Tribological study of hydrolytically stable S-containing alkyl phenylboric esters as lubricant additives

Article abstract of DOI:10.1039/c4ra01672f

Three novel S-containing alkyl phenylboric esters, i.e., DSPB, TSPB and TBPB, were prepared and characterized. Compared with common boric esters, these three additives exhibit significant hydrolytic stability, which can be attributed to their specific mol

Full text of DOI:10.1039/c4ra01672f

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Tribological study of hydrolytically stable
S-containing alkyl phenylboric esters as lubricant
additives
Cite this: RSC Adv., 2014, 4, 25118
ab
c
a
a
d
Zhipeng Li, Yilin Li, Yawen Zhang, Tianhui Ren* and Yidong Zhao
Three novel S-containing alkyl phenylboric esters, i.e., DSPB, TSPB and TBPB, were prepared and
characterized. Compared with common boric esters, these three additives exhibit significant hydrolytic
stability, which can be attributed to their specific molecular structures, in which a phenyl group
conjugates with an electron-deficient boron that helps improve the hydrolytic stability of boric esters.
The tribological properties were evaluated using a four ball tester, which suggests that all the synthesized
compounds act as anti-wear additives and have relatively high load-carrying capacities. In particular, it
should be noted that, among the synthesized compounds, DSPB exhibits both anti-wear and friction-
reducing properties. Furthermore, the worn surface was investigated by X-ray absorption near edge
structure spectroscopy (XANES). The results demonstrate that the resulting tribofilm consists of trigonal
and tetragonal coordination boron, iron sulfide and disulfide, and iron sulfate. Moreover, atomic force
microscopy (AFM) was used to characterize the morphology of the tribofilm and the large pads
elongated and orientated in the sliding direction were observed, which contribute to the anti-wear (AW)
and extreme pressure (EP) performances.
Received 26th February 2014
Accepted 19th May 2014
DOI: 10.1039/c4ra01672f
www.rsc.org/advances
phosphorus compounds, which can cause air pollution by
directly releasing these hazardous chemicals into the air or
1
. Introduction
It is well known that base oil does not satisfy all the stringent indirectly poisoning the emission-control catalysts and thus
5
requirements imposed on lubricants for use in heavy-duty block the lters in car exhaust system. Therefore, it is necessary
machineries, such as modern engines, hydraulic devices and to develop environmentally friendly lubricating additives to
large construction vehicles, which usually release a signicant substitute or reduce the use of environmentally hazardous
6
amount of heat and exhibit severe material consumption in the ZDDPs.
joint working parts. Such friction-induced heat release and
Boron-containing additives have been reported to be prom-
wear-induced material consumption accounts for about one- ising alternatives for substituting ZDDPs due to their compa-
1
,2
third of the world total energy consumption every year.
rable mechanical lubricating properties as well as low
7
Various compounds containing boron, sulfur, phosphorus, environmental impact. One of the typical boron-containing
halogens, nitrogen or metals as active components have been additives is organic boric esters, which are used because of their
used as lubricating additives for base oil to minimize wear and anti-oxidant properties as well as specic properties such as oil
control friction, and thus improve the working efficiency as well solubility, low-toxicity and biodegradability.
as prolong the lifetime of these heavy-duty machines. Zinc reported that the formation of a thin layer of boric oxide (B
dialkyldithiophosphates (ZDDPs) have been widely used as on the metal surfaces under extreme pressure is responsible for
8,9
It has been
2 3
O )
3,4
10,11
lubricant additives in many heavy-duty machines. However, its anti-wear properties.
Kapadia and co-workers proposed
there is a signicant environmental risk in using ZDDPs that amorphous networks of mixed three or four connective
because they produce zinc ash and large amount of sulfur and polymer units are formed on the metal surface due to the
12
dehydration of H
3
BO
3
.
However, this process is still unclear
a
and more effort is need to reveal the tribological behavior of
School of Chemistry and Chemical Engineering, Key Laboratory for Thin Film and
Microfabrication of the Ministry of Education, Shanghai Jiao Tong University, boron on a metal surface.
2
00240, China. E-mail: thren@sjtu.edu.cn; Tel: +86 21 5474 7118
Moreover, due to the electron deciency of the boron atom,
boric esters tend to hydrolyze under ambient conditions by
forming the oil-insoluble and abrasive boric acid, which is
considered as a major limitation for their further applica-
b
Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, China
c
Voiland School of Chemical Engineering and Bioengineering, Washington State
University, Pullman, WA 99164, USA
13–15
d
tion.
A common method to improve the hydrolytic stability
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese
of boric esters is to introduce an electron-rich nitrogen atom
Academy of Sciences, Beijing 100039, China
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into the molecular structure, which can coordinate with boron
All chemicals (purchased from Shanghai Chemical
7,15
for its stabilization.
Nevertheless, the N-containing boric Company, Shanghai, China) were of reagent grade and used
esters still exhibit relatively poor hydrolytic stabilities due to the without further purication. 4-Dodecylphenylboric acid was
weakness of the coordination bond between boron and prepared according to procedures reported in previous
19,20
nitrogen. Therefore, it is necessary to develop a new strategy by studies.
molecule design to improve the hydrolytic stability of boric The synthesized additives are bis(1-(butylthio)propan-2-yl) 4-
esters. In this report, we proposed an effective way to improve dodecylphenylboronate (TBPB), bis(1-(dodecylthio)propan-2-yl)
the hydrolytic stability of boric esters by introducing alkyl 4-dodecylphenylboronate (DSPB) and bis(t-(dodecylthio)
phenyl groups that are directly conjugated to the electron-de- propan-2-yl) 4-dodecylphenylboronate (TSPB). The synthetic
cient boron. The synthesized boric esters exhibit improved routes are shown in Fig. 1 and the related synthetic procedures
hydrolytic stabilities due to the p-p conjugating effect between can be described as follows:
the phenyl group and boron atom, and the long alkyl chain acts
Stoichiometric n-butanethiol (18.04 g, 0.20 mol), 1,2-epoxy-
as a hydrophobic group that prevents a nucleophilic attack of propane (11.62 g, 0.20 mol) and triethylamine (20.24 g, 0.20
water, which leads to hydrolysis. Moreover, S-containing borate mol) as the catalyst were dissolved in 150 mL of dichloro-
5,13,16,17
esters have been extensively studied in lubricant oils.
It methane in a 250 mL round-bottomed ask. The reaction
has been conrmed that a synergistic effect due to the forma- mixture was stirred at room temperature overnight. The
tion of a protective lm between S and B will occur when the resulting reaction mixture was extracted with dichloromethane,
element sulfur is introduced into the molecular structures of acidied with dilute hydrochloric acid and washed with water 3
18
organic boric ester. Therefore, three novel S-containing alkyl times. Then, the combined organic phases were dried over
phenylboric esters were developed as lubricant additives and anhydrous MgSO and ltered. The solvent was removed under
4
their tribological properties were evaluated. The results indicate reduced pressure to afford 1-(butylthio)propan-2-ol (26.24 g,
that they all exhibit excellent anti-wear and friction-reducing 0.18 mol) in 89% yield.
properties as well as relatively high extreme pressure (EP)
4-Dodecylphenylboric acid (14.51 g, 0.05 mol), 1-(butylthio)
properties. We also believed that their unique tribological propan-2-ol (14.83 g, 0.10 mol) and a catalytic amount of strong
properties come from the specic tribological chemistry that acid resin (Amberlite 732) were dissolved in 150 mL of toluene
occurs during the lubricating process under heavy-duty condi- in a 250 mL round-bottomed ask equipped with a Dean–Stark
tions. We utilized atomic force microscopy (AFM) and X-ray trap for separation of water. The reaction mixture was reuxed
absorption near edge structure spectroscopy (XANES) to char- for 6 h under a nitrogen atmosphere. The resulting reaction
acterize the tribological surface at both a morphological level mixture was ltered and rotary evaporated to remove the
and a composition level and thus reveal the mechanism of the toluene under vacuum resulting in TBPB (27.25 g, 0.05 mol) in
tribological chemistry.
99% yield. DSPB (or TSPB) can be prepared by a reaction
between n-dodecylmercaptan (or t-dodecylmercaptan) and 4-
dodecylphenylboric acid using a similar procedure. All of the
synthesized additives were characterized by infrared (IR) spec-
troscopy, nuclear magnetic resonance (NMR) and elemental
analysis. The results of the elemental analysis are provided
separately in Table 2 for the convenience of indicating the
percentages of sulfur or boron, which are important parameters
in subsequent discussion.
2. Experimental
2
.1 Base oil properties and synthesis of additives
A common mineral oil, hydro-isomerized and dewaxed base oil
HVI WH150, purchased from PetroChina Lanzhou Lubri-
(
cating Oil R&D Institute in Lanzhou, China), was used as the
base oil in this study without any further treatment. The
physical properties of the HVIW H150 mineral oil are given in
Table 1.
Table 1 Physical properties of the HVI WH150 mineral oil
Parameter
Value
0.844
Fig. 1 Synthesis of S-containing alkyl phenylboric esters.
Table 2 Elemental analysis result of synthesized additives
ꢀ
À3
Density (20 C, g cm
)
2
À1
Kinematic viscosity (mm s
)
ꢀ
4
1
0 C
29.95
5.512
125
ꢀ
00 C
Viscosity index
Chrominance number
Pour point ( C)
Open ash point ( C)
Neutralizing value (mg KOH per g)
Extrinsic feature
Evaporation loss, Noack 250 C, 1 h (%)s
Items C (wt%)
H (wt%)
S (wt%)
B (wt%)
0
ꢀ
À30
a
a
a
a
a
a
TBPB
DSPB
TSPB
69.37 (69.79)a
10.89 (10.80)a
11.66 (11.64)
1.83 (1.96)
1.36 (1.39)
1.29 (1.39)
ꢀ
226
a
74.08 (74.37)
11.99 (11.83)
7.72 (8.27)a
0.01
Transparence
10.15
a
a
74.04 (74.37)
11.85 (11.83)
7.90 (8.27)
ꢀ
a
Calculated value (balanced by oxygen).
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Bis(1-(butylthio)propan-2-yl)
4-dodecylphenylboronate the steel balls. The surface roughness (root mean square, S
q
) of
À1
(
TBPB). Molecular formula: C H BO S . IR (KBr) (cm ): the anti-wear lms was calculated by Image Analysis 2 soware.
3
2
59
2 2
2
957.46m (–CH ); 2926.53s, 2855.96m (–CH –); 1609.05w (C]
XANES data were collected at the Beijing Synchrotron Radi-
3
2
C); 1457.08m, 1377.10m (–CH ); 1341.85s (–CH –); 1310.81s (B– ation Facility (BSRF), which is situated at the 2.2 GeV storage
3
2
1
22
O); 758.54m (phenyl ring); 702.51w (C–B); 652.76w (C–S). H- ring, Beijing Electron Positron Collider (BEPC). The K-edge
NMR (d, ppm, CDCl ): 7.69–7.09 (4H, phenyl ring), 2.72–2.34 spectra of boron were recorded using the so X-ray optics
9H, alkyl chain), 1.58–1.22 (34H, alkyl chain), 0.93–0.80 (12H, beamline in the total electron yield (TEY) mode with a resolu-
3
(
alkyl chain).
tion of 1000 (E/DE). This beamline is monochromatized by a
À1
Bis(1-(dodecylthio)propan-2-yl) 4-dodecylphenylboronate Monk–Gillieson monochromator using 800 mm gratings to
À1
(DSPB). Molecular formula: C H BO S . IR (KBr) (cm ): cover the region of 190–210 eV. The K-edge spectra of sulfur
4
8
91
2 2
2
953.72m (–CH ); 2925.48s, 2854.45m (–CH –); 1609.35w (C] were obtained using the mid-energy spectroscopy beamline in
3
2
C); 1456.38m, 1377.10m (–CH ); 1336.75s (–CH –); 1318.60s (B– the uorescence yield (FY) mode with a double-crystal mono-
3
2
1
O); 759.51m (phenyl ring); 701.27w (C–B); 652.43w (C–S). H- chromator covering the energy of 2450–2500 eV. Both the K-
NMR (d, ppm, CDCl ): 7.69–7.09 (4H, phenyl ring), 2.73–2.35 edge of boron and sulfur provide chemical information about
9H, alkyl chain), 1.59–1.48 (9H, alkyl chain), 1.33–1.25 (60H, the surface of the reaction lm. The thermal lms were
3
(
alkyl chain), 0.89–0.86 (9H, alkyl chain).
prepared under the following parameters: steel ball; additive
ꢀ
Bis(t-(dodecylthio)propan-2-yl) 4-dodecylphenylboronate concentration, 2.0 wt%; temperature, 150 C; and time, 6 h. The
À1
(
TSPB). Molecular formula: C H BO S . IR (KBr) (cm ): tribolms for XANES evaluation were generated from the
4
8
91
2 2
2
959.62m (–CH ); 2928.79s, 2872.61m (–CH –); 1609.21w (C] tribological tests (the upper balls). All the examined samples
3
2
22
C); 1460.11m, 1375.26m (–CH ); 1342.51s (–CH –); 1319.25s (B– were gently rinsed in petroleum ether prior to XANES analyses.
3
2
1
O); 759.51m (phenyl ring); 701.27w (C–B); 652.43w (C–S). H- Some model compounds were also scanned and recorded for
NMR (d, ppm, CDCl ): 8.15–7.11 (4H, phenyl ring), 2.70–2.34 the purpose of identication and comparison.
7H, alkyl ring), 1.53–1.21 (50H, alkyl chain), 0.87–0.83 (30H,
alkyl chain).
3
(
3
. Results and discussion
2.2 Tribological evaluation
3.1 Hydrolytic stabilities of synthesized additives
The tribological properties of TBPB, DSPB and TSPB as addi- The hydrolytic stabilities of TBPB, DSPB and TSPB are listed in
tives in mineral oil were investigated using a MMW-1 four-ball Table 3, and for comparison, the additives TB (tributyl borate)
23
tester (manufactured by Ji'nan instrument manufacturer, and NBO (N-containing borate ester) have also been listed.
China). The steel balls used in the tribological measurement are Overall, the hydrolytic stability of each additive decreases with
1
2.7 mm in diameter and made of GCr 15 bearing steel with an increase in concentration, which is ascribed to the high reaction
HRC hardness of 59–61. General tribological tests were con- probability between the additive molecule and water when the
ducted under ambient conditions with a rotating speed of 1450 additive concentration is high. As shown in Table 3, at 0.5 wt%
5
rpm for a time duration of 30 min. The coefficient of friction and 2.0 wt% of TB, the solutions start to hydrolyze in a very
(
COF) was recorded automatically by a computer linked to the short amount of time of 10 h and 2 h, respectively; however,
four-ball tester, and the values of the wear scar diameter (WSD) NBO exhibits better hydrolytic stability than TB due to the
for the three lower balls were measured using an optical stabilization of the boron atom by coordination with nitrogen in
23
microscope with an accuracy of Æ0.01 mm. The maximum non- its molecular structure. The hydrolytic stabilities of the
seizure loads (P values) tests of all additives were conducted synthesized compounds is considerably improved compared
B
according to the Chinese national standard GB12583-98, which with that of TB and NBO, which can be attributed to the
is similar to ASTM D-2783.
.3 Hydrolytic stability test
following two major points: (1) the improved stability of boron
due the phenyl group through p-p conjugation, and (2) the
presence of a long alkyl chain that prevents a nucleophilic
attack by water. This is further supported by the experimental
observation that TSPB is more hydrolytically stable than DSPB
and TBPB because its alkyl chain is longer (12C) than that of
TBPB (4C) and more complicated (t-dodecyl) than that of DSPB
2
A wet heat treatment that causes accelerated hydrolysis was
used in the hydrolytic stability test. Generally, 150 g of each
lubricant sample (0.50 wt% or 2.0 wt% of each additive in the
base oil) in a 200 mL beaker was placed in a hot and humid oven
ꢀ
(
50 Æ 2 C, relative humidity >95%). Each lubricant sample was
monitored hourly until there was precipitation observed or the
oil was no longer transparent, which indicates the hydrolysis of
the additive. The nal time was recorded as a parameter for
Table 3 The hydrolytic stability of boric esters
0.5 wt%
2.0 wt%
21
describing the hydrolytic stability of each additive.
TB
NBO
TBPB
10 h
48 h
312 h
475 h
599 h
2 h
16 h
89 h
282 h
496 h
2
.4 Surface analysis
A NanoScope IIIa atomic force microscope (Digital Instruments) DSPB
was used to investigate the morphology of the tribosurfaces on TSPB
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(
n-dodecyl). Therefore, the order of hydrolytic stabilities of the abrasion caused by the more active sulfur in the branched
2
6
synthesized additives is TSPB > DSPB > TBPB. The observed chain t-dodecyl group than that of n-dodecyl group in DSPB,
experimental results and the trend of hydrolytic stabilities the compactness of the protective lm is impacted. However,
related to the molecular structures of the additives support our for TBPB, as the n-butyl chain is short, associating with the
rationale for the molecular design. In addition, compared with element analysis results of the synthesized compounds, the
21
a reported phenylboric ester, the synthesized additives also content of sulfur is relatively high, which results in more
show better hydrolytic stabilities due to the hydrophobic activity corrosive abrasion compared with DSPB when the additives
7
of the long alkyl chain that prevents the nucleophilic attack of are at the same concentration. Therefore, the WSD of TSPB
the boron atom by water.
.2 Anti-wear (AW) performance
and TBPB is higher than that of DSPB.
3
3
.3 Friction-reducing performance
Fig. 2 depicts the relationship between the additive concen-
tration and the wear scar diameter (WSD) of each additive
under 294 N. It is obvious that all the three additives are effi-
cient in improving the anti-wear properties of the base oil at
the tested concentration. Note that the anti-wear performance
of DSPB is the best, while TBPB and TSPB are similar to each
other. For DSPB, it can be seen that the WSD rst decreases
sharply when the additive is added to the base oil, and then
with increasing additive concentration, the decrease in WSD
becomes more gradual. In the case of TBPB and TSPB, the
WSD also decreases rapidly when the additives are added, and
then remains relatively stable with a slightly increasing trend.
These results can be explained by two phenomena: the
adsorption of the additive on the metal surface and the
corrosive abrasion caused by the sulfur in the additive mole-
The coefficient of friction (COF) of the synthesized compounds
at different concentrations under an applied load of 294 N is
exhibited in Fig. 3. It can be seen from the curves that the three
compounds show different friction-reducing properties. For
DSPB, the COF rst increases when the additive concentration
is low, then decreases with an increase in the additive
concentration, and nally remains relatively stable. These
results suggest that the adsorbed additives interact with the
freshly exposed worn surface on the friction pairs to form a
protective adsorption lm and/or reaction lm. When the
additive concentration is low, the protective lm is not
compact enough to resist the high shear force; thus, it could be
damaged easily, whereas the shear strength is high, resulting
in an increase in the COF. With the increase in additive
concentration from 0.5 wt% to 2.0 wt%, more additive mole-
cules adsorb on the metal surface, which leads to the forma-
tion of a more compact protective lm, and the adsorbed
additive molecules act as an adsorption lm, resulting in a
decrease in the shear strength, and therefore the COF
decreases. However, when the concentration is increased
continuously, the COF increases slowly, which indicates that a
component of the protective lm is transformed with the
increase in additive concentration. These details will be dis-
cussed in the surface analysis of the tribolm. For TBPB and
TSPB, the COF does not change much compared with the COF
of the base oil, which indicates that the shear strength and
roughness of the protective lm remains relatively stable with
the increase in additive concentration.
2
8
2
4,25
cules.
Note that TSPB contains a branched chain structure,
which leads to its poor adsorption on the metal surface;
however, DSPB contains a more hydrophobic dodecyl group
than TBPB, which leads to higher solubility in base oil,
resulting in more the enhanced adsorption of DSPB on the
metal surface. On the other hand, when the additive concen-
tration is low, the corrosion of the S element can be ignored,
but with increase in additive concentration, the corrosive
abrasion becomes gradually more signicant and nally
counteracts the generation of a protective lm, resulting in a
dynamic equilibrium of abrasion. Furthermore, the difference
in the anti-wear performance of the synthesized additives can
be ascribed to the difference in the structure of the additive
2
6
molecule. As for the TSPB, due to the more serious corrosive
Fig. 2 The effect of additive concentration on the wear scar diameter Fig. 3 The effect of additive concentration on the coefficient of
(
applied load, 294 N; rotary speed, 1450 rpm; duration, 30 min).
friction (applied load, 294 N; rotary speed, 1450 rpm; duration, 30 min).
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.4 Extreme pressure (EP) performance
Paper
3
reactions occurring on the metal surface of the friction pairs
during the rubbing processes. Thus, formation of a protective
lm covering the metal surface prevents direct metal-to-metal
contact between the surface asperities. The difference in the
components and the surface morphology of the protective lm
may be responsible for the difference in the tribological
performance. Therefore, it is worthwhile to investigate the
nature of the protective lm by performing surface analysis of
the worn surface.
The effect of these S-containing long chain alkyl phenylboric
esters under EP conditions was also investigated in this study.
In general, the purpose of the EP additive is to provide added
wear protection and anti-seizing properties to both metallic

26
surfaces and lubricants under higher loads and temperatures.
The maximum non-seizure loads (P values) of the base oil
HVIW H150), 2.0 wt% DSPB, 2.0 wt% TSPB and 2.0 wt% TBPB
B
(
in the base oil are presented in Fig. 4. Signicantly, all of the
synthesized additives are effective in enhancing the P value of
B
the base oil. It is also clear that TBPB is better than DSPB in 3.5 Surface analysis
increasing the P value, because, according to the element
B
3.5.1 Atomic force microscopy (AFM) analysis. AFM was
analysis, the sulfur content in TBPB is higher than that of DSPB,
leading to an increase in the maximum non-seizure load. Note
that TSPB is the best among the three additives in EP perfor-
mance. It has been reported that in the EP region, the surface
layer is easily removed and a fresh metal surface is exposed
continuously. Therefore, under these rigorous conditions, the
additive that exhibits the best EP performance should be the
one that reacts with the newly formed metal surface to generate
used to examine the general morphology of the reaction lms
generated from the additives under AW conditions. The 2D and
3D AFM topography images of tribolms generated from the
synthesized additives are shown in Fig. 5 and 6. In comparison
with the 2D and 3D AFM images of tribolms generated from
the synthesized additives, it clearly reects that the tribolms
are quite heterogeneous. They are composed of large pads and
are elongated and orientated in the sliding direction, which is
several tens of microns long and several microns wide. This
phenomenon is similar to the tribolm lubricated with ZDDPs,
29
the thickest protective lm. Therefore, TBPB exhibits better EP
performance than DSPB due to its higher sulfur content;
moreover, the P
B
value is also related to the reactivity of sulfur. It
is known that the reactivity of t-dodecylthio group is higher than
27
that of the n-dodecylthio group. Upon the generation of high
shear forces and triboheat due to the EP conditions, the C–S
bond is cleaved and the t-dodecylthio group is more likely to
react with the freshly exposed metal surface to form a protective

lm, resulting in an improvement in the maximum non-seizure
load. However, although the reactivity of TSPB is higher than of
DSPB, the anti-wear property of TSPB does not perform better
than that of DSPB, which is ascribed to a limitation of the
branched group with respect to access to the friction pairs.
Therefore, TSPB performs well in the extreme pressure property
(
P
B
value), but exhibits relatively poor performance in anti-wear
property.
As discussed above, all the three synthesized additives
exhibit good anti-wear and relatively high EP properties, espe-
cially DSPB, which also exhibits friction-reducing performance.
These results make us believe that there are tribological
Fig. 5 AFM topography images (20 Â 20 mm) of boundary films
generated from the synthesized additives along with the topographic
profiles images taken along the dotted lines: (a) DSPB; (b) TSPB; (c)
B
Fig. 4 Maximum non-seizure loads (P values) of the synthesized TBPB (2.0 wt% additive in base oil, applied load, 294 N; rotary speed,
compounds (four-ball machine, rotation velocity of 1760 rpm for 10 s). 1450 rpm; duration, 30 min).
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and DSPB are 145.3 nm, 163.1 nm and 245.2 nm, respectively.
This result demonstrates that a smoother metal surface leads to
a lower COF, which agrees with the data on the friction-
reducing performance shown in Fig. 3.
3.5.2 XANES Analysis of thermal lm and tribolm. The
AFM analysis of the tribolms indicates the formation of a
protective lm by the synthesized additives. In order to obtain
detailed information on the chemical state of the protective

lm on the surface of the frictional pairs, X-ray absorption
near edge structure (XANES) spectroscopy was used to inves-
tigate the thermal lms and tribolms generated from the
synthesized additives in base oil. Several model compounds
were also used for comparison to determine the chemical state
of the lm.
3.5.2.1 Thermal lm. During the rubbing process, the reac-
tions between the adsorbed additives and friction pairs proceed
with the help of triboheat, exoelectron, and a catalytic effect of
35
the renascent metal surface. It is generally accepted that
thermal lms may provide additional information about the
reactions taking place prior to friction. Note that the tempera-
ture at the contact point between the frictional pairs is about
ꢀ
36
1

50 C below moderate AW conditions; therefore, the thermal
ꢀ
lms were prepared at the temperature of 150 C. The B K-edge
(TEY) and S K-edge (FY) XANES spectra of the thermal lms
generated from synthesized additives at 2.0 wt% along with
some model compounds are presented in Fig. 7. Their peak
positions along with some relevant model compounds are listed
in Tables 4 and 5. Compared with the model compounds, the
boron element in the synthesized additives has mainly trans-
formed into trigonal and tetragonal coordination boron, while
the sulfur element has mostly transformed into a sulfate under
the thermal oxidation conditions. Especially in DSPB, a small
peak at 2473.7 eV, which corresponds to alkyl sulde, suggests
that the triboheat generated from the friction pairs plays an
important role in the formation of the protective lms. More-
over, iron sulde or iron disulde is not found in the thermal
Fig. 6 3D topography images (20 Â 20 mm) of boundary films
36
generated from the synthesized additives: (a) DSPB; (b) TSPB; (c) TBPB
(
2.0 wt% additive in base oil, applied load, 294 N; rotary speed, 1450
rpm; duration, 30 min).

lms, which can be attributed to the cover of iron oxide on the
3
0–32
which had been reported previously.
The pads formed by surface of the sample, preventing active sulfur to react with the
22
DSPB are larger and more compact than those of TSPB and metal iron to form FeS or FeS
2
.
TBPB, which explains why DSPB exhibits better anti-wear 3.5.2.2 Tribolm. The B K-edge (TEY) and S K-edge (FY)
properties (Fig. 2). The topography images also allow for an XANES spectra of the tribolms generated from the synthesized
accurate determination of the height prole of the anti-wear additives at 0.5 wt% and 2.0 wt% along with some model
3
2,33
pads
and the roughness of the metal surface of friction compounds are presented in Fig. 8. Their peak positions
34
pairs. A cross-sectional analysis of the pad was performed along with some relevant model compounds are listed in Tables
along the dotted line, which is shown in Fig. 5. The anti-wear 6 and 7.
pads of DSPB, TSPB and TBPB were found to be 540.4 nm, 732.0
From the boron K-edge XANES spectra, it can be seen that
nm and 1048.0 nm thick, respectively. Together with the EP the curves of the tribolms generated from the synthesized
properties presented in Fig. 4 and the 3D AFM topography compounds are similar to those of the thermal lms, and they
images, the results indicate that the TSPB and TBPB are more do not show a signicant difference with increase in additive
active in reacting with the fresh metal surface to generate a concentration except for the peak intensities. Similar to thermal
thicker protective lm, which leads to better EP properties. lms, the boron in the additive has mainly transformed into
Despite the thicker pad of TBPB, the more corrosive abrasion by trigonal and tetragonal coordination boron by the triboheat,
sulfur results in a slightly worse EP performance compared with which also demonstrates that the triboheat generated from the
that of TSPB. Moreover, due to the corrosive abrasion, the friction pairs plays an important role in the formation of
compactness of the protective lm is impacted, which leads to protective lms. The result is consistent with the previous
10,11,37
smaller and more sparse pads, resulting in poor anti-wear reports.
Furthermore, it can be seen that the peak intensity
properties. The values of surface roughness (S
q
) of DSPB, TSPB increases gradually with an increase in the additive
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Table 4 Peak positions of B K-edge (TEY) spectra of thermal films and
model compounds
K-edge
a
b
c
Samples
2
2
2
.0 wt% DSPB
.0 wt% TSPB
.0 wt% TBPB
193.9
193.9
193.9
199.2
199.2
199.2
203.2
203.2
203.2
Model compounds
Na
NaBO
B O
2 3
2
B
4
O
7
193.9
193.9
193.9
199.2
199.2
203.2
203.2
3
Table 5 Peak positions of S K-edge (FY) spectra of thermal films along
with model compounds
K-edge
a
b
c
d
e
Samples
2
2
2
.0 wt% DSPB
.0 wt% TSPB
.0 wt% TBPB
2473.7
2482.4
2482.2
2482.4
Model compounds
FeS
2470.7
2477.2
2477.2
FeS
Na
FeSO
2
2472.0
2
SO
3
4
2482.4
becomes more compact to prevent direct contact with the metal
surface leading to a decline in the WSD; moreover, the rough-
ness of the surface increases, resulting in an increase in the
COF.
With regard to the sulfur K-edge XANES spectra, it is
apparent that the lm chemistry changes under the boundary
conditions of high shear force and triboheat generated from the
rubbing process. This can be explained by the fact that during
the sliding process, the fresh metal surface was exposed
continuously and therefore the active sulfur could react with the
22
fresh metal surface. There are three apparent peaks observed
at 2470.7 eV, 2477.2 eV and 2482.4 eV. According to the spectra
of model compounds, it can be seen that the tribolm is mainly
composed of iron sulde and iron sulfate. In addition, the peak
at 2472.0 eV, attributed to iron disulde can also be found,
Fig. 7 XANES spectra of thermal films generated from synthesized although it is not signicant. For DSPB, it is obvious that the
additives and some model compounds: (a) B K-edge (TEY); (b) S K-
edge (FY).
peak of sulde and disulde is weaker than that of TSPB, while
the peak of sulfate is stronger. The sulde and disulde lms
are easily worn down during the rubbing processes, which leads
26
to inferior anti-wear performance of TSPB. Although the peak
of the sulde and disulde of TBPB is weak (panel F), due to the
corrosion caused by the high content of sulfur, the anti-wear
performance of TBPB is not as good as that of DSPB. This
explains why the anti-wear performance of DSPB is better than
that of TSPB and TBPB.
concentration, which suggests that the additional adsorbed
additive molecules react with the metal surface to generate a
thicker lms. It is known that boron oxide is a very hard
material, which accounts for the decrease in the WSD and an
increase in the COF when the additive concentration is high.
Because harder materials are generated, the protective lm
38
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Table 6 Peak positions of B K-edge (TEY) spectra for thermal films and
model compounds
K-edge
a
b
c
Samples
0
2
0
2
0
2
.5 wt% DSPB
.0 wt% DSPB
.5 wt% TSPB
.0 wt% TSPB
.5 wt% TBPB
.0 wt% TBPB
193.9
193.9
193.9
193.9
193.9
193.9
199.2
199.2
199.2
199.2
199.2
199.2
203.2
203.2
203.2
203.2
203.2
203.2
Model compounds
Na
NaBO
B O
2 3
2
B
4
O
7
193.9
193.9
193.9
199.2
199.2
203.2
203.2
3
Table 7 Peak positions of S K-edge (FY) spectra for thermal films and
model compounds
K-edge
a
b
c
d
Samples
0
2
0
2
0
2
.5 wt.% DSPB
.0 wt.% DSPB
.5 wt.% TSPB
.0 wt.% TSPB
.5 wt.% TBPB
.0 wt.% TBPB
2470.7
2470.7
2470.7
2470.7
2470.7
2470.7
2472.0
2472.0
2472.0
2472.0
2472.0
2472.0
2477.2
2477.2
2477.2
2477.2
2477.2
2477.2
2482.4
2482.4
2482.4
2482.2
2482.4
2482.4
Model compounds
FeS
2470.7
2477.2
2477.2
FeS
Na
FeSO
2
2472.0
2
SO
3
4
2482.4
under the boundary lubrication conditions, a complicated tri-
bolm is generated from the synthesized additives, which
prevents direct contact between the asperities on the surface of
friction pairs. As for the hydrolytic stability, directly linking a
long chain alkyl phenyl with boron is an efficient method to
enhance the hydrolytic stability of the borate ester. Moreover,
the structure of the group around the boron element also leads
to hydrolytic stability. A complicated group is benecial for
improving the hydrolytic stability of the borate ester. However,
for the tribological properties, the amount and activity of the
sulfur have a great inuence on the AW and EP performance.
Furthermore, there is an equilibrium for the amount of sulfur
contained in the additive molecule because of corrosive abra-
sion. With regard to the surface analysis of XANES and AFM, it
can be seen that the tribolm consists of large pads on the
asperities of the metal surface, composed of trigonal and
tetragonal coordination boron, iron sulde and disulde, and
iron sulfate. The compactness and roughness of the tribolm
correspond to the AW and friction-reducing properties. A more
Fig. 8 XANES spectra of tribofilms generated from synthesized addi-
tives and some model compounds: (a) B K-edge (TEY); (b) S K-edge
(
FY) (applied load, 294 N; rotary speed, 1450 rpm; duration, 30 min).
3.6 Discussion
Based on the hydrolytic stability test, tribological evaluation and compact and smooth tribolm results in better AW and friction-
surface analysis by XANES and AFM, it can be inferred that reducing performance.
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9
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25126 | RSC Adv., 2014, 4, 25118–25126
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