AQUEOUS LUBRICATING COMPOSITION COMPRISING MOS2 NANOSHEET

Abstract

 The present invention relates to a lubricating composition comprising:
- water, especially purified water, for example ultrapure water, demineralized water, osmotic water, deionized water;
- glycerol;
- nanosheets of h- MoS2, wherein the active content of h-MoS2 nanosheets is comprised between 0.01 and 0.07 wt%, preferably between 0.02 to 0.06 wt%, based on the total weight of the lubricating composition,
- wherein, the nanosheets of h-MoS2 are obtained in-situ in the lubricating composition by liquid-phase exfoliation of bulk MoS2 particleswater

Description

[0001] The present invention relates to lubricant compositions in particular for gear, bearing and transmission, in particular automotive gears and transmissions, for example for electric vehicle reducers and industrial gears and bearing, more particularly for manual gearboxes.

[0002] Gears, especially industrial gears, face extreme operating conditions that can lead to damage, for example, wear to the internal components of the gears. This damage reduces the life of the gears, for example industrial gears, and can lead to costly and prolonged maintenance, repair costs, unscheduled downtime for the equipment that contains the gears.

[0003] Lubricants are used in industrial gears to solve a part of that issues and also in drive elements, such as automotive gears and transmissions, roller bearings, and plain bearings or seals on rotating shafts. Lubricants are classically based on mineral oil or synthetic hydrocarbons, but also could be based on an aqueous basis. In particular, in roller bearings and plain bearings, the lubricants cause a separating, load-transferring lubricating film to be built up between the parts that slide or roll on one another. It is thus achieved that the metal surfaces do not touch, and thus, also reduced friction occurs.

[0004] There is an on-going need for improved lubricants that can provide better performance in and protection of gears and bearings, and thus extending the service life of the gears, the bearings and the equipment that contains them.

[0005] It is an object of the present invention to provide new lubricant compositions providing an improved wear resistance and also a better stability.

[0006] The present invention relates to a lubricating composition comprising:

• water, especially purified water, for example ultrapure water, demineralized water, osmotic water, deionized water;
• glycerol;
• nanosheets of h- MoS₂, wherein the active content of h-MoS2 nanosheets is comprised between 0.01 and 0.07 wt%, preferably between 0.02 to 0.06 wt%, based on the total weight of the lubricating composition,

wherein, the nanosheets of h-MoS₂ are obtained in-situ in the lubricating composition by liquid-phase exfoliation of bulk MoS₂ particles.

[0007] In h-MoS₂, the "h" refers to the hexagonal phase of MoS₂. Thus, in the present invention, the crystal structure of MoS2 is hexagonal.
The UV-visible absorption spectra of the resulting h-MoS₂ (h is for hexagonal) nanosheet aqueous dispersion displayed the characteristic bands A and B at 660 and 603 nm respectively corresponding to the excitonic transitions of hexagonal MoS₂ nanosheets. Absorbance at these characteristic wavelengths was used to estimate the concentration of MoS₂ nanosheets.

[0008] Preferably, the nanosheets of h-MoS₂ dispersed in the water has an average particle hydrodynamic size comprised between 25 and 65 nm, preferably between 30 and 60 nm, more preferably around 45 ± 5 nm . The average particle size of the nanosheets is measured by all techniques known by the skilled person, preferably by Transmission Electron Microscopy (TEM) for lateral size, Atomic Force Microscopy (AFM) for thickness or Dynamic Light Scattering (DLS) for hydrodynamic size.
The hydrodynamic size is the size of the dispersed particle with the solvation layer, it depends on the solvent (dielectric constant) and may be larger than the size of dry particle measured by microscopy.

[0009] Advantageously, the glycerol acts as a stabilizer of the h-MoS₂ nanosheet. The glycerol is an organic molecule comprising three free hydroxyl groups, of following formula:

Preferably, the lubricating composition of the invention comprises at least 20 wt% of glycerol, preferably from 20 to 75 wt%, more preferably from 30 to 50 wt%, based on the total weight of the lubricating composition.
According to another embodiment, it is possible to replace glycerol by one or more compounds of polyol type, for example chosen among glycol, polyalkylene glycol. It is also possible to use this glycol or polyalkylene glycol in combination with glycerol.

[0010] The water implemented in the invention is preferably purified water. Purified water can be chosen among ultrapure water, demineralized water, osmotic water, deionized water. The amount of water in the lubricating composition is greater than 35 wt%, preferably comprised between 35 and 80 wt%, preferably from 35 to 75 wt%, based on the total weight of the lubricating.
In the present invention, the water that has been purified to uncommonly stringent specifications. In ultrapure water the water is treated to the highest levels of purity for all contaminant types, including: organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile; reactive, and inert; hydrophilic and hydrophobic; and dissolved gases.

[0011] Advantageously, the in-situ process enables to obtain h-MoS₂ nanosheets directly dispersed in the aqueous solution and stabilized with glycerol.
The liquid phase exfoliation can be carried out by any method known by the skilled person, it is preferably carried out using a high-power tip sonicator or high-shear rotors. Preferably, the liquid phase exfoliation is carried out in reduced time, preferably from 4 to 8 hours.
The liquid -phase exfoliation is preferably followed by a centrifugation step that can be carried by any means known by the skilled person, it is preferably carried out by a centrifuge allowing for high rotation speeds for instance, a Thermo Scientific Sorvall Legend XT. Preferably, the centrifugation is carried out during 1 to 4 hours from 1000 to 10000 rpm. The higher the speed, the smaller the size and concentration of nanosheets will be. Preferably, in the present invention, the bulk MoS₂ is not functionalized. The bulk raw material is micron-sized (1-10 µm) MoS₂ powder (> 98.5% purity) from Acros Organics (CAS number: 1317-33-5)

[0012] The in-situ preparation of the h-MoS₂ nanosheet enables a single step preparation (there is no need of drying and isolating steps of h-MoS₂ nanosheets otherwise produced in a pre-exfoliating liquid media). Advantageously, the in-situ process requires a lower concentration of h-MoS₂ nanosheets (compared to process of the literature) to obtain improved properties, improved stability and reduction of wear and friction.

[0013] The lubricating composition according to the invention can also comprise polyalkylene glycol.
The polyalkylene glycols (denoted "PAG") are chosen from water-soluble polyalkylene glycols.
The term "water-soluble" is intended to denote a polyalkylene glycol having a solubility in water of at least 10 g / L, preferably of at least 500 g / L, in water at ambient temperature (approximately 25 °C).
The polyalkylene glycols can more particularly be formed of C1-C4, preferably C1-C3, more particularly C2-C3 alkylene oxide units.
Advantageously, a polyalkylene glycol used in an aqueous lubricating composition according to the invention comprises at least 50% by weight, in particular at least 80% by weight, more preferably at least 90% by weight of propylene oxide and / or ethylene oxide units. It may be an ethylene oxide / propylene oxide random copolymer.
Preferably, a polyalkylene glycol used in an aqueous lubricating composition according to the invention has a kinematic viscosity measured at 100 ° C (KV100), according to the ASTM D445 standard, between 100 and 5000 mm²/s, in particular between 150 and 3000 mm²/s.
Preferably, a polyalkylene glycol used in an aqueous lubricating composition according to the invention has a kinematic viscosity measured at 40 ° C (KV40), according to the ASTM D445 standard, between 500 and 30,000 mm²/s, more particularly between 1,000 and 25,000 mm²/s.
The flash point of a polyalkylene glycol used in an aqueous lubricating composition according to the invention is preferably greater than or equal to 160 °C, in particular greater than or equal to 220 ° C. The flash point can be measured by ISO 2592 or ASTM D92. Preferably, a polyalkylene glycol used in an aqueous lubricating composition according to the invention has a viscosity index measured according to the ASTM D2270 standard, of between 100 and 800, preferably between 250 and 550.

[0014] The lubricating composition according to the invention can also comprise various additives that are compatible with the aqueous solution.

[0015] Advantageously, the additives are used in a form soluble or emulsifiable in water, for example in the form of salts or ionic liquids.
Said additive(s) are of course chosen with regard to the intended application for the aqueous lubricant.
Of course, a person skilled in the art will take care to choose the optional additives and / or their quantity in such a way that the advantageous properties of the aqueous lubricating composition according to the invention, in particular the tribological properties, in particular of reduction of friction and protection of parts against wear, are not altered by the additives envisaged.
Such additives can be more particularly chosen from anti-foam agents, biocides, pH regulators, corrosion inhibitors, anti-wear and / or extreme pressure additives, sequestering agents, metal passivators, colorants, dispersants, emulsifying agents, and mixtures thereof. Advantageously, a lubricating composition according to the invention can comprise one or more additives chosen from anti-foam agents, extreme pressure agents, corrosion inhibitors, pH regulators, metal passivators, colorants, and their mixtures.

[0016] A lubricating composition according to the invention may more particularly comprise from 0.1 to 10% by mass of additives, in particular from 1.0 to 8.0% by mass of additives, relative to the total mass of the composition.

Corrosion inhibitor

[0017] An aqueous lubricating composition according to the invention can comprise at least one corrosion inhibitor agent. Corrosion inhibitors advantageously make it possible to reduce or even prevent the corrosion of metal parts. The nature of said corrosion inhibitor(s) can be chosen with regard to the metal to be protected against corrosion, such as aluminum, steel, galvanized steel, yellow metals, for example copper or brass.
Among the inorganic corrosion inhibitors may be mentioned nitrites, sulphites, silicates, borates, sodium, potassium, calcium or magnesium phosphates, alkali metal phosphates, hydroxides, molybdates, zinc sulphates, magnesium or nickel.
Among the organic corrosion inhibitors may be mentioned alkanolamines, such as triethanolamine, aliphatic monocarboxylic acids, in particular having 4 to 15 carbon atoms, for example octanoic acid, aliphatic dicarboxylic acids having 4 to 15 carbon atoms, for example decane dioic acid, undecane dioic acid, dodecane dioic acid or their mixtures, polycarboxylic acids optionally neutralized with triethanolamine, such as 1,3,5-triazine-2,4,6-tri-(6-aminocaproic) acid, alkanoylamidocarboxylic acids, in particular isononanoylamidocaproic acid, and mixtures thereof. Borated amides, products of the reaction of amines or amino alcohols with boric acid, can also be used.
An aqueous lubricating composition according to the invention may in particular comprise from 0.1% to 5.0% by weight of corrosion inhibitor (s), preferably from 0.5% to 4.0% by weight, more preferably from 1.0% to 2.5% by mass, relative to the total mass of the composition.

Anti-wear / extreme pressure additive

[0018] A lubricating composition according to the invention may comprise at least one anti-wear and / or extreme pressure additive. Their function is to reduce wear and the coefficient of friction, or to prevent metal-to-metal contact by forming a protective film adsorbed on these surfaces.
There is a wide variety of anti-wear additives, among which may be mentioned those chosen from phosphosulfur additives such as metal alkylthiophosphates or their salts.
Amine phosphates are also antiwear additives which can be used in a composition according to the invention.

[0019] Additives which do not provide phosphorus may also be suitable, such as, for example, polysulphides, in particular sulfur-containing olefins.
Among the extreme pressure additives suitable for the present invention, mention may be made of water-soluble extreme pressure additives, such as 2,5-dimercapto-1,3,4-thiadiazole (DMTD) or one of its salts, in in particular a disodium salt (NaDMTD).
An aqueous lubricating composition according to the invention may comprise between 0.01% and 10% by mass of anti-wear and / or extreme pressure additive(s) as defined above, preferably between 0.5% and 5.0% by mass, relative to the total mass of the composition.

Defoamer

[0020] An aqueous lubricating composition according to the invention may comprise at least one anti-foam additive. Antifoams help prevent foaming of the lubricating fluid.
It may, for example, be an anti-foaming agent based on polysiloxanes or on acrylate polymers.
Preferably, the anti-foaming agent is chosen from three-dimensional siloxanes.
The anti-foaming agents can also be polar polymers such as polymethylsiloxanes or polyacrylates.

[0021] In particular, a lubricating composition according to the invention can comprise from 0.001% to 3.0% by weight of anti-foaming additive (s), preferably from 0.005% to 1.5% by weight, more preferably from 0.01 % to 1.0% by weight, relative to the total weight of the lubricating composition.

PH regulator

[0022] A lubricating composition according to the invention can comprise at least one pH regulating additive, in particular an alkaline buffer. The pH regulator makes it possible to maintain the desired pH of the lubricating composition, in particular in order to preserve an alkaline pH, advantageously between 8 and 11, in particular to prevent corrosion of the metal surfaces. The pH regulator can be chosen from amines, in particular alkanolamines and amino alcohols.
It may in particular be a pH regulating additive chosen from ethanolamines, such as monoethanolamine (MEA), diethanolamine (DEA); triethanolamine (TEA), diglycolamine (DGA) isopropanolamines, such as mono-isopropanolamine (MIPA), diisopropanolamine (DIPA) and triisopropanolamine (TIPA), ethylene amines, such as ethylene diamine (EDA), diethylene triamine (DETA), triethylene tetramine (TETA) and tetraethylene pentamine (TEPA), alkanolamines, such as methyldiethanol amine (MDEA), cyclamines, such as cyclohexylamine, 2-amino-2-ethyl-1,3-propanediol, 2-amino-2-methyl-1-propanol and mixtures thereof.
An aqueous lubricating composition according to the invention can in particular comprise from 0.1% to 10% by mass of additive (s) regulating the pH, preferably from 0.5% to 5.0% by mass, relative to the total mass of the composition.

Metal passivators

[0023] A lubricating composition according to the invention can comprise at least one metal passivating agent. Metal passivators protect metal parts by promoting the formation of metal oxide on their surface.
The metal passivating agents can for example be chosen from triazole derivatives, such as tetrahydrobenzotriazole (THBTZ), tolyltryazole (TTZ), benzotriazole (BTZ), amines substituted with a triazole group, such as N,N-bis(2-ethylhexyl)-1,2,4-triazol-1-yl methanamine, N'-bis(2 ethylhexyl)-4-methyl-1H-benzotriazol-1-methyl-amine, N,N-bis (heptyl)-ar-methyl-1H-benzotriazole-1-methanamine, N,N-bis(nonyl)-ar-methyl-1H-benzotriazole-1-methanamine, N,N-bis(decyl)-ar-methyl-1H-benzotriazole-1-methanamine, N,N-bis(undecyl)-ar-methyl-1H-benzotriazole-1-methanamine, N,N-bis (dodecyl)-ar-methyl-1H-benzotriazole-1-methanamine , N, N-bis (2-ethylhexyl) -ar-methyl-1H-benzotriazole-1-methanamine, 1,2,4-triazoles, benzimidazoles, 2-alkyldithiobenzimidazoles, 2-alkyldithiobenzothiazoles, 2-(N,N-dialkyldithiocarbamoyl) benzothiazoles, 2,5-bis(alkyldithio)-1,3,4-thiadiazoles, such as 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole, 2,5-bis(tert nonyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-decyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-undecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-dodecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-tridecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-tetradecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-pentadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-hexadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-heptadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-octadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-nonadecyldithio)-1,3,4-thiadiazole, 2,5-bis (tert-eicosyldithio)-1,3,4-thiadiazole, 2,5-bis(N,N-dialkyldithiocarbamoyl)-1,3,4-thiadiazoles, 2-alkyldithio-5-mercaptothiadiazoles, and mixtures thereof.
Preferably, the metal passivating agents are chosen from tetrahydrobenzotriazole (THBTZ), tolyltriazole (TTZ), benzotriazole (BTZ), and their salts, alone or as mixtures.
A lubricating composition according to the invention may in particular comprise from 0.01% to 2.0% by weight of metal passivating agent (s), preferably from 0.1% to 1.0% by weight, relative to the total mass of the composition

Dyes

[0024] A lubricating composition according to the invention may comprise one or more dyes. The dyes can be natural or synthetic, generally organic.
The dyes which can be used in an aqueous lubricating composition can be more particularly chosen from natural or synthetic water-soluble dyes, for example the dyes FDC Red 4, DC Red 6, DC Red 22, DC Red 28, DC Red 30, DC Red 33, DC Orange 4, DC Yellow 5, DC Yellow 6, DC Yellow 8, FDC Green 3, DC Green 5, FDC Blue 1, betanin (beet), carmine, a chlorophyllin, methylene blue, anthocyanins (enocianin, black carrot and hibiscus), caramel and riboflavin.
An aqueous lubricating composition according to the invention may comprise between 0.01% and 2.0% by weight of dye(s), preferably between 0.01% and 1.5% by weight, more preferably between 0.02% and 1.0% by weight, relative to the total weight of the composition.

Emulsifying agents

[0025] A lubricating composition according to the invention can comprise one or more emulsifying agents, also called emulsifiers. Their function is to generate stable emulsions in water.
The emulsifying agents can be more particularly nonionic, such as, for example, ethoxylated fatty alcohols, ethoxylated fatty acids, ethoxylated fatty amides; anionic, for example soaps of KOH, NaOH; sulphonates; cationic, such as quaternary ammonium compounds; or else water-soluble or emulsifiable carboxylic acid esters.
In particular, an aqueous lubricating composition according to the invention can comprise from 0.01% to 10% by weight of emulsifying agent(s), preferably from 0.1% to 5.0% by weight, relative to the total weight of the lubricating composition.

Sequestering agents

[0026] A lubricating composition according to the invention can comprise at least one sequestering agent. Sequestering agents, also called chelating agents, make it possible to limit the encrustation of metal ions in the composition.
As examples of sequestering agents, there may be mentioned those derived from phosphonic acids and phosphonates, such as diethylenetriaminepentamethyl phosphonic acid (DTPMPA), amino tri(methylene phosphonic acid) (ATMP), acid hydroxyethane-diphosphonic acid (HEDP), 1-hydroxylethylidene 1,1-diphosphonate, 2-hydroxyethylamine di (methylene phosphonic) acid (HEAMBP), diethylene triamino penta (methylene phosphonic) acid (DTMP), acids multifunctional organic and hydroxy acids, such as ethylenediaminetetraacetic acid (EDTA), pteroyl-L-glutamic acid (PGLU), organic polyacids, such as maleic acid and polyaspartic acid, polysaccharides and carbohydrates, such as inulin, carboxymethylinulin and carboxymethylchitosan.
A lubricating composition according to the invention may comprise from 0.001% to 2.0% by weight of sequestering agent (s), preferably from 0.01% to 1.0% by weight, relative to the total weight of the composition.

Biocides and fungicides

[0027] A lubricating composition according to the invention can comprise at least one biocidal and / or fungicidal agent. Biocides and fungicides can be used to improve the biological stability of the composition by limiting the proliferation of bacteria, fungi and yeasts in the lubricating fluid.
Such biocides can be chosen from parabens, aldehydes, reactive acetylacetone compounds, isothiazolinones, phenolic compounds, acid salts, halogenated compounds, quaternary ammoniums, certain alcohols and their mixtures.
Preferably, the biocides can be chosen from optionally substituted benzisothiazolinones (BIT), such as N-butyl-1,2-benzisothiazolin-3-one, methylisothiazolinones (MIT), mixtures of methylisothiazolinone and chloromethylisothiazolinone (MIT / CMIT), orthophenyl-phenol (OPP) or its sodium salt, 3-iodo-2-propynylbutylcarbamate (IPBC), chloro-cresol and N, N-methylene-bis-morpholine (MBM); sorbic acid; preferably from orthophenyl-phenol (OPP) or its sodium salt, 3-iodo-2-propynylbutylcarbamate, chloro-cresol, benzisothiazolinones and N, N-methylene-isomorpholine.
An aqueous lubricating composition according to the invention can in particular comprise between 0.01% and 10% by weight of biocide (s) and / or fungicide (s), preferably between 0.5% and 5.0% by weight, relative to the total weight of the composition.

[0028] Preferably, the lubricating composition according to the invention comprises less than 5 wt% of non-water soluble oil, preferably less than 2 wt%, more preferably less than 1 wt%, based on the total weight of the lubricating composition. Preferably, the lubricating composition of the invention does not comprise non-water soluble oil.
Non-water soluble oil is oil that is not substantially solubilized in water at room temperature (around 25°C). In particular, non-water soluble oil has a solubility in water less than 0.2 g/l at room temperature (around 25°C). Such oil is for example lubricating base oil from groups I to V of the API classification (or equivalent in ATIEL classification) and their mixtures.

[0029] The present invention also relates to a process for preparing the lubricating composition according to the invention comprising the following steps:

• addition of bulk MoS₂ particles in a water solution comprising glycerol ;
• liquid-phase exfoliation of the bulk MoS₂ particles;
• Centrifugation to remove unexfoliated bulk MoS₂ particles.

[0030] The present invention also relates to a process for creating h-MoS₂ nanosheets in an aqueous solution comprising the following steps:

• addition of bulk MoS₂ particles in a water solution comprising glycerol ;
• liquid-phase exfoliation of the bulk MoS₂ particles;
• Centrifugation to remove unexfoliated bulk MoS₂ particles.

[0031] The water, the glycerol and the h-MoS₂ nanosheets are as decribed above.

[0032] Advantageously, the in-situ process enables to obtain h-MoS₂ nanosheets directly dispersed in the aqueous solution and stabilized with glycerol.
The liquid phase exfoliation can be carried out by any method known by the skilled person, it is preferably carried out using a high-power tip sonicator or a high-shear rotor. Preferably, the liquid phase exfoliation is carried out in reduced time, preferably from 4 to 8 hours.
The centrifugation step can be carried by any means known by the skilled person, it is preferably carried out by a centrifuge allowing for high rotation speeds for instance, a Thermo Scientific Sorvall Legend XT.
Preferably, the centrifugation is carried out during 1 to 4 hours at 1000 to 10000 rpm .

[0033] The present invention also relates to the use of the lubricating composition according to the invention for the lubrication of mechanical parts of a mechanic system, preferably of gear, bearing and transmission, in particular automotive gears and transmissions, for example electric vehicle reducers and industrial gears and bearing, particularly of manual gearboxes. Preferably, the use of the composition according to the present invention enables to reduce wear and friction.

[0034] The present invention also relates to a process for lubricating mechanical parts of a mechanic system, preferably of gear, bearing and transmission, in particular automotive gears and transmissions, for example electric vehicle reducers and industrial gears and bearing, particularly of manual gearboxes, comprising putting the mechanical parts of the mechanic system, preferably of automobile transmissions, particularly of manual gearboxes, in contact with the lubricating composition according to the invention.

[0035] The present invention also relates to a process for reducing wear and friction on mechanical parts of a mechanic system, preferably gear, bearing and transmission, in particular automotive gears and transmissions, for example electric vehicle reducers and industrial gears and bearing, particularly of manual gearboxes, comprising putting the mechanical parts of the mechanic system, preferably of automobile transmissions, particularly of manual gearboxes, in contact with the lubricating composition according to the invention.

[0036] The present invention will now be described with non-limitative examples.

Examples

Pin-on-disk tribometer measurements

[0037] The lubricating performance of formulations was investigated using a pin-on-disk high temperature tribometer (CSM Instruments THT).
The measurements were carried out in the following conditions:

• Load values : 5-10 N;
• Sliding speeds : 1-4 cm/s;
• Temperature : measurements were carried out at room temperature (25°C);
• Disk surfaces : the lubricants were tested on steel 100Cr6 disks from Optimal Instruments (24 mm diameter x 7.9 mm thick) with a Rockwell C hardness according to the supplier of 60 ± 2 HRC and an average roughness Ra=0.046 µm ± 0.003 µm, measured hardness was 66±1 HRC, higher than that given by the supplier
• Ball properties : steel 100Cr6 balls (6 mm diameter) purchased from Anton Paar were used for the pin-on-disk tribology measurements with a hardness according to the supplier of 60-66 HRC and an average roughness Ra<0.032 µm. Measured hardness was 68±1 HRC higher than that given by the supplier.

Wear analysis

[0038] The surface of steel 100Cr6 disks and balls after the pin-on-disk tribology measurements was examined by optical microscopy (Leica DM4000M) and confocal microscopy (Leica DCM3D) to evaluate wear of both contacting surfaces lubricated by the different formulations. The ball wear volume loss was estimated from the radius of the wear trace using the equation to calculate the volume of a spherical cap.

Stability

[0039] Stability refers to the time without observing sedimentation and preserving tribological performance.

Example 1: Preparation of lubricant composition

[0040] Aqueous solution of MoS₂ nanosheet were produced by liquid-phase exfoliation of bulk micron-sized MoS₂ powder (4 mg/ml) (99%, Acros Organics) in certain volume of aqueous solution of glycerol using a high-power tip sonicator (Dr. Hielscher UP 400S at the highest amplitude and frequency) for times ranging from 4 to 8 hours. Volumes used varied from 30 to 100 mL. After removing unexfoliated MoS₂ as sediment by centrifugation (Thermo Scientific Sorvall Legend XT, from 1 to 4 hours, at 3500 rpm), nanosized MoS₂ nanosheet are obtained directly dispersed in the aqueous lubricants as a result of the exfoliation process. The final concentration of MoS₂ nanosheets can be tuned by varying the sonication time and centrifugation speed and time in a way that the highest concentrations correspond to large sonication times and low centrifugation speed and time.

Example 2: Tribological measurements

[0041] The tribological measurements mentioned above are carried out using 1mL out of a 65 mL sample. The pin-on-disk tests were carried out on 66HRC 100CR6 steel disks under the following conditions: 60°C temperature, 10N load, 1 cm/s sliding speed and 72 m distance.

[0042] The results show a remarkable wear reduction ability of the composition of the invention.

Claims

1. Lubricating composition comprising:

- water, especially purified water, for example ultrapure water, demineralized water, osmotic water, deionized water;

- glycerol;

- nanosheets of h- MoS2, wherein the active content of h-MoS2 nanosheets is comprised between 0.01 and 0.07 wt%, preferably between 0.02 to 0.06 wt%, based on the total weight of the lubricating composition,

wherein, the nanosheets of h-MoS2 are obtained in-situ in the lubricating composition by liquid-phase exfoliation of bulk MoS2 particles.

2. Composition according to claim 1 wherein the active content of h-MoS2 nanosheets is comprised between 0.02 to 0.06 wt%, based on the total weight of the lubricating composition.

3. Composition according to claim 1 or 2 comprising at least 20 wt% of glycerol, preferably from 20 to 75 wt%, more preferably from 30 to 50 wt%, based on the total weight of the lubricating composition.

4. Composition according to anyone of claims 1 to 3 wherein the water is purified water, preferably ultrapure water, demineralized water, osmotic water, deionized water.

5. Composition according to anyone of claims 1 to 4 comprising more than 35 wt% of water, preferably from 35 to 80 wt%, preferably from 35 to 75 wt%, based on the total weight of the lubricating composition.

6. Composition according to anyone of claims 1 to 5, wherein the liquid phase exfoliation is carried out in reduced time from 4 to 8 hours.

7. Composition according to anyone of claims 1 to 6, wherein the liquid phase exfoliation is followed by centrifugation carried out during 1 to 4 hours at 1000 to 10000 rpm.

8. Process for preparing the lubricating composition according to anyone of claims 1 to 7 comprising the following steps:

- addition of bulk MoS₂ particles in a water solution comprising glycerol;

- liquid-phase exfoliation of the bulk MoS₂ particles;

- centrifugation to remove unexfoliated bulk MoS₂ particles.

9. Use of the lubricating composition according to anyone of claims 1 to 7 for the lubrication of mechanical parts of a mechanic system, preferably to reduce wear and friction.

10. Use according to claim 9 for the lubrication of gear, bearing and transmission, in particular automotive gears and transmissions, for example electric vehicle reducers and industrial gears and bearing.

11. Process for reducing wear and friction on mechanical parts of a mechanic system comprising putting the mechanical parts of the mechanic system in contact with the lubricating composition according to anyone of claims 1 to 7.

12. Process according to claim 11 for reducing wear and friction of gear, bearing and transmission, in particular automotive gears and transmissions, for example electric vehicle reducers and industrial gears and bearing.