The tribological properties of textured surfaces of brass and aluminum were compared with untextured surfaces of the same specimens. This was done to research the effect of micro-dimples and their geometry parameters in reducing the wear of samples tested under lubricated contact with a ball-on-flat reciprocating tribometer.

1. 1
R I T
Study of Tribological Properties of Textured Surfaces made by
Modulation Assisted Machining
Denny Sebastian1(*)
, P. Iglesias 1
1
Mechanical Engineering Department. Kate Gleason College of Engineering, Rochester
Institute of Technology, Rochester, NY, USA.
*dennygsk101@gmail.com
ABSTRACT
Surface texturing has long been viewed as a viable option for reducing friction in moving
parts, improving wear resistance and prolonging their life along with other tribological
improvements.
It has been shown that textured surfaces reduce the friction coefficient and wear volume
compared to untextured surfaces. The micro-dimples on the textured surfaces trap
lubricants and create an hydrodynamic pressure between surfaces, thus enhancing
lubrication ability. The dimples or grooves also function as receptacles for debris and
wear particles, eliminating potential scratching of the substrate surface during relative
motion of the interface parts. Several techniques are used for fabrication of textured
surfaces, however there are limitations in terms of control and scalability. Modulation
assisted machining (MAM) has been demonstrated as a viable approach to produce
such textures. MAM is shown to be cost effective for scalable production of these
features on component surfaces.
In this study, the effects of surface texturing and the influence of the dimple dimensions
on wear performance will be observed. Samples of brass 360, and aluminum 6061 T6
will be mated with AISI 440C stainless steel balls using a ball-on-flat reciprocating
configuration. Polyalphaolefin (PAO) oil will be used for lubrication. The wear analysis
will then be carried out using image analysis (optical microscope). Wear mechanisms
are discussed from optical microscopy observations.
KEY WORDS: surface texturing, wear resistance, micro-dimples, modulation
assisted machining, brass 360, aluminum 6061 T6
Fall-Spring, 2014-15
Project with Paper
••

2. 2
1.0. INTRODUCTION
Friction is caused by the dissipation of energy between two bodies in relative contact.
Wear, a byproduct of friction, leads to material losses between moving parts. Friction
may be wanted or unwanted while wear is always considered as unwanted [1]. In
industries, a significant proportion of total revenue is consumed by friction and wear.
Tribology is the science and technology of interacting surfaces in relative motion and
the practices related thereto. The objects of study of tribology include friction, wear,
lubrication, surface science and tribochemistry [2, 3]. Recently, tribology has been
widely recognized as a new general concept embracing all aspects of transmission and
dissipation of energy and materials in mechanical equipment, including the areas of
friction, wear, lubrication, and related fields of science and technology [4].
It has been estimated that approximately 11 % of the total energy annually consumed in
the U.S. in the four major areas of transportation, turbomachinery, power generation
and industrial processes is lost due to firction. These losses can be saved through new
developments in lubrication and tribology [3]. Friction is responsible for a major loss of
useful mechanical energy, and wear is a major reason for replacing equipment. Thus, a
better understanding and utilization of the principles of tribology is particularly important
for conservation of energy and materials in engineering design [4].
The efficiency, reliability, and durability of machine components depend on the friction
occurring at the sliding contact interface. In addition, there is always the desire to
increase the load capacity or the power density of engine elements, which of course will
lead to higher intensities of surface interactions. One method in reducing friction is by
surface lubrication. Surface lubrication involves many aspects of the physical and
chemical properties of the surface material and lubricant. Smoother surfaces have
better friction reduction under lubrication [5].
The improvement of surface finish is one of the most reasonable methods of reducing
friction. Engine friction loss accounts for 40% of the total energy developed by a typical
automotive engine. It is therefore necessary to reduce friction for improved fuel
consumption. This led to an increase in the number of automotive components whose
surfaces are finished with grinding and subsequent additional processes such as
lapping or superfinishing [6-8].
Surface texturing has gained attention in recent years due to the effect of dimples on
tribological performance of the surface. It has been identified that controlled porosity on
a tribological surface can contribute to friction reduction at sliding contact interfaces [7].
The presence of artificially created microfeatures can significantly affect the friction and
wear behaviour of lubricated surfaces [5]. Surface texturing is a refined surface
technology to form regular micro/nanometer-sized dimples or asperities on a smooth
surface by laser processing technology, reactive ion etching technology, photochemical
machining technology and so on [9].
Surface texturing is used as a method to reduce friction and wear and improve the
lubrication ability of various mechanical components; it presents different tribological

3. 3
functions under different lubrication conditions. Under hydrodynamic lubrication
conditions, each micro-dimple behaves as a tiny convergence wedge, which generates
additional hydrodynamic pressure to increase the total load carrying capacity [9]. Under
boundary or mixed lubrication conditions, micro-dimples act as oil reservoirs, the
lubricant is held back in these reservoirs and are considered as a secondary lubrication
source. Upon relative movement between the contacting surfaces, the lubricant is pulled
out of the dimples and permeates into the surrounding surface areas [10-12]. Under dry
friction conditions, micro-dimples also trap wear debris to prevent further abrasive wear,
thus reducing ploughing and deformation components of friction. The effect of micro-
dimples improves the dry running performance of textured surfaces[13].
There are various methods of surface texturing such as laser surface texturing (LST),
electro chemical machining (ECM), vibro-mechanical texturing (VMT), abrasive jet
machining (AJT), sand blasting, and photolithography. Most of these methods have
limitations related with cost effectiveness, use of specialized equipments (as in the case
for laser texturing), rate of production, reliability, environmental conditions, change in
material properties due to laser heating [6]. Recently, the use of Modulation Assisted
Machining (MAM) processes provides a cost effective approach for creating surface
textures over large areas and it offers high control over the geometry of the textured
surfaces [14].
There are few publications on the effect of micro-dimple parameters on the tribological
performances of textured surfaces. In 2003, Wakuda et al. [7] assessed the frictional
properties of textured ceramic surfaces assessed with a pin-on-disk tests. The study
concluded that micro-dimples reduced friction even under severe friction reduction due
to its lubrication retention ability. A dimple size of approximately 100 μm was
recommended; the distribution of micro-dimples is also an important factor. A dimple
density of 5-20% is recommended. The dimple geometry, rounded or angular profiles,
had little influence on the frictional properties of samples.
In the experimental work done by Tang et al. [8], change in dimple area fraction can
dramatically reduce friction and wear. A 5% optimal dimple area fraction generated the
greatest hydrodynamic pressure of all dimple fractions. It had the least wear and friction,
with reductions of 72% and 38%, respectively compared to non-textured surface [8].
Denkena et al. [13], however, found that an increase in dimple depth, ap (ap,max > 30
μm), leads to a parallel increase in friction coefficient. This occurs because large
dimples do not support the load carrying capacity in the same way as small dimples do
leading to higher surface contact in the mixed friction area. Recently, Hao et al. [9],
assessed the effects of dimple area density and diameter size effects on lubrication in
line contacts with cylinder-on-ring tests. The study found that textured specimens with
low dimple area density, 3 %, and large dimple diameter, 150 or 200 μm, reduced the
friction coefficient to some extent as compared to untextured specimens [9].
This work provides a better understanding on the effect of micro dimples in the friction
reduction of textured surfaces. It is important to understand the various parameters of
the dimple geometry to effectively reduce wear on the surface of specimens under
lubricated contact conditions. An optimal value of all dimple parameters would provide

4. 4
the best tribological properties for a textured specimen in achieving higher wear
resistance than an untextured surface of the same specimen.
2.0. MATERIALS & METHODS
In this work, the textured samples of Brass 360, Aluminum 6061 T6 are made by means
of Modulation Assisted Machining. There are two basic configurations: a) Sliding Type,
and b) Plunging Type. In the first configuration, the superimposed modulation is applied
in the direction of tool feed in turning. This configuration is referred to as sliding type,
and the resulting textures are caused by the local material removal occurring at the tool
nose radius as the tool reciprocates repeatedly over the work surface. In plunging type
texturing configurations, the superimposed modulation is applied perpendicular to the
tool field. The resulting textures are created by the repeated engagement-
disengagement of the cutting tool as it enters and exits the periphery of the work
cylinder. Figure 1 shows the schematic representation of the turning in configurations.
Fig.1. Schematic representation of (a) Sliding type texturing and (b) Plunging type texturing.
(c) Configuration to produce textured surface on cylinder face by plunging type texturing.
The samples were tested on a ball-on-flat reciprocating tribometer, shown in figure 2,
against AISI 440C stainless steel balls (3 mm spherical radius, 690 hardness HV). A
series of tests with frequency settings of 1.5 Hz and 3 Hz were carried out under
constant normal load of 23 N. The frequency is set by adjusting the rate of sliding
through an air pressure valve. Two sliding times of 20 minutes and 1 hour were made
for respective tests. A stroke length of 10.5 mm is kept constant for all tests. For
lubrication, 2ml of synthetic poly alpha-olefin oil (Synton PAO 40) is used.
a) b)
c)

5. 5
Fig.2. Schematic representation of ball-on-flat reciprocating tribometer
This project study explores the effect of dimple density on the tribological properties of
textured samples. The textured samples have three different dimple densities- low,
medium and high marked as LDD, MDD and HDD respectively. After testing, the disk
specimens are sent for analysis. The machining and modulation condition of the brass
360 samples and aluminium 6061 T6 samples are shown in table 1.
Table 1: Machining and modulation conditions for samples; where ho=feed rate; Ra= Roughness; App=
modulation amplitude; fm=modulation frequency.
S# Type
Fm
(Hz)
Vpp
(V)
Voff
(V)
App (mm)
C (Tool
offset
position)
h0
(mm/rev)
CSS (sfpm) Material Ra (µm)
6'4 CS Control Sample 0.01 1000RPM Brass 360 0.137
7'1 LDD 100 50 70 0.03 0 1.5 3 Brass 360 0.143
7'2 MDD 100 50 70 0.03 0 1 3 Brass 360 0.221
7'3 HDD 100 50 70 0.03 0 0.5 3 Brass 360 0.175
8'4 CS Control Sample 0.01 1000 RPM Al 6061 T6 0.121
8'1 LDD 100 50 70 0.03 0 1.5 3 Al 6061 T6 0.1
8'2 MDD 100 50 70 0.03 0 1 3 Al 6061 T6 0.175
8'3 HDD 100 50 70 0.03 0 0.5 3 Al 6061 T6 0.175

6. 6
Optical images of brass and aluminium samples are shown in figure 3 and 4
respectively.
Fig.3. Optical images of Brass 360 samples (DD represents dimple density)
Fig.4. Optical images of Al 6061 T6 samples (DD represents dimple density)
The wear measurement is done by image analysis using a Carl Zeiss optical
microscope to obtain optical micrographs of the wear track. An average of 45
measurements of the wear track width is taken. The wear volume is then calculated
according to Eq. 1 [15].
Vf = Ls [ arcsin( ) - (Rf - hf)] + ( 3Rf - hf) (1)
where,
W: wear width Ls: stroke length
Rf: radius of 440C steel ball hf: wear depth

7. 7
Fig.5. Schematic representation of wear track taken by an optical microscope [15].
The primary aim of this study is to observe the effect of dimple density on the
tribological properties of textured surface made by modulation assisted machining
(MAM). The dimple density of a textured sample is calculated by dividing the total sum
of the area of dimples per unit square area by area of the square viewed on a
microscope. Figure 6 (a) and (b) shows a medium dimple density Brass 360 sample and
a high dimple density Al 6061 T6 sample respectively.
Dimple Density= (2)
Fig.6. (a) Brass 360 MDD (b) Aluminium 6061 T6 HDD
3.0. RESULTS AND DISCUSSION- Brass 360
3.1. Effect of Sliding Frequency
The effect of frequency on the tribological properties of textured surface is studied. The
experimental analysis was performed under a constant load of 23 N and a time of 20
minutes. The samples were tested against a ball-on-flat reciprocating tribometer with
frequencies of 1.5 Hz and 3 Hz. Figure 7 compares the wear volumes of all the four
samples tested under both frequencies. At lower frequency (1.5 Hz), there was no major
differences observed between textured and un-textured surfaces. As frequency
increases, the wear volume showed significant differences between the textured

8. 8
samples and control sample (CS). From the data computed, the textured sample with
medium dimple density had the greatest wear reduction (~50%) compared to the control
sample.
Figure 8 shows the wear track of the MDD and CS samples after testing under the
same experimental conditions. The width of the wear track appears similar between the
MDD sample and control sample when tested under a frequency of 1.5 Hz. At higher
frequency, the wear track width is much wider for the control sample than the MDD
sample.
Fig.7. Effect of frequency on wear volume
Fig.8. Average width of wear track corresponding to conditions of varying frequency under constant load
and sliding time.
1 2 3 4
1.5 Hz 0.0140 0.01286 0.00989 0.01167
3 Hz 0.0359 0.0195 0.0183 0.0248
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
0.0350
0.0400
0.0450
0.0500
WearVolume(mm^3)
Effect of Frequency
- CS - LDD - MDD - HDD- CS - LDD - MDD - HDD
Sliding Time- 20 minutes
Normal Load- 23 N
49 % Wear Reduction

9. 9
3.2. Effect of Sliding Distance
To better understand the effect of sliding distance, the wear factor is taken into account.
Wear factor is defined as the ratio of wear volume of samples to the product of sliding
distance and load.
Wear Factor= (3)
The effect of sliding distance on the wear behavior of samples was carried out under
constant load and frequency. In one case, the sliding distance is kept constant at 76 m,
and tests are carried out. After testing, the sliding distance is increased to 227 m and
the tests are repeated. The wear factor of all samples at varying sliding distance was
compared.
At lower frequency (1.5 Hz), the wear factor analysis did not show significant wear
reductions between the textured samples and the control sample. However, it was
found that when the frequency was increased to 3 Hz, keeping the load constant, the
medium dimple density sample had the greatest wear factor reduction (74% reduction)
with respect to the control sample. The results points to the fact that the brass textured
sample with medium dimple density exhibits better wear resistance than the control
sample.
Fig.9. Effect of sliding distance on wear volume
1 2 3 4
76 m 0.0000410 0.0000112 0.0000105 0.0000142
227 m 0.0000280 0.0000222 0.0000218 0.0000315
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
3.00E-05
3.50E-05
4.00E-05
4.50E-05
WearFactor(mm^3/Nm)
Effect of Sliding Distance
- LDD- CS - MDD - HDD
Frequency- 3 Hz
Load- 23 N
74 % Wear Reduction

10. 10
3.3. Effect of Normal Load
The effect of normal load on the wear performance of the samples was studied under
constant frequency of 1.5 Hz and a sliding time of 20 minutes. As the normal load
increases, the wear volume of all samples increases. From the analysis, it was
observed that the medium dimple density sample again had better wear resistance with
a wear volume reduction of 41% compared to the control sample.
Figure 11 shows almost similar wear track width between the MDD sample and control
sample at normal loads. As the load increases, the wear track width of control sample is
much wider than the MDD sample.
Fig.10. Effect of load on wear volume
Fig.11. Average width of wear track corresponding to conditions of varying load under constant frequency
and sliding time.
The following tests confirm the theory that increasing the dimple density on the sample
improves the tribological characteristics until an optimum dimple density is reached [7-
9]. Beyond an optimal value of dimple density, the wear volume of the samples was
shown to increase.
1 2 3 4
23 N 0.0140 0.01286 0.00989 0.01167
34 N 0.0304 0.02076 0.01792 0.02616
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
0.0350
0.0400
0.0450
0.0500
WearVolume(mm^3)
Effect of Load
- CS
Frequency- 1.5 Hz
Sliding Time- 20 mins
- LDD - MDD - HDD
41 % Wear Reduction

11. 11
4.0. RESULTS AND DISCUSSION- Aluminum 6061 T6
4.1. Effect of Sliding Frequency
The experimental analysis was performed under a constant load of 23 N and a time of
20 minutes. The samples were tested against a ball-on-flat reciprocating tribometer with
frequencies of 1.5 Hz and 3 Hz. At lower frequency (1.5 Hz), it was shown that the high
dimple density sample exhibited better wear reduction than the control sample with a
wear reduction of 35%. This is somewhat consistent with the findings that an increase in
dimple density leads to reduced wear volume. At higher frequency, there was no
significant difference between the textured and un-textured surfaces.
Fig.13 shows the wear track measurement of both samples. Even though the wear track
of HDD sample seems higher than the control sample (CS), the micro-dimples reduce
the overall wear track width of the HDD sample thus leading to lesser wear volume.
Fig.12. Effect of Frequency on wear volume
Fig.13. Average width of wear track corresponding to conditions of varying frequency under constant load
and sliding time.
1 2 3 4
1.5 Hz 0.0098 0.00783 0.00722 0.00634
3 Hz 0.0092 0.0080 0.0090 0.0105
0.0000
0.0025
0.0050
0.0075
0.0100
0.0125
0.0150
WearVolume(mm^3)
Effect of Frequency
) CS ) LDD ) MDD ) HDD
Load- 23 N
Sliding Time- 20 minutes
35 % Wear Reduction

12. 12
4.2. Effect of Sliding Distance
The wear factor was taken into account to study the effect of sliding distance. Keeping
frequency constant at 1.5 Hz, and a normal load of 23 N, the experimental analysis was
carried out. At a sliding distance of 38 m, the high dimple density sample was again the
most wear resistant material, showing a wear volume reduction of 35% with respect to
the control sample. As sliding distance increases, the samples exhibited a marked
increase in the wear volume.
The results, once again supports the findings that textured samples with increasing
dimple density have the ability to act as a secondary lubrication source and reduce wear
volume by a significant margin as compared to untextured surfaces.
Fig.14. Effect of sliding distance on wear volume
4.3. Effect of Normal Load
The conditions for the test were a constant frequency of 1.5 Hz and a sliding time of 20
minutes. As can be seen from figure 15, an increase in the normal load increases the
wear volume of the samples. At lower normal loads, the wear volume of high dimple
density sample had 35% lower wear volume than that of control sample. Under higher
loads, the lower dimple density sample showed a better wear volume reduction of 25%
with respect to the control sample. The high dimple density sample had only 23% wear
reduction compared to the control sample at the higher load conditions.
Fig.16 shows the wear track measurement of LDD sample and the control sample.
Under low frequency conditions, the wear track width of the LDD sample is less than the
control sample. At higher frequency test, the wear track width of the LDD sample seems

13. 13
similar to the control sample; the micro-dimples reduce the overall wear track width of
the LDD sample, thus leading to lesser wear volume than control sample.
Fig.15. Effect of Load on wear volume
Fig.16. Average width of wear track corresponding to conditions of varying load under constant frequency
and sliding time.
The results of the aluminum 6061 T6 samples are consistent with the theory of dimple
density as an important parameter in improving the tribological characteristics of
textured surfaces. This study infers that an optimum value of dimple density is required
to obtain the best wear reduction. The optimum value of dimple density varies with
regard to material of specimen, experimental conditions and machining of textured
surfaces.
1 2 3 4
23 N 0.0098 0.00783 0.00722 0.00634
34 N 0.0199 0.01490 0.01833 0.01519
0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
WearVolume(mm^3)
Effect of Load
) CS ) LDD ) MDD ) HDD
Frequency: 1.5 Hz
Sliding Time: 20 minutes
25 % Wear Reduction

14. 14
5.0. WEAR MECHANISM
5.1. Plastic Deformation and Roller Abrasion
During the tests, it was observed that the samples exhibited plastic deformation along
the wear track. The extent of plastic deformation increases at higher load and frequency
conditions and was more pronounced on control samples as shown in figure 17.
For Brass 360 tests, the AISI 440C steel balls showed a layer of brass particles adhered
on the contact surface. For the Aluminum 6061 T6 test, there was no noticeable
adherence of aluminum particles; instead there was an abrasion of the steel material on
the contact surface. This occurred under a load of 34 N, at a frequency of 1.5 Hz and
sliding time of 1 hour.
Figure 18 (a) and (b) shows the adherence of brass particles and removal of roller
material at the contact surface respectively.
Fig.17. Plastic deformation along wear track
Fig.18. Optical microscopic images of roller; arrow indicates- (a) Adherence of brass particles
(b) Removal of roller material at contact surface after testing with Al 6061 T6 sample

15. 15
5.2. Wear Profile
The wear profile of the sample specimens was carried out using a Taylor-Hobson
profilometer. From figure 19, it is clearly evident that the textured sample of brass
(MDD) has a lower wear volume than the control sample.
Aluminum samples, however, did not have a noticeable difference in the wear profile, in
terms of wear reduction, between textured and untextured surfaces as shown in figure
20.
Fig.19. Wear Profile of Brass 360 samples
Fig.20. Wear Profile of Aluminum 6061 T6 samples

16. 16
6.0. CONCLUSION
This project explores the effects of surface texturing on the tribological properties of
sample specimens. It takes into the account the influence of dimple density on the wear
performance of the surface performed under lubricated contact conditions with a ball-
on-flat reciprocating tribometer. The concluding remarks of this project are outlined as
follows:
1. The experimental analysis showed that, under controlled machining conditions,
textured surfaces exhibited better wear performance than untextured surfaces. The
textured surfaces also achieved better wear reduction under increasing frequency
and load.
2. For Brass 360 samples, under high frequency and load conditions, the medium
dimple density sample was found to be the most wear resistant material achieving a
wear volume reduction greater than 40% with respect to the control sample. An
increase in dimple density improves the wear performance until a certain value
known as optimum dimple density is reached. Increasing the dimple density beyond
the optimum dimple density leads to an increase in the wear volume of the
samples.
3. Aluminum 6061 T6 samples showed similar results with textured samples having
better wear performance compared to the control sample. An increase in dimple
density led to better wear resistance with respect to control sample. Under
conditions of low frequency and increasing load, the high dimple density (HDD)
sample exhibited wear reduction of more than 23% as compared to an untextured
sample of the same specimen.
4. The lack of adherence of wear particles of aluminum material on to the contact
surface of the roller could be due to the softer metal matrix of aluminum. For brass
360 tests, the wear particles adhered well on the contactsurface of roller.
5. The softer metal matrix of aluminum could also be one of the reasons why the wear
profile of the high dimple density sample is deeper than that of the control sample.
The micro-dimples allowed easier penetration of the roller into the aluminum sample
during testing which led to a deeper wear track profile than the control sample.
6. The study confirms the findings made by [7-9] that the effect of wear reduction of
textured surface is characteristic of the dimples' ability to act as fluid reservoirs and
provide lubrication retention at the contact interface. It also shows that an optimal
value of dimple density on a textured sample specimen yields the best wear
reduction and the greatest hydrodynamic pressure at the contact surface, thus
improving the lubrication retention ability of the micro-dimples at higher load and
frequency under lubricated contact conditions.

17. 17
7.0. ACKNOWLEDGMENT
First of all, I would like to extend my sincere thanks to my advisor, Dr. Patricia Iglesias
Victoria, for her guidance in the development of this project. This project with paper
would not have been possible without her help. I would also like to thank Dr. Haselkorn,
and Mr. Maiola for their assistance.
I also acknowledge the financial support from the FEAD grant program at the Rochester
Institute of Technology.

18. 18
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