Biomedical Engineering Reference
In-Depth Information
uniform oxide layer over the entire surface.
6
The component, named further on as
OxZr, is finally burnished to produce an articular surface at least as smooth as CoCr
components.
By nature of the diffusion process, an oxygen gradient is created inward the sub-
strate. The gradient is functioning as a transition zone in mechanical properties from
metal substrate to oxide layer, the latter being a true ceramic. The difference in duc-
tility between ceramic and metal substrate is overcome in the transition zone. The
use of a transition zone, although on a more macroscopic scale, is also the basic
philosophy of a proposal for the perfect prosthesis, discussed in Chap. 11 and for
the development of gradient ceramics (see Chap. 9). A zone between the uniformly
grey top layer ZrO
2
in Fig.
5.3
and the metal substrate forms the transition zone.
The structure of the oxide is a dense predominantly monoclinic fine-grained (grain
size in at least one dimension of <100 m, in the limit qualified as nanostructural
zirconia [194].
An oxide layer of a few nanometer is always present on the surface of any metal
unless kept under inert atmosphere. To master the oxygen content, the metallurgical
processing of zirconium is conducted under vacuum or inert gas. The difference in
behavior of the alloys, however, is the velocity of the oxidation reaction and the
quality of the oxide as diffusion barrier for ions to or from the substrate. It is bad
for iron but improves in the following order: Al
2
O
3
,Cr
2
O
3
,TiO
2
,ZrO
2
and Ta
2
O
5
and/or Nb
2
O
5
to name only the oxides of interest in the present context.
Corrosion.
From the E-pH diagram we learn that zirconium is not at all a noble
metal. On the acid side at pH<1 it dissolves as Zr
C
4
,below3.5asZrO
C
2
and above
13 as HZrO
3
[195, pp. 226]. Fortunately in between ZrO
2
is stable and screens the
bulk from the outside world. At pH
D
7, the equilibrium potential Zr/ZrO
2
is about
2 V: contact of the bare metal with an aqueous medium will oxidize the metal
with evolution of hydrogen. The diagram is very similar to the one for titanium
with stable TiO
2
between pH
D
2 and 12 but it has at pH
D
7 a higher equilibrium
potential (about
1.8 V). The corrosion resistance of zirconium is exuberantly great:
it is hardly attacked by concentrated solutions of acetic acid in any combination
with reagents as FeCl
3
, HI, HCl, NH
3
, aqua regia, formic acid, H
2
O
2
,etc.(seeData
sheets of Wah Chang, Allegheny Technology Cy, Albany, Oregon). Much work has
been done on the electrochemical behavior of oxide films in aqueous media. The
results are less relevant here because in the current biomedical applications of OxZr
the oxide films are thicker and denser than those studied in most electrochemical
work (see [196]).
Biocompatibility.
The results of a set of current biocompatibility tests enumer-
ated hereafter were all negative as reported by Davidson et al. [197]:
Cytotoxicity: L929 MEM
7
Mouse Fibroblast Test
Sensitization: Kligman Guinea Pig Maximization Test
6
Oxidized zirconium components are exclusively produced and marketed under the trade name
OXINIUM
TM
by Smith & Nephew, Inc. (Memphis, TN).
7
Abbreviations: MEM: Minimal Essential Medium; USP: United States Pharmacopeia.
