Molybdenum Disilicide

Molybdenum Disilicide

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Request QuoteWritten by AZoMMay 282001

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 recognized, 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 the 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 economical. 

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.

Key Properties

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 of 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 is shown 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

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Applications

Molybdenum disilicide is most commonly used commercially in electric heating elements.  The material has the potential to be used in high-temperature structural components.

Heating Elements

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 the air at temperatures above 1600 ^oC (figure 1). Much 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 MoSi2heating 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 a 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