Biomedical Engineering Reference
In-Depth Information
C
co
C
dl
R
s
R
p
R
ct
FIGURE 4.39
Basic electrochemical equivalent circuit of DLC as coating materials.
Several reports about DLC coating in biomedical implant application have been published.
Nam et al. [196] studied DLC-Si coating stress effect in 0.89 wt.% NaCl-simulated body
environment, and Turcio-Ortega et al. [197] investigated the corrosion behavior. For elec-
trochemistry corrosion character, impedance plot is always used as a measurement for
related coating resistance
R
p
in the electrolyte. The electrochemical equivalent circuit of
DLC is introduced in Figure 4.39. Here,
R
s
is the electrolyte resistance corresponding to
the geometrical configuration of the electrodes.
R
ct
and
C
dl
represent the charge transfer
resistance and double-layer capacitance existing at the interface between the electrolyte
and substrate, whereas
R
p
and
C
co
are the resistance and capacitance of the DLC coating
related to the total effective coverage, material thickness, and other surface characteristics,
respectively. This model can be applied in most of the DLC materials, with combining or
simply changing some element parameters. In some previous reports, the resistance
R
p
can be replaced by
R
pf
, which represents the pore resistance on the sample surface [198]. A
higher
R
ct
implies a slow corrosion rate and good anticorrosion property.
In several previous reports,
C
co
and
C
dl
can be expressed with another impedance
Z
cpe
,
which is a CPE representing the deviation from the true capacitive behavior. The com-
plex impedance is given by
Z
cpe
=[
C
(
j
ω
)
α
]
-1
. The coefficient
α
is the deviation index related
to surface roughness and inhomogeneity of the electrode. The whole equivalent circuit
model is simplified (Figure 4.40) and widely applied in many DLC samples.
In 2002, Papakonstantinou et al. [199] applied this model on a-C:H with Si incorporation.
After the Si content grew onto the substrate, the corrosion resistance increased 11.8 %. Kim
et al. [200] studied DLC corrosion performance in simulated body fluid environment in
2005 (0.89% NaCl solution, pH 7.4 at 37°C) and concluded that Si incorporation in a-C:H
could increase corrosion resistance
R
p
, as well as charge transfer resistance
R
ct
.
Z
CPE(co)
= [
C
co
(
jω
)
α
co
]
-1
Z
CPE(dl)
= [
C
dl
(
jω
)
α
dl
]
-1
C
co
C
dl
R
s
R
p
R
ct
FIGURE 4.40
Combined electrochemical equivalent circuit of DLC.
