SiC Coatings Research Articles

Tribological properties of SiC coatings deposited by r.f. magnetron sputtering as a function of substrate temperature

Silicon carbide coatings have been deposited by r.f. magnetron sputtering on AISI 304LN stainless steel at four different substrate temperatures. At the lowest substrate temperature of 300°C, the film was amorphous whereas at a substrate temperature of 600°C a crystalline SiC phase was obtained. However, at substrate temperature of 800°C, the films were amorphous again. The tribological properties of the coatings were also found to change with substrate temperature. The coefficient of friction was found to be low for the crystalline films. Such a low value of coefficient of friction is indicative of superior wear resistance. A higher coating thickness enhanced the tribological properties while any surface roughness of the substrate and coating was found to degrade these properties. Micro-Raman spectra of the films varied with substrate temperature. Some phases were found to disappear and new phases were observed in the wear track with progressive sliding.

Deuterium retention in SiC coated graphite by D 2+ implantation

The doped graphite tiles bolted to the active cooling heat sink, made of GBST1308 (1% B 4C, 2.5% Si, 7.5% Ti) coated with SiC, are now being used as the only plasma facing material (PFM) for the EAST device since the campaign of 2008. From the plasma density and fueling point of view, it is important to study thoroughly the hydrogen isotope retention in this kind of SiC-coated doped graphite. D 2 + implantations into the SiC coated doped graphite were performed at Shizuoka University. The chemical states of Si and C were studied by means of X-ray photoelectron spectroscopy (XPS), and the thermal desorption behavior of deuterium was analyzed by thermal desorption spectroscopy (TDS). It was found that deuterium was trapped by both C and Si in the SiC coatings. In the previous studies, Oya et al. reported the deuterium retention behavior in polycrystalline β-SiC. In this paper, difference of retention behavior in β-SiC and SiC coating will be also discussed.

Rf‐GDOES analysis of composite metal/ceramic electroplated coatings with nano‐ to microceramic particles' size: issues in plasma sputtering of Ni/micro‐SiC coatings

AbstractNi matrix composite coatings reinforced with nano‐ and microceramic particles were analyzed by radio frequency glow discharge optical emission spectrometry (Rf‐GDOES). An interesting phenomenon related to the sputtering and excitation modes of this technique was observed. During plasma sputtering with Rf‐GDOES, the micro‐SiC particles were detached from metal matrix and did not contribute to the analytical signals. The same was not found in composite coatings containing nanoceramic particles. This anomalous behavior was confirmed by atomic force microscopy (AFM) investigation and scanning electron microscope (SEM) observations into Rf‐GDOES craters that showed the presence of residual non‐sputtered microparticles. Various attempts were done in order to minimize this problem, mainly by varying the analysis parameters of the used instrumentation, but without any relevant success. Some suggestions were then proposed for explaining the observed phenomenon, moreover possible solutions (e.g. by using a strong magnetic field or changing plasma gas to be more energetic) are discussed. Copyright © 2011 John Wiley & Sons, Ltd.

Fabrication of Silicon Carbide (SiC) Coatings from Pyrolysis of Polycarbosilane/Aluminum

The SiC coatings were fabricated by the pyrolysis of polycarbosilane (PCS)/aluminum, in which PCS acts as precermaic precursor of SiC and aluminum (Al) powder acts as an active filler to both compensate the volume shrinkage of SiC coatings during pyrolysis and enhance the adhesion of SiC coatings with Fecralloy substrate. SiC coatings as thick as ~35 μm without cracking can be fabricated through our approach. Microstructural analysis revealed that the SiC coatings were composed of α-Al2O3 and β-SiC. Hardness and modulus of the SiC coatings as measured by nano-indentation were 12.2 ± 4.0 and 153.7 ± 47.0 GPa, respectively.

Microstructure characteristics of silicon carbide coatings fabricated on C/C composites by plasma spraying technology

A functional gradient SiC coating on C/C composites has been developed using a novel process which is the combination of plasma spraying technology with reaction-formed heat-treatment. Microstructure observation and phase identification of the SiC coatings were analyzed by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Experimental results showed that a uniform silicon coating was deposited on C/C composite by plasma spraying technology. The reaction between the silicon coating and C/C substrate occurred during the heat-treatment at temperature of 1450 °C and 1600 °C in argon environment, respectively. A continuous SiC coating was formed on the surface of the C/C substrate. And a layer of SiC/C convention layer was formed on the near-surface area of the substrate, which was resulted from the molten silicon penetrating into the open pores and consequently reacting with the C/C composites. The thickness of the formed SiC coatings was closely related to the original silicon coatings.

Coating materials for fusion application in China

Thick SiC coatings of ∼100μm on graphite tiles, prepared by chemical vapor infiltration of Si into the tiles and the following reactions between Si and C, are used as plasma facing material (PFM) on HT-7 superconducting tokamak and Experimental Advanced Superconducting Tokamak (EAST). With increase in the heating and driving power in EAST, the present plasma facing component (PFC) of the SiC/C tiles bolted to heat sink will be replaced by W coatings on actively cooled Cu heat sink, prepared by vacuum plasma spraying (VPS) adopting different interlayer. The VPS-W/Cu PFC with built-in cooling channels were prepared and mounted into the HT-7 acting as a movable limiter. Behavior of heat load onto the limiter and the material was studied. The Cu coatings on the Inconel 625 tubes were successfully prepared by high velocity air–fuel (HVAF) thermal spraying, being used as the liquid nitrogen (LN2) shields of the in-vessel cryopump for divertor pumping in EAST.

SiC/W/Ir multilayer-coated grating for enhanced efficiency in 50–100 nm wavelength range in Seya–Namioka mount

A broadband [SiC/W/Ir](2) multilayer coating was deposited onto a diffraction grating to enhance the grating efficiency in the 50-100 nm wavelength range in a Seya-Namioka mount. The holographic ion-beam etched grating had a laminar profile with 1200 lines/mm. The coating was designed by using the subquarterwave multilayer theory. The measurement results show that the efficiency in the -1st diffraction order is greater than 9.3% throughout the wavelength range of 50-100 nm, which is obviously higher than that of single-layer gratings with SiC, Ir, or Au coatings.

Characterization of Silicon Carbide Films Prepared by Chemical Vapor Deposition

Silicon carbide prepared by chemical vapor deposition (CVD) is one of the important candidate materials for space mirror and high-power mirror such as laser mirror, because of its superior performances such as low density, high melting point and homogeneity. In this paper, the SiC coatings were deposited on the substrates of reaction bonded silicon carbide (RB-SiC) by CVD process. Then, the morphologies of the deposits were examined with scanning electron microscopy. The crystalline phase of the as-deposited films was confirmed with X-ray diffractometry. And the adhesion between the CVD film and the substrate was rated with scraping method. As a result, the morphologies of the deposits, i.e. whiskers at 1050°C or films at 1100°C, are different from that of the substrate. And the mean diameter of the deposits at 1100°C is larger than that at 1050°C. Furthermore, the crystalline phase of the as-deposited film is determined as β-SiC and the adhesion is firm enough not to be peeled off with the scraping test.

Yttrium silicate coatings for SiC coated C/C composites prepared by atmospheric plasma spraying

Different yttrium silicate coatings for SiC-coated C/C composites were designed and prepared by an atmospheric plasma spray method to prevent the C/C composites from oxidation.XRD profiles showed that the coating was composed of Y2SiO5,Y2Si2O7,and Y4Si3O12.SEM showed that the gradient SiC/Y4Si3O12/Y2Si2O7/Y2SiO5 coating had a more compact surface than the other coatings did.Isothermal oxidation tests at 1 500 ℃ in air showed that the weight loss curves of the as-prepared C/C composites showed a linear tendency with time and the weight loss rate of all coated samples increased significantly after 73 h.The coating could protect the C/C composites from oxidation at 1 500 ℃ for 73 h in air with a weight loss rate of 1.01×10-4 g·cm-2·h-1.The SiC/Y4Si3O12/Y2Si2O7/Y2SiO5 was the better coating at high temperature than other coatings.

Corrosion resistance of electroless Cu–P and Cu–P–SiC composite coatings in 3.5% NaCl

The Cu–P and Cu–P–SiC composite coatings on carbon steel substrates were deposited via electroless plating. The anti-corrosion properties of Cu–P and Cu–P–SiC coatings were studied in 3.5% NaCl solution. The anti-corrosion properties of Cu–P and Cu–P–SiC coatings were investigated in 3.5% NaCl solution by the weight loss, potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. It has been found that the shift in the corrosion potential (Ecorr) towards the noble direction, decrease in the corrosion current density (Icorr), increase in the charge transfer resistance (Rct) and decrease in the double layer capacitance (Cdl) values indicated an improvement in corrosion resistance with the incorporation of SiC particles in the Cu–P matrix. The effects of varying the SiC concentration on the corrosion resistance of carbon steel were investigated and it was found that the best anti-corrosion property of Cu–P–SiC is at 5gL−1 SiC in the bath formulation.

Effect of SiC on the corrosion resistance of electroless Cu–P–SiC composite coating

In this work, Cu–P–SiC composite coatings were deposited via electroless plating with the addition of sodium hypophoshite (NaH2PO2) as a reducing agent. The coating compositions deposited were determined by using energy dispersive X-ray spectroscopy (EDX). The surface morphology of the coatings that were analyzed using scanning electron microscopy (SEM) showed that SiC particles were uniformly distributed by virtue of surfactant addition and mechanical stirring. The anti-corrosion properties of Cu–P and Cu–P–SiC coatings in NaCl and HCl solutions were investigated by the weight loss and potentiodynamic polarization techniques. The results showed that the corrosion resistance of Cu–P–SiC coatings was superior to that of electroless Cu–P coatings and carbon steel substrates in various concentrations of NaCl and HCl solutions.

Effect of Microstructure on the Resilience of Silicon Carbide to Palladium Attack

The Pd/silicon carbide (SiC) reaction rate in tristructural isotropic (TRISO)‐coated fuel particles has been studied. We have shown that it is possible to reduce the attack by palladium significantly by controlling the SiC microstructure. Large grain sizes as well as high coating rates did not increase the attack by Pd as previously thought. The activation energy for the Pd/SiC reaction obtained for the optimized SiC coatings, 337 kJ/mol, is almost twice as high as for standard coatings (∼160–190 kJ/mol) and five times higher than for porous samples (67 kJ/mol). An increase of the (111) texture component and the characteristics of the grain boundaries are suggested as possible reasons for reduction of the Pd/SiC reaction.

Effect of Nano-Sized TiN Additions on the Electrical Properties of Vacuum Cold Sprayed SiC Coatings

Three different contents of nano-sized TiN powers of 20 nm in size were added to nano-sized SiC powders of 40 nm. Vacuum cold spray (VCS) process was used to deposit SiC-TiN composite coatings on Al2O3 substrates. Microstructure and phase structure analysis of the samples was performed by scanning electron microscopy (SEM) and x-ray diffraction (XRD). Sheet resistance of the VCS coatings was measured using a four-point probe method. The influences of TiN additions on the electrical resistivity of SiC-TiN composite coatings and the conductive mechanisms were investigated. The electrical resistivity of SiC-TiN coatings decreases with increasing TiN contents, reaching a minimum of 1.82 Ω m with 50 mol% TiN.

Development of multimaterial coatings by cold spray and gas detonation spraying

The basic objective is the development of multifunctional multimaterial protective coatings using cold spraying (CS) and computer controlled detonation spraying (CCDS). As far as CS is concerned, the separate injection of each powder into different zones of the carrier gas stream is applied. Cu–Al, Cu–SiC, Al–Al 2O 3, Cu–Al 2O 3, Al–SiC, Al–Ti and Ti–SiC coatings are successfully sprayed. As to CCDS, powders are sprayed with a recently developed apparatus that is characterized by a high-precision gas supply system and a fine-dosed twin powder feeding system. Computer control provides a flexible programmed readjustment of the detonation gases energy impact on powder thus allowing selecting the optimal for each component spraying parameters to form composite and multilayered coatings. Several powders are sprayed to obtain composite coatings, specifically, among others, WC–Co–Cr + Al 2O 3, Cu + Al 2O 3, and Al 2O 3 + ZrO 2.

Synthesis of nanostructured SiC coatings on carbon fibres by in situ reaction sintering with milled powders

Abstract A homogeneous and continuous nanostructured SiC coating on the surfaces of T700 and M40 carbon fibres was synthesized by in situ reaction sintering at 1450 °C with the milled Si and C powders, and the coating was characterized. The results show that the slurry is easier to stick to the surface of M40 carbon fibres than that of T700 carbon fibres. A homogeneous, continuous and crack-free nanostructured SiC coating is formed on the surface of these two kinds of carbon fibres. The SiC coating is composed of huge amounts of nanometric SiC crystals. Coatings with thickness of about 30 nm and 150 nm were fabricated. Carbon fibres with SiC coating whose thickness is about 30 nm display good flexibility.

Influence of deposition voltage on phase, microstructure and antioxidation property of cristobalite aluminum phosphate coatings

Cristobalite aluminum phosphate (C-AlPO4) coatings were prepared by a hydrothermal electrophoretic deposition process on SiC-coated C/C composites. Phase compositions and microstructures of the as-prepared coatings were characterized by XRD and SEM analyses. The influence of deposition voltage on the phase, microstructure and antioxidation property of the cristobalite aluminum phosphate coatings was investigated. Results show that the as-prepared coatings are composed of cristobalite aluminum phosphate crystallites. The thickness and density of cristobalite aluminum phosphate coatings are improved with the increase of deposition voltage. The deposition amount and bonding strength of the cristobalite aluminum phosphate coatings also increase with the increase of deposition voltage. The deposition mass per unit area of the coatings and the square root of the deposition time at different hydrothermal voltages satisfy linear relationship. The antioxidation property of the coated C/C composites is improved with the increase of deposition voltage. Compared with SiC coatings prepared by pack cementation, the multilayer coatings prepared by pack cementation with a later hydrothermal electrophoretic deposition process exhibit better antioxidation property. The as-prepared multi-coatings can effectively protect C/C composites from oxidation in air at 1 773 K for 37 h with a mass loss rate of 0.53%.

Microstructure and oxidation of multi-layer MoSi 2–CrSi 2–Si coatings for SiC coated carbon/carbon composites

Multi-layer MoSi 2–CrSi 2–Si anti-oxidation coatings with different compositional ratios were prepared on the surface of SiC coated carbon/carbon (C/C) composites by a two-step pack cementation method. The microstructure and anti-oxidation performance of the coating were studied. The results show that the multi-layered coatings could protect the C/C composites from oxidation in air at 1773 K for 1000 h or 1873 K for 750 h, respectively. The anti-oxidation performance of the multi-layer MoSi 2–CrSi 2–Si coating is mainly attributed to their dense and microcrack-free structure, appropriate thermal expansion coefficient and the well dispersed MoSi 2 and CrSi 2 in the coating.

Evaluation of Fracture Stress for the SiC Layer of TRISO‐Coated Fuel Particles Using a Modified Crush Test Method

Fracture stress data for the chemical vapor deposition SiC coatings of tri‐isotropic carbon/silicon carbide‐coated fuel particles were obtained using a newly developed testing and evaluation method, and their relationship with microstructure was investigated. A crush testing technique using a blanket foil at load‐transferring contact has been developed for hemispherical shell SiC specimens based on finite element analysis results. The mean fracture stress varied with test material in the range of 330–650 MPa, and was linked to the combined characteristics of inner surface roughness and porosity.

Effect of Temperature on the Synthesis of SiC Coating on Carbon Fibers by the Reaction of SiO with the Deposited Pyrolytic Carbon Layer

The influence of reaction temperature on the preparation of SiC coating on carbon fibers by the reaction of silicon monoxide with the deposited pyrolytic carbon (PyC) layer has been discussed. With rising reaction temperature, the thickness of SiC layer increases and the SiC grain is coarsening. The apparent activation energy for the synthesis of SiC layer is about 103.3 kJ/mol. The oxidation resistance of carbon fiber can be improved by the SiC/PyC layers significantly. The initial oxidation temperature of the SiC/PyC coated carbon fiber is about 300°C higher than that of the uncoated carbon fiber. The oxidation of the SiC/PyC coated carbon fiber is owing to the diffusion of oxygen through the cracks generated by the mismatch of thermal expansion.

Microstructure characterization of erosion resistant coatings on carbon‐bonded carbon fibre composites

The microstructure of as received and surface treated carbon-bonded carbon fibre composites has been examined by light microscopy, scanning and transmission electron microscopy. The microstructure of the as received material consists of a bonded together layered carbon fiber network, identified as graphitic carbon (hexagonal close packed). To improve the erosion resistance of the carbon-bonded carbon fibre composites composite, the SiC and silicate glass-ceramic coatings from the system SiO(2)-Al(2)O(3)-Y(2)O(3) were produced on carbon-bonded carbon fibre composites composites by a low-cost slurry technique. Transmission electron microscopy investigations of cross-section thin foils allowed for detailed analysis of the coatings microstructure. It was found that the SiC coating was consisting mainly of a nanocrystalline SiC (fcc). The multilayered glass-ceramic coating showed a complex microstructure consisting of an external SiO(2)-Al(2)O(3)-Y(2)O(3) layer and an intermediate nanocrystalline SiC layer. The SiO(2)-Al(2)O(3)-Y(2)O(3) layer was composed of SiO(2) (fcc), Y(2)Si(2)O(7) (op) and Al(4.644)Si(1.357)O(9.68) (op).