34th INTERNATIONAL CONFERENCE ON
PRODUCTION ENGINEERING
28. - 30. September 2011, Niš, Serbia
University of Niš, Faculty of Mechanical Engineering
INFLUENCE OF SUBSTRATE ROUGHNESS ON ADHESION STRENGTH OF HARD TiN
FILMS
Damir Kakaš, Branko Škorić, Aleksandar Miletić, Pal Terek, Lazar Kovačević, Marko Vilotić
Faculty of Technical Sciences, University of Novi Sad, Trg D. Obradovica 6, Novi Sad, Serbia
kakasdam@uns.ac.rs, miletic@uns.ac.rs
Abstract: Ion beam assisted deposition was used to prepare hard TiN coatings. In order to provide
appropriate adhesion of TiN film, a titanium nano-interfacial layer was introduced between coating and
base material. The interfacial layer was produced by ion beam mixing of titanium atoms with atoms of
steel substrate. The influence of substrate surface roughness on adhesion properties of TiN coatings was
studied in this research.
Adhesive strength of the coatings was characterized by commonly used scratch test technique. Coating
adhesion and toughness were evaluated qualitatively by HRC adhesion test. Optical microscopy was
implemented in order to examine channels generated during scratch test. Atomic force microscopy was
employed to determine sample roughness prior and after the deposition of TiN films. It has been found
that critical load of coating detachment increases with increase in substrate roughness.
Key words: Adhesion strength, Roughness, TiN, IBAD
1. INTRODUCTION
Hard coatings are used to improve surface properties of
components in a great number of applications, e.g. plastic
injection moulding, die casting tools, cutting and forming
tools, and others.
The most important parameter in practical application of
coated elements is adhesion between a coating and a
substrate. Regardless of its importance, there is no widely
accepted characterization technique for quantitative
measurement of adhesion strength of thin coatings.
Adhesion is usually evaluated by generally accepted
scratch test technique. During the scratch test a diamond
tip is dragged over the surface with the normal force
increasing linearly with the traveled distance. The critical
load, the load at which the coated film is removed from
the substrate, is influenced by many factors such as film
thickness, substrate hardness, substrate roughness,
interface bonding, and intrinsic properties of the deposited
film [1-3].
Thin hard coatings can be produced by great number of
different deposition techniques. Among them ion beam
assisted deposition distinguishes as a promising to
produce highly adherent films [4,5]. This technique
combines physical vapor deposition with ion beam
bombardment of growing films. Careful selection of
process parameters such are the ion angle of incidence,
ion energy, ion current and ion to atom ratio provides a
possibility to prepare thin film with desired properties.
This article presents the study of the effect of substrate
roughness on the adhesion of TiN coatings deposited on
carburized steel substrates. The references reporting on
influence of substrate roughness on adhesion of hard
coatings are scarce. While Bromark et al [6] did not find
any significant influence several researches [7-9] have
shown a decrease of the critical load with increase in
substrate roughness.
2. EXPERIMENTAL
TiN coatings were produced by using the Ion Beam
Assisted Deposition (IBAD) technique. The used IBAD
system is equipped with a Kaufmann ion source and an ebeam evaporation source. All depositions were performed
in a system with a base pressure of 1.5x10-6 mbar. The
coatings were deposited in mixed Ar and N2 atmosphere
with partial nitrogen pressure between 1.1 and 1.2•10 -5
mbar, while substrate temperatures were kept at 400 °C.
Hot Work Tool Steel (X38CrMoV5-1) was used as a base
material. Substrate roughness was varied by varying
grating of the sand paper used during grinding procedure.
The studied samples were denoted as sample 400, sample
1500, and sample 2000 where substrates were prepared
using 400, 1500, and 2000 grit silicon carbide sandpaper
respectively. The sample 2000 was additionally polished
by using a 3 μm diamond polishing paste. Prior to
deposition, all substrates were sputter-cleaned by using an
argon ion beam.
In order to provide appropriate adhesion a Ti interfacial
was introduced between TiN film and steel substrate. The
layer was produced by ion beam mixing of atoms of base
material with titanium atoms.
A scratch test was employed to characterize adhesion
properties of TiN coatings. During the test, the diamond
tip moved at a velocity of 10 mm/min, the load applied
was progressively increased from 1 to 100 N at rate of
100 N/min over the scratching distance of 10 mm.
Optical microscopy was used to identify failures along
scratch channels and to determine the critical loads at
which these failures occur.
Additionally, the friction coefficient curves were used to
determine the critical load for coating detachment.
In addition to scratch test, adhesion was quantitatively
evaluated by standard HRC adhesion test. Rockwell
indentations were made by applying 140 kg force.
Atomic force microscopy (AFM) was applied to measure
sample surface roughness prior and after the deposition.
All images were acquired in contact AFM mode using a
symmetrically etched silicon-nitride probe. The scan area
was 90x90 µm2 while scan rate and set point were kept at
0.5 Hz and 225 nN respectively.
3. RESULTS AND DICUSSION
Table 1. Surface roughness before and after the coating
deposition
Sample
Sa (nm)
Uncoated
Coated
Sq (nm)
Uncoated Coated
400
1500
25
11
38
15
33
15
48
19
2000
3
9
4
12
The surface roughness was characterized by arithmetic
mean (Sa) and root mean square roughness (Sq). The
values of roughness parameters are given in Table 1.The
AFM images of samples with and without TiN coating are
shown in Figure 1.
It can be seen from the Table 1 that surface roughness of
all samples increases after applying TiN coating. Change
in surface roughness can be attributed to several effects
directly connected with ion bombardment, among which
are adatom mobility, sputtering and ion incorporation.
There is a great number of parameters which influence the
roughness after the deposition such as the ion beam
energy [10,11], ion to atom arrival ratio [11], film
thickness [12-14], grain size [13,14], coating texture
[12,14], and others. In this study all process parameters
were kept constant for all samples. The coating thickness
was around 1 μm for all samples; therefore it should not
have influence on surface roughness of studied coatings.
During the deposition growing film was bombarded by
argon ion beam with energy of 1keV. Such ion
bombardment induces enough energy into growing film to
enhance the mobility of the surface atoms. Increased
adatom mobility leads to aggregation of large crystal
grains which results in increased surface roughness [15].
The surface topography of uncoated and coated samples
has been imaged by using an atomic force microscope.
Fig.1. AFM images of samples 400 and sample 2000
In order to find a relation between substrate roughness
and coating adhesion all samples were subjected to
scratch test. During this test a critical load which leads to
coating failure is determined. In the present study three
critical loads were identified: Lc1 - is the normal load at
which the first chipping of the film occurs, Lc2 - is the
normal load at which the substrate is exposed for the first
time, and Lc3 - is the normal load at which the complete
removal of the film occurs. Optical microscopy of
generated scratch channels was used to determine all three
critical loads. In addition, friction and acoustic emission
curves (Figure 2) were examined in order to evaluate the
critical load of complete coating detachment.
Curved, concentric, through - thickness cracks extending
to the edge of the scratch track or beyond are typical for
this type of failure [16]. Usually chipping failure occurs
around previously formed crack in the region of high
tensile stress within the coating. However, there was only
minor chipping formation on the two smoother samples,
while there was no coating chipping on the roughest
sample (sample 400). First coating delamination
(substrate exposure) occurs earlier on the smooth samples,
and is present on a much larger scale comparing to the
roughest sample.
a)
b)
Fig.2. Friction and AE curves recorded during scratch
tests a) sample 400, b) sample 2000. Vertical lines
indicate the critical loads (Lc)
Figure 3 shows critical loads Lc1, Lc2 and Lc3 in a
function of the substrate roughness. There was no
formation of chipping failure along the scratch channel of
the sample 400; therefore the critical load Lc1 for this
sample is not presented in the Figure 3. The highest
critical load Lc3 of 71 N was recorded for the coating
deposited on the roughest substrate. Decreasing trend of
the critical load with decrease in surface roughness was
observed.
Fig.4. Optical micrographs of scratch tracks of TiN
coatings, a) sample 400, b) sample 2000
High adhesion of studied coating was confirmed by
employing standard HRC adhesion test. Optical
micrograph of produced indents is shown in Figure 5.
There was no visible coating delamination for all samples.
According to the HRC test, there are six grades of
adhesion strength quality HF1 - HF6 (HF1 being the
best). All samples were characterized by high adhesion
strength of HF2. High adhesion might be explained by the
presence of ion beam mixed layer between TiN coating
and steel substrate.
a)
b)
Fig.3. Critical load of TiN coatings in function of
substrate roughness
Optical micrographs of generated scratch channels are
presented in Figure 4. All samples show remarkable
adhesion, with damage localized to do edge of the scratch
track. Buckling failure mode was observed for the
samples. Buckling failures appear as a result of the
compressive stress field in front of the moving slider.
Fig.5. HRC indents made on TiN coatings applying
standard 140 kg load, a) sample 400, b) sample 2000
The increase of the critical load with an increase of
substrate roughness can be explained by different
deformation modes of samples with different surface
morphology. Rougher coating resists a greater amount of
plastic deformation before formation of the damage. As
substrate roughness increases, the amount of energy
required for plastic deformation of surface asperities
increases as well. Therefore, higher normal load is
required to develop the same type of failure on rougher
samples. The increase of roughness induces growth of
interface surface area resulting in lower shear stress
values and an apparent increase in adhesion strength [17].
The adhesion trend found in this study is contrary to the
results reported by other authors [7-9] who studied
influence of substrate roughness on coating adhesive
properties. According to cited articles, the ridges on rough
substrates generate the stress concentration which leads to
easier crack formation and to decrease in adhesion.
The different trend can be explained by the difference in
absolute roughness of studied samples. Comparing to
cited articles this study was conducted on generally
smooth substrates. Apparently, there is an upper value of
roughness after which coating adhesion starts to decrease.
4. CONCLUSION
The surface roughness of all studied samples increases
after applying TiN coating.
High scratch resistance was observed for all samples.
Evidently, the ion beam mixed interlayer introduced
between the coating and steel substrate plays important
role in increasing the adhesion of TiN coatings.
Increase of adhesion strength with increase in substrate
roughness was found. This is contrary to previous
findings from studied area. It appears that there exists a
peak roughness after which adhesion starts to decay.
In order to confirm observed adhesion trend further
investigation should be directed to other types of hard
coatings applied to samples with surface roughness from
larger domain.
5. ACKNOWLEDGMENT
This research was supported by the Serbian Ministry of
Science and Technological Development which authors
gratefully acknowledge.
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CORRESPONDENCE
Damir KAKAŠ, prof.dr, Faculty of technical sciences,
Trg D. Obradovića 6, Novi Sad, kakasdam@uns.ac.rs
Aleksandar МILETIĆ, MSc, Faculty of technical
sciences, Trg D. Obradovića 6, Novi Sad,
miletic@uns.ac.rs