By: Dr. Akanksha Urade (Graphene & 2D Materials Science Writer)
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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.
Grandview Research predicts that the $130 billion lubricants business in will expand at a CAGR of 3.7% through , 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 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”.
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 ().
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.
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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 ()
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.
References
Berman, Diana, Ali Erdemir, and Anirudha V. Sumant. “Graphene: a new emerging lubricant.” Materials today 17.1 (): 31-42.
Berman, Diana, et al. “Macroscale superlubricity enabled by graphene nanoscroll formation.” Science 348. (): -.
Graphene has some interesting and beneficial solid lubricating properties in comparison to graphite, which is well known to work best in humid environments but does not offer low friction and wear in inert, dry or vacuum conditions
The mechanism behind graphite’s capability in humid environments is the intercalation of water molecules between the graphite sheets, which allows for shearing action that provides its low friction properties. Graphene however, is capable of providing lubricating properties regardless of a dry or humid environment.
Besides its potential application as a solid lubricant by itself, graphene can be applied into conventional lubricating fluids and composites. In dispersions in oils, functionalized graphene platelets have been shown to improve load-carrying capacity, wear reduction and coefficient of friction in base oil, with performance comparing favorably against graphite.
2-Dimensional: Graphene’s extreme thinness that is the result of its 2D nature means that it can gain entrance into tribo-interfaces.
Chemical inertness: Graphene’s two-dimensional property makes it chemically inert, which gives it unique ability to combat oxidation.
High mechanical strength: graphene provides excellent potential for use as an
ultra-thin protective layer for many precision components to suppress material wear owing to its greater strength.
Ease of surface functionalization: This property of graphene enhances the potential of graphene-based materials for aqueous lubrication by adding oxygen functionalities.
Thermal Conductivity: The excellent thermal conductivity of graphene is conducive to the heat dissipation under elevated temperature conditions for lubricants. The application of graphene as a lubricant or lubricant additive at elevated temperatures mainly includes solid lubrication and liquid lubrication.
Drilling equipment in the oil and gas industry tends to wear down very quickly. Oil and gas well pipes, normally consist of relatively low cost, low carbon steel susceptible to hydrogen embrittlement, hydrogen sulfide induced corrosion, and chloride stress corrosion and cracking.
The use of graphene in coatings and lubricants can improve the way companies drill and complete their wells through increasing strength, durability (e.g., surface coating to avoid erosion or scale attachment) and potentially provide completion design options not possible with existing technologies.
Lubricant Additives
The use of performance-enhancing additives is commonplace in modern lubricants, but nanoscale additives based on graphene can drastically improve performance in lubricants, resulting in potential reduction in petroleum consumption in the order of millions of barrels per day and reducing CO2 emissions by millions of metric tons each year.
Minimum Quality Lubrication
An important development in the machining of metal parts has been the rise of minimum quantity lubrication (MQL) systems, which involves the application of vegetable oil sprayed by nozzle onto the metal sheet surface as the metal is formed into functional parts. This process dramatically reduces the amount of lubricating oil required, as compared to conventional flood cooling machine processes, which use large quantities of metal working fluid.
The relatively low-cost impact of the oil component allows scope for functional additives such as graphene to be incorporated without adversely affecting the overall cost advantage of the MQL process. Graphene has been shown to improve both the wettability of the vegetable oil on the cutting surface and reduce the coefficient of friction, when added at small doses.
In milling of stainless steel for instance, cutting tool failures are often observed such as flank wear, crater wear, cracks, chipping, etc. due to their tendency to work-harden and from low thermal conductivity properties. Graphene-enhanced vegetable oil however shows reduced initial flank wear and as such, could be used to overcome some of these challenges.
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