TLT: What nanostructured solid lubrication solutions are you working with?
Berman: Among the solid lubricants of interest are two-dimensional (2D) materials such as graphene, molybdenum disulfide, boron nitride, etc. These materials usually can be applied as loose particles or as coatings that are evaporated or burnished onto the surface. Alternatively, they can be used as fillers for composite structures. Making lubricants that work under different environmental conditions often requires combining more than one material to take advantage of their different properties. These mixed lubricants, sometimes called chameleon coatings, adapt to changing environmental conditions. For example, mixtures of graphite and MoS₂ provide lubrication both in humid and dry environments, respectively. Graphite also is more effective at lower temperatures, while MoS₂ provides good lubrication at higher temperatures. During operations, the material in the chameleon coating that provides better lubrication under a particular set of conditions migrates to the top of the coating and plays a more active role in providing shear.

Alternatively, I’m looking into the ways of in situ generation of 2D-based tribofilms during sliding to improve the tribological properties of the systems during the operation. My recent studies demonstrated the tribo-assisted transformation of hydrocarbon sources, such as fuels and alcohols, into adaptive and self-replenishing low friction and wear coatings mostly made of graphitic carbon. The transformation can be activated by presence of catalytic metals, such as copper and magnesium, in the sliding interfaces. The characterization analysis revealed the onset of the hydrocarbon decomposition and reconstruction correlating with applied load and temperature conditions.

TLT: What kinds of techniques have you used to probe materials interactions at different scales?
Berman: In probing the interactions of 2D materials, we use a wide range of techniques, starting from the nanoscale analysis with atomic force microscopy (AFM) and quartz crystal microbalance (QCM) and going to macroscale evaluation with macroscale pin-on-disk tribometers and high frequency reciprocating rigs (HFRRs).

TLT: Please provide a couple of examples of successfully implemented nanostructured materials for tribological applications.
Berman: Upon discovery of graphene, 2D materials attracted lots of attention for many areas of applications, including tribology. The layered structure of the materials enables easy shearing, thus providing reduced friction and wear characteristics of the systems.

My colleagues and I discovered the unique tribological characteristics of the 2D form of carbon, graphene, when spray-coated the steel substrates. Graphene attaches to a steel surface with van der Waals forces. The relatively weak bonding allows the graphene to adapt to external stresses like shearing or sliding, and to accommodate those stresses to minimize friction. The graphene particles move around on the steel surface, but eventually they do get pushed to the sides, away from the points of contact. Replenishing the coatings by simply spraying the graphene-containing ethanol solution allows them to extend the lifetime of the coatings for as long as needed (one drop of their graphene suspension after about every 50 km of sliding). We also are looking at ways to manipulate the structures of these 2D materials to further improve their performance to achieve superlubricity—that is, systems where the coefficient of friction (COF) is less than 0.01—nonzero, but too low to measure using conventional instruments.


The particular focus is on components and mechanisms in devices and systems, which cannot be lubricated with conventional oils or greases due to extreme operational conditions, such as very low or high temperatures (cryogenics, turbine and aerospace technology); controlled gas atmosphere or vacuum (aerospace, energy, medical and vacuum technology); as well as for maintenance, safety, hygienic, environmental or health reasons (food, textile or paper technology).

Another example relates to the design of environment adaptive coatings based on 2D materials. Hybrid dual-phase coatings composed of an A356 aluminum alloy modified by plasma electrolytic oxidation (PEO) and burnished with graphite-MoS₂-Sb₂O₃ chameleon solid lubricant powders have been produced. In situ Raman spectroscopy revealed the chemical stability of the dual phase coatings at temperatures up to 300 C with no signs of oxidation or reduction of the chameleon components. COF values ranged from 0.2 at room temperature down to 0.02 at 300 C. The observed low friction values were attributed to the synergism between PEO and chameleon layers that promote defect healing and adaptive behavior of the coating. In situ Raman spectroscopy revealed that the lubricating phases, i.e. MoS₂ and graphite, were protected from oxidation by the porous PEO structure. These lubricious phases formed a transfer film in the wear tracks and the counterpart bodies as a result of the contact pressure (up to 1.4 GPa) and thermal energy, which led to an order of magnitude reduction in the COF at high temperatures. The low shear strength of MoS₂ and graphite and the good adhesion and integration of the chameleon coating with the PEO sublayer due to high contact pressures during sliding were responsible for the ultra-low friction behavior of the composite coating. The newly engineered chameleon coating would open a new window to protect an aluminum-based (Al) alloy against oxidation and, yet, does the self-lubrication even above 300 C, up to 500 C, which is far beyond the tribological capability of the Al alloys, thus providing lubrication for engine blocks and aerospace applications.

Currently, 2D materials are still under the exploration stage, but the recent efforts already demonstrate their benefits for being used in various machine components, including sliding and rolling bearings, gears and seals. The particular focus is on components and mechanisms in devices and systems, which cannot be lubricated with conventional oils or greases due to extreme operational conditions, such as very low or high temperatures (cryogenics, turbine and aerospace technology); controlled gas atmosphere or vacuum (aerospace, energy, medical and vacuum technology); as well as for maintenance, safety, hygienic, environmental or health reasons (food, textile or paper technology).

TLT: How critical is tribological and material testing and characterization to the implementation of novel solid lubricants for tribological applications?
Berman: Lab scale testing enables quick assessment of the novel lubrication solutions over the range of possible application parameters (contact pressure, velocity and environment conditions, such as humidity, temperature and atmosphere). As a result, it minimizes the risks of the failure once the materials are applied in applications.

TLT: Can nanostructured solid lubricants be used for friction reduction, antiwear or lubrication purpose?
Berman: Our previous results clearly demonstrated that 2D materials have a great potential of not just simply reducing friction and wear but almost completely eliminating them toward achieving the superlubricity regime (regime of almost zero friction and wear).

You can reach Diana Berman at diana.berman@unt.edu.