Biomedical Engineering Reference
In-Depth Information
as roughness, porosity, adhesion to the substrate, and
thickness, are a direct consequence of the choice and
interaction of the spray parameters.
With reference to the surrounding atmosphere, the
coating process can take place in various conditions, as
shown in the subclassification of
Fig. 9.1
. Atmospheric
plasma spraying (APS) is a widely used process, as it is
the most cost-effective. In the APS process, the parts to
be coated are usually cooled with air. This process is
used when coating material oxidation, up to a certain
degree, is considered acceptable. In shrouded plasma
spraying (SPS), the substrate is shrouded by an inert
gas flow that reduces oxidation phenomena. Vacuum
plasma spraying (VPS) or low-pressure plasma
spraying (LPPS) is very effective for oxidation-sensi-
tive materials. Furthermore, due to the reduced
chamber pressure (commonly between 50 and
250 mbar), the plasma jet reaches higher velocity. The
higher kinetic energy of the injected particles, together
with the reduced oxidation, leads to higher cohesive
strength of the coating, as well as higher adhesive
strength to the substrate.
One of the key factors that regulates the adhesion
to the substrate is the pretreatment of the surface.
Sandblasting is an inexpensive, simple, and rapid
technique that is widely used for this purpose.
However, sandblasting factors such as type, hardness
and size of abrasive particles, process pressure, and
process time must be taken into account and carefully
adjusted for different substrates.
Being a highly reactive metal, titanium powder
undergoes a change in chemical composition during
plasma spraying and gas adsorption onto particles,
oxidation, nitridation, and phase transitions may
occur. Through a careful optimization of the spraying
parameters and an appropriate choice of the sprayed
particles, a variety of plasma-sprayed Ti coatings
with different characteristics can be obtained.
Generally, APS coatings present a higher percentage
of oxidized areas than coating sprayed under special
atmosphere, and therefore lower adhesive and cohe-
sive strength. However, they match regulatory
requirements and compact rough APS coatings are
widely used in the clinical field with a successful
long-term implantation history.
Processes such as VPS and SPS are specifically
suitable for production of open-porosity structures,
which allow bone ingrowth providing remarkably
stable bone
e
implant interfaces.
Figure 9.4
shows
different titanium coatings that are successfully used
for metal prostheses made of Ti alloys or cobalt
e
chrome alloys.
Artificial hydroxyapatite (Ca
10
(PO
4
)
6
(OH)
2
), due
to its similarity to the mineral phase of natural bone
hard tissue, is considered a bioactive material. This
means that HA can participate directly in the natural
bone-forming processes as a source of calcium and
phosphate ions. Nevertheless, its poor mechanical
properties hinder the use of HA as a bulk material for
areas where high stress occurs. The combination of
bioactive HA coatings and mechanically strong
metals was the key to the successful fabrication of
surgical implants for load-bearing applications.
Among the different methods used to obtain HA
coatings, plasma spray is still considered the most
efficient technique in terms of cost/benefit
[28
e
30]
.
The idea of using plasma spraying for coating
endoprostheses was first employed in the 1970s
[31,32]
, and its clinical benefits have been demon-
strated in many scientific publications
[33
e
39]
.HA
undergoes chemical and phase variations during
plasma spraying: dehydration occurs above 800
C
and decomposition to tricalcium phosphate (
9.3 Biomedical Plasma-Sprayed
Coatings
There are mainly two types of plasma spray
coatings used in biomedical applications in order to
improve the osteointegration of joint replacement
prostheses: titanium (Ti) and hydroxyapatite (HA).
Due to the presence of a very protective oxide
layer, titanium is a well-known and clinically proven
metal used for the manufacture of implants. The
natural TiO
2
layer is self-protecting and is quickly
reformed after having been locally removed.
Provided the surface is not smooth, the titanium
oxide layer permits a close contact to the bone tissue
[26,27]
. The coating of artificial joint components
with plasma-sprayed titanium has been, therefore, the
logical solution in order to produce a rough surface
that facilitates bone apposition and long-term
stability of implants
[31]
.
a
-TCP
and
-TCP) and tetracalcium phosphate (TTCP)
takes place in the range of 1000
e
1400
C. Upon
rapid cooling, virtually all melted portions of HA turn
into a glassy phase in which calcium oxide (CaO) can
form. Therefore, the final plasma-sprayed HA
coating has different characteristics than the initial
powder, with alterations in both chemical composi-
tion and crystallinity
[28,38]
. It is worth remarking
b
