Solid lubrication is an emerging field that seeks to develop new lubrication concepts without using raw-oil resources. In dry-sliding tribological systems, two-dimensional (2D) materials and coatings have shown great potential to permit tailoring the respective friction and wear response. 

Established solid lubricant materials include graphene (i.e., an allotrope of carbon), graphene’s derivatives, transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN) and the new family of MXenes. MXenes are a class of 2D inorganic compounds that consist of atomic layers of transition metal carbides, nitrides or carbonitrides. However, all of these compounds offer both advantages and drawbacks depending on the specific applications. 

Material fabrication strategies 
Successfully designing hybrid nanocomposite coatings for solid lubrication requires considering both the materials and fabrication strategies. Materials selection needs to generate synergistic effects, while also ensuring a good interfacial connection between the involved nanomaterials. A poor interface could be detrimental to the resulting mechanical/tribological performance due to the generation of areas for crack initiation. 

When it comes to the fabrication of hybrid solid lubricant coatings, processing routes need to be developed that allow for the homogeneous deposition of the hybrid coatings. This involves the development and precise study of the involved dispersion characteristics of different nanomaterials in the used solvents. The key technical aspect to ensure the coating is homogeneous is the establishment of stable dispersions, which are prerequisites for a number of methods to deposit solid lubricant coatings. These methods include electrophoretic deposition, spray-coating and dip-coating, among others. The selection of the deposition technique to be used is critical as it directly determines the quality of the fabricated coatings and, therefore, the resulting friction and wear properties. 

The most promising nanomaterials 
Dr. Sebastian Suarez, chair of functional materials in the department of materials science and engineering at Saarland University in Saarbuecken, Saarland, Germany, says that identifying the most promising carbon nanomaterials hinges on the particular tribological system within which they will operate. In situations with low contact pressures, or loads, most carbon nanomaterials perform well, and can even display superlubricity in certain conditions. He notes that in high contact pressure situations, closed structures (e.g., carbon nanotubes [CNT], carbon nano-onions [CNOs]) have had good results regarding friction reduction and duty life. CNOs are carbonaceous nanostructures composed of multiple concentric shells of fullerenes, which are an allotrope of carbon. 

Suarez notes that temperature and humidity impact performance. Carbon nanoparticles (CNPs) perform well in a wide range of temperatures and humidities, as opposed to traditional solid lubricants. Surface finishing also is important for nanomaterials in general and for CNPs in particular. Suarez says that carbon onions and fullerenes are sensitive to high roughness, while CNTs benefit from a certain roughness that results in a continuous lubricant supply to the contact interface. 

Suarez observes: “Graphene has shown an outstanding ability to form hybrid solid lubricants, mainly due to its layered nature.” All technical solid lubricants are layered, which Suarez notes makes the integration of graphene quite simple. Suarez says, “This facilitates graphene’s alignment during motion, enabling the interlayer sliding and consequently, reducing friction.” Suarez cites work on a hybrid lubricant composed of graphene and nanodiamonds that enables a roller bearing effect to achieve outstanding lubricity in both dry and humid environments.¹ 

Pros and cons of carbon-based nanomaterials 
Carbon-based nanomaterials have a number of advantages and limitations compared to other families of 2D materials. Suarez notes, “The handling and processing of CNPs have been extensively studied and reported, therefore simplifying their application.” The chemistry of CNPs is relatively simple with large-scale synthesis methods already optimized. He says that CNPs also have shown very good performance in both diverse environments and loads, making them strong candidates for applications that present a wide variety of conditions during operation. 

Suarez finds the primary drawback of CNPs to be their price when compared to traditional solid lubricants (e.g., TMDs). In addition, he says, “Most of the [CNPs] come in agglomerate form due to their large specific surface, which favors van der Waals interactions between particles.” Because of these interactions, a pre-processing step (i.e., dispersion/de-clustering) is usually required. 

In contrast to CNPs, Suarez observes: “TMDs have the benefit of having been around for a while and their application range is well known.” In addition, he says, “They are widely used in high temperature applications, showing stable lubrication.” However, in humid environments TMDs oxidize, which increases their interlayer shear strength and compromises their lubricity. 

Suarez notes that comparing carbon- based nanomaterials to MXenes “is not straightforward since these are upcoming nanomaterials and the latest scientific articles are still dealing with the fundamentals (e.g., lubrication mechanism).” While there is still work to do to simplify the synthesis of MXenes without aggressive chemicals like hydrogen fluoride, he says that MXenes “are great solid lubricants, showing outstanding durability and stability.” 

Hybrid nanocomposites 
STLE member Dr. Max Marian is assistant professor in the department of mechanical and metallurgical engineering in the Engineering School at the Pontifical Catholic University of Chile in the Macul metropolitan region of Chile. Marian says, “Hybrid nanocomposites, where inorganic nanoparticles are incorporated into a larger organic matrix, feature improved electrical and thermal conductivity as well as mechanical properties like stiffness and strength due to the presence of the remarkably large interfacial ‘third’ phase in the resulting material.” He notes: “Hybrid nanocomposites have become a rapidly expanding field and also feature tremendous potential for solid lubrication.” 

Hybrid nanocomposites are formed by combining solid lubricant nanoparticles with other reinforcing nanoparticles or additives (i.e., graphene, CNTs, nanoparticles with specific functional properties). Marian says, “The resulting hybrid nanocomposites can have superior tribological performance, including enhanced wear resistance, improved load-carrying capacity and reduced friction.” 

Specific lubrication requirements can be met by tailoring the composition and structure of the hybrid nanocomposites. Marian notes: “By selecting appropriate matrix materials, solid lubricant nanoparticles and nanofillers, it is possible to achieve a wide range of properties, such as temperature stability, chemical resistance and self-lubricating behavior.” This means that hybrid nanocomposites can be structured and customized to “exhibit multifunctional properties beyond solid lubrication, for example self-healing capabilities or corrosion resistance, offering additional benefits to the lubricated systems.” 

Solid lubrication coatings 
Marian notes: “By applying solid lubricants as thin films or coatings on rubbing surfaces of mechanical components, essential lubrication properties can be maintained.” This type of application can reduce friction, minimize wear and enhance both service life and energy efficiency. These enhancements can be especially helpful in challenging conditions, where conventional oils and greases cannot be used because of extreme operating conditions. Marian says that these conditions can “include very high or low temperatures (e.g., cryogenics, turbine, aerospace technology), exposure to aggressive media (e.g., acids in processing technology), radiation, gas atmospheres, vacuum environments (e.g., aerospace, energy, medical, vacuum technology) and requirements related to maintenance, safety, hygiene, environment or health (e.g., food, textile or paper technology).” 

Aside from special and extreme conditions, Marian notes: “Solid lubrication coatings have the potential to reduce or even eliminate the need for external lubricants where they are traditionally employed.” This means that solid lubricant coatings have applications beyond niche applications or specialized products, especially as they offer significant social, environmental and economic benefits. These applications are important as they offer ways to attain environmentally friendly lubrication and green tribology practices. 

Dr. Diana Berman is associate professor in the department of materials science and engineering in the College of Engineering, University of North Texas in Denton, Texas. Her research group is currently working in two areas: “1.) advanced 2D material-based solid lubricants and 2.) in-situ generation of solid lubrication solutions from the environment.” 

In the case of advanced 2D lubricants, Berman is “looking at adaptive materials that are sensitive to changes in environment and operating conditions.” To do this, her team combines different 2D materials (e.g., graphene, molybdenum disulfide [MoS2 or Moly], boron nitride, MXene) to “use their unique characteristics to minimize friction-induced energy losses.” As an example, graphene “is a great choice for humid environment operation while MoS2 provides excellent tribological characteristics in dry environments.” MXenes offer the potential to enhance the tribological performance of these materials. As many systems experience a wide range of operating conditions, including ranges in temperature, humidity and corrosive environment, it is important to have materials capable of adapting to their surroundings. To enable such, the protective solid lubricant solutions are designed by mixing different components together to facilitate their performance based on the current system requirements. 

The second area of research focuses on alternative approaches to addressing the issues of friction and wear or tribocatalysis. When looking at an in-situ generation of solid lubrication solutions from the environment, Berman says that her team is “looking at how the tribologically induced heating and stresses can be used for good, to assist in repairing the damaged surfaces.” In other words, the presence of carbon sources (e.g., oil is easily used as it is made from hydrocarbons) during high friction sliding with the correct combination of metals on the surface (i.e., catalytically-reactive metals) can help to break carbon-based molecules to “facilitate the formation of protective carbon films in the damaged regions.” This process is called tribocatalysis and can be used in hydrocarbon-rich environments (e.g., oils [synthetic or mineral], fuels [diesel or gasoline] and gases [methane]). 

Tribocatalysis means that surface repairs occur on demand without requiring that the equipment be disassembled. Berman’s team has “succeeded in designing surfaces in the form of predeposited coatings that include such catalytic elements (e.g., copper, nickel, platinum), enabling very efficient operation of the systems.” The surfaces involved are constantly replenished with films of carbon-based solid lubricants (i.e., graphitic or amorphous carbon) during sliding. Berman says, “The liquid lubricant still works most of the time, but in case of its failure, additional protection comes from the solid lubricant formed at the contact interface.” 

Drawbacks to solid lubricant coatings 
Each 2D material offers its own advantages and drawbacks. For example, graphene and MoS2 can induce very low friction under solid lubrication; however, both are very sensitive to environmental conditions. 

Another limitation is that solid lubricants can offer limited wear resistance. Once these materials start to wear off, the system can experience a drastic increase in friction and the initiation of catastrophic wear. While the new family of MXenes has shown outstanding wear resistance caused by the formation of a beneficial tribolayer that may help to overcome this shortcoming, MXenes do not offer low friction. 

Marian notes: “It is crucial to achieve homogeneous coatings with consistent thickness and roughness, regardless of the component’s geometry.” The substrate- coating interface and adhesion during the fabrication of solid lubricant coatings are vital to prevent coating delamination during early operation stages. Otherwise, catastrophic wear and component failure can occur. The use of interfacial gradient layers or composite coatings can improve the coating-substrate interface and help “balance the mechanical and thermal properties between the coating and the underlying substrate, enhancing the adhesion and overall coating performance.” 

Research focus 
Marian says, “Further research is required in the deposition of solid lubricant coatings, particularly in the development of efficient, cost-effective and large-scale strategies.” He observes that research “should focus on developing efficient and reliable techniques that enable consistent and robust deposition of solid lubricant coatings, as well as strategies for optimizing the coating- substrate interface.” Addressing these challenges will improve durability of solid lubricant coatings and their performance in various industrial applications. 

The dynamic operating conditions of industrial applications can be a challenge. Marian says, “Solid lubricant coatings often demonstrate excellent performance during model testing under specific ambient and consistent operating conditions; however, practical applications are far more dynamic, as they operate under varying temperatures, ambient humidities and stresses.” As an example, Marian looks at gears where “the pressure and distribution of rolling-sliding, among other factors, will vary along the contact path.” This type of variability can result in uneven wear and potentially impact coating effectiveness. This type of wear impacts the accuracy of the tooth geometry and ultimately influences the smooth interaction of the gears. 

As previously described, new research is looking at combining different 2D materials to create hybrid nanocomposites. The idea is to combine graphene or its derivatives that offer low friction and poor wear resistance with MXenes that offer high friction and excellent wear resistance to create new low-friction and low-wear solid lubricant coatings. This is a highly prospective research topic that should continue to gain attention going forward. 

Berman says, “The major drawbacks are associated with transferring these exciting research results to the real applications.” Knowing how things work at the lab-scale is one thing, but scaling up and applying these research results in the real world can be challenging. Berman notes: “It is difficult to test without incorporating them in real systems.” 

However, Berman says that the research on “solid lubrication is critically important for the tribological community since we all know that use of oils is not always possible/relevant.” Solid lubricants can offer a wide range of characteristics and can be more flexible than oils in their use. Both researchers and industry need to be willing to explore these topics. 

Suarez says, “Developing a robust bond with the industry holds paramount significance for the academic tribology community.” Through this relationship, researchers can gain invaluable insights into industry needs, which enables them to identify the ideal solid lubricant solution. He sees that “this synergy between academia and industry not only accelerates technological developments but also empowers the industrial workforce with enhanced knowledge and expertise.”

REFERENCE
1. Berman, D., Deshmukh, S.A., Sankaranarayanan, S.K.R.S., Erdemir, A. and Sumant, A.V. (May 14, 2015), “Macroscale superlubricity enables by graphene nanoscroll formation,” Science, 348 (6239), pp. 1118-1122, https://doi.org/10.1126/science.1262024.

Andrea R. Aikin is a freelance science writer and editor based in the Denver area. You can contact her at pivoaiki@sprynet.com.