Tribological Properties of the Lubricant Containing [617740]

lubricants
Article
Tribological Properties of the Lubricant Containing
Titanium Dioxide Nanoparticles as an Additive
Filip Ilie1,*,†and Cristina Covaliu2,†
1Department of Machine Elements and Tribology, Polytechnic University of Bucharest,
313 Spl. Independentei, 060042 Bucharest, Romania
2Department of Biotechnical Systems, Polytechnic University of Bucharest, 313 Spl. Independentei,
060042 Bucharest, Romania; [anonimizat]
*Correspondence: ilie.filip@yahoo.com or filip@meca.omtr.pub.ro; Tel.: +4-021-402-9411
† These authors contributed equally to this work.
Academic Editors: Antolin Hernández Battez and Rubén González Rodríguez
Received: 4 January 2016; Accepted: 12 April 2016; Published: 21 April 2016
Abstract: To improve the oil-solubility of nanoparticles, a new technology was used to prepare
a kind of lubricant containing titanium dioxide (TiO 2) nanoparticles. The microstructures of the
prepared nanoparticles were characterized via transmission electron microscope (TEM) and infrared
spectroscopy (IR). Tribological properties of TiO 2nanoparticles used as an additive in base oil were
evaluated using four-ball tribometer and ball-on-disk tribometer. In addition, the worn surface of
the steel ball was investigated via polarized microscopy (PM) and X-ray photoelectron spectroscopy
(XPS). The TiO 2nanoparticles can be completely well-dispersed in the base oil under a new process
(NP), which has no significantly negative effect on the anti-oxidation property. The results of the
tribological tests show that TiO 2nanoparticles under the NP show a better anti-wear property and
friction-reducing property in base oil compared to TiO 2nanoparticles under the tradition process (TP).
The main aim of this paper lies in solving with the oil-solubility problem through the combination
effect of surface modification and special blend process of lubricating oil. This method was first used
to prepare lubricant containing TiO 2nanoparticles and then used as additives in engine oil, gear oil,
and other industrial lubricants. At the same time, tribological properties of TiO 2nanoparticles in
base oil as a lubricating additive were also studied.
Keywords: blended oil; lubricants; titanium dioxide (TiO 2) nanoparticles; oil-solubility;
tribological properties
1. Introduction
Conventional lubricant additives are generally thosecompounds containing sulfur, phosphor, or
chlorine, among others [ 1–3] that play an important role in gear lubrication and cutting lubrication.
However, the commercial applications of these kinds of additives are unsatisfactory because of the
pungent odor, extreme corrosion, and the poor thermal stability [ 4–7]. Therefore, it is necessary to
develop new kinds of additives that can be used as a substitute for traditional lubricant additives. The
use of nanoparticles as oil additives and lubricants is a recent idea. Numerous nanoparticles have
been investigated in recent years [ 8–22]. These oil additives contain small particles of solid material,
and their use is not straightforward and only recently has been recognized as feasible. The utilization
of oil additives as nanolubricants presents many advantages, as they are relatively insensitive to
temperature, and tribochemical reactions are limited, compared to traditional additives. Different
anti-friction and anti-wear mechanisms using the nanoparticulate additives have been explained in
previous papers [10,12–14,23,24].
Lubricants 2016 ,4, 12; doi:10.3390/lubricants4020012 www.mdpi.com/journal/lubricants

Lubricants 2016 ,4, 12 2 of 13
Experimental results with nanoparticles used as additives in oil lubricants show that they deposit
on the friction surfaces and improve the tribological properties of the base oil, displaying good
friction and wear reduction features, even at low concentrations. Inter alia , titanium dioxide (TiO 2)
nanoparticles as lubricant additive were studied with much more attention, because of their good
performance on anti-oxidant features, relatively low toxicity, pleasant odor, and non-volatility [ 25–27].
Unfortunately, nanoparticles are not well dispersed in nonpolar organic solvents due to their oleo-philic
property, which limits the nanoparticles applications in lubricants. Now, two approaches to solve this
frequently problem are taken. The first method is the addition of dispersant into base oil. The main
drawback of this method is that the sedimentation is unavoidable after a long-time storage and the
negative effect of same dispersants on tribological properties [ 28–31]. The second method is the usage
of surfactant [31–33].
Based on what we know metal nanoparticles, oxide nanoparticles and hydrate nanoparticles can
change their oil solubility utterly under the effect of surface modification—even transfer from water to
oil phase [ 34,35]. However, titanium (Ti) atoms of TiO 2coordinate with either two or three oxygen
atoms (O) to form TiO 2or Ti 2O3groups, so they are hybridized to a planar or three-dimensional
structure. Such structure units can comprise several different typical groups through various
combinations, which lead to a structure more complex and cause the difficulty of surface modification
of TiO 2. However, the transfer and adhesion of the nanoparticles accelerates surface modification,
self-reduction, and the formation of a fine TiO 2tribofilm that reduced the coefficient friction, pressure,
and temperature in contact area and hence wear. Thus, it can be concluded that both methods (listed
above) are classical and have their own defects (the addition of dispersant or usage of surfactant
into base oil) for solving the oil solubility of TiO 2nanoparticles. This claim is strengthened by the
works of several researchers [ 3,8–11,23,24,36–38]. Therefore, the application process of the two classical
methods was named the traditional process (TP). The TiO 2nanoparticles cannot be well dispersed
in the base oil after the TP , for the sedimentation is unavoidable in time, with negative effect on
tribological properties.
In this paper, a new preparation method—referred to here as the new process (NP)—is suggested
for solving the poor oil solubility of TiO 2. The new technology is no longer restricted to the surface
modification process, but solves the difficult problem through the combination effect of surface
modification and an oil-blending process. The new method was first used to prepare a lubricant
containing TiO 2nanoparticles, and it should be helpful for the TiO 2nanoparticles used as additives in
engine oil, gear oil, and other industrial lubricants [ 39–41]. At the same time, tribological properties of
TiO 2nanoparticles in base oil as a lubricating additive were also studied. Thus, the paper discusses
the anti-friction and anti-wear behavior of TiO 2nanoparticle suspensions in base oil (nanolubricant
or nanofluid) under mixed lubrication using a four-ball tribometer and ball-on-disk tribometer, and
presents PM and XPS analysis of the worn surfaces.
2. Experimental Details
2.1. Materials, Blend Oil Process, and Characterization
Oleic acid for chemical industry (Sigma-Aldrich Co. LLC., Wilmington, NC, USA, from office in
Germany), absolute ethanol (Sigma-Aldrich Chemie GmbH, Munich, Germany), TiO 2nanoparticles,
and petroleum ether (60C–90C, Xilong Chemical Co. Ltd., Guangzhou, Guangdong, China) were of
analytical grade. All the reagents were used without further purification. Distilled water was applied
for all synthesis and processes. API-1509 base oil came from Petro-Brazi Lubricating Oil R&D Institute.
Its typical properties are given in Table 1.

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Table 1. Properties of the API-1509 base oil.
Kinematic Viscosity [mm2/s]to:

40C

100C42.85
6.037
Viscosity index 80
Pour point [C]
9
Flash point [C] 199
Acid value [mgKOH/g] 0.05
Sulfur content [%} 0.03
Degree of saturation [%] 90
Color Yellowish
The typical experimental procedure for preparing surfaces modified particles is as follows:
Analytical-grade titanium tetrachloride (TiCl 4) was adopted as the source material and sodium
hydroxide (NaOH) as mineralizer, which by reaction is obtained TiO 2. A certain amount of TiO 2
(obtained from the reaction TiCl 4and NaOH) was first diluted in the mixture solution of absolute
ethanol and distilled water. An aqueous solution of TiO 2was obtained by mixing one molar
stoichiometric ratio of TiO 2in 50 mL of distilled water. The aqueous solution with 2–3 mol of NaOH
was stirred for several minutes (5 min), resulting in a white colloidal solution. The final volume was
adjusted to 90 mL using distilled water. Therefore, the 90-mL solution was transferred to a 100-mL
Teflon lined auto clave vessel. The sealed vessel was heated to 240C for 12 h, and the resultant
precipitate was dried at 450C for 2 h to obtain TiO 2nanoparticles. Then, TiO 2nanoparticles were
mixed with the petroleum ether containing oleic acid and stirred for 2 h at room temperature. The
white liquid suspended matter was dried by the rotary evaporation method, followed by filtering,
washing repeatedly with water and absolute ethanol to remove unreacted reactants and oleic acid,
followed by drying at 80C for 4 h with a conventional oven, in an ambient air environment.
A certain amount of prepared TiO 2nanoparticles was added to the base oil. The mixture (TiO 2
nanoparticles added in base oil) was exposed to direct insert ultrasound irradiation in ambient air for 10
min. Ultrasound irradiation was accomplished with a high-intensity ultrasonic probe (Ti-horn, 25 kHz,
1200 W/cm2) immersed directly in the reaction solution. Then, the solution was accomplished in an
ultrasonic cleaning bath in ambient air for 15 min. After the above two procedures, the appearance of
the base oil with nanoparticles was of semitransparent suspension. It was heated and stirred at 120C
for 30 min, followed by cooling and standing. Finally, the transparent stable lubricant was obtained.
All lubricants containing 0.1, 0.2, 0.3, 0.4, 0.5 wt% surface-modified TiO 2nanoparticles were
obtained. For comparison, lubricants under the TP of oil-blending were also prepared. The TP by
oil-blending used for comparison with the NP is as follows: the insert ultra-irradiation and ultrasonic
cleaning bath was used under the same condition as the above. Then, it was heated and stirred
at 120C for 30 min, followed by cooling and standing.
It should be noted that heating and stirring were necessary and carried out at the same temperature
for both lubricants (NP and TP) to obtain a transparent stable lubricant.
The morphology and size of the samples were observed using a transmission electron microscope
(TEM) by type JEM-2100F. The samples for TEM were dispersed in absolute ethanol and ultrasonicated
before observation. The adsorption behavior of oleic acid on the TiO 2nanoparticles was characterized
with a FTIR spectrometer on a Bio Rad FTS-165. The wave number ranges from 4000 to 500 cm
1at a
resolution of 4 cm
1.
The dispersing stability of lubricant was measured with TG20C high-speed centrifuge, in
accordance with previous works [ 20–22]. The conditions were: 10,000 rpm for 10 min. In total, 10 g of
sample was placed in the centrifuge tube and centrifuged. The centrifuged precipitate was dried in
muffle and weighted to the nearest 0.1 mg; in the end, the percentage of precipitate was obtained.

Lubricants 2016 ,4, 12 4 of 13
The antioxidation property of lubricant was characterized with a P/N 15300-3 rotating bomb
tester by Seta Corporation England. The test temperature was 150C. Angulations between bomb and
level were 30, and the rotation speed of bomb was 100 rpm.
Tribological properties of TiO 2nanoparticles used as an additive in base oil were evaluated on a
four-ball tribometer and a ball-on-disk tribometer, and the worn surface was investigated by polarized
microscope (PM) with a Leica DM LP type and with X-ray photoelectron spectroscopy (XPS) on a
PHI-5702 electron spectrometer using pass energy of 188 eV and Mg Ka line excitation source with the
reference C1s at 284.6 eV .
2.2. Measurement of Anti-Wear and Friction-Reducing Properties, Analysis of the Friction Surface
Wear tests were carried out with a four-ball tribometer. At the end of each test, the mean wear
scar diameter (WSD) on the three down balls was measured using a digital reading optical microscope
with an accuracy of 0.01 mm. The steel balls used in the tribological test had a diameter of 12.7 mm
made of Rul2 bearing steel with an HRC of 59–61 hardness. Rul2 bearing steel (which is similar to
AISI K 19,195 steel) has a composition according to Table 2.
Table 2. Composition Rul2 (similar to AISI K 19,195) steel.
Symbol C [%] Si [%] Mn [%] P [%] S [%] Cr [%] Mo [%]
Rul2 0.93–1.05 0.45–0.75 1.00–1.20 max 0.025 max 0.015 1.40–1.65 max 0.10
The friction coefficient of the oil was measured both with a model based on the principle of
reciprocating friction on a ball-on-disk tribometer and with a four-ball tribometer according to ASTM
D5183-95. The tribological conditions of model reciprocating friction were: a frequency of 25 Hz, a
stroke length of 1 mm, a temperature of 75C, an applied load of 100 N, a duration of the test of 30 min.
The friction test was conducted in a reciprocating “ball-on-disk” mode, through the oscillation of a
Rul2 steel ball ( F12.7 mm) over a Rul2 steel disk ( F258 mm) in the oil samples being tested. The
arithmetic average surface roughness (R a) of the disk was about 0.016 m with an oscillation of 1 mm
of the ball on disk.
The choice of the contact pair Rul2 against Rul2 is explained by the use of the same material
couples in experimental tests, both for four-ball tribometer and for the ball-on-disk tribometer, in order
to allow a comparison of the results. The major procedure is as follows:
– First, three 12.7 mm diameter steel balls are clamped together and covered with 12.7 mL of OMV
oil white 32 oil. The fourth 12.7-mm diameter ball, referred to as the “top ball”, is pressed with a
force of 400 N (p c2269.6 Pa) into the cavity formed by the three clamped balls and with three
points of contact. The temperature of the wear-in lubricant is regulated at 75C; then, the top
ball is rotated at 600 rpm ( v= 1.25 m/s) for 60 min.
– Second, white oil is discarded and balls cleaned. The WSD on each of the lower three balls is
examined. If the wear scars average of the balls is 0.63 mm 0.03 mm, then the 12.7 mL of test
fluid is added to the ball cup with the worn-in test balls in place. The temperature of the test
lubricant is regulated at 75C, and the top ball is rotated at 600 rpm ( v= 1.25 m/s) at 100 N
(pc= 1429.1 Pa) for 10 min.
– Third, the load is then increased by 100 N (p c= 1429.1 Pa) at the end of each successive 10 min
interval. The friction coefficient is measured at the end of each 10 min interval.
Nota bene: The balls were covered primarily with the base oil and were later added TiO 2
nanoparticles, because a comparative analysis of the wear scars morphology of the balls was necessary,
lubricated with base oil, and then lubricated with base oil containing nanoparticles TiO 2.
The balls after the four-ball test were cleaned using an ultrasonic bath in ligroin (laboratory
solvent similarly petroleum ether). Polarized microscope (PM) analysis was performed with a Leica

Lubricants 2016 ,4, 12 5 of 13
DM LP scope. XPS analyses of the elements on the wear scar were conducted on the PHI-5702
electron spectrometer.
3. Result and Discussion
3.1. Structure and the Dispersing Stability of TiO 2Nanoparticles
Representative TEM images of both kinds of nanoparticles are shown in Figure 1. It can be found
that the appearance of pure TiO 2was a hollow sheet with the size of around 100–200 nm (Figure 1a),
while the average size of surface-modified nanoparticles and thus the size distribution is in the range
of 50–100 nm (Figure 1b). Furthermore, the result indicates that the oil additive can effectively keep
TiO 2nanoparticles from agglomeration. This role of oil additive improves oleo-phobic property of the
nanoparticles and adapts it for use as lubricant additive.
Lubricants  2016, 4, 12  5 of 13 
3. Result and Discussion  
3.1. Structure  and the Dispersing  Stability of TiO 2 Nanoparticles  
Representative  TEM images of both kinds of nanoparticles  are shown in Figure 1. It can be 
found that the appearance  of pure TiO 2 was a hollow sheet with the size of around 100–200 nm 
(Figure 1a), while the average size of surface‐modified  nanoparticles  and thus the size distribution  
is in the range of 50–100 nm (Figure 1b). Furthermore,  the result indicates  that the oil additive can 
effectively  keep TiO 2 nanoparticles  from agglomeration.  This role of oil additive improves  oleo‐
phobic property  of the nanoparticles  and adapts it for use as lubricant  additive.  
 
(a)  (b)
Figure 1. Transmission  electron microscope  (TEM) images: (a) of pure titanium  dioxide (TiO 2); (b) of 
surface‐modified  TiO 2. 
To investigate  the dispersion  stability of nanoparticles  under different  processes,  the centrifuge  
test was carried out and the oil, containing  TiO 2 nanoparticles  under the NP (coded with (a)) was 
compared  with those containing  TiO2 nanoparticles  under the TP (coded with (b)). Surface‐
modified  nanoparticles  were added into the base oil at a fixed concentration  of 0.5 wt%, for both the 
NP and the TP. No precipitate  was detected to the two processes  (NP or TP) after the centrifuge  test, 
which indicates  that each kind of lubricant  has good stability.  
The dispersed  nanoparticles  of TiO 2 will start to stabilize (depending  on the densities  of the 
TiO 2 nanoparticles  and the base oil) as soon as the agitation  stops, and the dispersion  is said to be 
more or less stable, depending  on how long TiO 2 nanoparticles  remain in suspension.  The rate of 
settling is governed  by the density difference  between the TiO 2 nanoparticles  and the base oil, the 
viscosity  of the base oil, and particularly  by the size of the TiO 2 nanoparticles.  
Most commonly  the disperse phase (TiO 2 nanoparticles)  is denser than the continuous  phase 
(base oil), and the larger nanoparticles  will settle out under the action of gravity soon after agitation  
stops. The smaller particles will be subject to two influences:  
(1) Brownian  movement  caused by the impact of liquid molecules  reduces the effect of gravity so 
that at a limiting size the nanoparticles  will stay in suspension  indefinitely.  
(2) The surface energy of the solid/liquid  interface  (TiO 2 nanoparticles/base  oil) increases  with 
decreasing  nanoparticle  size. 
At the same time, the dispersing  stabilities  of nanoparticles  in lubricants  also were studied 
from the aspect of light transmittance  (as shown in Figure 2). It is well known that, the better 
dispersed  the nanoparticles  in the base oil are, the higher the light transmission  of the solution is. It 
can be seen from Figure 2 that the light transmission  of lubricant  (a) is better than that of lubricant  
(b). This phenomenon  indicates  that lubricant  (a) has better dispersion  stability than lubricant  (b). In 
addition,  the lubricant  (a) retains clarity, and no sediments  were detected after four months. 
Figure 1. Transmission electron microscope (TEM) images: ( a) of pure titanium dioxide (TiO 2); (b) of
surface-modified TiO 2.
To investigate the dispersion stability of nanoparticles under different processes, the centrifuge
test was carried out and the oil, containing TiO 2nanoparticles under the NP (coded with (a)) was
compared with those containing TiO2 nanoparticles under the TP (coded with (b)). Surface-modified
nanoparticles were added into the base oil at a fixed concentration of 0.5 wt%, for both the NP and
the TP . No precipitate was detected to the two processes (NP or TP) after the centrifuge test, which
indicates that each kind of lubricant has good stability.
The dispersed nanoparticles of TiO 2will start to stabilize (depending on the densities of the TiO 2
nanoparticles and the base oil) as soon as the agitation stops, and the dispersion is said to be more or
less stable, depending on how long TiO 2nanoparticles remain in suspension. The rate of settling is
governed by the density difference between the TiO 2nanoparticles and the base oil, the viscosity of
the base oil, and particularly by the size of the TiO 2nanoparticles.
Most commonly the disperse phase (TiO 2nanoparticles) is denser than the continuous phase
(base oil), and the larger nanoparticles will settle out under the action of gravity soon after agitation
stops. The smaller particles will be subject to two influences:
(1) Brownian movement caused by the impact of liquid molecules reduces the effect of gravity so
that at a limiting size the nanoparticles will stay in suspension indefinitely.
(2) The surface energy of the solid/liquid interface (TiO 2nanoparticles/base oil) increases with
decreasing nanoparticle size.
At the same time, the dispersing stabilities of nanoparticles in lubricants also were studied from
the aspect of light transmittance (as shown in Figure 2). It is well known that, the better dispersed the
nanoparticles in the base oil are, the higher the light transmission of the solution is. It can be seen from

Lubricants 2016 ,4, 12 6 of 13
Figure 2 that the light transmission of lubricant (a) is better than that of lubricant (b). This phenomenon
indicates that lubricant (a) has better dispersion stability than lubricant (b). In addition, the lubricant
(a) retains clarity, and no sediments were detected after four months.
Lubricants  2016, 4, 12  6 of 13 

 
Figure 2. Appearance  of (a) lubricant  under the NP and (b) lubricant  under the TP. 
The good dispersing  stability of lubricant  (a) can be explained  by the following:  
– First, the effect of surface modification  of oleic acid contributes  to the good dispersion  property.  
– Second, weaker agglomerates  were grossly eliminated  by means of the ultrasound  irradiation  
and direct insert during the novel blending  process. 
– Third, the higher temperature  accelerates  the Brownian  motion of lubricant,  which is helpful to 
the dispersion  of the nanoparticles.  
These factors meet the requirements  of the dispersing  stability of the nanoparticles  in the 
lubricant  [26,42]. Eventually,  the electric double layer of each nanoparticle  formed ensures good 
dispersing  stability of the lubricant.  Double‐layer electrical  charge is formed in the contact areas as 
a result of electrochemical  processes  in the early phase of friction operation  and as a result of 
absorption  of tensio‐active lubricant  under the circumstances.  Therefore,  the base oil containing  
TiO 2 nanoparticles  after the NP possesses  excellent  dispersing  stability.  
In the blending  process after the NP, oils were heated at higher temperature  than the usual 
blending  method, and it may influence  the anti‐oxidation  property  of the lubricant.  To justify the 
influence,  a rotary bomb oxidation  test was adopted to evaluate the oxidative  stability of lubricant  
(a) and of the base oil alone. The results show that the time of the rotary bomb oxidation  test of the 
base oil alone is 58 min and of the base oil containing  TiO 2 nanoparticles  under the NP is 50 min; 
thus, it can be concluded  that the oil blending  process under the NP does not have a significant  
negative  effect on the antioxidation  property  of the base oil. 
3.2. Anti‐Wear and Friction‐Reducing  Properties  of TiO 2 Nanoparticles  
WSD is an indication  of wear extent when sliding contact occurs. WSD results of four‐ball tests 
that were carried out in different  oil samples are shown in Figure 3. 
 
Figure 3. Effect of additive content on the wear scar diameter  (WSD), comparative  to the base oil 
under 200 N. 
Figure 2. Appearance of ( a) lubricant under the NP and ( b) lubricant under the TP .
The good dispersing stability of lubricant (a) can be explained by the following:
– First, the effect of surface modification of oleic acid contributes to the good dispersion property.
– Second, weaker agglomerates were grossly eliminated by means of the ultrasound irradiation
and direct insert during the novel blending process.
– Third, the higher temperature accelerates the Brownian motion of lubricant, which is helpful to
the dispersion of the nanoparticles.
These factors meet the requirements of the dispersing stability of the nanoparticles in the
lubricant [ 26,42]. Eventually, the electric double layer of each nanoparticle formed ensures good
dispersing stability of the lubricant. Double-layer electrical charge is formed in the contact areas
as a result of electrochemical processes in the early phase of friction operation and as a result of
absorption of tensio-active lubricant under the circumstances. Therefore, the base oil containing TiO 2
nanoparticles after the NP possesses excellent dispersing stability.
In the blending process after the NP , oils were heated at higher temperature than the usual
blending method, and it may influence the anti-oxidation property of the lubricant. To justify the
influence, a rotary bomb oxidation test was adopted to evaluate the oxidative stability of lubricant (a)
and of the base oil alone. The results show that the time of the rotary bomb oxidation test of the base
oil alone is 58 min and of the base oil containing TiO 2nanoparticles under the NP is 50 min; thus, it
can be concluded that the oil blending process under the NP does not have a significant negative effect
on the antioxidation property of the base oil.
3.2. Anti-Wear and Friction-Reducing Properties of TiO 2Nanoparticles
WSD is an indication of wear extent when sliding contact occurs. WSD results of four-ball tests
that were carried out in different oil samples are shown in Figure 3.
Under relatively low concentrations of additive, the WSDs of the base oil slightly increased after
adding TiO 2nanoparticles under the NP or the TP; with the increase in additive concentration, WSD
of the samples is greatly decreased. The anti-wear property of TiO 2under the NP is better than that of
TiO 2under the TP (except for the concentration of 0.1 wt%), which can be ascribed to the effect of the
good dispersing stability. The exception that occurs at relatively low concentrations of the additive
(here of 0.1 wt%) seemed to be due to the damming actions of TiO 2nanoparticles accumulated in the
front of the leading ball; they were hard to disperse in the base oil and could not easily penetrate into
the interface of the base oil and not reduce the shear stress, i.e., no friction force.

Lubricants 2016 ,4, 12 7 of 13
Lubricants  2016, 4, 12  6 of 13 

 
Figure 2. Appearance  of (a) lubricant  under the NP and (b) lubricant  under the TP. 
The good dispersing  stability of lubricant  (a) can be explained  by the following:  
– First, the effect of surface modification  of oleic acid contributes  to the good dispersion  property.  
– Second, weaker agglomerates  were grossly eliminated  by means of the ultrasound  irradiation  
and direct insert during the novel blending  process. 
– Third, the higher temperature  accelerates  the Brownian  motion of lubricant,  which is helpful to 
the dispersion  of the nanoparticles.  
These factors meet the requirements  of the dispersing  stability of the nanoparticles  in the 
lubricant  [26,42]. Eventually,  the electric double layer of each nanoparticle  formed ensures good 
dispersing  stability of the lubricant.  Double‐layer electrical  charge is formed in the contact areas as 
a result of electrochemical  processes  in the early phase of friction operation  and as a result of 
absorption  of tensio‐active lubricant  under the circumstances.  Therefore,  the base oil containing  
TiO 2 nanoparticles  after the NP possesses  excellent  dispersing  stability.  
In the blending  process after the NP, oils were heated at higher temperature  than the usual 
blending  method, and it may influence  the anti‐oxidation  property  of the lubricant.  To justify the 
influence,  a rotary bomb oxidation  test was adopted to evaluate the oxidative  stability of lubricant  
(a) and of the base oil alone. The results show that the time of the rotary bomb oxidation  test of the 
base oil alone is 58 min and of the base oil containing  TiO 2 nanoparticles  under the NP is 50 min; 
thus, it can be concluded  that the oil blending  process under the NP does not have a significant  
negative  effect on the antioxidation  property  of the base oil. 
3.2. Anti‐Wear and Friction‐Reducing  Properties  of TiO 2 Nanoparticles  
WSD is an indication  of wear extent when sliding contact occurs. WSD results of four‐ball tests 
that were carried out in different  oil samples are shown in Figure 3. 
 
Figure 3. Effect of additive content on the wear scar diameter  (WSD), comparative  to the base oil 
under 200 N. 
Figure 3. Effect of additive content on the wear scar diameter (WSD), comparative to the base oil
under 200 N.
Furthermore, there is an optimum concentration for TiO 2under the NP and the TP corresponding
to 0.4 and 0.5 wt% for their anti-wear abilities. The WSD of the oil containing TiO 2under the NP
or the TP increased when the concentration was beyond the optimum concentration. This can be
interpreted as granule abrasions that take place in the presence of excess nanoparticles in the friction
area [ 25,33,39]. However, it is observed that the anti-wear property of the TiO 2nanoparticles both
under the TP and under the NP is better than that of the base oil (see Figure 3). The morphology of
the surface is shown in Figure 4, where it can be seen that there are more scratches and deeper scars
by the wear due to excess nanoparticles in the friction area (see Figure 4a). Moreover, one can see the
difference in surface morphology with the traces of wear scars only in the presence of base oil, as can
be seen in Figure 4b, where we notice that the wear scars are in sliding direction in the friction zone
(approximately parallel).
Lubricants  2016, 4, 12  7 of 13 
Under relatively  low concentrations  of additive,  the WSDs of the base oil slightly increased  
after adding TiO 2 nanoparticles  under the NP or the TP; with the increase in additive concentration,  
WSD of the samples is greatly decreased.  The anti‐wear property  of TiO 2 under the NP is better 
than that of TiO 2 under the TP (except for the concentration  of 0.1 wt%), which can be ascribed to 
the effect of the good dispersing  stability.  The exception  that occurs at relatively  low concentrations  
of the additive (here of 0.1 wt%) seemed to be due to the damming  actions of TiO 2 nanoparticles  
accumulated  in the front of the leading ball; they were hard to disperse in the base oil and could not 
easily penetrate  into the interface  of the base oil and not reduce the shear stress, i.e., no friction force.  
Furthermore,  there is an optimum  concentration  for TiO 2 under the NP and the TP 
corresponding  to 0.4 and 0.5 wt% for their anti‐wear abilities. The WSD of the oil containing  TiO 2 
under the NP or the TP increased  when the concentration  was beyond the optimum  concentration.  
This can be interpreted  as granule abrasions  that take place in the presence  of excess nanoparticles  
in the friction area [25,33,39].  However,  it is observed  that the anti‐wear property  of the TiO 2 
nanoparticles  both under the TP and under the NP is better than that of the base oil (see Figure 3). 
The morphology  of the surface is shown in Figure 4, where it can be seen that there are more 
scratches  and deeper scars by the wear due to excess nanoparticles  in the friction area (see Figure 
4a). Moreover,  one can see the difference  in surface morphology  with the traces of wear scars only 
in the presence  of base oil, as can be seen in Figure 4b, where we notice that the wear scars are in 
sliding direction  in the friction zone (approximately  parallel).  
 
(a)  (b) 
Figure 4. Morphology  of the surface with traces of the abrasion  wear in the presence  of excess 
nanoparticles  (a) and only of base oil (b) on the friction area. 
The friction coefficient  is a presentation  of energy loss by friction. The results of base oil alone 
and with oleo‐phobic TiO 2 are shown in Figures 5 and 6. 
 
Figure 5. Effect of additive content onfriction  coefficient  under the ball‐on‐block tribometer.  
Figure 4. Morphology of the surface with traces of the abrasion wear in the presence of excess
nanoparticles ( a) and only of base oil ( b) on the friction area.
The friction coefficient is a presentation of energy loss by friction. The results of base oil alone
and with oleo-phobic TiO 2are shown in Figures 5 and 6.
As shown in Figure 5, the effect of TiO 2content as an additive in the friction coefficient has
the curvilinear trend obtained under the NP similar to that of TiO 2under the TP on the whole, but
with different values. Both kinds of additives (TiO 2nanoparticles under the NP and the TP) can
extraordinarily improve the friction-reducing ability of base oil at the concentration ranging from 0.1
to 0.75 wt%. The friction coefficients of the oils containing TiO 2under the NP or the TP have a steady
reduction with the rise in concentration. In addition, the friction-reducing ability of TiO 2under the
NP is better than that of TiO 2under the TP , except for the concentration of 0.1 wt%. Furthermore,

Lubricants 2016 ,4, 12 8 of 13
there is an optimum concentration for the additive under the NP or the TP , which corresponds to a
concentration of 0.5 wt%. Furthermore, the minimum friction coefficient of TiO 2after the NP or the TP
is 0.115 and 0.120, respectively. Figure 6 shows the relationship between applied loads and friction
coefficients at the same additive concentration of 0.5 wt%. The curvilinear trend of TiO 2under the NP
is very similar to that of TiO 2after under the TP . Both lubricants show the highest friction coefficient
at an approximate load of 300 N. With an increasing applied load, the friction coefficient gradually
becomes smaller.
Lubricants  2016, 4, 12  7 of 13 
Under relatively  low concentrations  of additive,  the WSDs of the base oil slightly increased  
after adding TiO 2 nanoparticles  under the NP or the TP; with the increase in additive concentration,  
WSD of the samples is greatly decreased.  The anti‐wear property  of TiO 2 under the NP is better 
than that of TiO 2 under the TP (except for the concentration  of 0.1 wt%), which can be ascribed to 
the effect of the good dispersing  stability.  The exception  that occurs at relatively  low concentrations  
of the additive (here of 0.1 wt%) seemed to be due to the damming  actions of TiO 2 nanoparticles  
accumulated  in the front of the leading ball; they were hard to disperse in the base oil and could not 
easily penetrate  into the interface  of the base oil and not reduce the shear stress, i.e., no friction force.  
Furthermore,  there is an optimum  concentration  for TiO 2 under the NP and the TP 
corresponding  to 0.4 and 0.5 wt% for their anti‐wear abilities. The WSD of the oil containing  TiO 2 
under the NP or the TP increased  when the concentration  was beyond the optimum  concentration.  
This can be interpreted  as granule abrasions  that take place in the presence  of excess nanoparticles  
in the friction area [25,33,39].  However,  it is observed  that the anti‐wear property  of the TiO 2 
nanoparticles  both under the TP and under the NP is better than that of the base oil (see Figure 3). 
The morphology  of the surface is shown in Figure 4, where it can be seen that there are more 
scratches  and deeper scars by the wear due to excess nanoparticles  in the friction area (see Figure 
4a). Moreover,  one can see the difference  in surface morphology  with the traces of wear scars only 
in the presence  of base oil, as can be seen in Figure 4b, where we notice that the wear scars are in 
sliding direction  in the friction zone (approximately  parallel).  
 
(a)  (b) 
Figure 4. Morphology  of the surface with traces of the abrasion  wear in the presence  of excess 
nanoparticles  (a) and only of base oil (b) on the friction area. 
The friction coefficient  is a presentation  of energy loss by friction. The results of base oil alone 
and with oleo‐phobic TiO 2 are shown in Figures 5 and 6. 
 
Figure 5. Effect of additive content onfriction  coefficient  under the ball‐on‐block tribometer.  
Figure 5. Effect of additive content onfriction coefficient under the ball-on-block tribometer.
Lubricants  2016, 4, 12  8 of 13 

 
Figure 6. Effect of different  loads onfriction  coefficient  under the four‐ball tribometer.  
As shown in Figure 5, the effect of TiO 2 content as an additive in the friction coefficient  has the 
curvilinear  trend obtained  under the NP similar to that of TiO 2 under the TP on the whole, but with 
different  values. Both kinds of additives  (TiO 2 nanoparticles  under the NP and the TP) can 
extraordinarily  improve  the friction‐reducing  ability of base oil at the concentration  ranging from 
0.1 to 0.75 wt%. The friction coefficients  of the oils containing  TiO 2 under the NP or the TP have a 
steady reduction  with the rise in concentration.  In addition,  the friction‐reducing  ability of TiO 2 
under the NP is better than that of TiO 2 under the TP, except for the concentration  of 0.1 wt%. 
Furthermore,  there is an optimum  concentration  for the additive under the NP or the TP, which 
corresponds  to a concentration  of 0.5 wt%. Furthermore,  the minimum  friction coefficient  of TiO 2 
after the NP or the TP is 0.115 and 0.120, respectively.  Figure 6 shows the relationship  between 
applied loads and friction coefficients  at the same additive concentration  of 0.5 wt%. The 
curvilinear  trend of TiO 2 under the NP is very similar to that of TiO 2 after under the TP. Both 
lubricants  show the highest friction coefficient  at an approximate  load of 300 N. With an increasing  
applied load, the friction coefficient  gradually  becomes  smaller. 
The mechanism  of this phenomenon  can be explained  as follows: the additive film is not 
formed in the initial stage; resistance  at friction is higher, so friction coefficient  increases  with the 
increase of the applied load (until 300 N). Once some tribochemical  reactions  have taken place (after 
300 N), the additive film is formed and resistance  at friction gradually  decreases,  which leads to the 
slow decrease  of friction coefficient.  In addition,  it is also observed  that the friction coefficient  of 
base oil with TiO 2 under the NP is better than that of the base oil with TiO 2 under the TP (as shown 
in Figure 5, where the variation  of friction coefficient  with the load applied to the base oil can be 
seen). 
It can be seen that the base oil containing  TiO 2 nanoparticles  under the NP and the TP provide 
much lower friction coefficients  than that of the base oil. This indicates  that TiO 2 nanoparticles  
under the NP and the TP markedly  improved  the friction‐reducing  ability of the base oil, which is 
especially  so for the TiO 2 nanoparticles  under the NP. It can therefore  be inferred that TiO 2 
nanoparticles  under the NP favors an enhancement  of the friction‐reducing  ability of TiO 2 
nanoparticles  in the base oil. 
Additionally,  we observe that, at a low concentration  of TiO 2 nanoparticles,  the base oil 
reduces the friction coefficients  (see Figures 5 and 6) and WSD (see Figure 3) to some extent, and the 
optimal friction‐reducing  and anti‐wear capacities  are obtained  at an TiO 2 nanoparticles  content of 
0.5 wt%. However,  elevating  concentration  of TiO 2 nanoparticles  above 0.5 wt% leads to an obvious 
increase in the friction coefficients  and WSD, which might be attributed  to the agglomeration  of 
TiO2 nanoparticles  under frictional  heat [42]. 
Based on the above analysis,  it can be deduced  that surface‐modified  TiO 2 nanoparticles  under 
the NP possess better anti‐wear and friction‐reducing  ability than that under the TP. These 
complicated  relations  may be attributed  to not only the presence  of Ti in the molecule  structure  but 
also the different  distribution  of Ti in the lubricant.  Two kinds of lubricants  have the same quantity 
of Ti, and the only difference  lies in the oil‐blending  process. Under the NP, surface‐modified  TiO 2 
Figure 6. Effect of different loads onfriction coefficient under the four-ball tribometer.
The mechanism of this phenomenon can be explained as follows: the additive film is not formed
in the initial stage; resistance at friction is higher, so friction coefficient increases with the increase of
the applied load (until 300 N). Once some tribochemical reactions have taken place (after 300 N), the
additive film is formed and resistance at friction gradually decreases, which leads to the slow decrease
of friction coefficient. In addition, it is also observed that the friction coefficient of base oil with TiO 2
under the NP is better than that of the base oil with TiO 2under the TP (as shown in Figure 5, where
the variation of friction coefficient with the load applied to the base oil can be seen).
It can be seen that the base oil containing TiO 2nanoparticles under the NP and the TP provide
much lower friction coefficients than that of the base oil. This indicates that TiO 2nanoparticles under
the NP and the TP markedly improved the friction-reducing ability of the base oil, which is especially
so for the TiO 2nanoparticles under the NP . It can therefore be inferred that TiO 2nanoparticles under
the NP favors an enhancement of the friction-reducing ability of TiO 2nanoparticles in the base oil.
Additionally, we observe that, at a low concentration of TiO 2nanoparticles, the base oil reduces
the friction coefficients (see Figures 5 and 6) and WSD (see Figure 3) to some extent, and the optimal
friction-reducing and anti-wear capacities are obtained at an TiO 2nanoparticles content of 0.5 wt%.
However, elevating concentration of TiO 2nanoparticles above 0.5 wt% leads to an obvious increase in
the friction coefficients and WSD, which might be attributed to the agglomeration of TiO2 nanoparticles
under frictional heat [42].

Lubricants 2016 ,4, 12 9 of 13
Based on the above analysis, it can be deduced that surface-modified TiO 2nanoparticles under
the NP possess better anti-wear and friction-reducing ability than that under the TP . These complicated
relations may be attributed to not only the presence of Ti in the molecule structure but also the different
distribution of Ti in the lubricant. Two kinds of lubricants have the same quantity of Ti, and the only
difference lies in the oil-blending process. Under the NP , surface-modified TiO 2nanoparticles have
good dispersing stability in base oil, so it can be supposed that both the presence and the distribution
of Ti on the surface play a major role in tribological behavior.
3.3. Tribological Mechanisms of TiO 2Nanoparticles
Figure 7 shows the wear scar morphology of the ball lubricated by base oil alone (Figure 7a) and
base oil containing 0.5 wt% TiO 2under the NP (Figure 7b), respectively under the TP (Figure 7c) and
under 100 N using ball-on-disk tribometer.
Lubricants  2016, 4, 12  9 of 13 
nanoparticles  have good dispersing  stability in base oil, so it can be supposed  that both the presence  
and the distribution  of Ti on the surface play a major role in tribological  behavior.  
3.3. Tribological  Mechanisms  of TiO 2 Nanoparticles  
Figure 7 shows the wear scar morphology  of the ball lubricated  by base oil alone (Figure 7a) 
and base oil containing  0.5 wt% TiO 2 under the NP (Figure 7b), respectively  under the TP (Figure 7c) 
and under 100 N using ball‐on‐disk tribometer.  
 
(a)  (b) (c) 
Figure 7. Micrographs  of worn surfaces lubricated  by base oil alone (a) and TiO 2 nanoparticles  
under the NP (b) and under the TP (c), respectively,  added in oil (0.5 wt%). 
It can be seen that there are more scratches  and deeper furrows on the wear scar obtained  from 
the metal surface lubricated  by base oil alone (see Figure 7a) than that of TiO 2‐added oil under the 
NP (see Figure 7b) and the TP (see Figure 7c). Through  further checks, it is observed  that TiO 2 both 
under the NP and the TP play an important  role in reducing  friction coefficient  and improving  the 
anti‐wear ability of the base oil; however,  that role is more significant  under the NP. Therefore,  the 
main role of the NP of oil‐blending  is to allow the evenly distribution  of Ti nanoparticles  in the 
friction area at the time when they are required.  
The element composition  of wear scar lubricated  with base oil containing  0.5 wt% TiO 2 
nanoparticles  was measured  using XPS. Titanium  (Ti2p), oxygen (O1s), iron (Fe 2p), and carbon (C1s) 
elements  were detected,  and the results are shown in Figure 8. 
Figure 7. Micrographs of worn surfaces lubricated by base oil alone ( a) and TiO 2nanoparticles under
the NP ( b) and under the TP ( c), respectively, added in oil (0.5 wt%).
It can be seen that there are more scratches and deeper furrows on the wear scar obtained from
the metal surface lubricated by base oil alone (see Figure 7a) than that of TiO 2-added oil under the NP
(see Figure 7b) and the TP (see Figure 7c). Through further checks, it is observed that TiO 2both under
the NP and the TP play an important role in reducing friction coefficient and improving the anti-wear
ability of the base oil; however, that role is more significant under the NP . Therefore, the main role of
the NP of oil-blending is to allow the evenly distribution of Ti nanoparticles in the friction area at the
time when they are required.
The element composition of wear scar lubricated with base oil containing 0.5 wt% TiO 2
nanoparticles was measured using XPS. Titanium (Ti 2p), oxygen (O 1s), iron (Fe 2p), and carbon (C 1s)
elements were detected, and the results are shown in Figure 8.
It can be seen that the binding energies of Ti 2plocating 464.6 (Ti 2p1/2 ) and 458.8 (Ti 2p3/2 ) eV
correspond to Ti 2O3and Fe 2O3, respectively, which suggests that oleo-phobic TiO 2was degraded to
Ti2O3and reacted with metal surface. The Ti 2pspectrum mostly appears in the Ti4+oxidation state
with the small contribution of Ti3+. Ti3+occurs due to oxygen deficiency in the TiO 2lattice [43]. Peak
shift is detected, which indicates a decrease of the coordination number of Ti and the shortening of
the Ti–O bond. This result supports that the presence of Ti4+is decreased. The Ti 2ppeak at 458.8 eV
demonstrates that TiO 2is deposited on a worn metal surface. Furthermore, it is clearly seen that TiO 2
nanoparticles have a Ti:O ratio different to a pure one. This can be ascribed to a presence of Fe 2pon
the surface of TiO 2. The shift of the binding energy resulted from the weak signal because the atomic
sensitivity factors of Ti 2pare much weaker than that of other elements [ 32], and the peaks cannot be
attributed to any other elements [29,32,39].

Lubricants 2016 ,4, 12 10 of 13
Lubricants  2016, 4, 12  10 of 13 

 
Figure 8. X‐ray photoelectron  spectroscopy  (XPS) of the worn surface. 
It can be seen that the binding energies of Ti2p locating 464.6 (Ti2p1/2) and 458.8 (Ti 2p3/2) eV 
correspond  to Ti2O3 and Fe2O3, respectively,  which suggests  that oleo‐phobic TiO 2 was degraded  to 
Ti2O3 and reacted with metal surface. The Ti2p spectrum  mostly appears in the Ti4+ oxidation  state 
with the small contribution  of Ti3+. Ti3+ occurs due to oxygen deficiency  in the TiO 2 lattice [43]. Peak 
shift is detected,  which indicates  a decrease  of the coordination  number of Ti and the shortening  of 
the Ti–O bond. This result supports  that the presence  of Ti4+ is decreased.  The Ti2p peak at 458.8 eV 
demonstrates  that TiO 2 is deposited  on a worn metal surface. Furthermore,  it is clearly seen that 
TiO 2 nanoparticles  have a Ti:O ratio different  to a pure one. This can be ascribed to a presence  of 
Fe2p on the surface of TiO 2. The shift of the binding energy resulted from the weak signal because 
the atomic sensitivity  factors of Ti2p are much weaker than that of other elements  [32], and the peaks 
cannot be attributed  to any other elements  [29,32,39].  
The Ti2p peaks are shifted positive from the baseline,  indicating  a formation  of Fe–O–Ti bonds. 
This can be attributed  to the formation  of binary TiO 2/Fe 2O3 oxides. Fe2p spectra showed a presence  
of Fe2+ and Fe3+. The binding energy of iron (Fe 2p) locates at 707.1 and 711.2 eV, respectively,  
corresponded  to Fe3+ (oxide), i.e. to Fe2O3 [30,32,40–42,44].  Fe2p spectra showed a presence  of Fe2+ and 
Fe3+. Peak at 711.2 eV suggests  the presence  of a minor portion of Fe2+ ions; by exposure,  surface Fe2+ 
lost their electron and changed their ionic state to Fe3+. An O1s peak appears at 531 eV, which is 
attributed  to a signal of oxygen in the TiO 2 lattice (Ti–O bonds). It can be seen that the O1s peak 
decreased  due to the formation  of Fe–O bonds on the surface of TiO 2. Another peak occurred  at 
531.5 eV, which corresponds  to the adsorption  of OH‐group.  
The Fe2p peak at 711.2 eV indicates  that iron is oxidized  into Fe2O3. Combined  with the binding 
energy of 531.5 eV in O1s spectra that decreased  due to formation  of Fe‐O bonds on the surface of 
TiO 2 and it could be further ascertained  that Fe2O3 must be produced.  The O1s peak at 531.5 eV and 
the C1s peak at 288.5 eV reveal and the existence  of carbonyl  group, and can be explained  by the 
high content of carbon absorbed  from the air.  
Figure 8. X-ray photoelectron spectroscopy (XPS) of the worn surface.
The Ti 2ppeaks are shifted positive from the baseline, indicating a formation of Fe–O–Ti bonds. This
can be attributed to the formation of binary TiO 2/Fe 2O3oxides. Fe 2pspectra showed a presence of Fe2+
and Fe3+. The binding energy of iron (Fe 2p) locates at 707.1 and 711.2 eV , respectively, corresponded
to Fe3+(oxide), i.e.to Fe 2O3[30,32,40–42,44]. Fe 2pspectra showed a presence of Fe2+and Fe3+. Peak
at 711.2 eV suggests the presence of a minor portion of Fe2+ions; by exposure, surface Fe2+lost their
electron and changed their ionic state to Fe3+. An O 1speak appears at 531 eV , which is attributed to
a signal of oxygen in the TiO 2lattice (Ti–O bonds). It can be seen that the O 1speak decreased due
to the formation of Fe–O bonds on the surface of TiO 2. Another peak occurred at 531.5 eV , which
corresponds to the adsorption of OH-group.
The Fe 2ppeak at 711.2 eV indicates that iron is oxidized into Fe 2O3. Combined with the binding
energy of 531.5 eV in O 1sspectra that decreased due to formation of Fe-O bonds on the surface of TiO 2
and it could be further ascertained that Fe 2O3must be produced. The O 1speak at 531.5 eV and the C 1s
peak at 288.5 eV reveal and the existence of carbonyl group, and can be explained by the high content
of carbon absorbed from the air.
From the above analysis, we can reasonably infer that the friction-reducing and anti-wear
mechanisms of the TiO 2nanoparticles under the NP in base oil are mainly attributed to two aspects.
First, oleic acid can be physically and chemically adsorbed on the sliding metal surfaces to form an
adsorption film, thereby reducing friction and wear of the metal sliding pair. Second, a complex
boundary lubrication film mainly composed of oxides of iron and titanium is formed on worn metal
surfaces through tribochemical reaction, which also accounts for reduced friction and wear of the
metal sliding pair.
Thus, it can be concluded that tribological reaction occurred between surface-modified TiO 2
nanoparticles and the metal surface during the sliding process, accompanied by the adsorption of

Lubricants 2016 ,4, 12 11 of 13
decomposed nanoparticles. Both these depositions and tribochemical reaction products (Ti 2O3, Fe 2O3)
mainly function with the good tribological properties of surface-modified TiO 2nanoparticles.
4. Conclusions

Nanoparticles are not well dispersed in nonpolar organic solvents due to their oleo-philic property,
which limits their applications in lubricant oils. A new technology for improving the poor oil
solubility of TiO 2nanoparticles in base oil is thus suggested.

Nanoparticles added in oil possessing excellent dispersing stability were obtained under the
new technology. The nanoparticle suspensions tested exhibited reductions of friction and wear
compared to the base oil. TiO 2suspensions under the NP and under the TP presented similar
friction and wear behavior as a function of nanoparticle content. Such research might be
helpful to overcome the difficulty of the usage of TiO 2nanoparticles in gear lubrication and
cutting lubrication.

The obtained results indicate that the average size of the prepared nanoparticles is in range
of 50–100 nm, and the surface of the nanoparticles was altered from oleo-philic to oleo-phobic. In
addition, the nanoparticles can be well dispersed in the base oil totally under the NP , which has no
significantly negative effect on the anti-oxidation property.

The results of the tribological experiments indicate that TiO 2nanoparticles under the NP show
friction-reducing and better anti-wear property in the base oil compared to TiO 2nanoparticles
under the TP .

Based on the results of PM and XPS, it can be deduced that a continuous resistance film containing
depositions and the tribochemical reaction products such as Ti 2O3and Fe 2O3formed during the
sliding process lead to excellent tribological properties of the nanoparticles in the base oil.

The main aspect of the novelty of this research lies in dealing with the oil-solubility problem
through the combination effect of surface modification and special blend process of lubricating
oil, and this method was first used to prepare lubricants containing TiO 2nanoparticles. It
should be helpful for the TiO 2nanoparticles used as additives in engine oil, gear oil, and other
industrial lubricants.
Author Contributions: Filip Ilie and Cristina Covaliu conceived and designed the experiments; Filip Ilie
performed the experiments; Filip Ilie analyzed the data; Cristina Covaliu contributed materials, prepared solutions,
and analysis tools; Filip Ilie wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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