Nicalon Fibers Research Articles

Ultrasonic Characterization of Oxidation Mechanisms in Nicalon/C/SiC Composites

The nonbrittle fracture of composites consisting of ex polycarbosilane SiC (Nicalon) fibers in a SiC matrix prepared by chemical vapor infiltration is strongly dependent on the presence of a pyrocarbon layer at the fiber/matrix interface (Nicalon/C/SiC composites). The mechanical properties of such materials are known to be influenced by oxidation reactions. Elastic modulus measurements, using ultrasonic wave propagation in the “long bar” mode, have been used to show the influence of the environmental parameters temperature, atmosphere, and pressure on the mechanical behavior of bidirectional Nicalon/C/SiC. In situ, measurements of elastic modulus performed in parallel with thermogravimetric analysis allow examination of oxidation mechanisms which affect interfacial properties. Results showed the modulus to be affected by two interfacial oxidation mechanisms: (1) oxidation of pyrocarbon coating and (2) closure of the resulting interphase gap by silica formation.

Subcritical Crack Growth in CVI Silicon Carbide Reinforced with Nicalon Fibers: Experiment and Model

Subcritical crack growth measurements of ceramic‐matrix composites have been conducted on materials consisting of CVI SiC matrix reinforced with Nicalon fibers (SiC/SiCf) having C and BN fiber–matrix interfaces. Velocities of effective elastic cracks were determined as a function of applied stress intensity in pure Ar and in Ar plus 2000 ppm O2 at 1100°C. A stage‐II regime, where the crack velocity depends only weakly on the applied stress intensity, was observed in the V–K diagrams over a range of applied stress intensities that correspond to the R‐curve of the materials. This stage‐II behavior was followed by a stage‐III, or power‐law, regime at higher stress intensity values. Oxygen was observed to increase the crack velocity in the stage‐II regime and to shift the stage‐II‐to‐stage‐III transition to lower stress intensity values. A 2D micromechanics approach was developed to model the time dependence of observed crack‐bridging events and is able to rationalize the measured effective crack velocities, the time dependence of the crack velocity, and the stage‐II‐to‐stage‐III transition in terms of the stress relaxation of crack‐bridging fibers.

Influence of microstructure on the strength of Nicalon-reinforced aluminium metal-matrix composites

The microstructure and mechanical properties of two aluminium-based composites reinforced with Nicalon fibre are investigated. During composite processing, aluminium carbide forms at the interface as a result of a reaction between aluminium and free carbon in the fibre. Magnesium, when present in the aluminium matrix, diffuses into the outer (~ 200 nm) layer of the fibre where it reacts with the silicon oxycarbide constituent to form magnesium-containing oxide and also to free carbon for the production of more interfacial aluminium carbide. These chemical reactions affect to differing degrees the strength of a fibre, as measured after extraction from the two composites, and influence the respective fibre/matrix interfacial friction stress and composite strength. A simple rule-of-mixtures approach based upon the measured strength of extracted fibres gave some agreement with longitudinal properties of the composite, but treatment of the fibres as bundles, using a Weibull probability distribution of properties, provided more accurate predictions.

Strength variability and size effect of Nicalon fibre bundles

Statistical strength and size effect of Nicalon fibre bundles are studied. The Weibull type of statistical theory underlying predictions of bounding Nicalon fibre bundle strength is presented and discussed. The relationship of bundle strength to single Nicalon filament strength and a model explaining the correlation are also discussed. The predicted values for Nicalon fibre bundles were in close agreement with the experimental data. Characterization of Nicalon fibres or bundles provides an insight into the ultimate mechanical performance of ceramic-matrix composites.

Future Trends and Recent Developments of Fabrication Technology for Advanced Metal Matrix Composites

Abstract Future trends and recent developments of reinforcing materials and fabrication processing for advanced metal matrix composites are discussed in this paper. A national R & D project is currently underway to improve the oxidation resistance and heat resistance to 1773 K of continuous ceramics Si-Ti-C-O fibers (Tyranno fiber) and SiC(PC)) fibers (Nicalon fiber) by electron beam irradiation curing techniques. Meanwhile, over the past seven years from 1981, a large national R & D project on MMC has been performed in Japan. Overviews of both projects are reported including the major fabrication methods (squeeze casting, plasma spraying and powder metallurgy) of advanced MMCs. The status and future trends of these methods are considered.

SIMS, EDX, EELS, AES, XPS study of interphases in nicalon fibre-LAS glass matrix composites

Secondary ion mass spectrometry (SIMS) and energy dispersive X-ray spectroscopy (EDX) were used to analyse interphases which develop at the fibre—matrix interface during fabrication of SiC nicalon fibre—LAS (Li2O, Al2O3, SiO2) or LAS+Nb2O5 glass matrix composites. SIMS was performed on fibres extracted from composites by dissolution of the matrix in hydrofluoric acid (HF). The composition of the interphases which remain on the fibre periphery was studied using sputter-depth profiling. EDX analysis of the interphases was performed on nicalon-LAS+Nb2O5 composite cross-sections. These investigations show that in both composites the reaction products at the fibre—matrix interface consist of a carbon layer and a silicate phase, rich in oxygen, located between the carbon layer and the fibre core. The analyses also demonstrate the efficiency of SIMS for analysing compositional changes in the near surface of fibres of small diameter.

Creep Anisotropy of a Continuous‐Fiber‐Reinforced Silicon Carbide/Calcium Aluminosilicate Composite

Creep studies conducted on a unidirectional silicon carbide calcium aluminosilicate composite indicate that the Nicalon fibers provide longitudinal creep strengthening at 1200°C. The deformation is transient in nature because grain growth in the fibers enhances their creep resistance. The‘ transverse creep strength is considerably smaller, being dominated by the matrix, resulting in appreciable creep anisotropy. This anisotropy leads to severe distortion when off‐axis loadings are imposed. Residual stresses develop upon unloading alter creep, and cause superficial matrix cracking.

Characterization of Dip‐Coated Boron Nitride on Silicon Carbide Fibers

Amorphous boron nitride thin coatings (∼0.2 μm) have been formed on Nicalon and C‐Nicaion (pre‐carbon‐coated Nicalon) yarns via dip coating in boric acid solution followed by heating and nitriding in NH3gas at 1000°C. X‐ray photoelectron spectroscopy (XPS) and Auger electron spec‐troscopy (AES) studies have shown the formation of boron nitride. The coating was boron rich and contains oxygen. The N/B and O/B ratios range from 0.6 to 0.8 and from 0.1 to 0.25, respectively. Tensile strength measurements revealed that the BN‐coated C‐Nicalon yarn maintained ∼85% of its original strength while BN‐coated Nicalon lost ∼85% of its original strength. Auger depth profiles showed that there was a consumption of carbon during the heating and nitridation process for both BN‐coated Nicalon and C‐Nicalon fibers. However, the depletion of carbon in BN‐coated Nicalon fibers was much more severe than that in BN‐coated C‐Nicalon fibers.

Mechanical Characterization of Si–C(O) Fiber/SiC (CVI) Matrix Composites witrTa BN–Interphase

The mechanical behavior of three CVT‐processed 2D woven SiC/BN/SiC composite materials with different initial BN interphase thicknesses has been investigated by means of tensile and impact tests. The results have established the efficiency of a BN interphase in promoting a nonlinear/non–catastrophic tensile behavior and high impact resistance. The effect of the initial BN interphase thickness on the resulting mechanical behavior has also been demonstrated. Characterization of the fiber/matrix interfacial zones by AES and TEM has revealed the presence of a SiO2/C double layer at the BN/fiber interface, which might result from a decomposition undergone by the Si–C(O) Nicalon fiber during processing. It has been suggested that the influence of the initial BN interphase thickness on the mechanical properties of the composites results from both changes occurring in the composition and morphology of the interfacial zones and modifications of the interfacial forces due to accommodation of the radial residual clamping stress.

Recent developments of the SiC fiber Nicalon and its composites, including properties of the SiC fiber Hi-Nicalon for ultra-high temperature

Recent developments of the SiC fiber Nicalon™ and its composites are reported. The first section concerns the development history and characteristics of the SiC fiber Nicalon, while the second deals with Nicalon-fiber-reinforced PMC, MMC and CMC. There are four grades of Nicalon fiber, namely the ceramic, HVR (high volume resistivity), LVR (low volume resistivity) and carbon-coated Nicalon fiber. The ceramic grade of Nicalon fiber has been investigated in relation to the tensile strength of the fiber after exposure at 800–1400°C in air. HVR or LVR fiber/epoxy resin composites have low and high dielectric constants, respectively. Nicalon/Al composite wire as an MMC is investigated with respect to tensile strength at high temperatures. The flexural strength, etc., of Nicalon/SiC composite, called Nicaloceram™, as a CMC is reported. A Nicalon/borosilicate glass composite used the carbon-coated Nicalon fiber having various thicknesses of the carbon layer, and the effect of the carbon layer thickness on the flexural strength of this composite has been investigated. The third section describes a low-oxygen-content SiC fiber referred to as Hi-Nicalon™ for ultra-high temperature use. This SiC fiber with excellent thermal stability has been developed by reducing the oxygen content. Polycarbosilane fiber is cured by electron beam irradiation under a He stream and pyrolyzed in an inert atmosphere. This SiC fiber, Hi-Nicalon, has a continuous, multi-filament form and consists of 0·5 wt% oxygen. It has high tensile strength and tensile modulus of 2·8 and 270 Gpa, respectively. Hi-Nicalon has good tensile properties at high temperature. The properties of SiC composites reinforced with Hi-Nicalon have also been investigated.

The Micromechanics of Ambient Temperature Cyclic Fatigue Loading in a Composite of CAS Glass Ceramic Reinforced with Nicalon Fibers

The behavior of a Nicalon fiber reinforced glass ceramic composite cyclicly loaded has been evaluated at ambient temperature using high-resolution micromechanical test methods. On this basis, the events leading to fracture have been found to be similar to those accompanying fracture in unidirectional tension tests. Matrix strains were determined locally at the point of matrix fracture. Crack opening displacements (CODs) were measured as a function of loading cycles, and fiber strains were determined, in some cases. It is concluded that debonding of fibers begins at the point of matrix cracking and rapidly increases. Most of the cyclic lifetime of the material is spent with fibers debonded over large distances (fractions of a millimeter); these fibers are pulled out of the matrix on each loading cycle. Final debond length, as determined by fractography, is a function of the number of cycles to fracture, and of the applied stress level.

Surface roughness of Nicalon fibre

Beside the chemical nature and general shape of the fibres, recent papers [1, 2] have shown that the fibre roughness can act on the mechanical properties of composites materials. For example, flexural and shear strength can be directly related to the surface topology of the fibres. Although there is a great deal of literature dealing with the properties of Nicalon fibres, few data seem to be available. To provide reliable numerical data for mechanical behaviour modeling, a study at a nanometric scale is needed. Therefore, both transmission electron microscopy (TEM, Philips CM12) and atomic force microscopy (AFM, Nanoscope II) were used on desized NL-200 Nicalon fibres. The fibres were prepared for TEM examination by embedding them in an epoxy resin

Elevated Temperature Static Fatigue of a Nicalon Fiber-Reinforced SiC Composite

ABSTRACTStatic fatigue tests of a Nicalon fiber-reinforced SiC matrix composite were conducted in four-point bending over a temperature range of 425° to 1150°C in air at selected stress levels. The composite consisted of a Nicalon cloth with a 0.3 μm graphite interfacial coating and a Forced Chemical Vapor Infiltration (FCVI) SiC matrix composite; samples were tested with or without a final protective SiC seal coat. The results indicated that the fatigue life of the Nicalon-SiC composite decreased with an increase in either applied stresses or test temperatures. However, the composite exhibited a fatigue limit of ∼ 100 MPa at temperatures ≤ 950°C which decreased to ∼ 70 MPa at 1150°C. Both electron microscopy and thermogravimetric studies suggested that the lifetime of the composites was dictated by the oxidation of graphite interfacial layer at temperatures ≤700°C and by oxidation of graphite coating accompanied by formation of silicate interfacial layer via oxidation of the Nicalon fiber (and the SiC matrix) at temperatures ≥ 950°C. Use of a SiC seal coat effectively retarded the oxidation reactions and increased the lifetime by at least one order of magnitude at 425°C. On the other hand, the SiC seal coat made little (if any) difference in fatigue life at 950 °C.

Raman spectroscopy for internal stress measurements in ceramic fibre reinforced glass composites

Raman spectroscopy has been used to investigated the internal stresses of carbon and polymer fiber reinforced epoxy composites. This is possible since the epoxy matrices are reasonably transparent, and the structure of the fiber is Raman active and shows peaks, which are shifted under the externally applied strain. During the composite fabrication, residual stresses are developed due to the thermal expansion mismatch which is usually present between the fiber and the matrix. The goal of this paper is to show that the same method can be applied to glass matrix composites, specifically to Nicalon fibers in a borosilicate glass matrix.

Mechanical Properties of Unidirectional HI-NICALON Fiber-Reinforced Si<sub>3</sub>N<sub>4</sub> Matrix Composites

Conventional NICALON fiber has a refractoriness of only about 1200°C because of its high oxygen content. Recently a new fiber HI-NICALON with improved refractoriness has been developed. The slurry process developed by the authors was adopted for the fabrication of HI-NICALON fiber-reinforced Si3N4 composites. Carbon-coated HI-NICALON fiber-reinforced composites showed high strength and toughness. At room temperature, samples hot-pressed at 1550°C showed a flexural strength of 890±10MPa and KIC of 29±8MPa√m, respectively. Samples hot-pressed at 1600°C showed high values at 1200°C; 640±70MPa of flexural strength and 18.3±0.8MPa√m of KIC. On the other hand, non-coated HI-NICALON fiber-reinforced composites showed a higher flexural strength at 1400°C than the coated ones, however, they showed a brittle fracture and KIC values were only 3 to 4MPa√m.

Properties of SiC-SiC Composites Produced Using CVR Converted Graphite Cloth to SiC Cloth

ABSTRACTNicalon fiber is the primary reinforcement in SiC-SiC composites currently produced by a variety of techniques including CVI and polymer infiltration. Low strength retention at high temperatures of the Nicalon fibers limits the choice of manufacturing processes which can be employed to produce low cost SiC-SiC composites. MER has developed a new SiC reinforcement based upon a conversion of low cost carbon fabric to SiC via a Chemical Vapor Reaction (CVR) process. This new SiC filaments exhibit an excellent creep resistance at temperatures up to 1600°C. Several SiC-SiC composites were fabricated using graphite fabric converted to SiC fabric utilizing the CVR process combined with a slurry infiltration and CVI densification. A correlation between processing conditions, microstructure and properties of the SiC-SiC composites are discussed in detail.

Metal Matrix-Ceramic Reinforcement Interaction in Long Fiber MMCs (Al-SiC) Obtained with Different Preparation Technologies/ Die Wechselwirkungen bei der keramischen Verstärkung von Metallmatrix-Verbundwerkstoffen (Al-SiC) nach unterschiedlicher Herstellungstechnologie

Various aspects of the interaction between metal matrix and ceramic reinforcing material in long- fiber MMCs (Al-SiC), obtained with different production technologies, were examined. Long fibers of SiC (Nicalon) were used for the processes of infiltration of the liquid metal and plasma spray. For these processes, the Nicalon fibers were woven in the form of fabric, and were subjected to a pretreatment of surface activation in order to increase the wettability of the fibers by the liquid aluminum. Strands of preformed fiber metallized with aluminum were used for the diffusion bonding process after various alloying elements had been deposited on the strands using electrochemical procedures

Fatigue behaviour of borosilicate glass-ceramic matrix, nicalon (silicon carbide) fibre composites

Uniaxial fatigue damage analyses were performed on borosilicate glass-ceramic matrix, Nicalon (silicon carbide) fibre reinforced unidirectional composites. The fibre volume fraction varied from about 0.25 to 0.60. Load-controlled tension-tension fatigue tests (R ratio = 0.1) were conducted at room temperature and 540°C (1000°F). The fatigue life was found to decrease with increasing cyclic stress level and a power-law relationship of the form σapp = σuts(2N f)b was established where σapp is the applied maximum stress, σuts the monotonic tensile strength, N f is the number of cycles to failure and b is the fatigue strength exponent. The fatigue damage evolution manifested itself as a decrease in stiffness of the composite with fatigue cycles. This stiffness drop was associated with matrix cracking followed by fibre-matrix debonding and fibre sliding breakage/pull-out, and final failure, respectively at 540°C. The damage evolution at room temperature was associated with degradation of the matrix followed by steady breakage of fibres with no debonding/pull-out, leading to eventual failure of the net section of the composite. In general, quantitative microscopic observations of debonded and pulled-out fibres showed a good correlation with the observed reduction in stiffness. A predictive model to interpret the drop in stiffness is presented and validated using experimental results from the current study.

Development of Silicon Carbide Composites for Fusion

The use of silicon carbide composites for structural materials is of growing interest in the fusion community. However, radiation effects in these materials are virtually unexplored, and the general state of ceramic matrix composites for nonnuclear applications is still in its infancy. Research into the radiation response of the most popular silicon carbide composite, namely, the chemically vapor-deposited (CVD) SiC-carbon-Nicalon fiber system is discussed. Three areas of interest are the stability of the fiber and matrix materials, the stability of the fiber-matrix interface, and the true activation of these [open quotes]reduced activity[close quotes] materials. Two methods are presented that quantitatively measure the effect of radiation on fiber and matrix elastic modulus as well as the fiber-matrix interfacial strength. The results of these studies show that the factor limiting the radiation performance of the CVD SiC-carbon-Nicalon system is degradation of the Nicalon fiber, which leads to a weakened carbon interface. The activity of these composites is significantly higher than expected and is dominated by impurity isotopes. 52 refs., 12 figs., 3 tabs.

High-temperature plasticity effects in bridged cracks and subcortical crack growth in ceramic composites

The time-dependent plasticity of the crack-wake bridging zone at high temperatures is found to control the rate of subcritical crack growth in continuous-fiber-reinforced ceramic composites. Subcritical crack growth measurements of ceramic matrix composites were conducted on materials consisting of chemical-vapor infiltrated SiC matrix reinforced with Nicalon fibers (SiC/SiCf) having C and BN fiber-matrix interfaces. Crack velocities were determined as a function of stress intensity in pure Ar and in Ar + 2000 ppm O2 at 1100 °C. A stage II regime, where the crack velocity is weakly dependent on the applied stress intensity, characterized the V − K data over a range of stress intensities corresponding to the R curve of the material. This stage II behavior was followed by a stage III, or power-law, regime at higher stress intensities. Oxygen increased the crack velocity in the stage II regime and shifted the stage II to stage III transition to lower stress intensities. A two-dimensional micromechanics approach modeled the time-dependence of crack bridging by allowing fiber creep and was able to rationalize the observed velocity measurements and the stage II to stage III transition.