The effect of sliding distance at different temperatures on the tribological properties of a palm kernel activated carbon–epoxy composite.
Tribology International 94 (2016) 352–359
Contents lists available at ScienceDirect
Tribology International
journal homepage: www.elsevier.com/locate/triboint
The effect of sliding distance at different temperatures
on the tribological properties of a palm kernel activated
carbon–epoxy composite
Noor Ayuma Mat Tahir a, Mohd Fadzli Bin Abdollah a,b,n, Rafidah Hasan a,b,
Hilmi Amiruddin a,b
a
b
Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
Centre for Advanced Research on Energy, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
art ic l e i nf o
a b s t r a c t
Article history:
Received 19 August 2015
Received in revised form
1 October 2015
Accepted 3 October 2015
Available online 14 October 2015
The aim of this study is to investigate the effect of sliding distance and temperature on the tribological
properties of a palm kernel activated carbon–epoxy (PKAC–E) composite. All specimens were formed
into 10-mm diameter pins of 30-mm length. Tribological tests were conducted using a pin-on-disc
tribometer. The results show that there is no significant effect on the coefficient of friction (COF) with the
sliding distance, although the wear rate is slightly increased. However, when the operating temperature
exceeds a critical limit, both the COF and wear rate rapidly increase with sliding distance. As compared
with synthetic and other agricultural waste-based polymeric composite, PKAC–E is considered as one of
the most potential self-lubricating materials at operating temperature below 90 °C.
& 2015 Elsevier Ltd. All rights reserved.
Keywords:
Sliding
Friction coefficient
Temperature
Polymer composite
1. Introduction
Many studies have been conducted to reduce wear and friction
by investigating different types of lubricants or coating materials
for tribological applications [1–14]. Several researchers have found
that composites activated by either graphite or carbon have the
potential to act as self-lubricating materials when reinforced with
other metal materials, such as aluminium [2,3,15,16]. Polymer
composite possess a huge potential in substituting monolithic
alloys due to its unique properties. Natural polymer composites
are environmentally friendly version of polymer composite as they
were reinforced with natural element such as corn fibre, kenaf
powder, and palm ash compared to synthetic fibre such as glass
and carbon fibres.
Recently, a substantial amount of research has shifted focus
from monolithic materials to composite materials to meet the
global demand for lightweight, renewable, high performance, ecofriendly, and wear and corrosion resistant materials. The advantages of composite materials include their permeability (optimise
waxing properties), cost effectiveness, and different strengthening
mechanisms [17–19]. For most wear applications, graphitic carbon
n
Corresponding author at: Faculty of Mechanical Engineering, Universiti Teknikal
Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.
Tel.: þ 60 6 234 6805/6914.
E-mail address: mohdfadzli@utem.edu.my (M.F.B. Abdollah).
http://dx.doi.org/10.1016/j.triboint.2015.10.001
0301-679X/& 2015 Elsevier Ltd. All rights reserved.
(mainly in sp2 bonding) has been used because its addition
reduces the coefficient of friction and increases wear resistance as
compared with the matrix. Zamri et al. [15,16] found that graphite
and porous carbon (also known as activated carbons) exhibited the
potential to act as a self-lubricating material when reinforced with
an aluminium alloy, which significantly improves the wear resistance by increasing the carbon content up to 10 wt%. Gomes et al.
[20] stated in their study of carbon–carbon composites that the
friction coefficient is relatively independent of the sliding speed,
but is highly affected by the test temperature.
Palm Kernel Activated Carbon (PKAC) is a waste from palm oil
extraction process. This waste contains carbon properties and
residual oils (natural lubricant), which has potential to become a
new self-lubricating materials. Current commercial selflubricating material is relatively expensive. Thus, implementing a
carbon material from agriculture wastes as new reinforcement
substitutes in the fabrication of polymer matrix composites, are
supposed to have large potential for a zero waste strategy in
improving tribological properties at an affordable cost [3,15–23].
In general, as observed from previous study, there is no study
regarding the potential of PKAC as solid lubricant in polymer
matrix composites. Thus, the present study was carried out to
examine the fundamental effect of sliding distance and temperature on the tribological properties of palm kernel activated carbon–epoxy (PKAC–E) composite under a dry sliding condition.
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
353
Fig. 2. (a) The mould used, (b) the PKAC–E composite after curing process and
(c) the cross-sectional micrograph of the PKAC–E composite.
Fig. 1. SEM micrograph of PKAC particles (a) before and (b) after sieving.
2. Methodology
2.1. Specimen preparation
The materials used in this study are PKAC and high-density
epoxy [West system 105 epoxy resin (105-B) and West system 206
slow hardener (206-B)]. The PKAC was obtained from manufacturer and the preparation of the PKAC is confidential. The PKAC
was blended using a blender and sieved with a 1-mm sieve. The
residual material (i.e., larger than 1 mm) was re-blended and resieved. The fine PKAC (size r1 mm) was then weighed to 70 wt%
and mixed with epoxy 30 wt% (at a resin to hardener ratio of 4:1).
Fig. 1 shows the SEM micrograph of the PKAC particles before and
after sieving. The mixed PKAC and epoxy were then placed into a
mould, hot-pressed at 80 °C with 2.5 MPa pressure for approximately 10 min and left to cool at room temperature for approximately 10 min before being pressed out from the mould. The pin
specimens, with 10-mm diameters and 30-mm heights, were left
to cure at room temperature for approximately one week. Fig. 2
shows the mould, the specimen and its cross-sectional image in
this study. The cross-sectional micrograph of the specimen shows
only a partial area of the whole cross-section of the composite pin.
This is due to the focusing image on the grain size and distribution.
The surface area analysis were calculated manually. The hollow
part was about 6% of the total area. By neglecting the hollow part,
the summation of the grain display was about 69%. Hence, it can
be said that from the surface analysis, the epoxy contain was about
31%. However, this value was subjected to change as the focus area
changed.
The density and hardness of the specimen were measured
using densitometer and Shore Hardness Durometer-D type,
respectively. Besides, the porosity was measured based on the
Archimedes principle.
The mechanical properties of the pin specimens and discs are
shown in Table 1.
2.2. Pin-on-disc test
The dry sliding test was performed following the ASTM standard [24] and used a pin-on-disc tribometer under different sliding distances between 500 m and 2000 m at temperatures ranging
from 27 °C to 150 °C. The heat was supplied to and measured on
the pin specimen. All of the tests were performed at a constant
sliding speed of 500 rpm (2.7 m/s) and an applied load of 49.05 N.
The average surface roughness was approximately 0.40 mm 70.02
for the pin specimen and approximately 0.13 mm 70.02 for the
disc. Prior to the sliding test, the discs were cleaned using acetone
in an ultrasonic bath. The pin was then mounted vertically on the
tester arm at one end, and the other pin surface was held against
the rotating disc. The schematic diagrams of the pin-on-disc
tribometer and the actual placement image are shown in
Figs. 3 and 4, respectively. The test conditions are summarised in
Table 2.
The coefficient of friction (COF) and frictional force were
measured using a PC-based data logging system. The following
354
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
formula was used:
F
μ¼
W
ð1Þ
where m is the COF, F is the frictional force in N, and W is the
applied load in N.
It was assumed that the disc had negligible wear because there
was no weight changes recorded after the test. The wear of the pin
was recorded by measuring the mass of the pin before and after
the pin-on-disc test. The mass loss was determined by taking the
mass differences, and the mass loss (precision of 0.001 g) was
converted to volume loss by dividing by the bulk density of the
specimen. The specific wear rate was determined as follows:
V loss ¼
k¼
mloss
ð2Þ
ρ
V loss
ðW LÞ
ð3Þ
where Vloss is the volume loss in mm3, mloss is the mass loss in g, ρ
is the bulk density in g/mm3, k is the specific wear rate in mm3/
Nmm, W is the applied load in N, and L is the sliding distance in
mm.
difference between the COFs and sliding distance below the
operating temperature of 90 °C. The COF was approximately 0.23–
0.33. This result is consistent with Rao and Das [25], who also
found that the average COF was unchanged with sliding distance.
However, the COF gradually increases with sliding distance as the
temperature exceeds 90 °C. The gradual increase of COF with
sliding distance at higher operating temperature may have been
due to the thermal degradation of the epoxy binder, which causes
ploughing of the test pin surface, as shown in Fig. 6. This finding is
in agreement with the research of Chowdhury and Helali [26]. The
ploughing effect causes the friction force to increase with rubbing
time. It can also be observed that after a distance of 1500 m at
150 °C, the COF was constant in the range of 0.71–0.74. In this case,
it is believed that tribofilm begins to form on the counter surface.
Fig. 7 clearly shows that the transfer film of the carbon element is
generated at the counter surfaces. A transfer film of the carbon
materials is formed on the counter surface and helps to stabilise
the friction between the surfaces. The tribofilm generated from the
preferential wear of the soft carbon material results in a carbon-
2.3. Surface morphology observation
Heat source
The surface roughness of the worn surfaces was measured
using a profilometer. Furthermore, the morphology of the particles
and worn surfaces were observed and analysed using scanning
electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDX).
3. Results and discussion
Heated
PKAC-E
composite
Fig. 5 shows the COF result at different sliding distances with an
increased operating temperature for a PKAC–E composite sliding
against EN-31 carbon alloy steel. There was no significant
EN-31 steel
disc
Fig. 4. Actual set up of a pin-on-disc test.
Table 1
Mechanical properties of the pin and disc materials before testing.
Properties
a
Pin (70 wt% PKAC þ30 wt%
Epoxy)
Hardness, H [GPa] 8.8
Porosity [%]
1.21
Density, ρ [g/cm3] 1.35
a
b
b
Disc (EN-31 carbon
alloy steel)
7.9
–
7.81
Table 2
Dry sliding test operating parameter.
Applied load, W
[N]
Sliding speed, v
[rpm]
Sliding distance, L
[m]
Temperature, T [°C]
49.05
500 (2.7 m/s)
500, 1000, 1500,
2000
27, 90, 150
Properties from laboratory measurements.
Properties from manufacturer.
Load
Stationary
PKAC-E
composite
Rotating
EN-31 steel
disc
Wear track
Fig. 3. (a) Schematic diagram of a pin-on-disc tribometer and (b) an illustration of the specimen placement.
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
Load = 49.05 N (0.65MPa)
Speed = 500 rpm (2.618m/s)
Distance = 2500 m
Temperature = 150 oC
1.0
Coefficient of friction, COF
oC
27oC
27
oC
90oC
90
0.8
355
EN-31 steel disc
oC
150oC
150
0.6
0.4
0.2
Transfer material
0.0
0
500
1000
1500
2000
2500
Sliding distance, L [m]
1.0
100µm
500 m
Coefficient of friction, COF
1000 m
0.8
1500 m
2000 m
0.6
0.4
0.2
0.0
0
20
40
60
80
100
120
140
160
Fig. 7. SEM micrograph and EDX spectrum showing that the transfer film of the
carbon element is generated at the counter surfaces.
Temperature, T [oC]
Fig. 5. Steady state coefficient of friction as a function of (a) sliding distance and
(b) operating temperature for the PKAC–E composite. The error bar is for standard
deviation.
Specific wear rate, k [mm3/Nmm] x 10-10
50
27 oC
27oC
90 oC
90oC
40
150 oC
150oC
30
20
10
0
0
500
1000
1500
2000
2500
Sliding distance, L [m]
Fig. 6. SEM micrograph showing the ploughing effect of the test PKAC–E composite
at 150 °C.
based tribofilm adhering to the counter surface, which breaks the
adhesive joints between the asperities and leads to low or maintained friction [21,27].
Fig. 8 shows the specific wear rate result of a PKAC–E composite at different sliding distances with increasing operating temperature. The wear rate gradually increases with the operating
temperature and sliding distance. The generated wear debris is
clearly observed in Fig. 9. At a lower operating temperature, it is
Specific wear rate, k [mm3/Nmm] x 10-10
50
500 m
1000 m
40
1500 m
2000 m
30
20
10
0
0
20
40
60
80
100
120
140
160
Temperature, T [oC]
Fig. 8. Specific wear rates as a function of (a) sliding distance and (b) operating
temperature for the PKAC–E composite. The error bar is for standard deviation.
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N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
Fig. 9. SEM micrograph of wear debris generated after the sliding test.
Unworn surface of PKAC-E composite pin
Ra = 0.40µm
Worn surface of PKAC-E composite pin
Ra = 0.78µm
Fig. 10. Optical micrograph of the (a) unworn surfaces and (b) worn surfaces of the PKAC–E composite at 90 °C.
10
PKAC-E composite pin
9
8.83
8.31
Hardness, H [GPa]
8
Crack
7
5.75
6
5
4
3
2
1
0
27
90
150
Temperature, T [oC]
20µm
Fig. 11. SEM micrograph of the worn surfaces of the PKAC–E composite showing
delamination wear occurring at 150 °C.
proposed that the increase in the wear rate with sliding distance
could be due to abrasive wear, where higher frictional heating
inhibits abrasion. Rao and Das [25] explained that as temperature
increases, the subsurface deformation also increases. This deformation will cause surface to be oxidised and releasing fragments.
It is clearly observed in Fig. 10 that parallel grooves are formed by
Fig. 12. Average hardness of the PKAC–E composite at different operating temperatures. The error bar is for standard deviation.
abrasive wear at 90 °C. At 150 °C, more adhesion was observed.
The ploughed surface, as shown in Fig. 6, is symptomatic of
adhesive wear. Surface fatigue occurred with crack formation to
induce delamination wear, as shown in Fig. 11. Nirmal et al. [28]
stated in their review that delamination wear was predominantly
due to polymer swelling. Delamination typically occurs with mild
signs of pitting, peel off, and cracks in the worn surface. As the
temperature increases, material hardness also decreases. This
phenomenon could potentially increase wear. From Fig. 12, it can
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
357
4.0
27 oC
27oC
Surface roughness, Ra [ µm]
3.5
90 oC
90oC
3.0
150 oC
150oC
2.5
2.0
1.5
1.0
0.5
0.0
0
500
1000
1500
2000
2500
Sliding distance, L [m]
Fig. 13. Average surface roughness of the PKAC–E composite at different operating temperatures and distances. The error bar is for standard deviation.
Table 3
Collected data from previous studies for the coefficient of friction and specific wear rate of synthetic and other agricultural waste-based polymeric composite under dry
sliding conditions that were tested at room temperature. The data for the PKAC–E composite is gathered from this study.
Materials
Testing apparatus
Sliding speed [m/s]
Load [N]
Sliding distance [km]
COF
Specific wear rate [mm3/Nmm] 10
Polyester
Coir/Polyester
Betelnut/Polyester
Glass/Polyester
Epoxy
Kenaf/Epoxy
Bambo/Epoxy
Glass/Epoxy
PKAC/Epoxy (27 °C)
PKAC/Epoxy (90 °C)
PKAC/Epoxy (150 °C)
Block-on-disc
Block-on-disc
Block-on-disc
Block-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.5
2.7
2.7
2.7
30
30
30
50
50
50
30
60
49
49
49
0.84–4.2
0.84–4.2
1–6
1–14
1–5
1–5
1–4
1–4.2
0.5–2
0.5–2
0.5–2
0.9–0.95
0.6–0.8
0.45–0.65
0.2–0.6
0.75
0.4–0.65
0.55–0.85
0.44
0.21–0.24
0.30–0.34
0.55–0.74
14–16
11–17
2–22
0.4 – 0.6
17–23
3–6
5.5–9
1.2–3
0.05–0.085
0.08–0.14
0.2–0.395
be observed that the average hardness of the PKAC–E composite
began to decease above an operating temperature of 90 °C. At
150 °C, the PKAC–E loses approximately 30% of its original hardness. It is believed that there is also a thermally activated process
between the PKAC and epoxy in the composite when the temperature is increased. The thermal activation process occurs
because of the thermal degradation of epoxy binder, which pyrolyses and oxidises. Hence, the epoxy element degraded as gas and
other degradation products. Epoxy is one of the ester groups. The
pour point of ester groups is lower at approximately 30–40 °C.
Reaching the pour point, epoxy tends to form a highly viscous
liquid. However, it is crystallisation-inhibited; therefore, epoxy is
rarely used above the pour point. The degradation of epoxy by this
mechanism typically occurs at temperatures of approximately 70–
80 °C. Bhatnagar [29] stated that epoxy has absorption and
degradation behaviours of its own. He also suspected that water
absorption can cause a decrease in glass transition (Tg) because of
strong hydrogen bonds as water was present at micro-cracks. This
absorption can cause atom bonds to weaken and influence the
mechanical properties. Thus, the rapid increasing of the wear rate
above an operating temperature of 90 °C was not only caused by
the thermal degradation and reduction of hardness but also by the
chemical and catalytic degradation due to the epoxy properties.
This result is supported by Brostow et al. [30], who found that
temperature has a direct impact on the wear of polymers.
The increase in the COF and wear rate with sliding distance
could also be due to the changing of surface roughness during the
sliding test, as shown in Fig. 13. It can be clearly observed that the
5
Ref.
[31]
[31]
[32]
[33]
[34]
[34]
[35]
[36]
This study
This study
This study
surface roughness gradually increased with sliding distance at
temperatures above 90 °C. However, at 27 °C, sliding distance does
not have a significant effect on surface roughness. These results are
proportional with the COF and wear rate results, as shown in
Figs. 5 and 9, respectively.
Table 3 lists the most recent studies on the tribological performance of commercial and other agricultural waste-based
polymeric composites for comparison purposes. The data in
Table 3 is extracted and represented in Fig. 14. The PKAC–E composite shows a lower COF and a higher wear resistance compared
to other synthetic polymers and their composites. However, after
the critical limit of 90 °C, the COF is slightly increased. Thus, PKAC–
E is most preferable in improving friction coefficient and wear
resistance at operating temperature below 90 °C.
4. Conclusions
In conclusion, there is no significant effect on the coefficient of
friction (COF) with sliding distance; although, the wear rate is
slightly increased. However, when the operating temperature
exceeds a critical limit, both the COF and wear rate increase rapidly
with sliding distance. Abrasive, adhesive and crack formations that
would induce delamination were identified as predominant wear
mechanisms. As compared with synthetic and other agricultural
waste-based polymeric composite, PKAC–E is considered as one of
the most potential self-lubricating materials at operating temperature below 90 °C.
358
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
Fig. 14. (a) Coefficient of friction and (b) specific wear rate of synthetic and other agricultural waste-based polymeric composite under dry sliding conditions and tested at
room temperature. The data for the PKAC–E composite is gathered from this study. The error bar is for maximum and minimum of mean value.
Acknowledgements
The authors are grateful for the contributions by members of
the Green Tribology and Engine Performance (G-TriboE) research
group. This research was funded by a Grant from the Ministry of
Higher Education Malaysia (Grant no.: ERGS/2013/FKM/TK01/
UTEM/02/04/E00016).
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Contents lists available at ScienceDirect
Tribology International
journal homepage: www.elsevier.com/locate/triboint
The effect of sliding distance at different temperatures
on the tribological properties of a palm kernel activated
carbon–epoxy composite
Noor Ayuma Mat Tahir a, Mohd Fadzli Bin Abdollah a,b,n, Rafidah Hasan a,b,
Hilmi Amiruddin a,b
a
b
Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
Centre for Advanced Research on Energy, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
art ic l e i nf o
a b s t r a c t
Article history:
Received 19 August 2015
Received in revised form
1 October 2015
Accepted 3 October 2015
Available online 14 October 2015
The aim of this study is to investigate the effect of sliding distance and temperature on the tribological
properties of a palm kernel activated carbon–epoxy (PKAC–E) composite. All specimens were formed
into 10-mm diameter pins of 30-mm length. Tribological tests were conducted using a pin-on-disc
tribometer. The results show that there is no significant effect on the coefficient of friction (COF) with the
sliding distance, although the wear rate is slightly increased. However, when the operating temperature
exceeds a critical limit, both the COF and wear rate rapidly increase with sliding distance. As compared
with synthetic and other agricultural waste-based polymeric composite, PKAC–E is considered as one of
the most potential self-lubricating materials at operating temperature below 90 °C.
& 2015 Elsevier Ltd. All rights reserved.
Keywords:
Sliding
Friction coefficient
Temperature
Polymer composite
1. Introduction
Many studies have been conducted to reduce wear and friction
by investigating different types of lubricants or coating materials
for tribological applications [1–14]. Several researchers have found
that composites activated by either graphite or carbon have the
potential to act as self-lubricating materials when reinforced with
other metal materials, such as aluminium [2,3,15,16]. Polymer
composite possess a huge potential in substituting monolithic
alloys due to its unique properties. Natural polymer composites
are environmentally friendly version of polymer composite as they
were reinforced with natural element such as corn fibre, kenaf
powder, and palm ash compared to synthetic fibre such as glass
and carbon fibres.
Recently, a substantial amount of research has shifted focus
from monolithic materials to composite materials to meet the
global demand for lightweight, renewable, high performance, ecofriendly, and wear and corrosion resistant materials. The advantages of composite materials include their permeability (optimise
waxing properties), cost effectiveness, and different strengthening
mechanisms [17–19]. For most wear applications, graphitic carbon
n
Corresponding author at: Faculty of Mechanical Engineering, Universiti Teknikal
Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.
Tel.: þ 60 6 234 6805/6914.
E-mail address: mohdfadzli@utem.edu.my (M.F.B. Abdollah).
http://dx.doi.org/10.1016/j.triboint.2015.10.001
0301-679X/& 2015 Elsevier Ltd. All rights reserved.
(mainly in sp2 bonding) has been used because its addition
reduces the coefficient of friction and increases wear resistance as
compared with the matrix. Zamri et al. [15,16] found that graphite
and porous carbon (also known as activated carbons) exhibited the
potential to act as a self-lubricating material when reinforced with
an aluminium alloy, which significantly improves the wear resistance by increasing the carbon content up to 10 wt%. Gomes et al.
[20] stated in their study of carbon–carbon composites that the
friction coefficient is relatively independent of the sliding speed,
but is highly affected by the test temperature.
Palm Kernel Activated Carbon (PKAC) is a waste from palm oil
extraction process. This waste contains carbon properties and
residual oils (natural lubricant), which has potential to become a
new self-lubricating materials. Current commercial selflubricating material is relatively expensive. Thus, implementing a
carbon material from agriculture wastes as new reinforcement
substitutes in the fabrication of polymer matrix composites, are
supposed to have large potential for a zero waste strategy in
improving tribological properties at an affordable cost [3,15–23].
In general, as observed from previous study, there is no study
regarding the potential of PKAC as solid lubricant in polymer
matrix composites. Thus, the present study was carried out to
examine the fundamental effect of sliding distance and temperature on the tribological properties of palm kernel activated carbon–epoxy (PKAC–E) composite under a dry sliding condition.
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
353
Fig. 2. (a) The mould used, (b) the PKAC–E composite after curing process and
(c) the cross-sectional micrograph of the PKAC–E composite.
Fig. 1. SEM micrograph of PKAC particles (a) before and (b) after sieving.
2. Methodology
2.1. Specimen preparation
The materials used in this study are PKAC and high-density
epoxy [West system 105 epoxy resin (105-B) and West system 206
slow hardener (206-B)]. The PKAC was obtained from manufacturer and the preparation of the PKAC is confidential. The PKAC
was blended using a blender and sieved with a 1-mm sieve. The
residual material (i.e., larger than 1 mm) was re-blended and resieved. The fine PKAC (size r1 mm) was then weighed to 70 wt%
and mixed with epoxy 30 wt% (at a resin to hardener ratio of 4:1).
Fig. 1 shows the SEM micrograph of the PKAC particles before and
after sieving. The mixed PKAC and epoxy were then placed into a
mould, hot-pressed at 80 °C with 2.5 MPa pressure for approximately 10 min and left to cool at room temperature for approximately 10 min before being pressed out from the mould. The pin
specimens, with 10-mm diameters and 30-mm heights, were left
to cure at room temperature for approximately one week. Fig. 2
shows the mould, the specimen and its cross-sectional image in
this study. The cross-sectional micrograph of the specimen shows
only a partial area of the whole cross-section of the composite pin.
This is due to the focusing image on the grain size and distribution.
The surface area analysis were calculated manually. The hollow
part was about 6% of the total area. By neglecting the hollow part,
the summation of the grain display was about 69%. Hence, it can
be said that from the surface analysis, the epoxy contain was about
31%. However, this value was subjected to change as the focus area
changed.
The density and hardness of the specimen were measured
using densitometer and Shore Hardness Durometer-D type,
respectively. Besides, the porosity was measured based on the
Archimedes principle.
The mechanical properties of the pin specimens and discs are
shown in Table 1.
2.2. Pin-on-disc test
The dry sliding test was performed following the ASTM standard [24] and used a pin-on-disc tribometer under different sliding distances between 500 m and 2000 m at temperatures ranging
from 27 °C to 150 °C. The heat was supplied to and measured on
the pin specimen. All of the tests were performed at a constant
sliding speed of 500 rpm (2.7 m/s) and an applied load of 49.05 N.
The average surface roughness was approximately 0.40 mm 70.02
for the pin specimen and approximately 0.13 mm 70.02 for the
disc. Prior to the sliding test, the discs were cleaned using acetone
in an ultrasonic bath. The pin was then mounted vertically on the
tester arm at one end, and the other pin surface was held against
the rotating disc. The schematic diagrams of the pin-on-disc
tribometer and the actual placement image are shown in
Figs. 3 and 4, respectively. The test conditions are summarised in
Table 2.
The coefficient of friction (COF) and frictional force were
measured using a PC-based data logging system. The following
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N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
formula was used:
F
μ¼
W
ð1Þ
where m is the COF, F is the frictional force in N, and W is the
applied load in N.
It was assumed that the disc had negligible wear because there
was no weight changes recorded after the test. The wear of the pin
was recorded by measuring the mass of the pin before and after
the pin-on-disc test. The mass loss was determined by taking the
mass differences, and the mass loss (precision of 0.001 g) was
converted to volume loss by dividing by the bulk density of the
specimen. The specific wear rate was determined as follows:
V loss ¼
k¼
mloss
ð2Þ
ρ
V loss
ðW LÞ
ð3Þ
where Vloss is the volume loss in mm3, mloss is the mass loss in g, ρ
is the bulk density in g/mm3, k is the specific wear rate in mm3/
Nmm, W is the applied load in N, and L is the sliding distance in
mm.
difference between the COFs and sliding distance below the
operating temperature of 90 °C. The COF was approximately 0.23–
0.33. This result is consistent with Rao and Das [25], who also
found that the average COF was unchanged with sliding distance.
However, the COF gradually increases with sliding distance as the
temperature exceeds 90 °C. The gradual increase of COF with
sliding distance at higher operating temperature may have been
due to the thermal degradation of the epoxy binder, which causes
ploughing of the test pin surface, as shown in Fig. 6. This finding is
in agreement with the research of Chowdhury and Helali [26]. The
ploughing effect causes the friction force to increase with rubbing
time. It can also be observed that after a distance of 1500 m at
150 °C, the COF was constant in the range of 0.71–0.74. In this case,
it is believed that tribofilm begins to form on the counter surface.
Fig. 7 clearly shows that the transfer film of the carbon element is
generated at the counter surfaces. A transfer film of the carbon
materials is formed on the counter surface and helps to stabilise
the friction between the surfaces. The tribofilm generated from the
preferential wear of the soft carbon material results in a carbon-
2.3. Surface morphology observation
Heat source
The surface roughness of the worn surfaces was measured
using a profilometer. Furthermore, the morphology of the particles
and worn surfaces were observed and analysed using scanning
electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDX).
3. Results and discussion
Heated
PKAC-E
composite
Fig. 5 shows the COF result at different sliding distances with an
increased operating temperature for a PKAC–E composite sliding
against EN-31 carbon alloy steel. There was no significant
EN-31 steel
disc
Fig. 4. Actual set up of a pin-on-disc test.
Table 1
Mechanical properties of the pin and disc materials before testing.
Properties
a
Pin (70 wt% PKAC þ30 wt%
Epoxy)
Hardness, H [GPa] 8.8
Porosity [%]
1.21
Density, ρ [g/cm3] 1.35
a
b
b
Disc (EN-31 carbon
alloy steel)
7.9
–
7.81
Table 2
Dry sliding test operating parameter.
Applied load, W
[N]
Sliding speed, v
[rpm]
Sliding distance, L
[m]
Temperature, T [°C]
49.05
500 (2.7 m/s)
500, 1000, 1500,
2000
27, 90, 150
Properties from laboratory measurements.
Properties from manufacturer.
Load
Stationary
PKAC-E
composite
Rotating
EN-31 steel
disc
Wear track
Fig. 3. (a) Schematic diagram of a pin-on-disc tribometer and (b) an illustration of the specimen placement.
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
Load = 49.05 N (0.65MPa)
Speed = 500 rpm (2.618m/s)
Distance = 2500 m
Temperature = 150 oC
1.0
Coefficient of friction, COF
oC
27oC
27
oC
90oC
90
0.8
355
EN-31 steel disc
oC
150oC
150
0.6
0.4
0.2
Transfer material
0.0
0
500
1000
1500
2000
2500
Sliding distance, L [m]
1.0
100µm
500 m
Coefficient of friction, COF
1000 m
0.8
1500 m
2000 m
0.6
0.4
0.2
0.0
0
20
40
60
80
100
120
140
160
Fig. 7. SEM micrograph and EDX spectrum showing that the transfer film of the
carbon element is generated at the counter surfaces.
Temperature, T [oC]
Fig. 5. Steady state coefficient of friction as a function of (a) sliding distance and
(b) operating temperature for the PKAC–E composite. The error bar is for standard
deviation.
Specific wear rate, k [mm3/Nmm] x 10-10
50
27 oC
27oC
90 oC
90oC
40
150 oC
150oC
30
20
10
0
0
500
1000
1500
2000
2500
Sliding distance, L [m]
Fig. 6. SEM micrograph showing the ploughing effect of the test PKAC–E composite
at 150 °C.
based tribofilm adhering to the counter surface, which breaks the
adhesive joints between the asperities and leads to low or maintained friction [21,27].
Fig. 8 shows the specific wear rate result of a PKAC–E composite at different sliding distances with increasing operating temperature. The wear rate gradually increases with the operating
temperature and sliding distance. The generated wear debris is
clearly observed in Fig. 9. At a lower operating temperature, it is
Specific wear rate, k [mm3/Nmm] x 10-10
50
500 m
1000 m
40
1500 m
2000 m
30
20
10
0
0
20
40
60
80
100
120
140
160
Temperature, T [oC]
Fig. 8. Specific wear rates as a function of (a) sliding distance and (b) operating
temperature for the PKAC–E composite. The error bar is for standard deviation.
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N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
Fig. 9. SEM micrograph of wear debris generated after the sliding test.
Unworn surface of PKAC-E composite pin
Ra = 0.40µm
Worn surface of PKAC-E composite pin
Ra = 0.78µm
Fig. 10. Optical micrograph of the (a) unworn surfaces and (b) worn surfaces of the PKAC–E composite at 90 °C.
10
PKAC-E composite pin
9
8.83
8.31
Hardness, H [GPa]
8
Crack
7
5.75
6
5
4
3
2
1
0
27
90
150
Temperature, T [oC]
20µm
Fig. 11. SEM micrograph of the worn surfaces of the PKAC–E composite showing
delamination wear occurring at 150 °C.
proposed that the increase in the wear rate with sliding distance
could be due to abrasive wear, where higher frictional heating
inhibits abrasion. Rao and Das [25] explained that as temperature
increases, the subsurface deformation also increases. This deformation will cause surface to be oxidised and releasing fragments.
It is clearly observed in Fig. 10 that parallel grooves are formed by
Fig. 12. Average hardness of the PKAC–E composite at different operating temperatures. The error bar is for standard deviation.
abrasive wear at 90 °C. At 150 °C, more adhesion was observed.
The ploughed surface, as shown in Fig. 6, is symptomatic of
adhesive wear. Surface fatigue occurred with crack formation to
induce delamination wear, as shown in Fig. 11. Nirmal et al. [28]
stated in their review that delamination wear was predominantly
due to polymer swelling. Delamination typically occurs with mild
signs of pitting, peel off, and cracks in the worn surface. As the
temperature increases, material hardness also decreases. This
phenomenon could potentially increase wear. From Fig. 12, it can
N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
357
4.0
27 oC
27oC
Surface roughness, Ra [ µm]
3.5
90 oC
90oC
3.0
150 oC
150oC
2.5
2.0
1.5
1.0
0.5
0.0
0
500
1000
1500
2000
2500
Sliding distance, L [m]
Fig. 13. Average surface roughness of the PKAC–E composite at different operating temperatures and distances. The error bar is for standard deviation.
Table 3
Collected data from previous studies for the coefficient of friction and specific wear rate of synthetic and other agricultural waste-based polymeric composite under dry
sliding conditions that were tested at room temperature. The data for the PKAC–E composite is gathered from this study.
Materials
Testing apparatus
Sliding speed [m/s]
Load [N]
Sliding distance [km]
COF
Specific wear rate [mm3/Nmm] 10
Polyester
Coir/Polyester
Betelnut/Polyester
Glass/Polyester
Epoxy
Kenaf/Epoxy
Bambo/Epoxy
Glass/Epoxy
PKAC/Epoxy (27 °C)
PKAC/Epoxy (90 °C)
PKAC/Epoxy (150 °C)
Block-on-disc
Block-on-disc
Block-on-disc
Block-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
Pin-on-disc
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.5
2.7
2.7
2.7
30
30
30
50
50
50
30
60
49
49
49
0.84–4.2
0.84–4.2
1–6
1–14
1–5
1–5
1–4
1–4.2
0.5–2
0.5–2
0.5–2
0.9–0.95
0.6–0.8
0.45–0.65
0.2–0.6
0.75
0.4–0.65
0.55–0.85
0.44
0.21–0.24
0.30–0.34
0.55–0.74
14–16
11–17
2–22
0.4 – 0.6
17–23
3–6
5.5–9
1.2–3
0.05–0.085
0.08–0.14
0.2–0.395
be observed that the average hardness of the PKAC–E composite
began to decease above an operating temperature of 90 °C. At
150 °C, the PKAC–E loses approximately 30% of its original hardness. It is believed that there is also a thermally activated process
between the PKAC and epoxy in the composite when the temperature is increased. The thermal activation process occurs
because of the thermal degradation of epoxy binder, which pyrolyses and oxidises. Hence, the epoxy element degraded as gas and
other degradation products. Epoxy is one of the ester groups. The
pour point of ester groups is lower at approximately 30–40 °C.
Reaching the pour point, epoxy tends to form a highly viscous
liquid. However, it is crystallisation-inhibited; therefore, epoxy is
rarely used above the pour point. The degradation of epoxy by this
mechanism typically occurs at temperatures of approximately 70–
80 °C. Bhatnagar [29] stated that epoxy has absorption and
degradation behaviours of its own. He also suspected that water
absorption can cause a decrease in glass transition (Tg) because of
strong hydrogen bonds as water was present at micro-cracks. This
absorption can cause atom bonds to weaken and influence the
mechanical properties. Thus, the rapid increasing of the wear rate
above an operating temperature of 90 °C was not only caused by
the thermal degradation and reduction of hardness but also by the
chemical and catalytic degradation due to the epoxy properties.
This result is supported by Brostow et al. [30], who found that
temperature has a direct impact on the wear of polymers.
The increase in the COF and wear rate with sliding distance
could also be due to the changing of surface roughness during the
sliding test, as shown in Fig. 13. It can be clearly observed that the
5
Ref.
[31]
[31]
[32]
[33]
[34]
[34]
[35]
[36]
This study
This study
This study
surface roughness gradually increased with sliding distance at
temperatures above 90 °C. However, at 27 °C, sliding distance does
not have a significant effect on surface roughness. These results are
proportional with the COF and wear rate results, as shown in
Figs. 5 and 9, respectively.
Table 3 lists the most recent studies on the tribological performance of commercial and other agricultural waste-based
polymeric composites for comparison purposes. The data in
Table 3 is extracted and represented in Fig. 14. The PKAC–E composite shows a lower COF and a higher wear resistance compared
to other synthetic polymers and their composites. However, after
the critical limit of 90 °C, the COF is slightly increased. Thus, PKAC–
E is most preferable in improving friction coefficient and wear
resistance at operating temperature below 90 °C.
4. Conclusions
In conclusion, there is no significant effect on the coefficient of
friction (COF) with sliding distance; although, the wear rate is
slightly increased. However, when the operating temperature
exceeds a critical limit, both the COF and wear rate increase rapidly
with sliding distance. Abrasive, adhesive and crack formations that
would induce delamination were identified as predominant wear
mechanisms. As compared with synthetic and other agricultural
waste-based polymeric composite, PKAC–E is considered as one of
the most potential self-lubricating materials at operating temperature below 90 °C.
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N.A. Mat Tahir et al. / Tribology International 94 (2016) 352–359
Fig. 14. (a) Coefficient of friction and (b) specific wear rate of synthetic and other agricultural waste-based polymeric composite under dry sliding conditions and tested at
room temperature. The data for the PKAC–E composite is gathered from this study. The error bar is for maximum and minimum of mean value.
Acknowledgements
The authors are grateful for the contributions by members of
the Green Tribology and Engine Performance (G-TriboE) research
group. This research was funded by a Grant from the Ministry of
Higher Education Malaysia (Grant no.: ERGS/2013/FKM/TK01/
UTEM/02/04/E00016).
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