Biomedical Engineering Reference
In-Depth Information
It is known that titanium carbides and nitrides have excellent mechanical and chemical prop-
erties, for instance, good wear resistance, inactivity with a number of chemical substances, and
outstanding hardness [118-124]. Titanium oxides are known to be fairly compatible with living
tissues [125-128]. It is also inactive to many chemical reactions. In the surface coating industry,
incorporation of C, O, and N into Ti alloys is quite common to improve both the mechanical and
the corrosion properties of the substrates using various methods [129-134]. The use of PIII is more
preferable for large samples, especially those with complex geometries, like orthopedic implants,
because PIII is a conformal treatment process when conducted under proper conditions. PIII as a
low-temperature process can enhance the mechanical properties of the specimens without altering
the original dimension. In addition, PIII is not subjected to normal thermodynamic constraints
such as impurity solubility. The implantation time is independent of the sample size. Using PIII,
a gradual transition can be formed in the NiTi near-surface region, decreasing the possibility of
delamination compared to the cases in which the coatings are put on. Here, the corrosion resistance
and tribological properties as well as the cytocompatibility of the N, C, and O plasma-treated NiTi
materials are illustrated. N
2
, C
2
H
2
, and O
2
plasmas are used to introduce N, C, and O into NiTi
substrates in the PIII system. The optimal implantation parameters of the N, C, and O PIII are listed
in Table 19.2.
Figure 19.25a shows that a 100 nm thick titanium nitride surface layer is formed on the N PIII
NiTi alloy. X-ray diffraction (XRD) and high-resolution x-ray photoelectron spectroscopy (XPS)
analyses reveal that TiN is the only secondary phase present in the N-implanted layer. With regard
to the acetylene-implanted layer, a titanium carbide layer with increasing Ti to C stoichiometric
ratios is detected underneath the surface oxide layer. Figure 19.25b shows a 75 nm thick titanium
carbide layer beneath a 25 nm surface oxide layer. The titanium chemical states are analyzed using
high-resolution XPS and the oxides of Ti
2
+
, Ti
3
+
, and Ti
4
+
are found in the implanted layer. The
results show that a titanium oxide layer about 120 nm thick beneath a 25 nm surface oxide layer
is formed on the O PIII NiTi alloy (Figure 19.25c). It should be noted that in all cases, the nickel
contents are suppressed to low levels in the near surface.
The hardness and the modulus profi les of the control sample, nitrogen-, acetylene-, and oxygen-
implanted samples are shown in Figures 19.27a through 19.27d. The hardness of the control sample
is 4.5 GPa and the Young's modulus is 57 GPa. All the surface-treated samples possess higher
surface hardness and Young's modulus values compared to the control. In the nitrogen-implanted
TABLE 19.2
Implantation and Annealing Parameters
NiTi with Nitrogen
Implantation
NiTi with Carbon
Implantation
NiTi with Oxygen
Implantation
Sample
Gas type
N
2
C
2
H
2
O
2
RF (W)
1000
—
1000
High voltage (kV)
-
40
-
40
-
40
Pulse width (μs)
50
30
50
Frequency (Hz)
200
200
200
Duration of implantation (min)
240
90
240
Base pressure (Torr)
7.0
×
10
-
6
1
×
10
-
5
7.0
×
10
-
6
Working pressure (Torr)
6.4
×
10
-
4
2.0
×
10
-
3
6.4
×
10
-
4
Dose/cm
2
9.6
×
10
16
5.5
×
10
16
1.0
×
10
17
Annealing pressure (Torr)
8.0
×
10
-
6
1.0
×
10
-
5
8.0
×
10
-
6
Annealing temperature (°C)
450
600
600
Duration of annealing (h)
5
5
5
