Page 177 - Handbook of Surface Improvement and Modification

172 The Coefficient of Friction

8
friction pairs. The friction coeffi-
cients at cryogenic temperatures
are lower than at room tempera-
8
ture. The friction coefficients fall
8
as the load increases.
The friction of poly(butylene
terephthalate) on dry sliding
against polyoxymethylene has
9
been studied. A reciprocating
pin-on-flat tribometer was
employed to measure the maxi-
mum initial and dynamic friction
coefficients using a PBT pin with
9
a spherical tip. The maximum ini-
tial and dynamic friction coeffi-
cients were strongly affected by
the applied load (50% reduction in
Figure 11.9. Optical microscope images of PBT pin showing the the values of COFs on load
actual contact area at different load vales: (a) 1 N, (b) 5 N (c) 10 N
9
and (d) 20 N, after 100 m sliding distance with a speed of 50 mm. increase from 1 to 20 N). Figure
[Adapted, by permission, from Chaudri, AM; Suvanto, M; 11.9 shows that the actual contact
Pakkanen, TT, Wear, 342-343, 189-197.]
between the probe and the sample
9
increases with the load increase.
Under a load of 60 N, the optimum content of PTFE in epoxy lies between 10 and 15
10
wt%, which yields an ultra-low coefficient of friction (0.095 with 12.5 wt%). After 2000
m of sliding, the gradual accumulation of the fractured SiO particles and back-transferred
2
steel on the worn composite surface leads to a significant increase of coefficient of fric-
10
tion. Smearing of the PTFE particles along the worn surface was observed together with
10
the fracturing of the SiO particles and cracking of the epoxy matrix. The energy disper-
2
sive x-ray microanalysis mapping shows the formation of a PTFE-containing third-body
tribolayer on the worn surface of the composite (20-30 nm on the surface of SiO particles
2
10
after sliding for more than 700 m) (Figure 11.10). After sliding for only 10 m, slight
10
spreading of F over C and Si concentrated regions is observed. With increasing sliding
distance, it is seen that more and more F is spread onto C and Si regions, indicating the
10
gradual build-up of PTFE-containing tribolayer on the worn surface. It is found that the
10
tribolayer on the SiO particles mainly contains PTFE. Several parallel F-concentrated
2
10
lines are clearly observed on the worn SiO particles. The tribotests of long sliding dis-
2
10
tance confirmed that the composite held a low coefficient of friction.
Tribological testing requires sophisticated tools to unravel complex features of sys-
11
tems subjected to friction. Silicon nitride atomic force microscopy tips have been used to
mimic the contact between amorphous polystyrene surfaces and a hard asperity to under-
stand how asperities behave in a macroscopic contact created during friction of materi-
11
als. The adhesion forces and the friction forces of four polystyrenes having different
11
molecular weights were measured. The average contact pressure was calculated using
11
the Johnson, Kendall and Roberts, JKR, contact theory. The nanotribological behavior of
polystyrene showed that the reaction of materials to the applied normal force and the slid-