Plasma & Thermal Coatings Ltd: Technologies
The demands for engineering coatings are becoming more and more stringent. Environmental concerns are also being considered as an integral part of the design process. For future economic competitiveness and a lower environmental impact we must therefore turn our attention to processes that use the minimum of resources. Thermal spraying is an attractive coating technique as it offers a wide choice of materials and processes that have a reduced impact on the environment when compared to conventional plating processes.
Thermal spray coating techniques such as flame spraying, wire arc spraying and plasma spraying, allow many problems of wear, corrosion and thermal degradation to be resolved by engineering the surface with tailor-made coatings.
Compared to traditional surface modification processes, thermal spraying offers greater thickness capability, no part size restrictions, it can be performed in situ, and it produces minimal noxious waste. High processing temperatures allow deposition of many high melting point materials onto a relatively cold substrate.
Click on any TOPIC below for further detail.
Principles of Thermal Spraying
High Velocity Oxy Fuel (HVOF)
Plasma Spraying
What is Plasma
Arc Wire
Combustion Spraying
Principles of Thermal Spraying
The Basics
All thermal spraying processes rely on the same principle of heating a feed stock, (Powder or Wire) and accelerating it to a high velocity and then allowing the particles to strike the substrate. The particles will then deform and freeze onto the substrate.
The coating is formed when millions of particles are deposited on top of each other. These particles are bonded by the substrate by either mechanical or metallurgical bonding.
The Process
1. The first step of any coating process is surface activation. This is done by cleaning and grit blasting the surface to be coated. Masking techniques are normally adopted for components that only need specific areas coated.
2. The second step is to melt the material, this is done by introducing the feed stock material into the hot gas stream. The hot gas stream is produced by either chemical reaction (Combustion) or by physical reaction (Plasma).
3. Thirdly the particles are then accelerated to the substrate by the gas stream and deform on impact to form a coating.
4. Finally the coatings are inspected and assessed for quality by either mechanical or microstructural evaluation.
High Velocity Oxy Fuel (HVOF)
HVOF is a thermal spray system utilising the combustion of gases, such as Hydrogen or a liquid fuel such as kerosene. Fuel and oxygen mix and atomise within the combustion area under conditions that monitor the correct combustion mode and pressure. The process creates a very high velocity which is used to propel the particles at near supersonic speeds before impact onto the substrate. One of the basic rules of spraying is that high combustion pressure = high gas velocity, high particle velocity and resulting high coating quality.
One of the key benefits of this system's high velocity is the extremely high coating density and low oxide content. The low oxides are due partly to the speed of the particles spending less time within the heat source and partly due to the lower flame temperature (around 3,000 °C) of the heat source compared with alternative processes.
Schematic of HVOF Combustion Chamber
As well as producing excellent bond strength, some HVOF coatings can be sprayed very thick due to the exceptionally high velocities producing coatings in compression instead of tension. This enables materials such as carbide to be applied very quickly in excess of 6mm.
Plasma Spraying
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.
Argon flows between the electrode and nozzle. A high frequency or high voltage alternating electric arc is struck between the nozzle and the electrode, which ionizes 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 1,000amps) 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.
What is Plasma
Plasma is often referred to as the fourth state of matter. Plasma, like the other three states of matter (Solid, Liquid and gas) has it's own unique properties. Just as most substrates will become solid if cooled enough, any substance will become a plasma if heated enough. In a plasma the electrons are stripped from the atoms creating a substance that resembles a gas but that conducts electricity. 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 it's magnetic field
The Plasmas created by this phenomena are called solar winds, most of which we are protected from by the earth's magnetic field.
Plasma & Thermal Coatings Ltd use this energy when creating a plasma by passing an electric current through a gas such as argon or nitrogen. This provides an energy heat source of around 15,000 °C under high pressure which heats and propels the coating material onto the substrate.
The utilisation of this technology enables Plasma & Thermal Coatings Ltd 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.
Arc Wire
This form of thermal spraying uses wire material as a feed stock. An electric arc is used to provide the heat source by utilising two current carrying wires. As the wires are fed towards each other the electric current short circuits between the wires creating a temperature of around 4,000°C.
This temperature causes the tips of the wire to melt and once molten, compressed air or inert gas is used to atomise and accelerate the feed metal towards the substrate.
One of the advantages of this system is that two different wires can be used simultaneously to produce a pseudo alloy. Cored wires are also available producing coatings with unique properties.
The process is often used when applying to large areas such as corrosion resistance on large components or for the building up of worn components.
Combustion Spraying
Flame Spraying
This is a useful process for applying relatively inexpensive coatings that typically contain high levels of oxides and porosity together with the option of achieving a rough surface finish.
The process relies on the chemical reaction between oxygen and a fuel of combustion to produce a heat source.
This heat source creates a gas stream with a temperature in excess of 3,000°C with correctly balanced conditions between oxygen and acetylene.
The feed stock material to be sprayed is fed into the flame in the form of a wire and compressed air is then used to atomise the molten metal and accelerate the particles onto the substrate.
The combustion powder process uses a similar technique, except that the wire feedstock is replaced with a powder.
Among others, the process is typically used for applying bond coat materials or materials for corrosion resistance applications.