Plasma Spray

Plasma spraying, a method of thermal spraying, is a materials processing technique for producing coatings and free-standing parts using a plasma jet. Deposits having thickness from micrometers to several millimeters can be produced from a variety of materials - metals, ceramics, polymers and composites.

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How it works

The material to be deposited (feedstock) - typically as a powder, sometimes as a liquid, suspension or wire - is introduced into the plasma jet, emanating from a plasma torch. In the jet, where the temperature is in the order of 10 000 K, the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. There are a large number of technological parameters that influence the interaction of the particles with the plasma jet and the substrate and therefore the deposit properties. These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc.

Deposit Properties

The deposits consist of a multitude of pancake-like lamellae called 'splats', formed by flattening of the liquid droplets. As the feedstock powders typically have sizes from micrometers to above 100 micrometers, the lamellae have thickness in the micrometer range and lateral dimension from several to hundreds of micrometers. 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 are generally mechanical properties, such as lower strength and modulus, higher strain tolerance, and lower thermal and electrical conductivity. Also, due to the rapid solidification, metastable phases can be present in the deposits. The first Vacuum Plasma Spray unit in the UK was installed by Prof. T. W. Clyne at the Dept. of Materials Science & Metallurgy, University of Cambridge.

Applications

This technique is mostly used to produce coatings on structural materials. Such coatings provide protection against high temperatures, corrosion, erosion, wear; they can also change the appearance, electrical or tribological properties of the surface, replace worn material, etc. When sprayed on substrates of various shapes and removed, free-standing parts in the form of plates, tubes, shells, etc. can be produced. It can be also used for powder processing - spheroidization, homogenization, modification of chemistry, etc. In that case, the substrate for deposition is absent and the particles solidify during flight or in a controlled environment (e.g. water).

Properties of Zirconia Based Plasma Sprayed Coatings

Thermal Conductivity = 0.7-0.9 W/m/K
In-plane Stiffness = 17-21 GPa
Porosity = 10-14 %

Wire Arc Spray

Wire Arc Spray is a form of plasma spraying where two consumable metal wires are fed independently into the spray gun. These wires are then charged and an arc generated between then. The heat from this arc melts the incoming wire which is then entrained in air jet from the gun and this entrained molten feedstock is then deposited onto a substrate. Used commonly in metallic, heavy coatings.

See also

• Vacuum plasmaspraying
• Solution precursor plasma spray
• Hard Coating with Thermal Spraying