Biomedical Engineering Reference
In-Depth Information
Tabl e 2. 5
Colors of oxide
layers on titanium as a
function of thickness
d (nm)
Color
d (nm)
Color
10-25
Golden
80-120
Yellow
25-40
Purple
120-150
Orange
40-50
Deep blue
150-180
Purple
50-80
Light blue
180-120
Green
Techniques and theoretical background of this art are exposed with Italian charm by
Pietro Pedeferri [56-58]. For the color palette offered by the various thicknesses,
see Table
2.5
(taken from Velten et al. [59]).
Titanium alloys are relatively young members of the metal alloy family and
not produced in quantity until the late 1940s. Their high melting point (1,678
ı
C),
low density and excellent mechanical properties, in particular at high temperature,
made them uniquely attractive to the aerospace industry. These properties together
with a high resistance to corrosion did not escape the attention of the biomedical
world. Moreover, and in this case much like steel, many titanium alloys undergo
allotropic transformations, usually between the high-temperature b.c.c. (ˇ)andlow-
temperature hexagonal (˛) phases, so that a wide variety of microstructures and
properties can be developed by heat treatments and thermomechanical processing
(further reading in Verlinden et al. [53]). Titanium alloys and implants proved to
be a happy and, thus far, a stable marriage. The use of stainless steels and cobalt-
chromium alloys in implant business in the foregoing sections was based on rather
restrained knowledge of the behavior in the biological environment and focused
merely on purely mechanical criteria. With so vast a body of materials properties
documented now, a selection of a material for a specific use can be made on more
solid grounds.
In Fig.
2.7
specific yield strength, YS divided by , and equivalent weight are
plotted for a set of common alloys. The equivalent weight was calculated for a bar
with square section as represented in Fig.
2.2
: constant load of 4,000 N, beam length
l
100 mm, thickness t calculated by (
2.1
) needed to limit the deflection ı to 1 mm.
The equivalent weight for Zr 705 is about equal to that of Ti CP4 and Mg CP. Zr and
Mg alloys are discussed in later chapters.
Inspection of the figure learns that tantalum is not a good partner (very high
density,
D
16:55, and low YS): a tantalum hip stem would be a cumbersome
block. After all a pity because it has a superior corrosion resistance. The other
extreme is beryllium: low density, high YS but completely shunted off by its toxi-
city. Nontoxicity as will be discussed later is an absolute
conditio sine qua non
in
implants! For this reason, aluminum alloys such as Alu6063 (about similar to F75)
are rejected because of their toxicity, moreover they suffer from various diseases:
toxic, mechanically not that excellent and a corrosion behavior biologically below
the limit. The others are quite acceptable but Ti4Al6V, or Ti6Al7Nb as preferred
today is the best choice, mechanically speaking (similar mechanical properties but
vanadium is toxic while niobium is not). A wrought Ti6Al7Nb (˛
D
ˇ) alloy T67 was
developed in the early 1980s [60]. We narrowed the selection by including corrosion
C
