Thermal spray is a generic term for a group of processes in which metallic, ceramic, cermet and some polymeric materials in the form of powder, wire, or rod are fed to a torch or gun with which they are heated to near or somewhat above their melting point. The resulting molten or nearly molten droplets of material are accelerated in a gas stream and projected against the surface to be coated (i.e., the substrate). On impact, 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 spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The “feedstock” (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). There are kinds of materials used in thermal spray coatings such as aluminum powder, copper powder, molybdenum powder, nickel powder, etc.
Thermal spray coatings are used in a wide spectrum of many useful applications, which can include protecting airplanes, buildings and other structures from extreme temperatures, chemicals or environmental conditions such as humidity and rain. Here are the 5 most common processes to generate thermal spray coatings today.
5 Types of Thermal Spray Coating Processes
1. HVOF (High-Velocity Oxy-Fuel Spraying)
HVOF is a process which makes use of a torch that allows the flame to spread whenever the nozzle is used. This creates rapid acceleration which speeds up the particles in the mixture. The end result is an exceptionally thin coating which is evenly applied. Despite being thin, this coating is strong and adheres well. Its resistance to corrosion is better than plasma coatings, but it is not well suited for high temperatures.
2. Combustion Flame Spraying
Combustion flame spraying is an excellent option for surfaces which aren’t designed to handle extreme stress. The coating which results from this process is not strongly attached to the surface since the spraying mechanism is powered by a lower flame velocity. The flame will be generated via oxygen which has been combined with fuel, and this will melt the mixture. Combustion flame spraying is popular for low-intensity applications due to its low cost.
3. Plasma Spraying
Plasma spraying makes use of the plasma torch as the main tool for heating and spraying the coating. After the powder material has been melted down, it is then placed on the product in a manner which is similar to combustion flame spraying. The coatings which result from plasma spraying may be a few micrometers thick to a few millimeters think. While powder is the most widely used material, metals and ceramics are also used. The plasma spraying process is highly popular due to its adaptability.
4. Vacuum Plasma Spraying
Vacuum plasma spraying is done in a controlled environment but utilizes low temperatures. This maintains the vacuum while also reducing damage on the material. A variety of gas combinations can be used to get the necessary pressure for spraying. Vacuum plasma spraying is used for items such as car bumpers, the dashboard or housings for door mirrors. This process can also be used for the pre-treatment of polyethylene moldings, which provides adhesion for epoxy adhesives which are water-based.
5. Two-Wire Electric Arc Spraying
This spraying method utilizes an arc point which is created between two wires which are electrically conductive. Melting will occur at the point where the wires connect. The arc allows for heating which in turn creates deposition and melting, similar to combustion flame spraying which is used with a torch. Compressed air will be used for spraying the coatings. This procedure is popular due to its cost-effectiveness, and will typically use aluminum or zinc as the base material.
Common Functions of Thermal Sprayed Coatings
Choosing a thermal spray material for an application is more complex than selecting a wrought or cast material for the same applications because coating properties are not as predictable as those of conventional materials.
However, now many applications are well established and new ones are continuously being developed. The optimal pairing of the base material and surface coating properties is now possible and it allows obtaining a combination of characteristics that would not be possible with homogeneous materials. The most common functions of thermally sprayed coatings are as follows.
- Wear-resistant coatings
Wear-resistant coatings against abrasion, erosion, cavitation wear, galling, fretting, friction, etc., and often more than one aspect of wear can be addressed. For example, cermets combine hardness, ductility, and acceptable thermal conductivity. Nonstick materials with low-friction coefficients can be combined with hard coatings. Self-lubricating materials can be deposited (for example, materials containing free carbon, or molybdenum disulfide). Very hard materials can be obtained with ceramics, which can also be combined (for example, zirconia and Al2O3). Corrosion and wear-resistant materials can be combined.
- Corrosion-resistant coatings
Many materials are used such as zinc, aluminum, nickel-base alloys, copper-nickel alloys, chemically inert ceramics, plastics, and noble metals. One of the big concerns with thermally sprayed coatings used against corrosion is the interconnected porosity. To make the coatings impervious, high-energy thermal sprays are used, or sealants, of course, depending on service temperature. Coatings can achieve protections against high-temperature sprayed on a superalloy bond coat protecting the substrate from oxidation or corrosion. It is worth to note that corrosion by hot gases decreases through the temperature drop achieved within the ceramic coating.
- Abradable and abrasive coatings
These coatings are used in gas turbine engines for clearance control. Blade tips are designed or coated to make grooves in the relatively soft abradable coating face. The coating thus creates a gas-path seal that prevents gases from bypassing the blades, increasing the engine performance. Nickel/graphite, nickel/bentonite, nickel/polyester, and aluminum/polyester are used or more generally a metal matrix with a nonmetallic filler (graphite, polyester, polyimide, boron nitride, or a friable mineral). The abrasive coatings made of oxides or carbides which can also be embedded in a metallic matrix are applied to the blade tip to reduce wear as it rubs against the abradable coating.
- Electrically conductive coatings
Electrical contacts are made of silver, copper, aluminum, tin alloys, and bronze alloys. Thus electrical conductivity depends on the spray technique used and is generally between 40 and 90 % of that of the bulk material.
Please visit http://www.samaterials.com for more information.