Plasma spray coating is the most flexible process out of all the thermal spray processes. Basically, it is the spraying of molten or heat-softened material onto a surface to provide a coating. The utilization of the powerful plasma spray coating technology can facilitate the spraying of almost any ceramic, cermet or metal onto a wide variety of materials with exceptional bond strength as well as a minimal distortion of the substrate.
Conventional plasma spray, commonly referred to as atmospheric plasma spray (APS), is the most versatile method for applying thermal spray coatings. The plasma arc that gives the process its name, is generated in the torch by superheating inert gas in a DC arc. The plasma temperatures in the powder heating region can range from approximately 6,000-20,000°C (11,000-36,000°F). These temperatures are beyond the melting range of most substances. This makes it a great choice for ceramics and other materials that have high melting points.
How Plasma Spray Works
The plasma spray process begins when the control console sends regulated inert gas (usually argon) to the rear of the plasma torch (SG100). The arc gas then flows through the gas injector and into the arc chamber, which consists of an axially aligned tungsten cathode and copper anode. Depending on process parameters, the configuration of the gas injector will impart a laminar or vortex flow to the gas. Once the gas is flowing, the control console then signals for water cooling to the torch.
While the tungsten cathode can operate near its melting point, the plasma torch is water-cooled to minimize cathode and anode erosion, as well as preventing the torch from melting down.
Next, a DC arc is struck between the cathode and anode by a high-frequency generator and then maintained by a DC power supply. As the inert gas passes through the arc, it expands rapidly, reaching temperatures that are much higher than those created by other thermal spray processes. Additionally, a secondary gas (helium, nitrogen, or hydrogen) is introduced to the arc gas, providing a significant energy boost to the plasma stream.
This expansion of gas accelerates through the anode and causes the plasma jet to become subsonic or supersonic depending on the torch configuration. The supersonic expansion increases particle velocity, while subsonic velocities slow particle velocity and increase particle dwell time.
The coating material, in the form of powder, is then metered out by a powder feeder and carried to the torch in suspension via a carrier gas. It is then injected into the plasma jet where it is softened and propelled onto the workpiece, causing a high integrity coating, with excellent bonding.
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