Page 360 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY
friction coefficient decreases with an increase of CNT that reaction products of SiC forming a soft layer
addition, while the wear loss shows a minimum value reduces surface stress, resulting in the better wear
as low as a half of that for pure alumina, but increases resistance at high temperatures.
by further addition. The decrease in the friction coef- Fig. 6.2.19 shows a mechano-chemical grinding
ficient is attributed to the lubrication effects by CNT, (MCG) process for preparation of a silicon nitride
which has the same structure as graphite, a well- base composite using a ball mill in nitrogen environ-
known solid lubricant. Initial addition of CNT ment. The composite, where Ti compound and self-
improves the mechanical properties such as hardness, lubricating particles are dispersed in the silicon
however further addition may cause poor cohesion nitride matrix, shows a low friction coefficient of 0.2
between alumina matrix and CNT particles, resulting level and a specific wear loss (wear volume divided
–1
2
in exhibition of the minimum value of the wear loss by load and sliding distance in a unit of m N ) nearly
by the CNT addition. two order lower in magnitude than conventional sili-
Addition of SiC particles suppresses the abrasive con nitride ceramics [5].
wear of alumina. It has been reported that this is Improvement of wear resistance by adding ceramics
because addition of nanoparticles refines the to plastic materials has also been investigated [6]. It
microstructure, decreasing the size of pulled-out has been reported that addition of alumina nanoparti-
particles [3]. In an alumina with addition of SiC cles to poly(ethylene) terephthalate (PET) leads to a
whisker by 10–20 vol%, the wear resistance is obvi- friction coefficient comparable to, or slightly higher
ously improved particularly in case of sliding along than PET without addition for a sliding against a steel.
the direction perpendicular to whickers at elevated The specific wear loss exhibits a half value of the PET
temperatures above 673 K [4]. It has been concluded with addition of 2 wt%, however, is slightly higher in
case of addition of 10 wt%. The improvement of the
wear resistance with a small addition of alumina par-
ticles is attributed to coherent transfer film formation
0.6 3 and decrease in crystallinity by adding the nanoparti-
cles. Further addition may accelerate abrasive wear by
0.5 2.5 aggregates of alumina particles contained in wear
debris.
Friction coefficient 0.4 2 1.5 Wear loss (mg) also been studied by dispersing nanoparticles in lubri-
Supplying nanoparticles to sliding interface has
cation oil [7]. When synthetic base oil (polyal-
0.3
faolefin) with addition of molybdenum base nanowire
(Mo S I ) is supplied to the friction surface, the fric-
1
0.2
6 3 6
tion coefficient drastically decreases under boundary
0.1 Friction coefficient 0.5 lubrication conditions where solid contact occurs by
Wear loss depletion of lubrication oil from the contact interface.
0 0 Since analysis of the friction surface has revealed the
0 5 10 15 presence of MoS , it has been concluded that
2
Amount of CNT addition (wt%) nanowire supplied on the friction surface transforms
into solid-lubricating substance on the surface. In this
Figure 6.2.18 case, dispersed particles in the lubrication oil exhibit
Friction coefficient and wear loss of hot-pressed alumina solid-lubricating effects by adhering to the friction
with addition of carbon nanotubes (CNT) [2]. surface where lubrication oil is poorly supplied.

Figure 6.2.19
Preparation process and microstructure of silicon nitride with dispersion of self-lubricating particles [5].

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