Monolithic MoSi2 Research Articles

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.

Abrasive wear behavior of [formula omitted] and [formula omitted] composites

The volume wear behavior of MoSi 2/SiC and MoSi 2/ZrO 2 composites was evaluated using 150 grit SiC particles in a pin-on-drum abrasion test. The addition of SiC whiskers or particles reduced the volume wear of the composite relative to monolithic MoSi 2 by about a factor of two, with the SiC whisker containing composite having a slightly lower volume wear rate than the SiC particulate reinforced composite. The addition of partially-stabilized (PS)-ZrO 2 particles lowered the volume wear of the composite relative to MoSi 2. The addition of unstabilized (US)-ZrO 2 or fully-stabilized (FS)-ZrO 2 particles to the MoSi 2 matrix had little effect of the volume wear relative to the unreinforced matrix. The difference in wear behavior of the ZrO 2 reinforced composites may be associated with the ability of the PS-ZrO 2 particles to transform reducing the fragmentation process during abrasion.

A comparative study of the abrasive wear behavior of MoSi 2

This study is a preliminary assessment of the abrasive wear behavior of monolithic MoSi[sub 2]. Comparisons with the wear behavior of other advanced materials, such as refractory metals, intermetallic compounds (i.e., TiAl, Fe[sub 3]Al), and ceramics (i.e., Si[sub 3]N[sub 4], ZrO[sub 2], and Al[sub 2]O[sub 3]), are made. In general, the wear behavior of MoSi[sub 2] is similar to oxide ceramics, due in large part to the high relative hardness of the compound. However, as with most brittle materials, as the hardness of the abrasive increases relative to the hardness of the wearing material (i.e., MoSi[sub 2] abraded on garnet at 13 GPa versus abrasion on SiC at 24 GPa), volume wear increases; and the dominant wear mechanism changes, from one of primarily edge fracture to one combining micro-cutting with significant micro-fracture and grain pull-out.

セラミックスの合成と評価 ＭｏＳｉ２／Ａｌ２Ｏ３／Ｎｉ／Ａｌ２Ｏ３／ＭｏＳｉ２系傾斜機能材料の作製と評価

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.

Abrasive wear behavior of MoSi 2-Nb composites

Previous research at the US Bureau of Mines has shown monolithic MoSi[sub 2] to be extremely wear resistant in a severe two-body abrasive environment. MoSi[sub 2] is attractive as a wear-resistant material because of its high hardness, high elastic modulus, low density, and excellent resistance to elevated temperature oxidation and hot corrosion. As with most intermetallic compounds, the low temperature fracture toughness is poor; however, reinforcing MoSi[sub 2] with SiC or Nb, for instance, has resulted in improved fracture toughness. It is also known that the wear rate of many materials can be reduced by a factor of up to ten by the introduction of a suitable reinforcement phase. While soft ductile phases have been effective in improving the fracture toughness of brittle matrices, it is intuitive that the addition of these phases should have a negative impact on wear resistance, i.e., the softer phase will wear at an accelerated rate. This study has been undertaken in order to make a preliminary assessment of the effect of adding a ductile reinforcement (Nb) on the abrasive wear resistance of MoSi[sub 2]. The wear behavior of the MoSi[sub 2]-Nb composites produced for this study are compared with the behavior of monolithic MoSi[submore » 2], Nb, aluminides, and ceramics.« less

Ductile Phase Toughening of MoSi2: Effect of Reinforcement Morphology

AbstractThe effects of ductile reinforcement morphology on the mechanical behavior of MoSi2 reinforced with 20 vol.% Nb are presented. While all the composites exhibit improved fracture toughness relative to the monolithic MoSi2, the most significant improvement is obtained in the composite with laminate reinforcement, followed by coarse fiber, fine fiber and particulate reinforcements. Bend strength measurements indicate a ductile-brittle-transition-temperature of ∼1250°C, and highlight the attractive properties of laminate reinforced composite. Preliminary studies of room temperature fatigue crack growth in particulate reinforced composites show a stable fatigue crack growth, which is not achieved in monolithic MoSi2. Possible ways of achieving a better combination of mechanical properties are also discussed.

The Effect of Deformation and Reinforcement Particles on the Grain Growth Behavior of MoSi2

AbstractThe grain growth behavior of polycrystalline MoSi2 and composites containing SiC particulates has been studied in the temperature range of 1200-1800°C during static annealing as well as under concurrent deformation conditions. Monolithic MoSi2, with ∼ 26 μm grain size appears to be extremely resistant to grain growth up to 1500°C. However, the grain growth rate above this temperature is quite rapid. When particulate reinforcements are used to reduce the grain size of MoSi2 to 4.4 μm, a stable microstructure is maintained up to 1500°C. Accelerated grain growth kinetics are observed at 1800°C under conditions of large plastic strain. This enhanced grain boundary mobility is associated with particle sweeping and particle agglomeration effects. At lower temperatures, where dislocation creep is the more dominant deformation mechanism these effects are absent. In the presence of a Si concentration gradient extremely high growth rates of columnar MoSi2 grains have been observed during reaction synthesis of MoSi2.

In-Situ Processing of Mosi2-Base Composites

AbstractThree different methods of preparing Mosi2 and composites reinforced with ceramic fibers by reactive in-situ processing are described. Reactive powder sintering (co-synthesis) of elemental powders, chemical vapor infiltration/deposition and reactive vapor infiltration are examined. Monolithic Mosi2, SiC particle-reinforced Mosi2 or fibrous Mosi2 composites reinforced with Nicalon fiber were prepared. The advantages and disadvantages of these processes relative to more traditional processing techniques such as HIPing of prealloyed powders, mechanical alloying and a reported in-situ displacement reactions are discussed.

The effect of microstructural scale on hardness of MoSi 2-Mo 5Si 3 eutectics

Recently there has been a renewed interest in molybdenum disilicide (MoSi2) as a possible high temperature structural material. Extensive research on the mechanical behavior of monolithic and MoSi2-based materials is being performed and structure property relationships are beginning to be addressed (e.g. 1-4). Of particular interest to this study is the relationship between hardness and the microstructural scale of the MoSi2. It has previously been shown that some intermetallic compounds, like FeCo, FeCo-V and Ni3Mn, (5) obey the Hall-Petch relationship and Tiwari et al (6) have suggested the same for MoSi2. In the case of MoSi2, the reported data cover a range of grain diameters between 7 and 30 Bm (1,2,7-12) and this information is tabulated with the reported hardness values in Table I. It is apparent that a smaller grain size is associated with higher hardness values, but the amount of data is too limited to suggest a specific structure-property relationship. Although the hardness and grain diameter database for monolithic MoSi2 is small, a wide range of values have been reported for MoSi2-SiC particulate composites (13). This data shows that as the SiC content is increased the hardness also increased and the grain size of the MoSi2 decreased. It is interesting to speculate that some of the increased hardness may have resulted from a decreasing grain size.

Strength and Toughness of Silicide Matrix Materials Consolidated by Hot Isostatic Pressing

AbstractSubstantial improvements have been reported in high temperature strength and creep resistance, and room temperature fracture toughness of molybdenum disilicide (MoSi2) reinforced with ductile or brittle reinforcements. The influence of Hot Isostatic Pressing (HIP) process parameters on the mechanical properties of MoSi2 based alloys was studied. Monolithic MoSi2 powder and MoSi2 powder blended with either niobium powder or silicon carbide whisker reinforcements were consolidated by HIP at 1200 − 1400°C, 207 MPa, and 1 - 4 hrs. The HIP'ed compacts were characterized for compression strength and creep resistance at 1100-1300°C. Fracture toughness was measured on single edge notched rectangular specimens at room temperature. Mechanical properties were correlated with post-HIP microstructural features.

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.

Atomization of rapidly solidified aluminium (RS) and magnesium alloy powders : D.G.C. Robertson et al, (University of Missouri-Rolla, Rolla, Missouri, USA.)

Monolithic MoSi2 and MoSi220vol.%SiC composites have been prepared by reaction hot-pressing Mo and Si or Mo, Si and SiC powder mixtures in the range 1450–1500°C. As Si atoms diffuse into the Mo lattice under external pressure, a complete reaction takes place between Mo and Si leading to the formation of MoSi2 phase. The initial Mo particle size affects the grain size of the MoSi2 that is formed. Grain boundaries and interfaces of MoSi2 with SiC were abrupt on the atomic scale, except for some MoSi2/SiC boundaries, where a 5–8 nm thick amorphous phase, believed to be SiO2, could be seen. MoSi2 was also processed by vacuum hot-pressing MoSi2 powder at 1700°C for comparison. The composites show a moderate increase in flexural strength and hardness over monolithic hot-pressed polycrystalline MoSi2. There is, however, a significant increase in the high-temperature compressive yield strength due to addition of SiC reinforcements. Grain sizes of the MoSi2 matrix strongly influence the fracture toughness and related micromechanisms.

Plasticity Enhancement of MoSi2 at Elevated Temperatures Through the Addition of TiC

ABSTRACTMonolithic MoSi2 and MoSi2-TiC particulate composites with 10 vol % and 15 vol % TiC were tested in compression between 950°C and 1200°C. The MoSi2-TiC composites can be deformed plastically at lower temperatures than MoSi2 can before brittle fracture occurs. The composites exhibit much lower strain hardening rates and attain zero strain hardening rate at much lower strains than monolithic MoSi2. The differences between the composites and monolithic MoSi2 in plasticity and in strain hardening behavior is attributed to efficient dislocation generation into the matrix from sources at the MoSi2-TiC interfaces.

The Structure and Properties of MoSi2-Mo5Si3 Composites

ABSTRACTThe addition of Mo5Si3 as a reinforcing second phase in a MoSi2 matrix has been investigated for possible high temperature strengthening effects. MoSi2 with up to 45 vol % Mo5Si3 was fabricated using powder metallurgy (PM) and arc-casting (AC) techniques. Effects of processing routes, which result in different microstructures, on their mechanical properties are given. PM composites, which have an equiaxed microstructure, exhibit a limited increase in hardness. Higher hardnesses are observed in script-structured AC eutectics and Er-modifiedeutectics throughout the temperatures studied (25–1300°C). Crack propagation paths induced by indentation show long transphase cracks in the AC materials vs short intergranular and interphase cracks in the PM composites at high temperatures.Transmission electron microscopy discloses that the interface in the AC composites has a low-index orientation relationship between the two phases and shows regularly faceted interfacial structures, while planar interfaces are found in the PM composites. These observations suggest the interface is stronger and lower in energy in the AC composites, which is consistent with the higher hardness values and long transphase cracks observed.Dislocation analysis shows the presence of ordinary dislocations (<100>, <110> and 1/2<111>) in MoSi2 in the as-fabricated composites. These types of dislocation are also responsible for the high temperature plastic deformation in compression in both the monolithic MoSi2 and the composites. <331> types of dislocation are only found in MoSi2 either near the interface of the AC composites or in materials deformed below 1000°C.

Characterization of the Plasticity Enhancement of MoSi 2Obtained at Elevated Temperatures by the Addition of TiC

Decremental step strain rate tests in compression were performed on MoSi 2 and MoSi 2 -10 vol % TiC at 1150°C and 1200°C. The composite exhibits a lower stress exponent than MoSi 2 between 10 -4 s- 1 and 10 -5 s -1 TEM observations reveal the deformation of the carbide phase, as well as the efficient generation of dislocations into the matrix from the carbide-matrix interfaces. The substructures of both the matrix of the composite and monolithic MoSi 2 appear to consist of mixed dislocations of predominantly edge character which occur on the same slip systems.

自己燃焼反応を利用したＭｏＳｉ２‐Ａｌ２Ｏ３系複合材料のｉｎ‐ｓｉｔｕ合成

MoSi2-Al203 composites with 0-50 vol% Al203 were in-situ fabricate by self-propagating high temperature synthetic process. The compacts of the powder mixture of MoO3, Al, Si and MoSi2 were encapsulated in Pyrex tube and then HIP'ed at 1573K under pressure of 196MPa. Self-propagation synthesis reaction occurred during elevated temperature and pressure.Relative density of HIP'ed samples decreased with increasing addition of MoSi2 pre-alloyed powder to the starting powder mixture.Vickers hardness and fracture toughness were performed to investigate the mechanical property of composites. Vickers hardness increased linearly with increasing volume fraction of Al2O3 tested. The indentation fracture toughness of 50 vol% Al2O3 composite was two times lager than that of monolithic MoSi2.