Biomedical Engineering Reference
In-Depth Information
Films and coatings were the irst commercial applications of the
sol-gel process. The development of sol-gel-based optical materials
has also been quite successful, and applications include monoliths
(lenses, prisms, and lasers), ibers (waveguides), and a wide variety
of optical ilms. Other important applications of sol-gel technology
utilize controlled porosity and high surface area for catalyst supports,
porous membranes, and thermal insulation.
4.1.2.2 Form in place processes
These include lithography, vacuum deposition processes such as
physical vapor deposition (PVD) and chemical vapor deposition
(CVD), and spray coatings. These processes are more geared to the
production of nanostructured layers and coatings, but can be used to
fabricate nanoparticles by scraping the deposits from the collector
[55].
Physical vapor deposition (PVD) is fundamentally a vaporization
coating technique, involving transfer of material on an atomic level.
It is an alternative process to electroplating. The process is similar
to chemical vapor deposition (CVD) except that the raw materials/
precursors, i.e. the material that is going to be deposited starts out
in solid form, whereas in CVD, the precursors are introduced to the
reaction chamber in the gaseous state. It incorporates processes
such as sputter coating and pulsed laser deposition (PLD).
PVD processes are carried out under vacuum conditions. The
process involved four steps: evaporation, transportation, reaction,
and deposition. During the irst stage, a target consisting of the
material to be deposited is bombarded by a high energy source such
as a beam of electrons or ions. This dislodges atoms from the surface
of the target, “vaporizing” them. The next process simply consists of
the movement of “vaporized” atoms from the target to the substrate
to be coated and will generally be a straight line affair.
In some cases, coatings will consist of metal oxides, nitrides,
carbides, and other such materials. In these cases, the target will
consist of the metal. The atoms of metal will then react with the
appropriate gas during the transport stage. For the above examples,
the reactive gases may be oxygen, nitrogen and methane. PVD
coatings are deposited for numerous reasons. Some of the main ones
are: improved hardness and wear resistance, reduced friction, and
improved oxidation resistance.
 
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