An Introduction to Thermal Spray Technology

The inception of thermal spray technology dates to the early 20th century. It refers to surface engineering techniques that involve the deposition of solid, semi-molten, or molten feedstocks on target substrates at extremely high speeds and elevated temperatures to create a uniform coating. The microstructure of thermally sprayed coatings contains successive, lamellar-like splat layers. The formation of thermal spray coatings involves extremely high quenching rates on the order of one million degrees per second. Thermal spray technology finds application in safeguarding the surfaces of engineering components against abrasive wear, high-temperature oxidation, and corrosion, thermal stresses, thermal and electrical insulation, etc. Metallic, ceramic, composite, and polymer-based coatings can be deposited by different thermal spray technologies.

Working Principles of Thermal Spray Technology

The thermal energy from an electric or combustion-powered heat source melts the feedstock. The spray torch then accelerates and propels the molten spray material towards a clean and prepared target surface. The particles flatten and cool upon their impact, and eventually solidify as a coating layer on this surface.

Types of Thermal Spray Technologies

Depending on the type of heat source, feedstock type, transport medium’s temperature and velocity, and the energy of the sprayed particles, thermal spray processes have different classifications. Both wires, and powders, can be employed as feedstock materials. With thermal spray technologies a wide range of materials such as oxides, carbides, pure metals, self-fluxing alloys, etc., can be deposited. The prominent thermal spray technologies include:

• Flame spraying: The feedstock is supplied to any oxygen-fuel flame where it is melted, atomized by compressed air, and directed on the substrate. Most commonly acetylene or propane are used in conjunction with oxygen.
• Electric wire arc spraying: It involves bringing two oppositely charged metallic wires in contact. An electric arc is generated and melts the feedstock wires. Compressed air atomizes the melt that is sprayed on the target substrate. 
• Plasma spraying: Extremely hot plasma plume gets generated by ionizing a plasma-forming gas mixture (Ar, H2, He, N2) by passing it between an anode and a cathode. This plasma melts the and accelerates it towards the substrate. 
• High-velocity oxy-fuel spraying: A confined combustion is used to create a supersonic gas flow where the feedstock is accelerated to very high velocities. Typical configurations use a combination of propane/kerosene/hydrogen and oxygen.

Due to different process parameters such as flame temperatures and spray velocities, coating properties like hardness, porosity, thickness, and bond strength vary across thermal spray technologies. 

Advantages and Applications

Compared to the conventional coating processes such as galvanizing, CVD and PVD, thermal spray technology enables coating complex geometries over large areas of varying thickness and characteristics. Thermal spray technology can produce coatings as fine as a few microns up to over 1000 microns with tailored porosity and phase composition. A careful selection of process parameters and feedstock of optimum shape and size distribution can help achieve coatings with desired properties, such as superior thermal conductivity, improved thermal expansion, and wear resistance.

Thermal spray technology has diverse applications in industries like aircraft, steel production, medical implants, textile machinery components, electronics, paper manufacturing, printing industry, engine, and gearbox components for automotive. 

Challenges

The two common microstructural defects associated with thermally sprayed coatings are porosity and oxide inclusions. Porosity depends on process parameters like particles’ impact velocity, temperature, and size. It is formed when subsequent lamellae do not fully conform with the previous ones. Oxide inclusions form due to the oxidation of molten metal particles in the air as they approach the substrate. Defects are typically detrimental for coatings because they can alter the desired coating properties. The oxide formation, however, can be prevented under inert atmospheric conditions. Thermal spray technology also necessitates a careful substrate preparation to enable a strong bonding between the coating and the substrate.

Saint-Gobain’s Expertise

Thanks to its engineering excellence and superior process technology, Saint-Gobain Coating Solutions is a world leader developing innovative and customized coating technologies for various industrial services. Our thermal spray solutions’ portfolio ranges from high-performance, high-grade, and cost-effective coating materials to the next-generation, highly-efficient flame, and ProPlasma spray guns. If you are interested in knowing more about our thermal spray technologies, contact us today.