The use of nanosize particles in lubricants and greases has been the focus of many research groups, laboratories and industries for the past 10 years. According to research by George Diloyan at Nanotech Industrial Solutions Inc., closed-caged nanoparticles of tungsten disulfide, also known as inorganic fullerene like nanoparticles (IF-WS2) hold promise as extreme pressure and antiwear additives in grease. In a presentation at the ELGI Annual General Meeting in Dubrovnik, Croatia, in April, he reviewed current progress in the performance of these additives.
Particle Development
Diloyan, who is based in Avenel, New Jersey, United States, explained, Closed-caged nanostructures of tungsten disulfide were first observed in 1992 by researchers at the Weizmann Institute of Science in Rehovot, Israel. The spherical nanosize particles attracted much attention because they were closely related structurally to carbon fullerenes.
The nanoparticles were described as inorganic closed-caged, onion-like nanostructures of metal dichalcogenides (the chemical elements in group 16 of the periodic table, also known as the oxygen family). Later, selenides and tellurides [compounds in which selenium or telluride serves as an anion] of tungsten were reported, Diloyan said, followed by compounds of molybdenum, vanadium, titanium, etc. The research showed that virtually any layered (two-dimensional) material can fold into closed-caged nanostructures: either quasi-spherical onion-like or nanotubes.
The unique structure of those particles was believed to produce properties different from those of layered nanoparticles, he added. Later, methods to synthesize larger quantities of closed-caged nanoparticles were developed, making possible the systematic study of their properties.
Since the traditional application of layered structures is lubrication, the new material was studied as a solid lubricant and as a component in lubricant formulations. Possible commercial application for closed-caged nanoparticles led to the establishment of Nanotech Industrial Solutions to commercialize the technology developed by the Weizmann Institute, said Diloyan.
He then compared fullerene-like particles of WS2 with platelet forms of WS2. First, nanosize IF-WS2 (inorganic fullerenes of tungsten disulfide) particles are in the range of 30 to 150 nanometers whereas platelet material is usually in the range of several microns. This small size increases surface area and makes the material better able to smooth a surface by covering roughness irregularities and asperities.
Second, their closed structure makes them chemically stable. And their spherical geometry with a hollow core provides high impact resistance (up to 35 GigaPascals), enabling the material to act as a damper in applications under high loads and impact.
Diloyan explained that the powder produced in chemical processes typically consists of so-called primary nanoparticles of tungsten disulfide stacked together forming agglomerates of about 50 to150 micrometers across. In most cases, intermediate size aggregates of several primary nanoparticles are also present.
Agglomerates cause particle sedimentation, reduce surface area and reduce dispersion stability, Diloyan noted. A number of deagglomeration techniques were tested, including ultrasound, high-shear mixing and treatment with surfactants. While individual nanoparticles do have a tendency to agglomerate, due to high surface energy, he said, some techniques have been adopted in laboratories and industry to deagglomerate particles to a primary size and prevent reagglomeration.
The availability of deagglomerated powders has boosted the development of solid lubricant products such as antifriction coatings and dry lubrication for bearings. In addition, deagglomerated powders have enabled the development of ready-to-use dispersions of IF-WS2.
Several formulations were developed with nanoparticles homogeneously dispersed in oils – for use as lubricant additives – or other media – as reinforcement for composite materials, said Diloyan. For instance, an aftermarket additive for engine oil enhanced with IF-WS2 showed the capabilities of nanoparticles to improve lubricant performance in the ASTM D2596 extreme pressure test.
Closed-caged IF-WS2 nanoparticles have been tested in lubricant applications since 1992. However, interest in the materials has increased since they became available in industrial size quantities.
This enabled a closer look at earlier data regarding the particles mode of operation. Diloyan said, Earlier studies suggested that the predominant lubricant mechanism of onion-like nanostructures was rolling of the nanoparticles between contact surfaces. However, recent studies suggest that the lubricity of nanoparticles may be explained by a variety of mechanisms, depending on operating conditions, mainly contact pressure.
In fact, he continued, the predominant lubricating mechanism is the formation of a tribofilm on the contact surfaces. The tribofilm consists of a thin layer of tungsten disulfide produced by exfoliation of external disulfide layers from the nanoparticles in the contact area, Diloyan explained. This mechanism was verified in a number of tests on oils and greases containing closed-caged nanoparticles of IF-WS2.
Making Stable Dispersions
One issue discovered early in research was that the test results could not always repeated when testing materials from different sources. First generation dispersions lacked stability, and the particles had a tendency to settle out over time, Diloyan said. The particle sediment not only looked bad, but performance also suffered.
So the challenge became finding a way to convert the powder into liquid dispersions with enhanced stability and uniform performance, he continued. In addition, the product had to be cost effective and compatible with a variety of oils and thickeners.
These aims were met in second-generation IF-WS2 dispersions that capitalized on an enhanced understanding of the nature of nanoparticles with advanced manufacturing techniques. The result was a new level of stability and performance that made possible the formulation of nanopowder dispersions targeting specific operation conditions, said Diloyan. In particular, additives for engine oils, gear oils, heavy industrial equipment have been developed.
Grease EP Additive
Research on greases formulated with dispersions of nanosize IF-WS2 resulted in the development of a grease additive that improves the extreme pressure properties of lithium greases and outperforms greases using molybdenum disulfide. Four-ball tests according to ASTM D2266 and ASTM D2596 showed that the additive improves both extreme pressure and antiwear properties of a wide range of greases, including lithium complex, calcium, aluminum complex and polyuria, Diloyan said.
In another study, the antiwear properties of a lithium-based grease containing nanosize IF-WS2 were compared to those of the same grease formulated with conventional micron-size tungsten disulfide. The grease containing nanosize IF-WS2 produced a wear scar diameter of 500 m compared to 820 m for the untreated grease and 682 m with the micron-size additive. Diloyan attributed the improved antiwear properties to the formation of a tribofilm on the contact surfaces that is visible in scanning electron microscope images.
Diloyan concluded by citing a study by a European grease manufacturer that compared the tribological behavior of grease enhanced with molybdenum disulfide and two type greases containing additives enhanced with nanosize IF-WS2. Tests performed in a Four-Ball machine confirmed that a treatment of only 1 percent IF-WS2 nanoadditives improves extreme pressure and antiwear properties of greases, and also outperform formulations based on molybdenum disulfide, he said.
The study also showed that the nanoparticles do not affect the dropping point of the grease. Also, while some nanoadditives may affect water wash-out properties, this tendency can be effectively controlled with other additives.