Dry Film Lubricants
Dry film lubricants are defined as one or more solids used between two surfaces in relative motion for the purpose of lowering friction or reducing wear.
Unlike greases (solid or semi-fluid based dispersions of a thickening agent in a lubricant) or the use of oils.
Also known as solid films, these are best utilized in extreme environments, such as very high temperature or pressure, where organic-based compounds could never survive.
Examples of the more frequently applied compounds include graphite, tungsten disulfide (WS2), and molybdenum disulfide (MoS2). Their material characteristics work similarly, in that each exhibits high ductile shear under an applied tangential force, like in the motion of sliding.
To have low friction, your dry film lube must have both low shear strength and adherence to the surface. These are materials having a special crystalline habit which causes it to shear into thin, flat plates, which readily slide over one another and thus produce low friction or a slipping effect.
For illustration purpose, imagine a series of vertically stacked plates (called basal planes) supported by long, wobbly legs (weak Van der Waals forces). Under an applied lateral force, the legs ‘collapse’. There is severe plastic flow. The plates shift out of place. Yet strong ionic bonds force the shear parallel with the basal planes, perpendicular to the crystal structure, their preferred orientation. Compounds of this type have an AX2 stoichiometry.
Here, friction is not the result of metal to metal contact. And it’s not the result of coating to metal contact. When dry film lubricants are applied to one or both surfaces, friction is determined within the lubricant itself, particle against particle.
To reduce the likelihood of wear, the coating must have low abrasivity. You want to be sure the compound or lubricating system is comparatively softer than the bearing material to avoid abrasive wear. Know, too, the thermal environment for your selection of stable high temperature lubricants.
Machinery lubrication or general metal lubrication are excellent areas for application. As are cutting tools, dies, and forming tools. This is especially true for gear lubrication, where contact stress or pressures can be very high, and organic-based compounds would creep or cold-flow.
Industrial lubricants may be applied in a variety of ways. From basic immersion, spray, brush, dip, burnish, impact or impingement, to sputtering and heat fusion alloys. Generally, they are applied in pure thin films (less than 0.0001 inch). Or, as resin-bonded systems whose thickness is typically 0.0002 inch or greater, using binders such as epoxy, phenolic, amide/imide systems, or ceramic. Other forms of application include plasma spray coatings (.003 inch and higher), where lubricating solids are interspersed in a wear resistant matrix. Or, for reaching internal surfaces beyond line of sight, the use of self-lubricating powder metallurgy composites.
From a tribological standpoint, preventing wear through solid compounds can play a crucial role, alongside hard coatings. Remember, in determining the number of systems to coat, the following should be considered. For a hard/soft bearing system, coating the softer of the two surfaces will extend the life of the system. However, the harder a substrate, the more effective the dry film lubricants will perform.
Generally, friction will be slightly higher by coating both surfaces, rather than coating one surface only. But wear life will increase coating both surfaces. So, while the differences may be negligible in your application, it is something you may have to determine: longer wear life or better performance.
Today, there are some exceptionally high performance products available for adverse conditions. Some perform in temperatures ranging from cryogenic to ultra high (1100 degrees Celsius). Others are compatible with liquid oxygen, perform in hard vacuum, withstand high frequency fretting wear and loads up to 250,000 psi.