EP2069462A1

EP2069462A1 - Grease composition for use in constant velocity joints comprising at least one tri-nuclear molybdenum compound

Info

Publication number: EP2069462A1
Application number: EP06806103A
Authority: EP
Inventors: Jisheng E; Frank Reher; Stefanie Rosenkranz
Original assignee: GKN Driveline International GmbH
Current assignee: GKN Driveline International GmbH
Priority date: 2006-10-07
Filing date: 2006-10-07
Publication date: 2009-06-17
Also published as: US20090247435A1; KR101124974B1; BRPI0622179A2; WO2008040381A1; CN101583701A; KR20090089846A; WO2008040381A8

Abstract

To solve the problem to provide for a new grease composition giving low wear and low friction primarily to constant velocity joints, a grease composition is suggested comprising a) a base oil composition; and b) 0,25 % by weight to 5 % by weight of at least one tri-nuclear molybdenum com pound of the formula Mo3SkLnQZ1 wherein L are independently selected legends having organo groups with a sufficient number of carbon atoms to render the compound soluble or dispersible in the oil, n is from 1 to 4, k varies from 4 though 7, Q is selected from the group of neutral electron donating compounds such as amines, alcohols, phosphines, and ethers, and z ranges from 0 to 5 and includes non-stoichiometric values.

Description

Grease Composition for use in Constant Velocity Joints comprising at least one

Tri-Nuclear Molybdenum compound

The present invention relates to a lubricating grease which is intended primarily for use in constant velocity universal joints, especially ball joints or tripod joints, which are used in the drivelines of motor vehicles.

The motions of components within constant velocity joints (CVJ) are complex with a combination of rolling, sliding and spinning. When the joints are under torque, the components are loaded together which can not only cause wear on the contact surfaces of the components, but also rolling contact fatigue and significant frictional forces between the surfaces. The wear can result in failure of the joints and the frictional forces can give rise to noise, vibration and harshness (NVH) in the driveline. NVH is normally "measured" by determining the axial forces generated in plunging type CVJ. Ideally the greases used in constant velocity joints need not only to reduce wear, but also have to have a low coefficient of friction to reduce the frictional forces and to reduce or prevent NVH.

Constant velocity joints also have sealing boots of elastomeric material which are usually of bellows shape, one end being connected to the outer part of the CVJ and the other end to the interconnecting or output shaft of the CVJ. The boot retains the grease in the joint and keeps out dirt and water.

Not only must the grease reduce wear and friction and prevent the premature initiation of rolling contact fatigue in a CVJ, it must also be compatible with the elastomeric material of which the boot is made. Otherwise there is a degradation of the boot material which causes premature failure of the boot, allowing the escape of the grease and ultimately failure of the CVJ. The two main types of material used for CVJ boots are polychloroprene rubber (CR) and thermoplastic elastomer (TPE), especially ether-ester block co-polymer thermoplastic elastomer (TPC-ET).

Typical CVJ greases have base oils which are blends of naphthenic (saturated rings) and paraffinic (straight and branched saturated chains) mineral oils. Synthetic oils may also be added. It is known that said base oils have a large influence on the deterioration (swelling or shrinking) of both boots made of CR and TPC-ET. Both mineral and synthetic base oils extract the plasticisers and other oil soluble protective agents from the boot materials. Paraffinic mineral oils and poly-α-olefin (PAO) synthetic base oils diffuse very little into especially boots made of rubber material causing shrinkage, but on the other hand naphthenic mineral oils and synthetic esters diffuse into boot materials and act as plasti- cisers and can cause swelling. The exchange of plasticiser or plasticiser compositions for the naphthenic mineral oil can significantly reduce the boot performance, especially at low temperatures, and may cause the boot to fail by cold cracking, ultimately resulting in fail- ure of the CVJ. If significant swelling or softening occurs, the maximum high speed capability of the boot is reduced due to the poor stability at speed and/or excessive radial expansion.

In order to solve the aforesaid problems, US 6,656,890 B1 suggests a special base oil combination comprising 10 to 35 % by weight of one or more poly-α-olefins, 3 to 15 % by weight of one or more synthetic organic esters, 20 to 30 % by weight of one or more naphthenic oils, the remainder of the combination being one or more paraffinic oils, and, further, a lithium soap thickener, and a sulphur-free friction modifier, that may be a or- gano-molybdenum complex, and molybdenum dithiophosphate, and a zinc dialkyldithio- phosphate and further additives such as corrosion inhibitors, anti-oxidants, extreme pressure additives, and tackiness agents. However, the friction coefficient and the wear of grease compositions according to US 6,656,890 B1 as measured in SRV (abbreviation for the German words Schwingungen, Reibung, Verschleiβ) tests needs to be improved.

Thus, it is the object of the present invention to provide for a grease composition, primarily for use in constant velocity joints, which has a good compatibility with boots made of rubber or thermoplastic elastomer, and which also gives low wear and low friction in use in CVJ.

Said object of the present invention is solved by a grease composition for use in constant velocity joints comprising

a) a base oil composition; and

b) 0,25 % by weight to 5 % by weight, preferably 0.3% by weight to 3% by weight, referred to the total amount of the grease composition, of at least one tri-nuclear molybdenum compound of the formula

Mo₃SkL_nQz,

wherein L are independently selected legends having organo groups with a sufficient number of carbon atoms to render the compound soluble or dispersible in the oil, n is from 1 to 4, k varies from 4 though 7, Q is selected from the group of neutral electron donating compounds such as amines, alcohols, phosphines, and ethers, and z ranges from 0 to 5 and includes non-stoichiometric values.

The number of carbon atoms present in the tri-nuclear molybdenum compound among all the ligands, organo groups is at least 21 carbon atoms, preferably at least 25, more preferably at least 30, and most preferably at least 35. Tri-nuclear molybdenum compounds usable in the present invention are disclosed in US 6,172,013 B1 , the disclosure of which is incorporated in the present invention insofar by reference. The inventors of the present invention have found that the presence of at least 0.25 % by weight of the tri-nuclear molybdenum compound according to claim 1 would significantly lower the friction coefficient as well as the wear of CVJ in use. Surprisingly, the presence of 0.2 % by weight or less of the tri-nuclear molybdenum compound would not lead to a much lowered friction coefficient nor to a lower wear when used in CVJs.

As a base oil composition according to the present invention, a base oil composition as disclosed in US 6,656,890 B1 , the disclosure of which is incorporated insofar herein by reference, may preferably be used. However, any further kind of base oil composition, especially a blend of mineral oils, a blend of synthetic oils or a blend of a mixture of min- eral and synthetic oils may be used. The base oil composition should preferably have a kinematic viscosity of between about 32 and about 250mm²/s at 40⁰C and between about 5 and about 25mm²/s at 100⁰C. The mineral oils preferably are selected from the group comprising at least one naphthenic oil and/or at least one paraffinic oil. The synthetic oils usable in the present invention are selected from a group comprising at least one poly-α- olefin (PAO) and/or at least one synthetic organic ester. The organic synthetic ester is preferably a di-carboxylic acid derivative having subgroups based on aliphatic alcohols. Preferably, the aliphatic alcohols have primary, straight or branched carbon chains with 2 to 20 carbon atoms. Preferably, the organic synthetic ester is selected from a group comprising sebacic acid-bis(2-ethylhexylester) ("dioctyl sebacate" (DOS)), adipaic acid-bis-(2- ethylhexylester) ("dioctyl adipate" (DOA)), and/or azelaic acid-bis(2-ethylhexylester) ("dioctyl azelate (DOZ)).

If poly-α-olefin is present in the base oil composition, preferably poly-α-olefins are selected having a viscosity in a range from about 2 to about 40 centistokes at 100⁰C. Naphthenic oils selected for the base oil compositions have preferably a viscosity in a range between 20 to 180 mm²/s at 40°C, whereas if paraffinic oils were present in the base oil composition, preferably the paraffinic oils have a viscosity in a range between about 25 to about 400 mm²/s at 40⁰C .

In a further embodiment of the present invention, the grease composition further com- prises at least one zinc compound additive, more preferably a zinc compound additive in an amount of about 0.1 % by weight to about 1.5 % by weight. Most preferred the zinc compound additive is selected from the group comprising at least one of zinc dithiophos- phates (ZnDTP) and/or zinc dithiocarbamates (ZnDTC)₁ ZnO and/or ZnS. The zinc dithio- phosphate is preferably selected from the group of zinc dialkyldithiophosphate of the fol- lowing general formula:

(R¹O)(R²O)SP-S-Zn-S-PS(OR³XOR⁴)

wherein each of R¹ to R⁴ inclusive may be the same or different and each represents a primary or secondary alkyl group having 1 to 24, preferably 3 to 20, most preferably 3 to 5 carbon atoms. In particular, excellent effects can be expected if the substituants R¹, R², R³ and R⁴ represent a combination of primary and secondary alkyl groups, each having 3 to 8 carbon atoms.

The zinc dithiocarbamate may be preferably selected from zinc dialkyldithiocarbamate of the following general formula:

wherein R⁵, R⁶, R⁷, and R⁸ may be same or different and each represents an alkyl group having 1 to 24 carbon atoms or an aryl group having 6 to 30 carbon atoms.

By adding at least one zinc compound additive to the grease composition according to the invention, the friction coefficient as well as the wear in CVJ are diminished further signifi- cantly. According to another embodiment of the invention, the grease composition further comprises a thickener selected from the group comprising lithium soaps, calcium soaps, lithium complex soaps, calcium complex soaps, and/or urea-derivative type thickener. The urea-derivative type thickener is not restricted to specific ones and maybe, for instance, also a diurea compound and/or a polyurea compound.

In the sense of the present invention, a lithium soap or a calcium soap is a reaction product of at least one fatty acid with lithium hydroxide or calcium hydroxide. Preferably, the thickener may be a simple lithium or calcium soap formed from stearic acid, 12-hydroxy stearic acid, hydrogenated castor oil or from other similar fatty acids or mixtures thereof or methylesters of such acids. Alternatively, a lithium and/or calcium complex soap may be used formed for example from a mixture of long-chained fatty acids together with a com- plexing agent, for example a borate of one or more dicarboxylic acids or a mixture of short and/or medium chained carboxylic acids. The use of complex lithium and/or calcium soaps allows the grease composition according to the present invention to operate up to a temperature of about 180⁰C, whereas with simple lithium and/or calcium soaps, the grease composition will only operate up to a temperature of about 120⁰C. However, mixtures of all of the aforesaid thickeners may also be used.

According to a further embodiment of the present invention, the grease composition further comprises an additive package selected from the group of agents comprising anti- oxidation agents, corrosion inhibitors, anti-wear agents, friction modifiers, and/or extreme pressure agents (EP agents).

The EP agent is preferably a metal-free, sulphurised fatty acid methyl ester agent with a viscosity of about 25mm²/s at 40⁰C being present preferably in an amount between about 0.1 to about 3% by weight, referred to the total amount of the grease composition. The total sulphur amount of the EP agent preferably ranges from about 8 to about 10% by weight and the active sulphur amount is about 1% by weight. Such EP agents exhibit ex- cellent effects with respect to the prevention of seizure of CVJ. If the sulphur content exceeds the upper limit defined above, it may promote the initiation of rolling contact fatigue and wear of the contacting metal components.

As an anti-oxidation agent, the grease composition of the present invention may comprise an amine, preferably an aromatic amine, more preferably phenyl-α-naphthylamine or di- phenylamine or derivatives thereof. The anti-oxidation agent is used to prevent deterioration of the grease composition associated with oxidation. The grease composition accord- ing to the present invention may range between about 0.1 to about 2% by weight, referred to the total amount to the grease composition, of an anti-oxidant agent in order to inhibit the oxidation degradation of the base oil, as well as to lengthen the life of the grease composition, thus prolonging the life of the CVJ.

Typically, the last operation before the assembly of CVJ is a wash to remove machining debris, and it is therefore necessary for the grease to absorb any traces of remaining water and to prevent the water from causing corrosion and adversely effecting the performance of the CVJ, it is therefore necessary to add a corrosion inhibitor. As a corrosion in- hibitor, the grease composition according to the present invention may comprise at least one metal salt selected from the group consisting of metal salts of oxidised waxes, metal salts of petroleum sulphonates, especially prepared by sulphonating aromatic hydrocarbon components present in fractions of lubricating oils, and/or metal salts of alkyl aromatic sulphonates, such as dinonylnaphthalene sulphonic acids, alkylbenzene sulphonic acids, or overbased alkylbenzene sulphonic acids. Examples of the metal salts include sodium salts, potassium salts, calcium salts, magnesium salts, zinc salts, and quaternary ammonium salts, the calcium salts being most preferred. Calcium salts of oxidised waxes also ensure an excellent effect.

Anti-wear agents according to the present invention prevent a metal-to-metal contact by adding film-forming compounds to protect the surface either by physical absorption or chemical reaction. ZnDTP-compounds may also be used as anti-wear agents. As anti- corrosion agents according to the present invention preferably calciumsulfonate salts are used, preferably an amount between about 0.5 to about 3% by weight, referred to the total amount of the grease composition.

Traditional friction modifiers such as fatty acid amides and fatty amine phosphates have been used in greases and other lubricants for many years (see, e.g., Klamann, Dieter - "Lubricants", Verlag Chemie GmbH 1983, 1^st edition, chapter 9.6). Their role is to give the lubricant stable but not necessarily low friction over a wide range of operating conditions.

In a further preferred embodiment of the present invention, a grease composition comprises about 55% by weight to about 97,5% by weight of the base oil composition, especially with a kinematic viscosity of between about 32 and about 250mm²/s at 4O⁰C and between about 5 and about 25mm7s at 100⁰C, about 0.3% by weight to about 3% by weight of at least one tri-nuclear molybdenum compound, about 0.1% by weight to about 1.5% by weight of at least one zinc compound additive and about 2% by weight to about 25% by weight of at least one thickener, in each case referred to the total amount of the grease composition. Preferably, an urea thickener may be present in a range between about 5 to about 20% by weight, a lithium soap thickener between 2 to 15% by weight and a calcium complex soap thickener between about 8 to about 25% by weight.

Further, the grease composition according to the present invention has a sliding friction coefficient of not more that 0,1 , as measured with a SRV test.

Best mode for carrying out the invention

In order to determine the effect of the lowering of the friction coefficient as well as the wear by the grease composition according to the invention, SRV tests are carried out using an Optimol Instruments SRV tester. Flat disc lower specimen made of the 100Cr6 standard bearing steel from Optimol Instruments Prϋftechnik GmbH, Westendstrasse 125, Munich, properly cleaned using a solvent are prepared and contacted with the grease composition to be examined. The SRV test is an industry standard test and is especially relevant for the testing of greases for CVJs. The test consists of an upper ball specimen with a diameter of 10 mm made from a 100Cr6 bearing steel reciprocating under load on the flat disc lower specimen indicated above. In tests for mimicking tripod joints a fre- quency of 40 Hz with an applied load of 200 N were applied for 60 minutes (including run- ning-in) at 80⁰C. The stroke was 1.5 mm and 3.0 mm, respectively. The friction coefficients obtained were recorded on computer. For each grease, the reported value is an average of four data at the end of tests in four runs (two runs at 1.5 mm stroke and two runs with 3.0 mm stroke). Wear is measured using a profilometer and a digital planimeter. By using the profilometer, a profile of the cross section in the middle of the worn surfaces can be obtained. The area (S) of this cross section can be measured by using the digital planimeter. The wear quantity is assessed by V=SI, where V is the volume of the wear and I is the stroke. The wear rate (W_r) is obtained from W_r=V/L [μm³/m], where L is the total sliding distance in the tests. For the running-in, it is started with an applied load of 50 N for 1 minute under the above-specified conditions. Afterwards, the applied load is increased for 30 seconds by 50 N up to 200 N.

The following substances are used in the examined grease compositions:

Base oil composition The base oil compositions used have a kinematic viscosity of between about 32 and about 250mm²/s at 40⁰C and between about 5 and about 25mm²/s at about 40⁰C. Two base oil blends are used in this invention. The base oil blend A is a mixture of one or more naphthenic oils in a range between about 10 to about 60% by weight, one or more paraf- finic oils in a range between about 30 to about 80% by weight and one or more poly- alpha-olefins (PAO) in a range between about 5 to about 40% by weight, referred to the total amount of the oil mixture. Oil blend A does not contain an organic synthetic ester, whereas oil blend B contains DOS in a range between about 2 to about 10% by weight referred to a total amount of the oil mixture.

The naphthenic oils are selected with a range of viscosity between about 20 to about 180 mm²/s at 40⁰C, paraffinic oils between about 25 to about 400 mm²/s at 4O⁰C, and PAO between about 6 and about 40 mm²/s at 100⁰C.

Tri-molecular molybdenum compound (TNMoS)

The tri-molecular molybdenum compound used in the grease compositions according to the present invention is a sulphur-containing tri-nuclear molybenium compound obtainable under the trade name C9455B by lnfineum International Ltd., USA. Its structure is defined in US 6,172,013 B1.

Further molybdenum compounds for comparative examples

For comparative examples, a molybdenum dithophosphate (MoDTP) sold under the commercial name RC3580 by Rhein Chemie Rheinau GmbH, Germany, with the chemical formula 2-Ethylhexyl molybdenum dithiophosphate, diluted with mineral oil, is used. Further, a molybdenum dithiocarbamate (MoDTC) sold under the trade name Adeka Sakur- alube 600 (S-600) in the solid state and Sakuralube 515 (S-515) in the liquid state, produced by Asahi Denka Co. Limited, Japan, is used. Further organo molybdenum com- plexes of organic amides (Organo Mo amide), sold under the trade name Molyvan 855 by R. T. Vanderbilt, USA, as well as one organo molybdenum complex of an amine (Organo Mo amine) sold under the trade name Sakuralube 700 (S-700), produced by Asahi Denka Co. Limited, Japan, are used.

Zinc compound additive As zinc compound additives, ZnDTP, sold by lnfineum International Ltd., UK, under the trade name Paranox-15 or sold by Rhein Chemie, Germany, under the trade name RC3038 are used, being a zinc dialkyldithiophosphate with primary and secondary alkyl groups, preferably diluted with mineral oil, is used. Further, ZnDTC sold under the trade name Vanlube AZ by RT. Vanderbilt, USA, as well as ZnO and ZnS are used as zinc compound additives.

Thickener

As a lithium soap (Li soap), a reaction product of a fatty acid, such as stearic or 12- hydroxystearic with lithium hydroxide monohydrate is used. Further, a calcium complex soap (Calcium complex soap) being a reaction product of calcium hydroxide with two car- boxylic acids, one with a short carbon chain length of 2 to 5 carbon atoms and one with a long carbon chain length of 16 to 20 carbon atoms, in which the short to long chain ratio is between 1 :2 and 1 :5 is used.

Additives

As an anti-oxidant agent (Anti-oxidant), a diphenylamine with butyl and/or octyl-groups is used, supplied by Ciba Specialty Chemicals, Switzerland under the trade name L-57 (Ir- ganox L57). As an EP agent, a sulphurised organic compound (fatty acid methylester) sold under the trade name DeoAdd MD10 by DOG Deutsche Oelfabrik, Gesellschaft fϋr chemische Erzeugnisse mbH und Co, Hamburg, Germany ("EP additive" in the examples), is used. Another example of an EP agent is a grease with calcium sulphonate thick- eners, as produced by Brugarolas S.A., Spain, under the trade name Ca-S Grease (Ca-S grease).

As a corrosion inhibitor, a calcium salt of dinonylnaphthalene sulfonate, distributed for example by King Industries Co. Ltd.,Norwalk, CT, U.S.A. under the trade name NaSuI 729 (Ca-sulphonate) is used.

First, the advantages of the grease composition according to the invention were examined by comparing the friction coefficient and wear of the same with other commercial organic molybdenum containing additives (example A). Six different grease compositions were produced, as listed in the following Table 1 : Table 1

The results from the SRV-measurement of the friction coefficient and the wear of examples A1 to A6 may be derived from Fig. 1. Only example A1 is a grease composition in accordance with the present invention, whereas examples A2 to A6 contain other commercial organic molybdenum-containing additives (A2 to A5) or no molybdenum- containing additive (A6). The friction coefficient for example A1 is clearly decreased when compared to the friction coefficient of the comparative examples, and is below 0.09. Further, the wear measured of example A1 is the lowest wear in the test series among examples A1 to A6, and is about 165 μm³/m.

In a further series of tests, further grease compositions in accordance with the present invention were prepared containing different concentrations of the tri-nuclear molybdenum compound containing sulphur (TNMoS), as listed in Table 2.

The results from SRV tests with respect to the friction coefficient and wear will be seen from Fig. 2.

As may be taken from Fig. 2a, surprisingly, the friction coefficient of grease compositions B1 to B3 is clearly about 0.1. Said grease compositions B1 to B3 are not in accordance with the present invention. One may easily see from Fig. 2a that concentrations of the sulphur containing tri-nuclear molybdenum compound of 0.2% by weight or less do not lower the friction coefficient significantly, whereas the grease composition B4 in accordance with the present invention shows a friction coefficient being lowered of at least about 25% when compared to examples B1 to B3. Thus, in accordance with the present invention only amounts of the sulphur containing tri-nuclear molybdenum compound of about 0.25% by weight, referred to the total amount of the grease composition, lead to an advantageously lowered friction coefficient and lower values for the wear, as will be seen from Fig. 2b.

In a third test series, the effect of the addition of a zinc compound additive to the grease composition according to the present invention was examined by preparing grease compositions in accordance with Table 3.

The results from the SRV tests carried out with respect to examples C1 to C6 are shown in Fig. 3.

As will be seen when comparing example C1 having no zinc compound additive with examples C2 to C6, one will see that especially the wear is significantly lowered (not measurable) when adding a zinc compound additive to the grease composition according to the present invention. Further, also the friction coefficients are lowered and do not exceed the value of 0.08. Especially preferred is the addition of ZnDTP (C2 and C3) or the addition of ZnS (C6).

Further, the effect of adding an EP agent to the grease composition according to the present invention is demonstrated by preparing different grease compositions in accordance with Table 4.

The results of the friction coefficient and wear measurements will be seen from Fig. 4.

The friction coefficient is lowered by the addition of the EP agents, especially by the addi- tion of about 0.4% by weight or less of the EP agent "EP additive", being a sulphurised organic compound sold under the trade name DeoAdd MD10. Further, the friction coefficient is also lowered by adding the further sulphur-containing compounds Ca-sulphonate 1 and 2 (examples D6 to D8). Also, the wear is lowered especially in the examples D3, D7 and D8. From example D4 one may derive that the addition of 0.5% by weight EP additive as an EP agent does not lead to a decrease in the value for the friction coefficient, but also gives higher values with respect to the wear measured. Thus, the addition of 0.5% by weight or more of EP additive as an EP agent shall be avoided.

Finally, the effects of using different thickeners in the grease composition in accordance with the present invention is demonstrated by preparing different grease compositions in accordance with Table 5.

Table 5

The results of the SRV tests of the friction coefficient and the wear will be seen from Fig. 5.

One may easily derive from the friction coefficient measurements shown in Fig. 5 the advantageous influence of adding a tri-nuclear molybdenum sulphur-containing compound to a grease composition in accordance with the present invention according to examples E1 and E3, when compared to the comparative examples E2 and E4. Further, the addition of a lithium-soap thickener leads to a decrease of the friction coefficient compared to a grease composition containing a calcium complex soap.

In summary, therefore, the grease composition according to the present invention has an advantageous significant influence on the friction coefficient and wear, leading to lower wear and lower friction in CVJ, and prevents the premature initiation of rolling contact fatigue in the joint.

Claims

1. A grease composition for use in constant velocity joints comprising a) a base oil composition; and b) 0,25 % by weight to 5 % by weight of at least one tri-nuclear molybdenum compound of the formula

Mo₃S_kL_nQ₂,

2. A grease composition according to Claim 1 , further comprising at least one zinc compound additive.

3. A grease composition according to Claim 2, comprising at least one zinc com- pound additive in an amount of between 0,1 % by weight to 2,5 % by weight, referred to the total amount of the composition.

4. A grease composition according to Claim 3 where the zinc compound is selected from the group comprising at least one of zinc dithiophosphates and/or zinc dithio- carbamates, zinc oxide and/or zinc sulfide.

5. A grease composition according to any one of the preceding claims, further comprising a thickener selected from the group comprising lithium soaps, calcium soaps, lithium-complex soaps, calcium-complex soaps and/or urea-derivative type thickener.

6. A grease composition according to any one of the preceding claims, characterised in that the base oil composition comprises poly-α-olefins, naphthenic oils, paraffinic oils and/or synthetic organic esters.

7. A grease composition according to any one of the preceding claims, further comprising an additive package selected from the group of agents comprising anti- oxidation agents, corrosion inhibitors, anti-wear agents, friction modifiers, and/or extreme pressure agents.

8. A grease composition according to any one of the preceding claims, comprising 55 % by weight to 97,5 % by weight of the base oil composition, 0,3 % by weight to

3 % by weight of at least one tri-nuclear molybdenum compound, 0,1 % by weight to 1 ,5 % by weight of at least one zinc compound additive, and between 2 % and 25 % by weight of at least one thickener in each case referred to the total amount of the grease composition.

9. A grease composition according to any one of the preceding claims, characterised in that the sliding friction coefficient of the grease composition is not more than 0,1.