micromachines
Article
Effects of Doping Elements on the Friction and Wear
of SUJ2 Steel Sliding against Aluminum Alloys
Yuh-Ping Chang *, Zi-Wei Huang and Huann-Ming Chou
Department of Mechanical Engineering, Kun Shan University, Tainan 710, Taiwan;
[email protected] (Z.-W.H.); [email protected] (H.-M.C.)
* Correspondence: [email protected]; Tel.: +886-6-272-7175
Academic Editor: Jeng-Haur Horng
Received: 7 February 2017; Accepted: 17 March 2017; Published: 23 March 2017
Abstract: Damage to mechanical components caused by wear is considered to be an important issue
for mechanical engineers. For the purpose of wear resistance, it is necessary to improve the material
properties of the mechanical elements. Furthermore, low friction plays an important role in saving
energy. Hence, it is important to establish a key technology for wear resistance and low friction
through appropriate materials science for related industries. In general, the tribological properties of
aluminum alloys are very different from those of steels. Hence, aluminum alloys should be specially
considered and clarified for their tribological properties before being applied industrially. This paper
therefore aims to further investigate the effects of the content of doping elements on the friction and
wear of the selected aluminum alloys. From the experimental results, it can be concluded that the
higher the Si content, the smaller the friction coefficient, and the milder the variation. The higher the
content of iron and copper, the more materials are removed, showing better machinability. Moreover,
three frictional models and wear mechanisms that describe the effects of the content of doping
elements on the friction and wear are proposed. The wear mechanisms change as the silicon content
increases, from the junction growth to the wedge and the ploughing particles. As a result, better
choices of aluminum alloys with regards to friction and wear can then be made. These results have
great practical importance.
Keywords: aluminum alloy; silicon; friction; wear
1. Introduction
With the rapid developments of traffic vehicles and key transmission components, the demand
for safe and lightweight new materials is growing, and with the global demand for energy saving and
carbon reduction, the industry urgently needs to develop lightweight, reliable and environmentally
friendly materials. Of the metal contained in the earth, aluminum is the second most abundant, behind
iron. Hence, aluminum and its chemical compositions are the materials with large development
potential [1–4]. In addition, due to its light weight, high strength, acceptable corrosion resistance and
good machinability, aluminum alloy is widely used in aerospace machinery [5], defense and people’s
livelihood industry. The early aluminum alloy material aimed at being lightweight, and did not
consider the enhancement of wear resistance, therefore, although aluminum alloy had high strength
and durability, its abrasion resistance was always significantly worse than that of steel [6]. In order to
apply aluminum alloy to advanced transport vehicles and key transmission parts, further improvement
of the wear properties of aluminum alloy materials is the main project of product development.
Since silicon is a very strong element, if its particles are added to the industrial material, the
abrasion resistance of the material can be greatly enhanced, and therefore its applicability is greatly
increased [7]. Sarkar and Prasad [8,9] found that the crystal structures of silicon will affect the wear
properties of aluminum alloys. If they are further mixed with an appropriate amount of Ni, Cu and
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Mg, their mechanical properties can then be improved. Over recent decades, these research topics
have been continuously investigated by national scholars [10–16].
Mg, their
can
then
be improved.
Overdegrade
recent decades,
these research
Somemechanical
researchersproperties
have found
that
certain
elements may
wear properties
such astopics
Fe or
have
been
continuously
investigated
by
national
scholars
[10–16].
Mn. It is generally believed that this is because Fe and Mn react with silicon to produce some
Some
researchers
havereduce
foundthe
thateffective
certain volume
elementsofmay
degrade
wear properties
such
Fe
special
compounds,
which
the silicon
particles
and reduce
the as
wear
or
Mn. It is
generally
believedrefers
that this
is gradual
because removal
Fe and Mn
reactsurface
with silicon
to produce
some
properties
[17].
Wear behavior
to the
of solid
material
by mechanical
special
which
reduce
the effective
volumetime
of the
silicon particles
andthe
reduce
the wear
actions;compounds,
if the wear rate
is lower,
a longer
experimental
is required
to identify
difference
[18].
properties
Wear
refers
to theLi
gradual
removal
of solid
surface material
mechanical
Moreover, [17].
Alpas,
Hu,behavior
and Zhang
[19–21],
and Tandon
[22],
and Subramanian
[23]by
designed
dry
actions;
if the
rate is
lower, a longer
is required toand
identify
theclearer
difference
[18].
wear tests
forwear
various
aluminum
alloys experimental
to speed up time
the experiments
obtain
results.
Moreover,
Alpas,
Hu, andthe
Zhang
[19–21],
Li and Tandon
[22], andtemperature,
Subramanianwear,
[23] designed
dry wear
Wang [24,25]
clarified
relations
between
the frictional
microstructures,
tests
for loads
various
aluminum
alloys of
to the
speed
up52100.
the experiments
and
clearer[26–28]
results. revealed
Wang [24,25]
normal
and
sliding speeds
steel
The studies
ofobtain
Moghadam
that
clarified
the
relations
between
the
frictional
temperature,
wear,
microstructures,
normal
loads
the addition of suitable elements to metals decreases both the friction coefficients and wear rateand
as
sliding
of the
52100.
The Moreover,
studies of Moghadam
[26–28]
that the addition
of
well as speeds
increasing
the steel
tensile
strength.
the composites
haverevealed
good tribological
properties
suitable
elements
to metalsconditions
decreases both
thegraphite
friction coefficients
and wear
well as increasing
under limited
lubricated
due to
particles acting
as a rate
solidaslubricant
on worn
the
tensile strength. Moreover, the composites have good tribological properties under limited
surfaces.
lubricated
duedocuments,
to graphite particles
acting as
a solid
lubricant
on worn surfaces.
Basedconditions
on the above
five aluminum
alloys
which
are frequently
used in industrial
Based
on the
documents,
fiveand
aluminum
alloys which
are frequently
industrial
circles
are used
in above
this study
for friction
wear experiments,
in order
to analyzeused
and in
compare
the
circles
used inelements
this study
orderagainst
to analyze
and compare
the
effects are
of doping
onfor
thefriction
frictionand
andwear
wearexperiments,
of SUJ2 steel in
sliding
the aluminum
alloys
effects
of doping
on theconditions.
friction and wear of SUJ2 steel sliding against the aluminum alloys
under the
selectedelements
experimental
under the selected experimental conditions.
2. Experimental Apparatus and Procedures
2. Experimental Apparatus and Procedures
Figure 1 shows a schematic diagram of the ball/disk friction tester and the measurement
Figure
shows acoefficient
schematic diagram
of the ball/disk
friction
tester andbythevariations
measurement
system.
system. The1 friction
is dynamically
measured
and exported
of electrical
The
friction
coefficient
is
dynamically
measured
and
exported
by
variations
of
electrical
voltage
voltage through the load cell. The electronic signals are captured by a data acquisition system, and
through
theare
loadprocessed
cell. The and
electronic
signals
captured computer.
by a data acquisition
and these
these data
analyzed
by are
a personal
Wear loss system,
is measured
and
data
are
processed
and
analyzed
by
a
personal
computer.
Wear
loss
is
measured
and
quantitatively
quantitatively analyzed by a microbalance, and microscopic wear particles are qualitatively
analyzed
microbalance,
and microscopic wear particles are qualitatively observed by the SEM.
observed by
by athe
SEM.
Figure 1. Schematic diagram of the ball/disk friction tester with the measuring system.
Figure 1. Schematic diagram of the ball/disk friction tester with the measuring system.
The SUJ2 steel ball is used as the Pin, and the five aluminum alloys are alternately used as the
The
SUJ2
steelofball
is used
the 6061
Pin, and
aluminum
alloys
arecontents
alternately
used
the
Disk, in the
order
1050,
5052, as
5083,
and the
7075five
aluminum
alloys.
The
of the
fiveas
disks
Disk,
in
the
order
of
1050,
5052,
5083,
6061
and
7075
aluminum
alloys.
The
contents
of
the
five
disks
are shown in Table 1. The size and geometry of the test specimens are shown in Figure 2. The sliding
are
shown
1. m,
Thethe
sizeload
andisgeometry
are shown
in Figure
The sliding
distance
is in
setTable
to 100
set to 30 of
N the
andtest
60 specimens
N, respectively,
and the
sliding2.speed
is 200
distance
is
set
to
100
m,
the
load
is
set
to
30
N
and
60
N,
respectively,
and
the
sliding
speed
is
200
mm/s
mm/s in this study. The response time of this measurement system is less than 1 ms and the accuracy
in
response
timeexperiments
of this measurement
system
is less
than
1 ms
and the
accuracy isRoom
0.1%
is this
0.1%study.
of theThe
full
scale. The
were carried
out
under
dry
friction
conditions.
temperature was maintained at 25 ± 2 °C, and relative humidity was maintained at 70 ± 5%.
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of the full scale. The experiments were carried out under dry friction conditions. Room temperature
was maintained at 25 ± 2 ◦ C, and relative humidity was maintained at 70 ± 5%.
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Table 1. Determination of the content of the five disks (wt %).
Micromachines 2017, 8, 96
Table 1. Determination of the content of the five disks (wt %).
Aluminum Alloys
Aluminum Alloys
Si
Si
0.09
Si
0.09
0.09
0.27
0.09
0.58
0.27
0.60
0.58
0.60
Fe
Fe
Cu
Cu
0.01
0.01
0.02
Cu
0.02
0.09
0.01
0.09
0.26
0.02
0.26
0.16
0.09
0.16
0.26
0.16
Mn
0.01
Mn
0.05
0.01
0.59
0.05
0.06
0.59
0.07
0.06
0.07
Mn
Table 1. Determination of the content of the five disks (wt %).
1050
0.09
1050
5052
Aluminum
5052 Alloys0.09
5083
0.27
1050
5083
6061
0.58
5052
6061
7075
0.60
5083
7075
6061
7075
0.23
0.23
0.26
Fe
0.26
0.38
0.23
0.38
0.62
0.26
0.62
0.20
0.38
0.20
0.62
0.20
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Mg
0.01 0.03
0.05 2.50
Mg
0.59 4.70
0.03
0.06 1.07
2.50
0.07 1.00
4.70
Mg
0.03
2.50
4.70
1.07
1.00
1.07
1.00
Figure 2. Size and shape of the specimens.
Figure 2. Size and shape of the specimens.
3. Results and Discussion
Figure 2. Size and shape of the specimens.
3. Results and Discussion
3.1.
Frictionand
Coefficient
3. Results
Discussion
3.1. FrictionThree
Coefficient
reproducibility experiments were conducted. The typical results are shown in Figures 3–7.
3.1. Friction Coefficient
Figure 3 shows that the average friction coefficient of 1050 is 1.1. Generally, the friction coefficient
Friction
coefficient
Friction
coefficient
,f ,f
Three reproducibility experiments were conducted. The typical results are shown in Figures 3–7.
Three
experiments
were
conducted.
typical
are shown
in Figures
3–7.
changes
in reproducibility
the range of 1.0
to 1.2, the
amplitude
is The
large,
and results
the friction
frequency
is high.
Figure Occasionally,
3Figure
shows
thatduring
the
friction
offriction
1050isis
1.1.
Generally,
the
friction
3 shows
that average
the
frictioncoefficient
coefficient
1050
1.1.
Generally,
the friction
coefficient
theaverage
experimental
process,
theof
coefficient
suddenly
decreases
tocoefficient
0,
changeswhich
in themay
range
1.0the
toof
1.2,
the
is large,
the
friction
frequency
isThe
high.
changes
in indicate
theofrange
1.0between
to amplitude
1.2, the interfaces
amplitude
isand
large,
and
thewear
friction
frequency
is Occasionally,
high.
jitter
resulting
from
the
particles.
oxidation
Occasionally,
the particles
experimental
process,
the friction
coefficient
suddenly
to 0,indicate
during film
the
experimental
the friction
coefficient
suddenly
decreases
to 0,decreases
which
has
brokenduring
into process,
small
between
the contact
interfaces,
and
consequently
there
aremay
some
which
may
indicate
the
jitter
between
the
interfaces
resulting
from
the
wear
particles.
The
oxidation
variations
between
the
frictional
interfaces.
The
vibration
amplitude
of
the
machine
is
larger
during
the jitter between the interfaces resulting from the wear particles. The oxidation film has broken into
filmexperimental
has broken into
small particles between the contact interfaces, and consequently there are some
the
small particles
betweenprocess.
the
contact interfaces, and consequently there are some variations between the
variations between the frictional interfaces. The vibration amplitude of the machine is larger during
frictional interfaces. The vibration amplitude of the machine is larger during the experimental process.
the experimental process.
1050-30N
1.2
1050-30N
1.1
1.2
1
1.1
0.9
0.81
0.9
0.7
0.8
0.6
0.7
0.5
0.6
0.4
0.5
0.3
0.4
0.2
0.3
0.1
0.2
0
0.1 0 10 20 30 40 50 60 70 80 90 100
0
Sliding
distance
(m) 80 90 100
0 10 20 30
40 50
60 70
Figure 3. Typical response of the
frictiondistance
coefficient(m)
of 1050 alloy at 30 N.
Sliding
Figure
3. Typical
responseofofthe
thefriction
friction coefficient
of 1050
alloyalloy
at 30 at
N.30 N.
Figure
3. Typical
response
coefficient
of 1050
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Figure 4 shows that the average friction coefficient of 5052 aluminum alloy is about 0.85. At the
Figure 4 shows that the average friction coefficient of 5052 aluminum alloy is about 0.85. At the
beginningFigure
of the4 friction
process,
the friction
coefficient
increases
from 0.15,
and the
maximum
friction
shows
that
theprocess,
average
friction
coefficient
of 5052
aluminum
about
At the
beginning of
the friction
the
friction
coefficient
increases
fromalloy
0.15,isand
the0.85.
maximum
coefficient
appears
at friction
a sliding
distance
80 m. coefficient
The large amplitudefrom
of the
friction
coefficient
means
beginning
of the
process,
theoffriction
0.15,
and the
maximum
friction coefficient
appears
at a sliding
distance of 80 increases
m. The large amplitude
of the
friction
that the
friction
is
very
severe,
and
because
of
the
sharp
change
in
the
friction
coefficient,
it
matches
friction
coefficient
appears
at
a
sliding
distance
of
80
m.
The
large
amplitude
of
the
friction
coefficient means that the friction is very severe, and because of the sharp change in the friction
coefficient
means
that the
thelarge
friction
is veryamplitude
severe,
and
because
of the
sharp
in the friction
the large
vibration
amplitude
of the
machine
during
thethe
experiment.
coefficient,
it matches
vibration
of
machine
during
thechange
experiment.
coefficient, it matches the large vibration amplitude of the machine during the experiment.
Friction
Frictioncoefficient
coefficient, f, f
5052-30N
5052-30N
1.2
1.2
1.1
1.11
1
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.10
0 0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80 90 100
Sliding distance (m)
Sliding distance (m)
Figure 4. Typical response of the friction coefficient of 5052 alloy at 30 N.
Figure
4. Typical
response
coefficientofof
5052
alloy
atN.
30 N.
Figure
4. Typical
responseof
of the
the friction
friction coefficient
5052
alloy
at 30
The average friction coefficient of 5083 is about 0.73 as shown in Figure 5. At the beginning of
Friction
coefficient
Friction
coefficient, f, f
The
average
friction
coefficient
5083increases
is about
about from
0.73
in in
Figure
5. coefficient
At
of
The
average
friction
ofof5083
is
0.73as
asshown
shown
Figure
5. the
At beginning
the swings
beginning
the friction
process,
thecoefficient
friction
coefficient
0.9
to
1.2. The
friction
at of
the
friction
process,
the
friction
coefficient
increases
from
0.9
to
1.2.
The
friction
coefficient
swings
at
the friction
frictiondistance
coefficient
increases
from
0.9 to 1.2.
Theamplitude
friction coefficient
swings
at
aroundprocess,
0.7 after the
the sliding
of 5 m.
Moreover,
it shows
a large
and high friction
around
0.7 which
after the
slidingthat
distance
of 5 m. Moreover,
it shows
largeparticles
amplitude
and high
friction
frequency
indicate
the oxidation
film has broken
intoasmall
between
thehigh
contact
around
0.7 after
the sliding
distance
of 5 m. Moreover,
it shows
a large
amplitude
and
friction
frequency
which
indicatethe
that the oxidation
film has
broken
into small
particles
between
the contact
interfaces.
Asindicate
a result,
amplitude
of the
machine
larger
during
the
experimental
frequency
which
that vibration
the oxidation
film has
broken
intoissmall
particles
between
the contact
interfaces.
As
a
result,
the
vibration
amplitude
of
the
machine
is
larger
during
the
experimental
process.
interfaces.
As
a
result,
the
vibration
amplitude
of
the
machine
is
larger
during
the
experimental
process.
process.
5083-30N
1.2 5083-30N
1.2
1.1
1.11
1
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.10
0 0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80 90 100
Sliding distance (m)
Sliding distance (m)
Figure 5. Typical response of the friction coefficient of 5083 alloy at 30 N.
Figure
5. Typical
response
coefficientofof
5083
alloy
atN.
30 N.
Figure
5. Typical
responseof
of the
the friction
friction coefficient
5083
alloy
at 30
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The
friction coefficient shown in Figure 6 is between that in Figures 4 and 5; it is representative
of
a few exceptions
in
the
series.
It
shows
that
the
average
friction
coefficient
of
6061
aluminum
alloy
is
The friction coefficient shown in Figure 6 is between that in Figures 4 and 5; it is representative
about 0.9,
and the
friction
high. coefficient
During the
Micromachines
2017, 8, 96 is
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of
athe
fewamplitude
exceptions
in large
the series.
It shows
thatfrequency
the averageisfriction
of experimental
6061 aluminum
the friction
changesisvery
Thisfrequency
may indicate
the
jitter the
between
the interfaces
alloycoefficient
is about 0.9,value
the amplitude
largedrastically.
and the friction
is high.
During
experimental
Thethe
friction
coefficient
shown
in changes
Figurethe
6 very
istemperature
between
that in
Figures
4indicate
andthe
5; itfriction
is representative
process,
the
friction
coefficient
value
drastically.
This
may
the
jitter process
between [24,25],
resulting
from
wear
particles.
Moreover,
rises
during
of
a
few
exceptions
in
the
series.
It
shows
that
the
average
friction
coefficient
of
6061 the
aluminum
the
interfaces
resulting
from
the
wear
particles.
Moreover,
the
temperature
rises
during
and the material is therefore softened, resulting in adhesion; and the softened surfacefriction
is ploughed,
alloy is about
0.9,and
the the
amplitude
is large
and thesoftened,
friction frequency
Duringand
the experimental
process
[24,25],
material
is therefore
resulting isinhigh.
adhesion;
the softened
generating
a
large
and
long
slip
tongue.
process, the friction coefficient value changes very drastically. This may indicate the jitter between
surface is ploughed, generating a large and long slip tongue.
the interfaces resulting from the wear particles. Moreover, the temperature rises during the friction
process [24,25], and the material is therefore softened, resulting in adhesion; and the softened
6061-30N
surface is ploughed, generating
1.2 a large and long slip tongue.
coefficient
,f
FrictionFriction
coefficient
,f
1.1
1 6061-30N
1.2
0.9
1.1
0.8
0.71
0.9
0.6
0.8
0.5
0.7
0.4
0.6
0.3
0.5
0.2
0.4
0.1
0.3
0
0.2 0 10 20 30 40 50 60 70 80 90 100
0.1
Sliding distance (m)
0
0
10
20
30
40 50
60 70of 6061
80 alloy
90 at
100
Figure 6. Typical response of the friction
coefficient
30 N.
Figure 6. Typical response of the friction coefficient of 6061 alloy at 30 N.
Sliding distance (m)
The average friction coefficient of 7075 aluminum alloy is about 0.7 as shown in Figure 7. At
Figure
6. Typical
response
of thealuminum
friction
coefficient
of 6061
alloy0.5,
at 0.7
30
N.as
the
beginning
of the
friction
process,
friction
coefficient
increases
and
theshown
maximum
The average friction
coefficient
ofthe7075
alloy
is from
about
in of
Figure 7.
friction coefficient appears at a sliding distance of 38 m. This may indicate that the temperature
At the beginning
of
the
friction
process,
the
friction
coefficient
increases
from
0.5,
and
the
maximum
of
average
frictionprocess,
coefficient
7075 aluminum
alloy
is about
0.7 as shown
in Figure
At
rises The
during
the friction
the of
material
is therefore
softened,
resulting
in adhesion,
and7.low
friction frequency
coefficient
appears
atand
a sliding
distance
ofcoefficient
38is m.
This
may
indicate
the
temperature
rises
the
beginning
of arises;
the friction
process,
the friction
increases
from 0.5,a that
and
the
maximum
of
wear
the softened
surface
ploughed,
generating
large
and
long slip
during the
friction
process,
the
material
is
therefore
softened,
resulting
in
adhesion,
and
low
frequency
friction
coefficient
appears
at
a
sliding
distance
of
38
m.
This
may
indicate
that
the
temperature
tongue. As a result, the response frequency of the friction coefficient is low. On the other hand, the
rises during
friction
the
material
is therefore
softened,
in adhesion,
and As
lowa result,
wear arises;
and
thethesoftened
surface
iswith
ploughed,
generating
a large
and long
slip
friction
coefficient
slightlyprocess,
increases
the sliding
distance
until
theresulting
experiment
ends.tongue.
frequency
wear
arises;
and
the
softened
surface
is
ploughed,
generating
a
large
and
long
the response frequency of the friction coefficient is low. On the other hand, the friction slip
coefficient
tongue. As a result, the response7075-30N
frequency of the friction coefficient is low. On the other hand, the
slightly increases with the sliding distance
until the experiment ends.
friction coefficient slightly1.2
increases with the sliding distance until the experiment ends.
coefficient
,f
Friction Friction
coefficient
,f
1.1
1 7075-30N
1.2
0.9
1.1
0.8
0.71
0.9
0.6
0.8
0.5
0.7
0.4
0.6
0.3
0.5
0.2
0.4
0.1
0.3
0
0.2 0 10 20 30 40 50 60 70 80 90 100
0.1
Sliding distance (m)
0
0 response
10 20of 30
40 50coefficient
60 70of 7075
80 alloy
90 100
Figure 7. Typical
the friction
at 30 N.
Sliding distance (m)
Figure 7. Typical response of the friction coefficient of 7075 alloy at 30 N.
Figure 7. Typical response of the friction coefficient of 7075 alloy at 30 N.
Micromachines 2017, 8, 96
6 of 11
According to the above results of the friction coefficient and the data in Table 1, it is shown that as
the Si content gradually increases, the friction coefficient of the material with a higher percentage of
Si decreases.
Micromachines 2017, 8, 96
3.2. Wear Loss
6 of 12
According to the above results of the friction coefficient and the data in Table 1, it is shown
as the Si contentexperiments
gradually increases,
friction coefficient
of the of
material
with a higher
Threethat
reproducibility
were the
conducted.
The results
the average
wear loss are
percentage of Si decreases.
shown in Figure 8. As shown in Figure 8a of 30 N, the average wear loss of 1050 aluminum alloy
is 37 mg;3.2.
51Wear
mgLoss
for 5052 aluminum alloy; 38 mg for 5083 aluminum alloy; 60 mg for 6061
aluminum alloy; and 38 mg for 7075 aluminum alloy. In Figure 8b of 60 N, the average wear loss of
Three reproducibility experiments were conducted. The results of the average wear loss are
1050 aluminum
78inmg
for8a5052
aluminum
57ofmg
5083 aluminum
alloy;
shown inalloy
Figureis8.44
As mg,
shown
Figure
of 30 N,
the averagealloy;
wear loss
1050for
aluminum
alloy is
153 mg for376061
aluminum
alloy;
and 58
mg38for
aluminum
Hence,
wear losses
mg; 51
mg for 5052
aluminum
alloy;
mg7075
for 5083
aluminumalloy.
alloy; 60
mg forthe
6061average
aluminum
alloy; andto38the
mg normal
for 7075 aluminum
alloy.indicates
In Figure that
8b of the
60 N,wear
the average
of 1050Moreover,
are proportional
loads. This
modeswear
are loss
regular.
aluminum alloy is 44 mg, 78 mg for 5052 aluminum alloy; 57 mg for 5083 aluminum alloy; 153 mg
the average
wear loss of 6061 aluminum alloy is especially larger. Compared to the data in Table 1,
for 6061 aluminum alloy; and 58 mg for 7075 aluminum alloy. Hence, the average wear losses are
the higherproportional
the contents
of iron and copper, the more materials are removed, representing better
to the normal loads. This indicates that the wear modes are regular. Moreover, the
machinability.
Since
Cu/Fe
pairs
are compatible,
this indicates
that
the wear
average
wearthe
lossFe/Fe
of 6061 and
aluminum
alloy
is especially
larger. Compared
to the data
in Table
1, the losses for
higher doping
the contents
of ironare
andlarger
copper,[18].
the On
morethe
materials
are removed,
representing
the compatible
elements
other hand,
it is worth
notingbetter
that the wear
machinability.
Since
the
Fe/Fe
and
Cu/Fe
pairs
are
compatible,
this
indicates
that
the
wear
losses
loss of 5083 is quite small. This indicates that a higher content of Mn or Mg is harmlessfor
to the wear
the compatible doping elements are larger [18]. On the other hand, it is worth noting that the wear
loss. Namely,
if
the
pairs
are
incompatible,
the
wear
losses
are
smaller
[18].
As
aluminum
alloys are
loss of 5083 is quite small. This indicates that a higher content of Mn or Mg is harmless to the wear
relatively soft
and
have
a
lower
melting
point,
the
wear
particles
produced
in
the
friction
process
easily
loss. Namely, if the pairs are incompatible, the wear losses are smaller [18]. As aluminum alloys are
relatively
soft andinterfaces,
have a lowerand
melting
the wear particles
in the heat
friction
processin material
deviate from
the contact
the point,
high temperature
dueproduced
to frictional
results
easily deviate
from the
contact interfaces,
the track
high temperature
to frictionalofheat
transfer. Therefore,
optical
microscopy
of theand
wear
and SEM due
observation
theresults
wearinparticle are
material transfer. Therefore, optical microscopy of the wear track and SEM observation of the wear
both required for observing the wear mechanisms.
particle are both required for observing the wear mechanisms.
Figure 8. Average wear loss of the five alloys: (a) 30 N; (b) 60 N.
Figure 8. Average wear loss of the five alloys: (a) 30 N; (b) 60 N.
3.3. Optical Microscopy of the Wear Track
Figure 9 shows that the wear surface of the 1050 aluminum alloy presents rough wavy scratching
at different widths, meaning that the vibration is relatively violent for the SUJ2 ball sliding against
Micromachines 2017, 8, 96
7 of 11
this material, i.e., there is significant material transfer and deformation during the friction process.
2017, 8, x FOR
PEER
REVIEW and adhere to areas outside the wear marks. This
8 of can
13
As a result,Micromachines
many materials
are
extruded
be used
to demonstrate that while the material is soft, the wear loss is not the largest.
X100
(a) 30 N
(b) 60 N
1050
5052
5083
6061
7075
Figure 9. Optical microscopy of the wear track of the five alloys: (a) 30 N; (b) 60 N.
Figure 9. Optical microscopy of the wear track of the five alloys: (a) 30 N; (b) 60 N.
The wear scratching of 5052 aluminum alloy is similar to the case of 1050 aluminum alloy, the
surfaces of which present rough and wavy scratching in different widths, the vibration is slightly
violent and there is some material transfer and deformation during the friction process. Thus, part of
the materials that are extruded adhere to areas outside the wear marks. Moreover, it is believed that
high temperatures result in material transfer and deformation during the friction process of 1050 and
5052 aluminum alloys, and the friction scratching presents many crash and melting patterns.
Micromachines 2017, 8, 96
8 of 11
For 5083 aluminum alloy, the friction marks are narrower but deeper than for 1050 or 5083
aluminum alloys. This means that the wear mechanism of 5083 aluminum alloy is quite different from
that of 1050 or 5083 aluminum alloys.
The wear surface of 6061 aluminum alloy is wide but with smooth friction tracks. Since the
7075 alloy sample has the highest hardness, it is to be expected that the scratched wear surface is the
smoothest and the friction tracks are the narrowest. Moreover, the scratched wear surfaces of 6061 and
7075 aluminum alloys look quite smooth. These results match the previous results of wear loss.
3.4. SEM Observation of Wear Particle
Figure 10 shows that the wear particles of 1050 and 5052 aluminum alloys are flake-like and
laminated; it can be reasonable explained by the fact that these materials are relatively soft and have
a lower melting point, thus resulting in deformation and material transfer due to high temperature
during the friction process. A large number of wear particles of large size can be found. The large wear
particles are caused by the junction growth during the adhesive wear. Therefore, in these experiments,
the friction coefficient is larger than the other cases due to the action of the junction growth [29].
Micromachines 2017, 8, x FOR PEER REVIEW
30N
X35
10 of 13
X100
X500
1050
5052
5083
6061
7075
Figure 10. SEM of wear particle of the five alloys.
Figure 10. SEM of wear particle of the five alloys.
3.5. Frictional Models and Wear Mechanisms
By considering
all of the
above aluminum
experimental alloy
results materials
as well as the
by the
Most of the
wear particles
of 5083
areprevious
similarstudies
to those
of 1050 and
authors
[29–31],
three
frictional
models
and
wear
mechanisms
that
describe
the
effects
of
the
content
5052 aluminum alloy materials. Moreover, it is worth noting that some thick ridge-like particles are
of doping elements on the friction and wear of SUJ2 steel sliding against aluminum alloys are
proposed. It is observed from Figure 11a that the flake-like particles resulted from the higher
adhesion at the junction growth region for the low content of silicon (only 0.09%): 1050 and 5052
aluminum alloys. Hence, more friction is caused. Figure 11b shows that medium adhesion is present
at the interfaces for the medium content of silicon (0.27%): 5083 aluminum alloy. The flake-like
particles with some wedge particles were developed from both the junction growth and the adhesive
transfer of the wedge. Figure 11c shows that the relatively large ploughing particles were caused by
Micromachines 2017, 8, 96
9 of 11
observed and the fracture mechanism of the wedge can be revealed. The wedge growth by plowing is
quite significant. It is believed that the thick ridge-like particles result from the adhesive transfer of the
wedge [30].
The 6061 and 7075 aluminum alloy materials produce strip-shaped wear particles. Several
examples of ploughed particles are shown. In these two cases, ploughing is comparatively significant
and large. It is believed that the ploughing wear particles are caused by grooving wear during the
wear test [31].
3.5. Frictional Models and Wear Mechanisms
By considering all of the above experimental results as well as the previous studies by the
authors [29–31], three frictional models and wear mechanisms that describe the effects of the content of
doping elements on the friction and wear of SUJ2 steel sliding against aluminum alloys are proposed.
It is observed from Figure 11a that the flake-like particles resulted from the higher adhesion at the
junction growth region for the low content of silicon (only 0.09%): 1050 and 5052 aluminum alloys.
Hence, more friction is caused. Figure 11b shows that medium adhesion is present at the interfaces
for the medium content of silicon (0.27%): 5083 aluminum alloy. The flake-like particles with some
wedge particles were developed from both the junction growth and the adhesive transfer of the wedge.
Figure 11c shows that the relatively large ploughing particles were caused by the grooving wear
mechanism for the high content of silicon (~0.6%): 6061 and 7075 aluminum alloys. Hence, less friction
is caused due
to the lower
Micromachines
2017, 8, 96 adhesion. These results correspond to the previous friction
10 of 12 coefficient
responses. Wear mechanisms change as the silicon content increases, from the junction growth to the
previous friction coefficient responses. Wear mechanisms change as the silicon content increases,
wedge particles,
and even to the ploughing particles.
from the junction growth to the wedge particles, and even to the ploughing particles.
Figure 11. Frictional models and wear mechanisms of SUJ2 steel sliding against Al–Si alloys: (a) Low
Figure 11. Frictional
models and wear mechanisms of SUJ2 steel sliding against Al–Si alloys: (a) Low
content of silicon: 1050 and 5052 aluminum alloys; (b) Medium content of silicon: 5083 aluminum
content of silicon:
and 5052
aluminum
alloys;
(b) Medium
content of silicon: 5083 aluminum
alloy; (c)1050
High content
of silicon:
6061 and 7075
aluminum
alloys.
alloy; (c) High content of silicon: 6061 and 7075 aluminum alloys.
4. Conclusions
From the variations of the friction coefficient with optical microscopy and SEM observations of
the wear, the following conclusions have been drawn:
1.
The materials with a higher Si content percentage show lower friction. Generally, the friction
coefficient decreases with increases in the Si content, and the variation of the friction coefficient
is milder.
Micromachines 2017, 8, 96
10 of 11
4. Conclusions
From the variations of the friction coefficient with optical microscopy and SEM observations of
the wear, the following conclusions have been drawn:
1.
2.
3.
4.
The materials with a higher Si content percentage show lower friction. Generally, the friction
coefficient decreases with increases in the Si content, and the variation of the friction coefficient
is milder.
The 6061 aluminum alloy has severe ploughing and wear particles. The wear losses for the
compatible doping elements are larger. Therefore, the higher the contents of iron and copper, the
more materials are removed, representing better machinability.
Three frictional models and wear mechanisms that describe the effects of the content of doping
elements on the friction and wear of SUJ2 steel sliding against aluminum alloys are proposed.
The wear mechanisms change as the silicon content increases, from the junction growth to the
wedge and the ploughing particles.
Based on the above three models, better choices of aluminum alloys with regards to friction
and wear can be made. These results have great practical importance for the precision
machinery industry.
Acknowledgments: The authors would like to express their appreciation to the National Science Council in Taiwan
for their financial support under grant number MOST 105-2622-E-150-001-CC2 and MOST 105-2221-E-168-004.
Author Contributions: Yuh-Ping Chang, Zi-Wei Huang and Huann-Ming Chou conceived and designed the
experiments; Zi-Wei Huang performed the experiments; Yuh-Ping Chang, Zi-Wei Huang and Huann-Ming Chou
analyzed the data; Yuh-Ping Chang and Zi-Wei Huang wrote the paper; Yuh-Ping Chang revised the paper.
All authors read and approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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