In particular, nanoparticles are well suited for tribological applications since lubrication takes place at the nanoscale level. To achieve boundary lubrication, for example, molecules can be selected that will form a thin carpet with the thickness of just one or two molecules to separate the surface asperities. For antiwear lubrication, molecules chemically attach to the metal surface, forming a thin barrier film. And in extreme-pressure lubrication, molecules react chemically with the metal surface, forming a sacrificial film of metallic salts to prevent catastrophic wear.

Nanoparticles fit these needs because they have a high surface affinity and chemical reactivity coupled with small size to penetrate wear crevices. There are promising applications for nanoparticles on the horizon now, especially as additive components in industrial lubricants such as greases, dry film lubricants and forging lubricants. This article will look at what nanoparticles are, how they are characterized and tested, and some of the challenges that must be overcome before nanolubrication can flourish.

Nanolube Types

A number of nanoparticles have been synthesized and many of them have been studied as lubrication oil additives. Particles which have drawn the most attention are metal oxides of silicon, titanium and zinc; fluorides of metals such as cerium, lanthanum and calcium; and zinc-, copper- and lead sulfides. Other research has involved straight metals nickel, zinc, copper, molybdenum compounds, and carbon nanotubes.

While it appears that some of these chemistries draw inspiration from traditional bulk lubricating materials which typically contain sulfur, chlorine and phosphorus, others are not so intuitive. Titanium, nickel and silicon, for example, are considered abrasive materials in their bulk form, with particle sizes between 3 to 10 microns, but have shown lubricating properties in the nanoscale range (generally considered to be less than 120 nanometers). For a sense of perspective, remember that a nanometer is one-billionth of a meter, and one human hair is over 50,000 nanometers wide.

While great progress has been made in less than a decade, nanoparticle lubrication technology is still in its infancy. A lot of research has taken place using Edisonian strategies by chemists with strong background in synthesizing nanoparticles but a weak understanding of tribology. They postulate that rigid spherical and cylindrical nanoparticles protect metal surfaces under low loads and slow speeds from wear by rolling actions – that is, they act as miniature ball bearings. (The spherical carbon atom buckminsterfullerene is a well-known example, as are carbon nanotubes.) At higher loads and speeds, the particles form a protective film.

However, current research merely indicates that nanoparticle films exist on worn surfaces, while specific chemical reactions and mechanisms for their anti-friction and antiwear properties have yet to be studied. The next stage of research should investigate these mechanisms so that materials beyond the current range of steel alloys can be understood (i.e. aluminum and titanium).

One Sticky Issue

One big challenge for nanolubricants research is how to control and optimize the particles solubility. Studies have shown that nanoparticles are attracted to and adsorb to surfaces due to Van der Waals forces at the molecular scale – and they likewise are attracted to one another. As a result, bare nanoparticles tend to agglomerate in both polar and non-polar solvents. When they agglomerate, they form clusters large enough to cause wear and fall out of dispersion.

The first steps to counter this phenomenon utilize non-ionic dispersants (such as OL100, OL300, T154, T101 or Aliquot 336) to help stabilize the suspension. These are moderately successful at extending the dispersive stability to several weeks. A number of studies have been conducted to find the optimal dispersant for this purpose.

Also promising are surface-capping techniques, which extend the longevity of nanoparticle dispersions to where they rival the stability of bulk organic lubricants, and can prevent agglomeration during synthesis. This technique binds organic molecules such as fatty acids and esters to the surface of the nano-core. When dispersed into petroleum oil, the long non-polar tails hang out into the solvent to keep the particles suspended. These non-polar coatings also repel the nanoparticles from one another to prevent agglomeration.

Surface capping is most often deployed during synthesis in situ, but has also been used to modify preexisting nanotubes.

Building a Nanoparticle

A number of synthesis methods have been developed for preparing nanoparticle lubricants, including reverse micro-emulsion mediated synthesis, arc spray nanoparticle synthesis, and catalytic synthesis of carbon nanotubes. In addition, carbon nanotubes can be surface-capped by acid catalyzed esterification after synthesis, and organic lubricant molecules can be mounted on to silica nanoparticles.

Of the above preparation methods, the most popular is reverse micro-emulsion mediated synthesis. An example of this six-step method is the synthesis of the inorganic nanoparticle molybdenum trisulfide (MoS3):

First, a reverse micro-emulsion (water-in-oil) is formed. Then ammonium molybdate salt is inserted into the aqueous core, which is subsequently reacted with hydrogen sulfide (H2S) to form tetrathiomolybdate salt. An appropriate surface-capping agent (an alkylated succinimide derivative) is added before the surfactant is removed, and finally the surface-capped MoS3 nanoparticle is extracted.

Tools and Tests

The method of choice for characterization of nanoparticles is transmission electron microscopy (TEM). This method provides information about particle morphology and size. Particle size is also confirmed by the use of X-ray diffraction (XRD).

Another method, Fourier transform infrared spectroscopy (FTIR), can be used to confirm the creation and loss of functional groups during the surface-capping reactions. For example, in one study an FTIR spectrum of a cis-9-octadecanoic acid-modified TiO2 nanoparticle demonstrated the gain of CH3 and CH2 alkyl groups; the conversion of a protonated COOH group to an ionized COO- group; and the existence of the nano-core. The spectrum also indicated the occurrence of an oleate side reaction, as signaled by an FTIR peak at 1739. Molecular structure is also confirmed by differential thermal analysis and thermogravity methods.

Tribological testing is also under way with nanoparticles. Both industry and academic researchers widely use 4-Ball Wear and 4-Ball EP test methods for tribological analysis, as these tests resemble the applications of real-world ball and roller bearing lubrication. Other methods used in the academic community to evaluate nanoparticles have included block-on-ring, reciprocal sliding disc and pin-on-disc configurations.

The 4-Ball Wear method is the most common of the above. It characterizes antiwear and anti-friction properties through measurement of wear scar diameter and depth, while wear surface morphology is characterized by scanning electron microscopy. In addition, elemental analysis of surface films on the worn region can be conducted by the energy dispersive x-ray spectroscopy and x-ray photo electron microscopy (XPS) methods.

Growing Opportunities

Nanotechnology represents a paradigm shift in the field of lubrication. Existing lubricating oil technology relies heavily on organic and organo-metallic chemistry in the bulk scale. While wide-scale commercialization of nanolubricants may still be a few years away in liquid lubrication, applications that traditionally utilize inorganic solids could prove to be a shorter leap.

Currently, many greases suspend solid lubricating particles of graphite, molybdenum disulfide, PTFE, polyethylene, zinc oxide and titanium dioxide, to boost their antiwear and extreme-pressure capabilities. Also, many forging lubricants today are water- or oil-based suspensions of graphite or other materials that remain as solids after the fluid evaporates. These solids do not burn off at the high temperatures; some provide boundary lubrication, some EP, and some have release properties. But in all cases they are there to provide metal movement and protect the die from wear.

In the short term, there also is a trend to move toward water-based, graphite-free non-black lubes. Regardless of the solid components, these stable dispersions are difficult to maintain (leading to short shelf-life) due to the particles size. This will likely be the first market to establish large-scale use of nanoparticles in industrial lubrication.

Indeed, Fuchs Lubricants, a leading forging lubricant manufacturer, has recently commercialized a high-performance water-based graphite forging lubricant that uses inorganic nanoparticle technology which is instrumental to its superior performance in the market. As with other emerging technologies, there are still a number of obstacles that need to be addressed. The impact of nanoparticles on worker health has only begun to be studied. For example, early studies suggest that nanotubes might behave similarly to asbestos when inhaled. The environmental consideration of dumping material at end of life has not even been initiated.

Despite these unknowns, nanoparticles offer an exciting new opportunity in the field of lubrication. Researchers have developed methods to prepare a wide range of nanoparticles. Dispersants and surface-capping methods are improving our ability to disperse them in solvents. Modern analytical instruments provide researchers with the capability to study their physical structures, functional groups and tribological properties.

Future efforts should be made to determine nanoparticle lubrication mechanisms, develop industrial scale preparation techniques, and study health and environmental impacts.