Angewandte
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into PAO stems from the inorganic NPs, though it is unclear if
the polymer brush lubrication mechanism operates here
owing to the much harsher conditions used compared with
those typical for brush lubrication studies. This is further
bolstered by the apparent superiority of lower molecular
weight samples in friction reduction, which is likely a result of
their high number density of NPs at 1 wt%. To investigate the
concentration effect of hairy silica NPs on lubrication
performance, tribological tests were carried out using three
lubricants exhibited lower COFs in the entire sliding process-
es compared with neat PAO, and there was a clear trend that
the COF decreased with decreasing brush molecular weight
(Figure 3G–I). At the end of sliding (1000 m), the COFs of
[8]
PAO lubricants containing 1 wt% of HNP-TiO -21.5k, -16.2k,
2
and -8.1k were 0.11, 0.10, and 0.08, respectively, in contrast to
0.14 for neat PAO. A ꢁ 40% reduction in friction was
observed for the lowest molecular weight sample, demon-
strating the exceptional lubricating performance of these TiO2
NPs. The COF reduction appeared to be larger than that of
additional dispersions of HNP-SiO -21.7k in PAO with
2
concentrations of 0.25, 2.0, and 4.0 wt%. The COF was
observed to decrease with increasing NP concentration, but
there was a limit; the 4.0 wt% lubricant performed only
hairy SiO NPs at similar molecular weights, which could be
2
caused by the differences in brush grafting density, the size,
shape, number, and chemical composition of core NPs. Like
the addition of 1 wt% hairy silica NPs, the wear was also
reduced significantly (Table 2); > 90% reductions in wear
volumes of iron flats were observed for the two lower
[
10]
slightly better than the 2.0 wt% sample.
Note that at
2
1
.0 wt% HNP-SiO -21.7k performed similarly to HNP-SiO -
1.8k and -4.1k.
Given these results, the inorganic NP core itself appears to
2
2
molecular weight hairy TiO nanoparticle samples. We note
2
play a critical role in friction reduction. To further demon-
strate the potential of hairy NPs as additives for lubricating
oils and to confirm the observed molecular weight effect, we
here that HNP-TiO -16.2k and HNP-SiO -4.1k performed
2
2
comparably to the commercially used ZDDP and amine-
[10,14c]
phosphate antiwear agents,
and more encouragingly
synthesized PLMA brush-grafted TiO NPs and studied their
HNP-TiO -8.1k exhibited better friction behavior than both
of the commercial ones.
2
2
[
10,14c]
tribological properties. Note that TiO NPs have previously
2
[
4a,6c,12]
been explored as a lubricant additive.
Anatase TiO2
The observed friction and wear reductions for hairy NP-
additized PAO lubricants are believed to result from the
function of NPs at the interfacial zone between the two
rubbing surfaces and the formation of tribofilms. To charac-
terize the tribofilm, we used the focused ion beam (FIB)
technique to lift out a small, thin cross-section from the wear
scar on the iron flat tested with the PAO lubricant containing
NPs with a size of 15 nm were functionalized with a triethoxy-
[
10,9b]
silane-terminated ATRP initiator.
Three PLMA brush-
values of 21.5, 16.2, and
n,SEC
grafted TiO NP samples with M
2
8
.1 kDa were made by SI-ATRP. Figure 1E–G shows the
TEM images of the three hairy TiO NP samples cast from
2
THF dispersions. In contrast to nearly spherical SiO NPs,
2
TiO NPs were irregular in shape with the presence of plate-
1 wt% HNP-SiO -4.1k and examined the area near the
2
2
and rod-like nanostructures. Like the hairy silica NPs, the
interparticle distance from TEM decreased and the weight
retention at 8008C of hairy titania NPs from TGA increased
surface zone with TEM. As shown in Figure 4A, a 200–
400 nm thick tribofilm can be clearly seen between the cast
iron substrate and the carbon layer (for protecting the surface
during the FIB process). A closer examination revealed that
the tribofilm is an amorphous matrix embedded with small
nanocrystals (Figure 4B). Interestingly, both the thickness
and morphology of the NP-formed tribofilm are similar to the
[10]
with decreasing brush molecular weight.
The character-
ization data for hairy TiO NP samples and the corresponding
2
free polymers are summarized in Table 1. To gain an idea
about the brush grafting densities, we assumed that the TiO2
NPs were spherical with a size of 15 nm, the size from the
manufacturer, and calculated the grafting densities, which
[2]
tribofilms formed by organic AW additives such as ZDDPs
[
13]
or ionic liquids.
Energy-dispersive X-ray spectroscopy
2
were in the range of 0.80–1.27 chains/nm . We also note here
(EDS) analysis showed that the tribofilm contained high
concentrations of silicon, iron, and oxygen. We believe that
under the rather harsh tribological testing conditions (a point
contact load of 100 N at 1008C), complex mechano-chemical
1
that the H NMR spectra of hairy TiO NPs and the free
2
[
10]
PLMA in CDCl were essentially identical, which clearly
3
demonstrated the high mobility of the grafted chains in a good
solvent.
All of the hairy TiO NPs could be well dispersed in PAO
2
and form stable dispersions as hairy silica NPs. To study the
stability in PAO, HNP-TiO -16.2k was selected and three
2
1
wt% dispersions in PAO were made (Figure 2C). The
slightly white color might be caused by the higher refractive
index of anatase TiO NPs (n = 2.488 in contrast to 1.460 for
2
D
silica). As for HNP-SiO -4.1k, the three dispersions were kept
2
at À20, 22, and 1008C, respectively. After 56 days there were
no changes in transparency observed, except the appearance
of a slightly yellow color for the dispersion kept at 1008C,
which was likely due to the oxidation of the residual ATRP
Cu/ligand complex in the hairy NPs.
Figure 4. A) Transmission electron micropscopy (TEM) micrograph of
the cross-section of the wear scar on the cast iron flat tested with
1
wt% HNP-SiO -4.1k-additized PAO. The element mapping data of Si,
2
C, Fe, and O of the selected area are shown on the left. The cross-
section TEM sample was prepared using the focused ion beam (FIB)
technique. B) Higher magnification TEM image of the area of the
tribofilm pointed to by the arrow.
The tribological properties of 1 wt% hairy TiO NPs in
2
PAO were investigated using the same tribo-test as that for
hairy silica NPs. All of the hairy TiO -additized PAO
2
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!
Wright, Roger A. E.
