2.3 Measurement of wear loss
The depth of the wear track was measured as the indicator of wear loss,
by using a laser microscope (S-neox, Sensofar Metrology, Scottsdale, AZ)
following the method reported by Yamamoto et al. (2013) [10] with
minor modifications. The laser wavelength used was 530 nm and the
resolution was 75 nm. The depth of the wear track and the
cross-sectional area profile were precisely determined. Figure 2 shows
typical data of the wear track profile obtained from the laser
microscope.
Observation of wax crystallization using Polarized Light
Microscopy
To determine how the environmental temperature affects the crystal
structures and subsequently physical properties such as coefficient of
friction and wear, the microstructure of the coating materials was
observed using PLM. Samples were prepared following the method reported
by Fei et al. [8] with minor modifications. A small amount of wax
was loaded on the glass microscopy slides and heated in an oven at 5 °C
above the melting temperature of waxes for 30 minutes. A preheated cover
slide was then slipped over the molten wax to produce a thin film. The
prepared slides were cooled at room temperature for one hour and then
analyzed with a Differential Interference Contrast (DIC) microscope
(Olympus BX53, Olympus Corporation, MA, USA) at 0, 25 and 50 °C using
the CellSens Dimension software (Olympus Corporation, MA, USA). The
crystal images were taken at 100× magnification.
Determination of relative crystallinity using DSC
The melting profile of these coating materials was determined by using a
differential scanning calorimeter (Discovery DSC, TA Instruments, New
Castle, DE, USA). The temperature program started with a 1-min
equilibration at 25 °C, following by 8 min conditioning at 0, 25 or 50
°C, and then heating to 110 °C at 10 °C/min and a 2-min hold at 110 °C.
The heat of fusion measured by DSC is used to characterize the
crystallinity as reported by Yao and Wang [11]. The degree of
crystallinity (%) of various waxes relative to paraffin wax at 25 °C
was estimated by the following equation:
RC (relative crystallinity, %) = 100 × ΔHS / ΔHB
where ΔHS is the heat of fusion of the tested wax and ΔHB is the heat of
fusion of paraffin wax conditioned at 25 °C.
Results and Discussion
3.1 Effect of normal force,
sliding velocity and environment temperature on coefficient of friction
The measurements were conducted directly on wax samples instead of
cardboard samples coated with the waxes. This is because that the tested
waxes are designed for bulk coating in which the wax forms a thick layer
on the cardboard surface, and testing on the wax samples and coated
cardboard samples would make negligible difference. Moreover, testing
directly on the wax samples help us understand the effect of these
boundary conditions on the material itself. Table 1a shows the
coefficient of friction of different waxes under various normal loads.
It can be seen that only paraffin’s μ was significantly affected by
normal load. The μ of paraffin decreased when the normal load was
increased from 5 to 10 N, and further increase of normal load to 15 N
did not lead to further decrease in μ (Table 1a). The decrease in μ of
paraffin is probably due to the formation of a paraffin film at the
interface which can serve as a solid lubricant with the increased normal
load. A paraffin layer was also observed on the stainless-steel balls
which may have changed the surfaces in contact and resulted in a reduced
μ. As 10 N already resulted a transfer film and a coating on the
stainless-steel balls, no further significant decrease in μ was observed
by further increasing normal load to 15 N. Zhou et al. [12] also
observed similar phenomenon when evaluating tribological properties of
polyurethane modified polyamide-based composites. As the applied load
increases, the friction heat increased quickly and a stable transfer
film which would act as a solid lubricant can be produced by the melting
material, thus reduced the friction coefficient [12]. FHSO and HCO
which are harder than paraffin generated powder during sliding and did
not form the transfer film, thus no significant variation in μ was
observed. EGMD and Estercoat are harder than paraffin but softer than
FHSO and HCO, and the wear-off material of EGMD and Estercoat generated
during sliding was not powdery. However, a smooth transfer film was
probably also not formed. Therefore, no significant changes in μ (Table
1a) for these two materials. Beeswax’s μ was not significantly affect by
the increased normal load either. It is the softest material compared to
the other five, and the stainless-steel balls were already fully coated
with the wax at a low normal load (5 N). With no significant change on
the surfaces in contact, no further formation of the transfer film, the
μ was not significantly affected with increased normal load. Comparing
the different materials, beeswax had the highest μ; EGMD, Estercoat and
paraffin behaved similarly; and FHSO and HCO had lower μ than the other
four materials at all three normal load levels (Table 1a). In addition
to the transfer film formation ability, the different μ of different
materials may also be attributed by their physical properties such as
hardness. By Bowden’s friction theory [13], the friction coefficient
may be expressed in terms of μ = Ars/N (where Ar is the real contact
area, which is directly proportional to the friction force, s is the
shear strength, and N is the applied load). Higher hardness may lead to
lower degree of deformation of the material at the contact point, and
thus smaller real contact area between the material and the
stainless-steel balls. Consequently, at the same applied load, materials
with higher hardness would have lower friction coefficient.
Table 1b shows the changes in μ of selected waxes with different sliding
velocities. For paraffin wax, its μ first decreased when sliding
velocity was increased from 1 to 5 rad/s, and then increased when
sliding velocity was further increased to 10 rad/s. The decrease in μ
can be explained by the same reason stated in the previous section, that
a transfer film was formed to serve as a solid lubricant [12].
However, at higher sliding velocity (10 rad/s), the paraffin film may
have been melted by the friction heat and transferred away. HCO had a
decreasing trend in μ when sliding velocity was increased, and a
significant reduction in μ was observed when sliding velocity was
increased to 10 rad/s (Table 1b). HCO powders which may have lubricant
effect were generated during the sliding. The HCO powder was tested in
pharmaceuticals as a lubricant to prevent ingredients from clumping
together and from sticking to the tablet punches or capsule filling
machine [14]. With the increase in sliding speed, the friction heat
may also accumulate quickly at the interface, leading to the softening
and spreading of the material, which can promote the formation of a
stable transfer film. The thin and stable transfer film then can act as
the solid lubricant and reduces the friction coefficient of HCO at 10
rad/s [12]. For beeswax, the increased sliding velocity resulted in
a significant increase in μ (Table 1b), this is likely due to the severe
deformation and an increase in the real contact area with the increased
sliding speed [15]. While for FHSO, EGMD and Estercoat, sliding
velocity had no significant effect on their μ (Table 1b) indicating that
they are less susceptible to shear. Overall, beeswax had the highest μ
at all sliding velocities tested. EGMD, Estercoat, and paraffin wax
again had similar frictional behavior, while FHSO and HCO had lower μ
compared to the other four materials (Table 1). Physical properties of
materials such as hardness again may have attributed to the friction
coefficient’s different responses to different sliding velocities
[13].
Table 1c shows the effect of temperature on coefficient of friction of
different waxes. The μ of paraffin and beeswax increased significantly
when the environmental temperature was increased to 50 °C. The elevated
temperature led to softening of paraffin and beeswax (melting point both
is about 62-66°C), which can subsequently cause severe deformation and
larger real contact area at the contact point. Severe deformation and
larger real contact area all can result in higher μ [13]. Increase
temperature may also lead to faster vibration of molecules and cause a
local uplifting of the surface, which could result in higher surface
roughness and subsequently increase friction coefficient [16]. Low
temperature did not have significant impact on paraffin, however, it
reduced the coefficient of friction of beeswax (Table 1c). Beeswax was
significantly hardened at freezing temperature, and the increased
hardness could result in a decreased μ and less wear [13]. For FHSO
and HCO, temperature (neither low nor high) had no significant effect on
μ (Table 1c). HCO has a much higher melting point (86-88 °C) compared to
the other five selected waxes, and it is expected that no significant
changes in μ would be seen at 50 °C. Although FHSO has a lower melting
point (about 66 °C), with a sharp melting profile, severe softening of
FHSO may not occur when the materials is heated at a temperature lower
than its melting. Thus, less significant impact of temperature on
coefficient of friction was observed when heated at 50 °C. The high
hardness of FHSO and HCO may also have attributed to their friction
coefficients’ low susceptibility to environmental temperature [17].
For EGMD and Estercoat, it was surprising to observe that both decrease
and increase in temperature resulted higher μ (Table 1c). The reasons
for this observation were not fully understood, and a study on the
changes of the crystal structure of the EGMD and Estercoat at low
temperature and high temperature was conducted and discussed in a later
section to help explain this.
3.2 Effect of normal force,
sliding velocity and environment temperature on wear loss
The sliding caused substrate worn and wear tracks with different depth
was observed. However, the weight difference of the sample before and
after sliding was little and cannot accurately represent the
significance of the boundary conditions on wear loss. Therefore, wear
loss was characterized by precisely measuring the depth of the wear
track, and Table 2a shows the surface wear loss of different coating
materials under different normal loads at fixed sliding velocity of 5
rad/s. It can be seen that all the materials had increased wear loss
when the normal load was increased. Others have reported similar trend
although different materials were used for the evaluation [12, 18].
As the normal load increases, the friction heat produced at the contact
surface is generated faster leading to a higher transient temperature,
and the waxes would melt and adhere to the surface of the counterface,
thus resulted in a more severe wear. Among the different materials,
beeswax at all normal load levels had the most severe wear, while HCO
had the least wear (Table 2a). The difference in wear of different
materials under same sliding conditions is also very likely determined
by their hardness. Among these materials, HCO has the highest hardness,
followed by FHSO, Estercoat, EGMD, paraffin and lastly beeswax. The wear
loss of these materials (from shallow to deep) seems to follow the same
order of their hardness, thus the higher hardness of the material, the
less wear. Others also have reported that materials with lower hardness
and softening temperature can be transferred away from the friction
surface more easily, resulting in more wear loss [12]. Overall, our
Estercoat which has higher hardness than paraffin had less wear
indicating that under similar frictional conditions, Estercoat may have
better performance durability than paraffin.
Table 2b shows the wear loss of different coating materials under
different sliding velocities with fixed normal load of 5N at room
temperature (25 °C). It was observed that the wear loss of all the
materials except that of EGMD increased as sliding velocity was
increased. When sliding velocity was increased, the sliding distance was
also increased as the sliding time was kept constant, and longer sliding
distance theoretically leads to more wear loss. However, the wear loss
of EGMD surprisingly decreased with the increased sliding velocity. The
wear behavior of coatings is strongly dependent on their ability to form
the transfer film at the interface [18, 19]. It is possible that
EGMD has formed a transfer film which prevented further contact of the
steel ball with the surface by the high shearing force under the high
sliding velocity. Such film can be useful for preventing fatigue
deformation and reducing plough effect for coatings [12]. Similar to
the data shown in table 1, harder material tends to be more wear
resistant. Beeswax having high coefficient of friction and being the
softest material had the most wear, while HCO which has high hardness
and low coefficient of friction had the least wear at different sliding
velocities. FHSO and Estercoat were similar at low sliding velocity,
while Estercoat has better performance at high sliding velocity. This is
probably due to Estercoat having better transfer film forming ability
than FHSO under high shearing force. The Estercoat also outperformed
paraffin at different sliding velocity, indicating that the Estercoat as
a surface coating probably has better performance durability.
The effect of environmental temperature on wear was also evaluated.
Samples went through sliding at fixed normal load and sliding velocity
(5 N and 5 rad/s, respectively) under different temperatures, and the
wear loss was quantified. Table 2c shows that the wear loss of paraffin,
FHSO, and beeswax all increased when the temperature was increased. The
wear loss of HCO, EGMD and Estercoat was at the lowest when at room
temperature, however, increased when the temperature was either lowered
or elevated. The low temperature may have negatively affected the
formation of transfer film, while the high temperature could have
increased the transfer away of the coating on the counterface, thus all
resulted in increased wear loss [12, 18, 19]. The wear loss of
paraffin, beeswax, EGMD and Estercoat all corresponded to their changes
in surface coefficient of friction with temperature. Low temperature did
not have significant impact on paraffin, however, it reduced the
coefficient of friction of beeswax. While the elevated temperature
resulted in higher surface friction coefficient of paraffin and beeswax
and may have led to softening and transferring away of paraffin and
beeswax at the interface [12, 18], thus more severe wear at high
temperature. For EGMD and Estercoat, their surface coefficient of
friction was higher at lowered and elevated temperatures than when at
room temperature, which matched their wear behavior. The coefficient of
friction of HCO and FHSO was not significantly affected by temperature,
however, their wear loss was significantly affected. It is suspected
that the different response of wear loss to temperature may relate to
the crystal structure and melting profile of the different materials.
Overall, the Estercoat would outperform paraffin according to their
observed friction and wear behaviors.