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Study of enhanced dry sliding wear behavior and mechanical properties of Cu-TiB2 composites fabricated by in situ casting process
Author’s Accepted Manuscript Study of enhanced dry sliding wear behavior and mechanical properties of Cu-TiB2 composites fabricated by in situ casting process Cunlei Zou, Zongning Chen, Huijun Kang, Wei Wang, Rengeng Li, Tingju Li, Tongmin Wang www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(17)31007-4 http://dx.doi.org/10.1016/j.wear.2017.09.016 WEA102252
To appear in: Wear Received date: 20 June 2017 Revised date: 20 September 2017 Accepted date: 22 September 2017 Cite this article as: Cunlei Zou, Zongning Chen, Huijun Kang, Wei Wang, Rengeng Li, Tingju Li and Tongmin Wang, Study of enhanced dry sliding wear behavior and mechanical properties of Cu-TiB2 composites fabricated by in situ casting process, Wear, http://dx.doi.org/10.1016/j.wear.2017.09.016 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 galley proof before it is published in its final citable 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.
Study of enhanced dry sliding wear behavior and mechanical properties of Cu-TiB2 composites fabricated by in situ casting process Cunlei Zou, Zongning Chen, Huijun Kang*, Wei Wang, Rengeng Li, Tingju Li, Tongmin Wang*
Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Material Science and Engineering, Dalian University of Technology, Dalian 116024, China Corresponding email: [email protected] (Huijun Kang) [email protected] (Tongmin Wang) Abstract: In this work, TiB2 particulate reinforced copper matrix composites were fabricated by casting method, based on in situ precipitation reaction between B and Ti elements to form TiB2 particles in molten copper. The microstructures of the Cu-TiB2 composites were characterized using SEM and TEM. The results show that TiB2 particles are successfully formed in the copper matrix and the interfaces between these particles and the matrix are clean and well bonded. The friction and wear characteristics of the Cu-TiB2 composites were determined by carrying out dry sliding tests on pin-on-disk machine under varying loads, sliding speeds and sliding distances. A comparison between the volume wear losses of the composites under different conditions shows a significant improvement in wear property of the composites with respect to the pure copper. Furthermore, the mechanical properties of the composites with different TiB2 levels were also investigated. Both tensile strength and hardness are significantly improved with the increasing amount of TiB2 in copper matrix, while compromises of the elongation and electrical conductivity nevertheless occur in all cases.
Key words: Copper matrix composite; Microstructure; Wear; Mechanical properties; Electrical conductivity 1. Introduction A large number of copper alloys are being developed for high performance applications in integrated circuit lead frames, the electrodes of resistance welding, rail transit contact wires and so on [1, 2]. However, the unwanted inverse relationship between wear resistance and electrical conductivity limits the expansion of their application fields. One alternative to address this issue is to introduce ceramics into copper or copper alloys to fabricate the so-called copper matrix composites (CMCs). There are many methods developed for preparing CMCs, such as mechanical alloying, powder metallurgy, spray deposition, in-situ casting synthesis, self-propagating high-temperature synthesis, etc. [3-6] Among all these methods, in situ synthesis based on traditional casting process can offer good particle/matrix interface as well as great potential for massive production owing to its low cost [7, 8]. Many kind of ceramic particles are considered as good candidates for reinforcing copper, i.e. TiC, Al2O3, Y2O3 and TiB2 [9-14]. Among those reinforcing phases, TiB2 is a typical ceramic with high melting point about 2970 °C, high hardness value about 34 GPa, high Young modulus, good creep resistance, good thermal conductivity (~65 W·m-1·K-1), high electrical conductivity (14.4 μΩ.cm) and considerable chemical stability [15, 16]. But above all, it is feasible to formTiB2 from Ti and B elements in copper melt, making TiB2 an ideal reinforcement for copper matrix composite [17]. Wear property, electrical conductivity and mechanical properties are required
simultaneously in service process in practice, such as rail transit contact wires, electrodes of resistance welding, etc. Copper matrix composites with well integrated properties are considered to be good candidates for friction and wear applications [18-22]. Moreover, it was reported that the addition of secondary reinforcements offers a barrier when the composites suffered frictional stresses [23-25]. Therefore, it is desirably expected that good TiB2/matrix interface is beneficial to the wear property of the materials. In this study, different percentages of TiB2 reinforcement from 0.5 % (weight percentage, all in the same unit unless otherwise specified) to 1.5 % were in situ synthesized in the copper matrix. The microstructures, electrical conductivities, mechanical properties were investigated. The effects of load, sliding speed and sliding distance on the wear performance of Cu-TiB2 composites were discussed. 2. Experimental procedure 2.1 Materials preparation The Cu-TiB2 composites were prepared using pure Cu (99.97% purity) as the matrix, Cu-5 wt.%B and Cu-10 wt.%Ti master alloys as the reactive agents. The composites were prepared in a vacuum medium frequency induction melting furnace. Firstly, pure copper was put in the crucible and melt to 1300 °C. Then, Cu-B and Cu-Ti master alloys were incorporated into the melt sequentially. After holding for 5 minutes, TiB2 particles were formed via chemical reactions between Ti and B. Then the melt was poured into a cylindrical graphite mould (45 mm in diameter and 220 mm in height). After homogeneous annealing at 960 °C for 3 h, the as-cast billets
were rolled at 850 °C with a 17 % reduction (from 30 mm to 25 mm) and then further rolled to 5.5 mm with a total of 78 % reduction at the room temperature. A series of Cu-TiB2 composites with the TiB2 mass fraction of 0.5, 1 and 1.5 were prepared. For comparison, a reference sample without TiB2 addition was also prepared. 2.2 Tests and analysis The rolled samples were ground, polished and then etched with a solution of 3 g FeCl3, 2 mL HCl and 95 mL C2H5OH for microstructures characterization. The microstructures of Cu-TiB2 composites were observed under a scanning electron microscopy (SEM, Zeiss Supra 55) operated at secondary electron mode. Phases in the Cu-TiB2 composites were identified using an X-ray diffractometer (XRD, EMPYREAN, Cu Kα radiation). The TEM and HAADF stem images were acquired in Talos F200x field emission transmission electron microscopy at an accelerating voltage of 200 kV. The tensile specimens, with a dimension of 28 mm gauge length, 6 mm width and 5 mm thickness, were machined from the samples according to the ASTM-E8 standard test methods for tension testing of metallic materials. Tensile tests with cross head speed of 2 mm·min-1 were conducted at room temperature. Vickers hardness tests were performed under a load of 100 g for 10 s using a Vickers hardness tester (MH-6L). For each test, ten measurements were performed at an interval of 1 mm and average experimental data were recorded. The electrical conductivities were measured by SIGMASCOPE SM-P350 eddy current conductivity meter. The size of the samples was bigger than 15 mm diameter and thicker than 2 mm. And five measurements were also performed for each test.
Friction and wear tests were conducted using a pin-on-disk apparatus (MVF-1A multifunctional friction and wear tester) at the room temperature. The specimens with ASTM G99-05 size were prepared from the rolled samples, as shown in Fig.1. The samples were machined into pin and the discs were made by a conventional 45# steel with a hardness of 465 HV. The tests were carried out at different loads, distances and sliding speeds. The volumetric loss was calculated by multiplying the cross section of the test pin with its loss of height. The worn surfaces and wear debris were observed using SEM and EDS.
Fig.1 The schematic diagram of pin-on-disk wear samples
3. Results and discussion 3.1 XRD Analysis The XRD patterns of the experimental composites with different contents of TiB2 reinforcing particles are given in Fig.2. With the increase of TiB2 contents, the reflection intensity of TiB2 peaks become more evident as seen in Fig.2, indicating that TiB2 particles are formed successfully via the in situ reactions between Ti and B atoms in molten copper. No other phases are observed except for the matrix Cu and TiB2 in the XRD patterns, which implies that TiB2 is the most thermally stable among
the possible Ti and B containing phases in copper.
Fig.2 XRD patterns of the CMCs with different mass fractions of TiB2
3.2 Microstructures The SEM images of Cu-1TiB2 are shown in Fig.3. It can be seen that TiB2 particles distribute uniformly in the copper matrix. The Fig.3 (b) and (c) are the point scanning results of EDS in point A and the corresponding element mapping results in Fig.3 (a). Only Ti and B elements are observed at the particulate sites. This is in accordance with the XRD analysis given above, confirming that only TiB2 is formed.
Fig. 3 SEM and EDS results of the Cu-1 wt.% TiB2 composite: (a) SEM image, (b) EDS point scanning pattern of point A in (a), (c) EDS mapping of image (a)
Transmission electron microscopic (TEM) and high angle annular dark field stem (HAADF-STEM) was used to study the interface between in situ synthesized TiB2 and the copper matrix, as shown in Fig.4. Fig.4 (a) presents the bright field image of the composite and the selected-area electron diffraction (SAED) pattern corresponding to the interface area. No obvious orientation relationship is observed between copper and the TiB2 particle. The high-resolution transmission electron microscopic (HRTEM) images of the interface shows that there exists a stacking fault structure of copper at the interface, which may compensate for the misfit between the TiB2 particle and the matrix. A very clean interface is clearly seen in the image, which shows that in situ formed TiB2 is well bonded to the copper matrix.
The Fig.4 (c) and (d) show the HAADF-STEM image and a line scan across the interface between copper and TiB2 particle, respectively. The dark gray part is TiB2 and the light gray part surrounding is identified to be copper matrix (Fig.4 (c)). The line scans of Cu and Ti in Fig.4 (d) reveals a well interface transition between TiB2 particle and copper matrix, which also confirms the well bonded interface.
Fig.4 TEM and HAADF STEM images of Cu-TiB2 composite (a) TEM image and interfacial electron diffraction of Cu-TiB2, (b) High resolution of interface between TiB2 and copper, (c) HAADF-STEM of Cu-TiB2, (d) Elements distribution of Cu and Ti in the marked position in (c)
3.3 Wear properties The wear macroscopic morphologies of copper and Cu-TiB2 composites under
different loads are shown in Fig.5. For pure copper, with the increase in load, the wear area is significantly enlarged, and the surface becomes macroscopically smooth. This is especially evident when the load is greater than 15 N. It is not surprising to see that the composites with more TiB2 have better wear resistance than pure copper, which indicates that the in situ formed TiB2 particles are beneficial to the composites when friction and wear behavior occur.
Fig.5 SEM images of Cu and Cu-TiB2 composites under different loads Fig.6 shows the effects of load, sliding speed and sliding distance on the volume loss rate of the Cu-TiB2 composites. The wear volume loss of pin can be determined by equation (1) as follows [26]: Pin volume wear loss = (πh/6)[3d2/4+h2]
(1)
where h = r-[r2-d2/4]1/2, d = wear scar diameter, and r = pin end radius. The wear scar diameters are measured using SEM images by Image-Pro Plus at a fixed magnification 64x. In addition, the wear loss of disk is neglected due to the very few
abrasion loss. Fig.6 (a) shows the variations of pin volume wear loss of copper and Cu-TiB2 composites at the loads of 15 N, 30 N, 45 N and a fixed sliding speed of 100 r/min (equal to 120.42 mm/s) and fixed sliding distance of 216.8 m. With the increase in load, it can be found from Fig.6 (a) that the volume wear loss of pure copper increases sharply compared to that of the Cu-TiB2 composites. When applying a relatively smaller load, the wear resistance of pure copper is nearly in a same level with the Cu-TiB2 composites. When increasing the load to a level higher than 15 N, however, the wear resistance of the composite tends to increase sharply with increasing of TiB2 fraction. This indicates that the TiB2 particles effectively improve the deformation resistance of the materials and Cu-TiB2 composites have better wear resistance than pure copper at the high load. Similar results have also been obtained in the cases with varying sliding speeds and distances, as presented in Fig. 6 (b) and (c). It is worth noting that the volume wear loss of pure copper shows a linear relationship with the sliding distance (Fig. 6 (c)), but once the sliding distance exceeds 120.42 m, the volume wear loss of Cu-TiB2 composites shows a relatively sharp escalating trend.
Fig.6 Volume wear loss of Cu and Cu-TiB2 composites under varying (a) load, (b) sliding speed and (c) sliding distance
Fig.7 shows the wear scar features of Cu and Cu-TiB2 composites. It can be seen that there exists distinct wear mechanism to explain the process for pure Cu and Cu-TiB2 composites. From Fig.5 and Fig.7 (a) ~(c), it is suggested that the wear mode of pure Cu should be adhesion and some fatigue wear at the beginning of wear process. As friction proceeds, the wear mode changes into abrasive wear and fatigue wear. Due to the low hardness of pure Cu, severe wear loss happens during the abrasive wear process, and the fatigue wear leads to the generation of a considerable mass of wear debris. On contrary to pure Cu, the Cu-TiB2 composites exhibit a different wear mode. At the very beginning, the adhesive wear dominates the wear mode of Cu-TiB2 composites, and no obvious fatigue wear is found, as shown in Fig. 7 (d) and (e). With the wear process continuing, the wear debris act as foreign particles, which will plow grooves on the wear surface, as shown in Fig. 7 (f). During
the wear process, crack nucleation and propagation take place constantly. Due to the well bonding between TiB2 and copper matrix (Fig.4), TiB2 particles will act as barriers when cracks occur. This can significantly delay the crack nucleation and growth. Therefore, smaller volume wear loss was obtained from Cu-TiB2 composites. Fig. 7 (g)~(i) shows the EDS analysis of wear scar of Cu-TiB2 composite. The Fe and O elements are detected, which shows that the materials transformation and oxidation may occur during the wear process.
Fig.7 Wear surface of Cu and Cu-TiB2 composites: (a)~(c) wear surface of pure Cu at load of 30 N, sliding speed of 120.42 mm/s and sliding distance of 108.38 m, 216.76 m and 325.13 m, respectively. (d)~(f) Wear surface of Cu-1 wt.% TiB2 at load of 30 N, sliding speed of 120.42 mm/s and sliding distance of 108.38 m, 216.76 m and 325.13 m, respectively. (g)~(i) EDS analysis of the wear surface from Cu-1 wt.% TiB2
The morphologies of wear debris for Cu and Cu-TiB2 composites are shown in Fig. 8. The wear debris of pure Cu are generally composed of massive platelike particles (10 to 150 μm in diameter), which are generated on the occurrence of the
severe wear loss by fatigue wear mode. A few equiaxed particles are occasionally observed in the Cu debris, as shown in Fig. 8 (a), which act as intermediary of third-body abrasion. Unlike pure Cu, the wear debris of Cu-TiB2 composites is mainly composed of small (<5 μm) equiaxed particles. It is fair to conclude that the dominant rate-controlling mechanism in the wear test of pure Cu with lower load and sliding speed is adhesive and fatigue wear modes, but with increasing load and sliding speed, abrasive and fatigue wear modes become predominant and severe wear loss occurs at this point. In particulate reinforced metal matrix composites, although the adhesive and oxidation wear exist in varying degrees, the abrasive wear occupies an important position. Under abrasive wear conditions, the wear resistance is proportional to the hardness of the material because hard particles act as the main bearing phases that significantly improve the wear resistance of the composites. The TiB2 particles in copper matrix prevent or delay the occurrence of fatigue wear so that severe wear loss of Cu-TiB2 composites is postponed. Consequently, Cu-TiB2 composites with better wear performance are obtained.
Fig.8 Wear debris and EDS for wear debris: (a) wear debris of pure Cu, (b) wear debris of Cu-TiB2 composites, (c) and (d) EDS analysis of Cu-TiB2 wear debris
3.4 Mechanical properties and electrical conductivity The mechanical properties of the composites with a variety of different TiB2 mass fractions are summarized in Table 1. It is clear that TiB2 particles enhance the hardness and strength of copper significantly. The ultimate tensile strength (UTS) increases from 397.7 MPa for pure copper to 442.5 MPa for the CMC with 1.5 wt.%TiB2. The variation of hardness as a function of TiB2 content has a similar tendency with respect to the UTS. The improvements in UTS and hardness are mainly attributed to the dispersed hard TiB2 particles. Upon solidification, strain fields are created in the vicinity of TiB2 owing to the difference between the thermal expansion coefficients of the two. The interaction between the strain field and the dislocation offers resistance to crack propagation during tensile loading. Furthermore, good interfacial bonding between copper matrix and TiB2 particles enhances the load bearing capacity. However, the elongation at maximum stress decreases with the
increase in TiB2 content. Table 1 Mechanical properties and electrical conductivity of Cu-TiB2 composites Composite Composition (wt.%)
σb (MPa)
σ0.2 (MPa)
Hardness (HV)
Elongation at Maximum Stress (%)
Electrical Conductivity (%IACS)
Cu
397.7
386.5
126.3±1.5
1.19±0.04
99.92±0.02
Cu-0.5TiB2
414.4
391.4
131.2±1.8
1.17±0.28
89.64±0.07
Cu-1TiB2
423.5
397.5
134.3±2.6
1.09±0.07
85.83±0.06
Cu-1.5TiB2
442.5
409.8
142.8±3.6
0.86±0.03
73.43±0.04
Based on Refs. [13, 27], contribution of second-phase particles to yield strength of PRMMC can be divided into four categories: grain refinement strengthening, Orowan strengthening, coefficients of thermal expansion (CTE) strengthening and load bearing strengthening. Extrinsic particles could act as heterogeneous nucleating sites, which refines the grain size, thus improving the strength of the composite according to the Hall-Petch relationship. On the other hand, the interaction between copper matrix and TiB2 particles enhances the strength of the composite through two mechanisms: dislocation multiplication induced by the difference of CTE between copper matrix and TiB2 particles, and Orowan strengthening when a dislocation passes through two adjacent particles. In addition, the grain size is refined significantly when in situ TiB2 particles were introduced into copper matrix, as shown in Fig. 9. The decrease of grain size may mainly attribute to the heterogeneous nucleation based on TiB2 particles and the probably unreacted Ti or B atoms. The decrease of grain size also contributes a lot to the yield strength of Cu-TiB2 composites.
Fig. 9 EBSD images of copper with different content of TiB2: (a) Cu, (b) Cu-0.5 wt.% TiB2, (c) Cu-1 wt.% TiB2, (d) Cu-1.5 wt.%TiB2
The electrical conductivities of copper and Cu-TiB2 composites are shown in Table 1. Somehow detriment of electrical conductivity is caused because of the incorporation of TiB2 into the matrix. The electrical conductivity is decreased from 99.92 %IACS for pure copper to 73.43 %IACS for the Cu-1.5 wt. %TiB2 composite. It has been reported by Matthiessen’s rule [28, 29] that the resistivity of a crystalline metallic specimen includes the resistivity resulting from thermal agitation of the metal ions and the lattice and imperfections in the crystal. This can be written as equation (2): 𝜌 = 𝜌(𝑇) + 𝜌𝑖
(2)
where ρ is the total resistivity of the metal or alloy, ρ(T) is the resistivity of the metal which varies with the temperature and ρi is the resistivity induced by chemical and
physical defects which are independent on the temperature. It is well known that the solute atoms usually result in lattice distortion which will have a detrimental influence on resistivity [30]. Furthermore, the phonons, impurities, point defects, dislocations, grain boundaries and surface are all scattering centers of conduction electrons and thus decrease the electrical conductivity. In Cu-TiB2 composite, TiB2 particles act as impurities and the electrical conductivity of the composites will slightly decrease. Moreover, grain refinement due to TiB2 addition will lead to an increase in grain boundaries, which will have an adverse effect on the electrical conductivity. Besides, the difference of CTE between copper and TiB2 particles will generate many dislocations during solidification, and this will also lead to a decrease in the electrical conductivity of the composites. Nevertheless, the Cu-TiB2 composites (especially the Cu-1.0wt.% TiB2) exhibit an adequate conductivity compared to other specifications of solid solution strengthening copper alloys, such as 45 %IACS of Cu-Ni-Si [31], 81.45 %IACS of Cu-Cr-Zr [32] and 60 %IACS of Cu2.5Fe1P [33]. Therefore, the Cu-TiB2 composites with adequate electrical conductivity, good mechanical properties and low cost fabrication method have a good application foreground.
4. Conclusions In-situ TiB2 particulate reinforced copper matrix composites were fabricated using traditional casting method from the synthesis between B and Cu elements in molten copper. Four samples with 0, 0.5, 1 and 1.5 wt.% TiB2 in copper matrix were prepared and the mechanical properties, electrical conductivities and wear behaviors have been
investigated. Based on the results, the following conclusions are drawn. (1) The TiB2 particles in situ formed were observed to have a good interfacial bonding to the copper matrix. Stacking faults may compensate for the interfacial misfit between TiB2 and copper. (2) The wear performance of the Cu-TiB2 composites is improved significantly due to the presence of TiB2 and the volume wear loss at different loads, sliding distance and sliding speed of the composite decreases with the increase in TiB2 content. (3) Compared with pure copper, whilst the UTS and hardness of Cu-TiB2 composites also recorded significant improvements (From 397.7 MPa and 126.3 HV of pure Cu to 442.5 MPa and 142.8 HV of Cu1.5 wt.%TiB2), the presence of TiB2 impairs the elongation and electrical conductivity, probably due to the dissevering effect of TiB2 on the particle/matrix interface.
Acknowledgements The authors gratefully acknowledge the supports of National Key Research and Development Program of China (Nos. 2017YFA0403803, 2016YFB0701203), the National Natural Science Foundation of China (Nos.51525401, 51774065, 51690163, 51601028), Dalian Support Plan for Innovation of High-level Talents (Top and Leading Talents, 2015R013) and the China Postdoctoral Science Foundation (2017T100174). References [1] Lu K. The Future of Metals. Science. 2010;328:319-20. [2] Deng B, Hsu PC, Chen GC, Chandrashekar BN, Liao L, Ayitimuda Z, et al. Roll-to-Roll
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Graphical abstract
Highlights
Cu-TiB2 composites were in-situ synthesized by conventional casting method.
The grain size of the Cu-TiB2 composites was refined by the in situ TiB2 phase.
TiB2 significantly diminished the wear volume loss Cu-TiB2 composites.
TiB2 made an effective improvement on mechanical properties of Cu-TiB2
composites.
PII: DOI: Reference:
S0043-1648(17)31007-4 http://dx.doi.org/10.1016/j.wear.2017.09.016 WEA102252
To appear in: Wear Received date: 20 June 2017 Revised date: 20 September 2017 Accepted date: 22 September 2017 Cite this article as: Cunlei Zou, Zongning Chen, Huijun Kang, Wei Wang, Rengeng Li, Tingju Li and Tongmin Wang, Study of enhanced dry sliding wear behavior and mechanical properties of Cu-TiB2 composites fabricated by in situ casting process, Wear, http://dx.doi.org/10.1016/j.wear.2017.09.016 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 galley proof before it is published in its final citable 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.
Study of enhanced dry sliding wear behavior and mechanical properties of Cu-TiB2 composites fabricated by in situ casting process Cunlei Zou, Zongning Chen, Huijun Kang*, Wei Wang, Rengeng Li, Tingju Li, Tongmin Wang*
Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Material Science and Engineering, Dalian University of Technology, Dalian 116024, China Corresponding email: [email protected] (Huijun Kang) [email protected] (Tongmin Wang) Abstract: In this work, TiB2 particulate reinforced copper matrix composites were fabricated by casting method, based on in situ precipitation reaction between B and Ti elements to form TiB2 particles in molten copper. The microstructures of the Cu-TiB2 composites were characterized using SEM and TEM. The results show that TiB2 particles are successfully formed in the copper matrix and the interfaces between these particles and the matrix are clean and well bonded. The friction and wear characteristics of the Cu-TiB2 composites were determined by carrying out dry sliding tests on pin-on-disk machine under varying loads, sliding speeds and sliding distances. A comparison between the volume wear losses of the composites under different conditions shows a significant improvement in wear property of the composites with respect to the pure copper. Furthermore, the mechanical properties of the composites with different TiB2 levels were also investigated. Both tensile strength and hardness are significantly improved with the increasing amount of TiB2 in copper matrix, while compromises of the elongation and electrical conductivity nevertheless occur in all cases.
Key words: Copper matrix composite; Microstructure; Wear; Mechanical properties; Electrical conductivity 1. Introduction A large number of copper alloys are being developed for high performance applications in integrated circuit lead frames, the electrodes of resistance welding, rail transit contact wires and so on [1, 2]. However, the unwanted inverse relationship between wear resistance and electrical conductivity limits the expansion of their application fields. One alternative to address this issue is to introduce ceramics into copper or copper alloys to fabricate the so-called copper matrix composites (CMCs). There are many methods developed for preparing CMCs, such as mechanical alloying, powder metallurgy, spray deposition, in-situ casting synthesis, self-propagating high-temperature synthesis, etc. [3-6] Among all these methods, in situ synthesis based on traditional casting process can offer good particle/matrix interface as well as great potential for massive production owing to its low cost [7, 8]. Many kind of ceramic particles are considered as good candidates for reinforcing copper, i.e. TiC, Al2O3, Y2O3 and TiB2 [9-14]. Among those reinforcing phases, TiB2 is a typical ceramic with high melting point about 2970 °C, high hardness value about 34 GPa, high Young modulus, good creep resistance, good thermal conductivity (~65 W·m-1·K-1), high electrical conductivity (14.4 μΩ.cm) and considerable chemical stability [15, 16]. But above all, it is feasible to formTiB2 from Ti and B elements in copper melt, making TiB2 an ideal reinforcement for copper matrix composite [17]. Wear property, electrical conductivity and mechanical properties are required
simultaneously in service process in practice, such as rail transit contact wires, electrodes of resistance welding, etc. Copper matrix composites with well integrated properties are considered to be good candidates for friction and wear applications [18-22]. Moreover, it was reported that the addition of secondary reinforcements offers a barrier when the composites suffered frictional stresses [23-25]. Therefore, it is desirably expected that good TiB2/matrix interface is beneficial to the wear property of the materials. In this study, different percentages of TiB2 reinforcement from 0.5 % (weight percentage, all in the same unit unless otherwise specified) to 1.5 % were in situ synthesized in the copper matrix. The microstructures, electrical conductivities, mechanical properties were investigated. The effects of load, sliding speed and sliding distance on the wear performance of Cu-TiB2 composites were discussed. 2. Experimental procedure 2.1 Materials preparation The Cu-TiB2 composites were prepared using pure Cu (99.97% purity) as the matrix, Cu-5 wt.%B and Cu-10 wt.%Ti master alloys as the reactive agents. The composites were prepared in a vacuum medium frequency induction melting furnace. Firstly, pure copper was put in the crucible and melt to 1300 °C. Then, Cu-B and Cu-Ti master alloys were incorporated into the melt sequentially. After holding for 5 minutes, TiB2 particles were formed via chemical reactions between Ti and B. Then the melt was poured into a cylindrical graphite mould (45 mm in diameter and 220 mm in height). After homogeneous annealing at 960 °C for 3 h, the as-cast billets
were rolled at 850 °C with a 17 % reduction (from 30 mm to 25 mm) and then further rolled to 5.5 mm with a total of 78 % reduction at the room temperature. A series of Cu-TiB2 composites with the TiB2 mass fraction of 0.5, 1 and 1.5 were prepared. For comparison, a reference sample without TiB2 addition was also prepared. 2.2 Tests and analysis The rolled samples were ground, polished and then etched with a solution of 3 g FeCl3, 2 mL HCl and 95 mL C2H5OH for microstructures characterization. The microstructures of Cu-TiB2 composites were observed under a scanning electron microscopy (SEM, Zeiss Supra 55) operated at secondary electron mode. Phases in the Cu-TiB2 composites were identified using an X-ray diffractometer (XRD, EMPYREAN, Cu Kα radiation). The TEM and HAADF stem images were acquired in Talos F200x field emission transmission electron microscopy at an accelerating voltage of 200 kV. The tensile specimens, with a dimension of 28 mm gauge length, 6 mm width and 5 mm thickness, were machined from the samples according to the ASTM-E8 standard test methods for tension testing of metallic materials. Tensile tests with cross head speed of 2 mm·min-1 were conducted at room temperature. Vickers hardness tests were performed under a load of 100 g for 10 s using a Vickers hardness tester (MH-6L). For each test, ten measurements were performed at an interval of 1 mm and average experimental data were recorded. The electrical conductivities were measured by SIGMASCOPE SM-P350 eddy current conductivity meter. The size of the samples was bigger than 15 mm diameter and thicker than 2 mm. And five measurements were also performed for each test.
Friction and wear tests were conducted using a pin-on-disk apparatus (MVF-1A multifunctional friction and wear tester) at the room temperature. The specimens with ASTM G99-05 size were prepared from the rolled samples, as shown in Fig.1. The samples were machined into pin and the discs were made by a conventional 45# steel with a hardness of 465 HV. The tests were carried out at different loads, distances and sliding speeds. The volumetric loss was calculated by multiplying the cross section of the test pin with its loss of height. The worn surfaces and wear debris were observed using SEM and EDS.
Fig.1 The schematic diagram of pin-on-disk wear samples
3. Results and discussion 3.1 XRD Analysis The XRD patterns of the experimental composites with different contents of TiB2 reinforcing particles are given in Fig.2. With the increase of TiB2 contents, the reflection intensity of TiB2 peaks become more evident as seen in Fig.2, indicating that TiB2 particles are formed successfully via the in situ reactions between Ti and B atoms in molten copper. No other phases are observed except for the matrix Cu and TiB2 in the XRD patterns, which implies that TiB2 is the most thermally stable among
the possible Ti and B containing phases in copper.
Fig.2 XRD patterns of the CMCs with different mass fractions of TiB2
3.2 Microstructures The SEM images of Cu-1TiB2 are shown in Fig.3. It can be seen that TiB2 particles distribute uniformly in the copper matrix. The Fig.3 (b) and (c) are the point scanning results of EDS in point A and the corresponding element mapping results in Fig.3 (a). Only Ti and B elements are observed at the particulate sites. This is in accordance with the XRD analysis given above, confirming that only TiB2 is formed.
Fig. 3 SEM and EDS results of the Cu-1 wt.% TiB2 composite: (a) SEM image, (b) EDS point scanning pattern of point A in (a), (c) EDS mapping of image (a)
Transmission electron microscopic (TEM) and high angle annular dark field stem (HAADF-STEM) was used to study the interface between in situ synthesized TiB2 and the copper matrix, as shown in Fig.4. Fig.4 (a) presents the bright field image of the composite and the selected-area electron diffraction (SAED) pattern corresponding to the interface area. No obvious orientation relationship is observed between copper and the TiB2 particle. The high-resolution transmission electron microscopic (HRTEM) images of the interface shows that there exists a stacking fault structure of copper at the interface, which may compensate for the misfit between the TiB2 particle and the matrix. A very clean interface is clearly seen in the image, which shows that in situ formed TiB2 is well bonded to the copper matrix.
The Fig.4 (c) and (d) show the HAADF-STEM image and a line scan across the interface between copper and TiB2 particle, respectively. The dark gray part is TiB2 and the light gray part surrounding is identified to be copper matrix (Fig.4 (c)). The line scans of Cu and Ti in Fig.4 (d) reveals a well interface transition between TiB2 particle and copper matrix, which also confirms the well bonded interface.
Fig.4 TEM and HAADF STEM images of Cu-TiB2 composite (a) TEM image and interfacial electron diffraction of Cu-TiB2, (b) High resolution of interface between TiB2 and copper, (c) HAADF-STEM of Cu-TiB2, (d) Elements distribution of Cu and Ti in the marked position in (c)
3.3 Wear properties The wear macroscopic morphologies of copper and Cu-TiB2 composites under
different loads are shown in Fig.5. For pure copper, with the increase in load, the wear area is significantly enlarged, and the surface becomes macroscopically smooth. This is especially evident when the load is greater than 15 N. It is not surprising to see that the composites with more TiB2 have better wear resistance than pure copper, which indicates that the in situ formed TiB2 particles are beneficial to the composites when friction and wear behavior occur.
Fig.5 SEM images of Cu and Cu-TiB2 composites under different loads Fig.6 shows the effects of load, sliding speed and sliding distance on the volume loss rate of the Cu-TiB2 composites. The wear volume loss of pin can be determined by equation (1) as follows [26]: Pin volume wear loss = (πh/6)[3d2/4+h2]
(1)
where h = r-[r2-d2/4]1/2, d = wear scar diameter, and r = pin end radius. The wear scar diameters are measured using SEM images by Image-Pro Plus at a fixed magnification 64x. In addition, the wear loss of disk is neglected due to the very few
abrasion loss. Fig.6 (a) shows the variations of pin volume wear loss of copper and Cu-TiB2 composites at the loads of 15 N, 30 N, 45 N and a fixed sliding speed of 100 r/min (equal to 120.42 mm/s) and fixed sliding distance of 216.8 m. With the increase in load, it can be found from Fig.6 (a) that the volume wear loss of pure copper increases sharply compared to that of the Cu-TiB2 composites. When applying a relatively smaller load, the wear resistance of pure copper is nearly in a same level with the Cu-TiB2 composites. When increasing the load to a level higher than 15 N, however, the wear resistance of the composite tends to increase sharply with increasing of TiB2 fraction. This indicates that the TiB2 particles effectively improve the deformation resistance of the materials and Cu-TiB2 composites have better wear resistance than pure copper at the high load. Similar results have also been obtained in the cases with varying sliding speeds and distances, as presented in Fig. 6 (b) and (c). It is worth noting that the volume wear loss of pure copper shows a linear relationship with the sliding distance (Fig. 6 (c)), but once the sliding distance exceeds 120.42 m, the volume wear loss of Cu-TiB2 composites shows a relatively sharp escalating trend.
Fig.6 Volume wear loss of Cu and Cu-TiB2 composites under varying (a) load, (b) sliding speed and (c) sliding distance
Fig.7 shows the wear scar features of Cu and Cu-TiB2 composites. It can be seen that there exists distinct wear mechanism to explain the process for pure Cu and Cu-TiB2 composites. From Fig.5 and Fig.7 (a) ~(c), it is suggested that the wear mode of pure Cu should be adhesion and some fatigue wear at the beginning of wear process. As friction proceeds, the wear mode changes into abrasive wear and fatigue wear. Due to the low hardness of pure Cu, severe wear loss happens during the abrasive wear process, and the fatigue wear leads to the generation of a considerable mass of wear debris. On contrary to pure Cu, the Cu-TiB2 composites exhibit a different wear mode. At the very beginning, the adhesive wear dominates the wear mode of Cu-TiB2 composites, and no obvious fatigue wear is found, as shown in Fig. 7 (d) and (e). With the wear process continuing, the wear debris act as foreign particles, which will plow grooves on the wear surface, as shown in Fig. 7 (f). During
the wear process, crack nucleation and propagation take place constantly. Due to the well bonding between TiB2 and copper matrix (Fig.4), TiB2 particles will act as barriers when cracks occur. This can significantly delay the crack nucleation and growth. Therefore, smaller volume wear loss was obtained from Cu-TiB2 composites. Fig. 7 (g)~(i) shows the EDS analysis of wear scar of Cu-TiB2 composite. The Fe and O elements are detected, which shows that the materials transformation and oxidation may occur during the wear process.
Fig.7 Wear surface of Cu and Cu-TiB2 composites: (a)~(c) wear surface of pure Cu at load of 30 N, sliding speed of 120.42 mm/s and sliding distance of 108.38 m, 216.76 m and 325.13 m, respectively. (d)~(f) Wear surface of Cu-1 wt.% TiB2 at load of 30 N, sliding speed of 120.42 mm/s and sliding distance of 108.38 m, 216.76 m and 325.13 m, respectively. (g)~(i) EDS analysis of the wear surface from Cu-1 wt.% TiB2
The morphologies of wear debris for Cu and Cu-TiB2 composites are shown in Fig. 8. The wear debris of pure Cu are generally composed of massive platelike particles (10 to 150 μm in diameter), which are generated on the occurrence of the
severe wear loss by fatigue wear mode. A few equiaxed particles are occasionally observed in the Cu debris, as shown in Fig. 8 (a), which act as intermediary of third-body abrasion. Unlike pure Cu, the wear debris of Cu-TiB2 composites is mainly composed of small (<5 μm) equiaxed particles. It is fair to conclude that the dominant rate-controlling mechanism in the wear test of pure Cu with lower load and sliding speed is adhesive and fatigue wear modes, but with increasing load and sliding speed, abrasive and fatigue wear modes become predominant and severe wear loss occurs at this point. In particulate reinforced metal matrix composites, although the adhesive and oxidation wear exist in varying degrees, the abrasive wear occupies an important position. Under abrasive wear conditions, the wear resistance is proportional to the hardness of the material because hard particles act as the main bearing phases that significantly improve the wear resistance of the composites. The TiB2 particles in copper matrix prevent or delay the occurrence of fatigue wear so that severe wear loss of Cu-TiB2 composites is postponed. Consequently, Cu-TiB2 composites with better wear performance are obtained.
Fig.8 Wear debris and EDS for wear debris: (a) wear debris of pure Cu, (b) wear debris of Cu-TiB2 composites, (c) and (d) EDS analysis of Cu-TiB2 wear debris
3.4 Mechanical properties and electrical conductivity The mechanical properties of the composites with a variety of different TiB2 mass fractions are summarized in Table 1. It is clear that TiB2 particles enhance the hardness and strength of copper significantly. The ultimate tensile strength (UTS) increases from 397.7 MPa for pure copper to 442.5 MPa for the CMC with 1.5 wt.%TiB2. The variation of hardness as a function of TiB2 content has a similar tendency with respect to the UTS. The improvements in UTS and hardness are mainly attributed to the dispersed hard TiB2 particles. Upon solidification, strain fields are created in the vicinity of TiB2 owing to the difference between the thermal expansion coefficients of the two. The interaction between the strain field and the dislocation offers resistance to crack propagation during tensile loading. Furthermore, good interfacial bonding between copper matrix and TiB2 particles enhances the load bearing capacity. However, the elongation at maximum stress decreases with the
increase in TiB2 content. Table 1 Mechanical properties and electrical conductivity of Cu-TiB2 composites Composite Composition (wt.%)
σb (MPa)
σ0.2 (MPa)
Hardness (HV)
Elongation at Maximum Stress (%)
Electrical Conductivity (%IACS)
Cu
397.7
386.5
126.3±1.5
1.19±0.04
99.92±0.02
Cu-0.5TiB2
414.4
391.4
131.2±1.8
1.17±0.28
89.64±0.07
Cu-1TiB2
423.5
397.5
134.3±2.6
1.09±0.07
85.83±0.06
Cu-1.5TiB2
442.5
409.8
142.8±3.6
0.86±0.03
73.43±0.04
Based on Refs. [13, 27], contribution of second-phase particles to yield strength of PRMMC can be divided into four categories: grain refinement strengthening, Orowan strengthening, coefficients of thermal expansion (CTE) strengthening and load bearing strengthening. Extrinsic particles could act as heterogeneous nucleating sites, which refines the grain size, thus improving the strength of the composite according to the Hall-Petch relationship. On the other hand, the interaction between copper matrix and TiB2 particles enhances the strength of the composite through two mechanisms: dislocation multiplication induced by the difference of CTE between copper matrix and TiB2 particles, and Orowan strengthening when a dislocation passes through two adjacent particles. In addition, the grain size is refined significantly when in situ TiB2 particles were introduced into copper matrix, as shown in Fig. 9. The decrease of grain size may mainly attribute to the heterogeneous nucleation based on TiB2 particles and the probably unreacted Ti or B atoms. The decrease of grain size also contributes a lot to the yield strength of Cu-TiB2 composites.
Fig. 9 EBSD images of copper with different content of TiB2: (a) Cu, (b) Cu-0.5 wt.% TiB2, (c) Cu-1 wt.% TiB2, (d) Cu-1.5 wt.%TiB2
The electrical conductivities of copper and Cu-TiB2 composites are shown in Table 1. Somehow detriment of electrical conductivity is caused because of the incorporation of TiB2 into the matrix. The electrical conductivity is decreased from 99.92 %IACS for pure copper to 73.43 %IACS for the Cu-1.5 wt. %TiB2 composite. It has been reported by Matthiessen’s rule [28, 29] that the resistivity of a crystalline metallic specimen includes the resistivity resulting from thermal agitation of the metal ions and the lattice and imperfections in the crystal. This can be written as equation (2): 𝜌 = 𝜌(𝑇) + 𝜌𝑖
(2)
where ρ is the total resistivity of the metal or alloy, ρ(T) is the resistivity of the metal which varies with the temperature and ρi is the resistivity induced by chemical and
physical defects which are independent on the temperature. It is well known that the solute atoms usually result in lattice distortion which will have a detrimental influence on resistivity [30]. Furthermore, the phonons, impurities, point defects, dislocations, grain boundaries and surface are all scattering centers of conduction electrons and thus decrease the electrical conductivity. In Cu-TiB2 composite, TiB2 particles act as impurities and the electrical conductivity of the composites will slightly decrease. Moreover, grain refinement due to TiB2 addition will lead to an increase in grain boundaries, which will have an adverse effect on the electrical conductivity. Besides, the difference of CTE between copper and TiB2 particles will generate many dislocations during solidification, and this will also lead to a decrease in the electrical conductivity of the composites. Nevertheless, the Cu-TiB2 composites (especially the Cu-1.0wt.% TiB2) exhibit an adequate conductivity compared to other specifications of solid solution strengthening copper alloys, such as 45 %IACS of Cu-Ni-Si [31], 81.45 %IACS of Cu-Cr-Zr [32] and 60 %IACS of Cu2.5Fe1P [33]. Therefore, the Cu-TiB2 composites with adequate electrical conductivity, good mechanical properties and low cost fabrication method have a good application foreground.
4. Conclusions In-situ TiB2 particulate reinforced copper matrix composites were fabricated using traditional casting method from the synthesis between B and Cu elements in molten copper. Four samples with 0, 0.5, 1 and 1.5 wt.% TiB2 in copper matrix were prepared and the mechanical properties, electrical conductivities and wear behaviors have been
investigated. Based on the results, the following conclusions are drawn. (1) The TiB2 particles in situ formed were observed to have a good interfacial bonding to the copper matrix. Stacking faults may compensate for the interfacial misfit between TiB2 and copper. (2) The wear performance of the Cu-TiB2 composites is improved significantly due to the presence of TiB2 and the volume wear loss at different loads, sliding distance and sliding speed of the composite decreases with the increase in TiB2 content. (3) Compared with pure copper, whilst the UTS and hardness of Cu-TiB2 composites also recorded significant improvements (From 397.7 MPa and 126.3 HV of pure Cu to 442.5 MPa and 142.8 HV of Cu1.5 wt.%TiB2), the presence of TiB2 impairs the elongation and electrical conductivity, probably due to the dissevering effect of TiB2 on the particle/matrix interface.
Acknowledgements The authors gratefully acknowledge the supports of National Key Research and Development Program of China (Nos. 2017YFA0403803, 2016YFB0701203), the National Natural Science Foundation of China (Nos.51525401, 51774065, 51690163, 51601028), Dalian Support Plan for Innovation of High-level Talents (Top and Leading Talents, 2015R013) and the China Postdoctoral Science Foundation (2017T100174). References [1] Lu K. The Future of Metals. Science. 2010;328:319-20. [2] Deng B, Hsu PC, Chen GC, Chandrashekar BN, Liao L, Ayitimuda Z, et al. Roll-to-Roll
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Graphical abstract
Highlights
Cu-TiB2 composites were in-situ synthesized by conventional casting method.
The grain size of the Cu-TiB2 composites was refined by the in situ TiB2 phase.
TiB2 significantly diminished the wear volume loss Cu-TiB2 composites.
TiB2 made an effective improvement on mechanical properties of Cu-TiB2
composites.