MoS2 Low Friction Coatings
MoS2 low friction coatings (also known as molybdenum disulfide, also spelled, disulphide) is regarded the most widely used form of solid film lubrication today. What makes it unique (with the other dichalcogenides) is the weak atomic interaction (Van der Waals) of the sulfide anions, while covalent bonds within molybdenum are strong.
Lubrication relies on slippage along the sulfur atoms. All the properties of the lamella structure are intrinsic. No external form of moisture is required. In fact, best performance from MoS2 low friction coatings is attained in the absence of water vapor, which are prone to surface adsorption. This makes them ideal under vacuum.
There are a number of application methods of MoS2 low friction coatings, including a simple rubbing or burnishing, air-spraying resin-bonded or inorganically bonded coatings, and more recently by sputtering through physical vapor deposition (PVD).
Thickness will vary, depending on form of MoS2 low friction coatings, but typically ranges between 5 to 15 micrometer. Sputtering techniques can produce thin films of 0.2 micrometer. While plasma sprays will result in higher builds, beginning at 0.003 inch or more.
Friction coefficient less than 0.05 is attainable, but will also vary with humidity and sliding conditions. Tests show friction decreases with increasing vacuum strength. Friction also lowers with higher load, faster surface speed, or both. In fact, MoS2 low friction coatings are superior to both graphite and tungsten disulfide (WS2). Friction with MoS2 low friction coatings is independent of particle size, though the larger particles can carry more load.
Dry lubrication for MoS2 low friction coatings remains superior at higher temperatures, with oxidation rates remaining relatively low at temperatures up to 600 degrees Fahrenheit. And in dry, oxygen-free atmospheres, lubricating performance, even with oxidation products, is stable to 1300 degrees Fahrenheit.
Higher air flow can affect oxidation kinetic rates in atmosphere. Molybdenum oxide products (MoO3) and sulfur dioxide. Since MoO3 alone offers dry lubrication, based on its relative softness, molybdenum disulfide coating are ideal in higher temperature environments. At higher temperatures, though, they are 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, using MoS2 low friction coatings. Generally, friction will be slightly higher by coating both surfaces, 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 non-lubricated systems, the static coefficient of friction is higher than the dynamic coefficient of friction. The resultant motion is often referred to as ‘stick-slip’. Basically, the two surfaces stick together until the elastic energy within the system has accumulated to some threshold, where a sudden, forward slip takes place. Under magnification, it’s apparent the union of two surfaces is often limited to intimate contact only at the tips of a few of the asperities (small scale, surface irregularities). At these point areas, pressures relating to contact may be near the hardness of the softer material. Thus, plastic deformation occurs on some localized scale. This is known as cold welding. Where bonded junctions are formed between two materials.
For lubrication to occur, these bonds, this adhesive component of friction, must be broken. And this is where products like MoS2 low friction coatings serve well.