"When should solid lubricants be used and what are their advantages?"
A variety of solid materials with inherent lubricating capability are available for use in solid‑film lubricants. The most commonly used are molybdenum disulfide, graphite and polytetrafluoroethylene. While these are the most common, you also may see such materials as tungsten disulfide, boron nitride, lead oxide, antimony oxide, lead, tin, silver, fluorinated ethylene propylene (FEP), perfluoroalkoxy copolymer (PFA), etc.
No one formulation can satisfy all of the requirements on a cost‑effective basis. Properties that should be considered are coefficient of friction, load‑carrying capacity, corrosion resistance (susceptibility to galvanic corrosion) and electrical conductivity. Furthermore, one must consider the environment in which the solid-film lubricant must perform.
Environmental factors include temperature, pressure, humidity, oxygen content, radiation, etc. Each of the above solid‑lubricant materials has strengths and weaknesses, and corresponding compromises must be made. Molybdenum disulfide generally has the highest load‑carrying capability with a corresponding low coefficient of friction. However, in an oxidative atmosphere in excess of 400 degrees C (750 degrees F), it begins to decompose.
Graphite has high‑temperature capability in an oxidative environment but tends to promote galvanic corrosion and will not function in high vacuum. Fluorinated polymers generally exhibit a low coefficient of friction and are aesthetically quite suitable for formulations with colored pigments. However, these fluorinated polymers cannot sustain high loading, nor do they have good radiation stability.
Solid lubricants are used primarily as extreme pressure (EP) or anti-wear type additives and are applied in one of three ways. The first and most popular application for solid lubricants, particularly molybdenum disulfide and graphite, is as an EP additive in grease formulations. The plate-like structure of these solid lubricant particles reduces friction by allowing the surfaces in motion to easily slide over each other. This application can be useful in pins and bushings.
Graphite is particularly beneficial where moisture is present. In fact, the presence of moisture is necessary to ensure graphite's full benefit as a friction reducer.
Molybdenum and polytetrafluoroethylene (PTFE) are frequently used as anti-wear additives blended with engine oils or other lubricants. There is some controversy over the effectiveness of these solid lubricants suspended in oil, but the practical evidence seems to suggest some benefit in the reduction of friction.
Perhaps the most practical and successful method of applying solid lubricants is the bonding or impregnation of these friction modifiers onto the surface of various machine components, such as cylinder liners and piston skirts in large stationary natural gas engines. The bonded film of solid lubricant forms a sacrificial wear film on the surface of the component subjected to friction. The bonding or impregnation process can be continually repeated for most components. Critical in these applications is the method used to bond the solid films onto the surfaces to be treated.