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Bioinspired, multiscale reinforced composites with exceptionally high strength and toughness Ningning Song, Yunya Zhang, Zan Gao, and Xiaodong Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02459 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018
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Nano Letters
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DOI: 10.1002/ ((please add manuscript number))
2 3 4 5 6 7
Bioinspired, multiscale reinforced composites with exceptionally high strength and toughness Ningning Song, Yunya Zhang, Zan Gao, Xiaodong Li*
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Department of Mechanical and Aerospace Engineering
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University of Virginia
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122 Engineer’s Way
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Charlottesville, VA 22904-4746 (USA)
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*Corresponding author. E-mail:
[email protected], Tel: 434-243-7762
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Keywords: bioinspired polymer composite, reinforcements, polyaniline
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Graphical Abstract
boron
carbide
18 19
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nanowires,
multiscale
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Abstract
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Nature’s multiscale reinforcing mechanisms in fabricating composite armors such as seashells
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provide lessons for engineering materials design and manufacturing. However, it is still a
4
challenge to simultaneously add both micro- and nano-reinforcements in a matrix material
5
since nano-fillers tend to agglomerate, decreasing their reinforcing effects. In this study, we
6
report a new type of micro/nano hybrid filler, synthesized by an unconventional cotton aided
7
method, which has B4C microplatelet as the core and radially aligned B4C nanowires as the
8
shell. To enhance the bonding between the B4C fillers and epoxy, the B4C micro-/nano-fillers
9
were coated with a layer of polyaniline (PANI). With a low concentration of the PANI
10
functionalized B4C micro-/nano-fillers (1 wt.%), this B4C/epoxy composite exhibited an
11
exceptional combination of mechanical properties in terms of elastic modulus (~3.47 GPa),
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toughness (2026.3 kJ/m3), and fracture strain (>3.6%). An analytical mechanics model was
13
established to show that such multiscale reinforcement design remarkably enhanced the load
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carrying efficiency of the B4C fillers, leading to the overall improved mechanical
15
performance of the composites. This new design concept opens up a new path for developing
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light-weight,
17
configurations.
yet
high-strength
and
tough
materials
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with
multiscale
reinforcing
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Biological materials often possess superlative properties1,2 which are still beyond the reach
2
of man-made materials. One of the best examples is nacre (mother-of-pearl), commonly
3
referred to as nature’s armor. Nacre is renowned for its unusual combination of strength and
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toughness. With only two constituent materials – aragonite and biopolymer cemented in a
5
brick-mortar architecture, nacre exhibits several times enhancement in strength and thousand
6
times amplification in toughness2-5. However, it has been proven that simply cloning the
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brick-mortar architecture in engineering materials cannot reproduce nacre’s accomplishments,
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because nature employs multiple multiscale reinforcing principles in nacre to achieve such
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exceptional mechanical prowess. The micro reinforcements (the aragonite bricks) act as
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in-plane load carriers while the nano reinforcements (the nanoasperities/nanobridges on
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nacre’s brick surfaces) lift the out-of-plane performance. Such hierarchical architecture
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facilitates the filler/matrix interfacial load transfer. Due to the large span of length scales and
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the overall complexity of nacre, the development of engineering biomimetic composites has
14
been largely hampered by the lack of micro/nano hybrid reinforcements2. Engineering
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composites often rely on employing reinforcements such as micro- or nano-fillers in a
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relatively soft matrix. However, simultaneously adding both micro- and nano-reinforcements
17
in a matrix material remains challenging since nano-fillers tend to loosely adhere
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(agglomerate) onto micro-fillers, decreasing their reinforcing effects. An immediate question
19
is raised: how to achieve multiscale (both micro- and nano-scales) reinforcing effects
20
simultaneously in the matrix material? Can we achieve multiple reinforcing mechanisms
21
using only one type of reinforcements? In this context, we have a pressing need for seeking
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new kinds of fillers with both micro- and nano-characteristics (for instance, nanowires
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decorated microplatelets), with the goal of replicating nature’s multiscale reinforcing
2
mechanisms in engineering composites.
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Boron carbide (B4C), one of the third hardest materials known to man, is a promising
4
reinforcement for composites. B4C possesses appealing physical and mechanical properties
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such as a low density (2.5 g/cm3), high melting point (exceeding 2400 oC), extreme hardness
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(27.4 - 37.7 GPa), high elastic modulus (460 GPa), chemical inertness, high thermal stability,
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and a high capture section for neutrons6-8. These properties make B4C a promising material
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for numerous applications such as lightweight armor sustaining extreme conditions, abrasives,
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electronics, and neutron absorbers in nuclear reactors7,9. Furthermore, armed with a unique
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rhombohedral crystal structure, B4C at reduced dimensions, especially 1-dimensional, often
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exhibits novel properties9 such as large specific surface area and high mechanical strength
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close to its theoretical value10. However, the outstanding properties make the multiscale
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structural design and hierarchical morphology control of B4C difficult. Although chemical
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vapor deposition (CVD)11, plasma-enhanced chemical vapor deposition (PECVD)12 and
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carbothermal reduction (CTR)13 have been used to grow B4C nanomaterials, synthesizing a
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B4C filler with both micro- and nano-characteristics remains a challenge.
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As shown in Figure 1a,b, B4C nanowires were uniformly grown on the carbon fiber cloth
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by a top-growth mechanism through a typical vapor-liquid-solid (VLS) process14, where
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amorphous boron served as source of boron, carbon-rich gases generated from pyrolysis of
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cotton served as carbon source, and nickel served as a catalyst (details in supplemental
21
materials). During the VLS growth process, catalyst-mediated precipitation led to the axial
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elongation of nanowires while microplatelets were deposited around the nanowires on the
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carbon fiber substrate (Figure S3). Each microplatelet with a size of 1 µm×1 µm contains
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around 20 nanowires which are 50-500 nm in diameter and around 5 µm in length. The
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typical x-ray diffraction pattern of the unaltered micro/nano hybrid fillers is presented in
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Figure 1c. The peaks at 32.9°, 34.8°and 37.6° can be respectively indexed to (110), (104) and
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(021) planes of rhombohedral B4C (JCPDS No. 6-0555), indicating that the microplatelets
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and nanowires are of the same material - B4C. The general peak broadening and shifting were
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observed because of the existence of nickel15. The other peaks resulted from the catalyst and
8
substrate. The electron energy-loss spectroscopy (EELS) spectrum (Figure 1b) revealed two
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ionization edges corresponding to the characteristic K-edges of boron and carbon,
10
respectively.
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The B4C micro/nano hybrid fillers were released and dispersed by simply processing the
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unaltered samples using ultrasonication, and characterized by transmission electron
13
microscopy (TEM), as shown in Figure 1d. The B4C nanowires, with “smooth surface”, are
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radially distributed on the B4C microplatelet. The primary structural unit cell of B4C has a
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rhombohedral arrangement ( R3m space group, a = 5.16Å and α = 65.7°16), consisting of
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12-atom icosahedra and 3-atom linear chains17,18, as shown in Figure S4a. HRTEM inspection
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(Figure 1e) revealed that B4C nanowires grew with a perfect crystal lattice that had the axial
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growth plane (201) , covered with a layer of amorphous carbon. After being coated with
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PANI (details in supplemental materials), B4C micro-/nano-fillers became rougher (Figure
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1g,h). Raman spectroscopy (Figure 1f) uncovered that the untreated B4C micro-/nano-fillers
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(without PANI coating) displayed peaks at 189, 376, 486, 529, 723, 812, 980 and 1067 cm-1,
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which all correspond to crystalline B4C19. The spectrum below 200 cm-1 and above 600 cm-1
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resulted from the intraicosahedral and intericosahedral modes. The bands at 376, 486 and 529
2
cm-1 are ascribed to the vibrations of chain structures linking icosahedra. PANI functionalized
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B4C micro-/nano-fillers (Figure 1f) displayed similar Raman peaks with slight offsets, which
4
could be caused by the introduction of PANI. In addition to these peaks from B4C, typical
5
bands of PANI were also observed, including C-N+ stretching vibration at 1367 cm-1, C=N
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stretching at 1505 cm-1 and C-C stretching in the benzene ring at 1584 cm-1 20,21. Clearly, the
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B4C micro-/nano-fillers were well functionalized by coating PANI. The general atomic
8
configuration of aniline polymerization was revealed by molecular dynamics (MD)
9
simulations (supplemental materials) and schematically described in Figure 1i, showing the
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adsorption of aniline cation radicals on the B4C and corresponding polymerization.
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The unfunctionalized and PANI functionalized B4C micro-/nano-fillers were separately
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dispersed into epoxy resin to fabricate B4C/epoxy composites (Figure 2a). The DIC
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deformation fields along the tensile direction obtained from the in-situ scanning electron
14
microscopy (SEM) tensile testing on pure epoxy, unfunctionalized B4C micro-/nano-filler
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reinforced composite, and PANI functionalized B4C micro-/nano-filler reinforced composite
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are respectively presented in Figure 2b-d. The strain line profiles along the tensile direction
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are illustrated in Figure 2e. The pure epoxy sample exhibited relatively uniform deformation
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(Figure 2b). For unfunctionalized B4C micro-/nano-filler reinforced composite (Figure 2c),
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the strain distribution was close to that of the pure epoxy sample (Figure 2b), except that the
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local region exhibited a slight strain reduction, due to the high stiffness of the reinforcements.
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The PANI functionalized B4C micro-/nano-filler reinforced composite (Figure 2d) displayed
22
similar, but even more pronounced micro-sized strain islands. Generally, due to the high
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surface-to-volume ratio and van der Waals force caused by π electrons on the amorphous
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carbon coating of the B4C22,23, the B4C micro-/nano-fillers tend to agglomerate (specimen ii
3
in Figure 3a) to form bundles/blocks susceptible to the formation of voids and cracks.
4
Therefore, the reinforcing potential of unfunctionalized B4C micro-/nano-fillers cannot be
5
fully achieved. The dangling carbon bonds on the B4C amorphous carbon layer are quite
6
reactive and can be stabilized by terminating the dangling bonds with hydrogen, hydroxy or
7
other functional groups24, enabling PANI to be well polymerized and strongly bonded with
8
the B4C fillers. Meanwhile, the introduced PANI layer formed covalent bonding with the
9
epoxy matrix, remarkably enhancing the load carrying capacity of the B4C micro-/nano-fillers.
10
According to the shear-lag theory25-27, for a discontinuous sheet reinforced composite with
11
perfect bonding interface, strain builds up to a plateau value in the middle of the sheet and
12
then dips down slightly at the edges. Thus, the PANI functionalized B4C micro-/nano-filler
13
reinforced composite exhibited localized oscillatory strains (Figure 2d and 2e), illustrating
14
that the introduction of PANI coating strengthened the interfacial bonding and accordingly
15
enhanced the load transfer efficiency of the fillers, and furthermore, unveiling that the PANI
16
functionalized B4C micro-/nano-fillers were dispersed uniformly in the epoxy matrix.
17
The pure epoxy exhibited a rather smooth fracture surface (Figure 3b), a typical feature of
18
the rapid crack propagation of a brittle thermoset polymer28. The fracture surface of the B4C
19
composites, no matter if the reinforcements were treated with PANI or not, were much
20
rougher with sea-wave like patterns (Figure 3c,d)). The energy consumption during the
21
fracture of the specimens can be evaluated according to the cross-sectional surface area of
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newly created fractures29, which were measured by atomic force microscopy (AFM). With
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the same projected area (10 µm×10 µm), the fracture surface area of the B4C composites
2
(122.6 µm2) was much larger than that of the pure epoxy (100.1 µm2), indicating more energy
3
consumption before failure. Fiber bridging and pulling-out were observed in the composites
4
(Figure 3e and Figure S5c,d), suggesting that the B4C hybrid fillers indeed played the role in
5
reinforcing the matrix30. The strength and toughness enhancements of the composites could
6
be attributed to the outstanding strength of B4C and B4C/epoxy bonding. Note that the PANI
7
functionalized B4C composite showed even rougher fracture surface (Figure 3d). Close-up
8
inspection (Figure 3f) revealed that the PANI functionalized B4C micro-/nano-fillers were
9
well dispersed in the epoxy matrix, and some broken fillers were visible on the fracture
10
surface where the surrounding epoxy was pulled up, suggesting stronger bonding between the
11
functionalized B4C and epoxy.
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Figures 4a and 4b show the stress-strain curves of B4C composites and epoxy. The strain
13
was measured and calibrated by DIC31. The general trend demonstrated that the tensile
14
strength increased with increasing B4C fillers. The 0.2 wt.%, 0.5 wt.% and 1.0 wt.% B4C
15
composites (without PANI functionalization) exhibited respectively the tensile strengths of
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56.0, 67.5 and 77.0 MPa, representing 14.5%, 38.0% and 57.5% enhancement compared to
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the pure epoxy sample (48.9 MPa), and elastic moduli of 2290.6, 2779.8 and 3329.9 MPa,
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17.9%, 43.1%, and 71.5% amplification over the epoxy control sample (1942.1 MPa). The
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strength and elastic modulus enhancements of untreated B4C reinforced composites are
20
mainly ascribed to the excellent mechanical properties of B4C. The toughness (here referred
21
to the area under stress-strain curve), however, appeared to decrease, because of the
22
decreased fracture strain. Whereas, the PANI-functionalized B4C composites exhibited an
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exceptional increase in toughness, 8.5%, 51.5%, and 63.1% amplification, respectively, for
2
0.2 wt.%, 0.5 wt.% and 1.0 wt.% PANI-functionalized B4C composites over the pure epoxy.
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Clearly, the PANI coating indeed enhanced the bonding between the B4C fillers and matrix.
4
Here, PANI acted like the protein in biomaterials, which increases toughness by redistributing
5
stress via the protein networks to avoid crack initiation and propagation at the interface32. The
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enhancements in elastic modulus and strength became pronounced by adding more (over 0.2
7
wt.%) PANI-functionalized B4C fillers (Tables S1 and S2). For comparison, Figure 4c,d
8
summarize the elastic modulus, fracture strain, tensile strength, and toughness values of 1 wt.%
9
B4C/epoxy composites, respectively, which are normalized by dividing the composites’
10
properties by the corresponding properties of pure epoxy. The 1 wt.% B4C micro-/nano-filler
11
(with/without PANI functionalization) reinforced composites exhibited 70% enhancement in
12
elastic modulus compared with other composites reported in literature. Importantly, the PANI
13
functionalized B4C composites demonstrated an exceptional performance of high elastic
14
modulus of 3474 MPa and large fracture strain of 3.5% (Figure 4c), thereby possessing a high
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strength-toughness combination (Figure 4d).
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To elucidate the reinforcing mechanisms of PANI functionalized B4C reinforced
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composites, a continuum micromechanics analysis was performed on a representative volume
18
element (RVE). According to our electron microscopy inspection (Figure 1), the B4C filler
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consists of a B4C microplatelet with irregular shape and B4C nanowires growing in various
20
directions. In order to better describe the B4C fillers and simplify the modelling process, the
21
multiscale RVE is assumed to be an individual B4C cylinder (Phase I) surrounded by a sheath
22
of B4C nanowire/epoxy nanocomposite (Phase II). It is assumed that the Phase II material are
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1
reinforced by radially aligned B4C nanowires, thereby exhibiting orthotropic properties. To
2
investigate the interfacial load transfer, a tensile stress σ0 was applied to the Phase II of the
3
RVE along z-direction (Figure 5a).33 The axisymmetric governing equations34, in terms of the
4
polar coordinates (r, θ, z), include the equilibrium equation in direction z, i ∂σ zzi 1 ∂ ( rσ rz ) + = 0 ; i = I and II ∂z r ∂r
5
6
the geometrical equations,
7
ε zzi =
8
and the constitutive equations,
9
σ zzi C11i C12i i i i σ θθ C12 C22 i σ rri C13i C23 i = 0 σ zθ 0 σ i 0 0 rz i 0 σ rθ 0
(1)
∂u zi ui ∂u i ∂u i ∂u i , ε θθi = r , ε rri = r , 2ε rzi = z + r ; i = I and II ∂z r ∂r ∂r ∂z
C13i
0
0
i C23 C33i
0 0
0 0
0 0
i C44 0
0 i C55
0
0
0
i 0 ε zz i 0 εθθ i 0 ε rr ; i = I and II 0 2ε ziθ 0 2ε rzi i i C66 2ε rθ
(2a-d)
(3)
10
In Eqs. (1-3), σmn and εmn are stress and strain components, ur and uz are, respectively, the
11
radial and axial displacement components, and [C] is the stiffness tensor with the indices 1, 2,
12
3 denoting the axes z, θ, and r, respectively. i is the material layer (i = I and II). The average
13
axial normal stresses over the cross-section are defined as
σ zzI =
14
15 16 17
1 π a2
∫
a
0
σ zzI ( r , z )( 2π r ) dr , σ zzII =
R 1 σ zzII ( r , z )( 2π r ) dr 2 ∫a π (R − a ) 2
The boundary conditions for this problem are given by
σ zzI
z =0
= σ zzI
z=L
= 0 , σ rrII
r=R
= σ rzII
r =R
=0
(5a,b)
and the interfacial continuity conditions are
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(4a,b)
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σ rrI r =a = σ rrII
2
where τint (z) is the transverse shear stress at the interface between Phase I and Phase II, a and
3
R are the inner and outer radii of Phase II, respectively, and L is the length of the cylindrical
4
RVE. Also, the force balance of the RVE cylinder along the z-direction is considered,
5
∫
a
0
r =a
, σ rzI
r =a
= σ rzII
= τ int ( z ) , urI
1
r =a
r =a
= urII
(6a-c)
r =a
σ zzI 2π rdr + ∫ σ zzII 2π rdr = π ( R 2 − a 2 ) σ R
(7)
a
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In addition, Phase I and II are assumed to be perfectly bonded, due to the one-step growth of
7
the B4C micro-/nano-fillers. According to the Eqs. (1-7), the tensile strain along z-direction
8
( ε zzi ), interfacial shear stress (τint) and the average axial normal stress in Phase I ( σ zzI ) can
9
be derived (see details in supplemental materials) as 1 2 r 2
10
2 2 C II ( R − a ) ε = 55I C55 a2
11
1 2 r r − a 2 ) − R 2 ln αL −α L ( a ( M + 1) e − 1 e−α z − e − 1 eα z ε zzII = M 0 ( R 2 − a 2 ) σ 0 1 − 2 α L −α L 32 1 2 R eα L − e−α L e − e R − a 2 ) − R 2 ln ( a 2
12
τ int ( z ) =
13
I zz
σ
I zz
(a
(R =
2
2
( M 32 + 1) M 0σ 0
eα L − 1 −α z e −α L − 1 α z − e e αL −α L 1 2 R eα L − e −α L − e e 2 2 ( R − a ) − R ln a 2
(10)
− a 2 ) σ 0 e−α L − 1 α z eα L − 1 −α z e e + 1 − α L −α L a2 eα L − e−α L e −e
(11)
where
15
M0 =
16
M 11 = C13II , M 13 = ( C23II − C33II )
17
M 22 = C13I − C13II , M 23 = C 23I + C 33I − C 23II + C 33II , M 24 = −2C33II ,
18
M 13 M 24 − M 14 M 23
( M 13 M 24 − M 14 M 23 )( − M 32 M 41 + M 42 ) + M 43 ( M 11M 24 M 32 + M 14 M 22 ) − M 44 ( M 11M 23 M 32 + M 13 M 22 )
M 32 =
I 55
(9)
− R 2 ) ασ 0 e−α L − 1 α z eα L − 1 −α z + e e α L −α L 2a eα L − e−α L e −e
2
14
(C
(8)
II a a a2 II M = C 1 − + C 1 + , , 14 23 33 2 2 R2 R R 2
− C55II )( R 2 − a 2 ) − 2C55I R 2 ln C55II ( R 2 − a 2 )
R a,
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1
R 3 4 1 4 R + a − 2a 2 R 2 − 2 R 4 ln a 2 M 41 = C11II 2 R R 2 − a 2 − 2 R 2 ln a
2
M 43 = 2a 2 ( C12II − C13II ) ln
3
2 ( R 2 − a 2 ) ( M 32 + 1) M 0 α =C 1 2 2 R R − a ) − R 2 ln ( 2 a 2
,
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R 1 1 - R 4 + a 4 + 2a 2 R 2 ln a 2 M 42 = C11II 2 R R 2 − a 2 − 2 R 2 ln a
,
R R R , M 44 = C12II R 2 − a 2 − 2 a 2 ln + C13II R 2 − a 2 + 2a 2 ln , a a a
II 55
4
The Mori-Tanaka (MT) model35 was used to estimate the effective elastic properties of the
5
B4C nanowire reinforced composite in Phase II. Unless otherwise mentioned, the overall
6
stiffness values of the composite in Phase I and II are given by
7
CI = CBC
8
CII = Cm +ν BC ( CBC − Cm ) T (ν m I +ν BC T)
9
−1
(12a,b)
in which −1 T = I + S ( Cm ) ( CBC − Cm )
10
−1
11
In Eq. (12), Cm and CBC are the stiffness tensor of the epoxy matrix and B4C nanowires,
12
respectively; I is an identity matrix; νm and νBC represent the volume fractions of the epoxy
13
matrix and B4C nanowires in Phase II, respectively; S is the Eshelby tensor, and the elements
14
were obtained from ref. [36]. According to the experimental results, the isotropic elastic
15
coefficients of the epoxy matrix, Cijm , are given by C11m = 2.96 GPa and C12m = 1.46 GPa.
16
The elastic coefficients of boron carbide, CijBC , were obtained from ref. [17] and [37] as
17
follows, C11BC = C 22BC = 542.8 GPa, C12BC = 130.6 GPa, C13BC = C 23BC = 63.5 GPa, C33BC = 534.5
18
GPa,
19
characterization, the following parameters were adopted to inform the analytical models: a =
C 44BC = C 55BC = 164.8 GPa,
C66BC = ( C11BC − C12BC ) 2 . Based on the experimental
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3 µm, R = 8 µm, L = 6 µm, and σo = 90 MPa.
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To better understand the reinforcing mechanisms of B4C micro-/nano-fillers, the strain
3
distributions of the RVE along the tensile direction (εzz) were calculated for three cases: (1)
4
Phase I and Phase II are both pure epoxy (Figure 5c); (2) Phase I is boron carbide and Phase
5
II is pure epoxy (Figure 5d); (3) Phase I is boron carbide and Phase II is B4C nanowire
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reinforced epoxy composite (νBC = 50%, Figure 5e). Consistent with the DIC deformation
7
fields obtained from the in-situ SEM tensile testing, the RVE of the pure epoxy exhibits
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relatively uniform strain distribution, whereas the RVEs of case (3) presents localized strain
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islands. As revealed by Figure 5d, the significant differences in stiffness between matrix and
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fillers lead to a sudden drop in strain at the interface susceptible to the formation of voids and
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cracks. One effective approach to mitigating this problem is to design the micro/nano hybrid
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fillers. It is observed in case (3) that Phase II, reinforced by B4C nanowires, serves as an
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intermediate layer, enabling strain gradually decreasing from the matrix to the filler (Figure
14
5e).
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Furthermore, the interfacial shear stress (τint) and average axial normal stress ( σ zzI ) were
16
calculated and normalized (Figure S6) to investigate load transfer efficiency in the B4C
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micro-/nano-filler reinforced composites. Due to symmetry, the middle of the RVE is shear
18
stress free, and the maximum stress transfer occurs near both ends (Figure S6a). Dedicated by
19
the global equilibrium requirement, the maximum axial stress is achieved in the middle of the
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RVE, whereas the minimum axial stress occurs at the ends (Figure S6b). The magnitudes of
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both interfacial shear stress and average axial normal stress increase as the volume fraction of
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B4C nanowires in Phase II (νBC) increases initially and decrease afterwards. Therefore, adding
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B4C nanowires to Phase II, that is the design of B4C micro-/nano-fillers, facilitates the load
2
applied on the matrix being efficiently transferred into the fillers and then carried by the
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fillers.
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As an index for evaluating the effectiveness of the reinforcing fillers embedded in the matrix, the effective length of the B4C fillers is introduced as38
Leff 6 7
I where σ zz
max
∫ =
L
0
σ zzI dz
σ zzI
max
(13)
is the maximum value of the average axial normal stress along the length
8
direction of the RVE. The results of the effective length associated with various volume
9
fractions of the B4C nanowires in Phase II are presented in Figure 5b. It is obvious that the
10
multiscale design of B4C fillers makes the effective length larger, which indicates higher load
11
carrying efficiency of the reinforcements, and, accordingly, better overall mechanical
12
performance of the composites38. Furthermore, the effective length can reach its peak by
13
adjusting the volume fractions of the B4C nanowires.
14
Assisted with cotton, we synthesized a new type of micro-/nano-filler which has B4C
15
microplatelet as the core and radially aligned B4C nanowires as the shell. Such B4C
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micro-/nano-fillers enabled multiple multiscale reinforcing effects in the epoxy matrix, and
17
largely enhanced the load carrying efficiency of the reinforcements, leading to the overall
18
improved mechanical performance of the composites. In addition, the radially aligned B4C
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nanowires on the B4C microplatelets acted as interlocks between the wires and matrix, further
20
pushing up the strength and toughness of the composites. To enhance the bonding between
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the B4C fillers and epoxy, the B4C micro-/nano-fillers were coated with a layer of PANI. With
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a low concentration of the PANI functionalized B4C micro-/nano-fillers (1 wt.%), this
2
B4C/epoxy composite exhibited an exceptional combination of mechanical properties in
3
terms of elastic modulus (~3.47 GPa), toughness (2026.3 kJ/m3), and fracture strain (>3.6%).
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Importantly, we provide a new technique to design lightweight yet strong and tough materials
5
that have enormous potentials in a multitude of fields including biomaterials, infrastructure
6
and armors.
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Associated Content
10
Supporting Information
11
(1) Experimental procedures and characterization methods, (2) Additional details on the
12
analytical models, (3) Atomic configurations of B4C and PANI, (4) Additional SEM image of
13
in situ tensile test, (5) AFM and SEM images description in reinforcing mechanisms, (6)
14
Results of the tensile tests.
15 16 17
Acknowledgements
18
Financial support for this study was provided by the U.S. National Science Foundation
19
(CMMI-1418696 and CMMI-1537021). The authors thank the staff members at the
20
University of Virginia NMCF for electron microscopy technical support.
21 22 23 24
Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) 15
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Figure 1. (a) SEM image of the morphology of B4C micro-/nano-fillers synthesized on carbon fibers. (b)
4
TEM image and EELS (inset figure) of B4C nanowires. (c) XRD pattern of the as-synthesized B4C
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micro-/nano-fillers on carbon fibers. (d) TEM image of untreated B4C micro-/nano-fillers. (e) HRTEM of
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untreated B4C micro-/nano-fillers. (f) Background-corrected Raman spectra of B4C micro-/nano-fillers
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excited with a 514-nm wavelength laser line. (g) TEM image of PANI functionalized B4C
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micro-/nano-fillers. (h) HRTEM image of PANI functionalized B4C micro-/nano-fillers. (i) Polymerization
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process of PANI and complexation mechanism between PANI and B4C.
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Figure 2. (a) Schematic showing the detailed processes for fabricating PANI functionalized B4C
3
micro-/nano-filler reinforced epoxy composites. (b-d) DIC strain maps of (b) pure epoxy, (c)
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unfunctionalized B4C micro-/nano-filler reinforced composite, and (d) PANI functionalized B4C
5
micro-/nano-filler reinforced composite. (e) Line profiles of the strains extracted from (b-d) along the
6
tensile direction.
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Figure 3. (a) Samples for mechanical testing ((i) epoxy control sample, (ii) unfunctionalized B4C
3
micro-/nano-filler reinforced composite, (iii) PANI functionalized B4C micro-/nano-filler reinforced
4
composite). (b) SEM image of the fracture surface of pure epoxy resin. (c) and (e) SEM images of the
5
fracture surface of unfunctionalized B4C micro-/nano-filler reinforced composite. (d) and (f) SEM images
6
of the facture surface of PANI functionalized B4C micro-/nano-filler reinforced composite.
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Figure 4. Reinforcing effects of unfunctionalized and functionalized B4C micro-/nano-fillers (a)
4
Stress-strain curves of untreated B4C micro-/nano-filler reinforced composites. (b) Stress-strain curves of
5
PANI functionalized B4C micro-/nano-filler reinforced composites. (c) and (d) Comparison of the
6
mechanical properties of 1 wt.% B4C composites with other typical polymer composites containing
7
different reinforcements.
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Figure 5. (a) Schematic diagram showing the cross section of the three-phase representative volume
3
element (RVE) for the composites. (b) The effective length of B4C fillers associated with different volume
4
fractions of B4C nanowires in Phase II. (c-e) Theoretically calculated normal strain distributions of the
5
RVE for three cases: (c) pure epoxy, (d) Phase I is B4C and Phase II is pure epoxy, and (e) Phase I is B4C
6
and Phase II is B4C nanowire reinforced epoxy composite. Half of the strain distributions of the RVE is
7
displayed, due to the symmetry.
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