Biomedical Engineering Reference
In-Depth Information
hip or knee prosthesis. The main disadvantages of Co-based alloys are
high moduli and density, which are almost twice those of Ti-alloys,
and this increase in modulus may have an effect on the load transfer to
bones in orthopedic devices [PAR 07].
The most widely used Co-based alloy in medical applications is the
Co-Cr-Mo alloy known as Vitallium (Table 3.1). The good corrosion
resistance of this alloy is attributed to the high chromium content (27
-
30%) which produces a Cr
2
O
3
passive film at the implant surface. The
structure of the alloy is a metastable face-centered cubic austenite
(Co-rich matrix) with interdendritic regions rich in Cr, Mo and C that
form carbides (primarily Cr
23
C
6
). For high-C alloy (0.35%), the
carbide regions are hardened during functional loading, and therefore
the cored structure is responsible for the material's high wear
resistance. Indeed, Co-Cr-Mo alloys are the most resistant metallic
biomaterials [PIL 09]. However, the carbide zones are sites for crack
initiation and may define propagation pathways along the grain
boundaries. In addition, Co-based alloys are difficult to machine due
to a low ductility resulting from the extensive carbide networks. As a
consequence, they are generally casted leading to coarse grain size
which gives weaker materials in comparison with wrought alloys
[DAV 03]. To reduce the grain size and obtain a higher strength,
molybdenum is added (5
-
7%). Moreover, to design the casted alloy, a
ceramic mold is used and the mold temperature impacts the grain size
(from hundreds of microns to the millimeter): high temperature leads
to larger grains and therefore inferior mechanical properties.
Nevertheless, high-temperature process gives large and far apart
carbide precipitates resulting in a less brittle material [PAR 07].
Finally, defects may arise from the casting due to ceramic inclusions
in the metal. These kinds of inclusions would be a site for crack
initiation and contribute to fatigue fracture of the device [BRU 04].
To avoid any problems that might be induced by casting processes,
Co-Ni-Cr-Mo and Co-Cr-W-Ni alloys
2
were developed with almost
2 Co-Ni-Cr-Mo composition: Co balancing, 33-37% Ni, 19-21% Cr, 9-10.5% Mo,
and max. 1% Ti, 1% Fe, 0.15% Si, 0.15% Mn, 0.025% C, 0.015% P, 0.01% S. Co-Cr-
W-Ni composition: Co balancing, 19-21% Cr, 14-16% W, 9-11% Ni, 1-2% Mn,
0.05-0.15% C and max. 3% Fe, 0.4% Si, 0.04% P, 0.03% S [BRU 04].
