Effect of Friction Testing of Metals on Particle Emission

Abstract

Metallic particles emitted during manufacturing processes can represent a serious danger for occupational safety. The mechanisms
responsible for these particle emissions include two- and three-body frictions; Moreover, such particles can also be emitted
during several other processes, including mechanical braking. To be in a position to devise ways to reduce these particle
emissions at the source, it is important to know their size, quantity, and distribution, as well as the relationships between
operating conditions and particle emissions. This article investigates nanoparticle and microparticle emissions during two
friction tests: one (setup 1: pin in rotation only) simulates the friction occurring during mechanical braking actions, and
another (setup 2: pin in rotation and translation) simulates the friction taking place at the tool-workpiece interface during
metal cutting processes. The materials tested were aluminum alloys (6061-T6 and 7075-T6), and the pin used was a carbide cylinder.
Particle emission was monitored using the Scanning Mobility Particle Sizer (SMPS) for nanoparticles, and the Aerosol Particle
Sizer (APS) for microparticles. It was found that friction produces more nanoparticles than microparticles, and that total
particle emission can be reduced by operating at low or at high sliding speeds.

Keywordsaluminum alloys–carbide–friction–microparticles–nanoparticles

Full text

Effect of Friction Testing of Metals
on Particle Emission
J. Kouam, V. Songmene, A. Djebara, and R. Khettabi
(Submitted May 14, 2010; in revised form April 28, 2011)
Metallic particles emitted during manufacturing processes can represent a serious danger for occupational
safety. The mechanisms responsible for these particle emissions include two- and three-body frictions;
Moreover, such particles can also be emitted during several other processes, including mechanical braking.
To be in a position to devise ways to reduce these particle emissions at the source, it is important to know
their size, quantity, and distribution, as well as the relationships between operating conditions and particle
emissions. This article investigates nanoparticle and microparticle emissions during two friction tests: one
(setup 1: pin in rotation only) simulates the friction occurring during mechanical braking actions, and
another (setup 2: pin in rotation and translation) simulates the friction taking place at the tool-workpiece
interface during metal cutting processes. The materials tested were aluminum alloys (6061-T6 and 7075-
T6), and the pin used was a carbide cylinder. Particle emission was monitored using the Scanning Mobility
Particle Sizer (SMPS) for nanoparticles, and the Aerosol Particle Sizer (APS) for microparticles. It was
found that friction produces more nanoparticles than microparticles, and that total particle emission can be
reduced by operating at low or at high sliding speeds.
Keywords aluminum alloys, carbide, friction, microparticles,
nanoparticles
1. Introduction
Researchers (Ref 1-5) have studied particle emissions, and
their results and proposed models become limited and unprac-
tical when the particle size is small. Ko et al. (Ref 6) modeled
the phenomenon of frictional particle production by proposing
a special microscopic roughness design in which surface
asperity shapes were considered as randomly spaced cylindrical
corrugations. Their study shows the effect of part surface
roughness on particle emission. Akarca et al. (Ref 7) found that
during the sliding wear of the A356 aluminum alloy, wear
particles are generated by the nucleation of voids and the
propagation of microcracks at a certain depth beneath the
surface. Fang (Ref 8) and Fang and Ho (Ref 9) proposed a
special surface texture design to control surface asperities to
identify the mechanism of particle detachment by friction, as
well as a predictive mathematical model. However, the Fang
(Ref 8) and Fang and Ho (Ref 9) model uses dimensions for the
surface texture that are not realistic since the shape and the
distribution of surface asperities of most mechanical parts are
random.
A very limited number of researchers, including Zemzemi
et al. (Ref 10), have studied the effect of the friction occurring
during machining on particle emission; Zemzemi et al. (Ref 10)
designed and used a special tribometer to study the friction
taking place during orthogonal turning operations. Zemzemi
et al. (Ref 11) were the first authors in the machining field to
isolate the friction effect during machining and to propose a
numerical model simulating this friction. They identified a
friction model capable of describing the friction effect on tool-
chip-workpiece interfaces. They used the new tribometer
designed by Zemzemi et al. (Ref 10) to simulate higher contact
pressures under high sliding velocities to study the forces,
friction coefficients, and heat partition coefficient. Their model
is suitable for turning processes, but not easy enough to be
applied for other processes, such as milling and drilling.
Balout et al. (Ref 12) and Songmene et al. (Ref 13)
identified the friction at the tool-workpiece interface as a
contributor to the total metallic particle emission occurring
during machining (Fig. 1), but the authors did not study the
friction alone. Songmene et al. (Ref 13) identified five particle
emission sources during drilling: shearing action, deformation
and friction of chips, deformation and friction on the tool-chip
interface, friction on tool-workpiece, and friction of chips in
drill flutes. They also recognized the difficulty of isolating each
source for a separate study. The experimental setup proposed in
this article addresses one of these difficulties by simulating
particle emissions at the tool-workpiece interface using a
friction test in which the tool is animated with two motions
(rotational speed and a translation, setup 2, Fig. 2). This study
will help in evaluating the amount of metallic particles emitted
in Mode 4.
The main objective of this research is to study the effect of
friction between the tool and workpiece on metallic particle
emission. Two setups were designed and tested on pairs of
materials consisting of aluminum alloy workpieces and a
carbide tool used as a pin. The first one (setup 1: pin in rotation
only) simulates the friction occurring during mechanical
braking actions, and the second (setup 2: pin in rotation and
translation) simulates the friction taking place at the tool-
workpiece interface during metal cutting processes. The main
J. Kouam, V. Songmene, A. Djebara, and R. Khettabi, Department of
Mechanical Engineering, E
´cole de technologie supe´rieure (E
´TS), 1100
Notre-Dame Street West, Montre´al, QC H3C 1K3, Canada. Contact
e-mails: Jules.Kouam@etsmtl.ca and victor.songmene@etsmtl.ca.
JMEPEG ASM International
DOI: 10.1007/s11665-011-9972-6 1059-9495/$19.00
Journal of Materials Engineering and Performance

difference between this study and that of Zemzemi lies in the
fact that this study involves two movements (rotation and
translation of the pin), which affords better simulation of the
milling process. The authors have also applied higher sliding
speeds (up to 1000 m/min), comparable to the speeds currently
used in the machining industry.
2. Experimental Procedure
Tests were carried out on two aluminum materials (6061-T6
and 7075-T6), and cutting forces were obtained using a table
dynamometer connected to a computer. Particle emission was
measured with an APS (Aerosol Particle Sizer, particle size
ranging from 0.5 to 20 lm) and an SMPS (Scanning Mobility
Particle Sizer, equipped with a nano DMA (Differential Mobility
Analyzer), particle size ranging from 7 to 300 nm). Their
experimental application has been described by Khettabi et al.
(Ref 14). These two units, which measure mass concentration,
particle number concentration and specific area concentration as
a function of aerodynamic diameter, are connected to a computer.
The machining unit used was a Computer Numerical
Control (CNC) milling machine (28000 rpm), to which a
30 930 920 cubic centimeter Plexiglas box was added on the
table, allowing the process to be carried out in a closed
environment. This increased the measurement efficiency, as
fewer particles were capable of escaping into the environment.
Polluted air within the closed box was driven into the particle
measurement unit through a polyester tube 10 mm in diameter.
The tube is short (about 304.8 mm), and kept upright to
minimize particle loss within.
The following conditions and parameters were used during
friction tests:
•Pin rotational speed (V): 100-1000 m/min
•Pin displacement speed ( f) for setup 2: 250 m/min
•Pin: uncoated carbide, 19.05 mm diameter
Figure 2presents the two different experimental setups used.
For setup 1, the pin is in rotation without displacement, and is
engaged in the workpiece material with a circular surface
contact. In setup 2, the pin rotates at different set spindle
speeds, and translates along the workpiece material at a
50 m/min linear speed.
3. Results and Discussion
3.1 Nanoparticle Emission
Figure 3(a) and (b) show typical particle emission results as
a function of the particle diameters obtained using SMPS. It is
seen that without displacement (Fig. 2, setup 1; Fig. 3a), the
particle number is higher than in the case where the pin rotates
and translates (Fig. 3b). The particle concentration becomes
smaller for larger aerodynamic particles.
In addition to the data presented in Fig. 3, the system also
computes the total particle number, mass, and specific surfaces
presented in Fig. 4-6, as a function of sliding speeds.
For the 7075-T6 aluminum alloy, the speed at which
maximum particle emission occurs (about 500 m/min), is
independent of the setup used (Fig. 4b, 5b, 6b). Conversely,
for the case of the 6061-T6 aluminum alloy, the maximum
particle emission during friction occurs at 200 m/min in setup 1
and at 400 m/min in setup 2. This observation indicates a
possible interaction between the materials and the setup at the
critical speed. On the other hand, their mechanical properties as
well as their hardness (53 HRA for 7075-T6 and 38 HRA for
6061-T6) could have played an important role.
Figure 7(a) and (b) compare the total particle number
concentrations obtained during friction and milling (grooving)
tests on 6061-T6 and 7075-T6. The milling tests were done at
0.165 mm/rev feed rate using a carbide endmill, 19.05 mm in
diameter. The depth of cut was 1 mm and the radial immersion,
100% of the tool diameter. Three different cutting speeds were
tested in milling: 300, 750, and 1200 m/min. For 6061-T6, the
particle emissions were higher in milling tests compared to
friction tests (setups 1 and 2). The same observation was made
for the 7075-T6 material, but particle emission was 10 times
higher for the 6061-T6 material in milling than in friction tests,
while it was only two times for 7075-T6. It thus appears that the
contribution of friction to particle emission during a machining
process such as milling can vary with the workpiece material
used. This observation can be explained by the fact that each of
the two materials has a different toughness, and the hardness of
the 7075-T6 is higher than that of the 6061-T6. This result is
being further investigated with other materials and conditions,
and taking into account the particle size distribution.
Fig. 1 Schematic representation of possible sources of metallic
particle emission during machining (Ref 12,13)
f
VV
Fig. 2 Different setups used during friction tests
Journal of Materials Engineering and Performance

3.2 Microparticle Emission
Figure 8(a) and (b) show typical particle emission results, as
a function of the aerodynamic diameter obtained from APS, for
particle diameters ranging from 0.5 to 30 lm. It is observed
that for microparticles, the particle number is approximately the
same in setup 1 as in setup 2.
Figures 9-11 present the total concentration for the particle
number, the specific area and the specific mass of microparticles
0.00E+000
1.00E+008
2.00E+008
3.00E+008
6061-T6 material : friction at 500 m/min cutting speed in setup 1
particle number concentration (#/cm3)
diameter (nm)
0.00E+000
1.00E+008
2.00E+008
3.00E+008
6061-T6 material : friction at 500 m/min cutting speed in setup 2
particle number concentration (#/cm3)
diameter (nm)
(a) (b)
0 102030405060708090100 0 102030405060708090100
Fig. 3 Particle number at different diameters for 6061-T6 at 500 m/min cutting speed from SMPS: (a) in setup 1 and (b) in setup 2
2.0x107
2.4x107
2.8x107
3.2x107
3.6x107
4.0x107
6061-T6 material : nanometric particle
total particle number concentration (#/cm
3
)
cutting speed (m/min)
friction in setup 1
friction in setup 2
2.0x107
2.4x107
2.8x107
3.2x107
3.6x107
4.0x107
7075-T6 material : nanometric particle
total particle number concentration (#/cm
3
)
cutting speed (m/min)
friction in setup 1
friction in setup 2
(a) (b)
0 200 400 600 800 1000 0 200 400 600 800 1000
Fig. 4 Total particle number concentration at different speeds obtained from SMPS. (a) 6061-T6 and (b) 7075-T6
3x1012
4x1012
5x1012
6x1012
7x1012
8x1012
6061-T6 material : nanometric particle
total specific area concentration (nm
2
/cm
3
)
cutting speed (m/min)
friction in setup 1
friction in setup 2
3x1012
4x1012
5x1012
6x1012
7x1012
8x1012
7075-T6 material : nanometric particle
total specific area concentration (nm
2
/cm
3
)
cutting speed (m/min)
friction in setup 1
friction in setup 2
(a) (b)
0 200 400 600 800 1000 0 200 400 600 800 1000
Fig. 5 Total specific area concentration at different speeds obtained from SMPS. (a) 6061-T6 and (b) 7075-T6
Journal of Materials Engineering and Performance

as a function of applied speeds. In general, the microparticle
emission obtained in setup 1 (pin in rotation only) is comparable
to that obtained in setup 2 (pin in rotation and translation). At
very low speeds, the amount of particles is low; it then increases,
reaches the maximum value, and eventually decreases. These
two speed regimes have been observed by Khettabi et al. (Ref 15)
in their study on turning, and by Kouam et al. (Ref 16) in their
study on drilling.
8.0x105
1.0x106
1.2x106
1.4x106
1.6x106
1.8x106
6061-T6 material : nanometric particle
total mass concentration (µg/m3)
cutting speed (m/min)
friction in setup 1
friction in setup 2
8.0x105
1.0x106
1.2x106
1.4x106
1.6x106
1.8x106
7075-T6 material : nanometric particle
total mass concentration (µg/m3)
cutting speed (m/min)
friction in setup 1
friction in setup 2
(a) (b)
0 200 400 600 800 1000
0 200 400 600 800 1000
Fig. 6 Total mass concentration at different speeds obtained from SMPS. (a) 6061-T6 and (b) 7075-T6
2.0x107
4.0x107
6.0x107
8.0x107
1.0x108
1.2x108
1.4x108
1.6x108
1.8x108
2.0x108
6061-T6 material: nanometric particle
total particle number concentration (#/cm3)
cutting speed (m/mn)
friction without displacement at 0 mm/rev feed rate
friction with displacement at 25 mm/rev feed rate
grooving test at 0.165 mm/rev feed rate
2.0x107
2.2x107
2.4x107
2.6x107
2.8x107
3.0x107
3.2x107
3.4x107
3.6x107
3.8x107
4.0x107
4.2x107
4.4x107
7075-T6 material: nanometric particle
total particle number concentration (#/cm3)
cutting speed (m/mn)
friction without displacement at 0 mm/rev feed rate
friction with displacement at 25 mm/rev feed rate
grooving test at 0.165 mm/rev feed rate
(a) (b)
0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200
Fig. 7 Comparison of total particle number concentration in friction and grooving tests. (a) 6061-T6 and (b) 7075-T6
0
2
4
6
8
10
12
14
6061-T6 material: friction at 500 m/min cutting speed in setup 1
particle number concentration (#/cm3)
aerodynamic diameter (µm)
012345678910 012345678910
0
2
4
6
8
10
12
14
6061-T6 material: friction at 500 m/min cutting speed in setup 2
particle number concentration (#/cm3)
aerodynamic diameter (µm)
(a) (b)
Fig. 8 Particle numbers at different aerodynamic diameters for 6061-T6 at 500 m/min cutting speed: (a) in setup 1 and (b) in setup 2
Journal of Materials Engineering and Performance

(a) (b)
0
5
10
15
20
25
30
6061-T6 material: micrometric particle
total particle number concentration (#/cm3)
cutting speed (m/min)
friction in setup 1
friction in setup 2
0
5
10
15
20
25
30 7075-T6 material: micrometric particle
total particle number concentration (#/cm3)
cutting speed (m/min)
friction in setup 1
friction in setup 2
0 200 400 600 800 1000
0 200 400 600 800 1000
Fig. 9 Total particle number concentrations at different speeds obtained from APS. (a) 6061-T6 and (b) 7075-T6
0
5
10
15
20
25
30
35
40
6061-T6 material: micrometric particle
total specific area concentration (µm2/cm3)
cutting speed (m/min)
friction in setup 1
friction in setup 2
0
5
10
15
20
25
30
35
40
7075-T6 material: micrometric particle
total specific area concentration (µm2/cm3)
cutting speed (m/min)
friction in setup 1
friction in setup 2
(a) (b)
0 200 400 600 800 1000 0 200 400 600 800 1000
Fig. 10 Total specific area concentrations at different cutting speeds obtained from SMPS. (a) 6061-T6 and (b) 7075-T6
0.00
0.02
0.04
0.06
0.08
0.10
6061-T6 material: micrometric particle
total mass concentration (mg/m3)
total mass concentration (mg/m3)
cutting speed (m/min)
friction in setup 1
friction in setup 2
0.00
0.02
0.04
0.06
0.08
0.10
7075-T6 material: micrometric particle
cutting speed (m/min)
friction in setup 1
friction in setup 2
(a) (b)
0 200 400 600 800 1000
0 200 400 600 800 1000
Fig. 11 Total particle number concentrations at different cutting speeds obtained from APS. (a) 6061-T6 and (b) 7075-T6
Journal of Materials Engineering and Performance

The critical speed required for microparticle emission to be
at a maximum is the same for the two setups (1 and 2) of both
aluminum alloys studied. The maximum particle emission
value for material types 7075-T6 and 6061-T6 occurs at speeds
of 600 and 500 m/min, respectively.
The results on particle emission (Fig. 6-7,9-11) indicate that
by operating at high speed (about 1000 m/min) for the
aluminum alloys tested, the emission of both nanoparticles
and microparticles due to friction is low. This is also the case
for very low speeds (below 200 m/min), but the use of the low
speed regime is not recommended, as it would lower the
productivity when machining.
3.3 Cutting Forces
Figure 12(a) and (b) present the forces (F
xy
) recorded during
the friction tests. F
xy
is obtained from the equation as follows:
Fxy ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
F2
xþF2
y
qðEq 1Þ
For both materials, the forces are slightly higher in setup 2 (pin
in rotation and translation) than in setup 1 (pin in rotation only).
This observation can be explained by the fact that as the pin
translates, it encounters asperities and peaks of newer surfaces.
In general, the forces obtained in friction tests are very low
(less than 10 N) as compared to milling forces (about 140 N),
Fig. 12. Therefore, the energy required for friction at the tool
workpiece interface is very limited compared to that used for
shearing and cutting. The load on the pin (applied force) was
kept low to simulate only the friction; otherwise, some material
work hardening might have taken place.
Figure 13 presents the roughness at different cutting speeds
for the 6061-T6 and 7075-T6 materials. The roughness values
were obtained using the Mitutoyo S-J400 equipment. These
figures show that the roughness decreases when the cutting
speed increases in setup 1 and in setup 2. This observation
confirms the fact that the surface finish improves as the cutting
speed increases, and is itself confirmed by the study of Fu et al.
(Ref 17), in which they show that the surface roughness
decreases when the cutting speed of aluminum is increased.
Their study involved the high speed (about 1500 m/min)
milling of aluminum, and they used the following equation for
the roughness, R
a
:
Ra¼CVb1fb2ab3
pab4
eðEq 2Þ
where Vis the cutting speed, fthe feed rate, a
p
the cutting
depth, and a
e
the cutting width.
0
20
40
60
80
100
120
140
160
milling force
6061-T6 material
Fxy(N)
cutting speed (m/min)
friction force in setup 1
friction force in setup 2
milling force at 1 mm depth
0
20
40
60
80
100
120
140
160
milling force
7075-T6 material
Fxy (N)
cutting speed (m/min)
friction force in setup 1
friction force in setup 2
milling force at 1 mm depth
(a) (b)
0 200 400 600 800 1000 0 200 400 600 800 1000
Fig. 12 Cutting forces (F
xy
) as a function of cutting speeds in setup 1 and setup 2. (a) 6061-T6 and (b) 7075-T6
0.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.32
0.36
0.40
before friction test
6061-T6 material
Roughness (Ra)
cutting speed (m/min)
setup 1
setup 2
Before friction
0.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.32
0.36
0.40
before friction test
7075-T6 material
Roughness (Ra)
cutting speed (m/min)
before friction
setup 1
setup 2
(a) (b)
0 100 200 300 400 500 0 100 200 300 400 500
Fig. 13 Roughnesses at different cutting speeds in setup 1 and setup 2. (a) 6061-T6 and (b) 7075-T6
Journal of Materials Engineering and Performance

C,b
1
,b
2
,b
3
, and b
4
(all positive) are materials and cutting
condition constants. The cutting speed (V) is the only parameter
with a negative exponent, and the roughness R
a
decreases when
the cutting speed increases.
Shaw (Ref 18) proposed a representative friction model for
machining, expressed in terms of the relationship between the
real contact area (A
r
) and the apparent contact area (A):
Ar
A¼1eBN ðEq 3Þ
where Bis a material constant, and Nis the applied load.
The Shaw model (Eq 3) was improved by Khettabi (Ref 19)
to take into account the effect of the interface temperature (DT):
Ar
A¼1eB1NþB2DTðÞ ðEq 4Þ
where B
1
and B
2
are constants.
The temperature involved in friction is proportional to the
sliding velocity, and so, by increasing the cutting speed, the real
surface interface will be affected as a result of the material
softening leading to asperities flattening. This improves the
surface finish.
Figure 14 shows the optical microscopic images obtained in
setups 1 and 2. The images were obtained using a type
Stereozoom 7 Bausch and Lomb microscope and a Firewire
type Clemex camera with 1280 91024 pixels. The magnifica-
tion used was 2.5 times. Figure 14 shows that at the same
cutting speed, the surface finish is better in setup 1 than in setup
2, the same as was observed in Fig. 13.
4. Conclusions
In this study, the effect of speed and pin motion on metallic
particle emission during friction was studied.
•Two distinct regimes of cutting speeds characteristic of
the particle emission during friction were observed:
(1) A low-speed regime, where particle emission is
low, and increases with speed;
(2) A high-speed regime, where particle emission
decreases as the speed increases. These two speed
regimes are delimited by a critical speed at which the
maximum particle emission rate is seen. For micro-
particles, the cutting speed at which the maximum
emission occurs does not change with the setups, the
conditions and the materials tested. A similar obser-
vation was found in the case of nanoparticles for the
7075-T6 material, but not for the 6061-T6 material.
•Overall, friction produced more ultrafine particles than
fine particles at all rotational speeds and for all materials
tested.
•As expected, the particle emission in friction was low
compared to that obtained in the machining process (mill-
ing). The particle number in friction was 2-10 times lower
than in machining, depending on the workpiece material
and conditions used.
•For the materials and conditions tested, the total particle
emission can be reduced by operating at very low or at
very high sliding speeds.
Future study will involve brittle materials and a comparison
with particle emissions occurring during machining to assess
the contribution of friction to particle emission during metal
cutting processes.
Acknowledgments
This research study is part of a project on nanoparticle emission
funded by the NanoQue´bec and the Institut de Recherche Robert-
Sauve´ en Sante´etSe´curite´ du Travail (IRSST). The authors also
acknowledge discussions with Y. Cloutier, M. Viens, S. Halle´, and
F. Morency.
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