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.     

Novel basalt fibre reinforced glass matrix composites

A novel hot-pressing technique for the manufacturing of basalt fibre reinforced glass matrix composites was investigated. Two-dimensional (2D) fibre mats were sandwiched between borosilicate glass powder layers, thus configuring a much simpler processing route than that commonly employed for the production of fibre-reinforced glasses. Besides economic benefits, the use of fibre mats may lead to technologic advantages due to the possibility of readily coating the fibres with a suitable material (e.g. titanium oxide) by means of the sol-gel method. The coating of basalt fibre mats with TiO₂ is proposed for preventing the fibres from an excessive adhesion to the glass matrix. The developed composites containing 15 vol% of 2D-fibre reinforcement exhibited promising bending strength (∼90 MPa) and desirable “graceful” fracture behaviour without catastrophic failure. Thus the present study represents a convenient approach for production of advanced low-cost fibre reinforced glass matrix composites for structural applications.     

Fibre-reinforced glasses: influence of thermal expansion of the glass matrix on strength and fracture toughness of the composites

The reinforcement of glasses by incorporation of fibres was considered to depend on the force transfer from the matrix on the fibres in order to obtain optimum strength and fracture toughness. This may occur by thermal shrinking of the matrix on the fibres after the hot-pressing procedure. It is shown that an optimum exists for strain and stress transfer from the matrix to the fibres if this shrinkage process is neither so strong that no pull-out and no bend-over effect is produced nor so weak that no stress transfer is possible. Therefore, experiments were performed with Nicalon-SiC fibres and with selected glasses which show different thermal expansion coefficients. In this way it was possible to produce fibre-reinforced glass composites with well-tailored special properties. Estimations of tensile stresses within the glass matrix led to values which are partly above those of the bulk glass. Because no cracks occurred during cooling and during heat shock treatment fromT _g, it was concluded that the strength of the thin glass layers between the very smooth surfaces of the Nicalon-SiC fibres cannot be compared with that of bulk glass but with that of protected (coated) glass fibres or thin sheet glass.     

Compressive failure of unidirectional hybrid fibre-reinforced epoxy composites containing carbon and silicon carbide fibres

The compressive failure of unidirectional hybrid fibre-reinforced epoxy matrix composites containing carbon (C) and silicon carbide (SiC) fibres has been investigated. In contrast to the case of flexural testing previously investigated by the authors, no significant increase in compressive strength, elastic modulus, or work of fracture was noted for the case of composites containing a mixture of C and SiC fibres. The specific compressive strength and elastic modulus generally decreased with increasing SiC fibre content due to the higher density of these fibres. Failure modes of tested specimens were classified into two main groups, namely compressive shear and compressive crushing, with the presence of fibre kinking and longitudinal splitting being noted in both cases.     

The properties of carbon fibre/SiC composites fabricated through impregnation and pyrolysis of polycarbosilane

Unidirectional carbon fibre reinforced SiC composites were prepared from four types of carbon fibres, PAN-based HSCF, pitch-based HMCF, CF50 and CF70, through nine cycles or twelve cycles of impregnation of polycarbosilane and subsequent pyrolysis at 1200°C. The polycarbosilane-derived matrix was found to be -SiC with a crystallite size of 1.95 nm. The mechanical properties of the composites were evaluated by four-point bending tests. The fracture behavior of each composite was investigated based on load-displacement curves and scanning electron microscope (SEM) observation of fracture surfaces of the specimens after tests. It was found that CF50/SiC and CF70/SiC exhibited high strength and non-brittle fracture mode with multiple matrix cracking and extensive fibre pullout, whereas HSCF/SiC and HMCF/SiC exhibited low strength and brittle fracture mode with almost no fibre pullout. The differences in the fracture modes of these carbon fibre/SiC composites were thought to be due to differences in interfacial bonding between carbon fibres and matrix. Values of flexural strengths of CF70/SiC and CF50/SiC were 967 MPa and 624 MPa, respectively, which were approximately 75% and 38% of the predicted values. The relatively lower strength of CF50/SiC, compared with CF70/SiC, was mainly attributed to the shear failure of CF50/SiC during bending tests.     

Effects of thermal cycling on thermal expansionand mechanical properties of SiC fibre-reinforced reaction-bonded Si3N4 composites

Thermal expansion curves for SiC fibre-reinforced reaction-bonded Si3N4 matrix composites (SiC/RBSN) and unreinforced RBSN were measured from 25 to 1400 °C in nitrogen and in oxygen. The effects of fibre/matrix bonding and cycling on the thermal expansion curves and room-temperature tensile properties of unidirectional composites were determined. The measured thermal expansion curves were compared with those predicted from composite theory. Predicted thermal expansion curves parallel to the fibre direction were between the measured curves for the strongly- and weakly-bonded composites, but those normal to the fibre direction for both bonding cases were similar to that of the unreinforced RBSN. Thermal cycling in nitrogen for both bonding cases resulted in no net dimensional changes at room temperature and no loss in tensile properties from the as-fabricated condition. In contrast, thermal cycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mechanical properties of the weakly-bonded SiC/RBSN composites the most, resulting in loss of strain capability beyond matrix fracture and catastrophic, brittle fracture. Increased bonding between the SiC fibre and RBSN matrix due to oxidation of the carbon-rich fibre surface coating and an altered residual stress pattern in the composite due to internal oxidation of the matrix are the main reasons for the poor mechanical performance of these composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of fibre pre-treatment on thermal characteristics of asbestos-nylon-epoxy composites

The objective of this study has been the enhancement of understanding the role of fibre pre-treatment in determining the thermal expansion behaviour and glass transition temperature of fibre-reinforced thermosetting polymers. Asbestos fibres were pre-treated with poly(hexamethylene adipamide) and used afterwards as filler reinforcement in a bisphenol-A-based epoxy matrix. Two alternative processes of fibre pre-treatment were used. Thermal expansion coefficients and glass transition temperatures for the asbestos-nylon-epoxy composites were experimentally determined. Results are qualitatively explained by theoretical models in which the concept of the is considered. The effects of asbestos and nylon content as well as of type of pre-treatment and adhesion between the phases are investigated.     

Mechanical behaviour of chemical vapour infiltration-processed two- and three-dimensional nicalon/SiC composites

The effect of two different fibre architectures on the mechanical properties of the Nicalon fibre-reinforced SiC composites processed by chemical vapour infiltration has been investigated. The microstructure, flexural strength, fracture toughness and failure mechanisms of both two-dimensional woven laminate and three-dimensional braided composites were characterized. It was found that the fibre placement in the preform will not only affect the infiltration of the SiC matrix, but also the mechanical property and failure behaviour of the composite. A strong, tough and damage-tolerant SiC matrix composite can be fabricated through the combination of a three-dimensional braided integrated fibre network and chemical vapour infiltration processing.     

Structure and chemistry of fibre-matrix interfaces in silicon carbide fibre-reinforced glass-ceramic composites: an electron microscopy study

Silicon carbide continuous fibre-reinforced glass and glass-ceramic matrix composites showing high strength and fracture toughness have been studied using thin-foil transmission electron microscopy and scanning transmission electron microscopy (AEM). The outstanding mechanical behaviour of these materials is directly correlated with the formation of a cryptocrystalline carbon (graphite) reaction-layer interface between the fibres and the matrix. A solid-state reaction involving relatively rapid diffusion of silicon and oxygen from fibre to matrix correlates well with the experimental observations. Silica activity in the glass-ceramic matrix is suggested to play a primary role in the ability to control the chemical reaction which creates the graphitic interface. AEM results are used to comment upon a possible mechanism for the high-temperature embrittlement behaviour noted for these materials when they undergo rupture in an aerobic environment.     

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. This revised version was published online in November 2006 with corrections to the Cover Date.     

Behaviour of coatings on reinforcements in some metal matrix composites

Coating on reinforcements affects the interface bonding of a composite, and is therefore usually used for improving the composite's properties. The behaviour of SiC coating on carbon fibre in reinforced aluminium metal castings, Fe on carbon fibre-reinforced copper and alumina coating on K₂O · 6TiO₂ whisker-reinforced aluminium composites were investigated, respectively, by modern techniques such as TEM, SEM etc. with the goal of controlling the interfacial interaction and wettability of reinforcement with the matrices. SiC coating produced by a polycarbosilane solution process effectively improved the strength because it successfully controlled oxidation of the carbon fibres themselves and the harmful reaction between the carbon fibres and molten aluminium during the fabrication process and heating process of the composites. The metal coating, Fe, made by electrical plating, strengthened the bonding of carbon fibres with copper by changing the bonding state of the interface from a mechanical one to a partly chemical one. Therefore the strengths of the resulting composites were improved. The alumina coating on K₂O · 6TiO₂ also controlled the diffusion of the K element from the whiskers into the aluminium matrix and altered the reaction with aluminium, and led to the optimization of interfacial bonding between the whiskers and a superior composite.     

The role of interfaces and matrix void nucleation mechanism on the ductile fracture process of discontinuous fibre-reinforced composites

The role of fibre morphology, interface failure and void nucleation mechanisms within the matrix on the deformation and fracture behaviour of discontinuous fibre-reinforced composites was numerically investigated. The matrix was modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. For the matrix, two materials exhibiting identical strength and ductility but having different void-nucleation mechanisms (stress-controlled and strain-controlled) were considered and fibres were assumed to be elastic. The debonding behaviour at the fibre interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. The results indicate that in the absence of interface failure, for a given fibre morphology the void nucleation in the matrix is the key controlling parameter of the composite strength and ductility, hence, of the fracture toughness. The weak interfacial behaviour between the fibres and the matrix can significantly increase the ductility without sacrificing strength for certain fibre morphology and for certain matrix void-nucleation mechanisms.     

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of ?31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of ?53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.     

Silicon carbide fibre-reinforced resin matrix composites

Resin matrix composites reinforced with silicon carbide yarn and silicon carbide monofilament were fabricated and evaluated. Both composite systems exhibited excellent mechanical properties. Composite thermal expansion behaviour, fibre electrical resistance, and fibre thermal oxidation resistance are also reported. Advantages with respect to carbon fibre-reinforced resins are discussed.     

High-strength silicon carbide fibre-reinforced glass-matrix composites

Silicon carbide fibre-reinforced glass-matrix composites have been fabricated and tested. Two fibre forms, a 140 μm diameter monofilament and a 10 μm diameter filamentary yarn, were incorporated into a matrix of borosilicate glass. The hot-pressing fabrication procedure resulted in fully dense unidirectionally reinforced specimens with excellent flexural strength and fracture toughness over the temperature range 22 to 700° C. In addition, composite thermal expansion was found to be nearly independent of fibre orientation indicating that multiaxially reinforced composites should be readily fabricable without the occurrence of extensive cracking.     

Oberflächenbehandlung von Kohlenstoffasern und mechanische Eigenschaften von Laminaten

Surface Treatment of Carbon Fibres and Resulting Composite Properties In composites carbon fibres are used as reinforcing fibres with thermosetting and thermoplastic resins as martices. These carbon fibres differ strong in their micro-structure and therefrom in fibre properties. To achieve sufficiant composite properties special carbon fibre surface treatment methods are necessary. This paper describes a systematic study on oxidative surface treatment of carbon fibres by wet-, dry- and anodic oxidation. Further investigations by matrix variation show us the influence of matrix strength on the mechanical composite properties. Finally it is shown that in case of impact load composite fracture behaviour is controlled only by the fibre itself.     

Fibre-matrix bond strength studies of glass,ceramic, and metal matrix composites

An indentation test technique for compressively loading the ends of individual fibres to produce debonding has been applied to metal, glass, and glass-ceramic matrix composites; bond strength values at debond initiation are calculated using a finite-element model. Results are correlated with composite longitudinal and interlaminar shear behaviour for carbon and Nicalon fibre-reinforced glasses and glass-ceramics including the effects of matrix modifications, processing conditions, and high-temperature oxidation embrittlement. The data indicate that significant bonding to improve off-axis and shear properties can be tolerated before the longitudinal behaviour becomes brittle. Residual stress and other mechanical bonding effects are important, but improved analyses and multiaxial interfacial failure criteria are needed to adequately interpret bond strength data in terms of composite performance.     

A compliant,high failure strain,fibre-reinforced glass-matrix composite

It is demonstrated that a unique form of composite material can be achieved by reinforcing glass matrices with discontinuous graphite fibres. The graphite fibres were utilized in the form of a paper, purchased in large sheets, and composites were formed by hot-pressing glass-powder-impregnated paper plys. The resultant composites exhibit high strength, high fracture toughness (compared to ceramics), low density and low thermal expansion coefficient. Of particular note is the unique tensile stress-strain curve achieved which exhibits both high strength and high failure strain. Its very non-linear shape differs markedly from that of either the unreinforced glass or a similarly reinforced epoxymatrix composite. In addition, the elastic modulus of the resultant composite, despite being reinforced with a high stiffness fibre, is lower than that of the parent matrix resulting in an unusually compliant ceramic material.     

Carbon fibre composites with ceramic and glass matrices

The fabrication, microstructure and some of the mechanical and thermal properties of a series of composites are described. The systems investigated were magnesia, alumina, soda-lime glass, borosilicate glass and a lithia alumino-silicate glass-ceramic incorporating high modulus, chopped carbon fibres and magnesia containing chopped, stabilized zirconia fibres. Fracture strengths were increased when the fibres were partially aligned, but decreased when the fibres were randomly oriented. In all cases, however, a substantial increase in work of fracture was observed compared to the non-reinforced matrices. The observed effects are discussed in terms of the volume fraction of fibre, the mismatch of thermal expansion coefficients between matrix and fibre and the nature of the interface.