Lapping or Polishing
March 20, 2019
In any industrial application, the purpose of lubrication is preventing friction between two rolling surfaces, or between the plain bearing and the shaft. Greases and oils are common types of lubricants used in industrial applications. However, these lubricants fail to perform in extreme environments. In such conditions, the dry film lubricants are used. How do these lubricants differ? How do they perform? This post aims to answer all important questions on dry film lubricants.
What are dry film lubricants and how do they work?
Unlike greases or the oils, 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.
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 structure that enables them 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 a 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 ensure that the compound or lubricating system is comparatively softer than the bearing material to avoid abrasive wear.
Machinery lubrication or general metal lubrication are excellent areas for application. However, cutting tools, dies, and forming tools are also ideal areas for application. 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.
How are dry film lubricants applied?
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 inches). 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 the line of sight, the use of self-lubricating powder metallurgy composites.
What are the factors to be kept in mind while coating a system with a dry film lubricant?
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.
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 a hard vacuum, and withstand high-frequency fretting wear and load up to 250,000 psi.