In situ Y2Si2O7 coatings on SiC fibers: Thermodynamic analysis and processing

It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for the formation of Y₂Si₂O₇ coatings on SiC. The proposed method is validated using an experimental procedure in which the in situ formation of Y₂Si₂O₇ on a commercial SiC-based fiber is demonstrated. The method involves the deposition of YPO₄ on preoxidized fine diameter SiC-based fibers, and heat treating the coated fibers within a calculated processing window of oxygen partial pressure, temperature, degree of preoxidation, and coating thickness. The results are promising for the development of environmentally resistant interfacial coatings for SiC-fiber reinforced SiC-based matrix composites. The proposed and validated approach allows a low-cost method to obtain continuous hermetic coatings on SiC fibers with interfacial properties adequate for tough composite behavior that resists degradation under turbine engine conditions.     

The yttria-stabilized zirconia and interfacial coating on Nicalon™ fiber

Sols of yttria-stabilized zirconia may be used as simple, readily processable and accurate controllable precursors for the ZrO₂ interfacial coatings on SiC-based Nicalon™ fibers. The ZrO₂ interfacial coatings of predictable crystal phase compositions were obtained in dependence of yttria dopant level. The morphology, composition and oxidation resistance of coated fibers were evaluated by SEM, EDS, XPS, XRD, and Raman analysis. All coatings obtained are uniform, continuous and adherent to substrates. The delamination within the ZrO₂ interfacial coating was found. Possible reasons of this phenomenon are discussed. The peculiarities of the behavior of Y-stabilized ZrO₂-coated fibers in air at elevated temperature are considered.     

The high temperature reaction of ammonia with carbon and SiC–C ceramics

This contribution aimed at developing a treatment under ammonia in order to eliminate free carbon from the surface of SiC-based fine ceramics like fibers or coatings. The reaction of NH₃ with graphitic and non-graphitic carbon was first investigated through kinetic measurements, in situ gas phase analysis and physicochemical investigations of the solid. The carbon etching rate is controlled by heterogeneous reactions involving active sites arising from bulk structural defects and the formation of HCN. A selection of SiC-based fibers and coatings with various carbon contents and (micro)structures was treated in ammonia in favorable conditions. The analyses of the tested SiC–C specimens revealed a reduction of the free carbon content and, simultaneously, a nitridation of the initial Si–C–(O) continuum over a reaction layer. The growth rate, composition and the volume change of this layer vary with the initial microstructure. The ammonia treatment is able to restore the adhesion of carbon-contaminated surfaces.     

Degradation of a SiC-SiC composite in water vapor environments

The article examines degradation of a SiC-based fiber composite containing Tyranno ZMI fibers in water vapor at elevated temperatures (800°C and 1100°C). Degradation is characterized through mechanical tests under cyclic and quasi-static tensile loading in the near-threshold regime, at stresses at or slightly above the matrix cracking limit. These tests are augmented by examinations of fracture surfaces and polished cross-sections, measurements of fracture mirror radii, and measurements of interfacial debond toughness and sliding resistance. Degradation involves highly localized consumption of fibers through reactions of water vapor with the fibers and the BN coatings in regions adjacent to the few matrix cracks present at low stresses; the global hysteresis response and the average interfacial properties are minimally affected. Boria formed by oxidation of BN appears to play a fluxing role; it combines with silica on the fibers to form a non-protective molten glass. Inhomogeneous fiber consumption leads to stress concentrations in the fibers and hence reduced fiber strength. Spatial variations in the degradation process occur at two length scales: at the macroscopic scale, because of cracking of the external CVI SiC overcoat and subsequent water ingress through the cracks, and at the tow-scale, because of cracking of the CVI SiC around the tows. Parsing the kinetic processes over the two length scales remains a significant challenge.     

The design of zirconium and hafnium germanate interphase in SiCf/SiC composites

Unidirectional SiC_f/SiC minicomposites with SiC matrix derived by polymer-impregnation pyrolysis (PIP), reinforced with SiC fibers coated with zirconium or hafnium germanate were fabricated. Microdebonding indentation tests for SiC_f/SiC composites with one- and multilayered germanate interphase were performed. Interfacial shear stress depending on the number of germanate interfacial layers and morphology was determined. The microstructure of the minicomposites and indented fracture surfaces were studied by scanning electron microscopy (SEM). It was stated that an increase in the number of interfacial coatings leads to a decrease in the interfacial frictional stress in SiC_f/SiC minicomposites with germanate interphases. The key factor of interphase weakening is the formation of a weak interlayer bonding within the interphase but not germanate layered crystal structure itself. Thus, bonding at the fiber/matrix boundary could be regulated by the number of layers of ZrGeO₄ or HfGeO₄ in the interphase zone.     

Effect of Hydrogen Atmosphere on Pyrolysis of Cured Polycarbosilane Fibers

SiC-based fibers with various chemical compositions were synthesized using an irradiation-curing process. Polycarbosilane (PCS) fibers were cured by irradiation with an electron beam in a helium atmosphere. The cured PCS fibers were pyrolyzed at 1300°C under controlled hydrogen or argon atmospheres, and SiC fibers with C/Si of 0.84 to 1.56 were obtained. The fibers consisted of <1.0 wt% O, <0.2 wt% N, <0.1 wt% H, with the balance being Si and C. The mechanism of pyrolytic transformation of cured PCS to SiC-based ceramics was investigated using TG/DTA analysis. Greater mass losses were observed during pyrolysis in a hydrogen atmosphere than in argon. This result suggests that the hydrogen atmosphere suppresses H₂ evolution and helps to remove excess carbon as CH₄ during pyrolysis. The microstructure and mechanical properties of the resulting SiC-based fibers were found to be very dependent on their C/Si chemical compositions.     

Monazite Coatings on SiC Fibers I: Fiber Strength and Thermal Stability

Commercially available SiC fibers were coated with monazite (LaPO₄) using a continuous vertical coater at 1100°C. Coated fibers were heat treated in dry air, argon, and laboratory air at 1200°C for 1–20 h. The tensile strengths of uncoated and coated fibers were measured and evaluated before and after heat treatment. Fiber coating did not degrade SiC fiber strength, but heat treatment afterwards caused significant degradation that correlated with silica scale thickness. Possible strength degradation mechanisms for the coated fibers are discussed. Coating morphology, microstructure, and SiC oxidation were observed with scanning electron microscopy and transmission electron microscopy. Monazite reacted with SiC to form lanthanum silicate (La₂Si₂O₇) in argon, but was stable with SiC in air. Despite the large coefficient of thermal expansion difference between monazite and SiC, micron thick monazite coatings did not debond from most types of SiC fibers. Possible explanations for the thermomechanical stability of the monazite fiber coatings are discussed.     

Appropriate thickness of pyrolytic carbon coating on SiC fiber reinforcement to secure reasonable quasi-ductility on NITE SiC/SiC composites

Pyrolytic carbon (PyC) coating of silicon carbide (SiC) fibers is an important technology that creates quasi-ductility to SiC/SiC composites. Nano-infiltration and transient eutectic-phase (NITE) process is appealing for the fabrication of SiC/SiC composites for use in high temperature system structures. However, the appropriate conditions for the PyC coating of the composites have not been sufficiently tested. In this research, SiC fibers, with several thick PyC coatings prepared using a chemical vapor infiltration continuous furnace, were used in the fabrication of NITE SiC/SiC composites. Three point bending tests of the composites revealed that the thickness of the PyC coating affected the quasi-ductility of the composites. The composites reinforced by 300?nm thick coated SiC fibers showed a brittle fracture behavior; the composites reinforced 500 and 1200?nm thick PyC coated SiC fibers exhibited a better quasi-ductility. Transmission electron microscope research revealed that the surface of the as-coated PyC coating on a SiC fiber was almost smooth, but the interface between the PyC coating and SiC matrix in a NITE SiC/SiC composite was very rough. The thickness of the PyC coating was considered to be reduced maximum 400?nm during the composite fabrication procedure. The interface was possibly damaged during the composite fabrication procedure, and therefore, the thickness of the PyC coating on the SiC fibers should be thicker than 500?nm to ensure quasi-ductility of the NITE SiC/SiC composites.     

Synthesis of a silicon carbide coating on carbon fibers by deposition of a layer of pyrolytic carbon and reacting it with silicon monoxide

A method for preparing a SiC coating on carbon fibers is presented. The SiC coating was generated from the reaction of silicon monoxide (SiO) with a pyrolytic carbon (PyC) layer deposited on the fibers. The influence of holding time on the microstructure of the SiC layer was discussed. The oxidation behaviors of the uncoated and SiC coated carbon fibers were compared. The formation mechanism of the SiC coating was evaluated. With increased reaction time, the SiC coating becomes thicker and its surface becomes rough. The oxidation resistance of the carbon fiber was improved by the SiC coating. The initial oxidation temperature of the SiC coated carbon fiber is about 200 °C higher than that of the uncoated carbon fiber. The growth of the SiC coating is mainly attributed to the indirect reactions of SiO with PyC in the SiO/SiC/C system, in which silicon is considered a critical intermediate product.     

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

To 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.     

In Situ Gas–Solid Reaction and Oxidation Resistance of Silicon Carbide Coating on Carbon Fibers

Silicon carbide (SiC) coating on carbon fibers was realized based on in situ low‐temperature gas–solid reaction processing in which carbon reacted with Si vapor at the temperature of 1200°C–1300°C. X‐ray diffraction (XRD), field‐emission scanning electron microscopy (FE‐SEM), and energy‐dispersive spectroscopy (EDS) analysis showed that the SiC coating was uniform and crystallized by beta‐SiC. The oxidation resistant properties of the SiC‐coated carbon fibers were significantly improved according to isothermal oxidation measurement. The initial oxidation temperature of the SiC‐coated carbon fibers was about 200°C higher than that of the raw carbon fibers. The SiC‐coating carbon fibers treated at 1250°C possessed higher antioxidant property than the one treated at 1300°C.     

Wet chemical deposition of BN,SiC and Si3N4 interphases on SiC fibers

Fiber coatings based on BN, BN/SiC and BN/Si₃N₄ were deposited on Hi Nicalon type S SiC fibers. The coating parameters were optimized using a design of experiments study. With optimized parameter sets, the coatings exhibited a high degree of coverage on the fibers and almost no fiber bridging could be observed. The coated fiber bundles are flexible and can be processed further by techniques such as filament winding. In comparison to a non-processed reference sample, the maximum tensile load of the fiber bundles with BN, BN/SiC and BN/Si₃N₄ coatings was reduced by only 5 %, 13 % and 10 %, respectively. The coated fiber bundles retained their tensile strength after thermal annealing up to 1650 °C in a nitrogen atmosphere for 0.5 h. SiC_f/SiC samples with BN/SiC fiber coatings exhibited higher values of bending strength and strain-to-failure as a reference sample without fiber coating indicating the functionality of the fiber coatings.     

Degradation of boron nitride interfacial coatings fabricated by chemical vapor infiltration on SiC fibers under ambient air/room temperature conditions

Hexagonal boron nitride (h-BN) interfacial coatings were deposited on SiC fibers by chemical vapor infiltration (CVI) and their degradation behavior under ambient air/room temperature conditions was studied with time. Degradation of the interfacial coatings with time was investigated by characterizing the morphology and microstructure of these materials with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Thermogravimetry coupled with differential thermal analysis (TG-DTA) and X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical reactions and phase transitions taking place in the BN coatings. The results showed that the as-deposited BN interfacial coatings fabricated by CVI were compact and well bonded to the SiC fibers. BN coatings remained relatively stable under ambient air/room temperature conditions for 50?h, while severe degradation was observed after 500?h of exposure. The degradation of BN interfacial coatings was mainly caused by two factors, namely, reaction with atmospheric air to produce boric oxide and amorphization of the hexagonal structure. The degradation observed under ambient air/room temperature might be due to incomplete crystallinity of BN interfacial coatings. Presence of water vapor may accelerate degradation of the coatings. The results of this degradation test can be used as a reference for the storage of BN coatings fabricated by CVI.     

Carbothermal Synthesis of Al-O-N Coatings Increasing Strength of SiC Fibers

Non-bridging Al-O-N coatings have been synthesized on the surface of Tyranno ZMI SiC fibers by a low-cost carbothermal nitridation method. First, a nanoporous carbide-derived carbon (CDC) layer is produced on the surface of SiC fiber by the extraction of Si with chlorine; the CDC layer on the fiber is then infiltrated by AlCl₃ solution, and finally nitrided in ammonia at atmospheric pressure to produce the coating. The intermediate carbon layer acts as a template for the coating, facilitates the formation of aluminum oxynitride, and helps to build a strong bonding between the fiber and coating. Optimization of the process parameters led to a more than 65% improvement in the tensile strength (up to ∼5.1 GPa) and a three-time increase in the Weibull modulus for the fiber with 200 nm coating compared to the as-received fibers. The coated fiber exceeds the strength of all other small-diameter SiC fibers reported in the literature. Al-O-N coating may also provide oxidation protection for the fibers in high-temperature applications.     

Strength degradation of SiC fibers with a porous ZrB2-SiC coating: Role of the coating porous structure

ZrB₂-SiC coatings with varied porous structures were deposited on SiC fiber tows using the sol-gel method and cured at 1400 ℃ in vacuum. Tensile strength of the coated SiC fibers were much lower than that of the uncoated fibers. The bimodal distribution in the Weibull plot of the coated SiC fibers demonstrated that the fracture of the coated fiber can be attributed to two types of defects: the porous structure of the coating and the fiber defects. Detailed morphology and microstructure characterization of the coating and fiber combined with strength calculation were carried out to investigate the individual contribution of the fiber defects and the porous coating layer respectively. The results revealed that apart from the fiber damage during the coating process the porous structure of the fiber coating has a non-negligible effect on the fiber strength, presumably due to a relatively strong bonding between the fiber and coating.     

Additive manufacturing of high mechanical strength continuous Cf/SiC composites using a 3D extrusion technique and polycarbosilane-coated carbon fibers

A novel additive manufacturing approach is herein reported for manufacturing high mechanical strength continuous carbon fiber-reinforced silicon carbide (C_f/SiC) composite materials. Continuous carbon fibers were coated with polycarbosilane (PCS) using a colloidal evaporative deposition process and then coextruded with high solid content SiC ink. The zeta potential of the SiC ink was adjusted to optimize the printing ability of the suspension. During sintering, small SiC grains and whiskers were generated in the gaps in and around the PCS-coated carbon fibers, which led to the improved flexural strength and density of the composites. Meanwhile, the PCS coating on the surface of the carbon fibers prevented the carbon fibers from reacting with SiO gas generated by reactions between the SiC matrix and SiO₂ and sintering additives (Al₂O₃ and Y₂O₃), effectively preserving the structural integrity of the carbon fibers. Compared to the SiC specimens containing uncoated carbon fibers, the density of the specimens fabricated with coated carbon fibers was increased from 2.51 to 2.85 g/cm³, and the strength was increased from 190 to 232 MPa.     

Low-temperature Pr3Si2C2-assisted liquid-phase sintering of SiC with improved thermal conductivity

A novel Pr₃Si₂C₂ additive was uniformly coated on SiC particles using a molten-salt method to fabricate a high-density SiC ceramics via liquid-phase spark plasma sintering at a relatively low temperature (1400°C). According to the calculated Pr–Si–C-phase diagram, the liquid phase was formed at ∼1217°C, which effectively improved the sintering rate of SiC by the solution–reprecipitation process. When the sintering temperature increased from 1400 to 1600°C, the thermal conductivity of SiC increased from 84 to 126 W/(m K), as a consequence of the grain growth. However, an increasing amount of the sintering additive increased the interfacial thermal resistance, resulting in a decrease of thermal conductivity of the materials. The highest thermal conductivity of 141 W/(m K) was obtained for the material having the largest SiC grains and an optimized amount of the additive at the grain boundaries and triple junctions. The proposed Pr₃Si₂C₂-assisted liquid-phase sintering of SiC can be potentially used for the fabrication of SiC-based ceramic composites, where a low sintering temperature would inhibit the grain growth of SiC fibers.     

TEM structure of (PyC/SiC)n multilayered interphases in SiC/SiC composites

Two generations of multilayered interphases, composed of carbon and silicon carbide, have been developed to act as a mechanical fuse in SiC/SiC composites with improved oxidation resistance. Pyrocarbon is an ideal interfacial material, from the mechanical point of view, whereas SiC has a good oxidation resistance. In the multilayered interphase, the carbon mechanical fuse is split into thin sublayers, each being protected against oxidation by the neighbouring SiC-based glass former layers. A first generation of multilayers as synthesised by means of isobaric-CVI with sublayers with micrometric thickness. Then, in order to push forward the concept, pressure pulsed-CVI was involved to deposit nanometric scale sublayers. In this work, transmission electron microscopy was developed to characterise the two generations of materials. The microstructure of the layers and the influence of the fibrous preforms on the structure of the layers were studied. Examinations were then performed on the loaded samples and damaging mode characterised at nanometric scale.     

Effect of Interfacial Shear Strength on Crack-Fiber Interaction Behavior in Ceramic Matrix Composites

Zircon matrix composites uniaxially reinforced with SiC fibers were fabricated with different interfacial properties by changing the fiber coatings. The phenomenon of crack interaction with fibers and/or fiber coatings and its dependence on the interfacial properties were studied using a microindentation technique. The influence of the fiber orientation relative to the crack extension direction on the crack-fiber interaction was also investigated. Crack deflection was observed at the fiber-matrix interface in composites having low interfacial shear strength, and the crack deflection was mostly single-sided, but double-sided deflection was also observed. Crack penetration into the fiber occurred in composites with high values of the interfacial shear strength. These observations were in general agreement with the theoretical predictions of the crack deflection behavior based on the bimaterial interfaces in ceramic composites, but additional observations were made on crack deflection at multiple fiber-matrix interfaces.     

Mechanical properties and oxidation resistance of SiCf/CVI-SiC composites with PIP-SiC interphase

SiC coatings were successfully synthesized on SiC fibers by precursor infiltration and pyrolysis (PIP) method using polycarbosilane (PCS) as precursor. The morphology of as-fabricated coatings was observed by SEM, and its structure was characterized by XRD and Raman spectrum. The SiC fiber reinforced chemical vapor infiltration SiC (SiC_f/CVI-SiC) composites with PIP-SiC coatings as interphase were fabricated. And, the effects of PIP-SiC interphase on mechanical properties of composites were investigated. The experimental results point out that the coating is smooth and there is little bridging between fibers. The coating is amorphous with SiC and carbon micro crystals. The flexural strength of composites with and without PIP-SiC interphase is 220 and 100 MPa, respectively. And the composites with PIP-SiC interphase obviously exhibit a toughened fracture behavior. The oxidation resistance of composites with PIP-SiC interphase is much better than that of composites with pyrolytic carbon (PyC) interphase.