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Fabrication and properties of the W-30wt%Cu gradient composite with W@WC core-shell structure
Accepted Manuscript Fabrication and properties of the W-30wt%Cu gradient composite with W@WC coreshell structure Qiao Zhang, Shuhua Liang, Longchao Zhuo PII:
S0925-8388(17)30833-2
DOI:
10.1016/j.jallcom.2017.03.064
Reference:
JALCOM 41102
To appear in:
Journal of Alloys and Compounds
Received Date: 16 December 2016 Revised Date:
3 March 2017
Accepted Date: 6 March 2017
Please cite this article as: Q. Zhang, S. Liang, L. Zhuo, Fabrication and properties of the W-30wt%Cu gradient composite with W@WC core-shell structure, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.064. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication and properties of the W-30wt.%Cu gradient composite with W@WC core-shell structure Qiao Zhanga,b, Shuhua Lianga,b*, Longchao Zhuoa*
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a. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
b. Shaanxi Province Key Laboratory of Electrical Materials and Infiltration
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Technology, Xi’an University of Technology, Xi’an 710048, China
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*Corresponding authors.
E-mail addresses: [email protected] (S. Liang); [email protected] (L. Zhuo) Tel/Fax: +86-29-82312181
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Abstract: In the present work, a novel W-Cu gradient composite with W@WC core-shell structure has been fabricated successfully through vacuum pulse carburization and copper infiltration. The W@WC core-shell structure was introduced
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by vacuum pulse carburization of tungsten skeleton at 950oC. Related microstructure
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and phase analysis revealed that the near surface region of the carburized tungsten skeleton was mainly covered by WC particles, the transition region with distributed WC particles, and the inner central region distant away from the surface without WC particles. The resulted W-Cu composite with W@WC core-shell structure after infiltration showed highly improved arc dispersion effect, smaller friction coefficient, more
stable
wear
resistance
and
enhanced
high-temperature
compressive
performances. In addition, the gradient structure also ensured the electrical 1
ACCEPTED MANUSCRIPT conductivity of the W-Cu composite and maintained connected strength with CuCr alloy as an integral contact. Key words: W@WC core-shell structure; W-Cu gradient composite; Arc erosion
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resistance; Wear resistance; High-temperature compressive performance
1. Introduction
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Tungsten-copper (W-Cu) composite is a typical pseudo-alloy exhibiting mutual
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insolubility or negligible solubility prepared by powder metallurgy process. It has been widely applied as the warhead, nozzle liner in rockets and missiles, high voltage switches, heat sink materials and deviator plates for fusion, benefiting from the good wear resistance and refractory characteristic of tungsten combined with the high
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thermal and electrical conductivity of copper [1-10]. However, the increasing demand for electric power systems requires high voltage switches to withstand ultrahigh voltages and possess larger capabilities or longer service lifetime [11]. This demands
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the W-Cu contact composite, as the core part of high voltage switches, to exhibit
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improved arc erosion resistance, high-temperature strength and wear resistance [12-15]. Nevertheless, traditional commercial W-Cu contact material does not meet the above requirements because its critical application focus lies in the arc erosion resistance, with ignorance of higher high-temperature strength and wear resistance. Therefore, exploiting novel contact material with excellent performances of improved arc erosion resistance, high-temperature strength and wear resistance is rather imperative. 2
ACCEPTED MANUSCRIPT It is well established that the failure of traditional commercial W-Cu contact material in ultrahigh voltage electric power system derives from the excessive erosion of arc; meanwhile, the strength and wear resistance rapidly decrease under the
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high-temperature and high-frequency opening-closing environment [12-15]. On the other hand, the metal-matrix composites reinforced by ceramic particles exhibit high thermal stability and good wear resistance, making them widely used as various
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high-temperature wear resistant components and coatings [16-20]. As tungsten
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carbide exhibits superior high-temperature properties with relatively low cost, it has been therefore attempted to reinforce the traditional W-Cu composite [20, 21]; however, the ceramic particles were easy to agglomerate and the bonding strength with matrix was terribly weak. Another attempt of complete replacement of W by WC
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to fabricate WC-Cu composite has also demonstrated its significantly improved arc erosion resistance and wear resistance; nonetheless, a fatal weakness of the poorly connected strength with CuCr alloy to form an integral contact material, gives rise to
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the possibility of separation of WC-Cu from the integral material.
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Therefore in this work, an idea of designing W-Cu gradient composite materials with W@WC core-shell structure is put forward to solve the problem. By vacuum pulse carburization process, the carbon source was introduced into tungsten skeleton. Depending on the concentration gradient of carbon, the carbonization reaction between tungsten and carbon occurred to form WC particles. The structure of the W-Cu gradient composite with W@WC core-shell structure is schematically presented as shown in Fig. 1. In the surface region (I) of the W-Cu composite, 3
ACCEPTED MANUSCRIPT tungsten powders were totally covered by WC particles, in the transition region (II) with distributed WC particles, while in the central region (III) distant away from the surface without WC particles. Therefore, under high-temperature and high-frequency
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opening-closing service, the W@WC core-shell structure for the outer tungsten skeleton would effectively improve the high-temperature strength and wear resistance; besides, the effect of dispersing arc erosion would be introduced [22, 23]. Even if the
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deposited carbon has not completely reacted with tungsten powders, the residual
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carbon would exhibit strong self-lubricating effect, thereby improving wear resistance of the composite [24, 25]. Additionally, the gradient composite with pure W-Cu in the center (region III as shown in Fig. 1)–could also maintain the connected strength between W-Cu composite and CuCr alloy as an integral contact without losing
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conductivity. With guidance of the above excogitation, the main objective of this study focuses on fabrication of the W-Cu gradient composite with W@WC core-shell structure and evaluation of its hardness, electrical conductivity, arc erosion and wear
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resistance, as well as high-temperature compressive properties.
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2. Experimental materials and methods Tungsten powders (purity ≥ 99.9 wt.%, average particle size of 6~8 µm) and
minor trace addition of nickel powders (purity ≥ 99.9 wt.%, average particle size of 25~30 µm) as the sintering activator were blended in a V-type mixer for 6 h, followed by pressing in a hydraulic machine to form tungsten green bodies (density = 70%). Carbon source is provided by acetylene gas (C2H2: purity ≥ 99.99 wt.%), with nitrogen gas (N2: purity ≥ 99.9 wt.%) as the protective atmosphere. The green bodies 4
ACCEPTED MANUSCRIPT were firstly heated up to 950oC in a vacuum carburizing furnace under inner protection of N2. At the target temperature, the samples were repeatedly conducted the pulse carburizing process for 40 min, and heat preservation for 100 min was taken to
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make an adequate carbonization or carbon diffusion. Followed by furnace cooling, the gradient tungsten skeleton with W@WC core-shell structure was obtained. Finally, through infiltration of an oxygen-free copper plate (purity ≥99.9 wt.%) into the
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gradient tungsten skeleton with W@WC core-shell structure in a sintering furnace at
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1350oC, the novel W-Cu gradient composite with W@WC core-shell structure was fabricated.
Morphologies of the tungsten skeleton with W@WC core-shell structure and cross section of the W-Cu gradient composite were observed on a JEOL-6700F or
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Zeiss-Merlin Scanning Electron Microscope (SEM). The phase composition was investigated using a 7000S X-ray diffraction (XRD) instrument with Cu Kα radiation. The structure was also confirmed on a JEM-3010 Transmission Electron Microscopy
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(TEM). Hardness and electrical conductivity were measured by a Brinell Hardness
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Tester and an Eddy Current Conductivity Meter. Vacuum electrical breakdown tests were conducted in a vacuum arcing indoor chamber modified by a TDR-40A Single-crystal Furnace under the voltage of 8 kV, to evaluate the arc erosion resistance of W-Cu composites. The compressive stress-strain curves at temperatures of 25oC, 300oC, 500oC, 700oC and 900oC were obtained on a Gleeble-3500 Thermal Cycle Simulation Testing Machine. For compression tests, the W-Cu composites were firstly machined into cylindrical samples with the dimension of φ6 mm × 9 mm and then 5
ACCEPTED MANUSCRIPT welded to thermo-couple to measure the real-time temperature. The samples were heated at a heating rate of 10 oC·s-1, and then compressed with a strain rate of 0.02 mm·s-1 at the target temperature. Wear resistance experiments (wear time of 180 min,
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wear radius of 8 mm, rotating speed of 80 r·min-1 and loading of 500 g) were carried out on a HT-1000 Ball-on-Disk Tester with the wear disk made of investigated material and the wear ball using high chromium stainless steel.
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3. Results and Discussion
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3.1 Morphologies and phase analysis of the carburized tungsten skeleton Figure 2 displays the microstructure and phase analysis results taken from the outer surface of the tungsten skeleton after vacuum pulse carburization. The overall morphology resembled closely with the partially sintered W powders as neck
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formation [6,9,12]. Nevertheless, the powder surface observation revealed that a layer of tiny particles was anchored evenly and densely over the W powder hosts, and the underlying W powders with sharp facets were hardly discernable. XRD analysis for
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clarification of the coated phases confirmed the introduction of WC, W2C and C
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phases after carburization, and the relative intensity of WC phase was much stronger than that of the other two phases. It can be thereby concluded that the main product over the tungsten skeleton is WC phase. According to the full width at half maximum of the (10-11) peak for hexagonal WC at 48.299o, the average grain size of the WC products was calculated to be approximately 26 nm, which would provide dramatically improved surface performance due to the presence of nano-WC [26-28]. To further reveal the detailed morphology change from the surface to the inner 6
ACCEPTED MANUSCRIPT center of the whole tungsten skeleton after carburization, three typical regions (I, II and III as schematically shown in Fig. 1) were experimentally validated as shown in Fig. 3. Figure 3(a, a1) shows the morphologies corresponding to the near surface
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region of the carburized tungsten skeleton. It can be observed that a layer of tiny WC particles covered evenly and densely over the surface of tungsten powders, forming a W@WC core-shell structure. Figure 3(b, b1) was taken from the transition region of
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the carburized tungsten skeleton. It can be seen that, with a few discernable sharp
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facets, the tungsten powders were not fully covered by WC particles. Figure 3(c, c1) presents the morphologies for the inner central region of the carburized tungsten skeleton. It is obvious that tungsten powders in this region present multiple sharp facets without WC coatings, which indicates that the carbon has seldom diffused into
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the central region of the tungsten skeleton. Consequently, the experimental fabrication of gradient tungsten skeleton with W@WC core-shell structure has been realized. Through further copper infiltrating and sintering process of the obtained skeleton, the
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W-Cu gradient composite with W@WC core-shell structure was fabricated
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successfully. Furthermore, the microstructure as well as properties including hardness, electrical conductivity, arc erosion resistance, wear resistance and high-temperature compressive performance were evaluated in the sections below, in comparison with those of the traditional commercial W-Cu composite. 3.2 Microstructure, hardness and electrical conductivity of the carburized composite Figure 4 shows the morphologies and phase constituents of the carburized W-Cu 7
ACCEPTED MANUSCRIPT composite near surface region, in comparison with the traditional commercial W-Cu composite. It can be seen from Figs. 4(a, b) that the size and distribution of W powders resemble closely between the two samples. However, it should be noticed
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that a few tiny particles were distributed surrounding the tungsten powders for the carburized W-Cu composite. The phase analysis by XRD (Figs. 4(c, d)) confirmed that the conventional W-Cu composite consisted of W and Cu phases, and the
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carburized W-Cu composite contained WC and W2C in addition to W and Cu phases.
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It should be noticed that the relative intensity of WC phase was still stronger than that of W2C phase after infiltration. Therefore, the distributed tiny particles surrounding tungsten powders are mainly WC particles. On the other hand, the additional C in tungsten skeleton prior to infiltration as revealed by Fig. 2(b) has disappeared in the
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resulted W-Cu composite (Fig. 4(d)), possibly resulting from the complete reaction between C and W during infiltration at 1350°C. In addition, TEM analysis in the vicinity of interfaces between W and Cu was carried out to further confirm the
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distributed tiny particles in carburized W-Cu composite, as shown in Fig. 5. The
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selected area electron diffraction (SAED) patterns (Fig. 5(b, c, d)) taken from regions B and C, as well as particle D as denoted in the bright filed image (Fig. 5(a)) confirms the existing phases of Cu, W and WC. It can be seen that the WC particles were distributed surrounding the tungsten powders with intact adhesive interfaces for the carburized W-Cu composite, which would behave improved performances, especially high temperature strength and wear resistance. In Table 1 a comparison of hardness and electrical conductivity between the 8
ACCEPTED MANUSCRIPT conventional and the carburized W-Cu composites is provided. The hardness of the carburized W-Cu composite increased by 19.34% compared with the conventional W-Cu composite, which is attributed to the introduction of nano-sized WC particles
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due to the occurrence of carbonization reaction. Contrariwise, the electrical conductivity of the carburized W-Cu composite decreased by 18.19%, owing to the higher electrical resistivity of tungsten carbide relative to pure tungsten. Besides, the
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tungsten carbide particles distributed over the tungsten skeleton would hinder the flow
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of copper liquid during infiltration, leading to a possible formation of blind holes. This would result in a decreased contiguity of copper phase, which further deteriorated the phenomenal electrical conductivity. However, it should be pointed out that the decreased electrical conductivity of the carburized W-Cu composite is still
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much higher than the national standard (GB/T8320-2003: 42%IACS) [12]. 3.3 Arc erosion resistance
The relationship between dielectric breakdown strength and breakdown times for
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respective conventional W-Cu composite and carburized W-Cu composite is shown in
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Fig. 6(a) and Fig. 6(b) for comparison. The calculated average breakdown strength of the carburized W-Cu composite (5.05×107 V/m) increased by 12.22% relative to that of the conventional W-Cu composite (4.50×107 V/m) [12]. Figure 6(c) and Figure 6(d) exhibit the surface morphologies for respective conventional W-Cu composite and carburized W-Cu composite after dielectric breakdown for 50 times. It can be seen from Fig. 6(c) that the most serious arc erosion with illegible and rough microstructure occurred in the central region just underneath the tungsten anode tip. 9
ACCEPTED MANUSCRIPT In contrast, for the carburized W-Cu composite, the erosion craters exhibited shallower and more scattered distribution. According to the work function theory, the breakdown would occur firstly on the phase with smaller work function during
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dielectric breakdown process [29, 30]. The work functions for tungsten, copper and tungsten carbide are 4.54 eV, 4.36 eV and 3.79 eV, respectively [31, 32]. Therefore, the breakdown at the interface of tungsten carbide and copper would occur in priority.
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Due to the profound distribution of nano-sized tungsten carbide, the erosion craters
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were more scattered as shown in Fig. 6(d), which effectively avoided the repeated breakdown. 3.4 Wear resistance
Figure 7 exhibits the friction coefficient curves of the conventional W-Cu
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composite and the carburized W-Cu composite. Both W-Cu composites accessed into the stable wearing stage after a precedent running-in wear of 30 min. It can be noticed that the fluctuation of friction coefficient for the carburized W-Cu composite is more
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stable and the average friction coefficient (0.64) is lower than that of the conventional
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W-Cu composite (0.72). Meanwhile, the mass loss rate for the carburized W-Cu composite (8.12×10-6) reduced by 80% relative to the conventional W-Cu composite (4.18×10-5). This could be attributed to the introduction of nano-sized tungsten carbide particles after carburization [26-28]. Besides, it also indicated that the tungsten skeleton was sufficiently strong to support the ceramic particles, contributing to the improved wear resistance of the carburized W-Cu composite. 3.5 High-temperature compressive properties 10
ACCEPTED MANUSCRIPT To further validate the superiority of mechanical properties of the carburized W-Cu composite, we also investigated the compressive behaviors at 300oC, 500oC, 700oC and 900oC, as well as room temperature for comparison. The compressive
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stress-strain curves at different temperatures for the conventional and the carburized W-Cu composites are shown in Fig. 8. It can be seen that, for both composites, the increased softening rate resulted in an obvious decrease of stress with the increase of
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temperature. The compressive strength of the carburized W-Cu composite was higher
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than that of the conventional W-Cu composite at any definite temperature. At room temperature especially, the compressive strength increased by 24% than the conventional W-Cu composite, which demonstrated the significant strengthening effect of dispersed nano-sized tungsten carbide particles. On the other hand, in
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comparison with the conventional W-Cu composite, the plasticity of the carburized W-Cu composite reduced obviously at room temperature and then recovered or even increased at high temperature. This is a consequence of ionic or covalent bonds for
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brittle ceramics, which makes the movement of dislocation at room temperature
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difficult; while at high temperature, significant ductility for ceramics can be realized when diffusion-controlled deformation mechanism becomes active [33, 34]. Therefore, the carburized W-Cu composite with dispersed nano-sized tungsten carbide particles showed highly improved high-temperature compressive strength and deformation ability. 4. Conclusions (1) The W-Cu gradient composite with W@WC core-shell structure was fabricated 11
ACCEPTED MANUSCRIPT successfully through vacuum pulse carburization of the tungsten green bodies at 950oC, followed by infiltration of copper. The surface region of the W-Cu composite was mainly covered by WC particles, the transition layer with distributed WC
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particles, and the central region distant away from the surface without WC particles. (2) In comparison with the conventional W-Cu composite, the carburized W-Cu composite with W@WC core-shell structure showed good electrical conductivity of
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46.55%IACS above the national standard (GB/T8320-2003: 42%IACS), highly
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improved arc dispersion effect, smaller friction coefficient, more stable wear resistance and improved high-temperature compressive performances. Acknowledgments
The authors would like to acknowledge the financial support of 863 Program the
National
Natural
Science
Foundation
of
China
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(No.2015AA034304),
(No.51371139 and 51604223), the Science and Technique Innovation Program of Shaanxi Province (No.2012KTCQ01-14), the Pivot Innovation Team of Shaanxi
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Electric Materials and the Infiltration Technique (No.2012KCT-25), the Science and
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Technology Project of Shaanxi Province (No.2016JQ5025), Key Laboratory Foundation of Shaanxi Provincial Department of Education (No.16JS064) and Shaanxi Provincial Project of Special Foundation of Key Disciplines for this research. References
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 Schematic diagram of the W-Cu gradient composite with W@WC core-shell structure used as contact material (yellow color indicates copper, black color indicates tungsten carbide, gray color indicates tungsten).
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Fig. 2 Surface morphology (a) and phase analysis (b) of the tungsten skeleton after vacuum pulse carburization
Fig. 3 SEM images taken from the near surface region (a, a1), transition region (b, b1)
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and inner central region (c, c1) of the tungsten skeleton after vacuum pulse carburization
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Fig. 4 SEM and XRD analysis of the conventional W-Cu composite (a, c) and the carburized W-Cu composite after infiltration (b, d)
Fig. 5 TEM bright field image of the carburized W-Cu composite after infiltration (a) and SAED patterns taken from region B (b), region C (c) and particle D (d)
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Fig. 6 The change of dielectric breakdown strength with breakdown times, and surface morphologies after breakdown for 50 times for the conventional W-Cu composite (a, c) and carburized W-Cu composite (b, d)
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Fig. 7 Friction coefficient curves of the conventional W-Cu composite (a) and
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carburized W-Cu composite (b)
Fig. 8 Compressive stress-strain curves at different temperatures for the conventional W-Cu composite (a) and carburized W-Cu composite (b)
Table Caption Table 1 Hardness and electrical conductivity of the conventional W-Cu composite and carburized W-Cu composite
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ACCEPTED MANUSCRIPT Table 1 Hardness and electrical conductivity of the conventional W-Cu composite and the carburized W-Cu composite Conventional
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Hardness (HB)
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216
Electrical conductivity (%IACS)
56.90
46.55
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ACCEPTED MANUSCRIPT Highlights The W@WC core-shell structure formed through vacuum pulse carburization. The W-Cu gradient composite with W@WC core-shell structure were obtained. Arc erosion and wear resistances of the carburized W-Cu composite highly
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improved.
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The high-temperature compressive strength and plasticity significantly improved.
S0925-8388(17)30833-2
DOI:
10.1016/j.jallcom.2017.03.064
Reference:
JALCOM 41102
To appear in:
Journal of Alloys and Compounds
Received Date: 16 December 2016 Revised Date:
3 March 2017
Accepted Date: 6 March 2017
Please cite this article as: Q. Zhang, S. Liang, L. Zhuo, Fabrication and properties of the W-30wt%Cu gradient composite with W@WC core-shell structure, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.064. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication and properties of the W-30wt.%Cu gradient composite with W@WC core-shell structure Qiao Zhanga,b, Shuhua Lianga,b*, Longchao Zhuoa*
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a. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
b. Shaanxi Province Key Laboratory of Electrical Materials and Infiltration
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Technology, Xi’an University of Technology, Xi’an 710048, China
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*Corresponding authors.
E-mail addresses: [email protected] (S. Liang); [email protected] (L. Zhuo) Tel/Fax: +86-29-82312181
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Abstract: In the present work, a novel W-Cu gradient composite with W@WC core-shell structure has been fabricated successfully through vacuum pulse carburization and copper infiltration. The W@WC core-shell structure was introduced
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by vacuum pulse carburization of tungsten skeleton at 950oC. Related microstructure
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and phase analysis revealed that the near surface region of the carburized tungsten skeleton was mainly covered by WC particles, the transition region with distributed WC particles, and the inner central region distant away from the surface without WC particles. The resulted W-Cu composite with W@WC core-shell structure after infiltration showed highly improved arc dispersion effect, smaller friction coefficient, more
stable
wear
resistance
and
enhanced
high-temperature
compressive
performances. In addition, the gradient structure also ensured the electrical 1
ACCEPTED MANUSCRIPT conductivity of the W-Cu composite and maintained connected strength with CuCr alloy as an integral contact. Key words: W@WC core-shell structure; W-Cu gradient composite; Arc erosion
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resistance; Wear resistance; High-temperature compressive performance
1. Introduction
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Tungsten-copper (W-Cu) composite is a typical pseudo-alloy exhibiting mutual
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insolubility or negligible solubility prepared by powder metallurgy process. It has been widely applied as the warhead, nozzle liner in rockets and missiles, high voltage switches, heat sink materials and deviator plates for fusion, benefiting from the good wear resistance and refractory characteristic of tungsten combined with the high
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thermal and electrical conductivity of copper [1-10]. However, the increasing demand for electric power systems requires high voltage switches to withstand ultrahigh voltages and possess larger capabilities or longer service lifetime [11]. This demands
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the W-Cu contact composite, as the core part of high voltage switches, to exhibit
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improved arc erosion resistance, high-temperature strength and wear resistance [12-15]. Nevertheless, traditional commercial W-Cu contact material does not meet the above requirements because its critical application focus lies in the arc erosion resistance, with ignorance of higher high-temperature strength and wear resistance. Therefore, exploiting novel contact material with excellent performances of improved arc erosion resistance, high-temperature strength and wear resistance is rather imperative. 2
ACCEPTED MANUSCRIPT It is well established that the failure of traditional commercial W-Cu contact material in ultrahigh voltage electric power system derives from the excessive erosion of arc; meanwhile, the strength and wear resistance rapidly decrease under the
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high-temperature and high-frequency opening-closing environment [12-15]. On the other hand, the metal-matrix composites reinforced by ceramic particles exhibit high thermal stability and good wear resistance, making them widely used as various
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high-temperature wear resistant components and coatings [16-20]. As tungsten
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carbide exhibits superior high-temperature properties with relatively low cost, it has been therefore attempted to reinforce the traditional W-Cu composite [20, 21]; however, the ceramic particles were easy to agglomerate and the bonding strength with matrix was terribly weak. Another attempt of complete replacement of W by WC
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to fabricate WC-Cu composite has also demonstrated its significantly improved arc erosion resistance and wear resistance; nonetheless, a fatal weakness of the poorly connected strength with CuCr alloy to form an integral contact material, gives rise to
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the possibility of separation of WC-Cu from the integral material.
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Therefore in this work, an idea of designing W-Cu gradient composite materials with W@WC core-shell structure is put forward to solve the problem. By vacuum pulse carburization process, the carbon source was introduced into tungsten skeleton. Depending on the concentration gradient of carbon, the carbonization reaction between tungsten and carbon occurred to form WC particles. The structure of the W-Cu gradient composite with W@WC core-shell structure is schematically presented as shown in Fig. 1. In the surface region (I) of the W-Cu composite, 3
ACCEPTED MANUSCRIPT tungsten powders were totally covered by WC particles, in the transition region (II) with distributed WC particles, while in the central region (III) distant away from the surface without WC particles. Therefore, under high-temperature and high-frequency
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opening-closing service, the W@WC core-shell structure for the outer tungsten skeleton would effectively improve the high-temperature strength and wear resistance; besides, the effect of dispersing arc erosion would be introduced [22, 23]. Even if the
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deposited carbon has not completely reacted with tungsten powders, the residual
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carbon would exhibit strong self-lubricating effect, thereby improving wear resistance of the composite [24, 25]. Additionally, the gradient composite with pure W-Cu in the center (region III as shown in Fig. 1)–could also maintain the connected strength between W-Cu composite and CuCr alloy as an integral contact without losing
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conductivity. With guidance of the above excogitation, the main objective of this study focuses on fabrication of the W-Cu gradient composite with W@WC core-shell structure and evaluation of its hardness, electrical conductivity, arc erosion and wear
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resistance, as well as high-temperature compressive properties.
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2. Experimental materials and methods Tungsten powders (purity ≥ 99.9 wt.%, average particle size of 6~8 µm) and
minor trace addition of nickel powders (purity ≥ 99.9 wt.%, average particle size of 25~30 µm) as the sintering activator were blended in a V-type mixer for 6 h, followed by pressing in a hydraulic machine to form tungsten green bodies (density = 70%). Carbon source is provided by acetylene gas (C2H2: purity ≥ 99.99 wt.%), with nitrogen gas (N2: purity ≥ 99.9 wt.%) as the protective atmosphere. The green bodies 4
ACCEPTED MANUSCRIPT were firstly heated up to 950oC in a vacuum carburizing furnace under inner protection of N2. At the target temperature, the samples were repeatedly conducted the pulse carburizing process for 40 min, and heat preservation for 100 min was taken to
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make an adequate carbonization or carbon diffusion. Followed by furnace cooling, the gradient tungsten skeleton with W@WC core-shell structure was obtained. Finally, through infiltration of an oxygen-free copper plate (purity ≥99.9 wt.%) into the
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gradient tungsten skeleton with W@WC core-shell structure in a sintering furnace at
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1350oC, the novel W-Cu gradient composite with W@WC core-shell structure was fabricated.
Morphologies of the tungsten skeleton with W@WC core-shell structure and cross section of the W-Cu gradient composite were observed on a JEOL-6700F or
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Zeiss-Merlin Scanning Electron Microscope (SEM). The phase composition was investigated using a 7000S X-ray diffraction (XRD) instrument with Cu Kα radiation. The structure was also confirmed on a JEM-3010 Transmission Electron Microscopy
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(TEM). Hardness and electrical conductivity were measured by a Brinell Hardness
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Tester and an Eddy Current Conductivity Meter. Vacuum electrical breakdown tests were conducted in a vacuum arcing indoor chamber modified by a TDR-40A Single-crystal Furnace under the voltage of 8 kV, to evaluate the arc erosion resistance of W-Cu composites. The compressive stress-strain curves at temperatures of 25oC, 300oC, 500oC, 700oC and 900oC were obtained on a Gleeble-3500 Thermal Cycle Simulation Testing Machine. For compression tests, the W-Cu composites were firstly machined into cylindrical samples with the dimension of φ6 mm × 9 mm and then 5
ACCEPTED MANUSCRIPT welded to thermo-couple to measure the real-time temperature. The samples were heated at a heating rate of 10 oC·s-1, and then compressed with a strain rate of 0.02 mm·s-1 at the target temperature. Wear resistance experiments (wear time of 180 min,
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wear radius of 8 mm, rotating speed of 80 r·min-1 and loading of 500 g) were carried out on a HT-1000 Ball-on-Disk Tester with the wear disk made of investigated material and the wear ball using high chromium stainless steel.
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3. Results and Discussion
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3.1 Morphologies and phase analysis of the carburized tungsten skeleton Figure 2 displays the microstructure and phase analysis results taken from the outer surface of the tungsten skeleton after vacuum pulse carburization. The overall morphology resembled closely with the partially sintered W powders as neck
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formation [6,9,12]. Nevertheless, the powder surface observation revealed that a layer of tiny particles was anchored evenly and densely over the W powder hosts, and the underlying W powders with sharp facets were hardly discernable. XRD analysis for
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clarification of the coated phases confirmed the introduction of WC, W2C and C
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phases after carburization, and the relative intensity of WC phase was much stronger than that of the other two phases. It can be thereby concluded that the main product over the tungsten skeleton is WC phase. According to the full width at half maximum of the (10-11) peak for hexagonal WC at 48.299o, the average grain size of the WC products was calculated to be approximately 26 nm, which would provide dramatically improved surface performance due to the presence of nano-WC [26-28]. To further reveal the detailed morphology change from the surface to the inner 6
ACCEPTED MANUSCRIPT center of the whole tungsten skeleton after carburization, three typical regions (I, II and III as schematically shown in Fig. 1) were experimentally validated as shown in Fig. 3. Figure 3(a, a1) shows the morphologies corresponding to the near surface
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region of the carburized tungsten skeleton. It can be observed that a layer of tiny WC particles covered evenly and densely over the surface of tungsten powders, forming a W@WC core-shell structure. Figure 3(b, b1) was taken from the transition region of
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the carburized tungsten skeleton. It can be seen that, with a few discernable sharp
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facets, the tungsten powders were not fully covered by WC particles. Figure 3(c, c1) presents the morphologies for the inner central region of the carburized tungsten skeleton. It is obvious that tungsten powders in this region present multiple sharp facets without WC coatings, which indicates that the carbon has seldom diffused into
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the central region of the tungsten skeleton. Consequently, the experimental fabrication of gradient tungsten skeleton with W@WC core-shell structure has been realized. Through further copper infiltrating and sintering process of the obtained skeleton, the
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W-Cu gradient composite with W@WC core-shell structure was fabricated
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successfully. Furthermore, the microstructure as well as properties including hardness, electrical conductivity, arc erosion resistance, wear resistance and high-temperature compressive performance were evaluated in the sections below, in comparison with those of the traditional commercial W-Cu composite. 3.2 Microstructure, hardness and electrical conductivity of the carburized composite Figure 4 shows the morphologies and phase constituents of the carburized W-Cu 7
ACCEPTED MANUSCRIPT composite near surface region, in comparison with the traditional commercial W-Cu composite. It can be seen from Figs. 4(a, b) that the size and distribution of W powders resemble closely between the two samples. However, it should be noticed
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that a few tiny particles were distributed surrounding the tungsten powders for the carburized W-Cu composite. The phase analysis by XRD (Figs. 4(c, d)) confirmed that the conventional W-Cu composite consisted of W and Cu phases, and the
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carburized W-Cu composite contained WC and W2C in addition to W and Cu phases.
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It should be noticed that the relative intensity of WC phase was still stronger than that of W2C phase after infiltration. Therefore, the distributed tiny particles surrounding tungsten powders are mainly WC particles. On the other hand, the additional C in tungsten skeleton prior to infiltration as revealed by Fig. 2(b) has disappeared in the
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resulted W-Cu composite (Fig. 4(d)), possibly resulting from the complete reaction between C and W during infiltration at 1350°C. In addition, TEM analysis in the vicinity of interfaces between W and Cu was carried out to further confirm the
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distributed tiny particles in carburized W-Cu composite, as shown in Fig. 5. The
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selected area electron diffraction (SAED) patterns (Fig. 5(b, c, d)) taken from regions B and C, as well as particle D as denoted in the bright filed image (Fig. 5(a)) confirms the existing phases of Cu, W and WC. It can be seen that the WC particles were distributed surrounding the tungsten powders with intact adhesive interfaces for the carburized W-Cu composite, which would behave improved performances, especially high temperature strength and wear resistance. In Table 1 a comparison of hardness and electrical conductivity between the 8
ACCEPTED MANUSCRIPT conventional and the carburized W-Cu composites is provided. The hardness of the carburized W-Cu composite increased by 19.34% compared with the conventional W-Cu composite, which is attributed to the introduction of nano-sized WC particles
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due to the occurrence of carbonization reaction. Contrariwise, the electrical conductivity of the carburized W-Cu composite decreased by 18.19%, owing to the higher electrical resistivity of tungsten carbide relative to pure tungsten. Besides, the
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tungsten carbide particles distributed over the tungsten skeleton would hinder the flow
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of copper liquid during infiltration, leading to a possible formation of blind holes. This would result in a decreased contiguity of copper phase, which further deteriorated the phenomenal electrical conductivity. However, it should be pointed out that the decreased electrical conductivity of the carburized W-Cu composite is still
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much higher than the national standard (GB/T8320-2003: 42%IACS) [12]. 3.3 Arc erosion resistance
The relationship between dielectric breakdown strength and breakdown times for
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respective conventional W-Cu composite and carburized W-Cu composite is shown in
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Fig. 6(a) and Fig. 6(b) for comparison. The calculated average breakdown strength of the carburized W-Cu composite (5.05×107 V/m) increased by 12.22% relative to that of the conventional W-Cu composite (4.50×107 V/m) [12]. Figure 6(c) and Figure 6(d) exhibit the surface morphologies for respective conventional W-Cu composite and carburized W-Cu composite after dielectric breakdown for 50 times. It can be seen from Fig. 6(c) that the most serious arc erosion with illegible and rough microstructure occurred in the central region just underneath the tungsten anode tip. 9
ACCEPTED MANUSCRIPT In contrast, for the carburized W-Cu composite, the erosion craters exhibited shallower and more scattered distribution. According to the work function theory, the breakdown would occur firstly on the phase with smaller work function during
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dielectric breakdown process [29, 30]. The work functions for tungsten, copper and tungsten carbide are 4.54 eV, 4.36 eV and 3.79 eV, respectively [31, 32]. Therefore, the breakdown at the interface of tungsten carbide and copper would occur in priority.
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Due to the profound distribution of nano-sized tungsten carbide, the erosion craters
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were more scattered as shown in Fig. 6(d), which effectively avoided the repeated breakdown. 3.4 Wear resistance
Figure 7 exhibits the friction coefficient curves of the conventional W-Cu
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composite and the carburized W-Cu composite. Both W-Cu composites accessed into the stable wearing stage after a precedent running-in wear of 30 min. It can be noticed that the fluctuation of friction coefficient for the carburized W-Cu composite is more
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stable and the average friction coefficient (0.64) is lower than that of the conventional
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W-Cu composite (0.72). Meanwhile, the mass loss rate for the carburized W-Cu composite (8.12×10-6) reduced by 80% relative to the conventional W-Cu composite (4.18×10-5). This could be attributed to the introduction of nano-sized tungsten carbide particles after carburization [26-28]. Besides, it also indicated that the tungsten skeleton was sufficiently strong to support the ceramic particles, contributing to the improved wear resistance of the carburized W-Cu composite. 3.5 High-temperature compressive properties 10
ACCEPTED MANUSCRIPT To further validate the superiority of mechanical properties of the carburized W-Cu composite, we also investigated the compressive behaviors at 300oC, 500oC, 700oC and 900oC, as well as room temperature for comparison. The compressive
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stress-strain curves at different temperatures for the conventional and the carburized W-Cu composites are shown in Fig. 8. It can be seen that, for both composites, the increased softening rate resulted in an obvious decrease of stress with the increase of
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temperature. The compressive strength of the carburized W-Cu composite was higher
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than that of the conventional W-Cu composite at any definite temperature. At room temperature especially, the compressive strength increased by 24% than the conventional W-Cu composite, which demonstrated the significant strengthening effect of dispersed nano-sized tungsten carbide particles. On the other hand, in
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comparison with the conventional W-Cu composite, the plasticity of the carburized W-Cu composite reduced obviously at room temperature and then recovered or even increased at high temperature. This is a consequence of ionic or covalent bonds for
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brittle ceramics, which makes the movement of dislocation at room temperature
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difficult; while at high temperature, significant ductility for ceramics can be realized when diffusion-controlled deformation mechanism becomes active [33, 34]. Therefore, the carburized W-Cu composite with dispersed nano-sized tungsten carbide particles showed highly improved high-temperature compressive strength and deformation ability. 4. Conclusions (1) The W-Cu gradient composite with W@WC core-shell structure was fabricated 11
ACCEPTED MANUSCRIPT successfully through vacuum pulse carburization of the tungsten green bodies at 950oC, followed by infiltration of copper. The surface region of the W-Cu composite was mainly covered by WC particles, the transition layer with distributed WC
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particles, and the central region distant away from the surface without WC particles. (2) In comparison with the conventional W-Cu composite, the carburized W-Cu composite with W@WC core-shell structure showed good electrical conductivity of
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46.55%IACS above the national standard (GB/T8320-2003: 42%IACS), highly
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improved arc dispersion effect, smaller friction coefficient, more stable wear resistance and improved high-temperature compressive performances. Acknowledgments
The authors would like to acknowledge the financial support of 863 Program the
National
Natural
Science
Foundation
of
China
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(No.2015AA034304),
(No.51371139 and 51604223), the Science and Technique Innovation Program of Shaanxi Province (No.2012KTCQ01-14), the Pivot Innovation Team of Shaanxi
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Electric Materials and the Infiltration Technique (No.2012KCT-25), the Science and
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Technology Project of Shaanxi Province (No.2016JQ5025), Key Laboratory Foundation of Shaanxi Provincial Department of Education (No.16JS064) and Shaanxi Provincial Project of Special Foundation of Key Disciplines for this research. References
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 Schematic diagram of the W-Cu gradient composite with W@WC core-shell structure used as contact material (yellow color indicates copper, black color indicates tungsten carbide, gray color indicates tungsten).
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Fig. 2 Surface morphology (a) and phase analysis (b) of the tungsten skeleton after vacuum pulse carburization
Fig. 3 SEM images taken from the near surface region (a, a1), transition region (b, b1)
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and inner central region (c, c1) of the tungsten skeleton after vacuum pulse carburization
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Fig. 4 SEM and XRD analysis of the conventional W-Cu composite (a, c) and the carburized W-Cu composite after infiltration (b, d)
Fig. 5 TEM bright field image of the carburized W-Cu composite after infiltration (a) and SAED patterns taken from region B (b), region C (c) and particle D (d)
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Fig. 6 The change of dielectric breakdown strength with breakdown times, and surface morphologies after breakdown for 50 times for the conventional W-Cu composite (a, c) and carburized W-Cu composite (b, d)
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Fig. 7 Friction coefficient curves of the conventional W-Cu composite (a) and
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carburized W-Cu composite (b)
Fig. 8 Compressive stress-strain curves at different temperatures for the conventional W-Cu composite (a) and carburized W-Cu composite (b)
Table Caption Table 1 Hardness and electrical conductivity of the conventional W-Cu composite and carburized W-Cu composite
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ACCEPTED MANUSCRIPT Table 1 Hardness and electrical conductivity of the conventional W-Cu composite and the carburized W-Cu composite Conventional
Carburized
Hardness (HB)
181
216
Electrical conductivity (%IACS)
56.90
46.55
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ACCEPTED MANUSCRIPT Highlights The W@WC core-shell structure formed through vacuum pulse carburization. The W-Cu gradient composite with W@WC core-shell structure were obtained. Arc erosion and wear resistances of the carburized W-Cu composite highly
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The high-temperature compressive strength and plasticity significantly improved.