Biomedical Engineering Reference
In-Depth Information
Table 2.4 Typical mechanical properties of CoCr alloys (Brunski, 2004)
ASTM
Condition
Young's
Yield
Tensile
Fatigue
designation
modulus
strength
strength
endurance
(GPA)
(MPa)
(MPa)
limit (at 107
cycle, R ÿ1)
(MPa)
F75
As-cast/annealed
210
448±517
655±889
207±310
Powder
253
841
1277
725±950
metallurgy
product, hot-
isostatically
pressed
F799
Hot forged
210
896±1200 1399±1586
600±896
F90
Annealed
210
448±648
951±1220 Not available
44% Cold worked
210
1606
1896
586
F562
Hot forged
232
965±1000
1206
500
Cold worked, aged
232
1500
1795
689±793
(axial tension
R 0:05,
30Hz)
R
max
remove these problems, powder metallurgical methods are usually used for
decreasing grain sizes as well as formulating finer distributions of carbides,
which leads to better fatigue properties and higher yield strength. F799 has a
similar chemical composition to F75 but has been treated by hot forging after
casting. As a result, its mechanical properties (such as yield, fatigue, and
ultimate tensile strength) increase by 100% compared with as-cast F75. The
mechanical properties of F90 are also twice as high as those for F75, due to the
addition of Ni and processed by cold work. F562, which is processed by cold
working and aging, is one of the strongest materials for implant applications.
Cobalt±chromium alloys are mostly used for bearing components due to its
excellent wear resistance. Table 2.5 lists clinical wear behavior for cobalt±
chromium alloys used in total hip replacements. Compared with currently used
metal-on-cross-linked UHMWPE bearings, second generation (reintroduced into
the field in the late 1980s) metal-on-metal bearings exhibit much better wear
properties. They can even compete with ceramic-on-ceramic bearings while they
are much stronger than ceramics for loading-bearing situations.
Biocompatibility
Although cobalt±chromium alloys are generally considered biocompatible, they
are not considered as optimal as titanium alloys in term of new bone tissue
