In this study, micro-scratch tests were conducted on a diamond-coated tungsten-carbide substrate to investigate the coating adhesion. High intensity Acoustic Emission (AE) signals were detected once the coating delamination initiated during the scratch test. It has also been found that the tangential force increased gradually with the normal force, but varied significantly when the critical load of coating delamination was reached. A finite element (FE) model with a cohesive-zone interface was developed to simulate the scratch process and the coating delamination phenomena. The preliminary results indicate that it is feasible to use the FE combined with scratch tests to evaluate the coating interface characteristics.

1. Proceedings of NAMRI/SME, Vol. 39, 2011
Micro-Scratch Testing and Simulations for Adhesion
Characterizations of Diamond-coated Tools
Ping Lu
Mechanical Engineering Department
The University of Alabama
Tuscaloosa, Alabama, USA
Xingcheng Xiao and Michael Lukitsch
Research & Development Center
General Motors Corporation
Warren, Michigan, USA
Kevin Chou
Mechanical Engineering Department
The University of Alabama
Tuscaloosa, Alabama, USA
ABSTRACT
In this study, micro-scratch tests were conducted on a diamond-coated tungsten-carbide substrate to investigate the coating
adhesion. High intensity Acoustic Emission (AE) signals were detected once the coating delamination initiated during the
scratch test. It has also been found that the tangential force increased gradually with the normal force, but varied significantly
when the critical load of coating delamination was reached. A finite element (FE) model with a cohesive-zone interface was
developed to simulate the scratch process and the coating delamination phenomena. The preliminary results indicate that it is
feasible to use the FE combined with scratch tests to evaluate the coating interface characteristics.
KEYWORDS
Adhesion, Cohesive zone interface, Diamond-coated tool, Delamination, Finite element, Scratch testing.
INTRODUCTION
Applying hard coatings such as diamond on cutting
tools is an ideal approach for enhancing tool lifetime and
improving the machining quality due to diamond’s high
hardness/strength, low friction coefficient, and chemical
stability, etc. Diamond-coated tools excel in cost and
flexibility to various tool geometry and size comparing to
synthetic polycrystalline diamond (PCD) tools (Hu, 2007),
which are also commonly used in the manufacturing
industry. Thus, diamond-coated cutting tools have a
potential to replace costly polycrystalline diamond tools.
However, in tooling applications, diamond coating
delamination remains the primary wear mode that often
result in catastrophic tool failures (Lu, 2009, Qin, 2009).
Coating delamination is due to insufficient adhesion
between the coating and substrate. Many approaches have
been developed to enhance the coating adhesion such as
etching cobalt (Co) phase off from the tool surface before
diamond depositions. In addition, reliable experimental
methods to quantitatively characterize the coating adhesion
are critical to compare and further optimize different
interface approaches.
There are several methods used to examine the adhesion
of coatings (Perry, 1983). For example, Bouzakis et al.
applied an inclined impact test to evaluate the cohesion and
adhesion properties of thin coatings (Bouzakis, 2004).
Scratch testing is one of the most practical means of
evaluating the adhesion of a hard, thin coating on a
substrate (Bull, 1992, Ollendorf, 1999), since it is reliable,
simple to perform and no special specimen shape or
preparation are required. Adhesion is measured when a
critical load is reached at which coating failure occurs.
Provided that the failure is adhesive, this critical normal
load is taken as a measure of the coating–substrate
adhesion, or the work of adhesion is derived from the

2. Proceedings of NAMRI/SME, Vol. 39, 2011
critical normal load (Jaworski, 2008, Nakao, 2007). Figure
1 is a schematic illustrating a scratch test on a coated
specimen. During scratch testing, a spherical indenter tip
slides over the surface of the coating to generate a groove
under incremental or constant normal loads. The tangential
force is measured during the test and the morphology of the
scratches can be observed simultaneously or afterwards.
When the mean compressive stress over an area in the
coating exceeds a critical value, the coating detaches from
the substrate to lower the elastic energy stored in the
coating (Liu, 2009). The work of adhesion at the interface
between the coating and substrate is equal to the energy
release rate from coating detachment and this rate is a
function of the mean compressive coating stress over the
detaching area at the instant of detachment. Thus, the
critical mean compressive coating stress responsible for the
detachment could be a measure of coating–substrate
adhesion. On the other hand, diamond coatings are very
brittle. While a coating can withstand compressive stresses
induced by the indenter to a certain extent, it may fracture if
a high tensile or shear stress field is induced
simultaneously, in particular, at the interface such as
delamination (Xie, 2002).
Figure 1. A schematic illustrating a scratch test on a coated
specimen (http://www.pvd-coatings.co.uk/scratch-adhesion-
tester.htm).
This study aims at better understanding the adhesion of
diamond-coated carbide tools by micro-scratch testing;
critical load for coating delamination and the corresponding
process signals were focused. In addition, a finite element
(FE) model was developed to simulate the scratch process,
with emphasis on the interface behavior, and to evaluate the
adhesion between the coating and substrate using interface
characteristics, which can be further applied in studies at
different loading conditions such as cutting.
EXPERIMENTAL DETAILS
The specimen was a diamond-coated tool. The substrate is
co-cemented tungsten carbide (WC-Co), submicron grains,
6% Co, with coating thickness around 4 µm, provided by
University of South Florida. The surface roughness of the
samples was about 0.38 µm Ra. A Micro-scratch tester
from CSM Instruments, model Micro-Combi, was used for
the experiments at the room temperature. Testing was
conducted in typical laboratory temperature/humidity
environments,
The diamond indenter with a tip radius of 50 µm was
utilized. The scratch speed was 2 mm/min with the
progressive loading method. The scratch length for each
test was 5 mm. During the scratch test, tangential forces,
acoustic emission (AE) signals, and the depth of the scratch
were acquired. A KEYENCE digital microscope (VHX-
600X) was used to observe the scratch marks and
delamination after the test. In addition, a white-light
interferometer (WLI) was used to acquire the morphology
of scratch grooves. To determine the critical load for the
tested diamond-coated tool, scratch tests were carried out
with a progressive load, maximum normal force of 10 N, 15
N, 20 N, 25 N and 30 N. Figure 2 below shows the overall
images of 5 scratch grooves at the corresponding load (1:
10 N, 2: 15 N, and 3: 20 N, etc.)
Figure 2. Digital microscopic images of scratch grooves on
the sample.
RESULTS AND DISCUSSION
Figure 3 shows the AE signal and tangential force (Ft)
vs. the applied normal load (Fn) during the scratch test with
a progressive load of maximum 10 N. From the figure, it is
observed that the tangential force increases smoothly when
the normal force is less than 5.4 N, but varies considerably
once the load exceeds 5.4 N. It is also observed that an
abrupt amplitude increase of AE signals (Spot 1) exists at
the load around 5.4 N, followed by a series of continuous
high-amplitude AE peaks. This implies coating
delamination initiation occurred around 5.4 N.

3. Proceedings of NAMRI/SME, Vol. 39, 2011
Figure 3. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum load of 10 N.
After testing, the scratched groove was observed in the
digital microscope at 500X. From the relation between the
load and the distance, the location corresponding to the high
force variation and sudden intensity increasing (Spot 1) can
be examined and verified whether coating delamination has
initiated at that point. Figure 4a shows the digital
microscopic image at Spot 1 and the corresponding normal
load is around 5.4 N. It can be clearly confirmed that
coating delamination has initiated at such a force, clearly
exposing the substrate layer of WC, near Spot 1. Figure 4b
shows the digital microscopic image at the end of scratch
test. It is shown that coating delamination continued, once
initiated, to the end of the final load, with a comparable
delamination width.
Figure 4. The digital microscopic images of (a) Spot 1
around 5.4 N, and (b) the end of the scratch for scratch test
under maximum load of 10 N.
For the conditions of different final normal loads, the
tangential force/AE data show similar behaviors as the 10 N
load, i.e., gradual increasing of tangential force and AE
signals followed by an abrupt jump, and followed by large
variations of force/AE magnitudes. For example, Figure 5
shows the AE signals and tangential force (Ft) vs. the
normal load (Fn) during the scratch with a 30 N final load.
Spot1
(b)
(a)

4. Proceedings of NAMRI/SME, Vol. 39, 2011
Figure 5. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum scratch load of 30 N.
Further, using the approach described earlier, the
corresponding critical load of delamination for each
condition can be estimated. It is found that the critical load
of delamination initiation is around 4 to 6 N for all tested
loads, indicating that the delamination critical load is not
sensitive to the load rates currently applied. Figure 6a
shows the digital microscopic image at Spot 1 (for 30 N
load scratching) with the normal load around 3.9 N. It is
clearly seen that coating delamination has been initiated at
this point (near Spot 1), an area of coating layer detached
from the WC substrate. Figure 6b shows the digital
microscopic image at the end of scratch test (maximum
load 30 N). It is also noted that coating delamination
continues spreading at the end of the scratches, and the
delamination width becomes larger compared to at the
beginning of delamination. Therefore, the coating
delamination becomes more severe with the increased
normal load. From the scratch test results, the evaluated
diamond-coated tool with a coating thickness of 4 µm has a
critical load for delamination in the range of 4 to 6 N.
Figure 6. Digital microscopic images of (a) Spot 1 around
3.9 N, and (b) the end of the scratch (30 N load).
The white-light interferometer has also been used to
obtain quantitative information of the scratched geometry.
Figure 7 below shows a 3D surface contour image of part of
the scratched track, at ~3 mm distance from beginning (20
N maximum load). It clearly shows coating delamination
propagated outward. Figure 8 is the 2D profile analysis of
the scratched mark at the same location. It is estimated that
the delamination width is about 88 µm and the depth of the
track is about 7 µm.
Figure 7. WLI image (3D) at 3 mm scratch distance for 20
N maximum load, showing coating delamination.
Spot1
(a)
(b)

5. Proceedings of NAMRI/SME, Vol. 39, 2011
Figure 8. 2D scratch track profile analysis at 3 mm scratch distance for 20 N maximum load.
SCRATCH PROCESS SIMULATIONS
A 3D FE model has been developed to study the micro-
scratch test by using ABAQUS 6.8. A half model is used
because of the symmetry of the problem. The geometric
model of the specimen shown in Figure 9a is 3000 µm in
length and 200 µm in width. The thickness of the diamond
coating and the carbide substrate is 4 µm and 60 µm,
respectively. The diamond indenter is assumed to be rigid
with a tip radius of 50 µm. Figure 9b shows the details of
the model around the indenter. The smallest mesh size
underneath the indenter is 0.6 µm for the coating and 5.0
µm for the substrate. Further, a cohesive layer with zero
thickness was included to model the mechanical behavior of
the interface between the coating and the substrate.
The materials of the specimen, both the coating and the
substrate, were modeled as an elastoplastic solid with
isotropic hardening. According to the Ramberg-Osgood
law, the stress-strain relation can be described as
,
where , and the strain-hardening exponent is
generally in the range of 0 to 0.5 (Xu, 2005). The yield
strength of WC-6wt%Co is in the range of 1.45 to 5.76 GP
(Renaud, 2009, Mittal, 2001), and the yield strength of the
WC-Co is assumed as 3.605 GPa in this study. Other
properties of WC are identical with those in a previous
study (Renaud, 2009). The yield strength and hardening
exponent of the diamond can be obtained by the method
proposed by Suresh (Giannakopoulos, 1999, Ramamurty,
1999, Venkatesh, 2000) based on the experimental data of
nanonindentation on diamond (Chowdhury, 2004). In this
study, the Young’s modulus of the coating is 1200 GPa,
other properties of the coating and substrate are listed in
Table 1. The friction coefficient was set as 0.1. The
cohesive zone properties such as the strength and
characteristic length are derived from a previous study (Hu,
2008) with adjusted property values: 543 MPa maximum
strength, 0.43 µm characteristic length and 117 J/m2
fracture energy density, to approximate the critical load
observed from the experiments.

6. Proceedings of NAMRI/SME, Vol. 39, 2011
Figure 9. 3D FEA model for the scratch test: (a) overall model, and (b) details around the indenter.
Table 1. Properties of the substrate and coating for the diamond-coated cutting tools
Part Material
Young’s
modulus/GPa
Poisson’s
ratio
Yielding
strength/GPa
Hardening
exponent
Substrate
WC-6 wt.
% Co
619.5 0.24 3.605 0.244
Coating Diamond 1200 0.07 26.7 0.23
Figure 10 shows the normal force and tangential force
development along with the indenter location from the
scratch simulation. The spherical indenter first moves
downward in contact with the coating, then slides along the
surface of the coating, at a speed of 500 µm/s, with a
linearly increased depth, and the sliding distance is 500 µm.
At the end, the slider moves upward until fully unloaded.
Figure 10. Normal and tangential forces at different
locations for the maximum load of 14 N.
Figure 11 shows (a) the maximum principal stress and
(b) the equivalent plastic strain in the specimen, around the
indenter, during the initial sliding period, corresponding to
around 2 N of the normal force. It can be seen that high
compressive stresses, around 6 GPa, occur near the indenter
and the maximum equivalent strain is 0.024. In addition,
there is a small area with tensile stresses. As to be shown
later, no delamination has been generated at this location.
Figure 12 shows (a) the maximum principal stress and (b)
the equivalent plastic strain in the specimen at the onset of
coating delamination. It can be observed that high
compressive stresses, around 20 GPa, occurred in the
coating near the indenter and the maximum equivalent
strain is 0.082. Moreover, high tensile stresses occur in the
wake of the slider. The corresponding load at this location
is about 4 N. And the delamination has just initiated, to be
shown later. Figure 13 shows (a) the maximum principal
stress and (b) the equivalent plastic strain in the specimen at
the end of the sliding period. It can be seen that
compressive stresses are as high as around 25 GPa,
occurred in the coating near the indenter and the maximum
equivalent strain is 0.34.
Indenter
Coating
substrate
Cohesive zone
(b)
(a)

7. Proceedings of NAMRI/SME, Vol. 39, 2011
Figure 11. (a) Maximum principal stress (unit: 1000 GPa)
and (b) Equivalent plastic strain at the initial scratching.
Figure 12. (a) Maximum principal stress (unit: 1000 GPa)
and (b) Equivalent plastic strain at the onset of
delamination.
Figure 13. (a) Maximum principal stress (unit: 1000 GPa)
and (b) Equivalent plastic strain at the end of scratching.
(a) (b)
(a)
(b)
(a)
(b)

8. Proceedings of NAMRI/SME, Vol. 39, 2011
Figure 14 illustrates the normal stress distribution in the
cohesive-zone layer at different times: (a) initial scratch, (b)
onset of delamination and (c) end of scratch. At the initial
period, though the normal stress of the cohesive zone has
reached the cohesive zone strength (543 MPa), the damage
evolution has not resulted in the cohesive zone failure.
Thus, the interface is still intact. On the other hand, at
around 4 N of normal load, the cohesive-zone layer has
shown some damage initiated; the cohesive zone normal
separation has reached its characteristic length (0.43 µm).
Figure 14c shows a large delamination area at the end of
scratch testing. The coating delamination crack has spread
in both the sliding and transverse directions.
Figure 14. Cohesive zone stress distribution (unit: 1000 GPa) at the interface: (a) initial scratch, (b) onset of delamination,
and (c) end of scratch.
Figure 15 further shows the cohesive-zone normal stress
vs. normal separation traced along the scratch time at two
different locations. Figure 15a is for a location where the
interface remains attached. The normal stress is first
compressive when the slider is approached to that location
and quickly changes to tensile and reaches the interface
strength. Then, the stress is reduced in a short time, but with
the normal separation increased. Finally, as the slider
moves away from this location, the stress is reduced, also
the separation, but does not return to zero at the end. On the
other hand, Figure 15b is for a location where the interface
has been delaminated. The normal stress vs. separation
curve follows the typical traction-separation behavior and
the normal separation has reached the maximum separation
and results in the interface failure. In a previous study of
(a) (b)
(c)

9. Proceedings of NAMRI/SME, Vol. 39, 2011
diamond-coated carbides (Hu, 2008), the ideal cohesive
zone properties (as an upper bound) were estimated from
the substrate (WC) toughness. For the current study, note
that the cohesive-zone properties have been modified (1/3
of the upper-bound fracture energy) to approximate the
critical load measured from the experiments. Therefore, the
estimated cohesive-zone characteristics may be used to
indicate the interface capacity related to the delamination
behaviors.
Figure 15. Normal stress vs. separation behavior along the
scratch time: (a) location without delamination, and (b)
location with delamination.
Note that, it is known that during machining, the
temperature will be substantially higher than the room
temperature testing conducted here. The temperature
increases may affect coating properties and the deposition
thermal stress (Bouzakis, 2010). However, as the first step,
this study attempted to establish the room-temperature
adhesion data for general evaluation purposes.
CONCLUSIONS
Scratch tests of diamond coatings deposited on a WC
substrate have been carried out using a micro-scratch tester.
During the scratch tests, the normal force, the tangential
force, the acoustic emission signals and the penetration
depth were acquired. After scratch tests, the scratch marks
were also observed in a digital microscope and analyzed by
a white-light interferometer. Moreover, to characterize the
interface mechanical behavior, an FE model including the
interface cohesive zone was developed to simulate the
scratch process. The results are summarized as the
following:
(1) Coating delamination can be clearly detected by AE
signals. It was observed that the abrupt AE peak jumps
followed by several continuous AE high-amplitude
peaks are associated with coating delamination. The
tangential force increases smoothly with the normal
force before the initiation of coating delamination, but,
varies considerably once coating delamination
initiated. Therefore, tangential force may also be used
to monitor the coating delamination during scratch
tests.
(2) The width of coating delamination would increase with
the increased loads after delamination initiated, this is
confirmed by the scratch images under digital
microscope, which show that the coating delamination
becomes more severe with the increased load.
(3) The critical load for the tested diamond-coated WC
tools was in the range of 4 to 6 N, which has been
confirmed by repeated tests.
(4) The simulation results indicate that it is feasible to use
the FE model combined with scratch tests to evaluate
the coating interface characteristics.
ACKNOWLEDGEMENTS
This study was conducted under an NSF-funded joint
project (CMMI 0928627) - GOALI/Collaborative Research:
Interface Engineered Diamond Coatings for Dry
Machining, between The University of Alabama, General
Motors and University of South Florida.
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