Plasma spray processing
Plasma Spray Processing is a method of giving a material a protective coating through the form of a plasma spray. The wide range of process temperatures allows you to use many different materials and compounds, from high to low melting temperatures. The process itself can be rather complex, because of all the interacting parameters. The applications for this process are increasing every year and will continue to increase because of the ability to vary coatings for different materials. The potential for this process is almost unlimited for substrate coatings.
The theory behind this process is called "Plasma Jet Theory" or "Plasma Flame Theory" even though there is no flame involved in the process.
A plasma is an electrically conductive gas which contains charged particles. When gases reach a high excited state, they can lose some of their electrons and become ionized which produces a plasma that is electrically charged with ions and electrons. In this plasma there are four gases, which are involved with the process:
The plasma spray process can produce temperatures around 7,000 K to 20,000 K. These temperatures are way above the melting point of any known material. This extreme temperature is not the only reason for effective heating properties. The plasma is able to provide large amounts of energy due to the energy changes that can happen with the dissociating molecular gases to atomic gases and ionization with very little change in the temperature.
N2 + E = 2N
This gives two free atoms of Nitrogen
2N + E = 2N+ + 2e-
This gives two Nitrogen ions and two electrons
The reverse of this process will provide the most energy for heating without a great loss in temperature
2N+ + 2e- = 2N + E 2N = N2 + E
Nitrogen and hydrogen are both diatomic gases while helium and argon are monatomic gases. The diatomic gases generally have a high energy contents for a set temperature than the monatomic gases. The monatomic gases generally have a lower energy but are higher in temperature than the diatomic gases.
The most favoured primary gas in this process, it is usually used with a secondary gas as well. This being one of the other three gases that can be used in the process. When used with a secondary gas, the argon wants to increase its energy. Argon is the easiest gas to form a plasma and it isn't as hard on the equipment used.
Mainly used as a secondary gas with argon. Helium is able to give a higher sensitivity to the control of a plasma energy. Mainly used for a high velocity plasma spray of carbide materials when process conditions are critical.
Generally used as a primary gas alone or used with hydrogen. This gas benefits the process because it is the cheapest plasma gas. Nitrogen is generally inert to most materials with the exception being Titanium.
Mainly used as a secondary gas. It is able to dramatically effect the heat transfer properties of a material and can act as an anti-oxidant. Just by adding very small amounts of hydrogen into the plasma gas, it can greatly change the properties of the plasma and energy levels. Therefore it is used as one of the controls for setting the voltage and energy.
How It Works
The plasma spray process is most commonly used in atmospheric conditions but there are some exceptions. Some spraying is done in protective vacuum environments, which have a gas backfilled at low pressure for protection of the process.
Plasma spraying is a Thermal sprayW process in which a molten or heat softened material is sprayed onto a surface to create a coating. Material that is in the form of a powder is injected at a high temperature plasma flame (no fire is actually involved in the process). This material is then heated rapidly and accelerated to a very high velocity. The hot material hits the surfaces and cools very quickly to form a coating. When the process is completed correctly it can be called a "cold process" because the temperature of the object being coated can be kept very low, which avoids damage during processing, metallurgical effects and distortion. 
The plasma gun consists of a tungsten cathode and a copper anode, which are both water cooled. The plasma gas consisting of nitrogen, argon, helium and hydrogen all flow around the cathode and through the anode with is shaped like a constricting nozzle. A high voltage discharge initiates the plasma with causes local ionization and a conductive DC arc to from between the cathode and anode. The arc causes resistance heating which enables the gas to reach extreme temperatures to dissociate and ionize to form a plasma. When the plasma exits the nozzle it is a neutral element (no charge is carried through it). The powder being used is so rapidly heated and accelerated that the spray distances can be from 25 to 150 mm.
Properties of Plasma Sprayed Coatings
There are four properties which are affected by plasma spraying which include Structure, Porosity, Strength and Adhesion, and Surface Quality. 
The size of the spray particles depend on the degree of melting. These spray particles have a spherical form to minimize surface tensions. On contact with the surface, these droplets spread out to a flat or laminate form. Spray particle velocity upon contacting the surface is the main ruling factor for flow. Varying coating quality is caused by differences in the degree of melting and the amount of kinetic energy that is in the spray particles.
During the spray coating application layers of laminate overlap one another. As the laminate forms on the surface, the particles con-solidify by rapid heat conduction. If the surface of the object is cooled by a jet with compressed air or carbon dioxide, cooling rates of 10^6 K/s can be reached; although residual stress caused by this rapid cooling can lead to a loss in strength over the bulk of the material. Along with the cooling, a mismatch in the CoefficientW of thermal expansion between the coating and the surface can also influence the stress state. This can, in turn, effects the strength and the properties of the sprayed coating.
PorosityW is a typical coating feature of plasma spray. Different plasma spray processes can be used and with different parameters a porosity of less than 1% may result, or inversely, the porosity may be very high for the case of porous coatings. Porosity in a sprayed coating can be controlled by the use of a very coarse spray powder. Introducing the powder far downstream in a plasma jet or by using a low-energy plasma can provide such a coarse spray powder.
Strength and Adhesion
In comparison to bulk materials, plasma spray coatings have different physical and mechanical properties. The majority of the plasma strength is formed is within the inter-particle bonding. The adhesion only applies to the surface roughness of the substrate. Increasing the ability for adhesion can be done by roughing or finishing off the substrate by grit blasting and removing the surface rust. The values of adhesion and coating strength can differ greatly (in the case of air plasma spray ceramics we can get 50 MPa and 100 MPa in a vacuum sprayed metal). Positive polarization is a process in which any gas that has been absorbed through the layers, and thin oxide films that appear on the surface are removed.
In many engineering cases a plasma coating of 0.5 mm is sufficient to protect the surface. The high residual stress in thick plasma coatings can reduce secure the bonds to the substrate with the increasing thickness. For thicker coatings it is optimal to minimize the residual stress that is in the coatings, this can be modified with changing the spray parameters and cooling conditions for the thin coatings. Powder size plays an effect in the roughness of the surface work piece, because the grit blasting before the spraying has roughened it up. To lower the magnitude of surface roughness on the substrate, there are finishing processes such as grinding to smoothen up the surface before plasma spraying is started.
Advantages vs. Disadvantages
An advantage of this process is the range of temperatures that can be used to make coatings on materials. This allows the range of high to low melting materials to be used. Also, the plasma coatings are generally denser, stronger, and much cleaner than a thermal spray process. The plasma spray process accounts for a large amount of the thermal spray processes, which makes it the most versatile. 
The major disadvantage to the plasma spray process is that the process itself can be quite complex due to the amount of interacting parameters. Also, the cost of this process could make it not as feasible as a cheap process which can grant almost the same results. Vortex gas flow can create a helical motion on the plasma jet as the gas is heated and accelerated. This can create a strong negative effect on the particle trajectory and the particle heating; this is due to the powder being dragged off by the jet axis when injected into the plasma jet. 
For the process of plasma spraying, a concern would be to create the greatest efficiency of spray on the substrate surface. This isn't only a concern for the process in itself but also for the tools used to complete the process. The quality of the coating is highly dependent on the gun. Designing the tools for the process is a form of art in the fact that it can greatly increase the productivity, efficiency, and profit for a company. 
To increase the efficiency of the deposition and coating quality would be to introduce a anti-vortex at the shroud gas flow area created by the argon gas. This is to counter the effects made by the vortex plasma flow, which in turn can be used to stabilize the cathode arc attachment and increase the life of the anode. This process keeps the particles closer to the torch axis and also reduces the amount of cold air entering the area. Due to this the deposition efficiency and coating quality is able to increase greatly.
The proper introduction of the powder into the plasma is very critical to the efficiency and the coating quality. This powder needs to be fed by a powder feeder with a carrier gas at a set feed rate to get the proper results. Improper introduction and too high of a feed rate will lower the deposition efficiency and if we have a feed rate that is too low the powder will have too much time to oxidize itself and in turn giving a very poor coating quality.
A plasma spray process to increase efficiency, we introduce a high velocity oxygen fuel for spraying, it includes a combustion chamber that creates heat flow to a nozzle downstream from the chamber. The nozzle and/or the chamber comprises of the first layer of material from the flow and a second layer that contacts the first to give the first layer a lower thermal conductivity. When used in the process to coat a substrate, this process lowers the heat loss along with increasing the durability. This causes the deposition efficiency to increase.
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- http://thermalsprayedcoatings.blogspot.com/2008/04/thermal-spray-tooling-efficiency.html , (14 November 2008)
- http://www.springerlink.com/content/942q7p036n353574/fulltext.pdf?page=1 , (17 November 2008)