Page 359 - Book Hosokawa Nanoparticle Technology Handbook

6.2 MECHANICAL PROPERTIES FUNDAMENTALS

Friction coefficient: =F/P with high threshold stress for microcracking and to
apply some lubrication to reduce the frictional stress.
When a hard material slides on a relatively soft
sliding Body A material, it often happens that material removal from
speed: V the hard material surface occurs by cutting or plough-
ing by fragment of hard material embedded on the
soft material surface. To reduce this type of abrasive
wear, it is effective to increase the hardness of the soft
Friction
force: F material for minimizing the invasive depth of the
fragments to the soft materials surface by using a
harder material for the mating component.
For improving the wear-resistant performance,
Load: P
there are some advantages in using composite materi-
als as described below.
Body B
1. Ceramic particles, dispersed in adhesive metals,
retard formation of metallurgical joints in the
Maximum stress
contact surface.
Figure 6.2.17 2. Addition of the second-phase particles enhances
Stress field around the surface of body B when body A deformation resistance and suppresses the plas-
slides on body B under a load of P at a speed of .
tic flow as well as the microfracture in the sur-
face. As an additional effect, the second-phase
particles increase the heat resistance to possible
wear. Wear often affects the life of machinery or temperature increase due to frictional heating in
equipment, for example, only a wear loss of a few a local area around the contact point.
milligrams may cause a serious trouble of a large 3. When adding second-phase particles with lubri-
machine of several tons. It is, therefore, critical to cating effects, the frictional force can be
suppress the wear in engineering design.
Wear, in general, depends on a number of factors reduced and the surface stress lowered.
including environment, temperature and materials as
well as the sliding speed and the applied load. In par- As described above, additions of the second-phase
ticular, material selection for components in the rela- particles often enhance the wear resistance, how-
tive motion is critical to the wear control. In the ever, it is also known that an excessive addition
frictional surface, true contact area is extremely degrades the mechanical properties as well as the
smaller than nominal contact area due to the micro- wear resistance. It suggests that the second-phase
scopic roughness or asperity, which is inevitably pres- particles have an optimized amount of addition.
ent on the surface. It results in a highly stressed field Microstructure with a fine grain size of a sub-
forming around the true contact point, suggesting that micrometer level generally exhibits better wear
metallurgical bonding may occur under a localized resistance. To improve the mechanical properties
high pressure during the sliding motion. Adhesive further as well as wear resistance, it needs nanopar-
wear, which is often observed in friction of metal con- ticles as the second-phase additives. If the nano-
tact, occurs by microfracture around the metallurgical sized second-phase particles enhance the
bond, generating debris fragments. An effective uniformity of dispersion, they also affect the adhe-
method to reduce this type of wear is, therefore, to sion resistance and lubrication effects.
select material pairs of less attractive combination for Several experiments have been reported on the sec-
bonding. ond-phase effects of nanoparticles. Addition of alu-
Ceramics, on the other hand, do not readily adhere mina particles of 50nm size by 1.11vol% to a
and are hard, and therefore are expected to exhibit a magnesium alloy improves the wear resistance of the
good performance as sliding components. However, alloy up to 1.8 times that of pure magnesium in slid-
the wear properties of ceramics are actually not so ing wear tests against a tool steel [1]. It has been sim-
excellent as expected when sliding against similar ply interpreted as the effect of reinforcement and
materials. It is mainly due to brittle microfracture hardness increase by the addition, however, suppres-
occurring beneath the contact point which is caused by sion of the wear by delamination in the surface area
resultant stress of contact pressure and frictional shear has also been observed.
force. Collision of asperities and microfracture at the It has been reported that alumina containing carbon
contact area causes a severe wear. In order to avoid nanotubes (CNT) exhibits a low friction coefficient
such wear, it is recommendable to choose materials and a low wear loss [2]. As shown in Fig. 6.2.18, the

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