The effects

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					Nanotechnology 17, (2006) 5759–5764




        The effects of interfacial bonding on mechanical
        properties of single-walled carbon nanotube
        reinforced copper matrix nanocomposites




                   Byengsoo Lim, Chul-ju Kim, Bumjoon Kim, Untae Shim,
                   Seyoung Oh, Byung-ho Sung, Jee-hoon Choi and
                   Seunghyun Baik
 Abstract
The effects of interfacial bonding on mechanical properties of single-
walled carbon nanotube reinforced copper matrix nanocomposites
were investigated. The nanocomposites were fabricated by means of
a powder metallurgy process, which consists of mixing carbon
nanotubes with matrix powder followed by hot-pressing. The mixing
process was carried out by ultrasonicating the nanotubes and copper
powder in ethanol. The interfacial strength between the nanotubes and
the copper matrix was improved by coating the nanotubes with nickel.
The displacement rate of the nanotube reinforced nanocomposites
was found to increase at 200 ◦C, whereas that of the nickel-coated
nanotube reinforced nanocomposites significantly decreased. The
incorporation of carbon nanotubes and nickel-coated carbon
nanotubes in the copper matrix composites improved tribological
properties compared with those of pure copper specimens. (Some
figures in this article are in colour only in the electronic version)
Introduction
   Nanosized reinforcements have received considerable
    attention because of the enhanced mechanical property that
    can be imparted to the metal matrix as compared with
    conventional micron-sized reinforcements
   The incorporation of carbon nanotubes in the development
    of advanced engineering composites with improved
    mechanical and tribological properties has become an
    interesting research area
   A weak mechanical bonding state was observed for the
    interface between the carbon fibre and the copper
    matrixdue to the poor wettability between carbon and
    copper
      Experiments            (copper powder mixture preparation)



   SWNT were dispersed in
    ethanol by an intensive
    sonication for 5 min at 540 W.
   Copper powder was added to
    the solution and mixed with
    nanotubes suspended in                     (a)                          (b)
    ethanol by ultrasonicating at    Figure 1. SEM images of copper powder. (a) A copper
                                     particle before the mixing process (b) nanotubes
    a power level of 5 W.            attached to the surface of a copper particle after the
   The solution was heated to a     mixing process.

    temperature of 50 ◦C and
    ultrasonicated until most of
    the ethanol was evaporated.
          Experiments               (Nickel electroless plating)


   Preactivation of carbon nanotubes was
    accomplished by ultrasonically dispersing
    carbon nanotubes in a solution of 0.1 M
    SnCl2/0.1 M HCl for 30 min.
   The Sn+2 sensitized carbon nanotubes
    were further activated in an aqueous
    solution of 0.0014 M PdCl2/0.25 M HCl for
    another 30 min.
   The activated nanotubes were then                   Figure 2. Nickel-coated carbon nanotubes.
    introduced into an electroless plating bath.
    The composition of the plating solution
    includes NiCl2·6H2O, NiSO4·6H2O,
    Na2HC6H5O7·1.5H2O, NaH2PO2·2H2O,
    NH4Cl and Pb(NO3)2.
   After the coating process, nickel coated
    nanotubes were mixed with copper
    powder by the mechanical mixing process.

                                                   Figure 3. EDX analysis of nickel-coated nanotubes.
         Experiments               (Hot-pressing of the nanotube-copper
                                    powder mixtures)


   Hot-pressing was performed in a
    graphite mould by uniaxial
    pressurization in a vacuum of 10−3
    Torr to prevent oxidization of the
    powder mixtures.


   The diameter of the specimens was                Figure 4. A schematic diagram of the
    15 mm and the thickness was 0.8                            hot-pressing apparatus.
    mm. The temperature was ramped to
    600 ◦C in 40 min and kept constant
    for 30 min. Finally, the temperature
    was cooled down to room
    temperature in 20 min.



                                           Figure 5. Temperature and pressure conditions for the hot-
                                           pressing process of specimens with a diameter of 15 mm.
       Experiments              (High temperature displacement rate
                                 tests and dry friction tests)


   The hot-pressed specimen was
    ground into the shape of a thin
    square (10 × 10 × 0.5 mm3), and a
    constant load of 245 N was applied
    to the specimen by a ceramic (Si3N4)
    ball with a diameter of 2.4 mm.
    Displacement of the specimen was
    measured by LVDT with an accuracy
    of 10−3 mm.

   The sliding tests were carried out
    without lubricant for 400 s at room
    temperature. All specimens were              Figure 6. A schematic diagram of the small
                                                 punch creep tester
    ultrasonically cleaned with acetone
    before the tests. A sliding speed of
    0.02 m s−1 (60 rpm) was used and a
    normal contact load of 5.2 N was
    applied in all tests.
         Results and discussion
   A pure copper specimen, hot-pressed
    using copper powder of about 10–15 μm,
    was used as a reference material.
   For the nanotube reinforced copper matrix
    composite (0.5 vol%), the displacement
    rate rapidly increased and the rupture time
    was shorter than that of the pure copper
    specimen. This might be due to the poor
    interfacial strength between carbon and
    copper.
   The nickel-coated nanotube reinforced
    copper matrix composite showed the
    longest rupture time which is six times
    longer than that of the pure copper           Figure 7. The effects of interfacial strength
    specimen. In addition, it showed the          on displacement rates.
    slowest displacement rate.
   Such remarkable strengthening by nickel-
    coated nanotube reinforcements was due
    to the high load transfer efficiency of
    nickel-coated nanotubes in the copper
    matrix.
          Results and discussion
   (a) shows the ruptured section of the pure copper specimen.
   (b) shows the ruptured section of the nanotube reinforced nanocomposites (0.5
    vol%). Nanotubes came into contact with each other rather than with the copper
    matrix. These aggregates prevented effective bonding between the copper particles
    and the nanotubes. The aggregates were detached from the copper matrix, and this
    type of interface was not strong enough to transfer the stress of the matrix to the
    reinforcements.
   (c) shows the ruptured section of the nickel-coated nanotube reinforced
    nanocomposites (0.5 vol%). Nickelcoated nanotubes were pulled out during the
    fracture process probably due to the increased interfacial bonding. Good interfacial
    strength was obtained due to dissolution of copper– nickel solid solution at the
    interface between nickel-coated nanotubes and copper particles.




                                                (b)                                    (c)
             (a)

     Figure 8. SEM images of ruptured sections of specimens (a) pure copper specimen (b) nanotube
     reinforced nanocomposites (0.5 vol%) (c) nickel-coated nanotube reinforced nanocomposites (0.5 vol%).
         Results and discussion

   The friction coefficients of the pure copper
    specimen started from about 0.2 and
    increased continuously with time.
   The incorporation of carbon nanotubes
    and nickel-coated carbon nanotubes in
    the copper matrix decreased the friction
    coefficients and increased intervals up to
    the onset of scuffing, compared with those
    of the pure copper specimen.
   The scuffing failure time of the nickel-
    coated nanotube reinforced
    nanocomposites was shorter than that of        Figure 9. The effects of carbon nanotubes and
    the nanotube reinforced nanocomposites.        nickel-coated carbon nanotubes on the
    This could be due to the increased             tribological properties of nanocomposites.
    adhesion wear between nickel and the
    steel ball. The excellent adhesion between
    nickel and steel is a well known
    phenomenon.
         Conclusion

   We have investigated the effects of interfacial bonding on mechanical
    properties of single-walled carbon nanotube reinforced copper matrix
    nanocomposites.
   Carbon nanotube – copper powder mixtures were prepared by a mechanical
    mixing process, and nanocomposites were fabricated by hotpressing.
   The interfacial bonding between the nanotubes and the copper matrix was
    significantly improved by coating the nanotubes with nickel.
   The displacement characteristics of nanocomposites at a high temperature
    were investigated to evaluate the interfacial strength between the copper
    powder and the carbon nanotubes.
   The displacement rate of the nanotube reinforced nanocomposites was
    found to increase, whereas that of the nickel-coated nanotube reinforced
    nanocomposites significantly decreased.
   The incorporation of carbon nanotubes and nickel-coated carbon
    nanotubes in the copper matrix composites improved tribological properties
    compared with those of pure copper specimens.

				
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posted:4/24/2011
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