Thermal Spray Coating System

Overview

A thermal spray coating system builds a protective layer on a part by melting a feedstock, usually a powder, and throwing the molten droplets at the surface, where they flatten, freeze, and stack into a coating. In the plasma version described here, a DC arc heats a flowing gas to roughly 15000 K, hot enough to melt almost any metal, ceramic, or carbide. The droplets hit the part at a few hundred metres per second, splat into thin lamellae, and cool in microseconds, so the coating grows splat by splat into a dense layer that resists wear, heat, or corrosion far better than the bare substrate.

The system is built around a small set of jobs: making the hot jet, feeding powder into it, supplying clean gas and power, cooling the gun, moving everything, and containing the noise and dust. The Plasma Spray Gun makes the plasma and melts the powder, the Plasma Power Supply drives the arc, and the Powder Feeder meters the feedstock. The Gas and Fuel Console supplies the plasma and carrier gases, the Gun Cooling Unit keeps the gun electrodes alive, the Motion Handling sweeps the gun across the part, the Control Console sequences the cycle, the Spray Booth contains the overspray and noise, and the Substrate Fixturing holds and prepares the substrate.

The spray gun

The Plasma Spray Gun is where the energy and the powder meet. An arc strikes between a Thoriated Tungsten Cathode of thoriated tungsten and a water-cooled Copper Anode Nozzle of copper. Gas from the Swirl Gas Injector passes through the arc, ionizes, and leaves the nozzle as a high-velocity plasma jet. The Inter-Electrode Insulator stands the cathode off from the grounded anode, and the Electrode Coolant Jacket floods both electrodes with the high-pressure water that lets a 40-80 kW arc run without melting its own copper. Powder enters through the Powder Injection Port just past the nozzle exit, where the jet picks it up, melts it, and carries it to the part. The Gun Body holds the electrodes in place and routes the gas, coolant, and current, sealed throughout by an O-Ring Set.

Power and arc control

The arc draws hundreds of amps at tens of volts, so the Plasma Power Supply is built for high current rather than high voltage. A Main Power Transformer steps the mains down and a Thyristor Rectifier Stack of thyristors rectifies it, with the firing angle set by the Arc Current Regulator so arc current holds to setpoint as the electrodes wear. A Smoothing Choke damps the ripple that would otherwise show up as variation in the coating. Because the gas gap will not conduct cold, the HF Arc Igniter fires a high-frequency burst through its HF Spark Gap to ionize the gap and let the main arc strike without the electrodes touching. The output reaches the gun through a Output Bus Bar, the line is switched by a Line Contactor, and a Power Supply runs the control electronics.

Powder, gas, and cooling

Coating quality depends on feeding powder at a steady rate, so the Powder Feeder meters it carefully. A pressurized Powder Hopper holds the feedstock, a grooved Metering Wheel driven by a Servo Motor picks up a fixed volume per turn, and a Hopper Vibrator keeps fine powder from bridging. The Carrier Gas Regulator sets the argon that sweeps the metered powder to the gun, and a Powder Mass-Flow Sensor confirms the feed rate. The Gas and Fuel Console meters every gas through mass-flow controllers: a Argon Mass-Flow Controller for the primary plasma gas, a Hydrogen Mass-Flow Controller or Nitrogen Mass-Flow Controller for the secondary gas that raises the jet enthalpy, and a Carrier Mass-Flow Controller for the powder carrier, all joined on the Gas Manifold and switched by the Solenoid Valve Bank. Fuel and oxygen lines carry a Flashback Arrestor, and a Pressure Sensor watches each supply. The Gun Cooling Unit protects the gun: a Coolant Pump drives deionized water at high pressure through the electrodes, a Deionizing Cartridge keeps the water non-conductive, a Refrigerated Chiller and Plate Heat Exchanger reject the heat, and a Flow Switch kills the arc if flow is lost.

Motion, fixturing, and containment

Even coating thickness comes from steady relative motion between gun and part. The Motion Handling handling carries the Plasma Spray Gun on a Spray Robot that sweeps it at constant standoff and speed, while a Part Turntable spins cylindrical parts for full coverage. The Substrate Fixturing holds the part in a Part Chuck, shields uncoated areas with a Masking Set, and prepares the surface: a Grit-Blast Head roughens it so the coating keys on, a Preheat Element with its Heating Element warms it to cut coating stress, and a Surface Pyrometer reads its temperature. The whole process is loud and dusty, so the Spray Booth contains it. The Booth Enclosure cuts the noise, the Exhaust Fan pulls air through, and the Dust Collector with its Filter Cartridge traps the metal-oxide overspray before the air leaves the building.

Process variants

The same handling, cooling, and control hardware backs several spray methods that differ mainly in how they make the hot jet. Atmospheric plasma spray, the version above, gives the highest temperature and suits ceramics and refractory carbides. High-velocity oxygen fuel, or HVOF, burns a fuel such as kerosene or hydrogen with oxygen in a chamber and accelerates the products through a converging-diverging nozzle, reaching particle speeds above 600 metres per second for very dense carbide coatings. Twin-wire arc spray strikes an arc between two consumable metal wires, melts their tips, and atomizes the melt with compressed air, a cheap high-output method for metal coatings. Flame spray burns oxy-acetylene to melt powder or wire at lower velocity for soft sacrificial layers. A Gas and Fuel Console sized for fuel and oxygen lets one system run more than one of these.

Use and limits

Thermal spray coatings go where a surface must outlast the part under it: thermal barrier coatings of yttria-stabilized zirconia on turbine blades, tungsten carbide on pump shafts and valve seats, abradable seals in compressors, and corrosion coatings of zinc or aluminium on bridges and offshore steel. The coating is applied cold enough that the substrate stays below a few hundred degrees, so heat treatment and temper survive, and worn parts can be rebuilt rather than scrapped. The limits are line-of-sight access, since the Plasma Spray Gun can only coat what it can see, and deposition efficiency in the 40-70 percent range, which wastes the powder that misses the part. Surface prep is unforgiving: a coating sprayed onto a surface the Grit-Blast Head did not properly roughen will spall off. Within those limits no other process lays down so wide a range of metals, ceramics, and carbides on so wide a range of parts.