Microstructure and micromechanics of the interface in carbon fibre reinforced Pyrex glass

Detailed microstructural studies have been carried out on composites consisting of Pyrex glass reinforced with carbon fibres. Analysis of the fibre-matrix interface showed that some reaction had taken place during fabrication of the composite and that a carbide or oxycarbide layer had formed between the glass and the carbon fibre. The measured interlaminar shear strength of the composite indicated that the layer was not a source of weakness and appeared to be well bonded to the matrix. Substantial fibre pull-out had occurred, however, to expose clean fibre surfaces and smooth sockets. These observations led to the conclusion that the interfacial shear process was confined substantially to the outer layers of the carbon fibre. Confirmatory evidence for the low interfacial friction stress was available from micro-indentation tests which showed fibre displacement relative to the matrix at loads of less than 10 kPa. Heat treatment of the composite at 500°C in air caused preferential oxidation of the carbon fibre. Where fibres met the specimen surface, oxidation had proceeded down the fibre to produce a smoothly tapering shape. The rate of oxidation was estimated to be 3 m h^–1 parallel to the fibre axis, but much less than this in a direction perpendicular to the fibre, 0.5 m h^–1, due to the relatively slow diffusion rate of oxygen through glass.     

Fibre-matrix adhesion and its relationship to composite mechanical properties

Two major areas of enquiry exist in the field of fibre-matrix adhesion in composite materials. One is the fundamental role that fibre-matrix adhesion plays on composite mechanical properties. The other is what is the best method used to measure fibre-matrix adhesion in composite materials. Results of an attempt to provide an experimental foundation for both areas are reported here. A well-characterized experimental system consisting of an epoxy matrix and carbon fibres was selected in which only the fibre surface chemistry was altered to produce three different degrees of adhesion. Embedded single-fibre fragmentation tests were conducted to quantify the level of fibre-matrix adhesion. Observation of the events occurring at the fibre breaks led to the documentation of three distinct failure modes coincident with the three levels of adhesion. The lowest level produced a frictional debonding, the intermediate level produced interfacial crack growth and the highest level produced radial matrix fracture. High fibre volume fraction composites made from the same material were tested for on- and off-axis, as well as fracture, properties. Results indicate that composite results can be explained if both differences in adhesion and failure mode are considered. It will be further demonstrated that fibre-matrix adhesion is an optimum condition which has to be selected for the stress state that the interface will experience. The embedded single-fibre fragmentation test is both a valuable measurement tool for quantifying fibre-matrix adhesion as well as the one method which provides fundamental information about the failure mode necessary for understanding the role of adhesion on composite mechanical properties.     

Transient liquid-phase sintering of ceramic-reinforced Fe-based composites

The microstructural development of ceramic-reinforced iron-based composites has been studied. The composites were fabricated via powder metallurgy and liquid-phase sintering, a processing route which achieves near-net-shape with good ceramic particulate dispersion. Two matrix alloys were used, Fe-1 wt% C-1 wt% Si and Fe-2 wt% Cu; up to 30 wt% (36 vol%) yttria-stabilized zirconia in the form of 20 m particles was added to these alloys. The microstructural evolution of these composite materials was studied by examining the densification rate and volume fraction of liquid phase as a function of time. Different particle/matrix interfaces developed in the two composites. A glassy silicon-rich layer formed in the Fe-1C-1Si-YSZ composites and a more limited crystalline layer was found in the Fe-2Cu-YSZ composites.     

Strength and toughness of jade and related natural fibrous materials

Room temperature mechanical properties measured for natural fibrous jade materials with random fibre orientations were similar to strengths (e.g. 100 MPa) and toughnesses,K _IC, (e.g. 3M Pa m^1/2) in other studies. However, nearly three- and five-fold higher values were found respectively for strength and fracture toughness of jade with highly aligned fibres for crack propagation perpendicular to the fibres. Further, the results indicate significantly higher strength and toughness with decreasing fibre diameter and increasing aspect ratio, and an accompanying increase in intrafibre fracture. However, failure was predominantly catastrophic in character for all fibre orientations, indicating some material (i.e. matrix) is necessary for non-catastrophic failure as found in fibre composites.     

Extrusion and mechanical properties of mixed powder and spray co-deposited Al 2014/SiC metal matrix composites

Al 2014/SiCp metal matrix composites have been produced by a powder metallurgy route that included mixing, canning, degassing and hot extrusion (300°C, 161). Two types of SiC (12 and 4 m), two SiC contents ( 10 vol. % and 15 vol. %) and three different degassing temperatures (350, 400, 500 °C) were used. The degassing temperature was found to have no significant effect on the final properties, which were affected by remnant porosity. Porosity was mostly attributed to SiC clustering, which was related to the relative reinforcement/ aluminium powder size distributions and volume fractions. The best mechanical properties were obtained for the composite with 10 vol. % SiCp and these were very similar to the properties of a spray co-deposited metal matrix composite billet of approximately the same composition, also extruded at 300 °C, 161.     

Microstructural analysis of the calcium aluminosilicate (CAS) glass-ceramic matrix in SiCf/CAS composites deformed at a high temperature

The calcium aluminosilicate (CAS: anorthite) glass-ceramic matrix materials in the SiCf/CAS composites deformed in compression at a high temperature were analyzed for microstructures. The matrix region right above the top of the SiC fiber ( 0°) showed high density of dislocations as well as twins. On the other hand, the matrix region remote from the top of the SiC fiber ( 90°) showed only a twinned structure. This experimental result was compared with the Meyer et al.'s mechanical modeling indicating that the interfacial traction of the matrix region right on the top of the fiber is significantly high compared to the remote stress and thus it rate limits the creep deformation of the composite.     

Microstructure and hardness of as-cast in situ TiB short fibre reinforced Ti-6Al matrix composites

XD^TM technique has been successfully used to prepare TiB short fibre reinforced Ti-6Al matrix composites. Macrostructure and microstructure have been observed by optical microscopy and SEM in order to study the influence of cooling rate on the morphology, size and distribution of TiB. Due to the cooling rate, there exist three kinds of macrostructure: fine grain zone, columnar grain zone and coarse equiaxed grain zone, corresponding to the cooling rate of 100–500 K/s, 20–50 K/s and less than 10 K/s respectively. In the fine grain zone, TiB distributes randomly in matrix with main rod morphology with 3 m in width and 50 m in length. In the columnar and coarse grain zone, a colony structure was observed in which TiB distributes with a special orientation direction with matrix. A lamellar TiB with up to 50 m width and 200 m length was also formed. It was indicated that the decreasing of the cooling rate changes the morphology of TiB from rod to lamellar shape, and markedly increase the length and aspect ration of TiB, from 50 m to 200 m and from about 15 to 200, respectively. TEM results show that the rod TiB has a hexagonal cross section. Vickers hardness testing shows a little reinforcement geometry dependence, but the average hardness of 484 MPa is much higher than that of unreinforced matrix alloy.     

Hydroxyapatite-316L fibre composites prepared by vibration assisted slip casting

To prepare hydroxyapatite (HA, or HAp)-stainless steel 316L fibre composites with up to 30 vol% 316L fibres (1 mm long and 50 m in diameter), slip casting assisted by vibration (frequency: 55 Hz; amplitude: 5 mm) was carried out, followed by both cold isostatic pressing (CIPing) and hot isostatic pressing (HIPing). With the addition of around 0.5 wt% sodium carboxymethylcellulose (Na-cmc), solids loadings up to 44 vol% were obtained in calcined HA powder-derived slips, which were castable only under the vibration. The slips were concentrated and viscous so that the preferential sedimentation of the dense and large 316L fibres could be avoided. Subsequent CIPing was able to increase the relative density of the cast and dried green compacts from 46% after casting to 60% after CIPing. With the dense and uniform green compacts of the HA-316L mixtures, final HIPing at 950 °C resulted in HA-316L fibre composites of 99% relative density. The HA-316L fibre composites had improved fracture toughness of 3.6 ± 0.3 MPa.m^0.5, due to the bridging effect of the ductile 316L fibres. However, the mechanical strength of the composites was limited by the presence of residual thermal stresses and circumferential microcracks. The HA-316L fibre composites were biocompatible and exhibited favourable bone-bonding characteristics.     

Effect of hot-pressing time and post-heat treatment on the microstructure and mechanical properties of SiC-fibre-reinforced glass-ceramic composites

Nicalon-SiC-fibre-reinforced (35 vol %) lithium-aluminosilicate (LAS) glass-ceramic composites were fabricated by a slurry-infiltration process followed by hot pressing at 1400°C and 10 MPa for varying soaking times. The ultimate strength and elastic modulus of the as-fabricated composites, as determined by four-point flexural tests, increased rapidly with the densification time, saturating after 30 min at 550 MPa and 130 GPa, respectively. Longer hotpressing times caused a decrease in the elastic modulus via fibre degradation. A carbon-rich interfacial layer formed between the fibres and the matrix, the thickness of which reached a maximum of 400 nm after 30 min soaking time. The flexural strength of post-heat-treated composites in air decreased by a factor of approximately four, due to oxidation and removal of the carbon content of the interfacial layer. The silica-rich bridges left behind between the fibres and the matrix contributed to brittle fracture of the composite.     

Squeeze cast aluminium reinforced with mild steel inserts

The bonding of a mild steel insert to an Al-7Si alloy during squeeze casting has been studied for a range of processing conditions. Assessment of the mild steel/Al-7Si alloy interface shear strength has been made with a push-out test, and the results have been correlated with microstructural observations and residual stress calculations. Uncoated inserts do not exhibit any significant reaction with Al-7Si because of rapid cooling of the melt during squeeze casting, giving a low interface shear strength of 30 MPa. Preheating the inserts to 900°C slightly improves the interface shear strength to 45 MPa, but reaction between the steel and Al-7Si is prevented by the formation of an Fe₃O₄ magnetite layer on the steel surface. Inserts hot-dipped in molten Al-10Fe before squeeze casting have a much greater interface shear strength of 110 MPa, with failure in the Al-7Si matrix rather than at the steel/Al-7Si interface. Inserts vacuum plasma spray coated with titanium have the greatest interface shear strength of 130 MPa, without any interface reaction, because of mechanical keying of the rough splat-quenched titanium surface combined with high residual stresses in the Al-7Si matrix.     

The strength of hybrid glass/carbon fibre composites

The tensile mechanical properties of hybrid composites fabricated from glass and carbon fibres in an epoxy matrix have been evaluated over a range of glass: carbon ratios and states of dispersion of the two phases. The failure strain of the carbon phase increased as the relative proportion of carbon fibre was decreased, and as the carbon fibre was more finely dispersed. This behaviour is commonly termed the hybrid effect, and failure strain enhancement of up to 50% has been measured. Only part of the effect may be attributed to internal compressive strains induced in the carbon phase by differential thermal contraction as the composite is cooled from its cure temperature. The laminae or ligaments of carbon fibre dispersed in the glass fibre phase show a multiple failure mode, and when the constitution is favourable catastrophic failure does not occur until a considerable number of ligament fractures have accumulated. Failure is thus progressive, and the material is effectively tougher than equivalent all-carbon fibre composites.     

High-temperature tensile load-bearing capacity of unidirectional continuous-fibre composites: significance of a specimen-end effect

A specimen-end effect which is of significance for the determination of high-temperature tensile load-bearing capacity of metal-matrix composites reinforced with continuous metal fibres has been indicated. The effect arises from the fact that differential axial straining of the fibre and matrix can occur at high temperatures due to viscous sliding at the fibre-matrix interface. A model-system study has been carried out using a tungsten fibre-copper composite, whose ultimate tensile stress (UTS) at temperatures up to 1000° C is determined indirectly from four-point bending data as well as directly from tensile test results. It is found that at temperatures above 0.6 of the matrix homologous temperature the UTS thus determined has much smaller values than those estimated on the basis of a simple rule-of-mixtures equation. Significance of the result is discussed in terms of potential turbine-blade applications of heat-resistant metal-matrix composites, such as tungsten fibre-reinforced superalloys.     

“Mimetic” molecular composites of Kevlar® aramid/poly(p-phenyleneterephthalamide)*

Mimetic molecular composites can be viewed as hybrids of conventional and molecular composites, and are prepared from a matrix and reinforcing fiber consisting of a single polymer composition. The aim of the work was to obtain a good chemical, physical and thermal property match at the interface for an overall excellent balance of composite properties. Kevlar® aramid 49/poly(p-phenyleneterphthalamide, an all-PPD-T composite, was used as a model system in the work, and, in theory, should be ideal for testing the merit of the mimetic molecular composite concept. The key to the successful preparation of all-PPD-T infusible composites was the acid catalyzed thermal transformation of a fusible precursor, poly(N,N-di-sec-butyl-p-phenyleneterephthalamide), into PPD-T. The composites were prepared by embedding Kevlar® aramid 49 fibers in poly(N,N-di-sec-butyl-p-phenyl-eneterephthalamide) resin, which, on heating in the presence of benzene sulfonic acid catalyst, dealkylated to a PPD-T matrix. In this way, Kevlar® aramid 49/PPD-T(8/92 to 40/60 v/o) composites with densities in the range of 0.2 to 1.2 g cm–3 (versus 1.4 g cm–3 for a fully consolidated PPD-T composite) have been prepared and their thermal and mechanical properties characterized. Some of the foamed composites prepared in this work bear a remarkable resemblance to wood, a natural fiber reinforced foam composite, but with the advantages of flame and rot resistance.     

The fracture energy of carbon-fibre reinforced glass

The fracture energy of carbon-fibre reinforced glass has been measured by the work of fracture technique, using specimens of varied geometry, Meaningful material properties were obtained only when crack propagation was controlled throughout failure. The work of fracture ( _F) depended on strain-rate and fibre volume fraction, and was typically 3 kJm^–2 for a 40 vol % specimen. Variations of work of fracture due to strain-rates have been related to the microstructure of the fracture surfaces and estimates have been obtained of the fibre-matrix interfacial shear stress during pull-out. Approximate estimates have been made of the fracture initiation energy ( _I) by fracture mechanics analyses, _I was less than _F and no strain-rate sensitivity was detected. An attempt has been made to explain _I in terms of the initial rate of release of strain energy during fibre fracture.     

Polyimide-silica microcomposite films

Planarizing films of fine-grained silica embedded in polyimide have been prepared by a spincoating technique and their structure has been investigated by optical and electron microscopy. The films were found to be diphasic. A silica content near 9 vol% yields a well mixed silica/polyimide composite which is homogeneous down to nanometre scale and has good planarization characteristics. Films with contents of 3 and 6 vol% silica have rough surfaces and exhibit two forms of segregation. In the 3% case, elongated inclusions of such silica/polyimide material appear at 100-m islands in a pure polyimide matrix. In the 6% case, the pure polyimide appears as 100-m elongated lakes surrounded by silica/polyimide material.     

Effect of porosity and alumina content on the high temperature mechanical properties of compocast aluminium alloy-alumina particulate composite

The effects of volume fraction of alumina and porosity on the tensile strength of Al-4 wt% Mg-alumina compocast particulate composite tested at various temperatures up to 623 K have0 been investigated with the help of a phenomenological model. The contribution of porosity on reduction of strength of composites at various levels of alumina content has been expressed as a linear function of porosity and the resulting equation contains two experimentally determined parameters, ₀, the ultimate tensile strength at zero porosity level and , a weakening factor. It is observed that decreases with an increase in volume fraction of alumine in the composite and it becomes more sensitive to alumina content of the composite with a rise in temperature. Ate given alumina contenta increases with a rise in test temperature but this effect is gradually countered by increasing alumine content of the composite. Finally, in a composite having 10.3 vol % alumina decreases with an increase in temperature. This may have occurred because the extent of particle-matrix debonding is determined by the plastic soak in the matrix and the fracture strain of a composite increases or decreases with temperature when the alumina content lies below or above 9.0 vol % of alumina respectively. At any test temperature ₀ of the composite decreases rapidly with an increase in volume fraction of alumina, but the rate of decrease of ₀ reduces at higher alumina levels. However at the elevated temperatures of 473K and 573K a sharp fall in ₀ is observed at alumina contents beyond 9.0 vol %. At a lower level of alumina content below about 8.98 vol % the fracture strain of the composites increases with an increase in temperature. However, in the case of higher alumina content beyond the level mentioned above the fracture strain of the composites decreases with the rise in temperature. At a given porosity level the fracture strain of a composite having about 9.4 vol % alumina decreases with an increase in temperature. Scanning electron microscopic observations show that the extent of the growth and linkage of voids before fracture become extensive at higher temperature. At ambient temperature the composites fail by a mixed mode of ductile and clevage fracture. At 573K a number of considerably small dimples along with the larger ones are observed in the fractured surface. At this temperature a large number of newly formed fine grains are observed in the matrix.     

Sol-gel prepared Ni-alumina composite materials

The sol-gel method has been utilized for the preparation of dense, homogeneous ceramic-metal composites with up to 50% Ni in Al₂O₂. Examination by SEM and TEM shows that the materials consist of micrometre-size Al₂O₃ with metallic Ni in isolated regions from 50 m down to nanometre size. The density ranges from 97% (10% Ni) to 74% (50% Ni) of the theoretical number. The hardness decreases from 18 GPa for pure alumina to 10 GPa for alumina containing 50% Ni. The fracture toughness increases significantly from K _1c=3–4 MPa m^1/2 to K _1c=8.5 MPa m^1/2. The elastic and shear moduli decrease from E=400 GPa and G=160 GPa for pure alumina to E=320 GPa and G=135 GPa when containing 50% Ni. The electrical resistivity is 10⁶m with 10 to 40% Ni but decreases drastically at 50% Ni content.     

Properties of high-alumina cement reinforced with alkali resistant glass fibres

The changes in the mechanical properties of cement composites made from high-alumina cement and Cem-FIL AR-glass fibres kept in three different environments up to 10 years are described. While the flexural and impact properties of the composite remained largely unaffected with time in a relatively dry atmosphere, in wet conditions a reduction in strength takes place. In natural weather the 10 year modulus of rupture and impact strength values are 22.8 MIN m^–2 and 6.7 KJ m^–2, respectively, corresponding to the 28 day values of 41.2 MN m^–2 and 22.8 KJ m^–2. These values are significantly better than the corresponding results obtained with Portland cement composites made from Cem-FIL fibres. High-alumina cement composites reinforced by E-glass fibre lose a very large proportion of their flexural and impact strength under wet conditions. The strength reduction with time observed for glass fibre reinforced high-alumina cement composites can be related to two sources: (a) the reduction in the strength of the glass fibre due to chemical corrosion and (b) conversion of the matrix. The latter has greater influence on those composite properties that are matrix controlled such as the Young's modulus whereas any significant reduction in fibre tensile strength is reflected in a corresponding loss in composite tensile and bending strength. Matrix conversion may also influence the fibre-matrix bond.     

Equal-channel angular pressing of an Al-6061 metal matrix composite

An Al-6061 metal matrix composite, reinforced with 10 vol % Al₂O₃ particulates, was subjected to equal-channel angular (ECA) pressing at room temperature to a total strain of 5. It is shown that the intense plastic straining introduced by ECA pressing reduces the grain size from 35 m to 1 m and this leads to an increase in the microhardness measured at room temperature. Inspection revealed some limit cracking of the larger Al₂O₃ particulates as a consequence of the ECA pressing. Tensile testing after ECA pressing gave a maximum ductility of 235% at a temperature of 853 K when testing at strain rates from 10^–4 to 10^–3 s^–1. It is suggested that high strain rate superplasticity is not achieved in this material after ECA pressing due to the presence of relatively large Al₂O₃ particulates.     

Adsorption of polyvinylpyrrolidone (PVP) and its effect on the consolidation of suspensions of nanocrystalline CeO2 particles

CeO₂ particles with an average size of 9 nm were synthesized under hydrothermal conditions. The adsorption of polyvinylpyrrolidone (PVP) on to the particle surfaces was measured in aqueous suspensions in the pH range of 3.7 to 11.5. The amount of adsorbed PVP decreased significantly with increasing pH value. For suspensions prepared at a pH value of 3.7, complete adsorption occurred for 2.5 wt% of PVP added to the suspension. Further additions of PVP produced a gradual increase in the adsorption until a limiting value was reached when the total amount of PVP added to the suspension was 10 wt%. At this PVP concentration, 6 wt% of the PVP was adsorbed and 4 wt% remained free in solution. The effect of the adsorbed PVP on the microstructural homogeneity of films deposited by spin coating of suspensions was investigated. With no addition of PVP, crack-like voids were prevalent in the dried and sintered films. Crack-free films were obtained from suspensions containing 10 wt% of PVP. Higher PVP additions (25 wt%) produced an increase in the viscosity of the suspension but no observable change in the microstructural homogeneity of the films. The use of adsorbed polymers for steric stabilization coupled with data from the adsorption isotherms is shown to provide a rational approach to the deposition of homogeneous films from suspensions of nanocrystalline particles.