Showing papers in "Journal of Materials Science in 1995"

Modelling studies applied to functionally graded materials

Abstract: This review contains a description of modelling studies relative to functionally graded materials (FGMs). Two principal topics are covered: models for microstructure-dependent thermophysical properties, and models for the design, processing, and performance of FGMs. The former is a particularly important input to FGM modelling because of the wide variety of microstructures that can exist across the graded direction of a single material. Based on the work described in this review, recommendations are made regarding areas in which additional modelling studies would be beneficial. Suggested approaches to the modelling include the application of a number of powerful techniques, such as percolation theory, fractal analysis, lattice-based microstructure models, the renormalization group, neural networks, and fuzzy logic.

Synthesis of bulk nanocrystalline nickel by pulsed electrodeposition

Abstract: Square-wave cathodic current modulation was used to produce nanocrystalline nickel electrodeposits with grain sizes in the range 40–10 nm from saccharin-containing Watts-type baths The optimum plating conditions to synthesize nanocrystals, namely pulse on- and off-time and peak current density, as well as bath pH and temperature, were identified At these plating conditions, the grain size of the electrodeposits was found to decrease with increasing saccharin concentration in the bath The preferred orientation of the deposits progressively changed from a strong (2 0 0) fibre texture for a saccharin-free bath to a (1 1 1) (2 0 0) double fibre texture for a bath containing 10 gl−1 saccharin Transmission electron microscopy showed that the electrodeposits consist of uniform structure with narrow grain-size distribution These deposits, as expected, were found to contain co-deposited sulphur and carbon impurities

Thermal and non-thermal interaction of microwave radiation with materials

Abstract: Industrial use of microwave radiation as an alternative to conventional thermal heating has generated interest recently mainly because of the drastic reduction in the processing time. In spite of its wide application, its chemical mechanism of interaction with materials has not been well understood. The current debate on the alternative use of microwave radiation to conventional thermal heating is on the involvement of a “microwave specific effect” other than the well accepted dielectric heating. There are reports of various reactions which show similar kinetics under both microwave and thermal methods at similar temperatures suggesting simple dielectric heating of materials by microwaves. There are also reports which show a clear reaction rate enhancement under microwave radiation compared to the thermal method under similar reaction conditions and temperatures indicating a “microwave specific effect” other than the simple dielectric heating of materials. This paper will discuss the above conflicting results reported in the literature.

Effect of low earth orbit atomic oxygen on spacecraft materials

Abstract: This review attempts to bring together the published data and analysis related to the effect of low earth orbit (LEO) atomic oxygen (AO) interaction with spacecraft materials. The basic interaction mechanism of AO with spacecraft materials and quantification of its effect on materials performance are briefly discussed. After providing a list of materials susceptible to the LEO environment, the paper focuses on the degradation mechanism of various spacecraft materials. Particular emphasis is given to the protective mechanisms for AO-susceptible materials and development of AO-resistant materials for long-term LEO spacecraft applications. Ground-simulation testing requirements and their present status are reviewed briefly. The need for further research is emphasized.

The positive temperature coefficient of resistivity in barium titanate

Abstract: Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

Laser welding of dissimilar metal combinations

Abstract: The ability to manufacture a product using a number of different metals and alloys greatly increases flexibility in design and production. Properties such as heat, wear and corrosion resistance can be optimized, and benefits in terms of production economics are often gained. Joining of dissimilar metal combinations is, however, a challenging task owing to the large differences in physical and chemical properties which may be present. Laser welding, a high power density but low energy-input process, provides solutions to a number of problems commonly encountered with conventional joining techniques. Accurate positioning of the weld bead, rapid heating and cooling, low distortion, process flexibility, and opportunities for product redesign are its principal characteristics. The review describes the principles underlying laser welding of dissimilar metal combinations and highlights the above benefits in a number of practical applications. It is concluded that there is potential for its application in many industrial sectors.

Monolayer films of diblock copolymer microdomains for nanolithographic applications

Abstract: Several techniques have been investigated for creating large-area thin films of diblock copolymers, with well-ordered two-dimensional periodic microstructure on the scale of a few tens of nanometres. Such structures might potentially be used as templates for lithography, at a length scale not easily accessed by electron-beam methods. Using a copolymer with a spherical microdomain structure, we find that it is quite easy to obtain large-area films consisting of a monolayer of spherical domains, arranged on a hexagonal lattice with a lattice constant of 33 nm. Copolymers with cylindrical microstructure typically orient parallel to the substrate and free surface, it has been found that the perpendicular orientation is metastable: if a well- or poorly-ordered film is initially prepared with the cylinders perpendicularto the surface, annealing results in a well-ordered film with the same orientation, with a lattice constant of 27 nm for the polymer used in this study. For both cylinders and spheres, grains measuring typically 30×30 lattice constants are readily obtained.

Role of magnesium in cast aluminium alloy matrix composites

Abstract: Wetting between the dispersoid and the matrix alloy is the foremost requirement during the preparation of metal matrix composites (MMC) especially with the casting/liquid metal processing technique. The basic principles involved in improving wetting fall under three categories: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy at the dispersoid matrix interface. The presence of magnesium, a powerful surfactant as well as a reactive element, in the aluminium alloy matrix seems to fulfil all the above three requirements. The role played by magnesium during the synthesis of aluminium alloy matrix composites with dispersoids such as zircon (ZrSiO4), zirconia (ZrO2), titania (TiO2), silica (SiO2), graphite, aluminium oxide (Al2O3) and silicon carbide (SiC), has been analysed. The important role played by the magnesium during the composite synthesis is the scavenging of the oxygen from the dispersoid surface, thus thinning the gas layer and improving wetting and reaction-aided wetting with the surface of the dispersoid. The combinations of magnesium and aluminium seem to have some synergistic effect on wetting.

Atomic oxygen resistant coatings for low earth orbit space structures

Abstract: This review presents research in the area of polymeric coatings developed for protecting low earth orbit (LEO) space structures from atomic oxygen. Following a brief description of the LEO environment, ground-based simulation facilities for atomic oxygen and evaluation of protective coatings are discussed. Atomic oxygen resistant coatings based on different polymeric systems such as fluorinated polymers, silicones, poly (carborane-siloxane)s and decarborane-based polymers are presented. Finally, the performances of different coating systems are compared and the scope for further research to improve the performance of some of the coating systems is discussed.

Morphology of thin anatase coatings prepared from alkoxide solutions containing organic polymer, affecting the photocatalytic decomposition of aqueous acetic acid

Abstract: Porous anatase coatings were prepared from alkoxide solutions containing organic polymer by a dip-coating technique. The morphology of the coatings, such as pore size, pore distribution and thickness, was controlled. The effects of the morphology of the porous anatase coatings on the photocatalytic activity for the photocatalytic decomposition of aqueous acetic acid were examined.

Effect of rhenium addition on fracture toughness of tungsten at elevated temperatures

Abstract: Fracture toughness tests of tungsten and tungsten-rhenium alloy specimens were carried out at elevated temperatures. Temperature dependence of fracture toughness and effect of rhenium content on fracture toughness were investigated. Although fracture toughnesses of three kinds of specimens with rhenium contents of 0, 5 and 10 wt% were almost identical at room temperature, fracture toughness at elevated temperatures increased with increasing rhenium content. The brittle-ductile transition, similar to steels, and subsequent transition of the fracture mode from ductile dimple to intergranular were observed for all three kinds of specimens. With increasing rhenium content, the transition temperatures increased. A significant grain growth was found, not for tungsten-rhenium alloy specimens, but for a tungsten specimen without rhenium in a temperature range higher than the recrystallizing temperature, which resulted in transition of the fracture mode from dimple to intergranular.

Observations of damage morphologies in nacre during deformation and fracture

Abstract: The deformation, fracture and toughening mechanisms of nacre from a kind of fresh-water bivalve mollusc (Cristaria plicata) were studied by SEM, TEM and microindentation tests. Experimental results revealed a strong anisotropy of the damage behaviour reflecting the microstructural character of nacre. The fractured surface parallel to the cross-sectional surface of nacre was much more tortuous than that parallel to the platelet surface. The crack line on the cross-sectional surface was step-like, while that on the platelet surface was polygonal. Sliding of aragonite layer combined with the plastic deformation of organic matrix is the main plastic deformation mechanism of nacre. Three main toughening mechanisms have been found acting in concert: crack deflection, fibre pull-out and organic matrix bridging.

Synthesis and high temperature mechanical properties of Ti3SiC2/SiC composite

Abstract: A high density Ti3SiC2/20 vol % SiC composite was hot pressed under a uniaxial pressure of 45 MPa for 30 min in an Ar atmosphere at 1600 °C. The grain size of the Ti3SiC2/SiC composite was finer than that of monolithic Ti3SiC2, though the composite was hot pressed at a higher temperature, due to the dispersion of SiC particles in the Ti3SiC2 matrix. Room temperature fracture toughness of the composite and Vickers hardness were measured as 5.4 MPa m1/2 and 1080 kg mm−2, respectively. A higher flexure strength of the composite compared to that of monolithic Ti3SiC2 was measured both at room temperature and up to 1200 °C. At 1000 °C, the composite showed a lower oxidation rate than that of monolithic Ti3SiC2.

Wear of ceramic particle-reinforced metal-matrix composites: Part I Wear mechanisms

Abstract: Pin-on-disc dry sliding tests were carried out to study the wear mechanisms in a range of metal-matrix composites. 6061-aluminium alloys reinforced with 10 and 20 vol % SiC and Al 2 O 3 particles were used as pin materials, and a mild steel disc was used as a counterface. A transition from mild wear to severe wear was found for the present composites; the wear rate increased by a factor of 10 2 . The effects of the ceramic particles on the transition load and wear with varying normal pressure were thoroughly investigated. Three wear mechanisms were identified: abrasion in the running-in period, oxidation during steady wear at low load levels, and adhesion at high loads. A higher particle volume fraction raised the transition load but increased the wear rate in the abrasion and adhesion regimes. Increase of particle size was more effective than increase of volume fraction to prolong the transition from mild wear to adhesive wear. The reasons for different wear mechanisms were determined by analyses of the worn surfaces and wear debris

Powder diffraction investigations of plasma sprayed zirconia

Abstract: The crystallographic and microstructural parameters of plasma sprayed pure zirconia powders were studied by X-ray and neutron powder diffraction. A significant influence of the flow rate of the quenching gas on the phase composition (the tetragonal to monoclinic ratio varied between 0.6 and 3.5) and on the micro-structure was observed, while structural parameters of both phases of all investigated samples remained essentially unchanged and were close to those reported in the literature. These results do not support the concept of a critical particle size effect as a stabilizing factor for the tetragonal phase. A transition from a tetragonal to a monoclinic phase was observed without any measurable change in the crystallite sizes by heating at 845‡C. A very high background on the neutron powder patterns may have been caused by the presence of pores in the samples.

Particle effects on friction and wear of aluminium matrix composites

Abstract: Particle effects on friction and wear of 6061 aluminium (6061 Al) reinforced with silicon carbide (SiC) and alumina (Al2O3) particles were investigated by means of Vickers microhardness measurements and scratch tests. Unreinforced 6061 Al matrix alloy was also studied for comparison. To explore the effect of heat treatment, materials subjected to three different heat treatment conditions, i.e. under-aged, over-aged and T6, were used. Multiplescratch tests using a diamond and a steel indentor were also carried out to simulate real abrasive wear processes. Vickers microhardness measurements indicated that T6 heattreated composites had the highest hardness. Single-scratch tests showed that the variation of friction coefficient was similar to that of Vickers hardness and the peak-aged composites exhibited the best wear resistance. The wear rate of fine particle-reinforced composites was mainly affected by hardness. However, the wear rate of large particle-reinforced composites was influenced by both the hardness and fracture of the particles.

Microfracture and material removal in scratching of alumina

Abstract: A bonded-interface sectioning technique is used to examine subsurface damage modes and to identify mechanisms of material removal in repeated single-point scratching of alumina as a function of grain size, load, and number of passes. In the fine grain alumina, the lateral and median crack system is observed, together with intergranular microcracks and intragrain twin/slip bands distributed within the plastic zone. The distributed form of damage, namely twin/slip bands and intergranular microcracks, are also observed in the coarse grain alumina; but no evidence is found for well-defined median and lateral cracks in this material. The mechanism of material removal in alumina is identified as grain dislodgement resulting from grain boundary microcracking, irrespective of the grain size. Extension of lateral cracks is found to contribute to the material removal process only in the fine grain alumina scratched under a large load and after several passes. A model for the microfracture-controlled material removal process is proposed that relates the volume of material removed to the applied load and material properties including grain size, elastic modulus, hardness, and short-crack toughness. Removal rate is shown to be proportional to grain sizeI1/2 and to loadP2. The model and the experimental results obtained in scratching are used to describe the action of an individual abrasive grit in grinding and other abrasive machining processes.

The possible role of the pre-exponential factor in explaining the increased reaction rates observed during the microwave synthesis of titanium carbide

Abstract: Reaction rates observed during the microwave synthesis of titanium carbide powder via the carbothermal reduction of the oxide are more than three times faster than those observed during conventional processing at the same temperature. Although not dissimilar to other reports of “microwave effects” in the literature, this result is of interest because the microwave heating mechanism is quite different compared to work using ionic ceramics. The initial powder mix consisted of low dielectric loss titania mixed intimately with very fine carbon black powder. It is the latter phase which couples with the microwaves via a conventional ohmic dissipation mechanism. Calculations have shown that the faster diffusion rates might be explained by an increase in the Arrhenius pre-exponential factor A, with no change in the activation energy.

The mechanical behaviour of PEEK short fibre composites

Abstract: Short glass (GF) and carbon fibre (CF) reinforced poly-ether-ether-ketone (PEEK) composites were prepared by injection moulding and then microstructurally characterized Their mechanical behaviour was determined by two different methods: a classical unidirectional tensile test and an immersion ultrasonic technique The reinforcing effect of fibres is discussed in the context of the theory of reinforcement of Bowyer and Bader Interfacial shear strength and critical fibre length at break are calculated for both PEEK/GF and PEEK/CF composites Examinations of fracture surfaces of uniaxial tensile specimens revealed a higher adhesion of carbon fibres to PEEK matrix in regards to the adhesion concerning glass fibre-PEEK interfaces, which is in agreement with the results provided by the model Compatibility of ultrasonic and tensile results is reported

An improvement in processing of hydroxyapatite ceramics

Abstract: Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in “softening” particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min−1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.

The hot compaction of polyethylene terephthalate

Abstract: A process is described for the successful compaction of polyethylene terephthalate fibres. The measurement of mechanical properties shows that a very high proportion of the original fibre properties are retained and that the compacted samples have a good degree of coherence. Electron microscopy studies of suitably etched samples reveals the effect of the compaction temperature on the structure of the compacted samples.

The effect of the moisture absorption on the interfacial strength of polymeric matrix composites

Abstract: Seven composite material systems have been studied to determine their potential suitability for structural applications for continuous immersion in sea water. The matrices of these composites have been found to absorb moisture with saturation occurring at 0.6%–2% of the matrix weight of additional moisture over approximately 1% present after fabrication, when soaked at ambient temperature in simulated sea water, with 20.7 MPa (3000 p.s.i.) hydrostatic pressure giving a very minor increase in moisture absorption. Pure water absorption gave a slightly higher saturation level than did simulated sea water. With the exception of the graphite/vinylester composites, the degradation in transverse tensile strength and interfacial shear strength due to moisture absorption has been found to vary from 0%–22%, with the thermoset/graphite systems and the vinylester/glass systems both showing sufficient promise to justify further study. The observed correlation in the decrease in interfacial shear strength due to moisture absorption with decreases in transverse tensile strength supports the hypothesis that the moisture-induced degradation is associated with a decrease in the interfacial strength rather than the degradation of matrix mechanical properties. In situ fracture observations in the scanning electron microscope further support this hypothesis.

Electrical properties of high-temperature oxides, borides, carbides, and nitrides

Abstract: High-temperature materials including oxides, borides, carbides, and nitrides encompass all types of conductors: metallic, semiconducting, and ionic. Their electrical conductivities are generally very sensitive to impurities regardless of the type of conductor. For large band-gap materials, which includes most of the oxides, the conductivities at low temperatures are frequently dominated by impurities or dopants, and intrinsic conduction only becomes significant above a temperature which depends largely on the level of dopant, the band gap and the defect structure of the base material. The borides, carbides, and nitrides of transition metals are metallic conductors with conductivities and temperature coefficients of resistivity comparable to that of their parent metals.

Microstructural modelling of auxetic microporous polymers

Abstract: A simple two-dimensional model for the deformation of auxetic microporous polymers (those with a negative Poisson's ratio) has been developed. This model network of rectangular nodules interconnected by fibrils has been further developed to include the possibilities of fibril hinging, flexure and stretching. Expressions for strain-dependent Poisson's ratio and Young's modulus have been derived and compared with experimental results on microporous PTFE and UHMWPE. A combination of the hinging mode followed by the stretching mode of deformation can be used to explain the general features of the experimental data for these auxetic polymers. The force coefficients governing the different modes of deformation are dependent on fibril dimensions and intrinsic material properties. By varying the geometry of the network, the model can be used to predict different combinations of Poisson's ratio with modulus, from large positive through to large negative values.

Inluence of water up-take on interlaminar fracture properties of carbon fibre-reinforced polymer composites

Abstract: Composite materials in practical use can be subjected to a wide variety of different loading conditions. The most important conditions are mechanical stresses and environmental attacks. An issue of major concern in the utilization of composites is associated with the occurrence of delaminations or interlaminar cracks, which may be related to manufacturing defects or are induced in service by low-velocity impacts. The main environmental attacks are temperature, humidity, radiation, and chemical exposure. Three materials were investigated; two thermosetting matrices (unmodified and toughness-modified epoxy, EP and EPmod) and one thermoplastic matrix (semicrystalline polyetheretherketone, PEEK), all reinforced with unidirectional continuous carbon fibres. Samples of these materials were exposed to water in baths of different temperatures; they were taken for mechanical testing after various time periods. As a result of absorbed moisture, GIC-values increased with moisture content of the samples, whereas GIIC-values decreased. By means of scanning electron microscopy, fracture surfaces were examined. Evidence was found that the increase of GIC-values was due to a greater ductility of the matrix (as a result of the moisture absorbed) and hence more energy-consumptive fibre-bridging. On the other hand, interface failure, as well as a loss of shear strength of the epoxy with increasing amount of moisture absorbed, were responsible for the decrease in the GIIC-values. The thermoplastic matrix system (CF/PEEK) exhibited no influence of moisture on the Mode I property, but a decrease of the values for Mode II.

Characterization of Na-X, Na-A, and coal fly ash zeolites and their amorphous precursors by IR, MAS NMR and XPS

Abstract: By fusion with sodium hydroxide followed by a hydrothermal reaction, fly ash and Alenriched fly ash were converted into Na-X and Na-A zeolites, respectively. The authentic Na-X, Na-A and fly ash zeolites as well as their amorphous precursors have been characterized by IR, 29Si and 27Al MAS NMR, XPS/AES, TG, and comparative ion-exchange studies of Cs and K with Na in zeolite samples. It appears that the same structural unit with a terminal OH, such as a sodalite unit, was formed in the induction period of the hydrothermal reaction for Na-X and Na-A, and then cross-linked through D4R and D6R external linkages to build up the zeolite framework of Na-A and Na-X, respectively.

Improvement in hydrogen storage characteristics of magnesium by mechanical alloying with nickel

Abstract: The hydriding and dehydriding kinetics of Mg are reviewed. It is reported that the hydriding and dehydriding reactions of Mg are nucleation-controlled under certain conditions and progress by a mechanism of nucleation and growth, and that the hydriding rates of Mg are controlled by the diffusion of hydrogen through a growing Mg hydride layer.

Hydrothermal stability of pure and modified microporous silica membranes

Abstract: The hydrothermal stability of microporous (0.6 nm) silica membranes prepared by the sol-gel process was studied at 600 and 800 °C in a 50 mol% steam atmosphere. The membranes remained microporous after calcination and hydrothermal treatment at 600 °C for 30 h but a substantial reduction in the specific surface area (48%) accompanied by a 77% decline in the micropore volume was observed. Hydrothermal treatment at 800 °C for 30 h resulted in complete densification of the membranes. The effect of alumina and magnesia on the hydrothermal stability of the membranes was investigated. Both Al2O3 and MgO were introduced into the membranes by doping the starting silica sol with controlled amounts of the corresponding nitrate salts. Alumina did not change the pore structure of the silica membranes which retained a large part of their microporosity after hydrothermal treatment at 600 °C compared to pure silica membranes. Doping with magnesia, however, resulted in lower specific surface areas relative to those of pure and alumina-doped silica membranes after drying and calcination. These effects on the stability of the membranes are explained by assuming structural changes in the membranes catalysed by magnesia.

Magnetic properties of hydrothermally synthesized strontium hexaferrite as a function of synthesis conditions

Abstract: Fine particles of strontium hexaferrite, SrFe12O19, with a narrow size distribution have been synthesized hydrothermally from mixed aqueous solutions of iron and strontium nitrates under different synthesis conditions. The relationship between the synthesis variables (temperature, time and alkali molar ratio) and the magnetic properties has been investigated. The results have shown that, as the synthesis temperature increases, the saturation magnetization of the particles increases up to a plateau and the coercivity decreases. As the alkali molar ratio R(=OH−/NO3−) increases, the coercivity decreases and goes through a local minimum, while the saturation magnetization increases and goes through a local maximum. Increasing the synthesis time from 2 h to 5 h has no significant effect on the saturation magnetization, but decreases the coercivity. An anisotropic sintered magnet with a high saturation magnetization value of 67.26 e.m.u g−1 (4320 G)‡ has been fabricated from the hydrothermally synthesized powders.