High temperature structural silicides represent an important new class of structural materials, with significant potential applications in the range of 1200-1600 °C under oxidizing and aggressive environments. Silicides, particularly those based on MoSi2, are considered to be promising due to their combination of high melting point, elevated temperature oxidation resistance, brittle-to-ductile transition, and electrical conductivity. Possible structural uses for silicides include their application as matrices in structural silicide composites, as reinforcements for structural ceramic matrix composites, as high temperature joining materials for structural ceramic components, and as oxidation-resistant coatings for refractory metals and carbon-based materials. The historical development of structural silicides, their potential applications, and important issues related to their use are discussed.

The mechanical and plastic behaviors of refractory silicide single crystals with Cllb (MoSi2), C40 (CrSi2, TaSi2 and NbSi2), D88 (Ti5Si3) and Cl (CoSi2 and (Co0.9Ni0.1)Si2) structures were investigated. The C40–type silicides were deformed by (0001)<1120> slip. Their yield stress decreased sharply with increasing temperature but NbSi2 and TaSi2 which were deformable even at low temperatures, exhibited anomalous strengthening around 1350°C. Deformation of Ti5Si3 whose ductile-brittle transition occurred around 1300°C was controlled by twins and the brittle fracture occurred on the basal plane. In CoSi2 the {001}<100> slip was only activated at ambient temperatures but addition of Ni activated {110}<110> slip as secondary slip system and improved the ductility. The creep behavior of MoSi2 and CrSi2 single crystals were also investigated and was found to be controlled by the viscous and glide motion of dislocations.

The temperature dependence of the flow stress and deformation mechanisms of single crystal MoSi2 have been determined for compression along three different orientations, [001], [021] and [771], at two different strain rates, 1 × 10−5/s and 1 × 10−4/s, and at temperatures between 900 and 1600°C. The flow stress along [021] is slightly higher than that along [771] while both orientations gave a much lower flow stress than that along [001]. Along [021], slip occurs on the {110} 1/2<111> slip system between 1000 and 1200°C, while at 1300-1400°C, slip occurs on the {013}<100> slip system. Along [771], deformation occurs by the [001]<100> slip system while cross-slip onto {013} and [011] planes is observed at 1000-1300°C except that slip occurs on the {013{1/2<331> slip system at 1000-1100°C for faster strain rates. Along [001], slip occurs on the {013}1/2<331> system at 900-1100°C while slip is observed on the {011} 1/2<111> system at 1300-1600°C. Strain rate jump tests from 1×10−5/s to 5x10−5/s at 1 100°C revealed a stress exponent of 7 along [771] and 20 along [021], while a rate jump test from lx10−5/s to 2x 10−5/s along [001] at 1400°C gave a stress exponent of 3.9.

A 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.

Compositionally-tailored, silica-free, MoSi2/SiC composites with SiC content ranging from 0 to 40 percent were synthesized through a novel processing scheme involving mechanical alloying and in-situ reactions for the formation of the reinforcement. Room temperature indentation fracture toughness and hardness measurements were obtained from these silica-free composites and were compared with values obtained from silica-containing, conventionally-processed MoSi2/SiC composites.

The 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.

Previous experimental evidence for the transformation of MoSi2 from the high temperature C40 structure to the low temperature C11b structure has been re-examined such that the fault character and the existence of the C40 structure is now questioned. New observations of faults in single crystals prepared over a range of growth rates from 1cm/hr to 30cm/hr are presented. Transformation-induced stacking faults, which were previously described as 1/4<111> lying on {110), have been re-identified as 1/6[001] condensation faults on (001). These faults were probably produced by silicon loss during crystal growth. The contribution that these faults may make to macroscopic strain under the action of diffusion-assisted mechanisms at elevated temperatures is discussed.

Three 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.

Molybdenum disilicide and its composites have been fabricated by a number of researchers in recent years through vacuum plasma spray (VPS) forming. VPS is capable of producing dense, fine grained deposits of these high temperature intermetallics and is a promising technique for near-net-shape manufacturing. Reviewed here is VPS forming of high temperature silicide intermetallics, principally Mosi2 and its composites. A discussion will be given of the processing-structure-properties relationship of the spray formed silicide.

Plasma-spray forming has been used to fabricate thick-wall tubes of Mosi2 and Mosi2 containing concentric layers of A12O3. This process is being investigated as a potential fabrication method for producing tubular components of Mosi2 and Mosi2 composites for use in high temperature fuel-burner applications. Results will be reported on the spray forming method used to produce tubes of various sizes. The room temperature strength of pure Mosi2 tubes in the as-deposited condition, and after heat-treating at 1500 °C for 2 hours in vacuum, will also be reported. The strength of plasma sprayed Mosi2 tubes were measured via diametral compression of 0-ring and C-ring sections in air at room temperature. Qualification of the strength distribution was based on Weibull statistical theory.

An overview of the application of the reactive hot compaction (RHC) process to fabricate various intermetallics such as silicides and aluminides is presented. Specific examples with the in-situ formation of diffusion barrier coatings on refractory metal reinforcements during RHC are also given. The processing involves blending the elemental powders with pre-treated refractory metal filaments and reactively synthesizing the mixture at elevated temperatures. During this process, the treated surfaces of the filaments react with one of the components (e.g. Al for aluminides or Si for silicides) to form in-situ a protective surface coating. The important influence of the RHC reaction sequence and rate on the consolidation of the composite are discussed. Finally, the fracture toughness of the composites are related to the various toughening mechanisms, with special emphasis on the role of the interfacial layer.

Despite the fundamental nature of sintering and its importance as a low cost fabrication process, little information exists on the sintering behavior of the structural silicide Mosi2. The sintering of commercial Mosi2 powders in the range of 1-10 μm was investigated as a function of sintering temperature, sintering time, and sintering atmosphere. Initial densities for uniaxially cold pressed powders were in the range of 47-56% theoretical. A maximum sintered density of 90% of theoretical was achieved for 1 μm Mosi2 powders after sintering for 100 hours at 1600 °C in an argon atmosphere. Larger 10 μm Mosi2 powders achieved lower sintered densities under these conditions. Avenues to optimize the sintering behavior of Mosi2 are suggested.

Mosi2 and Mosi2-composites are potential candidate materials for both high-temperature coatings and structural components. The application of combustion synthesis and hot pressing in the production of dense Mosi2 and Mosi2- composites, i.e., Mosi2-SiC, Mosi2 -Mo, is currently being studied. Selection of compositions was based on oxidation resistance, thermal expansion, thermodynamic stability, and compatibility. Functionally-graded materials (FGM's) of these composite material systems are also currently being studied.

A combustion wave originating from an exothermic reaction between Mo and Si propagates through the reactants converting Mo-2Si mixture to Mosi2. X-ray analysis of the product confirmed that the product is single phase Mosi2 with no second phase or reactants when combustion synthesis experiments were carried out in argon. The oxygen content of the product was 0.22 wt.%. When the mixture was reacted under pressure in a hot press, the product obtained was also Mosi2, and a theoretical density up to 92% could be achieved. Heating of a mixture of Mo and Si with C leads to the formation of Mosi2 with a dispersed SiC phase.

Co-reduction of mixtures of MoCl3(THF)3 and SiCls4 in THF using Li/C10H8 or both Li/C10Hs8 and LiBEt3H resulted in formation and separation of black powders which upon thermal annealing at temperatures ranging from 750°C to 1 100°C produced crystalline molybdenum silicide and silicon carbide composite. Co-reduction of mixtures of WCI4 and GeBr4 with LiBEt3H in THF formed W2C and elemental Ge which upon thermal treatment at 750°C for 4 hours generated a small amount of crystalline W5Ge3.

Mosi2 has a very favorable combination of materials properties, including a high melting point (2020°C), high strength at elevated temperatures, and resistance to high temperature oxidation and corrosion. These properties make it a good candidate for a hightemperature structural material; however, it has very poor ductility. A great deal of research has focused on improving the ductility of this alloy through various preparative routes. We have synthesized Mosi2, as well as WSi2, TaSi2, and NbSi2, using rapid solid-state metathesis reactions between a high oxidation state metal halide and an alkaline earth silicide. These reactions take advantage of the large exothermic heat of formation of the alkaline earth halide and can reach temperatures as high as the melting point of the product silicides. In addition, this approach yields crystalline products in seconds. The synthetic technique will be discussed along with characterization results.

Low-pressure plasma deposition (LPPD) and co-injection has been used to fabricate a MoSi2 composite reinforced with 15 µm SiC particles. The microstructure and creep behavior of the LPPD processed composite are reported and discussed. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed the structure of the composite to be lamellar and energy dispersive X-ray analysis (EDAX) identified the phases present in the material as: MoSi2, Mo5Si3, SiO2, and SiC. Density characterization revealed a porosity of less than 1.0 vol. %, indicating a nearly fully dense material. A high concentration of SiO2 (∼8.0 vol. %) present in the MoSi2/SiC composite may be attributed to possible contamination of the starting powders before or during LPPD. Sublimation of SiC during co-injection led to a low volume fraction (< 2.0 vol. %) of reinforcement in the composite. The creep rate of the LPPD MoSi2/SiC was higher relative to that of MoSi2/SiC composites fabricated by powder metallurgy (PM) techniques. On the basis of the results of this study it has become evident that alternative processing methods such as LPPD insitu processing may be better suited for the fabrication of elevated volume fraction MoSi2/SiC composites.

Intermetallic silicides and silicon carbide composites are potential useful materials in high temperature applications requiring superior strength combined with enhanced fracture toughness. This paper presents results obtained on these materials in nanostructured form, since structural reduction to this scale offers a way for achieving unique properties.

Chromium silicide / silicon carbide and molybdenum silicide / silicon carbide composite powders were readily synthesized by thermal conversion from spray dried precomposites at 1250°C. Basically, the process involves spray drying the aqueous chemical solution to a precomposite, followed by thermal conversion in a tube furnace reactor. The phases present in the composites are hexagonal Cr5−XSi3−XCX+Y (Cr5Si3Cx) / cubic β-SiC and tetragonal MoSi2, hexagonal Mo5Si3CXx/ cubic β-SiC. The volume fraction of these phases can be controlled through the solution preparation of the precursors. Characterization of composite powders include x-ray diffraction spectrometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Particle size distribution of the synthesized chromium silicide and silicon carbide composite was conducted by a computerized image analyzer and the statistical result was obtained.

A review of creep behavior of molybdenum disilicides and their composites is presented. Creep rates of these silicides are compared with those of other high temperature materials such as superalloys, ceramic-ceramic composites, intermetallics including aluminides (nickel and titanium), berylides (vanadium and niobium), and refractory metals(molybdenum and tungsten). Creep rates of silicides are shown to be very sensitive to grain size even in the power-law creep regime with grain size exponent of the order of five and above. In addition, the results show that with increase in volume fraction of reinforcements there is a decrease in creep rates for volume percentages less than 25%. To achieve significant improvement in creep strength volume percentages of reinforcements greater than 25% are required. This weakening effect at low volume percentages is related to accompanying decrease in grain size with the addition of reinforcements. Addition of carbon to MoSi2 eliminated the silica present at grain boundaries and converted it to SiC. The SiC thus formed, inhibited grain growth during hot pressing. Thus although addition of carbon enhanced creep resistance, its effect is masked by the accompanying decrease in grain size. Thus, grain size plays a dominant role in the creep of molydisilicide composites. It is shown that among all the materials molybdenum disilicides possess very high creep resistance comparable to ceramic-ceramic composites.

Low temperature plasticity enhancement processes observed in body-centered cubic metals and B2 ordered alloys can be observed in MoSi2 in appropriate stress-temperature-strain rate regimes. We illustrate effects of surface films (ZrO2) and dispersoids (TiC) in enhancing plasticity of MoSi2 in the ductile-to-brittle transition range of temperatures, 900-1400°C. We also show, through experiments involving high temperature (1300°C) prestrain, that effective operation of dislocation generation processes can extend the low temperature range to which MoSi2 can be plastically deformed. These results illustrate that approaches to enhance toughness of MoSi2 need not be limited to ductile phases characterized by weak interfaces.