PLASMA-SPRAY PROCESSING

General Plasma Spray Processing - Plasma-spray processing is a relatively specialised high temperature industrial process that utilises an electrically generated plasma to heat and melt the feedstock material. The process is capital intensive and requires significant electrical power. It offers a method of depositing a feedstock material (the material that is being plasma-sprayed) as a solid coating over an underlying target material. It can also be used to produce free standing parts made from just the plasma-sprayed material. Deposits having a thickness from just a few micrometers (µm) up to several millimeters can be produced using a variety of feedstock materials, including metals and ceramics.

 

The feedstock material is normally presented to the plasma torch in the form of a powder or wire. This feedstock melts rapidly within the plasma torch, where the typical operating temperature is ~10,000oC (18,000oF). It is then propelled as small molten droplets via a carrier gas towards the target material. When the molten droplets contact the target material they flatten, rapidly solidify to form a deposit that remains adhered to the target material. There are a large number of parameters that influence the interaction of the feedstock with the plasma jet and the substrate, and these parameters can lead to wide variations in the final product and product quality. These parameters include feedstock type and composition, feed rate, plasma gas composition and flow rate, energy input, torch geometry, nozzle design, nozzle offset distance, substrate cooling, etc.

 

The plasma-sprayed deposits consist of a multitude of pancake-like 'splats', formed by flattening of the liquid droplets. As the feedstock powders typically have sizes from micrometers to above 100 micrometers, the splats have thickness in the micrometer range and lateral dimension up to hundreds of micrometers. Small voids occur between the splats thereby trapping either air or the carrier gas whilst the splats cool rapidly leading to quite rapid crystallisation and large grain size. It is also possible for the deposited material to exist in metastable phases due to rapid cooling (i.e. crystalline structures that would normally only occur at very high temperature). Small micro-cracks and regions of incomplete bonding can also occur under certain plasma-spray conditions. The deposited material can therefore have properties that are significantly different from the bulk feedstock materials, eg. lower strength, higher strain thermal and electrical conductivity. By controlling plasma-spray parameters it is possible to control and make use of these changed material properties to suit specific applications. 

 

This technique is mostly used to produce coatings on structural materials. It can be used to provide protection against high temperatures, corrosion, erosion and wear, or to change the appearance, electrical or tribological properties of the surface. The technique can also be used to replace worn material. Free standing parts in the form of plates, tubes, shells, etc. can be produced. The technique can also be used for powder processing (spheroidisation, homogenisation,modification of chemistry, etc.). In this case, the substrate for deposition is absent and the particles solidify during flight or in a controlled environment such as water.

 

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