Such layers are able to slide relative to each other with minimal applied force, thus giving them their low friction properties.
These include certain soft metals (indium, lead, silver, tin), polytetrafluroethylene, some solid oxides, rare-earth fluorides, and even diamond.
[2] Limited interest has been shown in low friction properties of compacted oxide glaze layers formed at several hundred degrees Celsius in metallic sliding systems.
These materials may be added in the form of dry powder to liquid lubricants to modify or enhance their properties.
For applications where only a minor lubricity is needed and a more thermally insulating coating is required, then amorphous graphite would be chosen (80% carbon).
Particle size and film thickness are important parameters that should be matched to the surface roughness of the substrate.
After the solvent evaporates, the coating cures at room temperature to form a solid lubricant.
Use of free powders has its limitations, since adhesion of the solid particles to the substrate is usually insufficient to provide any service life in continuous applications.
After application and proper curing, these "slippery" or dry lubricants bond to the metal surface and form a dark gray solid film.
Many dry film lubricants contain special rust inhibitors which offer exceptional corrosion protection.
Most long-wearing films are of the bonded type but are still restricted to applications where sliding distances are not too long.
AF coatings are applied where fretting and galling is a problem (such as splines, universal joints and keyed bearings), where operating pressures exceed the load-bearing capacities of ordinary oils and greases, where smooth running in is desired (piston, camshaft), where clean operation is desired (AF coatings will not collect dirt and debris like greases and oils), and where parts may be stored for long periods.
[5] Self-lubricating composites: Solid lubricants such as PTFE, graphite, MoS2 and some other anti-friction and anti-wear additives are often compounded in polymers and all kind of sintered materials.