Graphite Low Friction Coatings
Graphite low friction coatings are regarded some of the more widely used forms of solid film lubrication today.
Similar to molybdenum disulfide (MoS2) or tungsten disulfide (WS2) dichalcogenides, graphite also has a layered lattice, a hexagonal structure, with stronger (covalent) bonds within the structure (closely packed carbon atoms within the basal plane) than with neighboring ‘sheets’ (carbon atoms between basal planes).
But unlike the other lubricants, carbon graphite low friction coatings are not intrinsically lubricated. Graphite properties rely on adsorption of moisture or other condensable vapor such as hydrocarbons to develop the capability of lubrication. Consequently, graphite lubricates well in a normal air atmosphere, but not at high altitudes or in vacuum. And once temperatures are high enough to create desorption, unless in the presence of relevant oxides, dry graphite will result in higher friction.
There are a number of application methods, including a simple rubbing or burnishing, impact or impingement (powdered graphite), or air-sprayed resin-bonded or inorganically bonded coatings. Coating thickness of graphite low friction coatings can vary from less than 0.0001 inch in pure form, and up to 0.0005 inch or so, as resin-bonded or ceramic-bonded lubricants.
Friction coefficient less than 0.05 is attainable, but will also vary with humidity and sliding conditions. Tests show friction lowers with load (up to approximately 50,000 psi), faster surface speed, or both. Similar to MoS2 or WS2, friction from graphite low friction coatings is independent of particle size, though the larger particles can carry more load.
Though oxidation products (carbon dioxide, CO2) begin to form above 900 degrees Fahrenheit, lubrication from graphite low friction coatings is feasible at service temperatures of 1200 degrees Fahrenheit.
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 from graphite low friction coatings will be slightly higher by coating both surfaces, rather than coating one surface only. But wear life will increase coating both surfaces.
Without friction, life would certainly be inconvenient, if not dangerous. For example, we would not be able to easily stop and start our motion, or change our direction. But when it comes to moving machinery, in the presence of friction, there is considerable inefficiency associated with energy loss, diminished performance, and certainly limitations on wear resistance.
Working systems abound in the absence of lubrication. With poor chosen combination, like hard/soft metals, or poor quality surfaces, static (stationary) friction can be exceptionally higher than dynamic (moving) friction. Oftentimes, the result is galling or seizure, the act of a ‘cold weld’. Best case, there is a ‘stick-slip’ phenomenon, the result of surfaces temporarily adjoined, bound sometimes by the smallest of areas, the most intimate of contact. Until the threshold of elastic energy is overcome between the two surfaces, forward movement can not occur.
Consequently, for dry lubrication to work successfully, these bonds (referred to as the adhesive component of friction) must be broken. This is where the dry film lubricants like graphite low friction coatings can serve you well.