Metal spray coating is a family of processes that deposit a layer of metal (or metal-ceramic composite) onto a surface by accelerating particles at high speed, where they flatten, interlock, and bond to form a protective or functional layer. The concept dates back over a century, but the range of techniques now available spans everything from simple flame-based systems to sophisticated cold spray processes that never melt the powder at all. What they share is a basic sequence: feed metal into a high-energy stream, accelerate it toward a prepared surface, and let the impact build up a coating one microscopic splat at a time.
How Metal Particles Actually Stick
The bonding question is less obvious than it sounds. You might picture molten metal landing on steel and welding itself in place, but the reality involves several competing mechanisms, and no single one tells the whole story. In thermal spray processes, droplets of molten or semi-molten metal hit the substrate, flatten into thin “splats,” and solidify in microseconds. Those splats lock into the peaks and valleys of the roughened surface below, creating a mechanical grip. But the substrate temperature during spraying matters enormously. Research has shown that a small increase in substrate temperature can shift the splat shape from a fragmented, splashed mess to a smooth, disk-like deposit with minimal splashing, which translates directly into a denser, better-bonded coating.1Materials Science and Engineering: A. Splat formation and microstructure development during plasma spraying: deposition temperature effects
Cold spray operates differently. Because the particles stay solid, there is no melting and no splat in the traditional sense. Instead, particles hit the surface at velocities between roughly 300 and 1,200 meters per second, deforming plastically on impact. The collision ruptures the thin oxide film that naturally coats metal powders, exposing fresh metal underneath and allowing metallic bonds to form at the interface.2Review of Materials Research. Comprehensive Survey Progress in cold spray solid-state deposition additive manufacturing technology One study on nickel particles deposited onto aluminum found something surprising: a single particle impact did not produce real metal-to-metal contact at the interface. It was only when subsequent particles pounded the surface, a peening effect, that the cracked oxide remnants were broken into tiny pieces and pushed aside, allowing discontinuous but genuine metallurgical bonding.3Scripta Materialia. New insights into the coating/substrate interfacial bonding mechanism in cold spray In other words, the coating partly bonds itself by hammering its own foundation into shape.
The Main Spray Methods and What Sets Them Apart
There is no single “metal spray” machine. The choice of process depends on the coating material, the part geometry, the performance requirements, and the budget. The methods fall into two broad camps: thermal spray (where heat is a primary ingredient) and solid-state spray (where it is not).
Thermal Spray Techniques
Flame spraying is the oldest and simplest variant. A fuel gas such as acetylene or propane is burned, and wire or powder is fed into the flame, melted, and blown onto the workpiece by compressed air. It is inexpensive and portable, making it popular for on-site corrosion protection of bridges, tanks, and pipelines. Arc spraying (also called twin-wire arc spray) uses an electric arc between two consumable metal wires as the heat source, with a jet of compressed gas atomizing the molten tips and driving the droplets toward the substrate. Compared to flame spray, arc spray tends to produce harder, denser coatings with less porosity. One comparative study found that twin-wire arc spray increased the surface hardness of carbon steel by about 50% and achieved average bond strengths around 21 MPa, outperforming wire flame spray on both counts.4Materials Science Forum. Physical and Mechanical Properties of Twin-Wire Arc Spray and Wire Flame Spray Coating on Carbon Steel Surface
Plasma spraying takes the energy level up considerably. An electric arc ionizes an inert gas (argon, helium, hydrogen, or a mixture) into a plasma jet that can reach temperatures above 10,000°C. That is hot enough to melt almost any material, including ceramics and refractory metals like tungsten. The process can be run in open air (atmospheric plasma spray) or inside a sealed, low-pressure chamber to reduce oxidation, which is especially useful for oxygen-sensitive materials. Low-pressure plasma spraying with ternary gas mixtures has been explored specifically for depositing dense tungsten and tungsten alloy coatings where porosity and unmelted particles need to be minimized.5ASM International. Effect of Ternary Gas Mixtures on the Microstructure of Refractory Metals and Alloys Deposited by Low Pressure Plasma Spraying
High-velocity oxy-fuel (HVOF) spraying occupies a middle ground: particle temperatures are lower than in plasma spray, but velocities are substantially higher. The combination produces extremely dense coatings with very high bond strength, and it has become the go-to process for carbide-based wear coatings.
Cold Spray
Cold spray sidesteps the thermal damage problem entirely. A heated, high-pressure inert gas (typically nitrogen or helium) is expanded through a converging-diverging nozzle, accelerating metal powder particles to supersonic speeds well above the critical velocity needed for bonding. Because the particles never melt, the process avoids oxidation, phase changes, and the grain growth that degrades many alloys at high temperatures.6PubMed Central. Advancements in Cold Spray Additive Manufacturing: Process, Materials, Optimization, Applications, and Challenges Keeping oxygen content in the powder below about 0.1% and maximizing plastic deformation at the interface are two of the most important levers for getting high-quality bonds.7Review of Materials Research. Comprehensive Survey Progress in cold spray solid-state deposition additive manufacturing technology
Why Surface Preparation Can Make or Break the Coating
A perfectly sprayed coating applied to a poorly prepared surface will peel off. Grit blasting with angular alumina or silicon carbide particles is the standard first step because it cleans the surface of oxides and contaminants while simultaneously roughening it. The rougher the surface, the more mechanical interlocking sites the incoming splats have to grip. Research on tungsten-carbide–cobalt coatings showed that increasing the blasting pressure from about 0.35 to 0.62 MPa raised the average surface roughness from roughly 10 to about 13 micrometers, and adhesion improved in step with that roughness, as measured by both pull-off testing and indentation methods.8Thin Solid Films. Effect of substrate roughness induced by grit blasting upon adhesion of WC-17% Co thermal sprayed coatings
Beyond roughness, cleanliness matters. Grease, moisture, or loose grit particles left on the surface act as weak boundary layers that prevent good contact. Most specifications call for spraying within a few hours of blasting because freshly exposed metal quickly re-oxidizes, especially in humid environments. For cold spray, where bonding depends on oxide rupture at the moment of impact, surface oxide thickness going into the process can directly limit the quality of the metallurgical bond.
Porosity, Residual Stress, and Other Things That Can Go Wrong
No thermally sprayed coating is perfectly dense. Tiny pores form between splats, around unmelted particles, and wherever gas gets trapped during deposition. The amount of porosity varies widely by process: HVOF coatings might have less than 1%, while arc-sprayed aluminum coatings can run much higher. One study that carefully measured the true porosity of arc-sprayed pure aluminum, using a composite polishing technique designed to avoid measurement artifacts, found porosity of about 9.9%, closely matching the roughly 10.2% measured by a bulk weighing method.9PubMed Central. Study on Porosity of Thermal-Sprayed Commercially Pure Aluminum Coating That level of porosity is perfectly acceptable for a sacrificial corrosion coating, but it would be a dealbreaker for a gas-turbine thermal barrier where interconnected pores let hot gases reach the metal underneath.
Residual stress is the other persistent headache. When molten splats solidify and contract on a cooler substrate, they want to shrink but are pinned in place, creating tensile “quenching stress.” After the part cools to room temperature, a second stress component develops from the mismatch in thermal expansion between the coating and the substrate. Together, these residual stresses can cause cracking, spallation (the coating literally flaking off in patches), or distortion of the underlying part.10Materials Science and Engineering: A. Measurement of residual stress in plasma-sprayed metallic, ceramic and composite coatings Managing these stresses is a constant balancing act, usually involving control of substrate temperature during spraying, choosing spray parameters that reduce splat temperature, and sometimes depositing in multiple passes with cooling periods in between.
Corrosion Protection for Marine and Industrial Steel
One of the oldest and most widespread uses of metal spray coatings is protecting steel from corrosion. Bridges, offshore platforms, port infrastructure, and water-treatment plants are all regularly coated with sprayed zinc, aluminum, or zinc-aluminum alloys. The two metals work by different mechanisms. Zinc is electrochemically active enough to sacrificially corrode in place of the steel underneath, providing cathodic protection even where the coating is scratched or damaged. Aluminum, by contrast, forms a passive oxide film that simply blocks the environment from reaching the steel, but it offers less sacrificial capability if the coating is breached.11CORROSION 1985. Characterization of the Corrosion Behavior of Zinc-Aluminum Thermal Spray Coatings
The trade-off is lifespan versus protection style. Zinc’s electrochemical activity means it corrodes faster, depleting the coating material over time. Aluminum lasts longer in calm seawater but may not protect a deep gouge. Zinc-aluminum alloys attempt to combine both benefits. In practice, these coatings are often sealed with a thin organic topcoat (paint or epoxy) that plugs the surface porosity and extends service life. The combination of a sprayed metallic base coat and a painted seal coat can protect marine steel for decades.
Wear Resistance and Thermal Barriers
Beyond corrosion, metal spray coatings are heavily used to fight mechanical wear. Industrial fan blades, for example, operate in environments full of abrasive dust, fly ash, or chemical particulate. Testing of different thermal-spray coatings on fan blades found that a cermet mixture of tungsten carbide and chromium carbide with a nickel binder delivered the best anti-erosion performance, outperforming simpler single-carbide coatings because it combined the favorable properties of both ceramic phases.12PubMed Central. Thermally Sprayed Coatings for the Protection of Industrial Fan Blades
At the extreme end, thermal barrier coatings on gas-turbine blades and combustion chambers use yttria-stabilized zirconia (a ceramic, not a metal, but applied with the same spray equipment) to insulate the metal underneath from flame temperatures that would otherwise destroy it. These coatings can be engineered in layers. One approach deposits a dense first layer by detonation spraying, achieving porosity as low as about 0.3%, followed by a much more porous second layer applied by atmospheric plasma spraying at around 38% porosity. The dense layer provides structural integrity, while the porous layer traps air and dramatically reduces heat conduction.13Ceramics International. Fabrication and characterization of multilayer YSZ thermal barrier coating by detonation spraying and atmospheric plasma spraying That kind of deliberate porosity engineering shows how far the technology has moved from simply blasting metal at a wall.
Replacing Hard Chrome Plating
For decades, hard chromium electroplating was the default way to give hydraulic rods, landing gear, and other high-wear components a hard, slippery surface. The problem is that hexavalent chromium, the form used in the plating bath, is a potent carcinogen and a serious environmental pollutant. Regulatory pressure to eliminate it has driven a search for alternatives, and HVOF-sprayed tungsten carbide coatings have emerged as one of the strongest contenders.14Surface and Coatings Technology. Effects of tungsten carbide thermal spray coating by HP/HVOF and hard chromium electroplating on AISI 4340 high strength steel
The performance comparison favors the spray coatings in several respects. In unlubricated sliding-wear tests, HVOF-sprayed cermet coatings outperformed electrolytic hard chrome in both wear resistance and friction coefficient, meaning less heat generation and less energy loss during service. The lower friction also opens the door to operating with reduced lubrication, which matters in applications where oil contamination is a concern.15Surface and Coatings Technology. Mechanical and tribological properties of electrolytic hard chrome and HVOF-sprayed coatings Aerospace maintenance shops and military depots have been among the first to adopt HVOF carbide coatings as chrome replacements, and the shift is gradually moving into broader industrial use.
Cold Spray for Repairs and Additive Manufacturing
Cold spray has found a particularly compelling niche in repairing expensive, long-lead-time parts rather than scrapping them. Corroded or worn areas on aluminum aerospace structures, magnesium gearbox housings, or steel shafts can be built back up with cold-sprayed material, then machined to the original dimensions. Because the process deposits material in the solid state, there is no heat-affected zone and minimal risk of distorting the part. Applications now extend across aerospace, defense, oil and gas, transportation, and heavy industry. The environmental argument is straightforward: refurbishing a part consumes far less energy and raw material than manufacturing a new one.16Procedia Manufacturing. Structural repair using cold spray technology for enhanced sustainability of high value assets
The same process is increasingly used as a form of additive manufacturing, building up near-net-shape components layer by layer. Unlike laser or electron-beam additive manufacturing, cold spray avoids the high temperatures that cause oxidation, columnar grain structures, and residual thermal stress. The deposited material retains the properties of the original powder feedstock to a much greater degree.17PubMed Central. Advancements in Cold Spray Additive Manufacturing: Process, Materials, Optimization, Applications, and Challenges The trade-off is that cold spray deposits often need post-processing (heat treatment or machining) to reach the mechanical properties of wrought or cast equivalents, and the process works best with ductile metals. Hard, brittle materials are poor candidates because they shatter on impact rather than deforming and bonding.
Measuring Whether a Coating Is Good Enough
A coating that looks smooth and shiny can still fail in service if its adhesion is weak or its internal structure is full of cracks. Industry relies on a handful of standardized tests to catch problems before parts leave the shop. The most common adhesion test is the tensile pull-off method (ASTM C633), where a cylindrical plug is glued to the coating surface and pulled until something breaks. If the coating separates at the interface with the substrate, you have an adhesion number. If it breaks within the coating itself, the cohesive strength is the weak link. If the glue fails first, you only know that the coating is at least as strong as the adhesive.
Typical bond-strength values span a wide range depending on the coating-substrate combination. Testing of flame-sprayed ceramic and metallic coatings on steel and titanium substrates yielded pull-off strengths from about 40 to 100 MPa, while a complementary interfacial indentation method gave toughness values from 1 to 4 MPa·m^0.5.18Surface Engineering. Adhesion tests for thermal spray coatings: Correlation of bond strength and interfacial toughness Both substrate roughness and coating thickness influenced the results, reinforcing the point that surface preparation and process control are not just process details but direct determinants of the coating’s structural integrity.
Health and Safety Concerns
Metal spray processes generate large quantities of fine aerosol, and the composition of that aerosol can be hazardous. Spraying stainless steel wire, for instance, produces airborne particles containing iron, chromium, nickel, and zinc in respirable size ranges.19PubMed Central. Development of a thermal spray coating aerosol generator and inhalation exposure system Chronic exposure to these metals is linked to lung disease, and hexavalent chromium formed during the spraying of chromium-containing alloys is a known carcinogen. Operators typically work with local exhaust ventilation, supplied-air respirators, and full protective clothing. Noise is another factor: plasma and HVOF guns produce sound levels that require hearing protection. In enclosed booths, the combination of high temperatures, metal fumes, ultraviolet light from the plasma arc, and noise makes the work environment one of the more demanding in industrial manufacturing.
The shift toward cold spray has a secondary benefit here. Because no melting occurs, cold spray generates far less airborne fume than thermal processes. The aerosol that does form consists mostly of undeposited powder rather than condensed metal vapor, and without the extreme temperatures of a plasma jet, there is no formation of toxic metal oxides in the plume. For facilities looking to reduce occupational exposure, this is a meaningful advantage on top of the technical benefits.
Spraying onto Soft and Temperature-Sensitive Materials
One of the more unexpected developments in the field is the ability to spray hard ceramic and cermet coatings onto heat-sensitive substrates like aramid fabric, the material in body armor. The challenge is obvious: the fabric melts or degrades at temperatures far below the melting point of the coating material. Researchers solved this by combining high particle velocities (which reduce the heat transferred per particle) with aggressive cryogenic cooling of the substrate during spraying. The result is a flexible, coated fabric that retains its textile properties while gaining a hard surface layer capable of providing stab protection on top of the fabric’s inherent ballistic resistance.20International Journal of Applied Ceramic Technology. Lightweight Ballistic with Additional Stab Protection Made of Thermally Sprayed Ceramic and Cermet Coatings on Aramide Fabrics
Similar cryogenic substrate-cooling approaches have been developed for other sensitive applications. The French Atomic Energy Agency, for example, developed an atmosphere-and-temperature-controlled plasma spray process that couples a standard inert plasma system with cryogenic cooling by atomized liquefied gas, allowing precise control of both the gas environment and the substrate temperature during deposition.21Annals of the New York Academy of Sciences. Measurements of Heat Flux in an Atmosphere‐ and Temperature‐Controlled Plasma Spray Process These techniques push the boundaries of what substrates can receive spray coatings, opening up applications in electronics, composites, and biomedical devices where thermal damage has historically been a disqualifier.