Frequently Asked Questions
Depending on the type of thermal spaying process employed, use of gas is for one or more of the following functions:
- carrying the powder consumable material
- atomising molten particles from the wire consumable
- creation of the energy source, flame or plasma
The carrier gas used to feed the powder consumable is mainly an inert gas such as argon or nitrogen.
For the arc spray process, air is often used as an atomising gas due to its low cost. By using an inert gas like nitrogen or argon as the atomising gas, the level of oxide in the coating can be reduced.
In the HVOF and flame spraying processes, combustion results from the reaction between oxygen and fuel. Fuels used are mainly in the gas form, although some liquid fuels like kerosene are also used with the HVOF process. A list of the main fuel gases used for the combustion process, together with their maximum flame temperatures, is given in the table.
As well as pure gases, mixtures of gases are used for the combustion process; examples of these mixtures are -
||- mixture of methylacetylene, propadiene, propylene and propane
||- mixture of methylacetylene, propadiene, propylene, propane and butane
||- mainly composed of methane
Plasma spraying relies on the generation of a jet of high temperature plasma. Gases ionised during the process are mainly:
- mixture of argon and helium
- mixture of argon and hydrogen
- mixture of nitrogen and hydrogen
Choice of gas is dictated mostly by the ability to melt the sprayed particles and obtain a narrow spray cone.
Thermal spraying processes- a guide to best practice.
See further information about Materials & Corrosion Management or please contact us.