Biomedical Engineering Reference
In-Depth Information
fused on to a metallic substrate. In history, enamels were first applied
on precious metals (such as gold and silver), then on copper and bronze,
and more recently on iron and steel. This technique was successfully pro-
posed in the past 20 years to produce bioactive glass and glass-ceramic
coatings on Co-Cr, titanium, and Ti-6Al-4V alloys, for orthopedic
applications [2, 4].
Metallic substrates should initially be polished with diamond and
cleaned in acetone and ethanol or chemically etched in acid solutions,
in order to remove the native surface oxide layer. Usually the glass is
synthesized by melting the raw materials in refractory crucibles, then
the melt is quenched in air or in water (see Figure 1 in colour section
and Chapters 1 and 2). The frit obtained by quenching is ground into
a powder by milling and sieved below an average grain size. In order
to apply the coating, the glass powders are often dispersed in a liquid
medium to obtain slurries, which can be applied to the substrate by
dipping, spraying, painting, and so on. Otherwise, substrates can be
covered by glass powders by controlled deposition of suspensions [2].
After drying of the powders, a thermal treatment is carried out, trying
to fulfill the following requirements:
(a) The firing process must be performed at an appropriate temper-
ature that will allow a good softening and sintering of the glass
powders (i.e., above the glass transition temperature, T g ) while
completely avoiding any degradation of the metal. For example,
in the case of titanium substrates, the firing temperature should
be below the
crystallographic transformation of Ti, which
occurs between 885 and 950 C for unalloyed Ti or between 955
and 1010 C for Ti-6Al-4V, and negatively affects the mechanical
properties of the metallic substrates.
(b) The firing time should be as short as possible, in order to prevent
the formation of undesired reaction layers at the interface between
the substrate and the coating, which can lead to poor adhesion
and ease delamination of the coating.
(c) The glass should have a thermal expansion coefficient matching
that of the metal that is being coated to prevent cracking as the
device cools after coating.
(d) The glass coating should maintain its bioactive properties, without
any contamination by metal ions diffused from the substrate, in
order to form hydroxycarbonate apatite (HCA) when in contact
with body fluids.
α β
Search WWH ::




Custom Search