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Air Plasma Spray
What is Plasma?
According to the Coalition for Plasma Science (CPS) groups, they explain the plasma
phenomenon as the following.
Plasma is often called the "Fourth State of Matter," the other three being solid, liquid and
gas. A plasma is a distinct state of matter containing a significant number of electrically
charged particles, a number sufficient to affect its electrical properties and behavior. In
addition to being important in many aspects of our daily lives, plasmas are estimated to
constitute more than 99 percent of the visible universe (see figure 1).
Figure 1: Lightning bolts are Plasma phenomenon
In an ordinary gas each atom contains an equal number of positive and negative charges;
the positive charges in the nucleus are surrounded by an equal number of negatively
charged electrons, and each atom is electrically "neutral." A gas becomes a plasma
when the addition of heat or other energy causes a significant number of atoms to release
some or all of their electrons. The remaining parts of those atoms are left with a positive
charge, and the detached negative electrons are free to move about. Those atoms and the
resulting electrically charged gas are said to be "ionized." When enough atoms are
ionized to significantly affect the electrical characteristics of the gas, it is a plasma.
In many cases interactions between the charged particles and the neutral particles are
important in determining the behavior and usefulness of the plasma. The type of atoms
in a plasma, the ratio of ionized to neutral particles and the particle energies all result in
a broad spectrum of plasma types, characteristics and behaviors. These unique
behaviors cause plasmas to be useful in a large and growing number of applications
important to our lives and to the world around us. [1]
Plasmas occur naturally on the earth in flames, electrical discharges, lightning bolts and
the aurora borealis (Northern Lights).
The Solar Winds
A naturally occurring Plasma phenomenon where the earth is protected by its
magnetic field (see figure 2).
Figure 2: Plasma phenomenon on Earth
The Plasmas created by this phenomenon are called solar winds, most of which we are
protected from by the earth's magnetic field. Flame Spray Technologies (FST) uses
this energy when creating plasma by passing an electric current through a gas such as
argon or nitrogen. This provides an energy heat source of around 15000°C under high
pressure which heats and propels the coating material onto the substrate. The
utilization of this technology enables Flame Spray Technologies (FST) to spray almost
any metallic or ceramic on to a huge range of materials with tremendous bond strength
and without heating the substrate to cause distortion. [2]
Air Plasma Spraying Ceramic onto Substrate [2]
In this process, a plasma jet melts the coating raw material in the form of powder. The
plasma is created in a plasma gun. Because this process conducted in air, it is called
the air plasma spray (APS) process [Shuda Bose’s book].
The plasma spraying process involves the latent heat of ionized inert gas (Plasma)
being used to create the heat source. The most common gas used to create the plasma
is Argon; this is referred to as the primary gas (see figure 3).
Figure 3: Schematic of Air Plasma Spray (APS) ceramic onto the substrate.
APS provides multi-layer ceramic with splat structure.
Argon flows between the electrodes through nozzle. A high frequency or high voltage
alternating electric arc is struck between the electrodes, which ionize the gas stream.
By increasing the arc current, the arc thickens and increases the degree of ionization.
This has the effect of increasing the power and also, due to the expansion of gas, an
increase in the velocity of gas stream.
With a plasma created by argon only, it requires a very large arc current (typically 800
to 1000amps) to create sufficient power to melt most materials. With this level of arc
current, the velocity may be too high to allow materials with a high melting point to be
made molten. Therefore, to increase the power to a level sufficiently enough to melt
ceramic materials it is necessary to change the thermal and electrical properties of the
gas stream. This is generally done by adding a secondary gas to the plasma gas stream
(usually Hydrogen). Once the appropriate gas stream has been established for the
material being sprayed, the feed stock (Material in various powder forms) is injected
into the gas stream [2]. Hydrogen is added to the gas to provide increased enthalpy
and higher plasma temperature. The additional enthalpy is derived from the
dissociated hydrogen atoms recombining into a molecule. Plasma gun operates at a
power rating of 40 kW, the plasma temperature just outside of the nozzle is very high,
and could be as high as 15000K (26500°F). Depending on the electrode shapes,
power input, the gas pressure and distribution, the plasma plume length varies
between 3 and 15cm. The gas velocity within the plasma ranges between several
hundred and 1000m/s. One attribute of the plasma is that the temperature drops off
very rapidly from the center to the outer periphery of the jet, thus requiring no
containment [Bose’s book].
Figure 4: Schematic of a plasma spray process.
Microstructure of ceramic coated by Air Plasma Spray (APS)
(a)
(b)
(c)
Figure 6: Microstructure of ceramic coated by Air Plasma Spray process.
(a) & (b): Microstructure of ceramic from literature
(c): Ceramic’s microstructure of sample with thickness measured.
References:
[1] Coalition Plasma Science “What is Plasma?”
http://www.plasmacoalition.org/what.htm
[2] Plasma & Thermal Coatings Applied Surface Technology, Ltd
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