Thermal spray is a family of coating processes in which material, typically in the form of powder, wire, or liquid feedstock, is heated and accelerated toward a surface, where it builds up as a protective or functional layer. The temperatures and velocities involved are extreme: particles can reach over 2,500 °C and travel at several hundred meters per second before flattening on impact and solidifying in microseconds. The result is a dense, adherent coating that can resist wear, corrosion, heat, or some combination of all three. Used across aerospace, energy, marine infrastructure, biomedical implants, and heavy industry, thermal spray has quietly become one of the most versatile surface engineering technologies available, and it continues to evolve in ways that push the boundaries of what coatings can do.
How the Process Works
At its core, every thermal spray variant follows the same logic. A feedstock material is introduced into a high-energy stream, melted or softened, and propelled toward a prepared substrate. When these particles strike the surface, they deform, spread out, and cool rapidly, forming thin disc-shaped deposits called “splats.” Thousands of overlapping splats build up to create a continuous coating, usually ranging from tens of micrometers to several millimeters thick.
The energy source varies by process. Plasma spray uses an electric arc to ionize a gas into plasma, generating temperatures well above the melting point of most ceramics. High-velocity oxy-fuel (HVOF) spray combusts a fuel-oxygen mixture to create a supersonic gas jet. Cold spray, the newest member of the family, relies on a heated high-pressure gas to accelerate particles to such high speeds that they bond on impact without ever fully melting. Each approach produces coatings with distinct characteristics, which is why engineers choose among them depending on what the coating needs to do.
What happens during the split-second that a particle hits the surface matters enormously. Research on splat formation has shown that the substrate temperature plays a critical role: on a relatively cool substrate, a disc-shaped splat forms in the center while the edges splash outward, creating irregular fingers. On a substrate heated above a transition temperature, the splat spreads into a clean disc without splashing, leading to a denser and more uniform coating.
Plasma Spray and Its Role in Extreme Environments
Plasma spraying is the workhorse for ceramic coatings, especially in gas turbines, where thermal barrier coatings (TBCs) protect metal blades from combustion gases that would otherwise melt them. A classic TBC system consists of a ceramic topcoat, typically yttria-stabilized zirconia, over a metallic bond coat on a superalloy blade. The ceramic layer insulates the metal beneath, letting turbine designers push operating temperatures higher for better fuel efficiency.
In-flight particle monitoring during plasma spraying shows just how intense the conditions are. Ceramic particles like yttria-stabilized zirconia have been measured traveling between roughly 490 and 680 m/s at temperatures ranging from about 2,535 to 2,636 °C, while metallic CoNiCrAlY particles reach around 500 m/s and 2,400 °C. These numbers give a sense of the energy involved: particles arrive at the surface as superheated, semi-molten projectiles, and the way they flatten and solidify determines the coating’s final structure.
TBCs do eventually fail, however, and one increasingly troublesome cause is environmental contamination. Calcium-magnesium-alumino-silicate, abbreviated CMAS, is a glassy deposit that forms when airborne dust and sand are ingested by jet engines. At high operating temperatures, CMAS melts and seeps into the porous ceramic coating, corroding its internal structure and ultimately causing it to spall off. This failure mode has become more of a concern as turbine inlet temperatures have risen, and it is an active area of materials research.
High-Velocity Oxy-Fuel Spray and the Move Away from Hard Chrome
HVOF spraying produces coatings that are notably dense and well-bonded because the particles travel at supersonic speeds but at somewhat lower temperatures than in plasma spray. This combination means metallic and cermet powders (blends of ceramic and metal, like tungsten carbide in a cobalt matrix) arrive at the surface fast enough to deform and pack tightly, while avoiding excessive oxidation. The resulting coatings routinely achieve porosity below one percent, bond strengths above 50 MPa, and oxide content under one percent even for reactive metals.
One of the biggest commercial drivers for HVOF has been the phase-out of hexavalent chromium plating. Hard chrome plating was once the standard for wear-resistant surfaces on pistons, valves, hydraulic cylinders, and landing gear. But hexavalent chromium is a potent carcinogen, and regulations in the European Union, including a directive published in 2000 that prohibits its use in vehicles put on the market after July 2003, have forced industries to find alternatives. HVOF-sprayed tungsten carbide coatings have emerged as the leading replacement, offering equal or better wear resistance without the toxic byproducts.
The tribology of these coatings is well studied. HVOF-sprayed tungsten carbide-cobalt coatings show a hardness of roughly 800 HV, and their wear behavior at elevated temperatures is more complex than a simple “hotter equals worse” story. In sliding tests, wear rates first increased with temperature and then decreased, with a minimum wear rate reached around 450 °C. The improvement at higher temperatures was attributed to the formation of oxide phases that act as solid lubricants on the contact surface. Optimizing HVOF deposition parameters has also shown improvements of more than 50 percent in erosion resistance compared to older detonation-gun coatings of the same composition.
Cold Spray and the Debate Over Bonding
Cold spray stands apart from every other thermal spray method because the feedstock never melts. Instead, particles are accelerated in a supersonic gas jet, typically nitrogen or helium heated to a few hundred degrees Celsius, and they bond to the substrate through severe plastic deformation on impact. Because the material stays solid throughout the process, cold spray avoids oxidation, phase transformations, and the residual tensile stresses that can plague high-temperature methods. The deposits preserve the original feedstock’s composition and microstructure remarkably well, making the process attractive for oxygen-sensitive materials like titanium and copper.
The mechanism by which particles actually stick during cold spray has been debated for over two decades. The long-standing explanation centered on adiabatic shear instability: the idea that when a particle hits hard enough, the material at the interface softens so rapidly from frictional heating that it flows in a localized shear band, jetting outward and creating intimate metallic contact. Computational modeling found that the minimum impact velocity needed to produce this shear localization correlated well with the experimentally observed critical deposition velocity across a number of metallic materials, lending strong support to the theory.
More recent work, though, has challenged the idea that adiabatic shear instability is strictly necessary. Researchers have argued that the large interfacial strain needed for bonding can also be triggered by the interaction of strong pressure waves with the free surface at the particle edges, a natural consequence of a sufficiently fast impact that produces hydrodynamic plasticity without requiring localized thermal softening. Supporting this alternative view, a proportionality between critical velocity and the bulk speed of sound has been proposed and confirmed experimentally. The reality is likely that both mechanisms operate, with their relative importance depending on the material pair, particle size, and impact conditions. Shear localization at the interface and within the particle itself has been identified computationally, and these regions appear to be responsible for both bonding and particle fragmentation at very high speeds.
Cold spray has also moved well beyond simple coatings. It is now used as an additive manufacturing technique, building up freestanding three-dimensional components layer by layer. Because the process produces oxide-free deposits without subjecting the substrate to extreme heat, it is well suited for repairs on heat-sensitive aerospace parts and for creating bimetallic structures that would be impossible with conventional welding or casting.
Feedstock Choices and How They Shape Coatings
The raw material fed into a thermal spray gun is not just a matter of chemistry. Particle shape, size distribution, and flowability all influence how evenly the powder feeds, how it behaves in the flame or jet, and what the final coating looks like. Studies on cold-sprayed titanium, for instance, have compared spherical, sponge-shaped, and irregularly shaped powders with mean particle sizes ranging from 20 to 49 micrometers. Spherical powder with a 29-micrometer mean size produced the lowest porosity coating and the best overall sprayability. The non-spherical powders had higher oxygen content, poorer flowability, and paradoxically higher average impact velocities, but their coatings were more porous.
Conventional thermal spray powders generally fall in the 10 to 100 micrometer range. Going finer than that with dry powder creates feeding problems: fine particles clump, clog feed lines, and do not flow consistently. This limitation spurred the development of liquid feedstock spraying, where the material is carried as a suspension of nano- or submicron particles in a liquid carrier, or dissolved as a chemical precursor solution. The liquid medium reduces friction and prevents clogging, and it opens up access to coating chemistries and microstructures that are difficult or impossible to achieve with conventional powders.
Suspension plasma spray (SPS) systems have demonstrated that ceramic coatings such as titanium dioxide, aluminum oxide, yttrium oxide, and yttria-stabilized zirconia can all be deposited using this approach, with some systems operating at relatively modest power levels of around 27 kW using only argon as the plasma gas. The microstructures produced range from porous columnar morphologies, which are desirable for thermal barrier coatings because they tolerate strain, to dense, well-sintered structures with low porosity for applications where impermeability matters. The specific feedstock formulation, including the solvent, particle loading, and particle size within the suspension, directly controls which type of microstructure you get.
Bond Strength and Why Coatings Sometimes Fall Off
A thermal spray coating is only useful if it stays attached. The bond between coating and substrate depends on mechanical interlocking, where splats grip the roughened substrate surface, and in some cases on metallurgical bonding, where atoms from the coating and substrate interdiffuse to form a continuous metallic joint. Most as-sprayed metallic coatings rely primarily on mechanical interlocking because a thin oxide film forms on the substrate during deposition, preventing true metal-to-metal contact.
Post-deposition heat treatment can dramatically improve adhesion. Research on vacuum heat treatment of thermally sprayed metallic coatings has shown that the oxide film at the coating-substrate interface gradually breaks down during heating, shrinking into isolated round-shaped oxide inclusions. Once the film is disrupted, metallurgical bonding forms between the coating and substrate, and adhesion strength increases significantly. The key finding is that the oxide film must actually break for metallurgical bonding to occur; simply heating without disrupting the oxide does not produce the same improvement.
Adhesion testing itself follows standardized protocols. The widely used ASTM C633 test involves gluing a cylindrical fixture to the coating surface and pulling it off in tension; the force at failure, divided by the area, gives the bond strength in megapascals. While straightforward in principle, this test has well-known quirks: the epoxy used to attach the fixture has its own strength limit (typically around 70 to 80 MPa), so very well-bonded coatings can exceed the test’s measurement ceiling. For thermal barrier coatings specifically, adhesion strength testing is complemented by thermal insulation capability measurements, since a coating that is well bonded but does not insulate is not doing its job.
Residual Stress, a Hidden Challenge
Every thermal spray coating carries internal residual stresses that develop during deposition and cooling. These stresses arise from the rapid quenching of splats as they solidify on a cooler substrate and from the mismatch in thermal expansion between the coating material and the substrate underneath. In ceramic-on-metal systems like thermal barrier coatings, this mismatch is large, and managing it is a central design challenge. Residual strain forms within both the coating and the substrate due to these thermo-mechanical processes and any microstructural phase changes that occur during spraying.
Measuring these stresses is not trivial. Techniques like neutron diffraction can probe strain deep inside a coating non-destructively, giving researchers a three-dimensional map of how stress varies through the thickness. Understanding the stress state helps predict where cracks will initiate and how a coating will behave under thermal cycling. In gas turbines, for example, coatings expand and contract with every startup and shutdown, and residual stresses can add to or subtract from the cyclic stresses, accelerating or delaying failure depending on their sign and magnitude.
Corrosion Protection in Marine Settings
Thermal spray has a long track record in marine corrosion protection. Steel structures exposed to seawater, splash zones, and salt-laden atmospheres are commonly coated with thermally sprayed aluminum, zinc, or zinc-aluminum alloys. These coatings protect the underlying steel both as a physical barrier and through cathodic protection: because aluminum and zinc are electrochemically more active than steel, they corrode preferentially, sacrificing themselves to keep the steel intact.
Long-term evaluations have tested commercially pure aluminum, commercially pure zinc, layered zinc-aluminum, and blended zinc-aluminum coatings on steel under marine atmospheric exposure, splash and spray zone conditions, and full seawater immersion. The performance varies considerably by alloy and environment. Aluminum coatings tend to excel under full immersion and in splash zones, while zinc coatings perform well in atmospheric exposure but can degrade faster in submerged conditions. These coatings are widely used on bridges, offshore platforms, port infrastructure, and ship hulls, often sealed with a paint topcoat to extend service life even further.
Biomedical Implants
One of the more surprising applications of thermal spray is in medicine. Plasma-sprayed hydroxyapatite coatings on titanium implants are used in orthopedic and dental surgery to promote bone integration. Hydroxyapatite is chemically similar to the mineral phase of natural bone, so coating an implant with it encourages the surrounding bone tissue to grow directly onto the surface, a property called osteoconductivity. This speeds up healing and improves long-term fixation compared to an uncoated metal surface.
The challenge with plasma-sprayed hydroxyapatite is that the adhesive bond between the coating and the titanium substrate can be weak. If the coating delaminates inside the body, loose particles can trigger inflammation and implant loosening, the opposite of what the coating was supposed to achieve. Researchers have explored compositionally graded coatings, where the composition transitions gradually from pure titanium at the substrate to hydroxyapatite at the surface, as a way to reduce the mismatch in thermal expansion and improve bond reliability.
Process Monitoring and Quality Control
Because so many variables affect coating quality, from gas flow rates and power levels to standoff distance and feedstock characteristics, real-time diagnostics have become critical in production settings. In-flight particle monitoring systems can measure the temperature, velocity, and position of particles as they travel through the thermal spray plume. Particle temperature is typically measured using a two-color pyrometry technique, which compares the thermal radiation emitted by a hot particle at two different wavelengths to determine its temperature without needing to know the exact particle size or perform an absolute intensity calibration.
These diagnostic tools let operators detect process drift before it shows up as a defective coating. If the average particle temperature drops because of a worn electrode or a change in gas supply pressure, the monitoring system flags the change. This has shifted thermal spray from a craft-based operation, where experienced operators adjusted parameters by eye and ear, toward a more controlled, data-driven process. In aerospace, where coating consistency can be safety-critical, such monitoring is increasingly a requirement rather than an option.
Worker Health and Aerosol Exposure
The same physics that make thermal spray effective also create occupational hazards that are only now receiving adequate attention. Heating metals to thousands of degrees and blasting them through a gas jet generates fine aerosols, including nanoparticles, that can linger in workplace air. Reports indicate that high concentrations of aerosols composed of toxic metals such as chromium are generated during thermal spray coating operations, and this has been flagged as an emerging risk despite the technology having been in industrial use for decades.
Animal inhalation studies have begun to characterize the biological effects. In one investigation, rats were exposed to stainless steel aerosols generated by an electric arc wire thermal spray system at a concentration of 25 mg per cubic meter for four hours a day over nine days, simulating an occupational exposure scenario. These controlled experiments help establish dose-response relationships for the complex metal oxide mixtures that workers actually inhale, which differ from the pure single-metal exposures that most older toxicology data are based on. For thermal spray operators, the practical takeaway is that engineering controls like enclosed spray booths with ventilation, along with proper respiratory protection, are not optional extras. They are essential, particularly when spraying alloys that contain chromium, nickel, cobalt, or other metals with known respiratory toxicity.
Cold Spray Additive Manufacturing
The expansion of cold spray from a coating technique into a full additive manufacturing method represents one of the more active frontiers in the field. Traditional additive manufacturing methods for metals, such as laser powder bed fusion and directed energy deposition, rely on melting and resolidification, which introduces residual stress, porosity, and microstructural changes that often require extensive post-processing. Cold spray additive manufacturing sidesteps these issues because the material never melts. The deposits are oxide-free, and the feedstock’s original grain structure and phase composition are largely preserved.
This matters most for repair applications. A turbine blade with localized wear can be rebuilt with cold spray, restoring the original geometry without subjecting the surrounding material to a damaging heat-affected zone. Military maintenance programs have been early adopters, using cold spray to repair magnesium gearbox housings and aluminum helicopter components that would otherwise require complete part replacement. The technology is also being explored for creating multi-material structures, depositing one alloy onto another to combine properties like corrosion resistance on the outside with high strength underneath, in a single near-net-shape part.