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PMMA/graphene/Bentonite and PVC/graphene/Bentonite hybrid composites were prepared by solution casting method. Thermal stability, electrical conductivity and EMI shielding effectiveness in X-band were studied in detail. PVC composite with 5 wt.% Bentonite exhibited an enormous improvement in the thermal stability compared to the PVC/graphene counterpart. However, increase in Bentonite content in the hybrid composites reduced their electrical conductivity and resulted in lower overall shielding effectiveness in X band. The hybrid nanocomposites were found to have a significantly stronger influence of absorption in the overall shielding effectiveness when compared with respective polymer/graphene composites. The detailed microstructural/mechanical characterization elucidates the underlying mechanisms. [ABSTRACT FROM AUTHOR]

Resistance spot welding of similar AISI-1008 steel plates and dissimilar AISI-1008/Al-1100 alloy joints were carried out using a graphene nanoplatelets interlayer and the underlying strengthening mechanisms were discussed. Weld strength of the joints depends on the welding current/time used for the resistance spot welding. An enhancement of ~ 49% and ~ 124% was reported at certain parameters for the similar and dissimilar joints, respectively. Optical microscopy, SEM, XRD, TEM and Raman spectroscopy was used for microstructural characterization. Microhardness studies at the weld nugget cross-section revealed an enhanced hardness at the fused zone for both the cases. Al-Fe rich intermetallics formation was reported at the interfacial zone of the dissimilar joints which played a role in hardness increment. Variation of nugget size with the increase in welding current was studied. Fractured surfaces were analyzed and it was concluded that shear dimple and a mix of the shear dimple and brittle fractures were dominant in cases of similar and dissimilar metal joints, respectively. [ABSTRACT FROM AUTHOR]

The present study aims to enhance the strength of friction stir lap welded aluminum alloys by using an interlayer of graphene nanoplatelets (GNPs) at the weld interface. With GNP interlayer, the weld strength and percentage elongation increased by 121 and 53%, respectively, as compared to the weld without GNP interlayer. The interlayer also changes the mode of fracture from brittle in the weld without GNP to ductile mode. Grain size in the weld with interlayer decreased by ~38% as equated to the weld without GNP. The height of the hook defect (HD) and cold lap defect (CLD) decreased by 26% and 41%, respectively, in the weld with GNP interlayer as compared to the weld without interlayer. In weld with interlayer, the bottom of the top plate on the retreating side acts as the potential site for fracture due to the presence of an interfacial defect, and undeformed GNP layer. The strengthening of the weld is attributed to various primary strengthening mechanisms like thermal mismatch, grain refinement, Orowan looping, and load transfer. Moreover, GNP interlayer also prevents the formation of the Al2O3 layer at the lap interface and thus contribute significantly in the weld strengthening. [ABSTRACT FROM AUTHOR]

Friction stir alloying is primarily employed for the fabrication of surface composite to improve surface properties like hardness, wear resistance, and corrosion resistance without significantly affecting the bulk properties of the alloy. The present study demonstrates the novel method for the fabrication of bulk aluminum-graphene nanoplatelets composite by using friction stir alloying. Here, the novelty is shown through the method of graphene nanoplatelets incorporation in the stir zone. For this purpose, a channel is fabricated on the cross-sectional surface of the aluminum plate and filled with graphene nanoplatelets. It is then covered by the cross-sectional surface of another aluminum plate of same dimensions and friction stir alloying is carried out. Reference material (RM) is also fabricated at the same parameters without any graphene nanoplatelet reinforcements for the performance evaluation of the nanocomposite. The microhardness of the fabricated composite increased by ∼57% as compared to the reference material. However, the tensile strength of the fabricated Al-graphene nanoplatelet composites decreased marginally as compared to reference material. The strengthening of the composite is explained systematically by various mechanisms. The results of microhardness and tensile test were corroborated with various characterization methods such as optical micrographs, scanning electron microscopy, atomic force microscope, and X-ray diffraction. [ABSTRACT FROM AUTHOR]

Graphene nanoplatelets (GNPs) were infused onto aluminum alloy (Al-1100) surface by a solid-state processing technique to form surface nanocomposites which enrich the mechanical and tribological properties of the aluminum (Al) surface. Electrical resistance heating which locally softens the Al surface, followed by hot pressing, mechanically impregnates and embeds GNPs onto the Al substrate. It was observed that the GNPs mainly occupy the grain boundaries in the matrix during subsequent solidification which leads to strengthening of the matrix through various mechanisms. A detailed tribological study was carried out, and it was found that the wear rate and coefficient of friction decreases by > 85%. Least wear depth in nanoscratch testing was obtained at processing parameters 2200 A and 0.5 s, which could be identified as the optimum current/time combination. The microhardness and Young's modulus results also show an improvement > 200%, corroborating the tribological studies. [ABSTRACT FROM AUTHOR]

Abstract: The fabrication and characterization of ultrathin composite films of surfactant-wrapped graphene nanoflakes and poly(vinyl chloride) is described. Free-standing composite thin films were prepared by a simple solution blending, drop casting and annealing route. A significant enhancement in the mechanical properties of pure poly(vinyl chloride) films was obtained with a 2wt.% loading of graphene, such as a 58% increase in Young’s modulus and an almost 130% improvement of tensile strength. Thermal analysis of the composite films showed an increase in the glass transition temperature of the polymer, which confirms their enhanced thermal stability. The composite films had very low percolation threshold of 0.6vol.% and showed a maximum electrical conductivity of 0.058S/cm at 6.47vol.% of the graphene loading. [Copyright &y& Elsevier]

Abstract: A simple and effective method for the preparation of a few layered graphene nanoflakes directly from graphite has been successfully demonstrated. Mild ultrasonication of highly ordered pyrolytic graphite, in presence of a cationic surfactant cetyltrimethylammonium bromide and acetic acid yielded graphene nanoflakes, which formed a stable colloidal suspension in organic solvent such as N,N-dimethyl formamide. Scanning and transmission electron microscopic analyses showed that the dispersed phase consist of mainly few layered graphene nanoflakes. Average thickness of the flakes was found to be ∼1.18nm. Energy dispersive X-ray analysis indicated the absence of graphene oxide. Field emission measurements for the nanoflakes showed a turn on voltage of 7.5V/μm and emission current densities of 0.15mA/cm2. [Copyright &y& Elsevier]

The influence of friction stir processing parameters and the subsequent evolution of microstructures and corrosion behaviour of Al–SiC surface composites fabricated in the presence of graphitic flakes are presented in this paper. The circumferential shear stress exerted by the rotating tool exfoliates the graphite flakes into few-layered graphene during processing. These graphene flakes preferentially occupy both the grain boundaries and the SiC–Al interface during recrystallization. The potentiodynamic polarization test, in 3.5 wt% sodium chloride (NaCl) solution, is conducted for samples processed at various tool rotational speeds and it is found that the presence of this thin graphitic layer at these locations reduces the intergranular and interfacial corrosion to a great extent. The tool rotational speed has a significant influence on processing, and samples fabricated at an optimum tool rotational speed of 2200 rpm shows better corrosion resistance and mechanical properties as compared to that at lower and higher speeds. Raman spectroscopic analysis, X-ray diffraction studies and microhardness tests corroborate the experimental results and the proposed mechanisms. [ABSTRACT FROM AUTHOR]

Polyvinyl chloride (PVC)/graphene and poly(methyl methacrylate) (PMMA)/graphene nanocomposites were made by solution casting technique with graphene weight fractions of 1, 5, 10, 15, and 20%. Multilayer structures of the composites were made by hot compression technique to study their electromagnetic interference shielding effectiveness (EMI SE). Tensile strength, hardness, and storage modulus of the nanocomposites were studied in relation with graphene weight fraction. There has been a substantial increase in the electrical conductivity and EMI SE of the composites with 15–20% filler loading. Differential thermal analysis of the composites shows improved thermal stability with an increase in graphene loading. PMMA/graphene composites have better thermal stability, whereas PVC/graphene composites have superior mechanical properties. About 2 mm thick multilayer structures of PMMA/graphene and PVC/graphene composites show a maximum EMI SE of 21 dB and 31 dB, respectively, in the X band at 20 wt % graphene loading. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47792. [ABSTRACT FROM AUTHOR]

Fabricating high wear resistance Al matrix composite without sacrificing its ductility and thermal properties is a critical issue in the designing of Al-based composites. In present study, a detailed wear mechanism of Al-SiC surface composite modified by the impregnation of Graphene, Graphite & carbon nanotubes through friction stir processing (FSP) is documented. The results revealed that with SiC and graphene nanoplatelets (GNP) reinforcements the friction coefficient and specific wear rate decreased by ~34% and ~50% as compared to as-received Al6061 alloy respectively. On the other hand, combination of SiC and carbon nanotubes (CNT) reinforcements severely deteriorates the wear resistance of the composite. The layered structure, large specific surface area and the wrinkled morphology of the graphene flakes are the primary reasons for the increased wear resistance. The exfoliation of GNP to few layered graphene (FLG) under the effect of plastic flow stresses during FSP also contributes significantly to the improvement of surface properties. The abrasion is identified as the dominant wear mechanism in Al-SiC-Graphite & Al-SiC-GNP hybrid composite. Whereas, delamination due to adhesion with the counter surface is the leading mechanism in Al-SiC-CNT hybrid composite. Various characterization strategies such as microstructural characterization through SEM, interfacial study through TEM and phase analysis through XRD corroborate the results. Finally, GNP is identified as the best reinforcement among the carbon family for improvement in wear resistance of Al6061-SiC surface composite. Unlabelled Image • Tribological study of Al-SiC-Graphite/Graphene/CNT fabricated via solid state FSP • Wear rate decreased by ~50% in Al-SiC-GNP hybrid composite. • GNPs identified as the best among carbonaceous materials for wear resistance • Abrasion, the dominant wear mechanism in Al-SiC-GNP/Graphite hybrid composite • Wrinkled and layered structure of GNPs with high specific area decreases wear rate. [ABSTRACT FROM AUTHOR]

Impregnation and homogeneous distribution of nanomaterials on metallic surfaces have always been a challenge for researchers in recent times. In the present work, authors demonstrate a surface modification of Aluminium by graphene impregnation through a novel powder metallurgy assisted friction surfacing (PMAFS) process. This modified friction surfacing technique involves two stages. In the first stage, an Aluminium-graphene composite tool is fabricated through a powder metallurgy route. In the second stage, this consumable tool is utilized for FS to obtain a composite coating layer on Aluminium surface. Experimental results show that the surface composite prepared by this modified friction surfacing technique does have impregnated graphene successfully in Al matrix. The obtained surface composites were characterized by using SEM, TEM, and XRD. Morphology and damage to graphene are studied by using Raman spectroscopy. Nanoindentation study was conducted for a various set of processing parameters with variation in graphene content. Further, the influence of graphene percentage and processing parameters on mechanical properties and microstructural changes were also studied. Composites prepared at relatively lower tool rotational speed with low content of graphene in the tool is found to have optimum mechanical properties with a homogeneous distribution of graphene in Al substrate. The nano-hardness of the surface composite is increased by 100% after impregnation of graphene nano platelets (GNP) with an optimum set of parameters. [ABSTRACT FROM AUTHOR]

In this study, graphene nanoplatelets (GNPs) were mechanically impregnated into aluminium substrates to form surface nanocomposites, which enhance the surface properties of aluminium. The impregnation was achieved by application of pressure on graphene coated aluminium plates which are locally softened by electrical resistance heating. The extent of softening of aluminium can be controlled by the process parameters current and time used for electrical resistance heating. Micro-structural characterization of the graphene impregnated aluminium surface was done through Raman spectroscopy, XRD, SEM and TEM. It was observed that GNPs were impregnated up to a depth of 200–220 μm on the aluminium surface. GNPs were mostly occupied along the grain boundaries which lead to strengthening of the composite by various mechanisms. Raman spectroscopic analysis indicates a noticeable shift and change in intensity ratios of the corresponding G, D and 2D bands of the impregnated graphene. A detailed study of surface and subsurface mechanical properties was done through nano-indentation and microhardness tests. > 400% improvement in surface hardness and 200% improvement in reduced Young's modulus were observed. [ABSTRACT FROM AUTHOR]

The present work investigated the resistance spot weldability of 0.85 mm thick galvanised CR210 steel sheets employing graphene nanoplatelets (GNPs) as an interlayer. The GNPs were drop-casted on the steel surface and resistance spot welding (RSW) was carried out at optimum welding current range of 6–9 kA with a constant weld time of 0.7 s. The lap-shear and cross-tensile tests were conducted to assess the mechanical behaviour of the weldments obtained with GNPs as an interlayer. An enhancement of ∼124% in the lap shear strength was observed in the specimen welded at 9 kA. However, the incorporation of GNPs led to the deterioration of the cross-tensile strength of the welded joints. The analysis of the different zones formed after welding was done using SEM and EBSD techniques which revealed the martensitic structure and finer grains obtained in the fusion zone. The presence of dislocation pileups, nano-precipitates, and interfacial shear stress transfer at the GNP-Fe interface was observed by TEM. Raman spectroscopy was carried out to get an insight into the stresses generated in the GNPs owing to RSW at different welding currents. Fracture surface analysis of lap shear specimens revealed the presence of shear dimples at the nugget zone welded at 7 kA and a mixed mode of fracture was observed in the specimen welded at 8 kA. However, a complete nugget pull-out was consistently found during the lap shear test of the specimen welded at 9 kA. Microhardness study revealed an increase in the fusion zone hardness at different welding parameters owing to the existence of GNPs and martensitic structure. • GNPs coating was used as interlayer for resistance spot welding of similar CR-210 steel sheets. • Enhancement of ∼124% in the lap shear strength was achieved at the best welding parameter. • Grain refinement, GNPs with dislocation tangles and nano-precipitates were observed in fusion zone. • Different failure modes and strengthening mechanisms of GNPs coated spot welds were investigated. [ABSTRACT FROM AUTHOR]