artikel 82005 buinsters SCRATCH Computer

Thin Solid Films 474 (2005) 186 – 196
www.elsevier.com/locate/tsf

Adhesion analysis of polycrystalline diamond films on molybdenum
by means of scratch, indentation and sand abrasion testing
J.G. Buijnstersa, P. Shankarb, W.J.P. van Enckevortc, J.J. Schermerd, J.J. ter Meulena,*
a

Applied Physics, IMM, Department of Applied Physics, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
b
Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India
c
Solid State Chemistry, IMM, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
d
Experimental Solid State Physics III, IMM, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
Received 27 November 2003; received in revised form 10 September 2004; accepted 10 September 2004
Available online 28 October 2004

Abstract
Diamond films have been grown by hot-filament chemical vapour deposition (CVD) on molybdenum substrates under different growth
conditions. The films grown with increasing substrate temperatures show a higher interconnection of diamond grains, whereas increasing
methane concentrations in the 0.5–4.0% range lead to a transition from micro- towards nanocrystalline films. X-ray diffraction analysis
shows Mo2C interlayer formation. Indentation, scratch and sand erosion tests are used to evaluate the adhesion strength of the diamond films.
Using steel ball indenters (F 750 Am), indentation and scratch adhesion tests are performed up to final loads of 200 N. Upon indentation, the
load values at which diamond film failure such as flaking and detachment is first observed, increase for increasing temperatures in the
deposition temperature range of 450–850 8C. The scratch adhesion tests show critical load values in the range of 16–40 N normal load for
films grown for 4 h. In contrast, diamond films grown for 24 h at a methane concentration of 0.5% do not show any failure at all upon
scratching up to 75 N. Film failure upon indenting and scratching is also found to decrease for increasing methane concentration in the CVD
gas mixture. The sand abrasion tests show significant differences in coating failure for films grown at varying CH4/H2 ratios. In contrast to
the other tests, here best coating performance is observed for the films deposited with a methane concentration of 4%.
D 2004 Elsevier B.V. All rights reserved.

PACS: 68.35.Gy; 68.55.-a
Keywords: Chemical vapour deposition; Diamond; Adhesion; Molybdenum

1. Introduction
The development of chemical vapour deposited (CVD)
diamond thin films has led to numerous applications.
Benefiting from the excellent material properties of diamond,
polycrystalline diamond films are used in cutting tools, as
protective coatings, composite additives, infrared windows
and by virtue of the wide band gap and the possibility of
doping, diamond films are ideal for high temperature and
high power electronic devices [1,2]. Additionally, the
negative electron affinity makes diamond an ideal candidate
for field emission display applications [3]. The electronic
* Corresponding author. Tel.: +31 24 3653022; fax: +31 24 3653311.
E-mail address: (J.J. ter Meulen).
0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2004.09.021

properties of diamond are also suited for field effect
transistors, piezoelectric effect devices, radiation detectors
and ultraviolet photodetectors. Till date, there is only a
limited number of substrates onto which polycrystalline
diamond films can be grown successfully without application
of interlayers. These include ceramics like WC, Si3N4 and
SiC, metallic substrates like Mo and W and the semiconductor Si.
The adhesion of the grown diamond films plays a crucial
role in the performance of the final product. Therefore, many
studies have been directed towards the improvement of the
adhesion by careful optimization of the various substrate

pretreatments as well as diamond growth conditions. For
example, substrate roughening by manual or ultrasonic
scratching is known to enhance the mechanical locking of


J.G. Buijnsters et al. / Thin Solid Films 474 (2005) 186–196

the grown diamond film [4]. The use of a bias-enhanced
nucleation step during the early stages of the diamond CVD
process also leads to an increased adhesion of the deposited
film [5]. The most important factor influencing the adhesion
is the interlayer formation during the diamond growth
process. The final adhesion of the diamond film with the
underlying substrate is determined by the number density of
interfacial bonds between atoms from the interface and
carbon atoms from the nucleated diamond together with the
corresponding chemical bond strength. These properties are
strongly influenced by the substrate surface phases and the
growth conditions, respectively.
In earlier work, molybdenum has been studied as a
model substrate because the use of molybdenum generally
leads to strongly adhering diamond layers [6]. Trava-Airoldi
et al. [7] reported on the surface nitridation of molybdenum
by ion sub-implantation. They found that the formation of a
thin molybdenum nitride surface layer reduced the indiffusion of carbon and hydrogen from the vapour phase
thereby leading to increased adhesion of the diamond films.
Experimental results have shown that the field emission
from diamond films deposited on molybdenum substrates is
much stronger as compared to uncoated Mo, thereby
emphasizing the importance of diamond film growth on

molybdenum for cold cathode applications [6].
It is known that diamond deposition at high temperatures
causes fragilization of the molybdenum structure, which is
unfavourable for most applications [7]. Therefore, the
diamond deposition has to be performed in the lower
temperature range, though the adhesive strength is known to
decrease with decreasing deposition temperatures. In the
present work, the adhesive properties of hot-filament CVD
grown diamond films on pure molybdenum substrates are
studied for varying growth conditions from 475 up to 850
8C. Indentation and scratch adhesion as well as sand
abrasion tests are performed and the outcomes are discussed
with respect to the applied deposition conditions.

2. Experimental details
The diamond films are grown in a conventional hotfilament-assisted CVD reactor on square, pure molybdenum
substrates with dimensions of 12Â12Â0.5 mm3. Diamond
deposition is carried out utilizing CH4/H2 volume ratios of
0.5–4% at substrate temperatures up to 850 8C. For all
deposition runs, the total pressure is kept at about 50 mbar
and the total flow rate at 300 standard cm3 minÀ1. The TaC
filament temperature as measured with an optical pyrometer
is kept constant at 2150F20 8C. A fixed filament-tosubstrate distance of 6–9 mm is used in all deposition runs.
To determine the substrate temperature, a K-type thermocouple is placed inside the substrate holder, approximately 1
mm below the actual deposition surface. In a separate study,
the temperature profile on the substrate surface perpendicular to the filament axis was evaluated using infrared

187

pyrometry (spot size ~3 mm), for a fixed filament-tosubstrate distance of 10 mm [8]. A maximum difference of

about 50 8C is measured for samples with dimensions up to
12Â12 mm2. Prior to diamond deposition, the molybdenum
substrates are manually scratched in a slurry of diamond
powder (1–2 Am) in glycerol, ultrasonically abraded in a
suspension of diamond powder (1–2 Am) in isopropanol
followed by ultrasonic and manual cleaning in isopropanol.
The mean surface roughness after the substrate pretreatment
procedure is in the order of R a=0.8 Am, as measured by a
Perthen Perthometer M4P.
Field emission scanning electron microscopy (JEOL JSM
6330 F) is employed to study the diamond film morphology
and film response after carrying out the different tests. The
diamond film quality is studied by means of micro-Raman
spectroscopy using an Ar ion laser (514.5 nm) with an
output power of 20 mW and a focused laser beam diameter
of about 2 Am (Renishaw System 1000). The Raman spectra
are taken in the 1055–1920 cmÀ1 range. The interlayer
phases formed upon diamond CVD are studied by X-ray
diffraction utilizing a Bruker AXS D5005 diffractometer
using CuKa radiation (k=1.5418 2) in the h–2h geometry.
The indentation and scratch adhesion tests are performed
in the 0–200 N normal load range using a scratch tester
(Revetest, CSM instruments) with steel ball indenters (750
Am in diameter). The acoustic emission signals are recorded
using a resonant detector (Vallen-System type SE150-M) set
at 125 dB gain in the 20–500 kHz range. The penetration
depth is measured by a depth sensor, which is positioned inbetween the indenter head and the force transducer (Interface
Force Measurements, Model SSB-AJ-50). On each specimen, a series of indentations is made applying normal loads
of 25–200 N. Constant indenter loading rates of 50 N minÀ1
are used for the b100 N indents and 100 N minÀ1 for the

z100 N indents. The steel ball is replaced after each
indentation series. Scratch tests are performed on the
diamond coated molybdenum samples using the steel ball
with a loading rate of 80 N minÀ1, a track length of 3 mm and
a scan speed of 3 mmd minÀ1. In the indentation tests, the
penetration depth, acoustic emission signal and normal load
are simultaneously recorded, while in the scratch tests the
lateral displacement, penetration depth, acoustic emission
signal, normal load and tangential force are recorded. Sand
abrasion tests are performed on the diamond coated samples
by a slightly modified standard rubber wheel test (ASTM
G65) using sand particles of 250 Am in diameter, a flux of 15
g minÀ1, a sliding speed of 0.1 m sÀ1 and a normal force of 50
N. All tests are performed at room temperature.

3. Results
3.1. Diamond film characterization
Fig. 1 shows the scanning electron microscopy (SEM)
micrographs of the diamond film surfaces for varying


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Fig. 1. SEM micrographs of the diamond film surfaces obtained at varying deposition conditions, i.e. at a substrate temperature of (a) 475 8C, (b) 550 8C, (c)
650 8C, (d) 750 8C and (e) 850 8C at a fixed CH4/H2 ratio of 0.5% and (f) using a CH4/H2 ratio of 4.0% at a fixed substrate temperature of 750 8C. The
deposition time for all films is 4 h.

deposition conditions. Fig. 1a–e shows the surface

morphologies of the grown diamond layers as obtained
at different substrate temperatures. It is apparent that at
475 8C the grain size is very small and that the
interconnection of the individual grains is poor. With
increasing substrate temperature, the average grain size as
well as the interconnection increases. However, at temperatures higher than 650 8C, secondary nucleation and
twinning lead to the formation of diamond films exhibiting grain sizes, which are varying between about 1 Am
and several tens of nanometers. Fig. 1d and f shows the
effect of different methane-to-hydrogen ratios on the
surface morphology of the grown diamond layers at a
fixed substrate temperature of 750 8C. Using CH4/H2
ratios of 0.5%, microcrystalline films are formed (Fig.
1d), whereas for 4.0% only nanocrystalline structures are
observed (Fig. 1f). For intermediate values, the film

morphology gradually changes from microcrystalline
towards nanocrystalline with increasing methane-to-hydrogen ratio. It has to be mentioned that the average grain
size for the films grown for 4 h at a CH4/H2 ratio of
0.5% is smaller as compared to that of the films grown
under the same conditions for 24 h (not shown). For the
latter films, the average surface grain size is about 2 Am.
It is known that the growth of grains within a continuous
polycrystalline film leads to the formation of columnar
grain structures. Due to the mutual competition between
these structures [9], the number of grains present at the
surface reduces as the film thickness increases. As a
result, the grains observed at the growth surface become
larger for longer deposition times. The thickness of the
films grown for 24 h is in the order of 20 Am, whereas it
varies from about 2 to 5 Am for the films grown for 4 h,

depending on the applied growth conditions.


J.G. Buijnsters et al. / Thin Solid Films 474 (2005) 186–196

In Fig. 2, the micro-Raman spectra of the diamond layers
are displayed. Although no remarkable difference in the
diamond film quality is seen as a function of the deposition
temperature from the micro-Raman peaks, the maximum
diamond peak intensity is found for the films grown at 650
8C. The peak position is shifted from 1333.0 cmÀ1 for the
films grown at 475 8C to 1336.5 cmÀ1 for those deposited at
850 8C. A broad band centred around 1500 cmÀ1, which can
be ascribed to amorphous carbon, is detected for all films
and its intensity is increasing with increasing temperature.
For temperatures less than 650 8C, a low intensity band at
about 1130 cmÀ1 is observed as well. This is commonly
attributed to nanocrystalline diamond [10], but recently
Ferrari and Robertson [11] assigned it to transpolyacetylene
segments at grain boundaries. In contrast to the temperature
series, the micro-Raman spectra show more significant
changes for varying methane concentrations. In Fig. 2b, the
spectra of the layers grown at 750 8C using varying methane
concentrations are displayed. From this figure, it is clear that
the quality of the deposited layers is decreasing with
increasing methane concentration. The diamond peak
intensity is drastically decreasing and is fully dominated
by the broad D-band of graphitic carbon (centred at ~1360
cmÀ1) for the film grown at CH4/H2=4%. For all samples

189

Fig. 3. X-ray diffraction pattern of a diamond coated molybdenum sample.
The applied growth conditions are Tsub=750 8C, CH4/H2=0.5% and t=4 h.

grown at CH4/H2z1.0%, the amorphous carbon band
(~1500 cmÀ1), G-band of graphite (~1580 cmÀ1) and
1150 cmÀ1 band are clearly distinguished. No significant
change in the diamond peak position (~1336 cmÀ1) is
observed with respect to the applied methane concentration.
In Fig. 3, the X-ray diffraction pattern of a diamond
coated molybdenum sample is shown. Diffraction peaks
from diamond, molybdenum and molybdenum carbide,
Mo2C, are apparent. The formation of a Mo2C intermediate
layer between the diamond film and molybdenum substrate
is the result of the carbide-forming tendency of the
molybdenum and is common for the applied deposition
conditions [12].
3.2. Indentation and scratch adhesion tests

Fig. 2. Micro-Raman spectra of the diamond layers as a function of (a)
substrate temperature and (b) CH4/H2 ratio. The deposition time for all
films is 4 h. For reasons of clarity, the subsequent spectra are shifted
vertically with respect to each other.

3.2.1. General description
One of the most frequently used methods to quantify
coating failure is based on indentation by a well-defined
indenter tip. Though in previous work, the radial crack
length has been used as a measure for the interfacial

cracking resistance of the film [13], the formation of such
radial and/or circumferential cracks is common to hard and
brittle films and is not a measure for adhesive failure. The
critical load for adhesive failure should only be taken as
the load at which failure of the interface occurs, thereby
leading to film delamination. Another commonly applied
tool to evaluate the adhesion of coatings is the scratch test.
It consists of sliding an indenter in a single scratch across
the coating surface with increasing normal load. A critical
load P cr at which stripping of the coating occurs, is
estimated and used as a measure of the adhesion [14]. In
this work, we have chosen for a blunt indenter type, i.e. a
spherical steel ball, for two main considerations. The first
was to avoid damage of the commonly applied and
expensive diamond indenters. Further, it has been shown
that, to suppress non-cohesive failure and to ensure only
adhesive failure of the films, it is necessary to use an
indenter with a large radius. Particularly for hard coatings


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on soft substrate materials, the Rockwell C indenter may
not have a radius sufficiently large enough to reliably
assess the adhesive failure of the interface [15].
The commercial instrument used in this study couples an
acoustic transducer with the indenter so that the critical load
might be identified by the acoustic emission signal from the

coating fracture. However, it appears extremely difficult to
determine the critical load by detection of the acoustic signal
from the steel ball indenter. The scratching of the ball on the
diamond layers results in the production of a very strong
acoustic background, thereby overlapping the signal from
the coating detachment. Therefore, in this study, the critical
load is derived from the plot of the tangential force ( F t)
versus track length, which shows an abrupt change at the
position of coating stripping.
3.2.2. Indentation test
In Fig. 4, the SEM micrographs of the indented regions
on diamond coated Mo substrates obtained at varying
substrate temperatures are shown. Careful examination of
all indented regions reveals the presence of concentric ring
cracks within the indents combined with radial cracks
running perpendicular outwards from the indents. Upon 200
N normal loading, diamond films grown at 550 8C show a
tendency for multiple, partial and concentric flaking at the
exterior of the indent. At 650 8C, the flaking results in the
delamination of a large single piece, reducing the number of
delamination events drastically. At 750 and 850 8C, no
delamination or coating flaking is observed at all. The

difference in film flaking behaviour of the 550 and 650 8C
specimens can be explained by the difference in the
interconnection of the individual grains within the coating.
As can be seen from Fig. 1, the interconnection increases for
increasing substrate temperature and, therefore, the flaking
results in larger film pieces being detached from the
substrate for increasing temperatures. For lower temperatures, the connection between the individual grains is

poorer and delamination results in the detachment of smaller
film parts.
On all samples, a series of indents is made with final,
normal loads of 25–200 N, which are stepwise increasing by
25 N. For the diamond layer grown at 550 8C, film
delamination is first seen at 75 N normal load, whereas it is
observed at 150 N for the layer deposited at a substrate
temperature of 650 8C. Upon close examination of the
indented regions, only interior concentric ring cracks and
exterior radial cracks are seen for temperatures of 750 and
850 8C. As there is no delamination at all, the adhesion
strength of the diamond layers grown at these temperatures
is higher than the stress field which is applied by the steel
ball at 200 N normal loading. Thus, an increasing diamond
film adhesion for increasing substrate temperatures is
concluded from Fig. 4.
In order to study the effect of the applied methane
concentration on the adhesion of the diamond films upon
indentation, 200 N indents are made on a series of samples
grown under varying methane concentrations, i.e. from
0.5% to 4.0%, at a fixed substrate temperature of 750 8C.

Fig. 4. SEM micrographs of the indented regions on diamond coated Mo substrates for different substrate temperatures during deposition, as indicated at the
upper left-hand side of each micrograph. The applied normal load is 200 N. The deposition time for all films is 4 h and a CH4/H2 ratio of 0.5% is applied.


J.G. Buijnsters et al. / Thin Solid Films 474 (2005) 186–196

For the films grown with 0.5% and 1.0% CH4/H2 gas ratios
(Figs. 4c and 5a), no delamination is observed at all. This

implies that the adhesion strength of these films is larger
than the stress field generated by the indentation. However,
for 2.5% and 4.0% methane gas mixtures (Fig. 5b,c), the
films show significant film detachment within the indented
regions. Critical load values of 125 and 100 N are derived
for film detachment at methane concentrations of 2.5% and

191

4.0%, respectively. At regions where the concentric ring
crack density is highest, the detachment is strongest. None
of the samples shows film delamination outside the indents.
In virtually all samples studied, radial cracks are formed
upon indentation at loads in the 50–200 N range. For these
relatively low loads, the radial crack length is of about the
same order of magnitude as the indent radius.
3.2.3. Scratch test
In Fig. 6, the applied normal load and resulting tangential
force are displayed as a function of the scratch track length
for three specimens grown at 750 8C. In all plots, the first
part shows a linear behaviour of the tangential force with
respect to the applied load. The abrupt change in tangential
force is detected at the moment at which the diamond layer
starts to flake and gets detached from the molybdenum
substrate, i.e. at the critical load. At CH4/H2=0.5%, the
critical load is 38.5 N, whereas it is 16.5 N at CH4/
H2=4.0%. It is clear that the critical load decreases for
increasing CH4/H2 gas ratio. SEM analysis at the positions
at which coating stripping and flaking is first observed
shows that at the exterior of the scratch channel the

formation of large radial cracks leads to the flaking of the
diamond film. Within the scratch channel, the diamond film
is stripped off by the steel ball indenter for slightly higher
loads. From the obtained scratch data, the coefficient of
friction for the steel-diamond sliding contact can be deduced
from the linear part of the plots prior to coating failure. The
ratio F t/F n (=l) is constant for loads bP cr and varies from
about 0.15 to 0.19 depending on the applied growth
conditions. After coating failure, the coefficient of friction
is not fully constant anymore, but values varying from 0.28
to 0.34 are obtained for these sliding contacts.
The scratch data for the film deposited for 24 h at 750 8C
with a CH4/H2 gas ratio of 0.5% do not reveal any abrupt
change in the tangential force at all. As the F t/F n ratio
(~0.20) is nearly constant over the entire load range (1–75
N), it can be concluded that the critical load for coating
delamination for this sample exceeds 75 N. This is also
confirmed by SEM analysis of the produced scratch track.
Only the deposition of steel ball indenter material onto the
diamond-coated surface is seen and no stripping or coating
failure of the film at all.
3.3. Sand abrasion wear test

Fig. 5. SEM micrographs of the indented diamond film surfaces at 200 N
normal loading. The films are grown at various CH4/H2 ratios, as indicated
at the upper left-hand side of each micrograph. The deposition time for all
films is 4 h and a substrate temperature of 750 8C is applied.

The experimental setup of the sand abrasion test is
shown schematically in Fig. 7a. The sample is pressed

against a rubber wheel and due to the elastic deformation
of the rubber, a nearly rectangular contact is formed.
During the test, a constant flux of dry sand particles is
introduced in between the sample and the rubber wheel.
Due to partial elastic embedding in the rubber, the
abrasive material (sand particles) reaches the contact area
between the sample and the rubber wheel and abrades the
stationary sample. In standard experiments, the weight


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bond strength, failure occurs most commonly by delamination at the interface. During each sand abrasion test, the
sample surface is investigated every minute by optical
microscopy in order to detect failure/detachment of the
diamond coating. In this way, lifetimes of the various
films are obtained, which correspond to the moments at
which the diamond layers are detached from the
molybdenum substrates and abrasion wear tracks are first
observed. In other words, the sand abrasion wear test is
used as a tool for evaluating the diamond film adhesion,
contrary to its application as a method to obtain the wear
resistance by probing the weight loss. In Fig. 7b, the
lifetimes of the diamond coated samples as obtained with
varying CH4/H2 ratios are displayed. After 10 min of
exposure to the abrasive sand particles, the diamond layer
grown at CH4/H2=0.5% detaches from the molybdenum
substrate at the contact area. For CH4/H2 ratios of 1.5%

and 2.5%, the lifetimes are 8 and 3 min, respectively.
Surprisingly, the diamond film grown at CH4/H2=4.0%
only detaches after 25 min.
In Fig. 8, the SEM micrographs of two tested samples
are displayed. Fig. 8a shows the wear track on the sample

Fig. 6. Tangential and normal load as a function of scratch length. The
deposition time is 4 h and a substrate temperature of 750 8C is applied for
all samples. The CH4/H2 gas ratio is stated at the upper left-hand side of
each graph.

loss of the sample is a direct measure for the wear
resistance of the sample. However, in the case of diamond
coated samples, since the diamond is much harder than
the abrasive used, it only leads to abrupt adhesion failure
of the diamond films. The sliding force at the surface of
the film results in a shear force component at the
interface. Due to differential deformation between the
substrate and the film, coupled with the lower interfacial

Fig. 7. A schematic representation of the sand abrasion test (a) and the
lifetime of the diamond coated molybdenum samples with respect to the
applied CH4/H2 ratio (b). All diamond layers are grown at 750 8C using a
deposition time of 4 h.


J.G. Buijnsters et al. / Thin Solid Films 474 (2005) 186–196

193

Fig. 8. The SEM micrographs of the sample surfaces exposed to the sand abrasion wear test: (a) overall view of the wear track on the sample grown at CH4/
H2=0.5%; (b) the boundary between the wear track and the diamond film (CH4/H2=0.5%); (c) the diamond layer morphology at ~200 Am from the wear track
(CH4/H2=0.5%); (d) view of the wear track on the sample grown at CH4/H2=4.0%; (e) the boundary between the wear track and the diamond film (CH4/
H2=4.0%); and (f) the diamond layer morphology at ~100 Am from the wear track (CH4/H2=4.0%). The white arrows indicate the sliding direction of the
abrading sand particles.

grown at CH4/H2=0.5%. It is about 2 mm in length and
750 Am wide. A magnified view of the boundary between
the wear track and the adherent diamond film is shown in
Fig. 8b. At the wear track, the diamond layer is fully
removed and the molybdenum substrate is exposed. As
molybdenum is less wear resistant than diamond, the
removal of substrate material at the wear track is
significant. However, outside the wear track the diamond
film is still adherent and only little wear of the diamond
grains is observed (see Fig. 8c). Due to the abrading
effect of the sand particles, the diamond film is
planarized, i.e. only the tops of the grains which are
sticking out from the surface are worn down. For the
sample grown at CH4/H2=4.0%, coating detachment is
first observed after 25 min. The corresponding wear track,

which is about 500 Am in length and 180 Am in width, is
shown in Fig. 8d. The boundary between the wear track
and the adherent diamond film is shown in Fig. 8e. As
can be seen from this figure as well as from the
magnified view of the dballasT diamond layer outside the
detached area (Fig. 8f), only the tops of the round
structures are affected by the abrasive sand particles. The
lower regions within the dballasT diamond layer still

exhibit the nanocrystalline features. Another striking
feature is the presence of openings behind the round
dballasT structures, which indicates that these ball-shaped
structures behave like one single cluster upon abrasive
contact. Surprisingly, apart from these openings, no
microcracks are observed at the abraded regions of all
the tested films.


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4. Discussion
4.1. Interfacial structure
Molybdenum is one of the strong carbide forming
materials. The X-ray diffraction pattern in Fig. 3 clearly
indicates the formation of a Mo2C interlayer. So, the
bonding structure of diamond films on molybdenum
substrates undergoes a transition from fully metallic (M–
M) in the molybdenum substrate via covalent metal carbidic
(M–C) in the interlayer region towards covalent carbon
bonds (C–C) in the diamond layer. Detailed SEM investigation of the delaminated areas upon indentation and
scratching in the present work shows that delamination of
the diamond films takes place at the carbide–diamond
interface. This agrees well with the findings of Bahr et al.
[12] who analysed the bonding structures within the
molybdenum carbide interlayer in fractured regions after
indentation using X-ray photoelectron spectroscopy and
Auger spectroscopy. They detected the presence of carbon–

carbon bonding and suggested that the extra carbon is most
likely present along the grain boundaries of the Mo2C
grains, thereby impacting the adhesion behaviour of the
film. Apart from applying optimum deposition temperatures
and methane concentrations, it is widely known that the
surface roughness of the substrate has a strong influence on
the adhesion. In this work, we have chosen for a substrate
pretreatment based on a combination of manual and
ultrasonic scratching. The manual scratching leads to an
increased surface roughness due to the formation of fine
scratches, thereby supplying good diamond nucleation sites.
Additionally, the use of diamond powder in both the manual
and ultrasonic scratching results in the presence of small
diamond seeds enhancing the nucleation rate and density.
After the pretreatment procedure, the overall roughness of
the molybdenum substrate surfaces is about R a=0.8 Am and
the nucleation density of the films grown at 750 8C and
CH4/H2=0.5% is in the order of 109 cmÀ2.
The presence of the brittle molybdenum carbide interlayer will also have a strong influence on the mechanical
behaviour of the grown diamond films when investigated by
indentation and scratch testing. From the present as well as
previous work [12], it is seen that diamond growth on
molybdenum can lead to carbide layers of even several
microns. The thickness of the carbide interlayer is strongly
determined by the applied substrate temperature and
methane concentration. A systematic study of the carbide
interlayer thickness and composition as a function of the
applied growth conditions together with their effect on the
adhesion and other mechanical properties is under progress.
4.2. Influence of composition and morphology on adhesive

strength
It can be concluded that the effect of the substrate
temperature on the carbon constitution of the grown diamond

films is relatively small (Fig. 2a). Considering that the
scattering cross-section for the sp2-bonded carbon structures
is even about 233 times larger than that of sp3-bonded
structures for the argon ion 514.5 nm radiation [16], the
grown films consist of more than 99% sp3-bonded carbon.
On the contrary, the CH4/H2 ratio applied during the hotfilament CVD process strongly affects the film morphology
and composition, as can be concluded from Figs. 1 and 2.
An increasing CH4/H2 ratio leads to the formation of smaller
grains and a higher percentage of non-sp3 bonded carbon
structures. By increasing the methane gas concentration in
the stock gas slightly, a gradual transition from microtowards nanocrystalline diamond films is observed. Though
the diamond peak is fully dominated by the graphitic Dband for films grown at CH4/H2z2.5%, the percentage of
sp3-bonded carbon will still be ample. The actual sp3/sp2carbon fraction within the diamond films strongly determines the mechanical properties of these films. For
example, the hardness, the elastic modulus, the cracking
behaviour (initiation and propagation) and the coefficient of
friction in many tribological systems will vary as a function
of the sp3/sp2 bonding ratio within the diamond films
[17,18]. Especially, the distribution of graphitic phases is
determinant for the cracking behaviour. As no additional
nucleation step is applied in the present work, increasing
methane concentrations will not only lead to lower sp3/sp2carbon fractions within the film, but also to the same at the
nucleation side. Consequently, the higher concentrations of
non-diamond phases at the nucleation side lead to higher
propensity for film delamination, which is clearly demonstrated in the indentation and scratch adhesion tests as the
critical load for coating delamination decreases for increasing methane concentration.
The detrimental effect of increasing methane concentration on the adhesion seems to be expressed in the

decreasing lifetime for sand abrasion as well. Up to methane
concentrations of 4.0%, the lifetime decreases significantly.
The higher resistance against sand abrasion of the film
grown at CH4/H2=4.0% cannot only be explained by the
difference in the percentages of the various carbon
structures within the diamond film and at the carbide
interface, as this will only gradually change as a function of
the methane concentration. For increasing methane concentrations, a gradual transition from sub-micron diamond
towards dballasT diamond is observed. At CH4/H2=4.0%, the
deposited layer consists of dballasT diamond exhibiting ballshaped clusters of nano-grained crystallites. During the
initial stages of the sand abrasion test, only the higher ballshaped clusters are in contact with the abrasive sand
particles and, therefore, the impact of the abrasive material
is only taking place at these topographically higher sites. An
exact explanation for the better coating performance as
obtained with CH4/H2=4.0% cannot be given. However, it is
known that apart from the reduced contact areas the
formation of thin, low shear strength films on hard coatings
or on the asperity tips of these coatings results in low


J.G. Buijnsters et al. / Thin Solid Films 474 (2005) 186–196

friction behaviour as well [19]. The higher sp2-ratio within
the diamond layer results in a higher propensity for the
formation of such nanofilms, ultimately leading to a lower
friction. Additionally, the slightly larger coating thickness
will also contribute to an increased coating lifetime.
Cleavage and crack propagation along crystallographic
planes is—due to the presence of a widespread network of
nano-twins—expected to be lower than in single-crystalline

diamond grains as well. These hypotheses will be verified in
the near future by means of atomic force microscopy
roughness measurements of diamond layers grown in a large
range of methane concentrations prior to and at several
intervals during the sand abrasion test.
4.3. Residual stress and coating thickness
The presence of residual stress in grown films generally
has a strong effect on the adhesion. For example, in the
range of 1.0–2.0 GPa residual, compressive stress the
cracking resistance and adhesion of plasma grown diamond
films was found to increase [20]. It is also known that the
residual stress acting on a diamond film is not homogeneously distributed along the film thickness. Especially at
the carbide–diamond interface the stress can be much larger
than at the surface layer of the diamond film. The higher
stress value measured for the thin film (4 h) indicates a high
stress close to the interface. Though the interfacial stress
cannot be measured directly for the thicker diamond layer
(24 h), it is believed to differ only slightly from that of the
thin film. A difference in interfacial stress will then not
explain the difference in adhesion of the two films as
measured by scratching. More likely, it is originating from
the difference in the load bearing capacity of the coatings. If
the diamond coating is thicker, it can, because of its
stiffness, carry part of the load and the deformation of the
molybdenum substrate will be smaller. The frictional
situation is more favourable as compared to the thin
diamond layer because ploughing or hysteresis effects due
to substrate deformation will be relatively minor [19].
Compressive stress values varying from about 0.3 to 1.8
GPa are derived for the films grown at 475–850 8C. For the

films grown at varying CH4/H2 ratios, the diamond peak
position seems to be almost unaffected indicating little effect
of the applied methane concentration on the residual stress.
As the adhesion increases for increasing temperatures and
decreases for increasing methane concentrations, a direct
correlation between the residual stress and the adhesion can
be ruled out. Certainly, the differences in carbon structures
and bonding densities at the carbide–diamond interface have
a much stronger effect on the adhesion of the diamond layers.

5. Conclusions
In this work, the effect of substrate temperature and
methane concentration on the adhesion of polycrystalline

195

diamond films grown by hot-filament CVD on molybdenum
substrates is investigated by means of indentation, scratch
and sand abrasion tests. Increasing substrate temperatures
lead to a higher interconnection of the individual diamond
grains and increasing methane concentrations in the 0.5–
4.0% range result in a transition from micro-towards
nanocrystalline films. Micro-Raman spectra taken from the
various films show an increasing level of non-diamond
phases for increasing methane concentrations, whereas only
a slight change in film composition is seen for increasing
deposition temperatures. X-ray diffraction analysis shows
that the diamond film growth is preceded by the formation
of a Mo2C interlayer. From the indentation and scratch
adhesion tests, it is concluded that higher deposition

temperatures lead to stronger adhesion, whereas increasing
methane concentrations result in a decrease of the adhesion.
However, for sand abrasion, the lifetime of films grown at a
methane concentration of 4.0% is about three to eight times
higher than that of films grown for lower methane
concentrations.
This work shows that, though the scratch, indentation
and sand abrasion tests differ largely, the coating performance or, more particularly, the film failure as visualized by
flaking, stripping and/or detachment in all three tests,
enables to class diamond films deposited from different
gas mixtures and at different substrate temperatures according to their adhesion strength.

Acknowledgements
The authors wish to thank Dr. Ir. Bert Huis in ’t Veld and
Cor Lossie for performing the sand abrasion tests and
Leander Gerritsen for his technical support. This work was
performed as part of the research program of the Netherlands Technology Foundation (STW) with financial support
from the Netherlands Organization for Scientific Research
(NWO).

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