Viscoplastic Properties and Tribological Behavior of Diamond-Like Carbon
  Films Using Nanoindentation and Nanoscratch Tests
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VISCOPLASTIC PROPERTIES AND TRIBOLOGICAL BEHAVIOR OF DIAMOND-LIKE 
CARBON FILMS USING NANOINDENTATION AND NANOSCRATCH TESTS 
V.Turq, J. Fontaine, J. L. Loubet, D. Mazuyer 
Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes, UMR CNRS 5513 
69134 Ecully Cedex, France 
TEL : 33-472-186-287, Email: Viviane.Turq@ec-lyon.fr 
Introduction 
Diamond-Like Carbon (DLC) films have been 
shown to demonstrate various tribological behaviors: in 
ultra-high vacuum (UHV), with either friction 
coefficients as low as 0.01 or less and very mild wear, 
or very high friction coefficients (>0.4) and drastic 
wear. These behaviors depend notably on gaseous 
environment, hydrogen content of the film [1], and on 
its viscoplastic properties [2,3]. A relation between 
superlow friction in UHV and viscoplasticity has indeed 
been established for a-C:H films and confirmed for a 
fluorinated sample (a-C:F:H). In this study, 
nanoindentation and nanoscratch tests were conducted 
in ambient air, using a nanoindentation apparatus, in 
order to evaluate tribological behaviors, as well as 
mechanical and viscoplastic properties of different 
amorphous carbon films. 
Experimental 
The samples were deposited on a Si (100) 
substrate by Plasma Enhanced Chemical Vapor 
Deposition (PECVD) process at different bias voltages, 
either from acetylene, cyclohexane precursors by d.c.-
PECVD, or hexafluorobenzene mixed with hydrogen 
precursor by r.f.-PECVD for the fluorinated sample (a-
C:F:H, noted FDLC). Details of the deposition process 
can be found in [4,5]. Thickness of the coating is 1µm, 
except for the FDLC, which is 0.4µm. 
Nanoindentation and nanoscratch tests were 
carried out in ambient air, at room temperature, with a 
MTS NanoIndenter® XP apparatus. A spherical (radius 
10µm) and a Berkovich diamond indenter were used. 
Mechanical and viscoplastic properties were evaluated 
from nanoindentation tests in continuous stiffness mode, 
using the Berkovich diamond indenter, with a maximum 
load of 100mN. As the load P is applied exponentially 
as a function of time, the ratio between loading rate P’ 
and load P is kept constant during indentation, and thus 
the strain rate ε  is also constant. Five different ratios 
P’/P, from 3.10-3 up to 3.10-1Hz, were used. 
Nanotribological evaluation of the samples was 
conducted from nanoscratch tests at ramping load (0.1 
to 10mN, 3 passes) and at constant load (5mN, 10 
passes) with spherical diamond indenter. 
Results 
The strain rate sensitivity of the materials is 
estimated and fitted by a Norton-Hoff law: 
xH H ε= ⋅  
where H is the hardness, H0  a constant, ε  the strain and 
x a constant called viscoplastic exponent (Table 1). 
Contrary to UHV, no evidence of correlation between 
friction coefficients in ambient air and viscoplasticity 
can be made. But even in this environment, some very 
low friction coefficient values, as low as 0.04 (FDLC), 
with very mild wear have been evidenced (Figure 1). 
Hardness H0 seems to be the key parameter: wear 
resistance in the air is improved with higher H0 and 
friction coefficient decreases with H0. Note that H0 is 
also roughly linked with the hydrogen content of the 
coating for the non fluorinated samples, as it has been 
shown in [2]. The number of passes seems also to lead 
to a decrease of friction coefficient. 
Sample H content 
(at. %) 
(GPa) 
x µ ramping 
load 
µ constant 
load 
Wear 
FDLC 5/18(F) 16 0.060 0.04-0.14 0.080 ~ none 
AC8 34 13 0.014 0.06-0.11 0.083 ~ none 
AC5 40 11 0.068 0.05-0.16 0.078 mild 
CY6.5 42 6.8 0.028 0.07-0.13 0.083 mild 
CY5 42 1.3 0.076 0.10-0.22 0.183 severe 
Table 1: Summary of nanofriction tests results 
and viscoplastic properties 
Conclusion 
This study shows that in ambient air, wear 
resistance and frictional behavior of a-C:H and a-C:F:H 
samples is improved with  hardness H0. In UHV, the 
achievement of super-low friction is linked with the 
viscoplastic character. Thus, intermediary coating, with 
high hardness and viscoplastic exponent, as a-C:F:H 
will demonstrate satisfactory tribological behavior both 
in ambient air and in UHV. 
References 
[1] C. Donnet et al., Tribo. Lett., 9 (2000) 137. 
[2] J. Fontaine et al., Tribo. Lett., 17 (2004) 709. 
[3] J. Fontaine et al., Thin Solid Films, (2005) in press. 
[4] C. Donnet et al., Surf. Coat. Tech., 94-95 (1997) 
456. 
[5] C. Donnet et al., Surf. Coat. Tech., 94-95 (1997) 
531. 
Figure 1: Constant load scratch micrograph
FDLC CY5
