Thermal Spray and Multiphase Flow Laboratories
Department of Mechanical and Industrial Engineering


Thermal Spray : Processes

Atmospheric Plasma Spray (APS)

Suspension Plasma Spray (SPS)

High Velocity Oxygen Fuel (HVOF)

 

Atmospheric Plasma Spray (APS)

In atmospheric plasma spray (APS) process, the plasma source is a DC (Direct Current) arc plasma torches. As the figure shows, the process is essentially composed of three functional subsystems

Figure 1 : Schematic of a conventional atmospheric plasma spray process

(1) plasma generation and its interaction with the environment : The plasma jet is generated by a continuous electric arc between two electrodes as the plasma gas with a pure gas or a binary mixture or a ternary mixture of monatomic gas (Ar, He), and / or diatomic (H2, N2). The plasma jet is produced by the flow of the gas mixture in a continuous electric arc between the two electrodes. At the arc column exit, the plasma formed is in extinction. This results in a jet of gas at high temperature to the maximum of the order of 12 000 to 14 000 K and at high speeds of 600 to 2200 m / s at atmospheric pressure

(2) powder injection : powder particles are injected into the plasma jet and are then melted, accelerated and crashed into a substrate previously prepared

(3) splat and coating formation : At impact onto the substrate, the molten or semi-molten particles flatten, solidify and form lamellae (splats) the coating resulting from their layering

 

Suspension Plasma Spray (SPS)

A suspension of nano-particles, with agglomerates, is injected into plasma jets. Suspension is the only way to inject such small particles with the suitable momentum. in APS, these particles would require carrier gas flow rates as high as 50 slm, which hugely perturb the plasma jet. The result is a fragmentation of the plasma droplets containing solid particles. After evaporation, the solid particles are melted and projected onto a prepared substrate

Figure 2 : Schematic of suspension plasma spray process

The SPS method presents the following advantages:

- the control of the coating porosity by adjusting the experimental parameters

- it enables multi-layer deposition gradient properties (composition, porosity...) using one or more suspensions

- working in ambiant air with minor modifications of conventional plasma spraying system

 

High Velocity Oxygen Fuel (HVOF)

HVOF is a is a high speeds spray process, was invented in 1958 by Union Carbride (now Praxair Surface Technologie, Inc.) and become commercially significant until the early 1980s with the introduction of the JetKote (Deloro Stellite, Goshen, IN) by James Browning. The process utilizes a combination of oxygen with various fuel gases (some systems using gas, some liquid) including hydrogen, propane, propylene, hydrogen, LPG, natural gas and even kerosene. In the combustion chamber, burning by-products (typical combustion between 2500ºC and 3500ºC)are expanded and expelled outward through an orifice where at very high velocities. Before powder injection, Typically 9 vivid "shock diamonds" can be seen at the nozzle exit. powders are injected axially into the expanding hot flame, where they are heated and accelerated onto a surface to form a coating.

Figure 3 : Schematic of HVOF process

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