Microstructural investigations in Cf–SiC composites in as-sintered state and after creep experiments

The high interest in ceramic matrix composites during the last decade has led to a considerable number of studies devoted to their thermomechanical properties and damage processes. Despite their sensitivity to oxygen partial pressure, carbon fibres appear to possess higher stability and better mechanical properties if they are treated under protective atmospheres than other ceramic fibres (especially classical silicon carbide fibres). The aim of this investigation is to characterize at the nanoscale the main microstructural parameters of C_f–SiC composites provided by the SEP (Division of SNECMA, Bordeaux, France). This material was fabricated from a 2.5D preform made of high strength polyacrylonitrile (PAN)-based carbon fibres densified according to the chemical vapour infiltration process. A pyrocarbon (PyC) interphase was deposited on the fibre prior to the β-SiC matrix infiltration. A careful high resolution electron microscopy (HREM) microstructural investigation focused on the fibre microstructure as well as on the different interfaces in the material: pyrocarbon/fibre and matrix/pyrocarbon interfaces. All these observations have been realized in longitudinal and transverse sections of the specimen. These observations are found in good agreement with Guigon's model for high strength ex-PAN carbon fibres. The PyC interphase texture was strongly anisotropic at the fibre/interphase and interphase/matrix interfaces over a mean thickness of 8–15 nm. Tensile creep tests were performed under partial pressure of argon between 1273 and 1673 K for stress levels ranging from 110 to 220 MPa. Scanning electron microscopy and high resolution electron microscopy were used to study the microstructural modifications inside the fibres and at the different interfaces. A discussion of the possible creep mechanisms based on the microstructural investigation and the creep results is presented.     

Microstructure-property relationships in silicate-matrix composites

Electron microscopy and associated analytical spectroscopic techniques have been used to characterize interfaces in SiC-fibre/silicate-matrix composites. Interface structure, formed via reaction during hot-press fabrication, is a function of time, temperature, matrix composition and fibre type. Interfaces with Nicalon or Tyranno fibres vary from amorphous carbon in fine precipitated form to continuous graphitic layers. Interface behaviour in a stressed composite, and hence the matrix microcracking stress, is a sensitive function of microstructure. Interface debond and shear properties have been assessed using indent-based ‘push-down’ and ‘push-through’ tests using a specially developed instrument within a scanning electron microscope. This uses piezoload measurement and translation, and is capable of dynamic image recording of the indentation sequence. Interface micromechanical (indent) measurements have been correlated with structure and macromechanical response in bend testing for a range of fibre/matrix types, processing and post-processing thermal treatments. An example is also given of interfaces prepared by fibre precoating.     

Microstructure and creep characteristics of experimental SiCf–YMAS composites

A unidirectional SiC_f –YMAS glass–ceramic composite has been developed by Céramiques-Composites (Bazet) and ONERA (Establishment of Palaiseau) in France. The matrix is totally crystalline and consists essentially of two main phases, cordierite and yttrium disilicate, with some minor phases, mullite, spinel, zirconium and titanium oxides. Image analysis methods have been used to characterize the homogeneity of the composite plates and to obtain granulometric information on the different matrix phases. Different interphase layers formed during the process by reaction between the matrix and the Nicalon NLM 202 fibres have been studied by using HREM and EDX. Their chemical composition has been determined by stepping the probe (8 nm) across the fibre–matrix interface. Two distinct nanoscale sublayers have been imaged. The sublayer on the matrix side has a light contrast in the TEM. The microstructure of this layer (≈ 80 nm) is typical of a turbostratic carbon. The carbon layer also contains Al, O, Mg and Si. The silicon content is low in the carbon layer. The sublayer on the fibre side (≈ 100 nm thick) has a dark contrast in the TEM. Profiles have been taken across this sublayer also. Tensile creep tests in air have been performed to investigate the tensile creep behaviour at 1223 K. They have been conducted in the 50–200 MPa stress range. Tensile creep results indicate that creep rates are of the same order of magnitude as for other glass–ceramic composites. Optical micrographs and SEM observations have revealed the damage in the composite. Changes occurring in the interface region have been studied at a finer scale by TEM and HREM at the surface of the sample and in the core. These observations enable us to explain the mechanical behaviour of the composite observed on a macroscopic scale.     

The microstructure of silicon carbide (Nicalon) fibre reinforced silicon carbide ceramic-matrix composites heat treated in vacuo and in pure oxygen

Silicon carbide (Nicalon) fibre reinforced SiC composites have been heat treated in vacuo and in pure oxygen environments at 1400°C for 100 h. The response of the microstructure and, in particular, of the interface between fibre, carbon interlayer and SiC matrix components has been studied. Microstructural modifications were observed by transmission electron microscopy, using imaging, electron energy-loss spectroscopy and electron diffraction techniques, and fibre stoichiometries were determined using a scanning Auger microprobe. Recrystallization of Nicalon fibres within composites heat treated in vacuo was found to result from decomposition of the metastable silicon oxy-carbide phase found in the fibres. No significant changes to the pyrolytic carbon interlayer were observed. Fibre recrystallization was considered to embrittle the composite. Samples heat treated in oxygen showed no appreciable fibre recrystallization. Study of interlayers in such aged samples often revealed decohesions, holes and narrow silica layers. In the most extreme cases, complete displacement of carbon by SiO₂ was found and such interfaces were identified as silica and α-cristobalite. Interfacial modifications were considered to be responsible for the retention of the small β-SiC grain size in Nicalon fibres and were also considered to be deleterious to the mechanical properties.     

Microcracking mechanism in a SiCf-SiBC composite creep-tested in argon

This paper addresses the creep behaviour of a woven SiC_f–SiBC composite, tested in tension under a partial pressure of argon, between 1273 K and 1473 K. It appears that the creep strain begins from 1273 K and becomes larger at higher temperatures. Moreover, the shapes of the creep curves led to the assumption of the existence of two competing deformation mechanisms depending on the temperature domain. The creep mechanism involved is microcrack damage-creep. From higher resolution studies at higher scales (scanning electron microscopy, transmission electron microscopy (TEM) and high resolution electron microscopy (HREM)), many types of damage were observed, for example matrix microcracking, fibre/matrix debonding and fibre/matrix sliding. The observations via TEM and HREM enabled us to specify the existence or not of the classical creep mechanism of the constituents of the composite, and also to characterize the behaviour and the role of the different interfaces and especially of the pyrocarbon interphase. These multiscale observations will be discussed in order to highlight the creep-damage mechanism as a function of temperature of the SiC_f-SiBC composites.     

Silicon carbide/SRBSN composites

Ceramic-matrix composites have been produced using unidirectionally aligned Textron SCS-6 fibres in a sintered reaction-bonded matrix. A tape-casting technique was used to produce a prepreg sheet that could be cut and stacked to form a layup. Al₂O₃ and Y₂O₃ were used as sintering aids, final sintering being carried out in a hot press at 1700°C. Matrix, fibre and interfacial microstructure has been characterized using analytical microscopical techniques. X-ray mapping of the carbon and silicon distribution at the fibre–matrix interface was carried out, and evidence of reaction between the outer carbon-rich layer of the fibre and the matrix was found. Micromechanical behaviour of interfaces has been investigated and compared with interfacial microstructure and macromechanical properties.     

The microstructure of experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS) materials

Two experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS)-type ceramic-matrix composite (CMC) materials fabricated (i) by the glass process and (ii) by chemical precursor infiltration have been studied by light microscopy, transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and energy-dispersive X-ray spectroscopy (EDS). The distribution of the fibres inside the composite as well as the average diameter of fibres have been determined by image analysis. The microstructure of the YMAS matrices has been characterized by TEM observations. YMAS matrices are formed of two main phases, cordierite and β-yttrium silicate (Y₂Si₂O₇). Two minor phases (mullite and spinel) have been found to crystallize inside the cordierite and the yttrium silicate crystals. Fibre-matrix interfaces have been observed in HREM. A thin turbostratic carbon layer (20–30 nm) has been imaged in both composites at the fibre-matrix interface. It crystallizes along the matrix interface and grows inside the fibre, forming a diffuse interphase. The carbon layer is believed to be the consequence of reaction between oxygen in the matrix and SiC nanocrystals of the Nicalon fibres.     

Interface compatibility in Nicalon-fibre reinforced metal and ceramic composites

Microstructural characteristics of the fibre–matrix interface of two composite systems which utilize Nicalon fibre reinforcement are analysed and discussed. An Al-based composite produced by liquid-metal infiltration was found to contain crystals of aluminium carbide and alumina at the fibre–matrix interface, which produced a strong interfacial bond, restricted fibre pullout, and resulted in an essentially brittle composite. A ceramic-matrix composite based upon calcium aluminosilicate and produced by hot pressing exhibited substantial fibre pullout during testing; microstructural analysis of the interface showed the presence of a C-rich layer. Treatment of the composite in air over a range of temperatures (600–1200°C) progressively oxidized the carbon and formed silica ‘bridges' between fibre and matrix, which resulted in increased brittleness. Electron-probe microanalysis combined with electron microscopy of the Nicalon fibre showed that approximately half the material consisted of microcrystalline β-SiC and the remainder was free carbon and silicon oxycarbide. Thus the carbon constituent was largely responsible for carbide formation in the Al-based material, which restricted fibre pullout, whilst free carbon, plus the additional free carbon formed by chemical reaction between silicon carbide in the fibre and the calcium aluminosilicate matrix, provided the interfacial carbon layer which gave enhanced fibre pullout in the ceramic-based composite; the decreased fibre pullout and increased brittleness of the latter after heat treatment in air could thus be explained by the removal of the carbon layer and the development of silica bridges between fibre and matrix.     

The use of a single fibre test technique to investigate interfacial phenomena in SiC/Ti3Al-based composites

A study of the chemical compatibility of a Ti₃ Al-based alloy (Ti–24Al–10Nb, at.%) with two silicon carbide continuous fibres (SCS-6, SM 1240) has been conducted. Owing to the difficulty in processing intermetallic matrix composites, this type of material has been simulated in the present work by sputtering a thin titanium aluminide layer onto fibres and heat treating at temperatures representative of fabrication conditions. The degradation of the fibre strength due to its interaction with the matrix was correlated with analytical studies of the fibre/matrix interface using a combination of SEM, TEM, EELS and a submicrometre ion probe.     

The compatibility of TiB2 protective coatings with SiC fibre and Ti-6Al-4V

TiB₂ coatings have been studied as prospective protective layers to inhibit the interfacial reaction between SiC fibres and Ti-alloy matrices. This protective coating has been deposited onto SiC monofilament fibres using a chemical vapour deposition (CVD) technique. The fibre-matrix compatibility of these TiB₂-coated SiC fibres in Ti-6Al-4V composites was evaluated by incorporating the coated fibres into Ti-6Al-4V using a diffusion bonding technique. The interfaces of this composite were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis, to evaluate the interfacial microstructures, chemical stability and the efficiency of TiB₂ as a protective coating for SiC fibres in Ti-alloy matrices, and to study the effects of deposition temperature on the interface of the coated fibre. Results show that stoichiometric TiB₂ coatings are stable chemically to both SiC fibres and Ti-6Al-4V and hinder the deleterious fibre-matrix reactions effectively. Boron-rich TiB₂ coatings should be avoided, as they lead to the formation of a needle-like TiB phase at the fibre–matrix interface. These findings provide promising evidence for the value of further exploration of the use of stoichiometric TiB₂ as a protective coating for SiC fibre in Ti-based composites.     

Microstructural evolution of the latest generation of small-diameter SiC-based fibres tested at high temperatures

The new generation of silicon-carbide-based fibres made from organosilicon precursors, cross-linked by electron irradiation, have been compared with the earlier fibres which have undergone cross-linking in air. The latest fibres, known as Tyranno Lox-E and Hi-Nicalon, possess a lower oxygen content (≈5 wt% and ≈0.5 wt%) whereas the NLM202 fibres contain 12 wt% and the Tyranno Lox-M 13 wt% of oxygen. The Tyranno fibres have been produced with a precursor similar to that used to produce the Nicalon fibres, but modified by the addition of titanium. All fibres possess a structure composed of β-SiC grains, free carbon aggregates, with no crystallized titanium compounds in the Tyranno fibres and an oxygen-rich intergranular phase, except in the Hi-Nicalon fibre. The Hi-Nicalon fibre has the largest grain size and its free carbon content is higher than in the NLM202 fibres. For all the fibres, the β-SiC grains grow when the temperature increases, whilst the strengths and Young's moduli decrease. The NLM202 shows the least change in grain size and tensile properties. The Hi-Nicalon is stiffer and stronger than the others at high temperature. TEM results show that grain growth is isotropic, even during creep tests. The growth depends on the nature and amount of the intergranular phase. Mechanical changes as a function of temperature can be explained by external oxidation during tensile tests in air and internal oxidation facilitated by the nanoporosity, which is greater in the Tyranno than in the Nicalon fibres. The presence of the oxygen-rich phase in the three fibres containing the most oxygen decreases the creep resistance. Titanium does not improve the mechanical properties and the creep resistance beyond 1523 K and does not have any positive influence in limiting the SiC grain growth. Tyranno fibres are less well stabilized than the Nicalon fibres. The Hi-Nicalon fibres have been shown to possess consistently better mechanical properties at all temperatures, including creep resistance, than the other fibres studied. All the fibres are sensitive to external oxidation at high temperature.     

X-ray tomographic imaging of Ti/SiC composites

In this paper, high‐resolution tomographic synchrotron X‐ray imaging is applied to study the occurrence and evolution of damage in Ti‐6Al‐4V/SCS6 SiC fibre composite materials. Three composite morphologies of increasing complexity have been studied, namely single fibre, single‐ply and multi‐ply composites. The single fibre composite was strained to full fibre fragmentation and the progressive introduction of damage monitored. For the single‐ply composite, damage was introduced deliberately by laser drilling to establish the effect of damaged fibres on their neighbours, whereas for the multi‐ply composite the morphology of a fibre bridging fatigue crack was studied. In addition to traditional mode I fibre fractures, subsequent fibre wedge cracks were observed presumably nucleating from damage introduced into the fibre surface by the first fracture event. In addition to these crack morphologies, spiral defects were observed for the single ply during failure. Finally, for the multi‐ply composite, the matrix crack front showed a number of characteristic features, including advancement in fibre‐free regions, crack bifurcation near fibres and different crack plane heights either side of a fibre.     

Analysis of SiC fibres and composites using Raman microscopy

The application of Raman spectroscopy in the analysis of the microstructure of SCS-6 silicon carbide fibres using a Renishaw Raman microscope is described. It is demonstrated that the technique allows a detailed study to be made of the point-to-point variation in microstructure across a fibre section. It has been possible to monitor the variation of the concentration of SiC and carbon in the fibre microstructure and to detect differences in the forms of carbon present. It is also shown that Raman spectroscopy can be used to follow the micromechanics of both the deformation of silicon carbide fibres and of the fibres within a model composite. Well-defined Raman spectra have been obtained from a variety of Nicalon and Tyranno fibres and the positions of the Raman bands shown to shift on the application of stress or strain. From such stress-induced Raman band shifts, the point-to-point variation of axial fibre stress or strain along an individual fibre in an epoxy matrix can be determined. An example is given of the use of the technique to map the distributions of axial fibre strain in a Nicalon/epoxy fragmentation test specimen and to model the failure processes at the fibre/matrix interface.     

Acoustic microscopy of ceramic-fibre composites

A variety of ceramic-fibre composites has been studied by acoustic microscopy, at 1·9 GHz with a resolution of 0·8 μm. The materials studied were Nicalon-reinforced borosilicate glass, SiC fibres in a magnesium–aluminosilicate matrix and a calcium-aluminosilicate matrix, and SiC monofilaments in a Ti–6Al–4V matrix. In all the specimens the contrast was dominated by strong excitation of Rayleigh waves in the surface. This gave strong contrast from different phases, and revealed interfaces and cracks by characteristic crack patterns. Quantitative agreement between observed and calculated fringe patterns was found, and values of shear and Young's modulus were measured. In the SiC monofilament specimens, various stages of progressive deterioration as a result of thermal ageing treatments were observed.     

The microstructure of spray-formed Ti-6Al-4V/SiCf metal-matrix composites

Spray-forming is a possible manufacturing route for the fabrication of Ti alloy fibre-reinforced metal-matrix composites (MMCs) because high rates of alloy-droplet cooling on impact with the fibres prevent excessive fibre-matrix reaction. Ti–6Al–4V matrix MMC monotapes containing TiB₂-coated SiC fibres have been manufactured by electric-arc spray-forming, and the key MMC microstructural characteristics in the as-sprayed monotapes have been investigated by optical and scanning electron microscopy. Fibre infiltration increases with decreasing spraying distance, decreasing atomizing gas pressure and increasing arc current, because of higher temperatures in the Ti alloy spray droplets on impact with the fibres. Too much binder in the fibre preform leads to poor fibre–matrix contact, while removing the binder leads to the fibres becoming misaligned during spraying.     

Tribological behaviour of glass fibre bundles reinforced epoxy gradient composites

Abstract

The short fibre bundles separated from the machining waste of a printed circuit board manufacturing plant were used in preparing functionally graded composites using polysulphide modified epoxy resin. Glass fibre bundles were thouroughly mixed with epoxy, which is getting polymerised with time and centrifugal force was applied to achieve graded dispersion of glass fibre bundles. The centrifugation time was varied to obtain different gradient profiles. Optical microstructures confirmed the graded dispersion of glass fibres bundles in the epoxy matrix. Increase in distance towards the centrifugation force direction increases the glass fibre concentration. Gradient characteristics in the composite have been observed in wear and friction measurements, which were conducted using a pin-on-disc machine. Worn surfaces of samples were analysed with the help of SEM. Both sliding (adhesive) and abrasive wear rates of glass fibre reinforced epoxy gradient composites reduced with increasing centrifugation time. Reduction in wear rate in glass fibre epoxy gradient composites has been attributed to the better interface bonding between epoxy coated fibre bundles and the epoxy matrix and hardening property of glass fibre. It has been found that capability to sustain pressure limit increased from 0·59 to 0·79 MPa on centrifuging the sample upto 2 min and reached to 1·19 MPa with increasing the centrifugation time to 30 min.     

Wear performance of a bulk liquid crystal polymer and its short-fibre composites

The wear behaviour of short-fibre reinforced composites having a liquid crystal polymer (LCP) matrix was investigated under two extremely different types of wear loading. Sliding wear tests against smooth steel at different values of sliding speed v and contact pressure p revealed that reinforcing LCP with carbon fibres, or glass fibres plus mineral filler and graphite flake, was much more effective in improving the composites' wear resistance than a reinforcement with glass fibres only. The same trend holds as far as the limiting pv factor was concerned. No improvement in wear resistance due to fibre reinforcement could be found under abrasive wear conditions. The layered structure of the unfilled LCP gave rise to a large anisotropy in wear rate in different sliding directions, but this was almost eliminated by the addition of short fibres under sliding wear conditions.     

A method for estimating the fibre length in fibre‐PLA composites

Wood pulp fibres are an important component of environmentally sound and renewable fibre‐reinforced composite materials. The high aspect ratio of pulp fibres is an essential property with respect to the mechanical properties a given composite material can achieve. The length of pulp fibres is affected by composite processing operations. This thus emphasizes the importance of assessing the pulp fibre length and how this may be affected by a given process for manufacturing composites. In this work a new method for measuring the length distribution of fibres and fibre fragments has been developed. The method is based on; (i) dissolving the composites, (ii) preparing the fibres for image acquisition and (iii) image analysis of the resulting fibre structures. The image analysis part is relatively simple to implement and is based on images acquired with a desktop scanner and a new ImageJ plugin. The quantification of fibre length has demonstrated the fibre shortening effect because of an extrusion process and subsequent injection moulding. Fibres with original lengths of >1 mm where shortened to fibre fragments with length of <200 μm. The shortening seems to be affected by the number of times the fibres have passed through the extruder, the amount of chain extender and the fraction of fibres in the polymer matrix.     

Moisture and gamma‐ray irradiation effects on the mechanical properties of carbon fibre–reinforced plastics

The effects of γ‐irradiation and moisture absorption on the mechanical properties of carbon fibres–epoxy resin composites were studied. The properties dominated by the matrix and fibre–matrix interface (interlaminar and in‐plane shear strength) were measured at room temperature using standard tests. These tests were carried out before and after exposures to gamma irradiation and before and after immersion in water at 80°C during 21 days. The dosage of gamma irradiation was up to 11.7 MGy. The micrographs of surfaces fractured in performed tests were observed on a scanning electron microscope. They were analyzed with consulting the stated effects on mechanical properties and the measured values of the glass transition temperature of tested coupons before and after irradiation and immersion in water. The obtained results show that moisture and irradiation, if they act one after the other, have a significant influence on the degradation of matrix‐dominated mechanical properties of the tested carbon–epoxy composite.     

Solid particle erosion of unidirectional fibre reinforced thermoplastic composites

In the present study, the solid particle erosion behaviour of neat PEEK matrix and unidirectional glass fibre (GF) and carbon fibre (CF) reinforced polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) composites has been studied. The erosion experiments have been carried out by using silica sand particles (200 ± 50 μm) as an erodent. Steady state erosion rates of these composites have been evaluated at different impact angles and impact velocities. The neat PEEK exhibited peak erosion rate at 30° impingement angle whereas the composites exhibited a semi-ductile behaviour with peak erosion rate at 60° impact angle. The erosion rate of the glass fibre reinforced composites was higher than that of the carbon fibre reinforced composites. The results show that the fibre orientation has a significant influence on erosion rate only at lower impact angles. The erosion rate of the composites was higher when the particles impact perpendicular to the fibre direction than parallel to the fibres. The morphology of eroded surfaces was observed under scanning electron microscope and damage mechanisms were discussed.