Superhard Copper Matrix Composite Reinforced by Ultrafine Boron for

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A Superhard Cu Matrix Composite Reinforced by Ultrafine Boron for Wear-resisting Bearings Shuo Zhao, Yuying Wu, Zuxin Sun, Bo Zhou, and Xiangfa Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01299 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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ACS Applied Nano Materials

A Superhard Cu Matrix Composite Reinforced by Ultrafine Boron for Wear-resisting Bearings Shuo Zhaoa, Yuying Wu*a, Zuxin Suna, Bo Zhoub and Xiangfa Liua a. Key Laboratory for Liquid−Solid Structural Evolution & Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, China b. Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing, 100124, China

Abstract The mechanical properties of traditional dispersion strengthened composites are greatly limited due to the weak interfacial bonding between metal matrix and ex-situ reinforcement. Here we report a Cu matrix composite with high hardness strengthened by high density dispersed nanoscale in-situ boron, having great potential in structure materials and functional materials, such as wear-resisting bearings. The hardness of the composite can reach 3.04GPa by nanoindentation test. The composite was fabricated by powder metallurgical process, in which the boron was produced in Cu melt directly. Nanotwins and lattice distortion zones observed in the phase boundaries may contribute to the ultrahigh hardness.

Keywords Ultrafine boron; Cu composite; Hardness; Interface; Nanotwins.

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Copper is widely used for structural materials and functional materials due to its high electrical and thermal conductivity, but the poor mechanical properties limit its application. Therefore, in pursuit of the high hardness and other mechanical properties, copper matrix composites reinforced by second-phase nanoinclusions have been ongoing1. In view of this, other elements or particles are introduced into copper as reinforcements2. For instance, carbides(B4C3, SiC4), borides(TiB25, MgB26) and oxides(Al2O37) have been widely investigated as reinforcing particles in copper and these ceramic particles do work indeed. To meet the requirements in some special fields, such as copper foils8, coatings9 and parts with high wear resistance10, the ultrahigh hardness of copper matrix composites is vital. In recent years, not only ceramic particles aforementioned can be used in metal matrix composites, but carbon nanotubes(CNTs) and graphene are candidates. Plenty of studies referring to Cu/C composites have been reported11-12. P.Jenei and his co-workers synthesized a copper-carbon nanotube composite, which showed the maximum hardness value of 2.5GPa11. However, there are some problems with these copper matrix composites, for example, the weak interfacial bonding between ex-situ phases and matrix can lead to the decrease in performance13-14. More importantly, some complex shaped parts are also needed to possess excellent mechanical properties, such as wear-resisting bearings, bending pipes and so on. In view of this, the formability of these parts may be affected by the weak interfacial bonding. In contrast, the composites strengthened by in-situ reinforcements have great potential in these applications. Boron possesses high hardness(Knoop:2160-2900), low density(2.340g/cm3) and high Young Modulus(380-400GPa)15, having great potential to be a superb candidate for reinforcement in metal. However, due to the high cost of pure boron and the difficulties in synthesis, there are rarely researches on boron as an enhancer in composites so far. Previously we prepared Cu-B alloys using a direct melt reaction, but coarse primary boron as well as eutectic boron along the grain boundary restricts the further application of the alloys16. Here we report a Cu/B composite fabricated by powder metallurgical process. The XRD and ACS Paragon Plus Environment

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EDX results reveal that the second phase in the composite is boron with a high atomic concentration of elemental boron(>98%). In contrast of the Cu matrix composites mentioned above, the interfacial bonding between copper matrix and boron might be stronger, because the in-situ boron was produced directly in Cu melt by a reduction reaction. In our study, the hardness of the composite was measured to 3.04GPa by nanoindentation test and a large number of nanoscale boron particles were observed. Besides, the high density boron might play an important role in the formation of nanotwins and lattice distortion placed in the phase boundaries, which are also related to the hardness of composite. It is well known that the higher content of the second phases or impurities, the greater impact on the conductivity and thermal conductivity of copper. This study focuses on the effect of the second phase on the microstructure of the matrix, thus greatly improving the mechanical properties of copper under the condition of low boron content. In this work, pure Cu(99.9wt.%), pure Al(99.9%) and B2O3 were used to prepare the raw Cu-B alloys, by which the reaction product Al2O3 floated in the melt surface and cleaned up as slags17. The Cu-B powders, which were synthesized by atomization of Cu-B alloys, were processed by high energy planetary ball milling for 18 hours and the ball-powder ratio is 4:1 with the rotary speed of 360rpm. In order to observe the microstructure of the powder, we embedded the powders into the bakelite and polished it. The powders were pre-compacted under the hydraulic pressure of 50MPa. The pre-compacted bulk having a diameter of 24mm and a height of 6mm was densified by pressureless sintering for 1 hour in vacuum and the applied temperature was 1273K. The preparation of the composite was briefly depicted in figure 1. More sintering time and higher temperature mean the coarser boron particle, while low density may occur under less sintering time and lower sintering temperature. Therefore, the applied parameters could achieve a balance between the boron size and the density of composites.



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Figure 1. The preparation of the Cu/B composite.

Thereafter, samples cut in the middle of the obtained bulk were polished to slices, which were divided into two. Therein, one sample was used in nanoindentation test, and the other was used in structure observation. In order to study the morphology and size of the second phase in powders, the Cu substrate was eroded using 50% HNO3 solution, and the obtained powders were extracted using a centrifuge. Microstructures characteristic was performed utilizing optical microscope, field emission scanning electron

microscope(FESEM,

SU-70

SEM,

Japan),

High-Resolution

Transmission

Electron

Microscope(HRTEM, ZEISS LIBRA 200FE, German) and EDX using FEI Titan G2 TEM(ETEM, German). Phase identification was characterized by RigaKuD/Max-Rb X-ray diffraction (XRD) using Cu Kα radiation. The nanoindentation tests were performed on the polished surface of the specimens by continuous stiffness method utilizing Nanoindenter(G200 AGILENT, America). In order to achieve the best enhancement effect and avoid the appearance of large size primary boron, the Cu-B alloy with 2wt.% boron was selected in this work. Meanwhile, a relative high content of boron will increase the viscosity and melt point of the melt, which makes the atomization more difficult. At the beginning, the raw Cu-B alloy was prepared by a direct melt reaction method as reported previously, in which high purity boron was obtained in copper melt due to the low solubility of boron in copper at room temperature16. In order to identify the phases, Cu-B powders are heated to 1273K, so that the atoms are diffused sufficiently. The XRD result of the powders extracted from the heated sample is shown in figure 2(c) and two main boron structures are characterized. Crystal faces of beta-rhombohedra boron and tetragonal boron are labeled by black and red round brackets, respectively, indicating that the second phase is boron ACS Paragon Plus Environment

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exactly. The spherical powders with the diameter of about 150µm and the microstructure of the Cu-B powders are shown in figure 2(a). In the Cu-B powders with high-speed cooling rate, the eutectic boron with sizes ranging from a few hundred nanometers to several microns is distributed along the grain boundary, but no nanoscale boron was observed in the field of view. Figure 2(b) shows a typical structure of boron with coral-shape extracted from the powders, implying the morphology of boron is not suitable as reinforcement, which is easy to lead to stress concentration at the weak grain boundaries. Therefore, it is difficult to improve the performance of Cu alloys greatly.

Figure 2. Microstructures and phase identification of Cu-B powders: (a)SEM image of the Cu-B powders after atomization; (b)SEM image of coral-shaped boron extracted from Cu-B powders by HNO3; (c) the XRD results of powders extracted from Cu-B powders heated at 1273K. Crystal faces of rhombohedra boron and tetragonal boron are labeled by black and red round brackets, respectively.

The insert image in figure 3(a) shows the shape of the powders after ball milling, implying that compared ACS Paragon Plus Environment


with spherical ones, the specific surface area of the powders with various shapes could increase, which is beneficial to sintering. As shown in figure 3(a), the morphology and distribution of boron are both changed obviously. Granular boron is distributed uniformly on Cu matrix after ball milling. Boron particles with sizes ranging from several nanometers to several microns are both dispersed on Cu matrix as shown in figure 3(b) and 3(c), indicating that the morphology and distribution of boron particles have little change compared with that before sintering, while the density of materials is increased. The SEM image exhibits the aggregated distribution of the particles with large size. The insert image in figure 3(b) is a highly magnified image of a coarse particles region, showing that ultrafine particles are still distributed around coarse particles. In other words, although a few particles have been coarsened in sintering at high temperature, ultrafine particles are still dominant, even in “coarse particle regions”. A typical EDX spectrum of the region in figure 3(e) uncovers that boron is still the dominant chemical composition, while small quantities of elemental Al, Cu and O could also be detected. Thus, it is suggested that the second phase of the composite can be treated as boron, although other dissolved elements should not be ignored. Figure 3(d) shows a typical curve obtained in nanoindentation test, indicating the relationship between displacement into surface and hardness of the specimen, in which the load changes as the displacement into surface increases and the hardness-displacement curve is obtained. The hardness data rarely change until the displacement values up to 50nm, so the hardness of one point is calculated by averaging the data of 50nm to 100nm. In this report, ninety measurements of Cu matrix were obtained by this method and the average was 3.04GPa, even higher than the pure nanotwinned Cu(~2.7GPa)18 and most reported Cu matrix composites strengthened by ceramic reinforcements as demonstrated in table 1.


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Figure 3. the characteristics of Cu/B composite by this method: (a)SEM image of the Cu-B powders after ball milling; (b)SEM image shows the dispersion of boron; (c)TEM image of Cu/B composite shows the nanoparticles in matrix; (d) A typical hardness curve of the composite by continuous stiffness method; (e) HAADF image of boron; (f)EDX analysis of the section in e. Table 1. The hardness of other Cu matrix composites reported previously and this work. Type of

Fraction of

hardness

Fabrication method

Refs

reinforcements

reinforcements

Al2O3

1.0 wt%

138HV

Hot extruded and ECAP

7

Al2O3

1.0 wt%

201.2HV

Hot extruded and HPT

7

Pb

5 vol%

285HV

High energy mechanical milling

19

Carbon nanotubes

3 vol%

2.5GPa

High energy mechanical milling and RT

12

TiB2 and Pb

10 wt% TiB2+10 wt% Pb

2GPa

Ball milling and SPS

20

In-situ TiB2

2.5 wt%

1.7GPa

Rapid solidification

21

In-situ boron

2 wt%

3.04GPa

Ball milling and sintering

This work

To further investigate the effect of ultrafine boron on hardness, the composite was observed by high resolution transmission microscope. The insert image in figure 4(a) shows the distribution of elemental Cu and B in the field, revealing that the red region is boron and the green one is Cu matrix, respectively. Copper ACS Paragon Plus Environment


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is a metal with a low stacking fault energy(SFE), stacking faults and twins thus tend to form. Actually, superior strength and ultrahigh hardness have been exhibited by nanotwinned copper, in which dislocation motion is effectively blocked by coherent twin boundaries22. In view of this, a large number of studies concentrating on the structure strengthen, such as nanotwins, stacking faults, are ongoing. Here a classical nanotwinned structure is also observed and the twin boundaries were labeled by green lines in figure 4(b), so it can be measured that the twin-boundary spacing is about 20nm according to the scale(10nm) on the photo. The magnified images of region A and B indicate that the upper and lower regions present the same lattice arrangement, while the middle region is featured by a typical twinning relationship with others. Furthermore, nanoscale twin-boundary spacing is also observed on copper matrix far from the phase boundary as shown in figure 4(c-d). In contrast to that near phase boundary, the twin-boundary spacing is larger, reaching about 50nm. Twins are easily formed in metals especially in copper, but the nanotwinned structure is hitherto hardly obtained in bulk materials. Zhao WS et al investigated a novel technique named Dynamic Plastic Deformation(DPD) for synthesizing bulk nanostructure metals23, in which high density nanotwins were observed in copper, suggesting that nanotwinned structure also can be induced in metals by plastic deformation. High energy ball milling is a process in which metallic powders are fully deformed and cold welded, while plastic deformation is happening rapidly, so high density twins may be induced by ball milling. Besides, annealing twins are likely to appear in deformed metals during heating process, especially in metals with low SFE24. On the one hand, some studies indicate that a small amount of second phase particles is beneficial to the formation of annealing twins25-26. On the other hand, heat treatment can lead to the disappearance of twins27, but the second phase particles could pin the grain boundary, blocking the diffusion of atoms, so that some twins remain at phase boundaries28. Therefore the nanoscale boron particles may play a more important role in the formation of high density twins. In spite of this, more research is still needed to confirm the effect of nanoscale boron on the formation of twins in copper. ACS Paragon Plus Environment

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Figure 4. The HRTEM micrographs of Cu matrix in Cu/B composite: (a) the TEM image of Cu/B composite; (b) the HRTEM image showing twins of Cu aside the phase boundary between Cu and boron; (c)(d) the nanotwinned structure in Cu matrix; (e) the magnified HRTEM image showing the structure with lattice distortion; (f) the HRTEM image showing the lattice distortion in Cu matrix aside PB.

Interestingly, a lattice distortion zone with a width of about 10nm on Cu matrix appears in the phase ACS Paragon Plus Environment


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boundary of copper and boron. The magnified images of region e and f labeled by white dotted frame are shown figure 4(e-f). The classical (111) planes of Cu is expressed by green solid line, the interplanar spacing of which is 0.207nm approximately, presenting a typical lattice structure of Face-Centered Cubic(FCC). However, the arrangement of Cu atoms becomes irregular near phase boundary in contrast to the adjacent region. To make it more convenient to observe, these two regions are roughly separated by a white solid curve. The initial (111) planes arrange along direction marked by the green arrow regularly, while in the right region the direction changes obviously as shown in figure 4(e). The orange dotted lines are marked to label the planes having been deformed, the spacing of which is about 0.162nm, but the other two planes labeled by purple and orange solid lines, seem still similar as the FCC structure in region A. Therein a typical structure of lattice distortion is characterized. The formation of lattice distortion zones adjacent to phase boundaries could be ascribed to the reduction of the interfacial energy. The red dotted line in figure 4(f) points out a plane of boron. The spacing was about 0.163nm, almost coherent with the lattice distortion zone completely. Thus it is speculated that the lower lattice mismatch may results in the appearance of the distortion region. Similarly, the transition zone induced by large angle grain boundary has been reported many times, such as “9R” phase in aluminum alloys29 and copper30, and it has been identified to play an important role in improving the mechanical properties. Moreover, high density staking faults and dislocations are also observed in this region, suggesting that boron particles might block the elimination of crystal defects during recrystallization. It is well known that work hardening could be able to cause the increase of the hardness due to high density lattice distortion zones via deformation in metals. Considering the ultrafine boron with sizes of only nanometer-scale or submicron-scale, the effect of the lattice distortion zones cannot be ignored thoroughly. In view of this, the high hardness of this composite is partly related to e lattice distortion zones placed at the phase boundaries. In summary, a novel Cu matrix composite strengthened by nanoscale in-situ boron was synthesized ACS Paragon Plus Environment

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successfully and high hardness was obtained. SEM and TEM images indicate that milling and sintering changed the size and distribution of boron in Cu-B alloy obviously. We mainly contribute the high hardness to the high density dispersed stiff boron. Meanwhile, the ultrafine boron particles might induce the formation of nanotwins and lattice distortion observed in the phase boundaries, which may also work on the mechanical properties of the composite.

Acknowledgment This work is supported by National Natural Science Foundation of China (Grant number: 51001065 and 51571133), Natural Science Foundation of Shandong Province (Grant number: ZR201702200041) and the Young Scholars Program of Shandong University (YSPSDU).

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the TOC graphic



Figure 1. The preparation of the Cu/B composite. 379x111mm (96 x 96 DPI)


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Figure 2. Microstructures and phase identification of Cu-B powders: (a)SEM image of the Cu-B powders after atomization; (b)SEM image of coral-shaped boron extracted from Cu-B powders by HNO3; (c) the XRD results of powders extracted from Cu-B powders heated at 1273K. Crystal faces of rhombohedra boron and tetragonal boron are labeled by black and red round brackets, respectively. 518x451mm (150 x 150 DPI)



Figure 3. the characteristics of Cu/B composite by this method: (a)SEM image of the Cu-B powders after ball milling; (b)SEM image shows the dispersion of boron; (c)TEM image of Cu/B composite shows the nanoparticles in matrix; (d) A typical hardness curve of the composite by continuous stiffness method; (e) HAADF image of boron; (f)EDX analysis of the section in e. 409x456mm (150 x 150 DPI)


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Figure 4. The HRTEM micrographs of Cu matrix in Cu/B composite: (a) the TEM image of Cu/B composite; (b) the HRTEM image showing twins of Cu aside the phase boundary between Cu and boron; (c)(d) the nanotwinned structure in Cu matrix; (e) the magnified HRTEM image showing the structure with lattice distortion; (f) the HRTEM image showing the lattice distortion in Cu matrix aside PB. 294x500mm (150 x 150 DPI)


Superhard Copper Matrix Composite Reinforced by Ultrafine Boron for

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