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Cold spraying
Cold spraying
Cold spraying
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Particle temperature and velocity for different thermal spraying processes[1]
Schematics of cold spraying
SEM image of a cold sprayed titanium particle bonded to steel surface

Gas dynamic cold spraying or cold spraying (CS) is a coating deposition method. Solid powders (1 to 50 micrometers in diameter) are accelerated in a supersonic gas jet to velocities up to ca. 1200 m/s. During impact with the substrate, particles undergo plastic deformation and adhere to the surface. To achieve a uniform thickness the spraying nozzle is scanned along the substrate. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be deposited using cold spraying.[2][3] The kinetic energy of the particles, supplied by the expansion of the gas, is converted to plastic deformation energy during bonding. Unlike thermal spraying techniques, e.g., plasma spraying, arc spraying, flame spraying, or high velocity oxygen fuel (HVOF), the powders are not melted during the spraying process.

History

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Cold spraying was developed by Russian scientists in the 1990s. While experimenting with the particle erosion of the target, which was exposed to a two-phase high-velocity flow of fine powder in a wind tunnel, scientists observed accidental rapid formation of coatings. This coating technique was commercialized in the 1990s.[1]

Types

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There are two types of CS. High pressure cold spraying (HPCS) in which the working gas is nitrogen or helium at pressures above 1.5 MPa,[4] a flow rate of more than 2 m3/min, heating power of 18 kW. It is used for spraying pure metal powders with the sizes of 5–50 μm. In low-pressure cold spraying (LPCS), the working gas is a compressed gas with pressure 0.5–1.0 MPa, flow rate 0.5–2 m3/min and the heating power 3–5 kW. It is used for spraying a mechanical mixture of metal and ceramic powders. The inclusion of a ceramic component in the mixture provides high-quality coatings with relatively low energy consumption.[5]

Basic principles

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The most prevailing bonding theory in cold spraying is attributed to "adiabatic shear instability" which occurs at the particle substrate interface at or beyond a certain velocity called critical velocity. When a spherical particle travelling at critical velocity impacts a substrate, a strong pressure field propagates spherically into the particle and substrate from the point of contact. As a result of this pressure field, a shear load is generated which accelerates the material laterally and causes localized shear straining. The shear loading under critical conditions leads to adiabatic shear instability where thermal softening is locally dominant over work strain and strain rate hardening, which leads to a discontinuous jump in strain and temperature and breakdown of flow stresses. This adiabatic shear instability phenomena results in viscous flow of material at an outward flowing direction with temperatures close to melting temperature of the material. This material jetting is also a known phenomenon in explosive welding of materials.[6][7][8]

Key parameters in cold spraying

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There are several factors that can affect the quality of cold-sprayed coatings and the deposition efficiency. Main influential factors are:

  • Gas type, e.g. air, nitrogen, helium
  • Gas pressure
  • Gas temperature (the maximum temperature in cold spraying is ca. 900 °C[1])
  • Particle size
  • Feedstock material properties, e.g. density, strength, melting temperature
  • Nozzle type
  • Substrate
  • Deposition kinetics (gun transverse speed, scan velocity, number of passes ...)
  • Standoff distance, i.e. the distance between the cold spray nozzle and the substrate.[9]

Cold spray parameters are selected with respect to the desired coating characteristics and economic considerations. This can be done by considering correlations between process parameters and final coating properties.[10] There are also software packages available for this purpose.

Advantages and disadvantages

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CS has many advantages that make the technology potentially very competitive. Being a cold process, the initial physical and chemical particle properties are retained and the heating of the substrate is minimal, resulting in cold-worked microstructure of coatings where no melting and solidification happen. Dynamic recrystallization with refined grains has been observed between particle and particle bonding region.[11][12] Furthermore, the technology allows to spray thermally sensitive materials and highly dissimilar materials combinations, due to the fact that the adhesion mechanism is purely mechanical.

Other advantages are:[13]

  • High thermal and electrical conductivity of coatings;
  • High density and hardness of coatings;
  • High homogeneity of coatings;
  • Low shrinking;
  • Possibility to spray micro-sized particles (5–10 μm);
  • Possibility to spray nanomaterials and amorphous materials;
  • Short stand off distance;
  • Minimum surface preparation;
  • Low energy consumption;
  • Possibility to obtain complex shapes and internal surfaces;
  • High productivity due to high power feed rate;
  • High deposition rates and efficiencies;
  • Possibility to collect and reuse 100% of particles;
  • No toxic wastes;
  • No combustion;
  • Increased operational safety due to the absence of high temperature gas jets and radiation.

The jet obtained is a high-density particle beam due to the small size of the nozzle (10–15 mm2) and the short stand-off distance (25 mm). This results in high focus of the jet and precise control over the deposition area. Finally, inducing compressive stresses allows to obtain dense uniform and ultra-thick (20 μm – 50 mm) coatings.

On the other side, some difficulties can be found. For instance, it is difficult to spray hard and brittle materials because, in this case, mechanical adhesion through plastic deformation could be not as effective as it is for ductile particles. Other problems could include:[13]

  • Near-zero ductility in the as-sprayed condition;
  • Need for ductile substrate;
  • Difficulty in processing pure ceramics and some alloys as work-hardening alloys;
  • high cost of Helium;
  • fouling and erosion of the nozzle.

Applications

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Coatings

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The ability for CS to deposit materials that are phase-sensitive or temperature-sensitive has positioned the technique to prepare coatings not possible with other thermal spray techniques. CS can generally be used to produce coatings of a wide variety of metals, alloys, and metal-based composites, including those materials that have an exceptionally high melting temperatures (e.g. tantalum, niobium, superalloys). The process is also valuable for depositing materials that are extremely sensitive to the presence of oxygen and will readily oxidize at modest elevated temperatures – a result which is deleterious to the performance of these materials. Some examples of oxygen sensitive coatings that are commonly produced with CS are aluminum, copper, titanium, and carbide composites (e.g. tungsten carbide),[14] as well as coatings made from amorphous alloys.[15]

Additional developments in CS are related to the deposition of ceramic materials on metals, notably titanium dioxide for photocatalytic effects,[16] and the use of CS in additive manufacturing.[17]

Repair

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Cold spraying is now used to repair machine parts in a matter of minutes. Metal (nickel alloys) particles travel in a blend of nitrogen and helium gas and gradually stack up on the damaged part to recreate the desired surface. A robot controls the movement of the sprayer. The U.S. Army uses the technology to repair a component in Blackhawk helicopters. General Electric is adapting the technology for civilian applications.[18] The US Navy has adopted cold spray welding across its global operations on an experimental basis.[19]

Manufacturing

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Additive manufacturing using cold spray technology can be used to develop parts and components rapidly with deposition rates as high as 45 kg/hour – much faster than other additive manufacturing methods.

Unlike other additive manufacturing methods such as selective laser melting or electron beam additive manufacturing, cold spraying does not melt metals. This means that metals are not affected by heat-related distortion, and parts do not need to be manufactured in an inert gas or vacuum sealed environment, allowing the creation of much larger structures. The world's largest and fastest metal 3D printer has a build envelope of 9×3×1.5 m and utilizes gas dynamic cold spray. Manufacturing with cold spray technology provides advantages such as the ability to create shapes with no shape or size constraints, more efficient buy-to-fly ratio when compared to machining, and capable of fusing dissimilar metals to create hybrid metal parts – materials such as titanium alloys, copper, zinc, stainless steel, aluminium, nickel, even hastelloy and inconel can be sprayed together.[20]

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Cold spraying

View on Grokipedia
from Grokipedia
Cold spraying, also known as cold spray deposition, is a solid-state material processing technique in which fine powder particles, typically metals or composites, are accelerated to supersonic velocities (often exceeding 500 m/s) using a high-pressure such as or , and propelled through a converging-diverging to impact and bond with a substrate at temperatures below the material's , forming dense coatings or bulk structures without significant thermal degradation or oxidation. The process relies on the of the particles to induce severe deformation upon impact, enabling through mechanisms like mechanical interlocking and localized metallurgical bonding, with particle sizes generally ranging from 1 to 50 micrometers and deposition rates up to several kilograms per hour. Developed in the late by Russian researcher Anatolii Papyrin and his team at the Institute of Theoretical and in , cold spraying emerged as an alternative to traditional thermal spray methods like plasma spraying, which involve high temperatures that can alter material properties. Initially patented in 1990, the technology gained traction in the 2000s through commercialization efforts, evolving from laboratory experiments to industrial systems categorized as low-pressure (5-10 bar) and high-pressure (>10 bar) variants, with the latter better suited for harder materials like and steels. By the , advancements in nozzle design and gas dynamics had improved process efficiency, leading to broader adoption in sectors requiring precise, high-integrity deposits. One of the defining advantages of cold spraying is its ability to produce coatings with low porosity (<1%), high bond strength, and no heat-affected zone, preserving the original microstructure and mechanical properties of both feedstock and substrate materials, which is particularly beneficial for heat-sensitive alloys like aluminum and copper. Unlike fusion-based processes, it avoids issues such as cracking, phase changes, or evaporation of alloying elements, while enabling the deposition of thick layers (up to centimeters) with deposition efficiencies often exceeding 90% for ductile metals. However, challenges include the need for high gas pressures (up to 50 bar), potential nozzle erosion from hard particles, and limitations in coating non-planar or temperature-sensitive substrates without post-processing like heat treatment to enhance density. In applications, cold spraying excels in additive manufacturing for building complex near-net-shape components, surface repair of worn or corroded parts (e.g., turbine blades in ), and functional coatings for resistance or thermal management in industries such as , automotive, biomedical, and . Notable examples include NASA's 2021 use of cold-sprayed liners for engines, demonstrating successful integration with hot-fire testing, and ongoing research into and hybrid processes combining cold spraying with cladding for enhanced performance. Recent developments, including for parameter optimization, the 2024 SAE International AMS7057 standard for applications, and the 2025 introduction of -assisted cold spray for improved efficiency on high-strength materials, continue to expand its versatility, positioning cold spraying as a key technology for sustainable manufacturing with reduced material waste.

History and Development

Origins and Early Research

The origins of cold spraying trace back to the late 1970s and early 1980s at the Institute of Theoretical and Applied Mechanics (ITAM) of the Siberian Branch of the in , where researchers were conducting experiments on supersonic two-phase gas flows and particle . During these studies, an unexpected phenomenon occurred: instead of eroding the target surface as anticipated, high-velocity metal particles adhered and formed deposits, leading to the accidental discovery of solid-state particle deposition without melting or significant heating. This serendipitous observation, initially noted with aluminum particles consolidating at velocities of 400-450 m/s in a low-temperature stagnation region, shifted focus from to potential applications. Key researchers, including Anatolii N. Papyrin, Anatolii P. Alkhimov, and Vladimir F. Kosarev, advanced this finding through systematic investigations in the mid-1980s. They filed initial in the as early as 1986 (issued 1990), describing methods to accelerate powder particles (1-200 μm) using unheated compressed gases to velocities exceeding 1000 m/s for formation. A pivotal , US5302414, was filed by Papyrin and colleagues in 1990 and granted in 1994, formalizing the gas-dynamic spraying process and marking the transition from exploratory erosion studies to deliberate deposition techniques. Early experiments utilized or as carrier gases to propel metal powders, such as and aluminum, through converging-diverging nozzles, achieving particle velocities typically in the 500-800 m/s range. These trials demonstrated successful solid-state bonding, producing coatings with low (around 1%) and strengths of 30-80 MPa, without the degradation associated with conventional spraying methods. Foundational s in the solidified cold spraying as a distinct non- technique. A seminal paper by Alkhimov, Kosarev, and Papyrin in 1990 detailed the cold gas-dynamic deposition method, emphasizing its reliance on for rather than . Subsequent works, such as Alkhimov et al. in 1994, reported depositions of various metals including , , iron, and , highlighting the process's versatility for ductile materials. These efforts laid the groundwork for further development, though commercial adoption emerged later.

Commercialization and Recent Advances

Commercialization of cold spray technology began in the late 1990s, driven by key players such as the Center for Powder Spraying Technology (OCPST) in , which developed the first commercial systems, including the DYMET series. Dr. Anatolii Papyrin founded Cold Spray Technology, LLC in the United States around 2003 after issuing early commercial licenses for high-pressure cold spray, contributing to broader industrial adoption. Early commercial systems predominantly utilized as the gas to achieve the high particle velocities necessary for effective deposition, enabling denser coatings compared to alternatives. The U.S. military played a pivotal role in advancing cold spray toward practical applications, initiating adoption for repairs around 2005 through efforts led by the Research Laboratory to address surface restoration on assets. By 2009, the U.S. certified cold spray for repairing specific components on UH-60 Black Hawk helicopters, including magnesium transmission parts, in collaboration with Sikorsky, marking a significant milestone for field-deployable repair technologies. From 2020 to 2025, cold spray has seen notable technological progress, including integration with for precise, automated deposition paths that enhance accuracy and in additive processes. Hybrid systems combining cold spray with in-situ have emerged, incorporating adaptive toolpath planning to repair complex defects while minimizing post-processing, thereby improving efficiency in industrial settings. Advancements in (ceramic-metal) deposits have also progressed, with 2024 studies demonstrating improved wear resistance through optimized particle embedding and bonding in cold-sprayed composites suitable for demanding environments. Standardization efforts have supported wider in , with ongoing development of guidelines to ensure consistent quality and performance. Additionally, the shift to nitrogen-only systems, often using on-site generation, has improved and accessibility across industries by reducing reliance on expensive , without compromising deposition efficacy for certain materials.

Process Variants

High-Pressure Cold Spraying

High-pressure cold spraying (HPCS) represents the conventional variant of cold spraying, distinguished by its use of elevated propellant gas pressures to generate supersonic particle velocities, making it particularly effective for depositing dense coatings of pure metals without significant thermal degradation. This setup relies on compressed gases like or at pressures exceeding 1.5 MPa, often reaching 2.5–4.5 MPa, to drive the process. Associated system demands include gas flow rates greater than 2 m³/min to sustain the high-velocity jet and power consumption around 18 kW primarily for gas preheating. The core component is the —a converging-diverging that accelerates the gas to Mach numbers of 1–3, ensuring particles achieve the necessary for adhesion upon substrate impact. Suitable feedstock particles for HPCS typically measure 5–50 μm in diameter and consist of ductile pure metals, such as aluminum, , or , which deform plastically under high-speed collision. These particles are accelerated to velocities up to 1200 m/s within the supersonic flow, enabling efficient bonding primarily through mechanical interlocking and localized deformation. The operational sequence begins with preheating the high-pressure gas, which then expands through the while entraining metal powder from a controlled feeder system at rates of 2–8 kg/h. The particle-laden jet is directed at the substrate with a standoff of 20–40 mm, optimizing retention and minimizing aerodynamic drag effects for uniform deposition. For ductile materials, this configuration yields deposition efficiencies of 70–95%, facilitating the buildup of coatings up to 10 mm thick in multiple passes while maintaining low and high integrity.

Low-Pressure Cold Spraying

Low-pressure cold spraying (LPCS) operates at significantly reduced gas pressures compared to high-pressure variants, typically ranging from 0.5 to 1.0 MPa (5 to 10 bar), which enables the use of standard or as the gas without requiring specialized high-pressure equipment. Gas flow rates in these systems generally fall between 0.5 and 2 m³/min, supporting particle acceleration through simpler, often converging designs that prioritize portability over extreme performance. Power consumption for LPCS setups is modest, typically 3 to 5 kW, allowing integration with portable compressors and making the process suitable for field deployment. This variant excels with particle sizes of 10 to 100 μm, particularly for metal-ceramic mixtures such as WC-Co cermets, where the ceramic phase enhances coating hardness and wear resistance. Particle velocities in LPCS achieve 300 to 600 m/s, sufficient for solid-state deposition of these composites but resulting in lower deposition efficiencies of 20 to 50% due to the moderated kinetic energy. The portability of LPCS systems facilitates on-site applications, including handheld units for rapid repairs in remote or operational environments. A notable example is the U.S. Navy's adoption of portable LPCS kits in 2023 for fleet maintenance, enabling deployable repairs of and ship components without extensive infrastructure. However, the reduced limits coating thicknesses to approximately 2 mm or less, as thicker builds risk incomplete bonding and . In contrast to high-pressure cold spraying, which offers higher efficiencies for pure metals, LPCS prioritizes mobility for composite materials in practical settings.

Other Variants

While HPCS and LPCS are the primary variants, emerging processes include vacuum cold spraying (VCS), which operates at sub-atmospheric pressures to reduce oxidation and enable deposition in low-gravity or sensitive environments, and pulsed-gas dynamic spraying, which uses intermittent gas pulses for improved control over particle deposition. These niche variants are under research as of but are not yet widely commercialized.

Fundamental Principles

Particle Acceleration and Deposition

In cold spraying, the acceleration of feedstock particles occurs through a supersonic gas jet generated in a , where compressed and heated gas—typically or at temperatures up to 900°C and pressures enabling adiabatic expansion—flows from a converging section to the and then diverges to produce supersonic velocities exceeding the . This isentropic expansion in the diverging section cools the gas while accelerating it, transitioning particles from subsonic speeds in the converging part to supersonic speeds upon exit, with particle velocities closely following the gas flow but lagging due to inertial effects. Particle velocity is determined by drag forces within the gas flow, typically reaching 300–1200 m/s at the nozzle exit. Upon exiting the , particles impact the substrate at velocities exceeding a material-specific critical threshold—such as approximately 600–640 m/s for aluminum—undergoing severe deformation that flattens them into splat-like structures without melting, as the process remains below the material's . The from this high-speed impact converts to localized heating and deformation at the particle-substrate interface, with rises confined to a narrow region reaching up to 60% of the in shear zones, ensuring solid-state deposition. The resulting spray plume exhibits a of 5–10°, influenced by and gas expansion, which affects particle and distribution downstream. is injected into the gas stream either at the entrance for high-pressure systems or at the for low-pressure variants, optimizing particle entrainment and acceleration. The standoff distance between the exit and substrate—typically 20–60 mm—plays a key role in plume characteristics, as excessive distance leads to particle deceleration from ambient , while optimal spacing promotes uniform build-up through sustained high velocities.

Bonding Mechanisms

In cold spraying, the primary bonding mechanism occurs through adiabatic shear at the particle-substrate interface upon high-velocity impact, which induces severe localized plastic deformation and generates sufficient heat for metallurgical without overall melting of the materials. This leads to a narrow shear band where temperatures can rise to up to 60% of the material's , enabling dynamic recrystallization and the formation of strong interfacial bonds in a solid-state process. Unlike thermal spray methods, no significant atomic takes place, as the process remains below the materials' recrystallization thresholds globally, relying instead on mechanical interlocking enhanced by metallurgical joining. The critical velocity also depends on (decreasing with smaller diameters), impact temperature (decreasing with higher temperatures), and substrate properties. A key concept in this bonding is the critical velocity, the minimum particle impact speed required to trigger adiabatic shear instability and achieve deposition. This velocity can be approximated by vcrit=Aσρ+Bcp(Tm−T),v_{\text{crit}} = \sqrt{\frac{A \sigma}{\rho} + B c_p (T_m - T)},
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