Size and synergy effects of nanofiller hybrids including graphene nanoplatelets and carbon nanotubes in mechanical properties of epoxy composites

We study mechanical reinforcement in a widely used epoxy matrix with the addition of graphene nanoplatelets (GnPs) and various mixture ratios of carbon nanotubes (CNTs) with GnPs. Two different dimensions of GnPs were used with flake sizes of 5 μm and 25 μm to investigate the influence of nanofiller size on composite properties. In GnP reinforced composites, bigger flakes showed greater reinforcement at all GnP concentrations as they actively control the failure mechanisms in the composite. In the mixture samples, highest CNT content (9:1) showed marked improvement in fracture toughness of 76%. The CNT:GnP ratio is an interesting factor significantly influencing the properties of the epoxy based nanocomposites. The combination of high aspect ratio of CNTs and larger surface area of GnPs contribute to the synergistic effect of the hybrid samples. Thermal conductivity consistently increases with incorporation of GnPs in the matrix. Transmission electron microscopy (TEM) images confirm the uniform nanofiller dispersion achieved in the composites. For the hybrid samples CNTs are seen to align themselves on the GnP flakes creating an inter-connected strong nanofiller network in the matrix. The homogeneous nanofiller dispersions have been achieved by high shear calendaring which is a method capable of being industrially scaled up.     

Direct observation of carbon nanotube induced strengthening in aluminum composite via in situ tensile tests

In situ tensile tests inside a scanning electron microscope chamber are conducted on spark plasma sintered Al and Al-1 vol.% CNT composites to understand the strengthening and deformation mechanisms due to long (25–30 μm) CNT reinforcement addition. Al–CNT composite shows 40% higher tensile strength, and 65% higher stiffness for mere 1 vol.% CNT addition. The failure occurs by CNT pullout from the matrix, which is directly imaged during the tensile testing. Telescopic sliding of CNT walls is also observed which aids to strengthening.     

Strengthening behavior of few-layered graphene/aluminum composites

Strengthening behavior of composite containing discontinuous reinforcement is strongly related with load transfer at the reinforcement–matrix interface. We selected multi-walled carbon nanotube (MWCNT) and few-layer graphene (FLG) as a reinforcing agent. By varying a volume fraction of the reinforcement, aluminum (Al) matrix composites were produced by a powder metallurgy method. Uniform dispersion and uniaxial alignment of MWCNT and FLG in the Al matrix are evidenced by high-resolution transmission electron microscope analysis. Although the reinforcements have a similar molecular structure, FLG has a 12.8 times larger specific surface area per volume more than MWCNT due to geometric difference. Therefore an increment of a yield stress versus a reinforcement volume fraction for FLG shows 3.5 times higher than that of MWCNT Consequently, for both reinforcements, the composite strength proportionally increases with the specific surface area on the composite, and the composites containing 0.7 vol% FLG exhibit 440 MPa of tensile strength.     

Reinforcing effect of multi-walled carbon nanotubes on microstructure and mechanical behavior of AA5052 composites assisted by in-situ TiC particles

In this research, the synergistic effects of multi-walled carbon nanotubes and in-situ synthesized titanium carbide (TiC) on the mechanical performance of aluminum hybrid composites were studied. The microstructural characterization involving the influence of both titanium carbide and carbon nanotubes was investigated. The reinforcing effects of both titanium carbide and multi-walled carbon nanotubes on the micro-hardness, tensile strength, and impact strength of the composites were also investigated. The microstructures of the fractured tensile and impact test surfaces were examined through FESEM (Field Emission Scanning Electron Microscope). Clear peaks of titanium carbide (TiC) and aluminum carbide (Al₄C₃) were observed in the X-Ray Diffraction (XRD) analysis. The micro-hardness of the aluminum composites was significantly improved after the reinforcement with CNT and TiC. The highest ultimate tensile strength and yield strength were found with Al–9%TiC-1%MWCNT and are equal to 206.44 MPa and 136.15 MPa, respectively, whereas a 16% decrease in the impact strength of Al–9%TiC-1%MWCNT was witnessed when compared to base alloy. The effects, such as CNT pull-out, CNT bridging was seen from tensile fractography of the composites. Further, the crack initiation from the pull-out cavity was also assumed to affect the fracture mechanism. Cleavage facets were associated both with the impact and tensile fracture surfaces of the composites. With the superior mechanical properties obtained, the aluminum hybrid composite can be replaced for different structural applications.     

Improving delamination resistance of carbon fiber reinforced polymeric composite by interface engineering using carbonaceous nanofillers through electrophoretic deposition: An assessment at different in-service temperatures

In this article, modification of carbon fiber surface by carbon based nanofillers (multi-walled carbon nanotubes [CNT], carbon nanofibers, and multi-layered graphene) has been achieved by electrophoretic deposition technique to improve its interfacial bonding with epoxy matrix, with a target to improve the mechanical performance of carbon fiber reinforced polymer composites. Flexural and short beam shear properties of the composites were studied at extreme temperature conditions; in-situ cryo, room and elevated temperature (−196, 30, and 120°C respectively). Laminate reinforced with CNT grafted carbon fibers exhibited highest delamination resistance with maximum improvement in flexural strength as well as in inter-laminar shear strength (ILSS) among all the carbon fiber reinforced epoxy (CE) composites at all in-situ temperatures. CNT modified CE composite showed increment of 9% in flexural strength and 17.43% in ILSS when compared to that of unmodified CE composite at room temperature (30°C). Thermomechanical properties were investigated using dynamic mechanical analysis. Fractography was also carried out to study different modes of failure of the composites.     

The use of flake powder metallurgy to produce carbon nanotube (CNT)/aluminum composites with a homogenous CNT distribution

A strategy called flake powder metallurgy (flake PM) was used to achieve a uniform distribution of carbon nanotubes (CNTs) in CNT/Al composites and thus realize the potential of CNTs as a reinforcement. It consists of the addressing of the incompatibilities of Al powders with CNTs, uniform adsorption of CNTs onto the Al nanoflake surface by slurry blending and consolidation of as-prepared CNT/Al composite powders by hot extrusion. By changing spherical Al powders to nanoflakes and surface modifying them with a polyvinyl alcohol hydrosol, flake PM achieved high compatibilities of Al powders with CNTs, in terms of both surface properties and geometries. Thus, it essentially exploits the fact that a homogeneous and individual distribution of CNTs in Al powders can be achieved simply by direct slurry blending. Moreover, the structural integrity of the CNTs was well maintained in the final composites since CNTs were protected from high energy physics force such as ballmilling. As a consequence, a strong and ductile CNT/Al composite with tensile strength of 435 MPa and plasticity of 6% was fabricated, which greatly surpasses values for materials fabricated by conventional methods.     

Synthesis of glass fiber‐multiwall carbon nanotube hybrid structures for high‐performance conductive composites

The conductive polyamide 66 (PA66)/carbon nanotube (CNT) composites reinforced with glass fiber‐multiwall CNT (GF‐MWCNT) hybrids were prepared by melt mixing. Electrostactic adsorption was utilized for the deposition of MWCNTs on the surfaces of glass fibers (GFs) to construct hybrid reinforcement with high‐electrical conductivity. The fabricated PA66/CNT composites reinforced with GF‐MWCNT hybrids showed enhanced electrical conductivity and mechanical properties as compared to those of PA66/CNT or PA66/GF/CNT composites. A significant reduction in percolation threshold was found for PA66/GF‐MWCNT/CNT composite (only 0.70 vol%). The morphological investigation demonstrated that MWCNT coating on the surfaces of the GFs improved load transfer between the GFs and the matrix. The presence of MWCNTs in the matrix‐rich interfacial regions enhanced the tensile modulus of the composite by about 10% than that of PA66/GF/CNT composite at the same CNT loading, which shows a promising route to build up high‐performance conductive composites. POLYM. COMPOS. 34:1313–1320, 2013. © 2013 Society of Plastics Engineers     

Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites

By using a catalytic growth procedure, carbon nanotubes (CNTs) are in situ formed on reduced graphene oxide (RGO) sheet at 600 °C. CNTs growing on RGO planes through covalent C–C bond possess lower interfacial contact electrical resistance. As a hybrid structure, the CNTs/graphene (CNT/G) are well dispersed into poly (dimethyl siloxane). The hybrid combining electrically lossy CNTs and RGO, which disperses in electrically insulating matrix, constructs an electromagnetic wave (EM) absorbing material with ternary hierarchical architecture. The interfacial polarization in heterogeneous interface plays an important role in absorbing EM power. When the filler loading is 5 wt.% and thickness of absorber is 2.75 mm, the minimum value of reflection coefficient and the corresponding frequency are −55 dB and 10.1 GHz, and the effective absorption bandwidth reaches 3.5 GHz. Therefore, combining the CNTs and graphene sheet into three-dimensional structures produces CNT/G hybrids that can be considered as an effective route to design light weight and high-performance EM absorbing material, while the effective EM absorption frequency can be designed.     

Interface microstructure and mechanical properties of Ni–Co–P alloy coatings modified carbon fibres reinforced 7075Al matrix composites

To optimize interface microstructure between 7075Al matrix and CFs, Ni–Co–P multi-component alloy coatings coated carbon fibres were prepared by electroless plating firstly and then Ni–Co–P coated CFs reinforced 7075Al matrix composites (CF/Al(Ni–Co–P)) with high relative density were fabricated by hot pressing sintering process. After modification of Ni–Co–P coatings, Al–Co–Ni Intermetallic compounds were formed stably between matrix and reinforcement because of the smaller mixing enthalpy values of Al–Co, Al–Ni and Co–Ni, which not only restrained the generation of Al₄C₃ but also improved interfacial bonding strength. Yield strength and ultimate tensile strength of CF/Al(Ni–Co–P) composites with 30 vol% CFs had maximum improvement compared with CF/Al(U) composites than other composites reinforced by 10 vol%, 20 vol% and 30 vol%CFs, which is up to 305.8 MPa and 668.7 MPa respectively, and the fracture mode of composites from accumulation fracture to non-accumulation fracture as the existence of Ni–Co–P coatings.     

Investigation of microstructure and mechanical properties of Cu/Ti–B–SiCp hybrid composites

In this study, Cu/Ti–B-SiC_p hybrid composite materials were produced by powder metallurgy method using three different sintering temperatures (950, 1000, 1050 °C). The optimum sintering temperature of Cu main matrix composites reinforced with Ti–B-SiC_p reinforcement materials at 2-4-6-8 wt.% were determined and their microstructure and mechanical properties were investigated. As a result of microstructure studies, it was determined that reinforcement elements have a homogeneous interface in the main matrix. The hardness of the produced composites was determined by the Brinell hardness method. The highest hardness value (77.74 HB) was determined in the sample with 6 wt% reinforcement ratio. In the tensile and three point bending tests, maximum strength values (112.96 MPa, 37.76 MPa) were found in samples with a reinforcement ratio of 4 wt%. It was determined that increasing reinforcement ratios and sintering temperature made a positive contribution to the hybrid composite materials produced.     

The interfacial strength and fracture characteristics of ethanol and polymer modified carbon nanotube fibers in their epoxy composites

Carbon nanotube (CNT) fibers spun from CNT arrays were used as the reinforcement for epoxy composites, and the interfacial shear strength (IFSS) and fracture behavior were investigated by a single fiber fragmentation test. The IFSS between the CNT fiber and matrix strongly depended on the types of liquid introduced within the fiber. The IFSS of ethanol infiltrated CNT fiber/epoxy varied from 8.32 to 26.64 MPa among different spinning conditions. When long-molecule chain or cross-linked polymers were introduced, besides the increased fiber strength, the adhesion between the polymer modified fiber and the epoxy matrix was also significantly improved. Above all, the IFSS can be up to 120.32 MPa for a polyimide modified CNT fiber, one order of magnitude higher than that of ethanol infiltrated CNT fiber composites, and higher than those of typical carbon fiber/epoxy composites (e.g. 60–90 MPa). Moreover, the composite IFSS is proportional to the tensile strength and modulus of the CNT fiber, and decreases with increasing fiber diameter. The results demonstrate that the interfacial strength of the CNT fiber/epoxy can be significantly tuned by controlling the fiber structure and introducing polymer to optimize the tube–tube interactions within the fiber.     

Plasma sprayed graphene/carbon nanotube reinforced lanthanum-cerate hybrid composite coating

Lanthanum cerate (LC: La₂Ce₂O₇) is a potential material for thermal barrier coating, whose improved toughness is a crucial necessity for the pathway of its industrialization. Herein, we demonstrated a promising approach to develop graphene/carbon nanotube hybrid composite coating using a large throughput and atmospheric plasma spraying method. Graphene nanoplatelets (GNP: 1 wt %) and carbon nanotube (CNT: 0.5 wt %) reinforced lanthanum cerate (LCGC) hybrid composite coatings were deposited on the Inconel substrate. Addition of 1 wt % GNP and 0.5 wt % CNT in LC matrix has significantly increased its relative density, hardness, and elastic modulus up to 97.2%, 2–3 folds, 3–4 folds, respectively. An impressive improvement of indentation toughness (8.04 ± 0.2 MPa m^0.5) was observed on LCGC coating, which is ～8 times higher comparing the LC coating. The toughening was attributed to the factors: such as the distribution of GNPs and CNTs in the LC matrix, synergistic toughening offered by the GNPs and CNTs; (i) GNP/CNT pull-out, (ii) crack bridging and arresting, (iii) splat sandwiching, mechanical interlocking, etc. Finally, this improved toughness offered an exceptional thermal shock performance up to 1721 cycles at 1800 °C, without any major failure on the coating. Therefore, the GNP and CNT-reinforced LC hybrid composite coating can be recommended to open a path for turbine industries.     

Synergistic strengthening by load transfer mechanism and grain refinement of CNT/Al–Cu composites

CNT/Al–Cu composites were fabricated by mixing of Al powders and CNT/Cu composite powders which were prepared by molecular level mixing process. The CNT/Al–Cu–Cu composites show a microstructure with a homogeneous dispersion of CNTs in the Al–Cu matrix and had a 3.8 times increase of yield strength and 30% increase of elastic modulus compared to Al–Cu matrix. The strengthening mechanism of CNT/Al–Cu composites was discussed by controlling the aspect ratio of CNTs and it was thought that the CNT/Al–Cu composites were strengthened by both load transfer from the Al matrix to the CNTs and dispersion strengthening of damaged short CNTs. At the same time, the addition of CNTs increases the grain refinement effect of the Al–Cu matrix which results in a grain size strengthening mechanism of the CNT/Al–Cu composites.     

Effect of dispersion conditions on the mechanical properties of multi-walled carbon nanotubes based epoxy resin composites

A suitable dispersion technique and quantitative evaluation of degree of dispersion of carbon nanotubes (CNT) in any solvent and matrix system has been one of the key issues for achieving enhanced performance of CNT reinforced composites. We report the use of UV–vis spectroscopy as a useful technique to ascertain the degree of dispersion of multiwalled carbon nanotubes (MWCNT) in the epoxy resin. The study has enabled to maximize dispersion of MWCNT in the epoxy resin using two different routes. As a result the composite samples prepared with only 0.3 wt.% amine functionalized MWCNT showed flexural strength of 140 MPa over the neat resin value of 55 MPa, an improvement of ~155% which is maximum reported so far for CNT-epoxy isotropic composites.     

An analysis of the factors affecting strengthening in carbon nanotube reinforced aluminum composites

Tensile strength data of Al/CNT composites from the literature is analyzed to understand the effects of CNT dispersion, processing technique, degree of deformation and CNT–matrix interface on the elastic modulus, strength and toughness of composites. Strengthening can be divided in three regimes which show decreasing strengthening effect with an increasing CNT content. The strengthening is highest for CNT content less than 2 vol.%. The applicability of the micromechanics models in predicting the strength and elastic modulus of CNT reinforced metal matrix composites is also analyzed. The rule of mixtures is effective in predicting the elastic modulus of the Al/CNT composites for low CNT content (<2 vol.%) whereas Halpin–Tsai and combined Voigt–Reuss models are better at intermediate CNT content (2–5 vol.%). Effect of degree of deformation such as extrusion ratio during processing on the load transfer to CNT and resulting strengthening is also discussed. Tensile data on Cu/CNT and Mg/CNT composites is compared with Al/CNT to show that strengthening is not effective when there is no chemical interaction between metal matrix and CNT. The analysis presented here would be very helpful in the future design of high strength CNT/metal matrix composites.     

Impact of enhanced interfacial strength on physical,mechanical and tribological properties of copper/reduced graphene oxide composites: Microstructural investigation

Copper/reduced graphene oxide (rGO) composites were prepared to improve the mechanical and tribological properties of copper without adversely affecting its physical properties in any significant manner. No hazardous chemicals were used for reduced graphene oxide production, which maintained the integrity of layers. For better dispersibility of rGO in the copper matrix, electroless deposition of copper was done on the activated and sensitized rGO surfaces. Different amounts of prepared Copper/rGO nanocomposites were then dispersed in bulk copper using ethanol and finally compacted using spark plasma sintering. The coefficient of friction of copper reinforced with 0.5 wt% of nanocomposite reduced by 77.5% compared to neat copper. The flexural strength of copper reinforced with 0.75 wt% of nanocomposite and modulus of 1 wt% of nanocomposite reinforced copper increased by 15.2% and 31.3%, respectively, with different strengthening mechanisms before and after yield point. The increase in hardness and strength of the material along with thin rGO films in the wear track accounted for the sharp decrease in the coefficient of friction for the composites. There was a minimal and gradual decrease in the physical properties (electrical and thermal conductivities) of the composites with an increase in the amount of reinforcement. The two-step composite fabrication process ensured better dispersion of rGO in the copper matrix, which resulted in even properties throughout the composite.     

Synergistic effects of hybrid carbon nanomaterials in room‐temperature‐vulcanized silicone rubber

This work examines nanocomposites based on nanofillers and room‐temperature‐vulcanized silicone rubber. The carbon nanofillers used were conductive carbon black (CB), carbon nanotubes (CNTs) and graphene (GE). Vulcanizates for CB, GE, CNTs as the only filler and hybrid fillers using CNTs, CB and GE were prepared by solution mixing. The elastic modulus for CNT hybrid with CB at 15 phr (4.65 MPa) was higher than for CB hybrid with GE (3.13 MPa) and CNTs/CB/GE as the only filler. Similarly, the resistance for CNT hybrid with CB at 10 phr (0.41 kΩ) was lower than for CB (0.84 kΩ) at 20 phr and CNTs as the only filler. These improvements result from efficient filler networking, a synergistic effect among the carbon nanomaterials, the high aspect ratio of CNTs and the improved filler dispersion in the rubber matrix. © 2016 Society of Chemical Industry     

Microstructural and mechanical behavior of multi-walled carbon nanotubes reinforced Al–Mg–Si alloy composites in aging treatment

Microstructural and mechanical behavior of heat treatable Al–Mg–Si (6XXX series) alloy composites reinforced with multi-wall carbon nanotubes (MWCNTs) fabricated by powder metallurgy process were investigated by SEM-EDS, XRD, tensile test and Vicker’s hardness test. As-extruded P/M 6063 alloy composites with CNT reinforcements indicated a small increment of mechanical strength compared to the monolithic 6063 alloy with no CNT before T6 heat treatment. When T6 heat treatment was applied to the specimens, the 6063 composite with CNTs showed a noticeable decrease of yield stress (YS) improvement, compared to the monolithic Al alloy. It means that Mg₂Si precipitates hardening effect by the artificial aging treatment was insufficient for the composite containing CNTs. This was mainly because Mg alloying elements were diffused around CNTs and consumed to form Al₂MgC₂ compounds, and resulted in the incomplete matrix strengthening behavior by Mg₂Si precipitation after the aging treatment.     

Nanostructured cigarette wrapper encapsulated PDMS-RGO sandwiched composite for high performance EMI shielding applications

Here, we report a unique hybrid sandwich-type composite structure consisting of waste cigarette wrapper (CW) inserted in polydimethylsiloxane (PDMS) and reduced graphene oxide (RGO) composite matrix that displays a high EMI shielding effectiveness (SE) of ~50.79 dB in the extended Ku-band. The function of the inserted CW is to facilitate the multiple reflections and heat dissipation as it contains an Al coating. The interior of CW in the composite as well as the three-dimensional conductive networks of RGO throughout the PDMS matrix facilitates the microwave absorption through conductive dissipation and heat dissipation. Thus, an absorption-multiple reflection-absorption pathway is followed due to the combination of three sandwiched layers, that is, PDMS-RGO, CW, and PDMS-RGO. The sandwich architecture of the hybrid PDMS nanocomposite containing both CW and RGO displays a superior EMI SE over the nanocomposite that only contains RGO as filler in PDMS matrix. Moreover, the fabricated composite displays a high thermal conductivity which helps to dissipate the radiated microwave energy via a Joule heating effect. Thus, the fabricated lightweight and flexible hybrid composite structure could be an efficient microwave absorber which offers an attractive and cost-effective alternative approach to metal based conventional EMI shielding materials.     

Effect of graphene and CNT reinforcement on mechanical and thermomechanical behavior of epoxy—A comparative study

Graphene‐nanoplateles (Gr) and multiwalled carbon nanotubes (CNTs) reinforced epoxy based composites were fabricated using ultrasonication, a strong tool for effective dispersion of Gr/CNTs in epoxy. The effect of individual addition of two different nanofillers (Gr and CNT) in epoxy matrix, for a range of nanofiller content (0.1–1 wt %), has been investigated in this study. This study compares mechanical and thermomechanical behavior of Gr and CNT reinforced epoxy. Gr reinforcement offers higher improvement in strength, Young's modulus, and hardness than CNT, at ≤0.2 wt %. However, mode‐I fracture toughness shows different trend. The maximum improvement in fracture toughness observed for epoxy‐Gr composite was 102% (with 0.3 wt % loading of Gr) and the same for epoxy‐CNT composite was 152% (with 0.5 wt % loading of CNT). Thorough microstructural studies are performed to evaluate dispersion, strengthening, and toughening mechanisms, active with different nanofillers. The results obtained from all the studies are thoroughly analyzed to comprehend the effect of nanofillers, individually, on the performance of the composites in structural applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46101.