WS2 Low Friction Coatings
WS2 low friction coatings (known as tungsten disulfide, also spelled disulphide) are also widely accepted forms of solid film lubrication today. Their uniqueness comes from their atomic structure, weak interaction (Van der Waals) of the sulfide anions, while covalent bonds within tungsten remain strong.
Thus, its lubrication relies on slippage along the sulfur atoms. No moisture is needed for lubrication, which makes them ideal under hard vacuum.
WS2 low friction coatings are applied in a variety of ways, but mostly by a simple rubbing or burnishing, or through impact or other form of mechanical impingement. Based on costs relating to this compound, WS2 low friction coatings are rarely used in the form of resin bonded lubricants, compared with MoS2.
Consequently, most thickness offerings of WS2 low friction coatings are 0.0002 inch or less.
Friction coefficients less than 0.05 are attainable, but will also vary with humidity and sliding conditions. Tests show, with increased vacuum strength, lower friction. Lower friction is also yielded in the presence of higher loads, faster speeds, or both.
Dry lubrication is superior at higher temperatures. Up to 1400 degrees Fahrenheit, which is 100 degrees more than MoS2. While oxidation rates are still relatively low.
Higher air flow can affect oxidation kinetic rates in atmosphere. Tungsten trioxide (WO3)and sulfur dioxide are the products of oxidation. Since WO3 alone offers dry lubrication, based on its relative softness, tungsten disulfide coating is ideal in higher temperature environments. At higher temperatures, though, it is better suited under vacuum. In atmosphere, they are prone to water adsorption from air based on their hygroscopic properties.
As with the other dry film lubricants, while differences may prove negligible, you will have to determine which is better for you: longer wear life or better performance. Generally, friction will be slightly higher by coating both surfaces with WS2 low friction coatings, rather than coating one surface only. But wear life will increase coating both surfaces.
Friction can be good in so many areas of life. Without it we could not easily stop and start our motion, or change direction. But in moving machinery, friction causes considerable loss of energy, poorer performance, not to mention limiting wear life.
As with many systems without lubrication, the static coefficient of friction is higher than the dynamic coefficient of friction. This is called ‘stick slip’, a situation where two surfaces are bound together until the system’s elastic energy has crossed some threshold. That is where the sudden, forward slip or movement takes place.
For lubrication to occur, these bonds or the adhesive component of friction between the two adjoining surfaces must be broken. This is where dry film lubricants like WS2 low friction coatings can serve so well.