MoSi2 Phase Research Articles

On the invariant reactions in the Mo-rich portion of the Mo-Si system

The Mo-Si system presents two invariant reactions (eutectic and peritectic) in the Mo-rich region; these reactions involve the Liquid, Moss, Mo3Si, and Mo5Si3 phases. The results presented in the literature show disagreement about the specific reactions, with two proposals being reported: (1) L + Moss ⇔ Mo3Si and L ⇔ Mo3Si + Mo5Si3; and (2) L + Mo5Si3 ⇔ Mo3Si and L ⇔ Moss + Mo3Si. In order to contribute to this subject, we have produced several Mo-Si alloys (21 to 29 at.% Si) via arc melting and evaluated the as-cast microstructures through x-ray diffraction (XRD), scanning electron microscope/backscattered electron image (SEM/BSEI), and energy dispersive x-ray analysis (EDS). The primary phases identified in the different samples were Moss, Mo3Si, and Mo5Si3. The results have indicated clearly the existence of a eutectic reaction involving the phases Mo3Si and Mo5Si3, confirming the existence of the L + Moss ⇔ Mo3Si and L ⇔ Mo3Si + Mo5Si3 invariant reactions. In addition, alloys with composition 26 at.% Si and 27 at.% Si presented Mo3Si and Mo5Si3 as primary phases, respectively, indicating that the liquid eutectic composition is located between those values.

Improvement of impact properties for Nb/MoSi2 laminate composites by the interfacial modification (II)

The thermodynamical estimation of the interfacial reaction and the impact properties of Nb/MoSi2 laminate composites containing SiC, NbSi2 or ZrO2 particles are investigated. Laminate composites, which comprise alternating layers of MoSi2 with the particle and Nb foil, were fabricated by the hot press process. It is clearly found out that the interfacial reaction of Nb/MoSi2 can be controlled by the addition of ZrO2 particle to the MoSi2 phase. The addition of ZrO2 particle increases both the impact value and the sintered density of Nb/MoSi2. The suppression of the interfacial reaction is caused by the formation of ZrSiO4 in MoSi2−ZrO2 matrix mixture.

A study of MoSi2MoS2 coatings fabricated by SHS casting route

Coatings were fabricated on a steel substrate by a self-propagating high temperature synthesis (SHS) casting route. The processing is described in detail. The phases in the coating were examined using X-ray diffraction (XRD). The microstructures of the coatings were analyzed with optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA). Tribological properties and microhardness of the coatings with 6–12 wt.% MoS2 were measured. The experimental results show that the coatings were composed of MoSi2 and MoS2 phases and coatings with 6–12 wt.% MoS2 had dense microstructure. Metallurgical bonding had formed between the coatings and the substrate. Friction of the coatings with 6–12 wt.% MoS2 is lower than its steel substrate. Microhardness and wear resistance of the coatings with 6–12 wt.% MoS2 are higher than that of the steel substrate. Near the interface microhardness of the coating varies with the distance to the interface.

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.

Formation of C54–TiSi2 in titanium on nitrogen-ion-implanted (001)Si with a thin interposing Mo layer

Formation of TiSi2 in titanium on nitrogen-implanted (001)Si with a thin interposing Mo layer has been investigated. The presence of a Mo thin interposing layer was found to decrease the formation temperature of C54–TiSi2 by about 100 °C. A ternary (Ti, Mo)Si2 phase was found to distribute in the silicide layer. The ternary compound is conjectured to provide more heterogeneous nucleation sites to enhance the formation of C54–TiSi2. On the other hand, the effect of grain boundary for decreasing transformation temperature was found to be less crucial. For Ti/Mo bilayer on 30 keV BF2+ or As+ + 20 keV, 1 × 1015/cm2 N2+ implanted samples, a continuous C54–TiSi2 layer was found to form in all samples annealed at 650–950 °C. The presence of nitrogen atoms in TiSi2 is thought to lower the silicide/silicon interface energy and/or the silicide surface energy to maintain the integrity of the C54–TiSi2 layer at high temperatures.

Growth of silicides and interdiffusion in the Mo-Si system

Solid-solid diffusion couples assembled with disks of Mo and Si were annealed at selected temperatures, over the temperature range from 900 °C to 1350 °C, for the development of diffusion structure and determination of interdiffusion coefficients for the silicides of Mo. Layers of MoSi2 and Mo5Si3 were observed to form in the diffusion zone; the MoSi2 layer was one to two orders of magnitude larger in thickness than the Mo5Si3 layer. The MoSo2 layer developed a columnar microstructure with evidence of texture and preferential growth of grains in the direction of diffusion. The Si-to-Mo ratio, determined by microprobe analysis across the thickness of the MoSi2 layer, varied within the approximate range from 1.9 to 2.0. From the concentration profiles, integrated interdiffusion coefficients as well as energies of activation for interdiffusion were determined for the silicide layers. On the basis of the observed stoichiometric range for the MoSi2 phase, average values of the interdiffusion coefficients were also estimated. Relations are derived between the growth-rate constant and the integrated interdiffusion coefficient for the MoSi2 phase. The evaluated activation energies for interdiffusion in the MoSi2 and Mo5Si3 phases are 130±20 and 210±10 kJ/mol, respectively.

Fabrication of Intermetallic Compounds by Spark Plasma Sintering

The characteristics of spark plasma sintering, a new method of powder processing, were investigated. Four systems of intermetallic compounds—Ti-Al, Ti-Al-Cr, Mo-Si, and Mo-Si-Nb—were fabricated, and the formation process of compounds, the formed phases, and the microstructure of samples were observed. During the sintering of all the compositions of mixed powders, most of the compounds were formed by combustion reaction which occurred at almost the same temperature as the conventional combustion reaction temperature. The fabricated samples were well densified, however, the relative densities of the Mo-Si samples were lower than the Ti-Al samples. Ultrasonic images show that no internal defects were found in any sample and the grain size became finer with the increase in the Cr content in the Ti-Al system and Nb content in the Mo-Si system. The formed phases of Ti:Al=1:1 composition samples were TiAl and Ti3Al phases, and Ti-Al added Cr samples consisted of TiAl, Ti3Al, Cr2Al, and Cr9Al17 phases. The sample synthesized with Mo:Si=1:2 mixed powders had only MoSi2 phases, and Mo-Si samples with added Nb consisted of four phases: MoSi2 with a small amount of Mo5Si3 phases in the matrix and Nb5Si3 with unreacted Nb for dispersed phases.

Structure and yield strength of directionally solidified Ni3Al intermetallic premelted with MoSi2 phase

Abstract Ni 3 Al and MoSi 2 intermetallic phases were arc melted in Ni 3 Al/MoSi 2 molar proportion of ten. Pure binary Ni 3 Al and the quaternary alloy were subjected to directional solidification using the floating zone method at growth rates of 10–50 mm h −1 . The phase composition and structure of the crystals were analyzed using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. The yield strength was studied in compression at 293–1293 K and in tension at room temperature. Compared to the binary Ni 3 Al crystals, the quaternary Ni–Al–Mo–Si crystals revealed up to 5 times higher compressive yield strength at 400–800 K. Ni–Mo–Si C14 Laves phase precipitation in the L1 2 Ni 3 Al+L1 0 NiAl matrix duplex phase was found in quaternary crystals. This precipitation is assumed to cause the observed mechanical behaviour.

Phase Transformation of MoSi<SUB>2</SUB> Phase in MoSi<SUB>2</SUB>/Mo<SUB>5</SUB>Si<SUB>3</SUB> Eutectic Alloy Containing Nb

Role of ternary solute element Nb on the C40→C11b phase transformation of the MoSi2 phase in MoSi 2 /Mo 5 Si 3 eutectic alloys which occurs during heat treatment is discussed in this paper. In the case of stoichiometric MoSi 2 a mixture of C40 and C11b phases can be prepared by low-pressure plasma spraying method, in which phase transformation from the metastable C40 to stable C11b is recognized during heating at around 800°C. It is found that the MoSi2 phase in an as-cast eutectic MoSi 2 /Mo 5 Si 3 alloy with 8.5 at%Nb consist only of metastable C40 phase. The transformation from C40 to C11b in this alloy does not occur by heating up to 1200°C but a portion transforms into C11b by heating at 1400°C. It is shown that the Nb concentration in C40 phase is higher than that in C11b phase in this alloy, which must be relevant to the observed transformation behavior.

Phase Characterization and Thermal Conductivity of Carbon Alloys Obtained by Interface Reaction between C/C Composites and Pressed Molybdenum Silicide

The C/C composites made by laying up the sheets of carbon fibers using a pitch matrix were used as startingmaterials. After cutting the composites in the parallel (PA) and the perpendicular (PE) directions to the sheets ofcarbon fibers, the surface of PA and PE substrates obtained were reacted with MoSi2, at 1450°C in argon gas. From XRDmeasurements, the four phases of graphite, MoSi2, MoO3 and α-SiC were observed at the surface of the substrates treatingfor 3 and 6h, irrespective of the direction to carbon fiber sheets. A concentration gradient of SiC in the as-prepared substrates was observed in both directions parallel and perpendicular; the concentration of SiC decreases along the depthfrom the surface of substrate. In the results of line analysis by EDX measurements, it was found that the reaction betweenC/C substrates and MoSi2 occurred in the portion of matrix in the substrates. The thermal conductivity of PA substratesdecreased with the increase of the treating time and was lower than that of PA substrate without treatment. However, the thermal conductivity of the PE substrates treating for 6h was higher than that of the PA substrtes treating below3h and without treatment.

酸化物の複合化によるＭｏＳｉ２材料の低温酸化特性の改善

MoSi2-oxides composites using fine aluminosilicate powder (<0.2, μm) have demonstrated excellent low temperature oxidation resistance and thermal shock resistance. These properties strongly depend on microstructural morphology and are obtained in composites that network-structures of both phases of MoSi2 and oxides are developed, i.e., in composites with oxides of 20-40 vol.%. When one phase is independently dispersed in the other phase, on the other hand, problems of low temperature oxidation and thermal shock occur. The low temperature oxidation problem occurs in the composites with oxides less than 15 vol.% and the thermal shock problem occurs in the composites with oxides more than 50 vol.%. These results will contribute to material design approaches for high temperature structural applications of MoSi2.

Growth Rate Constant and Chemical Diffusivity in Silicides Mo&lt;SUB&gt;5&lt;/SUB&gt;Si&lt;SUB&gt;3&lt;/SUB&gt; and Ta&lt;SUB&gt;5&lt;/SUB&gt;Si&lt;SUB&gt;3&lt;/SUB&gt;

The growth kinetics and chemical diffusion coefficients in intermetallic silicides M5Si3 (M=Mo and Ta ) were investigated at temperatures between 1273 and 1673 K in a vacuum capsule using Mo/MoSi2 and Ta/TaSi2 couples, where both disilicides were coated on Mo and Ta substrate by a CVD method. It was found that both the M5Si3 layers grew parabolically. Concentration profiles of Si, Mo and Ta were measured across cross-sectioned samples using an electron-probe microanalysis and chemical diffusion coefficients were obtained using Wagner’s equation on multilayer diffusion systems. Both the parabolic rate constant and chemical diffusion coefficient in the Mo5Si3 phase were approximately one order of magnitude larger than those of the Ta5Si3 phase. Activation energy for the parabolic rate constant was close to that of the chemical diffusion coefficient for each diffusion couple, showing 297 and 271 kJ/mol for the Mo5Si3 and Ta5Si3 phases, respectively. Microstructural observations indicated that Kirkendall voids formed within the Mo5Si3 layer close to the MoSi2 phase, whereas there is little formation of voids in the Ta5Si3 layer.

Ternary alloying study of MoSi2

Ternary alloying of MoSi2 with adding a series of transition elements was investigated by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Iron, Co, Ni, Cr, V, Ti, and Nb were chosen as alloying elements according to the AB2 structure map or the atomic size factor. The studied MoSi2 base alloys were prepared by the arc melting process from high-purity metals. The EDS analysis showed that Fe, Co, and Ni have no solid solubility in as-cast MoSi2, while Cr, V, Ti, and Nb exhibit limited solid solubilities, which were determined to be 1.4±0.7, 1.4±0.4, 0.4±0.1, and 0.8±0.1. Micro-structural characterization indicated that Mo-Si-MVIII (MVIII=Fe, Co, Ni) and Mo-Si-Cr alloys have a two-phase as-cast microstructure, i.e., MoSi2 matrix and the second-phase FeSi2, CoSi, NiSi2, and CrSi2, respectively. In as-cast Mo-Si-V, Mo-Si-Ti, and Mo-Si-Nb alloys, besides MoSi2 and C40 phases, the third phases were observed, which have been identified to be (Mo, V)5Si3, TiSi2, and (Mo, Nb)5Si3.

Siliconizing of Molybdenum Metal in Indium-Silicon Melts

Siliconizing of Mo to form a silicide layer was investigated in the In–Si melt alloy with a two phase mixture of a solid Si and an In–Si melt at temperatures between 1273 and 1473 K for up to 90 ks in a vacuum ampoule. The MoSi2 layer was grown in thick because an activity of silicon in the melt alloy is close to unity, accompanied by a formation of a thin Mo5Si3 layer between the MoSi2 layer and Mo substrate. These two layers grew in accordance with a parabolic rate raw.Chemical diffusion coefficients in both the MoSi2 and Mo5Si3 layers were obtained using Wagner analysis for the diffusion couple of a Mo/Mo5Si3/MoSi2/Si (in the melt). Activation energies for the growth rate constants are 157 kJ/mol for the MoSi2 and 350 kJ/mol for the Mo5Si3, respectively, and they are the same as those for chemical diffusion coefficients due to the fact that terminal compositions of 37.5 and 39.1 at%Si for both sides of the Mo5Si3 layers are almost independent of temperature. Composition of 39.1 at%Si was coincided with the measured one for the two MoSi2+Mo5Si3 phase alloy, which was smaller than 40 at%Si cited in the phase diagram. Duplex layer structure and its growth kinetics in the present In–Si melt method were consistent with those obtained in the CVD method.

Mechanical Behavior of MoSi2 Reinforced–Si3N4Matrix Composites

The mechanical behavior of MoSi2 reinforced–Si3N4 matrix composites was investigated as a function of MoSi2 phase content, MoSi2 phase size, and amount of MgO densification aid for the Si3N4 phase. Coarse‐phase MoSi2‐Si3N4 composites exhibited higher room‐temperature fracture toughness than fine‐phase composites, reaching values &gt;8 MP·am1/2. Composite fracture toughness levels increased at elevated temperature. Fine‐phase composites were stronger and more creep resistant than coarse phase composites. Room‐temperature strengths &gt;1000 MPa and impression creep rates of ∼10−8 s−1 at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi2‐Si3N4 mechanical properties. Internal stresses due to MoSi2 and Si3N4 thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi2‐Si3N4 composites.

Corrosion properties of thin molybdenum silicide films

The corrosion properties of sputtered molybdenum and molybdenum silicide films in hydrochloric acid (HCl) have been studied by means of potentiodynamic measurements. Contributions from the substrate to the corrosion behaviour was avoided by depositing the films on inert aluminium oxide (Al 2O 3). The compositions studied were Mo, MoSi 0.58, MoSi 1.04, MoSi 1.4 and MoSi 1.9–2.1. Characterisation of the sampies was made by X-ray diffraction (XRD) and scanning electron microscopy (SEM) before and after corrosion. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to analyse the polarised films. Corrosion of M 3Si was found in the molybdenum-rich samples (MoSi 0.58) containing the two phases Mo 3Si and Mo 5Si 3. Polarisation curves for these films showed one passivation peak at 228 mV vs. the saturated calomel electrode (SCE). The MoSi 1.9–2.1 films had the best corrosion properties of the films studied. This composition had three passivation peaks, at about 154, 305 and 1850 mV SCE, respectively. In the silicon-rich samples, containing the phases MoSi 2 and Mo 5Si 3, preferentiai corrosion of Mo 5Si 3 was found. All the samples containing the disilicide phase showed at least two passivation peaks. XPS and AES studies on the passive films formed on the samples at the two first passivation peaks indicate that both peaks are due to oxidation of silicon- and molybdenum-containing species. The amount of molybdenum in the outermost layer is increased after the second peak.

X-ray diffraction study of solid-state formation of metastable MoSi2 and TiSi2 during mechanical alloying

The formation of metastable C40–MoSi2 and C49–TiSi2 during the mechanical alloying of metal/silicon powder mixtures has been investigated by x-ray diffraction. The mechanical alloying of a Si-rich Mo/Si powder mixture results in the formation of both C11b–MoSi2 and C40–MoSi2. The C40–MoSi2 phase is metastable and transforms to C11b–MoSi2 after annealing at 850 °C. The mechanical alloying of a stoichiometric Ti/Si powder mixture leads to the incipient formation of C49–TiSi2 and the subsequent polymorphic transformation to C54–TiSi2. The lattice parameters of C49–TiSi2, as determined from a mechanically alloyed Ti/Si powder sample that has been annealed at 700 °C, are as follows: a0=3.56 Å, b0=13.57 Å, and c0=3.55 Å.

Fabrication and Microstructures of MoSi2 Reinforced–Si3N4 Matrix Composites

Details of the fabrication and microstructures of hot‐pressed MoSi2 reinforced–Si3N4 matrix composites were investigated as a function of MoSi2 phase size and volume fraction, and amount of MgO densification aid. No reactions were observed between MoSi2 and Si3N4 at the fabrication temperature of 1750°C. Composite microstructures varied from particle–matrix to cermet morphologies with increasing MoSi2 phase content. The MgO densification aid was present only in the Si3N4 phase. An amorphous glassy phase was observed at the MoSi2–Si3N4 phase boundaries, the extent of which decreased with decreased MgO level. No general microcracking was observed in the MoSi2–Si3N4 composites, despite the presence of a substantial thermal expansion mismatch between the MoSi2 and Si3N4 phases. The critical MoSi2 particle diameter for microcracking was calculated to be 3 μm. MoSi2 particles as large as 20 μm resulted in no composite microcracking; this indicated that significant stress relief occurred in these composites, probably because of plastic deformation of the MoSi2 phase.

金属間化合物の粉末冶金と諸物性 パルス通電加圧−燃焼合成法によるＭｏ‐Ｓｉ系金属間化合物の作製

In this study, Mo-Si and Mo-Si-Nb system were fabricated by the pulse discharge pressure-combustion synthesis method (Nb was added into Mo-Si, in order to improve ductility at room temperature and high temperature strength). During the synthesis of Mo-Si and Mo-Si-Nb mixed powders, samples abruptly shrunk at 1323K. It is clear that combustion reaction occurred at this temperature and compounds were formed by combustion reaction. The relative density of fabricated samples was 96 to 98% and no internal defects were foundby ultrasound images in the samples. The formed phases of the fabricated samples were as follows;only MoSi2 phases were formed for Mo:Si=1:2 mixed powders, and, for the Nb added samples, MoSi2, Nb5Si3 and Mo5Si3 phases were formed and unreacted Nb remained. Young's modulus and the bending strength of fabricated samples showed similar results. Monolithic MoSi2 sample had the highest value of Young's modulus and bending strength, and for Nb added samples, Young's modulus and bending strength decreased with the increase in Nb content.

Study of Biaxial Stress Induced by Cosputtered Thin Molybdenum Silicide Films in Silicon Single-Crystal Substrates

Bending of 25 mm-diameter (100) silicon wafers produced by 100 nm-thick cosputtered molybdenum–silicon films has been investigated just after deposition, after rapid thermal annealing and after etching of the deposits. Values of biaxial stress in the films were determined by the use of a widely used relation. The Mo:Si ratio in the as-deposited films was 89:11. Radii of curvature of substrates were measured by a double-crystal X-ray diffractometer. Blank wafers with three widely different radii of curvature (33, 62 and 250 m) were chosen as specimens and their curvatures were taken into account in the determination of the value of biaxial stress in wafers with deposits. All the wafers were convex when viewed from the polished surface side on which the films were deposited. Values of σ for wafers with as-deposited films were in the range 3 × 108 to 14 × 108 Nm−2 (tensile). Wafers with higher initial bending showed higher values of stress. Deposition led to degradation of perfection, as revealed by broadening of the diffraction curves and the contrast in the topographs. Rapid thermal annealing at 1273 and 1373 K (3–4 min) led to formation of the MoSi2 phase and to a notable relaxation of stress. The values of σ were in the range 1 × 107 to ~6 × 108 N m−2 (tensile). The value of the stress was lowest for the blank wafer with smallest bending. Annealing also improved the degree of crystalline perfection of the silicon. Experiments performed after etching of the annealed specimens showed no significant change in the radii of curvature.