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Tribological investigation of diamond-like carbon coated micro-dimpled surface under bovine
serum and osteoarthritis oriented synovial fluid
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2015 Sci. Technol. Adv. Mater. 16 035002
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National Institute for Materials Science
Science and Technology of Advanced Materials
Sci. Technol. Adv. Mater. 16 (2015) 035002 (11pp)
doi:10.1088/1468-6996/16/3/035002
Tribological investigation of diamond-like
carbon coated micro-dimpled surface under
bovine serum and osteoarthritis oriented
synovial fluid
Subir Ghosh1, Dipankar Choudhury2,3, Taposh Roy1, Azuddin Bin Mamat4,
H H Masjuki4 and Belinda Pingguan-Murphy1,5
1
Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala
Lumpur, Malaysia
2
Faculty of Mechanical Engineering, Brno University of Technology, Technická 2896/2, 616 69 Brno,
Czech Republic
3
Central European Institute of Technology, Brno University of Technology, Technická 3058/10, 616 00
Brno, Czech Republic
4
Department of Mechanical Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia
E-mail: um.subir@gmail.com, dipankar.choudhury78@gmail.com, taposh.um@gmail.com, azuddin@um.
edu.my, masjuki@um.edu.my and bpingguan@um.edu.my
Received 18 January 2015, revised 3 April 2015
Accepted for publication 4 April 2015
Published 14 May 2015
Abstract
Osteoarthritis-oriented synovial fluid (OASF), i.e., that typical of a patient with osteoarthritis,
has different physical and biological characteristics than bovine serum (BS), a lubricant widely
used in biotribological investigations. Micro-dimpled and diamond-like carbon- (DLC) coated
surfaces are key emerging interfaces for orthopedic implants. In this study, tribological
performances of dimpled surfaces, with and without DLC coating, have been investigated under
both BS and OASF. The friction tests were performed utilizing a pin on a disk tribometer,
whereas contact pressure, speed, and temperature were simulated to a ‘medium walking gait’ of
hip joint conditions. The mechanical properties of the specimen and the physical properties of
the lubricant were characterized before the friction test. Raman analysis was conducted to
identify the coating condition both before and after the test. The DLC-coated dimpled surface
showed maximum hardness and residual stress. A DLC-coated dimpled surface under an OASF
lubricated condition yielded a lower friction coefficient and wear compared to those of plain and
dimpled specimens. The higher graphitization of coated materials with increasing load was
confirmed by Raman spectroscopy.
Keywords: friction, wear, DLC, dimple, OA synovial fluid
1. Introduction
5
Improvements in medicine and healthcare have increased life
expectancy, especially among aging people for the last six to
10 decades [1]. However, elderly people are more likely to
suffer from osteoarthritis (OA); subsequently, the high
demand for joint replacement has become yet higher [2].
Although hip replacement is considered one of the most
successful orthopedic surgical procedures, satisfaction has not
Author to whom any correspondence should be addressed.
Content from this work may be used under the terms of the
Creative Commons Attribution 3.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the
title of the work, journal citation and DOI.
1468-6996/15/035002+11$33.00
1
© 2015 National Institute for Materials Science
Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
been achieved yet, especially as recent statistics showed that
the failure rate of hip joints is around 15% within 15 years of
their implantation [3]. Therefore, a number of studies have
been carried out based on different materials and designs to
increase the durability of implants by decreasing friction,
wear, and corrosion. The bearing clearance, which is the
difference between the radius of the prosthesis head and cup,
is an important factor of hip joint-bearing performance and
stability [4, 5]. The recommended clearance of a modern hip
joint prosthesis is usually 50–100 microns [6, 7]; however, it
is a big challenge for manufacturers to maintain the clearance
with an exact deviation, despite the availability of precise
manufacturing facilities. Furthermore, the clearance is a
variable parameter over time and mode of sliding. Therefore,
a super wear-resistant material along with a lower friction
yielding capability is desirable for the hip joints’ interface.
Ti–6Al–4V alloy is commonly used in biomedical,
aerospace, and automotive industries because of its excellent
mechanical and physical properties, such as high specific
strength, high melting point, high corrosion resistance, and
bioactivity [8]. However, it exhibits poor tribological performance, leading to research aiming to improve its tribological performance by surface modification techniques [9, 10].
One effective technique to do this is fabricating micro-dimples on a Ti–6Al–4V alloy substrate. It was reported previously that micro-dimples exhibit a low coefficient of friction
[11, 12] and wear rate [13, 14] because they improve lubrication, trap wear debris generated from the contacting surface,
and provide a hydrodynamic pressure [5]. Thus, surface
texturing, such as the use of micro-dimples, enhances tribological performance in the presence of a lubricant. However,
micro-dimples may rapidly wear out after an extended period
of cyclic loading. Such loading typically includes a number of
different lubrication modes; for example, near to the resting
period of a gait, dimples becomes less effective in forming
lubrication film thickness because of the low speed and high
pressure. In these circumstances, a boundary or a solid
lubricant can play a vital role. Diamond-like carbon (DLC)
films are considered one of the best solid lubricants and have
a high wear resistance capability. For example, DLC acts as a
solid lubricant by transforming into graphite where it builds
up a transfer layer on its counterpart [15, 16], so it causes an
easy slip between the contact surfaces. Furthermore, it has
been popular due to its biocompatibility, hardness, wettability, corrosion resistance properties [17, 18], and suitability
of fabrication in a complex shaped specimen. Amanov and
Sasaki [8] revealed a significant reduction of friction and wear
rate by utilizing a combination of DLC-coated micro-dimple
on a Ti–6Al–4V substrate under oil-lubricated sliding conditions. Very recently, Choudhury et al [3] revealed that
DLC-coated micro-dimples have dual benefits: The dimples
provide a thicker film thickness, and DLC enhances boundary
lubrication. However, in that study, only bovine serum (BS)
was used, and the conditions of post-experimental DLC were
not comprehensively studied. Therefore, it is still not clear if
the tribological behavior of DLC-coated micro-dimples works
under body fluids, which have similar properties to OASF.
The status of the coating after extended cyclic loading is also
unclear.
In order to establish likely in vivo tribological outcomes,
an appropriate lubrication of joint interfaces should be utilized
in vitro. Normally, researchers use water or BS in their
in vitro testing; however, such fluids do not accurately
simulate body fluids [19, 20]. Because BS does not have most
of the tribologically active proteins, such as (globulin, lubricin) and hyaluronic acid (HA), it is therefore very difficult to
predict the influence of body fluids on the friction and wear
mechanism. Moreover, the fundamental lubrication mechanism between natural joints and OA-affected joints is different,
since the effects of the protein—surface interactions play a
significant role [21, 22]. It is crucial to understand what is
happening in the actual contact zone of artificial joints. It is
also important to understand in vivo lubrication mechanisms
under a lubricant that matches the physiological conditions of
an OA patient. This study may posit better suggestions for
component design and implantation. To our knowledge, there
is no study conducted with OASF (considering all major
biological components with an appropriate concentration
ratio) in tribological investigation on the advanced interface
of hip joints. A successful DLC-coated micro-dimple interface can offer a single part of the stem and head without
having a tapper joint, which could solve the problem of
fretting wear. Therefore, the objective of this study is to
investigate the tribological influence of a dimpled surface and
a DLC-coated dimpled surface under BS and OASF conditions (a defined composition).
2. Materials and methods
2.1. Sample preparation
According to the tribometer (TR 283 Series, DUCOM, Bangalore, India) dimension, both disk and pin (rod) were prepared from commercially obtained titanium grade 5 alloy
(Nova Scientific, Malaysia), commonly known as Ti-6Al-4V.
The disk and pin were prepared in their respective dimensions: 15(L) mm × 15(W) mm × 6(H) mm and 6(L) mm × 6(∅)
mm. Each disk went through a series of polishing processes
using various grades of silicon carbide paper: 1000, 1200,
1500, and 2000, and finally with a diamond polycrystalline
suspension (0.02 micron) on a polishing cloth. In this
experiment, three types of samples were prepared, namely,
plain surface (T1), dimpled surface (T2), and DLC-coated
dimpled surface (T3).
2.2. Surface modification
2.2.1. Dimple fabrication. Briefly, the CATIA V5 design
software was used to draw the dimple array patterns. A CNC
micro drilling machine (Mikrotools DT110, Singapore) was
used to create the micro-dimples in a circular shape. (This is
advantageous, as it can be precisely controlled and applied to
curved surfaces without any significant change in bulk
material properties.) Our previous study also confirmed that
2
Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
the possibility of the presence of wear debris from a drill bit is
markedly reduced and thus also suitable for biomedical
applications [12, 14]. A diamond drill bit (UKAM Industrial
Superhard Tools, US) with a diameter of 400 μm was used to
fabricate dimples on titanium alloy disks.
Figure 1(b) shows the morphology of the dimpled surface
as measured by FESEM.
2.4.2. Coating characterization. FESEM was used to observe
the DLC-coated dimpled surface (figure 2(a)) and to measure
the thickness of the coating (figure 2(b)). The coating
thickness (1.10 ± 0.05 μm) was measured by a focused ion
beam technique. The highly energized ion beam was charged
on the coated surface to dig a groove.
2.2.2. Coating deposition. In the current research, dimpled
surfaces were coated using a physical vapor deposition
process. The machine (Milman Hybrid Decocoater) was
equipped with an integrated arc and magnetron sputtering
technology for greater flexibility to deposit hydrogenated
amorphous carbon (a-C:H) DLC [23]. The DLC coating on
dimpled titanium alloy disks was performed through five
steps, namely, evacuating, argon cleaning, chromium
deposition, DLC deposition, and, finally, cooling. First, the
samples were degreased, chemically etched, and then put into
a deposition chamber where samples were subsequently
evacuated to a vacuum of 2 × 10−5 mbar. Second, argon
cleaning was conducted. Third, chromium was deposited for
10 min as an interlayer. After that, the amorphous carbon a-C:
H was deposited for 90 min, where coating thickness was
controlled by deposition time. Finally, samples were cooled
for 30 min.
2.4.3. Mechanical properties. A 3D optical profiler was
applied for surface roughness measurement on different
surface conditions T1, T2, and T3. The hardness of each
sample was measured before and after coating. In general,
three non-dimpled areas were selected for measurement. For
samples T2 and T3, three different zones were selected
correspondingly near the edge of a dimple, 50 μm from the
edge of a dimple, and 100 μm from the edge of a dimple. A
dynamic ultra-micro hardness tester (Shimadzu DUH-211/
DUH-211S) was used to measure the hardness. In this study,
residual stress was calculated for both dimpled (T2) and DLCcoated dimpled surfaces (T3) on dimpled areas using an x-ray
diffractometer (XRD) (PANalytical Empyrean). The XRDsin2ψ technique was applied using CuKα radiation
(0.154 0598 nm) at 40 kV and 40 mA. Tilt angles (ψ) were
applied ranging from 0–35° by a computer-controlled Omegagoniometer. Each measurement was repeated five times for
accuracy.
2.3. Lubricants preparation
2.3.1. Osteoarthritis-oriented synovial fluid (OASF). The
OASF lubricant composition is summarized in table 1. The
formulation is based on the concentration and composition of
an OA patient’s synovial fluid [20, 24, 25]. The powder- form
composition was dissolved in a phosphate buffered solution
(PBS) at concentrations as stated in table 1 to prepare OASF
that represents body fluid affected by OA. Mucin, type III was
used, which is closest to lubricin protein, as it is difficult to
extract lubricin through the many steps of the purification
process.
2.4.4. Physical properties measurement. Prior to the friction
test, the viscosity of the lubricants was measured by using a
Brookfield Viscometer –LV (DV-11 + Pro EXTRA) at a
25 °C temperature at a definite shear rate 100 s−1.
Wettability measurement reveals the information about
surface energy of the material and the interaction of the
coated surface with lubricants in terms of contact angle. A
contact angle analyzer (OCA15EC, Data Physics Instruments,
Germany) was used to measure the contact angle. The static
sessile drop method was applied, where 2 μl lubricants were
dropped through a syringe onto a substrate. The three types of
sample (T1, T2, and T3) were subjected to a wettability test
with three types of lubricants, namely water, BS, and OASF.
Each measurement was repeated five times for better
accuracy.
2.3.2. Bovine serum (BS). The BS (HCL#SV30160.03;
HyClone Fetal Bovine Serum) was obtained commercially
from Life Technologies (Sigma-Aldrich #4417). The control
lubricant is made up of 30% BS with 70% distilled water. All
lubricants were stored at −20 °C, and, prior to the friction test,
the lubricants were kept in an oven at 37 °C for 1 h to achieve
normal body temperature.
2.5. Friction test
The samples, namely T1, T2, and T3, were subjected to a
friction test. A tribometer (TR 283 Series, DUCOM, Bangalore, India) was used in this experiment. The schematic diagram of the experimental setup has been shown in previous
studies [12, 14]. Loads of 10, 15, and 20 N were used, corresponding to mean Hertzian contact pressures of 107 MPa,
131 MPa, and 151 MPa, respectively. Pressures in a diarthrodial joint are usually around 5 MPa and can be as high as
18 MPa [26]. However, it is higher for artificial implants,
because Hertzian contact pressure depends on the modulus of
elasticity of the material, contact area, and applied load. Our
2.4. Characterization
2.4.1. Dimple profile. The dimple array patterns were
observed by field emission scanning electron microscopy
(FESEM), (AURIGA, Zeiss Singapore). A three-dimensional
(3D) optical profiler (Alicona Infinite Focus, Chicago, USA)
was used to measure diameter, pitch, and depth of the
dimples. The structure of the micro-dimple shown in
figure 1(a) was produced by a 3D optical profiler.
3
Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
Figure 1. (a) Structure of a micro-dimple produced by 3D optical profiler and (b) FESEM image of dimpled surface.
Table 1. The composition and concentration of OASF.
Product Name
Product Code
Solvent
Hyaluronic Acid (HA)
Bovine albumin
Bovine γ-globulin
Mucin, Type III
MP Biomedicals, USA #0215993350
MP Biomedicals, USA #0332
Sigma, US #G5009
Sigma, US #M1778
Final concentration (mg ml−1)
2.5
18.1
13.1
0.2
PBSa
a
Phosphate buffered solution (PBS) (Sigma-Aldrich # 4417)
Figure 2. (a) SEM image of DLC-coated dimpled surface and (b) coating thickness profile of a-C:H.
estimated contact pressure was kept as low as possible and
was lower compared to previous studies [27, 28]. Total running time for each loading condition was 90 min. Table 2
summarizes the experimental setup of tribology testing.
Parameters were selected based on simulated hip joints conditions [12, 27].
Table 2. Experimental parameters.
2.6. Raman analysis
A Raman microscope is commonly used to characterize the
structural changes of coated samples due to friction [29]. In
4
Items
Description
Specification of pin
Specification of disk
Speed
Hertz contact pressure
Temperature
Diameter—6 mm, Length—6 mm
L—15 mm, W—15 mm, H—6 mm
20 mm s−1
107 MPa, 131 MPa, 151 MPa
37 °C
Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
Table 3. Dimple parameters on two surface conditions (± shows the standard deviation of measured value).
Samples
Diameter,
∅ (μm)
Depth,
h (μm)
Aspect ratio,
λ = h/∅
Pitch (μm)
Dimple density,
A (%)
Total no. of
dimples
410 ± 5
410 ± 5
30 ± 2
30 ± 2
0.073
0.073
950 ± 20
950 ± 20
16.20
16.20
64
64
Dimpled surface (T2)
DLC-coated dimpled surface (T3)
Table 4. The mechanical properties of the material at different surface conditions.
Surface condition
Plain surface (T1)
Dimpled surface (T2)
DLC-coated dimpled surface (T3)
Surface roughness (nm)
Hardness (HV)
Residual stress (MPa)
50 ± 5
50 ± 5
50 ± 5
326
325
698
—
245
477
our study, Raman analysis was carried out by using an Ar+
laser operating at 514.5 nm, and Raman spectra were taken
from 800–2400 cm−1. The laser beam was focused to a 2 mm
spot size, and Raman spectra of the coatings were taken in
spot mode. The Raman analysis was only conducted for T3
samples, and this experiment was repeated after the T3 sample
had undergone friction tests. The Raman analysis was focused
on the as-deposited surface condition and wear track region
after friction tests for different loads.
deviation confirms the accuracy of producing micro-dimples
by a diamond drill bit. It also shows that there is no significant
difference in depth of dimple due to DLC coating, because
coating thickness was very low, at 1.10 μm on average.
3.2. Coating characterization
The FESEM image shows the morphology of the DLC-coated
dimpled surface at deposited condition (figure 2(a)). Carbon
should be the main composition on the coating layer; it also
contains chromium, as it was used as an interlayer during the
deposition process in order to increase the bonding strength
between coating and titanium alloy substrate [5]. Furthermore, some pores, as well as lighter-colored bumps, may exist
on the coating surfaces (figure 2(a)). The coating thickness
was 1.05–1.15 μm for DLC coating (figure 2(b)). Previous
studies on DLC coating also showed a similar coating
thickness value [5, 23].
2.7. Wear analysis
The weight of the pin and disk was measured both before and
after friction tests by a digital balance (Oertling VA304) than
can measure up to 0.010 ± 0.005 mg to retrieve weight loss
due to friction. Thus, the wear rate of the pin and disk was
calculated from the weight loss of the sample. After friction
tests, samples were dried to confirm no weight gain due to
lubricant contamination. Also, the surface morphologies of
worn surfaces after friction tests were analyzed by FESEM.
3.3. Mechanical properties
The mirror-polished surface is desirable to understand the
effect of lubrication in tribology. Higher values of surface
roughness have an adverse effect on tribological performance.
Our study represents a standard surface roughness profile,
which is comparatively lower than previous studies [19, 31].
The lower surface roughness was achieved by multistage
polishing of the titanium substrate. The mirror-polished surface was obtained by removing sharp edges from the substrate. Titanium alloy is comparatively softer and easier to
polish to a mirror surface. The mechanical properties of all
samples are presented in table 4.
DLC coating increases the hardness of a substrate
material and plays a significant role in friction [32]. It
enhances the ability to withstand load and provides lubricating ability. Table 4 illustrates that hardness increases significantly for DLC-coated dimpled surfaces. Previous studies
also showed that DLC coating produced a high hardness
[23, 33]. However, the Vickers hardness number was lower
near the dimple area (T2), which can be attributed to the
geometrical effect of the dimple and the changes in microstructure during dimple fabrication. However, the average
2.8. Statistical analysis
Statistical analysis was performed by the IBM SPSS statistics
21 software to identify whether there is any significant difference in friction coefficient value for the different lubricants. For this purpose, a two-way analysis of variance was
performed on all subsets of data in each study to compare
between lubricants over time.
3. Results and discussion
3.1. Dimple measurement
The dimple array is an important parameter for better lubrication in artificial joint prostheses. Recent studies revealed
that the dimple diameter, density, and depth have major
effects on the friction coefficient value [12, 30]. In our study,
the dimple parameters are similar for two surface conditions,
T2 and T3, as measured by a 3D optical profiler. The dimple
parameter is presented in table 3. The small standard
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Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
Table 5. Contact angle on different surface conditions before friction
tests.
Contact angle for different lubricants (°)
Surface condition
Plain surface (T1)
Dimpled surface (T2)
DLC-coated dimpled
surface (T3)
θwater
65 ± 3
70 ± 4
87 ± 3
θBS
35 ± 3
38 ± 4
57 ± 3
θOASF
50 ± 3
52 ± 4
72 ± 3
time under different lubrication condition of BS and OASF is
presented in figure 3, which identifies the tribological effect of
lubricants with three different surface conditions. It shows
that OASF lubricants provide the lowest friction coefficient
(0.157) for the T3 surface, and BS gives the highest friction
coefficient (0.248) for the T1 surface under the same contact
pressure of 131 MPa.
It is easily understood from figure 3 that OASF exhibits a
comparatively lower friction coefficient than BS lubricants,
which confirms the better lubricating abilities of OASF for all
surface conditions. However, the trend of the friction
coefficient profile over time for the same experimental
conditions stated in table 2 are quite stable for T3 surfaces,
while the other shows increasing order with increasing time.
The stability of the friction value for the T3 surface may be
attributed to the improved mechanical and surface properties
(details are in tables 4 and 5). In addition, there are
statistically significant differences (p < 0.05) in friction
coefficient values over time between the groups, and they
are homogenous (p > 0.05) across each group for the same
loading conditions.
Lubricant makes the difference in the friction coefficient
value because the composition of the lubricant plays a
significant role in reducing friction. Generally, fetal BS
contains 20–36 mg ml−1 albumin, which is also present in
normal synovial fluid (SF) in a range of 11.65–12.92 mg ml−1,
while OA patients carry 17.75–18.45 mg ml−1 albumin [25].
Moreover, albumin is considered an important biological
component that helps in boundary lubrication through
adsorption to joint materials and often provides a soft
boundary layer between the contact surfaces. BS does not
have other protein compositions (globulin and lubricin) and
HA, which are important for lubrication; however, OASF
contains important biological components such as globulin,
lubricin, and HA, along with albumin, that help in lubrication
and thus provide the lower friction coefficient. HA increases
the viscosity of a lubricant and thus helps in better lubrication
[35]. Other protein components also are relevant; thus,
globulin helps in artificial joints by blocking metal ion
removal from the surface layers, and lubricin reduces the
shear strength at the asperity contact interface [25]. HA plays
a major role in lubrication, similar to the role of additives in
commercial lubrication, and enhances the lubricating ability
of these protein components [25]. Thus it is considered a key
component of SF to reduce friction and wear. We found
viscosity 2.58 cP and 1.34 cP for the lubricant OASF and BS,
respectively. It confirms that HA increases the viscosity of
Figure 3. Friction coefficient profiles over time at 131 MPa under BS
and OASF lubricants.
value of the three positions was nearly the same as the plain
surface (T1).
The residual stress was measured to identify the stresses
that persist in a solid material after the original cause of the
stress is removed. The residual stress was focused on the
dimple area. The high value of residual stress confirms the
poor surface adhesion of the DLC coating, which can cause
early delamination of coated materials during friction at high
loads.
3.4. Wettability
Table 4 clearly illustrates the variation in contact angle for
different surface conditions over different lubricants. Prior to
the experiment, different lubricants were used to find their
interaction with solid materials. A DLC-coated surface is
considered mildly hydrophobic [33, 34]. In our studies,
similar trends are found. The contact angle for the T3 surface
has been increased compared to the T1 and T2 surfaces over
all lubricants. The increase in contact angle confirms the
reduction of surface energy. As a result, the coated surface
provides less attraction to water particles and thus helps in
protein adsorption. Protein can form a stable layer that
enhances lubricating ability.
Moreover, the contact angle for the OASF lubricant on
T3 is higher (72 ± 3°) than the BS lubricant (57 ± 3°), and this
confirms that the hydrophobic nature of the DLC-coated
surface is more intense for the OASF lubricant than BS, and
so results in a better interaction of fluid particles with coated
surfaces. Furthermore, OASF has a wider range of protein
composition than BS, and, eventually, it reduces the friction
coefficient during friction testing.
3.5. Friction tests
3.5.1. Effects of surface conditions. The friction coefficient is
considered a key parameter for tribological outputs, where a
reduction in friction value is desirable to reduce wear of a
joint prosthesis. The change in friction coefficient value with
6
Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
OASF significantly as compared with the viscosity of BS. The
higher viscosities of OASF reduce the friction between
contact surfaces by preventing two surfaces coming in
contact. In previous studies, it was found that lubricant
containing albumin, globulin, HA, and lubricin provided the
lowest friction coefficient, and this is similar to our
experimental results [25]. OASF along with DLC coating
exhibits the lowest friction coefficient, confirming the better
interaction of the DLC-coated surface with the biological
components of SF.
More specifically, T2 and T3 surfaces reduce the friction
coefficient by 16% and 33%, respectively, more than that of
the T1 surface under OASF lubricant at the same contact
pressure of 131 MPa. In addition, similar trends are found for
BS lubricants, where the T2 and T3 surfaces decreased the
friction coefficient by 10% and 26% more than that of the T1
surface, respectively. It confirms the better lubricating ability
of the DLC-coated dimpled surface. It was reported in
previous studies that a dimpled surface reduces the friction
because it increases hydrodynamic pressure under sliding
conditions [12, 36, 37]. Our study shows similar results. The
frictional behavior of micro-dimples may also be attributed to
the decreased contact area and the dimples supplying
lubricant continuously in the contact region. Micro-dimples
can improve anti-seizing ability by reserving lubricant,
improve additional lift by generating hydrodynamic pressure,
reduce adhesion by reducing contact area, and also prevent
abrasive wear by trapping wear debris generated by friction
[12]. In our studies, it was found that the DLC-coated
dimpled surface enhanced the lubricating ability and, thus,
reduced friction between the contact surfaces. The low
friction coefficient of the T3 surface can be attributed to its
higher hardness. Moreover, it forms a graphite transfer film
on the counter surface, due to the graphitization of coated
materials, which helps provide easy slip between contact
surfaces and thus minimizes friction.
Figure 4. Average friction coefficient at contact pressure for BS and
OASF lubricants.
similar phenomenon is found under OASF lubricants. Thus,
DLC coating improves the lubricating ability on dimpled
interfaces.
Clearly, for the T3 surface, with increasing load, the
graphitization of coated material increases and so reduces the
rate of increasing friction value. However, the friction
coefficient value increases dramatically when the load
increases to 151 MPa under different lubrication conditions.
This happens because of full delamination of coated materials
under high load. As the graphitization is directly proportional
to load, at a certain high load the coated materials split out
from the surfaces. Still, OASF lubricants provides a
comparatively lower friction coefficient value compared to
BS and prove the load-withstanding ability of OASF is better
than BS when it acts as a lubricant between the tribopairs.
3.5.2. Effects of different loads. The friction test was
3.6. Raman analysis
conducted on three different loads, stated in table 2.
Experimental conditions were similar to body movement
that represents medium walking conditions for hip joints. The
friction coefficient profiles were obtained at contact pressures
of 107, 131, and 151 MPa, respectively, applying normal
loads of 10, 15, and 20 N. The friction profiles for the three
loading conditions are presented in figure 4. It clearly
demonstrates that friction coefficients increase with
increasing load. The friction coefficient increases 26%,
27%, and 20%, respectively, for T1, T2, and T3 surfaces as
a result of increasing loads of 107–131 MPa in the presence of
the same lubricant, BS. With increasing load, the contact area
may increase, and the dimpled surfaces are not able to bear
high load. As a result, the increasing rate of the friction value
for the dimpled surface is quite higher than the plain surface.
The lower increasing rate of friction value for the T3 surface
with increasing load may be attributed to its high hardness
value. Furthermore, DLC enhances the lubricating ability due
to its solid lubricant activity during sliding. In addition, a
DLC is a combination of sp2 and sp3 hybridized bonds, where
tribological outcomes of these coatings depend on the sp2/sp3
bonding ratio [23, 38]. The relative ratio of the D peak and G
peak (Id/Ig) plays a significant role in Raman spectra to
characterize the sp2/sp3 bonding ratio [23]. The sp2/sp3
bonding ratio is also considered a dominant factor in determining the quality of DLC-coated surfaces. Both peaks are
decorated after the Gaussian fitting method. The Raman
analysis results of the T3 surfaces under different loading
conditions are presented in figure 5. There is a significant
change in the Id/Ig ratio when the load is applied. Under a
deposited condition, the Id/Ig ratio was 0.57. As the sample
goes through the loading condition, the Id/Ig ratio increases.
While the sample was tested at deposited and at 107 MPa
load, the Id/Ig ratio changed from 0.57 to 0.77. This confirms
the graphitization of the coated materials. Similar trends of
increasing Id/Ig ratio are found in previous studies that
revealed that the Id/Ig ratio increases at deposited condition to
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Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
Figure 5. Raman analysis of DLC-coated sample tested at different load conditions.
the wear track region [8, 23, 39]. It indicates a poor quality of
DLC coating in the wear track region, identified by a higher
value in the sp2/sp3 or Id/Ig ratio. Moreover, hardness and
modulus of elasticity vary, depending on these sp2/sp3
bonding ratios [8]. In addition, G peaks and D peaks move to
higher wave numbers with an increasing load, possibly due to
the microstructural modification in DLC film during sliding of
friction tests. Also, with a change in load from 107 MPa to
131 MPa, the Id/Ig increases from 0.77 to 0.81. Although the
change in peak wave number is significant from the deposited
condition to 107 MPa, there is a smaller change with an
increase in load from 131 MPa to 151 MPa. Moreover, peak
intensity also increases with increasing load.
Notably, there is a significant correlation between friction
coefficient value and Id/Ig ratio. The smaller increase in the Id/
Ig ratio increases the graphitization of the coated material.
Hence, the surface becomes smoother when a transfer of
graphite film forms on the counter surface. The friction
coefficient value decreases due to graphitization with
increasing Id/Ig ratio. The Id/Ig ratio is high (0.81) at 131 MPa,
which confirms a high degree of graphitization. Graphite-like
sp2 type content has a low shear strength, and, hence, coated
materials are progressively removed from the surface [38].
Interestingly, the friction coefficient increases rapidly over the
all lubricants at 151 MPa, because the transfer film also
moves from the contact area due to higher load. Furthermore,
no peak can be observed in the scanning region of the Raman
spectra (figure 5(d)), as the coated materials completely split
out at higher loads.
3.7. Wear analysis
A low wear rate is desirable for an artificial joint prosthesis, as
a high wear rate decreases its useful life. Researchers have
identified a correlation between friction coefficient and wear
rate [27, 31], and in previous studies, it has been found that
wear rate increases with increasing friction coefficient over
different types of lubricants [12]. Our experimental results
demonstrate similar trends. The wear rate was calculated by
the weight loss of the disk material both before and after
testing. The weight loss at 107 MPa was negligible; nevertheless, the weight loss became high for plain surface conditions over all lubricants and with different loads. The weight
losses of the disk material at three different test loads are
presented in figure 6.
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Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
confirms that lubricant-like body fluids cannot withstand high
load. Although the weight loss for OASF lubricant is lower
than for BS lubricant, it shows a higher wear rate for the T1
and T2 surfaces, which may be attributed to the poor loadwithstanding ability of the surface at higher loads, as well as
the hardness of the material. The wear resistance depends on
the ratio of hardness (H) and modulus of elasticity (E). Previous studies also reveal that a high value of H/E exhibits
high wear resistance of the material [8]. In our study, a lower
wear rate for the T3 surface can be explained by an H/E ratio
compared to the T1 and T2 surfaces.
However, the FESEM image also reveals the true morphology of the worn surfaces. It was quite impossible to
distinguish the worn surfaces for the same load with different
lubricants. The surface morphology due to applied different
loads is demonstrated in figure 7. Figure 7(a) describes the
deposited condition of the surface (T3), and figure 7(b)
describes the surface at applied load condition. The graphitization of the coated material happened with the applied load
and coated materials transferred from the surface at a lesser
rate at low load (figure 7(b)), but it increases rapidly at higher
loads. Previous studies also reported that the formation of film
transfer occurs during sliding contact of materials [23, 40].
Figure 6. Weight loss of disk materials after friction test.
However, the lower wear rate for T3 surfaces compared
to those of T1 and T2 is associated with a low friction coefficient. The reason of weight loss for T3 at higher loads can be
explained by the graphitization of the coated materials.
Nevertheless, the weight loss of disk material is very high at
151 MPa, whatever the lubricant in the friction test. It
Figure 7. FESEM images showing (a) as-deposited DLC-coated surface, (b) formation of film transfer due to load, (c) full delamination of
coated materials at higher loads, and (d) wear track on dimpled area.
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Sci. Technol. Adv. Mater. 16 (2015) 035002
S Ghosh et al
Figure 7(c) shows coated materials are fully delaminated from
the contact region due to higher loads. It confirms that the
DLC-coated sample can bear the loads only up to a certain
limit. After that, it pulls out from the surface with an
increasing transfer film formation rate. Furthermore,
figure 7(d) shows that there is damage to the structure of the
dimple at higher loads, which may be attributed to the lower
hardness of the titanium alloy. Also, there is some grain pullout during sliding contact at higher loads, as also seen in
previous studies [14].
This study has been conducted based on the friction
coefficient and wear rate of joint implants. Further investigation on film formation of lubricant and its adsorption and
desorption on coated surfaces would suggest guidelines for
the development of optimized surfaces with better control of
friction and wear in joint implants.
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4. Conclusions
The tribological performance of advanced interfaces, namely,
dimpled and DLC-coated dimpled surfaces, was investigated
under a formulated OASF and BS, using a pin-on-disk tribometer. The comparison between different surface conditions
revealed that the maximum reduction of friction coefficient
was 33% by a DLC-coated dimpled surface, whereas 16% by
a dimpled surface, compared to a plain surface for 131 MPa
contact pressure under OASF condition. The effects of
lubricant viscosity and surface wettability were found to be
significant on tribological performance. The hardness and
residual stress exhibited maxima at 698 HV and 477 MPa,
respectively, for a DLC-coated dimpled surface; hence, it
lowered the friction and wear rate. The friction coefficient
increased with increasing load; however, the rate of increase
was lower for DLC-coated dimpled surfaces due to the graphitization of the coated materials. The formation of transfer
graphite film was confirmed by Raman spectroscopy. Overall,
considering friction coefficient and wear rate, it can be concluded that the DLC-coated dimpled surface would be beneficial in artificial joint implants for OA patients.
Acknowledgments
The research was funded by Faculty of Engineering, University of Malaya, from the Ministry of Higher Education
High Impact Research Grant (UM.C/HIR/MOHE/ENG/44) at
Malaysia.
Conflict of interest
The authors declare that there is no conflict of interest
regarding the publication of this paper.
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