Use of tungsten disulfide nanoparticles as lubricant additives

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2012

Nanoparticles form a protective coating through exfoliation that provides better additive performance.

KEY CONCEPTS
• Tungsten disulfide nanoparticles function as antiwear and extreme pressure additives by forming protective coatings through an exfoliation process conducted under severe lubrication conditions.
• The nanospheres can be dispersed in mineral oil and synthetic basestocks and used in a variety of lubricant applications.
• Performance testing has shown that improved wear, friction and performance under extreme pressure conditions can be achieved through the use of these nanospheres.

AS LUBRICANT RESEARCH IS SHIFTING TO MORE CLOSELY EXAMINE how performance can be impacted at the molecular level, the use of nano-based additives and fluids is growing. The functions of the nanoparticles used cover the broad spectrum of lubricant additives.

One area that has been highlighted several times in this column is the use of nanoadditives to improve the thermal conductivity of heat transfer fluids and refrigeration lubricants. In a previous TLT article, the effect of using hexagonal boron nitride nanoparticles in transformer insulating fluids was discussed (1).

The researchers developed stable dispersions of hexagonal boron nitride nanoparticles with average diameters of 600 nanometers in naphthenic base oil. At a peak concentration of 0.1%, the boron nitride nanoparticle-based fluid exhibited a thermal conductivity enhancement of approximately 80%.

Preparation of nanoparticles exhibiting antiwear and extreme pressure (EP) additive characteristics is of interest because both of these additive types function on the metal surface. These smaller sized nanoparticles may provide better performance because they are better able to find their way to small spots between metal surfaces present in a boundary lubrication regime.

Baruch Halpert, chairman & CEO of NanoMaterials Ltd., in Yavne, Israel, says, “Many of the older generation of solid-based lubricant additives consist of large flat-lamellar shapes such as those seen with molybdenum disulfide. One of the problems with these materials is that because of their size and shape, they may create a risk of blocking the tubes and capillaries of the engines, as well as potentially blocking automotive engine filters. As a general rule of thumb, this lead to less use of these solid-based additives as compared to the regular liquid-based lubrication solutions.”

This effect can decrease the ability of the nanoparticle to impart antiwear and extreme pressure characteristics. A new type of nanoparticle structure is needed to overcome this deficiency and provide better additive performance. Such a material has now been developed.

CONCENTRIC NANOSPHERES
Halpert and his researchers have developed tungsten disulfide nanoparticles that are a series of progressively smaller concentric spheres that exhibit a diameter between 60 and 70 nanometers. He says, “We have been able to develop these nanoparticles that act in a similar fashion to an onion. As the layers encounter severe lubrication, they can peel off or exfoliate and form a protective coating that improves performance. This is also known as a tribofilm.”

This enables these nanoparticles to be well-suited for use as antiwear and extreme pressure additives. An image of a nanoparticle with concentric spheres is shown in Figure 2. Halpert adds, “There are typically 30 layers of concentric spheres in a typical nanoparticle.”


Figure 2. Tungsten disulfide nanoparticles are prepared with progressively smaller concentric spheres that can peel off under severe lubrication conditions to provide superior performance. (Courtesy of NanoMaterials, Ltd.)

These nanospheres are sold for various lubricant applications under the NanoLub® tradename. Key applications include engine oils, gear oils, chain oils and greases.

In these applications, the nanoparticles are successfully dispersed in mineral and synthetic oil. Halpert says, “Finding the proper technique for developing a stable dispersion is very complex. We have been able to disperse our nanoparticles in all types of basestocks. This includes low- and high-viscosity base oils.”

The dispersed tungsten disulfide nanoparticles at a concentration of 5% demonstrated a major improvement in wear through the use of the SRV tester. The SRV test conditions, as defined by the ASTM D5707 standard, were 200 newtons of applied load, 50 Hz of frequency, a stroke amplitude of 1 millimeter and a duration of two hours. When incorporated into standard 5W- 40 motor oil, a 21% drop in wear scar diameter was obtained.

Further testing was accomplished through the use of the 4-ball wear test, ASTM D4172. Halpert says, “We found a 30% improvement in wear when testing a 5W-40 engine oil.”

The tungsten disulfide nanoparticles can also provide beneficial performance in automotive gear oils. In a ball on cylinder test run under a pressure of 5.85 gigapascals, a 85W-140 oil used in heavy-duty engine oil applications displayed operated for 2.1 million cycles without failure when the tungsten disulfide nanoparticles were added to the engine oil. In the absence of these nanoparticles, failure was seen after 800,000 cycles. Micropitting was also seen in the absence of the nanoparticles.

More specifically, some independent laboratories evaluated friction properties under high pressure conditions (over 1 GPa) in a mixed lubrication regime. A coefficient of friction of 0.073 was obtained.

The tungsten disulfide nanoparticles also can impart extreme pressure characteristics to greases. At a 3% treat rate of a dispersion in a lithium grease, strong performance was seen in the 4-ball extreme pressure grease test (ASTM D2596).

The weld point for the base grease without any additive is 100 kg. In contrast, use of the tungsten disulfide nanoparticle dispersion boosted the weld point to 800 kg. Conducting the same experiment with molybdenum disulfide at a 3% concentration led to a weld point of 400 kg. Halpert says, “The tungsten disulfide nanoparticles are compatible with most grease thickeners.”

Another application for the tungsten disulfide nanoparticles is as a dry lubricant. Among the applications indicated by Halpert are antifriction coatings and impregnation in self-lubricating parts.

Halpert indicates that the mechanism for the synthesis of and the technology for the tungsten disulfide nanoparticles were invented by professor Reshef Tenne of the Weizmann Institute of Science.

In summary, this nanomaterial product offers a new formulation ingredient for designing high performance lubricants in terms of wear and friction reduction, even under the most difficult conditions. Further information can be found on the Nano-Materials Web site: www.apnano.com or by contacting Halpert at bhalpert@apnano.com.

REFERENCES
1. Canter, N. (2012), “Nano-Based Transformer Insulating Fluid,” TLT, 68 (5), pp. 10-11.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.