Plasma spray is a form of thermal spray coating process. It is the process of spraying a molten or heat softened material onto a surface to form a coating. The coating material, in the form of a powder, is injected into a very high temperature plasma flame which then heats the powder and accelerates it to very high speeds. When the now molten coating material comes into contact with the substrate (surface of the material which is to be coated) it then rapidly cools, bonding to the surface to form a coating.

How does plasma spray coating work?

Plasma is the term used to describe a gas that has been heated to such high temperatures that it ionizes and becomes electrically charged. The plasma spray gun achieves this by utilising the combination of a copper anode and tungsten cathode inside the gun, around and through which a gas (usually either argon, nitrogen, hydrogen or helium) flows. This causes the gas to become plasma where it is then forced out through a constricting nozzle where it is mixed with the coating substance in powder form. The powder then becomes molten and is fired out of the gun towards the substrate where it then cools and forms a coating.

Uses of plasma spray coatings

When carried out correctly the plasma spray coating process is known as a ‘cold process’, as even though the plasma stream itself is very hot, the actual temperature of the substrate material that is to be coated can be kept relatively cool. This means that damage, metallurgical changes and distortion to the substrate material can be avoided and so more delicate substrate materials can be successfully coated using this process. However the extreme heat of the plasma also means that materials with very high meting points, such as refractory metals e.g. tungsten and ceramics e.g. zirconia, can used as coating materials.

Advantages of plasma spray coatings

There are a number of advantages to using plasma spray coatings including:

• Wide variety of materials that can both be coated and be used to form coatings.
• Higher quality coatings can be achieved compared to other types of thermal spray processes.
• A broad range of powder particle sizes can be used, typically between 5-100µm.
• Well known process that is widely available and well understood.

Disadvantages of plasma spray coatings

Although this is a very popular type of thermal spray coating, plasma spray does still exhibit a number of disadvantages which include:

• The specialist equipment is expensive to buy at the outset.
• Due to the high temperatures involved the internal components of the guns experience rapid deterioration and so must be replaced at regular intervals.
• The high temperatures involved can result in carbide decomposition or excessive oxidation when spraying in air.

For more information about the suitability of plasma spray coatings for your project, contact the experts at IRS Ltd today.

High Velocity Oxygen Fuel Thermal Spraying, or HVOF for short, is a thermal spray coating process which relies on a high velocity but low temperature procedure to create extremely dense and strong coatings.

How does HVOF spraying work?

During the HVOF spraying process particles of the coating material are sprayed at the substrate (the material which is to be coated) at supersonic speeds. The extreme kinetic energy of these particles allows them to bond with the substrate to form a very dense and strong coating. To achieve this result fuel and oxygen are sent into a combustion chamber under pressure and in a continuous flow, which produces a jet of combustion products at extremely high speed. The coating material, in the form of powder particles, is then injected into this gas stream and becomes accelerated towards the substrate at a very high velocity. There are a number of different types of HVOF spraying guns and processes which differ in the types of fuel that they use (hydrogen, natural gas or kerosene) and their oxygen source (pure oxygen or compressed air).

What materials is HVOF suitable for?

HVOF is most suitable for use creating thermal coatings on the following materials:

• Tungsten
• Chrome & Nickel Carbides
• Titanium
• Super-Alloy Ranges
• Hastelloy
• Inconel
• Monel
• Iron based alloys, AISI 316L, etc

Advantages of HVOF spraying

In general HVOF is thought to result in an improved coating quality compared to other thermal spraying processes. Advantages include:

• Higher density (lower porosity) coatings due to greater particle impact velocities.
• Lower cost compared to other thermal spray methods.
• Better wear resistance as a result of harder, tougher coatings due to less degradation of carbide phases.
• Relatively cool coating process, due to the brief presence of particles in the hot gas stream.
• Lower compressive stress which results in thicker coatings being able to be applied.
• Higher strength bond to the underlying substrate and improved cohesive strength within the coating.
• Lower oxide content due to less in-flight exposure time.
• Smoother as-sprayed surface due to higher impact velocities and smaller powder sizes.

Disadvantages of HVOF Spraying

• HVOF sprayed coatings can be extremely complex, due to the numerous variables within the method.
• Powder sizes are restricted to a range of about 5 – 60µm.
• HVOF spraying usually needs to be undertaken in a specialised workshop, with suitable sound attenuation and dust extraction facilities.
• Equipment costs are high initially.
• HVOF is not suitable for creating coatings on many internal, or other restricted access surfaces as HVOF spraying needs line of sight to the surface and a spray distance of 150-300 mm.

To find out more about the HVOF and other Thermal Spraying Systems options, contact the experts at IRS Ltd today.