Molybdenum disilicide (MoSi2) is a refractory metal silicide that is primarily used as a heating element. In recent years, its potential for use as a structural ceramic has been recognised, as the material combines excellent oxidation resistance and high elastic modulus at elevated temperatures.
These applications have been limited as MoSi2 is inherently brittle below 1000 oC, and has poor creep resistance at temperatures above 1200 oC. Consequently, workers have concentrated on improving these properties by combining MoSi2 with a second material in a composite.
MoSi2 and associated composites are most commonly made by pressure assisted sintering techniques. Hot pressing and hot extrusion produce simple shapes and are relatively economic.
Hot isostatic pressing produces complex shapes, with uniform density and grain structures, but is a more costly process.
Many other techniques such as reaction sintering, mechanical alloying and self propagating high temperature synthesis are being investigated, but have yet to demonstrated reproducibility and commercial viability.
Molybdenum disilicide has gained attention due to its attractive properties:
• Moderate density
• High melting point
• Excellent oxidation resistance
• High modulus at elevated temperatures.
Its use as a structural ceramic is limited by poor characteristics in two key areas:
• Low toughness at temperatures less than 1000 oC means that the material is very brittle at room temperature, and only becomes plastic at elevated temperature.
• Poor creep resistance at temperatures above 1200 oC.
Typical properties of hot pressed molybdenum disilicide are shown in table 1.
Table 1. Typical Physical and Mechanical Properties of Molybdenum Disilicide.
|Melting Point (°C)
|Young’s Modulus (GPa)
|Bend Strength (MPa)
|Fracture Toughness K1C (MPa.m0.5)
|Resistivity (ohm.cm) (at room temp)
|Resistivity (ohm.cm) (at 1700 °C)
Molybdenum disilicide is most commonly used commercially in electric heating elements. The material has the potential to be used in high temperature structural components.
Molybdenum disilicide is a conductive silicide which resists oxidation through the formation of stable layers of silica on its surfaces at high temperatures.
MoSi2 has been developed as an electric heating element for use in air at temperatures above 1600 oC (figure 1). Many commercial MoSi2 heating elements known are in fact cermets, comprising a mixture of MoSi2 particles bonded together with an aluminosilicate glass phase, typically as 20% of the total volume.
Figure 1. A Split tube furnace (max. operating temperature 1800 °C) equipped with MoSi2 heating elements (Photo Courtesy of Radatherm Pty Ltd)
The best grade of MoSi2 elements are capable of operating up to 1800 oC but the material is extremely brittle and can suffer badly from creep in use. However, they offer process advantages as they can operate at high electrical loads without aging and do not show increasing electrical resistivity with use. They are resistant to oxidation in air, oxygen and oxygen-rich atmospheres. They can also be used with nitrogen, noble gases, hydrogen, ammonia and limited vacuum, however operating temperatures and campaign life may be reduced due to breakdown of the protective oxide layer.
MoSi2 elements are supplied as ready-made elements, and are produced as either straight or bent forms and in a wide range of dimensions. The elements are used mainly in laboratory furnaces and production furnaces in the glass, electronics, steel, ceramics and heat treatment industries.
High Temperature Structural Components
The attractive combination of oxidation resistance and high temperature elasticity makes molybdenum disilicide a most promising candidate for use in high temperature applications such as gas turbine engines. Many investigators have worked to improve low temperature ductility and high temperature creep resistance. However, they have yet to produce a commercially viable composite combining all the desired properties.
Primary author: Ceram Research
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