Zhmud said, Nanoadditives open new ways to maximize lubricant performance. However, there exists a certain gap … between university researchers and industry professionals concerning their willingness to [adopt new technologies].

One practical issue, of which university researchers are largely ignorant, he added, is that lubricant formulations must balance a number of sometimes competing properties. This has slowed the use of nanoadditives in lubricants, and large-scale introduction of nanofortified lubricants is still facing serious technical obstacles, Zhmud said.

He then described the technical challenges of using nanoparticles in lubricants, including fullerenes, nanodiamonds, ultradispersed boric acid and PTFE.

Fullerenes are claimed to enable a rolling lubrication mechanism, that has never actually been proven, Zhmud said. Besides 60-carbon (C60) fullerenes, inorganic fullerenes such as tungsten disulfide with an average size of about 50 nanometers have been developed.

Studies suggest that C60 fullerene soot and inorganic-fullerene/tungsten-disulfide nanoparticles form more stable dispersions in hydrocarbons compared to regular tungsten disulfide powders and graphite, Zhmud said. Apart from improved dispersion stability, inorganic-fullerene/WS₂ does not appear to offer any obvious performance benefits over regular WS₂ powder. For example, when used in grease, the nanoadditive scores below the regular powder in a number of tribological properties.

Inorganic-fullerene/WS₂is marketed as an extreme pressure/antiwear additive for engine oils, gear lubricants and greases, yet its applications so far are very limited, Zhmud noted. Among the chief limiting factors is uncertainty about the health and safety of fullerenes, copper corrosion and poor oxidation stability.

As a result, engine oils fortified with inorganic-fullerene/WS₂ are likely to fail ILSAC GF-2 Sequence L38 and GF-3 Sequence VIII. In addition, the additized oils may cause severe damage to engines with Nikasil cylinder bore coatings, and offer no advantage whatsoever for engines with Alusil bores and roller-follower valvetrains.

Nanodiamond usually refers to ultradispersed diamonds with an average particle size of 4 to 6 nm. As a lubricant additive, nanodiamonds are claimed to embed into the sliding surfaces, rendering them more resistant to wear, Zhmud explained. Alternatively, they are said to enable rolling lubrication between the surfaces, thus reducing friction and wear.

Our studies and formulation experience have led us to a different conclusion regarding the extreme pressure and antiwear efficiency of nanodiamonds, Zhmud said. The fact that [reduced] friction is observed when nanodiamonds are added to lubricant formulations is consistent with their micropolishing effect, resulting in faster running-in and smoother mating surfaces. However, the effect is insignificant for aged oil, where wear rate and surface roughness are dominated by other factors such as oil contamination.

Furthermore, since the abrasiveness of nanodiamonds does not [lessen] after the initial running-in period, there is a risk for excessive wear over a longer period of time, Zhmud claimed. Analysis of used engine oils doped with nanodiamonds shows unusually high levels of aluminum, copper and chromium, indicating accelerated wear of bearings and piston rings.

Nanodiamonds may also increase the risk of roller skidding in finger roller follower valvetrains, Zhmud said. On the other hand, micropolishing seems to improve the meshing of certain components after running-in. Therefore, nanodiamonds may prove useful in running-in oil, although more studies are needed to discern possible unintended consequences.

Boric acid once was a common additive in metalworking fluids because of its excellent extreme pressure, antiwear, bacteriostatic and bactericidal properties. Now, it has been largely phased out because of health and safety concerns.

However, Zhmud said that there may be renewed interest in boric acid because some studies mention boron-based nanoadditives that can reduce friction and wear in a variety of industrial and transportation applications. The boron additives replace sulfur and phosphorous, thereby eliminating the main sources of hazardous emissions and wastes.

Unfortunately, there are quite a few technical hurdles to mar that optimism, Zhmud cautioned. First, boric acid has no antioxidant effect, so it cannot replace ZDDP. Second, boric acid is not compatible with some essential lubricant additives, specifically TBN buffers, which may lead to corrosion and sludge problems. Engine oils containing boric acid are likely to fail ASTM D 6557 and D 6593 (ILSAC GF-3 Sequence VG) tests.

A recent proposal has combined electrochemical boriding with nanocolloidal boron nitride to improve the performance of drivetrain components in advanced wind turbines. This may be a promising future application of this new technology, Zhmud said.

Polytetrafluoroethylene has been used in greases, chain oils, dry-film lubricants, etc., to provide higher weld loads, higher load wear indexes and reduced stick-slip, Zhmud noted. Though PTFE nanodispersions are used in a number of aftermarket engine treatment products, its use in engine oils is rather limited because of the inherent instability of PTFE dispersions in oil, the risk of filter clogging, and difficulties with recycling. In addition, he cautioned, the use of PTFE in engine oils is discouraged even by the major PTFE producer, DuPont Co.

In conclusion, Zhmud said, Nanomaterials have potential for enhancing certain lubricant properties, yet there is a long way to go before balanced formulations are developed.