Superlubrious high temperature coatings

A low friction wear surface operable at high temperatures and high loads with a coefficient of friction in the superlubric regime including MoS2 and graphene-oxide on the wear surface is provided, and methods of producing the low friction wear surface are also provided. The low friction wear surface remains with a coefficient of friction in the superlubric regime at temperatures in between about 200° C. and 400° C.

BACKGROUND

Understanding and controlling the root causes of friction have long been a tireless pursuit of mankind mainly because friction impacts our safety, mobility, and environment in so many ways. Accordingly, those scientists who study friction across many scales and engineers who design, manufacture, and operate moving mechanical assemblies (“MMAs”), like motored vehicles, have all aimed at drastically reducing or even totally vanishing friction or achieving superlubricity at engineering scales. The superlubric regime is attractive because it would provide the highest levels of savings in energy, environment, and money. Despite the development and use of many kinds of solid and liquid lubricants in recent years, superlubricity is seldom achieved at macro or engineering scales. Friction coefficients of above 0.01 to 0.1 are considered low friction and above that transitions to high friction. Generally, friction coefficients of less than 0.01 are considered superlow, and hence fall in the superlubric regime. Such levels of friction coefficients are typical of those surfaces that are either aero- or hydro-dynamically separated or magnetically levitated where little or no solid-to-solid contact takes place. Under sliding regimes where direct metal-to-metal contacts prevail and high contact pressures are present, achieving superlubric friction coefficients (i.e., less than 0.01) is difficult due to the concurrent and often very complex physical, chemical, and mechanical interactions taking place at sliding surfaces.

Superlubricity is very difficult to achieve at macro-scale tribological tests and mechanical systems. Sliding mechanical systems at high temperatures and high loads are the ubiquitous in industries such as automobile, heavy machinery, surface and underground drilling, space and extraterrestrial applications. These applications typically experience high loads applied at high temperatures that result in aggravated frictional losses resulting in significant deterioration of overall efficiency of the engine systems. This is because all sliding mechanical systems are prone to wear/tear and their efficiency directly depends on the friction that exists between sliding components. Currently, high temperature greases and limited compositions of synthetic oil-based lubricants are being used in such applications; these not only need replenishment at periodic intervals but also significantly fall short of the expected levels of friction elimination. Replacement of lubricant oils also calls for either partially or completely shutting down operations, which results in further lowering of efficiency and increased man hours for continuous functioning of the mechanical systems.

Multiple reports have thoroughly established and explained superlubricity at room temperature; however, no reports have demonstrated such an effect at high temperatures or high contact pressures and/or as an dynamically adaptive mechanism. Very low friction between linearly sliding surfaces in the order of 0.02-0.06 has been shown but only in inert and ultra-high vacuum (“UHV”) environments. The lubricants themselves are applied via an elaborate chemical or physical deposition technique.

MoS2composites containing Ti, Au, or Sb2O3have shown low friction comparable to the aforementioned values but are developed using elaborate procedures such as magnetron sputtering and/or pulsed laser deposition. MoS2has been used in low wear solid lubricants widely in aerospace applications due to its ability to have its shear strength decrease with increasing temperature. However, the coefficient of friction (“COF”) is limited to about 0.02-0.06 in inert and UHV environments which is not in the range of superlubricity. MoS2is sensitive to water and oxygen contamination and can rapidly deteriorate with increasing temperatures. The low friction regimes were limited to a high of 200° C., beyond which the lubricants were often observed to render ineffective. The best performance of MoS2in ambient conditions has been shown to be between about 100° C. and about 250° C. Water vapor deteriorated the coating under 100° C., whereas oxidation deteriorated the coating above 250° C. due to interference and disruption of lamellar shear through physical bonding.

SUMMARY

One embodiment relates to a method of forming a low friction wear surface. The method comprises preparing graphene by chemical exfoliation of highly oriented pyrolytic graphite, suspending the graphene in a solvent to form a solution of at least 1 mg/L, adding at least 1 g/L of MoS2ultrafine nanocrystalline flakes to the solution, sonicating the MoS2and the solution to form a homogeneous solution, and disposing the homogenous solution on a substrate. Disposing the homogenous solution includes spraying the homogenous solution on a substrate via a process of air-spray coating, forming a wet film on the substrate, and evaporating the solvent component to form a dry coating layer. The substrate has a temperature at about 275° C., and the graphene-oxide and the MoS2are in a range of ratios in between (1±0.25):(1∓0.25) by weight.

One embodiment relates to a low friction wear surface. The low friction wear surface comprises a substrate, graphene-oxide in an oil-free solvent disposed over the substrate, and MoS2ultrafine nanocrystalline flakes disposed over the substrate. The graphene-oxide and the MoS2are in a range of ratios in between (1±0.25):(1∓0.25) by weight.

One embodiment relates to a method of forming sliding mechanical system on a low friction wear surface. The method comprises forming an oil-free homogenous solution comprising MoS2and graphene-oxide in a range of ratios in between (1±0.25):(1∓0.25) by weight, disposing the homogenous solution over a substrate to form a first sliding component, and sliding the first sliding component against a second sliding component in open air. Scrolls of MoS2are formed and encapsulated in the graphene-oxide in this sliding mechanical system.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various concepts related to, and embodiments of, inventive low friction and wear resistant graphene containing surfaces. It should be appreciated that various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the disclosed concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

Generally speaking, the various embodiments described herein include a low friction wear surface including two-dimensional (“2-D”) materials such as, but not limited to, MoS2, graphene-oxide (“GO”), WS2, MoTe2, and graphene. The wear surface may exhibit superlubricity in high temperatures (above 200° C., such as to a maximum of 400° C.) and under high loads (above 0.5 N, such as to a maximum of 9 N). Embodiments described herein may provide several advantages over conventional materials that demonstrate superlubricity, including, for example: (1) providing superlubricity (i.e., less than 0.01 COF) at high temperatures ranging from about 200° C. to about 400° C.; (2) significantly reducing wear, to the point of no measurable wear, at these high temperatures and at high loads with excellent longevity; (3) improving energy efficiency by reducing wear; (4) providing highly uniform surface finishes through a spray-coating application without limiting uniformity to flat surfaces; (5) reducing operational costs; (6) extending the service life of components; and (7) demonstrating the superlubricity and these improved properties without the restriction of a using a diamond like carbon counter face or a dry nitrogen environment as has been required by prior 2-D materials. Realization of superlubricity in sliding systems at high temperatures is extremely important not only from a scientific point of view but also from a technological point of view. It could make a significant positive impact on enhancing efficiency and durability of many rotating mechanical systems such as wind turbines and hence considerable savings on energy.

A low friction wear surface may be produced by any appropriate process. According to one embodiment, the process may include forming a homogenous solution from two 2-D materials. The 2-D materials may be MoS2and GO. The MoS2and GO form a homogenous suspension in a range of ratios in between (1±0.25):(1∓0.25), such as but not limited to 8:10 to 10:8, including 10:10. The homogenous suspension is disposed over a substrate heated to within the range of 250° C. to 300° C., such as 275° C. Water is a carrier medium to deposit the homogenous suspension of MoS2and GO on to the substrate. It is believed that, as perfectly spherical water droplets containing the solid nano-materials impinge on to the superheated surface, they instantaneously evaporate with a miniature yet high energy explosion akin to cavitation. This transfers the entire solid materials on to the solid substrate as the liquid evaporates. This high activation energy contained in the small water-droplet system forces the solid particles together. Since the two phases (MoS2and GO) are in equal or near-equal proportions in a homogenous suspension, they eventually form layered structure which is identified as the MoS2layer-level encapsulation. Specifically, with regard to the lower end-point of the temperature range, the boiling point of the carrier liquid (100° C. in this case), amount of solid phases present (such as 2 g/L), and the droplet size are believed to impact the minimum temperature at which this occurs. Notably, the solid phases in the water drop act as impurities, as well as the impinging jet effectively eliminate Leidenfrost effect. The process may be carried out at atmospheric pressures and temperatures, that is exposed to oxygen (in the atmosphere) and at a temperature of about 20-22° C. contrary to the prior art requiring a dry, inert, or nitrogen environment. The disposing of the homogeneous solution on the substrate may be achieved by any suitable process, such as a spray casting or a solution processed method.

FIG. 1is a schematic flow diagram of an example method100for forming a low friction wear surface including MoS2and GO. In this method100, MoS2may be added to a container (e.g., a vial) at 102. The MoS2may be in the form of ultrafine nanocrystalline flakes. The size of the MoS2flakes is 300-500 nm and must should be commensurate or be smaller than the size of the GO flakes. In some embodiments, at least 1 g/L of MoS2ultrafine nanocrystalline flakes is added. The MoS2powder can be a nanocrystalline in nature and does not have to be 2-D to begin. It is believed that during the sliding process, exfoliation of MoS2occurs, forming 2-D layers of MoS2.

A GO solution may be then added to the container at 104. The GO solution has a concentration between 1-15 g/L for forming the lubricant. The GO solution may be obtained by exfoliating graphene and disposing the resulting graphene flakes in a liquid. The graphene may be exfoliated by any appropriate chemical or mechanical exfoliation process, such as chemical exfoliation of highly oriented pyrolytic graphite in the case of graphene. The GO solution may be an aqueous with an oil-free solvent. In some embodiments, the GO will be suspended in water. An oil-free solution is more environmentally friendly, devoid of oil related hazards, and easy to strip after usage. In some embodiments, the coating may be easily removed by immersing the coated substrate into de-ionized water and sonicating for 3 minutes or instantaneously by pressure jet washing. In some embodiments, the GO is suspended in a solvent to form a solution of at least 1 g/L, preferably 5 g/L.

Ultra-low friction is achieved when GO and MoS2form heterostructures and lower the shear strength of layered 2-D materials with increasing temperature. The shear strength of MoS2decreases with increasing temperature in itself, but does not achieve superlubricity without the addition of GO. MoS2alone can produce a friction as low as 0.04 at 100° C. when tested in ambient atmospheric conditions. See Hare & Burris, “The Effects of Environmental Water and Oxygen on the Temperature-Dependent Friction of Sputtered Molybdenum Disulfide,” Tribology Letters 52(3), pp. 485-493 (2013). However, increase in temperature beyond 100° C. deteriorated lubricious properties due to rapid intercalation of MoS2films from oxygen and water, as evidenced by formation of MoO3compounds. It has been discovered that the addition of GO prevents such deleterious effects and prolongs the life of MoS2for longer durations. It has been observed that MoS2reorients the basal planes progressively, with increasing sliding distance and temperature. The signal from the A1gand Ag′B1grises sharply indicating that this basal plane reorientation contributes to lowering of friction. These morphological changes are permitted by the protective graphene oxide layers that seal MoS2from oxygen and vapor. In the absence of such a protection, MoS2would generate rapidly before any reorientation and consequent friction reduction kicks into effect.

The method100for forming the low friction wear surface includes sonicating at106the mixture of MoS2and GO in the container to form a homogeneous suspension108. The solid MoS2interacts with solid GO sheets in the suspension via van-der-Waals forces. The mixture includes solid graphene-oxide sheets suspended in water to form a solid in an liquid suspension. The reaction does not proceed by nucleation of MoS2/GO from a saturated/supersaturated solution, but rather evaporation of the carrier liquid and deposition of the solid materials onto the hot substrate. The deposition and instantaneous bond formation is aided by the cavitation type explosion as the water droplet impinges on to the surface as discussed previously. Sonication may be done in any device capable of applying sound energy to agitate particles in a sample, for example, but not limited to an ultrasonic bath or an ultrasonic probe. In one embodiment, the thin layers are mixed by sonication, “sonixing.” It is believed that physical agitation will not provide the necessary agitation for the materials to mix homogenously. However, the sonixing did not indicate any material modification of the parent phases. For example, the d-spacing of MoS2and GO have remained same after sonication.

At110, the 2-D materials may be introduced onto the surface via a process of air spray-coating by spraying a 2-D materials-containing solution (with a solvent such as water) over the substrate and then evaporating the solvent. In one embodiment, the coating can be applied using any technique that produces a mist. The mist droplets must contain the two phases in suspension. Alternatively, any physical deposition techniques wherein a carrier liquid is used to deliver the solid materials and the carrier liquid, but not require a chemical reaction, can evaporate without physically altering/changing/damaging MoS2and GO can be used. Such deposition differs from those remaining in solution, such as graphene suspended in oil, or those applied chemically. For example, those materials in solution are, obviously, in solution and not bound to the substrate surface (e.g., flowable oil with suspended particles). In contrast, solid materials that have been deposited as by spraying will not be in solution, rather such materials will be controlled by Van der Waals forces to attach the materials to the substrate. Further, there is also a structural difference between such materials and those formed as a solid on the surface of the substrate by chemical vapor deposition, atomic layer deposition, or the like. In such instances, the material is reacted (covalently bonded) with or chemisorbed to the substrate rather than merely held by Van der Waals forces. The thickness is controlled by altering the samples' exposure time to the mist. The pressure/flowrate can also effectively be used to change the amount of soli-bearing liquid carrier impinged on to the surface. The thickness of the coating “required” to produce superlubric properties depends on the test load as would be appreciated in the art. Lower test loads transition into superlubric regime easily, whereas thicker coatings are required at higher loads and for longer sliding distances. Subsequent coats must be applied after the initial layers have completely dried and have adhered to the substrate firmly.

Further, for more than mono layer, the additional layers are also bound, whereas the solution processed materials experience weaker Van der Waals forces, enabling the sloughing of outer layers and the improved lubricity.

In some embodiments, the method100of forming the low friction wear surface includes evaporating the solvent component and encapsulating the MoS2flakes in GO in one step (i.e., by simultaneous evaporation and consequent encapsulation). Encapsulating the MoS2flakes greatly helps the longevity and lubricity of the low friction wear surface because the flakes are passivated from ambient oxygen and moisture, increasing the temperature range of the MoS2flakes to above 250° C. The MoS2flakes are uniformly, fully coated. This is ensured by virtue of the composition (wt %) of the two phases chosen. Also, this coating process is scalable to larger surfaces and is not restricted to flat surfaces. A large scale application of such may be to utilize a scanning spray nozzle to cover a large area with the graphene in solution and then vaporize the solvent. The surface is required to have some anchoring elements. In one embodiment, a rough surface of at least (Ra˜0.2-0.4) provides sufficient anchoring points. In case of extremely smooth substrates, the surfaces would be treated to make them amenable to the deposition techniques. Examples of such treatments include, but are not limited to, ozone treatment and doping with binders that make bonds between the steel and the initial layers and higher substrates (up to 400° C.) temperatures.

At112, the substrate may be heated, for example to 275° C. In some embodiments, the substrate may be a steel surface, such as but not limited to self-mated hardened stainless steel, ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, duplex stainless steel, and precipitation hardened stainless steel. In some embodiments, the substrate can comprise of at least a portion of a metal working die, a wind turbine, a polymer injection molding die, a piston, a piston ring, a piston sleeve, a ball and roller bearing element, an oil-free air compressor, a gas compressor, a gas seal, a sliding rail guide, or a heavy load bearing wheel guide.

In some embodiments, the low friction wear surface may undergo subjecting the wear surface on the substrate for high temperature wear testing at114. The method100includes heating the temperature to about 200° C. to about 400° C. Normal loads may be applied to the low friction wear surface on the substrate at these high temperatures at116.

Superlubricity may be defined as a regime of motion in which friction vanishes or nearly vanishes, such as a COF of less than about 0.01. The superlubric friction is measured by sliding the low friction wear surface using a ball-on-disc configuration of wear testing, under unidirectional sliding. The low friction wear surface may be applied on to heated steel samples and immediately tested at high temperatures (e.g., 200° C., 300° C., and 400° C.) or lower temperatures (e.g., 22° C. and 100° C.). The coated substrates were evaluated for wear-friction evaluation in a high temperature tribometer with loads from about 0.5 N to about 8 N and a rotating speed from about 50 rpm to about 300 rpm.

Preparation of a Low Friction Wear Surface.

Solution-processed molybdenum disulfide was prepared by chemical exfoliation of bulk MoS2crystal and was then suspended in ethanol with 18 mg/L graphene. The resulting solution contained 1 to 8 monolayers thick MoS2flakes. The GO solution may be obtained by exfoliating graphene and disposing the resulting graphene flakes in a liquid. The graphene may be exfoliated by any appropriate chemical or mechanical exfoliation process, such as chemical exfoliation of highly oriented pyrolytic graphite in the case of graphene. The GO solution is suspended in an oil-free solvent to form a solution of at least 1 g/L.

The MoS2flakes and the GO solution are prepared in different concentrations are then mixed together and then sonicated using an ultrasonic bath for 2-15 minutes at 20-125 kHz frequency. The now homogenous solution is then sprayed or drop casted (100-10,000 nm in diameter) on a self-mated stainless steel substrate heated to about 275° C. This process results in a uniformly distributed coating on the substrate surface. The expected area of 2-D MoS2flakes per unit area of the substrate is in the range of 175-800 cm2per mm2of substrate.

In the ball-on-disc tests, the counterpart may be a stainless steel ball (440 C grade) of 3-10 mm diameter.

Tribological tests were performed in ambient environmental conditions at temperatures ranging from about 22° C. to about 450° C. using a CSM ball-on-disk macroscale tribometer. The normal load during the tribotests was kept at with loads from about 0.5 N to about 8 N and a rotating speed from about 50 rpm to about 300 rpm (where the radius of the wear track varied from 1 mm up to 15 mm). Zero calibration of the machine was performed automatically at the beginning of each test. All the tests were repeated at least 5 times at each temperature to confirm reproducibility of the results.

The wear volume of the flat was very difficult to assess, as wear was manifested as deep scratches and could not be fit into a reliable wear equation. To estimate the wear volume for the balls after the tribotests, we used the following equation:

h=r-r2-d24,
d is wear scar diameter, and r is the radius of the ball.

As shown inFIGS. 2A and 2B, low friction values for a low friction wear surface comprised of MoS2and GO, formed from a method of any of the previous embodiments discussed, are observed at both 22° C. and 100° C. under ambient environmental conditions (room temperature, atmospheric exposure). Also, it is determined that although the COF does not reach the superlubric threshold at these temperatures, the COF decreases with increasing temperature and still display values lower than 0.1, under a contact pressure of 1.3 GPa.

As shown inFIGS. 3A-3D, superlubricity is demonstrated by a low-friction wear surface formed from the method previously described herein at temperatures ranging from about 200° C. to about 400° C. These figures also demonstrate exceptional coating longevity: three hours at 200° C. (FIG. 3A), 1.4 hours at 300° C. (FIG. 3B), and 0.4 hours at 400° C. (FIG. 3C). The coatings also sustain maximum contact pressures from 0.5 GPa to 1.0 GPa, which are all much higher than the yield strength of steel.

As shown inFIGS. 4A-4F, initially, the tribosystem shows a very low friction value which further diminishes to superlubricity under dynamic load from temperatures ranging from about 200° C. to about 400° C. The tribopair remained in the superlubric regime for 67 minutes at 200° C., 100 minutes at 300° C., and 17 minutes at 400° C. with a coating thickness of a few hundred nanometers.

In comparison to the aforementioned friction values fromFIGS. 4A-4F,FIGS. 5A-5Cshows bare steel-on-steel tests without a coating of the low friction wear surface. The bare steel-on-steel tests were subject to similar loads and run at 200° C. (FIG. 4A), 300° C. (FIG. 4B), and 400° C. (FIG. 4C). At each temperature, the friction on the bare tests were several of orders magnitude larger than that of the steel tests with the low friction wear surface coating. Without the coating, the COF values ranged from about 0.2 to about 4.0, which is substantially larger than the COF of the steel with the coating, even at room temperature.

The wear on a steel surface in comparison to a steel surface with the low friction wear surface coating is further demonstrated inFIGS. 6A-6D. When tested on a self-mated SS440C tribopair, the wear volume loss on the tribopair (FIG. 6A) was 77 times more than that of the self-mated SS440C with the low friction wear surface coating (FIG. 6B).

The wear scars were imaged with an Olympus UC30 microscope and characterized by an Invia Confocal Raman microscope using the red laser light (λ=514 nm). The wear debris formed during the tribotests was imaged with a JEOL JEM-2100F transmission electron microscope (“TEM”), for which samples were picked up from the wear track with a probe and transferred to a copper grid. SEM images were imaged with FEI Quanta Scanning Electron Microscope. To gain further insight into the evolution of the carbon-based tribolayer within the wear track and identify the chemical state of the MoS2, Raman spectroscopy studies were carried out.

The longevity of a low friction wear surface is further demonstrated inFIGS. 7A-7BandFIGS. 8A-8C. As shown inFIGS. 7A and 7B, the Raman 2-D mapping of the characteristic peaks shows gradual yet distinct changes in tribochemistry and demonstrates a high degree of coating integrity. Similarly,FIGS. 8A-8Cdemonstrate the integrity of the sample and the wear track.FIG. 8Bdemonstrates Raman spectra acquired from the as-deposited coating whereas that from inside the wear track is presented inFIG. 8C. The Raman peak positions remain unchanged indicating that the coating did not deteriorate during the test.

TEM analysis fromFIGS. 9A-9Bof the low friction wear product demonstrates the lamination (i.e. the coating process via the one-step process described above of the MoS2flakes with GO. The flakes are encapsulated by large blankets of GO in order to passivate the MoS2from the ambient oxygen and moisture. This further shows in the TEM image fromFIG. 9Bthat scrolls of MoS2form. The concentration of scrolls formed results in a superlubric behavior.

It is believed that the reason for superlubricity is related to maintaining the inert layered structure of MoS2by encapsulation of GO flakes. The layered structure may be scrolls, but must encapsulate the 2-D material, such as MoS2. It is believed that during sliding at high contact pressures at high sliding velocity and elevated substrate temperatures, structural re-orientation of MoS2takes place resulting in layered structure as shown inFIG. 9B. This layered structure is preserved for longer time from oxygen induced degradation due to conformal encapsulation provided by large GO sheets as shown inFIGS. 9A-9B, thus lowering the friction to superlubric regime (below 0.01) and maintaining it for longer time duration.

In summary, a low friction wear surface includes a substrate, GO in an oil-free solvent disposed over the substrate, and MoS2ultrafine nanocrystalline flakes disposed over the substrate. The GO and the MoS2are in a range of ratios of (1±0.25):(1∓0.25) by weight, respectively. In order to form this low friction wear surface, the substrate has a temperature of about 275° C. when the GO and the MoS2are disposed on its surface. In some embodiments, the solvent mixed with GO is water.

As shown in the figures and descriptions above, the low friction wear surface displays a much lower COF than a surface without the low friction wear surface. In some embodiments, the low friction wear surface has a COF value less than 0.06 at a temperature in between about 200° C. and 400° C. for a duration of about 15 minutes to about 3.5 hours, inclusive. In some embodiments, the low friction wear surface has a maximum contact pressure in between about 0.1 GPa and about 1.0 GPa at a temperature in between about 200° C. and about 400° C. In some embodiments, the low friction wear surface has a COF less than 0.1 at a temperature in between about 22° C. and about 100° C. in ambient conditions.

In some embodiments, the substrate of the low friction wear surface is made of at least a portion of a metal working die, a wind turbine, a polymer injection molding die, a piston, a piston ring, a piston sleeve, a ball and roller-bearing element, an oil-free air compressor, a gas compressor, a gas seal, a sliding rail guide, or a heavy load bearing wheel guide. In some embodiments, the substrate of the low friction wear surface is a steel, for example but not limited to, a self-mated stainless steel, ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, duplex stainless steel, or precipitation hardened stainless steel.

Furthermore, a method of a forming a low friction wear surface includes preparing graphene by chemical exfoliation of highly-oriented pyrolytic graphite, suspending GO in a solvent to form a solution of at least 1 g/L, adding at least 1 g/L of MoS2ultrafine nanocrystalline flakes to the solution, sonicating the MoS2and the solution to form a homogenous solution and disposing the homogenous solution on a substrate. The method of disposing the homogenous solution includes spraying the homogeneous solution on a substrate via a process of air-spray coating, wherein the substrate has a temperature of 275° C.; forming a wet film on the substrate; and evaporating the solvent component to form a dry coating layer. The GO and the MoS2are in a range of ratios in between (1±0.25):(1∓0.25) by weight, respectively. The solvent in this method may be water, or the solvent may be oil-free. In some embodiments, the method further comprises evaporating the solvent component and encapsulating the MoS2flakes in large blankets of GO in one step.

In some embodiments, the method of forming the low friction wear surface includes achieving a COF value in between about 0.001 and about 0.06 at a temperature in between about 200° C. and about 400° C. In some embodiments, this method includes demonstrating a COF value less than 0.05 for a duration of about 15 minutes to about 3.4 hours in between about 200° C. and 400° C. In some embodiments, the substrate is a steel material. In some embodiments, the method further includes a COF value less than 0.1 at a temperature in between about 22° C. and about 100° C. in ambient conditions.

A method of forming a sliding mechanical system with a low friction wear surface includes forming a homogeneous solution of MoS2and GO in a range of ratios in between (1±0.25):(1∓0.25) by weight, disposing the homogeneous solution over a substrate to form a first sliding component, and sliding the first sliding component against a second sliding component in open air. Scrolls of MoS2are formed and encapsulated in the GO. In some embodiments, the second sliding component may be a steel material. In some embodiments, the method may further include sliding the first sliding component and the second sliding component at a temperature in between about 200° C. and about 400° C. In some embodiments, the solution uses an oil-free solvent. In some embodiments, the homogeneous solution is disposed over the substrate via a method of air spray coating when the substrate is heated to a temperature of about 275° C.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” Any ranges cited herein are inclusive.

The terms “substantially” and “about” used throughout this Specification are used to describe and account for small fluctuations. For example, they may refer to less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%, such as less than or equal to ±0.2%, such as less than or equal to ±0.1%, such as less than or equal to ±0.05%.