KEY CONCEPTS
• To qualify as a nanoparticle, there must be one dimension less than 100 nm. This small size gives it the advantage of easily entering into the contact area to enhance load bearing and lubrication.
• Beneficial properties of nanoparticles include rolling of nanospheres, tribofilm formation due to catalytic and tribochemical reactions, a mending effect due to minimal particle size and polishing. Because of this, healthcare sectors have rapidly adopted nanotechnology for drug delivery, monitoring, diagnosis and treatment.
• As with most substances containing microparticles, there are environmental and health issues that must be considered and mitigated. 

Nanoparticle-based lubricant additives and their formulations (nanolubricants) have been widely researched for their effective lubrication properties. As additives, these solid particles are complex because of their physical interactions, chemistries and size. 

Various types of lubricant chemistries, including soft metals, oxides, sulfides and carbon-based additives, are fundamentally different in their lubrication mechanism and applicability for various mechanical systems and components. Additionally, the dispersion stability of these additives, compatibility with base chemistries and multi-component functionalization pose additional optimization challenges.

Nanoparticles are defined as having one dimension less than 100 nm. Incoming STLE Vice President Dr. Hong Liang, professor of mechanical engineering, Texas A&M University, explains a few fundamentals: “As additives, nanoparticles would characteristically have a certain type, size and shape. Because of the small size, their surface has higher energy than a bulk material. The small size allows them to easily enter into the contact interface for load bearing and lubricating.”

She continues, “Nanoparticles are more active and prone to be reacted with the environment into a desired product. For example, they can be used as catalysts to generate tribofilms. Nanolubricants can be made effective for friction and wear reduction.”

STLE Fellow Dr. Raj Shah, director at Koehler Instrument Co. based in Holtsville, N.Y., adds, “The key mechanisms can differ from nanoparticle to nanoparticle, but important features that dictate the mechanisms can include morphology (spherical, solid, single crystal, multi-layered, mono-layered, platelet, etc.), chemistry and nanoparticle size. Morphology could provide shock absorbency for spherical solids with a hollow core, while the nanosize could contribute to a surface smoothening effect and high surface area. The key here is it all depends on the type of nanoparticle.”

There are several mechanisms proposed and investigated to describe the effect of nanoparticle additives in lubrication. “Based on my research, I found most evidence supporting the effect of nanoparticles coming from their physical interference between the contacting surfaces as a third body contact,” says STLE member Hamed Ghaednia, director of advanced technology and manufacturing, Gehring L.P. “This results in reduction of real area of contact between the two surfaces, which results in friction reduction.”

Ghaednia clarifies that size-dependent mechanical properties make nanoparticles stronger than their bulk counterparts. This helps reduce the real area of contact as an enhancement mechanism for nanoparticles. Depending on the nanoparticle composition and structure, other suggested nanoparticle mechanisms, such as providing solid lubrication or rolling motion of particles, come into play.

Nanolubricant formulations
Shape, size and material are three primary considerations for developing the best nanolubricant for the application. Dr. Mubashir Gulzar, associate professor at University of Engineering and Technology (UET Taxila), Taxila, Pakistan, believes that—for an optimal formulation—the choice of nanoparticle type and concentration in the base oil depends mainly upon contact geometry and other operating conditions.

Shah explains, “Certain metal oxides may be able to provide reductions in friction or rolling mechanisms. But they run the risk of possibly becoming abrasive in increased loads. In contrast, are spherical particles—if a nanoparticle like WS2 can provide friction reduction via laser exfoliation and shock absorption at medium to high loads and maybe for certain high-temperature applications.” 

As Liang mentions, the shape of nanoparticles is an important factor in determining functional properties. “In tribological studies, commonly mentioned shapes are granular, onion, sheet, spherical and tube,” she says. “Because of those shapes, they can slide, roll and exfoliate for favorable friction reduction.” 

Ghaednia explains that the average size of the particles and their distribution are key factors in the effectiveness of nanoparticles in the application. For a given concentration of nanoparticles in the lubricant, the smaller the size, the more particles are present in the contact. “In other words, the smallest particle size and sharpest distribution, which allow for triggering of the enhancement mechanism, is desired,” he says. “Otherwise, larger particles or wider distribution of size would result in a reduced number of particles and, hence, a reduced overall effect.” 

Solution stability is another factor that favors smaller particle size. “Nanoparticles, when suspended properly in lubricants, can have good suspension stability. However, this usually depends on the size of the particles, and smaller particles are favored to have a longer shelf life,” Ghaednia says. 

He adds, “In terms of lubricant properties, which could be tuned for nanoparticles, suspension stability and effect of nanoparticles on other additives are important. Nanoparticle lubricants usually contain surfactants or other chemical agents to increase the suspension stability. The effect of these additional chemical agents on overall effectiveness of the base lubricant and additive pack should be considered. Also, the additive package could contain several chemical components, such as antiwear agents, which compete for the surface in order to be effective. These could be in competition with nanoparticles for surface area, which would have hindering effects—all of this should be considered.”

Gulzar agrees. “The role of material properties including hardness, density, morphology, average particle size as well as suitable concentration of nanoadditives plays a significant role in the design of nanolubricants for specific applications. Similarly, contact geometry as well as the application plays a major role in deciding the suitable additive and related concentrations.” Gulzar adds that sliding contact geometry has been widely used for wear and friction behaviors, while four-ball tribotesting is used for analyzing the extreme-pressure characteristics.

Regarding the specifics of metal and metal oxide-based lubricant additives and their role in lubrication specific to structure property manufacturing, Ghaednia has worked on silver and copper oxide nanoparticles. “Silver nanoparticles, which I tend to think of as malleable nanoparticles, tend to have less negative effect on contact wear,” he says. “Therefore, while providing friction benefits, they don’t alter the wear rate of the lubrication system. Given this, more care should be given to particles, such as copper oxide, which can increase the overall wear rate of the contact. However, previous research has shown that the wear and friction effect of a particle can be controlled by controlling the particle size, distribution, concentration and surfactants, etc.”

Nanoparticle-based lubricants are typically dispersed in suitable base oils for lubrication. This requires compatibility on several levels and adds another layer of complexity to the formulation of nanolubricants.

“The compatibility of nanoparticles with base oils in terms of dispersion stability is the essential requirement after selection of the suitable type/size and concentration of nanoparticles,” Gulzar explains. “Thus, the variation in base oil yields different results for the same additives. The properties of a base oil, such as its organic/inorganic nature, affect the uniform and durable dispersion of nanoparticles. Therefore, the application-based solution is designed by surface modification of nanoparticles for dispersion in a specific base oil. For such purposes, chemical surface modifications of nanoadditives are designed, or suitable surfactants are used.”

Liang says that the first question most people would ask is: What is the suspension ability of nanoparticles in lubricants? “To achieve suspension, we need to formulate the nanolubricants with dispersant or to functionalize the particle surface. From what we have gathered, surfactant and alkoxysilanes are the primary chemical groups for surface modification.” 

She continues, “If using a surfactant, the nanoparticles need to be smaller than 50 nm. Otherwise, larger particles (>50 nm) would need to use alkoxysilanes. One would ask: Why 50 nm? For small particles, there is a minimal thickness of the grafted layer in surfactant that helps suspension. When particles are larger than 50 nm, silanization of alkoxysilane increased the grafting density. To maximize suspension, one would need to find a way to form a dense and complete grafting layer. To achieve this, one has a lot to play with: the particle type, size and shape with a variety of molecules.”

Liang adds, “Various lubrication regimes have different mechanisms of lubrication using nanoparticles. The boundary lubrication largely depends on the catalytic effects and/or tribochemical interactions leading to tribofilm formation. In a hydrodynamic regime, the viscosity modification or fluidic behavior consisting of nanoparticles dominates. These general physical chemical characteristics would dictate the selection of a suitable nanoparticle chemistry.” 

Reiterating the importance of shape, size, material and other properties, Shah concludes that, in order to optimize lubricants that use nanoadditives, the final formulation is usually a balance between a good understanding of the creation of a two-phase product, optimal dosage of the nanoparticles, optimal and correct test conditions to verify efficacy and the proper selection of surfactant chemistry that can help provide stability of the suspension/dispersion.

While there are many factors for choosing properties for specific nanolubricants, these experts agree that the choice is driven by the application and lubrication conditions. 

Benefits and drawbacks of nanolubricants
Nanoparticles are widely employed as friction modifiers, but they also are attractive as lubricant additives for a variety of reasons due to their remarkable properties at the nanoscale level—often outperforming bulk analogs. While research is ongoing and more benefits are continually uncovered, Liang cites the following four beneficial effects that have been widely accepted: rolling of nanospheres, tribofilm formation due to tribochemical reactions, mending effect due to minimal particle size and polishing to make the contact surface smoother. 

However, even though widely researched, the industrial application of nanoparticle-based materials creates specific manufacturing, reliability and safety challenges.

Ghaednia observes that nanoparticles in dry conditions tend to perform effectively as long as the enhancement mechanism of nanoparticles is reduction of the real area of contact. “However, main challenges and concerns are keeping particles in between the contacting bodies, possible aerial contamination and uniform dispersion of particles in contact over time,” he says. 

Regarding drawbacks, Shah believes that a firm grasp of the nanoparticle chemistry is necessary in order to have a comprehensive understanding of the potential environmental and health hazards. Depending on the application, the nanoparticles may need to be tested for acute, dermatological toxicity as well.

“From a usage perspective, in most cases, nanoparticles should not be treated very differently than, say, micron-sized particles that share the same chemistry,” Shah clarifies. “In regard to waste management, the same precautions absolutely should be taken. There is not really any standardization that I am personally aware of so far.”

Some of the potential issues that nanolubricants raise are reason enough for the slow adoption of nanolubricants across industries. “To date, there is still a lot of resistance, and companies are reluctant to use new nanoparticles as additives due to the concern of unknown factors such as environment, health and cost,” Liang says. “In reality, we have come a long way in using nanoparticles safely and cost effectively. The healthcare sectors have adopted nanotechnology rather quickly. We have seen the use of nanomaterials in drug delivery, monitoring, diagnosis and treatment. This is achieving a certain level of success.” 

Liang adds, “If needs in energy and the environment require us to make better lubricants, we should take advantage of the effectiveness of nanoparticles as additives.”

Simulation and modeling
Computerized simulation and modeling have paved the way for rapid progress in nanolubricant research and eventual widespread adoption. Molecular dynamic simulation has been used and proven effective in simulating the behavior of nanoparticle and particle-surface-fluid interactions.

Ghaednia observes that modeling of nanoparticles depends on the scale of the mechanism that is believed to be responsible for tribological effects of the nanoparticles. “Well-crafted contact mechanics models, which can capture the scale dependent properties of nanoparticles, have proven to be effective in predicting the behavior of nanoparticles,” he explains. “Molecular dynamics also is an effective tool in modeling different mechanisms associated with nanoparticle driven enhancements. Attention to the key contributing physics and chemistry of the system as well as scale dependent properties of particles are the key in the successful modeling of nanoparticle lubricants.”

Areas for further research 
Ghaednia summarizes that the main points, which should be addressed, are the effect of nanoparticles on filtration systems, shelf life of the lubricant, compatibility with other additives and environmental concerns.

Future research also could include the role of surface textures in dictating/controlling tribochemical reactions of nanolubricants, especially under boundary lubrication conditions.

Gulzar believes the main gap in understanding is the lack of knowledge about various types of lubrication mechanisms. He says, “While all the potential lubrication mechanisms for a specific application could be investigated, typically only mending and tribofilm formation are reported since they seem easy to justify by using post-analysis tools. In this regard, the literature should elaborate the possibility of various mechanisms, and researchers should be encouraged to report the possibility of various lubrication mechanisms for nanolubricants.”

“Nanoparticles have advanced significantly in last two decades, and there are many available candidates as potential lubricant additives,” Liang says. “As a matter of fact, for decades, lubricant scientists have used nanoparticles as additives or lubricant, i.e., phosphorus, sulfur and graphite. However, critical challenges remain. In manufacturing, large quantity production, quality control and safe handling are still some of the key issues. Fundamental research is needed to understand the effects, tribochemical interactions and performance of each particle, in each base oil, on each substrate. There is limited funding available for fundamental academic research. Industries have expended visible effort on this, but they don’t talk about it. To move forward and faster, we need industry research entities and universities to work together.”

Shah adds, “The size of the particle, in my humble opinion, is just as important as its morphology and chemistry. Challenges sometimes lie in how to de-agglomerate and disperse solids and formulate stable dispersions/suspensions. In addition, determining how to test them is a big challenge.”

Gulzar believes that the major challenge in designing and synthesizing stable nanolubricant formulations is dispersion stability. Gulzar then poses a few final questions for the consideration of the future application of nanolubricants:

1. How long is the shelf life without sedimentation of nanoadditives?
2. Will nanolubricants be suitable for automobile engines—which typically have long stationary periods?
3. Will the agglomerations affect the operation of the filter system in automobile applications?

Nanolubricants are not new. What is new is the increased understanding of their properties and their potential for outstanding performance in specific applications. They have many of the same environmental and health drawbacks of all substances containing microparticles; this means nanolubricants may not be the best choice for certain applications, but as research continues, they are certain to be game changers where they can be employed safely and effectively.

Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at jeanna@smartprcommunications.com.