Biomedical Engineering Reference
In-Depth Information
As cast material, as originally used in orthopaedics, Co-Cr alloys are substantially
inferior in strength to other implant metals. More recently post casting processing,
such as the Hot Isostatic Press (HIP) process substantially improves the strength of
Co-Cr alloys to the point where they approach that of Titanium [10].
Such improvement is necessary to minimize the risk of implant fractures [11].
Still the risk of intergranular corrosion leading to fracture [12] is significant unless
the casting and post casting processes are very well controlled.
c) Biocompatibility
Cobalt alloys have been used successfully for implants for more than a half
century. Clearly, then, they have an acceptable level of biocompatibility. This
biocompatibility, as in the case of titanium, stems from the formation of a hard
oxide film. Since cobalt is not as reactive as titanium this film is not as quick in
formation and is not as self healing. It does not provide corrosion protection equal
to the oxide film on titanium. As a result some small degree of corrosion, and
therefore, metal ion release takes place from Co-Cr implants.
These corrosion products are, to some degree, toxic and can provoke an allergic
reaction [13-16]. It has been observed by the authors that bone will not always grow
against Co-Cr alloys. There is usually a fibrous tissue layer between Co-Cr and bone
in vivo.
1.5.1.3 Stainless Steel
a) Introduction
Steel is an iron - carbon alloy. A simplified iron - carbon equilibrium diagram is
shown in Fig. 1.6 [1].
At room temperature it is a two phase alloy called “pearlite”, consisting mostly
of “ferrite”, which has a body centered cubic crystal structure and is magnetic, and
“cementite” a compound of iron and carbon. At high enough temperatures the
material becomes a single phase, “austenite”, which is face centered cubic and non
magnetic. The amount of cementite increases with the amount of carbon. The
corrosion resistance of steel declines with an increase in carbon content since there
is more of the second phase to promote corrosion.
Moderate and high (.04-1.0%) carbon steels may be hardened by heating at
temperatures in the austenitic region and then very rapid cooling (quenching) to room
temperature. This can produce steels with an ultimate strength in excess of 1000 MPa
and a hardness of RC65. The resulting microstructure is called “martensite”. Such
materials are then, usually heated at a temperature midway between melting and
room temperature for several hours (tempered) to minimize the internal stresses and
the brittle behavior of the “as quenched” material.
Other alloying elements are used to, alter the phase transition regions so as to
improve the results of heat treatment, improve mechanical properties and,
particularly, corrosion resistance. The focus here will be on the latter. A substantial
amount of chromium is used for all stainless steels since these steels develop a
