Ci a ion: Liñei a del Río, J.M.;
Guima ey, M.J.G.; Somoza, V.; Ma iño,
F.; Comuñas, M.J.P. T ibological
Pe o mance o a Pa a inic Base Oil
Addi i e wi h Coa ed and Uncoa ed
SiO
2
Nanopa icles. Ma e ials 2024,17,
1993. h ps://doi.o g/10.3390/
ma17091993
Academic Edi o : Csaba Balázsi
Recei ed: 15 Ma ch 2024
Re ised: 11 Ap il 2024
Accep ed: 22 Ap il 2024
Published: 25 Ap il 2024
Copy igh : © 2024 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
ma e ials
A icle
T ibological Pe o mance o a Pa a inic Base Oil Addi i e wi h
Coa ed and Uncoa ed SiO2Nanopa icles
JoséM. Liñei a del Río
1,
* , Ma ía J. G. Guima ey
1
, Vanesa Somoza
1
, Fá ima Ma iño
2
and Ma ía J. P. Comuñas
1
1Labo a o y o The mophysical and T ibological P ope ies, Na oma G oup, Depa men o Applied Physics,
Facul y o Physics and Ins i u o de Ma e iais (iMATUS), Uni e sidade de San iago de Compos ela,
15782 San iago de Compos ela, Spain; [email p o ec ed] (M.J.G.G.);
[email p o ec ed] (V.S.); [email p o ec ed] (M.J.P.C.)
2School o Enginee ing, Uni e si y o he Basque Coun y UPV/EHU, Plaza Ingenie o To es Que edo 1,
48013 Bilbao, Spain; [email p o ec ed]
*Co espondence: [email p o ec ed]
Abs ac : Elec ic ehicles (EVs) ha e eme ged as a echnology ha can eplace in e nal combus ion
ehicles and educe g eenhouse gas emissions. The e o e, i is necessa y o de elop no el low-
iscosi y lub ican s ha can se e as po en ial ansmission luids o elec ic ehicles. Thus, his
wo k analyzes he in luence o bo h SiO
2
and SiO
2
-SA (coa ed wi h s ea ic acid) nanoma e ials on he
ibological beha io o a pa a inic base oil wi h an ISO VG iscosi y g ade o 32 and a 133 iscosi y
index. A adi ional wo-s ep p ocess h ough ul asonic agi a ion was u ilized o o mula e eigh
nanolub ican s o pa a inic oil + SiO
2
and pa a inic base oil + SiO
2
-SA wi h nanopowde mass
concen a ions anging om 0.15 w % o 0.60 w %. Visual con ol was u ilized o in es iga e he
s abili y o he nanolub ican s. An expe imen al s udy o di e en p ope ies ( iscosi y, iscosi y
index, densi y, ic ion coe icien , and wea ) was pe o med. F ic ion analyses we e ca ied ou in
pu e sliding con ac s a 393.15 K, and a 3D op ical p o ilome e was used o quan i y he wea . The
ic ion esul s showed ha , o he SiO
2
-SA nanolub ican s, he ic ion coe icien s we e much lowe
han hose ob ained wi h he nea pa a inic base oil. The op imal nanopa icle mass concen a ion
was 0.60 w % SiO
2
-SA, wi h which he ic ion coe icien dec eased by a ound 43%. Rega ding
wea , he g ea es dec eases in wid h, dep h, and a ea we e also ound wi h he addi ion o 0.60 w %
SiO
2
-SA; hus, educ ions o 21, 22, and 54% we e ob ained, espec i ely, compa ed wi h he nea
pa a inic base oil.
Keywo ds: ic ion; wea ; nanolub ican s; nanopa icle su ace modi ica ion
1. In oduc ion
Ene gy needs a e cons an ly inc easing; consequen ly, he na u al en i onmen is
signi ican ly impac ed. This is he case wi h he anspo sec o , as i is esponsible
o a la ge pa o ca bon dioxide, CO
2
, gas emissions, and clima e change due o uel-
powe ed machine y [
1
]. Fo his eason, au omo i e indus ies need o de elop new
echnologies o p oduce highly e icien ehicles o indi idual and public mobili y [
2
,
3
].
Thus, elec i ica ion designs (hyb id o elec ic ehicles (EVs)) ha e eme ged as an op imal
solu ion o new p opulsion sys ems o educe g eenhouse gas emissions [
4
], al hough
a speci ic analysis o each coun y is necessa y o ensu e p ope emissions educ ion [
5
].
These g eenhouse gas educ ions, pa icula ly ha o CO
2
, g ea ly depend on he sou ce
o he elec ici y [
6
]. When elec ici y o igina es om enewable ene gy sou ces, he
CO
2
emissions o an EV a e 4.5 imes less han hose o a combus ion engine ca [
6
].
Al hough EVs a e e y e icien and p oduce e y low exhaus emissions, hey ha e
e iciency and endu ance issues ha a ec he mo ing componen s and hus hei ibology.
Thus, ibological solu ions such as new ma e ials o op imized lub ican s can help o
Ma e ials 2024,17, 1993. h ps://doi.o g/10.3390/ma17091993 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2024,17, 1993 2 o 12
inc ease he d i ing ange o EVs since ibology can help o enhance he e iciency by
lowe ing ic ion in elemen s like gea s and wheel bea ings [7].
E en hough EVs exhibi signi ican ly ele a ed e iciency in e ms o ene gy use, he e
a e s ill challenges ela ed o he need o u he enhance he e iciency; hence, he imp o e-
men o new luids and ma e ials [
7
] and he p og ession o new ba e ies [
8
] a e being
p omo ed. EVs equi e ansmission oil lub ican s wi h g ea e echnical equi emen s [
9
]
han hose o in e nal combus ion mo o s; his is because du ing he ope a ion p ocess
con ac is made wi h coppe wi es, senso s, and ci cui s [
10
]. Mo eo e , he high o a ion
a es o he elec ic mo o equi e he use o lub ican s wi h e y low iscosi y. I he oil
iscosi y is educed, iscous d ag and iscous hea ing d op and, he e o e, he hea ans e
is aised [
9
,
11
]. Howe e , i he iscosi y o a lub ican is lowe ed a shi om ull ilm o
bounda y lub ica ion occu s and a mo e c i ical su ace con ac and wea is p oduced. This
ci cums ance indica es ha enhanced an i-wea and an i- ic ion p ope ies a e equi ed.
The e o e, o mee he needs o u u e EV lub ican s, i is undamen al o use ad anced
addi i es [
12
]. The bes con en ional lub ican s used o ICEVs, wi h he chance o be used
in EV au omo i e elemen s, a e made o mine al-based oils p epa ed wi h se e al addi i es
o mee he igo ous equi emen s [7].
Recen ly, nano echnology-based an i- ic ion and an i-wea addi i es we e sugges ed
o ansmission luids o EVs [
7
]. Then, he esea ch on nanopa icles as oil addi i es was
able o suppo he ad ance o a no el p oduc ion o lub ican s wi h low iscosi y ha we e
speci ically modi ied o mee he necessi ies o EVs (elec i ied ansmissions), owing o
hei ou s anding an i- ic ion and an i-wea capaci ies, which can lead o an ex ended li e in
he ope a ing condi ions o EVs [
12
–
14
]. Fu he mo e, nanoma e ials a e mo e ecologically
iendly han o he con en ional addi i es [
15
,
16
]. A c ucial pa o achie ing a p ope
nanolub ican is he c ea ion o empo al s abili y o an ex ended ime; he sedimen a ion
o nano-addi i es can lead o a dec ease in e iciency and sys em damage owing o he
ab asi e wea [
17
]. To imp o e he s abili y o nanolub ican s, di e en p ocedu es can be
ca ied ou , using su ac an s, physical ea men , o chemical su ace modi ica ion [
18
,
19
].
The equi emen o s able nanolub ican s is pa icula ly impo an o hose lub ican s
composed o oils wi h low iscosi y owing o he poo s abili y o he nanopa icles in such
luids. E en hough lub ican s wi h addi i es con aining nanoma e ials ha e exhibi ed
good an i- ic ion and an i-wea pe o mances in con en ional lub ican s [
13
,
20
–
22
], he e
is sca cely any in es iga ion on nanolub ican s wi h ega d o EVs’ ibological needs.
Mus a a e al. [
9
] ha e ecen ly e iewed he ibological pe o mance o se e al low-
iscosi y lub ican s, based on di e en polyalphaole in low- iscosi y base oils and wa e .
Fo ins ance, Chou e al. [
23
] analyzed he e ec o adding Ni nanopa icles (20 nm) on
he ibological ac i i y o PAO6 base oil; hey obse ed a educ ion in ic ion be ween
7% and 30% and in wea be ween 5% and 45%, and hey achie ed he highes ic ion
and wea educ ions wi h he PAO6 + 0.5 w % Ni nanolub ican . Because o he needs,
i is necessa y o imp o e and s udy po en ial s able lub ican s o med h ough base oils
wi h low iscosi y and nano-addi i es. In his in es iga ion, a pa a inic G-III base oil was
selec ed o mee hose quali ies. The nano-addi i es used in his wo k, SiO
2
nanopa icles,
ha e exclusi e physical and chemical cha ac e is ics; he e o e, hey can be used in se e al
ields, such as in adhesi es, he ex ile indus y, and lub ica ion [
24
]. In ac , SiO
2
as
a lub ican addi i e usually shows an excellen an i-wea p ope y, due o he ac ha
SiO
2
has hyd oxyl and unsa u a ed bonds and can o m a solid chemical adso p ion
ilm o p o ec he me allic su ace, signi ican ly imp o ing he ic ion pe o mance o
he lub ica ing oil. Fu he mo e, i has good elec ical, op ical, and magne ic p ope ies
and has ecei ed conside able in e es in e ms o applica ions such as hose o ca alysis,
pha maceu icals, d ug deli e y, and pigmen s. SiO
2
nanopowde is a solid and colo less
c ys alline subs ance, which does no eac wi h wa e and is esis an o acids. Fu he mo e,
in his esea ch, comme cial SiO
2
nanopa icles we e chemically unc ionalized wi h s ea ic
acid (SA) h ough an es e i ica ion p ocess o enhance hei s abili y in G-III base oil.
SiO
2
nanopa icles we e s udied as lub ican addi i es and demons a ed good abili y as
Ma e ials 2024,17, 1993 3 o 12
ic ion and wea modi ie s [
25
–
30
]. Fo ins ance, Co és e al. [
25
] s udied he ibological
pe o mance o non-coa ed SiO
2
nanopa icles, such as he addi i es o a ege able oil,
achie ing dec eases o up o 77% in e ms o ic ion and 74% in e ms o wea olume.
Addi ionally, some au ho s [
17
,
26
] unc ionalized he SiO
2
nanopa icle su ace wi h he aim
o enhancing he empo al s abili y o he nano-dispe sions. Fo ins ance, Peng e al. [
17
]
coa ed he SiO
2
nanopa icles wi h oleic acid (SiO
2
-OA), eaching a empo al s abili y o
abou one mon h o a pa a in oil, wi h he mass pe cen ages shi ing om 0.05 o 1.0 w %
o he SiO2-OA.
In his a icle, we ocus ou a en ion on he use o comme cial SiO
2
and s ea ic acid-
coa ed SiO
2
(SiO
2
-SA) nanopa icles as addi i es o a G-III base oil, and hese nanolub ican s
we e ibologically analyzed a high empe a u e (393.15 K) in pu e sliding con ac s.
2. Ma e ials and Me hods
2.1. Base Oil and Nanopa icles
The pa a inic G-III base oil was p o ided by Repsol S.A. (Mad id, Spain); i possesses
a dynamic iscosi y and densi y o 28.9 mPa and 0.8234 g
·
cm
−3
a 313.15 K, espec i ely,
and a 133 iscosi y index. This oil was p e iously ully cha ac e ized h ough in a ed spec-
oscopy (FTIR) and Raman spec oscopy; peaks associa ed wi h CH
3
and CH
2
s e ching
we e obse ed using FTIR, and o he s a ibu ed o C-H and C-C s e ching we e ound
using Raman spec oscopy [
31
]. Rega ding he nano-addi i es, wo di e en ypes o SiO
2
nanopa icles we e used. The i s ones we e comme cial SiO
2
nanopa icles p o ided by
he company US Resea ch Nanoma e ials, Inc. (Hous on, TX, USA), wi h a pu i y o 99%
and a diame e o 8 nm. The second ones we e he same SiO
2
nanopa icles bu coa ed
in ou labo a o y wi h s ea ic acid (SiO
2
-SA). The SiO
2
nanopowde s we e cha ac e ized
by means o ansmission elec on mic oscopy (TEM); i can be seen in Figu e 1a ha he
s udied SiO2NPs ha e a oughly sphe ical shape. Th ough he TEM cha ac e iza ion, he
calcula ion o he a e age pa icle size was pe o med using Image J so wa e ( e sion
1.54h). Thus, as shown in Figu e 1b, a e age sizes o a ound 11 nm we e eached and
we e simila o he a e age size p o ided by he manu ac u e (8 nm). Fu he mo e, in
a p e ious wo k [
32
], in a ed spec a o SA, uncoa ed SiO
2
NPs, and SiO
2
-SA NPs we e
also epo ed; i was obse ed, among o he in o ma ion, ha he cha ac e is ic peaks o
SA also appea in he spec um o SiO
2
-SA, e idencing a p ope SA coa ing wi h he SiO
2
nanopa icles.
Ma e ials 2024, 17, x FOR PEER REVIEW 3 o 12
demons a ed good abili y as ic ion and wea modi ie s [25–30]. Fo ins ance, Co és e
al. [25] s udied he ibological pe o mance o non-coa ed SiO2 nanopa icles, such as he
addi i es o a ege able oil, achie ing dec eases o up o 77% in e ms o ic ion and 74%
in e ms o wea olume. Addi ionally, some au ho s [17,26] unc ionalized he SiO2 na-
nopa icle su ace wi h he aim o enhancing he empo al s abili y o he nano-dispe -
sions. Fo ins ance, Peng e al. [17] coa ed he SiO2 nanopa icles wi h oleic acid (SiO2-OA),
eaching a empo al s abili y o abou one mon h o a pa affin oil, wi h he mass pe cen -
ages shi ing om 0.05 o 1.0 w % o he SiO2-OA.
In his a icle, we ocus ou a en ion on he use o comme cial SiO2 and s ea ic acid-
coa ed SiO2 (SiO2-SA) nanopa icles as addi i es o a G-III base oil, and hese nanolub i-
can s we e ibologically analyzed a high empe a u e (393.15 K) in pu e sliding con ac s.
2. Ma e ials and Me hods
2.1. Base Oil and Nanopa icles
The pa affinic G-III base oil was p o ided by Repsol S.A. (Mad id, Spain); i possesses
a dynamic iscosi y and densi y o 28.9 mPa and 0.8234 g·cm−3 a 313.15 K, espec i ely,
and a 133 iscosi y index. This oil was p e iously ully cha ac e ized h ough in a ed
spec oscopy (FTIR) and Raman spec oscopy; peaks associa ed wi h CH3 and CH2
s e ching we e obse ed using FTIR, and o he s a ibu ed o C-H and C-C s e ching
we e ound using Raman spec oscopy [31]. Rega ding he nano-addi i es, wo diffe en
ypes o SiO2 nanopa icles we e used. The i s ones we e comme cial SiO2 nanopa icles
p o ided by he company US Resea ch Nanoma e ials, Inc. (Hous on, TX, USA), wi h a
pu i y o 99% and a diame e o 8 nm. The second ones we e he same SiO2 nanopa icles
bu coa ed in ou labo a o y wi h s ea ic acid (SiO2-SA). The SiO2 nanopowde s we e cha -
ac e ized by means o ansmission elec on mic oscopy (TEM); i can be seen in Figu e 1a
ha he s udied SiO2 NPs ha e a oughly sphe ical shape. Th ough he TEM cha ac e iza-
ion, he calcula ion o he a e age pa icle size was pe o med using Image J so wa e
( e sion 1.54h). Thus, as shown in Figu e 1b, a e age sizes o a ound 11 nm we e eached
and we e simila o he a e age size p o ided by he manu ac u e (8 nm). Fu he mo e,
in a p e ious wo k [32], in a ed spec a o SA, uncoa ed SiO2 NPs, and SiO2-SA NPs we e
also epo ed; i was obse ed, among o he in o ma ion, ha he cha ac e is ic peaks o
SA also appea in he spec um o SiO2-SA, e idencing a p ope SA coa ing wi h he SiO2
nanopa icles.
Figu e 1. Image o TEM (a) and mean size pa icle dis ibu ion o SiO2 NPs (b).
2.2. Fo mula ion o Nanolub ican s
The uncoa ed SiO2 nanolub ican s we e o mula ed wi h diffe en mass concen a-
ions o SiO2 (0.15, 0.30, 0.45 and 0.60 w %) in G-III base oil. Fo his pu pose, a con en-
ional wo-s ep me hod and a Sa o ius MC 210P mic obalance (±0.00001 g) we e u ilized.
Fu he mo e, an ul asonic me hod (Ul asonic ba h FB11203 Fishe b and om Fishe
Figu e 1. Image o TEM (a) and mean size pa icle dis ibu ion o SiO2NPs (b).
2.2. Fo mula ion o Nanolub ican s
The uncoa ed SiO
2
nanolub ican s we e o mula ed wi h di e en mass concen a ions
o SiO
2
(0.15, 0.30, 0.45 and 0.60 w %) in G-III base oil. Fo his pu pose, a con en ional
wo-s ep me hod and a Sa o ius MC 210P mic obalance (
±
0.00001 g) we e u ilized. Fu -
he mo e, an ul asonic me hod (Ul asonic ba h FB11203 Fishe b and om Fishe Scien i ic,
Hamp on, VA, USA) was used o 4 h o homogenize he SiO
2
-based nanolub ican s. On he
Ma e ials 2024,17, 1993 4 o 12
o he hand, o p epa e he SiO
2
-SA nano-dispe sions, comme cial SiO
2
nanopowde s we e
coa ed wi h SA ollowing he chemical eac ion gi en in Figu e 2a and hen he dispe sion
me hod displayed in Figu e 2b, o inally ob ain a 4 w % SiO
2
-SA nanolub ican . Mo e
de ails abou a simila unc ionaliza ion p ocess can be seen in ou p e ious a icle [32].
Ma e ials 2024, 17, x FOR PEER REVIEW 4 o 12
Scien i ic, Hamp on, VA, USA) was used o 4 h o homogenize he SiO
2
-based nanolub i-
can s. On he o he hand, o p epa e he SiO
2
-SA nano-dispe sions, comme cial SiO
2
na-
nopowde s we e coa ed wi h SA ollowing he chemical eac ion gi en in Figu e 2a and
hen he dispe sion me hod displayed in Figu e 2b, o inally ob ain a 4 w % SiO
2
-SA
nanolub ican . Mo e de ails abou a simila unc ionaliza ion p ocess can be seen in ou
p e ious a icle [32].
Figu e 2. Scheme o he nanopa icle unc ionaliza ion (a) and dispe sion me hod (b).
The e o e, dilu ions o he achie ed 4 w % SiO
2
-SA nanolub ican we e pe o med
by adding G-III base oil, un il eaching he desi ed (0.15, 0.30, 0.45, and 0.60 w %) SiO
2
-SA
nanolub ican s. A e he dilu ions, he nanolub ican s we e also homogenized ia an ul-
asonic ba h, as in he case o he ba e SiO
2
nanolub ican s. Fu he mo e, he empo al
s abili y o he nanolub ican s was e alua ed by isual con ol and e ac i e index e o-
lu ion o he samples o e ime.
2.3. The mophysical Cha ac e iza ion
The densi y o he nanolub ican s was examined om 278.15 o 373.15 K, u ilizing a
ib a ing densime e An on Paa (G az, Aus ia) SVM 3000 S abinge . The expanded (k =
2) unce ain y o he densi y measu emen s was 0.0005 g cm
−3
. The iscosi y a a mos-
phe ic p essu e and he iscosi y index (VI) o he nanolub ican s we e also analyzed wi h
he a o emen ioned densime e . This de ice can measu e kinema ic and dynamic iscos-
i ies be ween 278.15 and 373.15 K. A ela i e expanded (k = 2) unce ain y o 1% was cal-
cula ed o he dynamic iscosi y.
2.4. T ibological Cha ac e iza ion
F ic ion es s we e ca ied ou in pu e sliding con ac s wi h a heome e MCR 302
om An on-Paa , ki ed wi h a ibology uni T-PTD 200 and u ilizing a Pel ie hood H-
PTD 200 o an ideal empe a u e con ol. In his esea ch, a ball-on- h ee-pins es dispo-
si ion was u ilized; he ball is pu on a sha and se o u n by he heome e mo o , while
being pushed a he same ime agains he h ee pins. The heome e axial o ce is ans-
e ed in o a no mal o ce which p oceeds pe pendicula ly o he con ac posi ions on he
pins. In his case, he ball u ns on he pins below a 20 N no mal o ce, esul ing in a load
o 9.43 N in each pin, which co esponds o a maximum con ac p essu e o a ound 0.8
GPa. F ic ion expe imen s we e conduc ed a a cons an o a ional speed o 213 pm and
o 3400 s a 393.15 K. The specimens es ed we e polished AISI 52100 (100C 6) s eel balls
(Ra = 20 nm) and pins (Ra = 50 nm) wi h a ha dness o 62–66 HRC. The ball had a 12.7 mm
diame e , and he cylind ical pins had a diame e and heigh ha we e bo h 6 mm. The
balls and pins we e cleaned wi h ace one/hexane and d ied wi h ai p io o he ibolog-
ical es s. The pins we e comple ely looded by adding o e 1.2 mL o each es ed
nanolub ican o base oil. A leas h ee eplica es we e es ed o each concen a ion o
lub ican o ob ain ep esen a i e alues. Mo e in o ma ion in ol ing his ibological
Figu e 2. Scheme o he nanopa icle unc ionaliza ion (a) and dispe sion me hod (b).
The e o e, dilu ions o he achie ed 4 w % SiO
2
-SA nanolub ican we e pe o med by
adding G-III base oil, un il eaching he desi ed (0.15, 0.30, 0.45, and 0.60 w %) SiO
2
-SA
nanolub ican s. A e he dilu ions, he nanolub ican s we e also homogenized ia an
ul asonic ba h, as in he case o he ba e SiO
2
nanolub ican s. Fu he mo e, he empo-
al s abili y o he nanolub ican s was e alua ed by isual con ol and e ac i e index
e olu ion o he samples o e ime.
2.3. The mophysical Cha ac e iza ion
The densi y o he nanolub ican s was examined om 278.15 o 373.15 K, u ilizing a
ib a ing densime e An on Paa (G az, Aus ia) SVM 3000 S abinge . The expanded (k = 2)
unce ain y o he densi y measu emen s was 0.0005 g cm
−3
. The iscosi y a a mosphe ic
p essu e and he iscosi y index (VI) o he nanolub ican s we e also analyzed wi h he
a o emen ioned densime e . This de ice can measu e kinema ic and dynamic iscosi ies
be ween 278.15 and 373.15 K. A ela i e expanded (k = 2) unce ain y o 1% was calcula ed
o he dynamic iscosi y.
2.4. T ibological Cha ac e iza ion
F ic ion es s we e ca ied ou in pu e sliding con ac s wi h a heome e MCR 302 om
An on-Paa , ki ed wi h a ibology uni T-PTD 200 and u ilizing a Pel ie hood H-PTD
200 o an ideal empe a u e con ol. In his esea ch, a ball-on- h ee-pins es disposi ion
was u ilized; he ball is pu on a sha and se o u n by he heome e mo o , while being
pushed a he same ime agains he h ee pins. The heome e axial o ce is ans e ed
in o a no mal o ce which p oceeds pe pendicula ly o he con ac posi ions on he pins. In
his case, he ball u ns on he pins below a 20 N no mal o ce, esul ing in a load o 9.43 N
in each pin, which co esponds o a maximum con ac p essu e o a ound 0.8 GPa. F ic ion
expe imen s we e conduc ed a a cons an o a ional speed o 213 pm and o 3400 s a
393.15 K. The specimens es ed we e polished AISI 52100 (100C 6) s eel balls (Ra = 20 nm)
and pins (Ra = 50 nm) wi h a ha dness o 62–66 HRC. The ball had a 12.7 mm diame e ,
and he cylind ical pins had a diame e and heigh ha we e bo h 6 mm. The balls and
pins we e cleaned wi h ace one/hexane and d ied wi h ai p io o he ibological es s.
The pins we e comple ely looded by adding o e 1.2 mL o each es ed nanolub ican
o base oil. A leas h ee eplica es we e es ed o each concen a ion o lub ican o
ob ain ep esen a i e alues. Mo e in o ma ion in ol ing his ibological machine can
be ob ained om an ea lie a icle [
31
]. To inspec he wo n pins a e he ibological
s udies, a 3D Op ical P o ile was employed o measu e he wea c ea ed in he pins o
di e se pa ame e s, such as wea sca diame e (WSD), wea ack dep h (WTD), o wo n
a ea. These pa ame e s we e analyzed in he h ee di e en pins by means o a con ocal
Ma e ials 2024,17, 1993 5 o 12
mode (10
×
objec i e). Mo eo e , a WITec alpha300R+ con ocal Raman mic oscope (Ox o d
Ins umen s, Abingdon, UK) was u ilized o ob ain knowledge ega ding he sp eading o
he nanopa icles in he wo n pins.
3. Resul s
3.1. S abili y o he Dispe sions
The s abili y o he SiO
2
and SiO
2
-SA nanolub ican s was checked using wo di e en
echniques: isual obse a ion and empo al e olu ion o he e ac i e index using a
Me le Toledo RA-510 M e ac ome e (Columbus, OH, USA). Figu e 3a e eals ha
sedimen a ion does no happen o he i s 96 h a e he nanolub ican o mula ion, o he
coa ed SiO
2
-SA nanolub ican s. Con e sely, in he case o he uncoa ed SiO
2
nanolub ican s,
i can be obse ed in Figu e 3a ha 24 h a e he p epa a ion, he sedimen a ion akes
place. Thus, h ough he SA coa ing o SiO
2
nanopa icles a be e s abili y is eached.
Simila s abili y beha io was obse ed in o he s udies using he s ea ic o oleic acid as
he coa ing o he NPs, and s abili y imp o emen s we e achie ed [
19
,
33
]. Figu e 3b shows
he empo al e olu ion o he e ac i e index (n) o he base oil and 0.6 w % SiO
2
-SA
nanolub ican . As can be seen, he endencies o he e ac i e index e olu ion o he
SiO
2
-SA nanolub ican and base oil a e e y simila , con i ming a good s abili y agains
sedimen a ion.
Ma e ials 2024, 17, x FOR PEER REVIEW 5 o 12
machine can be ob ained om an ea lie a icle [31]. To inspec he wo n pins a e he
ibological s udies, a 3D Op ical P o ile was employed o measu e he wea c ea ed in
he pins o di e se pa ame e s, such as wea sca diame e (WSD), wea ack dep h
(WTD), o wo n a ea. These pa ame e s we e analyzed in he h ee diffe en pins by
means o a con ocal mode (10× objec i e). Mo eo e , a WITec alpha300R+ con ocal Raman
mic oscope (Ox o d Ins umen s, Abingdon, UK) was u ilized o ob ain knowledge e-
ga ding he sp eading o he nanopa icles in he wo n pins.
3. Resul s
3.1. S abili y o he Dispe sions
The s abili y o he SiO
2
and SiO
2
-SA nanolub ican s was checked using wo diffe en
echniques: isual obse a ion and empo al e olu ion o he e ac i e index using a Me -
le Toledo RA-510 M e ac ome e (Columbus, OH, USA). Figu e 3a e eals ha sedi-
men a ion does no happen o he i s 96 h a e he nanolub ican o mula ion, o he
coa ed SiO
2
-SA nanolub ican s. Con e sely, in he case o he uncoa ed SiO
2
nanolub i-
can s, i can be obse ed in Figu e 3a ha 24 h a e he p epa a ion, he sedimen a ion
akes place. Thus, h ough he SA coa ing o SiO
2
nanopa icles a be e s abili y is eached.
Simila s abili y beha io was obse ed in o he s udies using he s ea ic o oleic acid as
he coa ing o he NPs, and s abili y imp o emen s we e achie ed [19,33]. Figu e 3b shows
he empo al e olu ion o he e ac i e index (n) o he base oil and 0.6 w % SiO
2
-SA
nanolub ican . As can be seen, he endencies o he e ac i e index e olu ion o he SiO
2
-
SA nanolub ican and base oil a e e y simila , con i ming a good s abili y agains sedi-
men a ion.
Figu e 3. (a) Visual s abili y obse a ion and (b) empo al e olu ion o he e ac i e index o SiO
2
-
SA nanolub ican and base oil.
3.2. The mophysical Resul s
The expe imen al densi ies and dynamic iscosi ies acqui ed o he base oil and SiO
2
and SiO
2
-SA nanolub ican s a e epo ed in Tables S1 and S2 (Supplemen a y Ma e ials).
Figu e 4a shows he ela i e a ia ion in he densi ies o he nanolub ican concen a ion
wi h espec o he nea pa affinic base oil. Fo he SiO
2
nanolub ican s, a clea inc ease in
densi y a ia ion is obse ed as a unc ion o he mass concen a ion o he nanopa icle;
he highe he concen a ion, he highe he densi y o he nanolub ican . Thus, he SiO
2
nanolub ican s a 0.15, 0.3, 0.45 and 0.6 w % inc ease ela i ely wi h espec o he nea
base oil densi ies o 0.10, 0.20, 0.29 and 0.34%, espec i ely. The ise in nanolub ican den-
si y wi h he nanopa icle concen a ion is a ibu able o he agglome a ion phenomenon
[34]. Howe e , o he SiO
2
-SA nanolub ican s, he ela i e densi y inc ease is simila
(a ound 0.03%) o all he concen a ions o he unc ionalized nanopa icles. The ela i e
iscosi y a ia ion in he SiO
2
and SiO
2
-SA nanolub ican s compa ed o he nea G-III
Figu e 3. (a) Visual s abili y obse a ion and (b) empo al e olu ion o he e ac i e index o
SiO2-SA nanolub ican and base oil.
3.2. The mophysical Resul s
The expe imen al densi ies and dynamic iscosi ies acqui ed o he base oil and SiO
2
and SiO
2
-SA nanolub ican s a e epo ed in Tables S1 and S2 (Supplemen a y Ma e ials).
Figu e 4a shows he ela i e a ia ion in he densi ies o he nanolub ican concen a ion
wi h espec o he nea pa a inic base oil. Fo he SiO
2
nanolub ican s, a clea inc ease in
densi y a ia ion is obse ed as a unc ion o he mass concen a ion o he nanopa icle;
he highe he concen a ion, he highe he densi y o he nanolub ican . Thus, he SiO
2
nanolub ican s a 0.15, 0.3, 0.45 and 0.6 w % inc ease ela i ely wi h espec o he nea base
oil densi ies o 0.10, 0.20, 0.29 and 0.34%, espec i ely. The ise in nanolub ican densi y
wi h he nanopa icle concen a ion is a ibu able o he agglome a ion phenomenon [
34
].
Howe e , o he SiO
2
-SA nanolub ican s, he ela i e densi y inc ease is simila (a ound
0.03%) o all he concen a ions o he unc ionalized nanopa icles. The ela i e iscosi y
a ia ion in he SiO
2
and SiO
2
-SA nanolub ican s compa ed o he nea G-III pa a inic
base oil is shown in Figu e 4b. The dynamic iscosi y ises as he concen a ion o SiO
2
nanopa icles g ows om 1% o 12%. Rega ding he SiO
2
-SA nanopa icles, he g ow h
in iscosi y a ies be ween 12 and 18% o he 0.15 and 0.3 w % SiO
2
-SA nanolub ican s,
espec i ely.
Ma e ials 2024,17, 1993 6 o 12
Ma e ials 2024, 17, x FOR PEER REVIEW 6 o 12
pa affinic base oil is shown in Figu e 4b. The dynamic iscosi y ises as he concen a ion
o SiO
2
nanopa icles g ows om 1% o 12%. Rega ding he SiO
2
-SA nanopa icles, he
g ow h in iscosi y a ies be ween 12 and 18% o he 0.15 and 0.3 w % SiO
2
-SA nanolub i-
can s, espec i ely.
Figu e 4. Rela i e inc ease in he densi ies (a) and iscosi ies (b) wi h he mass concen a ion wi h
espec o he nea pa affinic base oil.
Addi ionally, wi h he a o emen ioned SiO
2
and SiO
2
-SA nanolub ican s, he impac
o concen a ion on he iscosi y index (VI) was analyzed, as shown in Figu e 5. A sui able
iscosi y index (VI) is essen ial in a lub ican since i helps o a e collisions and ic ion
among he mechanical de ice componen s du ing ope a ion, while also enhancing he
machine’s efficiency [35]. I can be obse ed ha all he samples ha e a highe iscosi y
index (VI) han he nea base oil, which con i ms ha he nanolub ican emains use ul
e en a ele a ed empe a u es wi h he p ese a ion o he hickness o he oil ilm. The
esul s show ha VI inc eased om 3% o 13% and om 11% o 15% o he SiO
2
and SiO
2
-
SA nanolub ican s, espec i ely, compa ed wi h he nea base oil.
Figu e 5. Viscosi y index (VI) o he nea pa affinic base oil and o he SiO
2
and SiO
2
-SA nanolub i-
can s.
3.3. T ibological Resul s
Figu e 6 and Table 1 p esen he mean alues o he coefficien o ic ion (µ) o all
he es ed lub ican s based on G-III pa affinic base oil. The ic ion coefficien s ound o
all he uncoa ed SiO
2
nanolub ican s a e qui e simila o ha eached o he nea G-III
base oil (wi hou addi i es). None heless, o he coa ed SiO
2
-SA nanolub ican s he ob-
ained ic ion coefficien s a e much lowe han ha p e iously epo ed using he nea G-
III base oil [31]. Speci ically, he op imal nanopa icle concen a ion was a ained o he
0.60 w % SiO
2
-SA nanolub ican , wi h a ic ion dec ease o a ound 43% (µ o 0.077 was
Figu e 4. Rela i e inc ease in he densi ies (a) and iscosi ies (b) wi h he mass concen a ion wi h
espec o he nea pa a inic base oil.
Addi ionally, wi h he a o emen ioned SiO
2
and SiO
2
-SA nanolub ican s, he impac
o concen a ion on he iscosi y index (VI) was analyzed, as shown in Figu e 5. A sui able
iscosi y index (VI) is essen ial in a lub ican since i helps o a e collisions and ic ion
among he mechanical de ice componen s du ing ope a ion, while also enhancing he
machine’s e iciency [
35
]. I can be obse ed ha all he samples ha e a highe iscosi y
index (VI) han he nea base oil, which con i ms ha he nanolub ican emains use ul
e en a ele a ed empe a u es wi h he p ese a ion o he hickness o he oil ilm. The
esul s show ha VI inc eased om 3% o 13% and om 11% o 15% o he SiO
2
and
SiO2-SA nanolub ican s, espec i ely, compa ed wi h he nea base oil.
Ma e ials 2024, 17, x FOR PEER REVIEW 6 o 12
pa affinic base oil is shown in Figu e 4b. The dynamic iscosi y ises as he concen a ion
o SiO
2
nanopa icles g ows om 1% o 12%. Rega ding he SiO
2
-SA nanopa icles, he
g ow h in iscosi y a ies be ween 12 and 18% o he 0.15 and 0.3 w % SiO
2
-SA nanolub i-
can s, espec i ely.
Figu e 4. Rela i e inc ease in he densi ies (a) and iscosi ies (b) wi h he mass concen a ion wi h
espec o he nea pa affinic base oil.
Addi ionally, wi h he a o emen ioned SiO
2
and SiO
2
-SA nanolub ican s, he impac
o concen a ion on he iscosi y index (VI) was analyzed, as shown in Figu e 5. A sui able
iscosi y index (VI) is essen ial in a lub ican since i helps o a e collisions and ic ion
among he mechanical de ice componen s du ing ope a ion, while also enhancing he
machine’s efficiency [35]. I can be obse ed ha all he samples ha e a highe iscosi y
index (VI) han he nea base oil, which con i ms ha he nanolub ican emains use ul
e en a ele a ed empe a u es wi h he p ese a ion o he hickness o he oil ilm. The
esul s show ha VI inc eased om 3% o 13% and om 11% o 15% o he SiO
2
and SiO
2
-
SA nanolub ican s, espec i ely, compa ed wi h he nea base oil.
Figu e 5. Viscosi y index (VI) o he nea pa affinic base oil and o he SiO
2
and SiO
2
-SA nanolub i-
can s.
3.3. T ibological Resul s
Figu e 6 and Table 1 p esen he mean alues o he coefficien o ic ion (µ) o all
he es ed lub ican s based on G-III pa affinic base oil. The ic ion coefficien s ound o
all he uncoa ed SiO
2
nanolub ican s a e qui e simila o ha eached o he nea G-III
base oil (wi hou addi i es). None heless, o he coa ed SiO
2
-SA nanolub ican s he ob-
ained ic ion coefficien s a e much lowe han ha p e iously epo ed using he nea G-
III base oil [31]. Speci ically, he op imal nanopa icle concen a ion was a ained o he
0.60 w % SiO
2
-SA nanolub ican , wi h a ic ion dec ease o a ound 43% (µ o 0.077 was
Figu e 5. Viscosi y index (VI) o he nea pa a inic base oil and o he SiO
2
and SiO
2
-SA nanolub ican s.
3.3. T ibological Resul s
Figu e 6and Table 1p esen he mean alues o he coe icien o ic ion (
µ
) o all he
es ed lub ican s based on G-III pa a inic base oil. The ic ion coe icien s ound o all
he uncoa ed SiO
2
nanolub ican s a e qui e simila o ha eached o he nea G-III base
oil (wi hou addi i es). None heless, o he coa ed SiO
2
-SA nanolub ican s he ob ained
ic ion coe icien s a e much lowe han ha p e iously epo ed using he nea G-III base
oil [
31
]. Speci ically, he op imal nanopa icle concen a ion was a ained o he 0.60 w %
SiO
2
-SA nanolub ican , wi h a ic ion dec ease o a ound 43% (
µ
o 0.077 was ound
agains 0.134). This p omising ic ion pe o mance can be explained by he syne ge ic
e ec be ween he SiO2nanopa icles and he coa ing o s ea ic acid.
Ma e ials 2024,17, 1993 7 o 12
Ma e ials 2024, 17, x FOR PEER REVIEW 7 o 12
ound agains 0.134). This p omising ic ion pe o mance can be explained by he syne -
ge ic effec be ween he SiO2 nanopa icles and he coa ing o s ea ic acid.
Figu e 6. Mean ic ion coefficien s, µ, o he p epa ed SiO2 and SiO2-SA nanolub ican s and o he
nea G-III base oil [31].
As ci ed p e iously, he wea o med in he pins a e he ic ion es s was es ima ed
h ough many pa ame e s o he wea ack: wid h, dep h, and a ea. Fo his goal, c oss-
sec ion p o iles and 3D mappings o he wea acks we e aken. The WSD, WTD, and
ans e sal a ea mean alues we e aken om he p o iles o he wo n acks on he pins
es ed wi h he nanolub ican s and base oil. The alues a e epo ed in Table 1.
Table 1. A e age coefficien s o ic ion, µ, and mean pa ame e s o wea wi h hei s anda d de i-
a ions o he s udied G-III base oil nanolub ican s a 393.15 K.
Sample μ σ WSD/μm σ/μm WTD/μm σ/μm A ea/μm2 σ/μm2
G-III base oil [31] 0.1351 0.0014 366 18 2.11 0.19 607 44
+0.15 w % SiO2 0.1319 0.0011 380 23 2.76 0.72 571 31
+0.30 w % SiO2 0.1370 0.0010 422 53 3.37 0.67 806 76
+0.45 w % SiO2 0.1410 0.0011 352 31 2.61 0.45 476 55
+0.60 w % SiO2 0.1332 0.0012 372 8.3 2.67 0.46 726 49
+0.15 w % SiO2-SA 0.0989 0.0014 356 23 2.00 0.49 439 99
+0.30 w % SiO2-SA 0.0905 0.0011 371 18 1.60 0.19 401 62
+0.45 w % SiO2-SA 0.0927 0.0010 296 54 1.80 0.38 279 98
+0.60 w % SiO2-SA 0.0766 0.0011 289 25 1.64 0.47 281 84
As wi h he ic ion esul s, he SiO2 nanolub ican s e ealed simila wea esul s o
hose o he G-III base oil. None heless, o all he SiO2-SA-based nanolub ican s, he p o-
duced wea was g ea ly in e io o ha achie ed wi h nea G-III base oil, pa icula ly in
he case o he wo n a eas (Figu e 7). Fu he mo e, he addi i e mass concen a ion used
in he nanolub ican design conside ably in luenced he lub ica ion pe o mance. Speci i-
cally, he g ea es dec eases in wid h and a ea we e eached wi h he G-III base oil + 0.60
w % SiO2-SA nanolub ican (Table 1), wi h educ ions o 21 and 54%, espec i ely (Figu e
8).
Simila imp o ed ibological pe o mances wi h SiO2 NPs we e p e iously ob ained
by o he au ho s. Thus, Sanuk ishna e al. [29] s udied he ibological p ope ies o SiO2
NPs as addi i es o a PAG lub ican , obse ing ic ion educ ions o a ound 38% and
Figu e 6. Mean ic ion coe icien s,
µ
, o he p epa ed SiO
2
and SiO
2
-SA nanolub ican s and o he
nea G-III base oil [31].
Table 1. A e age coe icien s o ic ion,
µ
, and mean pa ame e s o wea wi h hei s anda d
de ia ions o he s udied G-III base oil nanolub ican s a 393.15 K.
Sample µ σ WSD/µmσ/µm WTD/µmσ/µm
A ea/
µ
m
2σ/µm2
G-III base oil [31] 0.1351 0.0014 366 18 2.11 0.19 607 44
+0.15 w % SiO20.1319 0.0011 380 23 2.76 0.72 571 31
+0.30 w % SiO20.1370 0.0010 422 53 3.37 0.67 806 76
+0.45 w % SiO20.1410 0.0011 352 31 2.61 0.45 476 55
+0.60 w % SiO20.1332 0.0012 372 8.3 2.67 0.46 726 49
+0.15 w % SiO2-SA 0.0989 0.0014 356 23 2.00 0.49 439 99
+0.30 w % SiO2-SA 0.0905 0.0011 371 18 1.60 0.19 401 62
+0.45 w % SiO2-SA 0.0927 0.0010 296 54 1.80 0.38 279 98
+0.60 w % SiO2-SA 0.0766 0.0011 289 25 1.64 0.47 281 84
As ci ed p e iously, he wea o med in he pins a e he ic ion es s was es ima ed
h ough many pa ame e s o he wea ack: wid h, dep h, and a ea. Fo his goal, c oss-
sec ion p o iles and 3D mappings o he wea acks we e aken. The WSD, WTD, and
ans e sal a ea mean alues we e aken om he p o iles o he wo n acks on he pins
es ed wi h he nanolub ican s and base oil. The alues a e epo ed in Table 1.
As wi h he ic ion esul s, he SiO
2
nanolub ican s e ealed simila wea esul s
o hose o he G-III base oil. None heless, o all he SiO
2
-SA-based nanolub ican s, he
p oduced wea was g ea ly in e io o ha achie ed wi h nea G-III base oil, pa icula ly in
he case o he wo n a eas (Figu e 7). Fu he mo e, he addi i e mass concen a ion used in
he nanolub ican design conside ably in luenced he lub ica ion pe o mance. Speci ically,
he g ea es dec eases in wid h and a ea we e eached wi h he G-III base oil + 0.60 w %
SiO2-SA nanolub ican (Table 1), wi h educ ions o 21 and 54%, espec i ely (Figu e 8).
Simila imp o ed ibological pe o mances wi h SiO
2
NPs we e p e iously ob ained
by o he au ho s. Thus, Sanuk ishna e al. [
29
] s udied he ibological p ope ies o SiO
2
NPs as addi i es o a PAG lub ican , obse ing ic ion educ ions o a ound 38% and wea
educ ions o 41%. Also, Ras ogi e al. [
30
] s udied he e ec o SiO
2
nanopa icles on he
ibological cha ac e is ics o ja opha oil, ob aining impo an ic ion and wea educ ions
o di e en no mal loads.
Ma e ials 2024,17, 1993 8 o 12
Ma e ials 2024, 17, x FOR PEER REVIEW 8 o 12
wea educ ions o 41%. Also, Ras ogi e al. [30] s udied he effec o SiO2 nanopa icles on
he ibological cha ac e is ics o ja opha oil, ob aining impo an ic ion and wea e-
duc ions o diffe en no mal loads.
Figu e 7. Mean wo n a eas, ound o all he es ed SiO2 and SiO2-SA nanolub ican s and o he
nea G-III base oil.
Figu e 8. Mean educ ions in WSD and wo n a ea, ob ained o all he es ed SiO2 and SiO2-SA
nanolub ican s.
Addi ionally, i can be clea ly obse ed in he wo n p o iles in Figu e 9 ha he op i-
mal SiO2 nanolub ican ha con ains he s ea ic acid coa ing p esen s conside ably be e
an i-wea capaci ies wi h espec o he G-III base oil and he op imal uncoa ed SiO2
nanolub ican .
Figu e 7. Mean wo n a eas, ound o all he es ed SiO
2
and SiO
2
-SA nanolub ican s and o he nea
G-III base oil.
Ma e ials 2024, 17, x FOR PEER REVIEW 8 o 12
wea educ ions o 41%. Also, Ras ogi e al. [30] s udied he effec o SiO2 nanopa icles on
he ibological cha ac e is ics o ja opha oil, ob aining impo an ic ion and wea e-
duc ions o diffe en no mal loads.
Figu e 7. Mean wo n a eas, ound o all he es ed SiO2 and SiO2-SA nanolub ican s and o he
nea G-III base oil.
Figu e 8. Mean educ ions in WSD and wo n a ea, ob ained o all he es ed SiO2 and SiO2-SA
nanolub ican s.
Addi ionally, i can be clea ly obse ed in he wo n p o iles in Figu e 9 ha he op i-
mal SiO2 nanolub ican ha con ains he s ea ic acid coa ing p esen s conside ably be e
an i-wea capaci ies wi h espec o he G-III base oil and he op imal uncoa ed SiO2
nanolub ican .
Figu e 8. Mean educ ions in WSD and wo n a ea, ob ained o all he es ed SiO
2
and SiO
2
-SA
nanolub ican s.
Addi ionally, i can be clea ly obse ed in he wo n p o iles in Figu e 9 ha he
op imal SiO
2
nanolub ican ha con ains he s ea ic acid coa ing p esen s conside ably
be e an i-wea capaci ies wi h espec o he G-III base oil and he op imal uncoa ed SiO
2
nanolub ican .
Fu he mo e, he Raman spec a o he nanolub ican componen s e idence he ac
ha cha ac e is ic a eas o hese elemen s appea in he wo n su aces o he pins (Figu e 10).
Thus, in Figu e 10a blue a eas o Raman mapping a e associa ed wi h i on oxides, g een
a eas wi h he base oil, and ed a eas wi h he bu ned oil. In Figu e 10b, i can be seen om
he p esence o blue a eas associa ed wi h he SiO
2
-SA nanopa icles ha he spec um
o his a ea coincides wi h he SiO
2
-SA Raman spec um [
32
]. Conside ing hese Raman
analyses, a p o ec i e ibo ilm om he SiO
2
-SA in he ibo-con ac can be a possible
ibological mechanism ha pa icipa es in he dec ease in ic ion and wea . Thus, he
sphe ical SiO
2
NPs ha a e dispe sed in he pa a inic lub ican wi h he high con ac
p essu e can en e in o he in e space o con ac su aces and p og essi ely deposi on
su aces, causing he c ea ion o a physical ilm. This ibo ilm can sepa a e he wo me al
su aces and p e en di ec con ac [
36
]. Fu he mo e, some ibochemical eac ions can
Ma e ials 2024,17, 1993 9 o 12
occu , boos ed by he high empe a u es and p essu es caused by he ic ion p ocess.
Hence, hese condi ions could cause he b eaking o he bonds be ween SA and he coa ed
SiO
2
NPs, as was p e iously poin ed ou by Zhang e al. [
37
] o SA-modi ied TiO
2
NPs.
SiO
2
NPs can easily be adso bed on he wo n su ace, gene a ing a bounda y-lub ica ing
ilm, whe eas he SA can also be physically adso bed on he s eel su ace du ing he
ibo es s, gene a ing good lub ican p ope ies [
38
,
39
]. Likewise, owing o he sphe ical
na u e o SiO
2
NPs, hey a e mo e likely o oll be ween wo su aces, educing he ic ion
coe icien and wea . The e o e, olling and ibo ilm o ma ion a e he wo possible
ibological mechanisms. Simila esul s we e p e iously ob ained by Xie e al. [
40
] o SiO
2
NPs dispe sed in engine oil.
Ma e ials 2024, 17, x FOR PEER REVIEW 9 o 12
Figu e 9. Th ee-dimensional (a) and wo-dimensional (b) p o iles o wo n pins es ed wi h nea G-
III base oil [31], 0.60 w % SiO
2
, and 0.60 w % SiO
2
-SA.
Fu he mo e, he Raman spec a o he nanolub ican componen s e idence he ac
ha cha ac e is ic a eas o hese elemen s appea in he wo n su aces o he pins (Figu e
10). Thus, in Figu e 10a blue a eas o Raman mapping a e associa ed wi h i on oxides,
g een a eas wi h he base oil, and ed a eas wi h he bu ned oil. In Figu e 10b, i can be
seen om he p esence o blue a eas associa ed wi h he SiO
2
-SA nanopa icles ha he
spec um o his a ea coincides wi h he SiO
2
-SA Raman spec um [32]. Conside ing hese
Raman analyses, a p o ec i e ibo ilm om he SiO
2
-SA in he ibo-con ac can be a pos-
sible ibological mechanism ha pa icipa es in he dec ease in ic ion and wea . Thus,
he sphe ical SiO
2
NPs ha a e dispe sed in he pa affinic lub ican wi h he high con ac
p essu e can en e in o he in e space o con ac su aces and p og essi ely deposi on
Figu e 9. Th ee-dimensional (a) and wo-dimensional (b) p o iles o wo n pins es ed wi h nea G-III
base oil [31], 0.60 w % SiO2, and 0.60 w % SiO2-SA.
