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A Comparison of Tungsten Disulfide and Molybdenum Disulfide

Tungsten Disulfide (WS2) and Molybdenum Disulfide (MoS2) are two of the most popular industrial dry film lubricants. Both are similar in terms of appearance, color, and high chemical durability. Both are dry lubricants, non magnetic, and are compatible with liquids such as paint, oil, fuel, and solvents. They can be applied on almost all surfaces for industrial purposes including iron, steel, plastic, aluminum, and copper. While both come from the same chemical family, there are certain differences that make them unique and thus, suitable for different industrial applications.

Differences between Tungsten Disulfide and Molybdenum Disulfide

Though they are similar in many properties, one can find a few differences between the two lubricants that set them apart.

  • Co-efficiency of Friction: Tungsten disulfide is perhaps the most lubricious material known to man. It is known to be a very low Coefficient of Friction (COF) at 0.03. Molybdenum disulfide also comprises good friction capabilities. It can provide a friction coefficient of up to 0.05. Both can be easily applied onto various metallic surfaces. This is the reason why the automotive industry is one of the most popular applications of both Ws2 and MoS2.
  • Weight and Density: For a dry film lubricant coating, the higher the molecular weight, the more stable it is. Tungsten disulfide has been found to have a molecular weight of 248, while molybdenum disulfide is 160.08. A similarity is also found in their respective densities. Ws2 has a density of 7500 Kg.m-3, while MoS2’s density is 5060 Kg.m-3. As they come from the same chemical family, Mos2 can be replaced by Ws2 in an industrial application.
  • Thermal Stability: Tungsten disulfide has the ability to operate in high air conditions. Beginning with a low COF of 0.01, its air thermal stability rate can increase up to 1100oF. With MoS2, it is slightly different. The basic COF is also 0.01, but it only provides an air thermal stability rate of 600oF. However, MoS2’s coefficient can be raised to 0.05, in which case, the thermal stability rate of MoS2 increases to 1100oF.
  • Load Bearing Capacity: This is one area where tungsten disulfide has a bigger advantage over molybdenum disulfide. On an average, Ws2 can provide high load bearing capacities between 200,000 and 400,000 psi at COF of 0.024 for coated films. MoS2 has the capability to provide load bearing capacities of up to 250,000 psi.
  • Temperature Range: This feature is always considered when choosing one of the dry film lubricants for industrial applications. The temperature range of a dry film lubricant is based on two aspects – ambient and vacuum temperature. Tungsten disulfide can operate anywhere between -273oC to 650oC. As Ws2 performs better in extreme applications, it has the ability to provide a vacuum temperature range from -188oC to 1316oC. Molybdenum disulfide has a vast difference in its ambient and vacuum temperatures. In an ambient atmosphere, it operates between -185oC to 350oC. Its temperature range increases in a vacuum atmosphere between -185oC to 1100oC.

Miscellaneous Advantages of Ws2:

It is obvious to see that tungsten disulfide has a better edge over molybdenum disulfide. Other aspects that make Ws2 more suitable for industrial applications are:

  • It has a coating thickness of up to 0.5 microns.
  • Unlike MoS2, tungsten disulfide has the ability to provide good resistance against corrosion.
  • It has good electrical properties, making it a good option for semi conductor applications.
  • Tungsten disulfide can be used in a variety of applications including aerospace, military, semi conductor, automotive, and medicine.

Depending on the application and the expected results, one should always give preference to tungsten disulfide for its strength, versatility, and durability.

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