Effect of Basalt Fiber Content on Mechanical and Tribological Properties of Copper-Matrix Composites

Abstract

The basalt-fiber-reinforced copper-matrix composites were fabricated using spark plasma sintering. The effect of basalt fiber content on the mechanical and tribological properties of copper-matrix composites was investigated. The results show that the relative density decreased, the hardness increased and the tensile strength first increased and then decreased with the increase in fiber content. The optimal tensile strength of 276 MPa was obtained when the content of basalt fiber was 1.5 wt.%. The friction coefficient of the composites decreased, and the wear volume first decreased and then increased with the increase in the fiber content. The addition of basalt fibers reduced the plastic deformation of the copper matrix during the friction test due to its strengthening effect. Moreover, the detached basalt fibers from the Cu matrix were ground into fine particles and were helpful for forming a mechanical mixing layer, which can decrease the friction coefficient of composites.

1. Introduction

Copper exhibits a face-centered cubic crystal with a density of 8.92 g/cm³ and a melting point of 1080 °C and is a traditional metal material [1]. Copper is widely used in the electrical, marine, aerospace, automobile and railway industries because of its excellent electrical conductivity, thermal conductivity and good machinability [2,3,4]. In spite of these advantages, the low wear resistance and low strength limit its application to heavy-duty applications [5,6].

For improving the mechanical properties and controlling the friction coefficient, reinforcements are often added to copper or copper alloys to fabricate copper-matrix composites that combine the advantages of metal and ceramic. A variety of reinforcements have been added to a copper matrix, for which ceramic particles are the most widely used. Yao et al. [7] found that the Brinell hardness and friction coefficient of copper material can be improved when ZrO₂ particles are added into a copper matrix. Sadoun et al. [8] investigated the effect of Al₂O₃ on the mechanical properties and wear behaviors of a Cu-matrix composite. The results showed that the compressive strength increased and the wear rate decreased with the increase in Al₂O₃ content. It was reported that the SiC-particle-reinforced copper-matrix composites displayed better mechanical and sliding wear behaviors [9,10,11]. However, the large-sized ceramic particles may cause severe abrasive wear due to their high hardness, and the dispersion and surface modification of nanosized ceramic particles is difficult [12,13].

Compared with particles, fibers do not easily fall off the matrix due to a large length-diameter ratio. At the same time, fiber has a high strength due to its few defects. For example, carbon-fiber-strengthened Cu-based composites exhibit better comprehensive performance, such as a high strength and fracture toughness [14,15]. However, the price of fibers is too expensive. Basalt fibers are made by high-speed drawing a platinum-rhodium alloy wire drawing leakage plate after melting basalt at 1450 °C~1500 °C [16]. The production process requires hardly any additives, which makes processing them more sustainable and economic. Basalt fibers exhibit better performance than glass fibers and lower prices than carbon fibers and are the most cost-effective type of high-tech fiber [17].

Basalt fibers are widely used in strengthening composites because of their excellent chemical stability and mechanical and thermal properties [18]. One important application of basalt fibers is as the reinforcing material of composites to improve their mechanical and tribological properties. For example, Wang et al. [19] investigated the effect of basalt fiber on the tribological and mechanical properties of polyether-ether-ketone (PEEK) composites. They found that the tensile strength of basalt fiber (BF)/PEEK composites increased and the specific wear rate decreased with the increase in BF content. Vannan et al. [20] studied the dry sliding wear behaviors of basalt short fiber reinforced aluminum metal matrix composites. The results showed that the addition of basalt fibers reduced the friction coefficient of the composites. Ding et al. [21] studied the influences of infiltration parameters on the microstructure evolution and formation mechanism of the multi-layered structure of a continuous basalt fiber (CBF)/Al composite. However, there are few studies on the applicability of basalt fiber as a reinforcement for Cu-matrix composites.

In this work, basalt fibers were used in a reinforcing phase to fabricate copper-matrix composites using powder metallurgy technology. The effect of basalt fiber content on the hardness, tensile strength, friction coefficient and wear resistance of the copper-matrix composite was investigated.

2. Experiment

Electrolytic Cu powder (purity > 99.9 wt.%) produced by Beijing Nonferrous Metals Research Institute and basalt fibers (length: 2 mm, diameter: 12 μm) obtained from Jilin Huayang New Composite Materials Co., Ltd., (Changchun, China) were used for fabricating the basalt-fiber-reinforced copper-matrix composites. The SEM image and XRD pattern of basalt fiber are exhibited in Figure 1. As can be seen from Figure 1a, the basalt fibers’ surface is smooth, and the diameter of the basalt fibers is about 12 μm. As can be seen in Figure 1b, the basalt fibers have only one wide diffraction peak between 15° and 38°, indicating the amorphous nature of the original basalt fibers. The chemical composition of the basalt fibers is shown in

Table 1. Chemical composition of basalt fiber (wt.%).

SiO2	Al2O3	Fe2O3	MgO	CaO	Na2O	K2O	TiO2
52.8	17.5	10.3	4.63	8.59	3.34	1.46	1.38

.

In order to improve the interfacial bonding between the basalt fibers and the Cu matrix, the basalt fibers were coated with copper up to about 60 wt.% using the electroless method. The composition of the electroless bath is given in

Table 2. Technology conditions of electroless copper coating.

CuSO4·5H2O	EDTA·2Na	KNaC4H6O6	HCHO(Solution)	NaOH	Temperature
20 g/L	30 g/L	25 g/L	15 mL/L	15 g/L	45 °C

. The copper powder and short-basalt-fiber-coated copper were mixed using a planetary ball mill under an argon atmosphere at a rotational speed of 150 rpm for 5 h. After that, the basalt-fiber-reinforced copper-matrix composites were fabricated using spark plasma sintering (SPS) with a graphite die using 40 MPa uniaxial stress in a vacuum (10⁻⁴ Pa). A heating rate of 100 °C/min, a sintering temperature of 700 °C and a soaking time of 10 min were used.

The microstructures of the sintered samples (40 mm in diameter and 6 mm in height) were characterized using a scanning electron microscope (SEM, Gemini Supra 40, Zeiss, Jena, Germany). The tensile strength of the composites was measured using a universal testing machine (Model 1186, Instron, Norwood, MA, USA) with a stretching speed of 0.5 mm/min at room temperature. The fracture morphology was observed using the SEM after the tensile test. As shown in Figure 2, the friction coefficient of the composites was measured with a multi-functional friction and wear tester (MFT-5000, Rtec-Instruments, San Jose, CA, USA). The applied loads and friction frequency were 10 N and 1 Hz (0.01 m/s), respectively. The diameter of the steel ball manufactured by 440C stainless steel was 6.350 mm. In order to ensure experimental accuracy, the steel balls need to be replaced at the end of each experiment for each test specimen. The roughness change curve of the wear surface was measured using a laser scanning confocal microscope (Figure 3), and then the wear volume was calculated using V₃-V₁-V₂. After the friction test, the SEM was used to observe the wear surface morphology of the composites.

3. Results and Discussion

3.1. Microstructure of Composites

Figure 4 shows the SEM images of the copper-matrix composites reinforced with different contents of basalt fibers. No cracks were formed in the basalt-fiber-reinforced copper-matrix composites. The trend of fiber agglomeration increased with the increase in fiber content. In particular, when the fiber content reached 2%, an obvious agglomeration of basalt fibers was observed. In addition, the length difference of the basalt fibers indicated that the distribution direction of the basalt fibers was disordered.

The relative density of the copper-matrix composites with different fiber contents is shown in Figure 5. It can be seen that the relative density of the copper-matrix composites decreased from 97.35% to 96.75% with the increase in the basalt fiber content from 0.5% to 2%. In particular, a significant decline in the relative density was observed when the content of basalt fiber increased from 1.5% to 2%, indicating that more defects were formed in the composites with 2% basalt fibers.

3.2. Mechanical Properties of Composites

Figure 6 shows the Brinell hardness and tensile strength of the basalt-fiber-reinforced copper-matrix composites. The hardness of the composites increased from 35.1 HB to 41 HB with the increase in the basalt fiber content from 0.5% to 2%, which can be attributed to the high hardness of the basalt fiber [22]. The tensile strength of the composites first increased and then decreased with the increase in the mass fraction of the basalt fibers. When the content of basalt fibers was 1.5%, the maximum tensile strength of 276 MPa was obtained. The SEM images of the fractured surface of the composites are shown in Figure 7. The pulling out of fibers was found on the fracture surface, which indicated that the basalt fibers maintained excellent mechanical properties. However, when the content of basalt fiber increased to 2%, hole defects were observed around the basalt fiber. It is well-known that the dispersion of fibers in a metal matrix becomes poor with an increase in fiber content [15]. On the one hand, the agglomeration of the fibers weakened the role of dispersion strengthening of the fibers. On the other hand, the agglomeration of the fibers hindered the sintering of the copper powder and caused the formation of defects. The above two reasons led to the decrease in tensile strength of the composites with the addition of 2% basalt fiber.

3.3. Friction Coefficient and Wear Volume of Composites

Figure 8 shows the friction coefficient of the composites with the different contents of basalt fibers. It can be seen in Figure 8a that the coefficient of friction of the composites first increased rapidly and then tended to be steady with the increase in friction time. The average friction coefficient of the composites in the stable stage decreased from 0.66 to 0.55 as the basalt fiber content increased from 0.5% to 2% (Figure 8b). This indicates that the addition of basalt fiber can effectively reduce the friction coefficient of a composite. On the one hand, the addition of the basalt fibers prevented the plastic deformation of the copper matrix during the friction test due to its strengthening effect [23]. Thus, the smooth wear surface was easier to obtain, which reduced the sliding resistance of the grinding ball and then led to a decrease in the friction coefficient. On the other hand, during the friction process, the basalt fiber fell off from the copper matrix and was adsorbed on the wear surface, which hindered direct contact between the composite and the grinding ball, reducing the friction coefficient of the composite [24].

Figure 9 shows the wear volume of the composites with the different contents of basalt fibers. The wear rate of pure copper under a 10 N load is about 6.5 × 10⁻⁴ mm³/Nm [25]. The wear volume of the composites decreased first and then increased with the increase in basalt fiber content. When the content of basalt fibers was 1.5%, the wear rate reached the minimum of 4.57 × 10⁻⁴ mm³/Nm. The addition of the basalt fibers improved the hardness and strength of the composites. In this way, the basalt fibers acted as load-bearing elements and reduced the actual contact area between the counterface and the Cu matrix, which improved the wear resistance. However, the increase in wear volume in the sample containing 2% basalt fiber was attributed to fiber agglomeration and their separation from the matrix as debris.

3.4. Wear Morphology of Composite Materials

The SEM images of the worn surfaces of the copper–basalt fiber composites are displayed in Figure 10. Furrows and cracks can be observed on the wear surface. The depth of the furrow indicates the extent of plastic deformation. The cracks were formed due to poor interface bonding between the copper substrate and basalt fiber. The depth of the furrow and the roughness of the worn surface decreased with the increase in basalt fiber content, indicating that basalt fiber can reduce the plastic deformation of a copper matrix. According to the EDS pattern shown in Figure 11, the black phases in the wear surface can be identified as basalt fiber. By comparing the size of the black phase (1–5 μm) and that of the original basalt fiber (12 μm), it can be determined that the detached basalt fibers were ground into fine particles due to the brittleness of the basalt fibers [26]. The fine basalt particles are helpful for forming a mechanical mixing layer, which can decrease the friction coefficient of composites [27]. When the content of the basalt fiber was 0.5 wt.%, the main wear mechanism was delamination wear caused by plastic deformation. With the increase in basalt fiber content, the delamination wear gradually weakens and changes into abrasive wear for the composites with 2 wt.% basalt fibers.

4. Conclusions

The effects of basalt fiber content on the mechanical and tribological properties of copper-matrix composites were studied. The main conclusions are as follows:

(1)

The trend of basalt fiber agglomeration increased with the increase in fiber content, which led to the relative density of the composites decreasing.

(2)

The hardness increased and the tensile strength first increased and then decreased with increasing fiber content. When the content of basalt fiber was 1.5%, the maximum tensile strength of 276 MPa was obtained.

(3)

The friction coefficient of the composites decreased from 0.66 to 0.55 as the basalt fiber content increased from 0.5% to 2%.

(4)

The wear volume of the composites first decreased and then increased with the increase in fiber content. The composites with 1.5% basalt fiber showed the best wear resistance.