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The short-crack domain and contact damage resistances of silicon carbide (SiC) ceramics containing graphene fillers (graphene nanoplatelets -GNPs- or reduced graphene oxide sheets -rGOs) are investigated by performing Hertzian contact tests. A progressive deviation from the linear Hertzian elastic response with increasing graphene content takes place, the composite containing 20 vol.% GNPs being the most deformable material. When adding increasing amounts of GNPs, the damage beneath the contact zone turns from well-defined cone cracks of monolithic SiC to a widespread subsurface damage where microcracks are generated due to the matrix/graphene interface debonding by a shear faulting process. This mechanism enhances the contact damage resistance of the composites, redistributing the stresses at the contact and limiting the long-crack formation. The composite containing 5 vol.% rGOs fully precludes the cone cracks development and enlarges the quasi-plastic damage zone, extraordinarily enhancing the contact damage resistance that approaches to that of a ductile material. [ABSTRACT FROM AUTHOR]

Glass compositions in the Y2O3-Al2O3-SiO2 ( YAS) system are envisaged as promising coatings for high-temperature protection, in particular for the thermal protection systems ( TPS) looked for aerospace applications. Recently, thermally sprayed YAS hybrid coatings containing a small amount of graphene nanoplateletes ( GNPs) showed enhanced performance as compared to the blank YAS coating, demonstrated by the occurrence of unusual electrical conductivity for these glasses and the development of better mechanical compliance, both phenomena associated with the presence of GNPs. Nevertheless, a crucial issue is to demonstrate if these kinds of coatings would also have superior behavior under ablation conditions, particularly regarding the mentioned TPS applications. This work goes far beyond, exploring the ablative behavior of new YAS/ GNPs coatings flame sprayed over SiC substrates. These essential tests were carried out under laboratory conditions, reaching limit temperatures of 1350°C while blowing gas. Results evidence that hybrid coatings having just 1.05 vol% GNPs show enhanced ablation resistance, actually withstanding up to 30 thermal cycles (between 200°C and 1350°C) without apparent damage. This satisfactory performance is linked to the benefits of the GNP additions, and fundamentally to the higher emissivity and the directional thermal conduction characteristics of the hybrid coatings-produced by the formation of a GNP network with a preferential surface parallel arrangement-that preclude the creation of hot spots and also hinder heat propagation toward the substrate; accordingly, coating degradation is constrained to the uppermost layer of these coatings. [ABSTRACT FROM AUTHOR]

In-situ grown graphene/SiC composites developed by spark plasma sintering have emerged as a very interesting family of materials with expected high performance for advanced applications. In this work, the local functional properties of graphene/SiC ceramics are elucidated for distinct α- and β- SiC polytypes combining scanning probe microscopies. We unambiguously identify all composite constituents and demonstrate the formation of a three-dimensional graphene conductive network inside the composite. The investigated composites exhibit grains with different doping level depending on growth rate during sintering so that conduction paths associated to graphene and matrix networks may compete. The relevance of nanoscale characterization on functional graphene/semiconductor materials is proved as it evidences the type of doping and carrier concentration of the semiconductor and the critical role played by the graphene constituent in the formation of ohmic contacts. Both issues are of crucial importance for understanding the macroscale behavior of these materials and determine their applications. [ABSTRACT FROM AUTHOR]

Despite numerous works have recently reported the densification of silicon carbide (SiC) ceramics using the Spark Plasma Sintering ( SPS) technique, the effect of the localized electric current flow near the specimen over the liquid phase sintering process of SiC ceramics and on their microstructural features has not been completely addressed. In the present work, two different SPS setups affecting current flow are selected, one based on the ordinary die/punch setup configuration, and the other employing a BN electrically insulating coating on the inner wall of the die to force the electric current to locally flow through the inner graphite foil in contact with the ceramic compact. The effective electrical resistance and the energy consumed during the SPS runs for both setups, as well as the sintering behavior, microstructure, and mechanical properties of the SPSed materials are analyzed. The BN die coating considerably increases the effective resistance of the system, decreases the power consumption, and accelerates the SiC densification. Besides, ceramic specimens experience significantly higher real temperatures than the set values and, accordingly, coarser microstructures and tougher materials than those for the ordinary setting are produced. The thermoelectrical properties of SiC materials are proposed as fundamental in their SPS process, especially when electrical current is forced through the inner part of the SPS setting around the specimen. [ABSTRACT FROM AUTHOR]

Synthesizing 3D carbon nanotube (CNT) networks with multifunctional characteristics has stimulated the interest from the scientific community since the 1990s. Here, the fabrication of a novel composite material consisting of 3D covalently interconnected multiwalled CNT with silicon carbide (SiC) nano and microparticles is reported. The material is synthesized by a two-step process involving the coating of CNT with silicon oxide (SiO x) via chemical routes, followed by spark plasma sintering (SPS). SPS enables carbothermal reduction of SiOx and subsequent densification of the material into 3D composite blocks. Covalent interconnections of CNT are facilitated by a carbon diffusion process resulting in SiC formation as SiOx coated CNT are subjected to high temperatures. The presence of SiC in the sintered composite has been confirmed by Raman spectroscopy, as well as through energy filtered transmission electron microscopy maps. Interestingly, the 3D CNT composite exhibits high thermal conductivity (16.72 W m−1 K−1); and also a semiconducting behavior with an electron hopping mechanism associated to a 3D variable range hopping model. These findings demonstrate that it is indeed possible to fabricate SiC-CNT composites with enhanced physical properties that can be used as multifunctional materials. [ABSTRACT FROM AUTHOR]

The thermal conductivity of two-phase ceramics has been modelled by finite element method for a wide range of second phase contents, aspect ratios and thermal conductivities of the filler phase. The finite element method model has been validated with the experimental thermal conductivity data from exemplary ceramics to show the applicability of this model, namely alumina/silicon carbide composites and highly porous mullite materials, which show second phases of very different thermal conductivity (SiC and pores). The selection of the mesh element dimensions has been based on a schematic picture of the microstructure. For the first time, the finite element method simulation of a large number of random biphasic meshes has been used for estimating errors in the predicted thermal conductivity values that were comparable with the experimental error. The model can be very helpful in designing new materials with specific thermal conduction, limiting the more expensive and time consuming material preparation work. [ABSTRACT FROM AUTHOR]

Abstract: The outstanding electronic and physico-chemical properties of graphene make it an ideal filler in the fabrication of conducting and robust ceramic composites. In this study, a novel single-step approach for processing electrically conducting and well dispersed graphene/SiC nanocomposites is shown. These materials were processed by growing epitaxial graphene with either α- or β-phase SiC ceramics during their densification via spark plasma sintering (SPS). About 4vol.% of few-layer graphene domains were generated in situ during the SPS process, leading to a conducting graphene network that significantly enhanced the electrical performance of SiC. The in situ graphene SPS growth mechanism arose from the combined action of the electric current, high temperature and partial vacuum. This approach offers unprecedented opportunities for the fast manufacturing of graphene/SiC nanocomposites with superior electrical and mechanical properties, precluding the handling of potentially hazardous nanostructures. This method widens their possible applications, including micro-electromechanical systems, brakes, micro-turbines or micro-rotors. [Copyright &y& Elsevier]

Geometrically complex, three-dimensional (3-D) structures of SiC were produced by a colloidal printing method known as robocasting, followed by low-pressure spark plasma sintering ( SPS) to produce dense ceramic bodies. A concentrated, aqueous colloidal ink consisting of SiC, Al2 O3, and Y2 O3 particles in a dilute polymer solution with a total solids volume fraction of 0.44 was developed to have pseudoplastic behavior with yield stress rheology. Lattice structures consisting of extruded filaments deposited in an overall cylindrical or cuboid shape were printed through nozzles ranging in diameter from 150 to 330 μm. After printing, drying and calcining processes, the structures were sintered at 1700°C in argon by SPS. The final average grain size was 1-2 μm and samples displayed above 97% of theoretical density, showing ~22.8% linear shrinkage from green to sintered state. [ABSTRACT FROM AUTHOR]

A novel approach for fabrication of 3D interconnected carbon nanotube (CNT)–silicon carbide composites is reported by M. Terrones and co‐workers on page 4985. The two‐step fabrication route involves coating CNTs with silicon oxide via a sol‐gel process, followed by spark plasma sintering, during which the SiOx is carbothermally reduced to SiC. A covalently interconnected SiC–CNT network composite with high thermal and electrical conductivities is created. [ABSTRACT FROM AUTHOR]

The electrical discharge machining (EDM) performance of silicon carbide (SiC) ceramics containing graphene nanoplatelets (GNPs) is investigated for the first time. Under fine machining conditions, the material removal rate (MRR) dramatically increases up to 186% when 20 vol.% of GNPs are added to SiC ceramics, leading to reductions on the electrode wear rate of 132%. The EDMed nanocomposites exhibit surface roughness ≤ 0.8 μm. This outstanding EDM response of the graphene nanocomposites as compared to monolithic SiC is explained by their enhanced transport properties, establishing a direct dependence of MRR with the electrical conductivity. EDM performance of the nanocomposites also depends on the testing direction for materials with low GNPs connectivity (≤ 10 vol.%). Melting/evaporation are the main removal mechanisms, thermal spalling also operating for low thermal conducting materials. The employ of EDM on SiC/graphene nanocomposites allows machining microparts with a fine dimensional precision, opening new opportunities for SiC-based microcomponents. [ABSTRACT FROM AUTHOR]

The role played by graphene in the friction and wear behaviour of graphene/silicon carbide (SiC) composites, tested under dry sliding conditions and using silicon nitride balls as counterbodies, is investigated as a function of the graphene nanoplatelets (GNPs) content and the graphene source. GNPs composites show an enhanced wear resistance as compared to monolithic SiC, with maximum improvements of ∼70% for the material containing up to 20 vol.% of GNPs; whereas the friction performance depends on the sliding distance and GNPs content. The analysis of the wear debris by micro-Raman spectroscopy evidenced that the tribological behaviour of the GNPs/SiC materials is linked to the formation of an adhered lubricating and protecting tribofilm. Multilayered graphene fillers participate more actively in the protecting tribofilm than other graphene sources such as reduced graphene oxide or in-situ grown graphene flakes. [ABSTRACT FROM AUTHOR]

Dense silicon carbide (SiC)-based composites containing up to 20 vol.% of graphene nanoplatelets (GNPs) were manufactured via a liquid-phase Spark Plasma Sintering (SPS) process, employing Y 2 O 3 and Al 2 O 3 as sintering aids. The applied uniaxial pressure during SPS produced a preferential orientation of the GNPs, with the ab-plane perpendicular to the pressing axis, thus bringing about high thermal anisotropy. Thermal diffusivity/conductivity were measured as a function of the GNPs content, being enhanced by ∼30% in the perpendicular direction to the SPS pressing axis, whereas a moderate decrease was observed in the parallel direction owing to the much lower intrinsic conductivity of the aligned GNPs in the c-axis, with no significant contribution of the thermal resistance of the GNP-SiC interfaces. A non-monotonical temperature dependence for the thermal conductivity was detected in composites containing <10 vol.% GNPs, explained by the strong p-doping of these SiC ceramics. [ABSTRACT FROM AUTHOR]

Graphene nanoplatelets and reduced graphene oxide (rGO) were selected as fillers to develop reinforced silicon carbide (SiC)/graphene composites. The mechanical properties of the materials were investigated as a function of the type of graphene source and graphene content. Composites containing just 5 vol.% of rGOs exhibited an outstanding mechanical performance, increasing both the fracture toughness by ~ 162%, with a maximum value of 8.3 MPa · m 1/2 , and the strength by ~ 60% (600 MPa) when compared to monolithic SiC. The preferential alignment of the graphene fillers, their dimensions, and the graphene-SiC mechanical interlock are key factors to promote crack shielding mechanisms. [ABSTRACT FROM AUTHOR]

The paper describes the fabrication by liquid-phase spark plasma sintering (SPS) of composites of dense silicon carbide (SiC) with up to 20 vol.% graphene nanoplatelets (GNPs), and discusses the relationships between composition, microstructure and electrical conductivity. The structural integrity of the GNPs is preserved during the whole process, as observed by Raman spectroscopy. The effects of the applied pressure (50 MPa) during SPS result in the preferential orientation of the GNPs perpendicular to the pressing axis and anisotropic electrical behaviour. The electrical conductivity measured in the direction perpendicular to the SPS pressing axis is 4 to 6 times higher than the parallel counterpart. The conductivity increases up to three orders of magnitude with increasing GNPs fraction, reaching values of 4380 S m −1 at room temperature for materials with 20 vol.% GNPs. The conduction mechanism of the composite is analysed as a function of the GNPs content. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) cellular ceramic skeletons based on ceramic pillars of ∼250 μm in diameter and porous micro-channels of ∼700 μm were used as support to grow long and aligned crystalline carboxyl functionalized carbon nanotubes (COx) by floating catalyst chemical vapor deposition. The resulting hybrid COx/SiC material retained water and allowed the flow of oils and gasoline through the specimen, showing, thus, superhydrophobic and superoleophilic properties. In addition, the developed hybrid material exhibited high electrical conductivity with a good ohmic contact at the COx–SiC interface. [ABSTRACT FROM AUTHOR]

Abstract: Hertzian indentation was used to investigate for the first time the elastic–plastic behavior and contact damage evolution with temperature (in the 25–850°C range) in air of different in situ grown graphene/SiC composites. Three SiC starting powders differing in polytypes and particle sizes were liquid-phase densified by spark-plasma sintering, thus producing ceramic composites that contain about 4vol.% of epitaxial grown graphene flakes at the grain boundaries and have interesting differences in their microstructure. The general trend in the contact mechanical behavior is qualitatively similar among the different composites, with the elastic modulus, yield stress, and critical loads for the onset of quasi-plasticity and ring/cone cracking all decreasing with increasing temperature, with the differences at the quantitative level being explained in terms of the particular microstructure of each graphene/SiC composite. [Copyright &y& Elsevier]

The thermal conductivity of hot-pressed Al2O3/SiC platelet composites is determined as a function of the platelet content, from 0 to 30 vol.% of SiC. Existing heat conduction models are employed to discuss the experimental data. Data agree with the presence of an interfacial thermal resistance at the Al2O3/SiC grain boundaries, which precludes the effect of percolation on the thermal conductivity for the higher percentage of SiC platelets. The observed orientation effect on the thermal conductivity due to an alignment of the platelets is also modelled using the Hasselman''s approach. The thermal conductivity of the SiC platelets is calculated from the effective thermal conductivity of the composites. [Copyright &y& Elsevier]