Tribological and Antioxidation Synergistic Effect Study of Sulfonate-Modified Nano Calcium Carbonate
et al. (2013) Tribological and Antioxidation Synergistic Effect Study of Sulfonate-Modified Nano Calcium
Carbonate. PLoS ONE 8(5): e62050. doi:10.1371/journal.pone.0062050
Tribological and Antioxidation Synergistic Effect Study of Sulfonate-Modified Nano Calcium Carbonate
He Zhongyi 0
Xiong Liping 0
Han Sheng 0
Chen Aixi 0
Qiu Jianwei 0
Fu Xisheng 0
Chris Lorenz, King's College London, United Kingdom
0 1 School of Basic Science, East China Jiaotong University , Nanchang , P.R. China , 2 Shanghai Institute of Technology , Shanghai , China , 3 PetroChina Lanzhou Lubricating Oil R & D Institute , Lanzhou , P.R. China
A middle base number sulphonate-modified nano calcium carbonate (SMC) with an average size of 35 nm was synthesized, and its tribological and antioxidation synergistic behaviors with ashless antioxidant N-phenyl-a-naphthylamine (T531) in hydrogenated oil (5Cst) were evaluated. The results demonstrate that adding this synethesized additive even at a low amount (,2.0 wt.%) can evidently improve its load-carrying capacity by 1.5 times and enhance its antiwear performance; in addition, the friction-reducing effect of additive in the high load was better than that in low load. The SMC have a good synergistic antioxidation effect with T531, which verifies the nano calcium carbonate compound was a kind of multifunctional and high-performance additive. The chemical composition of the rubbing surface which formed on the boundary film was analyzed by using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results indicating that the excellent antiwear and load-carrying performance could be attributed to the forming of boundary lubrication film which composed of calcium carbonate, oxides, ferrites, sulphide and FeSO4, and so on. Its ability to increase oxidation free energy of base oil is the main reason for increasing its antioxidant collaboration property with ashless antioxidant T531.
Competing Interests: Authors QJ and FX are employees of PetroChina Lanzhou Lubricating Oil R & D Institute. There are no patents, products in development
or marketed products to declare. This does not alter the authors adherence to all the PLOS ONE policies on sharing data and materials.
Sulphonates are mainly used as metal detergents in lubricating
oil, and they are mainly formed with calcium sulphonate,
magnesium sulphonate, barium sulphonate, etc. They are applied
in engine oil, antirust oil, gear oil, metal-working fluid, automotive
grease, industrial grease and other fields according to different
structures [1,2]. Calcium sulfonates was introduced as detergents
which can scavenge acidic contaminants from the lubricant
originally. Some sulphonates (mainly sodium and calcium salts)
have been found not only possess very good extreme pressure
property in metal working fluid, but also possess very good
solubility of nano carbonate particle in gel since the 1980s. Such
an active film  that contains oxides and sulphides which come
from additive the lubricating surface can prevent the contact with
the substrate directly and plays an important role in extreme
pressure behaviors. Recently, the development and research of
inorganic nanoparticle as lubricant eliciting widespread interest,
many nano-materials were added into lubricating oil/grease to
improve load-carrying, anti-wear and friction-reduction properties
. Nanoparticles used as lubricant additive was considered an
effective approach to upgrade lubricants due to their small size and
physicochemical properties, which have been thoroughly
investigated during the past decades [5,7]. And these nanoparticles
mainly focused on nano metal and metallic oxide, such as copper,
nano calcium carbonate etc. , which modified with surfactant to
improve solubility in base oil, and have poor oil-soluble stability.
As to calcium carbonate, different crystalline forms possess
different tribological property. For exmple, cubic CaCO3 that
added in lithium grease can significantly improve its antiwear,
friction-reduction and load-carrying capacity .
With rapid industrial development, the modern equipment
requires strengthen lubricating oil with oxidation-resistance
property, such as high-temperature chain oil, synthetic aviation
lubricant and other high-temperature lubricants. Currently,
aromatic amine antioxidants, such as T531 and P, P-dioctyl
diphenylamine, were mainly used as high-temperature antioxidant
to suit the new requirements of modern machine. Many study
results showed that adding oil-soluble metal salts in aromatic
amine antioxidant is an effective way of to improve its
hightemperature thermal oxidation stability. The metal salts are
mainly alkali metal salts , which have no antiwear property,
and earth metal salts has apparently not been investigated.
High temperature turbocharger test (TEOST 33C) was used
according to the GF-5 specifications by the International
Lubricant Standardization and Approval Committee (ILSAC)
organization enforced in 2010,and test temperature is as high as
480uC, One pure antioxidant can not resist such a high
temperature. The resulting stringent demand for two or more
additives mix. According to this basis, adding oil-soluble metal salt
into lubricant  is an effective way to improve the thermal
oxidation stability at high temperature. Calcium sulphonate
possess good oil-solution, when it was used as a kind of surfactant
to modify CaCO3 nanopartices, it can improve solubility of
CaCO3 in base oil. They offer anextremely attractive performance
being as an additive for base oil, therefore, the study of CaCO3
nanoparticles is of great importance to us.
In this paper, alkyl benzenesulphonic acid was used as raw
material to obtain middle base number calcium sulphonate,
CaCO3 nanoparticles were prepared by the carbonation method.
Being as additive in 5Cst, its tribological performances were
investigated by using the four-ball tester, and the antioxidant
property with T531 in 5Cst was evaluated by using pressure
differential scanning calorimetry (PDSC). In addition, the
lubricating mechanism was also discussed.
Additive Synthesis and Analysis Methods
2.1 Base oil and chemical reagents
A commercial hydrogenated oil product 5Cst, which n100uC is
5.539 mm2?s21, n40uC is 31.60 mm2?s21, flashpoint is 238uC,
viscosity index is 110,made by Daqing Refinery Factory of China,
was used as the lubricating oil without any further treatment. All
chemical reagents (AP grade) are purchased from Shanghai Shen
Bo Chemical co., LTD of China, the benzenesulphonic acid and
HVI 150 base oil are made by Daqing Refinery Factory of China.
2.2 Additive synthesis
A four-flasks were installed with stirrer, thermometer, condenser
evaporator, then added gasoline and methanol containing a certain
amount of calcium oxide at 40uC, keep stirring, then added
benzenesulphonic acid (industrial, with acid value 106 mg KOH/g),
HVI150 base oil, heating to 120uC, leading gas containing carbon
dioxide into the reaction chamber for 4 h to maintain alkalinity.
After that removing the water and accelerant, when it cooled enough
centrifuged it, the upper clear liquid was distilled under vacuum to
obtain viscous bright red fluid. Calcium sulphonate which is middle
base number, was abbreviated to SMC (The elemental analysis
results: S% is 1.33, Ca% is 11.20, TBN is 282 mgKOH/g, n100uC
is 130.6). The peak at 881 cm21 in the IR spectra of the samples is
the absorption peaks of crystalline calcium carbonate.
2.3 Analysis methods
(1) Freeze-etching electron microscopy observation method: the
sample was froze to liquid nitrogen temperature (negative 196uC),
placed in a vacuum plating apparatus and then vacuumed. When
the residual voltage reached to 0.004 Pa, the temperature drops to
2150uC, the sample was fractured, after that, raise the
temperature to 290uC, kept for 10 min in order to etching. After that the
sample was withdrawn, washed with xylene, removed the film with
copper net, and investigated it with electron microscopy . The
type of electron microscopy is HFZ-1 type.
(2) Specimens and testing apparatus: All test balls (Q 12.7 mm)
which have been used in the test were made of GCr15 bearing
steel with hardness HRc of 5961. A microscope was used to
determine the wear scar diameters (WSD) of the 3 lower balls with
an accuracy 60.01 mm. The Chinese standard test method
GB3142-82, which is similar to ASTM D2783, was used to
evaluate the maximum non-seizure loads, the rotation speed was
conducted at 1450 rpm, keeping for 10 s, at room temperature.
Friction and wear tests were conducted on a 4-ball test machine
made in Ji nan Testing Machine Factory, China. The testing
rotation speed was at 1450 rpm under different loads for 30 min.
The worn scar was analysed by PHI-5702 type X-ray
photoelectron spectroscope (XPS). And the additive was analysed by
Thermo VG ESCA LAB 250 type X-ray photoelectron
spectroscope (XPS). The radiation source was Mg Ka with pass energy of
29.35 eV, and the binding energy of C1s (284.6 eV) was used as a
standard value. A JSM-5600LV type scanning electron
microscope (SEM) was also used to study the rubbed surface
Results and Discussion
3.1 Freeze-etching electron microscopy of the product
The enlarged 100,000 times freeze-etching electron microscopy
of synthesized nano calcium carbonate is shown in Figure 1. The
SMC shown like flake shape, instead of the ball shape, its size
range from 20 to 50 nm, and the weight average particle size is
about 35 nm. It is crystallization calcium carbonate, and it has a
different from traditional calcium sulphonate product. As a
detergent agent, the particle size of CaCO3 should be less than
80 nm , otherwise it will cause product to turbid, and the poor
colloid stability will affect its using performance. Some research
shown that the right particle size of load micelle should be under
20 nm, and uniformly distributed.
Its grain size analysis used the Zetaplus/90 plus type zeta laser
particle size analyser, images has been also shown in Figure 1. It
can be seen that the average size of SMC is about 35 nm.
3.2 Tribological properties
The maximum non-seizure load (PB value) results of base oil and
containing different concentration additive at room temperature
were shown in Table 1. The PB value of SMC is 1.5 times of base
oil at 2.0 wt %. With the additive concentration increasing, the PB
value increases accordingly, but high concentration does not
corresponding to high PB value when concentration over 2.0 wt%.
Figure 2 shows the wear scar diameter (WSD) at 392N and
196N. It can be seen that in the presence of SMC, the WSD
decrease compared to those of the base oil. The additive possesses
better antiwear property along with additives concentration
increasing, meanwhile, it become smoother when additive
concentrations increasing over 0.5wt%. And it possesses better
antiwear property at lower load than at higher load.
Figure 3 was shown the relation between the different load and
the friction coefficient, and that of the oil containing 2.0 wt%
SMC was investigated to reference to that of the base oil. The
friction coefficient shows low value at 392N after addition of SMC,
the friction-reducing effect of additive in the high load was better
than in the low load.
The oxidation induction period (ton) of PDSC was used to study
antioxidation property of SMC and T531. The oxidation process
of base oil is an exothermic reaction, when the base oil becoming
oxidized, it will appear apparent exothermic peak in the PDSC
curves under heating conditions with oxygen. The lower ton of
base oil, means the worse oxidation stability. The results of ton at
different temperatures have been tabulated Table 2.
It can be seen from Table 3 that with the temperature rising, the
ton of base oil is significantly reduced, which means that higher
temperature helps proceeding oxidation reaction.
The oxidation reaction of base oil with aromatic amine
antioxidant is a free radical reaction , and the activation
energy of the oxidation reaction can be calculated by using the
Arrhenius formula according to PDSC test results. Because the
chemical activation energy measure describes the difficulty of
chemistry reaction. And in general, the smaller activation energy,
the easier to proceeding the reaction. So the chemical activation
energy response to the difficulty of reaction degree, it can be used
to study its oxidation mechanism.
According to Arrhenius equation: lnk = 2Ea/RT+C, and k is
the oxidation rate constant, and k is inversely proportional with
the reaction time in the initial stages of oxidation, therefore, then it
can deduce the following equation : ln ton = Ea/RT2C9.
Then we used lnton as the y-axis, the 1/T as the x-axis, we can
obtained a straight line in theory, the slope equal to Ea/R. The
activation energy Ea of oxidation reaction have been calculated,
the result was shown in figure 4.
In Figure 4, it can be seen that the resulting curves were
essentially straight lines, which means that the oxidation curves of
base oil and containing additive base oil are in accordance with the
Arrhenius formula, and that the activation energy calculated
according to this method is feasible. The calculated values of
oxidation activation energy of the oil samples are shown in table 3.
The activation energy data shows that the synthesized calcium
sulfonate can improve the activation energy of oxidation reaction
very well, and they shall be able to improve the antioxidant effect
of base oil effectively.
3.4 The worn surface analysis of steel ball surface
In order to understand the boundary lubricating mechanism,
the worn surface of steel ball was studied by SEM. Figure 5 gives
the typical SEM images of the worn steel ball surface lubricated by
5Cst and the 5Cst containing 1.0 wt% SMC at 392 N for 30 min.
It can be seen that the surface lubricated by 5Cst alone is rough
and shows signs of severe scuffing (Figure 5a), taking on grain
abrasion characteristic. On the other hand, scuffing on the surface
of the tested steel ball lubricated by 5Cst containing 1.0 wt% SMC
was significantly inhibited (Figure 5b). Moreover, the wear scar of
the steel ball lubricated by 5Cst containing 1.0 wt% SMC is much
smaller and smoother than that lubricated by 5Cst alone.
Therefore, the results further testify that SMC has good anti-wear
Table 4 was atoms concentration of worn surface lubricated by
5Cst and 5Cst containing 1.0 wt % SMC at 392 N for 30 min. It
can be seen that the elements of C, O, Cr and Fe were present on
the worn scar surface lubricated only by 5Cst. The elements of Ca
and S were present on the worn scar surface lubricated by 5Cst
containing 1.0 wt % SMC, and its atomic concentration of Ca was
0.36%, S was 0.18%. This fact shows that Ca in CaCO3
nanoparticle, S in calcium sulfonate were deposited on the worn
steel surface in the process of friction. The presence of Ca gives
strong evidence that a lubricate film must be formed and probably
contains CaCO3 nanoparticle and/or calcium oxide, which can
prevent the steel-to-steel direct contact.
In order to understand the tribological mechanism of the SMC
in the lubricating oil, the additive-derived elements: carbon,
oxygen, sulfur, and calcium were detected by XPS analysis, and
1.0% SMC/5cst 34.97
these elements which analysed of worn surface lubricated with
SMC at 392 N for 30 min were also detected by XPS. The results
were shown in figure 6.
The binding energy peak of C1s appears at 289.8 eV, which is
identified as C in carbonate . Besides, the binding energies of
C1s at 284.9 eV directly correspond to C-H and C-O, which
existing in the additive and base oil, and it means that the base oil
and additive were adsorbed on the metal surface. For worn steel
ball surface, the binding energy peak of C1s appears at 284.9 eV,
which is identified as C in C-H and C-O the manuscript presented
in an intelligible fashion and written in standard English. The
weak peak at high binding energy of 289.8 eV is associated with
For additive SMC, the peak of O1s appears at 531.9 eV,
corresponding to calcium sulfonate and calcium carbonate, and
the peak at 529.0 eV, corresponding to calcium oxide, hasnt
appear, it means that calcium oxide perhaps wrapped in the
calcium carbonate, or maybe SMC does not cintain calcium
oxide. For the tested steel ball worn surface, the O1s peak appears
at 529.6 eV can attribute to oxygen in iron oxide and calcium
For additive SMC, the peak of S2p appears at 168.7 eV,
corresponding to sulfonate. But the peaks have changed to
168.8 eV, 167.8 eV, 161.4 eV and 159.5 eV on the worn surface
after lubricating process, which corresponding to FeSO4, FeS
respectively. The binding energy of S2p is 168.8 eV, which
corresponds to FeSO4 , and it means that S element of
sulfonate was adsorbed on the steel balls surface mainly in the
form of ferrous sulphate after tribological process.
For SMC, the main peaks of Ca2p appear at 351.0 eV and
347.3 eV were attributed to calcium in CaCO3 and CaO
respectively. Also Ca2p at 347.3 eV and Ols at 529.6 eV are
identified as calcium oxide. For the steel ball worn surface, the
main peak of Ca2p appears at 346.7 eV is attributed to calcium in
The Fe2p peak appears around 710.6 eV is attributed to the
generation of iron oxide. And this conclusion was also supported
by the binding energies of O1s being around 530 eV. Another
peak appeared at 723.5 eV was attributed to Fe in FeSO4.
The XPS analysis of the outside worn surface lubricated by
blank base oil only presents C, O, and Fe elements peaks ,
hadnt detected Ca and S elements. The XPS analysis mentioned
above indicated that thin boundary lubrication film have been
formed and this film contained CaCO3, CaO, iron oxide and
FeSO4,FeS etc., and some organic compounds coming from 5Cst
According to the above XPS analysis results, discussion on the
lubrication mechanism of the sulfonate-modified calcium
carbonate nanoparticals as additives in 5 Cst are as followings.
First, the calcium sulfonate was vertically adsorbed on steel ball
surface according to preferential orientation adsorption
mechanism , which taking part in a competitive adsorption with
base oil during lubricating process. And the sulfonate-modified
CaCO3 nanoparticles contained in the 5Cst are trapped inside the
contact and fill in the gap between the rubbing surfaces, the
additive facilitated reduction of friction and wear. The tribological
action between metals will produce high temperature, the alkyl of
calcium sulfonate was easily sheared under boundary lubrication
conditions. The nanoparticles of CaCO3 can deposite on the
shearing surface because of the high surface energy of the fresh
worn surface, and the complicated tribochemical reaction that had
occurred in reaction system. The additive molecules have been
decomposed , the sulphonic acid group reacts with the metal
surface forming sulphate, sulphide and ferrite film, the inorganic
film has high hardness, which made the tribological surface film
acquired higher load-carrying capacity. And the calcium
carbonate reacts with iron oxide and fresh iron on the worn surface to
generate calcium oxide and ferrite calcium. The tribochemical
reaction film  is composed of absorbed organic materials
which comes from additives or 5Cst itself, the generating CaO,
iron oxide, sulphate, sulphide etc.. Besides, the tribochemical
compounds which act as solid lubricants during the friction
process, and tribochemical reaction products on the worn surface
improve the tribological properties of 5Cst.
From the above results, the following conclusion can be drawn:
(1). The synthetic sulphonate-modified calcium carbonate can
improve the extreme pressure performances of base oil. The
extreme pressure performance increases with the increase of
additive concentration. The additive has a good antiwear
property, and the friction-reducing effect of additive at the high
load was much stronger than that at low load.
(2). The oxidation stability of additive is poor, but it can
improve the antioxidation property future of the base oil by
increasing the activation energy of oil sample during oxidation
reaction process, when the additive is mixed with the antioxidant
(3). As a lubricating oil additive, the sulphonate-modified nano
calcium carbonate occured tribochemical reaction with steel ball
in the friction process. The sulphonate formed inorganic salts such
as oxides, FeSO4 and sulphide, and the carbonate particle formed
calcium oxide and ferrites, which formed a complex boundary film
on the surface. The mixed reaction boundary film adsorbed on the
worn steel ball surfaces can improve the tribological properties of
the base oil.
Conceived and designed the experiments: HZ. Performed the experiments:
XL QL. Analyzed the data: QJ FX. Contributed reagents/materials/
analysis tools: CA HS. Wrote the paper: HZ.
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