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.
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.
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.
There are a number of advantages to using plasma spray coatings including:
Although this is a very popular type of thermal spray coating, plasma spray does still exhibit a number of disadvantages which include:
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.
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).
HVOF is most suitable for use creating thermal coatings on the following materials:
In general HVOF is thought to result in an improved coating quality compared to other thermal spraying processes. Advantages include: