Today's engineering products and systems are required to operate and survive in the most extreme environments and conditions as never before. Higher performances are demanded from materials, which in reality are not sterile.
Surface characteristics of existing materials have to be enhanced to meet this demand, as it would be impossible to invent new materials to keep pace with the ever-increasing surface property requirements and thermal spraying is one such kind to enhance the surface characteristics of materials.
Thermal Spraying
Thermal spraying is a process in which metals are melted and sprayed on a surface to form coating. A coating produced by this process in which molten or softened particles are applied by impact onto a substrate. Different types of thermal spraying includes flame spraying, metallizing, and hardfacing. The metals sprayed are pure metals or alloys that are generally in the form of powders, wires, or rods.
A combustion source or an electric arc provides the energy that is used to melt the metal. After the metal is melted, compressed air or another gas propels the droplets to the surface being coated. In many cases, thermal sprayed surfaces are machined / grounded, to provide the desired surface finish.
A common feature of all thermal spray coatings is their lenticular or lamellar grain structure resulting from the rapid solidification of small globules, flattened from striking a cold surface at high velocities.
Properties that Thermally Sprayed Coatings can provide
- Tribological - wear resistance and Corrosion resistance
- Heat resistance and Thermal barrier
- Electrical conductivity or resistivity
- Abradable or abrasive
- Textured surfaces
- Restoration of dimension
- Repair or build up of worn or damaged surfaces
Types of Thermal Spraying
Based on the coating material form and the heat source the thermal spray coating processes are classified as
- Powder flame spray coating
- Wire arc spray coating
- Plasma arc spray coating
- HVOF spray coating
- Detonation spray coating
Benefits of Thermal Spraying
- Last longer than conventional coating.
- Can reduce cost by producing high quality surface on parts that are formed from inexpensive metal.
- No cure time is needed.
- This proves a versatile and effective weapon in combating hazards like corrosion at high temperature, wear, erosion, heat, impact, radiation etc.
- The main advantage being that thermal spray coating can provide the surface properties and the component substrate material can be chosen from the bulk requirements be it strength, weight or cost without the need to consider its inherent wear resistance or other surface properties.
Comparison of Thermal Spray Processes
| Process | Material Used | Energy Source | Fuels Include |
| Flame spraying | Powder / Wire | Combustion | Oxygen-Fuel(Acetylene) |
| Plasma Arc Spraying | Powder | Electric Arc to produce plasma | Hydrogen, Nitrogen, Argon |
| High Velocity Oxy - fuel | Powder | Combustion | Oxygen - fuel (hydrogen, methane, propylene) |
| Detonation Gun | Powder | Spark Ignition of Gas Mixture | Oxygen-fuel(acetylene) |
Types of Coating Materials
Types of coating powders which are used in D-Gun Coating*
| Oxide based | Carbide based | Metals and alloys |
| Alumina Titania 13% | Tungsten carbide-Cobalt | Iron Boron |
| Alumina Titania 40% | Tungsten Carbide - Cobalt Chromium | Iron Alumina |
| Alumina Titania 3% | Tungsten Carbide - Nickel | Nickel Chromium |
| Alumina (99%) | Chromium carbide - Nickel Chromium | Nickel Aluminium |
| Stainless steel, Copper etc | ||
| Colmonoy |
Types of coating wires by metallizing
Copper, Bronze, Brass, Aluminium, Stainless steel 304, Stainless Steel 316
Coating Thickness Achievable
| Powder | Thickness acheivable |
| Oxides & Ceramics | 20 to 500 microns |
| Carbides | 50 to 1000 microns |
| Metals & Alloys | Upto few mm |
The industries that use thermal spraying
| Aerospace | Machine shops | Power plants |
| Agriculture | Marine | Pulp and paper |
| Automative | Medical | Printing |
| Computers | Military | Steel mills |
| Electronics | Oil gas and exploration | Textile |