SiC Coatings Research Articles

Improvement of oxidation resistance of carbon fibers by coating SiC

SiC-coated carbon fibers (CFs) were prepared by spark plasma sintering, and their phase compositions and microstructures were identified by x-ray diffraction and characterized by field-emission scanning electron microscope, respectively. The oxidation behaviors of the SiC-coated CFs was examined through thermo gravimetric analysis and compared with that of pristine CFs. The effects of Si content in the SiC coatings on the surfaces of the CFs were also investigated. SiC coating with 25 wt% Si contributing to the reaction was thicker than that with 10 wt% Si but had weaker oxidation-resistant properties. When coated with SiC, the CFs exhibited considerable improvement in their oxidation resistance, and the temperature for its oxidation increased nearly by 140 °C.

Manufacturing and Wear Properties of SiC Coating Layer on Zr alloyFabricated by Vacuum Kinetic Spray Process

Manufacturing and Wear Properties of SiC Coating Layer on Zr alloyFabricated by Vacuum Kinetic Spray Process

Atmospheric plasma spraying of nanocrystalline SiC particle reinforced MoSi2 prepared by mechanically activated annealing process

The mechanically activated annealing process was adopted to synthesize nanostructure MoSi2–SiC powders containing different SiC contents using Mo, Si, and SiC powders. Atmospheric plasma spraying technique was used to deposit the agglomerated powders of MoSi2–SiC nanocomposite under inert conditions on a nickel substrate. Phase identification and microstructural characteristics of the nanopowders and coatings were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and qualitative energy-dispersive spectroscopy (EDS). The microstructure of the plasma-sprayed coating consists of α and β-MoSi2, SiC, Mo5Si3, and a trace of SiO2 phases. It can be concluded that mechanically activated annealing synthesis with agglomeration is an effective method for the preparation of high-melting-point refractory compounds for using in plasma spraying process. The measured adhesion strength, microhardness, and roughness results of MoSi2–SiC coatings are also reported in the present work.

EXPERIMENTAL STUDIES FOR THE HIGH-TEMPERATURE OXIDATION OF THE HTGR FUEL PROTOTYPE

This paper presents the results of an experiment on high-temperature oxidation of SiC coating of a prototype HTGR fuel in an oxygen environment under the following experimental conditions: the maximum temperature of the samples with linear heating is 1400 °C; heating rate – 10 °С/min. According to the results of the experiments, dependences of the change in the composition of the gas mixture in the corrosion chamber with linear heating of the sample were obtained. A qualitative analysis of corrosion data and microstructural research data was carried out before and after the corrosion experiment. The results obtained allow us to estimate the corrosion resistance and the loss of the operational properties of SiC coatings on prototype HTGR fuel.

Strengthening of C/SiC Composites by Electrophoretic Deposition of CNTs on a SiC Coating

The current study reports the enhancement of mechanical properties of carbon fiber-reinforced silicon carbide ceramic matrix composites (C/SiC CMCs) by the application of a carbon nanotube/silicon carbide (CNT/SiC) coating. CNTs were deposited on the surfaces of C/SiC composites using electrophoretic deposition (EPD), after which infiltration by SiC was achieved through a chemical vapor infiltration process. An EPD duration of 5 min was associated with a 40% increase in the ultimate flexural strength relative to that of composites with a pure SiC coating. The observed enhancement was rationalized by the microstructural observations of SiC infiltration into the porous CNT morphology and the subsequent formation of CNT/SiC layers on the surfaces of the composites and by the inherent toughness of the SiC whiskers. The flexural strength decreased with EPD durations greater than 5 min due to the formation of thick CNT meshes, which decreased the open porosity and thereby obstructed further SiC infiltration. This is a viable methodology for the improvement of mechanical properties of CMCs by the introduction of a ceramic coating containing CNT.

Oxidation resistance of valve steels covered with thin SiC coatings, obtained by RF CVD

Oxidation studies at 1173 K under isothermal and thermal shock conditions performed on four different valve steels (X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8) covered with a 2 μm thick SiC coating revealed that they exhibit greater corrosion resistance compared to the uncoated steels. This is due to the formation of highly protective MnCr2O4 on coated samples in contrast to uncoated materials, on which Fe3O4 and Fe2O3 oxides with poor protective properties are detected. Studies performed in compressed natural gas also demonstrate the positive effect of SiC application. Obtained results enable tailoring a new generation of inexpensive coating for engine valve protection.

Mechanical properties of CVD-SiC coatings with Si impurity

SiC coatings with different atomic concentration of Si impurity were coated on graphite substrates by chemical vapor deposition (CVD) technique at 1150 °C in the present work. The mechanical properties, crystallinity of SiC and Si in the coating, and surface roughness of the coatings were found to decrease with the decrease of Si concentration. The results suggest that the mechanical properties of the SiC coatings were affected mainly by the crystallinity of the coatings, while the concentration of Si impurity played less important role.

Gradient HfB2-SiC multilayer oxidation resistant coating for C/C composites

The gradient HfB2 modified SiC coating was prepared on the surface of SiC-coated C/C composites by in-situ synthesis. Anti-oxidation behaviors of the coated C/C samples at 1773, 1873 and 1973 K were investigated. The results show that the gradient HfB2 modified SiC coatings possess excellent oxidation resistance, which can protect C/C substrates from oxidation for 800, 305 and 100 h at 1773, 1873 and 1973 K, respectively. In addition, with the oxidation temperature increasing, the evaporation of the Hf-Si-O glass layer and the active oxidation of SiC were accelerated, which is the reason for the worst oxidation resistance of the sample at 1973 K among the three temperatures.

Effect of free carbon on micro-mechanical properties of a chemically vapor deposited SiC coating

SiC coatings with different content of free carbon were deposited on polycrystalline silicon substrate by chemical vapor deposition (CVD) by using CH3SiCl3-H2-Ar reactive system. The composition, microstructure and micro-mechanical properties of coatings were investigated. The results show that, when H2/MTS is 10, there is no free carbon in coatings. However, at the ratio of H2/MTS of 8.89, 7.97 and 7.32, the content of free carbon is falling in the range of 0.025–0.065, 0.100–0.800 and 0.005–0.035 respectively. The supersaturation of [H] is a key factor to control the generation of free C and the nucleation and growth of SiC coating. As to mechanical properties of SiC coatings, there may be a critical value of the content of free carbon in the coating. When the content of carbon is less than the critical value, the free carbon can enhance the mechanical properties of the coating; otherwise, the free carbon weakens the mechanical properties.

The Ablation and Oxidation Behaviors of SiC Coatings on Graphite Prepared by Slurry Sintering and Pack Cementation

SiC coatings were generated on graphite using slurry sintering (SS) and pack cementation (PC). The samples’ ablation features were assessed by an oxyacetylene torch. The rates of mass ablation of the PC–SiC and SS–SiC coatings were approximated 2.17 × 10−3 and 9.52 × 10−3 g s−1, respectively, decreased by 84.1 and 29.6% compared to the uncoated samples. It was mainly attributed to the formation of a SiO2 layer on the surface. The continuous SiO2 molten film formed via the PC–SiC oxidation generates a sealing mechanism which can be an obstacle against the oxygen diffusion and hinder more ablation. This is while discontinuous SiO2 film formed from the thin SS–SiC cannot protect the graphite effectively. The non-isothermal oxidation test shows that without the SiC coating, the sample weight is lost largely from 25 to 1500 °C, and its weight loss was 2.2% after the TGA. However, after coating, the samples possessed excellent oxidation protection and weight losses of SS–SiC and PC–SiC coatings are down to 1.3 and 0.6%, respectively. The more oxidation of the graphite substrate occurred due to the formation of macrocracks in the coating during the TGA and also the formation of holes on SiO2 glass layer owing to release of CO or CO2.

The effects of carbon/carbon composites blasting treatment and modifying SiC coatings with SiC/ZrB2 on their oxidation and cyclic ablation performances

To enhance the oxidation and ablation performances of SiC coating, C/C composites with a porous surface layer were designed via blasting treatment and then followed by ZrB2 additive in the process of pressure-less reactive sintering. Cyclic ablation test using oxyacetylene torch from 1750 °C to room temperature showed that the coating possessed good ablation performance accompanying with the formation of Zr-O-Si glassy protective layer. The good sintering performance combined with the increased coating/substrate interfaces contributed to reduce the cyclic thermal stress and enhance the stability of the Zr-O-Si layer to the denudation of oxyacetylene torch, thus promoting the ablation performance.

Design of Corrosion Resistive SiC Nanolayers.

Recently, we have shown that the protecting layer of nanosize can be produced by means of ion beam mixing (IBM) of a Si/C multilayer system. The corrosion resistance of the layer correlated with the SiC amount and distribution, determined by Auger electron spectroscopy depth profiling. It has also been shown that the IBM of the Si/C system can be well described by TRIDYN simulation. By combining these two findings, it is possible to design protective layers for various arrangements of layer structure and irradiation conditions. Three different multilayer structures (with individual layer thicknesses falling in the range of 10-20 nm) have been irradiated by Ar+ and Xe+ ions at room temperature in the energy and fluence ranges of 40-120 keV and 0.25 × 1016 to 6 × 1016 ion/cm2, respectively. The carbon and silicon depth distributions have been calculated by TRIDYN simulation. From these profiles applying a simple rule for compound formation, the SiC in-depth distributions were calculated. The resulting corrosion resistance has been measured by potentiodynamic corrosion test in 4 M KOH solution. Excellent correlation between these results and the in-depth distribution (calculated by TRIDYN simulation) of SiC has been found. Thus, the design of a protective SiC coatings operating in harsh environments is possible by applying fast and cheap simulation techniques.

Oxidation protection and mechanism of the HfB2-SiC-Si/SiC coatings modified by in-situ strengthening of SiC whiskers for C/C composites

To improve the oxidation resistance and alleviate the thermal stress of the HfB2-SiC-Si/SiC coatings for C/C composites, in-situ formed SiC whiskers (SiCw) were introduced into the HfB2-SiC-Si/SiC coatings via chemical vapor deposition (CVD). Effects of SiCw on isothermal oxidation and thermal shock resistance for the HfB2-SiC-Si/SiC coatings were investigated. Results showed that the SiCw-HfB2-SiC-Si/SiC coatings exhibited excellent oxidation resistance for C/C composites with only 0.88% weight loss after oxidation for 468 h at 1500 °C, which was markedly superior to 4.86% weight loss for coatings without SiCw. Meanwhile, after 50 times thermal cycling, the weight loss of the SiCw-HfB2-SiC-Si/SiC coated samples was 4.48%, which showed an obvious decrease compared with that of the HfB2-SiC-Si/SiC coated samples. The SiCw-HfB2-SiC-Si/SiC coatings exhibited excellent adhesion to the C/C substrate and had no penetrating cracks after oxidation. The improved performance of the SiCw-HfB2-SiC-Si/SiC coatings could be ascribed to the SiCw, which effectively relieved CTE mismatch and remarkably suppressed the cracks through toughening mechanisms including whiskers pull-out and bridging strengthening. The above results were confirmed by thermal analysis based on the finite element method, which demonstrated that SiCw could effectively alleviate thermal stress generated by temperature variation. Furthermore, the SiCw-HfB2-SiC-Si/SiC coating can provide a promising fail-safe mechanism during the high temperature oxidation by the formation of HfSiO4 and SiO2, which can deflect cracks and heal imperfections.

The effect of HfB2 content on the oxidation and thermal shock resistance of SiC coating

SiC coatings modified by HfB2 were synthesized by in-situ reaction at 2173–2373 K. The effect of HfB2 content on the oxidation and thermal shock resistances of the modified SiC coatings was investigated. HfSiO4, the oxidation products of HfB2 during oxidation test, dispersed uniformly in the coating and efficiently promoted crack deflection and termination. Therefore, a proper amount of HfB2 is helpful for improving the oxidation resistance of the SiC coating, while excessive quantity of HfB2 caused some defects in the coating due to its coefficient of thermal expansion (CTE) mismatches with that of the substrate. 18.2 wt% HfB2 modified SiC coating has the best oxidation resistance. Compared to the SiC coating without HfB2, the mass losses of SiC coating modified by 7.3 wt% and 18.2 wt% HfB2 decreased by 67.5% and 76.8% after oxidation, respectively. Moreover, the mass loss of the modified sample declined just by 25.2%, 25.8% after thermal cycle, respectively. But SiC coating modified by 31.5 wt% HfB2 has worse resistance behavior to oxidation and thermal shock than SiC coating.

Effect of precoated carbon layer on microstructure and anti‐erosion properties of SiC coating for 2D‐C/C composites

AbstractTo improve the erosion resistant of carbon‐carbon composites, an SiC coating was synthesized on carbon‐carbon composites by the in situ reaction method. They are firstly coated with carbon layer by slurry, and then SiC coatings are obtained by chemical vapor reaction. The effects of precoated carbon layer on the microstructure and anti‐erosion properties of SiC‐coated C‐C composites were studied and characterized. The thickness of the SiC coating increased with the increase in the precoated carbon layer thickness. The different thickness of carbon layer affects hardness of the SiC coatings, resulting in diverse erosion resistance of the coatings. The SiC coating prepared with moderate thickness of precoated carbon layer exhibits the best erosion resistance, and show better resistance at an impact angle of 30° than 90°. The eroded surface revealed that coating cracking and brittle fracture, fiber‐matrix debonding, fiber breakage, and material removal, and the additional microcutting and microploughing at oblique impact angle are the major erosion mechanism of SiC coating for C/C composites.

First-principles study for strain effects on oxygen migration in zirconium

Oxygen interstitial atom diffusion in bulk α-Zr is studied using first-principles calculation and nudged elastic band method under various tetragonal strain conditions. Octahedral site is a stable position for an oxygen interstitial in α-Zr. An oxygen interstitial atom may diffuse from one octahedral site to another via a tetrahedral site or a crowdion site. An oxygen atom can travel around zirconium bulk with various combination of these diffusion processes. The diffusion from an octahedral site to a tetrahedral site or a crowdion site is the controlled step. The strain effects on the two diffusion mechanisms are different. In general, the diffusion rate decreases under certain tetragonal compressive strain conditions. As a result, the oxidation rate decreases under certain compressive strain conditions.Since surface coating may be used to protect zirconium metal surface from oxidation, the surface strain generated from the interfacial mismatch can be used for the material design. In our calculations, SiC, TiN, and Al2O3 coatings may slow down the oxidation rate of zirconium because they can induce compressive strain on zirconium near the interfacial region.

Si impurity concentration in nominally undoped Al0.7Ga0.3N grown in a planetary MOVPE reactor

The unintentional silicon incorporation during the metalorganic vapor phase epitaxy (MOVPE) of nominally undoped Al0.7Ga0.3N in a Planetary Reactor under various growth conditions was investigated. Dependent on growth temperature, pressure and V/III ratio, Si concentrations of below1 × 1016 up to 4 × 1017 cm−3 were measured. Potential Si sources are discussed and, by comparing samples grown in a SiC coated reactor setup and in a TaC coated setup, the SiC coatings in the reactor are identified as the most likely source for the unintentional Si doping at elevated temperatures above 1080 °C. Under identical growth conditions the background Si concentration can be reduced by up to an order of magnitude when using TaC coatings.

Thermal cycling resistance of ZrB2‒SiC coatings with MgO additive on SiC coated graphite

To improve the thermal cycling resistance of ZrB2–SiC/SiC dual layer coating, MgO was added into the pack powders used to prepare ZrB2–SiC outer coating by pack cementation. The composition, microstructure and thermal cycling resistance of these coatings were studied. The results show that as-received coatings were mainly composed of SiC, ZrB2 and Si. ZrB2–SiC outer coating contained 5wt%, 7wt%, 9wt% MgO additive showed better thermal cycling resistance with 97.6%, 98.8% and 96.5% weight retention after 15 thermal shock cycles between 1500°C and room temperature.

Ablation resistance of carbides-coated C/C composites

ABSTRACTTo improve the ablation resistance of C/C composites under oxyacetylene flame, SiC, TaC and HfC coatings were fabricated on composites surface. Ablation properties of coatings were detected and the ablation mechanism was discussed in the paper as well. The centre region of SiC and TaC coatings has failed during ablation and exhibited high ablative rates; it attributed to so high ablation temperature, which was much higher melting point of their oxides (Ta2O5 and SiO2). HfC coating exhibited lowest ablative rates. The reason is that the ablation temperature was just between sinter point and melting temperature of HfO2, the oxides could behave as the thermal barrier coating during ablation and provide efficient protection for C/C composites. It can be indicated that the carbide coating provides best ablation protection between the Tammann temperature and melting point of their oxides.

Surface Diversifying and Deuterium Retention Characteristics of Doped Graphite with SiC Coating Exposed to Plasma Discharges in EAST Tokamak

Surface Diversifying and Deuterium Retention Characteristics of Doped Graphite with SiC Coating Exposed to Plasma Discharges in EAST Tokamak