Multilayer Graphene (MLG)-Reinforced Al2O3 Nanocomposites Fabricated by High-frequency Induction-heat Sintering Technology

Nanocomposites of Al2O3 (alumina) ceramic reinforced with various multilayer graphene (MLG) contents (2 and 4 vol.%) were fabricated by novel rapid high-frequency induction-heat (HF-IF) sintering technology and their microstructures, interface and mechanical properties were meticulously investigated. MLG was synthesized through thermal exfoliation process and dispersed homogenously into Al2O3 matrix by colloidal chemistry technique. Novel HF-IF sintering consolidated nanocomposites to 99% relative densities without damaging the MLG nanostructures and contributed in forming good Al2O3/MLG interface. The fracture toughness (KIC) of the nanocomposites was increased by 41% and 25% whereas a 6% increase and 15% reduction in hardness values were observed at 2 vol.% and 4 vol.% MLG additions respectively, compared to unreinforced Al2O3. Rise in the KIC values of the nanocomposites was associated with, (i) higher densities due to novel HF-IH sintering, (ii) uniform dispersions of the MLG within Al2O3 matrix, (iii) fracture-mode alteration, (iv) grain refinement, (v) strong MLG/Al2O3 interface and (vi) MLG/Al2O3 grain bridging and grain sharing toughening mechanism.


Introduction
Graphene is 2D nanosheets of carbon atoms and is the stiffest and strongest carbon based nanomaterial and offer ceramics/polymer/metallic matrices for becoming super strong materials provided that the unique properties of graphene nanostructures could be transferred to the resulting nanocomposites [1,2]. Multifunctional nanocomposites are the hottest topic in contemporary materials because these are suitable as structural materials and also satisfy the needs for additional functionality requirements such as electrical, magnetic, optical, chemical, biological etc. Although, graphene can be promising contestant for converting the brittle ceramics into superior structural material, however less focused [3].
Alumina (Al 2 O 3 ) is an attractive ceramic for advanced high temperature structural applications and is currently used in electronics, aerospace and automotive industries [4][5][6][7][8]. Despite inspiring mechanical hardness and chemical inertness, the poor fracture toughness restricts Al 2 O 3 use for advanced structures of aircraft engine, rocket materials, armor materials, automobiles and other space engineering applications [9].
In contrast with CNTs, graphene offers higher specific surface area, mechanical strength/flexibility, fewer tendencies to tangle thus dispersion is far easy into a ceramic matrix, high yield [23][24][25]. Therefore, the graphene has potentials to substitute CNTs for preparing tougher Al 2 O 3 nanocomposites. Walker et al. reported impressive 235% higher in K IC of Si 3 N 4 after loading with 1.5 vol.% graphene whereas Dusza et al. could get moderate 45% increase at 3.0 vol.% graphene additions into Si 3 N 4 , compared to monolithic samples [26,27]. Moderate K IC improvements of 27%, 28% 33%and 53% in Al 2 O 3 ceramic after 0.3 vol.%, 0.8 vol.%, 0.22 wt.% and 2 wt.% graphene loadings have been reported [28][29][30]. It is fact that graphene is an effective nano-reinforcement to enhance mechanical performance of the Al 2 O 3 ceramics. Furthermore, thin graphene, quality distribution into base matrix, novel rapid sintering process and firm graphene/Al 2 O 3 interface could further upgrade Al 2 O 3 for structural part, thus interesting [31].
In this perspective, we prepared highly dense Al 2 O 3 nanocomposites reinforced with various MLG contents (2 vol.% and 4 vol.% ) using HF-IH sintering technology. The resulting nanocomposites were systematic investigated for structural analysis and mechanical properties evaluation. The leading objective of this study is to introduce HF-IH sintering, as a promising technique, for producing quality graphene-reinforced Al 2 O 3 nanocomposites. Fast HF-IH sintering indeed facilitated to form strong MLG/Al 2 O 3 interfacial connections and to retain the graphene nanostructures into nanocomposites thus better mechanical properties could be attained in the final nanocomposites.

Nanocomposites Preparation
Three-Dimensional graphite flakes (grade 3775-Asbury Graphite Mills, Inc., New Jersey, USA) were chemically modified to graphite oxide (GO) by process, as described in the Hummers method and others [32]. Subsequently, the thermal exfoliation of the GO was performed at 1800°C using a heating rate higher than 1000°C/min under high vacuum conditions to produce thin MLG [33]. For obtaining uniform MLG dispersion into the base matrix, a colloidal chemistry technique was opted and for this purpose, designed amounts of MLG (0.2 g) were added into distilled water (100 ml) and small amounts of sodium dodcyle sulfate (SDS, < 2 wt.% of the MLG) was also added. The aqueous MLG/SDS slurry was agitated with the assistance of ultra-sonic probe (Sonic Vibracell, VCX-750, manufactured by Sonics, Materials Incorporation, USA, power 750 watts, frequency 20 KHz) for 30 min and sufficient time (2 week) was given to slurry for thorough adsorption of SDS on MLG surface and again sonicated for 30 min. A separate slurry of aqueous/Al 2 O 3 nanopowder (Sigma Aldrich, UK having particle size < 30 nm) was also prepared and later both suspensions were mixed and ultrasonicated for 60 minutes and dried at 120°C. The dried nanocomposite were condensed by HF-IH sintering furnace (Korea) at 1500°C at a heating rate of 150°C/min under an uniaxial pressure of 50 MPa and high vacuum (4.5×10 -2 Tor) and the temperatures were consistently monitored by an optical pyrometer (Thermalert TX, Raytek GmbH, Germany). Pure monolithic Al 2 O 3 reference samples were also fabricated following the same processing conditions.

Physical Properties and Material Characterization
Well-known Archimedes method was adopted to measure the apparent densities of all sintered nanocomposites samples. For relative density assessment the theoretical densities of 3.9 g/cm 3 for Al 2 O 3 and 2.1 g/cm 3 for MLG were used [26,34]. Bruker D-8 Discover X-ray diffractometer using Cu Kα radiation, Germany was utilized to identify the phases present in the sintered samples. Structural features and grain size was analyzed by scanning electron microscope, SEM, (FEG-SEM JEOL JSM-7600F, Japan.). Fractured and polished/thermally etched, at 1400 °C for 15 min in inert atmosphere, were prepared for observation under SEM. For structural examination of MLG and interfacial investigations of the MLG/Al 2 O 3 nanocomposites, an advanced transmission electron microscope, TEM, (FEG-TEM 2100F, JEOL, Japan) was used. For TEM analysis, MLG sample was prepared by dispersing MLG into acetone and later transferred to holey-carbon TEM grid. For interfacial investigation, sintered nanocomposite samples were prepared by ion milling technology using ion−slicer (EM 09100 IS, JEOL, Japan).

Mechanical Testing
Microhardness of all polished sintered samples were measured at 9.8 N loads for 15s employing Buehler-micromet 5114 (Akashi corporation, Japan) and later the obtained Vickers hardness numbers (HV) were further converted to GPa [21]. Nanoindentation technique was adopted to determine the nanohardness and elastic modulus of all sintered samples. Nanoindentation measurements were performed using a NanoTest, (Micro Materials, and UK). The indenter was continuously loaded to peak load of 200 mN in 25-30 sec and unloaded after holding time 30 sec to allow full plastic deformation. Reduced modulus was directly obtained from machine whereas the modulus of elasticity was calculated by employing equation-1 [35].
where Vs and Vi are the Possion's ratio of the test piece (0.23 for monolithic Al 2 O 3 and composites) and the indenter (0.07 for diamond) respectively, Ei is the elastic modulus of the indenter (1.14×10 6 Nmm -2 for diamond).
The K IC of all samples was appraised using the direct crack measurement (DCM) method, the lengths of crack emerging from the corner of an indent generated during Vickers hardness tests were used to estimate the K IC . The crack lengths were carefully measured using SEM and the recorded images were analyzed using images processing and analysis software (Image). The K IC value was calculated using equation-2 [36].
where E is the modulus of elasticity, H is the micro-hardness (HV) and c is the radial crack length generated by the Vickers indentation. 10 indents were made on each sample.

MLG Synthesis and Dispersions
High-frequency Induction-heat Sintering Technology Electron microscopy revealed interesting structural features concerning (i) the quality of the MLG prepared by thermal exfoliation process and (ii) the homogenous dispersion of MLG within the base Al 2 O 3 matrix. SEM studies of the thermally exfoliated MLG, Figure 1a, showed two-dimensional sheet-like geometrical features associated with the MLG and the close observation of a MLG edge was appeared with nanometric dimension (~ 6 nm), as shown in Figure 1b. Furthermore, light contrast of the same MLG samples under TEM depicts thin or mono graphene sheet (white arrow) whereas relatively darker contract represents fewer MLG layers (black arrow) and the crumpling or wrinkling of thin MLG (black arrow) was also observed, as exhibited in Figure 1c.
The thermal reduction of GO is entirely a complex mechanism and the GO has to undergone stern chemical processes (eradication of the intercalates i.e. H 2 O molecules and oxides groups) and rigorous physical phenomenon (folding/unfolding and reshuffling of individual graphene layers), at the instant of rapid thermal shock, before its transformation to thin MLG, as already proposed [37]. These obvious microscopic details suggest that graphene maintained its two-dimensional sheet morphology, structure and nanoscale features throughout elevated temperatures exfoliation process.
Uniform distribution of MLG within the Al 2 O 3 grains is important to prepare superior quality nanocomposites. SEM fractured surfaces of the sintered samples in Figure 2b clearly shows the homogenously dispersion of MLG (black arrows) within the Al 2 O 3 grains. Indeed combined processing of ultrasonication, surfactant and time provided for surfactant adsorption to MLG surfaces jointly contributed in obtaining the uniform distribution of MLG within the parent ceramic matrix. Ultrasonication provided high energy sonic waves to detangle the MLG and the surfactant adsorbed on the MLG surfaces to create positive electrostatic repulsive force with each others to counter the Van der Waal attractions for even distributions of MLG within Al 2 O 3 nanoparticles [26].

Densification and Structural Features
Novel HF-IH sintering consolidated monolithic Al 2 O 3 to near theoretical densities (99.9%) however, additions of MLG (2 vol.% and 4 vol.%) resulted in minor decrease (0.5 and 1%) in the densities of nanocomposites, respectively as graphically represented in Figure 3. This indicates that novel sintering technique, owning rapid heating rate, simultaneous pressure application and short sintering duration, is promising to condense MLG/Al 2 O 3 nanocomposites to higher densities [38]. The novel HF-IH sintering process is done in a graphite die, somehow like hot pressing, but the heating is performed by a source of high-frequency electricity to drive a large alternating current through a copper (Cu) coil, around the graphite die. When the current is passed through this coil, a very intense and rapidly changing magnetic field is generates in the space within the work coil. The conductive graphite die containing ceramic composite powder is placed within this intense alternating magnetic field which induces a current flow in the conductive graphite die. Thus allows the right amount of heat for desired period of time which surely ensures controlled performance without damaging the MLG nanostructure and consolidate nanocomposite to higher densities [39]. Figure 2b also shows that the MLG are wrapped around (black circle) the grain boundaries and this might be the another reason behind the grain refinement effect in the nanocomposites because MLG existing around the grain boundaries can effectively prevent the grain growth during sintering, and act as pining points to slow down grain boundary movement during grain growth in consolidation process [28]. However, further addition on MLG barely influenced the grain size and this could be due to the agglomeration of MLG at high concentration (black arrow in Figure 4c), as getting homogenous dispersions of MLG at concentration greater than 2 vol.% is challenging. It means that MLG merely accumulate at grain boundaries rather than pinning them thus further reduction in grain size could not be obtained, as demonstrated graphically in Figure 3 and structurally in Figure 4.

Fracture Toughness (K IC ) and other Mechanical Properties
The K IC values of all monolithic Al 2 O 3 and MLG-reinforced Al 2 O 3 nanocomposites samples were estimated by direct crack method (DCM), as adopted in several nanocomposites studies [13,16,18,19,20]. Compared to monolithic Al 2 O 3 , a 47% and 25% rise in K IC values was obtained in nanocomposites samples containing 2 vol.% and 4 Vol.% MLG respectively, as shown in Figure 5a. Figure  5b represents a respective increase of 7 % hardness at 2 vol.% MLG whereas 18% decrease in hardness was recorded for composite containing 4vol.% MLG against monolithic Al 2 O 3 . With reference to benchmark monolithic Al 2 O 3 , 0.6% and 3% reductions in elastic modulus were also noticed for nanocomposites reinforced with 2 vol.% and 4 vol.% MLG contents, respectively. SEM investigations of the fractured surfaces of samples greatly helped in understanding the reasons behind the hike in fracture toughness, as shown in Figure 2. The freshly fractured nanocomposites samples showed interesting features which explained that the conventional toughening mechanisms derived for fiber reinforced composites are also applicable to Al 2 O 3 -MLG nanocomposites [21]. MLG grain sharing (Figure 6a) MLG pull-out (Figure 2a) and crack bridging (Figure 6b) mechanisms were also observed in fractured samples and these exiting toughening mechanisms are believed to have vital role in improving the K IC of the nanocomposites. Furthermore, Figure 6a shows that the MLG is perfectly embedded within the two Al 2 O 3 grains thus strengthened the grain boundaries and led to the changed the fracture mode, from intergranular in case of monolithic Al 2 O 3 (Figure 2a) to mixed inter/intra transgranular in the nanocomposites (Figure 2b). It is further that noted in Figure 2b that a large GNP securely wrapped/rolled around the Al 2 O 3 grain (black circle), possibly due to their flexibility and high aspect ratio, forming a large area of interface with matrix. This unique interaction associated with the two-dimensional geometry of the MLG generated anchoring mechanism which can lead to increased interfacial frictions between the MLG and the matrix, and increase the required energy to pull out such MLG. Therefore, the fracture accrued through the Al 2 O 3 grains rather than grain boundaries, as demonstrated in Figure 2b. These results mean that the MLG have played significant role in improving the fracture toughness, due to their distinctive microstructures by (i) increasing the required pull out energy during fracture (ii) anchoring around the Al 2 O 3 grains and (ii) producing higher contact area with grains. The nominal differences in the hardness and elastic modulus are possibly due to the graphene layer sliding in MLG during the course of deformation [23].
In ceramic materials, the grain size and grain morphology define the mechanical properties. High hardness value of monolithic Al 2 O 3 may be attributed with the elongated prismatic and needle-like grain shapes, as shown in Figure  4a. After addition of 2 vol.% MLG, no apparent change in the microstructure was observed except grain size reduction (see Figure 4b). It means that MLG has induced more grain boundaries by refining matrix microstructure. It is fact that grain boundary area and reinforcing constituents mutually hinder the onset of plasticity by impeding dislocation movement across the entire nanocomposite microstructure [34] thus led composites containing 2 vol.% MLG to a respective hike of 7 % and 45 % in hardness and K IC values, against unreinforced Al 2 O 3 . In contrast, an 18% reduction in hardness along with deprived K IC values were noticed in Al 2 O 3 /4 vol.% MLG sample, as shown in Figure 5a. This drop in mechanical properties of the nanocomposites containing higher MLG additions may be associated with change in grain shape, intrinsic lubrication characteristics of graphene and residual porosity, as appeared in the composite microstructure shown in Figure 4c. Compared with monolithic alumina blunt/round grains morphology was noticed in the Al 2 O 3 /4 vol.% MLG nanocomposites, Figure  4c, and presumably this phenomenal change has facilitated sliding of the grains over each other because the rounded grains could easily slide on each others compared to faceted ones [40]. Furthermore, the residual porosity (white arrows in Figure 4c) in the microstructure of the Al 2 O 3 /4 vol.% MLG nanocomposites could be another factor in declining hardness and K IC values because the ceramics are sensitive to residual porosity according to an empirical square root relationship (σ F ∞ l -0.5 ) which describes the fracture strength (σ F ) dependency on the pore size (c) [34]. Most probably the residual porosity would have facilitated the crack to initiate at much lower load thus nanocomposites has lower mechanical performance [34]. Adding on, Figure 5b also shows consistent drop in the elastic modulus (E) values of the nanocomposite samples with increasing MLG concentration. This decrease is elastic modulus is probably due to the presence of porous features and low modulus of the large MLG along both in-plane and out-of-plan directions, as suggested in earlier study [41].

Interfacial Investigations
Existence of a sturdy interface between the Al 2 O 3 and the MLG is imperative to transfer the exceptional mechanical properties of MLG to the nanocomposite for performing unique toughening mechanism. During loading sequence, weakly bonded MLG with Al 2 O 3 would pull-out easily without transferring the load from one Al 2 O 3 grain to another resulting in nanocomposite with meager mechanical performance. In depth atom level study of the nanocomposites by TEM is greatly helpful in revealing the interfacial details exist at Al 2 O 3 /MLG junctions. Figure 6c shows high-resolution lattice resolved TEM images where MLG with typical fringe separation of 0.34nm which corresponds to the graphitic (002) planes of MLG can be indentified and Al 2 O 3 can be recognized by its fridge distance of 0.26nm corresponds to (104) planes. In earlier studies, the formation of a strong connection at the Al 2 O 3 /CNT interface through an intermediate thin phase (Al 2 OC) has been reported [18,19].
Nevertheless, both the CNT and MLG are constituted by graphitic structure however, our FEG-TEM investigation reveals that MLG is directly adhere with the Al 2 O 3 without any intermediate phase. The reported interface in case of Al 2 O 3 /CNT is probably due the long duration sintering process i.e. hot-pressing which possibly allowed CNT/Al 2 O 3 to form an intermediate phase through slow diffusion mechanism [18]. In contrast, novel HF-IH technology adopted in this study is a rapid sintering technology and the sintering mechanism is entirely different (creep and related mechanisms) from the conventional (diffusion and mass transportation of materials across grain boundaries) sintering processes [18,38,39]. Absence of any interfacial second phase layer between MLG and Al 2 O 3 strongly suggests direct atomic level attachment between the MLG and Al 2 O 3 matrix, as shown in Figure 6c (white arrow). Based on the TEM results, strong MLG/Al 2 O 3 interfacial connections is suggested which allow MLG to play a direct contribution in absorbing more energy during crack propagations and led nanocomposite to higher toughness.

Conclusions
 In this repot, novel rapid high-frequency induction-heat (HF-IF) sintering technology was used to fabricate Al 2 O 3 (alumina) ceramic nanocomposites reinforced with 2 vol.% and 4 vol.% of the MLG contents. The resulting nanocomposites were appraised for structural investigation and mechanical properties evaluation by employing diverse analytical techniques.  The natural graphite was chemically reduced to prepare graphite oxide (GO) and then transferred to thin MLG by thermal exfoliation process. Electron microscopic investigation confirmed the preparation of very thin (~ 6 nm) graphene nanosheets (MLG). Later these MLG were evenly distributed into Al 2 O 3 ceramic matrix by adopting colloidal chemistry method.  New HF-IF sintering route, practiced in this study, provided several benefits such as (i) condensed the monolithic Al 2 O 3 and nanocomposites to > 99% relative sintered densities, (ii) short sintering duration protected the nanostructures of MLG from degradation by retaining its morphology/structure and (iii) contributed in forming strong Al 2 O 3 /MLG interfacial connections.  Structurally, 50% and 35% refinement into the grain size of monolithic Al 2 O 3 was obtained at the MLG loading of 2 vol.% and 4 vol.% respectively. Further, the incorporation of MLG altered the fracture mode from intergranular in monolithic Al 2 O 3 to mixed inter/transgranular in the nanocomposites.  Mechanically, the K IC of the nanocomposites was increased by 45% and 25% whereas 7% increase and 15% drop in hardness values was recorded at 2 vol.% and 4 vol.% MLG additions respectively, compared to unreinforced Al 2 O 3 .  Rise in the K IC of the nanocomposites was associated with, (i) higher densities due to novel HF-IH sintering, (ii) uniform dispersions of the MLG within Al 2 O 3 matrix, (iii) fracture-mode alteration, (iv) grain refinement, (v) strong MLG/Al 2 O 3 interface and (vi) grain bridging/grain sharing toughening mechanism in MLG/Al 2 O 3 nanocomposites.