Formation Of SiC Phase Research Articles

EFFECT OF LASER POST-TREATMENT ON MICROSTRUCTURAL AND SLIDING WEAR BEHAVIOR OF HVOF-SPRAYED NiCrC AND NiCrSi COATINGS

In this study, NiCrC and NiCrSi coatings are deposited on the MDN 310 steel using High-Velocity Oxy-Fuel (HVOF) process. Laser Surface Melting (LSM) post-heat treatment is carried out on as-sprayed coatings using Laser Engineered Net Shaping (LENSTM) with a power of 300[Formula: see text]W. The characteristics of both coatings in terms of mechanical and metallurgical properties have been investigated. The thicknesses of the as-sprayed NiCrC and NiCrSi coatings are in the range of 170–200[Formula: see text][Formula: see text]m. Laser-treated NiCrC and NiCrSi coatings exhibit a thickness range of 162–185[Formula: see text][Formula: see text]m, respectively. The microstructure of laser-treated NiCrC-300W coating clearly shows a dendrite-like structure, whereas the laser-treated NiCrSi coating exhibits hard layer and columnar homogeneity. Microhardness of as-sprayed NiCrC coating is [Formula: see text] [Formula: see text] and that of NiCrSi coating is [Formula: see text] HV[Formula: see text]. Microhardness of laser-treated NiCrC coating is [Formula: see text] HV[Formula: see text] and that of NiCrSi coating is [Formula: see text] HV[Formula: see text]. Dry sliding wear tests are conducted at room temperature (RT) and 400∘C with 10-N and 20-N loads. The wear rates at 400∘C temperature of laser-treated NiCrC and NiCrSi coatings produced are slightly below [Formula: see text][Formula: see text]mm3/m and [Formula: see text][Formula: see text]mm3/m, respectively. Laser-treated coatings produced better dry sliding wear behavior compared with as-sprayed coatings owing to dense microstructure. Formation of SiC phase in NiCrSi coating imparts high wear and frictional resistance compared to the NiCrC coating.

Structure and porosity of silicon oxycarbide/carbon black composites

This study describes the influence of carbon black (Cblack) on structural evolution and porosity of silicon oxycarbide (SiOC)-based ceramics obtained from pyrolysis of silicone-derived polymeric precursors. Initially, polymeric precursors were synthesized by hydrosilylation reaction between poly(methylhydrosiloxane) (PHMS) and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (D4Vi), with different Cblack contents (0, 1, 3, 5 and 10 wt%), followed by pyrolysis under argon at 1000 and 1500 °C, to give rise to the respective SiOC/Cblack ceramic composites. FTIR-ATR, TG, XRD, N2 gas physisorption at 77 K were used to characterize the carbonaceous phase, preceramic polymers and resulting ceramics. Cblack incorporation, together with the pyrolysis temperature, generated more porous ceramic materials and intensified the semiconducting SiC phase formation, maintaining the presence of conducting Cgraphitic structures embedded into SiOC matrices. Results concerning structural and textural features were associated to the different distributions of Cblack into polymeric precursors. Moreover, the availability of residual carbon to react with degradation products during pyrolysis and produce SiC and Cgraphitic crystalline phases was reported. Cblack immobilization into preceramic Si-containing polymers plays an important role to produce SiOC/Cblack ceramic composites with predominance of electroactive phases. The presence of Cblack also contributed to higher porosities on resulting composites, which are suitable features for electrochemical investigations in polymer-derived ceramic matrices.

Improvement on electrical conductivity and electron field emission properties of Au-ion implanted ultrananocrystalline diamond films by using Au-Si eutectic substrates

In the present work, Au-Si eutectic layer was used to enhance the electrical conductivity/electron field emission (EFE) properties of Au-ion implanted ultrananocrystalline diamond (Au-UNCD) films grown on Si substrates. The electrical conductivity was improved to a value of 230 (Ω cm)−1, and the EFE properties was enhanced reporting a low turn-on field of 2.1 V/μm with high EFE current density of 5.3 mA/cm2 (at an applied field of 4.9 V/μm) for the Au-UNCD films. The formation of SiC phase circumvents the formation of amorphous carbon prior to the nucleation of diamond on Si substrates. Consequently, the electron transport efficiency of the UNCD-to-Si interface increases, thereby improving the conductivity as well as the EFE properties. Moreover, the salient feature of these processes is that the sputtering deposition of Au-coating for preparing the Au-Si interlayer, the microwave plasma enhanced chemical vapor deposition process for growing the UNCD films, and the Au-ion implantation process for inducing the nanographitic phases are standard thin film preparation techniques, which are simple, robust, and easily scalable. The availability of these highly conducting UNCD films with superior EFE characteristics may open up a pathway for the development of high-definition flat panel displays and plasma devices.

Strengthening of Al2O3-C slide gate plate refractories with expanded graphite

Expanded graphite, fabricated by microwave irradiation of acidified flaky graphite, was used to partially replace graphite and carbon black in a commercial Al2O3-C refractory slide gate plate composition. The developed refractories exhibited enhanced cold and hot strength, as well as oxidation resistance. X-ray diffraction indicated the formation of SiC phase even at temperatures lower than the melting point of Si. Field emission scanning electron microscopy exhibited homogeneous formation of three dimensionally oriented nanostructured SiC whiskers in the samples fabricated with expanded graphite. The formation of the thicker primary whiskers and thinner secondary whiskers in the inter-aggregate space bridging two particles was observed frequently for the expanded graphite fortified samples. Whisker reinforcement of this nature has rarely been observed in refractory systems, which is credited for a considerable improvement in the hot modulus of rupture of the new compositions.

Photoluminescence and structural properties of Si/SiC core–shell nanowires growth by HWCVD

Si/SiC core–shell nanowires grown by hot-wire chemical vapor deposition were studied. Ni nanoparticles act as metal nano-templates to catalyze the growth of these core–shell nanowires. These nanowires were grown at different deposition pressures of 0.5 and 1mbar. The nanowires showed a tapered-like morphology at deposition pressure 0.5mbar. Increase in pressure to 1mbar leads to a formation of agglomerated grains at the root of the nanowires. The results show that these nanowires consisted of crystalline Si core and amorphous SiC shell nanowires. Increase in pressure enhanced the formation of SiC phase in the shell of the nanowires. On the other hand, the formation of the agglomerated grains attributed to an increasing of the SiC phase at higher deposition pressure. The presence of Si and SiC nano-crystallites embedded within an amorphous matrix exhibited a room temperature PL emission in the range of 400–1000nm. The effects of the deposition pressure on the optical and structural properties of the nanowires are also discussed.

Microstructures and mechanical properties of Al 2O 3–C refractories with silicon additive using different carbon sources

Graphite flake and carbon black were selected as carbon source in Al 2O 3–C refractories, respectively. The influences of carbon sources on the morphologies of in situ formed SiC and mechanical properties of Al 2O 3–C refractories were investigated in this work. The results showed that SiC of different morphologies can be produced via in situ reaction between silicon powder and different carbon sources. The spherical SiC particles were synthesized from carbon black, while SiC whiskers were produced from both graphite flake and derivative carbon pyrolyzed from phenolic resin. Meanwhile, the mechanical properties of Al 2O 3–C refractories were related to the formation of SiC phase in the matrix. At 1000 °C there is no obvious difference in mechanical properties of the two kind carbon source containing refractories, for they are bonded only by carbon derived from phenolic resin. With increase in temperature, ceramic bond also forms in Al 2O 3–C refractories gradually owing to the formation of SiC phase in matrix. Larger displacement and greater force can be observed in graphite flake containing Al 2O 3–C refractories than that in carbon black containing refractories, which is attributed to the formation of much more curved SiC whiskers and the sliding mechanism between graphite layers in the former refractories at higher temperatures.

Formation and analysis of SiC coating layer on carbon short fiber

AbstractSiC‐coated carbon fibers for aluminum–carbon composites were synthesized with carbothermal reduction. For the SiC layer formation on the surface of carbon fiber, silica sol was coated and heat treated at a temperature of 1460 and 1500 °C. SiC layer formation by carbothermal reduction complete at above 1500 °C for 1 h. From TGA in atmosphere, SiC coating effectively protected against the oxidation of graphite. The thickness of the SiC coating layer ranged from 150 to 300 nm and the layer was formed homogenously along the surface of carbon fiber. As the first step of reaction for SiC formation, it is generally accepted that SiO is formed and vaporized in interface between fiber and silica. It is analogized in this work that the SiO vapor reacts directly with carbon fiber, resulting in formation of SiC phase. From the TEM results, the SiC layer was confirmed as a ZnS‐type FCC structure. Copyright © 2010 John Wiley & Sons, Ltd.

Near coalescent submicron polycrystalline diamond films deposited on silicon: Hydrogen bonding and thermal enhanced carbide formation

The influence of high temperature annealing up to 1200 °C in vacuum on ∼100 nm nearly continuous thick diamond films consisting of 30–50 nm crystallites, deposited onto silicon substrates is reported. The hydrogen bonding and phase composition of the films were studied with Raman spectroscopy, while the surface microstructure and composition were studied with high resolution scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS), respectively. Annealing to 800–900 °C of ∼100 nm thick films results in a decrease in the intensities of the peaks associated with hydrogen bonding (Raman), as well as changes to the morphological microstructure at the film surface. Heating the films to 1000 °C resulted in the complete disappearance of the Raman peaks associated with hydrogen bonding at grain boundaries, and an increase in the relative intensity of the diamond peak relative to the graphite-related D and G Raman peaks, concomitant with changes to the microstructure (SEM and TEM). Ex situ XP analysis of the films annealed to 800 and 1000 °C provides clear evidence for the formation of SiC on the films surface and near surface region. However a sharp SiC Raman peak at 796 cm−1 appears only after annealing to 1200 °C and it is concomitant with a decrease in the Raman peaks associated with sp2 bonded carbon. Our results suggest that formation of SiC phase preferentially consumes sp2/sp hybridized carbon matrix, produced by thermal desorption of hydrogen atoms at diamond grain boundary and at the diamond film—silicon substrate interface.

Thin films by metal-organic precursor plasma spray

While most plasma spray routes to coatings utilize solids as the precursor feedstock, metal-organic precursor plasma spray (MOPPS) is an area that the authors have investigated recently as a novel route to thin film materials. Very thin films are possible via MOPPS and the technology offers the possibility of forming graded structures by metering the liquid feed. The current work employs metal-organic compounds that are liquids at standard temperature-pressure conditions. In addition, these complexes contain chemical functionality that allows straightforward thermolytic transformation to targeted phases of interest. Toward that end, aluminum 3,5-heptanedionate (Al(hd)3), triethylsilane (HSi(C2H5)3 or HSiEt3), and titanium tetrakisdiethylamide (Ti(N(C2H5)2)4 or Ti(NEt2)4) were employed as precursors to aluminum oxide, silicon carbide, and titanium nitride, respectively. In all instances, the liquids contain metal-heteroatom bonds envisioned to provide atomic concentrations of the appropriate reagents at the film growth surface, thus promoting phase formation (e.g., Si–C bond in triethylsilane, Ti–N bond in titanium amide, etc.). Films were deposited using a Sulzer Metco TriplexPro-200 plasma spray system under various experimental conditions using design of experiment principles. Film compositions were analyzed by glazing incidence x-ray diffraction and elemental determination by x-ray spectroscopy. MOPPS films from HSiEt3 showed the formation of SiC phase but Al(hd)3-derived films were amorphous. The Ti(NEt2)4 precursor gave MOPPS films that appear to consist of nanosized splats of TiOCN with spheres of TiO2 anatase. While all films in this study suffered from poor adhesion, it is anticipated that the use of heated substrates will aid in the formation of dense, adherent films.

Effect of microstructure on the mechanical properties of carbon nanofilms deposited on the Si(100) at high temperature under ultra high vacuum

Carbon nanofilms were deposited on the Si(100) wafers at substrate temperatures from room temperature (RT) to 700 °C by ultra-high-vacuum ion beam sputtering. The effect of substrate temperature on the evolution of microstructure and mechanical properties of carbon nanofilms was investigated using Raman spectra, X-ray diffraction, scanning electron microscopy (SEM) and nanoindentation. The correlation between microstructure and mechanical properties was discussed. Raman results showed that I D/ I G ratio increased with substrate temperature from RT to 500 °C due to graphitization effect with more sp 2 bonds and then decreased with temperature from 500 to 700 °C owing to the new SiC phase formation. In addition, the C and Si reaction was found at 600 °C with an asymmetrical Raman shift wavenumbers at 900–1050 cm − 1 and finished at 700 °C with the enhanced asymmetrical band as well as no C peak. A nanoweb-like morphology of the crystalline SiC formation was also observed on the surface of film. The evolution of both hardness and Young's modulus of films had the similar trend as the microstructure. They were 15.01 GPa and 185.11 GPa at RT, respectively, and decreased with substrate temperature to 4.50 GPa and 60.53 GPa at 500 °C, and then increased with temperature to 32.53 GPa and 200.82 GPa at 700 °C. The much enhanced hardness and Young's modulus at 700 °C are attributed to the SiC formation as well as nanoweb-like morphology.

Solid State Sintering of SiC-Ceramics

The most interesting feature in silicon carbide is the structure-property relation where the formation of different types of microstructure due to different structural modifications (polytypism) and grain-boundary/interfacial phase chemistry dictate the final properties of the monoliths. Since synthesis of SiC in last century, several methods such as hot pressing with a sintering aid (B, C), pressureless sintering with a sintering aid (B, C, Al) and reaction bonded (Si-SiC) were used to fabricate dense SiC. A newer method of fast sintering (spark plasma sintering) using pulsed current is also employed to consolidate nano/submicron size SiC with or without additives. The solid state sintered SiC materials have fine-grained equiaxed microstructure (grain size 1 to 4 µm) with thin layer of intergranular phases (amorphous film), exhibit moderate high-temperature creep and oxidation resistance, fracture toughness (3 to 4 MPam1/2) and have highly flaw-sensitive strength at room temperature. The high temperature mechanical properties are highly influenced by the presence of free C, Al and B + C containing grain-boundary phases. Moreover, during prolong processing, abnormal grain growth occurs resulting in anisotropic -SiC phase formation. The Si-SiC materials are poor candidates for high-temperature applications due to the limit set by the melting point of silicon, and the limitations of hot pressing (HPSiC) as a densification technique are well known. SPSed SiC without sintering additive revealed inferior mechanical properties attributed to poor bonding between adjacent grains. In the present survey, an overview of the new developments in silicon carbide processing and properties will be presented together with the information on structure-properties correlationship. Information on the structure of the grain-boundary/secondary phases and interfaces until now was not comprehensively analyzed.

Microstructure and mechanical characteristics of nanodiamond-SiC compacts

Superhard nanodiamond-SiC ceramics are prepared by infiltrating liquid Si into porous nanodiamond compacts under pressure. Synthesized samples are 2.2 mm thick and 3–4 mm in diameter. The effect of particle size of dynamically synthesized nanodiamond powders on silicon infiltration and SiC phase formation is studied. It is established that silicon does not penetrate into the pores of nanodiamond powders if the original particle size is smaller than 0.5–1.0 µm. The critical pore size for infiltration is 100–200 nm. A study of the microstructure of the samples showed the presence of the nanometer-and submicron-scale SiC phase. The ultrasound velocities are measured in the prepared compacts, and the elastic moduli are calculated.

Conversion of Si(100) to Si1-x-yGexCy Alloy by Hydrogen Plasma Containing Ge and C Species

The conversion of a crystal Si(100) surface to a Si1-x-yGexCy alloy layer by reaction with hydrogen plasma containing Ge and C species is studied at temperatures from 600°C to 900°C. At temperatures below 685°C, the Ge1-xCx layers are deposited and phase separation growth between Ge and C is observed. The phase separation results in the growth of crystalline Ge particles and amorphous C whiskers. At temperatures above 700°C, carbon layers are formed on the surface of Si(100) crystal. In a narrow temperature range of 685–700°C, the surface of the Si(100) crystal is converted into a smooth Si0.12Ge0.08C0.80 alloy layer in which no phase separation occurs. There are interdiffusions of atoms between the alloy layer and the Si crystal. The diffusion of Si atoms into the alloy layer leads to the formation of an amorphous SiC phase, which stops the phase separation growth and results in a SiC-stabilized Si1-x-yGexCy alloy layer with smooth morphology. The formation of the SiC phase results in a driving force for diffusion of Si atoms. On the other hand, the Ge and C atoms diffuse into the Si crystal during the conversion, and incorporate into substitutional sites. The atom interdiffusions may lattice match the Si1-x-yGexCy layer to the Si(100) crystal.

Ion beam induced reaction of carbon films on Si(1 0 0)

The irradiation effects of 2 MeV He + and Ar + ions on the film structure of the C–Si system were investigated with RHEED and XPS. The formation of SiC phase and/or the growth of epitaxial SiC were possible by He + irradiation for the carbon films up to 0.7 nm in thickness, which was thinner than that by Ar + irradiation. The He + irradiation could not grow the turbostratic graphite which could be grown by Ar + irradiation. The mechanism of the formation and the epitaxial growth of SiC by ion irradiation was discussed from the view point of the energy transfer from the irradiated ions.

Ion-beam-induced crystallization of carbon-implanted silicon studied by auger electron spectroscopy

The influence of high-energy Si+ irradiation on carbon-implanted silicon at low temperature was studied. C+ Ions of kinetic energy 25 keV were implanted into Si(100) at room temperature with a dose density of 1×1017 ions cm−2 after amorphization of the surface region by Ge+ implantation at 200 keV. Subsequently, the amorphous and substoichiometric silicon–carbon matrix was subjected to a bombardment with 300 keV Si+ at 400°C to generate ion-beam-induced epitaxial crystallization. Rutherford backscattering spectroscopy combined with channelling disclosed the migration of the recrystallization front from the underlying amorphous silicon substrate up to the carbon-rich layer in dependence on the Si+ dose. Auger electron spectroscopy (AES) in combination with rotational sputter depth profiling provided detailed information about the depth-dependent composition, the different chemical states of the elements and the synthesized phases. The electron spectra reveal SiC phase formation with a corresponding electron density derived from plasmon energy losses. The plasmon-loss features in the Si LVV and KL23L23 spectra indicate the presence of fine-dispersed silicon carbide precipitates of average size ∽0.5–3 nm in a silicon–carbon matrix. Although a beneficial effect of Si+ irradiation on crystallization of the entire implantation zone could not be observed for dose densities ⩽1×1017 ions cm−2, this study demonstrates the successful application of AES for the characterization of heterogeneous materials with embedded nanoparticles in the matrix. © 1998 John Wiley & Sons, Ltd.