Biomedical Engineering Reference
In-Depth Information
Long process times or high temperatures could cause extensive reaction
between the glass and the substrate, with the formation of oxides or
other products at the coating interface, and thus strongly affecting the
coating adhesion. These reactions can be observed when a silica-based
glass is used as the coating on Ti-based alloys or Co-Cr alloys according
to [2, 5]:
8Ti(substrate)
+
3SiO
2
(glass)
→
Ti
5
Si
3
(interface)
+
3TiO
2
(interface)
+
→
+
5Ti(substrate)
3SiO
2
(glass)
Ti
5
Si
3
(interface)
3O
2
3
2
Na
2
O(glass)
1
2
Cr
2
O
3
(interface)
Cr(substrate)
+
→
+
3Na(g)
1
2
SiO
2
(glass)
1
2
Si(interface)
Cr(substrate)
+
→
CrO(interface)
+
Additionally, several reactions have been observed between Ti substrates
and glasses containing P
2
O
5
:
16Ti(substrate)
+
6P
2
O
5
(glass)
→
4Ti
4
P
3
(interface)
+
15O
2
9Ti(substrate)
+
3P
2
O
5
(glass)
→
2Ti
4
P
3
(interface)
+
7O
2
+
TiO(interface)
17Ti(substrate)
+
6P
2
O
5
(glass)
→
4Ti
4
P
3
(interface)
+
14O
2
+
TiO
2
(interface)
These reactions can be successfully controlled by careful optimization
of the enameling treatment, and both time and temperature play equally
important roles for the final coating properties. The conventional enam-
eling theory proposes that the glass in contact with the alloy should be
saturated with the lowest valence oxide of the metal, without any inter-
facial layers [5]. In this way, a transition region should form between
the metallic bonding of the substrate and the ionocovalent bonding
of the glass, providing a ''continuity of electronic structure'' that will
result in a good bonding between the glass and the metal. Anyway,
a proper tailoring of the interfacial reaction layer could improve the
coating adhesion. For this purpose, Lopez-Esteban
et al
. [2] formulated
a new family of glasses in the SiO
2
-Na
2
O-K
2
O-CaO-MgO-P
2
O
5
sys-
tem. Glass coatings,
m thick, have been fabricated onto metallic
orthopedic implants. Care has been taken to avoid excessive interface
reaction, in order to prevent the formation of a thick reaction layer
accompanied by bubbles in the glass and loss of adhesion.
∼
100
μ
