Molybdenum disulphide as an oil additive
In past RET-Monitors for this keyword we have taken a general overview of oil additive technology, but it is worth looking at some additives in more detail, in this case molybdenum disulphide, often referred to as just moly.
With the chemical symbol MoS2, this black crystalline compound occurs as the mineral molybdenite, the principal ore from which molybdenum metal is extracted. It is commonly used as a solid lubricant, thanks to its low-friction properties, which are similar to those of graphite, as well as its high load-bearing capabilities and the fact that it is relatively unreactive, being unaffected by dilute acids and oxygen. Most usefully when it comes to engine applications, it has good thermal stability, up to 350-400 C in an oxidising environment.
Molybdenum disulphide was first discovered more than 250 years ago, when the lubricating properties of an unknown ore were noted in 1744 by Johann Alexander Cramer. The ore was similar to lead, galena and graphite, and these substances were labelled with the Greek word ‘molybdos’, meaning lead-like. In 1778, a Swedish scientist named Carl Wilhelm Scheele identified molybdenite as the sulphide of a distinct metallic element by heating it to yield a white oxide powder. At his suggestion, Peter Jacob Hjelm, another Swedish scientist, successfully isolated the metal in 1782 and named it molybdenum.
The first use for the material was as a strengthening agent in steel production, a use to which it is still put, but it was not until 1935 that it was used for its lubricating properties. A German engineer, Alfred Sonntag, had designed a huge machine to simulate aircraft vibrations, but it failed due to friction between the moving parts. He tried many lubricants to solve the problem, but none had sufficient load-bearing capability to be effective. However, he came across an 18th century text that mentioned the lubricating properties of molybdenite, and on using it as a lubricant found it to be highly effective. After this discovery, Sonntag developed a method of purifying molybdenite, which contains traces of quartz, into the powdered lubricant that is in use now.
Molybdenum disulphide takes the form of microscopic hexagonal platelets, with several molecules making up each platelet. These platelets are attracted to metal surfaces which, when combined with sliding force between metal parts, results in a thermo-chemical reaction, creating a protective coating of MoS2 on the parts in question. This coating can withstand pressures of about 500,000 psi, and as such makes MoS2 an attractive option for use on components where boundary lubrication is an issue, such as the interface between camshafts and tappets.
While the application of MoS2 as a dry-film lubricant was established in the mid-20th century, using it effectively in oil took longer to perfect. The problem was that the particles would not stay in suspension in oil, leading to the particles forming a sludge that could block oil passageways (while also negating the material’s lubricating benefits). However, once methods were found to prevent this, MoS2 has proved to be a highly effective anti-wear additive. For example, tests undertaken at the Argonne National Laboratory, in Illinois in 2012* showed that adding MoS2 nano-particles, 50 nm in size, to a polyalphaolefin base oil showed significant reductions in friction between the piston skirt and cylinder liner on heavy-duty industrial engines. These same benefits can also be realised in transmission oils, where boundary lubrication is far more common.
As with any technology though, the use of molybdenum disulphide as an oil additive is not a silver bullet. For example, it only acts a friction reducer under boundary lubrication conditions; in hydrodynamic and full-film regimes, the particles do not come into play and some studies have even shown that they can actually marginally increase friction. However, provided its limitations are recognised, it can have considerable benefits when used correctly.
* Nicholaos, G., Demas, Elena, V., Timofeeva, Jules L., Routbort, George R. Fenske, “Tribological effects of BN and MOS 2 nanoparticles added to polyalphaolefin oil in piston skirt/cylinder liner tests”, Argonne National Laboratory, 2012
Written by Lawrence Butcher