# Tribochemistry and EP Activity Assessment of  Mo-S Complexes in Lithium-Base Greases

Authors: Tarunendr Singh
Journal: Advances in Tribology
 (2008)

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## Abstract

The blends of bis(1,5-diaryl-2,4-dithiomalonamido)dioxomolybdenum(VI) complexes in lithium-base grease are evaluated for their extreme pressure activity in a “four-ball test” using 12.7 mm diameter alloy steel ball specimen. The additive, bis(1,5-di-p-methoxyphenyl-2,4-dithiomalonamido)dioxomolybdenum(VI) and bis(1,5-di-p-chloro-phenyl-2,4-dithiomalonamido)dioxomolybdenum(VI) exhibited lower values of wear-scar diameter at higher load and higher values of weld load, flash temperature parameter, and pressure wear index as compared with lithium-base grease without additives. The greases fortified with the developed additives prevent rusting and corrosion of bearings while grease containing no additives did not pass these tests as per the standard tests. These greases have also better oxidation protection as compared to the grease that has no additive. The topography and tribochemistry of the wear-scar surface are carried out by means of scanning electron microscopy and Auger electron spectroscopy techniques, respectively.


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## Body

## 1. Introduction

Lubricants are employed in
bearings to decrease wear and destructive heating and to increase mechanical
efficiency. Under extreme pressure, the boundary film of lubricant may fail
leading to deterioration of surface properties causing adhesion, seizure, and
excess wear. Using an appropriate extreme pressure, lubricant additive may
minimize the friction and damage, which form a chemical film with the material
of the surface. Certain lubricants containing EP additives find extensive
applications in hypoid gears and in metal cutting and forming operations. The
use of molybdenum disulphide as an excellent solid lubricant is well known,
however, its insolubility in oil prevents its use in liquid lubricants.
Oil-soluble organomolybdenum compounds, for example, molybdenum
dialkyldithiophosphate (MoDTP) and molybdenum dialkyldithiocarbamate (MoDTC),
have been used as excellent antifriction, antiwear, and extreme pressure
additives in lubricating oils and greases. These additives that are known to
increase the load-carrying capacity of lubricants and reduce fuel consumption
and power loss by reducing friction lubricants containing oil-soluble
organometallic compounds, sometimes referred as third-generation lubricants,
have been known to show outstanding advantages over conventional lubricants in
enhancing the component life, reducing operating temperatures and greatly
extending lubrication intervals [1–8].It is believed that the
molybdenum and sulphur present in the additive form a low-friction surface film
during operation of a machine under high loads. In view of these observations
and our interest in searching for better extreme pressure additives, we are
reporting, in this paper, few potential molybdenum-sulphur
complexes as potential extreme pressure additives for lithium-base lubricating
grease and their tribochemistry by AES and topography by SEM techniques.Lithium-base greases are widely
used in industrial machineries and automobiles [9]. These greases are
formulated with various compounds with a view to optimizing the end use. In
steel plant applications, lithium-12-hydroxy-stearate greases, formulated with
extreme pressure additives, have performed well and they vary from sinter plant to coke oven.
These greases have shown better performance, for example, high drop point,
better antiwear and extreme pressure properties, better oxidation stability,
pumpability as well as better wheel bearing performance as compared to
conventional calcium base grease. The use of certain S-P and Pb-S systems in the lithium-base
grease has been on record [10–13].


## 2. Experimental

### 2.1. Lithium Grease

Lithium-base grease (Grease A)
was prepared in situ by reacting 12-hydroxystearic with lithium hydroxide
monohydrate in presence of paraffinic mineral oil (see Table1).Table 1
TestResultsMethods(1)Nature of the base
oilParaffinic mineral
base oil—(2)Base oil viscosityASTM D 445at 40°C100.70at 100°C10.94(3)Viscosity index92ASTM D 2270(4)Flash point (COC),°C234ASTM D 92(5)Pour point,°C0ASTM D 97(6)Aniline point,°C104ASTM D 611


### 2.2. Base Oil

Typical characteristics of the
base oil used in the prepared lithium-base grease are given in Table1.


### 2.3. Additives

The Mo-S complex additives were prepared [14, 15] by reacting an ethanolic solution of an appropriate
1,5-di-aryl-2,4-dithiomalonamide (2 mol) with an acidic solution of ammonium
molybdate (1 mol) and digesting the reaction mixture for 15 minutes on a water bath.
(1)2Ar–NH–C||S–CH2–C||S–NH–Ar→EthanolAmmonium Molybdate;HMoO2(Ar–NH–C||S–CH2–C||S–NH–Ar)21,5-di-aryl-2,4-dithiomalonamideBis(1,5-diaryl-2,4-dithiomalonamido)dioxomolybdenum(VI)
Whereas, Ar=p-Methoxyphenyl-
and p-Chlorophenyl- groups.


### 2.4. Preparation of Bis(1,5-diaryl-2,4-dithiomalonamido) dioxomolybdenum (VI)

The following two Mo-S complexes
were prepared and used as extreme pressure additives in the lithium-base
grease.Bis(1,5-di-p-methoxyphenyl-2,4-dithiomalonamido) dioxomolybdenum(VI)
Molecular Formula: MoO2C34H36N4S4O4. Melting Point: >200°C (Dec.) MoO2(CH3O–H4C6–NH–C||S–CH2–C||S–NH–C6H4–OCH3)2.Bis(1,5- di-p-chloro-phenyl-2,4-dithiomalonamido) dioxomolybdenum (VI)
Molecular Formula: MoO2C30H24N4S4Cl4. Melting Point: >200°C (Dec.) MoO2(Cl–H4C6–NH–C||S–CH2–C||S–NH–C6H4–Cl)2.


### 2.5. Grease Additive Admixture

The optimized dosages of the above additives
were blended at a temperature of 80°C with the prepared Grease 1
(without additive) and with a view to achieve the following two main properties for the extreme
pressure greases.(a)
Timken OK load of 60 lbs minimum.(b)
Four-ball weld load of 315 kgf minimum.The following greases were used in this study.Grease A: Lithium-base grease without additive.Grease B: Grease A  +  bis(1,5-di-p-methoxyphenyl-2,4-dithiomalonamido)-dioxomolybdenum(VI).Grease C: Grease A  +  bis(1,5-di-p-chlorophenyl-2,4-dithiomalonamido)-dioxomolybdenum(VI).The rust and corrosion inhibitors were also incorporated in the above greases with a view to pass the following tests.(a)
Emcor rust test,
ASTM D 6138 [16].(b)
Corrosion
prevention test, ASTM D 1743 [16].


### 2.6. Test Balls

SKF steel bearing balls of 12.7 mm diameter (type
RB-12.7/111/E212) were used as the test specimen in the four-ball test.


### 2.7. Apparatus

(a) Extreme pressure lubricant test(i)
Four-ball machine. The tests were conducted on a four-ball
machine by following standard procedure [16]. The duration of the test was 60
seconds. A series of tests were performed until the welding point was reached,
and the determined parameters
were initial seizure load (ISL), 2.5-second seizure delay load (SDL), just
before weld load (JBWL), weld load (WL), wear-scar diameter (d) at ISL and JBWL, flash temperature
parameter (FTP), and pressure wear index (PWI).(ii)
Timken machine. The measurement of load-carrying capacity of
the prepared polyurea greases was carried out by Timken method by following
ASTM D 2509 test method [16]. The duration of the test was ten minutes. The
test results obtained with the Timken machine are recorded as Timken OK load in
Table 3.(b)Rust tests. These tests were performed by following ASTM D
6138 method in SKF Emcor test rig [16] and ASTM D 1743 test rig [16]. The
prepared polyurea greases (Grease 2 and Grease 3) pass the rust tests as per
ASTM D 6138 and ASTM D 1743.(c)Oxidation stability test. It
was carried out using oxidation bomb method, ASTM D 942, [16].(d)Topography of the wear-scar surface was studied by scanning electron microscopy (Phillips
XL-20) technique [17]. The wear scar obtained with Grease A, Grease B, and
Grease C after “four-ball” test at just before weld loads were selected for the
study. The stains of sludge or varnish found on the ball surface in the
vicinity of the wear scar were removed with cotton and then cleaned with
acetone in an ultrasonic bath before taking the micrographs.(e)Tribochemistry was performed to study the film, formed on the wear-scar
surface, obtained
after “four-ball” test using AES technique [18]. The presence of sulphur and
molybdenum in the film, formed during lubrication, was detected. The loads
selected for the investigation were just before weld loads (for Grease B and
Grease C). The tested ball surface was cleaned with acetone in an ultrasonic
bath before the AES analyses.


## 2.1. Lithium Grease

Lithium-base grease (Grease A)
was prepared in situ by reacting 12-hydroxystearic with lithium hydroxide
monohydrate in presence of paraffinic mineral oil (see Table1).Table 1
TestResultsMethods(1)Nature of the base
oilParaffinic mineral
base oil—(2)Base oil viscosityASTM D 445at 40°C100.70at 100°C10.94(3)Viscosity index92ASTM D 2270(4)Flash point (COC),°C234ASTM D 92(5)Pour point,°C0ASTM D 97(6)Aniline point,°C104ASTM D 611


## 2.2. Base Oil

Typical characteristics of the
base oil used in the prepared lithium-base grease are given in Table1.


## 2.3. Additives

The Mo-S complex additives were prepared [14, 15] by reacting an ethanolic solution of an appropriate
1,5-di-aryl-2,4-dithiomalonamide (2 mol) with an acidic solution of ammonium
molybdate (1 mol) and digesting the reaction mixture for 15 minutes on a water bath.
(1)2Ar–NH–C||S–CH2–C||S–NH–Ar→EthanolAmmonium Molybdate;HMoO2(Ar–NH–C||S–CH2–C||S–NH–Ar)21,5-di-aryl-2,4-dithiomalonamideBis(1,5-diaryl-2,4-dithiomalonamido)dioxomolybdenum(VI)
Whereas, Ar=p-Methoxyphenyl-
and p-Chlorophenyl- groups.


## 2.4. Preparation of Bis(1,5-diaryl-2,4-dithiomalonamido) dioxomolybdenum (VI)

The following two Mo-S complexes
were prepared and used as extreme pressure additives in the lithium-base
grease.Bis(1,5-di-p-methoxyphenyl-2,4-dithiomalonamido) dioxomolybdenum(VI)
Molecular Formula: MoO2C34H36N4S4O4. Melting Point: >200°C (Dec.) MoO2(CH3O–H4C6–NH–C||S–CH2–C||S–NH–C6H4–OCH3)2.Bis(1,5- di-p-chloro-phenyl-2,4-dithiomalonamido) dioxomolybdenum (VI)
Molecular Formula: MoO2C30H24N4S4Cl4. Melting Point: >200°C (Dec.) MoO2(Cl–H4C6–NH–C||S–CH2–C||S–NH–C6H4–Cl)2.


## 2.5. Grease Additive Admixture

The optimized dosages of the above additives
were blended at a temperature of 80°C with the prepared Grease 1
(without additive) and with a view to achieve the following two main properties for the extreme
pressure greases.(a)
Timken OK load of 60 lbs minimum.(b)
Four-ball weld load of 315 kgf minimum.The following greases were used in this study.Grease A: Lithium-base grease without additive.Grease B: Grease A  +  bis(1,5-di-p-methoxyphenyl-2,4-dithiomalonamido)-dioxomolybdenum(VI).Grease C: Grease A  +  bis(1,5-di-p-chlorophenyl-2,4-dithiomalonamido)-dioxomolybdenum(VI).The rust and corrosion inhibitors were also incorporated in the above greases with a view to pass the following tests.(a)
Emcor rust test,
ASTM D 6138 [16].(b)
Corrosion
prevention test, ASTM D 1743 [16].


## 2.6. Test Balls

SKF steel bearing balls of 12.7 mm diameter (type
RB-12.7/111/E212) were used as the test specimen in the four-ball test.


## 2.7. Apparatus

(a) Extreme pressure lubricant test(i)
Four-ball machine. The tests were conducted on a four-ball
machine by following standard procedure [16]. The duration of the test was 60
seconds. A series of tests were performed until the welding point was reached,
and the determined parameters
were initial seizure load (ISL), 2.5-second seizure delay load (SDL), just
before weld load (JBWL), weld load (WL), wear-scar diameter (d) at ISL and JBWL, flash temperature
parameter (FTP), and pressure wear index (PWI).(ii)
Timken machine. The measurement of load-carrying capacity of
the prepared polyurea greases was carried out by Timken method by following
ASTM D 2509 test method [16]. The duration of the test was ten minutes. The
test results obtained with the Timken machine are recorded as Timken OK load in
Table 3.(b)Rust tests. These tests were performed by following ASTM D
6138 method in SKF Emcor test rig [16] and ASTM D 1743 test rig [16]. The
prepared polyurea greases (Grease 2 and Grease 3) pass the rust tests as per
ASTM D 6138 and ASTM D 1743.(c)Oxidation stability test. It
was carried out using oxidation bomb method, ASTM D 942, [16].(d)Topography of the wear-scar surface was studied by scanning electron microscopy (Phillips
XL-20) technique [17]. The wear scar obtained with Grease A, Grease B, and
Grease C after “four-ball” test at just before weld loads were selected for the
study. The stains of sludge or varnish found on the ball surface in the
vicinity of the wear scar were removed with cotton and then cleaned with
acetone in an ultrasonic bath before taking the micrographs.(e)Tribochemistry was performed to study the film, formed on the wear-scar
surface, obtained
after “four-ball” test using AES technique [18]. The presence of sulphur and
molybdenum in the film, formed during lubrication, was detected. The loads
selected for the investigation were just before weld loads (for Grease B and
Grease C). The tested ball surface was cleaned with acetone in an ultrasonic
bath before the AES analyses.


## 3. Results and Discussion

The prepared greases were
evaluated for the extreme pressure, rust and corrosion inhibition, and
oxidation stability properties, and the results are reported in Tables2, 3.Table 2
Summary of the experimental results obtained with prepared Grease A, Grease B, and Grease C in
the four-ball test.
Sl. No.LUBRICANTISL* kgfd* at ISL mmSDL* kgfJBWL* kgfd* at JBWL* mmWL* kgfFTP* maxPWI* max(1)Grease A800.701001401.901601329(2)Grease B1120.701262802.6031521711(3)Grease C1120.501262802.5031527011d*  =  wear-scar diameter, ISL*  =  initial seizure load, SDL*  =  2.5-second seizure delay load, JBWL*  =  just before weld load, WL*  =  weld load, FTP*  =  flash
temperature parameter, PWI*  =  pressure wear index.Table 3
Typical test results recorded for the prepared Grease A, Grease B, and Grease C.
Sl. No.TestResults GREASE AResults GREASE BResults GREASE CMethods ASTM(1)NLGI grade222NLGI(2)Penetration, 60 X274272272D 217Penetration, 100,000 X299294295(3)Drop point,°C198198199D 2265(4)Oil
separation, % by wt
at 100°C, 30 h1.41.51.8D 6184(5)Oil
separation on
storage, % by wtNilNilNilD 1742(6)Oxidation stab. at 99±  1°C,
100 h pressure
drop, psi1521.5D 942(7)Water washout at80°C, % loss by wt4.04.24.0D 1264(8)Rust
testFAILS11D 1743(9)EMCOR
rust testFAILS0,00,0D
6138(10)Copper corrosion test at100°C,
24 hPASSPASSPASSD
4048(11)Deleterious particles, number of
scratchesNILNILNILD 1404(12)Wheel bearing test at105°C for 6 h,
leakage by wt, gms.1.21.21.3D 1263(13)Four-ball
wear test,
Wear-scar dia., mm, ~1200 rpm,
40 kg at75°C0.850.450.42D 2266(14)Four-ball
weld load, kg160315315D
2596(15)Timken Ok
Load, lbs204545D
2509Table2 presents a record of the
values of the initial seizure load (ISL), wear-scar diameter (d), 2.5-second
seizure delay load (SDL), just before weld load (JBWL), weld load (WL), flash
temperature parameter (FTP), and pressure wear index (PWI) for the prepared
Grease A, Grease B, and Grease C in the four-ball test.A series of test were conducted
with the prepared greases,
a “four-ball machine”,
and it was found that the wear-scar diameter values increase gradually up to
ISL, owing to the physiosorbed/chemisorbed thin layers of the additive/lubricant
on the rubbing surfaces. On increasing the load, there is a sharp increase in
the wear-scar diameter values, indicating that the adsorbed lubricant/additive
film has become partially desorbed. The transition from ISL and the consequent
rise in the temperature from ISL to SDL result in the chemisorption of additive
film on the contacting surfaces, and ultimately a chemical film of
iron-sulphide/iron-oxide/molybdenum-disulphide is believed to be formed. This
mixed film separates the contact surfaces and lower the friction coefficient
and wear-scar diameter values, even at much higher loads.Table3 records the typical test results for the
prepared greases (Grease A, Grease B, and Grease C). The additive fortified
greases (Grease B and Grease C) show better oxidation stability as well as
excellent rust and corrosion protection properties. The greases (Grease B and Grease C) show
higher load carrying properties in four-ball test (weld loads of 315 kgf) and
Timken test (Timken OK loads of 45 lbs) as compared to the Grease A (without additive) .Topography. The effectiveness of the prepared greases was examined by
the technique of scanning electron microscopy [17]. The wear scar obtained with
Grease A, Grease B, and Grease C after four-ball test at just before weld loads
was selected for the study. The stains of sludge or oil found on the surface of
the test specimens in the vicinity of the wear scar were removed with cotton.
The tested specimens were then cleaned with acetone in an ultrasonic bath. The
micrographs are shown in Figure 1. The micrographs of Grease B and Grease C
exhibit smoother surfaces in comparison to the micrograph obtained with Grease
A. The flow pattern at the leading edges indicates adhesive wear, which might
be due to the junction growth and subsequent rupture. The black lines and spots
are probably due to the formation of mixed chemical films of
iron-sulphide/iron-oxide/molybdenum-disulphide, which provide effective
lubrication at higher loads. Thus, the SEM study confirmed the wear scar
obtained with Grease B and Grease C is smoother in nature as compared to the
wear scar obtained with Grease A.Scanning electron micrographs. (a) Grease A at 140 kgf, (b) Grease B at 280 kgf, (c) Grease C at 280 kgf.
(a)(b)(c)Tribochemistry. Auger electron spectroscopy [18] was used toexplore the chemical composition of the film formed during lubrication in the wear tracks. The “Auger spectra” were
obtained by plotting the derivative of electron energy distribution against
energy. The Auger analyses of the used balls were analyzed using scanning
electron micro-probe. The Auger spectra obtained with line analyses for Grease B
(see Figure 2) and Grease C (see Figure 3) (at just before weld load) show the
presence of oxygen, carbon, sulphur, nitrogen, chlorine, and molybdenum. The
quantitatively detected elements are absent in the lithium grease without
additive (Grease A at just before weld load). It appears that the sulphur and
molybdenum are responsible for higher weld load for Grease B and Grease C, this
may probably be due to the formation of iron-sulphide as well as
molybdenum-disulphide at high temperature, which diffuses into the first atomic
layer of the metal and forms a new alloy, which then provides effective
lubrication to the bearing ball surfaces.Figure 2
AES of Grease B at just before weld load; 280 kg.Figure 3
AES of Grease C at just before weld load; 280 kg.Thus, it can be corroborated that
these elements are derived from the additives which are used in the lithium-base
grease and are responsible for the smooth efficiency of the prepared greases at
higher loads.


## 4. Conclusions

The prepared greases blended with
different additives were found to be more effective in reducing friction and
wear at sliding surfaces, and increasing the load carrying capacities as
compared to Grease A (grease without additives), Grease B, and Grease C showed
lower values of wear-scar diameter and higher values of weld loads.The scanning electron micrographs
showed that the
wear scar obtained with the Grease B and Grease C is smoother in nature than
Grease A. The tribochemistry by AES analyses confirmed the presence of sulphur
and molybdenum in the wear tracks of the test specimen obtained after the
four-ball test with Grease B and Grease C. The prepared greases (Grease B and
Grease C) also pass the rust and corrosion and oxidation stability tests.



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