Lubricants are substances enabling mechanical processes such as sliding to occur with a minimal amount of wear. Wear produced by friction, not only shears material, but it generates destructive heat.1 Part of the Merriam-Webster definition of ‘lubricity’ is “the capacity for reducing friction.” By way of example, the external surfaces of pistons rub up and down inside automotive engine cylinders with the aid of motor oil. The better the oil, the lower the friction, the cooler the operation, and the longer the engine life. To that end, modified or artificial oils are being developed.
A Microscopic View
But what physical or chemical characteristics make a lubricant slippery? To determine that requires a consideration of events at the microscopic level. We begin with diamond. At the “nano” level, using photoelectron emission microscopy (PEEM), it is found that diamond is quite slippery. Bonds at the surface level broken through sliding, are passivated by dissociative absorption of environmental moisture.
Weak surface bonding forces are often of the Van der Waals variety called London Dispersion forces. Another slippery solid material called Teflon® has a very smooth surface and only affords these dispersion forces which are in and of themselves weak, making Teflon one of the most slippery surfaces known.
Two other well-known solid lubricants are molybdenum sulfide and graphite. Graphite forms in large self-contained sheets or planes that slide across one another readily. Thus graphite is an excellent solid lubrication for some situations. However, for high-temperature applications, molybdenum sulfide displays superior stability lubricating properties. Molybdenum sulfide is characterized by flat layers of molybdenum metal, sandwiched between single atomic layers of sulfur atoms. Since the sulfur-sulfur planes interact only very weakly, the layers slide across each other with little frictional drag.
What about Liquid Lubricants?
What about motor oil molecules and synthetic oil substances? While nothing comprehensive will be discussed here, suffice it to say that lubricating oils tend to be viscous. Also, although molecules may be longer than most, they generally do not bond to each other except, once again, by the relatively weak dispersion forces. Hence, the molecules can glide across one another relatively unhindered. Substances weakening these molecules stability must be eliminated. A serious enemy of motor oil is oxidation to produce polar, bonding, corrosive carboxylic acids.
The common ground, then, is the reduction of internal forces such as molecules or planes of the lubricant, while maintaining for most applications, neutral, stable bulk properties.
1 Heat can cause a disabling warping of parts, damaging functionality, or even destructive welding.
Argonne National Laboratory – “Nano-boric acid makes motor oil more slippery”
University of Pennsylvania – What Makes Diamonds Slippery at the Nano-Scale?
University of Illinois at Champaign – “Illinois Chemists Spray Their Way to Better Catalysts”
For additional information about motor oils:
Motor Oil Engineers – “Motor Oils and Lubrication – A Comprehensive Practical and Technical Handbook,” by Dave Mann