Dry ball milling and wet ball milling for fabricating copper-yttria composites
来源期刊:Rare Metals2018年第10期
论文作者:Fei Huang Hang Wang Jin-Shui Chen Bin Yang
文章页码:859 - 867
摘 要:Yttria-reinforced copper matrix composites were prepared by dry ball milling (DBM) and wet ball milling(WBM), respectively, followed by spark plasma sintering(SPS). It is to determine which milling process is better for fabricating Cu-Y2O3 composites. It is found that Cu-Y2O3composites synthesized by DBM exhibit better densification, mechanical and electrical properties than those by WBM. Less agglomeration of reinforcements in the bulk composites by DBM is responsible for the better performances. To further understand the reason of less agglomeration of Y2O3 in the bulks by DBM, morphologies of prepared powders were investigated and analyzed. Higher ball’s impact energy and the formation of copper oxide on the matrix surface during DBM process contribute to small matrix particles, which is beneficial for less agglomeration.
稀有金属(英文版) 2018,37(10),859-867
Fei Huang Hang Wang Jin-Shui Chen Bin Yang
School of Materials Science and Engineering, Jiangxi University of Science and Technology
Editorial Office, Nonferrous Metals of Science and Engineering Jiangxi University of Science and Technology
Institute of Engineering Research, Jiangxi University of Science and Technology
School of Materials Science and Engineering,Jiangxi University of Science and Technology
作者简介:*Bin Yang e-mail: yangbin65@126.com;
收稿日期:4 April 2017/
基金:financially supported by the National Key Research and Development Program of China (No. 2016YFB0301400);the Organization Department of Jiangxi Province(No. 2012215);the Education Department of Jiangxi Province (No. KJLD13041);the Outstanding Doctoral Dissertation Project Fund of JXUST (No. YB2017011);
Fei Huang Hang Wang Jin-Shui Chen Bin Yang
School of Materials Science and Engineering, Jiangxi University of Science and Technology
Editorial Office, Nonferrous Metals of Science and Engineering Jiangxi University of Science and Technology
Institute of Engineering Research, Jiangxi University of Science and Technology
School of Materials Science and Engineering,Jiangxi University of Science and Technology
Abstract:
Yttria-reinforced copper matrix composites were prepared by dry ball milling (DBM) and wet ball milling(WBM), respectively, followed by spark plasma sintering(SPS). It is to determine which milling process is better for fabricating Cu-Y2O3 composites. It is found that Cu-Y2O3composites synthesized by DBM exhibit better densification, mechanical and electrical properties than those by WBM. Less agglomeration of reinforcements in the bulk composites by DBM is responsible for the better performances. To further understand the reason of less agglomeration of Y2O3 in the bulks by DBM, morphologies of prepared powders were investigated and analyzed. Higher ball's impact energy and the formation of copper oxide on the matrix surface during DBM process contribute to small matrix particles, which is beneficial for less agglomeration.
Keyword:
Cu-Y2O3 composites; Dry ball milling; Wet ball milling; Spark plasma sintering; Morphology;
Received: 4 April 2017/
1 Introduction
Particulate-reinforced copper matrix composites(PRCMCs) are a kind of artificial synthetic materials.Excellent electrical and thermal conductivities of copper and high strength,high modulus of the particle reinforcements are adopted for great performance of the composites.The most common particle reinforcements include oxides(e.g.,Al2O3),carbides (e.g.,SiC,TiC),nitrides (e.g.,AlN,Si3N4),borides (e.g.,TiB2,ZrB2).Oxides of trivalent rare earth oxides (RE2O3) exhibit excellent thermodynamic stability,but most of them are less used as inclusions in matrix for their fancy price
One of the most widely used methods in preparing PRCMCs is solid-state processing which involves many steps prior to final consolidation
Ball milling methods are usually pided into dry ball milling (DBM) and wet ball milling (WBM).DBM is mainly carried out in the environment of gas atmosphere,e.g.,argon ambient.WBM is mainly conducted in liquid environment,e.g.,ethanol.Both of the ball milling methods are used in fabricated metal matrix composites
2 Experimental
High-purity electrolytic copper powders and yttria powders were used as raw materials to fabricate CMCs.Average particle sizes of Cu and Y2O3 are 43 and 4μm,respectively.Contents of Y2O3 are 2 vol%,5 vol%,8 vol%and12 vol%.Powders are milled in horizontal ball mill (Xianyang Jinhong General Machinery Ltd.,GMJ/B) for 24 h.WBM was carried out in ethanol for it is easy to remove after milling.DBM was done in sealed nitrogen atmosphere,and nitrogen with the least protective effect was to represent commonly used atmosphere (H2,Ar and N2)
The milled composites powders were sintered in a SPS device (Sumitomo Electric,SPS-1050).The sintering temperature was up to 920℃for 25 min,holding for5 min at the maximum temperature,followed by cooling in the furnace to room temperature.The entire heating was done under a vacuum below 10 Pa with a uniaxial pressure of 45 MPa.The geometry of the samples was a pellet of30 mm in diameter and 5 mm in height.Pure Cu powders were also processed according to the same procedure for the purpose of comparison.
Phases of bulk samples were analyzed by X-ray diffractometer (XRD,Bruker's,D8-Advance).The distribution of yttria in bulk composites and morphology of milled powders were observed and analyzed by scanning electron microscope (SEM,TESCAN,MIRA3 LMH).Laser particle size analyzer (LPSA,Sympatec GmbH,Helos/Rodos) was utilized to measure the distribution of particle size of milled CuY2O3 composite powders by different milling methods.Details of milled particle micro structure and contents were also studied by transmission electron microscopy (TEM,FEI,Tecnai G2-20).
Density of the consolidated Cu-Y2O3 composites was determined by Archimedes drainage method.Electrical conductivity of each sample was tested 5 times by SIG-MASCOPE SMP10 eddy current electro-conductive device at room temperature.Average values were adopted in the analysis.The tensile test was carried out by low load small sample drawing machine (Shimadzu,EZ graph) with a strain rate of 1×10-3 s-1.Dimensions of tensile specimens for tensile tests are shown in Fig.1 according to Ref.
3 Results
3.1 Phase identification
XRD patterns of consolidated Cu-12 vol%Y2O3 microcomposites by different ball milling ways are illustrated in Fig.2.XRD patterns of the two samples both show the presence of pure copper and yttria only.No evidence of formation of any copper oxides in the composites is observed.Also,there is no signal for copper oxides in XRD patterns of 2 vol%,5 vol%,8 vol%Y2O3 samples,which are been shown here.
3.2 Distribution of Y2O3 particles
Back-scattered electron (BSE) images of Cu-Y2O3 composites by DBM and WBM are shown in Fig.3a-d,e-h,respectively.The bright regions in BSE images represent copper matrix,while the dark regions correspond to Y2O3particles.Morphology of Y2O3 particles in copper matrix is almost sphere-shape at low contents in Fig.3a,e.When the content of yttria increases,the shape of Y2O3 particles becomes more needle-like (Cu-12 vol%Y2O3 sample in Fig.3d,h).Taking WBM samples of CuY2O3 for example,agglomeration can be easy found with 5 vol%yttria,shown in Fig.3f,and the agglomeration size in this sample is approximately 10μm.More severe agglomeration is observed in the rest two samples with 8 vol%and 12 vol%Y2O3,as shown in Fig.3g,h.However,the Y2O3 particles exhibit slight agglomeration when DBM is introduced(Fig.3b,c).
Fig.1 Dimensions of specimen used for tensile tests (unit:mm)
Fig.2 XRD patterns of Cu-12 vol%Y2O3 composites by different ball milling ways
3.3 Densification
Figure 4a demonstrates density of Cu-Y2O3 composites against yttria content using DBM and WBM processes.The line with square stands for theoretical density,and those with circle and triangular symbols represent sintered density by DBM and WBM,respectively.With the increase in the content of Y2O3,both theoretical density and sintered density decrease.With the same Y2O3 content,the density of SPS sintered samples by DBM is obviously higher than that by WBM and closer to the theoretical density.Figure 4b shows densifications of SPS sintered samples.The densifications of DBM samples are about 98.5%,while those of WBM samples vary from 94%to 96%.This is due to less agglomeration of yttria particles in the bulk composites.It should also be noted that the densification values are higher than those prepared by traditional powder metallurgy technology (up to 94%)
3.4 Mechanical properties
True stress-strain curves of the CuY2O3 composites fabricated by different milling processes are shown in Fig.5.It is seen that the plastic property of the composites,no matter by DBM or WBM,decreases dramatically with the content of yttria increasing.In case of DBM,the yield strength of 8 vol%yttria-reinforced CMCs reaches the maximum value of about 239.8 MPa,and its elongation is7.34%.In case of WBM,the yield strength reaches the maximum of 138.2 MPa with yttria content of 12 vol%,and its elongation is only 2.78%.The results indicate that the elongation,yield strength and ultimate tensile strength of sintered samples by DBM are higher than those by WBM with the same content of yttria.
Fig.3 BSE images for distribution of Y2O3 in copper matrix by different ball milling processes:a DBM 2 vol%,b DBM 5 vol%,c DBM8 vol%,d DM 12 vol%,e WBM 2 vol%,f WBM 5 vol%,g WBM 8 vol%and h WBM 12 vol%
Fig.4 a Density and b densification of Cu-Y2O3 composites by different ball milling processes
Fig.5 True stress-strain curves of Cu-Y2O3 composites by a DBM and b WBM processes
The Vickers hardness with different contents of yttria is shown in Fig.6.The hardness values of DBM and WBM samples are on the rise with the content of yttria increasing.Note that the Vickers hardness of bulk samples by DBM can achieves the maximum of HV 112.8,increasing by73.5%compared with that of pure copper (HV 65.0).While the maximum Vickers hardness of WBM samples is HV91.3,increasing only 40.5%compared with that of pure copper.The results of microhardness of sintered samples are consistent with the stress-strain results.
Fig.6 Microhardness of Cu-Y2O3 composites by different ball milling processes
3.5 Electrical conductivity
Electrical conductivities of the Cu-Y2O3 composites produced by different ball milling methods are depicted in Fig.7.It illustrates that the conductivities of both DBM and WBM samples decrease along with the increase in the content of Y2O3.The conductivity of Cu-2 vol%Y2O3sample by DBM is 93.3 IACS%,while that by WBM is87.2 IACS%.When the content reaches 12 vol%,both conductivity of samples by DBM and WBM drops below80 IACS%.
Fig.7 Electrical conductivity of Cu-Y2O3 composites by different ball milling processes
4 Discussion
Properties of the sintered samples by two different ball milling processes are summarized in Table 1,e.g.,densification,yield strength,hardness and electrical conductivity.The properties of consolidated samples by DBM are generally superior to those by WBM.Also,the performance of the Cu-8 vol%Y2O3 sample by DBM is the best one—densification,Vickers hardness,yield strength and electrical conductivity of 99.07%,HV 112.8,239.8 MPa,and 83.0 I ACS%,respectively.This is obviously due to the distribution of Y2O3 in the matrix.However,it remains unclear why DBM leads to better distribution of yttria.
On the one hand,collision energy produced by falling impact of ball is one of the important parameters for ball milling.Therefore,during WBM,the impact force of the ball on powders is weakening due to the resistance of liquid to falling ball.The lower impact force of the ball influences particle size of both reinforcements and matrix.On the other hand,contamination of oxygen is unavoidable in the nitrogen atmosphere during DBM,so the copper powder surface tends to form small amount of oxides.These oxides are brittle and easy to crush to fine powders under the continuous impacts of the ball.Fine powders make less room for yttria to be aggregated.However,in the process of WBM,the copper powders are protected from oxygen in ethanol environment to form any oxide.Therefore,no brittle oxides effect can be expected in WBM case.The above explanation has been validated from two aspects:micro structure of copper powders after DBM and WBM and determination of oxygen content on the surface of copper powders.
SEM images for Cu-8 vol%Y2O3 composites powders of DBM and WBM before sintering are shown in Figs.8and 9,respectively.Figure 10 illustrates particle size distributions of the powder samples referring to Figs.8 and 9.In case of DBM,the particle size range (d10-d90) is between 4.19 and 30.50μm and the arithmetic mean size is13.94μm.Please note that dx is the cumulative particle size for x%of particles.The morphology of copper shows flat flake shape with 1-2μm in thickness.The surface of the flake is relatively smooth,and some Y2O3 particles (about1μm) are embedded on it.In the case of WBM,the morphology of copper is still flat flake shape,but it is much thicker and coarser than that of DBM.Meanwhile,there are some Y2O3 particles (around 2μm) embedded on the surface.The particle size range (d10-d90) is from 5.19 to43.41μm,and the arithmetic mean size equals 18.94μm.The greater the particle is,the larger space exists between these particles.The larger space between copper powders will become flat during sintering.Therefore,the morphology of yttria clusters in bulk composite becomes“needlelike”shape.
If brittle oxides are formed on the copper surface,it is easier to obtain small copper particles.Therefore,there will be more oxygen on small particles surface than big ones.TEM was used to find the small copper particle by DBM and WBM.Figures 11 and 12 show the typical morphology of the small copper particles and energy-dispersive spectroscope (EDS) results using DBM and WBM,respectively.The oxygen contents on the surface are about1.4 wt%and 0.1 wt%,respectively.The results convince that more brittle oxides have been found on the copper particle surface by DBM—beneficial for particle fragmentation.
Table 1 Characterization of Cu-Y2O3 by different ball milling processes and SPS
Fig.8 SEM images of 8 vol%Cu-Y2O3 powders after 24-h DBM with different magnifications
Fig.9 SEM images of 8 vol%Cu-Y2O3 powders after 24-h WBM with different magnifications
Fig.10 Histograms of particle size distribution of 8 vol%Cu-Y2O3 powders with a DBM and b WBM processes
Fig.11 a TEM image of copper particle and b EDS results using DBM showing oxygen contents of 1.5 wt%
Fig.12 a TEM image of copper particle and b EDS results using WBM showing oxygen contents of 0.1 wt%
As long as a layer of brittle oxide surrounded copper powders improves fragmentation,it can be speculated that DBM is better for ductile matrix powders,while WBM is better for brittle matrix powders.Table 2 demonstrates usage profiles of different properties of the matrix and reinforcements by DBM or WBM.There are in total 36papers under consideration.When the raw material or the matrix is brittle particles,the number of literatures using WBM is 13
Table 2 Different properties of materials by different ball milling processes
In the case of copper powders,almost all the studies
5 Conclusion
Two series Cu-Y2O3 composites powders with different contents of reinforcements were prepared by DBM and WBM,respectively.Microstructure,mechanical properties and electrical conductivity of these two series SPS sintered samples were analyzed.The mechanical properties and electrical conductivities of sintered samples fabricated by DBM with hydrogen reduction are better than those prepared by WBM.The performance of Cu-8 vol%Y2O3composites prepared by DBM is the best one among all the samples.The densification,Vickers hardness,tensile strength and electrical conductivity are 99.07%,HV 112.8,239.8 MPa and 83 IACS%,respectively.Less agglomeration of reinforcements in consolidated composites by DBM is attributed to the stronger ball's impact force and formation of copper oxide on the matrix surface during milling process.When the matrix is ductile material,finer particles and better properties of sintered materials can be obtained by DBM.However,if the matrix is brittle,it is better to use WBM.
Acknowledgements This study was financially supported by the National Key Research and Development Program of China (No.2016YFB0301400),the Organization Department of Jiangxi Province(No.2012215),the Education Department of Jiangxi Province (No.KJLD13041) and the Outstanding Doctoral Dissertation Project Fund of JXUST (No.YB2017011).One of the authors Fei Huang would like to thank Prof.Hao Chen and Dr.Xue-Hui Zhang for their advice.
参考文献
[1] Liu G. Rare Earth Materials. Beijing:Chemical Industry Press;2011. 44.
[42] Huang JY, Wu YK, Ye HQ. Ball milling of ductile metals.Mater Sci Eng A. 1995;199(2):165.