Graphite as a lubricant dates to antiquity. Its first referenced in the mid 1500’s with use as pencils. [Pencil lead is a blend of graphite and clay. Pencil is from the Roman word penicillum, which means “little tail” referring to fine point brushes.] Graphite is a layer lattice lamella crystal structure where the bond between the carbon atoms in the basal plane of the layer are stronger (1.42° A) than the carbon bonds between basal planes (3.35° A).
Graphite has excellent lubricating properties as long as moisture vapor is available. Graphite functions as a lubricant up to about 1450° F. and as a release and anti-seize up to about 2400° F. Graphite begins oxidation at about 900° F, but does not begin sublimation until approximately 3600° C. The oxidation product is CO2. Due to the requirement for entrained moisture vapor, graphite does not function well as a lubricant in a hard vacuum and is, therefore, seldom used in deep space applications. Graphite itself does not undergo plastic flow. Graphite loaded 90° to the basal plane would have a higher friction (and slightly abrasive quality) than when loaded with the basal or lubricating plane. Fortunately, all lamellas orient under load to their lubricating attitude. Graphite is an anti-flux and, therefore, inhibits welding and fretting. Substrate surface profile/finish and various pre-treatment’s have profound affects on graphite’s lubricating performance.
Graphite comes in the natural state or artificially produced in an electric furnace. The best grades of mined graphite are vein type which is structurally identical to artificial graphite. Impure graphite of any type is unacceptable as a lubricant. Blends of graphite used in dry lubrication must be pure and have grading consistency absolutely maintained. Electric furnace graphite was developed and patented by Dr. Edward Goodrich Acheson who started the Acheson Colloids Company in 1899. Many companies employ natural graphite or blends of graphite in various fluid lubricants and organic or inorganic dry lubricant systems. Artificial graphite is the easiest to grade control. Natural and artificial graphite are produced in the U.S. and are available in war time.
Graphite and graphite dry film lubricant systems are commonly used in applications such as hot and cold forming, wire drawing, billet coatings, on high speed cutting tools, as a mold release for some die cast, plastic and rubber mold applications, cylinder head and exhaust bolts, ammunition and armament applications, automotive engine and many common industrial applications. Graphite functions well in very high temperatures, radiation environments and in cryogenic applications. Great caution must be exercised in cryogenics regarding associated binders and system additives especially in LOX systems.
The Dept. Of Defense, NASA, and The American Tribology Society (formerly A.S.L.E.) have substantial volumes of information. Significant data on graphite are available in the literature. Most work dates from 1950 to date, however there is some dated back to the 1890’s.
