MoSi2 Composites Research Articles

Materials for temperatures above 1500°C in oxidizing atmospheres. Part II: Experimental results on the oxidation of MoSi2 composites and coated RSiC

The topic Materials for temperatures above 1500 °C in oxidizing atmospheres is discussed in a 3 part publication. In the first part a literature survey had been performed leading to the conclusion that either particle or fiber reinforced MoSi 2 -based materials or RSiC coated with a MoSi 2 -based layer are suitable for applications at such high temperatures. In the present part such material systems are investigated experimentally at 1500 and 1600 °C with respect to their oxidation resistance in air. Particular interest was given to the influence of the Mo 5 Si 3 content in MoSi 2 and the influence of second phase particles in MoSi 2 consisting of ZrB 2 and SiC. Furthermore, the oxidation resistance of several dip-coatings on RSiC which were manufactured from a polysiloxane precursor by subsequent pyrolysis were also investigated. Part III which reports about contact corrosion between different materials at these temperatures will follow in the next issue.

An improvement in processing and fabrication of SiC-whisker-reinforced MoSi2 composites

An improvement in processing and fabrication of SiC-whisker-reinforced MoSi2 composites

Effect of Processing Methods on Purity, Structure of Interfaces and High Temperature Strength of MoSi2 and MoSi2-SiC Composites

The effect of processing methods on the purity and structure of MoSi2 grain boundaries and MoSi2-SiC interfaces and their elevated temperature mechanical properties have been investigated. The processing techniques involve formation of MoSi2 by in-situ reaction, hot pressing of elemental Mo and Si powders and conventional hot pressing of commercial grade Starck MoSi2 powders. The SiO2 contents of the hot pressed products have been found to be sharply different, depending on the processing methods and the composition of the raw materials used. Transmission electron microscopy (TEM) examination has shown that reaction-hot-pressed MoSi2 based materials possess atomically abrupt grain boundaries (matrix) and interfaces (MoSi2-SiC composites), which are free of amorphous layers, while Starck MoSi2 contains globular SiO2 particles. Compression tests in the range of 1100°-1300°C have shown that the reaction-hot-pressed MoSi2 and MoSi2-SiC composite have superior properties. The mechanisms of high temperature deformation and serrated flow behaviour have been investigated using TEM of deformed specimens.

Grain Boundaries and Interfaces in MoSi2 and MoSi2-SiC Composites and Their Effect on High Temperature Strength

Abstract Molybdenum di-silicides (MoSi2) based materials have a strong potential for high temperature structural applications due to high melting point of 2030°C, outstanding elevated temperature oxidation resistance and limited ductility above a temperature range of 1100-1300°C. The major shortcomings of MoSi2 for structural applications are its poor room temperature fracture toughness and low high temperature strength. Sustained efforts including reinforcing MoSi2 with ceramic reinforcements, alloying and in-situ processing, have been made to improve these properties. The purity of grain boundaries and interfaces, which in turn depends on the processing method plays a significant role in the high temperature properties and this paper aims to show that. Intimately mixed Mo and Si powders (Mo:Si = 63:37 by weight fraction) were reaction hot pressed (“RHP“) in vacuum at 1500°C for 1 h, using a pressure of 26 MPa. During the hot pressing process, Mo and Si reacted to form MoSi2.

Microstructure and Mechanical Properties of MoSi<SUB>2</SUB>/Nb Composites Made by MA-Pulse Discharge Sintering

MoSi2 is one of the most promising candidates for high-temperature structural materials because of its high melting point and excellent oxidation resistance. However, the low room temperature fracture toughness and ductility, etc., still remain as major disadvantages to potential high temperature structural applications. MoSi2 composites containing Nb have shown a great improvement in mechanical properties, especially in fracture toughness, over the monolithic MoSi2. However, the addition of refractory metals such as Nb or W has not been that acceptable because of the high reactivity of these additions with the MoSi2 matrix, and the deleterious effect of reducing oxidation resistance and increasing density. Also, Nb reinforced MoSi2 composites resulted in a penalty with decreasing strength, both at room and elevated temperatures. In the present study, MoSi2 and MoSi2–Nb powders with a fine grain size were prepared by mechanical alloying. In order to retain the fine microstructure in the consolidated samples, pulse discharge sintering was used to consolidate these powders at a lower sintering temperature compared to that of the conventional hot pressing or HIP . The grain size of the consolidated MoSi2–10Nb composite was 0.18 to 1.2 μm, which was finer than that of monolithic MoSi2, 1.5 to 3.0 μm. The formation of the (MoNb)5Si3 phase was pronounced in the Nb added composites, and this phase seemed to be formed during the sintering process. Both monolithic MoSi2 and MoSi2–10Nb composites from the mechanically alloyed powders showed a significant amount of plastic deformation in a bending test at 1073 K . The yield stress of the Nb added alloy was higher than that of the monolithic MoSi2, which may be caused by the refinement of the microstructure.

Effect of MoSi2 and Nb reinforcements on mechanical properties of Al2O3 matrix composites

The room temperature mechanical properties of Al2O3 composites reinforced with 25 vol% of either MoSi2 or Nb particulates were investigated. It was found that addition of Nb particles resulted in a reduction in the elastic modulus, but caused a significant increase in both flexural strength and fracture toughness. On the other hand, the addition of MoSi2 particles resulted in only a marginal decrease in elastic modulus and marginal increase in both flexural strength and fracture toughness. The elastic modulus results were explained on the basis of Tsai - Halpin model. For both the composites, the increase in flexural strength was attributed to the grain refinement of the Al2O3 matrix as well as the load transfer to the reinforcement particles. The marginal increase in fracture toughness in Al2O3 / MoSi2 composites was attributed to crack deflection, whereas the threefold increase in fracture toughness in Al2O3 / Nb composites was attributed to crack blunting and bridging.

Synthesis of a (MoSi2, Mo5Si3)/SiC Composite Using an In Situ Solid-State Displacement Reaction between Mo2C and Si.

The intermetallic compound, MoSi2, has great potential as an elevated-temperature structural material but it exhibits some undesirable properties such as low fracture toughness and low strength at elevated temperatures. Improvement of these properties can be achieved by reinforcing MoSi2 with the addition of SiC particles or whiskers, or by alloying it with Mo5Si3. The preparation of MoSi2 composites from powders gives rise to an oxidation problem since SiO2 covers the raw powder. To avoid this problem, the material must be prepared in situ. In this work, multiphase composites based on the Mo-Si-C system have been prepared using a solid-state displacement reaction between Mo2C and Si. Samples were prepared from a mixture, containing different proportions of Si and Mo2C powders, which was pelletized under vacuum at 40MPa and heat-treated in argon. The pieces obtained exhibit different proportions of SiC, MoSi2 and Mo5Si3. The microhardness obtained ranges from 12 to 14GPa, and Kc is over 8MPa·m1/2, which is higher than that of pieces made of MoSi2 only.

Molybdenum Silicide Based Materials and Their Properties

Molybdenum disilicide (MoSi2) is a promising candidate material for high temperature structural applications. It is a high melting point (2030 °C) material with excellent oxidation resistance and a moderate density (6.24 g/cm3). However, low toughness at low temperatures and high creep rates at elevated temperatures have hindered its commercialization in structural applications. Much effort has been invested in MoSi2 composites as alternatives to pure molybdenum disilicide for oxidizing and aggressive environments. Molybdenum disilicide-based heating elements have been used extensively in high-temperature furnaces. The low electrical resistance of silicides in combination with high thermal stability, electronmigration resistance, and excellent diffusion-barrier characteristics is important for microelectronic applications. Projected applications of MoSi2-based materials include turbine airfoils, combustion chamber components in oxidizing environments, missile nozzles, molten metal lances, industrial gas burners, diesel engine glow plugs, and materials for glass processing. In this paper, synthesis, fabrication, and properties of the monolithic and composite molybdenum silicides are reviewed.

Mechanical Properties of SiC-Whisker-Reinforced MoSi<sub>2</sub> Composites

Molybdenum disilicide (MoSi2) is considered as a candidate material for high temperature structural applications, particularly in oxidizing environments. In the present study, SiC-whisker reinforcement has been applied to improve the mechanical properties such as fracture strength and fracture resistance at room temperature as well as creep resistance at elevated temperature. Almost dense SiC-whisker-reinforced MoSi2 composites containing up to 20vol% whisker were fabricated by hot-pressing at 1750°C. Due to the well-dispersed composite structure with fine grained matrix, the composites showed much higher bending strength than unreinforced materials; the average 4-point bending strength increased from 350MPa for MoSi2 to 870MPa for MoSi2-20%SiCw. The fracture resistance also considerably improved by the SiC-whisker reinforcement. The composites exhibited increasing R-curve behavior and the plateau values were about 23-34% higher compared to that of the unreinforced materials. Crack bridging was found to be mainly responsible for the toughening contribution. Creep resistance was remarkably increased by the suppression effect of SiC-whisker on the grain boundary sliding, which was confirmed to be an important deformation mechanism in MoSi2 polycrystals.

Simultaneously MoSi2 and SiC-reinforced α-SiAlON composites

Simultaneously MoSi2 and SiC-reinforced α-SiAlON composites

A study of the combustion synthesis of MoSi2 and MoSi2-matrix composites

The combustion synthesis of MoSi2 from elemental powders was investigated. Higher completion of the reaction was achieved by the use of prior compaction of powders and higher heating rates. The reaction mechanism was studied by performing experiments at a heating rate of 20 K/min. It was established that reaction occurred via capillary motion accompanying the flow of molten silicon to the molybdenum powder particles. During the synthesis of MoSi2/TiC composites, two ternary (Mo,Ti) Si2 phases were observed, in contradiction to the thermodynamic predictions of the Mo-Si-Ti-C system. Combustion synthesis of (Mo, W) Si2 yielded an inhomogeneous product where the composition of the solid solution varied throughout the sample. These observations were discussed in terms of the characteristics of the reaction process.

Fatigue and fracture behavior of Nb fiber-reinforced MoSi2 composites

Fatigue and fracture mechanisms in Nb fiber-reinforced MoSi2 composites are elucidated in this article. The effects of fiber diameter on fracture and crack-tip shielding mechanisms are discussed after a review of micromechanical models which are applied to the prediction of residual stress levels, toughening, and microcracking phenomena. Toughening is shown to occur by a combination of crack bridging and crack-tip blunting under monotonic and cyclic loading. However, the observed failure mechanisms are different under monotonic and cyclic loading. Composites with smaller (250-μm) fiber diameters are shown to have better fatigue resistance and lower fracture toughness than composites with larger (750-μm) fiber diameters. The occurrence of slower fatigue crack growth rates in the composites reinforced with smaller diameter Nb fibers is rationalized by assessing the combined effects of fiber spacing and interfacial crack growth on the average crack growth rates within the composites.

Abrasive Wear Behavior of a Si3N4‐MoSi2 Composite

MoSi2 particles (20 vol%) have been added to Si3N4 to form ceramic matrix‐intermetallic composites. Benefits associated with the addition of the MoSi2 to Si3N4 include higher strength, higher fracture toughness, no loss in oxidation resistance, and lower electrical resistivity. However, because the hardness of MoSi2 is approximately half that of Si3N4, a decrease in the specific wear rate of the Si3N4‐20 vol% MoSi2 composite is expected to result from the incorporation of the MoSi2 into the Si3N4. In this U.S. Bureau of Mines and Los Alamos National Laboratory study, it is found, however, that the specific wear rate of the composite during two‐body abrasion by SiC particles is equivalent in magnitude to the specific wear rate of monolithic Si3N4. The specific wear rates of both the Si3N4‐20 vol% MoSi2composites and monolithic Si3N4 are four to five times less than that of monolithic MoSi2.

Fabrication of SiC‐Whisker‐Reinforced MoSi2 Composites by Tape Casting

In this paper, we describe a procedure for the processing of SiC‐whisker‐reinforced MoSi2 composites via tape casting. Based on the characteristics of SiC whiskers and MoSi2 powder in aqueous and nonaqueous solvents, a slip formulation (solvent, dispersant, binder, etc.) has been developed. The formulation developed allows for a uniform distribution of SiC whiskers in the matrix, easy separation of the tapes from the polymeric carrier, convenient control of both tape thickness and orientation of SiC whiskers, and a low binder burnout temperature. The latter is important for the prevention of the oxidation of MoSi2 powder during the binder burnout in an oxidizing atmosphere.

Effect of B2O3 on the microstructure and strength of SiC/MoSi2 composite

The effect of boron oxide (B2O3) on the microstructure and strength of 20% (by volume) silicon carbide (SiC) reinforced molybdenum disilicide (MoSi2) composites was examined. Microstructural analysis using optical and electron microscopes was performed. A standard four-point bending test and Weibull statistics were applied to evaluate the flexural strength of the composites. The addition of boron oxide to composite was found to reduce the processing temperature and the grain sizes, result in formation of glassy boundary phase, and to increase the flexural strength of the composite.

Toughening MoSi2 with niobiúm metal—effects of morphology of ductile reinforcements

Morphology effect of ductile reinforcements was evaluated using a four-point bend test on chevron-notched MoSi2 composites reinforced with 20 vol. % niobium. The niobium used had three different morphologies, i.e., fibre, foil and particle. The thickness of the foils was 250 Μm, while the fibres and particles had diameters of 250 and 200 Μm, respectively. Toughness of MoSi2 composites was increased from 3.3 MPa m1/2 for the matrix to 15 MPa m1/2 with the incorporation of the Nb fibres or foils. The particulate composites also exhibited an increase in toughness (∼7 MPa m1/2). The toughening achieved was mainly attributed to ductile phase bridging in all the composites tested. The relatively small toughness improvement in the particulate composites was ascribed to the embrittlement of the Nb particles. The results indicate that toughening by crack bridging depends mainly on the intrinsic properties of the ductile bridging ligaments rather than on their morphology, and that the embrittlement of the bridging ligament is detrimental to the toughening of the composites.

Thermal Fatigue of MoSi2 Particulate and Short Fiber Composites

AbstractInduction heating of disk shaped specimens was used to compare and contrast the thermal fatigue behavior of MoSi2 and MoSi2-based composites. Specimens were subjected to 5 s heating and cooling cycles between temperature limits of 700°C and 1200°C. The monolithic material and a MoSi2- 10 vol% TiC composite exhibited poor thermal shock resistance and could not be thermally cycled according to this temperature-time profile. A 30 vol% TiC composite exhibited much better thermal shock and thermal fatigue resistance as compared to the monolithic material, but exhibited undesirable oxidation. MoSi2-10 and 30 vol% SiC particulate composites exhibited excellent thermal shock and thermal fatigue resistance compared to that of the monolithic material. A MoSi2-10 vol% SiC whisker composite did not show improved thermal fatigue resistance due to the initial processing defects present in the material. The monolithic material and the 10 vol% TiC composite were also subjected to 30 s heating and cooling cycles between temperature limits of 700°C and 1200°C. Both of these materials exhibited better thermal fatigue resistance at this temperature-time profile, but the 10 vol% TiC composite also exhibited undesirable oxidation. The fatigue results are discussed with reference to the initial microstructure of the specimens and the stress-strain history of the specimens which was obtained by a thermoelastic finite element analysis.

Wear Behavior of MoSi2 and MoSi2 Matrix Composites

AbstractThe U.S. Bureau of Mines is examining the wear behavior of a variety of advanced materials, including the intermetallic compound MoSi2. The high stiffness and hardness of MoSi2 make it attractive for use in applications requiring wear resistance. This research reports on the results of pin abrasion wear tests for a variety of powder processed MoSi2 and MoSi2 matrix composites. The effect of the addition of ductile (Nb) and brittle (SiC) reinforcements, as well as the influence of reinforcement type and orientation geometry, on abrasive wear is discussed. Comparisons of the wear behavior of MoSi2 and MoSi2-based composites with other materials, such as refractory metals, aluminides, and ceramics, are made.

Properties of Silicon Nitride–Molybdenum Disilicide Particulate Ceramic Composites

The physical and mechanical properties of hot‐pressed Si3N4–MoSi2 particulate composites containing 15 and 30 vol% MoSi2 were studied. The average room‐temperature four‐point bend strength, fracture toughness, and electrical resistivity are 522 MPa, 3.6 MPa·√m, and 6.3 × 105Χ·cm for the 15 vol% MoSi2 composite, and 487 MPa, 5.3 MPa·√m, and 0.31 Ω·cm for the 30 vol% MoSi2 composite. The mechanical properties of the composites are very close to those of hot‐pressed Si3N4 ceramics. The high electrical conductivity of the 30 vol% MoSi2 composite was attributed to the percolation effect of MoSi2 particles. Parabolic oxidation behaviors were observed for the 30 vol% MoSi2 composite during the 1200°C long‐term oxidation experiments.

Thermochemical Evaluation of Combustion Synthesis of MoSi2─SiC Composites

Thermochemical calculations were carried out for the selfpropagating high‐temperature synthesis (SHS) of SiC‐particulate‐reinforced MoSi2 composites. These composites can be prepared using the elemental powders of Mo, Si, and C by SHS. Adiabatic temperatures were calculated at different initial temperatures for the reactions forming MoSi2, SiC, and their composites. Amounts of liquid phase of MoSi2 formed at various temperatures and mole ratios were computed.