Design of a C/SiC functionally graded coating for the oxidation protection of C/C composites

To reduce the residual thermal stress between the carbon fiber-reinforced carbon (C/C) composites and the SiC coating layer, functionally graded materials (FGM) consisting of a C/SiC compositionally graded layer (C/SiC interlayer) were adopted. After designing the compositional distribution of the C/SiC interlayer which can relieve the thermal stress effectively, the deposition conditions of the entire compositional range of the C/SiC composites were determined using a thermodynamic calculation. According to the design and calculation the C/SiC interlayer and the SiC outer layer were deposited on the C/C composites by a low pressure chemical vapor deposition (LPCVD) method at deposition temperatures of 1100 and 1300 °C. The stress calculation and the experimental results suggested that the SiC-rich compositional profile in the FGM layer is the most effective for relieving the thermal stress and increasing the oxidation resistance.     

Molten salt synthesis of a SiC coating on graphite flakes for application in refractory castables

A silicon carbide coating was formed on the surface of graphite flakes by reaction of molten Si with carbon at 1100–1300 °C in a 95%KCl-5%NaF molten salt under Ar atmosphere. The effect of temperature and Si/graphite ratio in the initial mixture on the quality and the amount of SiC were investigated by XRD and SEM/EDS analyses. Also, the water wettability, oxidation resistance and zeta potential of un-coated and coated graphite were examined by TGA analysis and sedimentation test. The results show the amount of coating to increase in the coated flakes with increasing temperature and Si/graphite ratio. The SiC coating improves water wettability of graphite and acts as a protective layer to enhance oxidation resistance. The zeta potential of coated graphite was also increased which indicated a better dispersion in water based systems. These improvements in both the water dispersivity and oxidation resistance of SiC coated graphite would make it as promising candidate raw materials for application in C-containing refractory castables.     

High-Temperature Oxidation at 1500° and 1600°C of SiC/Graphite Coated with Sol–Gel-Derived HfO2

Oxidation of SiC compositionally graded (SCGed) graphite coated with HfO₂ derived from HfCl₄ by a sol–gel process was performed at 1500° and 1600°C in a flowing gas mixture of Ar and O₂ (80/20 kPa). SCGed graphite was produced by reaction of graphite with either molten Si or SiO gas at 1450°C. The sol–gel-derived HfO₂ precursor was deposited on SCGed graphite by a dip-coating method. Isothermal and cyclic oxidation of uncoated- and HfO₂-coated SCGed graphite was studied by monitoring overall weight change using an electro-microbalance. Scanning electron microscopy with energy-dispersive X-ray analysis was used to observe the surfaces and cross-sections of the oxidized HfO₂-coated SCGed graphite. The formation of HfSiO₄ was confirmed on the outer layer of the oxidized sample, beneath which a thin silica layer was formed. The improved oxidation resistance of SCGed graphite by coating with HfO₂ is discussed on the basis of the formation of these two layers.     

A novel ultra-light reticulated SiC foam with hollow skeleton

In this study, ultra-light reticulated SiC foam (SF) with hollow skeleton was prepared by applying chemical vapor deposition technique to deposit SiC layer on carbon foam (CF) skeleton, followed by high temperature oxidation of CF. The microstructures of materials were examined by SEM and SF samples show higher specific surface area (349 ± 13 m²/g), initial oxidation temperature (1000 °C) and compressive stress (0.6 MPa) than CF. The compression test results show that the compressive strength of SF increased with the CVD time. While the compressive strength decreased significantly, when the CVD temperature reached 1200 °C. Keeping in view superior observed related characteristics, the prepared SF with special structures was anticipated to be suitable for catalysis, energy storage or membrane science.     

Fabrication of Machinable Silicon Carbide-Boron Nitride Ceramic Nanocomposites

SiC/BN nanocomposite powders with the microstructure of micrometer-sized SiC particles coated with nanometer-sized BN particles were prepared via a chemical reaction, which used a mixture of boric acid (H₃BO₃) and urea (CO(NH₂)₂) as reactants coated on the surface of the SiC particles to react under a nitrogen-gas atmosphere. The results of XRD, TEM, and SAED studies showed that the coating layer (BN) was composed mostly of amorphous and nanometer-sized BN particles at the reaction temperature of 850°C. When the nanocomposite powders were hot-pressed at 1850°C, machinable SiC/BN ceramic nanocomposites with fine grain size and homogeneous microstructure were fabricated. The composite that contained 20 wt% BN exhibited high strength (the three-point bending strength was 588.4 ± 26.8 MPa) and excellent machinability.     

The improvement in oxidation resistance of carbon by a graded SiC/SiO2 coating

A dense functionally gradient SiC/SiO₂ coating has been developed to improve the oxidation resistance of carbon at elevated temperatures. SiC was coated on the surface of a graphite substrate by a reaction between thermally evaporated silicon and carbon at 1400 °C. The SiO₂ layer was deposited by exposing the SiC coated specimens next to a bed of Si powder in a flowing H₂–H₂O gas (P_H2O=2.6×10⁻² atm) at 1400 °C. The formed SiC/SiO₂ layers were dense and had gradient compositions with good adhesion to the carbon substrate. However, as the coating thickness increased, the coating layer became cracked and delaminated from the substrate due to thermal stress. The specimens with the continuous SiC/SiO₂ layer showed a remarkably improved oxidation resistance up to 1200 °C.     

Strengthening of Silicon Carbide Ceramics by Surface Nitridation during Hot Isostatic Pressing

Surface strengthening of SiC by in situ surface nitridation during post-hot isostatic pressing (post-HIPing) in N₂ was investigated. The formation of a thin (5–15 μm) layer of submicrometer β-Si₃N₄ on the surface of SiC was obtained at 1850°C and 200 MPa. While SiC HIPed in Ar attained a mean bending strength of 660 MPa, a significant increase in strength (with a maximum fracture stress above 1000 MPa) was observed for the SiC/Si₃N₄-layer composite material. Generation of residual compressive stresses on the surface layer caused by the differences in thermal expansion may account for the observed strengthening. Thus, in situ surface nitridation by post-HIPing in N₂ may offer an attractive way to improve surface-sensitive mechanical properties of complex-shaped SiC components.     

Preparing the SiC coated C/C composites with excellent mechanical and antioxidative properties using a buffer layer

The preparation of SiC coating on C/C composites via a pack cementation method would cause serious mechanical damage to C/C substrate due to the siliconization corrosion by molten silicon during the ultra-high-temperature preparation process (2173–2373 K). In order to prepare SiC coated C/C composites with excellent mechanical and antioxidative properties, we applied a buffer layer on the surface of C/C to inhibit siliconization corrosion and densify coating. Results showed that the siliconized area ratio of the C/C substrate was decreased from 60.9% to 24.8%, and its bending strength was increased from 36.9 MPa to 60.6 MPa. Moreover, the mass loss of the modified SiC coated C/C sample has reduced by ~4.14 times after oxidation for 144 h in air at 1773 K and decreased from 2.44% to ? 0.15% after suffering 50 thermal cycles between room temperature and 1773 K.     

碳纤维补强氧化铝陶瓷的研究

采用热压的方法制备了碳纤维／氧化铝复合陶瓷，与未加碳纤维的氧化铝陶瓷相比，其抗弯强度有明显提高，但断裂韧性提高不大。为了改善碳纤维与氧化铝基体的结合状态，利用化学气相沉积的方法分别在碳纤维表面沉积了ＳｉＣ，ＴｉＣ和ＳｉＯ２＋ＳｉＣ。研究结果表明，当采用有沉积层的碳纤维补强氧化铝陶瓷时，抗弯强度和断裂韧性都有明显提高。     

Enhancing thermal conductivity of C/SiC composites containing heat transfer channels

In this study, CNTs/SiC micro-pillars at controlled content ratios were introduced into C/SiC composites as heat transfer channels to improve the thermal conductivity in the thickness direction. The thermal conductivities and bending strengths before and after heat treatment at 1650 °C were investigated and the results were discussed. The theoretical calculations and finite element analyses confirmed that CNTs/SiC micro-pillars successfully worked as heat transfer channels. The theoretical thermal conductivity calculated by effective medium theory (EMT) model was 19.25 W/m⋅k and agreed-well with the experimental value. The measured thermal conductivity was estimated to 20.69 W/m⋅k and improved to 22.36 W/m⋅k after heat treatment. The latter was 3.56-fold higher than that of traditional C/SiC and attributed to increased grain growth during heat treatment. The optimal bending strength before heat treatment was recorded as 324.5 ± 23.74 MPa due to microstructure evolution caused by CNTs. After heat treatment, the bending strength improved by 138 % with ductile fracture mode attributed to ordered layer structure of PyC interphase and complex phase composition of the composites. These features benefited the abundant propagation of cracks and energy consumption. In sum, introduction of heat transfer channels into C/SiC composites provided a new way to improve the thermal conductivity in thickness direction of ceramic matrix composites.     

Joining of C/SiC composites by spark plasma sintering technique

CVD–SiC coated C/SiC composites (C/SiC) were joined by spark plasma sintering (SPS) by direct bonding with and without the aid of joining materials. A calcia-alumina based glass–ceramic (CA), a SiC + 5 wt% B₄C mixture and pure Ti foils were used as joining materials in the non-direct bonding processes. Morphological and compositional analyses were performed on each joined sample. The shear strength of joined C/SiC was measured by a single lap test and found comparable to that of C/SiC.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

Catalytic Fabrication of SiC/SiO2 coated graphite and its behaviour in Al2O3–C castable systems

SiC/SiO₂ coated graphite was prepared via a combined sol-gel coating and catalytic conversion route, using graphite flake and tetraethyl orthosilicate as the starting materials, and Fe(NO₃)₃·9H₂O as the catalyst precursor. X-ray diffraction analysis and microstructural examination revealed that a homogeneous coating comprising SiC and cristobalite (SiO₂) and covering the whole surface of graphite was formed. As prepared SiC/SiO₂ coated graphite exhibited better oxidation resistance and water wettability than its uncoated counterpart. Also, oxidation resistance and slag corrosion resistance of a model Al₂O₃–C castable using coated graphite as a carbon source were better than in the case of its counterpart using uncoated graphite.     

Preparation and high temperature oxidation of SiC compositionally graded graphite coated with HfO2

SiC compositionally graded (SCG) graphite was coated with sol-gel-derived HfO₂ films and oxidized at 1500 °C in air. SCGed graphite was produced by reaction of graphite with molten Si at 1450 °C for 10 h. The sol-gel HfO₂ precursor solution was prepared by dissolving HfCl₄ in ethanol and refluxing with diethanol-amine and HNO₃, and was coated on SCGed graphite by dipping. The HfO₂-coated SCGed graphite was produced by decomposition of the precursor under conditions determined from the results of TG, DTA, and MS analysis. Oxidation of HfO₂-coated SCGed graphite was performed at 1500 °C in air, revealing a small weight loss (0.6 mg cm⁻²) after 15 h. It was found that HfO₂-coated SCGed graphite exhibits extremely high oxidation resistance, which may be due to the formation of HfSiO₄ acting to heal pores or cracks.     

裂解碳包覆对SiC纳米纤维增强SiC陶瓷基复合材料性能的影响

以SiC纳米纤维(SiC_nf)为增强体,通过化学气相沉积在SiC纳米纤维表面沉积裂解碳(PyC)包覆层,并与SiC粉体、Al₂O₃-Y₂O₃烧结助剂共混制备陶瓷素坯,采用热压烧结工艺制备质量分数为10%的SiC纳米纤维增强SiC陶瓷基(SiC_nf/SiC)复合材料。研究了PyC包覆层沉积时间对SiC_nf/SiC陶瓷基复合材料的致密度、断裂面微观形貌和力学性能的影响。结果表明:在1 100 ℃下沉积60 min制备的PyC包覆层厚度为10 nm,且为结晶度较好的层状石墨结构;相比于纤维表面无包覆层的复合材料,复合材料的断裂韧性提高了35%,达到最大值(19.35±1.17) MPa·m^1/2,抗弯强度为(375.5±8.5) MPa,致密度为96.68%。复合材料的断裂截面可见部分纳米纤维拔出现象,但SiC_nf/SiC陶瓷基复合材料界面结合仍较强,纳米纤维拔出短,表现为脆性断裂。     

Insight into microstructural architectures contributing to the tensile strength of continuous W-core SiC fiber

The properties of CVD silicon carbide (SiC) fiber are closely correlated with its complex microstructure; however, the microstructural architectures contributing to its strength are not well understood. Here, two kinds of W-core SiC fibers (SiC-3200 and SiC-3800) coated with same C coating, possessing different tensile strength and discrepant microstructure in SiC portion, were fabricated by controlling deposition temperature during CVD process. The structure discrepancy contributing to the different tensile strength for both SiC fibers were explored by the combined Raman spectra, EPMA, SEM and HRTEM. It is revealed the high-crystallinity β-SiC columnar grains in the inner zone always evolve into a higher disorder character yet involving partial surviving columnar grains toward surface for both SiC fibers. However, as compared with SiC-3800 fiber, the slightly higher C over-stoichiometric composition yields lower-crystallinity SiC columnar grains across the radius for SiC-3200 fiber, further resulting in lower strength via forming smoother stepped fracture morphology.     

Effect of Carbon and Boron on the High-Temperature Oxidation of Silicon Carbide

Silicon carbide has good oxidation resistance, due to the formation of a protective silica layer. Although amorphous silica is an excellent oxygen barrier, it is very sensitive to impurity elements, which affect its viscosity, oxygen diffusivity, and crystallization kinetics. This paper compares the oxidation rates of CVD SiC, sintered α-SiC, and CVD SiC- coated graphite in 1 atm oxygen at 1500^deg;C to determine the effects of small additions of boron and carbon. The formation of bubbles in the silica scale formed on sintered α-SiC in oxygen between 1230° and 1550^deg;C is also discussed.     

Oxidation and erosion resistance of multi-layer SiC nanowires reinforced SiC coating prepared by CVD on C/C composites in static and aerodynamic oxidation environments

A multi-layer SiC nanowires reinforced SiC (SiCnws-SiC) coating was prepared in-situ on carbon/carbon (C/C) composites by three chemical vapor deposition (CVD) processes. The microstructure and phase composition of the nanowires fabricated on the first-layer SiCnws-SiC coating and the coatings were examined by SEM, TEM, and XRD. The bamboo-like SiC nanowires with a 50?nm diameter and a length of several tens of micrometers are straight, randomly orientated and distributed like a net on the first-layer SiCnws-SiC coating. The growth direction is [111], and the growth mechanism is VS. The multi-layer SiCnws-SiC coating has three layers: the thickness of the first-layer is roughly 400?µm, and the outer two layers are about 200?µm. Each layer has a sandwich structure. The isothermal oxidation and erosion resistance of the multi-layer SiCnws-SiC coating were investigated in an electrical furnace and a high temperature wind tunnel. The results indicated that the weight loss of the multi-layer SiCnws-SiC coated C/C composites was only 1.8% after oxidation in static air at 1773?K for 361?h. Further, the coated sample failed due to fracture of the coating at the clamping position (i.e. 80?mm) after erosion at 1873?K for 130?h in the wind tunnel. The weight loss of the coated C/C composites occurred due to the formation of penetrating cracks in the coating during the oxidation thermal shock. The maximum bending moment and the larger clamping force caused the coating fracture and resulted in intense oxidation of the substrate and the failure of the specimen.     

High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings

A nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction. The effects of the coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and flexural strength of silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the specimens, and the results indicated that the formed nanosized alumina coatings could retard cristobalite formation by inducing compressive stress on the fused silica particle surface. A mullite phase was also found due to the reaction of the nanosized alumina coating and the surface of the fused silica when the sintering temperature was increased to 1300 °C. Analysis using scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) suggested that alumina nanoparticles in the coated layer dispersed into a liquid phase and formed a barrier layer to impede the movement of the liquid phase, preventing the pore-filling process and increasing the open porosity of the ceramic specimens. Flexural strengths at room temperature were tested, indicating that increases in the sintering temperature of the specimens without coated fused silica powders had little effect on flexural strength. However, the flexural strength of the specimens with coated fused silica powders increased with increases in sintering temperature. The improvement in flexural strength was related to the reinforcement by sintering necks between particles and the improvement in the strength of the coated fused silica powder.     

Mechanical Behavior of Alumina–Silicon Carbide „Nanocomposites”

The fracture behavior of Al₂O₃ containing 5 vol% 0.15μm SiC particles was investigated using indentation techniques. A significant increase in strength was achieved by the addition of SiC particles to the base Al₂O₃. Specifically, the strength increased from 560 MPa for Al₂O₃ to 760 MPa for the composite samples (average values for unindented hotpressed bars tested in four-point bending). After annealing for 2 h at 1300°C, the average strength of the composite samples increased to about 1000 MPa. Toughness was estimated using indentation-strength data. While there was a slight increase in toughness, the increase was not sufficient to account for the increase in the unindented strength on SiC particle addition. It is suggested that the observed strengthening and apparent toughening were due to a machining-induced compressive surface stress.