SiC-based Fibers Research Articles

Oxidation embrittlement of SiC fibers at intermediate temperatures

SiC-based fibers are key constituents for the reinforcement of SiC/SiC thermo-structural composites. Their tensile strength is however known to decrease if oxidized because of stresses associated to the oxide scale growth, which in turns is affected by the fiber type (chemical composition, microstructure). This phenomenon is here reported for several types of fibers (five in total, Nicalon® or Tyranno®) between 450 and 950 °C in ambient air. The highest strength loss rate was noticed on Tyranno® Grade S, also known to be more sensitive to oxidation. The initial strength was nevertheless recovered when the oxide scale was removed. This report uses a simplistic empirical model to fit the embrittlement phenomenon, relating the tensile strength to x−1/2. The strength distribution and the projection to larger scale thicknesses are discussed. Assuming the same conditions (environment, scale crystallization rate), the oxidation kinetic could be used to estimate the embrittlement phenomenon.

Predicting the failure by slow crack growth of SiC-based multifilament tows: Batch-to-batch discrepancies as linked to fiber slack

This work investigates the scale effect observed on slow crack growth parameters for SiC-based fibers (Nicalon® and Tyranno® ZMI) and how it can be affected by the variability from a batch of tow to another one. A 10% difference on the stress exponents (nt) was numerically estimated using a dedicated Monte Carlo simulation. This effect is however marginal when compared to the lifetime scatter itself. This partly originates from the fiber slack in used tow specimen, assessed by tensile test completing the experimental work. The number of tensile tests to be performed is discussed. Specimen gauge length is an additional and significant source of discrepancies affecting the fiber alignment and subsequently the tow stress exponent.

Polymer derived ZrO2 reinforced SiC-ZrB2 polycrystalline fiber

Most polycrystalline SiC-based fibers were prepared at a sintering temperature higher than 1600 °C. In this work, a ZrO2 reinforced SiC-ZrB2 polycrystalline fiber was prepared at 1400 °C via the polymer-derived ceramic method from a new Zr- and B-containing polycarbosilane. The morphology and microstructure of the polycrystalline nanocomposite fiber were studied using XRD, XPS, EDS, SEM, and TEM. The results showed that t-ZrO2 was formed at relatively lower temperature (<1000 °C). The most interesting result is that the polycrystalline nanocomposite SiC-ZrB2 was generated after heat-treatment at 1400 °C, producing an excellent ZrO2 reinforced SiC-ZrB2 polycrystalline fiber. The present study provides a novel strategy for the fabrication of SiC-based polycrystalline fiber at a relatively low temperature.

Design and preparation of hollow SiO2 nanospheres/SiC-based nanocomposite fiber membrane for high temperature thermal insulators

Silicon carbide (SiC) fibers are attractive for their thermal stability and chemical erosion resistance. However, the practical application of SiC fiber in the fields of thermal insulation is limited owing to its high intrinsic thermal conductivity. Herein, a novel closed cell structured SiC-based nanocomposite fiber (NCF) using hollow SiO 2 nanospheres (HSNSs) as pore forming agent were designed by precursor conversion method combined with electrospinning technology. According to the content and size of HSNSs, NCFs with different pore size and porosity could be obtained. When the content of HSNSs reached 15%, a large number of closed cells were constructed inside the fiber, which broke the continuity of solid heat transfer and endowed it with excellent thermal insulation performance (∼0.105 W m −1 K −1 at 1000 °C). Meanwhile, the NCFs showed excellent high-temperature resistance and mechanical properties, which makes it have a wide application prospect as a high temperature thermal insulator. This research also offers a new insight for the development of SiC-based fiber with multiple functions to meet various applications, such as catalyst supporters and microwave absorbers. • A new type of porous insulation material with closed cell structure is designed. • Multiple phases enhance interface thermal resistance and infrared shielding performance. • Hollow SiO 2 spheres significantly reduces the thermal conductivity of the fibers. • The nanocomposite fibers show great application prospects as high temperature thermal insulators.

Effect of microstructure and chemical composition on subcritical crack growth in SiC-based fiber tows

SiC-based fibers, a key component for the reinforcement of thermostructural ceramic matrix composites, are subjected to subcritical crack growth (SCG) causing their delayed failure. Stress exponents <10 were reported for various fiber types, standing out from other types of ceramic or glass. The continuum of silicon oxycarbide (SiCO) and carbon free phases is suspected to govern the environmentally assisted crack growth phenomenon, highlighted by relationships between SCG parameters and oxygen or carbon free contents. Metallic heteroelements (Ti or Zr, found in Tyranno® fibers) play a critical role on this sensitivity as well. The existence of 2 distinct groups of fiber types, possibly associated to different SCG mechanisms, is discussed.

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.

Statistical data for the tensile properties and static fatigue of sic-based bundles.

Due to their high specific strength at elevated temperatures and resistance to oxidative environments, SiC-based fibers are of great interest for the reinforcement of ceramic matrix composites. They are however subjected to a slow crack growth (SCG) phenomenon causing their delayed failure under subcritical conditions. The testing of filaments, other than comprising handling difficulties, requires large sets of data (broadly dispersed), drawback alleviated by multifilament tow testing. The data available in the present paper correspond to a comprehensive mechanical characterization and static fatigue testing of various types of SiC-based fiber bundles. The initial non-linearity of load displacement curves were analyzed to reveal the tow structure originating from filament misalignment. Static fatigue tests were used to assess the lifetime prediction coefficients and its distribution parameters. These data may found interest for the interpretation of dispersion bundle testing can highlight under different solicitation mode. Such data are also prominent for the wealth of composite design and to guaranty long term performances over the broad application field offered.

Oxidation/corrosion of BN-based coatings as prospective interphases for SiC/SiC composites

The ceramic matrix composites envisaged as structural parts in the hot section of aircraft engines consist of SiC-based fibres and matrices with a BN interphase coating between them. To control their service life, the oxidation/corrosion behaviour of these materials must be determined. For this purpose, samples of different geometries were prepared by chemical vapour deposition and thermally subjected to wet air. The behaviour of BN alone depends on its crystallization degree, structural homogeneity and the considered direction in the coating. In the case of a thin layer of pure BN of interphase type, recession is significantly reduced at 800 °C by the formation of a sealing borosilicate phase in the confined space surrounded by SiC. At 1000 °C, the protection is no longer sufficient and a steady recession remains. The presence of aluminium added to BN improves protection at 1000 °C due to the formation of alumina combined with borosilicate.

In-beam creep of SiCf/SiC minicomposites under uniaxial tensile loading

A newly developed clamping system for ceramic specimen is described in the present report. By using this system, the uniaxial tensile loading was first successfully applied up to fracture (950 MPa) on a SiCf/SiC minicomposite. A minicomposite is a 1D model composite. It consists of a SiC-based fiber tow embedded in a SiC matrix, which has a diameter of about 0.5 mm. Afterwards, the irradiation creep properties of SiCf/SiC minicomposites were studied. In-situ creep was performed in an in-beam creep device under uniaxial tensile stresses from 40 to 382 MPa during homogeneous helium irradiation. Homogenous irradiation was carried out by helium implantation with energies varying from 0 to 45 MeV. The displacement dose rate was 1.2 × 10−6 dpa/s. The average temperature was controlled to 700 and 900 °C within ±15 °C. Irradiation creep compliance of minicomposite was measured to be 3.07 × 10−5 and 5.43 × 10−5 dpa−1 MPa−1 at 700 °C and 900 °C, respectively.

Static Fatigue of SiC Multifilament Tows at Temperatures up to 1200 °C in Air

SiC-based fibers are sensitive to delayed failure under constant load at high temperatures in air. Static fatigue at intermediate temperatures &lt; 800 °C was attributed to slow crack growth from flaws located at the surface of fibers, driven by the oxidation of free carbon at grain boundaries. The present paper examines the static fatigue behavior of SiC-based Hi-Nicalon fibers at high temperatures up to 1200 °C and Hi Nicalon S fibers at intermediate temperatures (500–800 °C). The degradation of stress- rupture time relation of multifilament tows with increasing temperature was investigated. Predictions of tow lifetime based on critical filament-based model of tow failure were compared to experimental stress-rupture time diagrams. Critical filaments are characterized by strength–probability relation. The critical filament-based model was found to describe satisfactorily the static fatigue behavior of fiber tows at these temperatures. The influence of various factors on lifetime as well as the origins of variability is analyzed.

Carbide derived carbon obtained from SiC-based fibers by phosphating-NaOH bath process

Abstract Carbide derived carbon (CDC) are unique materials with tunable micro and mesopore size distribution. They are produced selectively removing metallic elements from carbides. Halogenation or hydrothermal treatments were, therefore, extensively studied. The study reports a new route to produce such carbon, aforementioned CDCp, applied to SiC-based fibers by reacting phosphoric acid vapors at 580–800 °C range, followed by condensed silicophosphate dissolution in tempered NaOH bath. Fibers could be, this way, fully or partially transformed. Obtained CDCp show a broad pore size distribution extending from microporosity (7–9 A) to mesoporosity (up to 60 A), specific surface area >1000 m2 g−1 and pore volume >0.7 cm3 g−1. Mesoporosity may infer from partial carbon oxidation or in-situ silicophosphate and carbon demixion at nanoscale. CDCp burn-off kinetics and tensile strength of non-etched fiber core are discussed. Depending on phosphating conditions, monolayer or concentric CDCp tubes could coat each individual fiber within complex fabric geometry.

Carbon derived from silicon carbide fibers, a comparative study

SiC-based fibers, eleven in total, derived from polycarbosilane pyrolysis and belonging to the three different generations, were fully converted into microporous carbon after selective etching of silicon atoms under pure chlorine flow at intermediate temperatures (550–850 °C). In this work, relationship between as-received fibers and their subsequent carbide-derived carbon (CDC) properties, pore structure oxidation resistance and mechanical properties, was investigated. The resulting carbon is microporous or micro-mesoporous with equivalent specific surface area exceeding 1000 m2 g−1 and pore size distribution (PSD) related to the substrate chemical composition and microstructure of former fibers. Oxidation kinetics were found to depend on this PSD. The former sp2 carbon free network of fibers remained unaffected by the chlorination. Its amount and percolation were respectively correlated to Young modulus and tensile strength of CDC. This carbon thus displays a nano-composite-like behavior with a former etched carbide matrix (transformed SiC and SiCO) and carbon free reinforcement. The results demonstrate CDC allows the control over pore size and mechanical properties by selecting an appropriate multiphasic substrate, combining etched and non-etched compounds.

Flexibility of SiC-based Ceramic Fibers Synthesized from Polycarbosilane

Flexibility of SiC-based Ceramic Fibers Synthesized from Polycarbosilane

Improvement of silicon carbide fibers mechanical properties by Cl2 etching

Silicon carbide fibers combine structural and refractory properties making them good candidates for ceramic matrix composite reinforcement, driving their ultimate failure. The fractographic observation after tensile tests of various silicon carbide fibers, belonging to first, second or third generations, reveals the critical flaw location i.e. internal or surface. An etching treatment under pure chlorine at 500–850 °C can be used to transform the fibers surface on hundreds of nanometers and alter the surface-located flaw population. This way, first-generation SiC-based fibers successfully show an asymptotic improvement of tensile strength (up to 60%) and Weibull modulus (up to twice), when etched on 0.3–1 μm depth range. The lifetime of bundles, under static fatigue conditions at intermediate temperature, is consequently multiplied by a factor ranging from 10 to 40 with a narrower dispersion. Nevertheless, the tensile strength could neither be increased on second-generation Hi-Nicalon nor dramatically dropped on third-generation Hi-Nicalon-S fiber.

Amorphous fine-diameter SiC-based fiber from a boron-modified polytitanocarbosilane precursor

An amorphous SiC-based fiber was successfully prepared by the preceramic polymer route from a boron-modified polytitanocarbosilane. It was found that the addition of titanium and boron had adverse effect on melt-spinning, while the tensile strength of the obtained SiC-based fibers increased with the contents of titanium and boron increasing. It is worth mentioning that the decomposition of SiCO phase and the resultant β-SiC crystallization were retarded by the incorporation of titanium and boron into the SiC-based fiber.

Analytical Electron Microscopy of W-Core β-SiC Fibers for Use in an SiC-Based Composite Material for Fusion Applications

In this work, the interactions between tungsten (W) and silicon carbide (SiC) in Sigma SiC fibers at high temperatures were characterized using scanning and transmission electron microscopy. These fibers could have the potential for use in fusion-related applications owing to their high thermal conductivity compared with pure SiC-based fibers. The as-received fibers were composed of a 100-μm-thick shell of radially textured β-SiC grains and a 15-μm-thick tungsten core, composed of a few hundreds of nm-sized elongated tungsten grains. The interfaces between the tungsten and the SiC and the SiC and the outer coatings were sharp and smooth. After heat treatment at 1,600°C for 3 h in Ar, the tungsten core reacted with SiC to form a rough interface surface. Inside the core, W₅Si₃, W₃Si, and W₂C phases were detected using energy-dispersive X-ray spectroscopy and electron-diffraction techniques. The mechanical properties of the fibers deteriorate after the heat treatment.

Investigation of subcritical crack growth using load relaxation tests on fiber bundles

Many inorganic fibers are sensitive to subcritical crack growth activated by the environment. Failure occurs even though the applied stress is much smaller than the fracture stress. This mechanism has been extensively investigated on ceramics, glasses and glass fibers, essentially at room temperature, and has recently been shown in SiC-based fibers at high temperatures. The present paper proposes a very powerful approach to static fatigue. It is based on tests performed on multifilament tows under deformation-controlled conditions (the load relaxation technique). This technique, which has not been used previously owing to practical difficulties of deformation control during long-term tests, permits the application of identical constant stresses on all the fibers during a single test. Thus it provides a statistically significant rupture time database containing just as many data as there are fibers in the tows. Bundles of ∼2000 E-glass filaments were used in the present paper. The samples were immersed in water during the static fatigue tests. Crack velocity–stress intensity factor diagrams for single filaments were derived from experimental stress–rupture time data. A closed form expression for statistical distributions of fiber lifetimes was established, and was found to compare fairly well with the experimental results, which assessed the approach and validated new findings.

In-situ processing of carbon coatings on the surface of SiC-based fibers

Carbon coatings have been formed on the surface of free carbon containing Tyranno ZMI and Nicalon SiC-based fibers by reaction with chlorine gas (chlorination) in the temperature range 550–675 °C. The kinetics and growth mechanisms have been investigated for both fibers. Results have shown that, for similar experimental conditions, the reactivity of both fibers displayed noticeable discrepancies. It has thus been observed that the Nicalon fiber was more prone to chlorine-based treatment and displayed a thicker carbon coating than the ZMI fiber. These results have also revealed that, in both cases, these thicknesses can be closely monitored by physical parameters such as temperature and aging duration. The nature of the obtained coating has also been investigated by various experimental devices. AES, SEM and TEM analyses have thus evidenced that these carbon coatings were uniform and well adherent whereas AFM showed that the chlorine-based treatments resulted in a nanometer-range roughness at the fiber surface. Adsorption gas analyses also revealed a microporosity coupled with a high specific surface area. The chemical composition of the final products was determined by XPS and Mass Spectroscopy. Finally, the oxidation behavior which was investigated by thermogravimetric analysis revealed that oxidation occurs at a quite low temperature (225 °C).

The Study of Nanostructured Interfacial Coatings on SiC Fibers by Atomic Force Microscopy

Adhesion force measurements by atomic force microscopy (AFM) have been carried out to investigate the adhesion between the Si3N4 cantilever tip and the surface of the initial SiC-based Hi-Nicalon, Hi-Nicalon S, and Tyranno-SA fibers, as well as the ZrO2-coated and oxidized coated fibers. It was shown that the application of coating resulted in a change of the roughness parameters and a decrease of the adhesive forces between the Si3N4 cantilever tip and the fiber surface. Surface heterogeneity at the nanoscale could be responsible for the dispersion of adhesive forces. The results open the possibility of transferring nano mechanical information to meso or macro mechanical properties of ceramic matrix composites.

Study of TiC Coatings on Nicalon Fibre Prepared by Pressure-Pulsed Reactive Chemical Vapour Deposition at Low Pressure

TiC coatings on Nicalon SiC-based fibres were obtained in two steps; the first step consisted in converting the surface of the Nicalon fibre into nanoporous carbon by a chlorine treatment at 773 K, the second step consisted in converting this nanoporous carbon film into TiC by pressure-Pulsed Reactive Chemical Vapour Deposition at low pressure with a H2/TiCl4 mixture at 1373 K. The coatings were characterized by X-ray photoelectron spectroscopy, Auger electron spectroscopy and scanning and transmission electron microscopies. Mechanical assessment was achieved by tensile tests at room temperature. The conversion rate of C into TiC was studied varying the number of H2/TiCl4 pulses from 0 to 100. The formation of a thin interlayer between the fibre and the nanoporous carbon coating was revealed as soon as only 20 pulses are used. The conversion rate increases with the number of H2/TiCl4 pulses while the fibre failure stress decreases almost linearly.