respectively. Such a condition was anticipated as possibly due to
the optimal deformation of surface asperities at the surface
roughness R¼0.20 μm under the specific 30 N loading, which
subsequently reduced the exerting normal pressure between the
friction pair. The COF curve (Fig. 5a) under the spring-connecting
load of 25 N exhibited relatively severer fluctuation, within the
period of approaching steady-loading, than those under the
spring-connecting loads of 30 N and 35 N, respectively. This suggests
the COF to be more sensitive to low DHE loadings. Its
probable reason could be the low load amplitude and short dying
down time in each loading period (cf. Fig. 3), which resulted in
lighter and shorter indented deformation path within a period.
Hence a complete wear track on disc would likely be covered with
higher numbers of such periodic sub-tracks to create more fluctuations
(Fig. 5). Comparatively, the high spring-connecting loads
of 30 N and 35 N tended to have less numbers of periodic indented
tracks in a complete wear track on a disc specimen. As shown in
Fig. 5a and b, the oscillating loading and the excitation of the
spring-connecting load were likely to seriously change the morphology
of the contact surfaces by indenting the surface material,
which was displaced to densify surrounding. In addition, the
sliding initiation meant the materials upstream to the indenting
ball to be shorn off. Their severity level usually increased with DHE
loading, which was the main reason of the sharply increase of the
COF and IPCA at the beginning of excitation period. To identify
further the possible wear mode involved with such a friction pair
subjected to high transient loading, quickly shutting down of the
test at the commencement of excitation period would allow the
analysis of wear track morphology on the disc specimen. Aiming at
achieving this, the test under the spring-connecting load of 35 N
was terminated at its fifth excitation after about 95 min of sliding
as the influence of transiently high loading on the sharp increase
of IPCA values to be diminished (Fig. 6c).
IPCA curves of the wear processes, as monitored by OLVF, under
the three different loadings are plotted in Fig. 6. Trend of the curve
(Fig. 6a) showed the IPCA values under the spring-connecting load
of 25 N inclining to remain at a comparatively low level with some
mild fluctuations, and to be remarkably different from that of the
COF curve. These observations seemed to suggest that there was
not any obviously direct relationship between friction and wear. In
each excitation period, IPCA curves under the spring-connecting
loads of 30 N and 35 N gave IPCA peaks of at the commencement
of excitation, which was then followed by a sharply decreasing
trend. The highest peaks of different tests occurred in the range of
around 40–80 min. Thereafter, the influence of high transient load
on the wear rate decreased (Fig. 6b and c). Such wearing condition
corresponding to high transient loading showed a similar trend to
that of the running-in period in constant loading test (Fig. 10d),
and it appeared to be agreeable with the trend of the COF curves
(Fig. 5b).
Increasing the spring-connecting load from 25 N to 35 N (Fig. 3)
led to more than 10 N increments in the transient load, and
oscillation dying time to become almost 1.25 times longer. Such
increase in transient load likely overloaded the friction pairs,