Environmental Engineering Reference
In-Depth Information
tion into the surface is limited to less than 0.5
m with an approximately Gaussian
distribution. Heating of the substrate by such interactions is minimal; once the
ions are implanted, no diffusion occurs. The fraction of the implanted ions can
be as high as 0.30 locally. In high-temperature applications, its principal use has
been limited to the implantation of reactive elements such as Y, Ce, La, etc., into
metal/alloy substrate. Since the implanted element is confined initially to a near-
surface layer, its subsequent redistribution during oxidation can provide unique
information about atomic migration mechanisms. This technique has remained
as a research tool and has yet to find industrial applications.
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Flame and Plasma Spraying.
These methods are similar to the weld overlay
techniques. They are generally based on processes [73,74] by which a metallic
or nonmetallic powder or wire is injected into the flame or plasma, where it melts
down to form small molten droplets. These droplets are then projected to the
metal/alloy surface to be coated, splatting and freezing on impact. The integrity
of the coating depends on atomization, melting point of the particles, degree of
oxidation of the droplets, and velocity at impact. In plasma spraying (in atmo-
spheric air), a direct-current electric arc is struck between the nozzle and the
electrode, while a stream of mixed gases (commonly used are nitrogen, hydrogen,
argon, and helium or their mixtures) is passed through the arc. This results in
dissociation and ionization of the gases, thereby producing a high-temperature
plasma (temperatures up to 16273 K) stream from the gun nozzle, although in
practice most coatings are deposited with a flame temperature in the range of
6273-11273 K. The plasma torch acts as a high enthalpy heat source and acceler-
ates the powders to velocities upto 300 m/s. In the short residence time of a few
milliseconds, the powders transform to molten droplets, which hit and flatten on
the component to be coated. By repeated movement of the gun, the coating is
built up layer by layer. A cross-sectional view of the plasma spray gun head is
shown in Fig. 6.52 [73]. However, the coatings formed by this technique are
generally porous (porosity in the 2-10% range) and their bonding to the sub-
strates is often not satisfactory. Spraying in the air causes oxidation of the pow-
ders due to turbulent mixing of the ambient air with the plasma gas. For improved
bonding, one approach is to use a first layer of a material that undergoes an
exothermic reaction with the substrate, thus developing a metallurgical bond. The
intended coating material is then sprayed over the bond layer. Often a third layer
is also applied to seal the top surface of the coating. A typical micrograph of
plasma-sprayed ZrO
2
CaO
Al
2
O
3
coating showing lamellar structure is de-
picted in Fig. 6.53 [74].
The impetus to improve the adhesive strength and homogeneity of coatings
by increased particle velocity and to minimize the disadvantage associated with
spraying in air brought changes in plasma coating technology. The latest state
of technology is the low-pressure plasma spraying (LPPS) process, where the
