Thermal Spray Processes
The technologies of thermal spray are highly versatile devices that allow for the engineering of an almost a limitless number of coatings for an extremely wide range of industrial applications. A few examples of where thermal spray coatings can be found include: jet engines to protect components from high temperatures and oxidation, medical devices such as surgical instruments and orthopedic implants, agricultural components to enhance wear resistance, and electronic devices to provide electrical insulation or conductivity.
What is Thermal Spray?
Thermal spray is a generic term for a group of processes that utilize a heat source to melt material in powder, wire or rod form. The molten or semi-molten material is propelled toward a prepared surface by expanding process gases. The particles quench rapidly, upon impact with the surface, and bond with the part. Subsequent impacting particles create a coating buildup, see figure 1. The thermal spray processes include the following:
Plasma-Arc Spray:
Plasma spray is the most versatile of the all the thermal spray processes. Plasma is capable of spraying all materials that are considered sprayable.
Shrouded Plasma-Arc Spray:
Shrouded plasma spraying is plasma spraying with a shroud of protective gas. The shroud is used to help protect the spray plume from the atmospheric environment.
HVOF
High-velocity, oxy-fuel, (HVOF) is a subset of flame spraying. This technology is capable of producing very dense and well adhered coatings in the as-sprayed condition.
Electric-Arc Spray
Electric-arc spraying uses two conductive wires to carry electrical charges of opposite polarity. When the two charges meet at the front of the spray device an electrical arc is created and this arc melts the wire. The molten wire is the feedstock material for creating a coating. It has the capability of depositing coatings at very high spray rates.
Flame Spray
Flame spray uses combustion of gases to melt powder, wire or rod material and propel the molten droplets onto a surface to create a coating. Its gas velocities are much slower than HVOF. Flame spraying can create coatings out of a wide range of materials from plastics to oxide ceramics.
Cold Spray
Cold spray is the newest member to the technologies of thermal spray. The technology relies on very high gas velocities to propel particles and provide a high degree of kinetic energy. The kinetic energy causes plastic deformation of the particles upon impact, which creates very dense coatings.
Materials
The materials used for thermal spraying are almost limitless. This makes the technologies extremely powerful in their ability to provide engineering solutions to the design and protection of components.