Solid (Dry)-Film Lubrication            

It is widely believed that e xtreme conditions are uncommon; however nearly every manufacturing plant has at least one application in which the operating conditions could be characterized as extreme from a lubrication perspective. Common extremes could include high and low shaft speeds, high and low temperatures, high pressures, concentrated atmospheric and process contaminants, and inaccessibility. Various materials that protect interacting surfaces, after the fluid film is lost, have been either discovered or created. The more common types of materials include the following:

• Molybdenum disulfide (MoS2) – also known as moly
• Polytetrafluoroethylene (PTFE) – also known as Teflon®
• Graphite
• Tungsten disulfide 

These materials are characterized as dry film or solid film lubricants. Moly, graphite and Teflon are the most commonly recognized by practitioners of machinery lubrication. Molybdenum and graphite are agents that are extracted from mined ore. Teflon was created by DuPont Chemical Company and is manufactured by various companies for many purposes.  

Dry Film Lubrication

Dry film lubricants are solid materials that provide low frictional resistance between surfaces when applied directly to interacting surfaces. Each material has different properties. Crystalline lattice (lamella) structure materials, such as molybdenum disulfide, tungsten disulfide and graphite, are widely used as agents in lubricants and as stand-alone lubricants. These materials are used independently or in combination with other agents and metals (lead, copper) to achieve the desired results. Lamella lubricating powders have low shear forces between their crystalline lattice layers that minimize resistance between sliding surfaces.   

General Dry Lubricant Properties

Each dry lubricating material has different properties. Molybdenum disulfide, graphite and tungsten disulfide are oilioscopic. Their structure is unable to tolerate detergent. These layer lattice lamella structures are analogous to stacks of nonadherent plates which, with slight tangential loading, slip out of place. It is comparable to walking across a room full of flat slippery plates. The lamella materials have good load-bearing capacity in sliding and rolling mode. Graphite has high-temperature capability and functions well in radiation atmospheres. MoS2 and WS2 function well in hard vacuum and tolerate higher loads better than graphite. The solid lubricating materials tend to have upper temperature ranges well above the surface-protecting capabilities of mineral and most synthetic base stocks. Fluorinated hydrocarbons (Teflon) are stable in liquid or solid form to roughly 600°F, but will begin to degrade and may produce noxious fumes at that temperature. Graphite and molybdenum can operate in a similar temperature range, but molybdenum disulfide can also function in a vacuum without losing its slippery property.