Biomedical Engineering Reference
In-Depth Information
Excellent adhesion to the substrates has been achieved through the
formation of controlled 100-200 nm thick interfacial layers (Ti
5
Si
3
on
Ti-based alloys and CrO
x
on Co-Cr).
Glass coatings can crack if stresses arise due to the glass shrinking at
a different rate from the metal substrate as it cools. The shrinkage of the
glass can be matched to that of the metal by tailoring the composition
of the glass so that its thermal expansion coefficient matches that
of the metal. In fact, the glass should have a slightly lower thermal
expansion than the metal. This may induce only small compressive
stresses, avoiding the generation of tensile thermal stresses during cooling
from the processing temperature to room temperature, which may cause
coating cracking or delamination during processing. Bioactive glasses are
typically silica-based glasses, with silica content below 60wt% (glasses
with silica contents greater than 60wt% are no longer bioactive; see
Chapter 2). Most of these glasses have thermal expansion coefficients
much higher than those of Ti alloys. Thermal expansion of the glass can
be reduced by increasing the SiO
2
content, but this reduces bioactivity
as well [1]. A lower thermal expansion can also be reached by a partial
substitution of CaO by MgO, and of Na
2
ObyK
2
O, matching the
thermal expansion of the coating to that of Ti-based alloys. In that
way, coatings with silica contents below 60wt% that do not crack or
delaminate can be successfully prepared [2]. Another method to match
the thermal expansion of the coating to that of metallic substrates is
to prepare multi-layer coatings. A simple method to produce bioactive
glass-ceramic coatings on Ti-6Al-4V substrates by dipping and firing
has been developed, in which an SiO
2
-CaO-Na
2
O-MgO-P
2
O
5
-K
2
O
glass is used as first layer in direct contact with the metallic substrate
and an SiO
2
-Al
2
O
3
-P
2
O
5
-K
2
O-CaO-F
−
glass-ceramic is used as outer
bioactive layer [6]. The deposition of the intermediate layer was useful
to obtain a good adhesion of the coating to the substrate, to minimize
the reactivity between the substrate and the outer glass-ceramic coating,
and thus to preserve the nature of its crystalline phases. The optimized
coating method was then used to coat Ti-6Al-4V screws for dental
applications.
The reactivity of glasses at the temperatures involved in the enameling
process is often an issue when a bioactive coating with good mechanical
and biological properties is needed. Silica-based glasses have a random
network structure with many open pathways for ion diffusion. Specif-
ically, for bioactive compositions, this property is directly related to
their high ability to form HCA on their surfaces when in contact with
physiological fluids. On the other hand, their open network also makes
