MoSi2 Phase Research Articles

Correlation between microstructure and properties of fine grained Mo–Mo3Si–Mo5SiB2 alloys

Three phase α-Mo–Mo3Si–Mo5SiB2 alloys of various compositions, namely Mo–6Si–5B, Mo–9Si–8B, Mo–10Si–10B and Mo–13Si–12B (at.%) were processed by a powder metallurgical (PM) route. Increasing the Si and B concentration in these Mo–Si–B alloys resulted in increasing volume fractions of the intermetallic phases Mo3Si (A15) and Mo5SiB2 (T2) and the distribution of the three phases present in these alloys was dependent on the volume fractions of the individual phases. Above volume fractions of about fifty percent, bcc Mo solid solution (α-Mo) formed the matrix. Consequently, Mo–6Si–5B and Mo–9Si–8B alloys, which possessed a continuous α-Mo matrix provided increased fracture toughness at ambient temperatures. Additionally, a decreased BDTT of about 950 °C is caused by the homogeneous α-Mo matrix. In contrast, Mo–13Si–12B with 65 vol.% of the intermetallic phases that formed the matrix phase in this material had a BDTT value higher than 1100 °C, while the strength at elevated temperatures up to 1300 °C was significantly increased compared to alloys that have the α-Mo matrix. Alloy compositions with ≥50 vol.% of intermetallic phases (corresponding to alloys containing a minimum of 9 at.% Si and 8 at.% B) were oxidation resistant with minimal mass loss under cyclic conditions for 150 h at 1100 °C due to the formation of a dense borosilicate glass layer that protects the material surface.

Oxidation behavior of magnetron sputtered double layer coatings containing molybdenum, silicon and boron

Protective coating systems were applied to Mo–9Si–8B (at.%) alloys to prevent oxidation at elevated temperatures. The coatings produced by magnetron sputtering and subsequent annealing consisted of an outer oxidation protection layer and an interlayer between this and the substrate. Three amorphous outer layers with different compositions were deposited: Mo–45Si–25B, Mo–55Si–10B and Mo–29Si–15B (all in at.%). The interlayer was selected to give a diffusion barrier with the composition of the Mo5SiB2 (T2) phase. All coatings were dense and well-adherent. During vacuum annealing the amorphous as-deposited coatings became crystalline exhibiting mainly the intermetallic Mo5SiB2 compound as interlayer and the MoSi2, Mo5Si3 and MoB phases in the top layers. The samples were exposed to dry laboratory air in the pesting regime at 800 °C and above, i.e. at 1000 and 1300 °C for up to 100 h under cyclic conditions. All coatings were protective at 800 and 1000 °C for at least 100 h and showed a marked improvement in mass change compared to the uncoated substrate. For protection at 800 °C higher boron content is preferential, while at higher oxidation temperatures a lower boron content provides improved oxidation protection. At 1300 °C stress induced failures like cracking, spallation and buckling occurred due to the relatively high CTE mismatch between PVD coating and substrate. Even though, the mass change was still markedly reduced as compared to the bare substrate.

The interaction of carbon with Mo5Si3 and W5Si3 silicides. Nowotny phase synthesis

High-temperatures reactions of Mo5Si3 and W5Si3 silicides powders with the carbon soot were studied experimentally on sintered and melt-quenched samples. It was shown that Nowotny phase Mo4.8Si3C0.6 may be synthesized after annealing at 1600–2000 °C of initial mixture of Mo5Si3 with the carbon soot compacted at room temperature, simultaneously the small quantity of Mo2C phase was synthesized also. We have not found the Nowotny phase for W (like W4.8Si3C0.6) after reactions of W5Si3 or W5Si3 + WSi2 eutectic powders mixtures with carbon neither for the solid state reactions, nor for the melt-quenched samples. Only in some experiments with quenching of melted on the graphite substrate W5Si3 and WSi2 silicides eutectic there were found some evidences of the phase like W4.8Si3C0.6 possible presence along with W5Si3 and WSi2 silicides and WC carbide. In the melt-quenched Mo5Si3 + C sample Nowotny phase for Mo was distinguished from the Mo5Si3 phase in the back-scattered electron image, Mo2C carbides were revealed also.

Dense ZrB2–MoSi2 composite coating fabricated by low pressure plasma spray (LPPS)

Low pressure plasma spray was applied to fabricate ZrB2–MoSi2 composite coatings. Their microstructure and oxidation behavior were characterized and compared with those of the ZrB2–SiC coatings. The ZrB2–MoSi2 coating displayed compact and lamellar microstructure with porosity lower than 5%, where MoSi2 phase was uniformly distributed in the ZrB2 matrix. ZrB2–30 vol% MoSi2 coating exihibited much better oxidation-resistance at high temperature compared to the ZrB2–30 vol% SiC coating. It is thought that the fine distribution of sufficient MoSi2 in the matrix and dense microstructure of the ZrB2–MoSi2 coating contributed to its excellent high temperature oxidation-resistance.

Microstructure Characteristics and Oxidation Behavior of Molybdenum Disilicide Coatings Prepared by Vacuum Plasma Spraying

In this study, molybdenum disilicide (MoSi2) coatings were fabricated by vacuum plasma spraying technology. Their morphology, composition, and microstructure characteristics were intensively investigated. The oxidation behavior of MoSi2 coatings was also explored. The results show that the MoSi2 coatings are compact with porosity less than 5%. Their microstructure exhibits typical lamellar character and is mainly composed of tetragonal and hexagonal MoSi2 phases. A small amount of tetragonal Mo5Si3 phase is randomly distributed in the MoSi2 matrix. A rapid weight gain is found between 300 and 800 °C. The MoSi2 coatings exhibit excellent oxidation-resistant properties at temperatures between 1300 and 1500 °C, which results from the continuous dense glassy SiO2 film formed on their surface. A thick layer composed of Mo5Si3 is found to be present under the SiO2 film for the MoSi2 coatings treated at 1700 °C, suggesting that the phenomenon of continuous oxidation took place.

Connecting structural, mechanical and tribological characteristics of Al alloyed nanocrystalline molybdenum silicide coatings

In this study, novel nanocrystalline molybdenum silicide coatings with differing Al contents were deposited on a commercial titanium substrate using a double-cathode glow discharge apparatus. Their microstructures were characterized by x-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy. These coatings mainly consisted of MoSi2 and Mo5Si3 phases; the ratio of Mo5Si3 to MoSi2 gradually increased from the surface towards the interior of coatings, forming a graded structure. With the increase in Al content, the hardness and elastic modulus of the coatings slightly decreased, but the ratios of H/E and 1/E2H, as well as damage tolerance, increased. Furthermore, these newly developed coatings showed excellent wear resistance; their specific wear rates were not only considerably lower than that of the monolithic MoSi2 coating, but also decreased with increasing Al content. The plan and cross-sectional views of the worn surfaces and wear debris were analysed using SEM and energy dispersive x-ray spectroscopy. The relationships between coating structure, mechanical property and wear mechanism were then clarified, which will help in designing hard, tough and wear-resistant coatings for applications involving severe loading conditions.

Plastic Deformation of Directionally-Solidified MoSi2/Mo5Si3 Eutectic Composites

ABSTRACTDeformation behavior of the directionally-solidified MoSi2/Mo5Si3 eutectic composites has been investigated as a function of the average thickness of MoSi2 phase over a temperature range from 900 to 1500°C. The average thickness of both MoSi2 and Mo5Si3 phases in the directionally-solidified ingots with script-lamellar morphologies grown by optical floating zone method decreases with increasing the growth rate. Plastic deformation was observed above 1000°C for all the DS ingots grown at different growth rates when the loading axis is parallel to [1¯10]MoSi2 close to the growth direction. Yield stress decreases monotonically with increasing temperature. Yield stress at 1400°C increases drastically with decreasing the average thickness of MoSi2 phase.

Phase stability of $$\upbeta$$ -MoSi2−x prepared by the Na flux method against thermal, oxidative, and mechanical treatments

The single \(\upbeta\)-MoSi2 phase was prepared by the Na flux method and its stability against thermal, oxidative, and mechanical treatments was investigated. The X-ray diffraction results show that the single \(\upbeta\) phase is formed at 600 °C within 1 h using pre-mixed Mo and Si powders with a Si/Mo molar ratio of 2.00–2.25. By energy-dispersive X-ray spectroscopy, the produced powder is found to be Si-deficient with a Si/Mo molar ratio of 1.87–1.96. The differential thermal analysis shows that the \(\upbeta\) phase transforms into the \(\upalpha\)-MoSi2 phase at 815 °C at 10 K/min with the segregation of a small amount of Mo5Si3. The transformation heat is −5.5 kJ/mol and the activation energy calculated by the Kissinger method is 290 kJ/mol. Thermogravimetry reveals that the \(\upbeta\)-MoSi2−x powder oxidizes significantly at 400–600 °C via the pest oxidation mechanism while it is resistant to oxidation at 700 °C for 5 h similarly to the \(\upalpha\)-MoSi2 phase. At last, mechanical milling on the \(\upbeta\)-MoSi2−x powder with a planetary ball mill up to 216 h demonstrates that this powder is stable under a severe mechanical treatment.

Phase formation and impurity effects in Ar+ ion irradiated Mo/Si thin film bilayer system

We report a dose‐dependent phase evolution in Mo/Si bilayer system upon Ar+ ion beam irradiation and subsequent flash annealing at 800 °C for 60 s. Micro‐structural characterization with Grazing Incidence X‐ray Diffraction and Raman scattering reveals a dose‐dependent nucleation of polymorphic phases occurring at the amorphized interface region. The ion beam mixing process has been investigated by Secondary Ion Mass Spectrometry and Rutherford Backscattering Spectrometry. While low ion doses favour nucleation of only metastable MoSi2 phase, co‐existence of polymorphic phases are observed at high ion doses. The persistence of such polymorphic phases even after a high‐temperature anneal for high dose implanted specimen is indicative of phase retardation. The phase retardation of h‐MoSi2 to t‐MoSi2 is accounted in terms of nucleation and growth process. Copyright © 2012 John Wiley & Sons, Ltd.

Interaction of molybdenum with copper–silicon melts

The solubility of molybdenum in copper–silicon melts and the kinetics of growth of Mo5Si3 and MoSi2 layers on the molybdenum–melt interface at 1200°C are studied. At 0.20–0.36 at. parts Si, the solubility of molybdenum in the melts is described by the following equation: lgX Mo = –2.587 + + (3.282)X Si. The dependence of the growth rate constants of the Mo5Si3 layer on the activity of silicon in the 9–15 wt.% Si melt is established: kMo–Si = –0.3806 + (65.191) aSi. It is shown that two phases form at 20 wt.% Si: Mo5Si3 and MoSi2. Their thicknesses are 29.8 ± 2.9 μm and 25.7 ± ± 2.8 μm, respectively. The time dependence of the Mo5Si3 layer growth rate in Cu–Si melts is established to obey a parabolic law. The melt composition in three-phase equilibrium Mo–Mo5Si– melt is found to be (at. parts): XSi = 0.147; XMo = 7.86∙10–3; the rest is copper. At 35 wt.% Si, only the MoSi2 phase is formed at the interface.

Modification of parameters in mechanochemical synthesis to obtain - and -molybdenum disilicide

In this paper, molybdenum disilicide <alpha>- and <beta>-phases can be successfully synthesized during mechanical alloying (MA). Also, this method promote a self-propagating reaction (MSR) at balls to powder ratio (BPR) 10:1, shorter milling time with speed (400rpm) without subsequent heat treatment that was considerably lower energy than that used in conventional methods. Two different molar ratios of Mo:3Si and Mo:4Si were prepared in addition to the stoichiometric powder mixture Mo:2Si intermittent sampling was done from 4h to 20h. Increasing Si content clearly delayed the MSR and the reactants were gradually converted to both <alpha>- and <beta>-MoSi2 phases over a relatively long time. Samples were characterized by using X-ray diffraction (XRD)/scanning electron microscopy (SEM) analyses and grain size calculated based on the conventional Scherrer method. XRD patterns of stoichiometric powder samples milled with BPR 10:1 indicated the rapid formation of <alpha>- and <beta>-MoSi2 even after 4h milling. Samples milled with higher BPR lost their crystallinity after milling for 16h. SEM images in general showed considerable lowering in average particle size with milled samples. Crystallite size was found to decrease with milling time and with increasing BPR.

Fabrication and mechanical properties of SiCw/MoSi2–SiC composites by liquid Si infiltration of pyrolyzed rice husk preforms with Mo additions

Dense SiC ceramic matrix composites containing SiC whiskers (SiCw) and MoSi2 phase (SiCw/MoSi2–SiC) are fabricated by a liquid Si infiltration (LSI) method. Pyrolyzed rice husks (RHs) containing SiC whiskers, particles and amorphous carbon are mixed with different amounts of Mo powder to form preforms for the infiltration. Microstructure and mechanical properties of the composites are studied. Fracture mode of the composites is discussed. Results show that the SiC whiskers and fine particles in the pyrolyzed RHs were preserved in the composites after the LSI process. The amorphous carbon and Mo powder in the preforms reacted with molten Si, forming SiC and MoSi2 in the composites. The presence of MoSi2 in the composite increases the elastic modulus but lowers the flexure strength. Content of MoSi2 of ca. 20wt.% provides an enhanced fracture toughness of 4.1MPam1/2 for the composite. But too large amount of MoSi2 caused crack formation in the composite. The compressive residual stress introduced by the formation of MoSi2 and SiC, and the de-bonding of the fine SiC particles and SiC whiskers from the residual Si phase are considered to favor the fracture toughness of the composites.

Formation of MoSi2 and Al doped MoSi2 coatings on molybdenum base TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy

Formation of aluminium (Al) doped molybdenum di-silicide (MoSi2) coatings was studied to improve the high temperature oxidation behavior of TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy. The pack composition of the halide activated pack cementation process was successfully optimized to form silicide and Al doped silicide coatings on the TZM alloy substrates. Mo(Si, Al)2 phase was found to form at the outer layer of the coating prepared by doping Al in MoSi2. A change in composition of the phases with increase in coating temperature was detected with Al doping, whereas un-doped silicide coating process was dominated by the formation and growth of MoSi2 phase. Oxidation test and the characterization studies using SEM, EDS, XRD, and micro-hardness measurements indicated the improved performance of Al doped silicide coating during high temperature oxidation in dry air due to the formation of the protective alumina scale.

Oxidation Behaviors of Two and Three Phase Mo-Si-B Alloys via Si Pack Cementation

Mo-Si-B alloys have been received an attention due to the high temperature strength and phase stability. However, the nature of poor oxidation resistance often limits the application of the alloy system. In order to resolve the poor oxidation resistance of the alloy system, in this study, the oxidation behaviors of Si diffusion coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of the constituent of the phase combination of Mo-Si-B alloys. The oxidation tests performed at 1100 °C show that the produced MoSi2 phase, as a result of the coatings, give an excellent oxidation resistance at prolonged high temperature exposure in air. The oxidation behaviors of uncoated and Si coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

Synthesis and characterization of MoSi2-Mo5Si3 nanocomposite by mechanical alloying and heat treatment

The nanocomposite of MoSi2-Mo5Si3 powder was synthesized by mechanical alloying from Mo and Si powder mixture at room temperature. The phase evaluation of powder after various milling durations and heat treatments were assessed via X-ray diffraction (XRD) and a differential thermal analysis (DTA). Morphology and microstructure of powder particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that nanocomposite of MoSi2-Mo5Si3 powder was synthesized by combustion reaction of Mo and Si powder using ball milling. In the early stages of ball milling β-MoSi2 was produced. However with continued milling for 48h α-MoSi2 and Mo5Si3 phases were formed. DTA results of 24h and 48h as milled mechanical alloyed specimens showed a well-defined peak at 852°C and 920°C relating to the formation of α-MoSi2. The activation energy for 24h and 48h milled specimens were –128.6KJ/mol and –121.4KJ/mol respectively. Annealing the milled specimens at 1000°C for 2h revealed the phase transformation of β-MoSi2 to α-MoSi2 and the formation of Mo5Si3. The crystallite size of α-MoSi2 and Mo5Si3 were about 9nm and 12nm after 48h mechanical alloying. These values increased slightly to 18nm and 14nm after annealing at 1000°C.

Superplasticity of a multiphase fine-grained Mo–Si–B alloy

Mo–9Si–8B–3Hf alloy consisting of a Mo solid solution and intermetallic phases Mo 3Si and Mo 5SiB 2 was fabricated by hot pressing sintering to yield a fine microstructure with all three phases being in the size range of micrometer. The tensile properties of this alloy were evaluated in vacuum at elevated temperatures. This alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400 °C to 1560 °C with strain rate of 3 × 10 − 4 s − 1 . The tensile elongation of 410% was measured at 1560 °C. Grain boundary sliding was the main mechanism of plastic deformation for this alloy.

Deformation Behavior of a Multiphase Fine-Grained Mo-Si-B Alloy

Mo-9Si-8B-3Hf alloy consisting of a Mo solid solution and intermetallic phases Mo3Si and Mo5SiB2 was fabricated by hot pressing sintering to yield a fine microstructure with all three phases being in the size range of micrometer. The tensile properties of this alloy at elevated temperature were evaluated in vacuum at elevated temperatures. This alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400 °C to 1560 °C with strain rate of 3×10-4 s-1. The tensile elongation of 410% is measured at 1560 °C. Grain boundary sliding is the main mechanism of plastic deformation for this alloy.

Microstructure and Oxidation Behavior of Molybdenum Disilicide Coating Prepared by Air Plasma Sprayed

Intermetallics molybdenum dislicied has a great potential as a protective coating in aircraft engines and gas turbines in the elevated temperature. The suit for plasma spraying MoSi2 powders were prepared by spray drying process and vacuum sintered. The oxidation behaviors of the coating were determined at 1200 °C. The coatings as sprayed and oxidized were characterized by XRD, SEM and EDS. Results show that the flow ability and loose density of MoSi2 powder by sintered treatment, were 17.1 s/50g and 2.1g/cm3, respectively, ideal for air plasma spraying. During the course of spraying, some of molybdenum disilicide with a tetragonal lattice was converted into molybdenum disilicide with a hexagonal lattice. Also, part of MoSi2 phase oxidized and transformed to Mo5Si3 phase. A relative dense molybdenum disilicide coating was prepared by air plasma spraying. A protective SiO2 layer, seems to be glassy, with a thickness about 10 μm was formed on the surface of MoSi2 coating during MoSi2 coating oxidized at 1200°C for 200 h. The results of the oxidation tests show that MoSi2 coating prepared by air plasma spraying may be provide a protect layer for high temperature structure material.

Influence of MoSi2 content in suspension on the phase, microstructure and properties of hydrothermal electrophoretic deposited SiC n –MoSi2 coating for SiC pre-coated C/C composites

To protect carbon/carbon (C/C) composites from oxidation at high temperature, a nano SiC–MoSi2 (SiCn–MoSi2) coating on SiC pre-coated C/C composites was prepared by hydrothermal electrophoretic deposition. The phase composition, surface and cross-section microstructures of the prepared SiCn–MoSi2 coating deposited with different MoSi2/SiCn mass ratio were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The influence of MoSi2 content in the hydrothermal electrophoretic deposition suspension on the phase composition, microstructure and high-temperature oxidation resistance of the multi-layer coatings were investigated. Results showed that the content of MoSi2 phase in the prepared coating increases with the increase of MoSi2 content in the suspension. The density and oxidation resistance of the SiCn-MoSi2 coating improve with the increase of MoSi2 mass content from 20 to 60 wt% in the deposition suspension. However, micro-cracks and micro-holes in the coating are found when deposited with 80 wt% MoSi2, and a decrease in oxidation resistance was also detected. The multi-layer coatings deposited with suspension of 60 wt% MoSi2 exhibited the best anti-oxidation ability, which can effectively protect C/C composites from oxidation in air at 1,873 K for 90 h with weight loss of 2.08%.

Preparation of Spraying Powders and Microstructure of Molybdenum Disilicide Coating

With the size of 1-2μm MoSi2 powder as raw material,MoSi2 powder ideal for air plasma spraying was prepared by spray drying and vacuum sintering.By using MoSi2 powders(9.68μm) and agglomeration powders(38-72μm) as the spray feedstock,MoSi2 coatings were successfully prepared using air plasma spraying.The phase composition and microstructure of the coating were studied.The results show that the mobility and loose density of the agglomeration powders sintered at 1300℃ for 1h are 17.1s/50g and 2.1g/cm3,respectively.Compared with the unsin-tered powders,the mobility and loose density of the sintered agglomeration powders increase by 57.0% and 46%,re-spectively.XRD patterns shows the main phases are Mo and Mo5Si3 phases in the coating prepared by the powders with the size of 9.68 μm.In case of spraying with agglomeration powders,the main phase of the dense coating is MoSi2.The "mesh" microstructure of molybdenum-rich phase is produced in the coating prepared by the agglomeration powders.