Metal/Ceramic Thermal Spray Coatings
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Thermal Spray Coating Processes

plasma thermal spray coating

Plasma Coating Process

The flexibility of the plasma spray process comes from its ability to develop sufficient energy to melt almost any coating feedstock material in powder form. The plasma gun utilizes a chamber with one or more cathodes (electrodes) and an anode (nozzle). With process gases flowing through the chamber, direct current power is applied to the cathod, which arcs to the anode. The powerful arc strips the gas molecules of their electrons to form a plasma plume. As the unstable plasma ions recombine back to the gaseous state, a temendous level of thermal energy is released. The feedstock material is injected into the hot gas plume, where it is melted and propelled towards the target substrate to form the coating.

The process gases typically used are argon, hydrogen, nitrogen and helium, either individually or in mixtures of two, or even three of these gases. The gases used in combination with the current applied to the electrode controls the amount of energy produced. Since has flows and the applied current can be accurately regulated, repeatable and predictable coating results can be obtained. In addition, the shape and bore size of the nozzle, the point and angle that the materil is injected into the plume, as well as the distance of the gun to the target surface are also controlled. This provides a high degree of flexibility to develop reproducible parameters for materials with melting temperatures across a very large range.

HVOF thermal spray coatings

HVOF Coating Process

The HVOF (High Velocity Oxy-Fuel) process efficiently uses high kinetic energy and controlled thermal output to produce dense, low porosity coatings that exhibit high bond strengths, some of which exceed 83 MPa (12,000 PSI), low oxides and extremely fine as-sprayed finishes. The coatings have low residual internal stresses and therefore can be sprayed to a thickness not normally associated with dense, thermal sprayed coatings.

The HVOF process uses an oxygen-fuel mixture. Depending on user requirements, propylene, propane, hydrogen or natural gas may be used as the fuel in gas-fueled spray systems and kerosene as the fuel in liquid-fueled systems. The coating material, in powdered for, is fed axially through the gun, generally using nitrogen as a carrier gas. The fuel is thoroughly mixed with oxygen within the gun and the mixture is then ejected from a nozzle and ignited outside the gun. The ignited gases surround and uniformly heat the powdered spray material as it exits the gun and is propelled to the workpiece surface. As a result of the high kinetic energy transferred to the particles through the HVOF process, the coating material generally does not need to be fully melted. Instead, the powder particles are in a molten state and flatten plastically as they impact the work piece surface. The resulting coatings have very predictable chemistries that are homogeneous and have a fine granular structure.

HVOF coatings can survive harsh service conditions, particularly in wear and many corrosion applications, which greatly increase component service life. The smooth, as-sprayed surface, uniform chemistry and low porosity of the coating can be finished to very smooth surface profiles.

HVOF is considered a “Green Technology” as there are no volatiles, chemical baths, hazardous air emissions, or large volume of rinse water consumed. 

arc wire thermal spray coatings

Arc Wire Coating Process

The Electric Arc Wire Thermal Spray Coating process uses two metallic wires, usually of the same composition, as the coating feedstock. The two wires are electrically charged with opposing polarity and are fed into the arc gun at matched, controlled speeds. When the wires are brought together at the contact point, the opposing charges on the wires create enough heat to continuously melt the tips of the wires. Compressed air is used to atomize the now molten material and accelerate it onto the workpiece surface to form the coating.

In electric arc wire spray, the weight of coating that can be deposited per unit of time is a function of the electrical power (amperage) of the system and the density and melting point of the wire. Depending on the columnar strength of the wire, ‘push’, ‘pull’ or ‘push/pull’ mechanisms can be used to feed the wire at a constant rate.


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What is Thermal Spray?

Thermal spraying is a group of coating processes in which metallic and non-metallic materials are deposited in a molten or semi-molten condition to form a coating. Coating materials include metals, alloys, carbides, ceramics, plastics, cermets, composites and blended materials.

It is a versitile, cost effective solution to protect and extend the surface life of components in nearly every industry, from electronics, to nuclear, military and aerospace.

About ThermoCermet

ThermoCermet is a division of Hitec Products Inc., a global supplier of strain gages/sensors and proud to be an ISO 9001:2008 Certified company.

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