Diamond and Related Materials 5 (1996) 649-653
DIAMOND
ELTED
MATERIALS
Fretting friction and wear of polycrystalline diamond coatings
E. Liu, B. Blanpain, J.-P. Celis
Departement Metaalkunde en Toegepaste Materiaalkunde, Katholieke Universiteit Leuven, de Croylaan 2, B-3001 Leuven, Belgium
Abstract
The tribological behaviour of diamond coatings deposited by widely different processes has been investigated. Hot-flame and
microwave CVD diamond coatings show a homogeneous structure and morphology, whereas plasma jet diamond deposits display
a laterally changing structure and morphology. The friction and wear behaviour of these three coating types sliding against a
corundum counterbody showed a marked difference in a fretting test. A single-crystal diamond with a polished (100) surface was
involved for comparative tests. An attempt is made to clarify the tribological behaviour of diamond coatings sliding against
corundum in terms of the purity of diamond coating, its surface roughness and crystal orientation.
Keywords: Diamond
coating; Fretting; Friction; Wear
1. Introduction
Diamond has a wide variety of applications because
of its exceptional physical and mechanical properties
and excellent wear resistance [ 11. For single-crystal
diamond, the mechanical and tribological properties
may vary greatly depending on the crystallographic
plane and direction [ 2-41. However, for polycrystalline
diamond coatings, not only the crystal orientation but
also the surface morphology, the coating thickness and
the diamond quality may influence the friction and wear
behaviour [S-7].
A fretting test with a. small displacement stroke can
be used to identify the tribological behaviour on a
microscopic scale. In the fretting test, the static friction
and dynamic friction can be extracted from each tangential force-displacement loop. The friction regime can be
determined by the relationship between the static friction
and the kinetic friction with respect to sticking time and
sample surface speed [S], or by a relative change in the
static friction coefficient with respect to the sticking time
[9]. The contact geometry, material properties and
vibration conditions can determine the friction and wear
behaviour of the counterfaces corresponding to different
fretting regimes [lo].
This paper focuses on a comparative tribological study
of diamond coatings deposited by three different deposition processes and a reference material diamond with a
polished (100) surface in a fretting test under the grossslip conditions. The results are interpreted in light of
differences in surface roughness, crystal orientation and
quality of the diamond coatings.
0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved
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2. Experimental details on sample preparation and
characterization
Three deposition systems were used to deposit
diamond coatings, namely hot-flame CVD (HFCVD),
microwave CVD (MWCVD) and stationary d.c. plasma
jet (DCPJ). In the HFCVD process [ 111, a gas mixture
of acetylene and oxygen was used. The substrate, a
cemented carbide insert, was held at 850 _+20 “C. During
deposition, the substrate was translated. In the MWCVD
process [ 121, a gas mixture of methane and hydrogen
with addition of oxygen was used. The substrate,
also a cemented-carbide insert, was maintained in the
range 800-1050 “C. A MO substrate was used in the
DCPJ process [7] and the coating was obtained from
a gas mixture of methane and hydrogen. A substrate
temperature of 900 “C was maintained during deposition.
A bulk diamond with a polished (100) surface (De Beers
Industrial Diamond, WTOCD) was used as a reference.
The crystal orientation of the bulk diamond was identified with a Laue back-reflection X-ray diffraction
method. The polished lines are in the [OOl] direction.
The surface roughness was evaluated with a surface
profilometer (Taylor-Hobson) operated with a pyramid
diamond stylus of 2 pm x 2 pm width. The coating structure was measured using a micro-Raman spectroscope
(Jobin Yvon S3000) with a Spectra Physics Ar ion laser
(wavelength, 514.5 nm; power, 150 mW) in the range
from 1000 to 18OOcm-‘.
In fretting tests, the sample oscillated under a linear
displacement mode against a corundum ball of 10 mm
650
E. Liu et al.lDiamond
and Related Materiuls 5 ( 1996) 649-653
diameter, in order to evaluate the tribological behaviour
on a microscopic scale under the conditions of 2 N
normal load, 100 urn displacement stroke, 8 Hz, 20 “C
and 50% relative humidity [ 131. For the bulk diamond,
the direction of the oscillatory displacement was [011]
at an angle of 45” to the polishing lines. The surface
morphology of the samples was characterized by scanning electron microscopy (SEM) (Philips 515) and
atomic force microscopy (AFM) (Digital Instruments,
NanoScope III) with a triangular S&N, cantilever before
and after performance of the fretting tests. The coating
thickness was also measured with SEM from the crosssections of the diamond coatings.
3. Results and discussion
3.1. Microstructure of the diamond coatings and the bulk
diamond
Both HFCVD and MWCVD diamond coatings have
typical diamond characteristics although some contamination in the form of sp2 amorphous carbon was found
from micro-Raman spectroscopy. The typical morphology of these two coatings are shown in Figs. l(a) and
2(a) respectively. The crystallites of MWCVD diamond
coating have a predominant (111) orientation while the
grains of HFCVD diamond coating are more randomly
oriented. The diamond coating deposited in the
DCPJ process showed varying structural features from
centre to the edge of the deposit as was described
previously [14]. Large and well-faceted diamond crystallites with a predominant ( 111) orientation were found
in the centre of the diamond coated area (Fig. 3(a)). All
tests on the DCPJ coatings were performed in this
central area.
An overview of the diamond crystal size, the coating
thickness and the surface roughness for the different
coatings together with the roughness value of the bulk
diamond is given in Table 1.
(aj
3.2. Tribological behaviour
Gross-slip conditions were active in the fretting tests
performed. Three kinds of friction information can be
obtained from the on-line recorded tangential forcedisplacement loop (Fig. 4(a)), namely the tangential force
amplitude, the tangential force under gross-slip conditions, and the average tangential force derived from the
dissipated energy which will not be discussed in detail
here. The tangential force-displacement
loops corresponding to the beginning (cycle 2) and the end of
fretting tests (cycle 5 x 105) for the different coatings and
the bulk diamond are shown in Fig. 4(b). The tangential
force amplitude appearing at each turnabout point of
the tangential force-displacement loops reflects the static
Fig. 1. AFM images of the HFCVD diamond coating: (a) as-deposited
surface; (b) after fretting.
friction or on the slip part reflects the friction regime.
The general tendency of the coefficient of friction for the
samples in this study (except for the coefficient of friction
corresponding to the maximum tangential force for the
DCPJ diamond coating) is that the coefficient of friction
is higher at the beginning of the fretting tests; then the
651
E. Liu et al./Diamondand Related Materials 5 (1996) 649-653
0a
(b)
Fig. 3. SEM images of the DCPJ diamond coating: (a) as-deposited
surface; (b) after fretting.
(b)
Fig. 2. AFM images of the MWCVD
deposited surface; (b) after fretting.
diamond
coating: (a) as-
friction coefficient rapidly or gradually decreases to a
more or less constant value for the subsequent cycles
(Figs. 4(c) and 4(d)).
For the DCPJ diamond coating, the maximum tangential force is maintained at the reversal points during the
fretting test. For the other diamond coatings and the
bulk diamond, this is not the case. At the beginning of
the fretting tests, the maximum tangential forces occur
at the reversal points. In the steady state, typically after
several thousand cycles, the tangential force amplitude
may appear anywhere in the cycle. Therefore the tangential force amplitude can only be taken as the static
friction force at the beginning of the fretting tests
(Fig. 4(c)) for the HFCVD and MWCVD diamond
coatings.
The tangential force amplitude reflects the influence
of the surface roughness and crystal orientation. For the
rough surface, for instance the DCPJ diamond coating,
a high tangential force amplitude was maintained
throughout the test. This is due to the small real contact area between diamond coating and corundum
counterbody limited by a few sharp points formed by
four (111) planes of each diamond crystallite, There are
two possibilities to lower the static frictional force under
the given fretting test conditions: by increasing the real
contact area or by accumulation of wear debris as a
E. Liu et al./Diamond and Related Materials 5 (1996) 649-653
652
Table 1
Characteristics of the diamond coatings and the polished (100) diamond
Diamond
type
&
(w)
R,
(pm)
Thickness
(pm)
Grain size
(pm)
DCPJ
HFCVD
MWCVD
(100) diamond
13.54 + 16.34
4.62 f 0.80
3.3OkO.52
0.210*0.12
7.41 + 0.9
0.32 + 0.02
0.26 + 0.05
0.015 +0.004
50-90
10.1
5.5
-
40-90
3-8
l-5
lubricant in the contact. The high tangential force
induced by the relative motion of the counterfaces can
elastically or even plastically deform and scratch the
counterbody in the contact to increase the real contact
area. During the fretting test, the counterbody was
indeed scratched by the diamond coatings. An enlarged
real contact area and accumulation of wear debris are
therefore to be expected, but the increased real contact
area and induced wear debris due to a very few asperities
on the DCPJ diamond coating are apparently not
enough to reduce the static frictional force significantly.
For the relatively smoother MWCVD and HFCVD
diamond
coatings, more crystallites contact
the
counterbody, which leads to a relatively low tangential
force amplitude compared with the DCPJ diamond
coating at the beginning of the fretting tests. In the
steady state, the real contact area and amount of wear
debris are expected to increase relatively rapidly, owing
to scratches on the counterfaces. The corundum wear
debris trapped in between the counterfaces identified by
SEM and energy-dispersive spectroscopy also contributes to an enlarged real contact area; so the influence
from the surface roughness gradually becomes less
important. This explains the fact that the coefficient of
friction corresponding to the tangential force amplitude
gradually decreases to a more or less constant value of
0.05 for the MWCVD and 0.1 for the HFCVD diamond
coatings.
For the very smooth surface such as the polished
(100) surface of single bulk diamond, the slightly higher
tangential force amplitude at the beginning of the fretting
test is due to the polishing lines on the bulk diamond
and the corundum ball. The rapidly reduced tangential
force amplitude in the steady state may be due to fine
scratches on the corundum counterbody.
The friction coefficient under gross-slip conditions
(Fig. 4(d)) shows dependence on the diamond quality,
wear debris and interaction between counterfaces while
the surface morphology and roughness become less
important. However, at the beginning of the fretting
tests, the surface roughness and crystal orientation of
the samples are still active as seen from the inset in
Fig. 4(d). The HFCVD diamond coating has a comparatively high friction coefficient due to a well-developed
contribution from sp2 amorphous carbon. Amorphous
diamond-like carbon coatings against a corundum
counterbody showed a higher coefficient of friction in a
previous study [ 151. Also this diamond coating was
scratched during the fretting test (Fig. l(b)). Scratching
the diamond crystallites needs a relatively high tangential force.
After exposure to the fretting test, no damage on the
DCPJ diamond deposit can be identified while much
wear debris loosely accumulates in the valleys (Fig. 3(b)).
No contact pressure seems to act on the wear debris
during fretting test. Abrasive wear is dominant for the
DCPJ diamond coating. A few slight scratches on the
MWCVD diamond coating could be resolved on the
fretted area as shown in Fig. 2(b) while more clear
scratches are observed on the HFCVD diamond coating
(Fig. l(b)), probably owing to a higher proportion of
amorphous carbon in the HFCVD diamond. Both abrasive and adhesive wear occurred for the MWCVD and
HFCVD diamond coatings during the fretting tests,
since material transfer was also identified on the fretted
area on the diamond-coated partner. For the bulk
diamond, no damage was found on the surface after
performance of the fretting test.
4. Conclusions
Three types of diamond coating were investigated
under gross-slip conditions. HFCVD diamond coating
has a comparatively high coefficient of friction (about
0.1 ), whereas DCPJ and MWCVD diamond coatings
and bulk diamond show a relatively low friction coefficient in the steady-state regime. The relatively high
friction coefficient for the HFCVD diamond coating
may be related to the diamond purity.
The coefficient of friction corresponding to the tangential force amplitude shows dependence on the surface
roughness and the crystal orientation. For the rough
diamond surface of the plasma jet coating, a high
tangential force amplitude was maintained within the
fretting duration. For the relatively smooth diamond
surfaces of the HFCVD and MWCVD coatings and the
bulk diamond, the tangential force amplitude was rapidly
reduced to a low steady-state value.
After a fretting testing against corundum, no evidence
of damage was noticed on the bulk diamond and the
DCPJ diamond coating. Some slight scratches could be
653
E. Liu et al.lDiamond and Related Materials 5 (1996) 649-653
dissipatedenergy
20
40
Displacement
so
60
(100)
DCPJ
HFCVD
MWCVD
diamond
coating
coating
coating
100
(pm)
Displacement
(vm)
(a)
.g
0.6 -
2
b
0.5 -
0.4 -
lb)
0.45 -
,
q
0.40
-
:
9
0.35
-
:
I
I
I
- +
A.
I
I
HFCVD diamond coating
DCPJ diamond coating
MWCVD diamond coating
--------DCPJ diamond coating
-HFCbD diamond coating
I
...... thVC\ID
diamond
---
wattng
(1GO)diamond
-.-.-.-.--,--.
c
Fretting cycles
(c)
0.00
1
’
0.0
I
1.0x105
I
2.0x105
I
3.0x105
I
4.0x105
5.0x105
Fretting cycles
(d)
Fig. 4. (a) Typical tangential forctiisplacement
loop corresponding to the DCPJ diamond coating surface conditions; (b) tangential forcedisplacement loops corresponding to the number of fretting cycles; (c) friction coefficient corresponding to the tangential force amplitude vs.
number of fretting cycles; (d) friction coefficient vs. number of fretting cycles under gross-slip conditions.
resolved on the MWCVD diamond coating while the
HFCVD diamond coating was comparatively heavily
scratched, which can al.so be linked to the diamond
purity.
Acknowledgements
The authors are grateful to Dr. A. Alahelisten
(Uppsala University, Sweden), Dr. Th. Priem (CEREM,
France), Dr. C. Quaeyhaegens (Limburg University,
Belgium), and L. Vermeyen (WTOCD, Belgium) for
providing samples. One of the authors (B.B.) is grateful
to the National Fund for Scientific Research (NFWO),
Belgium for providing a postdoctoral fellowship. This
work has been partially financed by IUAP contract 4 of
the Belgian Government
and Brite/Euram
Project
Eufretting (BRE 2-CT92-0224).
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