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Wednesday, 19 December 2012

Solid-Film Lubricants Are Very Useful!

Where do you use solid-film lubricants and what are their advantages?

Solid-film lubricants are much more than low friction coatings: They can be used for metal wear protection, too. Especially when surface contact is severe and localized. This is the time when lubrication, not hardness alone, is key. Just think of the valves and seats in your car.

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 may also 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.

Consideration must be given to 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-film lubricant materials has strengths and weaknesses, and corresponding compromises have to 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, it begins to decompose. 
  • Graphite when used as a solid-film lubricant 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-film 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-film 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 a particularly good solid-film lubricant 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-film lubricants suspended in oil, but the practical evidence seems to suggest some benefit in the reduction of friction.

The best application process for solid film lubricants.

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 film lubricants 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.

As can be seen, solid-film lubricants have a very special role to play in special applications. If you have a special application you need to professional advice on, contact Habot Synthetic Lubricants for professional advice.