture of the silver nanowires and the diffused α-Cu phase. Therefore, silver can strengthen the matrix, graphite and interface by the different forms. As a conclusion, the strengthening of the modified Cu-Ni-graphite
4.2. Strengthening mechanism by silver Solid solution strengthening and precipitation strengthening are the 9
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Fig. 11. Schematic of modification mechanism (a) Before sintering, (b) Initial sintering (c) Deeply sintering (d) After sintering.
composites is attributed to the solid solution strengthening and precip itation strengthening for the matrix as well as the graphite and interface strengthening by the silver nanowires and nanoparticles. The use of silver to modify metal matrix composites holds great promise. In future experiments, the tribological properties of silver-modified Cu-Nigraphite composites will be assessed under the conditions of SSBs.
Project (GUIKEAA18242001); the Fundamental Research Funds for the Central Universities of China. We thank Mr. Zijun Ren, Mrs. Jiao Li, Mrs. Jiamei Liu and Mrs. Xiaoqing Wu at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with SEM, TEM, and XRD analyses. References
5. Conclusion
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1) Flake graphite is used to silver-coated graphite by Tollens’ re agent. The silver coating is composed of a pure silver phase with a rough and granular surface. The thickness of the layer deposited on the surface of the graphite is approximately 1.52 μm, and the silver coating made of nanoparticles exhibits densification and small distinct pores. 2) The silver-modified Cu-Ni-graphite composite is prepared by powder metallurgy. The composite consists of graphite, an α-Cu phase and a silver phase. The silver phase in the graphite exists as silver nano wires and nanoparticles, and a silver precipitate phase is generated in the matrix. 3) At the interface of the silver-modified Cu-Ni-graphite composite, silver nanowires and an α-Cu phase is generated in the frontier of the matrix and the graphite. These silver nanowires and the α-Cu phase that joint the interface can efficiently enhance the interface strength and dramatically improve the mechanical properties. 4) Through the silver modification process of the composite, the hard ness increases by approximately 49.9%, and the relative density is increased by approximately 2.1%. Additionally, the flexural strength increases by approximately 187.8% compared with the composites without modification. Acknowledgment This work was supported by the National Natural Science Foundation of China (51805408); the Science and Technology Project of Guangzhou City in China (201604046009); the Natural Science Foundation of Shaanxi Province of China (2018JM5002); the Science and Technology Project of Guangdong Province in China (2015B010122003, 2015B090926009); the Guangxi Innovation-Driven Development 10
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[17] Y. Tang, X. Yang, R. Wang, et al., Enhancement of the mechanical properties of graphene–copper composites with graphene-nickel hybrids, Mater. Sci. Eng. A 599 (2014) 247–254. [18] M. Gui, S.B. Kang, Aluminum hybrid composite coatings containing SiC and graphite particles by plasma spraying, Mater. Lett. 51 (2001) 396–401. [19] W. Yang, Y. Fu, A. Xia, et al., Microwave absorption property of Ni-Co-Fe-P-coated flake graphite prepared by electroless plating, J. Alloy. Comp. 518 (2012) 6–10. [20] S.W. Ip, R. Sridhar, J.M. Toguri, et al., Wettability of nickel-coated graphite by aluminum, Mater. Sci. Eng. A 244 (1998) 31–38. [21] H.Y. Chu, J.F. Lin, Experimental analysis of the tribological behavior of electroless nickel-coated graphite particles in aluminum matrix composites under reciprocating motion, Wear 239 (2000) 126–142. [22] M.L.T. Guo, C.Y.A. Tsao, Tribological behavior of aluminum/SiC/nickel-coated graphite hybrid composites, Mater. Sci. Eng. A 333 (2002) 134–145. [23] F. Li, Y.F. Yan, G.L. Yu, et al., The tribological behaviors of copper-coated graphite filled PTFE composites, Wear 237 (2000) 33–38. [24] Y. Yin, C. Du, Z. Zheng, et al., Effect of graphite coated with nickel on mechanical properties of copper-matrix/graphite composite materials, Chin. J. Nonferrous Metals 16 (2006) 1895. [25] S.D. Yas, I.V. Pavlenko, A.A. Zaporozhets, et al., Copper-graphite materials with copper-clad graphite granule additions, Sov. Powder Metall. Met. Ceram. 12 (1973) 280–283.
[26] I.V. Pavlenko, S.D. Yas, The copper plating of graphite powders in the manufacture of copper-graphite materials, Sov. Powder Metall. Met. Ceram. 15 (1976) 89–92. [27] Y. Zhan, G. Zhang, Y. Wu, Effect of surface metallization of graphite on the tribological properties of copper hybrid composites, Scand. J. Metall. 33 (2004) 80–84. [28] S.F. Moustafa, S.A. El-Badry, A.M. Sanad, Effect of graphite with and without copper coating on consolidation behaviour and sintering of copper-graphite composite, Powder Metall. 40 (1997) 201–206. [29] S.F. Moustafa, S.A. El-Badry, A.M. Sanad, et al., Friction and wear of coppergraphite composites made with Cu-coated and uncoated graphite powders, Wear 253 (2002) 699–710. [30] E. Rabinowicz, Influence of surface energy on friction and wear phenomena, J. Appl. Phys. 32 (1961) 1440–1444. [31] S. Chang, J. Lin, S. Lin, et al., Processing copper and silver matrix composites by electroless plating and hot pressing, Metall. Mater. Trans. A 30 (1999) 1119–1136. [32] S. Carroll, S. Pratontep, M. Streun, et al., Pinning of size-selected Ag clusters on graphite surfaces, J. Chem. Phys. 113 (2000) 7723–7727. [33] Y. Wang, G. Yimin, T. Jun, et al., Titanium-modified graphite reinforced Cu-Ni composite by multi-arc ion plating technology, Vacuum (2019), https://doi.org/ 10.1016/j.vacuum.2019.108829.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Investigation of modification of Cu-Ni-graphite composite by silver Yiran Wang a, b, *, Yimin Gao a, Jun Takahashi b, Yi Wan b, Mengting Li a, Bing Xiao b, Yunqian Zhang b, Xiangdong He b a b
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, PR China Department of Systems Innovation, School of Engineering, The University of Tokyo, Tokyo, Japan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Silver phase in graphite exists as silver nanowires and nanoparticles, and as silver precipitate phase in the matrix. � Silver nanowires and α-Cu phase are generated at the interface. � Silver efficiently improve the mechani cal properties.
A R T I C L E I N F O
A B S T R A C T
Keywords: Silver Surface treatment Interfacial modification Composite
Surface treatment of graphite by silver is applied to modify microstructure and mechanical properties of Cu-Nigraphite composites. In this study, flake graphite is used to create silver-coated graphite by Tollens’ reagent, and the silver-modified Cu-Ni-graphite composite is prepared by powder metallurgy. The microstructure character ization and modification mechanism of the composite are the emphases of this investigation. The results showed that the silver coating is composed of a pure silver phase with a rough and granular surface and the thickness of the layer is about 1.52 μm. The composite consists of graphite, an α-Cu phase and a silver phase. The silver phase in graphite exists as silver nanowires and nanoparticles, and a silver precipitate phase is generated in the matrix. At the interface of the composite, silver nanowires and α-Cu phase is generated in the frontier of the matrix and the graphite. These silver nanowires and the α-Cu phase that joint the interface can efficiently enhance the interface strength and dramatically improves mechanical properties. Through the silver modification process of the composite, hardness increases by approximately 49.9%, the improvement of relative density is increased by approximately 2.1%. Additionally, the flexural strength increases by approximately 187.8% compared with the composites without modification.
1. Introduction In China, high-speed railways are in the age of hypered development.
Switch slide baseplates (SSBs), one of the most significant railway components, urgently require new materials to increase the efficiency of railway capacity [1]. To date, Q235 steel, a type of low carbon steel,
* Corresponding author. State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, PR China. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.matchemphys.2019.121990 Received 26 April 2019; Received in revised form 21 July 2019; Accepted 7 August 2019 Available online 13 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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coated with chromium has been used for SSBs, and grease lubrication has typically been used to decrease the friction coefficient of the SSBs [2]. In outdoor operation conditions, rain fall, or contamination from sources such as ash and sand grit could easily make the lubrication decay. Therefore, self-lubricating materials are of interest to many re searchers. In recent years, many studies have focused on the use of self-lubricating materials for SSBs to overcome the shortcomings of current SSB materials [3–5]. Solid self-lubricating materials have a great potential to be considered for use in SSBs. Copper matrix composites have the advantages of excellent anticorrosion in the outdoors and unique tribology properties when sliding against steel compared with other self-lubricating composites. Until now, copper matrix composites have been widely used in bearings and others fields [6–11]. In our previous study [12–16], the tribology and mechanical properties of a Cu-Ni-graphite composite were tested under SSB operation conditions. The results show that due to the weak bonding strength at the interface of the graphite reinforced copper matrix composites, graphite as a lubricating phase severely reduces the mechanical properties in the composites. Due to debonding and pulling out of the composites, the graphite loses its capacity under service conditions. Ni plays a significant role in solution strengthening with the copper matrix in the Cu-Ni-graphite composite and enhance the corro sion properties in outdoor operation conditions. However, the Ni element in the matrix cannot effectively improve the weak interface bonding. Surface modification of graphite is one of the best ways to improve the metal and graphite interface in metal matrix composites. Many studies have confirmed that metal coated graphite is the most effective application for surface modification [17–23,33]. The electroless plating technology for copper and nickel metals has been used for many years, and therefore, many studies on copper- or nickel-coated graphite rein forced copper matrix composites have been published [24–29]. In this case, the modifying the Cu-Ni-graphite composites with nickel [14] and copper [16] has been attempted to increase both the mechanical and tribological properties. These metals enhance the mutual diffusion of copper and carbon atoms and increase the interface strength accord ingly. The mechanical properties of Cu-Ni-graphite composites are effectively improved; however, the tribological properties show limited increases. To achieve more improved properties, new metals are tested for Cu-Ni-graphite composites. In addition to nickel and copper, other functional metals have been considered to modify the surface of graphite. Silver, which is a soft metal, can play an all-important function as not simply a reinforced phase but also a lubrication phase. Compared with nickel and copper, the silver phase can decrease the friction coefficient when sliding against steel [30]. Up to now, few studies have focused on silver-coated graphite modified copper matrix graphite composites, and few researchers have studied the modification mechanism of silver in composites [31,32]. Therefore, it is worth studying the modification of Cu-Ni-graphite composites by silver and the associated strength mechanism. This study investigates the way in which a silver covering on graphite for modifies the microstructure and mechanical properties of Cu-Nigraphite composites. Silver-coated graphite was prepared with Toll ens’ reagent, and then a silver-modified Cu-Ni-graphite composite was constructed from silver-coated graphite, copper and nickel powders by powder metallurgy. The microstructure characterizations of the silvercoated graphite and silver-modified Cu-Ni-graphite composite were studied. Then, the interfacial morphologies and modification mecha nisms of the silver-modified Cu-Ni-graphite composite were investi gated. The mechanical properties of the silver-modified Cu-Ni-graphite composite with different contents were also discussed.
2. Experiments 2.1. Materials and preparation Flake graphite with irregular shapes was selected as a raw material. The microstructure of the flake graphite, which is shown in Fig. 1 (a) & (b), reveals a mean size of 45 μm and a thickness of 5 μm. The crosssection morphology is given in Fig. 1 (c), and it can be seen that flake graphite has a lamellar structure inside the particle. The XRD pattern in Fig. 1 (d) also shows a close-packed hexagonal structure and a layer substructure in the (002) crystal plane. In this study, silver-coated graphite was prepared with Tollens’ re agent. The specific constituents of Tollens’ reagent are reported in Table 1, and the principle of Tollens’ reaction can be explained by equations (1)–(3). AgNO3þ NH3⋅H2O →AgOH ↓ þNH4NO3
(1)
AgOH þ NH3⋅H2O→Ag(NH3)2 OH þ 2H2O
(2)
4Ag[(NH3)2]OH þ HCHO →4Ag þ 2H2O þ 6NH3 þ (NH4)2CO3
(3)
The preparation procedures can be divided into five steps, and the schematic diagram is illustrated in Fig. 2. First, flake graphite was pre cleaned for 30 min. The precleaning was conducted with a 20% NaOH aqueous solution to wash off the impurities attached to the surface of the graphite. Then, graphite was sensitized in a 15 ml/L HCl & 10 g/L SnCl2 aqueous solution. The sensitization procedure generated an Sn(OH)Cl film on the graphite. The activation procedure was conducted in a 15 ml/L HCl & 0.3 g/L PbCl2 aqueous solution to deposit a (Pb, Sn2þ) film on the surface of the graphite. Finally, a reduction procedure was carried out in a 40 g/L NaH2PO2⋅H2O aqueous solution. In the reduction, the activated Pb coating was attached to the surface of the flake graphite. Through the above procedures, the silver could be deposited more easily because graphite has an inactive surface onto which metal can hardly be deposited. The precleaning, sensitization, activation and reduction procedures could activate the surface of the graphite by attaching active ions (Pb). The last procedure was chemical plating, and the flake graphite with activated Pb was added into a Tollens’ reagent for plating. The graphite powder was plated at 323 K in an aqueous solution consisting of 36 g/L of AgNO3, 36 g/L of NH3⋅H2O, 24 ml/L of HCHO, and 95 ml/L of CH₃CH₂OH. The activated Pb surface facilitated deposition of the silver ion coating. After 1 h of the chemical plating process, the graphite was successfully coated with silver. The silver-modified Cu-Ni-graphite composite was prepared from silver-coated graphite, copper and nickel powders. Information about the raw materials and the composition of the silver-modified Cu-Nigraphite composites with different graphite contents is listed in Table 2. In the experiment, the 36 g/L of AgNO3 in the Tollens’ reagent and the 1-L aqueous solution reacted to yield approximately 17.8 g of silver. Moreover, the 1-L aqueous solution could plate 20 g of graphite, and the plating efficiency was approximately 30%. Therefore, 20 g of graphite could produce 5.34 g of silver, and the mass fraction of the silver in the silver-coated graphite was 21.1 wt.%. As a conclusion, the silver con tents introduced in the Cu-Ni-graphite composites with graphite con tents of 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%, 5 wt.% and 6 wt.% were approximately 0.2111wt.%, 0.42 wt.%, 0.63 wt.%, 0.84 wt.%, 1.06 wt. % and 1.27 wt.%, respectively. The silver modified Cu-Ni-graphite composite was prepared by a powder metallurgy process. The process consisted of ball milling, com pacting, sintering, recompacting and resintering. The specific prepara tion parameters are reported in Table 3. Finally, specimens with diameter and thickness of 44 mm and 6 mm, respectively, were prepared.
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Fig. 1. Raw flack graphite. (a) Low magnification morphology. (b) High magnification. (c) Cross-section morphology. (d) XRD pattern.
spectrum. An X-ray diffraction detector (XRD) scanned the samples with an the angular 2θ range from 20� to 90� and a step size of 0.02. Highresolution transmission electron microscopy (TEM) equipped with a top-entry goniometer which has a point-to-point resolution of 0.17 nm at 400 kV, was used. Thin foil specimens were prepared by punching 3 mm discs from wafers cut from large blocks and, mechanically grinding by hand to 50 μm. The specimens were twin-jet electropolished to electron transparency at 50 V in a mixture of 5% perchloric acid, 20% glycerol and 75% methanol, with the solution being cooled to 253 K with liquid nitrogen, following the standard techniques of thin foil used for TEM. The powder specimens were directly observed for TEM by
Table 1 Composition of Tollens’ reagent. Solution
AgNO3
NH3⋅H2O
HCHO
CH₃CH₂OH
Content
36 g/L
36 g/L
24 ml/L
95 ml/L
2.2. Microstructure characterization and mechanical properties The microstructure characterizations of the silver-coated graphite and the silver-modified Cu-Ni-graphite composites were investigated by a scanning electronic microscope (SEM), including the energy dispersive
Fig. 2. Schematic of preparation of silver-coated graphite. 3
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3. Results
Table 2 Composition of silver modified Cu-Ni-graphite composite. Element
Content/wt.%
Silver-coated graphite Ni Cu
1–6 8 Bal.
3.1. Microstructure characterization of silver-coated graphite Fig. 3 exhibits the morphologies of the silver-coated graphite with rough and granular surfaces. EDS tests were also conducted to examine the composition of the silver-coated graphite in Fig. 3 (a). The results of the EDS test are given in Table 4, and it can be observed that the coating is composed of the purest silver. The composition of the entire silver coating is uniform, and no other impurities could be detected in the coating. As shown in Fig. 3 (b), the coating is composed of silver nanoparticles with an intermediate size of 200 nm in the high magni fication range. The compact silver coating has a low porosity and is organized by the deposition process of silver atoms during chemical plating. From the TEM morphologies of the silver coating in Fig. 3 (c) & (d), it can been seen that the silver atoms deposited on the surface of the graphite rapidly crystallized, and afterward, the silver atoms grew at the preferred orientation of (200). In agreement with the deposition process, the silver nanoparticles were generated and bound to the thick silver coating. The XRD results given in Fig. 4 (a) also support the phases of silver coated graphite. It can be inferred that silver-coated graphite consists of a silver phase and a graphite phase. Moreover, no reaction was gener ated between the silver and the carbon, and no new phase was formed. Base on the cross-section microstructure shown in Fig. 4 (b), it can be easily summarized that the thickness of the coating is approximately
Table 3 Preparation parameters of silver modified Cu-Ni-graphite composite. Step
Process
Parameter
1 2 3 4 5
Ball-milling Compacting Sintering Re-compacting Re-sintering
15 h 600 MPa 1123 K þ 1 h 600 MPa 1073 K þ 0.5 h
being loaded in a copper grid. The Archimedes’ principle was measured to determine the relative density of silver modified Cu-Ni-graphite composites with different graphite contents. Hardness experiments were tested by Vickers hard ness meters by subjecting samples to 5 N loads for 15 s of dwell time. The flexural strength was examined by conducting three-point tests on a universal testing machine with specimens that had been cut into 30 mm � 6 mm � 6 mm bars. The results were calculated according to the following equation (4)
σf ¼
3PL 2bd2
(4)
Table 4 EDS results of silver coated graphite.
where P is the maximum load at fracture, L is the span length, b is the width of the samples, and d is the thickness of the samples. Thus, the microstructure of the fractures was observed by SEM.
Point 1 2 3 4
C
Ag
wt.%
at.%
wt.%
at.%
13.6 9.9 11.4 7.8
54.0 52.7 53.1 52.4
86.4 90.1 88.6 92.2
46.0 47.3 46.9 47.6
Fig. 3. Silver-coated graphite (I). (a) Low magnification morphology. (b) High magnification. (c) TEM morphology. (d) High-resolution TEM at (111) plane. 4
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Fig. 4. Silver-coated graphite (II). (a) XRD pattern. (b) Cross-section morphology. (c) Interface morphology. (d) Interface EDS results.
1.52 μm, and silver nanoparticles were deposited onto the surface of the graphite, exhibiting densification. From the EDS test shown in Fig. 4 (c) & (d), it can be found that the diffused intensity of silver is reduced as the diffused distance increases, and the distribution of silver in the inner graphite is relatively weak. Additionally, the carbon element diffused into the silver coating, and its diffusion intensity decreases as the diffused distance increases.
matrix, plenty of silver atoms were dispersed and dissolved in the ma trix. Additionally, copper and nickel element have no visible presence in the graphite zone, and a minimal amount of carbon is found in the matrix. Therefore, the silver coating on the graphite restrained the mutual diffusion of the matrix and graphite during the sintering process. As shown in Fig. 7, the microstructure characterization of the silvermodified Cu-Ni-graphite composite was further analyzed by TEM research. It can be deduced that the silver atoms formed not only silver nanoparticles but also silver nanowires in the graphite in Fig. 7 (a) & (c). These silver nanowires are as a kind of substructure that exists in the graphite phase. From the FFT image in Fig. 7 (b), the interplanar spacing of silver nanoparticle was d(111) ¼ 0.2418 nm, which is a slightly larger than the average value (d(111) ¼ 0.2359 nm). It can be asserted that the carbon atoms dissolved into the silver crystal lattice and resulted in lattice distortion. Furthermore, through observing the nanostructure in the matrix in Fig. 7 (d), it is indicated that silver atoms dissolved into the matrix (α-Cu) and made the interplanar spacing of α-Cu increase from d(111) ¼ 0.2086 nm to d(111) ¼ 0.2218 nm. Due to the extremely low solid solubility in the α-Cu phase, silver atoms finally dissolved out of the matrix and generate the precipitated phase. From Fig. 8 (a), the XRD pattern results of the silver-modified Cu-Ni-graphite composite also prove that the composite consisted of three phases: α-Cu, graphite and the silver phase.
3.2. Microstructure of silver-modified Cu-Ni-graphite composite Fig. 5 expresses the microstructure of the silver-modified Cu-Nigraphite composite. The black phase is graphite, and the gray zone is α-Cu as the matrix (Cu-Ni solid solution). Because it is affected by the compacting process, the graphite has two different morphologies in the composite. In Fig. 5 (a), (c), and (e), the graphite exhibited an irregular shape in the microstructure perpendicular to the compacting direction, whereas the graphite expressed a linear shape in the microstructure parallel to the compacting direction shown in Fig. 5 (b), (d), and (f). As the graphite content increases, the volume fraction of the graphite distributed in the composite also increases. According to our previous work, the graphite showed a continuous net distribution when the graphite content reached 6 wt.% in the unmodified Cu-Ni-graphite composite. However, the graphite shows a dispersed and uniform dis tribution in the composite utilizing the silver modification process. Further analysis of the microstructure of the silver-modified Cu-Nigraphite composite at high magnification and with the EDS test is shown in Fig. 6. From the high magnification microstructure in Fig. 6 (a), it can be seen that the silver phase existed and dispersed in both the matrix and the graphite. It is notable that the silver element was generated as silver nanoparticles in the graphite phase shown in Fig. 6 (b) and formed the precipitated phase in both the matrix and the interface expressed in Fig. 6 (c) & (d). The EDS test results given in Fig. 6 (e) ~ (f) imply that the amount of silver distributed in the matrix is more significant than the amount in the graphite. In addition to the precipitated phase in the
3.3. Interfacial characterization of silver-modified Cu-Ni-graphite composite Fig. 8 (b) express the interface of the silver-modified Cu-Ni-graphite composite, The black zone on the left is the graphite phase, the gray zone on the right is the matrix (α-Cu), and the white district is the silver phase. In Fig. 8 (b), EDS tests at three different regions were conducted to determine the element distribution and atom diffusion at the inter face. In terms of EDS line 1, the silver precipitated phase was located in the interface and joints to the graphite and matrix. From the results of 5
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Fig. 5. Silver-modified Cu-Ni-graphite composite(I). (a) 2 wt.% Top surface. (b) 2 wt.% Cross-section. (c) 4 wt.% Top surface. (d) 4 wt.%Cross-section. (e) 6 wt.% Top surface. (f) 6 wt% Cross-section.
Fig. 6. Silver-modified Cu-Ni-graphite composite (II). (a) High magnification morphology. (b) Silver nanoparticle. (c) Silver precipitated phase. (d) Silver precip itated phase at the interface. (e) EDS results of the silver element. (f) Carbon element. (g) Copper element. (h) Nickel element.
EDS line 1 in Fig. 8 (c), mutual diffusion with a width of 1.32 μm was generated at the interface. The diffusion layer of copper was approxi mately 1.35 μm as well as the carbon was approximately 1.06 μm. On
part of EDS line 2, there was no silver phase at the interface as shown in Fig. 8 (d). Only a small amount of mutual diffusion was generated at the interface. The width of mutual diffusion was the only 0.66 μm, and the 6
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Fig. 7. TEM morphology. (a) Silver nanowires/nanoparticles in the graphite. (b) Silver nanoparticles. (c) Silver nanowires. (d) Silver precipitated phase.
Fig. 8. Interface microstructure. (a) XRD pattern. (b) Interface morphology. (c) EDS line 1. (d) EDS line 2. (e) EDS line 3. (f) EDS of unmodified Cu-Nigraphite composite.
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diffusion layer of copper and carbon was approximately 0.92 μm and 1.04 μm, respectively. For EDS line 3, there was a silver nanoparticle in the graphite near the interface. The result in Fig. 8 (e) shows that the mutual diffusion in line 3 is the same as that of line 2, and the silver nanoparticle near the interface could hardly affect the diffusion of the graphite and the matrix. For comparison, the EDS result of the interface of the graphite/Cu composites without coating is shown in Fig. 8 (f), and the diffusion layer of carbon was only approximately 1.856 μm, and copper could hardly generate diffusion because the mutual solubility of Cu and C is low in combination with the absence of an intermediate compound, resulting in weak interface mechanical bonding between the matrix and the graphite. The TEM results shown in Fig. 9 further demonstrate the micro structure of the interface of the graphite and matrix. It is noticed that the graphite phase and the matrix are joined by an intermediate phase in Fig. 9 (a) according to the lattice structure of the high magnification image in Fig. 9 (b). The results show plenty of silver nanowires existed at the interface of the composite and play a significant role in the suture interface. It is further supported by the fact that these silver nanowires were generated from graphite and grew towards the matrix. During the sintering process, the silver atoms diffused from the silver coating to the matrix and the graphite. The silver atoms generated a precipitated phase in the matrix and the silver nanowires and nanoparticles formed in the graphite. The silver did not react with carbon and had no solid solubility in the graphite. In this case, the atoms diffused and aggregated on the surface of the graphite. The silver had the preferred orientation growth at the (111) plane and was elongated unidirectionally. The elongated silver atoms, which were incompatible with graphite, crystallized and grew to the silver nanowires. Therefore, the silver coating transformed to the many silver nanowires on the surface of the graphite through sintering. As surgical sutures, these silver nanowires formed metallurgical bonding between the graphite and the matrix. Therefore, they enhanced the interfacial bonding and transferred the mechanical bonding to metallurgical bonding. In addition, it is worth noting that a Cu-Ag solid solution (another kind of α-Cu) emerged at the interface. This Cu-Ag phase formed by the mutual diffusion of silver and copper in the fron tier of the matrix. Consequently, silver nanowires and Cu-Ag phase at the interface reinforced the bonding of graphite and the matrix and increased the interfacial strength. In Fig. 9 (c), a Cu-Ni solid solution (α-Cu) was observed mixing with the silver nanowires in the frontier of
the graphite phase. The Cu-Ni solid solution dispersed with the silver nanowires also enhanced the bonding strength at the interface. Never theless, there were still some pores at the interface found in Fig. 9 (d). 3.4. Mechanical properties of silver-modified Cu-Ni-graphite composite The hardness values of the silver-modified Cu-Ni-graphite composite with different graphite contents are illustrated in Fig. 10 (a). It can be concluded that the hardness of the Cu-Ni-graphite composite increased by 49.9% through the silver modification process. This result can be explained by the generation of a silver precipitate phase and the strength of interfacial bonding. The relative density of the Cu-Ni-graphite com posite was enhanced by 2.1% by utilizing the silver modification process in Fig. 10 (b). This result implies that the number of pores at the inter face decreases dramatically through the joint operation of the silver nanowire and the α-Cu phase. As shown in Fig. 10 (c), the flexural strength increased by approxi mately 187.8%. This phenomenon can be attributed to the improvement of the interface and the precipitation strengthening in the matrix. From Fig. 10 (d), it can be inferred that dimples exist in the matrix an no spelling pits from graphite emerging were founded. Fig. 10 (e) illustrates that the fracture of the graphite is a transgranular fracture. Moreover, it can be seen that plenty of silver nanoparticles existed in the graphite, which can resist the rupture efficiently. When the cracks spread along with the interface shown in Fig. 10 (f), some regions didn’t generate fracture at the interface for efficient bonding. The crack is deflected at 45� and propagated at another crack resources. It can be summarized that the work of rupture at the interface increases through the strength of interfacial bonding. In our previous study, the fracture mechanism of the Cu-Ni-graphite composite without modification was dimples and an intergranular fracture. Consequently, the matrix, graphite and interface are strengthened by the silver element with different phases. Further more, the flexural strength is significantly increased, and the fracture mechanism is changed from an intergranular to a transgranular fracture. 4. Discussion 4.1. Modification process by silver According to the previous results, the modification process of the CuNi-graphite composite by silver can be concluded as illustrated in
Fig. 9. Interface. TEM morphology. (a) Low magnification. (b) High magnification. (c) Silver nanowire. (d) Pores at the interface. 8
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Fig. 10. Mechanical properties (a) Vickers hardness. (b) Relative porosity. (c) Flexural strength. (d) Fracture morphology. (e) Fracture of the graphite. (f) Fracture of the interface.
Fig. 11. The entire process can be divided into four steps. As shown in Fig. 11 (a), in the initial stage, the graphite is plated by the silver coating before sintering. Then, after the ball milling and compacting process, the silver coated graphite copper powder, and nickel powder are sintered at 1123 K in Fig. 11 (b). During the sintering process, the silver atoms diffuse to the graphite and the copper powder separately. The silver atoms dissolve into the copper crystal lattice and form the solid solution in the matrix. At the same time, silver atoms diffuse and crystallize as nanowires in the graphite. With the further sintering process, silver atoms saturate and gradually dissolve out of the matrix. As a result, the silver precipitate phase is generated in the matrix. With further diffusion in the graphite, more silver atoms crystallize and grow to silver nano wires. These increasing nanowires form wet nanostructure in the graphite, and the initial nanowires grow to silver nanoparticles, which are shown in Fig. 11 (c). With the development of the sintering process, in the final stage illustrated Fig. 11 (d), plenty of silver nanowires diffuse from the graphite to the interface and join to the interface of the graphite and the matrix. The copper and nickel atoms also diffuse along the silver nanowires and form Cu-Ni and Cu-Ag solid solutions (α-Cu phase), which disperse in the frontier of the graphite and the matrix.
main strengthening mechanisms for the matrix of the silver-modified Cu-Ni-graphite composite. Silver atoms diffuse into the matrix to form the Cu-Ni and Cu-Ag solid solutions (α-Cu phase), and therefore, solid solution strengthening can strengthen the matrix. In addition, due to the low solid solubility, the silver precipitate phase is largely generated in the matrix and the interface. According to the Orowan mechanism, the dispersed precipitate phase can effectively hinder dislocations of the α-Cu phase and strengthen the matrix by second-phase reinforcement. In this study, the amount of sliver atoms forming the precipitate phase is much greater than the amount of silver diffusing into the matrix. Therefore, the effect of precipitation strengthening is greater than that of solid solution strengthening. The graphite phase is also strengthened by the formation of silver nanowires and nanoparticles. The flake graphite powder shows a low strength and brittle properties, whereas both ductility and toughness are increased by the surface modification of the silver coating. The facture of the graphite shows a transgranular fracture instead of spalling off. The silver nanowires and the α-Cu phase jointing the interface can efficiently strengthen the interface bonding and largely improve the mechanical properties. Metallurgical bonding is formed at the interface through the suture of the silver nanowires and the diffused α-Cu phase. Therefore, silver can strengthen the matrix, graphite and interface by the different forms. As a conclusion, the strengthening of the modified Cu-Ni-graphite
4.2. Strengthening mechanism by silver Solid solution strengthening and precipitation strengthening are the 9
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Fig. 11. Schematic of modification mechanism (a) Before sintering, (b) Initial sintering (c) Deeply sintering (d) After sintering.
composites is attributed to the solid solution strengthening and precip itation strengthening for the matrix as well as the graphite and interface strengthening by the silver nanowires and nanoparticles. The use of silver to modify metal matrix composites holds great promise. In future experiments, the tribological properties of silver-modified Cu-Nigraphite composites will be assessed under the conditions of SSBs.
Project (GUIKEAA18242001); the Fundamental Research Funds for the Central Universities of China. We thank Mr. Zijun Ren, Mrs. Jiao Li, Mrs. Jiamei Liu and Mrs. Xiaoqing Wu at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with SEM, TEM, and XRD analyses. References
5. Conclusion
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1) Flake graphite is used to silver-coated graphite by Tollens’ re agent. The silver coating is composed of a pure silver phase with a rough and granular surface. The thickness of the layer deposited on the surface of the graphite is approximately 1.52 μm, and the silver coating made of nanoparticles exhibits densification and small distinct pores. 2) The silver-modified Cu-Ni-graphite composite is prepared by powder metallurgy. The composite consists of graphite, an α-Cu phase and a silver phase. The silver phase in the graphite exists as silver nano wires and nanoparticles, and a silver precipitate phase is generated in the matrix. 3) At the interface of the silver-modified Cu-Ni-graphite composite, silver nanowires and an α-Cu phase is generated in the frontier of the matrix and the graphite. These silver nanowires and the α-Cu phase that joint the interface can efficiently enhance the interface strength and dramatically improve the mechanical properties. 4) Through the silver modification process of the composite, the hard ness increases by approximately 49.9%, and the relative density is increased by approximately 2.1%. Additionally, the flexural strength increases by approximately 187.8% compared with the composites without modification. Acknowledgment This work was supported by the National Natural Science Foundation of China (51805408); the Science and Technology Project of Guangzhou City in China (201604046009); the Natural Science Foundation of Shaanxi Province of China (2018JM5002); the Science and Technology Project of Guangdong Province in China (2015B010122003, 2015B090926009); the Guangxi Innovation-Driven Development 10
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