What is Thermal Spraying?
Thermal spraying is an industrial coating process where heated or melted materials are sprayed onto surfaces to create protective layers. This versatile technique applies metallic, ceramic, or polymer materials at high velocity to enhance wear resistance, corrosion protection, and thermal insulation of components.
Thermal spraying typically uses an electrical arc or high temperature combustion as the source of energy to create coatings from accumulated powdered material.
It is distinguished by its ability to deposit coatings of metals (such as stainless steels), cermets, ceramics and polymers in layers of substantial thickness, typically 0.1 to 10mm, onto substrate materials for engineering applications. Almost any material can be deposited so long as it melts or becomes plastic during the spraying operation. At the substrate surface, the particles form 'splats' or 'platelets' that interlock and build up to give the coating.
The deposit does not fuse with the substrate or have to form a solid solution to achieve a bond with the surface to be coated. This is a significant feature of thermal spraying compared to many other coating processes, particularly arc welding, brazing and laser coating processes.
The bond between a thermally sprayed coating and the substrate is primarily mechanical, and not metallurgical or fused. Adhesion to the substrate will depend on the condition of the substrate surface, which must be clean and roughened by grit blasting or machining prior to spraying.
Thermal spraying processes have been widely used for many years throughout all the major engineering industry sectors for component protection and reclamation. Recent equipment and process developments have improved the quality and expanded the potential application range for thermally sprayed coatings.
Types of Thermal Spraying
There are a number of different thermal spraying processes that are used for different applications, including:
Cold Spraying
Cold spraying is an advanced solid-state material deposition process where fine powder particles are accelerated to supersonic speeds using a high-pressure carrier gas and projected onto a substrate. Upon impact, these particles undergo severe plastic deformation and bond to the substrate without melting, forming dense deposits for applications such as coatings, repair, and near-net-shape additive manufacturing. The bonding mechanism is similar to explosion welding and, ensures strong adhesion and minimal thermal impact on the substrate, preserving the material's properties and avoiding issues such as oxidation or thermal distortion.
Detonation Spraying
This process creates a shock wave through the detonation of a mixture of acetylene and oxygen. This shock wave propels a powder consumable towards a substrate at high speeds. This process can create very high quality coatings that act as a benchmark for other coating processes.
Flame Spraying
This process uses the combustion of a mixture of oxygen with a fuel gas (acetylene, propylene, propane, hydrogen) to generate heat in a wire or powder consumable. Compressed air/inert gas is then used to propel the heated consumable towards a substrate. Although this process delivers a moderate spray rate at a low cost, flame sprayed coatings show a low bond strength, high oxide content and high porosity.
High Velocity Air Fuel (HVAF) Spraying
This technique uses the combustion of propane is a stream of compressed air. This creates a high velocity, uniform jet into which coating particles are injected so that they are propelled at the substrate. This process can deliver uniform, ductile coatings with a good mechanical bond strength (in excess of 12,000 psi).
High Velocity Oxy-Fuel Coating (HVOF) Spraying
This process takes the combustion of a gas or liquid fuel mixed with oxygen to generate heat and pressure in a chamber. As the mixture expands it forces exhaust gases mixed with particles from a nozzle at high speeds. The high velocities of the particles create high bind strengths and low porosity. HVOF spraying is often used to deposit wear and corrosion resistant coatings.
Plasma Spraying
A DC electric arc is used to create a high-temperature plasma gas that heats a powder consumable as it is fed into the plasma jet. Inert gas is fed into the torch, expanding to create a high velocity jet that can propel particles at speeds of around 200-300 m/s. This process delivers excellent coatings from high melting point materials. The use of a vacuum for vacuum plasma spraying (VPS) or low pressure plasma spraying (LPPS) can deliver even higher quality coatings, albeit at a higher cost.
Spray and Fuse
This process involves spraying a powder material against a substrate before following with an acetylene torch that melts the coating material and top layer of the substrate material together. This fuses the two together and increases the bond between the coating and the part, creating a metallurgical bond. The high temperatures involved can cause some distortion, but this technique can deliver highly wear and abrasion-resistant coatings.
Warm Spraying
This technique is a modification of HVOF spraying where the combustion gas temperature is lowered by adding nitrogen to it. The resulting process temperature is closer to cold spraying than HVOF, although the spray contains more water vapour, oxygen and unreacted hydrocarbons, making it dirtier than cold spraying. Despite this, the efficiency of the final coating is higher than with cold spray. The lower temperatures reduce the melting and chemical reactions of the powders compared to HVOF, making warm spraying ideal for spraying materials that oxidise or deteriorate at higher temperatures, like plastics, metallic glass or titanium.
Wire Arc Spraying
This process feeds two consumable metal wires into a spray gun where they are charged to create an electric arc between them. The heat from the arc melts the wire as it is fed through and the resulting material is then picked up by an air jet fired from the gun. The compressed air jet propels the molten feedstock at a substrate, where it creates a typically heavy, metallic coating.
The main benefits and features of thermal spraying as a coating process are summarised below:
- Comprehensive choice of coating materials: metals, alloys, ceramics, cermets, carbides, polymers and plastics
- Thick coatings can be applied at high deposition rates
- Thermal spray coatings are mechanically bonded to the substrate - can often spray coating materials which are metallurgically incompatible with the substrate
- Can spray coating materials with a higher melting point than the substrate
- Most parts can be sprayed with little or no preheat or postheat treatment, and component distortion is minimal
- Parts can be rebuilt quickly and at low cost, and usually at a fraction of the price of a replacement
- By using a premium material for the thermal spray coating, the lifetime of new components can be extended
- Thermal spray coatings may be applied both manually and mechanised.
Thermal spray coatings are extensively used in the manufacturing of gas turbines, diesel engines, bearings, journals, pumps, compressors and oil field equipment, as well as coating medical implants.
Thermal spraying is principally an alternative to arc welded coatings, although it is also used as an alternative to other surfacing processes, such as electroplating, physical and chemical vapour deposition and ion implantation for engineering applications.