WO2008040383A1

WO2008040383A1 - Grease composition for use in constant velocity joints comprising at least one tri-nuclear molybdenum compound and a urea derivative thickener

Info

Publication number: WO2008040383A1
Application number: PCT/EP2006/009718
Authority: WO
Inventors: E. Jisheng; Shinya Kondo; Frank Reher; Akira Taniguchi
Original assignee: Gkn Driveline International Gmbh; Kyodo Yushi Co., Ltd.
Priority date: 2006-10-07
Filing date: 2006-10-07
Publication date: 2008-04-10
Also published as: FR2907461A1; FR2907461B1; DE102007048091A1; CN101200669A; JP2008150579A; CN101200669B; DE102007048091B4; DE102007048091A8; US20080176776A1

Abstract

In order to provide for a grease composition which has a good compatibility with boots made of rubber or thermoplastic elastomer, and which also gives low wear and low friction, a grease composition for use in constant velocity joints is suggested, comprising a) a base oil composition; b) at least one tri-nuclear molybdenum compound of the formula Mo3SkLnQZ1 wherein L are independently selected ligands 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; c) at least one urea derivative thickener.

Description

Grease Composition for use in Constant Velocity Joints comprising at least one Tri-Nuclear Molybdenum compound and a urea derivative thickener

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

Mo₃S_kL_nQ_z, (I)

wherein L are independently selected ligands 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; c) at least one urea derivative thickener;

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 presence of at least 0.25% by weight of the tri-nuclear molybdenum compound according to claim 1 is preferred and significantly lowers the friction coefficient as well as the wear when used in CVJ.

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 mineral 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 4O⁰C 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)), adipic 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. The naphthenic oils selected for the base oil compositions have preferably a viscosity in a range between about 20 to about 150 mm²/s at 40⁰C, 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 170 mm²/s at 40⁰C. According to the present invention, the grease composition comprises at least one urea derivative thickener. The urea derivative thickener used in accordance with the present invention may also be a urea complex thickener, that is defined as a mixture of at least one urea derivative thickener with at least one other thickener not being a urea derivative thickener. Especially preferred urea complex thickeners in accordance with the present invention are mixtures of at least one urea derivative thickener with at least one calcium and/or lithium-based thickener and/or complex thickener.

As a urea-derivative type thickener in the present invention, especially a urea thickener manufactured by the company Kyodo Yushi Co., Ltd., Tokyo, Japan, is used as defined in US 5,589,444. The urea-derivative thickener is preferably a reaction product of at least one organic aliphatic amine with at least one organic phenyl isocyanate. However, the urea-derivative thickener is not restricted to specific ones and may be, for instance, also a diurea compound and/or a polyurea compound.

Examples of diurea compounds include those obtained from a reaction of a monoamine with a diisocyanate compound. Examples of useful diisocyanates include phenylendiiso- cyanate, dephenyldiisocyanate, phenyldiisocyanate, dephenylmethandiisocyanate, octa- decanediisocyanate, decanediisocyanate, and hexanediisocyante. Examples of useful monoamines include octylamine, dodecylamine, hexadecylamine, octadecylamine, oli- ylamine, aniline, t-toluidine, and cyclohexylamine.

Examples of useful polyurea compounds include those obtained from a reaction of a dia- mine with a diisocyanate compound. Examples of useful diisocyanate include those used for the formation of the diurea compounds as mentioned above, whereas examples of useful diamines include ethylendiamine, propanediamine, butanediamine, hexanediamine, octaπediamiπe, phenylenediamine, tolylenediamine, and xylenediamine. Most preferred examples of urea type derivative thickeners include those obtained through a reaction of arylamine such as aniline or p-toluidine, cyclohexylamine or a mixture thereof with a diisocyanate. The aryl group in the diurea compound has preferably 6 or 7 carbon atoms.

In a preferred embodiment of the present invention the urea derivative thickener is selected from the group comprising urea complex thickeners. Urea complex thickeners are defined as a mixture of at least one urea derivative thickener with any further kind of thickener, especially calcium-based thickeners. Especially preferred as a urea complex thickener in accordance with the present invention is a mixture of a urea derivative thick- ener as defined above with at least one calcium complex thickener and/or calcium thickener (calcium-based thickeners).

In the sense of the present invention, a calcium thickener (soap) is a reaction product of at least one fatty acid with calcium hydroxide. Preferably, the thickener may be a simple calcium soap formed from 12-hydroxy stearic acid or from other similar fatty acids or mixtures thereof or methylesters of such acids. Alternatively, a calcium complex thickener (soap) may be used formed for example from a mixture of long-chained fatty acids together with a mixture of short and/or medium chained carboxylic acids. However, mixtures of all of the aforesaid thickeners may also be used.

The urea derivative thickener may be present in the grease composition claimed in an amount of about 1 % by weight to about 25% by weight, referred to the total amount of the grease composition.

In a further embodiment of the present invention, the grease composition claimed further comprises an additive package comprising zinc dithiophosphates, molybdenum dithiocar- bamates and/or molybdenum dithiophosphates. Preferably, the amount of zinc dithiophosphates, molybdenum dithophosphates and/or molybdenum dithiocarbamates is in a range of between about 0.1 % by weight to about 5% by weight, more preferably about 0.3% by weight to about 2% by weight, in each case referred to the total amount of the grease composition. Most preferably, the weight percent added, referred to the total amount of the grease composition, of each of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is essentially identical. In such an em- bodiment of the present invention, preferably the amount of the zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is about 0.4% by weight, 0.5% by weight, 0.6% by weight, and/or 0.7% by weight, in each case referred to the total amount of the grease composition.

In an preferred embodiment of the present invention, the further molybdenum containing compound is selected from the group comprising molybdenum dithiocarbamates and/or molybdenum dithiophosphates. The at least one molybdenum dithiophosphate (MoDTP) and/or molybdenum dithiocarbamate (MoDTC) is preferably present in the grease composition according to the present invention in an amount in a range between about 0.3% by weight to about 3% by weight, in each case referred to the total amount of the grease composition. However, also any further molybdenum containing compound may be present in the grease composition according to the present invention as component c), of which organic molybdenum compounds are preferred. The grease composition according to the present invention may contain one or more MoDTC and/or MoDTP, and especially mixtures thereof. The MoDTP according to the present invention is of the following general formula:

wherein X or Y represents S or O and each of R¹ to R⁴ inclusive may be the same or different and each represents a primary (straight chain) or secondary (branched chain) alkyl group having between 6 and 30 carbon atoms.

The MoDTC according to the present invention is of the following general formula:

[(R⁵)(R⁶)N-CS-S]₂-Mo₂O_mS_n (III)

wherein R⁵ and R⁶ each independently represents an alkyl group having 1 to 24, preferably 3 to 18 carbon atoms; m ranges from O to 3 and n ranges from 4 to 1, provided that m+n=4.

The grease composition comprises in the additive package at least one zinc compound additive, more preferably a zinc compound additive in an amount of about 0.1 % by weight to about 2.5 %, preferably about 0.5% by weight to about 2.0% by weight, referred to the total amount of the grease composition. Most preferred the zinc compound additive is selected from the group comprising at least one of zinc dithiophosphates (ZnDTP) and/or zinc dithiocarbamates (ZnDTC), and ZnDTPs are most preferred. The zinc dithiophos- phate is preferably selected from the group of zinc dialkyldithiophosphate of the following general formula:

(R⁷O)(R⁸O)SP-S-Zn-S-PS(OR⁹XOR¹⁰) (IV)

wherein each of R⁷ to R¹⁰ 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 R⁷, R⁸, R⁹ and R¹⁰ 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 R¹¹, R¹², R¹³, and R¹⁴ 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 significantly.

According to a further embodiment of the present invention, the grease composition may further comprise 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 40°C 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 about 1% by weight. Such EP agents exhibit excellent effects with respect to the prevention of scuffing of contacting CVJ internal compo- nents. 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 and may lead to degradation of the CVJ boot material.

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 according 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 composition, 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, thus a corrosion inhibitor is required. As a corrosion inhibitor, 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 alkyl- benzene sulphonic acids. Examples of the metal salts include sodium salts, potassium salts, calcium salts, magnesium salts, zinc salts, 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 calcium sulphonate 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 used in the present invention such as fatty acid amides and fatty amine phosphates have been used in greases and other lubricants for many years (see, e.g., the modifiers disclosed in 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 preferred embodiment of the present invention, the grease composition claimed comprises about 50% by weight to about 98.9% by weight of the base oil composition, about 0.1 % by weight to about 5% by weight of at least one tri-nuclear molybdenum compound, about 1% by weight to about 25% by weight of at least one urea derivative thickener. Most preferred, the grease composition according to the present invention comprises about 55% by weight to about 98.1% by weight of the base oil composition, about 0.1% by weight to about 5% by weight, preferably about 0.3% by weight to about 2% by weight, of the tri-nuclear molybdenum compound, about 1% by weight to about 25% by weight of the urea derivative thickener, about 0.5% by weight to about 15% by weight of at least one calcium complex thickener, about 0.1% by weight to about 5% by weight of at least one ZnDTPs, about 0.1% by weight to about 5% by weight of at least one MoDTPs, and about 0.1 % by weight to about 5% by weight of at least one MoDTCs. A urea derivative thickener may be present in a range between about 5 to about 20% by weight. Further, the grease composition according to the present invention has a sliding friction coefficient of not more than 0.08, 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 CVJ. The test consists of an upper ball specimen with a diameter of 10 mm made from 100Cr6 bearing steel reciprocating under load on the flat disc lower specimen indicated above. In tests for mimicking tripod joints a frequency of 40 Hz with an applied load of 200 N were applied for 60 minutes (including running-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 pro- filometer, 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.

Further, the welding load exerted on CVJs with a different grease composition is meas- ured in accordance with a bear 4 ball EP test according to standard IP-239 (Energy Institute, London, UK). The following substances are used in the examined grease compositions:

Base oil composition (oil blend)

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 100⁰C. 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 mnfVs at 40°C, 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 molybdenum compound obtainable under the trade name C9455B by lnfineum International Ltd., UK. Its structure is defined in US 6,172,013 B1.

Further molybdenum compounds

A molybdenum dithophosphate (MoDTP) sold under the commercial name Sakuralube 300 (S-300) by Asahi Denka Co. Ltd., Japan, with the chemical formula 2-Ethylhexyl mo- lybdenum dithiophosphate, diluted with mineral oil, is used. Further, a molybdenum di- thiocarbamate (MoDTC) sold under the trade name Sakuralube 600 (S-600) in the solid state, produced by Asahi Denka Co. Limited, Japan, is used. Zinc compound additive

As zinc compound additives, ZnDTP, sold by lnfineum International Ltd., UK under the trade name Paranox-15, is used, being a zinc dialkyldithiophosphate with primary and secondary alkyl groups, preferably diluted with mineral oil.

Thickener

The urea thickener manufactured by the company Kyodo Yushi Co., Ltd., Tokyo, Japan, is used as defined in US 5,589,444 (hereinafter referred to as Thickener).

Further, a calcium complex thickener (Calcium complex thickener) being a reaction product of calcium hydroxide with two carboxylic 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. Examples having mixtures containing a urea thickener as well as a calcium complex thickener, thus, comprising a urea complex thickener in accordance with the definition in the present invention.

First, the advantages of the grease composition according to the present invention were examined by measuring the friction coefficient and the welding load. Six different grease compositions were produced, as listed in Table 1 :

Table 1

The results from the SRV-measurements of the friction coefficient as well as the welding load measurements of examples A1 to A6 may be derived from Fig. 1. Example A2 does not contain any calcium complex thickener and/or calcium thickener, and, thus, does not comprise an urea complex thickener, whereas the other examples comprise an urea complex thickener. Further, the amounts of the additive package as well as the composition of the same is amended in examples A1 to A6. The friction coefficient of example A1 is below 0.06, and is the lowest friction coefficient measured in said test series. The friction coefficient of example A2 is above 0.08, and is the highest friction coefficient measured. Further, also the friction coefficients of examples A4 and A5 are slightly higher than the friction coefficients of examples A1 , A3 and A6. One may derive from the friction coefficient measurements that the addition of an additive package containing at least one ZnDTP, at least one MoDTP, and at least MoDTC gives the lowest values for the friction coefficient. Further, the addition of at least one ZnDTP as well as at least one MoDTP, preferably in combination with each other (see example A6), is preferred.

From the measurements of the welding load in Fig. 1(b) one may derive that the welding load of example A1 as well as example A5 is higher than the welding load measured for the other examples. Thus, the grease composition according to example A1 shows the best values not only for the friction coefficient, but also with respect to the welding load, and, thus, exhibits a good extreme pressure performance. In a further series of tests, the amount of the TNMoS as well as the additive package composition is amended. Three grease compositions were prepared in accordance with Table 2.

Table 2

In all of the examples B1 to B3, the amount of the thickener remains unamended, whereas the amount of the TNMoS compound as well as the components of the additive package were amended to 0.1% by weight, 0.5% by weight and 1.0% by weight, respectively, in each case referred to the total amount of the grease composition. The results from the SRV measurements with respect to the friction coefficient as well as the welding load may be derived from Fig. 2.

Example B1 (=A1) shows the lowest friction coefficient and highest welding load, and, thus, exhibits a very good extreme pressure performance when compared to examples B2 and B3. Further, the lowering of the amount of the TNMoS compound as well as the components of the additive package at values around 0.1 % by weight clearly results in an increase of the friction coefficient and a decrease in the welding load. Thus, at least about 0.25% by weight of the TNMoS compound as well as at lest one of ZnDTPs, MoDTPs and MoDTCs should preferably be present in the grease composition.

In a third test series, the effect of the addition of a calcium complex thickener added to four grease compositions C1 to C4 in accordance with Table 3 is studied. Table 3

Example C2 is identical to example A2. One may derive from the SRV measurements of the friction coefficient as well as the measurement of the welding load (see Fig. 3) that the addition of 3% by weight calcium complex thickener resulted in the lowest friction coefficient values and a welding load above 3000 N. The welding load is especially increased by adding 15% by weight calcium complex thickener in accordance with example C4, however, also the friction coefficient is increased to values about 0.08. This third test se- ries indicates that the amount of calcium complex thickener used in the grease composition may be in a range of about 0.5% by weight to about 20% by weight, preferably to about 15% by weight, thus forming a urea complex thickener with the Thickener.

In summary, the grease composition according to the present invention has an advanta- geous significant influence on the friction coefficient and wear, leading to a good extreme pressure performance as well as a good NVH performance in CVJs.

Claims

1. A grease composition for use in constant velocity joints comprising a) a base oil composition; b) at least one tri-nuclear molybdenum compound of the formula

Mo₃SkL_nQz,

wherein L are independently selected ligands having organo groups with a sufficient number of carbon atoms to render the compound soluble or dis- persible 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; c) at least one urea derivative thickener.

2. A grease composition according to Claim 1 , characterised in that the urea derivative thickener is selected from the group comprising di-urea and/or polyurea compounds and mixtures of said compounds with calcium-based thickeners.

3. A grease composition according to Claim 1 , further comprising an additive package comprising zinc dithiophosphates, molybdenum dithiocarbamate and/or molybdenum dithiophosphates.

4. A grease composition according to any one of the preceding claims, characterised in that the amount of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is in a range of between 0,1% by weight to 5% by weight, referred to the total amount of the grease composition.

5. A grease composition according to Claim 4, characterised in that the weight percent added, referred to the total amount of the grease composition, of each of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is essentially identical.

6. A grease composition according to any one of the preceding claims, comprising 50% by weight to 98,9% by weight of the base oil composition, 0,1% by weight to 5% by weight of at least one tri-nuclear molybdenum compound, 1% by weight to 25% by weight of at least one urea derivative thickener, in each case referred to the total amount of the grease composition.

7. A grease composition according to any one of the preceding claims, characterised in that the weight percent added, referred to the total amount of the grease composition, of tri-nuclear molybdenum compounds is essentially identical with the weight percent of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates added.

8. A grease composition according to any one of the preceding claims, characterised in that the sliding friction coefficient is at most 0,08.