Microstructural Characterization of Sintered MoSi2/SiCP Composites

A MoSi₂/SiC_P composite was synthesized by in situ reactive sintering of a mixture of molybdenum, silicon, and carbon powders. Its microstructural features were studied by X-ray energy dispersive spectroscopy (EDS), conventional transmission electron microscopy (CTEM), and high-resolution transmission electron microscopy (HREM). It was determined that the composite was composed of α-MoSi₂ and β-SiC. There were no specific orientation relationships between the MoSi₂ matrix and SiC_P, because the MoSi₂ and SiC were formed at 1450°C by the reaction of solid Mo and C and liquid Si. The abrupt change occurring in the microstructure of the composite is explained by the presence of an interface between MoSi₂ and SiC_P, where no observable SiO₂ amorphous layer or particles were found. Microtwins and stacking faults were frequently observed in {111} planes of SiC_P.     

Simultaneous Synthesis and Densification of MoSi2 by Field-Activated Combustion

A method to simultaneously synthesize and consolidate MoSi₂ from powders of Mo and Si was investigated. Combustion synthesis was carried out under the combined effect of an electric field and mechanical pressure. Highly dense molybdenum silicide up to (99.2%) was produced from elemental powders in one step. Minor amounts of Mo₅Si₃ were present at the boundaries of MoSi₂ grains in the interior of samples made from stoichiometric reactants. The addition of 2.5 mol% Si excess, however, resulted in Mo₅Si₃-free, dense MoSi₂ products.     

Pb-Diffusion Barrier Layers for PbTiO3 Thin Films Deposited on Si Substrates by Metal Organic Chemical Vapor Deposition

The interfaces between metal organic chemical vapor deposited PbTiO₃ thin films and various diffusion barrier layers deposited on Si substrates were investigated by transmission electron microscopy. Several diffusion barrier thin films such as polycrystalline TiO₂, amorphous TiO₂, ZrO₂, and TiN were deposited between the PbTiO₃ thin film and Si substrate, because the deposition of PbTiO₃ thin films on bare Si substrates produced Pb silicate layers at the interface irrespective of the deposition conditions. The TiO₂ films were converted to PbTiO₃ by their reaction with diffused Pb and O ions during PbTiO₃ deposition at a gubstrate temperature of 410°C. Further diffusion of Pb and O induces formation of a Pb silicate layer at the interface. ZrO₂ did not seem to react with Pb and O during PbTiO₃ deposition at the same temperature, but the Pb and O ions that diffused through the ZrO₂ layer formed a Pb silicate layer between the ZrO₂ and Si substrate. The TiN films did not seem to react with Pb and O ions during the deposition of PbTiO₃ at 410°C, but reacted with PbTiO₃ to form a lead-deficient pyrochlore during postdeposition rapid thermal annealing at 700°C. However, TiN could effectively block the diffusion of Pb and O ions into the Si substrate and the formation of Pb silicate at the interface.     

High-Temperature Oxidation of Molybdenum Disilicide

The oxidation of MoSi₂ in air at atmospheric pressure was studied by electron diffraction, X-ray diffraction, and thermogravimetric analyses. The oxidation process occurs in two parts: (1) formation of MoO₃ and SiO₂ at temperatures below the boiling point of MoO₃, and (2) formation of Mo₅Si₃ and SiO₂ at higher temperatures. Evidence is presented which indicates that oxygen permeation through a silica layer, which may be of a mixed crystalline-glassy nature, controls reaction rate at high temperatures and that Mo₅Si₃ is present directly beneath the protective oxide. The activation energy for oxidation of MoSi₂ above 1200°C was calculated as 81.3 kcal mole⁻¹.     

Molten Glass Corrosion Resistance of Immersed Combustion-Heating Tube Materials in E-Glass

The corrosion resistance of molybdenum, molybdenum disilicide, and a SiC_{( p )}/Al₂O₃ composite to molten E-glass at 1550^deg;C was studied. Mo showed no tendency to oxidize as it was immersed in soda-lime silicate glass in a parallel study. MoSi₂ was corroded by soluble molecular oxygen, leaving a Mo₅Si₃ interface behind. The SiC_{( p )}/Al₂O₃ composite was corroded at a more rapid rate wherein the SiC component was oxidized to form amorphous silica and CO bubbles. Based on these results, the activity of soluble molecular oxygen in E-glass was determined to be in the range of 2.4 × 10^-14 to 2.0 × 10^-8.     

Reactive Hot Pressing of SiC/MoSi2 Nanocomposites

Dense SiC/MoSi₂ nanocomposites were fabricated by reactive hot pressing the mixed powders of Mo, Si, and nano-SiC particles coated homogeneously on the surface of Si powder by polymer processing. Phase composition and microstructure were determined by methods of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectrometry. The nanocomposites obtained consisted of MoSi₂, β-SiC, less Mo₅Si₃, and SiO₂. A uniform dispersion of nano-SiC particles was obtained in the MoSi₂ matrix. The relative densities of the monolithic material and nanocomposite were above 98%. The room-temperature flexural strength of 15 vol% SiC/MoSi₂ nanocomposite was 610 MPa, which increased 141% compared with that of the monolithic MoSi₂. The fracture toughness of the nanocomposite exceeded that of pure MoSi₂, and the 1200°C yield strength measured for the nanocomposite reached 720 MPa.     

High-Temperature Behavior of MoSi2 and Mo5Si3

The high-temperature stability and behavior of MoSi₂ was studied by heating dense sintered specimens under a vacuum of 10⁻⁵ mm Hg in the temperature range 1700° to 2000°C. The resulting material was examined using physical measurements, X-ray analysis, and metallographic techniques. The decomposition of MoSi₂ into Mo₅Si₃ is described. The Mo₅Si₃-MoSi₂ eutectic temperature was determined as 1900° C, and the melting points of MoSi₅ and Mo₅Si₃ were determined as 1980° and 2085° C, respectively.     

Processing of Molybdenum Disilicide Using a New Reactive Vapor Infiltration Technique

Processing of MoSi₂ via a lower-temperature reactive vapor infiltration route is explored. In this process a loosely compacted molybdenum powder is exposed to a gaseous silicon precursor. Initially, a surface MoSi₂ layer forms and, as the process proceeds, a distinct reaction front moves progressively inward. At 1200°C, the silicide layer grows at a rate of about 20 μm/h. The MoSi₂ layer is of the high-temperature tetragonal phase. No excess silicon or molybdenum is present, unlike in chemically vapor deposited molybdenum silicide. Irrespective to the volumetric increases involved in siliciding molybdenum to MoSi₂, none of the samples, silicided between 1100° and 1300°C, show swelling or surface cracks.     

Formation Kinetics of MoSi2 and Mo5Si3 by the Reactive Diffusive Siliciding of Molybdenum

The formation kinetics of products formed by the reaction between dense molybdenum and vapor-supplied silicon at an activity approximating that of solid silicon under open flowing gas conditions was studied at 1200°C. An outer MoSi₂ layer overlaid the much thinner Mo5Si3 that formed on the molybdenum. Both phases obeyed parabolic growth laws over a 22 h period, having parabolic rate constants of 6.8 × 10⁻¹⁰ cm²/s for the MoSi₂ and 1.3 × 10⁻¹³ cm²/s for the Mo₅Si₃ phases. These results were 2 orders of magnitude less than prior results, mostly obtained by another processing route. Possible explanations include enhanced growth rates from chemical contamination. Gross distortion and abnormal layer thicknesses at specimen edges and the 159% volume increase during siliciding suggest that the kinetics also are strain dependent.     

High-Temperature Multilayer Composites with Superplastic Interlayers

A high-temperature multilayer composite (MLC) with hot hard layers and superplastic layers was proposed in this communication. The hard layer can provide the MLC high-temperature strength; the superplastic layer can deform plastically at high temperatures, disperse the applied stress, and stop the crack from advancing. Such an MLC was prepared via tape casting in the Al₂O₃/MoSi₂+Mo₂B₅ system in the present work; in this system, Al₂O₃ was the hard layer and MoSi₂+Mo₂B₅ was the superplastic layer. The microstructures and the stress-displacement behaviors of the MLCs were investigated. Finally, the design rules for the high-temperature MLCs were discussed briefly.     

Aluminum Infiltration into Molybdenum Silicide Preforms

The presence of Mo₅Si₃ in MoSi₂ preforms hinders the reactive infiltration of aluminum. To understand the role of Mo₅Si₃, the kinetics of aluminum infiltration into pure Mo₅Si₃ is studied. Irrespective of the initial composition (MoSi₂ or Mo₅Si₃) of the preform, the final product always contains Mo(Al,Si)₂. However, the aluminum content in the two cases is different: when the preform is MoSi₂, the aluminum content is 14–18 at.%, and, when the preform is Mo₅Si₃, the aluminum content is 25–27 at.%. The activation energy for the reactive infiltration of aluminum into the Mo₅Si₃ preform is ∼26 kJ/mol.     

Influence of Molybdenum Silicide Additions on High-Temperature Oxidation Resistance of Silicon Nitride Materials

The influence of additions of molybdenum disilicide (MoSi₂) on the microstructure and the mechanical properties of a silicon nitride (Si₃N₄) material, with neodymium oxide (Nd₂O₃) and aluminum nitride (AIN) as sintering aids, was studied. The composites, containing 5, 10, and 17.6 wt% MoSi₂, were fabricated by hot pressing. All materials exhibited a similar phase composition, detected by X-ray diffractometry. Up to MoSi₂ additions of 10 wt%, mechanical properties such as strength, fracture toughness, or creep at 1400°C were not affected significantly, in comparison to that of monolithic Si₃N₄. The oxidation resistance of the composites, in terms of weight gain, degraded. After 1000 h of oxidation at 1400° and 1450°C in air, a greater weight gain (by a factor of approximately three) was obtained, in comparison to that of the material without MoSi₂. Nevertheless, after 1000 h of oxidation, the degradation in strength of the composites was considerably less severe than that of the material without MoSi₂. An additional layer was formed, caused by processes at the surface of the Si₃N₄ material, preventing the formation of pores, cracks, or glassy-phase-rich areas, which are common features of oxidation damage in Si₃N₄ materials. This surface layer, containing Mo₅Si₃ and silicon oxynitride (Si₂ON₂), was the result of reactions between MoSi₂, Si₃N₄, and the oxygen penetrating by diffusion into the material during the hightemperature treatment.     

Evaporation of Silicon from Molybdenum Silicides at High Temperature and in Hard Vacuum

The nature of the degradation of molybdenum disilicide coatings on molybdenum at 3110° to 4000°F in hard vacuum (10⁻⁷ torr) was investigated. Degradation occurred as a selective silicon loss by the successive transformations: MoSi₂→Mo₅Sia →Mo₃Si →Mo. Comparison of estimated emittance values with surface composition of the coating indicated an abrupt decrease in emittance from 0.5 to 0.3 corresponding to the transformation Mo₅Si₃→Mo₃Si.     

Stability of Molybdenum Disilicide in Combustion Gak Environments

The stability of MoSi₂ in combustion gas at 1370° and 1600°C was evaluated using SOLGASMIX-PV thermodynamic modeling, periodic weight measurements, and characterization via XRD, SEM, EDS, and image analysis. Passive oxidation occurred at both temperatures. During an initial stage of exposure, specimen surfaces oxidized to form MoO_3(g) and amorphous SiO₂ via reduction of CO₂ and H₂O. After a short time (<6.5 min at 1370°C, <1 min at 1600°C), the oxidation mechanism switched; Mo₅Si₃ and amorphous SiO₂ formed as oxidation products. The first mechanism esulted in the formation of 46.1 vol% at 1370°C and 42.6 vol% at 1600°C of the amorphous silica surface coating. The attainment of a near-terminal weight gain implied silica formation was limited by H₂O and CO₂ diffusion through the silica coating.     

Molten Glass Corrosion Resistance of Immersed Combustion-Heating Tube Materials in Soda-Lime-Silicate Glass

The corrosion resistance of molybdenum, molybdenum disilicide, and a SiC_(p)/Al₂O₃ composite to molten soda-lime-silicate glass was studied. The ASTM-C621–84 corrosion test method was modified because of inherent inaccuracies in the method and Si attack of platinum crucibles. Specimen-glass interfacial regions were characterized using XRD, SEM, and EDS. After 48 h of exposure at 1565°C, the half-down corrosion recessions of Mo, MoSi₂, and SiC_(P)/Al₂O₃ were 0.11, 0.316, and 0.26 mm, respectively. Mo oxidized to form a MoO₂ surface scale which cracked, allowing glass seepage and further oxidation. Silicon was leached out of MoSi₂ into the glass, leaving a Mo₅Si₃ interface and particles of Mo near the interface. For the SiC_(P)/Al₂O₃ composite, bubbles observed at the interfacial regions formed from oxidation of SiC to form CO. Thermodynamic modeling corroborated these experimental observations.     

Control of the Interfacial Reactions in Nb-Toughened MoSi2

Toughening of MoSi₂ for high-temperature applications can be achieved by incorporating ductile refractory-metal reinforcements, provided that a coating is applied to prevent interdiffusion and reaction between the matrix and the reinforcements. In the present study, three different coating techniques for applying a thin Al₂O₃ film on Nb reinforcements as a diffusion barrier have been studied. The techniques consisted of (i) sol-gel coating; (ii) physical vapor deposition (PVD); (iii) hot dipping in molten Al, followed by anodizing Al to form Al₂O₃. The processing parameters for the techniques were evaluated and the effectiveness of each coating as a diffusion barrier was assessed. For the present MoSi₂ matrix which contains SiO₂, PVD coatings provided the most effective diffusion barrier for processing MoSi₂/Nb composites.     

Electrical Properties of the Ta-RuO2 Diffusion Barrier for High-Density Memory Devices

The electrical properties of a Ta layer prepared with and without RuO₂ addition were investigated. The Ta + RuO₂/TiSi₂/poly-Si/SiO₂/Si contact system exhibited lower total resistance and ohmic characteristics up to 800°C. Meanwhile, the Ta/TiSi₂/poly-Si/SiO₂/Si contact system showed higher total resistance and nonohmic behavior after annealing at 650°C, attributed to the oxidation of both Ta and TiSi₂ layers. In the former case, a Ta + RuO₂ diffusion barrier showed an amorphous Ta microstructure and embedded RuO _x nanocrystals in the as-deposited state. The conductive RuO₂ crystalline phase in the Ta + RuO₂ film was formed by reaction between the nanocrystalline RuO _x and oxygen indiffused from air during annealing. When the Ta layer was deposited with RuO₂ addition, therefore, both the electrical properties and the oxidation resistance of the Ta + RuO₂ diffusion barrier were better than those of TiN, TaN, and Ta-Si-N barriers.     

Transmission Electron Microscopy Characterization of Hot-Pressed ZrB2 with MoSi2 Additive

Microstructure of the hot-pressed ZrB₂ with MoSi₂ additive was investigated by transmission electron microscopy (TEM). The effect of MoSi₂ addition on the microstructure of the ceramic was assessed. For the pure ZrB₂, the microstructure consisted of the equiaxed ZrB₂ grains and a few elongated ZrB₂ grains. For the ZrB₂ with MoSi₂ additive, the microstructure consisted almost entirely of equiaxed ZrB₂ grains. A few dislocations were present in the ZrB₂ grains. In addition, high-resolution TEM observations showed that the intergranular amorphous phase was absent at two ZrB₂ grain boundaries in the ZrB₂ with MoSi₂ additive.     

Thermal Oxidation Kinetics of MoSi2-Based Powders

The oxidation process of MoSi₂ is very complex, and controversial results have been reported, especially for the early-stage oxidation before the formation of passive SiO₂ film. Most oxidation studies have been carried out on bulk consolidated samples, and the early stage of oxidation has not been studied. In this investigation, very fine MoSi₂ powder with an average particle size of 1.6 μm was used. Such a fine particle size makes it easier to study the early stages of oxidation since a significant portion of the powder is oxidized before the formation of passive SiO₂ film. The oxidation kinetics of commercial MoSi₂-SiC and MoSi₂-Si₃N₄ powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500° to 1100°C in 0.14 atm of oxygen. X-ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO₃ phase and an associated weight gain at low temperatures (500° and 600°C). At temperatures higher than 900°C, Mo₅Si₃ phase formed first and was subsequently oxidized to solid SiO₂ and volatile MoO₃, resulting in an initial weight gain followed by subsequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi₂ and Mo₅Si₃ are proportional to their fractions in the system describes the experimental data well.     

Microstructure and Dielectric Properties of Textured SrTiO3 Thin Films

We investigate the relationship between microstructure and dielectric properties of textured SrTiO₃ thin films deposited by radio-frequency magnetron sputtering on epitaxial Pt electrodes on sapphire substrates. The microstructures of Pt electrodes and SrTiO₃ films are studied by transmission electron microscopy, atomic force microscopy, and X-ray diffraction. SrTiO₃ films grown on as-deposited and annealed Pt electrodes, respectively, consist of a mixture of (111)- and (110)-oriented grains. Temperature-dependent dielectric measurements show that differences in texture and microstructure are reflected in the Curie–Weiss behavior of the SrTiO₃ films. Phenomenological models that account for the effects of thermal mismatch strain on the dielectric behavior are developed for different film textures. The models predict that at a given temperature, paraelectric (111)-oriented films of SrTiO₃ on tensile substrates will have a higher Curie–Weiss temperature and a greater dielectric constant than (110)-oriented films or bulk SrTiO₃. The experimental dielectric behavior is compared with the predictions from theory, and different contributions, such as interfacial layers, film stress, and microstructure, to the Curie–Weiss behavior are discussed.