Metallic Biomaterials - Biomaterials - page 4

glow-discharge nitrogen implantation are widely used in order to improve corrosion resistance, wear
resistance, and fatigue strength of 316L stainless steel (Bordiji et al., 1996).

1.3 CoCr Alloys

There are basically two types of cobalt-chromium alloys: (1) the castable CoCrMo alloy and (2) the
CoNiCrMo alloy, which is usually wrought by (hot) forging . The castable CoCrMo alloy has been used for
many decades in dentistry and, relatively recently, in making artificial joints. The wrought CoNiCrMo
alloy is relatively new, now used for making the stems of prostheses for heavily loaded joints such as the
knee and hip.
The ASTM lists four types of CoCr alloys that are recommended for surgical implant applications: (1)
cast CoCrMo alloy (F75), (2) wrought CoCrWNi alloy (F90), (3) wrought CoNiCrMo alloy (F562), and
(4) wrought CoNiCrMoWFe alloy (F563). The chemical compositions of these alloys are summarized in
Table 1.3. Currently only two of the four alloys are used extensively in implant fabrications: the castable
CoCrMo and the wrought CoNiCrMo alloy. As can be noticed from Table 1.3, the compositions of these
alloys are quite different from one another.
The two basic elements of the CoCr alloys form a solid solution of up to 65% Co. Molybdenum is
added to produce finer grains which results in higher strengths after casting or forging. Chromium
enhances corrosion resistance as well as solid solution strengthening of the alloy.
The CoNiCrMo alloy originally called MP35N (Standard Pressed Steel Co.) contains approximately
35% Co and Ni each. This alloy is highly corrosion resistant to seawater (containing chloride ions)
under stress. Cold working can increase the strength of this alloy considerably as shown in Figure 1.3.
However, there is a considerable difficulty of cold working on this alloy, especially when making large
devices such as hip joint stems. Only hot forging can be used to fabricate a large implant with this alloy.
The abrasive wear properties of the wrought CoNiCrMo alloy are similar to those of cast CoCrMo
alloy (about 0.14 mm/year in joint simulation tests with ultra-high-molecular-weight polyethylene
acetabular cup); however, the former is not recommended for the bearing surfaces of joint prosthesis
because of its poor frictional properties with itself or other materials. The superior fatigue and ultimate
tensile strength of the wrought CoNiCrMo alloy make it suitable for such applications, which require

TABLE 1.3 Chemical Compositions of Co-Cr Alloys

CoCrMo
(F75)

CoCrWNi
(F90)

CoNiCrMo
(F562)

CoNiCrMoWFe
(F563)

Element Min. Max. Min. Max. Min. Max. Min. Max.

Cr 27.0 30.0 19.0 21.0 19.0 21.0 18.00 22.00

Mo 5.0 7.0 — — 9.0 10.5 3.00 4.00

Ni — 2.5 9.0 11.0 33.0 37.0 15.00 25.00

Fe — 0.75 — 3.0 — 1.0 4.00 6.00

C — 0.35 0.05 0.15 — 0.025 — 0.05

Si — 1.00 — 1.00 — 0.15 — 0.50

Mn — 1.00 — 2.00 — 0.15 — 1.00

W — — 14.0 16.0 — — 3.00 4.00

P — — — — — 0.015 — —

S — — — — — 0.010 — 0.010

Ti — — — — — 1.0 0.50 3.50

Co Balance

Source: Adapted from ASTM. 1992. Annual Book of ASTM Standards , Vol. 13,
Medical Devices and Services , F75-F87, p. 42; F90-F87, p. 47; F562-F84, p. 150.
Philadelphia, PA: ASTM.