- Email: [email protected]
ZrB2 hybrid composites with hierarchical architectures spanning several length scales
Accepted Manuscript Bioinspired high toughness graphene/ZrB2 hybrid composites with hierarchical architectures spanning several length scales Yumin An, Jiecai Han, Xinghong Zhang, Wenbo Han, Yehong Cheng, Ping Hu, Guangdong Zhao PII:
S0008-6223(16)30449-3
DOI:
10.1016/j.carbon.2016.05.074
Reference:
CARBON 11038
To appear in:
Carbon
Received Date: 29 February 2016 Revised Date:
16 May 2016
Accepted Date: 30 May 2016
Please cite this article as: Y. An, J. Han, X. Zhang, W. Han, Y. Cheng, P. Hu, G. Zhao, Bioinspired high toughness graphene/ZrB2 hybrid composites with hierarchical architectures spanning several length scales, Carbon (2016), doi: 10.1016/j.carbon.2016.05.074. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Bioinspired high toughness graphene/ZrB2 hybrid composites with hierarchical architectures spanning several length scales Yumin An*, Jiecai Han, Xinghong Zhang*, Wenbo Han, Yehong Cheng, Ping Hu,
RI PT
Guangdong Zhao National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150001, P.R. China
SC
Abstract:
M AN U
Natural bioinspired hierarchically ordered architectures from the nanoscale to the macroscale are achieved in graphene/ZrB2 hybrid composites to improve their toughness using graphene oxide. Two types of films containing different volumes of graphene oxide are self-assembled with ZrB2 or SiC micro particles through a
TE D
vacuum-assisted filtration method. Scanning electron microscopy images show that ceramic particles are homogeneously distributed in a continuous multilayer graphene oxide network, forming a nano-micro hierarchical structure. Tensile tests are
EP
employed to test the strength of these films. The spark plasma sintering method is
AC C
utilized to construct micro-macro structural order through densifying two types of alternately stacked films containing different volumes of graphene oxide. Indentation tests reveal alternately compressive and tensile layers are achieved after the sintering process. By combining different structural features spanning several length scales, the composites exhibit a unique combination of high strength (522 MPa) and toughness (9.5 MPa·m0.5); in particular, the fracture toughness is more than double that of the *
Corresponding author. E-mail: [email protected] (Yumin An); [email protected] (Xinghong
Zhang).
ACCEPTED MANUSCRIPT composite without the hierarchical architecture. The toughening mechanisms are also analyzed at different length scales. This bioinspired material-independent approach should be employed in the designing and processing of materials for structural,
RI PT
high-temperature and energy related applications. 1. Introduction
Because of the strong covalent bond between boron atoms, Zirconium diboride (ZrB2)
SC
exhibits a high melting point, high strength, hardness and chemical inertness and has
M AN U
been selected as a leading potential material for a variety of ultra-high temperature structural applications [1, 2]. With the aim of further improving the antioxidant and sintering properties of ZrB2, silicides, such as SiC, MoSi2, or ZrO2, have been chosen to fabricate ceramic composites [3, 4]. Moreover, the enhancement of flexural
TE D
strength for ZrB2-SiC composites can be ascribe to SiC limit the growth of ZrB2 grain during densification. However, the inherent brittleness and sensitivity to flaws of ZrB2 based ceramic composites severely obstruct their wide application in ultra-high
EP
temperature environments. One of the methods used to overcome this problem
AC C
involves the choice of the ingredients and the design of a hierarchically ordered architecture from the nanoscale to microscale to improve the damage tolerance of the composites [5]. This approach is inspired by naturally existing materials, such as nacre and bone. These natural materials generally possess mechanical performances far exceed those of their individual constituents through complex hierarchical designs using brittle minerals and organic molecules [6-8]. These hierarchically ordered structures are expected to resist crack initiation and propagation when usually
ACCEPTED MANUSCRIPT spanned from nano to micro scale [9-11]. For example, the ‘brick-and-mortar’ structure of nacre is 95 vol.% composed of microscale crystalline calcium carbonate ‘bricks’ and a nanoscale organic matrix ‘mortar’ between the mineral platelets [12, 13].
RI PT
While the overlap of the mineral platelets guarantee the strength, the high toughness is ensured by a combination of an intrinsic plasticity toughening and an extrinsic toughening mechanism. The first toughening mechanism is ascribed to the nanoscale
SC
mortar slip, and the latter one is mainly provided by the tortuous crack paths and the
M AN U
pull-out of the mineral platelets on the micro-macro scale.
Numerous techniques have been developed to mimic natural nacre with hierarchical architectures, such as gluing ceramic layers [14], ice templating [15], magnetic particle alignment [16] and thermal spray [17]. Freeze casting followed by a sintering
TE D
stage has enabled one to fabricate a biologically based high toughness alumina composite with 80 vol.% alumina platelets and sub-micrometer lubricating poly(methyl methacrylate) inter-layers [18,19]. To further achieve high strength
EP
materials, Valentina Naglieri, et al. [20, 21] have discussed the relationships between
AC C
SiC scaffold morphologies and the process of ice-templating, such as cooling rates. There is an important limitation in these materials that is a significant amount of organics material, between 20 vol%-60 vol%, is employed as the ductile phase between the ceramic scaffolds, which limits the application temperature of the composites. For ultrahigh temperature applications, new materials should be found for use as the ductile phase. By substituting the organic polymer with glass-phase precursors, an alumina based composite with a strength of 400 MPa and a fracture
ACCEPTED MANUSCRIPT toughness of 20 MPam0.5 has been achieved [22]. The most striking point is the replacement of the organic polymer with inorganic nanoparticles will expand the temperature range of application environment.
RI PT
Recently, graphene oxide (GO) with outstanding mechanical properties has easily been modified with different functional groups on its surface and self-assembled into films with hierarchical architectures [23-25]. Tremendous work has been performed to
SC
analyze the mechanical properties and toughness mechanisms of GO based layered
M AN U
materials [26, 27]. For example, ionic bond [28], hydrogen bond [29] and covalent cross linking [30] can form between adjacent GO nanosheets to improve the tensile strength of the layered GO based composites. Except for the chemical bond effect, the slippage of adjacent GO nanosheets is an efficient method for energy dissipation. In
TE D
addition, chemical reduction [31, 32] and thermal reduction methods [33] have been utilized to reduce these composites to graphene based materials to realize their functionalities. Furthermore, graphene nanosheets have been utilized as a toughening
EP
phase in ultrahigh temperature ceramics by the in situ thermal reduction of GO [34].
AC C
Inspired by natural structures, some simple layered ZrB2 based structures have been fabricated to enhance the toughness through the presence of residual compressive stresses that exist in the ceramic layers [35-37]. However, these composites are usually composed of structural feature at the micron scale, which will limit further improvement of the fracture resistance. Furthermore, an abundance of organics is needed to improve the rheological properties of the ceramic slurry to realize the shaping of green body. After that, a low temperature degradation process is needed to
ACCEPTED MANUSCRIPT decompose these organics to improve the mechanical properties of the materials before sintering. This step is time consuming and not easily achieved. It is expected that a simple and efficient method can be found to construct ZrB2 based composites
RI PT
with a combination of toughening mechanisms at different length scales. In this paper, we propose a simple and new method to fabricate graphene/ZrB2 based composites with hierarchical architectures to extend their application in severe environments.
SC
Here, two bioinspired graphene/ZrB2 and graphene/ZrB2-SiC composites are
M AN U
fabricated through a ‘bottom-up’ assembly method. The microstructures as well as the toughening mechanisms of the novel graphene/ZrB2 based ceramic composites are investigated and discussed in detail. The results clearly show that a combination of high strength and toughness can be achieved via properly fabricated hierarchical
2. Experimental 2.1 Materials
TE D
architectures spanning several length scales.
EP
Commercial graphite flakes (325 mesh, >99%, USA) were used to prepare GO
AC C
through a modified Hummers’ method. GO/ceramic films were self-assembly using ZrB2 (2 µm, >99%, Northwest Institute for Non-ferrous Metal Research, China), SiC (0.5 µm, >98%, Kaihua, China) and poly(vinyl alcohol) (PVA, >97%, Fuchen, China). Two types of GO/ceramic films containing 5 vol.% and 30 vol.% GO were utilized to fabricated bioinspired graphene/ceramic composites. 2.2 Self-assembly GO/ceramic hierarchical structures: The dried GO was dispersed in deionized water via sonication (Kunshan KQ5200E, China) for 2 h. ZrB2
ACCEPTED MANUSCRIPT and PVA (0.25 wt.%) were added to the GO solution (10 mg/ml), and the mixed solution was stirred (Huapuda JJ-1, China) at high speed for 30 min. After that, the solution was filtered to achieve the GO/ZrB2 hierarchical structures. The volumes of
RI PT
the GO solution were 30 vol.% and 5 vol.% for the GO30Z and GO5Z films. Furthermore, SiC was utilized to achieve the GO/ZrB2-SiC hierarchical structures. The fabrication process was the same as that of the GO/ZrB2 films using the same GO
SC
solution volumes (GO5ZS and GO30ZS films). SiC was added in order to further
M AN U
enhance the mechanical properties of graphene/ZrB2 based composites to expand their application.
2.3 Fabrication of graphene/ceramic composites: For the graphene/ZrB2 composite, the GO30Z and GO5Z films were alternately assembled in a graphite die until the
TE D
desired composites were achieved. The spark plasma sintering (SPS, FCT Systeme GmbH, Germany) method was utilized to fabricate the composite at 1950 °C for 15 min under a uniaxial load of 30 MPa in high vacuum. The GO was in situ thermally
EP
reduced to graphene during the sintering process. For the graphene/ZrB2-SiC
AC C
composite, GO5ZS and GO30ZS films were utilized, and the fabrication was the same as that of the graphene/ZrB2 composite. 2.4 Characterization and measurements: The microstructures of the composites were characterized by scanning electron microscopy (SEM, Nanolab 600i). The graphene structure was analyzed by micro-Raman spectroscopy (REMSHAW Invia, laser wavelength: 532 nm) and X-ray diffraction (Rigaku, Dmax-rb, Cukα 1.5425 Å). The nanoindentation was performed with a G200 Nano indenter (Agilent
ACCEPTED MANUSCRIPT Technologies, USA) and was utilized to analyze the residual stress in the layers of the bioinspired graphene/ZrB2-SiC composites. The tensile strength was recorded with a nanomechanical testing system (UTM, Agilent T150, USA) using the “UTM-Bionix
RI PT
standard Toecomp CDA” method. The lengths and thicknesses of the test samples were measured by a caliper and a screw micrometer, and the width was measured by optical microscopy. Four individual samples were prepared and tested for each of the
SC
synthetic conditions. The tensile strain rate was set as 1.0 × 10-3 s-1, and the harmonic
M AN U
force and the frequency were typically 4.5 mN and 20 Hz, respectively. The fracture toughness was determined by a single edge notched beam test on 1.5 (width) × 3 (height) × 16 (length) mm bars using a 12 mm span and a crosshead speed of 0.05 mm/min (ASTM E1820). Flexural strength was tested by three points bending on 3
TE D
(width) × 1.5 (heigth) × 25 (length) mm bars using a 20 mm span and a crosshead speed of 0.2 mm/min (ASTM C1161). The test bars were cut with the surface perpendicular to the hot pressing direction, and five specimens were tested for
EP
fracture toughness and flexural strength. The microstructures of the fracture surface
AC C
and indentation crack propagate paths were analyzed by SEM. 3. Results and discussions Through ‘bottom-up’ assembly method, we fabricated dense ceramic composites defined by four structural features, spanning several length scales (Fig. 1): the stacked graphene nanosheets ensuring crack deflection and delamination; the closely packed micro-scale ceramic particle units separated by the graphene nanosheets; alternating ceramic/graphene layers containing different volumes of graphene to redistribute the
ACCEPTED MANUSCRIPT load; and the macro structural order. The composite assembly process consisted of two steps: the self-assembly of the ceramic/GO films using a vacuum-assisted filtration method and the combination of two types (different volumes of GO) of
RI PT
alternately assembled ceramic/GO films in a graphite die. After that, the SPS method was utilized to densify the composite, and the GO was thermally reduced to graphene
M AN U
SC
during the high temperature sintering process.
Figure 1 The structural features of the bioinspired graphene/ZrB2 hybrid composites at different length scales.
TE D
In the fabrication of GO, several oxygen containing groups, such as C-O and C=O groups, formed on the surface of GO, which had been verified in a previous study
EP
[34]. Two important roles were played by these groups: one was guaranteeing the high stability of the dispersion with the ceramic particles in deionized water, and the other
AC C
was forming the hydrogen bonds between adjacent GO sheets during the self-assembly process. Furthermore, PVA (0.25 wt.%) was added to the solution to further improve the interaction between the ceramic particles and GO. Based on the above points, the GO/ceramic films could be formed during the vacuum-assisted filtration process. Here, four different types of GO/ceramic films were self-assembled. The size of the GO/ceramic films depended on the area of the filter that was used. For the GO/ZrB2 films, the color was black. After addition of the SiC particles, the color
ACCEPTED MANUSCRIPT of the GO/ceramic films changed to lightly khaki. The microstructures of these films were depicted in Fig. 2. For the GO/ZrB2 films, some holes in the microstructures of the GO/ZrB2 films were easily observed, and the structures were relatively loose.
RI PT
During the vacuum-assisted filtration process, adjacent GO sheets in the solution were firstly self-assembled into GO nanosheets, and then ZrB2 particles contacted the connected GO nanosheets. With further self-assembly of the GO nanosheets and the
SC
ZrB2 particles, a three dimensional GO network was formed and the ZrB2 particles
M AN U
filled in the network. It was worth mentioning that it was the size difference between the GO nanosheets and the ZrB2 particles prompted the formation of the three dimensional GO network. We also fabricated two types of GO/ZrB2-SiC films made with 5 vol.% and 30 vol.% GO solutions, which were shown in Fig. 2 (c) and (d),
TE D
respectively. Their structures were relatively compact after the introduction of the SiC particles. The reason was due to the size of SiC particles was much smaller than that of ZrB2 particles. Therefore, the SiC particles filled the holes between the ZrB2
AC C
EP
particles and GO networks in the self-assembly process.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 2 Microstructures of the GO/ZrB2 films made with 5 vol.% GO (a) and 30 vol.% GO (b), and microstructures of the GO/ZrB2-SiC films made with 5 vol.% GO (c) and 30 vol.% GO (d).
TE D
The tensile strength of each of the GO/ceramic films was tested. The films obtained after the self-assembly process were cut into strips with a length between 1-2 cm and
EP
a width of 0.2-0.5 mm. The thickness of the strips varied between 20-100 µm. The results shown that GO5Z and GO5ZS films had average tensile strengths of 41.6 MPa
AC C
and 25.4 MPa, respectively. When the volume fraction was increased to 30%, the tensile strengths of the films increased to 141.5 MPa (GO30Z) and 72.1 MPa (GO30ZS). In addition, the modulus of each film type was improved with the increase in GO, and the values were listed in Tab. 1. The improvement of the tensile strengths and modulus might be due to enhanced interactions in the GO networks and the interlocking geometry of the ceramic particles and GO networks.
RI PT
ACCEPTED MANUSCRIPT
SC
Figure 3 Tensile strengths of the GO/ceramic films containing different volume fractions of GO.
Materials
GO5Z
Strength (MPa) Modulus (GPa)
41.6±3.5 9.1±0.3
M AN U
Table 1 Tensile strengths and modulus of the GO/ceramic films containing different volume fractions of GO. GO30Z
GO5ZS
GO30ZS
141.5±7.9 14.2±0.3
25.4±6.2 10.7±1.1
72.1±4.2 12.5±0.5
By alternate assembling of the GO/ceramic films containing different volumes of GO,
TE D
layered structures at the macro-scale were achieved. There were three important issues to be solved to improve the mechanical properties of the novel composites with
EP
the SPS process. The first one was to densify the composites to obtain high strength. It was difficult to densify ZrB2 based composites at a low temperature. Therefore, we
AC C
densified the composites at 1950 °C under a uniaxial load of 30 MPa. At such a high temperature, the organics were generally carbonized and disappeared, even in high vacuum. However, the research by X.H. Zhang, et al [34] had shown that graphene retains a relatively intact structure during the sintering process. Furthermore, graphene could act as a sintering aid for the ZrB2 based materials [38], which was a reason for choosing graphene. The second issue to be solved was that residual stress should form in the graphene/ceramic layers during the cooling process. When loading,
ACCEPTED MANUSCRIPT compressive stress would arrest crack and prompted crack bifurcation. The third issue was that the graphene structures should be achieved by the in situ thermal reduction of GO. The microstructures of the graphene/ZrB2 and graphene/ZrB2-SiC composites at
RI PT
different magnification were clearly depicted in Fig. 4. It was easily found that nearly dense graphene/ceramic layers with different thicknesses were alternately stacked in the two composites, which was consistent with that shown in Fig. 1 (d). The
SC
thicknesses of each of the graphene/ceramic layers were substantially thinner than that
M AN U
of the GO/ceramic films, which revealed the effect of the high temperature sintering. In addition, it was also clearly observed that graphene retained its structure after such
AC C
EP
TE D
a high temperature treatment, which were confirmed by Fig. 4 (c) and (f).
Figure 4 Microstructures of the bioinspired graphene/ZrB2 composites at different magnifications (a), (b) and (c), and the graphene/ZrB2-SiC composites at different magnifications (d), (e) and (f).
The residual stress in different layers of the bioinspired graphene/ceramic composite was characterized by the indentation method. The load-displacement curves for the G5ZS layer and G30ZS layer in the bioinspired graphene/ZrB2-SiC composite were
ACCEPTED MANUSCRIPT depicted in Fig. 5 (a) and (b), respectively. In Fig. 5 (a), the final indentation depth in the G5ZS layer of the bioinspired composite was significantly deeper than that of bulk G5ZS composite, which indicated that tensile stress existed in the G5ZS layer [39, 40].
RI PT
In the G30ZS layer of the bioinspired composite, the indentation depth was shallower than that of bulk G30ZS composite (Fig. 5 (b)), which revealed that compressive stress was present in the G30ZS layer. The compressive stress was easily calculated
σcom = H / sinα(1− A0 / A) , where
H was hardness,
SC
by the expression
α
was
M AN U
inclination of a sharp indenter to the surface, A0 represented projected contact area of the reference materials, and A was contact area of the G30ZS layer. The calculated compressive stress was 1.2 GPa in the G30ZS layer. During the cooling process, the thermal expansion mismatch between adjacent graphene/ceramic layers induced a
TE D
residual tensile stress in layers with less volume fraction of GO and a compressive stress in layers with more GO. Alternating between tensile stress layers and
AC C
composites.
EP
compressive stress layers would further improve the fracture toughness of the
Figure 5. Force-displacement curves of the ZrB2-SiC composites made with 5 vol.% (G5ZS composite) and 30 vol.% (G30ZS composite) GO and the G5ZS layer and G30ZS layer of bioinspired graphene/ZrB2-SiC composite.
At a high temperature (1100 °C) and in high vacuum, GO was thermally reduced to
ACCEPTED MANUSCRIPT graphene [41]. We employed XRD and Raman to analyze the effect of SPS on the structure of GO. The XRD patterns of the GO/ceramic films and graphene/ceramic composites in different degree range were shown in Fig. 6. After oxidation of graphite
RI PT
(26.5º), the peak associated with GO shifted to approximately 10º. For GO/ZrB2 films, the peaks between 20-70º were assigned to ZrB2. The peak located at 9º was assigned to GO, as shown in Fig. 6 (b). However, the X-ray diffraction intensity of GO was
SC
significantly weaker than that of ZrB2. The same phenomenon was found in the XRD
M AN U
patterns of the GO/ZrB2-SiC films. The difference was that a shift of GO peak to 10.6º was observed for GO/ZrB2-SiC films, which revealed different GO stacked structures in the two types of films. After the SPS, the peak at approximately 10º disappeared for both of the graphene/ceramic composites. However, peaks located at
TE D
26.5º and 26.2º appeared in the graphene/ZrB2 and graphene/ ZrB2-SiC composites, respectively, revealing that most of the oxygen containing functional groups in GO were removed during the thermal reduction. The appearance of graphene nanosheets
EP
in the composite acted as a lubricant between the ceramic particles, which ensured
AC C
that more energy was consumed during the failure of the composites.
Figure 6 XRD patterns of the GO/ZrB2 films, bioinspired graphene/ZrB2 composite, GO/ZrB2-SiC films and the bioinspired graphene/ZrB2-SiC composite.
ACCEPTED MANUSCRIPT Raman spectroscopy was a powerful and effective tool to analyze the structure of GO and graphene. While the E1g symmetry oscillations of carbon atoms in six members ring yielded D peak, the E2g stretching oscillations of C-C bonds in sp2 hybrid carbon
RI PT
clusters generated G peak. Only the typical D peak (1338.8 cm-1) and G peak (1597.9 cm-1) were found for the original GO (Fig. 7). After the SPS process, D peak right shifted 12 cm-1 and G peak left shifted 13 cm-1 for graphene/ZrB2 composite.
SC
Furthermore, 2D peak appeared at 2697.8 cm-1 illustrated the thermal reduction of GO
M AN U
during the SPS process exhibited double resonance mode of graphene. Additionally, D peak, G peak and 2D peak located at 1349.4 cm-1, 1583.1 cm-1 and 2696.5 cm-1 for graphene/ZrB2-SiC composite. Compared with the nearly same intensity (ID/IG=0.98) of D peak and G peak for GO, ID/IG decreased to 0.19 and 0.32 for the graphene/ZrB2
TE D
and graphene/ZrB2-SiC composites, respectively. The graphitic domain La could be calculated by the expression, 2.4 × 10-10×Ilaser4×(ID/IG)-1 (Ilaser represent the wave length of laser) [42, 43]. After thermal treatment, the average crystallite size La of
EP
graphene notably increased for the two composites, revealing carbon structures in
AC C
graphene/ceramic composites became more orderly. These findings were consistent with the results of XRD. The detailed data was listed in Tab. 2. For monolayer or bilayer graphene, the I2D/IG was
1 for single and sharp 2D peak [44]. In this work,
the average measured single 2D peak possessed an I2D/IG in the range of 0.5–1 proving that multilayer graphene structures existed in the bioinspired composites after SPS process, which was confirmed by Fig.4 (c) and (f).
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 7 Raman shift spectra of GO, bioinspired graphene/ZrB2, and graphene/ZrB2-SiC composites. Table 2 D, G and 2D Raman spectral properties of GO, the bioinspired graphene/ZrB2 composite and bioinspired graphene/ZrB2-SiC composite. Materials
P(G) (cm-1)
P(2D) (cm-1)
ID/IG
I2D/IG
La (nm)
1338.8 1351.1 1349.4
1597.9 1583.1 1583.1
2697.8 2696.5
0.98 0.19 0.32
0.57 0.74
19.6 101.2 60.1
TE D
GO Bioinspired graphene/ZrB2 composite Bioinspired graphene/ZrB2-SiC composite.
P(D) (cm-1)
EP
Mimicking the structural features of natural materials to obtain their unique combinations of mechanical properties was the goal for engineering structural The
mechanical
AC C
materials.
properties
of
bioinspired
graphene/ZrB2 and
graphene/ZrB2-SiC composites were plotted in Fig. 8. Compared with the reference bulk ZrB2 and ZrB2-SiC composite, the flexural strength of bioinspired graphene/ZrB2 and graphene/ZrB2-SiC composites were high in the direction perpendicular to the ceramic layers, with the values of 284 MPa and 522 MPa, respectively. More importantly
though,
the
corresponding
fracture
toughness
of
bioinspired
graphene/ZrB2 (6.0 MPa·m0.5) and graphene/ZrB2-SiC (9.5 MPa·m0.5) composites
ACCEPTED MANUSCRIPT were triple and double those of the reference materials (Tab. 3), respectively. These values were also higher than the reported toughness of ZrB2/graphene composites [35] and ZrB2-SiC/graphene composites [34]. In these works, the volume fraction of
RI PT
graphene was in the range of 2-6 vol.%, and the raw grain sizes of ZrB2 and SiC were
M AN U
SC
between 1-2 µm and 0.5 µm, respectively.
TE D
Figure 8 Flexural strength (a) and fracture toughness (b) of four different composites (bulk ZrB2 composite, bioinspired graphene/ZrB2 composite, bulk ZrB2-SiC composite and bioinspired graphene/ZrB2-SiC composite, respectively).
EP
Table 3 Flexural strength and fracture toughness of bulk ZrB2, ZrB2-SiC composites and bioinspired graphene/ZrB2, graphene/ZrB2-SiC composites.
AC C
Bulk ZrB2 composite Bioinspired graphene/ZrB2 composite Bulk ZrB2-SiC composite Bioinspired graphene/ZrB2-SiC composite
Flexural strength (MPa)
Fracture toughness (MPa·m0.5)
222±17 284±31 465±23 522±42
1.85±0.11 5.98±0.47 4.51±0.21 9.45±0.74
The fracture behavior of bioinspired graphene/ceramic composite was employed to analyze the synergistic effects of the structural features, including the stacked graphene nanosheets, closely packed micro ceramic particle units and alternating ceramic/graphene layers. The stress-stain curves of bioinspired graphene/ZrB2 and
ACCEPTED MANUSCRIPT graphene/ZrB2-SiC composites were depicted in Fig. 9 (a). For bulk ZrB2 and ZrB2-SiC composites, only purely linear elastic response was exhibited until catastrophic failure occurs, with unstable crack propagation characteristic of brittle When
combined
these
structural
features
together
(bioinspired
RI PT
materials.
graphene/ceramic composite), we obtained stable crack propagation in the failure process, which was unusual behavior in pure ceramic materials. The corresponding
SC
fracture surface showed crack deflection in the crack propagation path and was
TE D
M AN U
depicted in Fig. 10 (a).
AC C
EP
Figure 9 Stress-strain curves for four different composites (bulk ZrB2 composite, bioinspired graphene/ZrB2 composite, bulk ZrB2-SiC composite and bioinspired graphene/ZrB2-SiC composite).
Figure 10 Fracture surface of a bioinspired graphene/ceramic composite revealing a long range crack deflection, as indicated by arrows(a); multiple cracking,crack branching and crack bridging towards the end of the crack path, which were also indicated by arrows (b); crack arrest and bifurcation in the crack propagation path (c).
In this study, we focused on the relationship between the toughening mechanisms and
ACCEPTED MANUSCRIPT the hierarchical architecture of the materials over different length scales. When the crack propagated in the bioinspired graphene/ceramic composite, crack deflection (Fig. 10 (a)), multiple cracks, crack branching and crack bridging (Fig. 10 (b)) served
RI PT
as effective paths to relieve the locally high stress around the crack tip. The stacked graphene nanosheets connected to each other formed a network in the composites (Fig. 4 (c) and (f)). Substantial amounts of energy would be consumed during graphene
SC
crack pulling out (Fig. 11 (a)) and graphene crack bridging (Fig. 11 (b)). Moreover,
M AN U
the closely packed micro-ceramic particle units were separated by the graphene nanosheets, which could act as a lubricant for the ceramic particle units. The sliding of graphene nanosheets between the ceramic particles (Fig. 11 (c)) was also an efficient energy dissipation pathway. The alternating graphene/ceramic layers
TE D
containing different volumes of graphene would result in alternating tensile and compressive stress layers because of the thermal expansion mismatch between adjacent layers. The compressive stress in the graphene/ceramic layers would arrest
EP
the crack tip and trigger crack bifurcation (Fig. 10 (c)). The improvement of
AC C
toughness was likely a product of the combination of these mechanisms. Further work would be necessary to quantify the relative contributions of all of the toughening mechanisms identified here.
RI PT
ACCEPTED MANUSCRIPT
Figure 11 Graphene crack pulling out (a), graphene crack bridging (b) and graphene sliding (c) in the crack propagation path.
4. Conclusions
SC
In this study, graphene oxide was utilized to fabricate bioinspired graphene/ZrB2
M AN U
ceramic materials with hierarchically ordered architectures spanning several length scales to improve the material’s toughness. The ‘bottom-up’ assembly process of the hierarchically arranged architecture consisted of two steps: one was the self-assembly of graphene oxide and ceramic particles into films, forming nano-micro hierarchical
TE D
architecture; the other was the spark plasma sintering of alternately assembled two types of films containing different volumes of graphene oxide, constructing micro-macro structural order. The tensile strength of the GO/ceramic films was
EP
improved with the increase in GO. After SPS process, alternating compressive and
AC C
tensile layers formed in the graphene/ceramic composites which could further improve the fracture toughness of the composites. The hierarchical structures formed by this method enhanced the mechanical properties of the composite, a unique combination of high strength (522 MPa) and toughness (9.5 MPa·m0.5). Specifically, the composite toughness was evidently enhanced by a combination of various toughness mechanisms that occur at different length scales, including the sliding of graphene nanosheets, graphene crack pulling out and bridging, crack branching, crack
ACCEPTED MANUSCRIPT arrest and crack deflection. This work expands the application of graphene and ceramic materials to high-temperature and severe environments and also provides an
RI PT
approach for designing structural materials for other applications.
Acknowledgments
The authors are grateful for the support from National Nature Science Foundation of
SC
China (NO. 11572105) and National Outstanding Youth Foundation of China (NO.
AC C
EP
TE D
M AN U
51525201).
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
Reference [1] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Mechanical Properties of Sintered ZrB2-SiC Ceramics. J. Eur. Ceram. Soc. 31(2011) 893-901. [2] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory Diborides of Zirconium and Hafnium. J. Am. Ceram. Soc. 90(2007)1347–1364. [3] J.C. Han, P. Hu, X.H. Zhang, S.H. Meng, W.B. Han, Oxidation-resistant ZrB2-SiC composites at 2200 ºC. Compos. Sci. Technol. 68(2008)799-806. [4] F. Monteverde, Beneficial Effects of an Ultra-fine α-SiC Incorporation on the Sinterability and Mechanical Properties of ZrB2. Appl. Phys. A 82(2006)329–337. [5] U.G.K. Wegst, H. Bai, E. Saiz, A.P. Tomsia, R.O. Ritchie, Bioinspired Structural Materials. Nature Mater. 14(2015) 23-36. [6] K.J. Koester, J.W. Ager, R.O. Ritchie, The True Toughness of Human Cortical Bone Measured with Realistically Short Cracks. Nature Mater. 7(2008) 672-677. [7] H. Peterlik, P. Roschger, K. Klaushofer, P. Fratzl, From Brittle to Ductile Fracture of Bone. Nature Mater. 5(2006)52-55. [8] H.S. Gupta, W. Wagernaier, G.A. Zickler, D.R. Aroush, S.S. Funari, P. Roschger, et al., Nanoscale Deformation Mechanisms in Bone. NanoLett. 5(2005)2108-2111. [9] M.E. Launey, M.J. Buehler, R.O. Ritchie, On the Mechanistic Origins of Toughness in Bone. Ann. Rev. Mater. Res. 40(2010)25-53. [10] S. Weiner, H.D. Wagner, The Material Bone: Structural-mechanical Functional Relations. Ann. Rev. Mater. Sci. 28(1998) 271-298. [11] R.Z. Wang, H.S. Gupta, Deformation and Fracture Mechanisms of Bone and Nacre. Annu. Rev. Mater. Res. 41(2011) 41-73. [12] X.D. Li, W.C. Chang, Y.J. Chao, R.Z. Wang, M. Chang, Nanoscale Structural and Mechanical Characterization of a Natural Nanocomposite Material: The Shell of Red Abalone. NanoLett. 4(2004) 613-617. [13] M.A. Meyer, J. McKittrick, P. Chen, Structural Biological Materials: Critical Mechanics-materials Connections. Science 339(2013) 773-779. [14] W.J. Clegg, K. Kendall, N.M. Alford, T.W. Button, J.D. Birchall, A Simple Way to Make Tough Ceramics. Nature 347(1990) 455-457. [15] S. Deville, E. Saiz, R.K. Nalla, A.P. Tomsia, Freezing As a Path to Build Complex Composites. Science 311(2006) 515-518. [16] R.M. Erb, R. Libanori, N. Rothfuchs, A.R. Studart, Composites Reinforced in Three Dimensions by Using Low Magnetic Fields. Science 335(2012) 199-204.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[17] G. Dwivedi, K. Flynn, M. Resnick, S. Sampath, A. Gouldstone, Bioinspired Hybrid Materials from Spray-Formed Ceramic Templates. Adv. Mater. 2015, 10.1002/adma.201500303. [18] M.E. Launey, E. Munch, D.H. Alsem, H.B. Barth, E. Saiz, A.P. Tomsia, et al., Designing Highly Toughened Hybrid Composites through Nature-inspired Hierarchical Complexity. Acta Mater. 57(2009) 2919-2932. [19] E. Munch, M.E. Launey, D.H. Alsem, E. Saiz, A.P. Tomsia, R.O. Ritchie, Tough, Bio-inspired Hybrid Materials. Science 322(2008) 1516-1520. [20] V. Naglieri, H.A. Bale, B. Gludovatz, A.P. Tomsia, R.O. Ritchie, On the Development of Ice-templated Silicon Carbide Scaffolds for Nature-inspired Structural Materials. Acta Mater. 61(2013)6948-6957. [21] V. Naglieri, B. Gludovatz, A.P. Tomsia, R.O. Ritchie, Developing Strength and Toughness in Bio-inspired Silicon Carbide Hybrid Materials Containing a Compliant Phase. Acta Mater. 98(2015)141-151. [22] F. Bouville, E. Maire, S. Meille, B. V. Moortèle, A. J. Stevenson, S. Deville, Strong, Tough and Stiff Bioinspired Ceramics from Brittle Constituents. Nature Mater. 13(2015) 508-514. [23] C.M. Chen, Q.H. Yang, Y.G. Yang, W. Lv, Y.F. Wen, P.X. Hou, et al., Self-Assembled Free-Standing Graphite Oxide Membrane. Adv. Mater. 21(2009) 3541–3541. [24] Q. Cheng, M. Wu, M. Li, L. Jiang, Z. Tang, Ultra Tough Artificial Nacre Based on Conjugated Cross-Linked Graphene Oxide. Angew. Chem., Int. Ed. 52(2013) 3750-3755. [25] S. Park, K.S. Lee, G. Bozoklu, W. Cai, S.T. Nguyen, R.S. Ruoff, Graphene Oxide Papers Modified by Divalent Ions-Enhancing Mechanical Properties via Chemical Cross-Linking. ACS Nano. 2(2008) 572–578. [26] Y. Tian, Y. Cao, Y. Wang, W. Yang, J. Feng, Realizing Ultrahigh Modulus and High Strength of Macroscopic Graphene Oxide Papers Through Crosslinking of Mussel-Inspired Polymers. Adv. Mater. 25(2013) 2980-2983. [27] W. Cui, M. Li, J. Liu, B. Wang, C. Zhang, L. Jiang, et al., A Strong Integrated Strength and Toughness Artificial Nacre Based on Dopamine Cross-Linked Graphene Oxide. ACS Nano. 8(2014) 9511-9517. [28] S. Park, K.S. Lee, G. Bozoklu, W. Cai, S.T. Nguyen, R.S. Ruoff, Graphene Oxide Papers Modified by Divalent Ions-Enhancing Mechanical Properties via Chemical Cross-Linking. ACS Nano 2(2008) 572–578. [29] Y. Xu, W. Hong, H. Bai, C. Li, G. Shi, Strong and Ductile Poly(vinyl alcohol)/Graphene Oxide Composite Films with a Layered Structure. Carbon 47(2009) 3538-3543. [30] S. Park, D.A. Dikin, S.T. Nguyen, R.S. Ruoff, Graphene Oxide Sheets Chemically Cross-Linked by Polyallylamine. J. Phys. Chem. C 113(2009) 15801– 15804. [31] X.D. Betriu, F.J. Mompeán, C. Munuera, J.R. Zuazo, R. Menéndez, G.R. Castro, et al., Graphene-oxide Stacking and Defects in Few-layer Films: Impact of Thermal and Chemical Reduction. Carbon 80(2014)40-49.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[32] H.J. Chu, C.Y. Lee, N.H. Tai, Green Reduction of Graphene Oxide by Hibiscus Sabdariffa L. to Fabricate Flexible Graphene Electrode. Carbon 80(2014)725-733. [33] X. Zhang, S.C. Han, P.G. Xiao, C.L. Fan, W.H. Zhang, Thermal Reduction of Graphene Oxide Mixed with Hard Carbon and Their High Performance as Lithium Ion Battery Anode. Carbon 100(2016)600-607. [34] X.H. Zhang, Y.M. An, J.C. Han, W.B. Han, G.D. Zhao, X.X. Jin, Graphene Nanosheet Reinforced ZrB2–SiC Ceramic Composite by Thermal Reduction of Graphene Oxide. RSC Adv. 5(2015) 47060-47065. [35] P. Zhou, P. Hu, X.H. Zhang, W.B. Han, Laminated ZrB2-SiC Ceramic with Improved Strength and Toughness. Scr. Mater. 64(2011) 276-279. [36] X.H. Zhang, P. Zhou, P. Hu, W.B. Han, Toughening of Laminated ZrB2–SiC Ceramics with Residual Surface Compression. J. Eur. Ceram. Soc. 31(2011) 2415-2423. [37] C.C. Wei, X.H. Zhang, P. Hu, W.B. Han, G.S. Tian, The Fabrication and Mechanical Properties of Bionic Laminated ZrB2-SiC/BN Ceramic Prepared by Tape Casting and Hot Pressing. Scr. Mater. 65(2011) 791-794. [38] G.B. Yadhukulakrishnan, S. Karumuri, A. Rahman, R.P. Singh, A.K. Kalkan, S.P. Harimakar, Spark Plasma Sintering of Graphene Reinforced Zirconium Diboride Ultra-High Temperature Ceramic Composites. Ceram. Int. 39(2013) 6637-6646. [39] J. Jang. Estimation of residual stress by instrumented indentation: A review. Ceram. Process. Res., 2009, 10, 391-400. [40] A.V. Zagrebelny, C.B. Carter, Indentation of strained silicate-glass films on alumina substrates. Scripta Mater., 1997, 37, 1869-1875. [41] H.A. Becerril, J. Mao, Z.F. Liu, R.M. Stoltenberg, Z.A. Bao, Y.S. Chen, Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano. 2(2008)463–470. [42] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, R. Saito, Studying Disorder in Graphite-based Systems by Raman Spectroscopy, Phys. Chem. Chem. Phys. 9(2007) 1276–1291. [43] Y.M. An, J.C. Han, W.B. Han, G.D. Zhao, B.S. Xu, K.F. Jin, et al., Chemical Vapor Deposition Synthesis Carbon Nanosheet–Coated Zirconium Diboride Particles for Improved Fracture Toughness, J. Am. Ceram. Soc. 99(2016) 1360-1366. [44] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, et al., Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 97(2006) 187401.
S0008-6223(16)30449-3
DOI:
10.1016/j.carbon.2016.05.074
Reference:
CARBON 11038
To appear in:
Carbon
Received Date: 29 February 2016 Revised Date:
16 May 2016
Accepted Date: 30 May 2016
Please cite this article as: Y. An, J. Han, X. Zhang, W. Han, Y. Cheng, P. Hu, G. Zhao, Bioinspired high toughness graphene/ZrB2 hybrid composites with hierarchical architectures spanning several length scales, Carbon (2016), doi: 10.1016/j.carbon.2016.05.074. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Bioinspired high toughness graphene/ZrB2 hybrid composites with hierarchical architectures spanning several length scales Yumin An*, Jiecai Han, Xinghong Zhang*, Wenbo Han, Yehong Cheng, Ping Hu,
RI PT
Guangdong Zhao National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150001, P.R. China
SC
Abstract:
M AN U
Natural bioinspired hierarchically ordered architectures from the nanoscale to the macroscale are achieved in graphene/ZrB2 hybrid composites to improve their toughness using graphene oxide. Two types of films containing different volumes of graphene oxide are self-assembled with ZrB2 or SiC micro particles through a
TE D
vacuum-assisted filtration method. Scanning electron microscopy images show that ceramic particles are homogeneously distributed in a continuous multilayer graphene oxide network, forming a nano-micro hierarchical structure. Tensile tests are
EP
employed to test the strength of these films. The spark plasma sintering method is
AC C
utilized to construct micro-macro structural order through densifying two types of alternately stacked films containing different volumes of graphene oxide. Indentation tests reveal alternately compressive and tensile layers are achieved after the sintering process. By combining different structural features spanning several length scales, the composites exhibit a unique combination of high strength (522 MPa) and toughness (9.5 MPa·m0.5); in particular, the fracture toughness is more than double that of the *
Corresponding author. E-mail: [email protected] (Yumin An); [email protected] (Xinghong
Zhang).
ACCEPTED MANUSCRIPT composite without the hierarchical architecture. The toughening mechanisms are also analyzed at different length scales. This bioinspired material-independent approach should be employed in the designing and processing of materials for structural,
RI PT
high-temperature and energy related applications. 1. Introduction
Because of the strong covalent bond between boron atoms, Zirconium diboride (ZrB2)
SC
exhibits a high melting point, high strength, hardness and chemical inertness and has
M AN U
been selected as a leading potential material for a variety of ultra-high temperature structural applications [1, 2]. With the aim of further improving the antioxidant and sintering properties of ZrB2, silicides, such as SiC, MoSi2, or ZrO2, have been chosen to fabricate ceramic composites [3, 4]. Moreover, the enhancement of flexural
TE D
strength for ZrB2-SiC composites can be ascribe to SiC limit the growth of ZrB2 grain during densification. However, the inherent brittleness and sensitivity to flaws of ZrB2 based ceramic composites severely obstruct their wide application in ultra-high
EP
temperature environments. One of the methods used to overcome this problem
AC C
involves the choice of the ingredients and the design of a hierarchically ordered architecture from the nanoscale to microscale to improve the damage tolerance of the composites [5]. This approach is inspired by naturally existing materials, such as nacre and bone. These natural materials generally possess mechanical performances far exceed those of their individual constituents through complex hierarchical designs using brittle minerals and organic molecules [6-8]. These hierarchically ordered structures are expected to resist crack initiation and propagation when usually
ACCEPTED MANUSCRIPT spanned from nano to micro scale [9-11]. For example, the ‘brick-and-mortar’ structure of nacre is 95 vol.% composed of microscale crystalline calcium carbonate ‘bricks’ and a nanoscale organic matrix ‘mortar’ between the mineral platelets [12, 13].
RI PT
While the overlap of the mineral platelets guarantee the strength, the high toughness is ensured by a combination of an intrinsic plasticity toughening and an extrinsic toughening mechanism. The first toughening mechanism is ascribed to the nanoscale
SC
mortar slip, and the latter one is mainly provided by the tortuous crack paths and the
M AN U
pull-out of the mineral platelets on the micro-macro scale.
Numerous techniques have been developed to mimic natural nacre with hierarchical architectures, such as gluing ceramic layers [14], ice templating [15], magnetic particle alignment [16] and thermal spray [17]. Freeze casting followed by a sintering
TE D
stage has enabled one to fabricate a biologically based high toughness alumina composite with 80 vol.% alumina platelets and sub-micrometer lubricating poly(methyl methacrylate) inter-layers [18,19]. To further achieve high strength
EP
materials, Valentina Naglieri, et al. [20, 21] have discussed the relationships between
AC C
SiC scaffold morphologies and the process of ice-templating, such as cooling rates. There is an important limitation in these materials that is a significant amount of organics material, between 20 vol%-60 vol%, is employed as the ductile phase between the ceramic scaffolds, which limits the application temperature of the composites. For ultrahigh temperature applications, new materials should be found for use as the ductile phase. By substituting the organic polymer with glass-phase precursors, an alumina based composite with a strength of 400 MPa and a fracture
ACCEPTED MANUSCRIPT toughness of 20 MPam0.5 has been achieved [22]. The most striking point is the replacement of the organic polymer with inorganic nanoparticles will expand the temperature range of application environment.
RI PT
Recently, graphene oxide (GO) with outstanding mechanical properties has easily been modified with different functional groups on its surface and self-assembled into films with hierarchical architectures [23-25]. Tremendous work has been performed to
SC
analyze the mechanical properties and toughness mechanisms of GO based layered
M AN U
materials [26, 27]. For example, ionic bond [28], hydrogen bond [29] and covalent cross linking [30] can form between adjacent GO nanosheets to improve the tensile strength of the layered GO based composites. Except for the chemical bond effect, the slippage of adjacent GO nanosheets is an efficient method for energy dissipation. In
TE D
addition, chemical reduction [31, 32] and thermal reduction methods [33] have been utilized to reduce these composites to graphene based materials to realize their functionalities. Furthermore, graphene nanosheets have been utilized as a toughening
EP
phase in ultrahigh temperature ceramics by the in situ thermal reduction of GO [34].
AC C
Inspired by natural structures, some simple layered ZrB2 based structures have been fabricated to enhance the toughness through the presence of residual compressive stresses that exist in the ceramic layers [35-37]. However, these composites are usually composed of structural feature at the micron scale, which will limit further improvement of the fracture resistance. Furthermore, an abundance of organics is needed to improve the rheological properties of the ceramic slurry to realize the shaping of green body. After that, a low temperature degradation process is needed to
ACCEPTED MANUSCRIPT decompose these organics to improve the mechanical properties of the materials before sintering. This step is time consuming and not easily achieved. It is expected that a simple and efficient method can be found to construct ZrB2 based composites
RI PT
with a combination of toughening mechanisms at different length scales. In this paper, we propose a simple and new method to fabricate graphene/ZrB2 based composites with hierarchical architectures to extend their application in severe environments.
SC
Here, two bioinspired graphene/ZrB2 and graphene/ZrB2-SiC composites are
M AN U
fabricated through a ‘bottom-up’ assembly method. The microstructures as well as the toughening mechanisms of the novel graphene/ZrB2 based ceramic composites are investigated and discussed in detail. The results clearly show that a combination of high strength and toughness can be achieved via properly fabricated hierarchical
2. Experimental 2.1 Materials
TE D
architectures spanning several length scales.
EP
Commercial graphite flakes (325 mesh, >99%, USA) were used to prepare GO
AC C
through a modified Hummers’ method. GO/ceramic films were self-assembly using ZrB2 (2 µm, >99%, Northwest Institute for Non-ferrous Metal Research, China), SiC (0.5 µm, >98%, Kaihua, China) and poly(vinyl alcohol) (PVA, >97%, Fuchen, China). Two types of GO/ceramic films containing 5 vol.% and 30 vol.% GO were utilized to fabricated bioinspired graphene/ceramic composites. 2.2 Self-assembly GO/ceramic hierarchical structures: The dried GO was dispersed in deionized water via sonication (Kunshan KQ5200E, China) for 2 h. ZrB2
ACCEPTED MANUSCRIPT and PVA (0.25 wt.%) were added to the GO solution (10 mg/ml), and the mixed solution was stirred (Huapuda JJ-1, China) at high speed for 30 min. After that, the solution was filtered to achieve the GO/ZrB2 hierarchical structures. The volumes of
RI PT
the GO solution were 30 vol.% and 5 vol.% for the GO30Z and GO5Z films. Furthermore, SiC was utilized to achieve the GO/ZrB2-SiC hierarchical structures. The fabrication process was the same as that of the GO/ZrB2 films using the same GO
SC
solution volumes (GO5ZS and GO30ZS films). SiC was added in order to further
M AN U
enhance the mechanical properties of graphene/ZrB2 based composites to expand their application.
2.3 Fabrication of graphene/ceramic composites: For the graphene/ZrB2 composite, the GO30Z and GO5Z films were alternately assembled in a graphite die until the
TE D
desired composites were achieved. The spark plasma sintering (SPS, FCT Systeme GmbH, Germany) method was utilized to fabricate the composite at 1950 °C for 15 min under a uniaxial load of 30 MPa in high vacuum. The GO was in situ thermally
EP
reduced to graphene during the sintering process. For the graphene/ZrB2-SiC
AC C
composite, GO5ZS and GO30ZS films were utilized, and the fabrication was the same as that of the graphene/ZrB2 composite. 2.4 Characterization and measurements: The microstructures of the composites were characterized by scanning electron microscopy (SEM, Nanolab 600i). The graphene structure was analyzed by micro-Raman spectroscopy (REMSHAW Invia, laser wavelength: 532 nm) and X-ray diffraction (Rigaku, Dmax-rb, Cukα 1.5425 Å). The nanoindentation was performed with a G200 Nano indenter (Agilent
ACCEPTED MANUSCRIPT Technologies, USA) and was utilized to analyze the residual stress in the layers of the bioinspired graphene/ZrB2-SiC composites. The tensile strength was recorded with a nanomechanical testing system (UTM, Agilent T150, USA) using the “UTM-Bionix
RI PT
standard Toecomp CDA” method. The lengths and thicknesses of the test samples were measured by a caliper and a screw micrometer, and the width was measured by optical microscopy. Four individual samples were prepared and tested for each of the
SC
synthetic conditions. The tensile strain rate was set as 1.0 × 10-3 s-1, and the harmonic
M AN U
force and the frequency were typically 4.5 mN and 20 Hz, respectively. The fracture toughness was determined by a single edge notched beam test on 1.5 (width) × 3 (height) × 16 (length) mm bars using a 12 mm span and a crosshead speed of 0.05 mm/min (ASTM E1820). Flexural strength was tested by three points bending on 3
TE D
(width) × 1.5 (heigth) × 25 (length) mm bars using a 20 mm span and a crosshead speed of 0.2 mm/min (ASTM C1161). The test bars were cut with the surface perpendicular to the hot pressing direction, and five specimens were tested for
EP
fracture toughness and flexural strength. The microstructures of the fracture surface
AC C
and indentation crack propagate paths were analyzed by SEM. 3. Results and discussions Through ‘bottom-up’ assembly method, we fabricated dense ceramic composites defined by four structural features, spanning several length scales (Fig. 1): the stacked graphene nanosheets ensuring crack deflection and delamination; the closely packed micro-scale ceramic particle units separated by the graphene nanosheets; alternating ceramic/graphene layers containing different volumes of graphene to redistribute the
ACCEPTED MANUSCRIPT load; and the macro structural order. The composite assembly process consisted of two steps: the self-assembly of the ceramic/GO films using a vacuum-assisted filtration method and the combination of two types (different volumes of GO) of
RI PT
alternately assembled ceramic/GO films in a graphite die. After that, the SPS method was utilized to densify the composite, and the GO was thermally reduced to graphene
M AN U
SC
during the high temperature sintering process.
Figure 1 The structural features of the bioinspired graphene/ZrB2 hybrid composites at different length scales.
TE D
In the fabrication of GO, several oxygen containing groups, such as C-O and C=O groups, formed on the surface of GO, which had been verified in a previous study
EP
[34]. Two important roles were played by these groups: one was guaranteeing the high stability of the dispersion with the ceramic particles in deionized water, and the other
AC C
was forming the hydrogen bonds between adjacent GO sheets during the self-assembly process. Furthermore, PVA (0.25 wt.%) was added to the solution to further improve the interaction between the ceramic particles and GO. Based on the above points, the GO/ceramic films could be formed during the vacuum-assisted filtration process. Here, four different types of GO/ceramic films were self-assembled. The size of the GO/ceramic films depended on the area of the filter that was used. For the GO/ZrB2 films, the color was black. After addition of the SiC particles, the color
ACCEPTED MANUSCRIPT of the GO/ceramic films changed to lightly khaki. The microstructures of these films were depicted in Fig. 2. For the GO/ZrB2 films, some holes in the microstructures of the GO/ZrB2 films were easily observed, and the structures were relatively loose.
RI PT
During the vacuum-assisted filtration process, adjacent GO sheets in the solution were firstly self-assembled into GO nanosheets, and then ZrB2 particles contacted the connected GO nanosheets. With further self-assembly of the GO nanosheets and the
SC
ZrB2 particles, a three dimensional GO network was formed and the ZrB2 particles
M AN U
filled in the network. It was worth mentioning that it was the size difference between the GO nanosheets and the ZrB2 particles prompted the formation of the three dimensional GO network. We also fabricated two types of GO/ZrB2-SiC films made with 5 vol.% and 30 vol.% GO solutions, which were shown in Fig. 2 (c) and (d),
TE D
respectively. Their structures were relatively compact after the introduction of the SiC particles. The reason was due to the size of SiC particles was much smaller than that of ZrB2 particles. Therefore, the SiC particles filled the holes between the ZrB2
AC C
EP
particles and GO networks in the self-assembly process.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 2 Microstructures of the GO/ZrB2 films made with 5 vol.% GO (a) and 30 vol.% GO (b), and microstructures of the GO/ZrB2-SiC films made with 5 vol.% GO (c) and 30 vol.% GO (d).
TE D
The tensile strength of each of the GO/ceramic films was tested. The films obtained after the self-assembly process were cut into strips with a length between 1-2 cm and
EP
a width of 0.2-0.5 mm. The thickness of the strips varied between 20-100 µm. The results shown that GO5Z and GO5ZS films had average tensile strengths of 41.6 MPa
AC C
and 25.4 MPa, respectively. When the volume fraction was increased to 30%, the tensile strengths of the films increased to 141.5 MPa (GO30Z) and 72.1 MPa (GO30ZS). In addition, the modulus of each film type was improved with the increase in GO, and the values were listed in Tab. 1. The improvement of the tensile strengths and modulus might be due to enhanced interactions in the GO networks and the interlocking geometry of the ceramic particles and GO networks.
RI PT
ACCEPTED MANUSCRIPT
SC
Figure 3 Tensile strengths of the GO/ceramic films containing different volume fractions of GO.
Materials
GO5Z
Strength (MPa) Modulus (GPa)
41.6±3.5 9.1±0.3
M AN U
Table 1 Tensile strengths and modulus of the GO/ceramic films containing different volume fractions of GO. GO30Z
GO5ZS
GO30ZS
141.5±7.9 14.2±0.3
25.4±6.2 10.7±1.1
72.1±4.2 12.5±0.5
By alternate assembling of the GO/ceramic films containing different volumes of GO,
TE D
layered structures at the macro-scale were achieved. There were three important issues to be solved to improve the mechanical properties of the novel composites with
EP
the SPS process. The first one was to densify the composites to obtain high strength. It was difficult to densify ZrB2 based composites at a low temperature. Therefore, we
AC C
densified the composites at 1950 °C under a uniaxial load of 30 MPa. At such a high temperature, the organics were generally carbonized and disappeared, even in high vacuum. However, the research by X.H. Zhang, et al [34] had shown that graphene retains a relatively intact structure during the sintering process. Furthermore, graphene could act as a sintering aid for the ZrB2 based materials [38], which was a reason for choosing graphene. The second issue to be solved was that residual stress should form in the graphene/ceramic layers during the cooling process. When loading,
ACCEPTED MANUSCRIPT compressive stress would arrest crack and prompted crack bifurcation. The third issue was that the graphene structures should be achieved by the in situ thermal reduction of GO. The microstructures of the graphene/ZrB2 and graphene/ZrB2-SiC composites at
RI PT
different magnification were clearly depicted in Fig. 4. It was easily found that nearly dense graphene/ceramic layers with different thicknesses were alternately stacked in the two composites, which was consistent with that shown in Fig. 1 (d). The
SC
thicknesses of each of the graphene/ceramic layers were substantially thinner than that
M AN U
of the GO/ceramic films, which revealed the effect of the high temperature sintering. In addition, it was also clearly observed that graphene retained its structure after such
AC C
EP
TE D
a high temperature treatment, which were confirmed by Fig. 4 (c) and (f).
Figure 4 Microstructures of the bioinspired graphene/ZrB2 composites at different magnifications (a), (b) and (c), and the graphene/ZrB2-SiC composites at different magnifications (d), (e) and (f).
The residual stress in different layers of the bioinspired graphene/ceramic composite was characterized by the indentation method. The load-displacement curves for the G5ZS layer and G30ZS layer in the bioinspired graphene/ZrB2-SiC composite were
ACCEPTED MANUSCRIPT depicted in Fig. 5 (a) and (b), respectively. In Fig. 5 (a), the final indentation depth in the G5ZS layer of the bioinspired composite was significantly deeper than that of bulk G5ZS composite, which indicated that tensile stress existed in the G5ZS layer [39, 40].
RI PT
In the G30ZS layer of the bioinspired composite, the indentation depth was shallower than that of bulk G30ZS composite (Fig. 5 (b)), which revealed that compressive stress was present in the G30ZS layer. The compressive stress was easily calculated
σcom = H / sinα(1− A0 / A) , where
H was hardness,
SC
by the expression
α
was
M AN U
inclination of a sharp indenter to the surface, A0 represented projected contact area of the reference materials, and A was contact area of the G30ZS layer. The calculated compressive stress was 1.2 GPa in the G30ZS layer. During the cooling process, the thermal expansion mismatch between adjacent graphene/ceramic layers induced a
TE D
residual tensile stress in layers with less volume fraction of GO and a compressive stress in layers with more GO. Alternating between tensile stress layers and
AC C
composites.
EP
compressive stress layers would further improve the fracture toughness of the
Figure 5. Force-displacement curves of the ZrB2-SiC composites made with 5 vol.% (G5ZS composite) and 30 vol.% (G30ZS composite) GO and the G5ZS layer and G30ZS layer of bioinspired graphene/ZrB2-SiC composite.
At a high temperature (1100 °C) and in high vacuum, GO was thermally reduced to
ACCEPTED MANUSCRIPT graphene [41]. We employed XRD and Raman to analyze the effect of SPS on the structure of GO. The XRD patterns of the GO/ceramic films and graphene/ceramic composites in different degree range were shown in Fig. 6. After oxidation of graphite
RI PT
(26.5º), the peak associated with GO shifted to approximately 10º. For GO/ZrB2 films, the peaks between 20-70º were assigned to ZrB2. The peak located at 9º was assigned to GO, as shown in Fig. 6 (b). However, the X-ray diffraction intensity of GO was
SC
significantly weaker than that of ZrB2. The same phenomenon was found in the XRD
M AN U
patterns of the GO/ZrB2-SiC films. The difference was that a shift of GO peak to 10.6º was observed for GO/ZrB2-SiC films, which revealed different GO stacked structures in the two types of films. After the SPS, the peak at approximately 10º disappeared for both of the graphene/ceramic composites. However, peaks located at
TE D
26.5º and 26.2º appeared in the graphene/ZrB2 and graphene/ ZrB2-SiC composites, respectively, revealing that most of the oxygen containing functional groups in GO were removed during the thermal reduction. The appearance of graphene nanosheets
EP
in the composite acted as a lubricant between the ceramic particles, which ensured
AC C
that more energy was consumed during the failure of the composites.
Figure 6 XRD patterns of the GO/ZrB2 films, bioinspired graphene/ZrB2 composite, GO/ZrB2-SiC films and the bioinspired graphene/ZrB2-SiC composite.
ACCEPTED MANUSCRIPT Raman spectroscopy was a powerful and effective tool to analyze the structure of GO and graphene. While the E1g symmetry oscillations of carbon atoms in six members ring yielded D peak, the E2g stretching oscillations of C-C bonds in sp2 hybrid carbon
RI PT
clusters generated G peak. Only the typical D peak (1338.8 cm-1) and G peak (1597.9 cm-1) were found for the original GO (Fig. 7). After the SPS process, D peak right shifted 12 cm-1 and G peak left shifted 13 cm-1 for graphene/ZrB2 composite.
SC
Furthermore, 2D peak appeared at 2697.8 cm-1 illustrated the thermal reduction of GO
M AN U
during the SPS process exhibited double resonance mode of graphene. Additionally, D peak, G peak and 2D peak located at 1349.4 cm-1, 1583.1 cm-1 and 2696.5 cm-1 for graphene/ZrB2-SiC composite. Compared with the nearly same intensity (ID/IG=0.98) of D peak and G peak for GO, ID/IG decreased to 0.19 and 0.32 for the graphene/ZrB2
TE D
and graphene/ZrB2-SiC composites, respectively. The graphitic domain La could be calculated by the expression, 2.4 × 10-10×Ilaser4×(ID/IG)-1 (Ilaser represent the wave length of laser) [42, 43]. After thermal treatment, the average crystallite size La of
EP
graphene notably increased for the two composites, revealing carbon structures in
AC C
graphene/ceramic composites became more orderly. These findings were consistent with the results of XRD. The detailed data was listed in Tab. 2. For monolayer or bilayer graphene, the I2D/IG was
1 for single and sharp 2D peak [44]. In this work,
the average measured single 2D peak possessed an I2D/IG in the range of 0.5–1 proving that multilayer graphene structures existed in the bioinspired composites after SPS process, which was confirmed by Fig.4 (c) and (f).
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 7 Raman shift spectra of GO, bioinspired graphene/ZrB2, and graphene/ZrB2-SiC composites. Table 2 D, G and 2D Raman spectral properties of GO, the bioinspired graphene/ZrB2 composite and bioinspired graphene/ZrB2-SiC composite. Materials
P(G) (cm-1)
P(2D) (cm-1)
ID/IG
I2D/IG
La (nm)
1338.8 1351.1 1349.4
1597.9 1583.1 1583.1
2697.8 2696.5
0.98 0.19 0.32
0.57 0.74
19.6 101.2 60.1
TE D
GO Bioinspired graphene/ZrB2 composite Bioinspired graphene/ZrB2-SiC composite.
P(D) (cm-1)
EP
Mimicking the structural features of natural materials to obtain their unique combinations of mechanical properties was the goal for engineering structural The
mechanical
AC C
materials.
properties
of
bioinspired
graphene/ZrB2 and
graphene/ZrB2-SiC composites were plotted in Fig. 8. Compared with the reference bulk ZrB2 and ZrB2-SiC composite, the flexural strength of bioinspired graphene/ZrB2 and graphene/ZrB2-SiC composites were high in the direction perpendicular to the ceramic layers, with the values of 284 MPa and 522 MPa, respectively. More importantly
though,
the
corresponding
fracture
toughness
of
bioinspired
graphene/ZrB2 (6.0 MPa·m0.5) and graphene/ZrB2-SiC (9.5 MPa·m0.5) composites
ACCEPTED MANUSCRIPT were triple and double those of the reference materials (Tab. 3), respectively. These values were also higher than the reported toughness of ZrB2/graphene composites [35] and ZrB2-SiC/graphene composites [34]. In these works, the volume fraction of
RI PT
graphene was in the range of 2-6 vol.%, and the raw grain sizes of ZrB2 and SiC were
M AN U
SC
between 1-2 µm and 0.5 µm, respectively.
TE D
Figure 8 Flexural strength (a) and fracture toughness (b) of four different composites (bulk ZrB2 composite, bioinspired graphene/ZrB2 composite, bulk ZrB2-SiC composite and bioinspired graphene/ZrB2-SiC composite, respectively).
EP
Table 3 Flexural strength and fracture toughness of bulk ZrB2, ZrB2-SiC composites and bioinspired graphene/ZrB2, graphene/ZrB2-SiC composites.
AC C
Bulk ZrB2 composite Bioinspired graphene/ZrB2 composite Bulk ZrB2-SiC composite Bioinspired graphene/ZrB2-SiC composite
Flexural strength (MPa)
Fracture toughness (MPa·m0.5)
222±17 284±31 465±23 522±42
1.85±0.11 5.98±0.47 4.51±0.21 9.45±0.74
The fracture behavior of bioinspired graphene/ceramic composite was employed to analyze the synergistic effects of the structural features, including the stacked graphene nanosheets, closely packed micro ceramic particle units and alternating ceramic/graphene layers. The stress-stain curves of bioinspired graphene/ZrB2 and
ACCEPTED MANUSCRIPT graphene/ZrB2-SiC composites were depicted in Fig. 9 (a). For bulk ZrB2 and ZrB2-SiC composites, only purely linear elastic response was exhibited until catastrophic failure occurs, with unstable crack propagation characteristic of brittle When
combined
these
structural
features
together
(bioinspired
RI PT
materials.
graphene/ceramic composite), we obtained stable crack propagation in the failure process, which was unusual behavior in pure ceramic materials. The corresponding
SC
fracture surface showed crack deflection in the crack propagation path and was
TE D
M AN U
depicted in Fig. 10 (a).
AC C
EP
Figure 9 Stress-strain curves for four different composites (bulk ZrB2 composite, bioinspired graphene/ZrB2 composite, bulk ZrB2-SiC composite and bioinspired graphene/ZrB2-SiC composite).
Figure 10 Fracture surface of a bioinspired graphene/ceramic composite revealing a long range crack deflection, as indicated by arrows(a); multiple cracking,crack branching and crack bridging towards the end of the crack path, which were also indicated by arrows (b); crack arrest and bifurcation in the crack propagation path (c).
In this study, we focused on the relationship between the toughening mechanisms and
ACCEPTED MANUSCRIPT the hierarchical architecture of the materials over different length scales. When the crack propagated in the bioinspired graphene/ceramic composite, crack deflection (Fig. 10 (a)), multiple cracks, crack branching and crack bridging (Fig. 10 (b)) served
RI PT
as effective paths to relieve the locally high stress around the crack tip. The stacked graphene nanosheets connected to each other formed a network in the composites (Fig. 4 (c) and (f)). Substantial amounts of energy would be consumed during graphene
SC
crack pulling out (Fig. 11 (a)) and graphene crack bridging (Fig. 11 (b)). Moreover,
M AN U
the closely packed micro-ceramic particle units were separated by the graphene nanosheets, which could act as a lubricant for the ceramic particle units. The sliding of graphene nanosheets between the ceramic particles (Fig. 11 (c)) was also an efficient energy dissipation pathway. The alternating graphene/ceramic layers
TE D
containing different volumes of graphene would result in alternating tensile and compressive stress layers because of the thermal expansion mismatch between adjacent layers. The compressive stress in the graphene/ceramic layers would arrest
EP
the crack tip and trigger crack bifurcation (Fig. 10 (c)). The improvement of
AC C
toughness was likely a product of the combination of these mechanisms. Further work would be necessary to quantify the relative contributions of all of the toughening mechanisms identified here.
RI PT
ACCEPTED MANUSCRIPT
Figure 11 Graphene crack pulling out (a), graphene crack bridging (b) and graphene sliding (c) in the crack propagation path.
4. Conclusions
SC
In this study, graphene oxide was utilized to fabricate bioinspired graphene/ZrB2
M AN U
ceramic materials with hierarchically ordered architectures spanning several length scales to improve the material’s toughness. The ‘bottom-up’ assembly process of the hierarchically arranged architecture consisted of two steps: one was the self-assembly of graphene oxide and ceramic particles into films, forming nano-micro hierarchical
TE D
architecture; the other was the spark plasma sintering of alternately assembled two types of films containing different volumes of graphene oxide, constructing micro-macro structural order. The tensile strength of the GO/ceramic films was
EP
improved with the increase in GO. After SPS process, alternating compressive and
AC C
tensile layers formed in the graphene/ceramic composites which could further improve the fracture toughness of the composites. The hierarchical structures formed by this method enhanced the mechanical properties of the composite, a unique combination of high strength (522 MPa) and toughness (9.5 MPa·m0.5). Specifically, the composite toughness was evidently enhanced by a combination of various toughness mechanisms that occur at different length scales, including the sliding of graphene nanosheets, graphene crack pulling out and bridging, crack branching, crack
ACCEPTED MANUSCRIPT arrest and crack deflection. This work expands the application of graphene and ceramic materials to high-temperature and severe environments and also provides an
RI PT
approach for designing structural materials for other applications.
Acknowledgments
The authors are grateful for the support from National Nature Science Foundation of
SC
China (NO. 11572105) and National Outstanding Youth Foundation of China (NO.
AC C
EP
TE D
M AN U
51525201).
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
Reference [1] S.C. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Mechanical Properties of Sintered ZrB2-SiC Ceramics. J. Eur. Ceram. Soc. 31(2011) 893-901. [2] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory Diborides of Zirconium and Hafnium. J. Am. Ceram. Soc. 90(2007)1347–1364. [3] J.C. Han, P. Hu, X.H. Zhang, S.H. Meng, W.B. Han, Oxidation-resistant ZrB2-SiC composites at 2200 ºC. Compos. Sci. Technol. 68(2008)799-806. [4] F. Monteverde, Beneficial Effects of an Ultra-fine α-SiC Incorporation on the Sinterability and Mechanical Properties of ZrB2. Appl. Phys. A 82(2006)329–337. [5] U.G.K. Wegst, H. Bai, E. Saiz, A.P. Tomsia, R.O. Ritchie, Bioinspired Structural Materials. Nature Mater. 14(2015) 23-36. [6] K.J. Koester, J.W. Ager, R.O. Ritchie, The True Toughness of Human Cortical Bone Measured with Realistically Short Cracks. Nature Mater. 7(2008) 672-677. [7] H. Peterlik, P. Roschger, K. Klaushofer, P. Fratzl, From Brittle to Ductile Fracture of Bone. Nature Mater. 5(2006)52-55. [8] H.S. Gupta, W. Wagernaier, G.A. Zickler, D.R. Aroush, S.S. Funari, P. Roschger, et al., Nanoscale Deformation Mechanisms in Bone. NanoLett. 5(2005)2108-2111. [9] M.E. Launey, M.J. Buehler, R.O. Ritchie, On the Mechanistic Origins of Toughness in Bone. Ann. Rev. Mater. Res. 40(2010)25-53. [10] S. Weiner, H.D. Wagner, The Material Bone: Structural-mechanical Functional Relations. Ann. Rev. Mater. Sci. 28(1998) 271-298. [11] R.Z. Wang, H.S. Gupta, Deformation and Fracture Mechanisms of Bone and Nacre. Annu. Rev. Mater. Res. 41(2011) 41-73. [12] X.D. Li, W.C. Chang, Y.J. Chao, R.Z. Wang, M. Chang, Nanoscale Structural and Mechanical Characterization of a Natural Nanocomposite Material: The Shell of Red Abalone. NanoLett. 4(2004) 613-617. [13] M.A. Meyer, J. McKittrick, P. Chen, Structural Biological Materials: Critical Mechanics-materials Connections. Science 339(2013) 773-779. [14] W.J. Clegg, K. Kendall, N.M. Alford, T.W. Button, J.D. Birchall, A Simple Way to Make Tough Ceramics. Nature 347(1990) 455-457. [15] S. Deville, E. Saiz, R.K. Nalla, A.P. Tomsia, Freezing As a Path to Build Complex Composites. Science 311(2006) 515-518. [16] R.M. Erb, R. Libanori, N. Rothfuchs, A.R. Studart, Composites Reinforced in Three Dimensions by Using Low Magnetic Fields. Science 335(2012) 199-204.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[17] G. Dwivedi, K. Flynn, M. Resnick, S. Sampath, A. Gouldstone, Bioinspired Hybrid Materials from Spray-Formed Ceramic Templates. Adv. Mater. 2015, 10.1002/adma.201500303. [18] M.E. Launey, E. Munch, D.H. Alsem, H.B. Barth, E. Saiz, A.P. Tomsia, et al., Designing Highly Toughened Hybrid Composites through Nature-inspired Hierarchical Complexity. Acta Mater. 57(2009) 2919-2932. [19] E. Munch, M.E. Launey, D.H. Alsem, E. Saiz, A.P. Tomsia, R.O. Ritchie, Tough, Bio-inspired Hybrid Materials. Science 322(2008) 1516-1520. [20] V. Naglieri, H.A. Bale, B. Gludovatz, A.P. Tomsia, R.O. Ritchie, On the Development of Ice-templated Silicon Carbide Scaffolds for Nature-inspired Structural Materials. Acta Mater. 61(2013)6948-6957. [21] V. Naglieri, B. Gludovatz, A.P. Tomsia, R.O. Ritchie, Developing Strength and Toughness in Bio-inspired Silicon Carbide Hybrid Materials Containing a Compliant Phase. Acta Mater. 98(2015)141-151. [22] F. Bouville, E. Maire, S. Meille, B. V. Moortèle, A. J. Stevenson, S. Deville, Strong, Tough and Stiff Bioinspired Ceramics from Brittle Constituents. Nature Mater. 13(2015) 508-514. [23] C.M. Chen, Q.H. Yang, Y.G. Yang, W. Lv, Y.F. Wen, P.X. Hou, et al., Self-Assembled Free-Standing Graphite Oxide Membrane. Adv. Mater. 21(2009) 3541–3541. [24] Q. Cheng, M. Wu, M. Li, L. Jiang, Z. Tang, Ultra Tough Artificial Nacre Based on Conjugated Cross-Linked Graphene Oxide. Angew. Chem., Int. Ed. 52(2013) 3750-3755. [25] S. Park, K.S. Lee, G. Bozoklu, W. Cai, S.T. Nguyen, R.S. Ruoff, Graphene Oxide Papers Modified by Divalent Ions-Enhancing Mechanical Properties via Chemical Cross-Linking. ACS Nano. 2(2008) 572–578. [26] Y. Tian, Y. Cao, Y. Wang, W. Yang, J. Feng, Realizing Ultrahigh Modulus and High Strength of Macroscopic Graphene Oxide Papers Through Crosslinking of Mussel-Inspired Polymers. Adv. Mater. 25(2013) 2980-2983. [27] W. Cui, M. Li, J. Liu, B. Wang, C. Zhang, L. Jiang, et al., A Strong Integrated Strength and Toughness Artificial Nacre Based on Dopamine Cross-Linked Graphene Oxide. ACS Nano. 8(2014) 9511-9517. [28] S. Park, K.S. Lee, G. Bozoklu, W. Cai, S.T. Nguyen, R.S. Ruoff, Graphene Oxide Papers Modified by Divalent Ions-Enhancing Mechanical Properties via Chemical Cross-Linking. ACS Nano 2(2008) 572–578. [29] Y. Xu, W. Hong, H. Bai, C. Li, G. Shi, Strong and Ductile Poly(vinyl alcohol)/Graphene Oxide Composite Films with a Layered Structure. Carbon 47(2009) 3538-3543. [30] S. Park, D.A. Dikin, S.T. Nguyen, R.S. Ruoff, Graphene Oxide Sheets Chemically Cross-Linked by Polyallylamine. J. Phys. Chem. C 113(2009) 15801– 15804. [31] X.D. Betriu, F.J. Mompeán, C. Munuera, J.R. Zuazo, R. Menéndez, G.R. Castro, et al., Graphene-oxide Stacking and Defects in Few-layer Films: Impact of Thermal and Chemical Reduction. Carbon 80(2014)40-49.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[32] H.J. Chu, C.Y. Lee, N.H. Tai, Green Reduction of Graphene Oxide by Hibiscus Sabdariffa L. to Fabricate Flexible Graphene Electrode. Carbon 80(2014)725-733. [33] X. Zhang, S.C. Han, P.G. Xiao, C.L. Fan, W.H. Zhang, Thermal Reduction of Graphene Oxide Mixed with Hard Carbon and Their High Performance as Lithium Ion Battery Anode. Carbon 100(2016)600-607. [34] X.H. Zhang, Y.M. An, J.C. Han, W.B. Han, G.D. Zhao, X.X. Jin, Graphene Nanosheet Reinforced ZrB2–SiC Ceramic Composite by Thermal Reduction of Graphene Oxide. RSC Adv. 5(2015) 47060-47065. [35] P. Zhou, P. Hu, X.H. Zhang, W.B. Han, Laminated ZrB2-SiC Ceramic with Improved Strength and Toughness. Scr. Mater. 64(2011) 276-279. [36] X.H. Zhang, P. Zhou, P. Hu, W.B. Han, Toughening of Laminated ZrB2–SiC Ceramics with Residual Surface Compression. J. Eur. Ceram. Soc. 31(2011) 2415-2423. [37] C.C. Wei, X.H. Zhang, P. Hu, W.B. Han, G.S. Tian, The Fabrication and Mechanical Properties of Bionic Laminated ZrB2-SiC/BN Ceramic Prepared by Tape Casting and Hot Pressing. Scr. Mater. 65(2011) 791-794. [38] G.B. Yadhukulakrishnan, S. Karumuri, A. Rahman, R.P. Singh, A.K. Kalkan, S.P. Harimakar, Spark Plasma Sintering of Graphene Reinforced Zirconium Diboride Ultra-High Temperature Ceramic Composites. Ceram. Int. 39(2013) 6637-6646. [39] J. Jang. Estimation of residual stress by instrumented indentation: A review. Ceram. Process. Res., 2009, 10, 391-400. [40] A.V. Zagrebelny, C.B. Carter, Indentation of strained silicate-glass films on alumina substrates. Scripta Mater., 1997, 37, 1869-1875. [41] H.A. Becerril, J. Mao, Z.F. Liu, R.M. Stoltenberg, Z.A. Bao, Y.S. Chen, Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano. 2(2008)463–470. [42] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, R. Saito, Studying Disorder in Graphite-based Systems by Raman Spectroscopy, Phys. Chem. Chem. Phys. 9(2007) 1276–1291. [43] Y.M. An, J.C. Han, W.B. Han, G.D. Zhao, B.S. Xu, K.F. Jin, et al., Chemical Vapor Deposition Synthesis Carbon Nanosheet–Coated Zirconium Diboride Particles for Improved Fracture Toughness, J. Am. Ceram. Soc. 99(2016) 1360-1366. [44] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, et al., Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 97(2006) 187401.