Overview of the Characteristics of Thermal Spray Powders
The characteristics of thermal spray powders, including the size and shape of the particles, are critical to their performance. They can affect coating formation and microstructure, oxidation in service, corrosion properties, wear protection and the ability to be machined.
There are several types of thermal spray powders available on the market. Each powder has its own unique characteristics and processes of application.
Particle Size
The particle size of thermal spray powder is a very important characteristic of this spraying material. It has a direct effect on the coating properties and is a vital factor in determining whether thermal spray powders can be used to form a plasma-sprayed coating that has good plasma etching resistance like for nickel alloy powder.
Generally, the average particle size of the granulated and sintered yttria particles in thermal spray powders should be preferably 20 mm or greater, more specifically 22 mm, still more specifically 25 mm and even more specifically 28 mm. In this case, the powder will have a high flowability and will be able to produce a coating that has a high plasma etching resistance.
In the present study, three powders of different sizes (S-powder, M-powder and L-powder) were prepared using plasma jet as an external heat source. The resultant coating microstructures were examined by SEM backscattered electron imaging.
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Particle Shape
Particle shape is an important characteristic of thermal spray powder. It determines a variety of factors, including consistency and reproducibility, inter-particle interaction, and coating microstructure.
In thermal spray processes, particles are accelerated to high velocities toward the substrate. This is achieved by feeding powders in powder or wire form into a jet of a gas, such as nitrogen, helium, or mixtures, and heating them to a molten or semi-molten state.
Plasma spraying is a common thermal spraying process in which molten particles are sprayed onto the surface of a substrate. These sprayed particles are impacted and shaped into coatings by a high-temperature plasma jet.
The particle size is also an important consideration in thermal spraying. As explained above, small powders experience a higher oxidation rate than larger powders because of the larger specific surface area.
Particle Density
The particle density of a thermal spray powder is one of the most important factors in determining its characteristics. It directly affects the deposition efficiency of a coating with stainless steel metal powder.
It is a function of the powder size, size distribution, and morphology. It also has a relation to the powder packing density.
Particles with high apparent density are difficult to transport through the spraying gun, and they tend to block the flow of gas. This can lead to reduced feed rate and poor repeatability.
The particle temperature of the sprayed powder is also an important factor in determining the properties of a coating. The particles' surface temperature varies widely, depending on the powder size and spray distance. Small particles heat up more rapidly and can vaporize when overheated. The particles' temperature is further influenced by the plasma arc power.
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Particle Flow Rate
The particle flow rate is one of the most significant for copper powder characteristics of thermal spray powders. It is a function of the plasma flow, injection velocity and size of particles. The flow rate is also affected by the gun's pressure and temperature, and by the injected gas (argon, hydrogen, or helium).
A plasma spray process consists of a gas gun nozzle at a high velocity and elevated temperatures. Software LAVA-P-3D simulates the plasma flowing out of the nozzle, trajectories and temperatures of powder particles injected into the plasma from the powder port, and the in-flight mean particles' characteristics (MPCs) found within a 1-cm wide window located just before the substrate to be sprayed.
In-flight particle characteristics determine coating quality as well as the sprayed material's physical and chemical properties. This is because particles are accelerated by the plasma jet, heated to fully molten or semi-molten state, and then projected onto a prepared substrate to form the sprayed coating through successive stacking of molten droplets.
