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.     

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.     

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.     

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.     

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.     

Initiation of Self-Propagating Combustion Waves in Dense Mo + 2Si Reactants through Field-Activation

Because of kinetic limitations, self-sustaining combustion synthesis reactions cannot be initiated in dense powder compacts. In compacts of Mo + 2Si, self-propagating waves can be initiated in samples with less than 78% relative density. At this and higher densities, no waves could be initiated without field-activation. In the presence of an electric field (at values of 7 and 13 V·cm^-1), reactant compacts with densities up to 95% could sustain a combustion wave to produce MoSi₂. In the absence of a field (for lower-density samples) the wave propagated in a non-steady-state (pulsating) mode, while under the influence of a field the wave propagated in a steady-state mode. The dependences of wave velocity and combustion temperature on the relative density of the reactants were qualitatively similar, showing maxima at a relative density of about 65%. These observations are explained in terms of the contribution of a liquid phase in the MoSi₂-Mo₅Si₃ binary to the synthesis kinetics. Although not detected by X-ray diffraction analysis, small amounts of Mo₅Si₃ were discerned at the grain boundaries of the MoSi₂ product. The particle size of the silicide synthesized from 95% dense reactants was significantly smaller than those synthesized from reactants with lower densities, but the reason for this observation is not well understood at this time.     

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.     

Dense Layered Molybdenum Disilicide–Silicon Carbide Functionally Graded Composites Formed by Field-Activated Synthesis

Dense, layered, single- and graded-composition composites of MoSi₂ and SiC were formed from elemental powders in one step, using the field-activated pressure-assisted combustion method. Compositions ranging from 100% MoSi₂ to 100% SiC were synthesized, with relative densities ranging from 99% to 76%, respectively. X-ray diffractometry results indicated the formation of pure phases when the concentration of MoSi₂ was high and the appearance of a ternary phase, Mo_4.8Si₃C_0.6, when the concentration of SiC was high. Electron microprobe analysis results showed the formation of stoichiometric and uniformly distributed phases. A layer-to-layer variation in composition of 10 mol% was sufficient to prevent thermal cracking during formation of the layered functionally graded materials.     

Processing Temperature Effects on Molybdenum Disilicide

A series of MoSi₂ compacts were fabricated at increasing hot-pressing temperatures to achieve different grain sizes. The materials were evaluated by Vickers indentation fracture to determine room-temperature fracture toughness, hardness, and fracture mode. From 1500° to 1800°C, MoSi₂ had a constant 67% transgranular fracture and linearly increasing grain size from 14 to 21 μm. Above 1800°C, the fracture percentage increased rapidly to 97% transgranular at 1920°C (32-μm grain size). Fracture toughness and hardness decreased slightly with increasing temperature. MoSi₂ processed at 1600°C had the highest fracture toughness and hardness values of 3.6 MPa.m^1/2 and 9.9 GPa, respectively. The effects of SiO₂ formation from oxygen impurities in the MoSi₂ starting powders and MoSi₂–Mo₅Si₃ eutectic liquid formation were studied.     

Processing, Microstructures, and Properties of Molybdenum Aluminosilicide

A ternary intermetallic compound-molybdenum aluminosilicide, Mo(Si_0.8,Al_0.2)₂-was synthesized via micropyretic synthesis. The reaction constituents were elemental molybdenum, silicon, and aluminum powders in an atomic mixture of Mo + 1.6Si + 0.4Al. With the appropriate processing parameters, a highly dense product (up to 94% theoretical density) was obtained in a short time without applying external pressure. The synthesized product had a uniform microstructure, with a grain size of ∼16 μm. X-ray diffractometry and energy-dispersive spectroscopy confirmed the formation of Mo(Si_0.8,Al_0.2)₂. In addition, small amounts of secondary phases (Mo₅Si₃ and Al₂O₃) were also identified in the product. The synthesized Mo(Si_0.8,Al_0.2)₂ was then subjected to various property measurements, including elastic moduli, Vickers hardness, indentation fracture toughness, and oxidation resistance. The measured property data, whenever possible, were compared to the corresponding values for MoSi₂.     

Tribological Behavior of Mo5Si3-Particle-Reinforced Silicon Nitride Composites

The tribological behavior of Mo₅Si₃-particle-reinforced silicon nitride (Si₃N₄) composites was investigated by pin-on-plate wear testing under dry conditions. The friction coefficient of the Mo₅Si₃–Si₃N₄ composites and Si₃N₄ essentially decreased slowly with the sliding distance, but showed sudden increase for several times during the wear testing. The average friction coefficient of the Si₃N₄ decreased with the incorporation of submicrometer-sized Mo₅Si₃ particles and also as the content of Mo₅Si₃ particles increased. When the Mo₅Si₃–Si₃N₄ composites were oxidized at 700°C in air, solid-lubricant MoO₃ particles were generated on the surface layer. Oxidized Mo₅Si₃–Si₃N₄ composites showed self-lubricating behavior, and the average friction coefficient and wear rate of the oxidized 2.8 wt% Mo₅Si₃–Si₃N₄ composite were 0.43 and 0.72 × 10⁻⁵ mm³ (N·m)⁻¹, respectively. Both values were ∼30% lower than those for the Si₃N₄ tested in an identical manner.     

Microstructure of Molybdenum Disilicide-Silicon Carbide Nanocomposite Thin Films

Composite thin films of molybdenum disilicide-silicon carbide (MoSi₂-SiC) have been deposited via rf magnetron sputtering onto molybdenum substrates. An intermediate layer was deposited in the presence of nitrogen gas and evaluated as a potential diffusion barrier layer. The composite films have been characterized using X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and Auger electron spectroscopy. The as-deposited films were amorphous but crystallized into nanometer-sized grains after annealing under vacuum at 1000°C for 30 min. There was a significant amount of interdiffusion between the film and substrate, which resulted in the formation of subsilicides such as Mo₅Si₃ and MoSi₃, as well as Mo₂C. The films that were deposited via reactive sputtering in a nitrogen ambient were amorphous in both the as-deposited and annealed conditions. Significantly fewer second phases were detected with the presence of the intermediate layer, which suggests the potential use of the nitrided (MoSi _x N _y C _z ) layer as a high-temperature diffusion barrier layer for the silicon and carbon.     

Oxidation Behavior of Boron-Modified Mo5Si3 at 800°–1300°C

Mo₅Si₃ shows promise as a high-temperature creep-resistant material. The high-temperature oxidation resistance of Mo₅Si₃ has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo₅Si₃ exhibits pest oxidation at 800°C. Mass loss occurs in the temperature range 900°–1200°C due to volatilization of molybdenum oxide, indicating that the silica scale that forms does not provide a passivating layer. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1050°–1300°C. The oxidation rate of Mo₅Si₃ was decreased by 5 orders of magnitude at 1200°C by doping with less than 2 wt% boron. Boron doping eliminates catastrophic pest oxidation at 800°C. The mechanism for improved oxidation resistance of borondoped Mo₅Si₃ is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.     

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.     

Fabrication and High-Temperature Phase Stability of Mo(Al,Si)2—MoSi2 Intermetallics

A two-step processing technique was used to make dense, homogeneous intermetallics in the Mo(Al,Si)₂—MoSi₂ system. A variation of self-propagating high-temperature synthesis was used, in which starting pellets were nucleated at room temperature to make intermetallic powders from metallic precursors, followed by uniaxial hot pressing at 1600°—1800°C to achieve densification. The samples were held at the hot-pressing temperatures for several hours; therefore, this study also provided qualitative phase-stability information. The solubility limit of Al for Si in MoSi₂ was <5% at 1800°C. Samples that had 10% Al substituted for Si yielded approximately equal amounts of MoSi₂ and Mo(Al,Si)₂.     

Carbon Additions to Molybdenum Disilicide: Improved High-Temperature Mechanical Properties

C addition (2 wt%) to MoSi₂ acted as a deoxidant, removing the otherwise ubiquitous siliceous grain boundary phase in hot-pressed samples, and causing formation of SiC and Mo₅Si₃C₁ (a variable-composition Nowotny phase). Both hardness and fracture toughness of the C-containing alloy were higher than those of the C-free (and oxygen-rich) material; more significantly, the fracture toughness of the MoSi₂+ 2% C alloy increased from 5.5 MPa·m^1/2 at 800°C to ∼11.5 MPa·m^1/2 at 1400°C.     

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.     

Post-Hot-Pressing and High-Temperature Bending Strength of Reaction-Bonded Silicon Nitride-Molybdenum Disilicide and Silicon Nitride-Tungsten Silicide Composites

A hot-pressing technique was used for the further densification of reaction-bonded silicon nitride-molybdenum disilicide and silicon nitride-tungsten silicide (Si₃N₄-MoSi₂ and Si₃N₄-WSi₂, respectively) compacts that were prepared via a presintering step and a nitriding process from silicon-molybdenum or silicon-tungsten powders. After hot pressing was performed at 1650°C (25 MPa for 1 h), most of the alpha-Si₃N₄ that formed during the reaction-bonding process was transformed to β-Si₃N₄ and, moreover, a very small amount of Mo₅Si₃ (W₅Si₃) was formed in addition to MoSi₂ (WSi₂). Three- and four-point bend tests were performed at room temperature (25°C), 1000°C, 1200°C, and 1400°C. The bend strength of the Si₃N₄-WSi₂ composite increased slightly from room temperature up to 1000°C, whereas the Si₃N₄-MoSi₂ composite showed a more-pronounced increase up to 1200°C. Microstructural analysis was performed on the fracture surfaces of both composites that were tested at different temperatures.     

Kinetics and Products of Molybdenum Disilicide Powder Oxidation

In this study, we investigated the kinetics and products of the oxidation of MoSi₂ powder with an average particle size of 1.6 μm at 900°, 1000°, and 1100°C, using a small sample size of 0.5 g. Such a small sample size allowed us to minimize the effect of oxygen transportation through the powder volume, while maintaining a good relative weighing accuracy. X-ray diffraction of oxidized samples indicated the formation of Mo₅Si₃ and Mo metal. Analysis of the oxidation kinetics suggested that gaseous MoO₃ formed initially and amorphous SiO₂ film later. The oxidation kinetics and products observed in this study differ from those reported in an early study, in which a larger sample size was used.     

Oxidation Kinetics and Composite Scale Formation in the System Mo(Al,Si)2

The oxidation kinetics of hot-pressed Mo(Al_0.01Si_0.99)₂ and Mo(Al_0.1Si_0.9)₂ were measured at 480°C, and between 1200° and 1600°C. The qualitative oxidation of arc-melted Mo(Al_0.1Si_0.9)₂, Mo(Al_0.3Si_0.7)₂, Mo(Al_0.5Si_0.5)₂, and Mo₃Al₈ was examined after 600°C for 1000 h in air. At all temperatures, the compositional difference between the materials yielded very different oxidation rates and scale microstructures. At 1400° and 1500°C, microstructural evolution of the oxide scales resulted in improved oxidation resistance at long times (>400 h). At these temperatures, a significant reduction in the long-time oxidation kinetics was correlated with the in situ formation of an inner mullite scale. At 480° and 600°C, oxidation resistance improved significantly with increasing aluminum concentration. Contrary to the behavior of MoSi₂, samples of Mo(Al_0.01Si_0.99)₂ did not demonstrate catastrophic oxidation, and samples of Mo(Al_0.1Si_0.9)₂ were very oxidation resistant.