Nicalon SiC Research Articles

Investigation of Specimen Size Effects on P-Quantile Diagrams and Normal Distributions of Critical Flaw Strengths in Fiber Tows

The present paper proposes a model of the specimen size effect on the critical flaw strength distribution in fiber tows for composite reinforcement. The model is based on the basic assumption of brittle fracture that the failure probability at a given strength increases with specimen size in the p-quantile vs. strength relation and on the normal distribution. Empirical results derived from force–strain curves determined on tows made of 1000 and 500 Nicalon SiC filaments and with various gauge lengths show some discrepancy with predictions using the model. The empirical p-quantile diagrams and cumulative distributions of critical flaw strengths exhibited excellent reproducibility at longer gauge lengths, which suggests the absence of a size effect above a critical tow size. The reproducibility of flaw strength distributions at gauge lengths above 60 mm and the higher strengths obtained at lower gauge lengths despite structural effects were related to the features of the critical flaw distribution in tows of parallel fibers.

Use of Electrophoretic Impregnation and Vacuum Bagging to Impregnate SiC Powder into SiC Fiber Preforms

Techniques based on vacuum bagging (VB) and electrophoretic impregnation (EPI) have been investigated for the impregnation of SiC powder into layered Nicalon SiC fabric preforms. The aim was to produce preimpregnated samples for subsequent chemical vapor infiltration (CVI) with reduced intertow porosity that arises from the construction of the fabric layers while leaving unblocked the intratow porosity that is so indispensable for a successful infiltration. Because the goal was simply to learn about the ability to impregnate the samples, no interphase coating was applied to the fibers as would normally be used when producing SiCf/SiC composites. While the VB process generally yielded much stronger preforms, depending on the pressure used and the powder particle size, it resulted in powder becoming located in the intratow rather than the intertow porosity. In contrast, provided an appropriate electrode arrangement was used, EPI offered the potential for a more controlled impregnation process with the powder primarily found in the intertow porosity; however, the preforms were very weak and delaminated easily. The combination of the two processes resulted in a very successful approach, with greater uniformity of particle infiltration and higher green strengths, while largely avoiding impregnating the intratow porosity.

Effect of different sintering aids on thermo–mechanical properties and oxidation of SiC fibers – Reinforced ZrB2 composites

Reinforced zirconium diboride composites containing 15 vol% of Hi Nicalon SiC chopped fibers were hot pressed with addition of various sintering additives, Si3N4, ZrSi2 or MoSi2. Depending on the sintering aid, different densification temperatures were set in the range 1650–1750 °C. Temperature and additive strongly influenced the matrix/fiber interface, which in turn had a strong impact on the mechanical properties and the oxidation behavior at 1650 °C. Even the workability, performed either by conventional machining or electro discharge machining, varied depending on the sintering additive and the secondary phases formed in the system.The system containing Si3N4 turned out to have the highest mechanical properties, but intermediate oxidation resistance; the composite containing ZrSi2 had the lowest sintering temperature, but displayed the worst oxidation resistance, and finally the composite containing MoSi2 showed intermediate mechanical properties, but the highest oxidation resistance and lowest degree of damage upon machining. Preliminary measurements of thermal shock resistance by the water quenching method were also carried out.

Effect of Nicalon SiC fibre heat treatment on short fibre reinforced β-sialon ceramics

SiC Nicalon fibre yarn was heat-treated at elevated temperature in a gas pressure furnace under CO atmosphere. Weak surface coating is essential for ceramic matrix composite (CMC) reinforcement. Therefore Nicalon SiC fibres were coated after CO heat treatment and then used for β-sialon ceramic reinforcement. The heat treated fibres were chopped about 1–2 mm, and β-sialon z = 1 starting powders were prepared with conventional ball milling. The sialon starting composition and the short fibres were mixed with the certain amount of water to obtain a plastically formable mud. This mud was unaxially cold-pressed to form green bodies and to decrease water content. The green bodies were hot pressed at elevated temperatures for half an hour to produce CMC samples. Vickers hardness test showed that heat-treated fibre reinforcement of β-sialon composites provided higher fracture toughness. Uniform fibre distribution, fibre coating, matrix densification and phase transformation were examined by SEM and XRD analysis.

Delayed Failure of Hi-Nicalon and Hi-Nicalon S Multifilament Tows and Single Filaments at Intermediate Temperatures (500°-800°C)

Previous results have shown that tows of SiC Nicalon fibers are sensitive to the phenomenon of delayed failure, at temperatures below 700°C. The present paper examines the static fatigue of Hi-Nicalon and Hi-Nicalon S when subjected to constant load, at temperatures between 500° and 800°C in air. Multifilament tows and single filaments were tested. Experimental data show that the rupture times of tows depend on the applied stress according to the conventional power law tσn=A. In contrast, the stress-rupture time data obtained on single filaments exhibit significant scatter. A model based on slow crack growth in single filaments shows that the stress-rupture of fiber tows follows the conventional time power law. The dependence on temperature was introduced. The model allowed sound calculations of tow lifetimes and characteristics of the slow crack growth phenomenon to be extracted from the tow stress-rupture time data.

Effect of Fibre Heat-Treatment on Nicalon SiC Fibre Reinforced β-SiAlON Matrix Composites

Nicalon SiC fibre tows have excellent properties for ceramic matrix reinforcement but residual oxygen within the fibres degrades fibre properties when these are incorporated into ceramic matrices at elevated temperatures. β-SiAlON ceramics also have excellent mechanical and physical properties, especially fracture toughness. However, sintering of β-SiAlON is generally carried out at 1650-1750°C, considerably higher than the temperatures above which fibre degradation occurs (>1200°C). In the present study, the refractoriness and strength of Nicalon fibres were improved by high pressure CO heat treatment, and densification temperatures of β-SiAlON were lowered by using different kinds of sintering additives. Heat-treatment of the fibres under 45 bar CO pressure at 1500-1650°C led to an increase in fibre strength and to the formation of a thin carbon layer on the surface of the fibres. These improvements in the Nicalon SiC fibres allowed them to be incorporated successfully into β-SiAlON matrices. The as-received and heat-treated fibres were infiltrated with β-SiAlON starting powder mixes and hot-pressed with low temperature sintering additives at 1600-1700°C for 30 min. Bending strength and fracture toughness measurements showed that samples containing heat-treated fibres provided a significant strength and fracture toughness increase compared with similar samples prepared using as-received fibres, and massive pull-out was observed because of the weak interface resulting from the surface carbon coating on the fibres.

Mechanical property improvements in Nicalon SiC fibre reinforced silicon nitride ceramics by oxide coating of Si3N4 starting powders

Ceramic matrix composites are attractive as candidate materials for high-temperature applications offering some advantages compared to monolithic ceramics and high-temperature metal alloys. SiC fibre reinforced silicon nitride is one such composite system. However, the processing route is critical to the production of a reliable composite. In this study, silicon nitride matrix densification was improved and sintering temperature was lowered by coating of Si3N4 particles with oxides deposited from hydrolysed metal alkoxides. The solution containing oxide coated Si3N4 powders was used as a slurry to infiltrate Nicalon SiC fibre tows. Following previous studies, the fibres were heat-treated in carbon monoxide to improve mechanical and surface properties. Infiltrated green bodies were hot-pressed at elevated temperatures to produce dense composites. The results showed that particle coating accelerated densification kinetics, eliminated pores and reduced the required hot-pressing temperature. There was also less fibre degradation as a result of the lower temperature of densification. Bending strength and fracture toughness of the composites were measured and fractography was conducted using scanning electron microscope. Composites manufactured using coated Si3N4 powders showed improved properties, specifically matrix stiffening and delayed crack initiation under load.

Microstructures of ceramic composites with glass–ceramic matrices reinforced by SiC-based fibres

A lot of studies have been carried out on the fibre/matrix interfaces in glass–ceramic matrix composites reinforced by SiC based fibres. Chemical and structural analyses at the nanometer scale have shown that the fibre/matrix interface has a very complex structure consisting of several sublayers. The most important point is the existence of a thin carbon layer which is often found textured close to the fibre. This carbon layer acts as a mechanical fuse with a low or extremely low interfacial debonding energy. The mechanism responsible of the carbon formation and of the complex interfacial microstructure is still mater of controversy. This review will be focused on the microstructure of the interfacial region and on the different techniques which have been used to obtain chemical and microstructural parameters of the fibre/matrix interfaces in a large variety of glass–ceramic composites. The approaches concerning the SiC/BN dual-coated Nicalon SiC fibre-reinforced BMAS matrix composites will be described as well as the thermomechanical properties of this class of glass–ceramic composites and future research.

High-performance SiC-fibre reinforced β-sialon CMCs prepared from heat-treated Nicalon fibres

Abstract In a previous paper (Demir, A. and Thompson, D. P., Heat-treatment of Nicalon SiC fibres in various atmospheres and pressures. In Proc. 3rd Int. Turkish Ceramic Congress, Vol. 2 , Engineering Ceramics , ed. V. Gunay, H. Mandal and S. Ozgen. Turkish Ceramic Society, Istanbul, 1996, pp. 180–185) it was reported that a high-pressure, high-temperature carbon monoxide treatment of Nicalon SiC fibres resulted in significantly improved strength and refractoriness, and at the same time deposited a thin carbon coating on the surface of the fibres. In the present study, fibres heat-treated in this way have been used to reinforce a range of β-sialon ceramic matrices, and the resulting mechanical properties (bend strength, fracture toughness) have been compared with results for similar materials prepared from non-heat-treated Nicalon fibres. All samples showed significant increases in strength and density, with in general, not much change in fracture toughness. The presence of the coating on the fibres assisted the densification of the composites by decreasing friction during hot-pressing, thereby increasing density and matrix strength. SEM micrographs indicated that the carbon coating is still present after fabrication and is sacrificed during fracture, assisting in easy pull-out of fibres.

Improvements in refractoriness and properties of Nicalon fibres by high-temperature heat-treatment

Nicalon SiC fibres were heat-treated in various atmospheres and at various pressures. Initially CO, nitrogen and air were used as the heat-treatment environment at one atmosphere pressure. Microstructural changes and any related strength degradation or improvement were measured for the heat-treated fibres. After heat-treatment in the temperature range 1000°C–1600°C, each sample showed different weight changes. Thus, in air, a weight gain was observed with increasing temperature, whereas in CO and N2, weight losses were observed but with a smaller weight loss observed for CO. Moreover, carbon monoxide had a significant effect on the strength retention of the fibres. Since the lowest weight loss was observed after heat-treatment in CO at one atmosphere, high pressure CO gas was used to heat-treat Nicalon fibres between 1000°C and 1700°C and the resulting fibres were analysed by single-filament strength testing, scanning electron microscopy, and X-ray diffraction. The results were completely different compared with those in one atmosphere of CO. As the temperature increased, weight and strength increased whereas at one atmosphere pressure, both weight and strength had decreased. The weight increase was because of surface reaction between the CO atmosphere and the SiC fibre and/or because of deposition of carbon from the pressurised CO gas, giving the fibre a surface carbon coating. Carbon coating of a fibre is a beneficial property for CMCs since it provides a weak interface which facilitates pull-out during fracture.

Recent advances in Nicalon ceramic fibres including Hi-Nicalon type S

Polymer-derived Nicalon SiC fiber produced commercially has been widely applied in high temperature Ceramic Matrix Composites (CMCs). Recently, SiC fiber with a low oxygen content (Hi-Nicalon) was developed and produced commercially using an electron beam curing. This fiber has a higher elastic modulus and creep resistance, and much higher thermal stability up to 1600 ° C than that of Nicalon fiber. High temperature mechanical properties of CMCs with Hi-Nicalon have much improved. However, creep and oxidation resistance of Hi-Nicalon CMCs are not satisfactory for high temperature structural materials, because Hi-Nicalon mainly consists of SiC micro-crystals and excess carbon. Therefore, we have developed an SiC fiber (Hi-Nicalon type S) with stoechiometric SiC composition and high crystallinity. Hi-Nicalon S fiber has high elastic modulus (420 Gpa), good oxidation resistance at 1400 °C, and excellent creep resistance. In this paper, the recent development of these oxygen free SiC fibres including physical and mechanical properties, thermal stability and an environmental resistance are reported.

Corrosion, oxidation and strength properties of Nicalon SiC fibre under loading

Hot corrosion and oxidation behavior of Nicalon fibres in air under loading were investigated and compared. Degradation of Nicalon fibres exposed at temperatures over 900°C in the presence of Na 2SO 4 was extremely severe compared with that in oxidation environment without Na 2SO 4. This is attributed to the rapid oxidation-dissolution of the fibres due to the formation of the liquid Na 2SiO 3. Room temperature strength of Nicalon fibre after oxidation decreased with increasing exposure temperature. Reduction of tensile strengths was most significant under the combined condition of exposure temperature and applied loading. This evident strength reduction is caused by the combination of applied load and the reaction between the inherent SiO 2 and free carbon in the fibre. Fracture surfaces of oxidized fibres exhibited mirror-hackle regions, whereas the fracture surfaces of fibres corroded were planar.

Chemical and mechanical alterations of SiC Nicalon fiber properties during the CVD/CVI process for boron nitride

To improve the interfacial properties in SiC/SiC composites, BN is an appropriate interphase material to control the fiber/matrix bond. Unfortunately, the gaseous phase (NH 3,BF 3,HF,Ar) used to deposit BN acts aggressively upon Si–C–O (ex-PCS) Nicalon fiber surfaces, and weakens that bond through the formation of a complex interfacial sequence (SiO 2/C), which actually controls the localization of debonds. The reactions between each gas involved and the fiber surface have been studied. Further, if the fiber surface consists of SiC or any silicon-containing compound, the BF 3 gas reacts through a substitution of the silicon by boron in the initial fiber composition. Then, the surface evolves from a (C,O,Si) composition to a (B,C,O,Si) glassy layer. Such a reaction occurs mainly before the BN nucleation, and it alters the reinforcing potential of fibers in composites. This boron-containing glass is shown to be very unstable in the presence of HF gas (the main reaction product).

Comparison of Two Processes for Manufacturing Ceramic Matrix Composites from Organometallic Precursors

A commercial polysilazane is used as a silicon carbonitride matrix precursor for the manufacture of ceramic matrix composites using bi-directional SiC Nicalon fabrics as reinforcing material. The objective is to develop a simple and fast process leading to materials able to compete with SiC/C/SiC composites obtained by the Chemical Vapour Infiltration (CVI) route. Two processes are investigated: (1) a ‘conventional’ process using the densification of a SiC fibre preform by several cycles of impregnation of the preform with the polymer followed by pyrolysis and (2) a ‘modified’ process consisting in a powder filling of the fibre preform prior to the precursor impregnation and pyrolysis. This paper describes the different steps of both processes. The materials obtained are characterised in terms of their porosity, microstructure and mechanical properties. ©

Optimization of ELNES processing using a four‐parameter function: application to SiC fibers

In the explanation of EELS spectra in terms of molecular orbitals, where the local band structure is approximated as a linear combination of atomic orbitals of the excited atom and its immediate neighbours, K and L core-loss spectra are decomposed using a four-parameter function that models the ionization continuum in a better way than ‘classical’ step functions. The peaks corresponding to particular chemical bondings are extracted and then fitted by Gaussian functions. The accuracy of such a technique is discussed in the case of Nicalon SiC fibre compared to pure α-SiC. Because of this ionization continuum subtraction we found, in agreement with previous XPS data, that the fibre contains a particular SiCO bonding that is different from SiC and SiO and was not visible without this treatment. Moreover, the CK-edge analysis shows the presence of graphite-like carbon. The existence of CH bonding is also discussed. © 1999 John Wiley & Sons, Ltd.

Optimization of ELNES processing using a four-parameter function: application to SiC fibers

In the explanation of EELS spectra in terms of molecular orbitals, where the local band structure is approximated as a linear combination of atomic orbitals of the excited atom and its immediate neighbours, K and L core-loss spectra are decomposed using a four-parameter function that models the ionization continuum in a better way than ‘classical’ step functions. The peaks corresponding to particular chemical bondings are extracted and then fitted by Gaussian functions. The accuracy of such a technique is discussed in the case of Nicalon SiC fibre compared to pure α-SiC. Because of this ionization continuum subtraction we found, in agreement with previous XPS data, that the fibre contains a particular SiCO bonding that is different from SiC and SiO and was not visible without this treatment. Moreover, the CK-edge analysis shows the presence of graphite-like carbon. The existence of CH bonding is also discussed. © 1999 John Wiley & Sons, Ltd.

Effect of high temperature heat treatment in vacuum on microstructure and bending properties of SiC f/SiC composites prepared by CVI

Thermal stabilities of SiC f/SiC composites prepared with chemical vapor infiltration have been examined. Three types of SiC fibers of commercial grade Nicalon, Hi-Nicalon and stoichiometric Hi-Nicalon Type-S with oxygen concentrations of respectively 11.7%, 0.5% and 0.2% were used as preforms. After heat treatment at 1273–2273 K in vacuum, changes in oxygen content of the composites were measured. Bending properties such as flexural strength and fracture toughness were also examined at room temperature. Deoxidation was clearly observed for the commercial grade Nicalon SiC f/SiC composite (SiC f(CG)/SiC) at temperatures above around 1700 K. Furthermore, decomposition of fibers completely occurred above 1973 K for SiC f(CG)/SiC. A slight degradation was also observed for Hi-Nicalon SiC f/SiC composite. On the other hand, Hi-Nicalon Type-S SiC f/SiC did not show an apparent degradation even after heat treating at 2200 K. The decomposition of the commercial grade Nicalon SiC fibers can be mainly explained by the sublimation of SiO which is formed according to the reaction SiC x O y → SiO+SiC.

SiC Nicalon fibre-glass matrix composites: interphases and their mechanism of formation

The fibre-matrix reaction that occurs during the processing of a Nicalon SiC fibre-Pyrex glass matrix composite is analysed. Interphases are characterized by means of various complementary techniques: electron diffraction, HRTEM, EDX, EELS, SIMS and XPS. Neither the results of the present study nor those previously obtained for Nicalon SiC fibre-LAS (Li2O-Al2O3-SiO2) glass or LAS + MAS (MgO-Al2O3-SiO2) glass-ceramic matrix composites support the available model of reaction. An alternative reaction mechanism is suggested whereby the dissolved and non-bridging oxygen atoms of the matrix diffuse and oxidize SiC and SiOxCy in the fibre. The reaction yields carbon and silicon oxycarbide in the fibre and SiO2 which dissolves into the matrix. When the oxygen in excess in the matrix is consumed, the reaction stops and the phases undergo a reorganization in the reaction layer which generates two interphases, one carbon rich and the other silicon oxycarbide rich. These interphases are observed at the fibre peripher...

98/02832 Oil conversion process for manufacturing low-sulfur fuel oil from spend lubricating oil and coal: Kuzara, J. K. and Klinger, L. D. U.S. US 5,676,711 (Cl. 44–622; C10L1/02), 14 Oct 1997, Appl. 604,399, 21 Feb 1996, 6 pp

A commercial polysilazane is used as a silicon carbonitride matrix precursor for the manufacture of ceramic matrix composites using bi-directional SiC Nicalon fabrics as reinforcing material. The objective is to develop a simple and fast process leading to materials able to compete with SiC/C/SiC composites obtained by the Chemical Vapour Infiltration (CVI) route. Two processes are investigated: (1) a ‘conventional’ process using the densification of a SiC fibre preform by several cycles of impregnation of the preform with the polymer followed by pyrolysis and (2) a ‘modified’ process consisting in a powder filling of the fibre preform prior to the precursor impregnation and pyrolysis. This paper describes the different steps of both processes. The materials obtained are characterised in terms of their porosity, microstructure and mechanical properties. ©

SiC Nicalon fibre-glass matrix composites: Interphases and their mechanism of formation

SiC Nicalon fibre-glass matrix composites: Interphases and their mechanism of formation