Showing papers in "Journal of Materials Research in 1991"
TL;DR: In this paper, the authors summarize recent developments concerning Fe3Al-based aluminides, including alloy development efforts and environmental embrittlement studies, and review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.
Abstract: Fe3Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However, limited room temperature ductility (<5%) and a sharp drop in strength above 600 °C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe3Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.
643 citations
TL;DR: In this article, the bulk mechanical properties of samples of nanocrystalline Cu and Pd consolidated from powders prepared by inert gas condensation were studied. But the results of the study were limited.
Abstract: This report gives results of a study of the bulk mechanical properties of samples of nanocrystalline Cu and Pd consolidated from powders prepared by inert gas condensation. Fourier analysis x-ray diffraction techniques, used to determine average grain size and mean lattice strains of the as-consolidated samples, show grain sizes in the range of 3–50 nm and lattice strains ranging from 0.02–3%. Sample densities range from 97–72% of the density of a coarse-grained standard. Microhardness of the nanocrystalline samples exceeds that of annealed, coarse-grained samples by a factor of 2–5, despite indications that sample porosity reduces hardness values below the ultimate value. Uniaxial tensile strength of the nanocrystalline samples is similarly elevated above the value of the coarse-grained standard samples. Restrictions on dislocation generation and mobility imposed by ultrafine grain size are believed to be the dominant factor in raising strength. Residual stress may also play a role. Room temperature diffusional creep, predicted to be appreciable in nanocrystalline samples, was not found. Instead, samples appear to show logarithmic creep that is much smaller than the predicted Coble creep.
459 citations
TL;DR: The silicon oxycarbide structure deduced from these results is a random network of silicon-oxygen tetrahedra, with some silicons bonded to one or two carbons substituted for oxygen as discussed by the authors.
Abstract: Silicon oxycarbide glass is formed by the pyrolysis of silicone resins and contains only silicon, oxygen, and carbon. The glass remains amorphous in x-ray diffraction to 1400 °C and shows no features in transmission electron micrographs (TEM) after heating to this temperature. After heating at higher temperature (1500–1650 °C) silicon carbide lines develop in x-ray diffraction, and fine crystalline regions of silicon carbide and graphite are found in TEM and electron diffraction. XPS shows that silicon-oxygen bonds in the glass are similar to those in amorphous and crystalline silicates; some silicons are bonded to both oxygen and carbon. Carbon is bonded to either silicon or carbon; there are no carbon-oxygen bonds in the glass. Infrared spectra are consistent with these conclusions and show silicon-oxygen and silicon-carbon vibrations, but none from carbon-oxygen bonds. 29Si-NMR shows evidence for four different bonding groups around silicon. The silicon oxycarbide structure deduced from these results is a random network of silicon-oxygen tetrahedra, with some silicons bonded to one or two carbons substituted for oxygen; these carbons are in turn tetrahedrally bonded to other silicon atoms. There are very small regions of carbon-carbon bonds only, which are not bonded in the network. This “free” carbon colors the glass black. When the glass is heated above 1400 °C this network composite rearranges in tiny regions to graphite and silicon carbide crystals. The density, coefficient of thermal expansion, hardness, elastic modulus, index of refraction, and viscosity of the silicon oxycarbide glasses are all somewhat higher than these properties in vitreous silica, probably because the silicon-carbide bonds in the network of the oxycarbide lead to a tighter, more closely packed structure. The oxycarbide glass is highly stable to temperatures up to 1600 °C and higher, because oxygen and water diffuse slowly in it.
355 citations
TL;DR: Silicon polymers were pyrolyzed to form silicon oxycarbides that contained only silicon, oxygen, and carbon, and NMR showed that the polymers had a silicon-oxygen backbone with branching and ring units as discussed by the authors.
Abstract: Silicone polymers were pyrolyzed to form silicon oxycarbides that contained only silicon, oxygen, and carbon. The starting polymers were mainly methyl trichlorosilane with a small amount of dimethyl dichlorosilane. NMR showed that the polymers had a silicon-oxygen backbone with branching and ring units. When the polymer was heated in hydrogen, toluene and isopropyl alcohol, used in production of the polymer, were given off in the temperature range 150 °C to 500 °C. Substantial decomposition of the polymer itself began only above about 700°by evolution of methane. The network of silicon-oxygen bonds and silicon-carbon bonds did not react and was preserved; the silicon-carbon bonds were linked into the silicon-oxygen network. The silicon oxycarbide was stable above 1000 °C, showing no dimensional changes above this temperature. The interior of the silicon oxycarbide was at very low effective oxygen pressure because oxygen diffused slowly in it. There was also a protective layer of silicon dioxide on the surface of the silicon oxycarbide.
229 citations
TL;DR: In this paper, electron micrographs of indents in (111) silicon reveal that a thin layer of material immediately adjacent to the indenter is plastically extruded, which indicates that the material can be deformed in this way.
Abstract: Scanning electron micrographs of indents in (111) silicon reveal that a thin layer of material immediately adjacent to the indenter is plastically extruded. The fact that the material can be deformed in this way indicates that it has metallic-like mechanical properties. This is presented as new evidence that a pressure-induced phase transformation to the metallic state occurs during the indentation of silicon.
208 citations
IBM1
TL;DR: The microindenter has proven to be a powerful device in the characterization of the mechanical properties of thin films as discussed by the authors, which allows for other mechanical properties, in addition to hardness, to be acquired.
Abstract: The microindenter has proven to be a powerful device in the characterization of the mechanical properties of thin films. The machine has both high resolution in the applied load and penetration depth measurements, as well as the versatility to perform different types of testing. The former provides the capability to deal with extremely thin films, while the latter allows for other mechanical properties, in addition to hardness, to be acquired. Four types of tests, namely indentation, scratch, load relaxation, and indentation fatigue tests can currently be conducted using the microindenter via different operating procedures. Only the scratch and load relaxation techniques will be covered in this paper. In a microscratch test, the normal load, tangential load, scratch length, and acoustic emission are monitored simultaneously during an entire scratch process for the purposes of measuring the critical load and studying the failure mechanisms of the deposited films. The adhesion strength, scratch hardness, fracture toughness, and friction are the mechanical properties which are possible to obtain by using this technique. Results from aluminum, carbon, and zirconia coatings will be discussed. The load relaxation test provides information on the creep properties of the films and results in an empirical constitutive relation between the applied stress and plastic strain rate. The creep properties of DC sputtered Al films will be used as an illustration of this.
197 citations
TL;DR: In this paper, a thorough investigation of the microstructure of single SCS-6 SiC fibers widely used as reinforcements in metalmatrix and ceramic-matrix composites has been made.
Abstract: A thorough investigation of the microstructure of single SCS-6 SiC fibers widely used as reinforcements in metal-matrix and ceramic-matrix composites has been made. Various techniques of electron microscopy (EM) including scanning (SEM), conventional transmission (TEM), high resolution (HREM), parallel electron energy loss spectroscopy (PEELS), and scanning Auger microscopy (SAM) have been used to analyze and characterize the microstructure. The fiber is a complicated composite consisting of many different layers of SiC deposited on a carbon core and different carbonaceous coatings covering the SiC layers. The structural and chemical aspects of each layer are characterized and discussed.
190 citations
TL;DR: Growth conditions for YBaCuO thin films are investigated in this article, and it is found that in conditions of lower oxygen, pressure films with average composition off the 1-2-3 stoichiometry have higher Tc's.
Abstract: Growth conditions for YBaCuO thin films are investigated. Films have been made by reactive e-beam coevaporation using three metal sources. In the best cases, as-made films are superconducting with Tc's (R = 0) up to 90 K and Jc's (at 4.2 K) above 107 A/cm2. Oxygen pressure, deposition temperature, as well as compositional dependencies of the films are presented. It is found that in conditions of lower oxygen, pressure films with average composition off the 1–2–3 stoichiometry have higher Tc's. For pressure 100 mTorr). The expanded c0's for these films cannot be reduced by a low temperature oxygen anneal. We suggest that metal-atom point-like defects are quenched into these films and we discuss a particular Ba-for-Y substitution model.
170 citations
TL;DR: Ion beam treatment studies have been carried out to investigate the potential for improvements in surface-sensitive properties of polymers as discussed by the authors, and the implanted materials were characterized by optical microscopy, transmission electron microscopy and nano-hardness indentation.
Abstract: Ion beam treatment studies have been carried out to investigate the potential for improvements in surface-sensitive properties of polymers. Kapton, Teflon, Tefzel, and Mylar have been implanted with boron, nitrogen, carbon, silicon, and iron ions, singly or simultaneously with dual or triple beams. The implanted materials were characterized by optical microscopy, transmission electron microscopy, nano-hardness indentation, wear testing, scanning tunneling microscopy, x-ray analysis, nuclear reaction analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Although the polymers showed a color change and varying degrees of measurable surface depression in the bombarded area, the implanted surface revealed substantial improvements in surface smoothness, hardness, and wear resistance. In particular, B, N, C triple-beam implanted Kapton showed over 30 times larger hardness than unimplanted material, making it more than three times harder than stainless steel. Sliding wear properties were characterized using an oscillating nylon or high carbon steel wear ball. Severe wear tracks were observed in virgin Kapton, but no appreciable wear was observed in ion implanted Kapton. Mechanisms underlying the improved surface properties are addressed.
152 citations
TL;DR: In this article, the authors used X-ray diffraction from a synchrotron source to identify the crystal structures in zirconia ceramics produced by the sol-gel method.
Abstract: X-ray diffraction from a synchrotron source was employed in an attempt to identify the crystal structures in zirconia ceramics produced by the sol-gel method. The particles of chemically precipitated zirconia, after calcination below 600 °C, are very fine, and have a diffracting particle size in the range of 7-15 nm. As the tetragonal and cubic structures of zirconia have similar lattice parameters, it is difficult to distinguish between the two. The tetragonal structure can be identified only by the characteristic splittings of the Bragg profiles from the “c” index planes. However, these split Bragg peaks from the tetragonal phase in zirconia overlap with one another due to particle size broadening. In order to distinguish between the tetragonal and cubic structures of zirconia, three samples were studied using synchrotron radiation source. The results indicated that a sample containing 13 mol% yttria-stabilized zirconia possessed the cubic structure with a0 = 0.51420 ± 0.00012 nm. A sample containing 6.5 mol% yttria stabilized zirconia was found to consist of a cubic phase with a0 = 0.51430 ± 0.00008 nm. Finally, a sample which was precipitated from a pH 13.5 solution was observed to have the tetragonal structure with a0 = 0.51441 ± 0.00085 nm and c0 = 0.51902 ± 0.00086.
147 citations
TL;DR: In this article, the porosity control in ceramic membranes has been achieved by controlling particle packing densities in sol-gel processing and the principle of controlling porosity is to make spongy aggregates and to control the degree of aggregation.
Abstract: Porosity control in ceramic membranes has been achieved by controlling particle packing densities in sol-gel processing. TiO2 xerogels with two mean pore radii of 0.7 and 1.7 nm and a porosity varying from 30% to 52% have been obtained. ZrO2 xerogels with a mean pore radius of 0.7 nm and a porosity varying from 7% to 34% have also been prepared. The principle of controlling porosity is to make spongy aggregates and to control the degree of aggregation. Experiments have been conducted to show that spongy aggregates can be produced by gradually removing protons from the strongly charged particles. Viscosity techniques have been used to measure the relative volume fraction of the dispersed phase which, in turn, provides information on aggregate structures. Two aggregation models have been proposed to explain different structural aggregates formed through thermal destabilization in the highly charged system and through charge neutralization by gradually removing charge from the particles in the system.
TL;DR: In this article, a model previously used to interpret the behavior of aluminum films proves to be, with minor modification, also applicable to copper, where the thermal expansion of copper is closer to that of silicon and, consequently, the thermally induced strains are smaller.
Abstract: Since copper has some advantages relative to aluminum as an interconnection material, it is appropriate to investigate its mechanical properties in order to be prepared in advance for possible problems, such as the cracks and voids that have plagued aluminum interconnect systems. A model previously used to interpret the behavior of aluminum films proves to be, with minor modification, also applicable to copper. Although the thermal expansion of copper is closer to that of silicon and, consequently, the thermally induced strains are smaller, the much larger elastic modulus of copper results in substantially higher stresses. This has implications for the interaction of copper lines with dielectrics.
TL;DR: In this paper, a dissolution-diffusion-reaction-like cycle of hydrating cement particles is directly simulated using cement particles packed around a simple nonreactive aggregate particle.
Abstract: Researchers using backscattered scanning electron microscopy, along with quantitative image analysis techniques, have clearly demonstrated the existence of a highly porous interfacial region between aggregate particles and the cement paste matrix in ordinary Portland cement concrete. This paper presents the results of a digital-image-based simulation model of this interfacial zone. A dissolution-diffusion-reaction-like cycle of hydrating cement particles is directly simulated using cement particles packed around a simple nonreactive aggregate particle. The model is two-dimensional, as we are comparing to experimental results obtained on two-dimensional images of polished sections. The qualitative features seen experimentally, such as large amounts of porosity and calcium hydroxide in the interfacial zone, are accurately reproduced. A new mechanism, one-sided growth, is proposed, along with the more usual particle-packing ideas, as an explanation of the origin of the characteristic features of the interfacial zone.
TL;DR: In this article, the reduction of dislocation density in heteroepitaxial III-V compound films on Si substrates has been studied using MOCVD (Metal-Organic Chemical Vapor Deposition).
Abstract: The reduction of dislocation density in heteroepitaxial III-V compound films on Si substrates has been studied using MOCVD (Metal-Organic Chemical Vapor Deposition). High-quality GaAs films on Si, with a dislocation density of about 106 cm−2, have been obtained by combining strained-layer superlattice insertion and thermal cycle annealing. Reduction of dislocation density in the III-V compounds on Si is discussed based on a simple model, where dislocation annihilation is assumed to be caused by dislocation movement under thermal and misfit stress. As a result of dislocation density reduction, high-efficiency GaAs-on-Si solar cells with total-area efficiencies of 18.3% (AM0) and 20% (AM1.5), and red and yellow emissions from InGaP-on-Si light-emitting diodes have been realized. Moreover, future prospects of heteroepitaxy of III-V compounds on Si are also discussed.
TL;DR: In this article, the authors used both continuous indentation and x-ray diffraction techniques to characterize stress relaxation in aluminum films of several thicknesses and found that the results of the indentation data from thicker films do not compare well to the x-rays data due to the presence of a residual stress distribution.
Abstract: Stress relaxation in aluminum films of several thicknesses was characterized by using both continuous indentation and x-ray diffraction techniques. Results of the indentation and x-ray stress measurements compare closely for films of small thicknesses. Indentation data from thicker films do not compare well to the x-ray data due to the presence of a residual stress distribution.
TL;DR: In this paper, an early stage of diamond growth in hot-filament chemical vapor deposition on silicon substrates was examined by the high resolution electron microscope, where diamond powder abrading was found to plant diamond seed crystals with a density of as high as 1011/cm2.
Abstract: An early stage of diamond growth in hot-filament chemical vapor deposition on silicon substrates was examined by the high resolution electron microscope. “Pretreatment” of the substrate surfaces by diamond powder abrading was found to plant diamond seed crystals with a density of as high as 1011/cm2. These crystals provide sites for subsequent growth of diamond films. The CVD grown diamond particles tend to be cuboctahedra. Smaller particles in nanometer size are faultless, but larger ones of several tens of nanometers develop crystal faults. Some of them may originate from the seed crystals. Degradation of the diamond seed crystals due to the electron beam irradiation is discussed in terms of fabrication of diamond film.
TL;DR: In this paper, single crystal silicon was selected as a model material to study the correlation of fracture surface features as characterized by their fractal dimension for two different orientations of fracture with the fracture toughness of the material as measured using the strength-indentation and fracture surface analysis techniques.
Abstract: The quantitative description of surfaces that are created during the fracture process is one of the fundamental issues in materials science. In this study, single crystal silicon was selected as a model material in which to study the correlation of fracture surface features as characterized by their fractal dimension for two different orientations of fracture with the fracture toughness of the material as measured using the strength-indentation and fracture surface analysis techniques. The fracture toughness on the {110} fracture plane of single crystal silicon was determined to be 1.19 MPa m1/2 for the {100} tensile surface and 1.05 MPa m1/2 for the {110} tensile surface using the indentation-strength three-point bending method. The fracture surface features of these two orientations are correspondingly different. Within our limitations of measurements (1–100 μm), the fractal dimension appeared different in different regions of the fracture surface. It has a higher value in the branching region and a lower value in the pre-branching and post-branching regions. The fractal dimensions are about the same in the pre-branching regions and post-branching region for these two orientations (D = 1.01 ± 0.01), i.e., nearly Euclidean (smooth); but the fractal dimensions are higher in the branching region for these two orientations. The fractal dimension is 1.10 ±0.4 for the {100} tensile surface and is 1.04 ±0.3 for the {110} tensile surface. If we select the highest dimension on a surface to represent the dimensionality of the surface, then a material with a higher fracture toughness has a higher fractal dimension in the branching region.
TL;DR: In this article, a simple analytical model for the effect of mechanical strength and line width (for the case of narrow lines) on the electromigration failure of metallic interconnects is presented.
Abstract: A simple analytical model for the effect of mechanical strength and line width (for the case of narrow lines) on the electromigration failure of metallic interconnects is presented. Because the line width/grain size ratio and the diffusivity enter differently in the model, application of the resulting failure time equation to published data can provide insight into the mechanisms of enhancement of electromigration resistance by grain structure optimization and alloying.
TL;DR: In this article, the nucleation and growth mechanisms during high temperature oxidation of liquid Al−3% Mg and liquid Al −3% mg and Mg−3%, Mg-Al2O4 alloys were studied with the aim of enhancing the understanding of a new composite fabrication process.
Abstract: The nucleation and growth mechanisms during high temperature oxidation of liquid Al−3% Mg and Al−3% Mg−3% Si alloys were studied with the aim of enhancing our understanding of a new composite fabrication process. The typical oxidation sequence consists of an initial event of rapid but brief oxidation, followed by an incubation period of limited oxide growth after which bulk Al2O3/Al composite forms. A duplex oxide layer, MgO (upper) and MgAl2O4 (lower), forms on the alloy surface during initial oxidation and incubation. The spinel layer remains next to the liquid alloy during bulk oxide growth and is the eventual repository for most of the magnesium in the original alloy. Metal microchannels developed during incubation continuously supply alloy through the composite to the reaction interface. During the growth process, a layered structure exists at the upper extremity of the composite, consisting of MgO at the top surface, MgAl2O4 (probably discontinuous), Al alloy, and finally the bulk Al2O3 composite containing microchannels of the alloy. The bulk oxide growth mechanism appears to involve continuous formation and dissolution of the Mg-rich oxides at the surface, diffusion of oxygen through the underlying liquid metal, and epitaxial growth of Al2O3 on the existing composite body. The roles of Mg and Si in the composite growth process are discussed.
TL;DR: For Y1Ba2Cu3O7, using only reported monocrystal measurements and some analysis-theory, the authors estimated the complete nine-component orthotropic-symmetry elastic-stiffness matrix, the Voigt Cij matrix Comparison with very high-frequency tetragonal symmetry phonon dispersion results shows good agreement (9% on average), except for C12.
Abstract: For Y1Ba2Cu3O7, using only reported monocrystal measurements and some analysis–theory, we estimated the complete nine-component orthotropic-symmetry elastic-stiffness matrix, the Voigt Cij matrix Comparison with very-high-frequency tetragonal-symmetry phonon-dispersion results shows good agreement (9% on average), except for C12
TL;DR: Two types of platinum group metal particles were found in borosilicate nuclear waste glasses: needle-shaped RuO2 particles and spherical PdRhxTey alloys as mentioned in this paper.
Abstract: Two types of platinum group metal particles were found in borosilicate nuclear waste glasses: needle-shaped RuO2 particles and spherical PdRhxTey alloys. They form a dense sediment of high electrical conductivity and relatively high viscosity at the bottom of the ceramic melting furnace. The sludge shows a non-Newtonian flow behavior. The viscosity and conductivity of the sludge depend not only on the platinum group metal content but also on the texture and morphology of the RuO2 particles. RuO2 forms long, needle-shaped crystals which are caused by alkalimolybdate salt melts that formed in the calcine layer. The salt melts oxidize the Ru present as small RuO2 particles after calcination to higher oxidation states. Ruthenium (VI) compounds are formed, presumably, which are not stable with respect to RuO2 under the melting conditions. RuO2 precipitates and crystallizes into long, needle-like particles.
TL;DR: The mechanism underlying hydrogen-induced embrittlement effect in FeAl has been investigated using a local density functional total energy approach in this article, where the bonding characteristics, the bond and cleavage strength between iron and aluminum layers, and the surface energy with and without interstitially absorbed H were calculated from first-principles bandstructure and atomic-cluster methods.
Abstract: The mechanism underlying the hydrogen-induced embrittlement effect in FeAl has been investigated using a local density functional total-energy approach. The bonding characteristics, the bond and cleavage strength between iron and aluminum layers, and the surface energy with and without interstitially absorbed H are calculated from first-principles band-structure and atomic-cluster methods. Our unique combination of techniques permits the simultaneous study of the metallic and localized bonding effects on an equal footing. Results from this study show that FeAl (in the absence of H) is intrinsically highly resistant to cleavage fracture in terms of the high theoretical cleavage strength. Hydrogen locally dilates the Fe–Al lattice, and this is accompanied by a sizable decrease in Fe–Al cleavage (or cohesive) strength. Our results suggest that the underlying mechanism of H-embrittlement in aluminides is a depletion of d-bonding charge on the Fe site resulting from the charge transfer from Fe to H. Results also indicate that the H-embrittlement effect is greater for H adsorbed in Fe-rich sites.
TL;DR: In this paper, two MPMG-processed YBaCuO materials with and without Y2BaCCuO5 (211) inclusions were investigated by transmission electron microscopy.
Abstract: Microstructures of two MPMG processed YBaCuO materials with and without Y2BaCuO5 (211) inclusions were investigated by transmission electron microscopy. Using the MPMG process, it is possible to change the quantity of the 211 inclusions in the YBa2Cu3O7 (123) matrix. We prepared two YBaCuO samples with 0 and 30 vol. % 211 and with respective critical current density values of 2000 and 30 000 A/cm2 at 77 K and 1 T (magnetic field parallel to the c-axis). As possible pinning centers, we found stacking faults in the 123 matrix. However, we observed no appreciable change in their number and structure by introducing the 211 inclusions. Therefore, the difference in Jc values can be attributed to the 211 inclusion itself.
TL;DR: In this paper, a two-step process was developed to form highly oriented thin films in material systems with dissimilar crystal structures and interatomic spacings, and the growth of highly (100) oriented thin film of cubic ZrO2 (25 mol % Y2O3) on (0001) single crystal substrates was demonstrated.
Abstract: A two-step process has been developed to form highly oriented thin films in material systems with dissimilar crystal structures and interatomic spacings. This processing method utilizes current polycrystalline thin film deposition techniques. In this method, a polycrystalline thin film is first deposited and heat treated to promote its breakup into isolated grains. The breakup process favors those grains that have a low substrate interfacial energy and so produces a film of highly oriented but isolated grains. In the second process step, another polycrystalline thin film is deposited. The remnant grains act as seeds for the growth of a highly oriented thin film. The process is demonstrated through the growth of highly (100) oriented thin films of cubic ZrO2 (25 mol % Y2O3) on (0001) Al2O3 single crystal substrates, a material system in which film and substrate have dissimilar structures and interatomic spacings. Implications for the growth of epitaxial films using this method are discussed.
TL;DR: In this article, solid state reactions between SiC ceramics and Co, Ni, and Pt metals have been studied at temperatures between 800 and 1200 °C for various times under He or vacuum conditions.
Abstract: Solid state reactions between SiC ceramics and Co, Ni, and Pt metals have been studied at temperatures between 800 and 1200 °C for various times under He or vacuum conditions. Reactions between the metals and SiC were extensive above 900 °C. Various metal silicides and carbon precipitates were formed in layered reaction zones. Interfacial melting was also observed at certain temperatures; teardrop-shaped reaction zones, porosity, and dendritic microstructure resulting from melting/solidification were evident. The metal/ceramic interfaces exhibited either planar or nonplanar morphologies, depending upon the nature of the metal/ceramic reactions. Concave interfacial contours were observed when interfacial melting occurred. By contrast, planar interfaces were observed in the absence of interfacial melting. In all cases, the decomposition of SiC was sluggish and may serve as a rate limiting step for metal/ceramic reactions. Free unreacted carbon precipitates were formed in all the reaction zones and the precipitation behavior was dependent upon the metal system as well as the location with respect to the SiC reaction interface. Modulated carbon bands, randomly scattered carbon precipitates, and/or carbon-denuded bands were formed in many of the reaction zones, and the carbon existed in a mixed state containing both amorphous and graphitic forms.
TL;DR: In this paper, the formation of tetragonal-like mullite coincides with that of the Al-Si spinel, which is determined mainly by the processing conditions of the sol-gel precursors.
Abstract: The mullite formation process in both single phase and diphasic sol-gel precursors to mullite was studied using dynamic x-ray diffraction (DXRD). A metastable, tetragonal-like mullite phase was observed in all the single gels at temperatures from 980 °C to 1200 °C, but not in any of the other precursors. The tetragonal to orthorhombic mullite transformation was very slow as the lattice parameters, a and b, split and moved gradually away from each other as a result of a gradual decrease of alumina content in the mullite solid solution with increasing temperature from 1100 °C to 1200 °C. The formation of tetragonal mullite coincides with that of the Al–Si spinel. The occurrence of tetragonal mullite or the spinel (or both) is determined mainly by the processing conditions of the sol-gel precursors.
TL;DR: In this article, transmission electron microscopy (TEM) and conversion electron Moessbauer spectroscopy (CEMS) have been used to determine the structure of {alpha}--Al{sub 2}O{sub 3} implanted with iron at room temperature.
Abstract: Rutherford backscattering (RBS)-ion channeling, transmission electron microscopy (TEM), and conversion electron Moessbauer spectroscopy (CEMS) have been used to determine the structure of {alpha}--Al{sub 2}O{sub 3} implanted with iron at room temperature. Changes produced by post-implantation annealing in oxidizing and reducing atmospheres were followed using the same techniques. Implantation of 160 keV Fe at room temperature produces a damaged but crystalline microstructure for fluences as high as 1{times}10{sup 17} Fe/cm{sup 2}. The iron resides in a variety of local environments: three Fe{sup 2+} components, one Fe{sup 0} component, and two Fe{sup 4+} components. The relative amount of each component varies with implantation fluence. Only the Fe{sup 0} component seems to be associated with second-phase formation. In this case, 2 nm diameter {alpha}-iron particles were detected by TEM studies. Recovery of implantation-induced disorder in the Al- and oxygen-sublattices occurs in two stages for annealing in oxygen and in one continuous stage for hydrogen-annealing. The end state for iron is Fe{sup 3+} for oxygen anneals and Fe{sup 0} for hydrogen anneals. The precipitated phases observed are those to be expected from the equilibrium phase diagrams.
TL;DR: In this article, the effect of electron-ion interactions on the dynamics of the thermal spike in Cu displacement cascades has been investigated with a modified embedded atom method potential. And the results point toward the important effect that inelastic energy losses might have on the dynamic stability of the subcascade energy regime where the lifetime of the spike is expected to exceed the electron-phonon coupling time.
Abstract: We present results of a molecular dynamics simulation study of the effect of electron-ion interactions on the dynamics of the thermal spike in Cu. Interatomic forces are described with a modified embedded atom method potential. We show that the electron-ion interaction acts to reduce the lifetime of the thermal spike and therefore the amount of atomic rearrangement that takes place in energetic displacement cascades in Cu. The results point toward the important effect that inelastic energy losses might have on the dynamics of displacement cascades in the subcascade energy regime where the lifetime of the thermal spike is expected to exceed the electron-phonon coupling time.
TL;DR: In this paper, polycrystalline diamond thin films have been deposited on single-crystal silicon substrates at low temperatures (not above 600 C) using a mixture of hydrogen and methane gases by high-pressure microwave plasma-assisted chemical vapor deposition.
Abstract: Polycrystalline diamond thin films have been deposited on single-crystal silicon substrates at low temperatures (not above 600 C) using a mixture of hydrogen and methane gases by high-pressure microwave plasma-assisted chemical vapor deposition. Low-temperature deposition has been achieved by cooling the substrate holder with nitrogen gas. For deposition at reduced substrate temperature, it has been found that nucleation of diamond will not occur unless the methane/hydrogen ratio is increased significantly from its value at higher substrate temperature. Selective deposition of polycrystalline diamond thin films has been achieved at 600 C. Decrease in the diamond particle size and growth rate and an increase in surface smoothness have been observed with decreasing substrate temperature during the growth of thin films. As-deposited films are identified by Raman spectroscopy, and the morphology is analyzed by scanning electron microscopy.
TL;DR: Ion channeling and transmission electron microscopy (TEM) techniques were used to examine the microstructure of single-crystal Y2O3 stabilized cubic cubic zirconia (YSZ) after implantation with 240 keV Xe+ ions.
Abstract: Ion-channeling and transmission electron microscopy (TEM) techniques were used to examine the microstructure of single-crystal Y2O3 stabilized cubic zirconia (YSZ) after implantation with 240 keV Xe+ ions. The observed microstructure was related to Knoop indentation hardness measurements. These measurements showed an increase in hardness for low ion-doses, reaching some maximum value, then a decrease in hardness at higher doses. In the hardening regime, below 7.5 × 1015 Xe+/cm2, point defects and dislocation networks were observed by TEM. Ion-channeling showed a corresponding increase in damage as a function of ion-dose. For doses between 7.5 × 1015 and 3 × 1016 Xe+/cm2 the hardness falls, and the amount of damage, measured with ion-channeling, reaches a limiting value at less than complete damage. In this dose range the Xe concentration continues to increase beyond the dose where the amount of damage saturates. For high doses, greater than 3 × 1016 Xe+/cm2, where softening of the zirconia occurs, additional reflections appear in the electron diffraction pattern that are consistent with the lattice parameter of solid Xe. A diffuse ring is also visible; this is believed to be due to the presence of fluid Xe. Both ion-channeling and TEM show that a significant amount of monocrystalline zirconia remains even up to doses of 1 × 1017 Xe+/cm2. There is also evidence for the presence of recrystallized zirconia at the high doses. Since so much crystalline material remains, it seems that amorphization of the zirconia is not the dominant cause of the softening at high doses.