Effect of Carbon Addition and Mixture Method on the Microstructure and Mechanical Properties of Silicon Carbide.
ABSTRACT: High dense (>99% density) SiC ceramics were produced with addition of C and B4C by spark plasma sintering method at 1950 °C under 50 MPa applied pressure for 5 min. To remove the oxygen from the SiC, it was essential to add C. Two different mixture method were used, dry mixing (specktromill) and wet mixing (ball milling). The effect of different levels of carbon additive and mixture method on density, microstructure, elastic modulus, polytype of SiC, Vickers hardness, and fracture toughness were examined. Precisely, 1.5 wt.% C addition was sufficient to remove oxide layer from SiC and improve the properties of dense SiC ceramics. The highest hardness and elastic modulus values were 27.96 and 450 GPa, respectively. Results showed that the 4H polytype caused large elongated grains, while the 6H polytype caused small coaxial grains. It has been observed that it was important to remove oxygen to achieve high density and improve properties of SiC. Other key factor was to include sufficient amount of carbon to remove oxide layer. The results showed that excess carbon prevented to achieve high density with high elastic modulus and hardness.
Project description:SiBCN ceramics were introduced into porous Si₃N₄ ceramics via a low-pressure chemical vapor deposition and infiltration (LPCVD/CVI) technique, and then the composite ceramics were heat-treated from 1400 °C to 1700 °C in a N₂ atmosphere. The effects of annealing temperatures on microstructure, phase evolution, dielectric properties of SiBCN ceramics were investigated. The results revealed that α-Si₃N₄ and free carbon were separated below 1700 °C, and then SiC grains formed in the SiBCN ceramic matrix after annealing at 1700 °C through a phase-reaction between free carbon and α-Si₃N₄. The average dielectric loss of composites increased from 0 to 0.03 due to the formation of dispersive SiC grains and the increase of grain boundaries.
Project description:I 4 ¯ -carbon was first proposed by Zhang et al., this paper will report regarding this phase of carbon. The present paper reports the structural and elastic properties of the three-dimensional carbon allotrope I 4 ¯ -carbon using first-principles density functional theory. The related enthalpy, elastic constants, and phonon spectra confirm that the newly-predicted I 4 ¯ -carbon is thermodynamically, mechanically, and dynamically stable. The calculated mechanical properties indicate that I 4 ¯ -carbon has a larger bulk modulus (393 GPa), shear modulus (421 GPa), Young's modulus (931 GPa), and hardness (55.5 GPa), all of which are all slightly larger than those of c-BN. The present results indicate that I 4 ¯ -carbon is a superhard material and an indirect-band-gap semiconductor. Moreover, I 4 ¯ -carbon shows a smaller elastic anisotropy in its linear bulk modulus, shear anisotropic factors, universal anisotropic index, and Young's modulus.
Project description:A systematic investigation of structural, mechanical, anisotropic, and electronic properties of SiC₂ and SiC₄ at ambient pressure using the density functional theory with generalized gradient approximation is reported in this work. Mechanical properties, i.e., the elastic constants and elastic modulus, have been successfully obtained. The anisotropy calculations show that SiC₂ and SiC₄ are both anisotropic materials. The features in the electronic band structures of SiC₂ and SiC₄ are analyzed in detail. The biggest difference between SiC₂ and SiC₄ lies in the universal elastic anisotropy index and band gap. SiC₂ has a small universal elastic anisotropy index value of 0.07, while SiC₂ has a much larger universal elastic anisotropy index value of 0.21, indicating its considerable anisotropy compared with SiC₂. Electronic structures of SiC₂ and SiC₄ are calculated by using hybrid functional HSE06. The calculated results show that SiC₂ is an indirect band gap semiconductor, while SiC₄ is a quasi-direct band gap semiconductor.
Project description:We investigated the high-P,T annealing and mechanical properties of nanocomposite materials with a highly nitrided bulk composition close to Ti₃N₄. Amorphous solids were precipitated from solution by ammonolysis of metal dialkylamide precursors followed by heating at 400-700 °C in flowing NH₃ to produce reddish-brown amorphous/nanocrystalline materials. The precursors were then densified at 2 GPa and 200-700 °C to form monolithic ceramics. There was no evidence for N₂ loss during the high-P,T treatment. Micro- and nanoindentation experiments indicate hardness values between 4-20 GPa for loads ranging between 0.005-3 N. Young's modulus values were measured to lie in the range 200-650 GPa. Palmqvist cracks determined from microindentation experiments indicate fracture toughness values between 2-4 MPa·m1/2 similar to Si₃N₄, SiC and Al₂O₃. Significant variations in the hardness may be associated with the distribution of amorphous/crystalline regions and the very fine grained nature (~3 nm grain sizes) of the crystalline component in these materials.
Project description:Mechanical properties of three different coatings comprising of pure silicon, 36 mol% HfO2-doped silicon and 60 mol% HfO2-doped silicon on SiC substrate material were obtained by nanoindentation. The coatings aim at oxidation protective layers in environmental barrier coating systems for SiC/SiC ceramic matrix composites (CMC). The examined coatings were produced by physical vapour deposition (magnetron sputtering) and have been tested under cycling conditions between room temperature and 1523?K until 100?h accumulated hot time. In order to measure the hardness and the reduced Young's modulus, a Berkovich tip has been used with a constant depth modulus of 100?nm and 160?nm. Two depth moduli have been chosen to investigate a possible volume impact on the mechanical properties. Six successfully produced indents have been averaged to examine the hardness and reduced Young's modulus. The testing has been done on polished cross sections thereby minimising the impact of the substrate material on the measured values. This article provides data related to "Hafnia-doped Silicon Bond Coats manufactured by PVD for SiC/SiC CMCs".
Project description:It would be desirable to remove volatile organic compounds (VOCs) while we eliminate the dusts using silicon carbide (SiC)-based porous ceramics from the hot gases. Aiming at functionalizing SiC-based porous ceramics with catalytic capability, we herein report a facile strategy to integrate high efficient catalysts into the porous SiC substrates for the VOC removal. We demonstrate an aqueous salt method for uniformly distributing fine platinum (Pt) particles on the alumina (Al2O3) layers, which are pre-coated on the SiC substrates as supports for VOC catalysts. We confirm that at a Pt mass loading as low as 0.176% and a weight hourly space velocity of 6000?mL?g-1 h-1, the as-prepared Pt/SiC@Al2O3 catalysts can convert 90% benzene at a temperature of ca. 215?°C. The results suggest a promising way to design ceramics-based bi-functional materials for simultaneously eliminating dusts and harmful VOCs from various hot gases.
Project description:In view of the difficulty in obtaining the mechanical properties of shale, the multiscale analysis of shale was performed on a shale outcrop from the Silurian Longmaxi Formation in the Changning area, Sichuan Basin, China. The nano-/micro-indentation test is an effective method for multiscale mechanical analysis. In this paper, effective criteria for the shale indentation test were evaluated. The elastic modulus was evaluated at a multiscale and the engineering validation of drilling cuttings was performed. The porosity tests showed that the pore distribution of shale from the nanoscale to macro-pore could be better displayed by the nuclear magnetic resonance test. The micro-scale elastic modulus and hardness increased nonlinearly with the increase in the clay packing density. It was observed that the size effect of the micro-hardness was based on porosity and composition. The partial spalling of shale at the micro-scale could lead to irregular bulges or steps in a load-displacement curve. The elastic modulus of pure clay minerals was 24.2 GPa on the parallel bedding plane and 15.8 GPa on the vertical bedding plane. The contact hardness (pure clay minerals) was 0.51 GPa. The indentation results showed that the micro-elastic modulus of shale obeyed the normal distribution, and the statistical average could predict the macro-mechanical properties effectively. The present work can provide a new way to recognize the mechanical behaviour of shale.
Project description:Carbon fiber-reinforced multi-layered pyrocarbon-silicon carbide matrix (C/C-SiC) composites are widely used in aerospace structures. The complicated spatial architecture and material heterogeneity of C/C-SiC composites constitute the challenge for tailoring their properties. Thus, discovering the intrinsic relations between the properties and the microstructures and sequentially optimizing the microstructures to obtain composites with the best performances becomes the key for practical applications. The objective of this work is to optimize the thermal-elastic properties of unidirectional C/C-SiC composites by controlling the multi-layered matrix thicknesses. A hybrid approach based on micromechanical modeling and back propagation (BP) neural network is proposed to predict the thermal-elastic properties of composites. Then, a particle swarm optimization (PSO) algorithm is interfaced with this hybrid model to achieve the optimal design for minimizing the coefficient of thermal expansion (CTE) of composites with the constraint of elastic modulus. Numerical examples demonstrate the effectiveness of the proposed hybrid model and optimization method.
Project description:In situ grown C0.3N0.7Ti and SiC, which derived from non-oxide additives Ti3SiC2, are proposed to densify silicon nitride (Si3N4) ceramics with enhanced mechanical performance via hot-press sintering. Remarkable increase of density from 79.20% to 95.48% could be achieved for Si3N4 ceramics with 5 vol.% Ti3SiC2 when sintered at 1600 °C. As expected, higher sintering temperature 1700 °C could further promote densification of Si3N4 ceramics filled with Ti3SiC2. The capillarity of decomposed Si from Ti3SiC2, and in situ reaction between nonstoichiometric TiCx and Si3N4 were believed to be responsible for densification of Si3N4 ceramics. An obvious enhancement of flexural strength and fracture toughness for Si3N4 with x vol.% Ti3SiC2 (x = 1~20) ceramics was observed. The maximum flexural strength of 795 MPa for Si3N4 composites with 5 vol.% Ti3SiC2 and maximum fracture toughness of 6.97 MPa·m1/2 for Si3N4 composites with 20 vol.% Ti3SiC2 are achieved via hot-press sintering at 1700 °C. Pull out of elongated Si3N4 grains, crack bridging, crack branching and crack deflection were demonstrated to dominate enhance fracture toughness of Si3N4 composites.
Project description:We systematically studied the physical properties of a novel superhard (t-C₃N₄) and a novel hard (m-C₃N₄) C₃N₄ allotrope. Detailed theoretical studies of the structural properties, elastic properties, density of states, and mechanical properties of these two C₃N₄ phases were carried out using first-principles calculations. The calculated elastic constants and the hardness revealed that t-C₃N₄ is ultra-incompressible and superhard, with a high bulk modulus of 375 GPa and a high hardness of 80 GPa. m-C₃N₄ and t-C₃N₄ both exhibit large anisotropy with respect to Poisson's ratio, shear modulus, and Young's modulus. Moreover, m-C₃N₄ is a quasi-direct-bandgap semiconductor, with a band gap of 4.522 eV, and t-C₃N₄ is also a quasi-direct-band-gap semiconductor, with a band gap of 4.210 eV, with the HSE06 functional.