Silicon Carbide Coatings Research Articles

Ceramic Coatings on Graphite via Polymer Vapor Pyrolysis Deposition Process

Silicon carbide coatings on graphite were prepared through polymer vapor pyrolysis deposition process (PVPD) under N2 atmosphere. During this process, some low molecular weight substances that polycarbosilane (PCS) pyrolyzed can be deposited on graphite, and they can convert into SiC in high temperature. The results of XRD showed that amorphous SiC coatings were formed on graphite when the pyrolysis temperature was 1000°C, andβ-SiC phase formed in the coatings when the temperature up to 1250°C. Effects of the coatings on the microstructure and properties were investigated. It was shown that the uniform dense SiC coatings could be obtained by carefully controlling the pyrolysis temperature and ramping rate when the number molecular weight of PCS was in the range of 1,000~1,500.

Silicon Carbide Coated Graphite Derived from Chlorine Free Preceramic Polymer Precursor

Graphite is widely used in electronic industry due to its excellent electrical and thermal properties. However, graphite starts to oxidize around 350°C which seriously degrades its properties. SiC coating can be applied to graphite to improve its high temperature oxidation resistance. In this research, SiC coating on graphite was made via preceramic polymer using a polymethylphenylsilane. 20% of polymethylphenylsilane in hexane solution was coated onto graphite by dip coating method. After coating, thermal oxidation was carried out at 200°C for crosslink of the preceramic polymer and the sample were pyrolysized at 1200°C under nitrogen to convert the preceramic polymer to SiC film. The phase formation, stiochiometry and microstructure of the SiC coating after pyrolysis were investigated using low angle XRD and FESEM etc. Oxidation resistance up to 600°C was evaluated.

Synthesis and characterization of non-oxide ceramic coatings on graphite substrates by solid–vapor reaction process

Silicon carbide (SiC) and silicon carbide/silicon nitride (SiC/Si 3N 4) were successfully synthesized on graphite substrates by the use of solid–vapor reaction (SVR) process. Layers of SiC and SiC/Si 3N 4 were synthesized on graphite substrate through the reaction between SiO and substrate the (SiO (vapor) + 2C (from graphite)) and N 2 and substrate the (3SiO (vapor) + 2N 2 (vapor) + 3C (from graphite)), respectively. With the increase of dwell time and synthesis temperature the thickness of SiC layers increases up to 1500 °C temperature. Also, with the increase of synthesis temperature hardness value of SiC coatings is increased, which is 10–15 times higher than the substrate. The critical load of SiC coatings for wear resistance is about 22 N, which was observed by scratch tests. The synthesized SiC coatings appear to consist of a β-SiC phase mixed with a minor amount of an α-SiC phase, and its thickness is mainly affected by porosity of the substrate. The thickness of SiC/Si 3N 4 coatings is much thinner than that of SiC coatings, but gives higher value of surface hardness.

Preparation of Silicon Carbide Coatings from Liquid Carbosilanes by Chemical Vapor Deposition

Liquid carbosilanes which were synthesized from polydimethylsilane (PDMS) were analyzed by infrared spectra (IR) and H-nuclear magnetic resonance (NMR). Silicon carbide coatings were prepared by chemical vapor deposition from liquid carbosilanes onto several different substrates. The coatings were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and IR. The results show that liquid carbosilanes are mixtures of several oligomers which have Si-C backbone. The coatings prepared at 850 °C contain some organic segments, and those prepared at 900 °C are comparatively pure SiC which is partly crystallized. The coatings are glazed and very hard.

Comparison between single and multilayered a-SiC:H coatings prepared by multi-frequency PACVD: Mechanical properties

Due to their low density and good mechanical properties, titanium alloys are commonly used to lighten structures in aeronautical applications. Unfortunately, their friction properties are poor due to titanium oxide layers which naturally occur. To confer to the surface both high mechanical resistance under load and low friction/low wear against other metals, amorphous silicon carbide (a-SiC:H) coatings are deposited on plasma treated substrates, using a multi-frequency plasma CVD device. The present study is devoted to compare scratch properties of single and multilayered a-SiC:H layers including up to 125 asymmetric periods of soft/hard coatings. Standard scratch-testing measurements reveals that the critical load corresponding to the formation of cohesive/adhesive cracks can be significantly increased (by a factor of 2.5) when using multilayered coatings. When optimized coatings are deposited, the substrate is not visible at the bottom of the flaked area until loads higher than 60 N are applied.

Silicon Carbide Coating on Graphite and Isotropic C/C Composite by Chemical Vapour Reaction

The application of Carbon and graphite based materials in unprotected environment is limited to a temperature of 450 o C or so because of their susceptibility to oxidation at this temperature and higher. To over come these obstacles a low cost chemical vapour reaction process (CVR) was developed to give crystalline and high purity SiC coating on graphite and isotropic C/C composite. CVR is most effective carbothermal reduction method for conversation of a few micron of carbon layer to SiC. In the CVR method, a sic conversation layer is formed by reaction between carbon and gaseous reagent silicon monoxide at high temperature. Characterization of SiC coating was carried out using SEM. The other properties studied were hardness density and conversion efficiency.

Advanced characterization techniques for SiC and PyC coatings on high-temperature reactor fuel particles

Enhancing the safety of high-temperature reactors (HTRs) is based on the quality of the fuel particles, requiring good knowledge of the microstructure of the four-layer particles designed to retain the fission products during irradiation and under accidental conditions. This paper focuses on the intensive research work performed to characterize the micro- and nanostructure of each unirradiated layer (silicon carbide and pyrocarbon coatings). The analytic expertise developed in the 1970s has been recovered and innovative advanced characterization methods have been developed to improve the process parameters and to ensure the production reproducibility of coatings.

Compressive Behavior of Al2O3—SiC Ceramic Composite Foams Fabricated by Decomposition of Aluminum Sulfate Aqueous Solution

In recent years, a new class of ceramic foams with porosity levels up to 95% has been produced by the chemical method with aeration of a suspension containing foaming agents. The method of foamed suspensions has originated from the chemical reactions of water-soluble salts. In this study, sintered cellular ceramic composites with varying degrees of reticulation are prepared using coarse and fine silicon carbide (SiC) powder. A new process has been developed in order to produce SiC particles containing alumina (Al2 O3) foam by chemical route. An aqueous solution of aluminum sulfate and ammonium sulfate plus SiC particles is fired to prepare ceramic foams up to 1200°C in a ceramic crucible. During heating, the viscous suspension is foamed, sulfate ions are volatilized, and an Al 2O3—SiC composite cake is obtained. The resulting Al2O3 are in networking morphology with flaky struts whereas SiC particles are encapsulated in the flaky alumina. These 3D connections of the struts are responsible for macropores and Al2O3 and SiC grains within the struts are responsible for micropores. Thus, a ceramic composite foam containing bimodal pore distribution is obtained. After sintering at 1550°C, compression tests are applied for the ceramic composite foams. High compressive strength is achieved after post sintering. Both sintering temperature and addition of SiC particles increase the compressive strength of the foam. Both SEM image and XRD analyses are carried out to examine composite microstructures and properties.

Shear properties at the PyC/SiC interface of a TRISO-coating

The failure behavior of TRISO-coated fuel particles depends significantly on the shear strength at the interface between the inner pyrolytic carbon (PyC) and silicon carbide (SiC) coatings. In this study, a micro-indentation fiber push-out test was applied to measure the interfacial shear properties of a model TRISO-coated tube. Of particular emphasis is that this study developed a non-linear shear-lag model for a transversely isotropic composite material due to insufficiency in the existing isotropic models as applied to layered TRISO-coating systems. In the model, the effects of thermal residual stresses and the roughness-induced clamping stress were identified as particularly important. The rigorous model proposed in this study provides more reasonable data on two important interfacial parameters: the interfacial debond shear strength and the interfacial friction stress. The modified model coupled with experiments yields an interfacial debond shear strength of 240 ± 40 MPa. This high interfacial strength, though slightly lower than that obtained by the existing isotropic model (∼280 MPa), allows significant loads to be transferred between inner PyC and SiC in application. Additionally, an interfacial friction stress of 120 ± 30 MPa was determined. This high friction stress is attributed primarily to the roughness at the cracked interface rather than clamping effects due to differing coefficients of thermal expansion.

Spheroid Growth during Sintering of Copper Coated Silicon Carbide Particles in the Fabrication of Nanocomposite

Copper coated silicon carbide clusters were used to fabricate nanocomposite. Compacts were isostatically pressed and heated in nitrogen atmosphere. Microstructure observations were carried out to show the spheroid growth of the coated clusters. Spheroid growth was found to proceed through coalescence of smaller spheroids of the coated Cu/SiC composite particles. The densification process contains steps of agglomeration – kernel-shell formation – slumping movement. Grain growth of the adherent Cu particles is suppressed due to the constraint of rigid SiC particles. This is the deterministic characteristic of the coated composite particles.

Open-air laser-induced chemical vapor deposition of silicon carbide coatings

This study demonstrates the open-air deposition of amorphous hydrogenated silicon carbide (a-SiC:H) by laser-induced chemical vapor deposition (LCVD) using an enclosureless (open-air) reactor system. Films are deposited on fused quartz substrates using the precursor gas trimethylsilane (TrMS). Based on Auger electron spectroscopy (AES), Fourier transform infrared absorption spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), the film's chemical composition and microstructure is determined to be composed of a form of silicon carbide (SiC:H) with organic moiety. To understand the temperature-dependence of film growth during deposition, varying deposition conditions were employed to correlate the SiC film deposition rate and deposition temperature.

Microstructural characterization using orientation imaging microscopy of cellular Si/SiC ceramics synthesized by replication of Indian dicotyledonous plants

Woods from three dicotyledonous plants of local origin (mango Mangifera indica), jackfruit ( Artocarpus integrifolia) and teak ( Tectona grandis) were transformed by pyrolysis into carbonaceous preforms and subsequently converted into cellular Si/SiC ceramics by liquid Si-infiltration and reaction. The pyrolyzed mango, jackfruit and teak were characterized in terms of pyrolysis weight loss, shrinkages, bulk density and microstructures. The Si-infiltrated pyrolyzed woods were found to have densities and porosities in the range of 2.46–2.60 gm cm −3 and 1.5–3.6 vol.% respectively. SEM imaging confirmed the preservation of microcellular tissue anatomy of the precursor wood structure in the morphologies of final ceramics. The end ceramics were investigated for the phases present and for crystallographic microtexture. A combination of XPS (X-ray photoelectron spectroscopy), XRD (X-ray diffraction) and OIM (orientation imaging microscopy) was used to establish the presence and the relative locations of silicon (Si), silicon carbide (SiC) and graphite (C). Fine SiC grains did typically surround coarse crystals of Si – the latter had significant presence of Σ3 twin boundaries. Graphite was primarily present in the regions containing SiC and was more textured than the SiC. Distinct orientation relationship could be established between the graphite crystals and the Si grains containing them.

Determining the shear properties of the PyC/SiC interface for a model TRISO fuel

The fracture behavior of TRISO-coated fuel particles is dependent on the shear strength of the interface between the inner pyrolytic carbon (PyC) and silicon carbide coatings. This study evaluates the interfacial shear properties and the crack extension mechanism for TRISO-coated model tubes using a push-out technique. The interfacial debond shear strength was found to increase with increasing sample thickness and finally approached a constant value. The intrinsic interfacial debond shear strength of ∼280 MPa was estimated. After the layer is debonded, the applied load is primarily transferred by interfacial friction. A non-linear shear-lag model predicts that the residual clamping stress at the interface is ∼350 MPa, and the coefficient of friction is ∼0.23, yielding a frictional stress of ∼80 MPa. These relatively high values are attributed to the interfacial roughness. Of importance in these findings is that this unusually high interfacial strength could allow significant loads to be transferred between the inner PyC and SiC in application, potentially leading to failure of the SiC layer.

Stability of a fluidized bed in the deposition of coatings on nuclear fuel kernels

The rates of fluidization of solid particles in a fluidized bed are estimated under conditions under which the mass, diameter, and density of particles change significantly as pyrolytic carbon and silicon carbide coatings are deposited on them. A relationship is revealed between the scale of turbulent pulsations of the gas flow and the stability of the fluidized bed.

Generation and oxidation of wear debris in dry running tests of diamond coated SiC bearings

Diamond coated silicon carbide (SiC) bearings can be used in pumps to enable dry running or mixed lubrication conditions. Dependent to different test parameters the coefficient of friction and the wear behaviour shows big differences. In this work α-SiC bearings were coated in a hot filament chemical vapour deposition process (HF-CVD) with a diamond layer thickness of 4 μm. Diamond coated rings were tested in a tribometer under dry running conditions in a ring–ring geometry. During tribological tests under various atmospheres (oxygen, argon, nitrogen, ambient air) the coefficient of friction shows values in a range from 0.1 to 0.4. These values can be ascribed to different chemical and mechanical processes between the mating surfaces. Tribological tests of self mating diamond layers generate wear debris between the mating surfaces. This diamond wear debris can convert from sp 3 hybridized to sp 2 hybridized carbon. Against common expectations tribological tests at ambient air with high sliding velocities and normal loads did not show wear debris, despite of high wear rates. Under inert gas (argon or nitrogen) the sliding surface shows agglomerated areas with sp 2 hybridised carbon. The reason is the absence of oxygen, which is necessary for combustion of carbon species. The coefficient of friction remains nearly constant during the test in oxidizing atmosphere, but shows high fluctuations in inert gas atmospheres. The authors explain this tribological behaviour by a new combustion wear model for self mating diamond coated bearings.

Preparation and tribological properties of SiC/rice bran carbon composite ceramics

Silicon carbide (SiC) ceramics have good wear resistance but poor friction properties under dry sliding conditions. To lower the friction of SiC, a novel porous carbon material called rice bran carbon (RBC) was added into SiC to make SiC/RBC composite ceramics. The SiC/RBC composites were prepared by mixing one of three kinds of RBC powders having different particle sizes and a fine SiC doped with Al4C3and B4C additives and sintering at 1600 °C for 5 min by a pulse electric current sintering (PECS) method. The mechanical and tribological properties of the SiC/RBC composites were evaluated and compared with those of monolithic SiC, monolithic RBC bulk material, and SiC/graphite composite. The SiC/RBC composites not only had superior fracture strength (3–4 times as high as that of the monolithic RBC material) but also showed low friction coefficients (around 0.25) and high wear resistance (at a level of 10−6mm3N−1m−1) when slid against a silicon carbide ceramic counterface during block-on-ring sliding tests under dry conditions. Compared with the conventional SiC/graphite composite, the SiC/RBC composites had higher mechanical strength, lower friction coefficients, and better wear resistance.

The effect of Zircaloy-4 substrate surface condition on the adhesion strength and corrosion of SiC coatings

Chemical vapor deposition (CVD) coatings of silicon carbide were deposited on various Zircaloy-4 substrates having different surface preparations to increase the corrosion resistance. The effects of several different surface treatments of the Zircaloy-4 substrate, such as surface roughness, the presence of interlayer, and pickling, on the adhesion and corrosion resistance of the SiC coatings have been evaluated using a scratch test method, electrochemical impedance spectroscopy and scanning electron microscopy. The scratch test was found to be a good tool for qualitative measurement of adhesion strength of thin coating films. Higher adhesion strengths were obtained for a moderate level of substrate roughness and the corrosion resistance of these films was closely related with the adhesion of the film on the substrate, as measured by impedance.

Microwave synthesis of phase-pure, fine silicon carbide powder

Fine, monophasic silicon carbide powder has been synthesized by direct solid-state reaction of its constituents namely silicon and carbon in a 2.45 GHz microwave field. Optimum parameters for the silicon carbide phase formation have been determined by varying reaction time and reaction temperature. The powders have been characterized for their particle size, surface area, phase composition (X-ray diffraction) and morphology (scanning electron microscope). Formation of phase-pure silicon carbide can be achieved at 1300 °C in less than 5 min of microwave exposure, resulting in sub-micron-sized particles. The free energy values for Si + C → SiC reaction were calculated for different temperatures and by comparing them with the experimental results, it was determined that phase-pure silicon carbide can be achieved at around 1135 °C.

Microstructural properties of SiC based hardfacing on low alloy steel

The effects of tungsten inert gas arc-processing (TIG) parameters on the microstructure and microhardness of silicon carbide coatings were investigated experimentally. Coatings were developed with TIG processing on an SAE 1020 steel substrate. Depending on the processing parameters (e.g., heat input, powder content, and processing speed), silicon carbide particles completely dissolved during the processing and different microstructures were produced. At lower powder contents, the microstructures consisted of dendrites. However, at high powder contents, the microstructures consisted of M 7C 3 primary carbides, Fe 3Si, SiC, and the graphitic carbon precipitates were generated. The microhardness of the surface layer was measured as a function of process parameters. Microhardness of the surface layer was significantly improved to as high as 1135 H V as compared to 220 H V of the substrate. The highest hardness measured 1250 H V, which produced high heat input and low powder contents of 2.310 kJ mm − 1 and 1.5 g, respectively. The lower hardness of dendritic microstructure was related to the presence of primary dendrites and relatively low concentrations of Si and C.

Synthesis of Silicon Carbide Coating from Precursor Polymers by EB Curing

Synthesis of Silicon Carbide Coating from Precursor Polymers by EB Curing