Enhanced lubrication by core-shell TiO2 nanoparticles modified with gallic acid ester

Journal Pre-proof Enhanced lubrication by core-shell TiO2 nanoparticles modified with gallic acid ester Frank T. Hong, Ameneh Schneider, S. Mani Sarathy PII:

S0301-679X(20)30105-5

DOI:

https://doi.org/10.1016/j.triboint.2020.106263

Reference:

JTRI 106263

To appear in:

Tribology International

Received Date: 31 August 2019 Revised Date:

9 February 2020

Accepted Date: 9 February 2020

Please cite this article as: Hong FT, Schneider A, Sarathy SM, Enhanced lubrication by core-shell TiO2 nanoparticles modified with gallic acid ester, Tribology International (2020), doi: https://doi.org/10.1016/ j.triboint.2020.106263. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Enhanced Lubrication by Core-Shell TiO2 Nanoparticles Modified with Gallic Acid Ester Frank T. Honga,*, Ameneh Schneiderb, S. Mani Sarathya,* a

Physical Sciences and Engineering (PSE) Division, Clean Combustion Research Center, King

Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia b

Optimol Instruments Prüftechnik GmbH, Munich 81369, Germany

Corresponding Author: Frank T. Hong; S. Mani Sarathy Address: Clean Combustion Research Center, Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia Mail: [email protected]; [email protected]

1

Abstract: Nanoparticles (NPs) added to oils have been demonstrated to enhance engine

2

lubrication, but sedimentation issues are barriers to their implementation. Here, we engineer

3

TiO2 NPs with polyphenol derivatives (2-octyldodecyl gallate, ODG), not only to improve

4

dispersion stabilities but also to introduce phenol-related tribochemical reactions. To that end, we

5

compare dispersion and lubrication performances of ODG modified TiO2 (ODG@TiO2) NPs

6

with unmodified ones in two base oils, polyalphaolefin and commercial engine oil. Surface

7

analysis results reveal the triboactive role played by ODG in ODG@TiO2 NPs, which helps

8

them to reduce more friction than with non-modified NPs, to generate thicker tribofilms, and to

9

strengthen surface mechanical properties. Thus, we expect further explorations of polyphenol

10

derivatives to improve NPs properties in lubricant applications.

11

Keywords: Nanoparticles; Polyphenol; Friction; Tribofilm.

1

1

1. Introduction

2

The automotive industry is in urgent need of improved fuel economy, particularly in

3

applications in which mechanical friction causes energy losses. Advancements in lubricants can

4

result in less frictional energy loss and improve mechanical efficiency [1]. Oil lubrication

5

minimizes interfacial friction in internal combustion engines in boundary lubrication, mixed

6

lubrication, and hydrodynamic (or film) lubrication regimes [2,3]. Reducing friction in boundary

7

lubrication and mixed lubrication regimes is particularly important. Friction caused by rough

8

surface contact during boundary lubrication and mixed lubrication regimes is far greater than

9

friction in the hydrodynamic lubrication regime [4]. By blending 0.5% to 1.0% nanoparticles

10

(NPs) into the lubricating oil, frictional energy loss and severe wear tracks were effectively

11

reduced [5]. Metal-based NPs (Fe, Cu, and Co [6]), and metal oxide-based NPs (ZnO, CuO,

12

ZrO2 [7]), in sizes ranging from 20 nm to 100 nm, effectively reduced friction in the boundary

13

lubrication regime [8]. Adding NPs is believed have the following benefits (1) polish surface

14

asperities when rolling over the contact area [9], (2) lower friction by rolling over the surface

15

like ball-bearing objects [10], and (3) supply additional cations in tribochemical reactions,

16

thickening the tribofilm on the contacting surfaces [11].

17

TiO2 NPs have been of great interest for the lubrication applications because of their proven

18

friction-modifying functionality [12–15], low toxicity [16], and ease of their synthesis [17].

19

Nonetheless, the undesired wear behavior caused by TiO2 NPs formulation in oil has been

20

concerned [15]. One possible explanation for this wear behavior caused by TiO2 NPs is that the

21

soft metal-organic tribofilm could be easily worn off by the NPs, resulting in inadequate

22

tribofilm formation during reciprocation in the boundary lubrication regime [18]. Currently,

23

Kumara et al. discussed that the lubrication behaviors can vary because of the component,

2

1

structure, and size of NPs applied [19]. The triboactive part used to modify the surface properties

2

of NPs can contribute to wear protection [20]. However, the tribological application of surface

3

modified TiO2 NPs focuses on dealing with NPs sedimentation issues using surfactants, such as

4

fatty acids [21,22]. In this work, we seek to investigate whether surface-functionalized TiO2 NPs

5

with specific triboactive components could help reduce friction, eliminate the micro-abrasions in

6

boundary lubrication, and follow similar tribological behaviors/mechanisms reported by Ye et al

7

[23].

8

Aggregation/sedimentation of NPs in oil lubrication has long been a concern for industrial

9

applications [24]. NPs tend to minimize the surface free energy by aggregating (reduce surface

10

area) when being blended in the non-polar base oils. Enhancing the dispersion stability of NPs by

11

formulating them with surfactants or detergents could be an easy solution, but have raised

12

concerns of stability due to weak electrostatic interactions or possible undesirable interactions

13

with other additives [25]. Surface modification has been considered to be a more reliable

14

technique but should be carefully applied, considering that he affinity of the surface modifying

15

agent could be either too strong to maintain the original structure of NPs, or too weak to graft

16

stably on the NPs surfaces [26]. Surface modifying agents, such as carboxylic acids,

17

polymer/copolymer, silanes, and organophosphorus, have been shown to suitable to improve

18

dispersion in lubrication [27]. However, little attention was paid to the versatile, non-toxic, and

19

abundant resources from polyphenols to modify NPs for lubricant applications.

20

The building block of polyphenol, gallic acid derivatives have been shown to effectively

21

modify the NPs, such as ZnO [28], Fe3O4 [26], and TiO2 [29]. In this work, we select the building

22

block of polyohenol, gallic and esterify it into 2-octyldodecyl gallic acid ester (ODG), for which

23

hydroxyl, benzyl, and alkyl groups serve different roles in modifying TiO2 NPs. The hydroxyl

3

1

groups form chemical bonds between the metal/metal oxides and ODG[30,31], which assures

2

the surface modification stability. The aliphatic of ODG alters NPs from hydrophilic to

3

lipophilic, which enhances dispersion stability. The benzyl rings of ODG act as triboactive

4

components, which could help tribological properties of NPs [19].

5

In this study, we compare lubrication and dispersion performances of ODG modified TiO2 NPs

6

(ODG@TiO2 NPs) with unmodified ones in two different oil lubrication systems,

7

polyalphaolefin (PAO, no other additives) and Helix 10W30 engine oil (HEO, containing

8

ZDDPs). An Optimol-SRV®5 reciprocation tribometer with ball-on-disk configuration was

9

applied to generate the coefficient of friction (COF) curves in the boundary lubrication regime.

10

Besides, this work determines the tribological performances of each blend test via the following

11

surface analysis tools: Zygo NVP7300 Optical Profiler for quantizing of ball and flat wear scar

12

volume; Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) for investigating

13

tribochemical reactions ; the scanning electron microscopy (SEM) coupled with energy-

14

dispersive X-ray spectroscopy (SEM-EDS) for understanding wear scar morphology; the

15

transmission electron microscopy (TEM) coupled with EDS and focused ion beam (FIB) for

16

analyzing generated tribofilm cross-sectional morphologies and their elemental profiles.

17

2. Experimental Method

18

2.1. Materials

19

All solvents and chemicals used in this work were analytical grades, purchased from Sigma-

20

Aldrich. Anatase TiO2 NPs, with single-particle sizes ranging from 20 to 25 nm, were used in

21

this study. The surface modifying agent, 2-octyl dodecyl gallic acid ester (ODG) was synthesized

22

from gallic acid monohydrate and 2-octyldodecanol, with catalyst p-toluene sulfonic acid

23

(PTSA). The base oil, polyalphaolefin 10 (PAO) was obtained from Anhui Jinao Chemical Co.,

4

1

Ltd. Fully formulated engine commercial engine oil used was Helix engine oil with viscosity

2

grade 10W-30 (HEO).

3

2.2. Synthesis of surface modifying agent and surface-modified NPs

4

Similar procedures for preparing ODG were adopted from a previous study [32]. Gallic acid

5

monohydrate was vacuum-dried under 100

for two hours to eliminate other side reactions

6

caused by moisture. Gallic acid (5.0 g, 30 mmol), 2-octyldodecanol (26.8g, 90 mmol), and PTSA

7

(0.05 g) were reacted in a mixture of anisole and nitrobenzene (25 mL, molar ratio, 15:1) under

8

reflux condition at 160

9

prevent possible oxidation (Scheme 1). After cooling, the prepared solution was dissolved in

10

ethyl acetate (30 mL) and washed successively with saturated solutions of NaHCO3 (100 mL)

11

and NaCl (100 mL). The organic layer was recrystallized in petroleum ether and dried overnight

12

at 60

13

petroleum ether/ethyl acetate (3:7) as the eluent.

for 12 hours, where nitrogen was purged into a double-necked flask to

under a vacuum. The residue was then purified by column chromatography, using

14 15

Scheme 1 Esterification of gallic acid with 2-octyldodecanol (R-OH, R=20)

16

The method to prepare ODG@TiO2 NPs was modified from a previous study [33]. Purified

17

ODG (2.245g, 50 mmol) was added by droplets into an ethanol/water solution (50 mL, ratio=1:4)

18

containing TiO2 (0.79 g, 10 mmol), where the TiO2 solution turned from milky white to an

19

orange colloidal solution within a few minutes (Scheme 2). The ethanol/water solution contained

20

TiO2 was pretreated with concentrated sulfuric acid to PH=4 before adding the ODG. The acidic

21

solution promotes surface modification since acid-catalyzed surface reactions facilitate

5

1

interactions between the surface of the NPs and the hydroxyl groups on ODG [29]. The mixed

2

solution (containing ODG and TiO2) was stirred for 12 hours. 120 mL methanol (ca. 150 mL)

3

was added to the mixed solution (containing ODG and ODG@TiO2) and centrifuged to collect

4

the precipitates. This washing procedure was repeated three times to ensure no unreacted ODG

5

remained the synthesized ODG@TiO2 NPs. The remaining orange/dark slurry was dried under

6

vacuum overnight.

7 8

Scheme 2 Straightforward in-situ surface polymerization of ODG@TiO2: (a) adsorption of ODG

9

on the TiO2 surface (b) polymerization of ODG at the weak-acid condition

10

2.3. Characterizations of synthesized ODG@TiO2 NPs

11

The size of TiO2 NPs before surface modification was firstly imaged using SEM to confirm the

12

single-particle size as specified by the provider. Fourier transform infrared (FTIR) analysis was

13

performed on both ODG@TiO2 NPs and synthesized ODG, using a Thermo Nicolet iS-10

14

instrument. Samples were prepared for transmission mode analysis using a KBr window and

15

scanned between 400 and 4000 cm−1 with 4 cm−1 resolution. The weight percentage of ODG

16

coating on the TiO2 NPs, and the thermal stability, were evaluated using Netzsch TG209F

6

1

thermogravimetric analysis (TGA) by 10 °C/min heating rate from 25°C to 800 °C flowing with

2

20 mL/min nitrogen.

3

The determination of NP sedimentation kinetics can be evaluated by various methods;

4

applying UV-Vis spectroscopy has been suggested as a more reliable tool for such evaluations

5

[24]. The relative concentration of suspended NPs in the base oil is proportional to the light

6

adsorption intensity, according to the Beer-Lambert Law in Eq. 1:

7

= ∙ ∙

8

, where A is the absorbance, ε is the molar extinction coefficient (M-1 m-1), c is the concentration

9

(mol L-1), and l is the path length (m). 0.5 w.t.% NPs (with and without surface modification) in

10

n-hexadecane was sonicated for 10 minutes before measurement using UV-VIS-Lambda 950

11

(Perkin Elmer) with 220 nm transparent cuvettes. Using hexadecane as dispersion fluid gives

12

advantages in evaluating the dispersion stability of NPs in non-polar solvents without the

13

concerns of involving unknown dispersants in HEO, the structural effects brought by the PAO10,

14

and physical properties characterized by different base oils. Detailed calculation methods and

15

related data calculation tables can be found in Table S1.

(1)

16

Zeiss Merlin scanning electron microscope (SEM) imaging was performed to study the

17

nanostructure of synthesized ODG@TiO2 NPs; further characterization on synthesized

18

ODG@TiO2 nanostructure was performed using core-loss electron energy loss spectroscopy

19

(EELS) data for elemental mapping and transmission electron microscopy (TEM). ODG@TiO2

20

NPs were prepared in ethanol solution for better dispersion on the TEM grid. An ultrathin carbon

21

film on Holey Carbon support film, 400 mesh, Cu (grid) was used to minimize carbon noise

22

background for EELS data analysis. Elemental mapping was performed at 300 kV with a Titan

23

(60-300kV) TEM, equipped with Probe Cs (spherical aberration) corrector, a high brightness

7

1

electron gun (x-FEG), and a Gatan Quantum 966 imaging filter (GIF). Core-loss spectra of

2

carbon, titanium, and oxygen were acquired in STEM mode with about 20 mrad semi

3

convergence angle and a 400 pA beam current.

4

2.4. Evaluation of the tribological performance of ODG@TiO2 in PAO and HEO

5

NPs blends were prepared at 0.5 w.t.% and 1.0 w.t.%. The 1.0 % TiO2 NPs blending ratio has

6

been set as a benchmark for comparison since previous work on using TiO2 has shown that this

7

is the best blending concentration for enhanced tribological performance [15]. All testing balls

8

and disks were cleaned with petroleum ether and acetone in a sonication bath, and dried before

9

testing to prevent contamination. The measurements of coefficient of friction (COF) for each

10

formulated sample were conducted by Optimol Instruments using an SRV®5 Tribometer with a

11

linear oscillating movement. The tests parameters were as follows: Load = 100 N (Specific mean

12

pressure, Pmean = 1.45 GPa and maximum pressure, Pmax =2.17 GPa), frequency = 25 Hz, Stroke

13

= 1 mm, test temperature = 50°C, test duration = 30 min. The test specimens were 10 mm steel

14

balls (polished, material: 100Cr6) and 24 x 7.9 mm disks (both surfaces lapped, surface

15

roughness 0.50-0.65 µm Rz, material: 100Cr6, hardened). After friction evaluations, the surface

16

wear volumes of tested disks and balls were studied using a Zygo NVP7300 Optical Profiler.

17

Surface elemental mapping of wear tracks was performed using a Zeiss Merlin SEM, coupled

18

with the Oxford Instrument energy-dispersive X-ray spectroscopy (EDS) with the electron high

19

tension and probe current set to be 12 keV and 2 nA, respectively. For the surface cross-section

20

analysis, the wear scar on the disk was lifted 10 mm by a gallium ion source using the Helios

21

focused ion beam (FIB), where the thickness of the tribofilm was quantized and imaged with the

22

qualitative mapping of elemental compositions using TEM-EDS. Before cross-sectional analysis

23

on the wear scars using FIB, the tribofilm was covered with protective platinum (ca 0.5 µm)

8

1

using a microscopy pen followed by electron beam deposition to form a protective platinum

2

layer.

3

Raman Spectroscopy (Horiba LabRAM ARAMIS) with Cobolt-source visible (blue) light, 473

4

nm was used to determine the chemical profile of TiO2 derived tribofilms. Spectrum was

5

collected using an objective lens (x50) with 1800 grating, 25% filter, and 300 µm confocal hole,

6

10 second/point acquisition time, 5 accumulation, and 1-second real-time display. XPS studies

7

were carried out in a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al

8

Ka X-ray source (hν = 1486.6 eV) operating at 150 W, a multi-channel plate and delay line

9

detector under a vacuum of ~10-9 mbar. All spectra were recorded using an aperture slot of 300

10

µm x 700 µm. Survey spectra were collected using a pass energy of 160 eV and a step size of 1

11

eV. A pass energy of 20 eV and a step size of 0.1 eV were used for the high-resolution spectra.

12

For XPS analysis, samples were mounted in floating mode in order to avoid differential

13

charging. Charge neutralization was required for all samples. Binding energies were referenced

14

to the C 1s binding energy of adventitious carbon contamination which was taken to be 284.8

15

eV.

16

Nanoindentation mechanical properties of the derived tribofilms were determined using the

17

Nanotest Vantage with a standard diamond Berkovich tip; calibration of the tip was performed

18

before the measurements, in which the calibration curves were used to construct the function for

19

fitting the data and determining the hardness and reduced modulus of contacting substrates. 25

20

sampling indentation points were set with indentation depths from 10 nm to 200 nm. The initial

21

loading of the diamond Berkovich tip on the sample was set at 0.03 mN; the loading and

22

unloading rate was 1 mN/s. After measuring, suitable unloading curves within the tribofilm were

23

collected and calculated to determine hardness and reduced modulus.

9

1

3. Results and Discussions

2

3.1. Synthesis and characterization of ODG@TiO2 NPs

3

Fig. 1a indicates the core-shell structure of ODG@TiO2 NPs, wherein carbon signal can be

4

assigned to the destructed ODG layer. The high electron energy (300 kV) used for elemental

5

mapping in TEM-EELS could lead to thinner shelled organic layer. SEM images are also

6

provided in Fig. S1 for confirming the nanostructure of ODG@TiO2 NPs and its original

7

thickness of shelled organic layer (5 nm to 10 nm). The shelled organic layer on ODG@TiO2

8

NPs can be attributed to the polymerization of gallic acid derivatives, ODG [29].

9

The TG and DTG curves in Fig. 1b confirm the successful surface medication. Both TG and

10

DTG curves of ODG@TiO2 NPs differentiate from those of TiO2 NPs in Fig. S1c. The TG and

11

DTG curves of TiO2 NPs in Fig. S1c indicate that the major weight loss happens below 100 °C,

12

which can be trapped water. On the other hand, the thermal degradation of ODG@TiO2 NPs

13

occurs majorly at 280 °C and 420 °C, which can be assigned to the ODG layer stably coating on

14

TiO2 NPs. The TG curve in Fig. 1b suggests that the amount of ODG coated on TiO2 NPs is

15

about 3 to 4 w.t.%, by counting the weight loss from 200 °C to 700 °C. Although the single NP

16

in Fig. S1c is shelled by a thick layer of ODG, the aggregated ODG@TiO2 NPs explain the

17

limited amount of coated ODG because of the reduction in surface area for modifications.

18

Fig. 1c and d reaffirm the successful surface modification of TiO2 NPs. The hydroxyl group

19

signal (between 3450 cm-1 and 3350 cm-1) of ODG disappeared after covering the surface of

20

TiO2. ODG and TiO2 can be linked in bidentate mononuclear, or binuclear, structure through the

21

hydroxyl groups as demonstrated in Scheme 2 [34]. The bands observed at 2953 cm-1 and 2869

22

cm-1 are caused by the C-H stretching of ODG. Broad peaks from 1320 to 1200 cm-1 can be the

23

result of the carbonyl functionality of the aromatic ester or aryl ether, which corresponds to the

10

1

polymer linkage reported in Tóth et al.’s study [29]. The broad signal vibrations from 1400 to

2

1600 cm-1 are caused by aromatic ring stretching. In Fig. 1d, the Raman shift peaks at 141 cm-1

3

and 638 cm-1 can be assigned to Eg phonic mode while peaks at 394 cm-1 and 514 cm-1 can be

4

attributed to A1g and B1g phonic mode for both TiO2 and ODG@TiO2 NPs [35]. For the spectra

5

of ODG@TiO2 NPs, the additional peaks at 1372 cm-1 and 1502 cm-1 can be assigned to the

6

ODG functionality.

7

Fig. 1e demonstrates the dispersion stability of the TiO2 NPs is improved in non-polar solvent

8

after being modified by ODG. Non-modified TiO2 NPs quickly settle in hexadecane in the first

9

15 minutes and settle out within 30 minutes. Meanwhile, ODG@TiO2 NPs began to appear at the

10

bottom after 40 minutes and fully settled down after 120 minutes. Although the alkyl group

11

characterized by ODG@TiO2 NPs helps them to slower down the sedimentation kinetic, the

12

limited dispersion stability ODG@TiO2 can be attributed to the aggregated NPs before and after

13

the surface modification as shown in Fig. S1a and b. The images that show dispersion stability

14

for each blend in two base oils can be found in Fig.S1d (a)

(b)

(c)

11

(d)

(e)

1

Fig. 1. Characterization results: (a) Core-shell ODG@TiO2 nanostructure and elemental

2

mapped by EELS (blue: titanium; red: carbon) (b) TGA results of ODG@TiO2 obtained at

3

heating rate 10 °C/min with the N2 purged at 20 cm3/min; (c) IR spectra of the ODG, TiO2, and

4

ODG@TiO2; (d) UV-vis dispersion stability evaluation of 0.5 w.t.% TiO2 (milky white) and 0.5

5

w.t.% ODG@TiO2 (orange) in n-hexadecane; (e) Raman spectra of anatase TiO2 and Core-shell

6

ODG@TiO2 before the tribological tests.

7

3.2. Friction and wear behavior

8

Fig. 2a shows that frictions of ODG@TiO2 NPs formulations are lower than those of TiO2 NPs

9

formulations and neat base oils in two oil lubrication systems, PAO and HEO. Increasing

10

concentrations of ODG@TiO2 NPs in both base oils, PAO and HEO reduces friction. By setting

12

1

PAO as the benchmark in the PAO system in Fig. 2b, the mean COF is lowered by 18.5% in

2

1.0% ODG@TiO2 NPs blend. In the HEO system, formulating 1.0% ODG@TiO2 NPs reduces

3

mean COF by 7.4% when setting HEO as the benchmark. The COF curves and mean COF

4

values in Fig. 2a and b suggest that the presence of ODG coated on the TiO2 NPs aids the friction

5

reduction of TiO2 NPs. The friction results correspond to the conclusion drawn from Kumara et

6

al [19]. However, the benefits of adding TiO2 NPs to the base oils, both PAO and HEO, are

7

negligible.

8

Fig. 2b also summarizes different wear behaviors of ODG@TiO2 NPs and TiO2 NPs in two

9

different base oil systems. Note that the PAO testing system is used to study the behaviors of

10

ODG@TiO2 when no additives are given and to investigate whether the shelled organic layer

11

could eliminate the micro-abrasions of TiO2 NPs. The wear tracks increase with increasing

12

ODG@TiO2 NPs concentrations in the PAO system. By comparing the wear performances of

13

1.0% TiO2 NPs blend with 1.0% ODG@TiO2 NPs, ODG@TiO2 NPs generate more wear. For the

14

HEO system, coupling ODG with TiO2 NPs reduces both wear and friction. The wear volumes of

15

0.5 % ODG@TiO2 and 1.0% ODG@TiO2 in the HEO formulation test are reduced by 0.7% and

16

2.1%, respectively while the addition of 1.0% TiO2 NPs in HEO dramatically increases the wear

17

volume by 19.9%. (a)

13

(b)

1

Fig. 2. (a) Coefficient of friction (COF) curves; (b) summary of mean COF and total wear

2

volume in PAO (left) and HEO (right) formulations

3

3.3. Surface morphologies of wear tracks

4

The wear tracks generated from ODG@TiO2 NPs as shown in Fig. 3a are covered by a large

5

amount of TiO2 NPs agglomerates, which could be a collection of iron oxides, ODG@TiO2 NPs,

6

and degraded organic compounds. Fig. 3a also demonstrates that ODG@TiO2 NPs in PAO

7

system generates wear track similar to TiO2 NPs as shown in Fig. S2a. As the NPs agglomerates

8

deposit on the surface, they could serve as additional asperity contacts that harm the surface

9

protection. The NPs agglomerates can explain why both ODG@TiO2 NPs and TiO2 NPs cannot

10

eliminate the surface wear in PAO system as summarized in Fig. 2b.

11

Fig. 3b shows beneficial effects of blending ODG@TiO2 NPs in HEO lubrication system. The

12

EDS elemental mapping in Fig. S3b shows the elements that help to polish the contact surface. In

13

Fig. 3b ODG@TiO2 NPs seem to synergize with anti-wear/friction agents contained in HEO and

14

generate a more polish surface than TiO2 NPs, which can be linked to the triboactive role of

15

ODG coating on ODG@TiO2 NPs. On the other hand, TiO2 NPs generated surface morphology

16

in Fig. S2b shows a severe wear track, similar to the surface morphology as shown in Fig. S2a.

14

1

As SEM-EDS results provide information only on the very superficial surface, further cross-

2

sectional surface analysis using TEM-EDS-FIB is required to reveal the elemental profiles and

3

thickness of the derived tribofilms. (a)

(b)

15

1

Fig. 3. SEM-EDS surface elemental and morphology analysis on wear track under the ball-on-

2

disk tribotest using the 1.0% ODG@TiO2 additive in (a) PAO; (b) HEO

3

3.4 Chemical and elemental profile of tribofilms

4

3.4.1 Cross-sectional morphology of tribofilms generated in HEO lubrication system

5

Fig. 4a and b show different tribofilm cross-section morphologies generated by TiO2 NPs and

6

ODG@ TiO2 NPs in HEO lubrication system. Note that in Fig. S4c1, the derived tribofilms in

7

neat HEO lubrication system are composed of mainly P, S, and Zn, which can be attributed to the

8

ZDDPs. The tribofilm thickness derived from 1.0% TiO2/HEO (c.a. 10nm, Fig. S4a) was

9

measured to be much thinner than that of 1.0% ODG@TiO2/HEO (c.a. 50nm~75nm, Fig. S4b)

16

1

and neat HEO (c.a. 50nm, Fig. S4c). When formulating the HEO with 1.0% TiO2 NPs, minor

2

amounts of Ti are found to fill the ZDDP derived tribofilm as shown in both the thin area (Fig.

3

4a) and the thick area (Fig. S4a1). In Fig. 4b, a much higher amount of Ti incorporates into the

4

ZDDP-derived tribofilm in 1.0% ODG@TiO2 blend, which could explain different wear

5

behaviors between ODG@TiO2 and TiO2 blends in the HEO lubrication system in Fig. 2b.

6

3.4.2 Cross-sectional morphology of tribofilms generated in PAO lubrication system

7

Fig. 4c and d demonstrate different cross-section morphology due to the presence of TiO2 NPs

8

and ODG@ TiO2 NPs in PAO lubrication system. The EDS result provided in Fig. 4c and S4d1

9

show that TiO2 NPs sinters into the contacting surface, instead of forming a complete tribofilm.

10

In Fig. S4d, the cross-sectional tribofilm morphology of the TiO2 NPs in PAO lubrication

11

system, is sporadic across the contacting surface line, which can serve as additional contacting

12

asperities, or a sintering center for aggregation and subsequent rigid NPs contacts. Although the

13

ODG@TiO2 NPs in PAO forms a complete tribofilm over the contacting surface, the generated

14

tribofilm is on average thicker than that from TiO2 NPs as shown in Fig. 4d and Fig. S4b1.

15

Surface properties resulted from ODG@TiO2 NPs may also have different mechanical or

16

chemical properties as compared with that caused by TiO2 NPs.

17

1

Fig. 4. Cross-sectional surface analysis on tribofilm, with elemental mapping on tribofilm

2

using TEM-EDS-FIB: (a)1.0% TiO2 in HEO; (b) 1.0% ODG@TiO2 in HEO; (c) 1.0% TiO2 in

3

PAO; (d) 1.0% ODG@TiO2 in PAO

4

3.4.3 Chemical composition profiles of tribofilms

5

Fig. 5 demonstrates TiO2 NPs and ODG@TiO2 NPs generate different surface chemical

6

profiles. In Fig. 5a and b, peaks at 213, 295, 296, 408, 411, 421, 425, and 495 cm-1 can be

7

assigned to the spectra of different iron oxides [36,37]. For the Raman spectra characterized by

8

TiO2, peaks at 141 cm-1 and 638 cm-1 are the symmetric stretching vibration of O-Ti-O (Eg); the

9

peak at 514 cm-1 is the symmetric bending vibration of O-Ti-O (B1g); the peak at 394 cm-1

10

corresponds to the anti-symmetric bending vibration of O-Ti-O (A1g ) [38]. The A1g peak and B1g

11

peak for both anatase TiO2 NPs and ODG@TiO2 NPs disappears after tribological tests. The

12

characteristic Eg phonic mode of TiO2 slightly shifts from original 141 cm-1 to 146 cm-1 or 148

13

cm-1 while the other characteristic Eg phonic mode of TiO2 relocates from 638 cm-1 to 643, 644,

14

645, or 647 cm-1.

15

In Fig. 6b, the peaks at 458.1 eV and 463.8 eV indicate the presence of Ti4+ species for

16

ODG@TiO2 NPs in both lubrication systems. Besides, the absence of any peaks from 453 eV to

17

456 eV excludes the possibility of having Ti2+ or Ti3+. Although Ti4+ species, which could be the

18

original binding state of TiO2 NPs or the binding with other species.

18

(a)

1 2

(b)

Fig. 5. Raman spectra of ODG@TiO2 and TiO2 NPs generated tribofilms from different oil lubrication system (a) HEO and (b) PAO (a)

(b)

19

1 2

Fig. 6. XPS spectra of ODG@TiO2 NPs from HEO (left) and PAO (right) lubrication system: (a) survey spectra and (b) Ti 2p

3

3.5 Nanoindentation on tribofilms

4

In Table 1a, small variances of measured hardness and reduced modulus indicate good surface

5

homogeneity, which is in agreement with the results shown in Fig. 4a and b. Without blending

6

NPs, the measured surface mechanical properties correspond to the range of measured hardness

7

(1 to 9 GPa) and reduced modulus (approximately 100 GPa) of ZDDP-derived tribofilms [39–

8

41]. The increases in hardness and reduced modulus can be attributed to the incorporation of

9

TiO2 NPs into ZDDP-derived tribofilms. The increases in hardness and reduced modulus may

10

suggest a synergistic effect similar to ionic liquids/ZDDPs formulations [39,40], which explains

11

the better friction and wear reduction contributed by ODG@TiO2 NPs. In Table 1b, the

12

indentation results with large variances from measured surface mechanical properties reaffirm

13

the rough surface morphologies as observed Fig. 3a and Fig.S2a. (a) HEO formulations Hardness (GPa)

Reduced modulus (GPa)

Without NPs

9.83±0.86

220.24±13.40

1.0% TiO2

11.14±1.63

226.23±20.55

1.0% ODG@TiO2

12.00±0.99

230.38±13.32

20

1

(b) PAO formulations Hardness (GPa)

Reduced modulus (GPa)

Without NPs

15.26±4.84

208.62±49.94

1.0% TiO2

10.61±3.54

208.80±54.14

1.0% ODG@TiO2

14.72±3.97

246.14±40.16

Table 1. Surface mechanical properties of (a) HEO; and (b) PAO formulations

2

3.6 Discussion on tribological performances of ODG@TiO2 and TiO2 NPs

3

To discriminate the different tribological behaviors between ODG@TiO2 NPs and TiO2 NPs,

4 5

we have to consider two major factors: •

6 7

The tribochemical reactions related to pi-pi intermolecular interactions of aromatic rings on ODG [19];

•

Micro ball-bearing rolling TiO2 NPs eliminated asperity contacts and separated two

8

connecting surfaces from direct contact, while the aggregated NPs hindered the ball-

9

bearing rolling dynamics [42].

10

Our results as summarized in Fig. 2b are in agreement with the conclusion drawn from recent

11

studies on surface-modified NPs, wherein the organic layer covering the nanoparticles enhance

12

friction reduction and dispersion [19], while the selected elements of NPs are key to tribofilm

13

growth [20]. Our analysis results in Fig. 3 and Fig. 4 further show that TiO2 NPs cause more

14

wear tracks regardless of the variety of base oil and ODG in ODG@TiO2 NPs is triboactive in

15

helping NPs to synergize with other tribofilms. The chemical composition profiles of generated

16

tribofilms as analyzed by Raman spectroscopy in Fig. 5 and XPS in Fig. 6 suggest that, when

17

blending the modified and unmodified TiO2 NPs, different tribochemical reactions occur, rather

18

than a direct transfer of NPs to the derived tribofilms.

21

1

Predicated on the chemical hardness concept [43], TiO2 itself could be regarded as a

2

chemically-hard species that destroys the chemically-soft ZDDPs derived tribofilm as shown in

3

Fig. S2b. The ODG@TiO2 NPs could be seen as chemically-soft species, which prevents the

4

scuffing of ZDDPs derived tribofilms in HEO lubrication system as shown in Fig. 3b. In

5

addition, the cross-section morphology results as shown in Fig. 4 and Fig. S4 shows that TiO2

6

NPs without the shelled ODG layer attenuates the ZDDPs derived tribofilm thickness while the

7

ODG@TiO2 NPs are able to incorporate into ZDDPs derived tribofilms. On the contrary, when

8

no anti-wear agents contained in the base oil, such as in PAO formulations, the ODG@TiO2 NPs

9

become chemically-hard because of pi-pi interactions among ODG@TiO2 NPs that build up

10

aggregations (Fig. 4), cause tribofilm scuffing (Fig. 5) and initiate undesired tribochemical

11

reactions (Fig. 6).

12

4. Conclusions

13

The straightforward surface modification, in-situ surface polymerization using ODG improves

14

the TiO2 NPs dispersion stability in non-polar oils. The core-shell ODG@TiO2 NPs enhance

15

tribological performances in two different lubrication base oil systems, PAO and HEO.

16

Formulating 1.0% ODG@TiO2 in PAO brings friction reduction by 18.5% as compared with

17

neat PAO but shows increased wear volume by 16.7%. On the other hand, 1.0% ODG@TiO2

18

NPs blend reduces friction by 7.4% and wear volume by 2.1% as compared with neat HEO. The

19

XPS and Raman spectra suggest that each NPs blend in two base oils, PAO and HEO undergoes

20

different tribochemical reactions. In this work, we can further conclude that:

21 22

•

The additional ODG layer on TiO2 NPs, as tested in both PAO and HEO lubrication system helps to thicken tribofilms;

22

•

1 2

If no other additives are present, π-π intermolecular interactions provided ODG may initiate undesirable tribochemical reactions, which cause more wear;

•

3

The additional ODG layer on TiO2 NPs helps TiO2 NPs to incorporate with ZDDP

4

derived tribofilms while non-coated TiO2 NPs tends to destroy the ZDDP derived

5

tribofilms by specific tribochemical or tribomechanical reactions.

6

Acknowledgment

7

The research reported in this publication was funded by the Office of Sponsored Research

8

(OSR) at King Abdullah University of Science and Technology (KAUST). Francesco Tutino in

9

the Clean Combustion Research Center helped with synthesizing the surface modifying agent

10

ODG. Dr. Sergei Lopatin in KAUST Imaging and Characterization Core Lab performed imaging

11

of the core-shell ODG@TiO2 NPs using TEM-EELS. Dr. Long Chen in KAUST Imaging and

12

Characterization Core Lab assisted with the experimental setup for the nanoindentation of

13

derived tribofilms.

14

Characterization Core Lab characterized the cross-sectional tribofilm using TEM-EDS-FIB. Dr.

15

Mohamed Hedhil and Dr. Nimer Wehbe in KAUST Imaging and Characterization Core Lab for

16

carrying out the XPS analysis. We would also like to appreciate reviewers’ valuable comments

17

and suggestions.

18

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Author’s contribution section In this study, the conceptualization of using ODG to modify the surface of TiO2 NPs and applying for lubrication, methodology for understanding the lubrication performances of TiO2 and ODG@TiO2, respectively, in different lubrication systems, validation of tribological and surface analysis results, and investigation of different lubrication behaviors were done by Frank T. Hong. The methodology, as well as investigation of tribological performances done by different blends using SRV5 tribometer, were carried out by Ameneh Schneider. The supervision, project administration, and funding acquisition for this study were responsible for S. Mani Sarathy. The original draft was prepared by Frank T. Hong and mainly reviewed/edited by both Frank T. Hong and S. Mani Sarathy.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: