Biomedical Engineering Reference
In-Depth Information
Table 3.2.9-1 Chemical compositions of stainless steels used for implants
Material
ASTM designation
Common/trade names
Composition (wt.%)
Notes
Stainless steel
F55 (bar, wire)
AISI 316 LVM
60-65 Fe
F55, F56 specify 0.03 max for P,S.
F56 (sheet, strip)
316L
17.00-20.00 Cr
F138, F139 specify 0.025 max for
P and 0.010 max for S.
F138 (bar, wire)
316L
12.00-14.00 Ni
F139 (sheet, strip)
316L
2.00-3.00 Mo
LVM ΒΌ low vacuum melt.
max 2.0 Mn
max 0.5 Cu
max 0.03 C
max 0.1 N
max 0.025 P
max 0.75 Si
max 0.01 S
Stainless steel
F745
Cast stainless steel
cast 316L
60-69 Fe
17.00-20.00 Cr
11.00-14.00 Ni
2.00-3.00 Mo
max 0.06 C
max 2.0 Mn
max 0.045 P
max 1.00 Si
max 0.030 S
substrate to about one-half or more of the alloy
0
s melting
temperature, which is meant to enable diffusive mech-
anisms to form necks that join the beads in the coating to
one another and to the implant's surface (
Fig. 3.2.9-2
).
Such temperatures can also modify the underlying me-
tallic substrate.
An alternative surface treatment to sintering is plasma
or flame spraying a metal onto an implant's surface. Hot,
high-velocity gas plasma is charged with a metallic
powder and directed at appropriate regions of an implant
surface. The powder particles fully or partially melt and
then fall onto the substrate surface, where they solidify
rapidly to form a rough coating (
Fig. 3.2.9-3
).
Other surface treatments are also available, including
ion implantation (to produce better surface properties),
nitriding, and coating with a thin diamond film. In ni-
triding, a high-energy beam of nitrogen ions is directed
at the implant under vacuum. Nitrogen atoms penetrate
the surface and come to rest at sites in the substrate.
Depending
cast, and therefore is frequently machined even though
titanium in general is not considered to be an easily ma-
chinable metal.
Another aspect of fabrication, which comes under the
heading of surface treatment, involves the application of
macro- or microporous coatings on implants, or the de-
liberate production of certain degrees of surface rough-
ness. Such surface modifications have become popular in
recent years as a means to improve fixation of implants in
bone. The surface coating or roughening can take various
forms and require different fabrication technologies. In
some cases, this step of the processing history can con-
tribute to metallurgical properties of the final implant
device. For example, in the case of alloy beads or ''fiber
metal'' coatings, the manufacturer applies the coating
only over specific regions of the implant surface (e.g., on
the proximal portion of a femoral stem), and the means
by which such a coating is attached to the bulk substrate
may involve a process such as high-temperature sintering.
Generally, sintering involves heating the coating and
on
the
alloy,
this
process
can
produce
