The plasma spraying process involves the generation of a plasma jet, the injection and treatment of particles within the plasma jet and finally the formation of the coating.
To generate the plasma jet, a working gas such as an argon/hydrogen mixture is passed through a powerful electric arc discharge formed in the gap between a cathode and anode. The energy released rapidly heats up the gas mixture, converting it to high-temperature plasma at about 14 000 K. Rapid expansion occurs, lifting the speed of the jet, giving it a very high nozzle speed of up to 800 m/s. The coating material in a fine powder form (within the range 20–90 mm) is then injected into the plasma jet. Molten droplets form, which are propelled at high speed towards the object to be coated (substrate).
Coating the substrate
On hitting the substrate, each molten droplet splats onto the surface, forming a pancake-like structure that rapidly solidifies. It is important that the droplet thoroughly ‘wets’ the substrate surface, and attention to the composition of the coating material needs to be made to ensure that this happens.
Each splat has a thickness in the micrometre range and a length that varies across the range from several to above 100 micrometres. Splats overlap one another as the deposit builds up to the required thickness. Often, there are small voids present as well as inclusions of rogue materials such as metal oxides. These can interfere with the mechanical strength of the coating and lead to poor adhesion to the substrate.
The properties of the surface of the substrate also need to be taken into account. In most industrial settings, the pieces arriving at the spraying unit are new or covered with old coatings. Each piece needs to be thoroughly cleaned and then the surface roughened by abrasive grit blasting. Thorough surface preparation ensures that a good mechanical bond between the coating and the substrate can be achieved.
Another factor to be considered is the temperature at the particle’s interface with the substrate on impact. This contact temperature influences the adhesion of the splats as well as the adhesion of the coating to the substrate.
Recent research has shown that, if a molten droplet, on hitting the substrate surface, forms a disc-shaped splat rather than a splashed splat, the coating formed tends to have good adhesion and cohesion with reduced void space.
Adhesion is the force of attraction between molecules of different substances while cohesion is the force of attraction between molecules of the same substance.
Atmospheric plasma spraying
There is a wide range of plasma spray techniques used in the coatings industry. One of these techniques – atmospheric plasma spraying – is extensively used to produce coatings on structural materials. Such coatings provide protection against high temperatures, corrosion and wear. For example, in aircraft jet engines, many of the component parts are subjected to very high temperatures as well as a corrosive and erosive environment. To limit wear and tear on these components and also give them thermal protection, a thin coating of a ceramic material called yttria stabilised zirconia (Y2O3 and ZrO2) is plasma sprayed onto the component surfaces.
In order to produce molten particles of the correct size, speed and temperature, a number of factors need to be controlled. Some of these are:
- composition of the working gas stream and its flow rate
- electric arc discharge power
- powder particle size, composition and injection rate
- spray distance
- speed and number of spraying passes.
For example, in the coating of a generator bearing housing with a ceramic material, an argon/hydrogen working gas mix is chosen. This not only increases the plasma temperature but also minimises oxide levels without compromising the integrity of the coating.