This paper investigates the effects of different surface pretreatments on the adhesion and performance of CVD diamond coated WC–Co turning inserts for the dry machining of high silicon aluminum alloys. Different interfacial characteristics between the diamond coatings and the modified WC–Co substrate were obtained by the use of two different chemical etchings and a CrN/Cr interlayer, with the aim to produce an adherent diamond coating by increasing the interlocking effect of the diamond film, and halting the catalytic effect of the cobalt present on the cemented carbide tool. A systematic study is analyzed in terms of the initial cutting tool surface modifications, the deposition and characterization of microcrystalline diamond coatings deposited by HFCVD synthesis, the estimation of the resulting diamond adhesion by Rockwell indentations and Raman spectroscopy, and finally, the evaluation of the dry machining performance of the diamond coated tools on A390 aluminum alloys. The experiments show that chemical etching methods exceed the effect of the CrN/Cr interlayer in increasing the diamond coating adhesion under dry cutting operations. This work provided new insights about optimizing the surface characteristics of cemented carbides to produce adherent diamond coatings in the dry cutting manufacturing chain of high silicon aluminum alloys.

1. Journal of Materials Processing Technology 212 (2012) 523–533
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology
journal homepage: www.elsevier.com/locate/jmatprotec
Adhesion analysis and dry machining performance of CVD diamond coatings
deposited on surface modified WC–Co turning inserts
Humberto Gomeza,e
, Delcie Durhama
, Xingcheng Xiaob,∗
, Michael Lukitschb
, Ping Luc
, Kevin Chouc
,
Anil Sachdevb
, Ashok Kumara,d,∗∗
a
Department of Mechanical Engineering, University of South Florida, Tampa, FL 33620, USA
b
Chemical Sciences & Materials Systems Laboratory, General Motors R&D Center, 30500 Mound Road, Warren, MI 48090, USA
c
Mechanical Engineering Department, The University of Alabama, Tuscaloosa, AL 35487, USA
d
Nanotechnology Research and Education Center (NREC), University of South Florida, Tampa, FL 33620, USA
e
Departamento de Ingeniería Mecánica, Universidad del Norte, Barranquilla, Colombia
a r t i c l e i n f o
Article history:
Received 11 October 2011
Accepted 21 October 2011
Available online 26 October 2011
Keywords:
Diamond
Adhesion
Dry machining
Surface modification
a b s t r a c t
This paper investigates the effects of different surface pretreatments on the adhesion and performance
of CVD diamond coated WC–Co turning inserts for the dry machining of high silicon aluminum alloys.
Different interfacial characteristics between the diamond coatings and the modified WC–Co substrate
were obtained by the use of two different chemical etchings and a CrN/Cr interlayer, with the aim to
produce an adherent diamond coating by increasing the interlocking effect of the diamond film, and
halting the catalytic effect of the cobalt present on the cemented carbide tool. A systematic study is
analyzed in terms of the initial cutting tool surface modifications, the deposition and characterization
of microcrystalline diamond coatings deposited by HFCVD synthesis, the estimation of the resulting
diamond adhesion by Rockwell indentations and Raman spectroscopy, and finally, the evaluation of the
dry machining performance of the diamond coated tools on A390 aluminum alloys. The experiments
show that chemical etching methods exceed the effect of the CrN/Cr interlayer in increasing the diamond
coating adhesion under dry cutting operations. This work provided new insights about optimizing the
surface characteristics of cemented carbides to produce adherent diamond coatings in the dry cutting
manufacturing chain of high silicon aluminum alloys.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
There is an uprising trend in the dry machining of composite
metals such as aluminum–silicon alloys and aluminum matrix
composites as the result of the environmental impact of coolants
used in traditional wet machining operations. The amount of
coolants disposed in the form of mist, waste, and coolant-coated
chips have been reported to produce a significant harmful effect to
the environment (Adler et al., 2006). The estimated global market
of over $1500 million in 2007 (increasing 6% annually) related
to equipment used in filtration and separation of cutting fluids
(Sutherland, 2008), provides an important reason to companies in
seeking new strategies to reduce fluids consumption by using min-
imum quantity lubrication (MQL) systems or remove them entirely
from the machining operation. Additionally, the development of
∗ Corresponding author. Tel.: +1 248 912 8132.
∗∗ Corresponding author at: University of South Florida, 4202 East Fowler Ave, ENB
118, Tampa, FL, USA. Tel.: +1 813 974 3942; fax: +1 813 974 3610.
E-mail addresses: xingcheng.xiao@gm.com (X. Xiao), kumar@usf.edu (A. Kumar).
new automated high-speed machine centers and the use of novel
composite materials in complex designs, create new challenges for
coated cutting tools that need to function under these aggressive
machining conditions.
Roy et al. (2009) found that the chemical inertness of CVD dia-
mond coatings is the key factor to enhancing the performance of
cutting tools in the dry machining of Al–Si alloys, outperforming
uncoated tools along AlON, TiC, TiB2, TiN, and Al2O3 coated tools.
Particularly, aluminum alloys are very abrasive and extremely dif-
ficult to dry machine with conventional TiN PVD coated materials
due to the formation of built-up layer (BUL) or built-up edge (BUE)
over the rake surface of the tool as concluded by Gangopadhyay
et al. (2010).
Köpf et al. (2006) discussed the initial substrate pretreatments
required to deposit adherent diamond coatings in WC–Co tools for
the machining of non-ferrous metals and fiber reinforced plastics.
However, the machining performance of diamond coated tools
is not yet robust due to a non-optimized adhesion between the
carbide tool and the deposited diamond film. The insufficient
diamond adhesion with the cutting tool substrates would render
them inadequate or lead to unpredictable behavior and even
0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2011.10.020

2. 524 H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533
possible catastrophic failure during dry cutting operations. More-
over, the adhesion and machining performance of CVD diamond
coated tools need to be optimized based upon considering the
particular manufacturing chain in terms of the substrate condition,
surface pretreatments, workpiece materials, and cutting operation
conditions (Uhlmann and Koenig, 2009).
Carbides enriched with 3–13 wt% of cobalt binder provide high
fracture toughness and are the most common substrate materials in
coated tools used for dry machining applications. However, when
diamond is deposited on these substrates, the diamond (carbon)
solubility of 0.2–0.3 wt% in cobalt degrades the adhesion of the film
by forming a graphitic layer at the interface at the conventional
CVD deposition temperatures of ∼800 ◦C, which prevents diamond
nucleation.
Several studies have been reported on enhancing the growth
mechanisms and behavior of adherent diamond films deposited on
different kinds of substrates, however, the majorities do not take
into account the practical substrate surface conditions encountered
when using commercial carbide tools existing in the market. It has
been shown by Li and Hirose (2007) that film and substrate operate
as a “composite” system and the interface between them plays an
important role in the durability of the coating. In order to optimize
the final performance of diamond coated tools under the harsh con-
ditions developed during dry machining operations, the resulting
tribological interface must be understood as a system in terms of
the fundamental coating adhesion and the wear mechanisms at the
cutting edge of the tool.
A systematic study between the shape and characteristics of the
WC–Co cutting tool, the CVD diamond deposition process, and the
dry machining parameters, was proposed by Chou et al. (2010) as
an optimal approach to achieve adherent diamond coatings for dry
drilling applications. Haubner and Kalss (2010) concluded that the
lifetime of diamond coating tools is influenced by the interaction
of many factors. As a consequence, an optimization of diamond
coated tool performance is needed for particular manufacturing
applications.
With the aim to analyze the dry cutting behavior of CVD dia-
mond coated tools in specific manufacturing chains, this study
utilized different surface pretreatments applied to commercial
WC–Co 6% turning inserts to modify their surfaces prior to the
diamond deposition.
Substrate pretreatments were focused on providing a surface
that facilitates the diamond film interlocking effect and at the same
time eliminates the effect of the cobalt by increasing nucleation
density, both of which are reported to improve film adhesion and
tool life. Two chemical etching methods and a CrN/Cr buffer inter-
layer were evaluated in the present study. The pretreated tools will
exhibit differences in their substrate surface textures and integrity,
providing different interfacial characteristics with a direct effect on
the diamond adhesion and dry machining performance.
The wear behavior and adhesion improvement of diamond by
using a Cr–N interlayer was evaluated by Glozman et al. (1999),
using a fretting test rig and compared with indentation and scratch
tests. Tribological and mechanical properties of HFCVD diamond
coatings deposited on WC–Co substrates with different Cr interlay-
ers were measured by indentation techniques and correlated with
the substrate roughness and hardness by Chou et al. (2008). Flu-
idized Bed modified Cr/CrN interlayers on WC–Co substrates were
proposed by Polini et al. (2010), as a mechanism to enhance the
diamond film nucleation by forming a highly adherent diamond
coating and evaluated by dry ‘pin-on-disk’ tribological tests.
Of particular interest is the behavior of the CrN/Cr inter-
layer when compared with chemical etching as diamond adhesion
improvement methods in dry machining conditions. In the present
study, diamond coatings were deposited using the same growth
characteristics and thicknesses (25–30 ␮m) than the commercial
microcrystalline diamond (MCD) tools found in the market. The
adhesion characteristics of the diamond coated tools were eval-
uated by indentation techniques and Raman spectroscopy, and
compared with the diamond wear failure under a particular dry
turning machining operation on A390 aluminum workpiece. It was
found that chemical etching methods surpassed the effect of the
CrN/Cr interlayer in increasing the diamond coating adhesion dur-
ing dry cutting operations. The wear failure mechanism of the
CrN/Cr interlayer on the diamond adhesion is discussed, thus, pro-
viding insights about the use and optimization of these interlayers
to increase the dry machining performance of WC–Co diamond
coated tools.
2. Materials and experiments
Commercial tungsten carbide–6 wt% cobalt square positive
angle turning inserts (WC–6%Co/SPG-422) were used as tool sub-
strates for further MCD coating depositions. Different surface
pretreatments were selected in order to evaluate the most reported
common technical approaches, which have been claimed to be able
to overcome the detrimental effect of the cobalt binder, includ-
ing the removal of the cobalt at the surface by chemical etching
(Zhang et al., 2000) and by pre-depositing an inter-diffusion bar-
rier layer (Polini and Barletta, 2008) to suppress the interaction
between cobalt and carbon.
2.1. Chemical etching
WC–Co inserts were first cleaned in an acetone ultrasonic bath
for 10 min, followed by an ultrasonic rinse for 5 min in methanol
to remove any contamination from previous processes. After that,
samples were ultrasonically treated with Murakami’s solution
(1:1:10 KOH + K3[Fe(CN)6] + H2O) for 10 min and then rinsed with
deionized water. In Method E-1, Murakami’s step was followed
by immersion of the samples in an ultrasonic bath containing 10%
HNO3 + 90% H2O2 for 60 s. Method E-2 included the initial etching
with the same Murakami’s solution followed by immersion in an
ultrasonic bath containing 3 ml of H2SO4 and 88 ml of H2O2 for 60 s.
After chemical etching, samples were ultrasonically rinsed with
deionized water and dried with nitrogen gas.
2.2. Cobalt inter-diffusion barrier interlayer
A buffer interlayer was deposited to prevent the diffusion of
carbon into the underlying cobalt binder phase and to act as a
stress relaxation layer to reduce the thermal expansion coefficient
mismatch between the diamond and the substrate material as con-
cluded by Sarangi et al. (2008). In this work, as-received WC–Co (6%)
inserts were coated with an initial layer of CrN (1.5 ␮m) followed by
a top layer of Cr (1.5 ␮m) using a commercial cathodic-arc Physical
Vapor Deposition (PVD) system. Then, the interlayers were further
roughened to promote diamond nucleation and interlocking of the
diamond film. Two methods were used to nucleate the diamond:
Method I-1 is an additional step of media blasting on the top Cr
surface during 1 min with 50 ␮m diamond particles at a pressure
of 40 psi as reported by Xu et al. (2007); Method I-2 is an additional
surface scratching process to the Cr interlayer during 60 min in the
ultrasonic bath containing a solution of 2.4 g of 50 ␮m diamond
powders dispersed in 50 ml of methanol.
The surface roughness of the substrates and the morphology
before and after each surface pretreatment were measured by a
Vecco Wyko-NT9100 white-light interferometer and a Hitachi S-
800 Scanning Electron Microscope in conjunction with an Energy
Dispersed Spectroscopy (EDS) system attached to the SEM. The
cross section before and after each treatment was prepared by dic-
ing the inserts with a refrigerated diamond saw, then followed by

3. H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533 525
Table 1
Surface modification treatments applied to the WC–Co(6%) turning inserts.
Surface denomination Treatment characteristics
As-received (AG) Ultrasound cleaning in acetone and methanol
Murakami (M) Ultrasound bath in (1:1:10 KOH + K3[Fe(CN)6] + H2O)
Method E-1 Murakami + ultrasound bath in 10% HNO3 + 90% H2O2
Method E-2 Murakami + ultrasound bath in a solution of 3 ml of
H2SO4 and 88 ml of H2O2
PVD coated (I-AG) Deposition of 1.5 ␮m of CrN and 1.5 ␮m of Cr (top)
Method I-1 PVD coated + shoot peening (diamond powder)
Method I-2 PVD coated + scratching in diamond solution
grinding and polishing the resulting surface mounted in conductive
epoxy resin with an automatic preparation Struers Prepmatic-
2 system. Supplementary SEM analysis was conducted on the
metallographically prepared samples by immersing the cross sec-
tioned samples for 5 min in a solution of Murakami reagent (10 g
K3Fe(CN)6 + 10 g NaOH in 100 mL H2O) in order to reveal the modi-
fied cemented carbide microstructure. All abovementioned surface
pretreatments are summarized in Table 1.
2.3. Diamond deposition and characterization
Commercial diamond films were synthesized in the SP3 M650
Hot Filament CVD system using hydrogen and methane as gas
precursors at a pressure of ∼40 Torr and a substrate temperature
of ∼850 ◦C to form a continuous microcrystalline diamond film
approximately 25 ␮m for all pretreated inserts. The diamond sur-
face characteristics were measured by white-light interferometer
and SEM analysis. X-ray diffraction (XRD) patterns were recorded
before and after the diamond deposition with a Bruker-AXS D8
Discover diffractometer (Cu K␣ wavelength ∼1.544 ˚A) operated at
40 kV and 40 mA. Data were collected between 25◦ and 85◦ 2Â,
using an integration time of 7 s per step and a step size of 0.01◦
2Â. Raman spectroscopy of the diamond coated-pretreated tools
was performed with a Renishaw 1000 Raman spectrometer with
an Argon laser at a wavelength of 514.5 nm, and a laser spot size
of 1 ␮m at a power of 25 mW. Rockwell C indentations were con-
ducted with a Wilson/Rockwell Hardness Tester (Series 500) to
obtain a qualitative representation of the diamond coating adhe-
sion at different load levels. The resulting diamond delaminations
from the center of the indentation zone were measured with a
Keyence VHX-500 digital microscope while the fracture patterns
were analyzed with an SEM. The film compositions in the indenta-
tion zone were determined by electron probe microanalysis (EPMA)
with a Cameca Instruments model SX100 electron probe. Electron
beam conditions were typically 15 kV and 40 nA during the analysis.
2.4. Dry machining performance test
Dry machining experiments were performed using a computer
numerical control (CNC) lathe Hardinge Cobra 42 equipped with
a Kistler dynamometer (9257B) to monitor the cutting forces dur-
ing the machining, and a Kistler 8152B piezotron acoustic emission
sensor to collect AE-RAW (raw data) and AE-RMS (root-mean-
square) values at a sampling rate of 500 kHz during the machining.
Fig. 1 shows the dry machining performance test setup. Work-
pieces were round bars made of A390 aluminum alloy. Machining
parameters were kept constant at a cutting speed of 10 m/s, feed
of 0.8 mm/rev, and a depth of cut of 1 mm. These conditions were
selected in accordance to previous experiments performed by one
of the coauthors on diamond coated tools (Hu et al., 2007a) which
represent the most aggressive parameters in terms of wear and
tool life out of a set of machining conditions on a similar work-
piece material. During the machining test, the diamond coated
inserts were periodically inspected to measure the flank wear-land
Fig. 1. Dry machining performance setup used for the MCD coated tools experi-
ments.
with optical microscopy. Worn tools after the machining tests were
cleaned with 10 vol% hydrochloric acid to remove the aluminum
alloy deposited on the cutting area and then examined by SEM and
EPMA.
3. Results and discussion
3.1. Pretreated surfaces before diamond deposition
Fig. 2 depicts the surface maps and roughness parameters
obtained by white-light interferometry corresponding to the sur-
face of as-received WC–Co (6%) commercial turning inserts (AG)
and the surface after the Murakami treatment (M), methods E-1, E-
2, I-1, and I-2, respectively. Roughness parameters values such as Rz,
Rt, Rp, and Ra were recorded and averaged from six measurements
on different top surface points. The scan size was determined by the
optical magnification of the system (50×) in a resulting surface area
of 126 ␮m × 94 ␮m containing 640 × 480 data points. These val-
ues were kept constant for all roughness measurements in order to
decrease the systematic error in the resulting roughness parameter
values as suggested by Poon and Bhushan (1995).
The surface map AG represents the as-ground original surface
depicting the directional marks (feed marks) left by the grinding
process on the surface of the tool, which also confers a non uni-
form texture in terms of the distribution of the WC grains and the
Co binder due to the amount of surface damage after the grinding
process. These feed marks display an average surface roughness
(Ra) of 0.16 ␮m measured perpendicular to them across a length
of 126 ␮m. Murakami (M) treated surfaces displayed an increased
surface roughness parameter Ra of 0.24 ␮m with a partial removal
of the original surface features. Previous experiments revealed an
increase in the surface roughness with an increase in the exposure
time in the Murakami solution; Ra values of 0.45 ␮m were obtained
for 30 min under this treatment.
The surface topography after treatment method E-1 reveals
a uniform surface with a Ra value of 0.51 ␮m, with a complete
removal of feed marks compared with Method E-2, which exhibits
some directional surface features with a corresponding Ra value
of 0.45 ␮m. The increase in roughness after the acid treatment for
methods E-1 and E-2 is attributed to the Murakami initial treat-
ment, which attacks the WC grains, thus reconstructing the initial
surface, while the final etching of the acid (for both etching meth-
ods) oxidizes the cobalt binder to soluble Co2+ compounds that
get washed out during ultrasonication (Polini, 2006). This washing

4. 526 H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533
Fig. 2. Surface maps obtained by white-light interferometry corresponding to as-received WC–Co (6%) turning inserts topography (AG), Murakami treated surface (M), and
surface textures after treatment method E-1, method E-2, method I-1, and method I-2, respectively. The surface area is 126 ␮m × 94 ␮m containing 640 × 480 data points.
action may also leach weakly bonded WC grains from the surface
promoting an additional reconstruction at the surface and creating
a new surface texture.
The effect of media blasting and diamond scratching of the
chromium after the deposition of the initial PVD CrN/Cr interlayer
are also characterized in Fig. 2. The PVD process produced a sur-
face (surface map not shown) that still displays features with a
preferential direction resulting from the conformal PVD coating
over the initial feed marks in the as-ground surface. Furthermore,
other features resulting from the original cathodic-arc PVD process
(microdroplets) were observed and will be discussed later in Fig. 4c.
Consequently, the roughness value for the PVD top chromium inter-
ferometry surface map are considerably affected by the combined
effect of these features, and are not suitable for a quantitative com-
parison purpose.
Method I-1 produced a uniform surface roughness with a Ra
parameter of 0.28 ␮m, few visible directional marks (at a bigger
surface area scale of 250 ␮m × 190 ␮m) and complete removal of
microdroplets on a microscale.
Method I-2, on the other hand, generated a surface with a Ra
value of 0.11 ␮m, with less Cr microdroplets compared with the
original PVD coating, and did not completely reconstruct the top
surface in terms of the initial as-ground feed marks, with resulting
roughness parameter values close to the original as-ground surface
characteristics, as shown in Fig. 3. The data presented in this last
figure summarizes all surface roughness parameters for each sur-
face denomination, including the maximum and minimum values
with respect to the average.
Fig. 4a–d is SEM micrographs of the substrate top surfaces and
their cross sections after metallographic preparation. Fig. 4a shows
the feed marks on the surface of the tool, while the microstructure
(insert) of the cemented carbide corresponds to the WC grains with
an average diameter of 1.4 ␮m embedded in the Co matrix. Fig. 4b
shows the surface reconstruction after pretreatment E-1 with a Co
removal depth of ∼9.5 ␮m along the cross section of the tool.
The surface modification after pretreatment E-2 is shown in
Fig. 4c, and shows a cobalt binder depletion band of ∼8.0 ␮m below
the cross section. It can be seen that both the E-1 and E-2 pre-
treatments, reconstruct the surface at the microscale in terms of
eliminating the directional features from the as-received sample
(in a rougher surface for method E-1), which are detrimental to
the final adhesion of the diamond coating due to the debonding of
the film that will occur along the preferential direction of the feed
marks. The microstructure morphology of the chemically etched
samples depicts the removal of the cobalt binder, which produces
single WC grains in the cross section of the tool for different Co
removal rates. The CrN/Cr interlayer surface deposited on top of
the as-received WC–Co inserts, including its cross section, is shown
in Fig. 4d, which depicts a uniform coating with the presence of
Cr particles (microdroplets) entrained on the surface, typical for a
cathodic arc-PVD process described by Warcholinski and Gilewicz
(2009), with heights ranging from 1.2 to 4.5 ␮m and diameters
ranging between 1.5 and 3.0 ␮m based on interferometer measure-
ments. The cross section (inset) shows the conformal deposition
and morphology of both, CrN and Cr interlayers on top of the WC–Co
substrate.
3.2. Diamond coated surfaces after the surface pretreatments
Diamond, with a coating thickness of approximately 25 ␮m,
was deposited on the different surface modified substrates.
The diamond coatings on samples corresponding to method I-2

5. H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533 527
Fig. 3. Roughness characteristics for each surface designation represented by the ten-point height (Rz), maximum peak-to-valley height (Rt), highest peak (Rp), and arithmetical
mean deviation (Ra) texture roughness parameters.
Fig. 4. (a) SEM micrograph sowing the finishing feed-marks at the surface of WC–Co (6%) as received turning inserts and the WC grains distribution (inset) in the Co binder.
(b) SEM micrograph at the surface of substrates after E-1 pretreatment including the Co depletion layer (inset) in the cross section. (c) SEM micrograph at the surface of
substrates after E-2 pretreatment including the Co depletion layer (inset) in the cross section. (d) SEM micrograph at the surface of substrates after the CrN/Cr PVD deposition
and the cross section (inset) showing the layer interfaces and morphology.

6. 528 H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533
Fig. 5. Morphology (SEM) of HFCVD grown diamond coatings deposited on surfaces
pretreated samples corresponding to methods E-1, E-2, and I-1.
delaminated from the substrate immediately after deposition,
while no delamination was observed for samples E-1, E-2, and
I-1. The surface characteristics of the various diamond coatings
are shown in SEM micrographs in Fig. 5. The diamond surface
corresponding to pretreatment method E-1 shows a continuous
diamond film with well defined polycrystal facets due to the
1 0 0 {1 1 1} texture on the surface. Optical interferometry mea-
surements showed that the grains were ∼2–4 ␮m and had an
average roughness Ra of 0.50 ␮m. The surface characteristics of
the diamond coatings on substrates from method E-2 displayed
a smaller diamond grain size ∼1–2 ␮m with a roughness Rz of
6.53 ␮m, Rt of 2.70 ␮m, Rp of 1.35 ␮m, and Ra of 0.38 ␮m. The
coating had sub-micron facetted crystals and a greater amount of
non-diamond carbon phases.
Fig. 6. XRD patterns of as-ground WC–Co (6%) samples (AG) prior to diamond depo-
sition, diamond coated sample after method E-1, diamond coated sample after
method E-2, PVD CrN/Cr coated sample (I-AG), and diamond coated sample after
method I-1.
The diamond coatings on samples I-1 consisted of a combination
of small crystal facets in ball-like agglomerated deposits, a crystal
size of ∼1 ␮m, and roughness parameter values Rz of 4.80 ␮m, Rt of
2.43 ␮m, Rp of 1.24 ␮m, and Ra of 0.31 ␮m. A comparison between
the surface morphology and roughness parameters of coated sam-
ples from the three methods E-1, E-2 and I-1 confirms that the
final roughness and morphology of the diamond coatings are cor-
related to the initial roughness and characteristics of the substrate
as conclude by Mallik et al. (2010). The higher Rz value (areal sur-
face data corresponding to the average absolute value of the five
highest peaks and the five lowest valleys) obtained in coated sam-
ples E-2 and I-1, compared with sample E-1, confirms that diamond
coatings deposited on pretreated surfaces with methods E-2 and I-1
partially contain some of the surface features related to the original
feed marks of the as-ground (AG) sample, and are visible (figures
not shown) in the interferometry results of the diamond coated
surface texture maps.
Fig. 6 shows the XRD patterns of the as-ground WC–Co (6%)
samples (AG) prior to diamond deposition, and following diamond
coating with methods E-1, E-2, PVD CrN/Cr coating (I-AG), and
after method I-1. These XRD patterns present a significant peak
due to the WC (hexagonal) {1 0 1} reflection with a characteristic
high intensity peak at 2Â = 48.2◦, which overlaps with the ␣-cobalt
(hexagonal) {1 0 1} peak normally present at ∼2Â − 47.0◦ due to the
scattering efficiency of W compared with that of cobalt, which is
only present at a level of 6% (Mallika and Komanduri, 1999). How-
ever, Co peaks corresponding to the {1 1 1} and {1 0 1} reflections

7. H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533 529
Fig. 7. The Raman spectrum of samples E-1, E-2, and I-1 after diamond deposition.
can be distinguished at 2Â = 44.4◦ and 2Â = 76.9◦, respectively. Addi-
tionally, the Co {1 1 1} reflection may overlaps with the peak from
a possible presence of Cr in the AG sample, detected in sample
AG during initial EPMA measurements, which is normally added
to the Co binder as Cr3C2 during the sintering process to limit the
WC grain growth (Okada et al., 1998). After deposition, samples E-
1 and E-2 show diamond peaks corresponding to the {1 1 1} that
is present at 2Â = 43.85◦ and the {2 2 0} at 2Â = 75.35◦ that might
overlap with the WC peak at 2Â = 75.2◦ shown in the AG sample.
The small Co peak present in samples E-1 and E-2 may be due to
Fig. 8. Lateral crack lengths present in diamond coated samples E-1, E-2, and I-1,
resulting from discrete indentations levels at 45, 60, 100, and 150 kg.
small amounts of the binder as Co–Cr stable compound (Delanoë
et al., 2004), not removed by the etching process at some points,
providing some binding to the WC grain. The XRD spectrum of sam-
ple I-AG shows broad peaks attributable to chromium nitride at
2Â = 38.2◦, 44.4◦, 64.65◦, and 77.65◦. Diamond coated samples after
pretreatment method I-1show XRD patterns with additional peaks
representing Cr3C2 and Cr7C3, suggesting intermediate chromium
carbide compounds formed during the diamond deposition
process.
Fig. 9. Backscattering SEM micrographs (left column) and W, Cr, and C Ka X-ray mappings (center and right columns, respectively) for samples E-1, E-2, and I-1.

8. 530 H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533
Fig. 10. Flank wear-land (VB) time evolution of diamond coated turning inserts
pretreated under methods E-1, E-2, and I-1.
Raman spectra of diamond films deposited on the three afore-
mentioned pretreated samples are shown in Fig. 7. The Raman
spectrum of MCD samples E-1, E-2, and I-1 show sharp diamond
peaks centered at 1335.1, 1336.6, and 1333.5 cm−1, respectively,
and shifted about 2.7 cm−1, 4.2 cm−1, and 1.1 cm−1 with respect
to natural diamond (1332.4 cm−1 at atmospheric pressure and
25 ◦C). These results correspond to biaxial residual compressive
stresses of 2.91, 4.53, and 1.19 GPa, calculated using the approach
proposed by Ager and Drory (1993) and used by Cristofanilli
et al. (2010) for diamond coatings deposited on WC–Co substrates
treated with CrN interlayers. The obtained thermal stresses are
expected to be the same due to a uniform diamond deposition
temperature for all samples by using a substrate heater and a
constant relative position of the samples with respect to the fila-
ments during the HFCVD deposition process. The residual stresses
present in the samples may be solely attributed to the intrinsic
stresses induced during the growth phase, responsible for the dif-
ferent diamond morphologies shown in Fig. 5. Furthermore, an
apparent stress relief mechanism provided by the CrN/Cr inter-
layer can be observed from the lower biaxial compressive stress
value.
Diamond coatings adhesion was evaluated in terms of the lateral
crack lengths resulting from discrete indentations levels at loads of
45, 60, 100, and 150 kg. Three single indentations were performed
at each load to ensure a delamination in the films, and ten crack
length measurements were recorded from the center of each inden-
tation. When the indentation force was sufficiently high, lateral
cracks were initiated, which propagated between the coating and
the substrate. Fig. 8 shows the average lateral crack lengths in the
diamond coated samples for the various pretreatment methods E-
1, E-2, and I-1. Crack lengths were higher for sample E-1 and no
delamination was observed at some load levels for samples E-1 (60
and 45 kg) and E-2 (45 kg). In contrast, delamination was observed
at all indentation load levels for sample I-1, which displayed simi-
lar crack length values with respect to sample E-2 at 60 and 100 kg.
The relative error (with respect to the average) is different for all
samples at each discrete indentation load, i.e. ∼10% for sample E-
1 at 150 kg, 7% for sample E-2 at 60 kg, and 15% for sample I-1 at
45 kg.
Moreover, when no fracture or delamination was observed at
some indentation loads for samples E-1 and E-2, white annular
marks appeared on the diamond surface, which suggest that the
coating was still not adherent, and that the crack energy was
Fig. 11. SEM micrographs of the worn samples after the machining test.
insufficient to promote a complete delamination. As a result, it
may be inappropriate to make a quantitative comparison about
the interfacial toughness of the diamond coatings for the different
surface pretreatment effects based on the data from Fig. 8, by
calculating the critical strain energy release rate Gc for crack
propagation as proposed in previous works as proposed by Drory
et al. (1995). However, it is quite interesting to find that for sample
E-1, perhaps it has the highest value of crack lengths for a corre-
sponding load of 150 and 100 kg, no delamination and no annular
marks appeared after indentations at load levels of 60 and 45 kg.
Contrary, diamond delamination was achieved for all indentation
loads in sample I-1 in conjunction with lower lateral crack lengths.
Fig. 9 presents the backscattered SEM micrographs (left col-
umn) of samples E-1, E-2, and I-1 after the Rockwell indentations,
and shows the white spallation area of the diamond coatings
and the lateral cracks when an indentation load of 100 kg was
applied to all samples. The Ka X-ray maps from W for samples

9. H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533 531
Fig. 12. Digital microscopy images (left column), backscattering SEM micrographs and EPMA chemical composition mappings (center and right columns, respectively)
corresponding to the worn cutting edges of samples E-1, E-2, and I-1.
E-1 and E-2, Cr for sample I-1, and carbon for all samples are also
shown.
The diamond coating surface features can be distinguished from
the carbon maps shown at the right column of Fig. 9, where ball-
like agglomerates are present in sample I-1, directional marks in
sample E-2, and a uniform diamond surface in sample E-1. It can
be also observed that diamond delamination for samples E-1 and
E-2 occurred from the Co-free WC substrate, whereas the diamond
delamination for sample I-1 occurred at the diamond/interlayer
interface, which suggested that the adhesion of the interlayer with
the substrate was stronger than the adhesion of the interlayer to the
diamond coating. Some non Cr areas after delamination of diamond
coated sample I-1 are depicted as blue regions, which correspond to
the highest stresses in the vicinity of the indentation, strong enough
to promote the delamination of both, the diamond coating and the
interlayer. A semi-quantitative analysis of some of the failure sur-
faces for sample I-1 show about 77 wt% Cr with less than 1% of W
and Co, which suggests that the flaking of the diamond coating may
have occurred mainly at the diamond/CrN interface.
3.3. Dry machining performance test
Fig. 10 shows the flank wear-land width (VB) time evolution of
diamond coated turning inserts pretreated with methods E-1, E-2,
and I-1, including previous results of commercial MCD, PCD, and
nano-diamond coated inserts done by Hu et al. (2007b) in a simi-
lar workpiece material and machining conditions. The fluctuations
observed in the wear-land width values (VB) in Fig. 10 are due to
some aluminum built-up edge (BUE) produced during the cutting.
This results show that sample I-1 suffered a sudden increase of
wear-land in a short period of time (0.49 mm in 26 s), suggesting
that coating delamination occurred abruptly and resulted in rapid
wear of the exposed interlayer/carbide substrate.
A better behavior was observed for samples E-1 and E-2, which
failed at lower VB levels and higher cutting times; 0.3 mm in 293 s
and 0.34 in 104 s, respectively. The failure of the diamond film was
also detected by the change in the intensity of the AE-RAW signal
compared with the initial cutting pass. This data confirmed that the
diamond coated turning inserts made with pretreatment method
E-1 are more effective and demonstrate better performance under
dry machining conditions.
Fig. 11 shows SEM micrographs of the worn samples after the
machining test and confirms the coating delamination for samples
E-2 and I-1. A gradual degradation of the diamond coating was
observed for sample E-1 where flank wear-land was the major fea-
ture observed, compared with sample E-2, which displayed sharp
tool wear growth. Sample I-1, on the other, hand suffered delam-
ination at the diamond and CrN/Cr interlayer interface, leading to
the diamond coating becoming detached from the substrate during
the very early stage of machining, which exposes the carbide as the
only remaining cutting surface.
A detailed analysis of the worn tool cutting edges after dry
machining tests is shown in Fig. 12 for samples E-1, E-2, and I-1;
the results from digital microscopy images, EPMA backscatter-
ing electron (BSE) analysis, and chemical compositional maps are
shown in columns from left to right, respectively. These results

10. 532 H. Gomez et al. / Journal of Materials Processing Technology 212 (2012) 523–533
evidenced that the sample E-1 displays a gradual wear-land for-
mation, no diamond debonding, and some Al built-up edge, typical
characteristics of an adherent diamond coating. The results from
the sample E-2 wear patterns represent a tool failure character-
ized by an abrasion wear mechanism at the top of the cutting
edge followed by a diamond peeling at the bottom of the flaking
area. This wear pattern is associated to an early coating debond-
ing at the top of the tool edge which promotes the subsequent
abrasion of the WC substrate during the cutting operation, which
is also confirmed by the W compositional maps. In sample I-1,
the failure sequence is similar to the sample E-2; however, the
initial diamond coating debonding begins at its interface with
the top Cr interlayer promoting an abrasion of the latter fol-
lowed by the CrN layer abrasion, and finally exposing the WC–Co
substrate as confirmed by the BSE image and the W chemical
map.
The failure of sample E-2 under dry machining conditions might
be attributed to the undesirable directional surface features charac-
terized in Fig. 2 which are perpendicular to the cutting direction at
the tool edge. These directional features provide preferential paths
for crack propagations which finally promote the diamond coating
debonding as evidenced in Figs. 9 and 12.
The interlayer failure evidenced in sample I-1may be mainly
originated from a weak chemical binding energy between the dia-
mond coating and the top Cr surface where the blasted diamond
particles are not sufficient enough to provide a strong bonding.
Due to the wide difference in machining performance between the
CrN/Cr interlayer and the chemical etching pretreated tools, any
additional optimization in the distribution or amount of diamond
blasted/impinged particles at the top of the interlayer could not
be sufficient to get closer to the performance of chemical etched
samples, and it may constitute a reason of why still there are
no commercial solutions in the market of diamond coated tools
intended for high silicon aluminum dry machining applications
which incorporate the use of CrN/Cr interlayers as an adhesion
improvement method.
The CrN/Cr lack of adhesion can be also attributed to its mini-
mal carbide formation ability due to its interdiffusion with carbon
and the formation of additional Cr multiphase compounds (Cr3C2,
Cr7C3, and Cr23C6) discussed by Xiao et al. (2009) and evidenced in
the XRD patterns shown in Fig. 6, which also provide a preferential
direction for crack propagation.
The definition of adhesion refers to a system where the work or
force of detachment is measured by the application of an external
load capable of causing failure to the system under investiga-
tion. Based on this concept, different methodologies, which are
essentially destructive, have been developed to characterize the
adhesion, including Rockwell C indentation, scratch, superlayer
test, bulge, and blister test (Volinsky et al., 2002). These meth-
ods are very useful for routine quality control. However, based
on our experimental results, it is not appropriate to correlate
the adhesion measured by the Rockwell C indentation evalua-
tion with the dry machining performance. Furthermore, evaluating
the adhesion of diamond coated cutting tools by pin-on-disc tri-
bological tests (Polini and Barletta, 2010), impact tests (Bouzakis
et al., 2007), scratch, indentation, or sand abrasion testing meth-
ods (Buijnsters et al., 2005) which are intended for relatively thin
coatings may not be suitable for the diamond thickness range
(25–30 ␮m) normally used in CVD commercial diamond coatings
for dry machining applications. We believed the force applied
and stress field to cause the delamination in Rochwell C inden-
tation test is different from the one which leads to the coating
delamination in the dry machining test. The former is a more
complicated and more aggressive test. In the later, the delamina-
tion is most due to the shear force and stresses generated during
the machining operation. Our future work would be to develop
a more reliable approach to characterize the adhesion, which is
based on the specific applications and this particular manufacturing
chain.
4. Conclusions
The surface characteristics of the WC–Co substrates in terms
of texture features and roughness play an important role in the
final performance of diamond coated tools, especially when com-
mercial substrates are used for further diamond depositions. The
removal of feed marks present in the as-ground samples leading
to a uniform surface texture and the cobalt removal depth are the
key factors to achieve an adherent diamond coating when chemi-
cal etching methods are used. Perhaps smaller lateral crack lengths
values were obtained in sample I-1 after Rockwell indentations, no
significant adhesion under dry machining conditions was achieved,
as revealed by the sudden coating delamination. This suggests that
under the present experimental conditions, no correlation can be
established between a small crack length value and a better dry
machining performance. In fact, sample E-1, exhibit higher crack
length values after Rockwell indentation under 100 and 150 Kg
loads, and displays the best dry machining performance. The failure
of the CrN/Cr interlayer is currently being analyzed by the authors in
order to promote a significant improvement under real dry machin-
ing scenarios, and compared with other interlayer architectures
reported previously.
Acknowledgements
This research is supported from the NSF GOALI/Collaborative
Research Award #: 0928823. Part of this research is also supported
from the USF/GM Research Collaboration Grant.
The authors would like to thank to Richard A. Waldo, Nicholas
Irish, and Curtis A. Wong at the GM R&D Center.
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