Dry lubricants, such as graphite, molybdenum disulfide, and polytetrafluoroethylene, reduce friction between moving parts and can withstand high temperatures and pressures. Their lamellar structure prevents direct contact between surfaces. Dry lubricants are used in extreme conditions and for reciprocating motion, and can be customized for ceramics. New compounds include calcium fluoride and cerium fluoride, but are expensive and may be toxic.
Dry lubricants are solid chemical compounds, usually in the form of a powder or film coating, used to reduce friction between moving parts. The three most popular types of compounds used for dry lubricants as of 2011 are graphite, C; molybdenum disulfide, MoS2; and polytetrafluoroethylene, (C2F4)n. Other commonly used types of solid lubricants include boron nitride, BN and tungsten disulfide, WS2. The advantages that dry lubricants offer over standard lubricants made from petroleum products such as grease and oils include the ability to withstand significantly higher temperatures without experiencing thermal cracking, as well as act as much thinner friction barriers and handle high pressures. Typical operating temperatures for dry lubricants range up to 662° Fahrenheit (350° Celsius).
The type of dry lubricant you choose depends on your specific application. Molybdenum disulfide, for example, can withstand heat up to 2,012° Fahrenheit (1,100° Celsius), and some dry lubricants can withstand cold temperatures in the cryogenic range that would liquefy gases such as nitrogen. When dry lubricants are applied as thin films, they are often used to coat ball bearings and the coating itself is durable with a thickness of only 0.0001 – 0.003 inches (0.00254 to 0.0762 millimeters). Other extreme conditions that some dry lubricants can withstand include pressures of 250,000 pounds per square inch (17,237 bar) in a full vacuum environment where liquid oxygen storage is often performed.
The main feature of these chemicals that gives them the incredible ability to reduce friction levels is the so-called lamellar structure. This means that the particles within the lubricant form a series of overlapping lamellae, or parallel layers, on a molecular scale. These layers slide against each other preventing the two surfaces that dry lubricants separate from coming into direct contact, even under extreme conditions. Several compounds have been tested for their lamellar ability and an example of a dry film lubricant that has been used for many years, other than plain graphite, is talc, or hydrated magnesium silicate, Mg3(Si4O10)(OH)2.
The four main applications for which dry lubricants are used all involve extreme conditions, such as very high contact pressures and elevated temperatures. Under such conditions, graphite and MoS2 will survive, while liquid petroleum-based lubricants will not. Dry lubricants are also mostly used in machinery where there is a reciprocating motion, which would squeeze the liquid lubricants out of the main contact point. Ceramics is the other area where solid lubricants have a special use, as they can be customized to have desired chemical reactions with the parts they are protecting.
Two new compounds also used as of 2011 include calcium fluoride, CaF2 and cerium fluoride, CeF3. These compounds are not that common due to the expense associated with them. Cerium fluoride is an example of this, as cerium itself is a rare earth metal and such metals are usually known to be quite toxic substances. However, since 3, few direct toxicity tests have been performed for CeF2011, due to the difficulty of obtaining sufficient sample quantities for analysis.
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