Thermal Spray Basics
Although thermal spray may not be a familiar process, we benefit from this technology on a regular basis. Examples of thermal spray products that we benefit from in our everyday lives are the non-slip surface in bathtubs and the Teflon coating in our cookware. Thermal spray is also associated with transportation, the energy industry and the art and architecture fields. Thermal spray is a “friend” that we all benefit from, even though we may not be familiar with the technology.
Consider a simple structural solid such as a bar or cube. Whether constituted of metal, plastic or ceramic, it consists of three parts:
- The central body which provides the basic form or shape and the strength and rigidity.
- The surface which is associated with color, texture, patina, sheen, etc.
- A layer between the outer surface and the main body. This layer may be a “barrier” for a number of actions including :
- Heat or cold
- Stress due to differential thermal expansion between the surface and the body.
Some materials create their own barrier. Aluminum is an example. Aluminum is very reactive with oxygen, rapidly forming aluminum oxide when exposed to air. However, the aluminum oxide produced on the surface of an aluminum structure becomes a beneficial barrier that prevents oxygen from diffusing further into the structural solid. Weathering steel is another example of a self produced barrier. In this case, the outer surface of weathering steel becomes oxidized (rust), but, as in the case of aluminum, the layer of rust protects the subsurface from rust.
In most cases, the barrier needs to be added to the core or central body to achieve needed protection. Paint is applied to materials for color and to serve as a barrier, usually against moisture along with some resistance to wear. However, paint fails as a barrier when high temperatures, severe chemical conditions and abrasive conditions are encountered. In such instances, the barrier needs to be a high-temperature, wear resistant material such as a metal or ceramic coating.
In other cases, such barriers are provided by a coating produced using a chemical bath such as zinc and chrome. However, even these processes can be limited due to the complexity of the operation in addition to environmental concerns.
THERMAL SPRAY IS THE SOLUTION
Although thermal spray may not be a familiar process, we benefit from this technology on a regular basis. The non-slip surface on bath tubs and the Teflon coating in our cookware are examples of products of Thermal Spray. In addition, many medical implants, such as those for teeth and hips, incorporate a special “barrier” coating, a compatible interface between the prosthesis and bone.
Thermal spray is also heavily associated with transportation. Oxygen sensors in combustion engines depend on thermal spray for the coating. Cylinder bores in aluminum blocks are possible because of a thermally sprayed, abrasion resistant coating. Thermally sprayed zinc provides rust-proofing of resistance-welded joints in exhaust systems. The high efficiency of jet engines is directly attributable to thermally sprayed special coatings that allow the operating of engines at elevated temperatures. The efficiency of these engines is also enhanced through use of abradable coatings that minimize internal leakage in these engines.
The energy industry also benefits from thermal spray: from the abrasion resistant coatings used on drilling tools to similar coatings used in hydro-turbines to prevent wear from sediment.
Finally, the art world has discovered thermal spray to be a new medium that encourages the economic production of works of art using steel structures with a coating of brass, bronze or aluminum.
In summary, thermal spray is a “friend” that we all benefit from, even though we may not be familiar with the technology.