A materials properties on a large scale may differ from its properties at the nanoscale. For example, smaller particles have an overall lower melting temperature than larger particles. Other differences observed with various nanoparticles include a higher affinity for binding to foreign molecules or atoms, negative heat capacity, lower electrical conductivity, altered chemical behavior and moments of magnetism in normally non-magnetic elements.

One of the more popular and extensively researched nano-additives for lubricants is molybdenum disulfide. MoS2 is a solid that exhibits a layered, planar structure and a low coefficient of friction. This structure allows energy from applied shear stress to be dissipated, reducing the amount of heat generated. In addition, the sulfur atoms in the molecules help create a protective tribofilm that further decreases friction between contact surfaces. As a result, the MoS2 molecule reduces wear and tear through both physical and chemical properties.

Unfortunately, MoS2 is mostly insoluble and produced in micron size ranges, which makes it difficult to use as an additive in liquid lubricants. However, work is ongoing, and multiple studies have been performed aimed at synthesis and use of soluble sulfur-containing molybdenum complexes exhibiting high lubricating and antioxidant properties.

Metal oxides are another commonly used nano-additive in lubricants. According to an article from a 2018 issue of the Russian Journal of Applied Chemistry, a team of researchers led by E. Yu. Oganesova of the Russian Academy of Sciences modified copper oxide molecules with oleic acid and blended them into mineral oil, causing a significant decrease in the wear of contact surfaces to which the lubricant was applied.

Zinc oxide has historically been used for strong adsorption since it has a low melting point, which is ideal for improving tribological properties of lubricants. Although the addition of zinc oxide to a lubricant does not influence the antiwear properties, it does affect extreme load carrying ability.

Zinc meta-aluminate has been researched as a nano-additive because of its high heat resistance, mechanical stability and low surface acidity. Results show that it produces lower friction and wear than zinc oxide, aluminum oxide and iron oxide.

Oganesovas team concluded that pure metal nanoparticles also can be used as nano-additives in lubricants. Their research revealed that copper nanoparticles at both 25 and 60 nm reduced the friction coefficient in a lubricant sample by 51 percent and 69 percent, respectively, at a load of 2200 newtons, with only a slight decrease in kinematic viscosity. A soft copper film formed between the two surfaces in contact, protecting them from direct contact and significantly reducing wear.

In addition, copper nanoparticles can have a mending effect on friction surfaces. The friction coefficient of a lubricant with copper nanoparticles was measured using the pin-on-disk test. Results showed that the copper nanoparticles operated at a lower friction coefficient than the control lubricant, as expected. However, when the experimental lubricant was wiped away and replaced with the control lubricant, the coefficient of friction remained lower than it did with the control lubricant alone. This phenomenon suggests that copper nanoparticles deposited themselves into worn-out spots on the disk, effectively mending the machinery.

The copper particles themselves can be altered to further improve their tribological properties. Copper nanoparticles modified with tetradecyl-hydroxamic acid have even greater antiwear and mending properties. Zinc particles around 20 nm have similar antiwear and mending properties to copper when suspended in a 6 centistoke polyalphaolefin base oil.

Bucky Additives

Recent recommendations for using molybdenum dithiocarbamates-which produce MoS2 in service-instead of phosphorodithioates stem mainly from rigorous restrictions imposed on the phosphorus content of motor oils.

An alternative to MoS2 exists in the form of the inorganic fullerene tungsten disulfide (IF-WS2) particle, discovered by Reshef Tenne of the Weizmann Institute of Science in 1992. This relatively new nanoparticle is gaining popularity due to its tribological properties and potential for use as a solid lubricant as well as in various liquid dispersions. It has a near spherical geometry and hollow core, similar to carbon fullerenes (also known as Buckyballs), which provides high impact resistance of up to 35 gigapascals. Further, its closed nanostructure makes the material chemically stable.

In a 1999 article published in Wear journal, researchers reported that synthesis methods for IF-WS2 had poor size control of the manufactured molecules, impacting its performance as an additive. Since then, technology has been developed that allows for control of the shape and size of the particles.

While standard solid chalcogen particles-WS2, MoS2 and other metal sulfides-have platelet-like structures that create their tribological properties, the spherical IF-WS2 particles have tens of closed concentric layers, making these multi-layered particles excel under extreme pressure or load. Under impact, the outer layers of the caged spheres exfoliate and bond with the working surfaces.

The peeled off layer then fills in asperities and other surface irregularities to create a continuous, protective lubricating coating that significantly reduces equipment wear and premature equipment failure. The process can repeat itself to form multiple protective layers. As a result, additives with IF-WS2 particles have excellent extreme pressure performance that can lead to reduced friction, lower operating temperatures, improved fuel efficiency and reduced wear.

In their research, Oganesovas team added IF-WS2 particles modified with trioctylamine to mineral oil to improve its dispersibility and tribological properties, and the results were promising. It was clearly shown that the chemical composition of nanoparticles plays an important role in friction and wear reduction, with metal sulfides showing the highest performance due to the effect of sulfur atoms on tribological behavior.

Further, stability of the particles is an important point, and stabilization methods along with surfactants such as trioctylamine play a vital role in performance.

An article published in Tribology International in July indicated that researchers have developed a novel nano-additive by modifying reduced graphene oxide with beta-lactoglobulin. The addition of BLG-RGO reduced friction by 37 percent and wear by 45 percent, and the nanoparticles remained dispersed within the medium for over eight months. These improvements may make water based lubricants a viable alternative to oil based lubricants.

Water based lubricants with various nano-additives such as titanium dioxide, lanthanum fluoride and cubic boron nitride sheets also show promising results, but the best performing lubricants are being delivered through the addition of IF-WS2 particles. The particles improve heat transfer by up to 20 percent, according to Oganesovas team, and can be used for metalworking as a nonflammable, water-soluble lubricant with exceptional cooling properties and surface finishing. An added bonus is that water based IF-WS2 additives can be added to all types of water-dilutable metalworking fluids, including soluble oils, semi-synthetics and synthetics.

Subtracting Additive Problems

Until recently, manufacturing nanoparticles inexpensively and reliably was one of the challenges for this developing technology, and there are still some difficulties. For example, copper nanoparticles are treated with oleic acid and milled in a ball mill. This method creates macroscopic particles that are uneven in size, potentially damaging their tribological performance.

The process also causes intense interaction between nanoparticles and the dispersion medium. Metals and their various alloys can be dispersed through electro­erosion, but the same problems that arise during milling may occur here as well. Electrochemical generation can produce a very large number of nanoparticles, but they are very small in comparison to most nanoparticles (1-2 nm), which may affect performance.

Nanoparticles can also be manufactured through chemical reactions. The procedure starts with metal-containing compounds such as metal carbonyls and organometallic compounds. One method involves exposing the metal-containing compounds to high amounts of heat or ultraviolet radiation, causing them to decompose. Copper nanoparticles have been synthesized from a copper sulfate solution using potassium tetrahydroborate, a powerful reducing agent.

In contrast, IF-WS2 can be produced in powder and in pre-dispersed, highly concentrated forms in both water and oil. The nanoparticles are produced from tungsten oxide through solid-gas synthesis in a hydrogen disulfide environment. This process allows industrial scale production, making the end product more cost-effective. Due to the higher surface area coverage, nanoparticles are only needed at low concentration, reducing if not closing the price gap with macro-size analogs.

Further, the magnetic properties of some nanoparticles may allow them to be recovered from a spent lubricant, decreasing the overall cost of recycling the used oil. In this way, using nanoparticles as additives can be economically viable.

George Diloyan is CEO of Nanotech Industrial Solutions. He holds a Ph.D. in mechanical engineering from Temple University with a focus on nano-technology, electrochemistry and material science as well as two Master of Science degrees in physics and computer science.

Raj Shah, Ph.D., is a director at Koehler Instrument Co. A fellow of the Society of Tribologists and Lubrication Engineers, the National Lubricating Grease Institute, the Royal Society of Chemistry and the Energy Institute, he has been active in the lubricants industry for over 25 years. He can be reached at rshah@
koehlerinstrument.com.