What is Thermal Spray ?
Thermal Spray is a group of coating processes in which metallic, ceramic, cermet, and some polymeric materials in the form of wire, powder, or rod are fed to a gun which results in heating to near or somewhat above their melting point. The resulting molten or nearly molten droplets of material are then accelerated in a gas stream and projected against the surface to be coated. When the hot energetic particles impact the surface of the part being coated, they rapidly solidify. As subsequent particles impact the surface, a deposit thickness is built up. The droplets flow into thin lamellar particles adhering to the surface, overlapping and interlocking as they solidify. The total coating thickness is usually generated in multiple passes of the coating device. Thermal spray devices used to accomplish this work are commonly called as guns or torches.
Advantages of Thermal Spray
- The major advantage of the thermal spray processes is the extremely wide variety of materials that can be used to make a coating. Virtually any material that melts without decomposing can be used.
- Second major advantage is the ability of most of the thermal spray processes to apply a coating to a substrate without significantly heating it. Thus, materials with very high melting points can be applied to finally machined, fully heat-treated parts without changing the properties of the part and without thermal distortion of the part.
- The third advantage is the ability, in most cases, to strip and recoat worn or damaged coatings without changing the properties or dimensions of the part.
Selection of the appropriate thermal spray method is typically determined by:
- Desired coating material
- Coating performance requirements
- Economics
- Part size and portability
The Process Basics
All methods of thermal spraying involves the projection of small molten particles onto a prepared surface where they adhere and form a continuous coating. To create the molten particles, a heat source, a spray material and an atomisation/projection method are required. Upon contact, the particles flatten onto the surface, freeze and mechanically bond, firstly onto the roughened substrate and then onto each other as the coating thickness is increased.
The metal spray equipment falls into four main categories, Powder Flame Spray, Wire Flame Spray, Arc Spray, High Velocity Oxygen Fuel (HVOF) and Plasma Spray.
The metal spray equipment falls into four main categories, Powder Flame Spray, Wire Flame Spray, Arc Spray, High Velocity Oxygen Fuel (HVOF) and Plasma Spray.
