简介概要

Hot-pressed sintering of W/Cu functionally graded materials prepared from copper-coated tungsten powders

来源期刊：Rare Metals2020年第10期

论文作者：Pei Tian Yi Feng Meng Xia Lan Zhao Cheng-Yu Cai Ke Liu Xiao-Chen Huang

文章页码：1229 - 1236

摘要：The three-layered（W-60 vol%Cu/W-40 vol%Cu/W-20 vol%Cu） W/Cu functionally graded material（FGM） containing a Cu network structure was fabricated at different temperatures by hot-pressed sintering produced from copper-coated tungsten powders.The effects of various sintering temperatures on relative density,microstructure,thermal conductivity,hardness and flexural strength were investigated.Scanning electron microscopy（SEM） and X-ray diffraction（XRD） analysis show that a Cu network extends throughout the W/Cu FGM specimens sintered at 1065℃ and the graded structure can be retained perfectly,and W particles are distributed homogeneously.The low-temperature sintering densification of W/Cu FGM arises because the sintering mode of the copper-coated tungsten particles includes just sintering Cu to Cu,rather than Cu to W,Cu to Cu and W to W,as required for conventional powder particles.The relative density of W/Cu FGM sintered at 1065℃ for 3 h under a load of25 MPa is 96.1%.The thermal conductivity is up to204 W·m^-1·K^-1 at normal temperature and 150 W·m^-1·K^-1at 800℃.And the Vickers hardness varies with the gradient of different layers from 3.34 to 4.05 GPa.

稀有金属(英文版) 2020,39(10),1229-1236

Hot-pressed sintering of W/Cu functionally graded materials prepared from copper-coated tungsten powders

Pei Tian Yi Feng Meng Xia Lan Zhao Cheng-Yu Cai Ke Liu Xiao-Chen Huang

School of Materials Science and Engineering,Hefei University of Technology

Department of Material Engineering,Zhejiang Industry and Trade Vocational College

作者简介：*Yi Feng,e-mail:fyhfut@163.com;

收稿日期：31 July 2016

基金：financially supported by the Wenzhou Public Welfare Technology Research Industry Project(No.G20140042);

Hot-pressed sintering of W/Cu functionally graded materials prepared from copper-coated tungsten powders

Pei Tian Yi Feng Meng Xia Lan Zhao Cheng-Yu Cai Ke Liu Xiao-Chen Huang

School of Materials Science and Engineering,Hefei University of Technology

Department of Material Engineering,Zhejiang Industry and Trade Vocational College

Abstract：

The three-layered(W-60 vol%Cu/W-40 vol%Cu/W-20 vol%Cu) W/Cu functionally graded material(FGM) containing a Cu network structure was fabricated at different temperatures by hot-pressed sintering produced from copper-coated tungsten powders.The effects of various sintering temperatures on relative density,microstructure,thermal conductivity,hardness and flexural strength were investigated.Scanning electron microscopy(SEM) and X-ray diffraction(XRD) analysis show that a Cu network extends throughout the W/Cu FGM specimens sintered at 1065℃ and the graded structure can be retained perfectly,and W particles are distributed homogeneously.The low-temperature sintering densification of W/Cu FGM arises because the sintering mode of the copper-coated tungsten particles includes just sintering Cu to Cu,rather than Cu to W,Cu to Cu and W to W,as required for conventional powder particles.The relative density of W/Cu FGM sintered at 1065℃ for 3 h under a load of25 MPa is 96.1%.The thermal conductivity is up to204 W·m^-1·K^-1 at normal temperature and 150 W·m^-1·K^-1at 800℃.And the Vickers hardness varies with the gradient of different layers from 3.34 to 4.05 GPa.

Keyword：

Copper-coated tungsten powders; Hot-pressed sintering; W/Cu functionally graded materials; Microstructure; Performance;

Received： 31 July 2016

1 Introduction

The pertor is one of the most significant components in future fusion reactors,such as international thermonuclear experimental reactor (ITER) and demonstration fusion reactor (DEMO).They have to bear a heat flux of about20 MW·m^-2 during transient phases.Tungsten is a prospective PFM and spallation target materials (SPMs)which has high fusion point,high sputtering resistance,low tritium/deuterium retention and low coefficient of thermal expansion [ 1, 2, 3, 4] .Copper has high thermal conductivity and some other advisable characteristics.So combining the advantages of W and Cu becomes more and more attractive lately [ 5, 6, 7, 8] .Nevertheless,there is mutual insolubility and a big difference of the coefficients of thermal expansion between W and Cu.High thermal stress exists under heat flux impacting,ultimately leads to material failure [ 9, 10, 11, 12, 13] .Moreover,it is difficult to fabricate the two materials because of insolubility and the big fusion temperature difference between the two materials.For the sake of reducing the risk of bonding failures,it is suggested to replace the soft layer by a continuous functional graded material [ 14, 15, 16] .As an interlayer,W/Cu FGMs have been designed between PFMs and heat sink materials,which avoid direct interfaces and improve the thermo-mechanical resistance [ 17, 18, 19, 20] .Some methods have been developed to manufacture W/Cu FGM,including microwave sintering [ 21, 22, 23] ,laser sintering [ 24] ,atmosphere plasma spraying(APS),vacuum plasma spraying (VPS) [ 25] ,chemical vapor deposition (CVD) and infiltration-welding methods [ 26] .But most methods work at a relatively high temperature or high pressure [ 21, 22, 23, 24, 25, 26] ,so that some molten copper is extruded easily.It usually leads to the composition of migration and the aggregation of W and Cu.Consequently,the graded structure of W/Cu FGM cannot retain entirely.

As well known,a network structure of Cu and homogenous distribution of W powders play a vital role on fabricating W/Cu FGM with excellent properties.Cu can be homogeneously and densely coated on the surface of W powders.The interfacial bonding strength between W and Cu is improved after coating,which solves the problem of heterogeneous mixing due to their extremely different densities [ 27] .The sintering mode of the copper-coated tungsten particles includes just sintering Cu to Cu,rather than Cu to W,Cu to Cu and W to W.Thus,a uniform distribution of the two phases with a fine Cu network structure and excellent overall properties can be prepared.

To our best knowledge,few attempts have been made on the fabrication of W/Cu FGM with copper-coated tungsten powders by hot-pressed sintering so far.To solve the aforementioned problem,relatively low-temperature densification has been attempted and copper-coated tungsten powders have been prepared for fabricating the W/Cu FGM.In the present work,a three-layer W/Cu FGM was fabricated by hot-pressed sintering method without adding any sintering aids.At the same time,the effects of various sintering processes on relative density,microstructure,thermal conductivity and hardness of W/Cu FGM specimens were studied.

2 Experimental

The W (purity 99.9%,particle size of 3μm) powders were used as received by the supplier which was plated Cu all over the powder particles homogeneously and densely.The three-layered W/Cu FGM specimens were designed with W content of 80 vol%,60 vol%and 40 vol%in each layer.Each layer is around 1 mm in thickness.

In the present work,copper-coated tungsten powders with different W contents were produced by electroless plating using the bath composition given in Table 1.pH was controlled within the range of 11-13 by adjusting the amount of sodium hydroxide during the plating process in an ultrasonic wave cleaner whose temperature was also controlled at 60℃.Ultrasonic wave helped to control the dispersity and improve the quality of copper-coated tungsten powders.The obtained copper-coated tungsten powders were cleaned with deionized water and dried in a vacuum oven.By controlling the electroless plating process(time,pH,temperature,the amount of sulfate pentahydrate,etc.),the thickness of Cu and the content of copper in each layer of W/Cu FGM can be controlled.

Copper-coated tungsten powders with various W contents were stacked and cold-prepress layer by layer into a graphite mold with the diameter of 45 mm.The load of prepressing was 2.39×10⁴ N.Hot-pressed sintering was applied to fabricating the W/Cu FGM specimens at different temperatures (1050,1065,1080,1095℃).Owing to the fact that the melting temperature of Cu is about1083℃,sintering at 1050 and 1065℃was solid-phase sintering.The soaking time was set as 3 h under a constant axial pressure of 25 MPa.Sintering at 1080 and 1095℃was liquid-phase sintering.In case of the liquid Cu spill,the soaking time and the pressure were reduced.The soaking time and the pressure of the liquid-phase sintering were 1 h and 20 MPa,respectively.The entire hot-pressed sintering was in the atmosphere of high-purity argon protection.At the temperature of below 800℃,the heating rate was controlled as 10℃·min^-1,while it was 10.0/1.5 min from 800℃to the target sintering temperature.After the soaking time of 3 or 1 h,the specimen was cooled with furnace.Figure 1 shows the schematic illustration of the experimental process.

The sample was incised into three rectangle specimens with dimensions of 3 mm × 8 mm × 35 mm used for the three-point bending test and three specimens with 6 mm in diameter used for thermal conductivity tests by wire-electrode cutting equipment.The density (ρ) of the hot-pressed sintered W/Cu FGM was measured by the Archimedes’ method.The phases in each layer of sintered samples were identified by X-ray diffractometer (XRD,PANalytical X'Pert PRO MPD).The microstructure was observed and analyzed by a field emission scanning electron microscope (FESEM;Hitachi SU8020) equipped with an energy-dispersive spectroscopy (EDS) system.And the Vickers hardness test was performed on the fine polished surface by HXD-1000 tester (Shanghai Second Optical Ltd,China) at the load of 0.98 N with the dwell time of 10 s.The flexuralstrength of bulk W/Cu FGM specimens with dimensions of 3 mm × 8 mm × 35 mm was tested by the three-point bending method.In order to analyze the strengthening mechanism of bulk W/Cu FGM,the fracture morphology of the specimens after three-point tests was also observed by SEM.Thermal conductivity (λ) was calculated on the basis of the formula:

下载原图

Table 1 Composition of bath used to plate Cu

^*EDTA-C₁₀H₁₆N₂O₈

Fig.1 Schematic illustration of experimental process

where C_p and oc are specific heat and thermal diffusivity,respectively,which were measured by the laser-flash diffusivity system (LFA457 Microflash,NETZSCH) at temperature from 25 to 800℃.The specific heat (C_p) was calculated by theoretical rule of mixtures according to the following formula:

where V_Cu and V_W are the content in volume fraction and C_pCu and C_pW are the specific heat of Cu and W,respectively.Meanwhile,the reference values of C_pCu and C_pW at various temperatures were taken from Ref. [ 28] .

3 Results and discussion

3.1 Electroless copper plating

Figure 2 shows SEM images of uncoated and coated W particles.The morphology of the as-received W powders is presented in Fig.2a.It is obvious that the average particle size of the W powders is about 3μm.It can also be seen that the surface of the original W powder is flat and smooth.Figure 2b shows the morphology of copper-coated W powders.From Fig.2c,it can be seen that the single copper-coated W particle is close to spherical in shape but not smooth.Figure 3a shows backscattered SEM image in which W particles are uniformly and densely coated.Figure 3b,c shows EDS elemental mappings of the cross section of copper-coated W particles.The dark gray outer layer is copper and the light gray particle is W,indicating that W particles have been coated by a layer of copper.

3.2 Microstructure and properties of W/Cu FGM

Figure 4 exhibits relative density of W/Cu FGM specimens produced by hot pressing at various temperatures.Density is calculated from the average of three times repeated experiments.From the sintering temperature of1050-1065℃,the relative density increases.But from1065 to 1095℃,the relative density decreases.Because the melting temperature of copper is about 1080℃,the relative density increases with temperature increasing at solid-phase sintering and decreases at liquid-phase sintering in other words.As known to all,the relative density of metallic composite materials increases with the increase in sintering temperature at a limited range.Nevertheless,when the temperature is above the melting temperature of copper,some liquid copper was extruded from the graphite mold,which changes the proportion of copper in each layer.Cu migrated and diffused inevitably,leading to the composition deviation of each layer.Moreover,the maximum value of relative density is 96.1%at the sintering temperature of 1065℃.

Fig.3 Backscattered SEM image a and EDS elemental mappings (b Cu and c W) of cross section of copper-coated W particles

Fig.4 Relative density of W/Cu FGM specimens with different sintering temperatures

Figure 5 shows the microstructures of the specimens in W-40Cu layer at different sintering temperatures.At the sintering temperature of 1065℃,it obviously indicates that W particles are distributed homogeneously in copper matrix,and copper matrix has formed a continuous network structure.Moreover,few pores are detected in Fig.5a,c.Instead,a number of pores appear in Fig.5b,d because of the extrusion of liquid copper when sintered at1095℃above the melting point of copper,leading to the aggregation of W and the discontinuity of Cu network.And it is corresponding to the trend of the relative density at different sintering temperatures.Compared to that in Fig.5a,the copper content at 1095℃in Fig.5b decreases obviously,which is far different from the original design.From Fig.5d,the entire Cu network structure can hardly be found.It indicates that the performance of the W/Cu FGM specimen cannot accord with the requirement.

On account of the sintering mode changing,the lowtemperature sintering densification of W/Cu FGM has occurred.By electroless plating,copper is homogeneously and densely coated on the surface of W particles,forming a W-Cu core-shell structure.Uniform distribution of copper coating and excellent sinter ability of copper promote the densification of W/Cu composite powders.Therefore,the sintering mode of copper-coated W powders becomes merely sintering of Cu to Cu instead of sintering of W to Cu,Cu to Cu and W to W existing in conventional powder metallurgy methods like mechanical alloying by using W and Cu powders [ 29] .In a word,sintering process and filling green-pressing pores of copper occur much more easily and the variational sintering mode largely decreases the difficulty of W-Cu sintering.Furthermore,the density of W/Cu FGM specimens is improved and the sintering temperature is reduced.

Figure 6a-c clearly shows that W particles are distributed homogeneously in copper matrix,and copper matrix has formed a network structure in different layers.The W-W adjacency is apparently restricted by the surrounding Cu network.No pores or aggregation of W or Cu are observed in the images.The continuous Cu network throughout the composite is a crucial factor for high-performance W/Cu FGM.Figure 6d-f shows the fracture surface morphologies of each layer in the W/Cu FGM specimens,which demonstrates the Cu network formed throughout the composites.It can be observed that there are no pores in W-60Cu and W-40Cu layers,indicating that the two layers are highly dense.But with the decrease in Cu content,few pores in the W-20Cu layer are observed.Therefore,the content of copper in each layer is the critical factor influencing the relative density of the entire W/Cu FGM specimens.The fracture surface consists of ductile failure of Cu matrix,together with some intergranular fracture of W particles,which demonstrates that the surfaces of W particles are fully covered by copper layer.

For the sake of further investigating the distribution of W particles and Cu network,the analysis of EDS elemental mapping in different layers of W/Cu FGM specimen sintered at 1065℃is also performed in Fig.7.It evidently demonstrates that Cu network distributes continuously between W particles and W particles.

SEM images and corresponding EDS line scan figure (Fig.8a,b) indicate qualitatively the content of tungsten and copper corresponding to the scanning distance from left to right across the polished cross section of the W/Cu FGM specimen sintered at 1065℃for 3 h at25 MPa.It is observed that the interfaces are obvious in Fig.8a.Figure 8b shows that tungsten and copper distribute homogeneously in each other in different layers,and the content of W and Cu at interface varies sharply.It indicates that the original gradient of W/Cu FGM remains very well for the reason that the solid copper does not migrate obviously during the whole hot-pressing process.

Fig.5 SEM images of polished surface and fracture surface of W-40Cu layer at different sintering temperatures with different magnifications:a,c 1065℃and b,d 1095℃

Fig.6 SEM images of polished surface and fracture surface of different layers in W/Cu FGM specimen sintered at 1065℃for 3 h at 25 MPa:a,d W-60Cu layer,b,e W-40Cu layer and c,f W-20Cu layer

Figure 9 shows thermal conductivity of W/Cu FGMspecimens at various sintering temperatures within temperature range of 25-800℃.The thermal conductivity decreases with temperature increasing from 25 to 800℃.However,it is noticed that the thermal conductivity of W/Cu FGM samples fabricated at 1080 and 1095℃is lower than that fabricated at 1050 and 1065℃.And the thermal conductivity decreases with the decrease in sintering temperature above the melting point of copper.It is mainly because plenty of liquid copper is extruded from the graphite mold when sintered above the melting temperature of copper,leading to pores in the samples.As known,the thermal transmission in metals is mainly the movement of free electrons.The pores can scatter electrons and prevent the transmission of the free electrons in the two metals of W and Cu.By electroless plating,copper is continuously and uniformly plated over the surfaces of W powders,and Cu network is well formed.On the other hand,copper has excellent conductivity,so the intact Cu network structure helps increase the conductivity of the W/Cu FGM specimens.Although the density of the sample fabricated at1065 is 96.1%,its thermal conductivity reaches204 W·m^-1·K^-1 at normal temperature and 150 Wrm^-1·K^-1at 800℃,which is much higher than the value of140 W·m^-1.K^-1 (at room temperature) and basically flat with the value of 151 W·m^-1-K^-1 of the W/Cu FGMproduced by spark plasma sintering (SPS) method [ 30] .As a result,hot pressing of W/Cu FGM prepared from coppercoated tungsten powders is a powerful way for developing high-performance W/Cu FGM.

Fig.7 EDS elemental mappings of Cu (red) and W (green) in various layers of W/Cu FGM specimen sintered at 1065℃for 3 h at 25 MPa:a,d W-60Cu layer,b,e W-40Cu layer and c,f W-20Cu layer

Fig.8 a SEM image of whole polished cross section of W/Cu FGM sample sintered at 1065℃for 3 h at 25 MPa and b EDS line scan of W and Cu

Fig.9 Thermal conductivity at various testing temperatures of W/Cu;GM specimens sintered at different temperatures

Figure 10 shows the flexural strength of W/Cu FGM samples sintered at different temperatures whose compression faces are rich copper layer and rich tungsten layer,respectively.It can be seen that the flexural strength increases first and then decreases with the increase in temperature from 1050 to 1095℃.The result is in good accordance with the trend of the relative density in Fig.4and the structures in Fig.5.Figure 10 shows that the flexural strength whose compression face is rich tungsten layer is higher than that of rich copper layer.The reason is that during the three-point bending test,the layer contacting with the indenter is bearing the axial compressive stress,while the layer of the other side is bearing tensile stress.Cu is plastic metal and W is brittle metal.Thus,when rich copper layer was pressed,it is easily for rich tungsten layer to crack.The maximum flexural strength of the W/Cu FGM specimen whose compression face is rich tungsten is up to 1014.8 MPa,while that of rich copper layer reaches 737.5 MPa.

In addition,the Vickers hardness of various layers in the W_/Cu FGM specimen sintered at 1065℃for 3 h at25 MPa is presented in Fig.11.The hardness depends on the rule of mixtures on the basis of the following formula:

where H_C,H_Wand H_Cu are the hardness and V_w is the content in volume fraction of tungsten.As shown in Fig.11,the hardness of various layers decreases with the increase in copper content,which is corresponding to the theoretical formula.

Fig.10 Flexural strength at various testing conditions of W/Cu FGM specimens sintered at different temperatures

Fig.11 Vickers hardness of various layers in W/Cu FGM specimen sintered at 1065℃for 3 h at 25 MPa

4 Conclusion

The W/Cu FGM specimens with three-layered structure(W-60 vol%Cu/W-40 vol%Cu/W-20 vol%Cu) prepared from copper-coated tungsten powders were manufactured by hot-pressed sintering method at different sintering temperatures.Microstructural analysis shows that the continuous network of copper is throughout the entire W/Cu FGM specimens sintering at the relatively low temperature (1065℃),and W particles are homogeneously distributed.The graded structure of the W/Cu FGM specimens can be well retained,which can eliminate and relax the thermal stress and reduce the risk of bonding failures.The relative density of the specimen sintered at1065℃for 3 h at 25 MPa is about 96.1%,and thermal conductivity at normal temperature reaches 204 W·m^-1·K^-1.And the flexural strength of the W/Cu FGM specimen whose compression face is rich tungsten is up to1014.8 MPa,while that of rich copper layer reaches737.5 MPa.The Vickers hardness varies with the gradient of different layers from 3.34 to 4.05 GPa.