Showing papers in "Journal of Materials Science in 1989"

Organic aerogels from the polycondensation of resorcinol with formaldehyde

Abstract: The polycondensation of resorcinol with formaldehyde under alkaline conditions results in the formation of surface functionalized polymer “clusters”. The covalent crosslinking of these “clusters” produces gels which are processed under supercritical conditions to obtain low density, organic aerogels ( ⩽ 0.1 g cm−3). The aerogels are transparent, dark red in colour, and consist of interconnected colloidal-like particles with diameters of approximately 10 nm. The polymerization mechanism, structure and properties of the resorcinol-formaldehyde aerogels are similar to the sol-gel processing of silica.

Toughening mechanisms in elastomer-modified epoxies

Abstract: The role Of matrix ductility on the toughenability and toughening mechanism of elastomer-modified, diglycidyl ether of bisphenol A (DGEBA)-based epoxies is investigated Matrix ductility is varied by using epoxide resins of varying epoxide monomer molecular weights These epoxide resins are cured using 4,4′ diaminodiphenyl sulphone (DDS) and, in some cases, modified with 10 vol% carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN) Fracture toughness values for the neat epoxies are found to be almost independent of the monomer molecular weight of the epoxide resin used However, the fracture toughness of the elastomer-modified epoxies is found to be very dependent upon the epoxide monomer molecular weight Tensile dilatometry indicates that the toughening mechanism, when present, is similar to the mechanism found for piperidine cured, elastomer-modified epoxies studied previously Scanning electron microscopy and optical microscopy techniques corroborate this finding

The structure and mechanical properties of sheets prepared from bacterial cellulose

Abstract: A preliminary experiment has shown that a sheet-shaped material prepared from bacterial cellulose has remarkable mechanical properties, the Young's modulus being as high as >15 GPa across the plane of the sheet. The mechanical properties were little affected by the fermentation conditions of pellicles and the preparation conditions of the sheets, i.e. the pressing and drying of pellicles. From structural investigations, the high Young's modulus has been ascribed to the unique super-molecular structure in which fibrils of biological origin are preserved and bound tightly by hydrogen bonds. It has also been found that a “pulp” obtained from bacterial cellulose gives a strong paper and is useful for reinforcing conventional pulp papers and enabling paper-making from some fibrous materials.

Zirconia-toughened alumina (ZTA) ceramics

Abstract: In the previous decade, a considerable amount of work has been done on the alumina-zirconia ceramic composite system with a particular emphasis on improving the mechanical properties utilizing the recognized toughening mechanisms. Zirconia-toughened aluminas (ZTA) can be regarded as a new generation of toughened ceramics; for example, a toughness of >12 MPa m0.5 has been obtained, compared with 3 MPa m0.5 for commercial alumina ceramics. The fracture strength of ZTA is also greatly in excess of that for alumina. The mechanical properties of ZTA are critically dependent on their microstructures, which can be designed in terms of specific applications and controlled by means of powder preparation and densification processes. This review also includes details of the possible future development of ZTA; these are expected to involve the development and measurement of the mechanical properties for high-temperature engineering applications.

Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing

Abstract: Single phaseβ-sialon ceramics, Si6−z Al z O z N8−z , have been prepared from carefully balanced powder mixtures, also taking account of any excess oxygen in the starting materials. Sintering powder compacts in a nitrogen atmosphere (0.1 MPa) at 1800° C or higher transforms the starting mixture into aβ-sialon solid solution atz-values up to about 4.3, but the sintered material has an open porosity. Addition of 1 wt% Y2O3 to the starting mix improved the sintering behaviour somewhat and the density of the sintered compacts reached 95% of the theoretical value. By glass-encapsulated hot isostatic pressing at 1825° C, however, sintered materials of virtually theoretical density could be obtained, with or without the 1 wt% Y2O3 addition. These latter samples have been studied by X-ray diffraction and electron microscopy, and their hardness and indentation fracture toughness have been measured. It was found that the maximum extension of theβ-sialon phase composition at 1825° C and 200 MPa pressure is slightly below 4,z∼ 3.85 and about 4.1 at atmospheric pressure, and that the hexagonal unit cell parameters are linear functions of thez-value. The single-phaseβ-sialon ceramics had no residual glassy grain-boundary phase. The grain shape was equi-axed and the grain size increased from about 1μm at lowz-values to 5μm at highz-values. At lowz-values the hardness at a 98 N load was 1700 and the fracture toughness 3, whereas an increase inz above 1 caused both the hardness and fracture toughness to decrease significantly. Addition of 1 wt % Y2O3 to the starting mix prior to the HIP-sintering gave rise to a small amount of amorphous intergranular phase, changes in grain size and shape, a clear increase in fracture toughness and a moderate decrease in hardness.

Study of Nicalon-based ceramic fibres and powders by EXAFS spectrometry, X-ray diffractometry and some additional methods

Abstract: The Nicalon SiC-based ceramic fibres and their precursors have been investigated by complementary techniques: EXAFS, WAXS, ESCA and NMR. For SiC-Nicalon fibres, an alternative model is proposed based on a microcrystalline structure where pureβ-SiC is embedded in a continuum of tetrahedral SiCxOy(x+y=4). We have found that the inorganic material skeleton is already contained in the carbon and silicon structure of the polymer precursor.

Evidence for a silicon oxycarbide phase in the Nicalon silicon carbide fibre

Abstract: The Nicalon silicon carbide fibre has been studied by X-ray photoelectron spectroscopy. Elements entering the fibre are carbon, silicon and oxygen. In addition to previously reported chemical entities (silicon carbide, silica and graphitic carbon) evidence is found of the presence of a new supplementary phase which is attributed to an intermediate silicon oxycarbide phase. As this phase is found to participate in very appreciable proportions to the composition of the fibre, some influence on the properties of this fibre can be anticipated.

Oxidation resistance of intermetallic compounds Al3Ti and TiAl

Abstract: The oxidation kinetics and morphological features of Al3Ti and TiAl were investigated. The oxidation resistance of Al3Ti is much better than that of TiAl, for example, by a factor of about 30 at 1000° C for 48 h. The big difference in the oxidation resistance is related to the characteristics of the external oxide scales of a protective Al2O3 or a mat of crystalline TiO2 formed on Al3Ti or TiAl, respectively. Sufficient aluminium transport from Al3Ti assists the formation of the Al2O3 scale which acts as a protective film against oxidation. The poor aluminium content of TiAl produces Ti3Al phase at the interface of TiAl and oxide scales and increases the diffusivity of titanium in the Al2O3 scale. The external crystalline TiO2 scale produced by the diffusion of titanium through the Al2O3 scale enhances oxidation of TiAl.

Structure-properties relationships for densely cross-linked epoxide-amine systems based on epoxide or amine mixtures

Abstract: The solubility and diffusivity of water at 100° C, 95% relative humidity were studied for 14 stoichiometric epoxide-amine networks based on epoxide or amine mixtures Neither the packing density nor the glass transition temperature nor the crosslink density seemed to play a significant role The water absorption is essentially linked to the concentration of polar structures, but also decreases with the extent of intramolecular hydrogen bonding This can be accurately predicted using a simple additive relationship The diffusivity decreases with the hydrophilicity and packing density, but in a complex way probably involving the nature of hydrogen bonds between the water and the substrate

Properties and microstructures of Lanxide Al2O3-Al ceramic composite materials

Abstract: A study has been made of four Al2O3-Al composite materials fabricated by the directed oxidation of molten aluminium alloys. Their microstructures are described and measurements of density, coefficient of thermal expansion, thermal conductivity, hardness, elastic constants, compressive strength, flexural strength, fracture toughness, work of fracture, and thermal shock resistance are reported. Compared to a typical dense sintered Al2O3, such as Durafrax® 1542, which is somewhat harder, stiffer, and stronger in compression, the new composites can be stronger in flexure, particularly at high temperatures, far tougher, and considerably more resistant to thermal shock. Attempts are made to relate their differences in properties to microstructure.

On the estimation of the tensile strength of carbon fibres at short lengths

Abstract: Generally, to determine the fibre-matrix interfacial properties in fibre reinforced plastics, it is necessary to know the tensile strength of the fibre at very short lengths, for which direct measurements are not possible. Accordingly, in this study, the determination of the tensile strength of high strength carbon fibres and their gauge length dependence are analysed by means of the Weibull model. The influence of the estimator chosen and of the sample size on the calculated value of the tensile strength of the fibre are first determined. Secondly, the accuracy of the three- and the two-parameter Weibull distributions is examined. Finally, it is shown that the most appropriate extrapolation at short length is performed by means of a linear logarithmic dependence on gauge length of the tensile strength. This method seems to be valid for untreated as well as for surface-treated high strength carbon fibres.

Stress transfer in single-fibre composites: effect of adhesion, elastic modulus of fibre and matrix, and polymer chain mobility

Abstract: The stress transfer in single-fibre composites is studied experimentally by determining the critical fibre length to diameter ratio,I c/d, in carbon fibre-epoxy resin or poly (ethylene vinyl acetate) systems. Our results and a great number of others available in the literature are compared with the predictions given, on the one hand, by the analytical approach by Cox and, on the other hand, by the theoretical study using finite element technique by Termonia. First, the influence of the fibre-matrix adhesion is analysed and it is observed, in agreement with Termonia, thatI c/d strongly decreases when the bonding efficiency between the two components is increased. Secondly, assuming a perfect fibre-matrix adhesion, it is shown that the critical fibre aspect ratio is proportional to the square root of the ratio of fibre to matrix elastic modulus, as predicted by Cox. However, two linear relationships are established: the first corresponds to the thermosetting and thermoplastic matrices, while the second corresponds to the elastomeric matrices. The difference between these two kinds of materials is attributed to the great difference in polymer chain mobility as shown by a study of the temperature dependence ofI c/d, particularly in the glass transition temperature zone of the matrices. However, in the case of elastomeric materials, the existence of an interphase layer between the fibre and the matrix, having an elastic modulus close to that of the elastomer in its glassy state, can also explain this particular behaviour.

Analysis of the d.c. and a.c. properties of K2O-doped porous Ba0.5Sr0.5TiO3 ceramic humidity sensor

Abstract: A humidity sensor using K2O-doped porous Ba0.5Sr0.5TiO3 ceramic is investigated. This ceramic humidity sensor exhibits a porous structure. The porous ceramic easily absorbs water vapour throughout the pores. The log-conductance against relative humidity (r.h.) sensitivity of this sensor is greater than 4 orders of magnitude in the range of 15∼95% r.h. at 400 Hz and 25°C. The adsorption process of the sensor is very fast. Its adsorption response time in r.h. variation from 15 to 95% is within a few seconds. Charging-discharging and complex impedance analysis techniques are used to analysis the direct current (d.c.) and alternating current (a.c.) response of this device under 50∼95% r.h. The sample can be polarized like electrolytes on charging process due to electrode space charge and grain surface water molecular polarization effects. The degree of polarization is enhanced with increasing r.h. The conduction carriers of this sensor in a humid atmosphere are ions and electrons, and the dominant conduction carrier is the ion. Using complex impedance analysis techniques, an equivalent circuit model associated with “non-Debye” capacitance is built. This model separates the sample into three regions: crystal grain, grain surface and electrode surface. The grain surface resistance and electrode surface resistance decrease sharply with increasing r.h., but crystal grain resistance is not affected by water vapour.

Preceramic Polymer Pyrolysis. I. Pyrolytic Properties of Polysilazanes.

Abstract: The physicochemical behaviour of characterized polysilazanes has been examined during their pyrolytic transformation into amorphous silicon-based ceramics. Selected polysilazanes bearing different substituents at silicon and nitrogen were synthesized by ruthenium catalysed dehydrocoupling of Si-H bonds with N-H bonds. The relationships between the structure and chemical content of polymers and their pyrolysed ceramic compositions and yields are discussed. Possible reactions occurring during pyrolysis are described in terms of a set of mechanisms based on known behaviour of silazane monomers. The decomposition product patterns at different temperature levels and the compositions of the final ceramics suggest specific kinetically or thermodynamically controlled thermolysis pathways. Additional chemical reactivity has been observed when the amorphous ceramic products at 800° C are heated and crystallized at 1600° C.

Structure of oxide gels and glasses by infrared and Raman scattering

Abstract: Optically clear monolithic gels (and powders) have been synthesized using various silicon alkoxides and different hydrolysis conditions. Thermal treatments lead to optically clear (transluscent if compacted powders are used) glasses and ceramics. The structure of the gel and its transformation in glass and in crystalline ceramic have been studied by X-ray, DTA, TGA, IR and Raman spectroscopies. Mullite gels show broad spectra similar to alumina gels, whereas germanium substitution leads to narrower bands located at lower frequencies according to the mass effect. The gel structure is of spinel-type. Use of alkoxides with long organic chains favours heterogeneities. Formation of the glass and also of the true spinel structure, does not modify the Raman spectra to any great extent whereas large modifications are observed on IR spectra with the evolution of protonic species. Continuous transformation of spinel into mullite spectra between 1000 and 1600°C, together with band narrowing, indicates a progressive evolution which explains the large nonstoichiometry range. In particular the narrowing of the SiO4 stretching mode (v=970 cm−1) seems to indicate a progressive ordering (history dependent) of the SiO4 environment.

Effect of substrate temperature and film thickness on the surface structure of some thin amorphous films of MoO3 studied by X-ray photoelectron spectroscopy (ESCA)

Abstract: X-ray photoelectron spectroscopy (XPS) core level spectra of MoO3 substoichiometric amorphous thin films in the thickness range 100 to 670 nm were studied as a function of thickness. Some samples 500 nm thick were studied for different substrate temperatures in the range 293 to 543 K. It was observed that with the increase of thickness of the samples no change in the electron spectrum was observed in the material. Under vacuum conditions, MoO3 turned blue when the substrate temperature was higher than 373 K. XPS spectra supported the formation of the Mo5+ oxidation state in the blue samples. Blue coloration was observed after heating in vacuum and this was attributed to an internal electron transfer from oxygen to metallic orbitals by thermal ionization creating an Mo5+ oxidation state.

Analyses of the mechanical properties and microstructure of bamboo-epoxy composites

Abstract: Bamboo reinforced epoxy possesses reasonably good properties to waarrant its use as a structural material, and is fabricated by utilizing bamboo, an abundant material resource, in the technology of fibre composites. Literature on bamboo-plastics composites is rare. This work is an experimental study of unidirectional bamboo-epoxy laminates of varying laminae number, in which tensile, compressive, flexural and interlaminar shear properties are evaluated. Further, the disposition of bamboo fibre, the parenchymatous tissue, and the resin matrix under different loading conditions are examined. Our results show that the specific strength and specific modulus of bamboo-epoxy laminates are adequate, the former being 3 to 4 times that of mild steel. Its mechanical properties are generally comparable to those of ordinary glass-fibre composites. The fracture behaviour of bamboo-epoxy under different loading conditions were observed using both acoustic emission techniques and scanning electron microscopy. The fracture mode varied with load, the fracture mechanism being similar to glass and carbon reinforced composites. Microstructural analyses revealed that natural bamboo is eligibly a fibre composite in itself; its inclusion in a plastic matrix will help solve the problems of cracking due to desiccation and bioerosion caused by insect pests. Furthermore, the thickness and shape of the composite can be tailored during fabrication to meet specific requirements, thereby enabling a wide spectrum of applications.

Methods for improving the mechanical properties of oxide glasses

Abstract: Methods are reviewed for improving the mechanical properties of oxide glasses. These are divided into surface and bulk techniques and include thermal and chemical strengthening, controlled crystallization, and particle, fibre and whisker reinforcement. The merits and limitations of individual techniques are compared and discussed. In conclusion, a number of applications for these materials are briefly outlined.

A predictive model for particulate-filled composite materials

Abstract: Our predictive model for particulate filled composite materials is applied to epoxy resin toughened by rubber spheres. Good agreement is found between predicted values of stiffness and experimental measurements. The variability of yield stress with volume fraction is explained. The observed fracture processes, including rubber cavitation, are explored; the importance of thermal stresses is highlighted. The fracture behaviour of these materials is discussed in the light of these predictions.

Study of copper-alumina bonding

Abstract: Bonding between copper and alumina can be obtained by the “solid state bonding” process and the “liquid phase bonding” process The strength of interfaces has been tested mechanically using shear tests, tensile tests and fracture toughness tests The effects of bonding parameters on bond strength have been studied Observations by transmisssion electron microscopy have been performed to detect and analyse the nature and evolution of interfacial compounds as a function of copper oxidation and bonding time Chemical reactions lead to the formation of the binary oxide CuAlO2 The stability of this compound and the reversibility of chemical reactions appear to be very dependent on the amount of oxygen present in the system

Toughening mechanisms in a multi-phase alloy of nylon 6,6/polyphenylene oxide

Abstract: Toughening mechanisms in a nylon 6,6/polyphenylene oxide (PA/PPO) alloy are studied. This alloy consists of well dispersed PPO particles (containing an elastomeric phase) in a PA matrix. Both crazing and shear yielding mechanisms are found to be sequentially operative in this alloy. When a crack propagates in the material, a crazed zone forms ahead of the crack tip. This crazed zone then transforms into a shear yielded zone as the crack propagates through it. The crazes inside the original crazed zone are closed or distorted by the shear yielding process. In the transformed region, the shear yielded material is oriented at different angles as the crack propagates. These findings are consistent with the notion that hydrostatic tension has to be dissipated in order for the remaining deviatoric stress to reach the critical value for yielding. It is possible to partially attribute the increase in toughness to both crack blunting and zone shielding mechanisms.

Toughness properties of a three-dimensional carbon-epoxy composite

Abstract: The three-dimensional (3D) orthogonal interlocked fabric contains through-the-thickness rein-forcement in order to enhance the interlaminar fracture toughness of the composite. The interlaminar fracture toughness of a carbon-epoxy orthogonal interlocked fabric composite was experimentally determined by use of the recently developed tabbed double cantilever beam specimen. The data reduction methods applicable to these tests and materials and the interpretation of the results were discussed. The results of critical strain energy release rate,GIc, were compared to those of a two-dimensional (2D) laminate having the same in-plane structure. The energy-dissipating crack propagation processes were described. The in-plane fracture toughness of the 3D fabric was experimentally measured and compared to that of the 2D laminate. The through-the-thickness fibres were found to create a ten-fold increase in interlaminar toughness, and a 25% improvement in the in-plane fracture toughness.

A model for development of orientation of vapour deposits

Abstract: The texture of vapour deposits changes from the orientation that places the lowest energy crystal facets parallel to the substrate under the condition of low atom or ion concentration adjacent to the deposit, to the orientation that places the higher energy crystal facets parallel to the substrate as the atom or ion concentration adjacent to the deposit increases.

Synthesis of continuous silicon carbide fibre: Part 6 Pyrolysis process of cured polycarbosilane fibre and structure of SiC fibre

Abstract: Polycarbosilanes which were synthesized by three methods were melt-spun and cured by heating at low temperatures in air. The curing mechanism and the structure of these cured fibres were studied and the relationship between the structure and the pyrolysis process is discussed. The structure of the cured fibre is represented by means of five structural elements and the rate of oxidation of the methyl group. The pyrolysis process of the cured fibre is discussed in five stages, and the effect of oxygen introduced into polycarbosilane fibre by curing on the pyrolysis process is clarified. The structure of the fibre obtained during the pyrolysis process strongly depends on the molecular weight of polycarbosilane.

Factors affecting particle-coarsening kinetics and size distribution

Abstract: The Lifshitz-Slyozov-Wagner (LSW) theory was developed to model kinetics of precipitate growth from supersaturated solid solutions. The theory corresponds to a zero volume fraction approximation but has been modified for finite volume fractions in order to correspond to real situations. The LSW theory has been applied to study coarsening of grains in liquid-phase sintering and to the coarsening of pores in solid-state sintering systems. There are some additional factors not considered in the LSW theory which can influence the coarsening kinetics depending on the system. It is important, therefore, to incorporate these factors into a coarsening model for better analysis of experimental data. The experimental evidence for the effects of these additional factors is reviewed together with the theoretical modifications made to the basic LSW theory in order to incorporate these factors.

Work of adhesion and contact-angle isotherm of binary alloys on ionocovalent oxides

Abstract: Using the monolayer approximation for metal-vapour and metal-oxide interfaces and Bragg-Williams statistics, a simple thermodynamic model has been constructed to calculate the variation in contact angle and work of adhesion as a function of composition in binary alloyionocovalent oxide systems. This model has been used to classify the curves of contact angle and the work of adhesion as a function of composition into three main types of isotherm. Model predictions and experimental results are compared using data on binary alloy-monocrystalline alumina systems.

The microstructure and mechanical properties of unidirectionally solidified Al−Si alloys

Abstract: Hypereutectic Al−Si alloys with minor additions of Sr were directionally solidified with a temperature gradient of 125°C cm−1 in the liquid. Silicon in the range 14–17 wt%, Sr in the range 0.0–0.5 wt% and specimen traction velocities between 1 and 1500 μm sec−1 were used. The relationship between hardness and traction velocity and spacing in eutectic silicon morphologies is defined and shown to be of the same form as that for yield stress. The possibility of using hardness measurements to be of the same form as that for yield stress. The possibility of using hardness measurements to indicate mechanical properties is discussed. The complex regular silicon structure makes a significant contribution to the hardness of hypereutectic alloys. This makes the relationship between hardness and traction velocity more complex adding difficulties to the use of hardness to measure mechanical properties.

Wetting improvement of carbon or silicon carbide by aluminium alloys based on a K2ZrF6 surface treatment: application to composite material casting

Abstract: A surface treatment with aqueous solutions of K2ZrF6 has been carried out to improve, in dramatic manner, the wetting of carbon (or SiC)-base ceramics by liquid light alloys at low temperatures (ie within the 700 to 900°C range) The mechanism which is thought to be responsible for the wetting improvement involves two steps: (i) K2ZrF6 reacts with aluminium with the formation of K3AlF6, other complex fluoride species and intermetallics, (ii) K3AlF6 dissolves the alumina thin layer, coating the liquid light alloy and enables the wetting of the ceramics The mechanism has been worked out from sessile drop experiments, solid state chemistry experiments and composite casting The K2ZrF6 surface treatment appears to be particularly suitable for processing composite materials made of carbon (or SiC) fibrous preforms and aluminium-base matrices according to techniques directly derived from the light alloy foundry

Intrinsic toughness of interfaces between SiC coatings and substrates of Si or C fibre

Abstract: The critical energy release rate for separation of SiC coatings from single crystal Si substrates or surfaces of carbon fibres, along their well-defined interfaces can be determined quite accurately from analysis of the spontaneous delamination of coatings under bi-axial stress, when such coatings exceed a critical thickness. Direct evaluations have been made of the specific work of delamination along the interface for SiC coatings from single crystal Si substrates, for both the case of coatings under bi-axial compression, as well as under bi-axial tension. The critical energy release rate for coatings in tension was 5.1 J m2, and that for coatings under compression was 5.9 J m2. The higher value of the latter is attributed to relative slippage between coating and substrate before lift-off of the former. Corresponding determination of the critical energy release rate for delamination of SiC coatings under bi-axial tension from surfaces of anisotropic Pitch-55 carbon fibres gave an answer of 5.5 J m2. These values compare very well with expectations from surface energies of strong solids.

Temperature-programmed reduction of Ni-Mo oxides

Abstract: Temperature-programmed reduction (TPR) has been employed to study Ni-Mo mixed oxides which were previously used as model hydrodesulphurization (HDS) catalysts, using compositions ranging from pure MoO3 to pure NiO. An assignment of TPR signals to the different bulk phases was attempted. Good agreement between TPR spectra and structural data obtained previously from X-ray and electron diffraction was observed. TPR traces were consistent with proposed mechanisms of reduction of the bulk oxides MoO3, MoO2, NiO and NiMoO4. The variations in TPR spectra were interpreted in terms of effects such as crystallite size, ageing of the samples, hydrous state and chemical interactions between the different species. The significance of these reducibility results for HDS catalysis is discussed.