MoSi2 Layer Research Articles

Formation of epitaxial CoSi2 by a Cr or Mo interlayer: Comparison with a Ti interlayer

The influence of Cr and Mo on phase formation and preferential orientation of CoSi2 is reported. Three different regimes are distinguished, depending on the thickness of the interlayer. For a thin interlayer or a capping layer, CoSi forms first, as in the standard Co/Si reaction. The remaining Cr or Mo can be considered as a contaminant that is present in the CoSi layer, causing a delay in CoSi2 nucleation and inducing preferential (220) and (400) nucleation. For interlayers with intermediate thickness, epitaxially (400) oriented CoSi2 is formed. For a thick interlayer, a polycrystalline layer of CrSi2 (or MoSi2) is formed first, followed by CoSi formation on top of the CrSi2. At higher temperatures, the CoSi layer is transformed into a polycrystalline, continuous layer of CoSi2 on top of the CrSi2 or MoSi2 layer, while some grains of CoSi2 are formed underneath the CrSi2. A similar behavior for Ti interlayers is observed, although a much thicker Ti layer is needed before the third regime is reached.

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.

Impact fracture characteristics on fabricating process of Nb/MoSi2 laminate composite (I)

Nb/MoSi2 laminate composites have been successfully fabricated by hot pressing in a graphite mould. Lamination of Nb foil and MoSi2 layer showed a sufficient improvement in the absorbed impact energy compared to that of monolithic MoSi2 material. The impact value of Nb/MoSi2 laminate composites obviously is reduced when sintered at temperatures higher than 1523K, even if the composite density contributing to impact load increased along with fabricating temperatures. Impact value of laminate composites was also drastically decreased with the growth of reaction layer after the heat treatment. However, it was effective to increase the pressure at the same sintering temperature for the improvement of the impact value.

One-step sintering of SiGe thermoelectric conversion unit and its electrodes

Dense p-type and n-type SiGe thermoelectric conversion units were fabricated with a double-layer electrode of W/TiB2 or W/MoSi2 by using glass encapsulation hot-isostatic-pressing process. The TiB2 and MoSi2 layers were used to prevent the chemical reaction between the tungsten and SiGe materials. Si3N4 ceramic particles were added into the electrode materials to reduce the mismatch of the thermal expansion between the electrode and the SiGe. Finite element analysis showed that the addition of 40 vol% Si3N4 into the TiB2 layer and 55 vol% Si3N4 into the MoSi2 layer reduced the thermal residual stress to a much lower value than the strength of individual layer. Sintered units had electrical resistivities of (1.5–2.0) × 10−3 Ω cm in the SiGe zone and 10−4 Ω cm in the electrodes. The comparison of the thermoelectric properties of the SiGe sintered with and without electrodes confirmed that the electrodes did not deteriorate the Seebeck coefficient of the SiGe alloys.

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.

Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

A new kind of oxidation protection coating of Si-MoSi2 was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550 °C. The overall oxidation behavior could be divided into three stages: (i) 500 °C 1100 °C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi2 layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550 °C for 50 h in air and 1550 °C ⇔ 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.

Oxidation of MoSi2/SiC nanolayered composite

The oxidation behavior of a nanolayered MoSi2/SiC composite material was determined at the temperature range of 400–900 °C in wet oxidation conditions. The samples were produced in the form of thin films using a sputtering technique from two different sources, and a rotating substrate holder, onto silicon single crystals and low carbon steel. For comparison, the oxidations of both constituents, MoSi2 and SiC, produced with the same sputtering technique, were measured separately. The microstructure of the MoSi2/SiC samples was determined with high resolution transmission electron microscopy (HRTEM), and the composition of the sputtered samples was measured using backscattering (BS) of protons. For quantitative determination of oxidation, the nuclear reaction 16O(d, p)17O was utilized. Oxide layers were also analyzed using a secondary ion mass spectrometry (SIMS) and the appearance of the oxidized surface with a scanning electron microscopy (SEM). As expected, the SiC films had both the lowest initial oxidation and steady state oxidation rate. The results show that the oxidation behavior of the MoSi2/SiC nanolayered composite material differs from that of both its constituents and involves a degradation mechanism of its own, resulting in the highest oxidation during the initial phase of the oxidation. A steady-state oxidation rate was observed after the initial transient phase to be the highest for the metastable C40 structure of the single MoSi2 layer. The oxidation rate of the nanolayered structure was retarded by the SiC layers. No signs of pest disintegration were observed on either of the MoSi2 containing coatings during the steady-state phase of the oxidation at 500 °C up to 40 h. Our results show that the oxidation of nanolayered structures can be only in part explained by the oxidation behavior of the constituents and that during the steady-state oxidation of the nanolayered structure the oxidation rate is largely determined by the constituent with the lowest oxidation rate and by the layered structure.

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.

Characteristics of residual stress generated by MoSi2 plasma thermal spraying

Summary This paper describes an investigation of plasma thermal spraying of MoSi2 offering a high melting point together with excellent high-temperature oxidation resistance. MoSi2 powder was deposited by low-pressure plasma thermal spraying on the surfaces of blasted SS400 substrate specimens precoated with 50%Ni-50%Cr. The residual stress generated in the specimens was then determined. Compressive stress is generated in the MoSi2 sprayed deposit. Tensile stress is generated in the 50%Ni-50%Cr sprayed deposit except at the SS400 substrate surface. A series of experiments was conducted to clarify the validity of the residual stress distribution obtained. These results suggest the following: 1. When only blasting treatment is performed, compressive stress is generated from the upper surface to 0.3 mm inside. Minor tensile stress is also generated inside. 2. When 50%Ni-50%Cr is deposited after blasting treatment, the residual stress is tensile in the 50%Ni-50%Cr layer and compressive on the substrate upper surface. Tensile stress is found inside. This is due to the fact that the substrate restrains the contraction of the sprayed deposit with a high thermal shrinkage. The validity of this result is also supported by the fact that the compressive stress near the substrate upper surface is greater than when only blasting treatment is performed. 3. When MoSi2 is deposited after blasting treatment, compressive stress is generated in the MoSi2 layer. This is due to the fact that, despite the MoSi2 layer having a higher temperature than the substrate, the linear thermal expansion coefficient of MoSi2 is much smaller than that of the substrate, so that the MoSi2 layer restrains the contraction of the substrate. 4. During estimation of the residual stress in materials deposited by plasma thermal spraying, it is important to ensure that the modulus of elasticity of the sprayed deposit is measured separately for each set of thermal spraying conditions.

MoSi2‐Based Sandwich Composite Made by Tape Casting

The mechanical properties of a novel laminar composite made by tape casting have been studied. The composite consists of three layers in which an inner core of pure molybdenum disilicide (MoSi2) is sandwiched between two layers of MoSi2 reinforced with alumina platelets. Monolithic MoSi2 exhibits poor room‐temperature strength and a brittle indentation strength response, indicative of the absence of R‐curve behavior. The flexural behavior of the sandwich composite (both strength and toughness) is dominated by the properties of the outer layer, so long as the thickness of this layer exceeds a critical value. A model has been developed which successfully predicts the critical thickness required.

High-temperature structural stability of MoSi2-based nanolayer composites

A systematic study of the high-temperature structural stability is reported for the following three nanolayered composites: (1) MoSi2–MoSi2Nx, (2) Mo–MoSi2Nx, and (3) Mo–MoSi2Nx–MoSi2, where x∼3–4. The alternating layers with layer thickness varying from 1 to 50 nm were synthesized by sputtering techniques. The structural evolution in these composites has been studied by cross-sectional transmission electron microscopy as a function of annealing temperature. As-deposited Mo layers exhibit nanocrystalline structure, while the other types of layers are amorphous in structure. With increasing annealing temperature, MoSi2 crystallizes to form a metastable C40 phase at ∼500 °C and then transforms to the stable C11b phase at ∼900 °C, while MoSi2Nx remains amorphous up to temperature as high as 1000 °C. No difference in crystallization behavior was observed for the constituents in either single phase or multilayered form. One way to improve the toughness of MoSi2 is through the addition of a ductile phase, e.g., Mo. However, the Mo and MoSi2 layers react and the layer structure deteriorates. Previous study has shown that the compound, MoSi2Nx, stays amorphous at 1000 °C. This suggests that it could function either as a stable second-phase reinforcement or as a diffusion barrier between Mo and MoSi2. The MoSi2–MoSi2Nx, Mo–MoSi2Nx, and Mo–MoSi2–MoSi2Nx nanolayers are found to remain stable up to 900 °C (highest temperature tested).

Structure and mechanical properties of MoSi2-Sic nanolayer composites

Abstract A systematic study of the structure-mechanical properties relationship is reported for both single phase and nanolayer composites of MoSi2 and SiC. Single phase or alternating layers of MoSi2 and SiC were synthesized by d.c.-magnetron and r.f.-diode sputtering, respectively. Cross-sectional transmission electron microscopy was used to examine several distinct reactions in the specimens when annealed at progressively higher temperatures: crystallization and phase transformation of MoSi., crystallization of Sic, layer spheroidization, and grain growth. Nanoindentation was employed to characterize the mechanical response of the materials as a function of the structural changes. As-sputtered material exhibits an amorphous structure in both single phase and multilayer forms. Heat treatment induces similar recrystallization behaviour in MoSi, in both single phase and multilayers. Sic, in single phase form, remains amorphous up to 900°-1 h anneal, while in multilayers it starts to crystallize at 700°C. Annealing at 900°C for 2 h causes the spheroidization of the layering which results in the formation of a nanocrystalline equiaxed microstructure. Abnormal grain growth is observed after the spheroidization. The crystallization process is directly responsible for the hardness and modulus increase in both single phase and multilayered films. A maximum hardness of 25.5 GPa and a modulus of 382 GPa can be achieved through crystallizing both MoSi2 and Sic layers. Prolonged high temperature exposure causes hardness degradation due to grain growth but the modulus remains almost constant. The layered geometry offers better elastic properties (higher modulus) than the single phase films.

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The symmetric gradient materials of MoSi2/Al2O3/Ni/Al2O3/MoSi2 were fabricated by SHS/HIP compaction. Due to the thermal expansion mismatch between the outer MoSi2 and inner Al2O3-Ni layers, the compressive residual stress as much as 100MPa was induced in the MoSi2 surface, which enhanced the toughness to 5.8MPa·m1/2 comparing to 3.8MPa·m1/2 of the monolithic MoSi2. The flexural strength of the FGMs was 580MPa which is slightly higher than the monolithic MoSi2. It was found that Al2O3 in the FGMs acted as a diffusion barrier to Ni even under heat treatment at 900°C for 8hr.

Phase transformation and layer evolution in molybdenum disilicide-silicon carbide nanolayered coatings

MoSi2 is a potential matrix material for high temperature structural composites due to its high melting temperature and good oxidation resistance at elevated temperatures. The two major drawbacksfor structural applications are inadequate high temperature strength and poor low temperature ductility. The search for appropriate composite additions has been the focus of extensive investigations in recent years. The addition of SiC in a nanolayered configuration was shown to exhibit superior oxidation resistance and significant hardness increase through annealing at 500°C. One potential application of MoSi2- SiC multilayers is for high temperature coatings, where structural stability ofthe layering is of major concern. In this study, we have systematically investigated both the evolution of phases and the stability of layers by varying the heat treating conditions.Alternating layers of MoSi2 and SiC were synthesized by DC-magnetron and rf-diode sputtering respectively. Cross-sectional transmission electron microscopy (XTEM) was used to examine three distinct reactions in the specimens when exposed to different annealing conditions: crystallization and phase transformation of MoSi2, crystallization of SiC, and spheroidization of the layer structures.

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.

Characterization of Structure and Mechanical Properties of MoSi2-SiC Nanolayer Composites

AbstractA systematic study of the structure-mechanical properties relationship is reported for MoSi2-SiC nanolayer composites. Alternating layers of MoSi2 and SiC were synthesized by DCmagnetron and if-diode sputtering, respectively. Cross-sectional transmission electron microscopy was used to examine three distinct reactions in the specimens when exposed to different annealing conditions: crystallization and phase transformation of MoSi2, crystallization of SiC, and spheroidization of the layer structures. Nanoindentation was employed to characterize the mechanical response as a function of the structural changes. As-sputtered material exhibits amorphous structures in both types of layers and has a hardness of 11GPa and a modulus of 217GPa. Subsequent heat treatment induces crystallization of MoSi2 to form the C40 structure at 500°C and SiC to form the a structure at 700°C. The crystallization process is directly responsible for the hardness and modulus increase in the multilayers. A hardness of 24GPa and a modulus of 340GPa can be achieved through crystallizing both MoSi2 and SiC layers. Annealing at 900°C for 2h causes the transformation of MoSi2 into the Cllb structure, as well as spheroidization of the layering to form a nanocrystalline equiaxed microstructure. A slight degradation in hardness but not in modulus is observed accompanying the layer break-down.

Mechanical properties and microstructures of metal/ceramic microlaminates: Part I. Nb/MoSi2 systems

Artificial multilayers, or microlaminates, composed of alternating layers of Nb and MoSi2 of equal thickness were synthesized by d.c., magnetron sputtering. Four different modulation wavelengths, λ, were studied: 7, 11, 20, and 100 nm. The compositions, periodicities, and microstructures of the microlaminates were characterized by Auger electron spectroscopy and transmission electron microscopy. Structural characterization revealed that the as-deposited Nb layers are polycrystalline, while the MoSi2 layers are amorphous. The hardnesses and elastic moduli of the films were measured using nanoindentation techniques. Neither a supermodulus nor a superhardness effect could be identified in the range of wavelengths investigated; for each of the microlaminates, both the hardness and modulus were found to fall between the bounds set by the properties of the monolithic Nb and MoSi2 films. Nevertheless, a modest but a measurable increase in both hardness and modulus with decreasing wavelength was observed, thus indicating that behavior cannot be entirely described by a simple rule-of-mixtures. The hardness was found to vary linearly with Δ−1/2 in a manner similar to the Hall–Petch relationship. Annealing the microlaminates at 800 °C for 90 min produces significant increases in hardness and modulus due to chemical interaction of the layers.

Characterization of MoSi2 coatings obtained by atmospheric pressure and low‐pressure plasma spraying

AbstractThe high‐temperature oxidation resistance property of MoSi2 coatings is strongly influenced by the chemical composition, density and structure of the deposit. The aim of this work is to determine the role of the deposition process in the microchemistry and microstructure of the coating. In order to achieve this task, we compare MoSi2 produced by LPPS (low‐pressure plasma spraying) and conventional APPS (atmospheric pressure plasma spraying) techniques with the aid of a complementary use of characterization methods such as SEM, EPMA (electron probe microanalysis), CMA (computer‐aided microanalysis), GDOS (glow discharge optical spectroscopy) and XPS. From the overall results it is evident that the application of plasma spraying under low pressure considerably improves MoSi2 layers. They are characterized by a higher density, with fewer pores and discontinuities, and have a more uniform chemical composition and a better coating‐substrate interface.

Formation of Bilayer Shallow MoSi2/CoSi2 Salicide Contact Using W/Co-Mo Alloy Metallization

A very simple process to form a bilayer shallow MoSi2/CoSi2/Si silicided contact using W/Co54Mo46/Si bilayer alloy metallization has been developed. Two-step annealing in a conventional flowing-nitrogen furnace is employed. The first annealing is performed at a temperature of 500-600°C, with the W film passivating the underlying Co-Mo alloy layer from being oxidized and consuming some Co. A shallow contact is feasible by selectively etching away the remaining outer layer of the Mo-dominant Mo-Co alloy (or W-Co alloy) phase to deduct the metal source which would finally be transformed into silicides. Silicide lateral growth which is observed during the silicidation of Co on Si does not occur in the present system. This is presumably because the formation of CoSi2 is not preceded by Co2Si or CoSi due to the nonrich Co source in the Co-Mo alloy system. The second annealing performed at a higher temperature of 700-950°C is to further transform the CoSi2 and mixed-silicide layers obtained after the first annealing into separate layers of MoSi2 and CoSi2.

Structure, Mechanical Properties, and Oxidation Behavior of Nanolayered MoSi2/SiC Coatings

ABSTRACTThe structure, hardness and elastic properties, and oxidation behavior of a nanolayered MoSi2/SiC structure were examined. DC magnetron and rf diode sputtering were used to deposit MoSi2 and SiC, respectively. The total number of sublayers of each kind was 90. The nominal thickness of the MoSi2 sublayers was 10 and that of the SiC 3 nm. The microstructure of the as-deposited samples was amorphous whereas annealing at 500 °C for I h resulted in the transformation of MoSi2 into C40 type hexagonal structure as determined using cross-section transmission electron microscopy (TEM). However, silicon carbide still remained amorphous. Hardness and Young's modulus were determined by using a nanoindentation technique. Hardness of 11.6 and 20.8 GPa were obtained for the asdeposited and annealed structure, respectively. Corresponding Young's moduli were 220 and 290 GPa, which yield values of 232 and 332 GPa for Young's moduli of amorphous and crystalline MoSi2. Oxidation tests were carried out in wet oxidation conditions at 400 and 500 °C. The degree of oxidation was determined by measuring oxygen concentration on the surface using a nuclear reaction 16O(d,p) 17O at a deuterium energy of 950 keV. The MoSi2/SiC coating gave good protection against oxidation for unalloyed steel. As compared to a single MoSi2 coating a slightly higher degree of oxidation and a significantly different mechanism of oxidation was observed. This was also supported by secondary ion mass spectroscopy (SIMS) measurements. Cracking of the coating caused by a thermal mismatch and transformation stresses was worse in the case of the single MoSi2 layer than in the case of the composite coating. Thus cracking of the coating can be significantly reduced by using a nanolayered structure.