Environmental Engineering Reference
In-Depth Information
WF
6(g)
3H
2(g)
→
W
(deposit)
6HF
(g)
(6.57)
and
TiCl
4(g)
2Mg
(g)
→
Ti
(deposit)
MgCl
2(g)
(6.58)
The gas phase may be composed of metal halides, metal carbonyls, metal
hydrides, organometallic compounds, etc. Reactions generally take place in the
temperature range of 423-2473 K and frequently between 773-1373 K. Both
open-loop and closed-loop reactor systems are used to produce CVD coatings.
Most of the CVD processes are carried out in open-loop systems, where the re-
actant gases are continually supplied from one end of the reactor and removed
from the other. The major advantages of CVD processes are:
1.
Possibility of forming various types of deposits (elemental metal/alloy, TiC,
TiN, Al
2
O
3
etc.)
2.
Good-quality deposits with widely varying structures (amorphous, crystal-
line, epitaxial, whiskers, etc.)
3.
High rates of deposition
4.
Possibility of coating complex-shaped bulk components.
Hybrid processes like plasma-assisted CVD (PACVD) and laser-assisted CVD
(LACVD) are presently gaining popularity. PACVD is being used on an industrial
scale in the fields of microelectronics, optics, solar energy devices, and so forth.
In this technique, chemically reactive species, such as ions and free radicals, are
created from the gas phase with the help of a plasma. In LACVD, the laser is
used as a heat source, and under appropriate conditions only the surface of the
substrate is heated to the CVD reaction temperature.
6.8.3 Thermal Barrier Coatings
In gas turbine operation, a considerable amount of energy derived from the fuel
combustion process is dissipated through the engine structure and the cooling
system. During combustion in a diesel engine, components like pistons, valves,
liners, cylinder covers, etc., attain high surface temperatures. The exchange of
heat occurs by convection and radiation or by direct contact with the flame. The
absorbed heat must be transported away to retain the mechanical, thermal, and
corrosion resistance properties of the component materials. The energy balance
of a cylinder unit in a diesel engine demonstrates that nearly 50% of the energy
produced in the combustion process is removed with cooling water/air and
through the exhaust gas. To save energy, it is essential and advantageous to pro-
tect the hot parts of an engine by a thermally insulating layer. A ceramic layer
would prevent heat transfer from the combustion zone to the coolant and sur-
roundings. It would not only contribute to the reduction in temperature of the
metallic components during service but provide protection against corrosion. Re-
