Effect on Microstructure and Performance of B₄C Content in B₄C/Cu Composite

Abstract

In this paper, boron carbide (B₄C) ceramics were added to a copper (Cu) base, to improve the mechanical properties and wear resistance of pure copper. The B₄C/Cu composites with different B₄C contents, were obtained by mechanical mixing and discharge plasma sintering methods. Scanning electron microscopy (SEM), energy spectrum analysis (EDS), and electron probe microanalysis (EPMA) were used, to observe and analyze the microstructures of the B₄C/Cu composites. The influences of the B₄C content on the hardness, density, conductivity, and wear resistance were also studied. The experimental results show that B₄C has an important effect on Cu. With increasing B₄C content, both the density and conductivity of the B₄C/Cu composites gradually decrease. The hardness of the Cu-15 wt.% B₄C composite has the highest value, 86 HBW (Brinell hardness tungsten carbide ball indenter), which is 79.2% higher than that of pure copper. However, when the B₄C amount increases to 20 wt.%, the hardness decreases due to the metallic connection being weakened in the material. The Cu-15 wt.% B₄C composite has the lowest volume loss, indicating that it has the best wear resistance. Analyses of worn B₄C/Cu composite surfaces suggest that deep and narrow grooves, as well as sharp ridges, appear on the worn pure Cu surface, but on the worn Cu-15 wt.% B₄C composite surface, the furrows become shallow and few. In particular, ridge formation cannot be found on the worn Cu-15 wt.% B₄C composite surface, which represents the enhancement in wear resistance.

1. Introduction

Metallic copper (Cu) is widely applied in the fields of electrical, transportation, construction, and aerospace, similarly to electric cable, heating, connector, lead frame, radiator, commutator, and brake facing, because of its excellent conductivity and thermal conductivity and the low price of raw materials. However, the hardness, strength and wear resistance of Cu are low, and cannot meet the increasing requirements of modern science and technology on material properties. To overcome the low hardness and low wear resistance of pure Cu, Cu-based composites have been widely studied [1,2,3,4]. For example, Shaik et al. [1] studied the mechanical properties and tribological characteristics of Cu–zirconium diboride (ZrB₂) composites against silicon carbide (SiC) emery paper. The results suggested that, when the ZrB₂ content was 10 wt.%, the Cu–ZrB₂ composites possessed the maximum hardness and the maximum yield strength, and the steady-state friction coefficient of Cu decreased, from 0.56 to 0.16, when ZrB₂ was added. In particular, the wear coefficient of the Cu-3 wt.% ZrB₂ composites drastically decreased by 7.33 times. Guiderdoni et al. [2] researched the effect of double-walled carbon nanotubes on the hardness and friction performance of Cu matrix composites. The results showed that, compared to pure copper, the Vickers microhardness was doubled when double-walled carbon nanotubes were added, and the wear was very low.

The hardness of boron carbide (B₄C) ceramics ranks third among known materials in the world, with the advantages of low density, high oxidation resistance, high acid and alkali corrosion resistance, high wear resistance, etc. In addition, it has stable high-temperature strength and can absorb neutrons [5,6,7,8,9]. B₄C has been added to the matrix of many metal materials, as a strengthening phase to improve their performance [10,11,12,13,14,15,16]. For example, Lyu et al. [10] added B₄C particles into a Q235 steel substrate and found that the wear resistance could be greatly improved. In addition, Yahya et al. [11] found that, with increasing B₄C ratio in aluminum (Al)-based composites, the hardness and wear resistance of B₄C/Al composites increased, and the Al-16% B₄C composites had the highest hardness and wear resistance. Moreover, Joshi et al. [12] introduced B₄C particles into a magnesium (Mg) alloy matrix, and found that the wear properties and hardness of B₄C/Mg composites were largely improved. Additionally, Hynes et al. [15] found that the mechanical properties of the AA6061 matrix composites were enhanced by adding B₄C reinforcement, and the wear resistance was also superior. At the same time, Aherwar et al. [16] studied the impact of B₄C and porcelain particulates on aluminum (Al) matrix composite properties. They found that both the mechanical and tribological properties were improved, and the wear properties of the Al matrix composites containing 4 wt.% B₄C and 12 wt.% porcelain particulates were the best.

The B₄C + Me → Me_xB_y + C graphite reaction occurs between boron carbide and most metals, but copper is one of the few metals that do not have such a reaction. If a new material can be composed of hard B₄C and Cu with good plasticity, it is expected to have high wear resistance and be applied to wear-resisting products, for example, automobile brake clutches. However, so far, there are only a few studies on the impact of B₄C on Cu matrix composites. Moreover, the research is relatively scattered, from different angles, and a lack of systematicness. For example, Prajapati1 et al. [17] fabricated Cu–B₄C composites by cold powder compaction, followed by conventional sintering at 900 °C for 1 h under argon atmosphere, and studied the effect of B₄C content (lower than 15 wt.%) on microstructure, mechanical properties, and electrical conductivity, but the effect of B₄C on the wear-resisting property of the B₄C/Cu composite was not involved. Balalan et al. [18] studied the effect of B₄C on the microstructure and properties of Cu matrix composites that were prepared by hot pressing sintering, but the mass fraction of B₄C was limited to 1.5~6%. Qin et al. [19] prepared Cu-based friction materials with different B₄C mass fractions (3%, 6%, 9%, and 12%) by high-energy ball milling and vacuum sintering, and studied the friction and wear properties of the pair of B₄C/Cu and T10 steel. However, the composite materials also included titanium (Ti), iron (Fe), graphite, rare earth lanthanum, SiC, and other components, which might influence the evaluation of the role of B₄C. Bai et al. [20] prepared B₄C/Cu composites with B₄C volume fractions of 40%–70% by copper plating on B₄C particles and spark plasma sintering, but the main research of the work was the effect of the B₄C surface copper plating process on the thermal conductivity of B₄C/Cu composites.

It is reasonable to infer, from the above analysis, that it is very necessary to further research the impact of B₄C on Cu matrix composites, so as to provide some useful information to extend the project application of B₄C/Cu composites. In this work, B₄C particles were introduced into the Cu matrix to form B₄C/Cu composites by vacuum spark plasma sintering, and the influences of B₄C content (0~20 wt.%) on the microstructures, hardness, density, conductivity, and wear resistance of Cu matrix composites, were systematically studied.

2. Experimental Procedures

2.1. Material Preparing

Raw materials (B₄C powder and Cu powder) were commercially purchased from Nangong Xindun Alloy Welding Material Spraying Co., Ltd., Xingtai, China. The average particle sizes of B₄C powder and Cu powder are 2.5 μm and 2 μm, respectively. The specimen numbers of the B₄C/Cu composites and their corresponding components (wt.%) are presented in

Table 1. The specimen numbers of the B₄C/Cu composites and their corresponding components (wt.%).

	BCu0	BCu5	BCu10	BCu15	BCu20
Cu	100	95	90	85	80
B4C	0	5	10	15	20

.

According to the predesigned composition, an electronic balance was used to weigh powders for five different compositions. Then, each powder mix was ball-milled in a bottle. In the preparation process of the B₄C/Cu composites, we used a laboratory small horizontal mixer to mix the raw materials, and the volume of the cylinder was 10 L. Due to the small amount of powder used in this study, we put it into a 500 mL plastic wide-mouth bottle with dispersant and grinding balls, tightened the bottle lid and fixed it in the barrel of the mixer for mixing. The diameter of the bottle was 80 mm, and the rotation speed was set as 105 r/min. The milling media were absolute ethyl alcohol and aluminum oxide (Al₂O₃) balls. The mixing time was 24 h. After mixing, the absolute ethyl alcohol was thoroughly dried away from the mixture in a vacuum drying oven, and the drying temperature was set as 50 °C. When the composite powder was ready, the powder was placed into a graphite mold and vacuum sintered by using a spark plasma sintering furnace (model: SPS-20T-10). The sintering pressure was 30 MPa, the sinter temperature was 980 °C, and the heat preservation and pressure holding time was 10 min. A B₄C/Cu composite sintering process diagram is shown in Figure 1.

The density of sintered B₄C/Cu composites was determined by the Aquimid drainage method. The hardness of sintered B₄C/Cu composites was tested using a digital display Brinell hardness tester (model: 320HBS-3000, Laizhou Huaxing Test Instrument Co., Ltd., Laizhou, China). The electrical conductivity was tested by a digital eddy current metal conductometer (model: Sigma2008B1, Xiamen Tianyan Instrument Co., Ltd., Xiamen, China). Five to ten measurements were made for each performance, and the average value was taken as the final value.

2.2. Wear Resistance Test

An ML-100-type abrasive wear tester was used to test the wear resistance of the sintered B₄C/Cu composite. The schematic diagram of the machine is shown in Figure 2. In the testing process, sintered B₄C/Cu composites served as pin samples to slide against 400^# SiC abrasive papers at room temperature. In the wear tester, a B₄C/Cu composite pin specimen that was prefixed in a pin holder was pressed down to contact a 400^# SiC abrasive paper that was firmly attached to a holder. The size of the B₄C/Cu composite pin specimens was 5 mm in diameter and 12.5 mm high. One end of the B₄C/Cu composite pin was first ground to a 5-mm-diameter hemisphere surface by a grinding wheel and then polished. The surface roughness (Ra) was less than 1 μm. Before sliding, pins were immersed in acetone, and after 15 min of ultrasonic irrigation, they were thoroughly vacuum dried. The sliding time was 10 min under a load of 10 N in a humidity (50% relative humidity (RH)) and temperature (25 °C) environment.

A laser scanning profilometer (model: VK-9710K, Keenes Ltd., Osaka, Japan) was used to evaluate the wear volume loss of pin specimens. Two orthogonal diameter lengths on the pin worn surface were first measure and then averaged, and the pin wear volume loss was calculated according to the average length of the orthogonal diameters and the initial tip radius. To make a rigorous conclusion, three repeated wear tests were carried out under identical experimental conditions, and the average value was taken as the final value of pin wear volume loss.

Scanning electron microscopy (SEM, TESCAN CHINA, Ltd., Shanghai, China), energy spectrum analysis (EDS, TESCAN CHINA, Ltd., Shanghai, China) and electron probe microanalysis (EPMA, JEOL Ltd., Akishima, Japan) were used to observe and analyze the microstructure and worn surface topographies.

3. Results and Discussion

3.1. Microstructures

Figure 3 presents the B₄C/Cu composite powder morphologies with different amounts of B₄C. As observed in Figure 3a, pure copper powder agglomerates obviously, and extends outward in a dendritic form with poor dispersion. After adding B₄C particles, the B₄C particles would interact with copper powder aggregates during ball milling, to inlay and cut the copper powder aggregates. We can observe, from Figure 3b, that when the B₄C content is 5 wt.%, the dispersibility of the composite powder has shown some improvement. With increasing B₄C content, the dispersibility of the composite powders of Cu-10 wt.% B₄C (see Figure 3c), Cu-15 wt.% B₄C (see Figure 3d), and Cu-20 wt.% B₄C (see Figure 3e), becomes better and better.

EPMA analysis was carried out for the small angular and irregular black particles (cross mark in Figure 3e), and the result is shown in

Table 2. EPMA analysis result of the Cu-20 wt.% B₄C powder (cross mark in Figure 2e).

Element	B	C
Content, at.%	80.8	18.2

. The B atom percent content is approximately 81%, and the C atom percent content is approximately 18%. The B-to-C atomic ratio is approximately 4:1, so the black particle is in the B₄C phase.

Figure 4 presents the morphologies of the B₄C/Cu composites under different B₄C contents. The black particles in the morphologies are B₄C particles. To make sure of that, three points (A–C) in the Cu-10 wt.%B₄C composite microstructure were qualitatively analyzed, by using EDS, and the results are shown in Figure 5. A conclusion can be drawn from Figure 5 that the black particles are B₄C particles. As observed in Figure 4, the distribution of B₄C particles is roughly uniform. However, when the content of the B₄C particles was over 15 wt.%, aggregation of B₄C particles occurred.

3.2. Properties of Density, Hardness, and Conductivity

Figure 6 shows the density, hardness, and conductivity of B₄C/Cu composites with different amounts of B₄C. As observed in Figure 6a, the density of pure Cu is 99.1%, and with the increase in the content of B₄C particles, the relative density of the B₄C/Cu composite gradually decreases. The variation trend of the density of B₄C/Cu composites, with the increase in the B₄C content, is consistent with the literature [18]. When the content of B₄C is 20 wt.%, the relative density decreases to 92.3%. There is a significant difference between pure Cu and B₄C. For example, the density of pure Cu is approximately 8.9 g/cm³, but the density of B₄C is only 2.52 g/cm³. The hardness of pure Cu is approximately 50 HV, but the hardness of B₄C can reach 55 GPa (approximately 5610 HV). The performance difference may affect the material preparation process, especially the densification of the material. The internal friction between the powders increases with increasing B₄C content, so the resistance to densification increases, resulting in a decrease in the relative density.

As observed in Figure 6b, the B₄C/Cu composite hardness first increases and then decreases. The hardness of pure Cu is 48 HBW, but due to the extraordinary hardness of B₄C, when only 5 wt.% is added into the soft Cu matrix, during indentation, B₄C produces resistance to the motion of dislocations, and the hardness of the B₄C/Cu composite increases considerably, from 48 to 75 HBW. In addition, the large differences in the thermal expansion coefficients of copper (17 × 10⁶/°C) and B₄C (5 × 10⁶/°C) would cause a great thermal expansion mismatch in the sintering process, resulting in the occurrence of a significant amount of dislocation, which is also conducive to an increase in hardness [17]. When the content of B₄C is 15 wt.%, the hardness of the B₄C/Cu composites reaches 86 HBW, which is the highest value. The hardness of the Cu-15 wt.% B₄C composite was 79.2% higher than that of pure Cu. However, when the B₄C amount increases to 20 wt.%, the hardness decreases to 81 HBW. As observed in Figure 6a, the relative density of the Cu-20 wt.% B₄C composite is lower than that of other contents. The voids or pores in the sintered microstructure of the Cu-20 wt.% B₄C composite (see Figure 7) were more, which would weaken the metallic connection in the material, resulting in the reduction in hardness.

The conductivity test results shown in Figure 6c, suggest that pure copper has the best conductivity, at 57 mega Siemens per meter (MS/m). However, the conductivity of the B₄C/Cu composites decreases much more quickly with increasing B₄C content, especially between 0 and 10 wt.% B₄C, and 15 and 20 wt.% B₄C. The main reason is that the conductivity of B₄C is extremely low, the Cu matrix continuity is broken, and B₄C particle networks are formed with increasing B₄C content. Hence, the conductivity of the B₄C/Cu composite largely decreases. The variation trend of the conductivity of B₄C/Cu composites with the increase of B₄C content is consistent with the literature [17].

3.3. Wear-Resisting Property

The wear volume losses of B₄C/Cu composites with different amounts of B₄C are shown in Figure 8, and pure copper has the largest wear volume loss. With increasing B₄C amount, the wear volume loss of the B₄C/Cu composite pin samples decreases first and then increases, so the corresponding wear resistance first increases and then decreases. The volume loss of the Cu-15 wt.% B₄C composite is the lowest, indicating that the Cu-15 wt.% B₄C composite has the best wear resistance.

To understand the underlying wear mechanisms, the worn morphologies of B₄C/Cu composites under different B₄C contents, after sliding against SiC abrasive papers, were investigated, as shown in Figure 9. As observed in Figure 9a,b, deep and narrow grooves exist on the worn pure Cu surface, indicating severe plastic deformation due to hard granules on the SiC abrasive papers. Additionally, sharp ridges were produced by the plowing mechanism. The worn pure Cu surface EDS analysis shows that the oxygen content is only 2.5 wt.% (see Figure 10), indicating that, during the process of sliding, tribo-oxidation seldom occurred as SiC removed the pure Cu material, and a new surface would be exposed each time hard SiC slid past the pure Cu sample, which caused a high material removal rate of the pure Cu sample.

As observed in Figure 9c–e, compared with the worn pure Cu morphology, when the B₄C amount is lower than 15 wt.%, the furrows on the worn surfaces of the B₄C/Cu composite pin samples become shallow, and the ridge amount that is formed by plastic deformation decreases with increasing B₄C content. The reason for this is easy to understand. Cu deformation should be restricted by B₄C particles during the sliding process, resulting in the improvement of the wear-resisting property of the B₄C/Cu composite. In particular, ridge formation cannot be found on the worn Cu-15 wt.% B₄C composite surface (see Figure 9e), which represents the enhancement in wear resistance.

When the B₄C content was higher, B₄C particles in the composite easily aggregated (see Figure 4d). In the wear process, these aggregated B₄C particles easily flake off (see Figure 9f), due to the lack of copper phase binding. On the one hand, the flayed B₄C particles between the pin sample and the abrasive paper acted as abrasives, reducing the wear resistance of the composite material. On the other hand, the spalling of B₄C reduces the protective effect on the Cu matrix, and increases the contact area between the Cu matrix and the microconvex peak, which aggravates the wear of the composite material. However, when the B₄C content is 20 wt.%, the impaired sites increase (see Figure 9f). B₄C particles are loosened during the wear process, with the agglomeration of B₄C. As shown in Figure 9f, the entire worn surface of the Cu-20 wt.% B₄C composite shows severe damage. During sliding against the hard SiC abrasives on the abrasive papers and loosened hard B₄C-reinforced particles, spalling phenomena of the Cu matrix occur, which would exhibit more wear loss.

4. Conclusions

In this paper, B₄C/Cu composites with different B₄C contents were obtained by mechanical mixing and discharge plasma sintering methods. The influences of B₄C content on the microstructures, hardness, density, electrical conductivity, and wear resistance of the B₄C/Cu composites, have been studied. Conclusions can be drawn as follows:

(1)

The B₄C/Cu composite microstructure analysis shows that, in the Cu matrix, B₄C ceramics are well distributed. However, when the B₄C content is over 15 wt.%, aggregation of the B₄C particles occurs;

(2)

Both the relative density and conductivity of the B₄C/Cu composites gradually decrease as the B₄C content increases;

(3)

The hardness of the B₄C/Cu composites first increase and then decrease. When the content of B₄C was 15 wt.%, the hardness of the B₄C/Cu composites reached the highest value, 86 HBW, which was 79.2% higher than that of pure Cu. However, when the B₄C amount increases to 20 wt.%, the hardness decreases, due to the metallic connection being weakened in the material;

(4)

The Cu-15 wt.% B₄C composite has the lowest volume loss, indicating that it has the best wear resistance. Analyses of worn B₄C/Cu composite surfaces suggest that deep and narrow grooves, as well as sharp ridges, appear on the worn pure Cu surface, but on the worn Cu-15 wt.% B₄C composite surface, the furrows become shallow and few. In particular, ridge formation cannot be found on the worn Cu-15 wt.% B₄C composite surface, which represents the enhancement in wear resistance.

Based on the above research results and analysis, we can see that B₄C particles have important effects on the microstructures and properties. Some properties, such as hardness and wear resistance, can be improve effectively, but the conductivity performance deteriorates significantly, indicating that B₄C/Cu composites are more suitable to be used in uncharged wear parts. In this work, the wear resistance of the B₄C/Cu composites was evaluated by an ML-100-type abrasive wear tester. However, this testing machine cannot obtain the friction properties (friction coefficient) of the material. To expand the application range of B₄C/Cu composites, in the next step of our work, we will use another friction and wear tester, to further evaluate the friction and wear properties of the B₄C/Cu composites sliding against different metal or ceramic couples under different conditions.