4.2. Effects of excitation periodEf

4.2. Effects of excitation period
Effect of the period of excitation on tribological properties of
the friction surfaces was also examined by exciting the springconnecting
load to oscillate with different excitation intervals. The
test under the spring-connecting load of 30 N generally gave a
smaller maximum COF peak value compared with the tests of
other two loads, i.e. 25 and 35, respectively. Therefore, the springconnecting
load of 30 N with a preloading of 5 N at the end of
cantilever (Fig. 2) was selected for such investigation with various
excitation periods as tabulated in Table 4. Results were than
compared with that of a test with similar loading system without
any excitation (see test P4 in Table 4).
Curves of their COF with these different excitation periods
(Table 4) and their no excitation counterpart are plotted and
compared in Fig. 9. The excited curves demonstrated insignificantly
increasing amplitudes of the COF value within the first
5 min (approx.) excitation time when compared with their
increasing trend beyond 5 min excitation. The reason could be due
to the existence of troughs in the initial asperities reducing the
actual contact area and the lubricant originally entrapped within
these troughs being squeezed to play its role of lubrication relatively
effectively at the early phase of the ball-on-disc tests.
Comparison of curves in Fig. 9 also suggested that the peaks of the
COF curves with the excitation periods of 20 min and 40 min,
respectively, were generally higher than the counterpart with
10 min excitation period; particularly, a secondary peak occurs
after the second excitation for the 40 min excitation period
(Fig. 9c). Furthermore, their increment in maximum peak COF
amplitude beyond 5 min excitation was almost 42% higher than its
counterpart below 5 min excitation, which also occurred for the
20 min excitation period. Such experimental phenomena could be
as a result of creation of higher irregularity on the wear track of
disc surface at larger excitation period. However, COF values for
non-excited loading seemed to fluctuate very mildly and very little
about mean value of 0.435 approximately in a continuous and
smooth manner after a short period of running in (Fig. 9d). These
monitored results served to indicate the sensitivity of COF of the
contact surfaces to the long excitation intervals of DHE loading.
Their corresponding IPCA curves, as monitored by OLVF, are
plotted in Fig. 10. Appearance of highest IPCA peak was observed
after the second excitation period, showing similar characteristics
to those of COF curves (cf. Figs. 9 and 10). These results suggested
that (i) IPCA values were remarkably influenced by the excitation
period under DHE loading; (ii) both peak and trough values of
IPCA tended to increase within the first few periods of excitation,
and then decrease until the values were down to a final level
almost equal to that without excitation; and (iii) maximum peak
value occurred with the excitation period of 40 min and the value
was higher than its counterpart of the other two excitation conditions.
Results suggested that: (i) excitation frequency had a
highly important role in the wear process of the ball-on-disc test;
and (ii) influence of DHE loading on the wear process was more
significant with friction pairs to be subjected to low loading within
running-in duration, typically in the excitation time in range of
40–80 min (Fig. 10c).
SEM images (Fig. 11) of wear tracks tested with various excitation
periods suggested that the damages and/or wear modes