Thermal spray is rapidly gaining recognition as a contemporary fabrication process.
One of the most common uses of thermal spray is the application of a zinc coating. The need for such a coating is often seen at overpasses or on school grounds where fabricated fencing leaves a trail of rust. In these cases, the fabricator neglected to apply a zinc coating to the weld between the galvanized fence post and the galvanized mounting plate.
Defining Thermal Spray
Understanding thermal spray starts with knowledge of the basics of solids. Solids can be thought of as having three parts:
The central body provides the basic form or shape and the strength and rigidity.
The surface is associated with color, texture, patina, and sheen.
The barrier creates a layer between the outer surface and the main body.
In many cases, there is no distinction between the three areas as they are all of the same material. However, it is often necessary to fabricate a part for which the body needs a coating or barrier with properties not present in the body material.
This is where thermal spray comes in. Thermal spray can provide a barrier against chemicals, extreme temperatures, abrasion, erosion, light, radiation, radar (such as with stealth aircraft), and stress related to differential thermal expansion between the surface and the body.
In addition, thermal spray is used for repair, rebuild, and clearance control and to modify the friction factor of the surface.
Thermal spray is a key part of advanced fabricated products such as prostheses and gas turbines. For prostheses, thermal spray is used to apply the hydroxyapatite ceramic coating for bone bonding. For gas turbines, thermal spray provides the thermal barrier coating for high-temperature operations and for clearance control, both essential to the high performance of these systems.
The term thermal spray
describes a family of processes with a wide range of thermodynamic properties . In all cases, material is heated and sprayed on the part being coated, the substrate. A combustion process, either an electric arc or a plasma process, provides the heat. Material can be introduced in powder form or as a wire. If a wire is used, it can be solid or can be filled or cored with a powder.
Thermal spray does have its limits. A key consideration is that it is a line-of-sight process. Unlike plating, it cannot coat deep bores, slots, or hidden areas.
• Combustion Powder.
This is the earliest thermal spray design. The material in powder form is melted in a flame, usually oxyacetylene, and then propelled to the surface using an air jet. Material is fed from a gun-mounted canister into the gun. Powder is fed into the side of the flame. Both the temperature and velocity are relatively modest.
• Combustion Wire
. These guns generally use an oxyacetylene flame for heating. Wire is introduced from the back of the gun and through the flame, where the wire is melted, forming droplets. An air jet then propels the droplets to the substrate to form a coating. Typical combustion wire guns use a gun-mounted, air-operated turbine to pull the wire into the gun. Temperatures and velocities are similar to those for the combustion powder process.
• Arc Wire
. This is a two-wire system in which an electric arc is created between the two conductive wires that are fed into the gun as the wire is melted by the arc. Jet air propels the droplets to the surface. Wire feed can be from a wire supply cart (push), from a gun-mounted motor drive (pull), or a combination of the two. The two wires are fed at the same rate using a variable-speed control adjusted to match the melt rate to the feed rate. Both the temperature and velocity associated with the arc wire process are typically slightly higher than for the combustion processes.
• High-velocity Oxyfuel (HVOF)
. Many advanced coatings for wear and erosion resistance depend on this HVOF process, which typically uses oxyacetylene. Combustion takes place in a chamber and the resulting gases exit the gun through a converging/diverging nozzle, similar to a jet engine. Gas velocities are supersonic. Powder is injected into the side of the gas stream as it exits the nozzle. Powder velocities, at this point, are significantly higher than for other combustion processes. The particles’ higher velocity and the ensuing impact result in a dense coating for hard materials such as tungsten carbide.
• High-velocity Air Fuel (HVAF)
. This is a newer process in which air is used as the oxidizer. It is similar to HVOF but different in that the process is not self-sustaining in the gun; a separate ignition source is needed to maintain the combustion. Because of the lower combustion temperature, coatings have a lower oxide content with improved performance.
These guns are used for very advanced processes involving exotic materials. In the plasma spraying process, the feedstock, typically powder, is introduced into the plasma jet from the gun. In the jet, where the temperature is on the order of 10,000 degrees K (17,540 degrees F), the material is melted and propelled toward a substrate. The plasma coating process can be carried out in the open (atmospheric conditions) or in a vacuum chamber under controlled conditions of the environment.
There are some variations for these processes, but they all are similar in that a material is melted or softened and then propelled to the substrate by a high-velocity gas stream.
Feedstock is available in different chemistries and forms. Materials include steel, zinc, copper, tungsten carbide, ceramics, and plastics. Powders can be produced by gas atomization, water atomization, and grinding. Powders can be agglomerated, sintered, fused, crushed, or a combination of these. A good relationship with a supplier is essential for selecting the right materials.