[0001] The present invention relates to the use of friction modifiers to reduce micropitting
of metal surfaces such as gear teeth, and to lubricant compositions comprising friction
modifiers.
[0002] Micropitting is a type of surface damage which occurs predominantly in rolling-sliding
contacts of hard steel surfaces. Sometimes called "frosting", "greystaining" or "peeling"
it typically occurs in rolling element bearings and most often on gear teeth, where
it poses a significant practical problem. Micropitting may lead to higher noise, to
significant rapid wear and to more serious surface damage, such as scuffing and even
to tooth fracture in gears. Conventional lubricants are used to reduce friction when
metal surfaces move in contact with each other but they do not prevent the occurrence
of micropitting. Original equipment manufacturers require lubricants which can lead
to a reduction in the amount of micropitting when compared with the conventional lubricants.
It is an object of the present invention to meet this need.
[0003] The awareness of micropitting within the lubricant additives industry has increased
significantly. A micropitting test has been established by the FZG Institute in Germany
and is called the FZG micropitting test. This test is run on gears sets with the same
metallurgy and surface profile/roughness as gears used in the field. The conditions
of the test (high load/low speed) are the optimum conditions for micropitting to occur.
Equipment manufacturers believe that the FZG micropitting test correlates well with
field experience.
[0004] The FZG micropitting test is carried out using a standardized FZG test rig according
to CEC L-07-A-71, with C type case hardened gears of minimum 0.4 Ra surface roughness.
The test has a stepwise phase to investigate build up of micropitting and an endurance
phase to investigate resistance to micropitting. The stepwise phase runs from load
stage 5 to load stage 10, each stage lasting 16 hours. The profile of the gears is
measured prior to testing and during the test. The variation from the original gear
profile (the profile deviation) is calculated. Also evaluated are the percentage micropitting
(the percentage of gear tooth which is micropitted) and the weight loss from the gears.
After the stepwise phase the endurance phase is run for 80 hours at load stage 8 and
then at load stage 10 until failure. Again, the deviation from the original profile
(maximum 20 microns), the level of micropitting and the weight loss are measured.
A result which would be particularly acceptable to the industry would be a pass at
load stage 10 in the stepwise phase of the test. This corresponds to a profile deviation
of less than 7.5 µm, micropitting of less than 15% (approx) and weight loss of less
than 15 mg (approx) after load stage 10. Extended performance in the endurance phase
is also desirable.
[0005] The present invention is based on the surprising appreciation that certain friction
modifiers may be included in lubricant compositions with the result that an improvement
in micropitting performance is observed when the lubricant compositions are used,
i.e. there is reduced micropitting. Accordingly, the present invention concerns the
use of at least one friction modifier to reduce micropitting of a metal surface, which
comprises lubricating the metal surface with a lubricant composition comprising the
at least one friction modifier, wherein the at least one friction modifier is selected
such that micropitting is reduced when the metal surface is so lubricated.
[0006] The metal surface may be the surface of a gear tooth, in which case the at least
one friction modifier may be added to a formulated gear lubricant composition.
[0007] In the present specification the term "friction modifier" is used to describe additive
compounds which are conventionally used in lubricant compositions to reduce friction.
The friction modifiers which are useful in practising the present invention are all
known in the art.
[0008] In accordance with the present invention it has been found that only certain friction
modifiers may be used to give the desired technical effect of reduced micropitting.
The efficacy of any given friction modifier in reducing micropitting may be assessed
by comparing the amount of micropitting observed when a metal surface is lubricated
with a lubricant composition comprising the friction modifier with the amount of micropitting
observed when an identical metal surface is lubricated (under the same conditions)
using the corresponding lubricant composition from which the friction modifier of
interest has been omitted. The FZG micropitting test may be used to assess the relative
performance of lubricant compositions.
[0009] Another way of identifying suitable friction modifiers is by reference to the friction
coefficient of lubricants including them. It has been found that the at least one
friction modifier may be selected such that, when measured at 130°C using a high frequency
reciprocating rig (HFRR) under the conditions described in SAE Technical Paper 961142,
a lubricant which comprises the friction modifier and which has a viscosity grade
of ISO 220 has a coefficient of friction of 0.100 or less. The HFRR test may thus
be employed as a screen for useful friction modifiers. Lubricant compositions which
have a viscosity grade of ISO 220 and which are useful in screening friction modifiers
may be prepared by blending a conventional sulphur-and phosphorus-containing gear
additive package with a base oil having a viscosity of between 1.98 x 10
-4 to 2.42 x 10
-4 m
2/s (198 to 242 cSt) at 40°C. Suitable additive packages include those comprising from
15-75 wt%, preferably from 45-65 wt%, of a sulfurized isobutylene, from 0-25 wt%,
preferably from 3-15 wt%, of a phosphorus-containing antiwear agent, from 0-60 wt%,
preferably from 5-25 wt% of a carboxylic-type or Mannich-type ashless dispersant,
from 0-20 wt%, preferably from 1-10 wt% of corrosion and rust inhibitors, from 0-20
wt%, preferably from 1-10 wt%, of surface active agents and diluent oil. Such additive
packages are commercially available. The additive package is used at conventional
treat rates. A suitable base oil to use in formulating the compositions includes a
blend of 51 wt% ESSO 600SN and 49 wt% of 2500 Brightstock. Useful additive package
are described in EP-A-0744456 and EP-A-0812901.
[0010] A number of different classes of friction modifiers have been found to be useful
in the present invention. Mention may be made of phosphonate esters, phosphite esters,
aliphatic succinimides, molybdenum compounds and acid amides.
[0011] Useful phosphonate esters include O,O-di-(primary alkyl) acyclic hydrocarbyl phosphonates
in which the primary alkyl groups are the same or different each independently containing
1 to 4 carbon atoms and in which the acyclic hydrocarbyl group bonded to the phosphorus
atom contains 12 to 24 carbon atoms and is a linear hydrocarbyl group free of acetylenic
unsaturation. These compounds thus comprise O,O-dimethyl hydrocarbyl phosphonates,
O,O-diethyl hydrocarbyl phosphonates, O,O-dipropyl hydrocarbyl phosphonates, O,O-dibutyl
hydrocarbyl phosphonates, O,O-diiso-butyl hydrocarbyl phosphonates, and analogous
compounds in which the two alkyl groups differ, such as, for example, O-ethyl-O-methyl
hydrocarbyl phosphonates, O-butyl-O-propyl hydrocarbyl phosphonates, and O-butyl-O-isobutyl
hydrocarbyl phosphonates, wherein in each case the hydrocarbyl group is linear and
is saturated or contains one or more olefinic double bonds, each double bond preferably
being an internal double bond. Preferred are compounds in which both O,O-alkyl groups
are identical to each other. Also preferred are compounds in which the hydrocarbyl
group bonded to the phosphorus atom contains 16 to 20 carbon atoms. A preferred friction
modifier in this class is dimethyloctadecyl phosphonate. Phosphonate esters useful
in the present invention are described in USP 4,158,633.
[0012] Useful phosphite esters are described in WO88/04313. These include dihydrocarbyl
hydrogen phosphites in which the hydrocarbyl groups are the same or different linear
aliphatic hydrocarbyl groups free of acetylenic unsaturation each independently containing
8 to 24 carbon atoms, and amine salts of these phosphites. The phosphites typically
contain linear aliphatic hydrocarbyl groups, each of which contains 12 to 24, preferably
16 to 20 carbon atoms. It is also preferred that at least 50% of the hydrocarbyl groups
in the dihydrocarbyl hydrogen phosphite contain at least one internal double bond.
It is preferred to use dioleylphosphite.
[0013] Preferred amine salts of the foregoing dihydrocarbyl hydrogen phosphites are those
in which the aliphatic group of the amine is a linear primary aliphatic group having
8 to 24 carbon atoms, for example 16 to 20 carbon atoms, and in which at least 50%
of the aliphatic groups contain one or more internal double bonds.
[0014] Useful succinimides include those of formula:

in which Z is a group R
1R
2CH- in which R
1 and R
2 are the same or different each independently representing straight- or branched-chain
hydrocarbon groups containing from 1 to 34 carbon atoms and the total number of carbon
atoms in the groups R
1 and R
2 is from 11 to 35. Such compounds are described in EP-A-0020037, EP-A-0389237 and
EP-A-0776964.
[0015] The radical Z may be, for example, 1-methylpentadecyl, 1-propyltridecenyl, 1-pentyltridecenyl,
1-tridecylpentadecenyl or 1-tetradecyleicosenyl. Preferably the number of carbon atoms
in the groups R
1 and R
2 is from 16 to 28 and more commonly 18 to 24. It is especially preferred that the
total number of carbon atoms in R
1 and R
2 is about 20 or about 22. Preferably, the succinimide is a 3 - C
18-24 alkenyl-2,5-pyrrolidindione. A sample of this succinimide contains a mixture of alkenyl
groups having from 18 to 24 carbon atoms.
[0016] Useful molybdenum compounds are described in USP 5,650,381. These compounds are typically
substantially free of active sulphur. Examples of suitable compounds include glycol
molybdate complexes as described in U.S.P. 3,285,942, overbased alkali metal and alkaline
earth metal sulfonates, phenates and salicylate compositions containing molybdenum
such as those disclosed in U.S.P. 4,832,857, molybdenum complexes prepared by reacting
a fatty oil, a diethanolamine and a molybdenum source as described in U.S.P. 4,889,647,
molybdenum containing compounds prepared from fatty acids and 2-(2-aminoethyl)aminoethanol
as described in U.S.P. 5,137,647, overbased molybdenum complexes prepared from amines,
diamines, alkoxylated amines, glycols and polyols as described in U.S.P. 5,143,633,
and 2,4-heteroatom substituted-molybdena-3,3-dioxacycloalkanes as described in U.S.P.
5,412,130.
[0017] Molybdenum salts such as the carboxylates are a preferred group of molybdenum compounds.
The molybdenum salts used in this invention may be completely dehydrated (complete
removal of water during preparation), or partially dehydrated. They may be salts of
the same anion or mixed salts, meaning that they are formed from more than one type
of acid. Illustrative of suitable anions there can be mentioned chloride, carboxylate,
nitrate, sulfonate, or any other anion.
[0018] The molybdenum carboxylate is preferably that of a monocarboxylic acid such as those
having from about 4 to 30 carbon atoms. Such acids can be hydrocarbon aliphatic, alicyclic
or aromatic carboxylic acids. Monocarboxylic aliphatic acids having about 4 to 18
carbon atoms are preferred, particularly those having an alkyl group of about 6 to
18 carbon atoms. The alicyclic acids may generally contain from 4 to 12 carbon atoms.
The aromatic acids generally contain one or two fused rings and contain from 7 to
14 carbon atoms wherein the carboxyl group may or may not be attached directly to
the ring. The carboxylic acid can be a saturated or unsaturated fatty acid having
from about 4 to 18 carbon atoms. Examples of carboxylic acids that may be used to
prepare the molybdenum carboxylates include butyric acid, valeric acid, caproic acid,
heptanoic acid, cyclohexanecarboxylic acid, cyclodecanoic acid, naphthenic acid, phenyl
acetic acid, 2-methylhexanoic acid, 2-ethylhexanoic acid, suberic acid, octanoic acid,
nonanoic acid, decanoic acid, undecanoic acid, lauric acid, tridecanoic acid, myristic
acid, pentadecanoic acid, palmitic acid, linolenic acid, heptadecanoic acid, stearic
acid, oleic acid, nonadecanoic acid, eicosanoic acid, heneicosanoic acid, docosanoic
acid and erucic acid. The preferred molybdenum carboxylate is molybdenum octanoate.
[0019] Useful carboxylic acid amides include aliphatic monocarboxylic acid amides. These
may be represented by the formula (R
3CO)N(R
4)(R
5) in which R
3 represents an alkyl or alkenyl group having 8 to 24 carbon atoms and R
4 and R
5 which may be the same or different are each independently hydrogen or alkyl of up
to 7 carbon atoms. Typically, R
3 represents a C
14-18 alkyl radical. Amides of this type are described in USP 4,280,916. A preferred friction
modifier falling within this class is oleyamide.
[0020] Friction modifiers which are useful in the present invention are commercially available
or may be prepared by the adaptation or application of known methods.
[0021] The amount of the at least one friction modifier which is used is at least sufficient
for it to exert its intended function of reducing micropitting. The friction modifier(s)
is/are generally used at conventional treat rates. Typically, the total concentration
of friction modifier used is 0.125 to 1% by weight based on the total weight of the
lubricant composition. Preferably, the total amount of friction modifier is 0.15 to
0.75% by weight, more preferably about 0.5% by weight.
[0022] Mixtures of friction modifiers may be used. In this case friction modifiers of the
same or different type may be used in combination. For example, satisfactory results
have been obtained using combinations of dimethyloctadecyl phosphonate and a 3-C
18-24 alkenyl-2,5-pyrrolidindione. When mixtures of friction modifiers are employed the
total amount of friction modifier is as described above.
[0023] It is important that the at least one friction modifier employed is sufficiently
soluble in the lubricant composition at the treat rate at which it is used. It is
also important that the at least one friction modifier is sufficiently compatible
with the additional components commonly found in lubricating compositions. Such components
include dispersants, detergents, antioxidants, extreme pressure agents, antiwear agents,
foam inhibitors, viscosity index improvers and pour point depressants. These additives
are themselves used in conventional amounts.
[0024] The base oil which is used to formulate lubricant compositions useful in the present
invention may be natural or synthetic, or a blend thereof. Useful base oils are known
in the art. The lubricant compositions are formulated in known manner by blending
the individual components. The at least one friction modifier responsible for improving
the micropitting performance may be added at the time the lubricant is formulated.
Alternatively, the at least one friction modifier may be added as a top treat to improve
or boost the micropitting performance of an existing formulated lubricant composition.
[0025] The invention also provides lubricant compositions which exhibit excellent micropitting
performance relative to conventional lubricants. In one embodiment the invention provides
a lubricant composition comprising an O,O-di-(primary alkyl)acyclic hydrocarbyl phosphonate
as described above and a succinimide as described above. Preferably, the composition
comprises dimethyloctadecyl phosphonate and a 3-C
18-24 alkenyl-2-pyrrolidindione. In another embodiment the composition comprises a molybdenum
carboxylate, such as molybdenum octanoate, and a sulfurized isobutylene extreme pressure
agent. The compositions may also include one or more of the other additive components
described above.
[0026] The invention is illustrated in the following examples.
EXAMPLES 1-7
[0027] Lubricant compositions were prepared by blending the components listed in Table 1
below. The sulphur- and phosphorus-containing gear additive package had the following
composition:
- 50 wt%
- sulfurized isobutylene (extreme pressure agent)
- 8 wt%
- mixed phosphite and phosphate anti-wear agent
- 17.5 wt%
- phosphorylated, boronated succinimide dispersant
- 9 wt%
- rust inhibitor package
- 2.6 wt%
- corrosion inhibitor
- 0.5 wt%
- defoamer
- 0.15%
- demulsifier
- 1.5 wt%
- 3-C18-24 alkenyl-2,5-pyrrolidindione
- balance
- base (diluent) oil
The base oil was ESSO ISO 220. The viscosity grade of each composition was ISO 220.
TABLE 1
COMPONENT |
RUN |
|
1 |
2 |
3 |
4 |
5 |
6 |
7 |
Molybdenum octanoate |
- |
- |
- |
0.50% |
|
|
|
Dimethyloctadecyl phosphonate |
- |
- |
- |
- |
0.50% |
0.25% |
0.50% |
Dioleylphosphite |
- |
- |
- |
- |
- |
- |
- |
3-C18-24 alkenyl 2,5-pyrollidindione |
- |
- |
0.25% |
- |
- |
0.25% |
0.50% |
Oleyamide |
- |
- |
0.20% |
- |
- |
- |
- |
S/P Containing gear pack |
2.5% |
2.0% |
2.0% |
2.0% |
2.0% |
2.0% |
2.0% |
[0028] The coefficient of friction for each composition was measured at 130°C using an HFRR
operated under the conditions described in SAE Technical Paper 961142 (ball diameter
6mm, load 4N, frequency 20 Hz, stroke length 1mm; ball and flat ANSI 52100 steel).
The HFRR coefficient of friction for each composition is given in Table 2. Each composition
was also subjected to the FZG micropitting test in accordance with CEC L-07-A-71.
The results obtained in this test are also shown in Table 2.
[0029] The solubility/compatibility of the friction modifier(s) within the lubricant compositions
tested the appearance of the compositions was assessed visually. The presence of precipitate
indicates poor solubility/compatibility. Table 2 reports the extent of the solubility/compatibility.
[0030] The percentage micropitting and weight loss were assessed after load stage 10. The
weight loss was determined by comparing the initial weight of the gears under test
with the weight of the gears after load stage 10. The results are also shown in Table
2.
TABLE 2
RUN |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
HFRR |
0.110 |
0.109 |
0.102 |
0.087 |
0.094 |
0.098 |
0.091 |
FZG |
10 FAIL |
9 FAIL |
ABORTED |
10 PASS |
10 PASS |
10 PASS |
10 PASS |
SOLUBILITY/COMPATIBILITY |
GOOD |
GOOD |
BAD |
SATISFACTORY |
SATISFACTORY |
SATISFACTORY |
SATISFACTORY |
%MICROPITTING* |
- |
18.0 |
- |
12.0 |
8.62 |
8.43 |
9.40 |
WEIGHT LOSS* (mg) |
- |
22 |
- |
9 |
10 |
11 |
15.0 |
[0031] In this table the FZG result is given as a load stage result (in the stepwise phase).
A profile deviation of 7.5 µm is used to differentiate a pass or fail result at any
given load stage. For example, Runs 1 and 2 give "10 fail" and "9 fail" results respectively
which means that the profile deviation exceeded 7.5 µm after load stage 10 (Run 1)
and load stage 9 (Run 2). Runs 4-7 on the other hand give an FZG result of "10 pass"
which means that the profile deviation has not exceeded 7.5 µm after load stage 10.
[0032] The results in Table 2 show that the friction coefficient obtained in the HFRR test
may be used to predict which friction modifier(s) is/are useful in improving micropitting
performance. HFRR results of less than 0.100 are predictive of friction modifiers
which give improved micropitting performance.
[0033] The lubricant composition used in Runs 1 and 2 is a conventional gear lubricant.
This gives reasonable micropitting protection, the 7.5 µm threshold being exceeded
after load stage 10 (Run 1) or 9 (Run 2). For Run 3 the micropitting test was aborted
after load stage 6 because the composition tested was found to contain precipitate.
This emphasises the need for the friction modifiers used to be fully soluble/compatible
in lubricant at the treat rate at which they are used. The compositions used in Runs
4-7 illustrate the present invention and give improved FZG results of "10 pass" when
compared with the conventional lubricant compositions of Runs 1 and 2. A consistent
"10 pass" result would be very acceptable in the industry. Runs 4-7 also showed acceptably
low levels of percentage micropitting and weight loss.
Example 8
[0034] To confirm the accuracy of the friction modifier screening procedure glycerol monooleate,
a friction modifier which is known to exhibit poor micropitting protection, was included
in a lubricant composition having a viscosity grade ISO 220 and the resulting composition
tested using the HFRR test (in accordance with SAE Technical Paper 961142). The composition
gave an HFRR result of 0.114, i.e. well above the threshold value of 0.100.
Examples 9 and 10
[0035] The HFRR screening procedure was repeated using compositions using a different sulphur-
and phosphorus-containing gear additive package. The base oil was ESSO ISO 220. The
viscosity grade of the formulated compositions was ISO 220. The treat rate of the
various components and the HFRR results obtained are shown in Table 3 below.
TABLE 3
COMPONENT |
RUN |
|
9 |
10 |
Dimethyloctadecyl phosphonate |
- |
0.25% |
3-C18-24 alkenyl 2,5-pyrollidindione |
- |
0.25% |
S/P Containing gear pack |
2.0% |
2.0% |
HFRR |
0.105 |
0.070 |
[0036] The HFRR result for Run 9 of in excess of 0.100 is consistent with the HFRR results
for Runs 1 and 2 in Table 1. The composition used in Run 10 included a combination
of friction modifiers which are known to give improved micropitting performance (see
the result for Run 6 in Table 2). The HFRR result for Run 10 is less than 0.100. This
is consistent with the HFRR result obtained for Run 6 in Table 2 where a different
gear additive package was used in formulating the composition under test. This shows
that the HFRR screening procedure remains predictive of useful friction modifiers
even when different gear additive packages are used in formulating the lubricant compositions.
1. Use of at least one friction modifier to reduce micropitting of a metal surface, which
comprises lubricating the metal surface with a lubricant composition comprising the
at least one friction modifier, wherein the at least one friction modifier is selected
such that micropitting is reduced when the metal surface is so lubricated.
2. Use according to claim 1, wherein the lubricant composition is a gear lubricant composition.
3. Use according to claim 1 or claim 2, wherein the at least one friction modifier is
selected such that, when measured at 130°C using a high frequency reciprocating rig
(HFRR) under the conditions described in SAE Technical Paper 961142, a lubricant which
comprises the at least one friction modifier and which has a viscosity grade of ISO
220 has a coefficient of friction of 0.100 or less.
4. Use according to any one of claims 1 to 3, wherein the friction modifier is an O,O-di-(primary
alkyl) acyclic hydrocarbyl phosphonate in which the primary alkyl groups are the same
or different each independently containing 1 to 4 carbon atoms and in which the acyclic
hydrocarbyl group bonded to the phosphorus atom contains 12 to 24 carbon atoms and
is a linear hydrocarbyl group free of acetylenic unsaturation.
5. Use according to claim 4, wherein the friction modifier is dimethyloctadecyl phosphonate.
6. Use according to any one of claims 1 to 3, wherein the friction modifier is a dihydrocarbyl
hydrogen phosphite in which the hydrocarbyl groups are the same or different linear
aliphatic hydrocarbyl groups free of acetylenic unsaturation each independently containing
8 to 24 carbon atoms, or an amine salt thereof.
7. Use according to claim 6, wherein the friction modifier is dioleylphosphite.
8. Use according to any one of claims 1 to 3, wherein the friction modifier is a succinimide
of formula:

in which Z is a group R
1R
2CH- in which R
1 and R
2 are the same or different each independently representing straight- or branched-chain
hydrocarbon groups containing from 1 to 34 carbon atoms and the total number of carbon
atoms in the groups R
1 and R
2 is from 11 to 35.
9. Use according to claim 8, wherein the friction modifier is a 3 - C18-24 alkenyl-2,5-pyrrolidindione.
10. Use according to any one of claims 1 to 3, wherein the friction modifer is a molybdenum
compound.
11. Use according to claim 10, wherein the friction modifier is molybdenum carboxylate.
12. Use according to claim 11, wherein the friction modifier is molybdenum octanoate.
13. Use according to any one of claims 1 to 3, wherein the friction modifier is an aliphatic
monocarboxylic acid amide of formula (R3CO)N(R4)(R5) in which R3 represents an alkyl or alkenyl group having 8 to 24 carbon atoms and R4 and R5 which bay be the same or different are each independently hydrogen or alkyl of up
to 7 carbon atoms.
14. Use according to claim 13, wherein the friction modifier is oleyamide.
15. Use according to any one of claims 1 to 14, wherein the at least one friction modifier
is used in an amount of 0.125 to 1.0% by weight based on the total weight of the lubricant
composition.
16. A lubricant composition comprising an O,O-di-(primary alkyl)acyclic hydrocarbyl phosphonate
as defined in claim 4 and a succinimide as defined in claim 8.
17. A lubricant composition according to claim 16 comprising dimethyloctadecyl phosphonate
and 3-C18-24 alkenyl-2-pyrrolidindione.
18. A lubricant composition comprising a molybdenum carboxylate and a sulfurized isobutylene
extreme pressure agent.