Intelligent Information Ma nagement, 2009, 1, 179-194
doi:10.4236/iim.2009.13027 Published Online December 2009 (
Copyright © 2009 SciRes IIM
An Investigation into the Adhesion Strength of
Diamond Like Carbon Multilayer Coating
Manufacturing Engineering Section, Department of Mechanical Engineering,
Indian Institute of Technolog y Madras, Chennai, India
Abstract: Advancement in vacuum technologies and vapor deposition processes during last decades has led
to the introduction of many modern coatings on metal cutting tools. Even in such an advanced vacuum coat-
ing techniques, the failure is not due to the wear of the coating but rather due to the lack of coating adhesion
to the substrate. In this work, the coating adhesion test results were performed which is based on the Rock-
well indentation tests. This coating adhesion tests were performed as per VDI standards 3198, 1991 for d.c.
magnetron sputter deposition of diamond like carbon multilayer coating (DLC / TiN/ Ti / Cu / Ni) on tool
substrate. Multilayer coating was deposited on tool substrates at different sputtering parameters/conditions
such as power density, partial p ressure, substrate temperature and reactive gases. The coated multilayer films
were characterized by experimental techniques such as X-ray diffractometer which measures the material
deposited, micro Raman spectroscopy and TEM to check DLC, Rockwell indentation to examine adhesion
strength, optical profilometer to measure thickness of coating. Ni increases the Cu adhesion on tool substrate.
Cu accommodates the shear stress induced by the films / substrate and the mismatch in thermal expansion
coefficient, while Ti and TiN promote better DLC bonding. As the target power was increased the adhesion
strength, micro hardness and depo sition rate were observed to impro ve. Increase in target power and substrate
temperature enhances adhesion strength. Proper substrate preparation and sequence of cleaning processes are
the crucial factors for the enhancement of adhesion streng th. The sputter deposition condition s for the above
mentioned multilayer coatings are identified in this work to get improved quality with particular reference to
adhesion and surface finish.
Keywords: cleanliness, sputtering, multilayer, adhesion, micro hardness
1. Introduction to Surface Engineering
In the recent past, the coating technology has advanced
tremendously and it is performed with enormous skill
and efficiency to produce quality components for many
engineering applications. With advances in the last two
decades, chemical vapour deposition (CVD), physical
vapour deposition (PVD) processes are widely used to
deposit hard, super hard, soft and combining hard/soft
coating applications such as dies, jigs, cutting tools and
bio compatible materials used in medical applications.
Hard wear resistant coatings are generally used in
tooling applications where it enhances tool lifetime of
two to ten times [1]. Since last two decades coating tech-
nologies like CVD, PVD and other coating processes
have been developed for depositing various types of hard
coatings on dies, jigs, cutting tools and machine compo-
nents in order to improve their resistance to seizure and
wear. The durability and quality of any such coating de-
pends not only on the hardness, toughness, seizure and
wear resistance of the coatings but also strongly on the
adhesion to the substrate. Hence it is v ital to consider the
quality of adhesion strength of the coating. The adhesion
strength of a coating is the amount of energy required to
delaminate the coating from the sub strate, but th at energy
is not easily measurable. Th erefore, the force required to
delaminate the coatings from the substrate is considered
as a standard indication of the adhesion strength. The
mismatch in thermal expansion coefficient of multilayer
coating material and substrate is responsible for the
above cited shortcomings [2]. Even when using well de-
veloped vacuum coating technology in hard coatings on
tool steel, the failure mode is not due to wear of coating
but due to adhesive failure [3].
The structure and properties of multilayer deposition
of Cr–B and Cr–B–N films by magnetron sputtering
process showed that, under optimal parameters, hardness
and properties were enhanced [4]. The indentation tech-
niques and its applications to thin film characterization
are very important for determining the mechanical per-
formance of coated substrate [5].
The multilayer coating of Cr/Cr2O3 were deposited
using reactive magnetron sputtering with a total thick-
ness of 7 μm on a tool steel substrate showed that grain
size increased with the increase in coating thickness [6].
The multilayer TiCrBN/WSex coatings were deposited
using sputtering process; ion implantation was employed
at the initial stage of deposition fo r 5 minutes to enhance
film adhesion. The coatings were characterized in terms
of their adhesion strength, micro hardness and friction.
The wear test verifies the decrease in the friction coeffi-
cient [7]. A multilayer gradient Ti (C, N) coating were
synthesized using magnetron sputtering process, better
wear resistance is observed in continuous turning rather
than in interrupted cutting [8].
A multilayer coating of TiN/TiBN with different bi-
layer thicknesses were carried out on a AISI M42 tool
steels at room temperature using magnetron sputtering in
an Ar – N2 gas mixture, the highest adhesion strength
was observed at an optimum combination and thickness
of multilayer deposition [9].
Comprehensive studies on seven different metallic in-
terlayers (W, Mo, Nb, Cr, Ti, Ag and Al) using sputtering
process reported that the interlayer in TiN on HSS
showed better adhesion properties [10]. A thin diamond
like carbon films were deposited onto a steel substrate
using plasma immersion ion implantation process and a
nitrogen interlayer was deposited on the substrate sur-
faces before depositing the DLC films which enhanced
the adhesion strength and wear resistance [11].
Thus, in order to design tailored coating / substrate
systems successfully, the most important aspect to be
considered is the mechanism of adhesion.
Adhesion and micro hardness are considered to have
crucial role on coating processes [12] and DLC coating
has in general good adhesion and hardness properties.
The direct coating of DLC on many substrate materials
may not render the requisite adhesion due to mismatch in
properties such as thermal expansion coefficient etc. The
coated layers are chosen in such a way to take care of the
thermal expansion mismatch and shear stress induced to
get better adhesion. Since the top layer is DLC, normally
the hardness of the coating is achieved will be very high
which is likely to be a major advantage for many engi-
neering applications. However, till now, little investiga-
tion has been reported about the properties of DLC/
TiN/Ti/Cu/Ni multilayer coating on tool substrates with
DLC coating as the top layer. In this present work, adhe-
sion test results are based on the Rockwell indentation
tests as per VDI standard 3198, 1991 for a d.c. magne-
tron sputtering process. A graded interlayer approach is
tried out to enhance the coating adhesion. The coated
films were characterized and results are reported and
2. Basic Aspects of Adhesion
2.1. The Adhesion Mechanisms
The mechanisms of adhesion can be mainly divided into
two groups: 1) mechanical interlocking 2) chemical
In all substrate/coating systems, these adhesion mech-
anisms individually or together are responsible for adhe-
sion. Very often, one of the mechanisms plays a domi-
nant role. These mechanisms can be subdivided further
and are shown Figure 1. Mechanical interlocking can be
divided into locking by friction and locking by dovetail-
ing. Chemical bonding is divided into ionic, covalent and
metallic bonding. The forces that can be transmitted by
mechanical interlocking depend on the size and geometry
of the locking sites. These sites can vary from mechani-
cally machined dovetails, grooves and other macroscopic
shapes to undercut porosity by pickling and micro
roughness produced by grit blasting. Depending on the
topographical results, three different effects are possible.
The first is the increase in surface area by which any type
of physical or chemical bonding is more effective. The
second possibility consists of sites which give rise to
friction between coating and substrate material. This is
particularly evident where the frictional force Ff =µFN is
high owing to a high coefficient of friction µ and high
normal stresses FN. These stresses can be raised by the
coating shrinking on the substrate because of the high
coating temperature and high coefficient of expansion of
the coating material. The third effect is the form of lock-
ing in which the forces transmitted depends on the me-
chanical properties of the materials involved. The mag-
nitude of locking effect is highly influenced by the wet-
ting properties of the coating. It is very sensitive to the
Figure 1. Schematic illustration of the different mechanical
and chemical bonding mechanisms [3]
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type of coating tech nology, the materials used and mostly
to the coating parameters. Physical bonds are v ery weak.
Their interaction energies are less than 50 kJ mol -1 over
a distance of 0.3 to 0.5 nm. Chemical bonds are com-
paratively strong with energy levels between 110 and
260 kJ mol -1 and in the case of metallic bonds up to 1000
kJ mol -1 with atomic and ionic bonds of 0.1 to 0.2 nm.
The forces of adhesion resulting from these energies
vary by at least one order of magnitude for physical and
chemical bonding, e.g. 500 N mm-2 for hydrogen bond-
ing and 500 0 N mm-2 for chemical bonding [13–16].
2.2. Importance of Graded Multilayer Coating
Multilayered materials with individual layers of less
than a micrometer in thickness are widely used in thin
film coatings, which can exhibit su bstantial enhancement
in hardness or strength. The enhancements in hardness
can be as much as 100% when compared to the value
expected from the rule of mixtures, which would have
been a weighted average of the hardness for the con-
stituents of the two layers.
There are many factors/theories that contribute/ex plain
enhancement of hardness and strength in multilayer.
These can be summarized as,
1) Hall-Petch behavior
2) Qrowan strengthening
3) Image effects
4) Coherency and thermal stresses
5) Composition modulation
Hall-Petch behavior is related to dislocations pile-up at
grain boundaries. This pile-up at grain boundaries im-
pedes the motion of dislocations. For materials with a
fine grain structure there are many grain boundaries
which block the movement of dislocations across the
grain boundaries. In polycrystalline multilayer, the grain
size decreases with the decrease in layer thickness. The
effect of an increased strengthening with decrease in
grain size dg is described by the well known Hall-Petch
relation (HPR).
HP g
YY kd
 (1)
where Y is the enhanced yield stress, Y0 is the yield
stress for a single crystal, and kHP is a constant and grain
size reduction has a negative effect on creep strength,
especially for metallic materials.
There is still an ongoing argument on the Hall-Petch
behaviour in nano structured multilayer. The basic model
assumes many dislocations pile-up, but such large dislo-
cations pile-up is not seen in small grain s and is unlikely
to be present in multilayer. As a direct consequence,
studies have found a range of values, between 0 and -1,
for the exponent in (Equation 1), rather than the - 0.5
predicte d fo r Hall-Petch behavior.
Orowan strengthening is due to dislocations in layered
materials being effectively pinned at the interfaces. As a
result, the dislocations are forced to bow out along the
layers. In narrow films, dislocations are pinned at both
the top and bottom interfaces of a layer and bow out par-
allel to the plane of the interface. Forcing a dislocation to
bow out in a layered material requires an increase in the
applied shear stress beyond a limit that is required to
bow out a dislocation in a homogeneous sample. This
additional shear stress would be expected to increase as
the film thickness is reduced.
Image effects were suggested by Koehler [17] as a
possible source of enhanced yield stress in multilayered
materials. If two metals, A and B, were used to make a
laminate and one of them, A, has a high dislocation line
energy, but the other, B, has a low dislocation line energy,
then there will be an increased resistance to dislocation
motion due to image forces. However, if the individual
layers are thick enough that there may be a dislocation
source present within the layer, then dislocations could
pile-up at the interface. This will create a local stress
concentration point thereby resulting in an enhancement
in strength. If the layers are thin en ough, there will be no
dislocation source present; the enhanced mechanical
strength may be substantial. The consequence of image
effects in reducing the thickness of the individual layers
in a multilayer is that it prevents dislocation sources from
being active within th e layer.
For many multilayer systems there is an increase in
strength as the thickness of coating is reduced, but there
is a critical thickness (e.g. 3 nm for the W/NbN multi-
layer) below which the strength falls. One explanation
for the fall in strength involves the effects of coherency
and thermal stresses on dislocation energy. While the
energy of dislocations is maximum or minimum in the
centre of layers for image effects, the energy maxima and
minima are at the interfaces for coherency stresses.
Combining the effects of varying moduli and coherency
stresses show that the dependence of strength on layer
thickness has a peak near the repeat period where a co-
herency strain begins to decrease.
Another source of deviations in behavior is the imper-
fect nature of interfaces. With the exception of atomi-
cally perfect epitaxial films, interfaces are generally not
atomically flat and there is some inter diffusion [18–21].
3. Experimental Procedure
3.1. Substrate Surface Preparation
For this study WC (a typical commercially available WC
turning indexable inserts) was used as substrate. The
substrate surface preparation involves prolonged step by
step sequential careful operations. Substrate surface pre-
paration processes involve two phases, grinding phase
and polishing phase.
The purpose of the grinding phase is to remove dam-
age from cutting, planerize the substrate and to remove
Copyright © 2009 SciRes IIM
material approaching the area of interest. It involves,
sequentially hand grinding of substrates with SiC abra-
sive paper of mesh sizes 120, 220, 320, 400, 600, 800,
1000 and 1200. The polishing phase involves the use of
alumina abrasives powder of 600 mesh sizes with water
slurry, used as a primary polishing abrasive and subse-
quently using a diamond paste of 14,000 mesh size with
a specially prepared lubricant as a final step of polishing
for obtaining a mi rror fi ni shed substrates (Ra ~ 0.01 4µm).
Extreme care has taken such that, never drying out of
lubricant takes place while polishing the substrate with
diamond paste, because it will damage the surface of the
polishing cloth with the polishing motions and create
scratch on the substrate surface. These operations in the
polishing phase were carried out using polishing ma-
A mirror finished substrate surface is mandatory for
thin film coating. If not, the impingement of species will
incident on the substrate surface at an oblique angle, in-
stead of falling normally on the substrate. This occurs
due to the shadowing effect of the neighboring columns
oriented towards the incident species. This shadowing
effect promotes enhancement of surface roughness. A
smoother coating will improve wear resistance whereas
high micro projections in a rough coating can be easily
knocked off during sliding, resulting in a catastrophic
failure of the coating [22,23].
3.2. Substrate Cleaning Process
Adhesion quality depends on the process of surface
preparation and surface cleanliness. Substrate cleaning
process consists of four steps; 1) ultrasonically degassing
2) ultrasonically d egreasing 3) deion ized water rinsing 4)
ultrahigh vacuum heating.
The surface of the substrate being cleaned has been
considered as flat but in reality surfaces are seldom flat,
instead being comprised of hills, valleys and convolu-
tions of all description. The mechanism of ultrasonic
cleaning is by cavitations and the solvent liquid selection
was made on the basis of higher surface tension and
lower vapor pressure.
Degassing is the process of removing small suspended
gas bubbles and dissolved gases from a solvent liquid
prior to using it as a vehicle for ultrasonic degreasing.
Dissolved gases, if not removed migrate into cavitations
bubbles during their formation and prevents them from
imploding violently, thus reducing the cleaning effect
and also these gas bubbles absorb ultrasonic energy re-
ducing the sound intensity. These dissolved gases act as a
cushion to the imploding cavitations bubble, which is
much like an air bag in a car. Trichloroethylene, acetone
and isopropyl alcohol were degassed by operating with
ultrasonic vibrations for a period of 15 minutes. If small
bubbles were not seen rising to the liquid surface during
ultrasonic degreasing operation, then it indicates that the
solvent was completely degassed.
The degreasing process involves, ultrasonic agitation
cleaning of substrate in trichloroethylene, acetone and
isopropyl alcohol for about 15 minutes each and rinsing
in deionized water. After each process, substrates were
dried with hot air. Rinsing in deionized water removes
residues of the cleaning chemistry and the contaminants
which were loosened to leave the substrate surface and
made the substrate completely free of residue.
This mirror finished substrates have nascent surface,
which is chemically very active and extremely easy to
scratch, hence special care was taken to keep the sub-
strates absolutely clean during cleaning process [24–33].
Finally, for ultra-high vacuum cleaning, the vacuum
chamber was evacuated to a pressure less than 5x10-6
mbar and the substrates were heated for about 45 minutes
at a substrate surface temperature of 400oC. Proper metal
tooling and hand gloves were used for handling of the
substrates during and after cleaning to avoid the transfer
of human body oil to substrate.
3.3. Applied Coating Technology
Five layers of different materials were deposited: DLC/
TiN/Ti/Cu/Ni. Each layer was coated for 30 minutes du-
ration to produce same layer thicknesses within an ap-
proximate range of 0.4 - 0.5 μm. The target to substrate
distance was kept at 5 cm. The substrate material used in
the experiments was cemented carbide turning inserts of
geometry with 6% cobalt and they were arranged on a
substrate holder, which was integrated with a heater.
Schematic illustration of the sputtering apparatus used is
shown in Figure 2.
The deposition processes were carried out in two target
planar magnetron sputtering system, where two different
layers of material can be coated under the same vacuum
conditions. This system contains two cathodes, placed on
upper portion of the chamber, with the corresponding
magnets behind the cathodes in a water-cooled bell jar
vacuum enclosure. Magnets were arranged to produce
closed magnetic fields for confining the plasma around
the target area, thus giving intense ion bombardment to
the growing film on the surface of the substrate. This
equipment consists of a rotary pump and a diffusion
pump, capable to create vacuum up to 1x10-6 mbar.
Pure nickel, copper, titanium and graphite discs of
2-inch diameter and 3 mm thickness were used as target
material for the deposition of DLC/TiN/Ti/Cu/Ni coating
respectively. The TiN was deposited using sputtering of
pure titanium with argon gas and nitrogen as reactive
gases. Initially the Ni and Cu targets were fixed on the
target holder. The substrate was kept below the Ni target
position and Ni layer deposition was carried out for 30
minutes duration. Then the substrate was moved to Cu
target position and second layer namely Cu deposition
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Figure 2. Schematic illustration of the sputtering apparatus used
Table 1. Deposition conditions used in experiments for multilayer coating (Ni - first layer, Cu - second layer, Ti- third
layer, TiN- fourth layer and the DLC - fifth layer) each layer 30 minutes deposition duration on WC substrates
No. Target material Sputtering/
Reactive gas Material
(W) Substrate temperature (OC)
1 Ni Ar Ni 4x10-3
2 Cu Ar Cu 4x10-3
3 Ti Ar Ti 4x10-3 100-400
4 Ti Ar/N2 TiN 5x10-3
5 Graphite Ar DLC 4x10-3
100-300 No substrate heating during
DLC deposition
was performed for 30 minutes. Then both the targets
were removed. Thereafter Ti and graphite targets were
placed on the target holder. The third layer deposited was
Ti; and then the fourth layer TiN deposition was carried
out with Ti as target in nitrogen reactive environment.
Then the substrate was moved to graphite target position
and the final fifth layer namely the DLC was deposited.
For each layer 30 minutes deposition duration was main-
DLC/TiN/Ti/Cu/Ni interlayer are supposed to accom-
modate the mismatch in shear stress induced by the
films/substrate and their thermal expansion coefficient.
To meet these requirements and to enhance DLC adhe-
sion, each layer should have enough thickness and
A systematic study was made in order to assess the in-
fluence of the process parameters on the final mechanical
properties of the coatings. Layers were deposited at a
low argon partial pressu re of 4 x10 -3 mbar, in all the cases.
Table 1 shows the relevant deposition conditions used
during deposition.
3.4. Characterization Methods of the Deposited
The surface topography of the coated specimens was
examined using a surface profilometer. A commercial
surface profilometer (VEECO VSI) was used to measure
thickness of the coating by providing a step on the glass
substrate surface. Qualitative coating adhesion test was
performed as per VDI standards 31981991 [22] using a
Rockwell indentation hardness testing equipment (Zwick
& Co. KG) and then the specimens were examined using
an optical microscope and SEM.
Micro hardness of the coating was evaluated using a
Future Tech - FM 700 equipment, at a load of 50 grams,
for a dwell time perio d of 1 5 seco nds .
Table 1 Deposition conditions used in experiments for
multilayer coating (Ni - first layer, Cu - second layer, Ti-
Third layer, TiN- fourth layer and the DLC - fifth layer)
each layer 30 minutes deposition duration on WC sub-
Thin films were then examined using a Buker AXS
(D8 discover) diffractometer. It is incorporated with th ird
generation Gobel mirrors providing the highest X-ray
flux density, which is essential for thin films. The CuKα
line of a conventional X- ray source, powered at 35 kV
and 25 mA was used for the experiment. The phase at-
tribution was p erf ormed JC-PDF databa se so ft ware.
Coefficient of friction of the coated film was estimated
using a standard pin on disc machine (Ducom, TR – 201).
The coated substrate was subjected to dry sliding wear
test at a constant load of 50 N and at a sliding speed of
150 rpm. The test duration was fixed as 900 sec. The disc
material used was AISI 316 L stainless steel, hardened to
55 HRc.
The characterization of DLC film was done using a
WITec-CRM200 confocal Raman instru ment an d TEM.
4. Results and Discussions
4.1. Film Orientation-Diffractometer
The deposited films were characterized by glancing an-
gle X-ray diffractometry (GAXRD). The Figure 3 (vary-
ing power) and Figure 4 (varying substrate temperature)
represent the GAXRD spectra obtained from the surface
of the coated substrates. The power at the target was in-
creased up to 250 W and the vacuum chamber pressure
was kept at a low value of 4x10-3 mbar. The growth of
atoms on the substrate surface involves:
1) Atoms arriving at a distribution which depends on
the self shadowing of the coating atom’s arrival direc-
tions and on the peak and valley of the substrate surface.
2) Atoms diffuse over the surface until they become
trapped in low energy lattice sites and incorporated into
the growing coating.
Finally, the deposited atoms may readjust their posi-
tions within the coating lattice b y recovery and recrystal-
lization. When the sputtering is performed at a higher
vacuum chamber pressure, it results in collision among
the ejected atoms and argon ions. This causes the sput-
tered atoms to scatter in the chamber and reach onto the
substrate surface with a low energy level in randomized
directions, which promotes shadowing. The structure
promotes shadowing because the high peak points on the
growing surface receive more coating flux than the val-
leys [23].
The atoms ejected from the target material possess
high energy level and the sputtering was performed at a
low vacuum chamber pressure. Therefore the loss of at-
oms energy was negligible due to collision with Ar ions.
Hence the ejected target atoms were transported to the
substrate surface at a high energy level. A few of them
penetrated into the substrate while others bounced back
to the chamber itself leaving behind roughness on the
substrate surface, which would affect the preferred ori-
entation and growth of the grains which can result in
poor adhesion [22,34].
No strong textures were developed in any of the sam-
ples, but rather, a mixture with variable relative amounts
of Ni :-( 111), (200), (220); Cu :-( 111), (200), (220); Ti:
- (110), (220); TiN: - (200), (220), (222) orientations
were formed. The sputtering deposition process was per-
formed at a vacuum chamber pressure of 4x10-3 mbar,
which is a low pressure. When sputtering process was
operated at low pressures, there was little collisional
scattering between the sputtered atoms and the argon
ions. Therefore, the loss of energy was minimal during
30 40 50 60 70 80 90
Ni (200)
Ni (220)
Ni (111)
Cu (111)
Ti (220)
Cu (220)
Cu (200)
TiN (200)
Intensity (Arb.unit)
Mu ltilaye r depo sitio n at 4 0 0 oC,
0.005m bar, 30 min.
Ti (110)
Figure 3. XRD pattern of multilayer deposition of (DLC/
TiN/Ti/Cu/Ni) coating coated at 400 oC, 0.004 mbar, each
layer of deposited for a duration of 30 minutes and at 250 W,
200 W, 150 W
30 40 50 60 70 80 90
Substrat e
Cu (200)
TiN (200)
Substrate Ti (110)
M ultilayer depo sition at 200W
0.005mb ar, 30 m in.
Intensity (Arb.unit)
Figure 4. XRD pattern of multilayer deposition of (DLC/
TiN/Ti/Cu/Ni) coating coated at 200 W, 0.004 mbar, each
layer of deposited for a duration of 30 minutes and at a
substrate temperature of 200 oC, 300 oC
transportation from the target to the substrate. This re-
sults in an enhanced surface mobility provided by higher
ion energy. Also it results in denser packing and en-
hanced crystal grain grow th on the substrate surface.
Nickel formation was observed at 400 oC, 200 W, 150
W, but it was not observed at lower substrate tempera-
tures of 300 oC and 200 oC. Ni is a magnetic material that
needs more surface energy to move along the substrate
surface to form crystalline structure and grow layer by
layer, which was observed at 400 oC. At a lower surface
temperature they do not have enough surface mobility,
therefore Ni formation was restricted. Even at 250 W,
400 oC Ni was not formed because at higher target power
and the decrease in sputtering yield. This is due to the
deep penetration of argon ions into the target and the
consequent decrease in energy of atoms deposited at the
target surface which decrease the sputtering yield. This
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result in a decrease in the number of atoms ejected from
the target and consequently low denser packing of the
material. Ni (JCPDS – 87 0712) crystalline structure
formation was obs er ved at 400 oC, 2 00 W and 15 0 W.
At higher target power (250 W) the sputtering yield
decreased. Since the ions penetrated deep into the target,
the energy deposited at the surface decreased, thereby
decreasing the sputtering yield. This results in th e forma-
tion of non-uniform and less dense microstructure at the
substrate surface. This leads to the decrease in micro
hardness, formation of defects etc. at a higher operating
power of 250 W. The micro hardness is hence less as
compared to the target power 200 W and the maximum
micro hardness is observed at 200 W and the micro
hardness values were in the range of 2700 – 3500
kg/mm2 [22,34].
At 250 W, 400 oC Copper (200) orientation was ob-
served without maximum intensity peak because at
higher target power, the deep penetration of argon ions
into the target and the consequent decrease in energy of
atoms deposited at the target surface decrease the sput-
tering yield. This result in a decrease in the number of
atoms ejected from the target resulting in a low denser
packing of the material. At all deposition conditions Cu
(JCPDS – 85 1326) crystalline structure formation was
observed. Figure 6 shows the variation in the micro hardness of
multilayer (DLC/TiN/Ti/Cu/Ni) coating, coated on a WC
substrate at various substrate surface temperatures rang-
ing from 200oC to 400oC at a constant power of 200 W
and at 0.004 mbar chamber pressure. High surface tem-
perature on the substrate surface enhances the diffusion
and surface mobility of atoms, which leads to denser
packing of the sputtered atoms on the substrate surface.
While depositing DLC, substrate heating was not pro-
vided. This is necessary for the formation of sp3-hybrid-
ised bonds, which determine the hardness of the DLC
coating. All these contributing factors together lead to the
improvement in micro hardness of the coated film.
Titanium Nitride (TiN) crystalline structure (JCPDS 71
0299) was formed at all deposition condition except at
200 oC. This is because, at a lower substrate surface
temperature the energy level of atoms was low and thus
the mobility of the atoms along the surface was restricted.
Hence crystalline structure formation was no t observed.
Ti (JCPDS 65 5970) crystalline structure formation
was seen at all deposition conditions with maximum in-
tensity peak.
4.2. Microhardness The micro hardness was measured and the composite
hardness was observed to be in the range of 2200 – 3600
kg/mm2 as shown in Figure 6. The measurement could
not eliminate the effect of the substrate. Hence, the
hardness of the DLC films will be definitely more than
the composite hardness. This finding is in agreement
with other rese archers observations.
Figure 5 shows the variation in the micro hardness of
multilayer (DLC/TiN/Ti/Cu/Ni) coating coated on a WC
substrate with various target powers ranging from 150 W
to 250 W at a constant temperature of 400oC, each layer
for 30 minutes deposition duration and 0.004 mbar
chamber pressure.
130 150 170 190 210 230 250 270
Target Power (W)
Vick ers m icroh ad n ess (Kg/ m m2)
Figure 5. Variation in the Vickers micro hardness of multilayer coating (DLC/TiN/Ti/Cu/Ni) on WC substrate with target
power at a constant temperature of 400 oC, 0.004 mbar, and each layer deposited for duration of 30 minutes
150 200 250 300 350 400 450
Vickers microhardness (Kg/m m 2)
Figure 6. Variation in the Vickers micro hardness of multilayer coating (DLC/TiN/Ti/Cu/Ni) on WC substrate with substrate
temperature at constant power 200 W, 0.004 mbar, and each layer deposited for a duration of 30 minutes
Figure 7. Optical Profilometer (3-D) result showing the rough surface of a typical multilayer (DLC/TiN/Ti/Cu/Ni) coated ma-
terial on a glass substrate coated at 200 oC, 200 W, 0.004 mbar, each layer deposited for a duration of 30 minutes
4.3. Surface Optical Profilometer
An optical profiler is used to measure the thickness of
the coated film by measuring a step from the top of the
film to the bare substrate. The optical profiler is a non
contact type instrument, provides three dimensional sur-
face profile measurements.
A typical optical profiler result is shown in Figure 7 at
200 oC the multilayer coating on a glass substrate be-
comes rough because mobility of atoms along the sub-
strate surface becomes limited at low temperature. Thus
the atoms grow in an isolated fashion such as islands
resulting in a rough substrate surface.
A typical 2-D optical profiler output is presented in
Copyright © 2009 SciRes IIM
Figure 8, which shows the total thickn ess of coating (2.6
µm) of a typical multilayer (DLC/TiN/Ti/Cu/Ni) coated
on a glass substrate. Each layer deposition was per-
formed at constant duration of 30 minutes.
4.4. Indentation Test Evaluation
The well known Rockwell ‘C’ indentation test is pre-
scribed by the VDI standards 3198, as a destructive
qualitative test for coated materials. The princip le of this
method is presented in Figure 9. A conical diamond in-
denter penetrates into the coated surface inducing mas-
sive plastic deformation to the substrate thereby fractures
the coating. The coated specimen was then evaluated
using conventional optical microscopy.
The contact geometry, in combination with the intense
load transfer, induces extreme shear stresses at the inter-
face. Well adherent coatings will withstand these shear
stresses and prevent extended delamination circumferen-
tially. The four different textures (HF1, HF 2, HF3, and
HF4) illustrate the indentation shapes (Figure 9) that
guarantee strong interfacial bonds between the coating
and the substrate. But the delamination (HF5, HF6) at the
vicinity of the indentation depicts a poor interfacial ad-
hesion (HF is the German short form of adhesion
strength) [19].
It is well known that each layer of graded coating has
its own specific function, basically coating acts as a dif-
fusion barrier. The Ni increases the Cu adhesion on the
substrate. Cu accommodates the shear stress induced by
the films /substrate and mismatch in thermal expansion
coefficient, while Ti and TiN promote DLC bonding.
The surface onto which the atoms deposited must have
the correct surface energy to promote chemical and
physical bonding. In addition, th e atoms must arrive with
sufficient energy to provide free mobility on the surface.
The surface cannot have contaminants that tie up poten-
tial bonds that are intend ed for the atoms depo siting on it.
This was observed in this investigation at 20 0 W, 400 oC
and 0.004 mbar.
Figures 10, 11, 12, 13, 14 show the acceptable level of
adhesion strength of multilayer (DLC/TiN/Ti/Cu/Ni)
coating. The power was increased from 150 W to 250 W
at a constant substrate surface temperature of 400 oC and
then the substrate temperature was varied from 200 oC to
400 oC at a constant power of 200 W. Firstly, the strong
interfacial adhesion due to intense surface diffusion was
promoted by the substrate heating and high impact en-
ergy of the bombarding species. These contributing fac-
tors together led to higher effective substrate surface
temperature, which promotes diffusion. Secondly, it is a
well-known fact that impingement islands were nucle-
ated before a continuous film forms. The impingement
island size is the size of islands when they begin to im-
pinge on each other. The smaller the impingement island
size, the higher the number of grains per unit area which
results into smaller grain size, thereby enhances the me-
chanical strength. Apart from this, due to high power at
the target temperature goes up. The impurity from the
target surface degasses and desorbs at a rapid rate which
enhances coating purity. This resulted in a cleaner sub-
strate surface and a better adherent coating was observed.
The sputtering depo sition processes were performed at
a low-pressure level of 0.004 mbar. At lower sputtering
pressures, the sputtered atoms experience fewer collisions
Figure 8. Optical profilometer (2-D) out put measures the total thickness (2.6 µm) of typical multi layer (DLC/TiN/Ti/Cu/Ni)
coated on a glass substrate at 400 oC, 200 W, 0.004 mbar, each layer deposited for a duration of 30 minutes
Copyright © 2009 SciRes IIM
Acceptable level of interfacial adhesion (HF1, HF2, HF3, HF4)
poor interfacial adhesion (HF5, HF6)
Figure 9. Classification of the acceptable level of interfacial adhesion of thin films based on typical indentation results, ac-
cording to VDI standard [19]
Indentation no cracks / delamination of coating
Figure 10. Indentation results indicating better adhesion of
multilayer (DLC/TiN/Ti/Cu/Ni) coatings coated with 250 W
at a constant substrate (WC) temperature of 400 0C, 0.004
mbar, each layer deposited for duration of 30 minutes
scattering with the Ar ions during travel and thus hit on
the film surface with more energy leading to a smooth-
ening effect. Besides this, a less disperse ion flux is nor-
mally associated to a decrease in self-shadowing effects
Indentation no cracks / delamination of coating
Figure 11. Indentation results indicating better adhesion of
multilayer (DLC/TiN/Ti/Cu/Ni) coatings coated with 200 W
at a constant substrate (WC) temperature of 400 0C, 0.004
mbar, each layer deposited for duration of 30 minutes
which also leads to less surface roughening [20,23]. This
low chamber pressure deposition process restricts the
diffusion of argon ions and other contaminating elements
into the film. All these contributing factors together re-
sulted in a stronger adhesion.
Copyright © 2009 SciRes IIM
Apart from this, an important requirement is that while
depositing DLC coating there should not be any heating
of the substrate surface. Since DLC is amorphous and no
grain boundaries exist, DLC grows as a hetrolayer above
each phase (i.e. nucleate a new phase and grow further).
The phase should have the right interfacial energy so
Indentation no cracks / delamination of coating
Figure 12. Indentation results indicating better adhesion of
multilayer (DLC/TiN/Ti/Cu/Ni) coatings coated with 150 W
at a constant substrate (WC) temperature of 400 0C, 0.004
mbar, each layer deposited for duration of 30 minutes
no cracks/delamination of coating Indentation
Figure 13. Indentation results indicating strong adhesion of
multilayer (DLC/TiN/Ti/Cu/Ni) coating coated on WC sub-
strate at 300 0C and target power of 200 W, 0.004 mbar,
each layer deposited for duration of 30 minutes
Indentation no cracks/delamination of coating
Figure 14. Indentation results indicating better adhesion of
multilayer coating (DLC/TiN/Ti/Cu/Ni) coated on WC sub-
strate at 200 0C and target power of 200 W, 0.004 mbar,
each layer deposited for duration of 30 minutes
that, it cannot form islands. Hence, carbon phase nucle-
ate everywhere on the substrate surface, which prevents
the formation of islands since nucleation barrier and
carbon atoms do not have enough mobility during depo-
sition process. Therefore, a smooth, amorphous, denser
and better adherent coating was formed [35–41].
Figure 15(a), (b), (c) presents the variation in the local-
ized surface defects at various substrate temperatures
ranging from 200 oC to 400 oC and at a constant power of
200 W. Substrate temperatures higher than 200 °C are
required to achieve strong adhesion and denser grain
growth. Higher the substrate temperature more the diffu-
sion and that helps stronger chemical bonding. However,
at 450 oC an embrittiling eta phase formation was re-
ported at the coating substrate interface, which is notori-
ous [42]. Low energy deposition promotes crystallite
island formation in the early stages of nucleation due to
low surface mobility of atoms. With sufficient surface
energy, mobility is high enough to promote coalescence
to a continuous film of small thickness, thus increasing
the packing density. If the kinetic energy delivered to the
surface is below a certain threshold and the ion current
density is low, mobility across the surface is limited and
empty zones are developed around the larger crystallites
as local atoms were captured. This would result in pore
and localized defects formatio n on th e films. This is illus-
trated in the microphotographs shown in Figure 16 (b),
(c). The rough surface formed at 200 oC is clearly seen in
the surface profilometer results as well (Figure 8) [43].
SEM micrographs (Figure 16 (a), (b)) show the localized
surface pore on the substrate surface at a temperature of
Copyright © 2009 SciRes IIM
Copyright © 2009 SciRes IIM
(a) X1000 (b) X1000
(c) X1000
Localized defects
Localized defects
Smooth, uniform coatin g
Figure 15. Variation in the localized surface defects occurred in multilayer coating DLC/TiN/Ti/Cu/Ni, each layer deposited
for a duration of 30 minutes at constant power of 200 W, 0.004 mbar with various substrate (WC) temperatures of (a) 400 oC
(b) 300 oC (c) 200 oC
Coated surface Porosity Localized porosity
(a) (b)
Figure 16. a, b Localized pore observed on substrate (WC) surface temperature of 200 oC, at a constant power of 200 W, 0.004
mbar, (DLC/TiN/Ti/Cu/Ni multilayer coating, each layer deposited for a duration of 30 minutes)
200 oC and at 200 W. Substrate temperature higher than
200 °C are necessary to achieve better adhesion and
denser grain growth. At substrate temperature 200 oC and
200 W, the atoms` arrive on the substrate surface with
low mobility across the surface and hence densification
was inhibited ending up with localized pores. Localized
defects and pore are in general beneficial for annihilating
the crack propagation.
4.5. Transmission Electron Microscopy (TEM)
The transmission electron microscope uses high-energy
electron beam transmitted through a very thin sample to
The representative TEM images of the DLC film are
presented in Figure 17(a). which indicates a uniform, pore
free, smooth film. The corresponding diffraction pattern is
incorporated in Figure 17(b). Microanalysis (EDAX) is
image and analyze the microstructure of materials with
atomic scale resolution. Philip s CM12 transmission elec-
tron microscope was used for the experiments. Speci-
mens were used approximately1500 Å or less in thick-
ness in the area of interest. The thin film specimen were
prepared and put into copper grid arrangement for mea-
surements. The electrons were focused with electromag-
netic lenses and the image was observed on a fluorescent
screen, and recorded on a black and white film. TEM is
destructive and is generally time consuming.
(a) (b)
Element Weight % Atomic%
C 100 100
Figure 17. TEM images of the DLC film (a) uniform, pore free, smooth film (b) No sharp diffraction rings in the diffraction
pattern indicates that the material is amorphous (c) EDAX microanalysis indicates 100% C. DLC deposited on a glass sub-
strate at 0.004 mbar, 1500 Å, 100 oC, 200 W
Copyright © 2009 SciRes IIM
0200 400 600 80010001200
Sliding Distance (M)
Fri cti o n Co effi cient ( µ)
Figure 18. Coefficient of friction of (DLC/TiN/Ti/Cu/Ni) multilayer coated on WC substrate obtained using pin on disc wear
test at 200 W, 0.004 mbar, 400 oC, each layer deposited for a duration of 30 minutes
1000 1250 1500 17502000
sp2 carbon (1580 cm-1 )
Microcrystalline sp3
carbon(1335cm-1 )
DLC/TiN/Ti/Cu/Ni coating: 400
C, 0.004mbar, 30m in.
250 W
Intensity (arbitary unit)
Raman shift (cm -1)
Figure 19. Raman Spectroscopy output showing DLC characterization. multilayer (DLC/TiN/Ti/Cu/Ni) coated on glass sub-
strate at 400 0C, 0.004 mbar, each layer deposited for duration of 30 minutes
presented in Figure 17(c), which indicates the element of
carbon present in the film. No sharp diffraction rings in
the diffraction pattern suggest that the material is in
amorphous form. Thus, it can be concluded that the
coating investigated in the present case is an amorphous
diamond like carbon [44].
4.6. Pin on Disc Wear Test
The variation of coefficient of friction (µ) of the multi-
layer (DLC/TiN/Ti/Cu/Ni) coating on WC substrate with
sliding distance is shown in Figure 18. DLC coatings
were amorphous, continuous, and pore free and had no
grain boundaries, which resulted in a very smooth sur-
face. The coefficient of friction values were continuously
recorded and the output was obtained during the entire
rotation cycle of the disc (Figure 18).
Initially an increase in coefficient of friction was ob-
served. This is due to the anchoring between micro pro-
jections present on the substrate surface and disc surface
and subsequently this was worn out. But further it was
seen that coefficient of friction reached to a steady state
Copyright © 2009 SciRes IIM
level. The coefficient of friction is maintained in the
range 0.2 - 0.3 in all the cases matching with the results
of earlier researchers.
4.7. DLC Characterization by Micro Raman
Figure 19 shows Multilayer DLC/TiN/Ti/Cu/Ni coating
on glass substrate at 400 oC, 0.004 mbar, deposited for
duration of 30 minutes. The Raman spectrum of DLC
films deposited on the glass substrate was smooth and
dense. The spectra display the characteristic Raman dia-
mond peak (sp3 hybridized carbon atoms) positioned at
1,355 cm-1. In addition, a broad band indicates the pres-
ence of amorphous carbon phases (sp2 hybrid carbon
atoms) observed at around 1,580 cm-1.
It is observed that, at 250 W, sp3 hybrised car bon bond
formation is not significant. The Figure 5 shows the mi-
cro hardness variation with target power, which clearly
indicates that the micro hardness decreased at 250 W.
This is due to the penetration of argon ions into the target
and the consequent decrease in energy of atoms depos-
ited at the target surface. All these put together decrease
the sputtering yield and a decrease in the number of at-
oms ejected from the target surface resulting in low sur-
face mobility and low denser deposition on substrate
surface. Therefore, sp3 formation is restricted which gov-
erns the increase in micro hardness. At 150 W, 250 W,
sp3 formation is significant, therefore maximum micro
hardness is observed, wh ich is clearly seen in Figure 5
5. Conclusions
Based on a comprehensive experimental investigation
carried out on coating of DLC/TiN/Ti/Cu/Ni multilayer
using d.c magnetron sputtering, the following observa-
tions are presented.
Surface preparation and cleanliness are very important
requirement for achieving better adhesion. The micro
hardness of coating is the major mechanical property that
influences the life and high performance of a surface
engineered cutting tool.
The DLC films deposited are fully amorphous, smooth,
and pore free and did not show crystalline structure as
evidenced by TEM images.
Uniform, continuous, crack free and pore free and
strongly adhered multilayer coating could be success-
fully deposited on WC tool substrate using d.c magne-
tron sputtering.
The sputter deposition conditions for DLC/TiN/
Ti/Cu/Ni multilayer coatings are identified to achieve
improved quality with particular reference to adhesion
and surface finish.
Due to the limitation of the sputtering deposition
equipment the coating of the cutting tools and their sub-
sequent machining performance studies could not be
carried out in this work. Therefore, with sophisticated
coating equipment, there is a wide scop e for furthe r work
to carry out different combinations of layers of coating
and machining studies.
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