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The investigation of microstructure and properties of in-situ Cu/TiC nanocomposite prepared by high energy ball milling and spark plasma sintering

Nghiên cứu cấu trúc vi mô và tính chất của nanocomposit nội sinh Cu/TiC chế tạo bằng nghiền bi năng lượng cao và thiêu kết xung điện plasma

NGUYEN THI HOANG OANH*
School of Materials Science and Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam

E-mail: oanh.nguyenthihoang@hust.edu.vn

Ngày nhận bài: 4/5/2020, Ngày duyệt đăng: 22/6/2020

ABSTRACT

In this study, in-situ Cu/TiC nanocomposites with 30 vol.% TiC reinforcement particles were synthesized via high energy ball milling and spark plasma sintering methods. After milling for 10 h at 500 rpm, composite powders were sintered at 800, 900 and 1000 °C in vacuum environment under an applied pressure of 50 MPa. The microstructure of composite samples was investigated by optical microscopy, scanning electron microscopy and high-resolution transmission electron microscopy techniques. Scanning electron microscopy images of samples sintered at 900 and 1000 °C showed that the reinforcement particles were distributed uniformly in the copper matrix. The appearance of nano TiC grains can be observed in HRTEM. The highest relative density and microhardness for Cu/TiC nanocomposite sintered at 1000 °C are 95.5 % and 364 HV, respectively.

Từ khóa: Spark plasma sintering, copper, high energy ball milling, insitu nanocomposite

TÓM TẮT

Trong nghiên cứu này, composit nội sinh cấu trúc nano Cu/TiC có 30 % thể tích TiC được tổng hợp bằng phương pháp nghiền bi năng lượng cao và thiêu kết xung điện plasma. Sau khi nghiền 10 giờ với tốc độ 500 vòng/phút, bột composit được thiêu kết ở 800, 900 và 1000 °C trong môi trường chân không dưới áp suất 50 MPa. Tổ chức tế vi của các mẫu composit được nghiên cứu bằng kỹ thuật hiển vi quang học, hiển vi điện tử quét và hiển vi điện tử truyền qua có độ phân giải cao. Ảnh hiển vi điện tử quét cho thấy các hạt gia cường phân tán đồng đều trong nền đồng. Sự có mặt các hạt nano TiC quan sát được qua ảnh hiển vi điện tử truyền qua phân giải cao. Tỷ trọng tương đối cao nhất và độ cứng tế vi của mẫu nanocomposit Cu/TiC thiêu kết ở 1000 °C tương ứng là 95,5 % và 364 HV.

Key words: Thiêu kết xung điện plasma, đồng, nghiền bi năng lượng cao, nano composit nội sinh

 1.  INTRODUCTION

Pure copper possesses excellent thermal and electrical conductivity, and good corrosion resistance, and so is widely used in optical and electronic industries [1, 2]. However, even with these advantages, pure copper has poor strength, wear and fatigue resistance, therefore it is unsuitable for applications requiring high strength and wear resistance such as contact terminals of electrical switches. Hence, copper matrix composites are primarily developed using ceramic-based rein-forcements as TiB2, TiC, NbC, Al2O3 [2-7]. Among ceramic particles used as reinforcement for copper matrix, TiC with high modulus, hardness, melting temperature, and chemical stability is a desirable choice [8-10]. In addition to good mechanical properties, TiC also has high electrical conductivity, comparable to that of metals. However, a challenge is the high mismatch between the thermal coefficient of expansion of ceramics and copper matrix, that leads to poor thermal fatigue and high dimensional instability of the composites under high cyclic thermal loading [11]. Several approaches and techniques have been applied to improve properties of metal matrix nanocomposites (MMnCs), which include pre-heat treatment of reinforcing particles, coating the particles with wetting agents, using nano and sub-micrometer particles, and using hybrid particle reinforcement [2, 6, 8, 12, 13]. The properties of the metal matrix composite are controlled by the nature of matrix-reinforcement interfaces. Efforts have led to the development of in-situ metal matrix composite, in which the reinforcement phase is synthesized in a metallic matrix by chemical reactions of elements or between an element and compound during composite fabrication. In the Cu-Ti-C mixture, a reaction between Ti and C °Ccurs during sintering process. High-energy ball milling (HEBM) proved to be a suitable technique for production of in-situ MMnCs [6, 8, 12]. In situ synthesis by HEBM also offers advantages such as more uniform distribution of reinforcement particle within the matrix. The milled composite powders were then consolidated with spark plasma sintering (SPS) technique. With fast heating rate and short sintering time, SPS proved a suitable technique for consolidating amorphous, nanocomposite powders [6, 13-15].

The present work aims to investigate the microstructure and properties of in-situ Cu/TiC nanocomposite prepared by HEBM and SPS. With a combination of optical, scanning and high-resolution transmission electron microscopy, the microstructure of sintered nanocomposite samples and the appearance of nano TiC particles will be presented.

2.  EXPERIMENTAL

The copper (with average particle size of 75 µm), titanium (average particle size of 45 µm) and graphite (average particle size of 5 µm) powder (≥99 % purity, from HIGH PURITY CHEMICALS Co., Ltd., Chiyoda, Japan) were used as starting materials. The powder mixture of Cu-Ti-C (with calculated mixing ratio of 30 vol % TiC) was milled in a high-energy planetary ball mill (P100-Korea). Milling was operated for 10 h at the rotational speed of 500 rpm and 0.5 wt % stearic acid was used as the milling process control agent. Balls and vials are made of stainless steel, the diameter of the balls was 5 mm and the powder-to-ball ratio was  1:10.  The  vial  was  evacuated  and subsequently filled with argon up to 0.3 MPa. The asmilled composite powders were then placed into a graphite die with an inner diameter of 10 mm. Before sintering, the SPS chamber was pumped to a pressure below 5 Pa. The sintering experiments were conducted using a spark plasma sintering facility (DR. SINTER LAB Model: SPS-515S, Sumitomo Coal Mining, Tokyo, Japan). The sintering temperatures are 800, 900 and 1000 °C with 5 min holding time. A pressure of 50 MPa was applied through the sintering cycle. The microstructure of the composites was studied by optical microscopy (OM), scanning electron microcopy and energy-dispersive spectroscopy (SEM/EDS) using a field-emission JEOL JSM7600F microscope (JEOL Ltd., Tokyo, Japan) and transmission electron microscopy (TEM) using a JEOL JEM-2100 microscope (JEOL Ltd., Tokyo, Japan). The phase composition of the SPSed sample was studied by means of X-ray diffraction (XRD) using a RIGAKU RINT-2000 diffractometer with Cu-Kα radiation (Rigaku Corporation, Tokyo, Japan). The relative density of the composites was determined by Archimedes’ method. The hardness of the sintered Cu/TiC composites was measured using a Vickers hardness instrument (Mitutoyo MVK-H1 Hardness Testing Machine, Mitutoyo, Japan) under a load of 100 g.

3.  RESULTS AND DISCUSSION

Fig. 1. SEM images of composite powder milled for 10 h (a) at magnification X500 and (b) X2000

The morphology of the milled Cu-Ti-C mixture powders prepared by HEBM at 500 rpm with milling time of 10 h was shown in Fig. 1 (a). There is an agglomeration of powder particles, which can be seen at higher magnification of 2000 (Fig. 1b). It is well known that cold welding and fracture are the two essential processes involved in the mechanical milling process where powder particles are repeatedly flattened, cold welded, fractured, and re-welded [16].

Fig. 2. Change in shrinkage of Cu/TiC composite powder compacts during SPS process

The green sample was heated from room temperature to 1000 °C and hold for 5 min. The change in shrinkage of Cu/TiC composite powders during the SPS process is illustrated in Fig. 2. Increasing the sintering temperature, the shrinkage of sample also increases. In holding time, the shrinkage of compacted sample seems to be unchanged. That means the densification of the sample is not affected if prolonging holding time over 5 min. XRD pattern of the Cu/TiC composite sample sintered at 1000 °C showed only the copper and TiC peaks (Fig. 3). No oxide peak was observed in the X-ray diffractogram of Cu/TiC composite.

Fig. 3. XRD pattern of nanocomposite sample SPS-ed at 1000 oC

Table 1. EDS analysis of the composite sample sintered at 1000 °C

Element Concentration, wt. %
Spectrum 2 Spectrum 3 Spectrum 4
C 10.77 10.49 9.60
O 11.85 13.15 9.23
Ti 15.18 24.66 10.49
Cr 0.42 0.57
Fe 1.13 1.72 0.94
Cu 60.66 49.41 69.75
Fig. 4. SEM images of composite sample sintered at 1000 oC and its EDS spectra

SEM/EDS analysis of composite sample sintered at 1000 °C is shown in Fig. 4 and listed in table 1. The results show that the light areas in the mechanical mixed layer such as spectrum 3 (Fig. 4 c) contain more copper but less titanium and carbon elements than dark region in spectrum 2 (Fig. 4 b). The content of copper of the spectra 3 and 2 is 69.75 and 49.41, respectively. The concentration (wt. %) of carbon in three spectra is nearly similar to each other that means the homogeneous distribution of carbon in copper matrix. However the concentration (wt. %) of iron is rather lower than value Jian Zhuang et al. reported in [17]. Fig. 5 presented the microstructure of the insitu Cu/TiC composite samples sintered at 900 and 1000 °C. The OM images of the polished samples in Fig. 5. (a, d) show the lowest porosity in composite sample sintered at 1000 °C. With high percentage of reinforcement particle of 30 vol%, the fracture surface of sintered composites as displayed in Fig. 5. (b,e) belongs to ductile mode. The TEM images of microstructure of the SPS-ed Cu/TiC nanocomposite samples sintered at 900 and 1000 °C are shown in Figure 5 (c,f). The particles of titanium carbide in the image are indicated with the arrows. Selected area diffraction patterns taken from this sample confirmed the presence of TiC, that is in consistence with XRD pattern from figure 4. Very fine TiC particles of about 10 nm distributed on the copper matrix can be seen from HRTEM images. Table 2 summarizes the relative density and microhardness of the Cu/TiC nanocomposites SPS-ed from the milled powders. At 800 °C, the relative density and hardness values of composite sample are about 74 % and 250 HV, respectively. The low value of relative density and hardness may be due to low sintering temperature, at which Ti still does not react with C to from TiC. Y. F. Yang et al. reported the formation of TiC from Ti and C can occur at 1130÷1300 °C. H. Liang and et al. also found that the initial temperature of TiC formation could be prophesied at about 1233 K in the 20 wt.% Cu-Ti-C system [18].

Fig. 5. (a) OM, (b) FE-SEM, (c) HR-TEM images of Cu/TiC nanocomposite sample sintered at 900 oC (inset SAED corresponding to a TiC phase), and (d) OM, (e) FE-SEM (f) HR-TEM images of Cu/TiC nanocomposite sample sintered at 1000 oC (inset SAED corresponding to a TiC phase)

Table 2. Relative density and hardness of composite samples sintered at different temperatures

Sintering temperature (oC) Hardness (HV) Relative density (%)
800 251 74
900 330 92
1000 364 95.5

Increment in the sintering temperature from 800 to 1000 °C leads to higher number of TiC particles which can be formed in SPS process, and enhanced the hardness of the Cu/TiC nanocomposite samples. The uniform distribution of the reinforcement nanoparticles in the matrix causes high hardness of the composite. Due to the refined microstructure of the as-milled powder, the SPSed composite powders show more effective compaction of powder samples with the hardness three times higher than that of the samples consolidated by hot press sintering the mixture of CuTi3SiC2/Al2O3 powders obtained by Xuelong Fuetal [19]. The highest relative density and microhardness of the Cu/TiC nanocomposites obtained for sample sintered at 1000 °C are 95.5 % and 364 HV, respectively.

4.  CONCLUSION

The in-situ Cu/TiC nanocomposites was synthesized by HEBM and SPS. Composite sample shrunk while increasing sintering temperature from room temperature to 1000 °C and nearly unchanged during holding time. XRD analysis of Cu/TiC composite sample sintered at 1000 °C indicates only diffraction peaks of TiC and Cu phases without any oxidation phase formed during sintering process. TiC nanoparticles were detected and indentified by HRTEM and electron diffraction. The size of the TiC particles distributed in the copper matrix of the SPS product is about 10 nm. The relative densities of Cu/TiC composite increased from 74 to 95.5 % for sintering temperatures of 800, 900 and 1000 °C, respectively. The microhardness of in-situ composites also increased from 251 to 364 HV. At higher sintering temperature of 900 and 1000 °C, the in-situ reaction between Ti and C to form nano TiC particles could be promoted. The more TiC nano particles are produced the higher relative density and hardness of composite can be obtained.

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