By: Dr. Akanksha Urade (Graphene & 2D Materials Science Writer)
Graphene is increasingly being used as an additive to transform a growing number of products in practically every industry.
In this article, I will explore the use of graphene as a lubricant.
Graphene flakes have practically endless applications. It is added to other materials to improve strength, water resistance, flexibility and electrical conductivity. A tiny amount – typically, between 0.01%-0.5% – can produce dramatic improvements.
Graphene can be an inexpensive replacement for many incumbent materials.
The problem has been finding a reliable source for industrial volumes of the right quality graphene for specific applications.
Global Lubricant Market
Grandview Research predicts that the $130 billion lubricants business in 2021 will expand at a CAGR of 3.7% through 2030, led by increasing global demand for higher-performance lubricants. Graphite is the primary incumbent material for lubricants. But graphite has a number of drawbacks – including that it only works in humid environments. Another disadvantage of graphite is the tendency of lamallae to rupture under severe mechanical loads, resulting in a limited lifetime and a higher coefficient of friction.
There are other problems with lubricants, including the use of ecologically hazardous additives or solid lubricants (such as molybdenum disulfide or boric acid). Both oil-based and solid lubricants do not bond well to the surfaces it lubricates and must be reapplied on a regular basis. Even under the best conditions, most lubricant oils eventually degrade over time due to oxidation.
Different forms of graphene have been extensively tested as a lubricant additive. Graphene’s use as a lubricant is attributed to a number of different physical-chemical properties. For example, graphene’s exceptional mechanical strength prevents material wear. Second, graphene has been demonstrated to be impermeable to liquids and gases like water and oxygen, slowing down the oxidative and corrosive processes that normally cause damage to rubbing surfaces. Furthermore, because graphene is an atomically smooth 2D material with low surface energy, it can replace the thin solid films that are typically used to reduce the adhesion and friction of various surfaces.
Graphene as an Additive in Oils
Graphene can also be utilized as an additive in lubricants to increase fuel economy and engine stability. Companies such as Graphenoil, Graphene-XT, HydroGraph, Versarien, NTherma and others have added different forms and quality of graphene to lubricating oil to enhance performance and stability, resulting in less wear and tear.
“The addition of graphene improves the oil’s tribological properties, making it more suitable for high-pressure, high-stress environments”, notes Simone Ligi, the Chief Executive Officer of Graphene-XT. “But the benefits of graphene do not stop there. Graphene has good heat transfer properties, essential to make lubricants safer at higher temperatures. All of these effects combined reduce engine noise and fuel consumption”.
Graphene as a Solid-State Lubricant
People commonly associate lubricants with the fluids found in automobiles and industrial machines. While fluids make up the vast majority of modern lubricants, a subset of lubricants known as solid-state lubricants also exists. Argonne National Lab has been researching solid lubricants based on graphene as a cheaper, more efficient and longer-lasting alternative to oil.
Image Courtesy: Berman, Diana, et al. Science (2015).
The use of graphene and carbon nanodiamonds as a solid-state lubricant to better preserve ball bearings is a field of study that has progressed rapidly in recent years, from an intriguing idea to a nearly practical reality. When graphene flakes and nanodiamond particles brush against a large diamond-like carbon (DLC) surface, the graphene encapsulates the nanodiamond by wrapping itself around it. As nanodiamonds are spherical in shape, the graphene-nanodiamond combination may travel freely between the two surfaces while providing lubrication. In addition to their lubricating and corrosion-preventative properties, they have also demonstrated super lubricity effects in which friction is reduced to nearly zero.
“That’s a significant improvement over any other existing solid lubricants coating available today,” says Argonne’s Prof. Anirudha V. Sumant. “Also, the amount of graphene needed is very small and therefore cost is much lower and eliminating oil waste would be more environmentally friendly, which is a great side benefit.”
The same research team revealed graphene to be an excellent steel lubricant. A few atomic layers of graphene not only reduce the degree of friction in steel rubbing against steel by seven times and the amount of wear by 10,000 times, but can also significantly lower the risk of corrosion.
The advantage of graphene-based solid lubricant coatings over standard lubricants is their simplicity of application. It is applied by spraying a solution over a vast surface area and can coat virtually any shape or size.
Graphene Oxide vs. Real Graphene
In our earlier piece titled “Fake Graphene: Let the Buyer Beware,” I made it abundantly clear that high-quality, defect-free graphene enjoys superior properties to their oxidized counterparts, such as graphene oxide (GO). However, in their marketing and on the labels of their bottles, many companies that sell GO and reduced graphene oxide (rGO) call these materials graphene. Even with lubricant applications, this is still the case.
Image courtesy: Berman, Diana, Materials Today (2014)
When compared to the wear rate of graphene layers, the wear rate of GO is between one and two orders of magnitude higher. As can be seen in the figure, oxidized graphene has dramatically inferior coverage compared to high-quality graphene, and the presence of oxygen in GO may cause corrosion of steel, which, in turn, increases wear. Because of this, GO does not offer anything approaching the same level of wear protection as high-quality graphene.
Contemporary lubricants contain ecologically hazardous chemicals or are solid lubricants (such as molybdenum disulfide or boric acid). Both oil-based and solid lubricants degrade over time and must be replenished on a regular basis. Real, high-quality graphene, on the other hand, can persist for a long period because the flakes realign themselves during initial wear cycles. Graphene, which is entirely composed of carbon, is environmentally friendly. In specific applications, do I think that graphene lubricants could serve as a suitable alternative to the more traditional oils and fluids? Yes. Would graphene lubricants be a universal replacement for oils? No. There are a number of reasons for this, but the key factor is the lack of supply of high-quality graphene. Nevertheless, we cannot deny that graphene-based lubricants and oils are making their way onto the market. However, whether or not they will come to dominate the market depends upon the ability to manufacture industrial volumes of high-quality graphene.
Berman, Diana, Ali Erdemir, and Anirudha V. Sumant. “Graphene: a new emerging lubricant.” Materials today 17.1 (2014): 31-42.
Berman, Diana, et al. “Macroscale superlubricity enabled by graphene nanoscroll formation.” Science 348.6239 (2015): 1118-1122.