Showing papers in "Journal of Materials Science in 1980"

Ultra-high-strength polyethylene filaments by solution spinning/drawing

Abstract: This paper deals with ultra-high-strength monofilaments of linear polyethylene that are produced by solution spinning and subsequent hot drawing at 120° C. The influence of the draw ratio on the mechanical and thermal properties of the fibres was investigated. Some salient features of a polyethylene filament with a draw ratio of 31.7 are: tensile strength at break=3.0 GPa, Young's modulus=90 GPa and DSC-melting point at a scan speed of 10° C min−1=145.5° C. The modulus was found to depend linearly on the draw ratio. The tensile strength tended, by contrast, to approach an upper limit at high draw ratios. Additional morphological and X-ray studies revealed an extremely good orientation of the macromolecules in the fibre direction of the highly drawn polyethylene monofilaments.

The Contribution of Structural Disorder to Diffuse Phase-Transitions in Ferroelectrics

Abstract: Simple crystal-chemical arguments were used to suggest that in the ferroelectric perovskite lead-scanium-tantalate (PbSc0.5Ta0.5O3) the B-site cations in this simple ABO3 structure should be close to the boundary between order and disorder. Both polycrystal ceramic and single crystal materials of this composition have been grown. In this study X-ray powder diffraction is used to identify the strong superlattice reflections associated with the ordering of scandium and tantalum ions in the B-site, and to demonstrate quantitatively how the degree of ordering can be modified by suitable thermal treatment. Thermal changes associated with the ferroelectric Curie temperature have been measured by differential scanning calorimetry and show very clearly the manner in which the diffuse (broadened) transition in this crystal is sharpened by increase in the B-cation ordering.

Thermal expansion and swelling of cured epoxy resin used in graphite/epoxy composite materials

Abstract: The paper presents results of experiments in which the thermal expansion and swelling behavior of an epoxy resin system and two graphite/epoxy composite systems exposed to water were measured. It was found that the cured epoxy resin swells by an amount slightly less than the volume of the absorbed water and that the swelling efficiency of the water varies with the moisture content of the polymer. Additionally, the thermal expansion of cured epoxy resin that is saturated with water is observed to be more than twice that of dry resin. Results also indicate that cured resin that is saturated with 7.1% water at 95 C will rapidly increase in moisture content to 8.5% when placed in 1 C water. The mechanism for this phenomenon, termed reverse thermal effect, is described in terms of a slightly modified free-volume theory in conjunction with the theory of polar molecule interaction. Nearly identical behavior was observed in two graphite/epoxy composite systems, thus establishing that this behavior may be common to all cured epoxy resins.

A model for the toughness of epoxy-rubber particulate composites

Abstract: Epoxy resins are toughened significantly by a dispersion of rubber precipitates. Microscopic examinations of propagating cracks in epoxy-rubber composites reveal that the brittle epoxy matrix cracks, leaving ligaments of rubber attached to the two crack surfaces. The rubber particles are stretched as the crack opens and fail by tearing at large, critical extensions. This fracture mechanism is the basis of a new analytical model for toughening. An increase in toughness (ΔGIC) of the composite is identified with the amount of elastic energy stored in the rubber during stretching which is dissipated irreversibly (e.g. as heat) when the particles fail. The model predicts the failure strain of the particles in terms of their size. It also relates the toughness increase to the volume fraction and tearing energy of the rubber particles. Direct measurements of the tearing strains of rubber particles, and toughness data obtained from epoxy-rubber composites, are in good agreement with the model. The particle-stretching model provides a quantitative explanation, in contribution to existing qualitative theories, for the toughening of epoxy-rubber composites.

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

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

On the formation of thermally sprayed alumina coatings

Abstract: A model for the fomation of thermally sprayed alumina coatings is proposed. The spreading and crystallization of liquid droplets on impact with the substrate are analysed and the thermal history of individual particles related to the kinetics of nucleation of γ-Al2O3 to other forms. The results suggest that under the usual spraying conditions undercooling of the liquid droplets is such that γ-Al2O3 nucleates in preference to α-Al2O3 and the cooling rate after solidification is sufficiently rapid to prevent transformation to δ-Al2O3 or α-Al2O3. Transformation of initially formed γ-Al2O3 to α-Al2O3 appears to be possible only if the lamellae formed on impact are thicker than about 10 μm if the substrate is heated to about 1000° C, or if the thickness is greater than about 20 μm on an unheated substrate. The α-Al2O3 generally observed in thermally sprayed coatings is the result of crystallization from pre-existing nuclei arising from incomplete melting of the feed material.

Synthesis of continuous silicon carbide fibre: Part 2 Conversion of polycarbosilane fibre into silicon carbide fibres

Abstract: The polycarbosilane (PC-470) synthesized by thermal decomposition of polydimethylsilane was melt-spun. The conversion process of the fibre into silicon carbide fibre was investigated by chemical analysis, TG-DTA and infra-red spectrum analysis, and measurements of the mechanical properties and densities. The conversion process of polycarbosilane (PC-TMS) synthesized by Fritz was examined and compared with the conversion process of PC-470. It is shown that the process is divided into three stages; condensation at the first stage, thermal decomposition at the second stage and crystallization at the third stage. The mechanical properties and density of the SiC fibre obtained by heattreatment were affected by the molecular weight and structure of the polycarbosilane of the starting material.

Crack blunting mechanisms in polymers

Abstract: A quantitative model has been developed to account for the degree of blunting that occurs at crack tips in epoxy materials prior to the onset of crack propagation. This mechanism controls the subsequent mode of crack growth and, to a large extent, the toughness as defined by the stress intensity factor for crack initiation. From this model a unique fracture criterion is derived which is applicable over all modes of crack propagation.

Principles of photoelectrochemical, solar energy conversion

Abstract: Photoelectrochemical devices for conversion of solar energy into both electrical energy and chemical energy are discussed with emphasis on how the various material properties of the photoactive electrodes influence device efficiency and stability. The similarity between photoelectrochemical cells (PECs) and solid state devices is used to model their behaviour and optimize such parameters as band gap, doping level, minority carrier lifetime, etc. A model is presented which calculates the electron affinity of any semiconductor and allows the prediction of the open circuit voltage of wet photovoltaic cells and optimum biasing forchemical producing cells. The effects of absorbed ions at the semiconductor/electrolyte interface are reviewed. The temperature dependence of the energy levels in the semiconductor and the electrolyte are considered and the implications of these results to operation of PECs at elevated temperature are discussed. The major differences between PECs and solid state devices are the stability considerations. The thermodynamics of this problem is discussed. Other important degradation mechanisms and some solutions to these problems are reviewed. Finally, a prognosis of the future of this field is presented.

Melting behaviour and metastability of yttrium aluminium garnet (YAG) and YAlO3 determined by optical differential thermal analysis

Abstract: The melting point of yttrium aluminium garnet (YAG), reinvestigated by optical differential thermal analysis (ODTA), was found to be 1940±7° C. Above this temperature YAG liquids are opaque, suggesting the presence of two immiscible liquids. In the composition range 10.0 to 47.5 mol% Y2O3, crystallization of the equilibrium phases can only occur in the presence of YAG nuclei; otherwise solidification of YAlO3 and Al2O3 will take place. A metastable phase diagram has been defined with a metastable eutectic at 23 mol% Y2O3-77 mol% Al2O3 and 1702±7° C. YAlO3 (perovskite) was found to melt incongruently with a peritectic temperature of 1916±7° C and a liquidus temperature of 1934±7° C. YAlO3 formed during metastable solidification transforms to YAG in the presence of Al2O3 at 1418±7° C. It is suggested that the metastability arises from the difficulty of the aluminium to attain four-fold co-ordination in the YAG structure.

Atomically sharp cracks in brittle solids: an electron microscopy study

Abstract: The issue of bond rupture versus microplasticity as an essential mechanism of crack propagation in brittle solids is addressed. A detailed survey of existing theoretical and experimental evidence relating to this issue highlights the need for direct observations of events within the crack-tip “process zone”, at a level approaching 10 nm. Transmission electron microscopy is accordingly used to study arrested cracks about sharp-contact (Vickers indentation and particle impact) sites in Si, Ge, SiC and Al2O3. The nature of the deformation which accommodates the irreversible contact impression is first investigated, in the light of Marsh's proposal of an “equivalence” between indentation and crack-tip zone processes. Interfacial and tip regions of the surrounding cracks are then examined for any trace of a plasticity-controlled fracture process. Dislocation-like images are indeed evident at the crack planes, but these are shown to be totally inconsistent with any conventional slip mechanism. The close connection between the dislocation patterns and moire fringe systems along the cracks points to “lattice mismatch” contrast in association with a partial closure and healing operation at the interface. Analysis of all other details in the crack patterns, e.g. the presence of a crack-front contrast band indicative of a residual strain field and the disposition of interfacial fracture steps relative to the dislocation/moire system, reinforces this interpretation. It is concluded that the concept of an atomically sharp crack provides a sound basis for the theory of fracture of brittle solids.

Shear deformation under pyramidal indentations in soda-lime glass

Abstract: The nature of the flow lines that occur in the deformed zone in soda-lime glass under pyramidal indentations has been investigated. A close examination of the deformed zone shows that the spiral flow lines meet at 110°, instead of the 90° required by the ideal rigid-plastic behaviour. The flow lines are indeed shear faults (of negligible thickness) produced by genuine shear displacements which are apparent at the intersection points of some of the flow lines. Evidence of void or crack formation at the intersection points or along the flow lines is also presented, along with a possible hardening effect from the suppression of slip at the intersection points of the flow lines.

Self-diffusion and impurity diffusion in oxides

Abstract: An updated bibliography of diffusion data in oxides is provided for the materials scientist who requires a convenient source of published results. The scope of the review has been enlarged to include data for the diffusion of the host and impurity species in both binary and multiple oxides. Brief descriptions of terminology, diffusional behaviour and new measurement techniques are followed by tables of selected results and associated experimental details.

Compressive creep of Si 3 N 4 /MgO alloys

Abstract: Highly localized strain fields are observed at grain boundaries in crept specimens of Si3N4/MgO alloys which were frozen under stress. These fields disappear upon annealing. Unresolved asperities between the grain pairs appear to give rise to the strain field during deformation. Viscoelastic effects responsible for primary creep and strain recovery are explained in terms of grain-boundary sliding on the glassy interphase which is accommodated by the elastic strain arising at the asperities. Each boundary containing an asperity can be modelled as a simple Kelvin element. The spectrum of these boundaries within the bulk gives rise to a spectrum of relaxation times that is observed for the strain recovery effect. The highly stressed region at the asperity also gives rise to the higher chemical potential required to drive diffusional creep. Although the source of the asperities was not observed, the possibility of opposing ledges of either single or multiple interplanar height is discussed.

The effect of particle shape on the mechanical properties of filled polymers

Abstract: The existing models for predicting the elastic moduli of polymers dispersed with particles of shape other than spheres and continuous fibres are reviewed. The applicability and limitation of these equations are discussed. The emphasis of the review is to seek a unified understanding and approach to the effect of particle shape at finite concentration on the elastic moduli, thermal expansion coefficient, stress concentration factor, viscoelastic relaxation modulus and creep compliance of filled polymers. The effects of anisotropic particle shape on mechanical properties of polymeric composites are clearly illustrated. Attention is also drawn to the relationship between elastic moduli, thermal expansion, creep elongation and stress relaxation moduli.

The mechanical properties of epoxy resins

Abstract: Crack propagation in a series of epoxy resins described in Part 1 has been studied as a function of testing rate and temperature. It has been found that crack propagation is continuous at low temperatures but that as the temperature is raised the mode of propagation becomes unstable (stick/slip). Features on the fracture surfaces at the crack arrest lines have been shown to be of the same dimensions as those expected for a Dugdale plastic zone. It has been suggested that the “slip” process takes place by slow growth of a crack through the plastic zone followed by rapid propagation through virgin material. It has been shown that the stick/slip behaviour is due to blunting of the crack which is controlled by the yield behaviour of the resin. A unique fracture criterion has been shown to be applicable to epoxy resins which is that a critical stress of the order of three times the yield stress must be achieved at a critical distance ahead of the crack. Electron microscope replicas of the fracture surfaces have been obtained and an underlying nodular structure can be resolved. However, no direct correlation between the nodule size and fracture properties has been found.

Preparation of glass fibres of the ZrO2-SiO2 and Na2O-ZrO2-SiO2 systems from metal alkoxides and their resistance to alkaline solution

Abstract: Glass fibres of the ZrO2-SiO2 and Na2O-ZrO2-SiO2 systems containing up to 33 wt% ZrO2 were prepared by a non-melting technique using zirconium n-propoxide, sodium methoxide and silicon tetraethoxide as raw materials. The mixed alkoxide solutions were exposed to moist air for hydrolysis. The fibrous gels were drawn from these solutions in the course of hydrolysis, and converted to the corresponding oxide glass fibres by heating at 500 to 700° C. It was found that chemical durability of the fibres toward alkaline solution increased with ZrO2 content. The weight loss due to the corrosion by 2 N NaOH solution at 96° C for 4 h was around 14 mg dm−2 for the fibres containing 17 to 26 wt% ZrO2, which was comparable to the alkali-resistant glasses so far obtained by the conventional melting technique. The glass fibres containing 33 wt% ZrO2 showed higher resistance.

The wetting of alumina by copper alloyed with titanium and other elements

Abstract: The wettability of alumina by ternary alloys of copper, titanium and aluminium, gallium gold, indium, nickel or silver has been investigated using sessile drop tests conducted in vacuum at 1050–1250° C. Substantial additions of titanium are known to induce copper to wet alumina due to the formation of a titanium rich reaction product at the alloy/ ceramic interface, but the present work has shown that the concentration of titanium can be reduced by the addition of ternary alloying elements. Additions of indium are very beneficial, of aluminium, gold or silver are moderately beneficial, and of gallium or nickel are of negligible benefit or detrimental. These observations, and previous work with copper-tin-titanium alloys [1] can be interpreted in terms of effects on the activity of titanium which it is argued will be enhanced if the ternary alloying element has a low surface energy and is readily saturated by titanium. The correlation of the experimental wetting observations with the surface energy and titanium solubility data for the ternary alloying elements provides a basis for the rational development of reactive metal brazes for joining unmetallized ceramics.

Characterization of Al-Si-alloys rapidly quenched from the melt

Abstract: Aluminium-silicon alloys with compositions in the range 0 at% to 33.9 at % Si were rapidly quenched from the melt at cooling rates between 106 and 107 K sec−1 using the melt-spinning technique. The resulting ribbons were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction methods. Metastable solid solubilities of silicon in aluminium were determined from lattice parameter and DSC data. The values found were strongly dependent on specimen thickness and a maximum of about 5 at % Si was reached for an alloy composition of 15 at % Sl (maximal equilibrium solid solubility of silicon in aluminium is 1.58 at % Si). Discrepancies between published values of metastable silicon solid solubities were related to the interpretation of the lattice parameter data. Alloy composition was shown to determine the lattice parameter of the silicon-rich phase. The crystallite sizes and the lattice distortions in the aluminium-rich and silicon-rich phases were determined by X-ray diffraction line profile analysis. From the aluminiumrich phase only strain broadening was observed whereas the silicon-rich phase gave rise to both size and strain broadening. The origin of the lattice strains was discussed. Changes in solidification behaviour are reflected in the structure parameters measured.

Compression strength of carbon, glass and Kevlar-49 fibre reinforced polyester resins

Abstract: The compression behaviour of a series of polyester resins of various compositions and in different states of cure has been investigated. Their mechanical characteristics having been established, the same range of resins was then used as a matrix material for a series of composites reinforced with carbon, glass and aromatic polyamide fibres. The composites were unidirectionally reinforced, having been manufactured by pultrusion, and were compression tested in the fibre direction after a series of experiments to assess the validity of a simple testing procedure. “Rule of Mixtures” behaviour occurred in glass-polyester composites up to limiting volume fractions (V f) of 0.31 for strength and 0.46 for elastic modulus, the compression modulus being equal to the tensile modulus, and the apparent fibre strength being in the range 1.3 to 1.6 GPa at this limiting V f. At a V f of 0.31 the strengths of reinforced polyesters were proportional to the matrix yield strength, σ my, and their moduli were an inverse exponential function of σ my. For the same matrix yield strength a composite with an epoxy resin matrix was stronger than polyester based composites. At V f=0.30, Kevlar fibre composites behaved as though their compression modulus and strength were much smaller than their tensile modulus and strength, while carbon fibre composites were only slightly less stiff and weaker in compression than in tension. The compression strengths of the polyester resins were found to be proportional to their elastic moduli.

Constitutional and structural studies of the intermetallic phase, ZrCu

Abstract: Constitutional and structural studies have been carried out on the equiatomic percent alloy ZrCu, using magnetic susceptibility measurements, metallography and X-ray diffraction. The high-temperature magnetic susceptibility measurements and the metallographic studies indicate the presence of two solid state transformations in this alloy, a martensitic transformation with Ms=440±5 K and a eutectoid transformation with TE=985±5 K. Thus, if the high-temperature ZrCu phase is metastably retained to lower temperatures then the martensitic transformation is observed, whereas prolonged annealing just below TE produces a eutectoid mixture; the metallographic studies indicate that this mixture is lamellar in nature. The X-ray diffraction studies indicate that the high-temperature ZrCu phase has the b c c CsCl-type structure.

Fracture behaviour of composites based on Al2O3-TiC

Abstract: The critical stress intensity factor and other related fracture parameters have been measured in three-point bending for pure Al2O3-TiC composites containing 4 to 35 volume fractions of TiC. An increase has been observed for all the parameters with increasing volume fraction of TiC. Following a study of the mode of fracture, the results are explained in terms of a linear variation of the fracture energy with the volume fraction of TiC.

Mechanical behaviour of polycrystalline BeO, Al2O3 and AlN at high pressure

Abstract: The mechanical behaviour of various types of BeO, Al2O3, and AlN have been investigated at confining pressures up to 1.25 GPa, at 25° C, and at strain rates of 3 to 7×10−5 sec−1. The stress-strain data taken in uniaxial compressive-stress loading indicate the BeO aggregates undergo a transition from brittle fracture at low pressures to plastic flow at high pressures. Depending on the fabrication process, this transition pressure in BeO occurs at 0.4 to 0.7 GPa. Concurrently, the ultimate compressive strength of BeO increases from 1.0 to 1.9 GPa at 0.1 MPa pressure to over 4.0 GPa at 1.O GPa. Alumina remains brittle at all pressures up to 1.25 GPa; its strength increases from 4.5 GPa at 0.1 MPa pressure to over 6.0 GPa at 1.25 GPa. Aluminium nitride behaves similarly to BeO, having a brittle-ductile transition at 0.55 GPa. Its ultimate strength increases from 3.2 GPa at 0.1 MPa pressure to 4.7 GPa at 0.8 GPa. The distortional strain energy (proportional to the area under the stress-strain curve) absorbed by each material during compression at pressure was calculated and compared to available data from the literature. Alumina shows a degraded energy absorption with pressure, but both BeO and AlN yield a strongly enhanced performance at moderate pressures. Beryllium oxide and AlN thus appear to be promising structural materials for certain applications where high strengths and ductilities are required at moderate pressures.

Detachment of an elastic matrix from a rigid spherical inclusion

Abstract: An approximate theoretical treatment is given for detachment of an elastomer from a rigid spherical inclusion by a tensile stress applied to the elastomeric matrix. The inclusion is assumed to have an initially-debonded patch on its surface and the conditions for growth of the patch are derived from fracture energy considerations. Catastrophic debonding is predicted to occur at a critical applied stress when the initial debond is small. The strain energy dissipated as a result of this detachment, and hence the mechanical hysteresis, are also evaluated. When a reasonable value is adopted for Young's modulus E of the elastomeric matrix, it is found that detachment from small inclusions, of less than about 0.1 mm in diameter, will not occur, even when the level of adhesion is relatively low. Instead, rupture of the matrix near the inclusion becomes the preferred mode of failure at an applied stress given approximately by E/2. For still smaller inclusions, of less than about 1 μm in diameter, rupture of the matrix becomes increasingly difficult, due to the increasing importance of a surface energy term. These considerations account for the general features of reinforcement of elastomers. Small-particle fillers become effectively bonded to the matrix, whereas larger inclusions induce fracture near them, or become detached from the matrix, at applied stress that can be calculated from the particle diameter, the strength of adhesion, and the elasticity of the matrix material.

Self-diffusion of 30Si in polycrystalline ?-SiC

Abstract: AbstractThe 30Si lattice self-diffusion coefficients in high-purity β-SiC are reported for the temperature range 2283 to 2547 K and may be represented by the expression $$D_{Si}^* = (8.36 \pm 1.99) \times 10^7 \exp \left[ {\frac{{(9.45 \pm 0.05 eV atom^{ - 1} }}{{kT}}} \right]cm^2 \sec ^{ - 1} $$ . Decomposition of the sample at the grain boundaries prevented detection of diffusion along these paths of fast transport. Lattice diffusion of Si was concluded to occur by a mechanism involving a direct jump to the nearest Si vacancy without the previous occupation of a normally unfilled position. A comparison of C and Si diffusion in this material is also given.

The effect of moisture on the physical and mechanical integrity of epoxies

Abstract: The effect of specific combinations of moisture, heat, and stress on the physical structure, failure modes, and tensile mechanical properties of diaminodiphenyl sulphone (DDS)-cured tetraglycidyl 4,4′diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (27 wt% DDS)] are reported. Sorbed moisture plasticizes TGDDM-DDS epoxies and deteriorates their mechanical properties in the range 23 to 150° C. Studies of the initiation cavity and mirror regions of the fracture topographies of these epoxies indicate that sorbed moisture enhances the craze initiation and propagation processes. The effect of tensile stress-level, applied for 1 h on dry epoxies, on the subsequent moisture sorption characteristics of the epoxies was also investigated. Such studies indicate that the initial stages of failure that involve both dilatational craze propagation and subsequent crack propagation enhance the accessibility of moisture to sorption sites within the epoxy to a greater extent than in the latter stages of failure which involve crack propagation alone. The amount of moisture sorbed by TGDDM-DDS epoxies is enhanced by ∼ 1.6 wt% after exposure to a 150° C thermal spike, as a result of moisture-induced free volume increases in the epoxies that involve rotational—isomeric population changes.

Wood fibre-reinforced cement composites

Abstract: The use of wood fibres as reinforcement for a structural composite material has been investigated. Although wood fibres have relatively poor mechanical properties compared with synthetic fibres, they have the advantages of low density, low cost and low energy demand during manufacture. A number of possible matrix materials were considered and Portland cement was chosen for further investigation. An examination was made of the effect of the pulping technique used to prepare the fibre on the strength of the composite and on the stability of the wood fibres in the cement matrix. A chemically pretreated high-temperature thermomechanical pulp and a pulp produced by the kraft process were selected for further study. The effect of the water-cement ratio of the matrix and the weight of fibre in the composite on the strength of the composite and the rate of increase in strength and fracture energy of composite are reported for composites containing these pulps. The results indicate that the kraft pulp is suitable for applications where slurry dewatering can be employed during the forming operation and that the thermomechanical pulp is more suited to applications where low water-cement ratio slurry is used.

Maximum strength and drawing mechanism of hot drawn high molecular-weight polyethylene

Abstract: An investigation of the hot drawing of high molecular weight polyethylene fibres containing a substantial amount of solvent, and of fibres with a porous structure that were dried after solution spinning, was conducted. It was found that the tensile strength as well as the Young's modulus of the polyethylene fibres were linearly related to the applied stress up to values of 0.3 GPa. Drawing stresses exceeding 0.3 GPa could produce polyethylene fibres with a tensile strength of 3.7 GPa and a Young's modulus of 120 GPa. The ultimate mechanical properties appeared to be affected by the nature of the solvent, the polymer concentration and the spinning temperature. Chain extension in the hot drawing and the ultimate draw ratio depended substantially on the number of intermolecular entanglements initially present in the fibre.