Silicon Carbide Coatings Research Articles

Poster #S175 THE ERYTHROCYTE MEMBRANE LIPID ABNORMALITIES OBSERVED IN SCHIZOPHRENIA PATIENTS SUPPORT THE OXIDATIVE STRESS HYPOTHESIS IN SCHIZOPHRENIA

Interface between ceramic particulate and matrix is known to control the response of the materials and functionality of the composite. Among numerous physical properties, grain structure of the materials has also played a significant role in defining the behaviour of metal matrix composites. Usually, silicon carbide (SiC) particles show poor interfacial wettability in aluminium melt. Herein, we were successfully synthesized magnesium oxide (MgO) and nanocrystalline magnesium aluminate (MgAl2O4) spinel coated silicon carbide (SiC) core-shell micro-composites through sol-gel technique to improve the wettability of dispersoids. Core-shell structures of submicron size were thoroughly investigated by various characterization techniques. Further, aluminium matrix composites incorporated with pristine SiC, MgO grafted SiC and MgAl2O4 grafted SiC particles were fabricated by stir casting technique, respectively. Additionally, as-cast composites were processed via friction stir processing (FSP) technique to observe the influence of grain refinement on mechanical and damping properties. Electron back scattered diffraction (EBSD), Field emission scanning electron microscopy (FE-SEM) and X-ray energy dispersion spectroscopy (EDX) analysis were conducted for investigating grain size refinement, adequate dispersion, stability and de-agglomeration of encapsulated SiC particles in aluminium matrix. The mechanical as well as thermal cyclic (from − 100 to 400 °C) damping performance of the as-cast and friction stir processed composites were studied, respectively. Finally, the enhanced properties were attributable to reduced agglomeration, stabilization and proper dispersion of the tailored SiC particles Al matrix.

Silicon carbide multilayer protective coating on carbon obtained by thermionic vacuum arc method

Thermionic vacuum arc (TVA) method is currently developing, in particular, to work easily with heavy fusible material for the advantage presented by control of directing energy for the elements forming a plasma. The category of heavy fusible material can recall C and W (high-melting point materials), and are difficult to obtain or to control by other means. Carbon is now used in many areas of special mechanical, thermal, and electrical properties. We refer in particular to high-temperature applications where unwanted effects may occur due to oxidation. Changed properties may lead to improper functioning of the item or device. For example, increasing the coefficient of friction may induce additional heat on moving items. One solution is to protect the item in question by coating with proper materials. Silicon carbide (SiC) was chosen mainly due to compatibility with coated carbon substrate. Recently, SiC has been used as conductive transparent window for optical devices, particularly in thin film solar cells. Using the TVA method, SiC coatings were obtained as thin films (multilayer structures), finishing with a thermal treatment up to 1000°C. Structural properties and oxidation behavior of the multilayer films were investigated, and the measurements showed that the third layer acts as a stopping layer for oxygen. Also, the friction coefficient of the protected films is lower relative to unprotected carbon films.

Effect of Electroless Copper Coating on the Corrosion Behavior of Aluminium Based Metal Matrix Composites Reinforced with Silicon Carbide Particles

The present paper involves firstly in the studies of corrosion properties of Al 6061 metal matrix composites reinforced with varying percentage of uncoated and Cu coated silicon carbide in 3.5 wt% NaCl solution using weight loss method. The processing route used for the composites was stir casting technique. Mass loss and corrosion rate measurements were utilized as criteria for evaluating the corrosion behavior of composites. The surface morphology of composites after the corrosion test is determined by using scanning electron microscopy. It has been observed that copper as reinforcement coating reduces the corrosion resistance of composites in comparison to uncoated reinforcements and monolithic alloy.

Effect of C/Si Ratio on the Electrochemical Behavior of a-SiCx:H Coatings on SS301 Substrate Deposited by PECVD

Amorphous hydrogenated silicon carbide (a-SiCx:H) coatings were deposited on stainless steel 301 (SS301) using plasma enhanced chemical vapor deposition with the methane gas flow ranging from 30 to 90 sccm. XRD spectra confirmed the amorphous structure of these coatings. The as-deposited coatings all exhibited homogenous dense feature, and no porosities were observed in SEM and AFM analysis. The a-SiCx:H coatings remarkably increased the corrosion resistance of the SS301 substrate. With the increase of the C concentration, the a-SiCx:H coatings exhibited significantly enhanced electrochemical behavior. The a-SiCx:H coating with the highest carbon concentration acted as an excellent barrier to charge transfer, with a corrosion current of3.5×10-12 A/cm2and a breakdown voltage of 1.36 V, compared to2.5×10-8 A/cm2and 0.34 V for the SS301 substrate.

Low-temperature chemical vapour curing using iodine for fabrication of continuous silicon carbide fibres from low-molecular-weight polycarbosilane

In order to manufacture fine and continuous silicon carbide (SiC) fibres from low-molecular-weight polycarbosilane (PCS), a new chemical vapour curing process based on the use of iodine was developed. Its main advantages are short processing-time (∼1 h) and low processing-temperature (∼100 °C). The underlying curing mechanism was investigated by performing TG-DTA, GC-MS, solid-state NMR, FTIR, FE-EPMA, ESR, elemental analysis, and XRD. The results indicate that the curing reaction occurs upon diffusion of iodine into the PCS, where iodine plays the critical role of accelerating the cleavage of Si–H (mainly), Si–Si, and C–H bonds. This cleavage coincides with recombination to form cross-linked networks of –Si–C– and –CC–. The by-products were identified as oligomeric silanes, iodomethane, and aromatics. When the curing is conducted in air, the surface region in contact with oxygen is heavily oxidized to form a Si–O–Si network that is localized on the surface.

Mechanical and tribological properties of silicon carbide coating on Inconel alloy from liquid pre-ceramic precursor

Liquid polycarbosilane (LPCS) derived hard coatings of silicon carbide (SiC) were deposited on Inconel alloy at three different moderately high temperatures by chemical vapour deposition. The deposited films were characterized by X-ray diffractometry and Field emission scanning electron microscopy. Liquid PCS yielded a mixture of α-SiC and β-SiC during decomposition having uniform round-shaped particles of dimension around 200–300nm without extensive cracking and few discrete shaped particles were also found to form at higher temperature (i.e. 1100°C and 1200°C) deposited films. The coated samples showed substantial increment in hardness and fracture toughness as compared to the uncoated sample. The fracture toughness (KIC) values of the deposited films were in the range of 6.7–10.7MPa(m)1/2. The tribological properties and hardness of the films were also found to vary with deposition temperature. The scratch tracks of the films revealed that brittle failures occurred in all SiC coated substrates.

Protective SiC Coating on Carbon Fibers by Low Pressure Chemical Vapor Deposition

High-quality <TEX>${\beta}$</TEX>-silicon carbide (SiC) coatings are expected to prevent the oxidation degradation of carbon fibers in carbon fiber/silicon carbide (C/SiC) composites at high temperature. Uniform and dense <TEX>${\beta}$</TEX>-SiC coatings were deposited on carbon fibers by low-pressure chemical vapor deposition (LP-CVD) using silane (<TEX>$SiH_4$</TEX>) and acetylene (<TEX>$C_2H_2$</TEX>) as source gases which were carried by hydrogen gas. SiC coating layers with nanometer scale microstructures were obtained by optimization of the processing parameters considering deposition mechanisms. The thickness and morphology of <TEX>${\beta}$</TEX>-SiC coatings can be controlled by adjustment of the amount of source gas flow, the mean velocity of the gas flow, and deposition time. XRD and FE-SEM analyses showed that dense and crack-free <TEX>${\beta}$</TEX>-SiC coating layers are crystallized in <TEX>${\beta}$</TEX>-SiC structure with a thickness of around 2 micrometers depending on the processing parameters. The fine and dense microstructures with micrometer level thickness of the SiC coating layers are anticipated to effectively protect carbon fibers against the oxidation at high-temperatures.

High temperature oxidation of two- and three-dimensional hafnium carbide and silicon carbide coatings

Abstract The difficulty in using C/C composites as structural components above 2000 °C in an oxidizing atmosphere is their poor lifetime. The solution proposed here consisted in combining two refractory carbides, hafnium and silicon carbides, in coating with a complex architecture, named a three dimensional coating, over a C/C substrate. Such a coating protects the C/C composite at 2000 °C under air. The oxidation of the coating leads to the formation of a SixOyHfz hafnium-containing silicate liquid, combined with HfO2(s). This liquid limits oxygen diffusion more than pure SiO2 does, so it is a better protection against oxidation. Furthermore, HfO2(s) acts as a frame holding SixOyHfz in place. From these results, an oxidation mechanism is proposed and discussed.

Thermally Sprayed SiC Coatings for Offshore Wind Turbine Bearing Applications

Tribological tests were conducted on thermally sprayed silicon carbide (SiC) coatings to investigate its potential on reducing wear in offshore wind turbine bearings. The tests were carried out under dry conditions, 3.5 wt.% NaCl solution, and polyalfaolefin (PAO)-lubricated conditions. In order to obtain good quality SiC coatings, it is compulsory to modify the feedstock to limit SiC decomposition during atmospheric spraying process. The SiC feedstock used in this research has been modified with yttrium aluminum garnet (Y3Al5O12) oxide additives that originated from its metal salt precursors. High-frequency pulse detonation (HFPD) technique has been utilized to produce coatings of around 100 μm in thickness. The sliding tests have recorded the lowest coefficient of friction (COF) of 0.15 in PAO condition and the highest COF of 0.50 in dry sliding. The wear tracks morphology show that during dry sliding test, the coatings experience abrasive wear accompanied by tribo-oxidation reaction that initiates crack formation along the splat boundaries. On the other two sliding test conditions (NaCl and PAO), polishing of asperities and some grain plowing from the splats were observed in the wear tracks. Tribochemical wear was found to be the main mechanism producing smooth surfaces. Nevertheless, in all cases, the wear losses were negligible.

Influence of SiC coating thickness on mechanical properties of SiCf/SiC composite

Silicon carbide (SiC) coatings with varying thickness (ranging from 0.14μm to 2.67μm) were deposited onto the surfaces of Type KD-I SiC fibres with native carbonaceous surface using chemical vapour deposition (CVD) process. Then, two dimensional SiC fibre reinforced SiC matrix (2D SiCf/SiC) composites were fabricated using polymer infiltration and pyrolysis (PIP) process. Influences of the fibre coating thickness on mechanical properties of SiC fibre and SiCf/SiC composite were investigated using single-filament test and three-point bending test. The results indicated that flexural strength of the composites initially increased with the increasing CVD SiC coating thickness and reached a peak value of 363MPa at the coating thickness of 0.34μm. Further increase in the coating thickness led to a rapid decrease in the flexural strength of the composites. The bending modulus of composites showed a monotonic increase with increasing coating thickness.

Silicon carbide coating of the aluminum joined boron carbide by using polycarbosilane

Silicon carbide (SiC) coatings on a joined Boron carbide (B4C), aimed at improving the corrosion resistance, were prepared by using polycarbosilane (PCS) in order to increase its corrosion resistance. Some B4C was diffused to PCS membrane in the curing process and the PCS membrane was made into SiC including crystal one in spite of 1473K pyrolysis. The 4-point bending test of the samples after corrosion test shows that the sample without any coating was close to 0MPa, while the samples with SiC coating maintained more than 300MPa.

Influence of fiber coating thickness on microstructure and mechanical properties of carbon fiber-reinforced zirconium diboride based composites

Abstract Unidirectional carbon fiber-reinforced zirconium diboride based composites were fabricated by hot pressing. The fiber–matrix interface was tailored by pre-coating the as-received carbon fibers with pyrolytic carbon (PyC) coatings of various thickness or silicon carbide (SiC) coating. The effects of the interfacial characteristics on mechanical properties and microstructure of the composites were studied. The results indicated that both the composites reinforced with as-received carbon fibers and SiC coated carbon fibers showed inferior flexural strength and fracture toughness. With optimized PyC coating thickness, the mechanical properties of the composites had been remarkably improved, i.e. a flexural strength of 309.6 MPa was achieved when the thickness of PyC coating was 0.1 μm, and a fracture toughness of 6.72 MPa m 1/2 was obtained when the PyC coating was 0.7 μm thick.

Preparation and characterization of surface modified silicon carbide/polystyrene nanocomposites

AbstractA simple method is reported to coat silicon carbide (SiC) nanoparticles with polystyrene (PS) to improve the interfacial adhesion between polymer matrix and SiC nanoparticles. The morphology of untreated SiC nanoparticles, PS coated SiC (p‐SiC) nanoparticles, SiC/PS nanocomposites, and p‐SiC/PS nanocomposites are observed. The HRTEM image of p‐SiC shows that the thickness of PS on the surface of SiC is about 1.5–2.0 nm, which is consistent with the TGA results. With 24.7 vol % untreated SiC nanoparticles dispersed into PS matrix, the thermal conductivity (λ) of the SiC/PS composites increases by about 192%. However, when the same volume fraction of p‐SiC nanoparticles is used, the increase is about 353%. This big difference could be attributed to the promoted dispersion of the p‐SiC in the PS matrix. The measurements of glass transition (Tg), dielectric constant (ε), and tensile strength at break (σb) also support this explanation. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Molten salt synthesis and characterization of SiC coated carbon black particles for refractory castable applications

Silicon carbide (SiC) coatings were prepared on carbon black (CB) particles after 6 h at 1100 °C by a molten salt synthesis (MSS) technique. By controlling the Si to CB ratio (Si/C) in the initial batch mixture, the SiC coating thickness could be readily tailored to meet practical requirements in real castable systems. True densities of CB particles after SiC coating increased with Si/C. The zeta potential values indicated that their dispersivity was improved evidently over a wide range of pH. At a given shear rate, the apparent viscosity of a water suspension containing SiC-coated CB was significantly lower than that of a water suspension containing as-received uncoated CB, indicating the improved flowability of SiC-coated CB in water. The improvements in both the water-dispersivity and flowability of SiC coated CB particles would make them a promising candidate carbon material for future castable applications.

Laser Alloying Nickel on 4H-Silicon Carbide Substrate to Generate Ohmic Contacts

To obtain ohmic contacts on silicon carbide semicondanneuctors commonly a thin metal layer is deposited onto the semiconductor wafer, followed by a rapid thermal process to transform the resulting Schottky diode into an ohmic contact. Here, we report on an alternative laser based process replacing the conventional rapid thermal treatment. A continuous wave infrared fiber laser is employed to locally alloy nickel coated silicon carbide to an ohmic contact. Our results show that ohmic contacts are achieved with laser powers between 100 W and 300 W over an irradiated area on the order of 7 mm diameter. Electrical resistances of the generated ohmic contacts are measured down to 6.5 Ω, depending on laser power. Temperature dependent measurements of the resistance reveal a negative temperature coefficient of 11 mΩ/K.

Microstructure and isothermal oxidation of 3Al2O3·2SiO2/SiC coating on high and low density carbon-carbon composites

Carbon-carbon composite (C/C) materials are prone to severe oxidation and volatilization problems. To address these issues, mullite (3Al2O3·2SiO2)/silicon carbide (SiC) coatings were deposited on C/C composite substrates characterized into high and low densities. The coatings were applied by a two-step approach: pack cementation and silica sol based slurry coating processes. The relationship between the microstructure of 3Al2O3·2SiO2/SiC coatings and C/C substrates during isothermal oxidation cycle at 1 500 °C was investigated using X-ray diffractometer (XRD) and scanning electron microscope (SEM) mounted with energy dispersive spectrometer (EDS). The results indicate that the density of the substrates has a marked effect on the coatings. Dense, thick and well-bonded coatings are obtained in the high density substrate. After 106 h of isothermal oxidation, the high density substrate with 3Al2O3·2SiO2/SiC coating offers effective protection as compared to low density substrate suffering recession.

Effect of Multilayer Technology on Surface Properties of Diamond Coated Silicon Carbide with Surface Defects

Low surface roughness of diamond films is beneficial for the widespread applications in the mechanical field. But brittle ceramic substrate easily forms surface defects, which can be detrimental for surface properties of diamond films. In this work, multilayer technology combining conventional hot filament chemical vapor deposition (HFCVD) is proposed to eliminate the influence of surface defects in the substrate on diamond films. Then multilayer diamond films are deposited on silicon carbide with surface defects by alternately repeating the processes of diamond films growth and surface polishing. Each layer of multilayer diamond films is evaluated by field emission scanning electron microscope (FE-SEM), surface profilometer and Raman spectrum. The results show that multilayer technology is supposed to be a novel deposition method of improving the surface properties of diamond coated the silicon carbide ceramic substrate with surface defects.

Study of thermal degradation behavior of dense and nanostructured silicon carbide coated carbon fibers in oxidative environments

The oxidation resistive properties of bare, dense SiC and nanostructured SiC coated carbon fibers were examined in different oxidative environments from room temperature to 1350°C. Chemical vapor deposition process was used for the preparation of dense and nanostructured silicon carbide (SiC) coating on carbon fiber using methyltrichlorosilane. SEM, TEM and XRD analysis inferred dense and nanostructured β SiC coatings. The dense and nanostructured SiC coated carbon fibers have shown much improved oxidation property in oxygen as well as in static air environment. Possible mechanism of oxidation behavior of dense and nanostructured SiC coated carbon fibers and role of SiO2 have been discussed.

Analysis of the anisotropy, stoichiometry and polytypes in pyrolytic carbon and silicon carbide coatings

Silicon carbide (SiC) and pyrolytic carbon (PyC) coatings in tristructural isotropic (TRISO) coated fuel particles were characterised by a combination of 2-modulator generalised ellipsometry microscopy (2-MGEM), Raman spectroscopy and transmission electron microscopy. We compared the values of anisotropy obtained from 2-MGEM and Raman spectroscopy to investigate the effect of sampling area and microstructure. No linear correlation in anisotropy was found between these two techniques despite both sampling areas of 2–5μm. The largest deviations were observed for highly anisotropic samples with optical anisotropy factors (OPTAFs) above 1.1. For medium and low anisotropy samples (OPTAF<1.1) the relationship is close to linear. The limited use of only the average value of diattenuation does not give an accurate representation of the characteristics of the coatings as significant areas can exist with considerably higher diattenuations that could increase the probability of failure under neutron irradiation. We also observe a change in the diattenuation of SiC due to the presence of stacking faults as confirmed by Raman spectroscopy. Raman spectroscopy was also used to perform semiquantitative analysis of the Si and carbon excess in SiC in four TRISO particles.

Study of Resistance Characteristics of Silicon Carbide Resistor Materials

Experimental program and preparation technology of highly densified reaction bonded silicon carbide resistance materials with different resistivity were investigated by using alumina or silicon nitride as aids for improving of the volume resistivity. The microstructure of the materials was observed by scanning electron microscope. According to the experimental results, the resistivity of the material could be controlled by the particle size and the content of the aids. Besides, alumina could reduce silicon penetration and increase resistivity greatly, and 10wt.% of alumina could satisfy the requirement of resistance. In addition, different particle size of matrix materials had a great impact on the resistivity. Compared with the coarse silicon carbide, the room temperature resistivity value by using the fine silicon carbide could increases for six times.