SiC Alloys Research Articles

Preparation of polymer-derived SiOC-Cu ceramic composites and their tribological performance

Cu-containing silicon oxycarbide (SiOC) ceramic composites have been prepared by thermal pyrolysis of polymeric precursors and subsequent spark plasma sintering at 1600 °C. The resulting composites consist of SiO2, SiC, free carbon, and Cu-Si alloys. The microstructure of the composites has been identified via optical microscope and SEM. The mechanical properties, including the Vickers hardness, nano-indentation hardness, and Young’s modulus, of the SiOC composites have been assessed via indentation methods. The effects of Cu content on the tribological properties of the composites have additionally been investigated under different sliding frequencies against Al2O3 spheres at room temperature. It is shown that the introduction of the Cu-Si alloys is effective in reducing the friction coefficient of the material system, which decreases linearly from 0.27 of monolithic SiOC to 0.15 of SiOC with 7 wt% Cu content at 1 Hz. Raman spectroscopy has highlighted the friction-driven evolution of the free carbon phase in the SiOC ceramics. Finally, a correlation between the sliding frequency, Cu-Si content, and tribological properties of SiOC is established.

Multiscale atomistic modelling of CVD: From gas-phase reactions to lattice defects

Growth from vapour/gas/plasma phases is a key process to produce high-quality nanostructures and thin films. The quest for high performances at low cost calls for the development of modelling strategies able to accurately predict growth rates and structure morphology under a variety of process conditions. In the semiconductor nanotechnology, Lattice Kinetic Monte Carlo (LKMC) is considered an advanced approach for simulating selective epitaxy of semiconductors by Chemical Vapor Deposition (CVD). However, state-of-the-art LKMC tools often neglect fundamental aspects such as lattice defects and chemical reactions, both in the vapor phase and around the evolving surface. We present a multiscale workflow for modelling CVD growth and etching processes also accounting for these critical phenomena. We implement it in the open-source KMC super-Lattice (KMCsL) code MulSKIPS , whose peculiar design allows for the generation and evolution of point-like and extended defects in tetrahedrally-bonded materials, such as Si, SiC or SiGe alloys. Gas-phase reactions at the meso-scale are considered by coupling with an external thermodynamic simulator, while surface reactions involving the equilibrium gas species are described by an analytical continuum model. We perform experiments to calibrate and validate the KMCsL model. We then apply the methodology to simulate nanoscale morphology modifications in planar, nanostructured and constrained geometries, unveiling the role of temperature, precursors’ pressures, surface coverage and defects kinetics in the CVD process.

Investigation of Dry Sliding Wear Behavior of Mg-SiC Alloys Produced by Powder Metallurgy Method

In these days, Mg alloys have great promise for weight-saving applications, because of their superior properties over monolithic metals, in the field of automotive and aerospace industries. In this study, the dry sliding wear behaviour of Mg- SiC alloys with different sintering temperature (375 °C, 400 °C, 425 °C) were investigated. Mg with %10 weight SiC powders were mechanically mixed and alloyed and then fabricated successfully via fast heating under vacuumed argon atmosphere. The friction coefficient, hardness value, and wear resistance of the alloys were higher than those of the pure Mg. The increase in the sintering temperature of the Mg-SiC alloys up to 425 °C caused an increase in the friction coefficient and a decrease in the wear mass loss respectively.

A New Strategy for Dissimilar Material Joining between SiC and Al Alloys through Use of High-Si Al Alloys

Joining metals and ceramics plays a crucial role in many engineering applications. The current research aims to develop a simple and convenient approach for dissimilar material joining between SiC and Al alloys. In this work, Al alloys with Si contents varying from 7 wt.% to 50 wt.% were bonded with SiC at a high temperature of 1100 °C by a pressure-less bonding process in a vacuum furnace, and shear tests were carried out to study the bonding strength. When using low-Si Al alloys to bond with SiC, the bonding strength was very low. The bonding strength of Al/SiC joints increased significantly through the use of high-Si Al alloys with 30 wt.% and 50 wt.% Si. The shear strength achieved (28.8 MPa) is far higher than those reported previously. The remarkable improvement in bonding strength is attributed to the suppression of brittle interfacial products and reduced thermal stresses. This research provides a new strategy for joining between SiC and a wide range of Al alloys through the use of high-Si Al alloys as the interlayers.

Si–C alloys with direct band gaps for photoelectric application

In this paper, the novel two Si–C alloys with direct band gap of semiconductor material, namely I-43d SiC and P2/m SiC, are proposed by using density functional theory. To our knowledge, this is the first report that Si–C alloys with a stoichiometric ratio of 1:1 have direct band gaps. The Si–C alloys in the I-43d and P2/m phases have dynamical stability and mechanical stability observed through phonon spectra and elastic constants. The varies of relative enthalpies for I-43d and P2/m SiC with pressure and their stability at 50 GPa and 800 K, suggest the synthesis possibility under high temperatures and pressures. The elastic moduli (B, G, and E) of I-43d and P2/m SiC are slightly larger than the elastic muduli of C2/m-16 SiC and Si3C, C2/m-20 Si3C2 and Si4C. In addition, the distribution of electrons effective mass of I-43d SiC in space is equivalent. I-43d SiC has the smallest maximum electrons effective mass and holes effective mass, and the minimum value of holes effective mass appear in b axis. The two SiC alloys have direct band gaps of 0.99 and 1.44 eV, and exhibit excellent optical properties, indicating the feasibility for optoelectronic devices.

Ultra-reduced phases in ophiolites cannot come from Earth’s mantle

Abstract Several recent papers have purported to find ultra-reduced minerals—as natural examples—within ophiolitic mantle sections, including SiC moissanite, Fe-Si alloys, various metal carbides, nitrides, and borides. All those phases were interpreted to be mantle derived. The phases are recovered from mineral concentrates and are assigned to the deep mantle because microdiamonds and other ultrahigh-pressure (UHP) minerals are also found. Based on these findings, it is claimed that the mantle rocks of ophiolite complexes are rooted in the transition zone (TZ) or even in the lower mantle, at redox states so reduced that phases like SiC moissanite are stable. We challenge this view. We report high-temperature experiments carried out to define the conditions under which SiC can be stable in Earth’s mantle. Mineral separates from a fertile lherzolite xenolith of the Eifel and chromite from the LG-1 seam of the Bushveld complex were reacted with SiC at 1600 K and 0.7 GPa. At high temperature, a redox gradient is quickly established between the silicate/oxide assemblage and SiC, of ~12 log-bar units in fO2. Reactions taking place in this redox gradient allow us to derive a model composition of an ultra-reduced mantle by extrapolating phase compositions to 8 log units below the iron-wüstite equilibrium (IW-8) where SiC should be stable. At IW-8 silicate and oxide phases would be pure MgO end-members. Mantle lithologies at IW-8 would be Fe° metal saturated, would be significantly enriched in SiO2, and all transition elements with the slightest siderophile affinities would be dissolved in a metal phase. Except for the redox-insensitive MgAl2O4 end-member, spinel would be unstable. Relative to an oxidized mantle at the fayalite-magnetite-quartz (FMQ) buffer, an ultra-reduced mantle would be enriched in enstatite by factor 1.5 since the reduction of the fayalite and ferrosilite components releases SiO2. That mantle composition is unlike any natural mantle lithology ever reported in the literature. Phases as reduced as SiC or Fe-Si alloys are unstable in an FeO-bearing, hot, convecting mantle. Based on our results, we advise against questioning existing models of ophiolite genesis because of accessory diamonds and ultra-reduced phases of doubtful origin.

The effect of core composition on iron isotope fractionation between planetary cores and mantles

We have conducted high-pressure, high-temperature isotope exchange experiments between molten silicate and molten Fe–Si–C-alloys to constrain the effect of Si on equilibrium Fe isotope fractionation during planetary core formation. The values of Δ57FeMetal-Silicate at 1850°C and 1 GPa determined by high-resolution MC-ICP-MS in this study range from −0.013±0.054‰ (2SE) to 0.072 ± 0.085‰ with 1.34–8.14 atom % Si in the alloy, respectively. These results, although not definitive on their own, are consistent with previous experimental results from our group and a model in which elements that substitute for Fe atoms in the alloy structure (i.e., Ni, S, and Si) induce a fractionation of Fe isotopes between molten silicate and molten Fe-alloys during planetary differentiation. Using in situ synchrotron X-ray diffraction data for molten Fe-rich alloys from the literature, we propose a model to explain this fractionation behavior in which impurity elements in Fe-alloys cause the nearest neighbor atomic distances to shorten, thereby stiffening metallic bonds and increasing the preference of the alloy for heavy Fe isotopes relative to the silicate melt. This fractionation results in the bulk silicate mantles of the smaller terrestrial planets and asteroids becoming isotopically light relative to chondrites, with an enrichment of heavy Fe isotopes in their cores, consistent with magmatic iron meteorite compositions. Our model predicts a bulk silicate mantle δ57Fe ranging from −0.01‰ to −0.12‰ for the Moon, −0.06‰ to −0.33‰ for Mars, and −0.08‰ to −0.33‰ for Vesta. Independent estimates of the δ57Fe of primitive mantle source regions that account for Fe isotope fractionation during partial melting agree well with these ranges for all three planetary bodies and suggest that Mars and Vesta have cores with impurity (i.e., Ni, S, Si) abundances near the low end of published ranges. Therefore, we favor a model in which core formation results in isotopically light bulk silicate mantles for the Moon, Mars, and Vesta. The processes of magma ocean crystallization, mantle partial melting, and fractional crystallization of mantle-derived melts are all likely to result in heavy Fe isotope enrichment in the melt phase, which can explain why basaltic samples from these planetary bodies have variable δ57Fe values consistently heavier than our bulk mantle estimates. Additionally, we find no clear evidence that Fe isotopes were fractionated to a detectable level by volatile depletion processes during or after planetary accretion, although it cannot be ruled out.

Density Functional Study of the Carbon Dependence of the Structural, Mechanic, Thermodynamic, and Dynamic Properties of SiC Alloys

Using a density functional scheme, for the first time the carbon dependence on the structural, dynamic, thermodynamic, and dynamic properties of hbox {Si}_{1-x}hbox {C}_{x} alloys (x=0.0 to 1.0 in steps of 0.125) has been investigated. The structural properties of these materials, in particular, the composition dependence of the lattice parameter and bulk modulus, are in excellent agreement with experimental data and follow a quadratic law in (x). A nonlinear relationship is found between the elastic constants C_{11}, C_{12}, and C_{44} and the carbon concentration (x). The behavior of the acoustical and optical phonon frequencies at high-symmetry points varGamma , X, and L is predicted. Through the quasi-harmonic Debye model, in which the photonic effects are taken into account, the Debye temperature, the heat capacity, the Helmholtz free energy, the internal energy, and the entropy are determined for the hbox {Si}_{1-x}hbox {C}_{x } compounds.

Influence of structure and thermodynamic stability on electronic properties of two-dimensional SiC, SiGe, and GeC alloys

The energetics and thermodynamic properties of two-dimensional binary graphene-like alloys made from graphene, silicene, or germanene are investigated by combining first-principles total energy calculations, and a statistical approach to account for disorder and composition effects. For the electronic properties the calculations are performed within the GGA-1/2 approach for an approximate quasiparticle bands. We derive lattice constants, first-neighbor distances, and buckling parameters as a function of composition $x$. The ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ system is the only stable random alloy at usual growth temperatures. For ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$, we observe strong distortions of the lattice making the random configurations less favorable and leading to a pronounced tendency for phase separation. The situation for ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ alloys is completely different. An ordered structure with composition $x=0.5$ is stable up to $T\ensuremath{\approx}1000$ K, while intermediate compositions are mainly realized by silicongraphene and graphene or silicene. The ordering and decomposition effects have a strong influence on the average fundamental energy gap versus composition. Whereas large gaps appear for ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ systems they almost vanish for ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ and ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$. Moreover, the dependence of the ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ energy gap on growth temperature is also obtained. The results can be very useful for chemical vapor deposition growth of these materials.

Density of Electron States and Volume of Solution of Hydrogen in SiC Alloys

Density of Electron States and Volume of Solution of Hydrogen in SiC Alloys

Dissimilar joint characteristics of SiC and WC-Co alloy by laser brazing

SiC and WC-Co alloys were joined by laser brazing with an active braze metal. The braze metal based on eutectic Ag-Cu alloy with additional Ti as an active element ranging from 0 to 2.8 mass% was sandwiched by the SiC block and WC-Co alloy plate. The brazing was carried out by selective laser beam irradiation on the WC-Co alloy plate. The content of Ti in the braze metal was required to exceed 0.6 mass% in order to form a brazed joint with a measurable shear strength. The shear strength increased with increasing Ti content up to 2.3 mass%Ti and decreased with a higher content.

Grain refinement and mechanical properties enhancement of AZ91E alloy by addition of ceramic particles

Improved mechanical properties and structural uniformity of Mg-based alloys can be achieved by use of grain-refining additives prior to casting. Ceramic particles of α-Al2O3 and SiC can serve as such additives to refine the microstructure of Mg–Al-based alloys. However, direct introduction of ceramic particles into Mg matrix is limited by the poor wetting of those particles by liquid Mg and their massive agglomeration. Mg/α-Al2O3 and Mg/SiC master alloys were prepared using a method based on the insertion of the ceramic particles into a molten Mg bath through a Mg-nitride layer formed on the surface of the molten bath. The mixture of Mg/ceramic particles was cooled to room temperature under a nitrogen atmosphere. Mg-15%Al2O3 and AZ91E + 10%SiC master alloys were obtained. These master alloys were used to refine AZ91E alloys by introducing various amounts of ceramic particles to manufacture AZ91E + 1%Al2O3, AZ91E + 1%SiC, and AZ91E + 3%SiC alloys. These were cast using high-pressure die casting and gravity die casting. The alloy AZ91E + 1%Al2O3 was grain refined to ~20 μm and the alloys AZ91E + SiC were grain refined to ~50 μm as against 110 μm in non-refined counterparts. The mechanical properties of the modified alloys are substantially better than those of a non-refined AZ91E alloy which is the result of a combination of grain refinement and reinforcement of the matrix by ceramic particles. Alloy AZ91E + 1%Al2O3 exhibited the best mechanical properties.

Ni60-SiC coating prepared by plasma spraying, plasma re-melting and plasma spray welding on surface of hot forging die

In order to produce the hear-resistant inner layer of hot-forging die, plasma spraying and plasma re-melting and plasma spray welding were adopted. Substrate material was W6Mo5Cr4V2, including 10%, 20%, 30% SiC ceramic powder used as coating material to obtain different Ni-based SiC alloys coating. Micro-structure and micro-hardness analysis of the coating layer were followed, as well as thermophysical properties for the coating layer were measured. The experimental results show that the coating prepared with 70% Ni60, 30% SiC powder has best properties with plasma spray welding, in which the micro-hardness can achieve 1100 HV, meanwhile can improve the thermal property of hot-forging die dramatically.

Characterization of electrodeposited Ni–Fe–SiC alloys for microelectromechanical applications

Ni–Fe–SiC alloy is a promising material for the fabrication of microactuators. In this article, the electrolytic codeposition technique is used to deposit the Ni–Fe–SiC composite onto stainless-steel substrates, where nickel becomes alloyed with iron as the binder phase, and SiC becomes alloyed as dispersed particles. Analysis of the morphology indicates that the deposited SiC nanoparticles are compact, with the orientation of the deposited crystal planes indexed as (111), (200), (220), (311), and (222). The resistivity of the deposited SiC nanoparticles is about 30×10−8 Ω m. When the loading of Fe (wt %) ranges from 10% to 50% in the deposit, the electrodeposit shows a strong paramagnetism with a lowest value of coercivity of 2.75×10−2 A/m. In addition, the remanence shows a monotonic decrease with an increasing iron content in the deposit. It is demonstrated that the electroformed Ni–Fe–SiC alloy has better electromagnetic properties and a higher corrosion resistance (with a corrosion rate of 0.17 mg/dm2 h 2M HCl) than the electroformed Ni–Fe alloy (with a corrosion rate of 0.23 mg/dm2 h).

Highly Conductive p-Type Silicon Carbon Alloys Deposited by Hot-Wire Chemical Vapor Deposition

P-type microcrystalline silicon carbide (µc-SiC:H) alloys for application as a window layer in silicon based thin film solar cells were grown by hot-wire chemical vapor deposition using hydrogen diluted monomethylsilane and trimethylaluminum. Conductivities up to 0.1 S/cm were obtained for p-type material. The optical properties were studied by photothermal deflection spectroscopy. At photon energies below 1.25 eV, both free carrier and defect absorption lead to a high absorption coefficient. For photon energies >2.0 eV, the absorption coefficient is affected by the crystallinity and the structural composition. The structure of Al-doped µc-SiC:H thin films were investigated by infrared and Raman spectroscopy. It was found that increase of the deposition pressure can compensate for the loss of crystallinity caused by Al-doping. At high deposition pressure (>100 Pa), increasing contributions of hexagonal SiC alloys, and separated carbon phases are observed.

Aluminum doped silicon‐carbon alloys prepared by hot wire chemical vapor deposition

AbstractAluminum doped stoichiometric microcrystalline silicon carbide (µc‐SiC:H) thin films deposited by Hot Wire Chemical Vapor Deposition (HWCVD) technique have been studied. Without Al‐doping, highly crystalline µc‐SiC:H thin films prepared from monomethysilane (MMS) and hydrogen are mainly cubic polytype and are unintentionally doped into n‐type. Trimethyaluminum (TMAl) is used as a source of Aluminum. By increasing the TMAl/MMS ratio (rdoping), the dark conductivity of µc‐SiC:H thin film firstly decreases and then increases. This variation indicates the compensation from n‐type to p‐type, which is confirmed by the thermo‐power measurements. It is observed in the infrared spectra that the crystallinity of µc‐SiC:H thin film deceases with increasing rdoping. In order to improve the crystallinity of this conductive p‐type µc‐SiC:H thin film, the effect of deposition pressure (pdepo) has been investigated. With increasing pdepo, the crystallinity is improved and the structure of SiC alloys changes according to the Raman spectra. A maximum dark conductivity of 1.3×10–1 S/cm has been achieved at rdoping = 0.0620 and pdepo = 100 Pa. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Full-Band Monte Carlo investigation of hole mobilities in SiGe, SiC and SiGeC alloys

Hole transport properties in relaxed and biaxially strained Si 1- x Ge x , Si 1- y C y and Si 1- x- y Ge x C y alloys are investigated using a Full-Band Monte Carlo simulator. Our results allow the extraction of bulk, in-plane and out-of-plane hole drift mobilities. Doping effects in SiGeC alloys are taken into account through an efficient ionized-impurity scattering model. This model is based on inverse momentum relaxation times derived from phase-shift theory. 1 1 Also known as partial-wave theory. A new alloy scattering model relevant to the case of random ternary alloys is presented. It involves two effective alloy potential parameters, which account for the respective scattering strengths of Ge and C in the crystal lattice. From our mobility results we have derived an analytical hole mobility model which includes dependencies upon doping concentration, doping type, Ge content and C content.

Sublimation growth of aluminum nitride on silicon carbide substrate with aluminum nitride–silicon carbide alloy transition layer

The advantages of depositing AlN–SiC alloy transition layers on SiC substrates before the seeded growth of bulk AlN crystals were examined. The presence of AlN–SiC alloy layers helped to suppress the SiC decomposition by providing vapor sources of silicon and carbon. In addition, cracks in the final AlN crystals decreased from ∼5 × 106/mm2 for those grown directly on SiC substrates to less than 1 × 106/mm2 for those grown on AlN–SiC alloy layers because of the intermediate lattice constants and thermal expansion coefficient of AlN–SiC. X-ray diffraction confirmed the formation of pure single-crystalline AlN upon both AlN–SiC alloys and SiC substrates. X-ray topography (XRT) demonstrated that strains present in the AlN crystals decreased as the AlN grew thicker. However, the XRT for AlN crystals grown directly on SiC substrates was significantly distorted with a high overall defect density compared to those grown on AlN–SiC alloys.

UV and blue light emission from SiC nanoclusters in annealed amorphous SiC alloys

Photoluminescence characteristics of hydrogenated amorphous SiC thin films prepared with various gas ratio R=[CH4]/[SiH4] were investigated. Red light emission (600–800nm) was observed at room temperature from as-deposited samples. The changes of chemical bonding configurations and structures of amorphous SiC alloys were monitored before and after thermal annealing at 500°C. An intense UV and blue light emission (300–500nm) can be obtained from annealed SiC films prepared with R>10. Raman and FT-IR absorption measurements demonstrated the formation and enhancement of 4H-SiC clusters in annealed films which may results in the UV and blue light emission from annealed SiC samples.

Liquid metal compatibility issues for test blanket modules

Liquid metal compatibility issues are being investigated for two different test blanket modules involving either Li or Pb–17 at.%Li. A solution to the magnetohydrodynamic (MHD) problem for the V–Li concept may be attainable using multi-layer coatings or a flow channel insert with vanadium in contact with the flowing Li instead of a ceramic insulating layer. These strategies rely on the expected excellent compatibility of vanadium alloys which is being further investigated. For systems using Pb–17Li, capsule testing of SiC and various alloys is being conducted. Monolithic SiC specimens exposed for 1000 h in Pb–Li at 800 and 1100 °C showed no mass change after cleaning and no detectable increase in the Si content of the Pb–Li after the test. In order to investigate the behavior of corrosion resistant aluminide coatings, initial capsule testing at 700 °C has been used to establish baseline dissolution rates for 316 stainless steel, FeCrAl, Fe 3Al and NiAl. The samples containing Al showed significantly less mass loss than 316 stainless steel, suggesting that aluminide coatings will be beneficial in this temperature range.