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Characterization of In-Situ Cu–TiH2–C and Cu–Ti–C Nanocomposites Produced by Mechanical Milling and Spark Plasma Sintering

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This study focuses on the fabrication and microstructural investigation of Cu–TiH2–C and Cu–Ti–C nanocomposites with different volume fractions (10% and 20%) of TiC. Two mixtures of powders were ball milled for 10 h, consequently consolidated by
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  metals  Article Characterization of In-Situ Cu–TiH2–C and Cu–Ti–CNanocomposites Produced by Mechanical Millingand Spark Plasma Sintering Nguyen Thi Hoang Oanh  1, *, Nguyen Hoang Viet  1  , Ji-Soon Kim  2 andAlberto Moreira Jorge Junior  3,4,5,6,7 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 Department of Materials Science and Engineering, Federal University of S ã o Carlos, Via Washington Luiz,km 235, S ã o Carlos, SP 13565-905, Brazil; Jorge.Moreira@simap.grenoble-inp.fr 4 University of Grenoble Alpes, Science et Ing é nierie des Mat é riaux et Proc é d é s (SIMAP),F-38000 Grenoble, France 5 Centre National de la Recherche Scientifique (CNRS), Science et Ing é nierie des Mat é riaux etProc é d é s (SIMAP), F-38000 Grenoble, France 6 University of Grenoble Alpes, Laboratoire d’Electrochimie et de Physico-chimie des Mat é riaux et desInterfaces (LEPMI), F-38000 Grenoble, France 7 Centre National de la Recherche Scientifique (CNRS), Laboratoire d’Electrochimie et de Physico-chimie des Mat é riaux et des Interfaces (LEPMI), F-38000 Grenoble, France *  Correspondence: oanh.nguyenthihoang@hust.edu.vn; Tel.: +84-4-3868-0409Academic Editor: Manoj GuptaReceived: 5 February 2017; Accepted: 27 March 2017; Published: 29 March 2017 Abstract:  This study focuses on the fabrication and microstructural investigation of Cu–TiH2–C and Cu–Ti–C nanocomposites with different volume fractions (10% and 20%) of TiC. Two mixtures of  powders were ball milled for 10 h, consequently consolidated by spark plasma sintering (SPS) at 900 and 1000  ◦ C producing bulk materials with relative densities of 95–97%. The evolution process of TiC formation during sintering process was studied by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). XRD patternsof composites present only Cu and TiC phases, no residual Ti phase can be detected. TEM imagesof composites with (10 vol % TiC) sintered at 900  ◦ C show TiC nanoparticles about 10–30 nmprecipitated in copper matrix, most of Ti and C dissolved in the composite matrix. At the highersintering temperature of 1000  ◦ C, more TiC precipitates from Cu–TiH2–C than those of Cu–Ti–C composite, particle size ranges from 10 to 20 nm. The hardness of both nanocomposites also increased with increasing sintering temperature. The highest hardness values of Cu–TiH2–C and Cu–Ti–C nanocomposites sintered at 1000  ◦ C are 314 and 306 HV, respectively. Keywords:  spark plasma sintering; Cu–TiC; in-situ composites; mechanical milling 1. Introduction Metal matrix composites (MMCs) are advanced materials which combine ductility and toughness of metal and high strength and modulus of ceramic particles. The unique properties of MMCs are high specific strength, specific modulus, and good wear resistance compare to unreinforced metal [ 1 ]. In many type of MMCs, copper matrix composites (CMCs) have received a lot of interest because of super toughness and wear resistance which are used for structural application in wear industry [ 2 ].  Metals  2017 ,  7 , 117; doi:10.3390/met7040117 www.mdpi.com/journal/metals   Metals  2017 ,  7 , 117 2 of 12 Generally, there are two routes to produce particulate-reinforced CMCs, which are ex-situ and in-situ. In the ex-situ method, ceramic particles such as TiB2, TiC, and oxide are introduced into the metal matrix via powder metallurgy or conventional casting methods [3,4]. However, the CMCs fabricated  by these methods revealed a drawback because of poor interfacial bonding between reinforcement particles and copper matrix [ 5 ]. In order to improve the wettability of the Cu matrix and reinforcement phase, nano-ceramic particles were used [ 6 ]. Nevertheless, ceramic nanoparticles have tendency to segregate into clusters in milling process leading decrease strength of composite. The distribution of  reinforced particles are non-uniform in the copper matrix, the mechanical and electrical features of the composite will be affected negatively [ 7 ]. On the contrary, ceramic particles synthesized by thein-situ method were dispersed more homogeneously in the copper matrix. The interfaces between reinforcement particles and matrix are clean, and very fine reinforcement particles are formed. Among CMCs, Cu–TiC system is attracted more attention due to their potential applications as electricalsliding contacts, resistance welding electrodes [ 8 ]. In the in-situ method, TiC nanoparticles were produced by the reaction between Ti and C during sintering process. In order to prevent grain growth of reinforcement and copper particles occur at high sintering temperature a fast sintering process need to be carried out. Spark plasma sintering (SPS) has some advantages such as rapid sintering, uniform sintering, low running cost, easy operation proves a suitable sintering technique for consolidation nano-structure, nanocomposite, and amorphous materials. In SPS, very high temperature over melting temperature may be attained in the contact area of powder particles which enhances interparticle  bonding without considerable grain growth occurring [9–13]. The replacement of Ti powder in Cu–Ti–C composite by another powder such as TiH2 is considerablebecauseofhighpriceofTipowder. Inadditionto, dehydrogenationofTiH2occursduring sintering process is always accompanied by formation of high concentration of lattice defects and the highly activated Ti atoms. Released hydrogen from TiH2 will react with oxygen on the surface of TiH2powders in the form of H2O which affect positively on the electrical conductivity of the composite [ 14 ]. The objectives of the present work are to explore the possibility of synthesizing Cu–TiC in-situcomposites made from Cu–TiH2–C and Cu–Ti–C powder mixtures by mechanical milling and SPS.The effect of reinforcement content and sintering temperature on microstructure and hardness properties of composites was investigated. 2. Experimental Procedure The copper (with average particle size of 75  µ  m), titanium (average particle size of 45  µ  m), TiH 2 (averageparticlesizeof40 µ  m)andgraphite(averageparticlesizeof5 µ  m)powder( ≥ 99%purity, from HIGH PURITY CHEMICALS Co., Ltd., Chiyoda, Japan) were used as starting materials. The powder mixtures of two composites Cu–TiH2–C and Cu–Ti–C with mixing ratio of 10 and 20 vol % TiC were mechanically milled in a high-energy planetary ball mill (P100-Korea). Milling was operated for 10 h atthe 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. A 1.5 g amount of as-milled powder was loaded into a cylindrical graphite die with 10 mm-inner and was subjected to a pulsed current using a spark plasma sintering equipment, (SPS-515 apparatusSumitomo Coal Mining, Tokyo, Japan). The chamber was pumped to low vacuum (<5 Pa). The composite powders were spark plasma sintered at 900 and 1000  ◦ C under a pressure of 50 MPa for 5 min with a heating rate of 50  ◦ C/min. X-ray diffraction patterns of the composites were recorded by a SIEMENS D5000 diffractometer (Siemens Industry Inc., Karlsruhe, Germany) using Cu K α  radiation ( λ  = 1.5418 Å). Microstructural analysis of powders and composite samples was carried out by using Scanning Electron Microscopy (SEM/EDX-JEOL JSM-7600F, JEOL Ltd., Tokyo, Japan) and Transmission Electron Microscopy (TEM-JEOLJEM-2100,JEOLLtd.,Tokyo,Japan). Relativedensitiesofbulkcompositesweredetermined   Metals  2017 ,  7 , 117 3 of 12  by Archimedes method. The indexation of such selected area electron diffraction (SAED) patterns was performed using JEMS software [15]. Microhardness measurements were performed using a Vickers hardness instrument (Mitutoyo MVK-H1 Hardness Testing Machine, Mitutoyo, Japan) under a load of 100 g. To analyze the surface of fracture, samples were simply fractured by gripping the halves of the composite with pliers and bending them apart. 3. Results and Discussion 3.1. Characterization of the Powders Figure 1a,b shows SEM images of starting powders, respectively for TiH 2  and copper powders. TiH 2  particles have an irregular shape while Cu powder particles have a dendritic shape. Figure 1c–f  show SEM images of composite powders formed after 10 h of milling of Cu–TiH2–C and Cu–Ti–C mixtures, with different amounts of reinforcement particle content. As one can observe, the increase of  the reinforcement particle content the particle size of milled powders decreases for both composites. Cu–Ti–C composite presented finer particles than those for the Cu–TiH2–C composite, with the same reinforcement particle content. As can be seen from SEM images shown in Figure 1g,h. Some large particleswereformedduetoagglomerationofsmallparticlesreachingasizeof10–30 µ  m. EDSanalyses (Figure 2) were performed on particles such as in Figure 1g,h which presents spectra relative to such analyses, of which it is worth noting that there was no contamination of Fe either from the millingtools (Table 1). However, such contamination was already observed even in the ex-situ method as reported in [16].   Figure 1.  Cont.   Metals  2017 ,  7 , 117 4 of 12   Figure 1.  SEM of starting powders in ( a ) TiH 2  and ( b ) copper powders. SEM images of compositepowders milled for 10 h, with 10 vol % TiC in ( c ) Cu–TiH2–C and ( d ) Cu–Ti–C; and with 20 vol % TiC in ( e ) Cu–TiH2–C and ( f ) Cu–Ti–C. Higher-magnification SEM images; the rectangles mark areas; from which EDS spectra were taken ( g ) Cu–TiH2–C and ( h ) Cu–Ti–C.   Figure 2.  Typical EDS analyses acquired from particles presented in Figure 1e (Cu–Ti–C) in ( a ) and Figure 1f  (Cu–TiH2–C) in ( b ), both with 20 vol % TiC. Table 1.  EDS analysis of the ball milled powders with 20 vol % of TiC reinforcement particles. CompositeConcentration, wt %Cu Ti C O Cu–TiH2–C 73.78 8.75 13.42 4.05Cu–Ti–C 70.73 7.89 16.49 4.89 3.2. Characterization of Compacts after SPS X-ray diffraction patterns of as-sintered nanocomposites are illustrated in Figure 3. After SPSat 900  ◦ C (Figure 3a), XRD patterns presents only diffraction peaks related to pure copper. There   Metals  2017 ,  7 , 117 5 of 12 is a shift peak of copper in sintered composite compare to starting Cu powder. This fact clearlyevidences that most of the Ti and C have dissolved in the copper matrix and also that the sintering temperature was not high enough to precipitate the TiC phase. Conversely, by increasing the sintering temperature to 1000  ◦ C, Ti reacts with C, and TiC precipitates, as it is clearly noticeable in the XRDpatterns of Figure 3 b for mixtures with 20 vol % TiC. However, there is reason to believe that thesame has occurred for mixtures with 10 vol % of TiC because this is a thermodynamical condition. TiC weight percentages in bulk composites sintered at 1000  ◦ C produced from starting powders was calculated by Rietveld refinement method as shown in Table 2. The weight percentages of in-situ TiC nanoparticles with 10 vol % reinforcement particles for Cu–TiH2–C and Cu-Ti–C composites are 1.43 and 2.86%, respectively. Furthermore, as it will be under mentioned, TEM analyses confirm thepresence of TiC nanoparticles in mixtures with 10 vol % of TiC. At higher reinforcement particles of  20 vol %, the amount of TiC precipitated from Cu–TiH2–C and Cu-Ti–C composites also increases to 6.9 and 6.45 wt %, respectively. Additionally, it is important to observe that there was no precipitationof intermetallic phases during sintering process at any of the sintering temperatures. If one considers the Cu–Ti–C system, normally Cu, Ti, and C may interact to form several products through chemical reactions [ 17 ]. However, the Gibbs free energy of TiC formation at the temperature of 1273 K is about 84.4 kJ/mol, which is much lower than those to form other intermetallic phases of Ti and Cu, implying that the precipitation of TiC is thermodynamically preferred relative to other possible reactions. 2 Theta (deg.) (a)   Cu  Cu-TiH2-CCu-TiH2-CCu-Ti-C10 %-TiC10 %-TiC20 %-TiC    I  n   t  e  n  s   i   t  y   (  a .  u .   ) 20 %-TiCCu-Ti-C 900 oC  304050607080 Cu  304050607080 0   Cu   TiC   1000 oC (b)    I  n   t  e  n  s   i   t  y   (  a .  u   ) 2 Theta (deg.) Cu-TiH2-C20%-TiCCu-Ti-C20%-TiCCu-TiH2-C10%-TiCCu-Ti-C10%-TiC    Figure 3.  XRD patterns of nanocomposites ( a ) Spark Plasma Sintered at 900  ◦ C; ( b ) Spark Plasma Sintered at 1000  ◦ C. Table 2.  Fraction of phases of Cu–TiH2–C and Cu–Ti–C composites sintered at 1000  ◦ C calculated by Rietveld refinement. PhaseFraction of Phases (%)Cu–TiH2–C Cu–Ti–C10 vol % TiC 20 vol % TiC 10 vol % TiC 20 vol % TiC TiC 2.86 6.9 1.43 6.45Cu 97.14 97.14 98.57 93.55 Figure 4a–d present backscattered electron (BSE) SEM images of Cu–TiH2–C and Cu–Ti–Cnanocomposites sintered at 1000  ◦ C, showing details of the surface of samples. From these images,one can observe the presence of three gray tones, where black regions indicate the presence of someclosed porosity on the surface, dark-gray regions correspond to the solid solution Cu(Ti, C), and,finally, white regions correspond to Cu-richer regions. It is interesting to note that the porosity is minimal for Cu–TiH2–C nanocomposites for any amount of TiC. The porosity increases for Cu–Ti–C nanocomposites but reduces by increasing the amount of TiC. However, apparently, Cu-richer regions are thinner and better distributed for Cu–Ti–C nanocomposites than for Cu–TiH2–C ones, diminishing
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