Showing papers in "Journal of Materials Science in 1986"

Toughening mechanisms in elastomer-modified epoxies Part 1 Mechanical studies

Abstract: Proprietes mecaniques de melange resines epoxyde-caoutchoucs liquides (copolymeres d'acrylonitrile et de carboxybutadiene). Mecanisme etudie par dilatometrie en etirage uniaxial

Hydrolysis of titanium alkoxide and effects of hydrolytic polycondensation parameters

Abstract: Titanium alkoxides hydrolyse vigorously with water producing polycondensates whose equivalent oxide content varies from ∼ 70% to over 90%. This variation reflects the average molecular size and the nature of the terminal bonds. Hydrolysis conditions also affect the molecular and particle morphologies modifying the crystallization and sintering behaviour of oxide materials derived from polycondensates. It is also possible to form clear polymer solutions under excess water hydrolysis with the addition of certain acids. Investigations show that there is a window in the acid concentration which provides long-term solution stability. The gels formed from these clear solutions shrink in their liquor under certain conditions and the introduction of hydrogen peroxide into the liquor (surrounding the gel) causes vigorous gel shrinkage.

Combustion synthesis of titanium carbide: Theory and experiment

Abstract: The combustion synthesis of titanium carbide from elemental powders has been theoretically and experimentally studied as a model system for self -propagating high temperature synthesis (SHS) of refractory compounds. Calculations of the adiabatic temperature of combustion of graphite and titanium powders to form TiC x have been made to show the effects of stoichiometry, dilution and the initial temperature of the reactants. Experimental observations on the stability of the combustion front, combined with theoretical predictions, lead to an estimated activation energy of ≤ 117 kJ mol−1 for the process. This value is at least a factor of about four too low to correspond to a diffusion-controlled process. The combustion of graphite and titanium powders was accompanied by the evolution of gases whose primary constituent was found to be hydrogen. This observation was attributed to the reaction of adsorbed moisture with titanium powder. The titanium carbide phase resulting from the combustion of compacted mixed powders of the elements was highly porous (∼ 50% porosity). It can be obtained in high density (∼5% porosity) when pressure is applied during the combustion process.

Ion-based methods for optical thin film deposition

Abstract: The optical properties of the dielectric oxide films SiO2, Al2O3, TiO2, ZrO2, CeO2 and Ta2O5 produced by ion-based techniques have been reviewed. The influence of ion bombardment during deposition is discussed in some detail and the various production techniques are described. Recent results on the deposition and properties of diamond-like carbon films are also reviewed. Finally, some examples of the practical applications of high quality dielectric oxide films are given.

Formation of a high-strength bioactive glass-ceramic in the system MgO―CaO―SiO2―P2O5

Abstract: Formation of a high-strength bioactive glass-ceramic in the system MgO-CaO-SiO2-P2O5 was investigated by observing the microstructure of the crystallized products. Crystallization of the parent glass in a bulk form led to the occurrence of large cracks in the crystallized product. This was attributed to the precipitation of fibrousβ-wollastonite crystals growing perpendicular to the outer surfaces of the glass after uniform precipitation of fine oxyapatite/fluoroapatite crystals. On the other hand, crystallization of the same glass in a powder compact led to the formation of a crack-free dense crystallized product due to uniform precipitation of both apatite and wollastonite fine crystals throughout the glass article. The uniform precipitation of the wollastonite crystals was attributed to the simultaneous formation of fine crystals in the individual glass particles.

Influence of grain size on the mechanical behaviour of some high strength materials

Abstract: Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

Intrinsic size dependence of the phase transformation temperature in zirconia microcrystals

Abstract: It is argued that because the tetragonal (t) to monoclinic (m) transformation in zirconia is exothermic, this guarantees that the surface free energy of the former is less than that of the latter structure. It is argued further that for pristine, unconstrained, single crystals,A s ∼M s ∼ 1447 K. It follows then, from a thermodynamic analysis that the reciprocal crystallite size is a linear function of the transformation temperature. Quantitative agreement was obtained between calculated and experimental crystallite size-temperature data which ranged over three orders of magnitude.

Synthesis of continuous silicon carbide fibre: Part 4The structure of polycarbosilane as the precursor

Abstract: The structure of polycarbosilane is represented by three structural elements, but their quantification is difficult. Polycarbosilanes were synthesized by three methods and the respective molecular structures were examined by measurements of the molecular weight and the intrinsic viscosity, infrared, ultraviolet,1H-,13C- and29Si-NMR spectral measurements, and chemical analysis. The three structural elements (SiC4, SiC3H, SiCxSi4−x) in the polycarbosilane molecule were determined quantitatively. By the comparison between1H-NMR spectral data and calculation assuming a linear chain structure, the number of linkages in the unit consisting of ten silicon atoms was estimated to be 3 to 4. This result is in agreement with the result from the intrinsic viscosity; it was found that the shape of the polycarbosilane molecule is planar.

Microstructural evolution in reaction-bonded silicon carbide

Abstract: A detailed microstructural investigation of reaction-bonded silicon carbide has been performed using both fully-bonded and quenched samples and other specially prepared specimens containing large original single crystals of known crystallographic habit. The development of the epitaxial SiC overgrowth on the original SiC particles has been followed and found to proceed by the progressive growth and coalescence of identically-oriented nuclei. This epitaxial layer grows with a habit characteristic ofβ (cubic) SiC and then transforms toα-SiC in the high temperature region behind the reaction front. The formation of faceted grain boundaries is explained by this growth morphology. Furthermore, SiC:SiC grain boundaries, SiC:SiC epitaxial boundaries and SiC:Si interfaces have all been characterized by TEM techniques. The grain boundaries are of particular interest since they usually comprise a thin (∼1 nm) layer of amorphous SiC with occasional suicide and graphite inclusions. The general cleanliness of the vast majority of interfacial area is a result of the removal of the impurities insoluble in SiC by liquid silicon moving through the sample. The overall distribution of impurities is discussed. Other microstructural features have been characterized and texturing due to original particle alignment during fabrication of the green compact investigated. The control of the mechanical properties of reaction-bonded silicon carbide by these various microstructural features is discussed. A basis for an explanation of the interesting trace-impurity-controlled contrast seen in secondary electron images of these materials is also established.

Tension and flexural strength of silicon carbide fibre-reinforced glass ceramics

Abstract: The use of silicon carbide-type fibres to reinforce lithium aluminosilicate glass ceramics results in composites with exceptional levels of strength and toughness. It is demonstrated that composite strength and stress-strain behaviour depend onin situ fibre strength, matrix composition, test technique and atmosphere of test. Both linear and non-linear tensile stress-strain curves are obtained with ultimate strengths at 22° C approaching 700 MPa and failure strains of 1%. Flexure tests performed at up to 1000° C in air are compared with data obtained in argon to demonstrate a significant dependence of strength and failure mode on test atmosphere. Finally, glass ceramic matrix composite performance is compared with a silicon carbide fibre-reinforced epoxy system to demonstrate the importance of matrix failure strain on strength and stress-strain behaviour.

The structure of thermotropic copolyesters

Abstract: Highly oriented polymeric products have been produced over the past fifteen years by two very different processing routes; from conventional polymers processed to highly oriented extended chain structures, and from “rod-like” polymers which exhibit liquid crystalline behaviour. Gel spun polyethylene is an example of such a conventional polymer. There are three main types of liquid crystalline polymers (LCP) which have high orientation and modulus: lyotropic aramids, such as poly(ρ-phenylene terephthalamide) (PPTA); lyotropic, aromatic heterocyclic polymers, or “ordered polymers”; and the family of thermotropic aromatic copolyesters. Extensive characterization of the thermotropic copolyesters has resulted in the delineation of a fibrillar, hierarchial structural model which accounts for the structures observed in a broad range of oriented fibres, extrudates and moulded articles. Three distinct fibrillar species are observed: microfibrils that are about 50 nm, fibrils about 500 nm, and macrofibrils about 5μm, in size. Superimposed on the structural hierarchy is a defect hierarchy, defined by the regular meander of the molecular chain and a localization of defects within a microfibril at about a 50 nm periodicity. Orientational variations, layering and skin core structures, in thick specimens, are the result of local flow fields on the basic structural units during solidification. The fibrillar textures appear to be present prior to any preparation for microscopy. A wide range of specimen preparation methods, i.e. fractography, sonication, microtomy and etching, and microscopic techniques, i.e. optical, scanning and transmission electron microscopy, were applied to the characterization of the aromatic copolyesters and PPTA. Interestingly, the same basic hierarchy is observed for both the lyotropic and the thermotropic LCPs and the microfibrillar structures of all the highly oriented polymers, including polyethylene, appear quite similar.

Failure of fibre-reinforced composites by pull-out fracture

Abstract: A simple model is proposed to predict the ultimate tensile strength of fibre-reinforced composites when the failure is governed by fibre debonding. The theoretical analysis is based on the concept of fracture mechanics where the debonded zone is considered as an interfacial crack. The analysis is first applied to the classical pull-out test in order to determine the specific work of interfacial cracking. Using this value, the uniaxial tensile strength of the composites can be predicted from an approximate, closed-form equation proposed here. The theoretically predicted results seem to compare favourably with experimental values for fibrere-inforced cement based composite.

Structure and properties of some vegetable fibres

Abstract: The stress-strain curve for sisal fibres has been experimentally determined. Ultimate tensile strength (UTS), initial modulus (YM), average modulus (AM) and per cent elongation at break of fibres have been measured as function of fibre diameter, test length and test speed. UTS, YM, AM and per cent elongation lie in the range 530 to 630 MN m−2, 17 to 22 GN m−2, 9.8 to 16.5 GN m−2 and 3.64 to 5.12 respectively for fibres of diameters ranging between 100 and 300μm. No significant variation of mechanical properties with change in diameter of the fibres was observed. However, with increase in test length of the fibres, the UTS and per cent elongation are found to decrease while YM and AM increased in the test length ranging from 15 to 65 mm. With the increase in speed of testing from 1 to 50 mm min−1, YM and UTS are found to increase whereas per cent elongation and AM do not show any significant variation. At a test speed of 500 mm min−1 the UTS value decreases sharply. The above results are explained in terms of the internal structure of the fibre such as the cell structure, microfibrillar angle, defects, etc. Scanning electron microscope (SEM) studies of the fractured tips of the sisal fibres reveal that the failure of the fibre is due to the uncoiling of microfibrils accompanied by decohesion and finally tearing of cell walls. The tendency of uncoiling seems to decrease with increasing speed of testing.

Crystal chemistry of the NaZr2(PO4)3, NZP or CTP, structure family

Abstract: The NaZr2(PO4)3 type structure (abbreviated as NZP or CTP, CaTi4(PO4)6), has emerged as a new family, which has extraordinary technological utility in three fields: fast-ion conductors, radwaste solidification and zero expansion ceramics. NZP or CTP is formed by an extraordinary range of discrete compositions and crystalline solutions. In this paper these compositions are classified according to their crystal chemical substitution scheme, and some uncommon trends in the systematic variation of their lattice parameters are shown. Some of the major trends are explained by correlation with the rotation of polyhedra in the structure.

Thermal expansion of the cubic (3C) polytype of SiC

Abstract: Thermal expansion of the cubic beta or (3C) polytype of SiC was measured from 20 to 1000° C by the X-ray diffraction technique. Over that temperature range, the coefficient of thermal expansion can be expressed as the second order polynominal: α11=3.19×10−6+ 3.60×10−9 T−1.68×10−12 T 2 (1/° C). It increases continuously from about 3.2×10−6/° C at room temperature to 5.1×10−6/° C at 1000° C, with an average value of 4.45 × 10−6/° C between room temperature and 1000° C. This trend is compared with other published results and is discussed in terms of structural contributions to the thermal expansion.

Preparation of spherical fine ZnO particles by the spray pyrolysis method using ultrasonic atomization techniques

Abstract: An ultrasonic atomizer was used in the spray pyrolysis method to prepare fine, spherical and uniform ZnO particles. Almost spherical particles were obtained successfully which had a mean particle size of 0.15 μm and had a very narrow particle size distribution. By using alcohol as the solvent, it was found that the particles do not have hollow shell layers which could usually be observed in the spray pyrolysis process by using water as the solvent. The morphology of the ZnO particles was strongly affected by the concentration of the starting solution.

Zirconium oxides formed by hydrolytic condensation of alkoxides and parameters that affect their morphology

Abstract: When zirconium oxides are formed via hydrolytic condensation of zirconium alkoxides, the particle size and morphology of the resultant zirconia is strongly affected by certain parameters during the condensation. These parameters include: the type of alkyl group in the alkoxide, water/alkoxide ratio, molecular separation of species, and the reaction temperature. The particle size and the morphology in turn affect the sintering behaviour and crystalline transformation of ZrO2, In this work the parameters that affect the formation of ZrO2 from zirconium alkoxides are investigated. It has been shown that the alkyl groups and molecular separations during the hydrolytic polycondensation have particular significance in the modification of monoclinic → tetragonal transformation of the resultant ZrO2. Tetragonal phase can also be stabilized by copolymerization of ZrO2 with SiO2 producing tough ceramic materials.

Structure and properties of some vegetable fibres: Part 2Pineapple fibre (Anannus Comosus)

Abstract: The stress-strain curves for pineapple leaf fibre have been analysed. Ultimate tensile strength (UTS), initial modulus (YM), average modulus (AM) and elongation of fibres have been calculated as functions of fibre diameter test length and test speed. UTS, YM, and elongation lie in the range of 362 to 748 MN m−2, 25 to 36 GN m−2, and 2.0 to 2.8%, respectively for fibres of diameters ranging from 45 to 205μm. UTS Was found to decrease with increasing test lengths in the range 15 to 65 mm. Various mechanical parameters show marginal changes with change in speed of testing in the range of 1 to 50 mm min−1. The above results are explained on the basis of structural variables of the fibre. Scanning electron microscope studies of the fibres reveal that the failure of the fibres is mainly due to large defect content of the fibre bo1h along the fibre and through the cross-section, The crack is always initiated by the defective cells and further aggravated by the weak bonding material between the cells.

Thermal-shock crack patterns explained by single and multiple crack propagation

Abstract: The phenomenon of thermal-shock cracking in ceramics is approached theoretically and experimentally. Sintered slabs made from a glass-quartz powder mix were quenched in water in order to generate crack patterns of various types, depending on the severity of the shock. The patterns serve as evidence for a scheme set up with the aim of explaining the variety of ways in which materials respond to thermal shock. The fracture-mechanical explanations are based on the time-dependent thermal load and on the concept of an energy release rate, with single and multiple crack propagation starting from randomly distributed initial flaws on the surface.

On the influence of silicon on the growth of the alloy layer during hot dip aluminizing

Abstract: The reactions between a low alloyed steel and a pure as well as a 2% silicon containing aluminium melt where studied at 780 and 792‡ C, respectively. The velocity of the iron enrichment was the same for both cases, whereas the alloy layer growth was much faster in the case of the pure aluminium melt. After hot dip aluminizing in the 2% silicon containing aluminium melt silicon was found to be incorporated in the alloy layer. From that it is clear that silicon acts on the solid state side, when reducing the alloy layer thickness during hot dip aluminizing.

The surface tension of liquid pure aluminium and aluminium−magnesium alloy

Abstract: This paper discusses the results of several experiments designed to further illustrate the recent findings of the present authors according to which, if a virtually leak-fee maximum bubble pressure system is used to measure the surface tension of liquid aluminium, a surface tension around 1100 mJ m−2 is first obtained, decreasing to the oxidized value (around 865 mJ m−2) as the experiment proceeds and oxygen enters the system mainly through the capillary walls. The peculiarities and difficulties inherent to the maximum bubble pressure method are illustrated. For instance, a study of the time needed for the surface tension to decrease to the oxidized value as a function of temperature reveals the important role played by the vapour pressure in the process. This is further illustrated by considering Al-Mg alloys, as magnesium has a different heat of vaporization and a much larger vapour pressure than aluminium at the measuring temperatures. Results for the changes in density and surface tension for the oxidized and unoxidized cases induced by magnesium (up to 8 wt%) are also presented and compared to previous data.

A TEM study of ordering in the perovskite, Pb(Sc1/2Ta1/2)O3

Abstract: Ordering behaviour and the factors which influence the ordering have been studied in both single crystals and hot-pressed ceramics of Pb(Sc1/2Ta1/2)O3 using TEM techniques, including low-temperature microscopy. The characteristics of the structurally-ordered domains are described and also the observation by electron diffraction of new superlattice reflections and diffuse scattering in both the paraelectric and ferroelectric phases. The superlattice reflections are interpreted in terms of Pb2+ cation displacements. The non-interaction of ordered domains and ferroelectric domains is explained within this model.

Creep of chemically vapour deposited SiC fibres

Abstract: The creep, thermal expansion, and elastic modulus properties for chemically vapour deposited SiC fibres were measured between 1000 and 1500°C. Creep strain was observed to increase logarithmically with time, monotonically with temperature, and linearly with tensile stress up to 800 MPa. The controlling activation energy was 480 ± 20 kJ mol−1. Thermal pretreatments near 1200 and 145O° C were found to significantly reduce fibre creep. These results coupled with creep recovery observations indicate that below 1400°C fibre creep is anelastic with negligible plastic component. This allowed a simple predictive method to be developed for describing fibre total deformation as a function of time, temperature, and stress. Mechanistic analysis of the property data suggests that fibre creep is the result of β-SiC grain boundary sliding, controlled by a small percentage of free silicon in the grain boundaries.

The evaluation of the strength distribution of silicon carbide and alumina fibres by a multi-modal Weibull distribution

Abstract: The strengh distributions of silicon carbide and alumina fibres have been evaluated by a multimodal Weibull distribution function. This treatment is based on the concept that the fracture of the fibre is determined by competition among the strength distributions of several kinds of the defect sub-population. Since those fibres were observed to have two types of fracture mode, the evaluation of a bi-modal Weibull distribution was performed in comparison with the single Weibull distribution usually employed. The accuracy of the fit for these two distributions was judged from maximum logarithm likelihoods and cumulative distribution curves. The result showed that the logarithm likelihood calculated using the bi-modal Weibull distribution function gave a larger value, as compared with those using the single Weibull distribution function. The curve predicted from the former function was also in good agreement with the data points. In addition, the strength distribution and the average value at a different gauge length were extrapolated from the Weibull parameters estimated at the original gauge length. In this case, also, the bi-modal Weibull distribution gave a more accurate prediction of the data points.

Stiff and strong polyethylene with shish kebab morphology by continuous melt extrusion

Abstract: In a previous work, it was shown that highly oriented fibres with 10 GPa modulus could be obtained by continuous single-stage melt extrusion of a medium molecular weight polyethylene to which 3% ultra-high molecular weight (M w ∼ 3 to 5 × 106) material had been added by solution blending. It was demonstrated that a special interlocking shish kebab structure was responsible for the favourable mechanical properties. In the present work, we succeeded in achieving the same effect from an unblended polyethylene by choosing starting materials with an inherently suitable molecular weight distribution. Both the high and low molecular weight tails of the distribution are very influential: the high tail contributes to the formation of extended-chain fibrils (which constitute the backbones of the shish kebabs), while the low tail affects melt extrudability and strength. Melt strength is important because unusually high tensile stresses are required during wind-up. The wind-up stress was measured and found to be an order of magnitude greater than that encountered in conventional melt spinning — where no shish kebabs are formed. The implications of the above findings for polymer processing, crystal morphology and melt rheology are discussed.

Reaction diffusion in heterogeneous binary systems

Abstract: A system of non-linear differential equations describing the growth kinetics of the two compound layers at the interface between two elements is proposed. A number of well-known experimental and theoretical relationships follows from this system in the limiting cases.

Sunhemp fibre-reinforced polyester

Abstract: This paper describes the tensile and impact behaviour of polyester composites reinforced with continuous unidirectional sunhemp fibres of plant origin. The tensile strength and Young's modulus of sunhemp fibre were found to be 389 MPa and 35.4 GPa, respectively. Tensile strength of composites containing up to 0.4 fibre volume fraction (V f) were found to increase linearly with (V f) and the results showed good agreement with the rule of mixtures. The work of fracture, as determined by Izod impact test, was also found to increase linearly with (V f) and the work of fracture for 0.24 (V f) composite was found to be approximately 21 kJ m−2. The analysis of various energy absorbing mechanisms during impact fracture showed that fibre pull out and interface fracture were the major contributions towards the high toughness of these composites. The results of this study indicate that sunhemp fibres have potential as reinforcing fillers in plastics in order to produce inexpensive materials with a high toughness.

Direct visualization of microstructural deformation processes in polyethylene

Abstract: The deformation morphology of high density polyethylene was investigated by electron microscopy of oriented thin films. A melt-drawing process was used to produce chain axis oriented and planar textured thin film, which was subsequently deformedin situ at room temperature in a scanning transmission electron microscope. Both as-drawn and annealed films were studied. Deformation along the orientation direction is initially accomodated by the interlamellar regions, which cavitate and form microfibres. For annealed films it is possible to directly observe strain-induced crystallization at about 300% strain in the fibrils. With increased deformation, suitably oriented lamellar crystals deform by two clearly visualized chain slip systems: {1 0 0}, 〈0 0 1〉 and {0 1 0}, 〈0 0 1〉. Thesec axis shear processes could be further distinguished as fine slip or as block shear. Still higher deformation causes more breakup of blocks by shear; when the block size is less than some critical size, the blocks decrystallize. The deformation leads toward a fibrillar morphology consisting of oriented crystals from crystallized amorphous material at high elongations, crystal blocks broken out of lamellae, and chains drawn out of lamellae and recrystallized. The deformation behaviour of the as-drawn films is somewhat different from that of the annealed films. Initially as for the annealed films the lamellae are separated with increasing strain as the interlamellar regions are deformed but there is less voiding. Higher deformation causes the lamellae in the as-drawn films to shear apart at significantly lower strain levels (50% as opposed to 300%) than in the annealed films. At about 100% deformation, the as-drawn film no longer has a recognizable lamellar structure. Although generalization is tempered by the simplicity of this model texture, these deformation results are highly relevant to the current microstructural understanding of lamellar deformation in different regions of a spherulite, to the morphology of commercial extruded and blown films, and to specially prepared textured polymers, such as rolled and annealed films or capillary melt flow and solidification methods which can produce texture approaching that of a single crystal.

Erosive wear of polymer surfaces by steel ball blasting

Abstract: The erosion behaviour of a variety of polymeric materials has been studied using steel balls at 57 m sec−1 in an air-blast rig. It is shown that the softer polymers (polyethylene, polypropylene, polybutene-1) exhibit an incubation period prior to stabilizing to a linear erosion rate, here defined as reduction in thickness per testing time. The more brittle polymer, polystyrene, on the other hand, shows no incubation time and possesses the highest erosion rate. Further effects can arise from the morphology of semicrystalline polymers. In particular, it was found that a coarse spherulitic microstructure in polypropylene wears much faster than a fine spherulitic one. A decrease in testing temperature generally increases the wear rate. The individual mechanisms of erosive wear are illustrated by SEM micrographs of the worn surfaces. It is suggested that a “brittleness index” of the form (hardnessH/fracture energyG Ic) is a good indicator for the erosion resistance of polymeric materials.