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Structural Investigations of TiC–Cu Nanocomposites Prepared by Ball Milling and Spark Plasma Sintering

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In this work, TiC–Cu composites containing 20 and 30 vol % of nano-sized titanium carbide (TiC) particles were prepared by powder metallurgy using copper powders with micrometer-sized and nanometer-sized particles. Mixtures of TiC and Cu powders were
  metals  Article Structural Investigations of TiC–Cu NanocompositesPrepared by Ball Milling and Spark Plasma Sintering Oanh Nguyen Thi Hoang  1, *, Viet Nguyen Hoang  1  , Ji-Soon Kim  2 and Dina V. Dudina  3,4 1 School of Materials Science and Engineering, Hanoi University of Science and Technology, No 1 Dai Co Viet, Hanoi 100000, Vietnam; viet.nguyenhoang@hust.edu.vn 2 School of Materials Science and Engineering, University of Ulsan, San-29, Mugeo-2 Dong, Nam-Gu,Ulsan 680-749, Korea; jskim@ulsan.ac.kr 3 Lavrentyev Institute of Hydrodynamics, Siberian Branch of the Russian Academy of Sciences,Lavrentyev Ave. 15, Novosibirsk 630090, Russia; dina1807@gmail.com 4 Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of the Russian Academy of Sciences, Kutateladze str. 18, Novosibirsk 630128, Russia *  Correspondence: oanh.nguyenthihoang@hust.edu.vn; Tel.: +84-4-3868-0409Academic Editors: Mieczyslaw Jurczyk and Manoj GuptaReceived: 17 January 2017; Accepted: 31 March 2017; Published: 3 April 2017 Abstract:  In this work, TiC–Cu composites containing 20 and 30 vol % of nano-sized titanium carbide (TiC) particles were prepared by powder metallurgy using copper powders with micrometer-sized and nanometer-sized particles. Mixtures of TiC and Cu powders were ball milled for 10 h and spark plasma sintered at 800–900  ◦ C under an applied pressure of 50 MPa. The relative density of the sintered composites was 95.0%–96.5%. The composites fractured in a ductile mode. The crystallite size of the copper matrix in the composites prepared using the nanometer-sized copper powder was smaller than that in composites prepared using the micrometer-sized copper powder, which wasconfirmed by transmission electron microscopy (TEM). The hardness of the composites increasedas the sintering temperature was increased from 800 to 900  ◦ C. When the TiC content increasedfrom 20 to 30 vol %, the hardness of the composites obtained from the micrometer-sized copper powder and sintered at 900  ◦ C increased from 284 to 315 HV, while in composites obtained from the nanometer-sized copper, the hardness decreased from 347 to 337 HV. Keywords:  spark plasma sintering; titanium carbide; copper; ball milling; hardness 1. Introduction Copper is widely used as a material for electrical contacts due to its high electrical and thermal conductivities, low cost, and good corrosion resistance. However, the poor mechanical propertiesof copper—low hardness and strength—narrow the range of its possible applications. Inherent limitations of copper stimulate the development of copper matrix composites reinforced with ceramic particles [ 1 – 6 ]. To maintain high electrical conductivity, reinforcements that are thermodynamically stable in copper should be used. This avoids the dissolution of other elements in copper and maintains its high electrical conductivity. Titanium carbide (TiC) can be used as a reinforcing phase in copper matrix composites due to its high modulus, high hardness, and high melting temperature. In addition, TiC has negligible solubility in copper such that the TiC/Cu interface remains free from intermetallic compounds or solid solutions [7,8]. Discontinuously reinforced metal matrix composites can be produced by powder metallurgy,casting, self-propagating high-temperature synthesis, and other techniques [ 4 , 5 , 9 – 11 ]. All of these techniques are based on the addition of ceramic reinforcements to the matrix materials, which are in the liquid or solid (powder) state. In practice, it is rather difficult to distribute reinforcing nanoparticles  Metals  2017 ,  7 , 123; doi:10.3390/met7040123 www.mdpi.com/journal/metals   Metals  2017 ,  7 , 123 2 of 11 in metallic melts [ 12 , 13 ]. Therefore, in order to improve the dispersion of TiC in the Cu matrix, high-energy ball milling is used to produce composite powders. Until now, most studies have focused on the possibilities of reducing the size of the reinforcing particles down to the nanoscale. The main issue in the synthesis of TiC–Cu composites is low wettability of the reinforcing particles by the matrix. It is generally accepted that a finer size of the reinforcing particles is desirable for improving the mechanical properties of the metal matrix composites. Another possibility to achieve a better mechanical performance of TiC–Cu is to use copperpowders with different particle sizes. It should be noted that the effect of reducing the particle sizeof the copper powders down to the nanoscale on the microstructure and mechanical properties of  TiC–Cu composites has been much less investigated. In this work, TiC–Cu composites were prepared by high-energy ball milling and spark plasma sintering (SPS). This sintering technique has gained a reputation of a versatile method of fastconsolidation of powder materials [ 14 – 18 ]. The SPS method proves effective for consolidatingcomposite powders. This works is aimed at investigating the influence of the size of the starting copper powders on the microstructure and hardness of the spark plasma sintered TiC–Cu composites. 2. Experimental Procedures Two copper powders with average particle sizes of 75  µ  m and 40 nm (US1090, US ResearchNanomaterials, Inc., Houston, TX, USA) were used as the staring materials. Titanium carbide TiC powder (US2052, US Research Nanomaterials, Inc.) with particles in the 40–60 nm range was used as a reinforcement. The TiC–Cu powder mixtures containing 20 and 30 vol % of TiC were mechanically milled in a high-energy planetary ball mill (P100, Taemyong Scientific Co., Ltd., Seoul, South Korea) for10 h under an argon atmosphere. The ball-to-powder weight ratio was 10:1, and the ball diameter was 5 mm. To disperse the nanometer-sized TiC particles in the copper matrix homogeneously, the milling conditions (vial rotation speed and milling time) were optimized. At a rotation speed of 500 rpm, stickingof the powder to the milling vials and balls prevailed, and the iron contamination of the powder mixturewas significant. At 300 rpm, mixing of the Cu and TiC powders was poor, which resulted in low hardness of the sintered materials. Conditions of milling established at 400 rpm were found to be optimal for the selected powder mixtures. A milling time of 10 h was determined to be sufficient for achieving a uniform distribution of TiC nanoparticles in the copper matrix. Composites obtained from the 75- µ  m copper powder are designated as “nanocomposite A”, while those obtained from the 40-nm copper powder are designated as “nanocomposite B”. The ball-milled powders were 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. SINTERLAB Model: SPS-515S, Sumitomo Coal Mining, Tokyo, Japan). The samples were heated from room temperature to 800–900  ◦ C by pulsed DC current passing through the graphite die, punches, andthe sample itself. The samples were held at the maximum temperature for 5 min. A pressure of  50 MPa was applied through the sintering cycle. The microstructure of the composites was studied by scanning electron microcopy and energy-dispersive spectroscopy (SEM/EDS) using a field-emission JEOL JSM-7600F 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 sintered samples was studied by means of X-ray diffraction (XRD) using a RIGAKU RINT-2000 diffractometerwith Cu K α  radiation (Rigaku Corporation, Tokyo, Japan). The relative density of the composites was determined by Archimedes’ method. The hardness of the sintered TiC–Cu 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 Mechanical milling of the TiC–Cu powder mixtures resulted in the formation of composite agglomerates. The SEM images showing the morphology of the agglomerates of nanocomposites A   Metals  2017 ,  7 , 123 3 of 11 and B containing 20 and 30 vol % of TiC are presented in Figure 1. With increasing TiC content from 20 to 30 vol %, the size of the agglomerates decreased in both nanocomposites. The agglomeratesof nanocomposite B were finer than those of nanocomposite A at the same TiC content. Most of the particles of nanocomposite B were smaller than 10 µ  m, as can be seen from the SEM images shown inFigure 2. Some large particles were formed due to agglomeration of small particles, reaching a size of  10–30 µ  m.   Figure 1.  Morphology of the milled TiC–Cu powders ( a ) 20 vol % TiC, nanocomposite A;  ( b ) 20 vol % TiC, nanocomposite B; ( c ) 30 vol % TiC, nanocomposite A; and ( d ) 30 vol % TiC, nanocomposite B (SEM images).   Figure 2.  Morphology of the milled powders ( a ) 30 vol % TiC, nanocomposite A; and ( b )  30 vol % TiC, nanocomposite B (higher-magnification SEM images; the rectangles mark areas from which energy-dispersive spectroscopy (EDS) spectra were taken). In the ball-milled powders, iron was detected by the EDS analysis (Table 1). The rectangles inthe micrographs shown in Figure 2 mark the areas analyzed by the EDS. The presence of iron was   Metals  2017 ,  7 , 123 4 of 11 due to contamination of the powders from the milling vials and balls. The measured concentrationsof iron were much smaller than those detected in the powders subjected to treatment in a horizontal miller [19]. Table 1.  EDS analysis of the ball milled TiC–Cu powders, nanocomposites A and B containing 30 vol % TiC. Composite Concentration, wt %Cu Ti C O Fe Nanocomposite A 63.83 11.32 18.26 5.75 0.83Nanocomposite B 64.23 11.32 14.04 7.78 2.63 Figure3showstheXRDpatternsofnanocompositesAandBcontaining20vol%TiCsparkplasmasintered at 800 and 900  ◦ C. After sintering, the nanocomposites retained the phase composition of the  ball-milled powder mixtures and consisted of two phases, Cu and TiC, as no chemical reaction took place between Cu and TiC. No copper oxides were found in the sintered nanocomposites. The values of the lattice parameters of TiC and Cu are presented in Tables 2–5. The lattice parameter values obtained from each crystallographic plane were plotted against the Nelson–Riley function  f  ( θ ) [20]:  f  ( θ ) =  c os 2 θθ  +  c os 2 θ sin θ  where θ isBragg’s angle. Inthismanner, astraight linewas obtained. The valueof thelatticeparameterwas estimated by extrapolating the straight line to  f  ( θ ) = 0. The obtained values of the lattice parameter of TiC were 4.312 Å and 4.306 Å for nanocomposites A and B, respectively. These values are only slightly smaller than the value reported in the Joint Committee on Powder Diffraction Standards file(JCPDS 311400)—4.33 Å. The values of the lattice parameter of Cu in nanocomposites A and B were 3.613 Å and 3.616 Å, respectively. These values agree with that reported in JCPDS 040836 file (3.615 Å).Good agreement of the calculated lattice parameters with those of pure TiC and Cu phases indicate the absence of any chemical interactions or compositional changes in the phases during sintering.   304050607080 TiC  composite B, 900°C Composite A, 900°C composite B, 800°C Composite A, 800°C Intensity (a.u.) 2 theta (deg.) Cu   Figure 3.  XRD patterns of nanocomposite A and nanocomposite B containing 20 vol % TiC sintered at 800 and 900  ◦ C.   Metals  2017 ,  7 , 123 5 of 11 Table 2.  Structural parameters of TiC in nanocomposite A with 20 vol % TiC sintered at 900  ◦ C. ( hkl )  h 2 +  k 2 +  l 2 2 θ  Sin θ  a (Å) f( θ ) a (Å) from theNelson–Riley Plot (111) 3 36.088 0.309 4.312 2.9704.312(200) 4 41.88 0.357 4.316 2.483(220) 8 60.77 0.505 4.312 1.496 Table 3.  Structural parameters of TiC in nanocomposite B with 20 vol % TiC sintered at 900  ◦ C. ( hkl )  h 2 +  k 2 +  l 2 2 θ  Sin θ  a (Å) f( θ ) a (Å) from theNelson–Riley Plot (111) 3 36.091 0.309 4.312 2.9704.306(200) 4 41.885 0.357 4.315 2.483(220) 8 60.868 0.506 4.306 1.493 Table 4.  Structural parameters of Cu in nanocomposite A with 20 vol % of TiC sintered at 900  ◦ C. ( hkl )  h 2 +  k 2 +  l 2 2 θ (deg.) a (Å) f( θ ) a (Å) from theNelson–Riley Plot (111) 3 43.2 3.628 2.3893.613(200) 4 50.4 3.622 1.956(220) 8 74.1 3.620 1.075 Table 5.  Structural parameters of Cu in nanocomposite B with 20 vol % of TiC sintered at 900  ◦ C ( hkl )  h 2 +  k 2 +  l 2 2 θ (deg.) a (Å) f( θ ) a (Å) from theNelson–Riley Plot (111) 3 43.309 3.620 2.3823.616(200) 4 50.41 3.622 1.956(220) 8 74.139 3.618 1.074 The fracture surfaces of nanocomposites A and B sintered at 800  ◦ C and 900  ◦ C are shown inFigures 4 and 5. The sintered nanocomposites fractured in a ductile mode. The relative densities of  nanocomposites A and B increase slightly with an increase in the sintering temperature from 800 to 900  ◦ C (Table 6). As the concentration of TiC increased from 20 to 30 vol %, the relative densities of the nanocomposites decreased. A similar effect was reported by Reddy et al. for the TiC–Cu composites produced by microwave processing [21]. Table 6.  Relative densities of the spark plasma sintered (SPS) TiC–Cu nanocomposites. SPS Temperature,  ◦ CRelative Density, %A20 vol % TiC–CuB20 vol % TiC–CuA30 vol % TiC–CuB30 vol % TiC–Cu 800 96.4 96.2 95.2 95.8900 96.5 96.4 95.5 96.0 The hardness values of nanocomposites A and B are presented in Table 7. For all concentrations of  TiCandsinteringtemperaturesstudied,nanocompositeBshowedhigherhardnessthannanocompositeA. An increase in the content of TiC from 20 to 30 vol % in nanocomposite A sintered at 900  ◦ C resulted in an increase in the hardness from 284 to 315 HV. Such an increase can be expected because hard particulate reinforcements act as barriers to the dislocation movement within the copper matrix. In thecase of nanocomposite B, an increase in the TiC content led to a slight decrease in the hardness. Indeed,
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