Nicalon SiC Research Articles

Characterization of Nicalon fibres with varying diameters: Part I Strength and fracture studies

Experimental studies have been conducted to examine the strength and fracture behaviour of monofilament Nicalon SiC fibres with diameters ranging from 8 to 22 μm. The effects of varying fibre diameter, flaw location and flaw population on the mechanical response of individual fibres were investigated by recourse to extensive fractographic analysis performed on fibres fractured under tensile loading. Results indicate that variations in fibre diameter influence the apparent fibre fracture toughness (K1c), with higher K1c values observed for decreasing fibre diameters. Observations also suggest that the location of the critical flaw may play a role in the fracture of Nicalon fibres. Tensile strength values are shown to increase as the normalized distance of the critical flaw from the fibre centre increases, while critical flaw population appears to be strongly dependent on location. The ratio of K1c to geometry factor (Y) is observed to remain constant with varying flaw location. In addition to surface flaws, three distinct internal flaw populations are seen to cause fracture in Nicalon fibres. Based on these experimental findings, a statistical characterization of the strength of Nicalon fibres with varying diameters is presented in Part II of this paper. © 1998 Chapman & Hall

Characterization of Nicalon fibres with varying diameters Part II Modified Weibull distribution

Diameters vary significantly in a tow of commercial NicalonTM fibres, which is one of the most attractive ceramic reinforcements for structural composites. It was found that the strength distribution of Nicalon fibres could not be adequately characterized using either single- or bi-modal Weibull distribution. A recently proposed modified Weibull distribution can account for the effect of varying diameter in the characterization of fibre strength. To verify the validity of the modified Weibull distribution, comprehensive mechanical testing and fractographic studies have been conducted on Nicalon SiC fibres with diameters varying from 8 to 22 μm. The experimental results have been reported in Part I. Part II of this paper further modifies the derivation of the modified Weibull distribution to yield a relationship which is similar in form, but soundly based on experimental findings. Factors considered in the modified Weibull distribution include the dependence of fracture toughness and flaw density on fibre diameter, both of which may vary with fibre diameter, as reported in Part I. Comparison with experimental data shows that the current modified Weibull distribution works very well, while both single-modal and bi-modal Weibull distributions are inadequate for describing Nicalon fibres with varying diameters. © 1998 Chapman & Hall

Micro-Raman study of SiC fibre-oxide matrix reaction

Micro-Raman analysis is a non-destructive technique used to provide information on the compressive/tensile strain applied to a fibre embedded in a ceramic matrix composite. Carefull analysis of this information helps to understand the physical-chemical evolution of the fibre. Example of continuous SiC Nicalon® fibres reinforcing various ceramic matrices (mullite, NASICON) shows how the comparison between fibre core and fibre periphery spectra-using wavenumber, bandwidth and intensity—provides independant tools to check the fibre evolution and to distinguish thermally and chemically induced transformations.

Analysis of Size Dependence of Ceramic Fiber and Whisker Strength

The strengths of ceramic fibers and whiskers have been observed to increase with decreasing fiber diameter and length. Typically, both surface flaws and volume flaws exist in ceramic fibers and whiskers, which makes it impossible to characterize the strength dependence of both the diameter and the length with a single‐modal Weibull distribution function. Our data also show that the single‐modal Weibull distribution is inadequate to characterize the strength of fibers with varying diameters even in the case of a constant fiber length. In addition, experimental data also show that, for sapphire whiskers whose surface flaws were removed by chemical polishing, the whisker strength has a much stronger size dependence on diameter than predicted by the single‐modal Weibull function, which indicates that factors other than those characterized by the Weibull function also play a role in the strength of sapphire whiskers. In this paper, the factors affecting the strengths of ceramic fibers and whiskers are analyzed in terms of Weibull statistics, fracture mechanics, and flaw size density variation with varying fiber diameters. A three‐parameter modified Weibull distribution, which combines the above strength‐affecting factors, is proposed to characterize both the diameter and the length dependence for ceramic fibers and whiskers with or without surface flaws. Characterization of the strength data of sapphire whiskers and Nicalon SiC fibers with varying diameters shows the validity of the modified Weibull distribution function.

Raman Study of the Chemical and Thermal Degradation of As-Received and Sol-Gel Embedded Nicalon and Hi-Nicalon SiC Fibres Used in Ceramic Matrix Composites

Raman microprobe analysis of Nicalon NLM 202 and Hi-Nicalon fibres as a function of thermal treatments in various atmospheres was carried out in order to understand the fibre evolution during the synthesis of composites and their use in combustion environments. Assignments are discussed and a description of the fibre structure on the nanoscale is proposed. A comparison is made between surface and core evolution of as-received fibres and of fibres used in the sol–gel route. The plateau observed between 1200 and 1600°C for Hi-Nicalon bands is consistent with a better thermal stability. © 1997 by John Wiley & Sons, Ltd.

Raman Study of the Chemical and Thermal Degradation of As‐Received and Sol–Gel Embedded Nicalon and Hi-Nicalon SiC Fibres Used in Ceramic Matrix Composites

Raman microprobe analysis of Nicalon NLM 202 and Hi-Nicalon fibres as a function of thermal treatments in various atmospheres was carried out in order to understand the fibre evolution during the synthesis of composites and their use in combustion environments. Assignments are discussed and a description of the fibre structure on the nanoscale is proposed. A comparison is made between surface and core evolution of as-received fibres and of fibres used in the sol–gel route. The plateau observed between 1200 and 1600°C for Hi-Nicalon bands is consistent with a better thermal stability. © 1997 by John Wiley & Sons, Ltd.

Interfacial characterization of Nicalon SiC fibre reinforced oxynitride glass matrix composites

Nicalon SiC and Hi-Nicalon SiC fibre oxynitride glass and glass–ceramic composites were prepared and the interface between the fibres and matrix characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy. It was found that the formation and thicknesses of interfacial layers were primarily determined by the type of fibre reinforcement, but the role of these interfaces in influencing composite properties was dependent on the thermal properties of the matrix. For Nicalon SiC composites, the carbon-rich layer did not promote fibre debonding and toughening unless the matrix had a smaller thermal expansion coefficient than the fibres. For Hi-Nicalon SiC composites, the absence of oxygen in the fibre significantly encouraged chemical reaction between fibre and matrix, resulting in no strengthening or toughening.

Elastic Properties of Laminated Calcium Aluminosilicate/Silicon Carbide Composites Determined by Resonant Ultrasound Spectroscopy

The elastic properties of unidirectional and 0°/90° crossply Nicalon‐SiC‐fiber‐reinforced calcium aluminosilicate (CAS/SiC) ceramic‐matrix composites have been measured using a resonant ultrasound spectroscopy (RUS) technique. This approach has allowed the nondestructive determination of the complete set of independent second‐order elastic stiffness constants of these ceramic composites. These stiffness data have been used to obtain the orientation dependence of Young's modulus and the shear modulus. The results are in reasonably good agreement with the limited experimental data obtained from mechanical testing. The RUS measurements reveal that the unidirectional CAS/SiC composite is well modeled by transverse isotropic symmetry, indicating relatively isotropic fiber spacing in the transverse plane. The analysis indicates that the overall elastic anisotropy is also small for unidirectional and 0°/90° laminated CAS ceramic‐matrix composites, a result that can be attributed to the relatively low modulus ratio of the Nicalon SiC fiber to the CAS matrix and to the moderate fiber volume fraction.

Fracture behaviour of SiC fibre-reinforced nitrogen glass matrix composites

Both Nicalon and Hi-Nicalon SiC fibre-reinforced nitrogen glass composites were prepared by slurry infiltration and hot-pressing, and the interfacial features, fracture behaviour and mechanical properties of these composites were investigated. It was found that the interfacial characteristics were mainly dictated by the thermal expansion properties of the matrix and the type of SiC fibre. Yttrium sialon glass has a higher thermal expansion coefficient than SiC fibres, so a radial compressive stress on the fibre due to thermal mismatch caused a larger interfacial frictional stress between fibre and matrix. As a result, the composite failed in a brittle manner with no effective strengthening and toughening. Strong reaction between the Hi-Nicalon SiC fibre and matrix also resulted in relatively poor performance of these composites. In contrast, lithium sialon glass provided a matrix for these composites with significantly improved mechanical properties.

Microstructure-property relationships of SiC fibre-reinforced magnesium aluminosilicates—II. Mechanical properties and failure characteristics

Interfacial frictional shear stresses, flexural properties and failure mechanisms are reported for two magnesium aluminosilicates unidirectionally reinforced with Nicalon SiC fibres. Composites A and B were hot-pressed at 1500 and 920°C, respectively. High values of interfacial frictional shear stresses inferred from Marshall's analysis of the micro-indentation technique could be attributed in part to the presence of compressive radial stresses at the fibre-matrix interfaces. Although both composites failed non-catastrophically in symmetrical four point bend testing at room temperature, the failure modes were different. Extensive matrix microcracking, fibre failure and then fibre pull-out were commonly observed in composite A. Failure modes in composite B included the formation of a limited number of matrix cracks, the failure of fibres in the matrix crack front and progressive delamination. Our observations demonstrate that the mechanical properties, the interfacial frictional shear stresses and the failure mechanisms of both composites are governed by their microstructural features, in particular the chemistry and structure of the matrix-fibre interfacial region.

The Influence of Fiber/Matrix Interface on the Mechanical Behavior of Nicalon SiC Fiber Reinforced Glass-Ceramic Composites

ABSTRACTThe mechanical properties of unidirectional Nicalon SiC fiber reinforced calcium aluminosilicate (CAS/SiC) and magnesium aluminosilicate (MAS/SiC) glass-ceramic composites have been investigated by tensile testing and a nondestructive laser-ultrasound technique. The barium-stuffed MAS was either undoped or doped with 5% borosilicate glass. The degradation of the elastic stiffness constant Cu in the transverse direction due to interface damage was monitored in-situ by measuring the laser-generated ultrasound wave velocity. The three composite materials show distinctly different macroscopic deformation characteristics, which are correlated strongly to the interface degradation. A stronger reduction trend of the elastic constant ?? is associated with a larger degree of inelastic deformation. Observations of the fracture surfaces also reveal the close relation between fiber pullout length and interfacial characteristics. Interfaces of these composites have been studied by TEM, and their influence on inhibiting and deflecting matrix cracks is discussed.

A microdebonding study of the high-temperature oxidation embrittlement of a cross-ply glass-ceramic/SiC composite

A constant cross-section specimen with adhesively bonded tabs has been used for an investigation of the high-temperature tensile behavior of a cross-plied glass-ceramic-matrix composite consisting of CAS-II reinforced with Nicalon SiC fiber. Oxidation of the exposed interfaces along matrix cracks in 0 ° plies lowers the composite failure strain at 800 °C to the 0 ° ply matrix-cracking strain. Scanning electron microscopy and microdebonding analysis of the fracture surfaces indicate that the embrittlement process is the result of oxidation of the carbon-rich interphase as the matrix crack encounters 0 ° ply fibers, the interphase subsequently fuses with a higher bond strength and the crack grows through the fibers. Planar cracks grow inwards from the surface, covering the entire fracture surface given enough time (or sufficient strain). Degradation of the fibers does not appear to contribute to the embrittlement. Transverse plies crack at a lower strain than does the matrix in the 0 ° plies. However, it appears that oxygen does not enter 90 ° ply cracks in sufficient quantity to produce oxidation embrittlement, at least up to the 0 ° matrix-cracking strain. The strain to crack the 90 ° plies does not decrease significantly at high temperatures despite the fact that the cracks are primarily in the fiber/matrix interphase as they grow across the 90 ° plies.

Effect of matrix liquid phase on interphase formation in SiC fibre-reinforced Si2N2O-Al2O3-CaO composites

Matrix compositions based on Si2N2O, with Al2O3 and CaO additions, were used to hot press Nicalon SiC fibre-reinforced composites at 1600 °C. With both CaO and Al2O3 additions, eutectic melting formed an appreciable volume of liquid phase during hot pressing, which remained as a stable glassy phase in the cooled composites. This liquid phase fostered formation of ∼240 nm thick carbon-rich interphases between the fibres and the matrix. These interphases showed relatively low interfacial shear strength and resulted in composites which showed non-catastrophic, notch-independent fracture. Matrices using either Al2O3 or CaO did not form adequate liquid phase to form coarse interphases, and fracture was catastrophic in nature. Post-heat treatment of the composites at 1000 °C showed peripheral oxidation (removal of the carbon content of the interphase) indicating limited protection afforded when glassy phase was present in the matrix. Controlled cooling in the hot press did not cause the liquid regions to devitrify.

High‐Temperature Behavior of Silicon Carbide Fibers in a Si‐Al‐Ca‐O‐N Ceramic Matrix

A Nicalon SiC fiber‐reinforced Si‐Al‐Ca‐O‐N composite was fabricated by a slurry infiltration process followed by hot pressing at 1600°C. A carbon‐rich interfacial layer (∼100 nm) as well as a crystalline silicon‐rich layer (∼15 nm) was observed between the fiber and matrix. Based on this interfarcial phenomenology, the following behabior of SiC fibers in the matrix was proposed: fine SiC grains (diameter of ∼ 1.7 nm in as‐received fibers) decomposed at fiber surfaces (SiC → C + Si), followed by silicon migration into the glasshy phase of the matrix. The glassy phase was interpreted to play a key role as a silicon consumer in fostering the formation of the carbon‐rich layer. The presence of silicon implied that the oxygen activity in the matrix was low enough to avoid SiC oxidation.

Microstructure and compression behaviour in the semisolid state of short-fibre-reinforced A356 aluminium alloys

This paper is concerned with the microstructural characterization and compressive behaviour of partially remelted A356 aluminium alloys reinforced with 10 vol.% chopped SiC Nicalon fibres. The composites were fabricated by the compocasting technique and subsequently solidified under pressure, some materials being also extruded at high temperature. It is shown that compocasting leads to breakage of the fibres, which are further fragmented by extrusion and aligned along the extrusion direction. Isothermal holding of the extruded composites in the semisolid state leads to rapid globularization of the structure of the solid phase, with globules coarsening as the holding time increases. The globularization is associated with a decrease in the compression stress at constant strain rate, this stress increasing with increasing strain rate according to a classical power low σ = K ε ̇ m with values of m close to 0.3. Comparison with the unreinforced alloy and with as-cast composites clearly demonstrates the influence of the presence of the reinforcing elements on the compressive stress of the material together with that of extrusion before partial remelting.

Fabrication of ceramic composites: forced CVI

Forced chemical vapor infiltration (FCVI) is being developed as an efficient means for producing thick-wall ceramic matrix components. Recent efforts have focussed on process scale-up, modeling and the properties of the resultant material. Disks 24.6 cm in diameter and 1.27 cm thick have been produced. Composites of Nicalon TM fibers and SiC matrix have exhibited 480 MPa strength in flexure.

Effects of radiation on SiC-based Nicalon fibers

This paper presents neutron and ion radiation effects in the Nicalon SiC polymer precursor fiber. It is shown that the serious structural degradation of this fiber and its composites (e.g., CVD SiC/Nicalon) previously reported for the standard grade of Nicalon is primarily due to the presence of the silicon oxycarbide phase. Results supporting this interpretation include microstructural analysis as well as post irradiation mechanical property measurements. Preliminary results of the effects of irradiation on low-oxygen Nicalon fibers are presented. The reduced oxygen content fibers exhibit radiation-induced density, strength, and Weibull's and Young's moduli changes typical of monolithic ceramic materials. This contrasts sharply with the poor irradiation behavior of the standard Nicalon fiber and suggests that improved radiation resistance can be expected in SiC/SiC composites fabricated with low oxygen Nicalon fibers.

Oxide ceramic matrix/oxide fibre woven fabric composites exhibiting dissipative fracture behaviour

The effects of a zirconia interphase on the mechanical properties of long, continuous, oxide fibre-reinforced oxide ceramic matrix (mullite) composites prepared through a sol-gel route have been studied by scanning electron microscopy, optical microscopy, X-ray diffraction analysis and three-point bending tests. The achievement of a non-brittle composite was related to the absence of chemical reaction between the zirconia interphase and the oxide fibres. Comparison was made with SiC Nicalon fibre woven fabricreinforced oxide matrix composites.

Tailoring interfacial shear strength of SiC fiber-reinforced brittle matrix composites

The interfacial shear strength of Nicalon SiC fiber-reinforced glass-ceramic matrix composites was aimed to be tailored via two methods: (1) varying of the thickness of the carbon-rich interfacial layer between the fiber and the matrix by controlling hot pressing period and (2) formation of the secondary interfacial layer, TaC, at the carbon/matrix boundary by doping the Ta 2O 5 matrix addition. In the series of composites with varying carbon-rich layer thickness, fiber/matrix debonding mostly occurred at the carbon/matrix boundary and hence the increase in the carbon-rich layer thickness did not cause any apparent changes in the interfacial shear strength. In the TaC formed series of composites, the interfacial shear strength was affected considerably by the presence of the TaC phase at carbon/matrix boundary. The Ta 2O 5 addition to control the quantity of the TaC phase has shown to be a useful method to tailor the interfacial shear strength of SiC fiber/glass-ceramic composites.

SiC fibre-reinforced LAS glass-ceramic via sol-gel technique

A modified process to fabricate Nicalon SiC fibre-reinforced LAS glass-ceramic composite is presented, which is based on further development of the slurry method and the sol-gel technique. The bending strength, fracture toughness and shear stress of composites fabricated by the new process and fibre maintaining strength were tested, the interfacial structures were investigated by transmission electron microscopy (TEM), selected-area diffraction (SAD) and electron energy loss spectroscopy (EELS) and the effect of the type of sol-gel solution on them was studied