Thermal spray coating involves heating a material, supplied in powder or wire form to a molten or semi-state. The material is atomized using a stream of process gases to deposit on various substrates, creating coating with thickness between a few micrometers to several millimeters.

Upon impact, they form platelets that bond to the surface, creating a dense, protective coating with no alternation to the substrate structure. This combination of coating and substrate allows designers and engineers to produce products with enhanced surface properties not inherent in the original material.

Thermal spray coatings are used to improve surface properties such as wear, corrosion (even for high temp), friction, and resistance from other damage, and as a result, the life span of machine parts is enhanced. It is a versatile process that can use a large range of materials starting from pure metals to alloys, carbides, ceramics and even composites. It is economically viable and has wide scope of applications.

Reduced Cost. The cost of repairing the component is less than buying a new one. Often, the coating actually lasts longer than the original material used.

Low Heat Input. With few exceptions, the thermal spray process leaves the component’s thermal history alone.

Versatility. Almost any metal, ceramic or plastic can be thermal sprayed.

Thickness Range. Depending on the material and spray system, coatings can be sprayed from 0.001 to more than 1 inch thick. The thickness typically ranges from 0.005-0.1 inch.

Processing Speed. The spray rates range from 3-60 lb/hr depending on the material and the spray system. Typical rates for material application are 1/2 -2 lb of material per sq ft per 0.01 inch thickness.

Initially thermal spray processes were classified according to the type of heat source. For example, processes were classed as flame, plasma or detonation gun spraying. Modifications to the heat source environment further classed these materials into the precise category by which these processes are known today.

• Cold spraying
• Flame spraying
• High velocity oxy fuel (HVOF) spraying
• Arc spraying
• Plasma spraying
• Detonation gun

Materials that can be plastically formed either in the solid or liquid state can be deposited. Where heating is involved, only those materials that remain stable upon heating can be sprayed. Instability may refer to oxidation or decomposition of the material. These materials may, however, be deposited in the form of composites where a secondary material is used to protect the unstable or reactive material. Spraying into a special atmosphere, use of a metallic or polymeric binder to form a composite, or encapsulation of these powder are means of protecting the thermally sensitive materials.

Thermal spray feedstocks can take several forms which are suited to various materials such as:

• Powder – plastic, metal, composite, ceramic
• Wire – metal, composite
• Rods – ceramic
• Liquid

• Wear and abrasion resistance coatings – mining
• Biomedical – orthopaedics (e.g. hydroxyapatite), dentistry, cancer therapy
• Thermal barrier coatings – combustion engines
• Anticorrosion coatings – infrastructure and marine environments
• Abradables – aviation industry
• Ink transfer rolls – printing industry
• Reclamation of worn components
• Art – glass colouring, bronze application
• Electronic applications

• Able to deposit high melting temperature materials
• Fast coating deposition
• No volatile organics are employed as is the case with many paints
• Fast heating and cooling produced inequilibrium phases and may avoid decomposition of certain materials.

A molten particle or a particle able to deform plastically is transported at high speeds within a heat source towards a surface upon which deposition occurs. The droplet or particle undergoes spreading and may create a chemical bond with the underlying surface. With materials that are not able to produce a chemical bond, the substrate is roughened to create a mechanical bond. Each droplet or particle impacts a roughened surface and mechanically interlocks with the asperities on the underlying surface.

Oxidation can be overcome by the use of a shroud placed onto the torch or by placing the thermal spray process into a chamber with a controlled atmosphere. With plasma spraying, the controlled atmosphere most commonly is a vacuum.

The residual stress remaining within the deposited particles mostly influences ceramic coatings. The cooling of such materials needs to be optimised to avoid excessive residual stress levels.

The coatings thickness is dictated by the size of the feedstock for powders, the size of the droplets for arc spraying or the size of the atomised droplets created by the liquid spray process. Typically, flattening of the material by factor of three of the particle size can be expected. To create thin coatings one requires a very fine particle size, usually at sizes between 10 and 20 microns. It is not uncommon to find coatings as thin as 30 microns. Liquid spray processing is able to decrease the thickness even more

Bond strength is dictated by the speed of the particle, temperature within the thermal spray plume, substrate roughness and reaction with the underlying substrate. Bond strength up to 60-80 MPa is not uncommon for thermally sprayed materials. The bonding ability is material and process dependant.