Solid and Dry Lubricants

Solid and dry lubricants differ significantly from liquid lubricants. Liquid lubricants reduce friction due to their fluidity and viscosity. However, while solid lubricants have neither of these properties, they are still capable of reducing friction and wear.
 

Use of Solid Lubricants

Solid and dry lubricants work in situations where liquid lubricators have no practical value. For instance, a solid lubricant may be sprayed on the outer hull of a boat to improve water resistance. Similarly, some of your kitchen pans also feature solid lubricants, namely in the form of Teflon®, to minimize interaction between the food you are cooking and the pan itself.
 
Solid and dry lubricants are also used in temperatures where liquid lubricants would boil away. A Teflon®-coated pan, for instance, provides lubrication at high temperatures, allowing you to adequately heat food on a non-stick surface.
 
When two objects roll across each other or when the two objects must move against each other under extreme pressure, solid lubricants can also be more effective than other types of lubricants. In such cases, a liquid lubricant would be squeezed out from between the two objects, but a dry lubricant will continue to operate.
 
The best solid and dry lubricants work due to forces at the molecular level. The molecular construction of the solid lubricant allows molecules to easily slide over each other.
 
When Not to Use a Solid Lubricant
In spite of its high tolerance for heat, a solid lubricant does not make a suitable cooling agent: Cooling agents need to flow from hot areas to cooler areas of a mechanism and then back again. For cooling purposes, a liquid lubricant is much more effective.
 
As a result, temperature thresholds tend to be the primary factor restricting the use of certain solid and dry lubricants. Graphite, for instance, is a commonly used solid lubricant. Graphite requires surrounding water vapor to maintain its lubricating qualities. Place graphite in a vacuum, such as outer space, and this normally reliable solid lubricant becomes abrasive.

Graphite: A Common Solid Lubricant

Graphite solid lubricants come in the form of either natural or synthetic lubricants. While natural graphite is mined, synthetic graphite is a man-made substance. Of the two, natural graphite is of higher quality, although high-grade synthetic graphite comes close to matching its natural counterpart.
 
Graphite, an effective solid lubricant up to 450˚C, can withstand higher temperatures. As noted above, however, this solid lubricant does not work in a vacuum.
 

PTFE (Teflon®) Solid Lubricants
Commonly known as Teflon®, PTFE is one of the most commonly used solid lubricants. Teflon-coated non-stick pans are commonplace household tools. As a solid lubricant, Teflon is limited: PTFE breaks down at temperatures higher than 260˚C.

Other Solid and Dry Lubricants

Graphite and PTFE are not the only solid and dry lubricants. Materials such as boron nitride, molybdenum disulfide and tungsten disulfide are also used as solid lubricants. Other materials that may be used as dry lubricants include:
  • lime
  • silver iodine
  • talc
  • white lead.

Dangers of Solid Lubricants

Overheating of PTFE cookware can cause the solid lubricant to break down. If this occurs, the ensuing airborne particles can cause respiratory problems. People with asthma or other respiratory disorders should take care not to overheat PTFE pans and cookware.
 
Solid lubricants are often sprayed or painted onto mechanical parts. As solid and dry lubricants typically contain toxic or carcinogenic substances, take care to avoid inhaling solid lubricants when using them.
 
Resources
 
Imagineering Finishing Technologies. (n.d.). About Dry Film Lubrication. Retrieved January 29, 2008, from the Imagineering Finishing Technologies Web site: www.imagineering-inc.com/solutions/dryfilmlubricants.htm.

SynLube. (updated 14 April 2007). Tribology. Retrieved January 29, 2008, from the Synlube Web site: www.synlube.com/tribolog.htm.

Tribology-abc.com. (n.d.). Solid Lubricants / Dry Lubrication. Retrieved January 29, 2008, from the Tribology-abc Web site: www.tribology-abc.com/abc/solidlub.htm.