Papers by Keyword: Molybdenum Disilicide

Abstract: In this study, oxidation behavior of ZrB₂-MoSi₂-SiC composite was investigated in the hot-pressed 5-20 vol% SiC-containing ZrB₂-20 vol% MoSi₂-based composites which were exposed to dry air between 1100°C and 1500°C up to 10 hours. The effects of SiC additive on the oxidation behavior were assessed. Experimental results showed that the weight gain due to oxidation exposure in air increased with increasing exposure temperature and exposure time. Parabolic oxidation behavior was observed for all the compositions composites. On the other hand, the weight gain decreased with increasing amount of SiC added. The addition of SiC improved the oxidation resistance of the composites, and the improvement was enhanced with increasing amount of SiC added. In addition, X-ray diffraction was used to identify major crystalline phases present in both the as-received and the post-oxidized composites. The oxidized sample surface was characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The microstructure of the post-oxidized composites consisted of two characteristic regions: oxidized reactive region and unreactive bulk material region. Furthermore, the oxidized reactive region divided into an outermost dense silica-rich scale layer and oxidized reactive mixture layer. The improvement of the oxidation resistance due to the addition of SiC is associated with the presence of the thicker dense outermost scale layer which inhibited inward diffusion of oxygen through it.

Abstract: In this study, the basic limitations of superalloys in high temperature performances will be explained and then after giving the important properties of MoSi₂ such as oxidation behaviour at relatively low temperatures (500°C-700°C) , some interesting composites of this material will be discussed as a candidate structural material in gas turbine engines.

Abstract: An experimental study on the preparation of molybdenum disilicide (MoSi2) was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts with different pelletizing pressures (0, 50, 100 and 200MPa). The composition of test specimens is Mo:Si = 1:2. The results showed that the propagation time of the reaction front, the density of the combustion products, the particle size and the morphology of MoSi2 were significantly influenced by pelletizing pressure of the reactant compact. In addition, all the samples obtained with different pelletizing pressures were single-phase MoSi2.

Abstract: AlN-ZrO₂ system composites were prepared with Ca-stabilized zirconia (Ca-PSZ, particle size d₅₀=29.3 μm) and AlN power (d₅₀=0.5 μm) as raw material, MoSi₂ (d₅₀=2 μm) and Si₃N₄ (d₅₀=44 μm) as additives respectively, in N₂ atmosphere by pressureless sintering at 1600 °C. The phase compositions were investigated by XRD. The sintering performance of the composites was measured by the checking of the bending strength and the apparent porosity of samples. The anti-oxidation performance of the composites was analyzed by the checking of oxidation weight increased of samples. The results showed that phase compositions of the composites consisted of ZrO₂, ZrN, CaAl₁₂O₁₉ and CaA_l2Si₂O₈when the content of AlN was 8 wt %. The additives promoted the sintering of AlN-ZrO₂ system composites. As a result, introduction of the additives increased the oxidation of the composites due to oxidation expansion of phases ZrN.

Abstract: MoSi2 based composites have been recognized as a potential candidate materials for high temperature structural applications. Although, MoSi2 has been reinforced by a variety of ceramic particles, ductile phase toughening of MoSi2 has received only limited attention. In the present investigation, ductile refractory metal (Mo and Ta) foils were used as reinforcements in MoSi2 matrix to prepare the laminated composites. 20 vol% SiC particles (SiCp) were also added in MoSi2 matrix to reduce the thermal expansion mismatch between MoSi2 and the ductile metal foils. The improved bonding between MoSi2-SiCp matrix layer and Mo / Ta foils due to reduced residual thermal stresses resulted in significant improvement in the room temperature fracture toughness of both the composite systems over the monolithic MoSi2.

Abstract: Provided in this article are the quantitative and qualitative morphological results describing the action of several nanostructured surfaces for bactericidal and bacteriostatic action. Results are also provided to illustrate microbial corrosion and its impact. Biofilm formation is correlated to colony formation. Nanostructured surfaces, i.e. surfaces with welded nanoparticles are noted to display biocidal activity with varying efficacies. Porous nanostructures, on stainless steel and copper substrates, made of high purity Ag, Ti, Al, Cu, MoSi2, and carbon nanotubes, are tested for their efficacy against bacterial colony formation for both gram-negative, and gram-positive bacteria. Silver and Molybdenum disilicide (MoSi₂) nanostructures are found to be the most effective bactericidal agents with MoSi2 being particularly effective in both low and high humidity conditions. Bacteriostatic activity is also noted. The nanostructured surfaces are tested by controlled exposures to several microbial species including (Gram+ve) bacteria such as Bacillus Cereus and (Gram-ve) bacteria such as Enterobacter Aerogenes. The resistance to simultaneous exposure from diverse bacterial species including Arthrobacter Globiformis, Bacillus Megaterium, and Cupriavidus Necator is also studied. The nanostructured surfaces were found to eliminates or delay bacterial colony formation, even with short exposure times, and even after simulated surface abrasion. The virgin 316 stainless steel and copper substrates, i.e. without the nanostructure, always displayed rapid bacterial colony evolution indicating the lack of antimicrobial action. The efficacy of the nanostructured surface against colony formation (bacterial recovery) for E-Coli (two strains) and virus Phi 6 Bacteriophage with a host Pseudomonas Syringae was also studied. Preliminary results are presented that also show possible anti-fungal properties by the nanostructured MoSi₂. When comparing antimicrobial efficacy of flat polished surfaces (no curvature or nanostructure) with nanostructure containing surfaces (high curvature) of the same chemistry, shows that bacterial action results from both the nanostructure size and chemistry.

Abstract: Molybdenum disilicide (MoSi₂) is an interesting material for high-temperature applications. It has a high melting temperature, good thermal and electrical conductivity and an excellent oxidation resistance. For many years the primary use of MoSi₂ has been in heating elements, which can be used for temperatures up to 1800°C. Since the 1990s the potential of MoSi₂ as a high-temperature structural material has been recognized as well. Its brittleness at lower temperatures and a poor creep resistance above 1200°C have hindered its use as in load-bearing parts. These disadvantages may be offset at least partly by using it together with a second material in a composite or an alloy. Projected applications of MoSi₂-based materials include, e.g. stationary hot section components in gas turbine engines and glow plugs in diesel engines. For future research and development directions of MoSi₂-based composites diffusion is a crucial property because creep is closely connected with diffusion. This paper is devoted to the basic diffusion and defect properties of MoSi₂. Data of Si and Mo as well as Ge diffusion from the Münster laboratory for both principal directions are briefly summarized. For all three kinds of atoms diffusion perpendicular to the tetragonal axis is faster than parallel to it. The diffusivities of Mo in both directions are many orders of magnitude slower than those of Si and Ge. The huge asymmetry between Mo and Si (or Ge) diffusion suggests that atomic motion of each constituent is restricted to its own sublattice. Positron annihilation studies on MoSi₂ from the Stuttgart laboratory are reviewed as well. They show that formation of thermal vacancies occurs primarily on the Si sublattice but cannot exclude vacancy formation on the Mo sublattice at higher temperatures. Correlation factors for Si and Mo diffusion via sublattice vacancies in the respective sublattices of MoSi₂ have been calculated recently mainly by Monte Carlo simulation techniques and are also briefly described. Diffusion, in particular self-diffusion, is discussed in connection with literature data on high-temperature creep, which is diffusion-controlled. Grain-size effects of creep have been reported and can be attributed to Nabarro-Herring and Coble creep. Power-law creep is attributed to diffusion-controlled dislocation creep. Some details are, however, not completely understood, presumably due to a lack of theoretical concepts for creep in uniaxial, stochiometric compounds and due to missing information on grain-boundary diffusion.

Abstract: Al substituted MoSi₂ composites were prepared by using microwave assisted synthesis method, which is a self-propagation synthesis ignited by microwave energy. The composition and microstructure of as combusted powder had been detected by XRD and SEM analysis. Result shows that the pre-heating temperature and phase composition of microwave assisted combustion synthesis is largely influenced by the content of aluminum. The phase structure of Mo(Si, Al)₂ alter from a mixture of MoSi₂ and Mo₅Si₃ to nearly pure Mo (Si, Al)₂ with the increasing of Al substitution. The main mechanism is dissolution separation..

Abstract: 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.

Abstract: Pest oxidation phenomena is the major drawback of MoSi2 for the applications of this material over gas turbine elements. Therefore, addition of Si3N4 does not only eliminate the structural disintegration behaviour which can only be observed between relatively low temperatures (673-873 K) but also improves the fracture toughness at elevated temperatures. In this study, after giving some significant properties of MoSi2, effect of processing conditions to microstructural morphology and oxidation characteristics at gas turbine operating temperatures will be discussed within a composit approach.