May 28 2001
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Molybdenum disilicide (MoSi2), a refractory metal silicide, is mainly used as a heating element. Of late, it has been recognized as a potential structural ceramic, since it exhibits high elastic modulus and exceptional oxidation resistance at very high temperatures.
The applications of MoSi2 have been limited as it is naturally brittle below 1000 °C, and exhibits weak creep resistance at temperatures over 1200 °C. Therefore, workers have focused on enhancing these properties by mixing MoSi2 with another material in a composite.
MoSi2 and related composites are usually prepared through pressure-assisted sintering methods. Hot extrusion and hot pressing create basic shapes and are comparatively cheaper. Hot isostatic pressing creates intricate shapes, with even density and grain structures. However, this process is more expensive.
Several other approaches like mechanical alloying, reaction sintering, and self-propagating high-temperature synthesis are being examined, but the commercial viability and reproducibility of these approaches are yet to be proven.
Key Properties
MoSi2 has gained a lot of interest because of its attractive properties:
- High melting point
- Moderate density
- High modulus at higher temperatures
- Exceptional oxidation resistance
Characteristics that restrict its key application areas when used as a structural ceramic:
- Low toughness at temperatures below 1000 °C signifies that the material is highly brittle at room temperature, and turns plastic only at higher temperatures.
- Poor creep resistance at temperatures over 1200 °C
Common properties of hot-pressed MoSi2 are illustrated in Table 1.
Table 1. Typical physical and mechanical properties of molybdenum disilicide
| Property |
Value |
| Density (g.cm-3) |
6.29 |
| Melting Point (°C) |
2230 |
| Young’s Modulus (GPa) |
430 |
| Bend Strength (MPa) |
250 |
| Fracture Toughness K1C (MPa.m0.5) |
3 |
| Hardness (GPa) |
9 |
| Resistivity (ohm.cm) (at room temp) |
3.5 x10-7 |
| Resistivity (ohm.cm) (at 1700 °C) |
4.0 x10-6 |
Applications
Commercially, MoSi2 is widely used in electric heating elements. The material can be potentially applied in high-temperature structural parts.
Heating Elements
MoSi2 is a conductive silicide that is oxidation-resistant because of the formation of stable silica layers on its surfaces at high temperatures.
MoSi2 has been produced as an electric heating element for use in air, at temperatures beyond 1600 °C. A known commercial MoSi2 heating element is cermet, which is made by bonding a mixture of MoSi2 particles together with an aluminosilicate glass phase, usually as 20% of the total volume.
Although top-grade MoSi2 elements can function at temperatures of up to 1800 °C, the material is highly brittle and can suffer severely from creep during use. However, they offer process benefits as they can work at high electrical loads without aging, and do not exhibit increasing electrical resistivity during use. They are oxidation-resistant in air, oxygen, and oxygen-rich atmospheres.
Moreover, they can be used with ammonia, nitrogen, hydrogen, noble gases, and limited vacuum, but operating temperatures and service life may be decreased because of the failure of the protective oxide layer.
MoSi2 elements come as readymade elements and are manufactured as either bent or straight forms, in an extensive range of dimensions. The elements are primarily used in production furnaces and laboratory furnaces in the ceramics, glass, steel, electronics, and heat-treatment sectors.
High-Temperature Structural Components
The striking blend of oxidation resistance and high-temperature elasticity makes MoSi2 a very promising material for use in high-temperature applications, like gas turbine engines. Many researchers have strived to enhance the high-temperature creep resistance and low-temperature ductility. However, a commercially feasible composite integrating all the preferred properties is yet to be developed.