What Is Thermal Spraying?

Thermal spraying is a group of surface coating processes in which feedstock material, supplied as powder, wire, or rod, is heated to a molten or semi-molten state and propelled at high velocity onto a substrate to form a protective or functional coating. The particles flatten into thin lamellae on impact, building up a layered deposit whose thickness can range from 20 micrometers to several millimeters depending on the process and the application. Thermal spraying is a line-of-sight process: the spray stream must have a clear geometric path to the surface being coated, which shapes how fixtures and robotic spray systems are designed. The bond between the coating and substrate is primarily mechanical, formed by the interlocking of solidified particles within surface asperities, though some processes also produce metallurgical bonding at the interface.

Thermal spray technology draws from plasma physics, combustion engineering, fluid mechanics, and materials science. The family of processes covers a wide range of energy densities and particle velocities, each suited to different coating material classes and performance requirements. Common processes include flame spraying, electric arc spraying, plasma spraying, high-velocity oxygen fuel (HVOF) spraying, and detonation gun spraying.

Spray Processes and Heat Sources

The various thermal spray processes differ primarily in the source of thermal energy and the velocity at which particles are accelerated toward the substrate. As documented by TWI Global's technical overview of thermal spray processes, flame spraying uses the combustion of a fuel gas, typically acetylene or propylene mixed with oxygen, to produce temperatures in the range of 2,500°C to 3,100°C, and is the lowest-cost process with correspondingly higher porosity in the resulting coating. Electric arc spraying melts two wire electrodes using a DC arc and atomizes the melt with compressed gas, achieving high deposition rates for metallic coatings. Plasma spraying generates a plasma jet at temperatures exceeding 15,000 K by passing an inert gas through a DC arc discharge; the extreme temperature allows refractory ceramics and oxides that cannot be melted by combustion-based methods to be deposited as coatings. HVOF spraying combusts fuel and oxygen in a high-pressure chamber, producing supersonic particle velocities of 800 to 1,000 meters per second; the high kinetic energy produces coatings with very low porosity and high bond strength without the extreme temperatures of plasma spray.

Coating Properties and Materials

The microstructure and performance of a thermal spray coating depend on the particle temperature, velocity, and composition at impact. Coatings with high particle velocity and short time at temperature, as produced by HVOF, have low oxide content and low porosity, making them well-suited for wear and corrosion protection. Plasma spray coatings, while more porous, can deposit ceramic compositions such as yttria-stabilized zirconia that are used as thermal barrier coatings (TBCs) on turbine blades, where their low thermal conductivity insulates the metallic substrate from combustion gas temperatures that would otherwise exceed the alloy's service limit.

Progressive Surface's overview of thermal spraying processes describes how combining substrate material and coating composition creates a system with properties superior to either component alone. Wire arc spray deposits zinc and aluminum corrosion-protection coatings on structural steel, while HVOF spray applies tungsten carbide-cobalt coatings to aircraft landing gear pistons for extreme wear resistance. The range of sprayable materials includes metals, alloys, ceramics, cermets, and polymers, giving thermal spraying versatility across a broad set of performance requirements.

Quality Control and Process Standards

Coating quality is characterized by porosity, bond strength, hardness, and microstructural uniformity, measured by metallographic cross-section analysis, adhesion pull-off tests, and microhardness indentation. Porosity is the most commonly controlled parameter because it directly affects corrosion penetration and thermal conductivity. International standards governing thermal spray practice include those published by the International Thermal Spray Association and HVOF spray process standards documented by industry technical bodies, which specify feedstock purity, surface preparation requirements, and post-spray inspection criteria. Substrate surface preparation, typically by abrasive grit blasting to a defined roughness profile, is the single factor most strongly affecting coating adhesion and is specified in every qualified thermal spray procedure.

Applications

Thermal spraying has applications in a wide range of fields, including:

• Gas turbine engine components requiring thermal barrier coatings for temperature isolation
• Aerospace structural parts needing wear-resistant or corrosion-resistant surfaces
• Industrial pump and valve components coated for chemical corrosion resistance
• Bridge and offshore steel infrastructure protected by zinc or aluminum arc-sprayed coatings
• Biomedical implants with hydroxyapatite coatings to promote bone integration