Thermal Spray Coating methods are processes projecting selected materials onto a target surface at high velocities. A precursor is heated by plasma, arc or by a flame. Metals, alloys, ceramics and composites are the main materials used. In summary, the material is input as a powder or wire and then the the molten or heated substance is accelerated towards the target substrates. Combustion or electrical arc is the primary energy source. The compounding accumulation of sprayed particles builds the resulting coating.
What are the different types of Thermal Spray Coating?
Fundamentally, the components of a thermal spray capability include a spray gun and its function is to melt and accelerate the particles. Secondly, a feeder which provides the source powder or wire to the gun and the media supply – gases or liquids generate a flame or plasma jet to carry the powder. The gun is robotically controlled. Finally, a control console which manipulates the complete system.
High velocity oxy-fuel coating spray (HVOF)
A mixture of gaseous or liquid fuel and oxygen is ignited within a combustion chamber. A high temperature gas proceeds in a converging–diverging nozzle. Examples of gases include hydrogen, methane, propane, propylene and acetylene. The jet velocity is supersonic. A powder is introduced into the gas stream, which accelerates the powder. The resultant powder melts and deposits on the target material. High bond strength is a favourable characteristic.
High velocity air fuel (HVAF)
HVAF involves the combustion of propane in a compressed air stream. This produces a high velocity jet. HVAF includes a heat baffle, aiding stability. The maximum flame temperature is between 3,560° to 3,650° F. This is close to the melting point of most spray materials, which results in a more uniform, predictable coating. The final coatings generally have an even greater bond strength. HVAF materials include; tungsten carbide, chrome carbide, stainless steel, and inconel. These coatings reduce and resist cavitation damage.
The plasma process uses a powder, sometimes as a liquid or wire and brings this into the plasma jet. The jet, with a temperature over 10,000 K melts the material. The molten droplets flatten and solidify. Variable parameters include feedstock type, plasma gas, flow rate and torch offset distance.
Lamellae form the deposit, a flattening of the liquid droplets. The thickness is in the micrometer range. Between these lamellae, there are small voids, such as pores, cracks and regions of incomplete bonding. As a result of this unique structure, the deposits can have properties significantly different from bulk materials. These mechanical properties include lower strength, higher strain tolerance, and lower thermal conductivity.
A water-cooled barrel with inlet valves for gases and powder is used. Oxygen and fuel are introduced into the barrel along with a charge of powder. Nitrogen pulses purge the barrel after each detonation. This process is repeated and the high kinetic energy of the particles with the substrate results in a very dense and robust coating.
Wire arc spray
Wire arc spray occurs when two consumable metal wires are fed into the gun. An arc is generated between the wires and the heat melts the incoming wire and the molten feedstock is then deposited onto a substrate.
Cold spraying utilises and benefits from solid particles with sufficient kinetic energy deforming plastically and bonding to form a coating. Metals, polymers, ceramics, and composite materials can be deposited. Metals such as Cu and Al are ideal for cold spraying.
Thermal Spray applications and benefits
A range of applications across numerous industries include, crankshaft reconditioning and corrosion protection. Wear control is also considered either in hardfacing or abradable coating. It is successful for repairing damaged surfaces and temperature/oxidation protection by creating a thermal barrier coating.
Thermal Spray Advantages
There are multiple benefits associated with these techniques. The wear and corrosion resistance make this a very favourable way to extend the lifetime of specific components. The value is shown uniquely when components are vulnerable and exposed within highly aggressive environments.
Potential limitations, disadvantages and hazards
There are a number of potential hazards specific to thermal spray. Equipment should be within enclosures, suitable to extract fumes and minimise noise levels. Large volumes of dust and fumes make up very fine particles.
Safety and prevention
The use of respirators fitted with suitable filters is highly recommended. Metal particles are potentially pyrophoric and harmful to the human body. Aluminium and zinc may react with water to evolve hydrogen, which is potentially explosive. Fumes of copper alloys cause metal fume fever.
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