I~ Fi? 1 -: Restoration of cast aluminum engine heads by cold spray. (Photo courtesy of Supersonic Spray Technologies, a division of Center-,,- Lme Windsor Ltd.)

.

A th I Ar ew solid-state spray process unique for preserving the original charac-erma (Fig. 1) is quickly being adopted teristics of the feedstock materials. It is

y many industries. Cold spray is especially attractive for the processing of --capable of providing restoration, sealing, advanced industrial materials, such as i

snrau-like Process surface mod~fication, w~ar resista~ce, those based on n,anotechnology.r '.J' thermal barrIers, corrOSion protection, The process is known as cold gas dy-

heat dissipation, rapid proto typing, near namic spray, gas dynamic spray, kineticnet shapes, aesthetic coatings, and many energy metallization, kinetic spraying, "

(Without the heat) other applications without the undesir- high-velocity po~der depositi.on, or sim-able effects of process temperatures or ply cold spray. It is a new tool m the largemetallurgical incompatibilities among family of thermal spray processes.materials. Similar to conventional thermal spray

, ! \ I oHers exciting ~old spray is capable of producing processes,. cold sprarproduces coatings or I~J' coatmgs or components made of metals, freestandmg deposits for a large number

cermets, polymers, or composites. Its abil- of applications in a wide range of indus-ity to prevent thermal effects such as oxi- tries. However, unlike conventional ther-

P ossibilities in dation, vaporization, melting, recrystal- mal spray processes, cold spray technol-lization, grain growth, and residual ther- ogy can deposit metallic and nonmetallicmal stresses makes this new process materials onto a diversity of surfaces at

c~" industry JULIO VILLAFUERTE (jvillafuerte@cntrline.com) is Corporate Product Develop-ment Manager for CenterLine (Windsor) Ltd" Windsol; Ont., Canada.

- MAY 2005 I

Thermal spray processes based on combustion.Fig. 2

Fig. 3 Electric arc thermal spray processes.

much low'er temperatures, virtually avoid-ing thermal effects. This article reviewsthe evolution of cold spray and describesits technology, applications, and benefits.

processes (arc wire spray, plasma spray)(Ref. 1).

Combustion Processes

History of Thermal Spray

protective coatings. It became more im-portant during and after World War II torepair tank and aircraft components.

Today, the number of applications forthermal spray is countless. Thermal spray-ing is used to improve resistance to cor-rosion, elevated temperatures, oxidation,and wear, as well as to enhance lubricity,restore lost material and provide aestheticsurfaces. The applications for thermalspray are found in aerospace, automotive,power generation, biomedical, heavyequipment, nuclear, mining, chemical,and electronics. The aircraft industry isperhaps one of the largest beneficiariesof thermal spray technology in applica-tions such as the deposition of yttria-stabilized zirconia on engine componentsfor thermal protection.

Based on the nature of the heat source,traditional thermal spray processes can beclassified into two groups: combustionprQcesses (powder flame spray, wire/rodflame spray, detonation spray, and high-velocity oxygen fuel) and electric arc

Flame spraying uses the heat gener-ated by the combustion of fuel gases, typ-ically acetylene, hydrogen, propane, orpropylene, to melt the feedstock material,which can be fed into a spraying gun aspowder, wire, or rod - Fig. 2. In powderflame spraying, powder is fed directly intothe flame by a stream of compressed airor inert gas. In wire flame spraying, solidwire or rod feedstock is fed into the flameat a controlled feed rate. The combustionprocess is external, and the molten par-ticulates are accelerated to relatively lowvelocities « 100 m/s) by a compressedannular air jet.

The detonation gun or D-Gun TM, a pro-prietary technology of Praxair SurfaceTechnologies, uses a long water-cooledbarrel with inlet valves for gases and pow-der. Oxygen and acetylene are fed into the

Thermal spray encompasses a familyof coating processes used to apply metal-lic and nonmetallic coatings to a varietyof metal substrates. In traditionalprocesses, a feedstock material (typicallyin powder, wire, or rod form) is heated bycombustion or electric arcing to formmolten particulates, which are then con-fined and accelerated toward the sub-strate by a jet of hot gas. Upon collisionwith the surface, the particles becomesplats that conform and adhere to the sub-strate, solidifying and building up into alaminar structure.

Thermal spray, invented in the late1800s, was originally known as flamespraying or metallizing. It was primarilyused to restore worn metal parts and apply

Fig. 5 - Schematic diagram of one cold spray system configura-tion (Ref 3).

Fig. 4 - Process temperature and particle velocities for thermal

spray processes.

Table 1 - Existing Applications for Cold Spraying

Applications Examples

Metal restoration and sealing

Thermal barriers

Engine blocks, castings, molds, dies, weld joints, auto body, HV AC, refrigeration, cryogenicequipment, heat exchangers (Ref. 8)

Aluminum piston heads, manifolds, disc brakes, aircraft engine components

Heat dissipation Cu or AI coatings on heat sinks for microelectronics

Microelectronics components and printed circuit boards

Cu or AI patches on metal, ceramic, or polymeric components (Ref. 12)

Soldering priming

Electrically conductive coatings

Dielectric coatings

Antistick properties

Ceramic coatings for aerospace, automotive, and electronic packaging

Friction coatings

Localized corrosion protection

DepositsImpregnated with release agents such as PTFE or silicone

Rolls for papermaking

Zn or AI deposits on affected helms, weldments, or other joints in which the original protectivelayer has been affected by the manufacturing process

Rapid protot.fping and near-net

manufacturing

Well-defined footprints. Fabrication of parts with custom composite or gradientstructures

Biomedical Biocompatible/bioactive materials on orthopedic implants, protheses, dental implants. Porouscoatings of these materials on load-bearing implant devices facilitate implant fixation and bone in.growth, replacing cements and screws

Wear-resistant and decorative coatings Numerous applications

oxygen fuel (HVOF). It uses the combus-tion of acetylene, hydrogen, propane, orpropylene inside a pressurized chamberto produce a hot high-pressure flame. Theflame is forced through a DeLaval nozzleto accelerate the carrier gas to supersonicvelocities - Fig. 2. Feedstock powder canbe fed axially into the high-pressure com-bustion chamber or directly through theside of the nozzle. The HVOF process hasbeen established industry-wide as an al-ternative to the D-Gun TN and other ther-mal spray processes.

Electric Arc Processes

Arc wire spray uses a DC electric arcbetween two continuously fed consumablewires to produce molten spray material-Fig. 3. Compressed air or inert gas pro-duces a spray of fine molten particulatesthat are accelerated toward the substrateat subsonic velocities « 300 m/s). Theprocess is simple and economically feasi-ble for the deposition of conductive ma-terials and the limited selection of ceramicmaterials that are available in wire form.

barrel along with a charge of powder -Fig. 2. A spark is used to ignite the gas mix-ture and the resulting detonation heats andaccelerates the powder at high velocitiesdown the barrel. A pulse of nitrogen isused to purge the barrel after each deto-nation.

This process is repeated many timesper second. The high kinetic energy of thehot powder particles results in the buildupof dense coatings on impact.

A recent addition to the thermal sprayfamily is a process known as high-velocity

Plasma arc spray uses a DC electric arcbetween two nonconsumable electrodes(a thoriated-tungsten cathode and a cop-per anode within the torch) to ionize theinert gas to produce the high-temperatureplasma jet - Fig. 3. Powder feedstock isintroduced into the plasma jet where itmelts and is accelerated to high speeds.The extremely high temperature of theplasma (> lO,OOO°C) makes this processsuitable for spraying elevated-temperature materials, including a wideselection of ceramic materials.

How Different IsCold Spray?

Fig. 6 - Left: Particles of AA2618 aluminum alloy with equiaxed (top left) and columnar(bottom left) grain structures. Right: Cold spray deposit produced with a blend of these twopowders showing the preservation of the original grain structure for each of the powder con-stituents (Ref. 5) (Photo courtesy of the University of Ottawa, Canada.)

into a pressurized chamber and heated to300°-700°C - not to heat the particles,but rather to increase the velocity of thegas jet. Powder feedstock is introducedinto the gas stream, which is not hotenough to melt the particles. The solidpowder/gas mixture is then passedthrough a DeLaval nozzle where the par-ticles are accelerated to supersonic veloc-ities (Ref. 3). The particles impact the sub-strate with enough kinetic energy to pro-duce mechanical/metallurgical bondingwithout melting and/or solidification.

Upon deposition, sprayed materials donot exhibit significant microstructuralchanges, as illustrated in Fig. 6. Cold spraypreserves the original microstructure offeedstock materials making it ideal for de-positing nanograined materials -:- Fig. 7.

For a given material, there appears tobe a range of super-sonic velocities abovewhich bonding mostlikely occurs, i.e., thedeposition efficiency isconsiderably higher.Below this critical ve-locity range, particlesare likely to be de-

1taditional thermal spraying processescan produce high-quality coatings on awide selection of materials, but their in-herent high process temperatures exposethe deposits and substrates to oxidation,metallurgical transformations, and resid-ual thermal stresses caused by the uncon-trolled solidification rates of individualparticles as they hit the substrate.

The invention of cold spray is creditedto a group of Russian researchers in the1980s working at the Institute of Theoret-ical and Applied Mechanics at the Russ-ian Academy of Sciences in Novosibirsk(Ref. 2). They demonstrated that whensmall particles (1-50 microns) struck asubstrate at supersonic velocities, a bondcould occur on impact. This differs fromtraditional thermal spray processes whereparticle velocities are generally lower, butparticle temperatures can be quite high,as illustrated in Fig. 4.

How Does ColdSpray Work?

In one configuration (Fig. 5), heliumor nitrogen is injected at high pressure

flected from the surface causing surfaceerosion similar to shot or abrasive blast-ing. At velocities above this critical range,the deposition efficiency increases andthe quality of the coatings improves (Ref.4). The critical velocity ranges for puremetals such as Cu, Fe, Ni, and Al havebeen estimated at greater than 550 mls(Ref. 2).

In a variation of this technology (Ref.7), the powder feedstock is introduceddownstream into the diverging section ofthe nozzle (Fig. 8). This eliminates the re-quirement for injecting powder into apressurized compartment and avoids po-tential nozzle erosion and clogging.Through this approach, the developmentof portable cold spray equipment has beenmade possible - Fig. 9. The process onlyrequires clean dry air at 80-90 Ib/in.2 forits operation.

In this process, air is preheated to200°-400°C inside a DYMETTM gun (Ref.7) to maximize supersonic velocities at thediverging section of the nozzle. By usingthis method, a wide selection of materialscan be deposited, including metal mix-tures, metal alloys, cermets, epoxy resins,polyurethane, and thermoplastic poly-

Fig. 7 - Transmission electron microscope photos showingnanograined structure of AA5083 aluminum deposited by coldspray (Ref. 6). (Photograph courtesy of the University of Ottawa,Canada. )

Fig. 8 - Schematic diagram of a variant of the cold spray process, where

the powder is injected downstream in the DeLaval nozzle (Ref 7).

Fig. 9 - Portable cold spray machine using the configuration wherethe powder is injected directly into the downstream of the DeLaval noz-zle. This design uses regular compressed air as a carrier gas. (Photocourtesy of SS1; a division of CenterLine Windsor Ltd.)

Fig. ll-AI-AI2Ojpowder mixture on cast iron deposited byair cold spray to repair a shrinkage void in an engine block.(Photo courtesy of the University of Windsol; Canada.)

cladding or shock wave powder com-paction (Ref. 10). Plastic deformation ofthe powder material also helps enhancethe mechanical properties of the depositby work hardening.

mers, ovei'a wide range of substrates, asillustrated in Figs. 10, 11 (Ref. 8).

Bonding Mechanisms

The exact bonding mechanisms forcold spray are not clearly understood.Some propose that the high-velocity im-pact disrupts thin oxide films present onparticles and substrate surfaces, pressingtheir atomic structures into intimate con-tact with one another under momentarilyhigh interfacial pressures, which promoteslocalized atomic bonding (Ref. 9). On theother hand, the critical supersonic veloc-ity for bonding appears to correlate withthe velocities required to trigger a mate-rial instability known as "adiabatic shearinstability," which often occurs in a plas-tically deforming material as it undergoesrapid shear straining (Ref. 10). This maybe similar to the interfacial deformationmechanism associated with explosive

Applications for Cold Spray

ing environment. However, the super-sonic velocities obtainable with nitrogenare lower than with helium, which limitsthe range of coatings that can be pro-duced. Air, on the other hand, offers theleast-expensive alternative but has thenarrowest range of known applications.Air is generally limited to applications notextremely sensitive to the presence of oxy-gen at low temperatures.

Localized plastic deformation at theparticle-substrate interface appears to benecessary for the kinetic energy transfor-mation. For this reason, the successfulpowders and substrates for cold spray aremostly metals with relatively high ductil-ity. Nevertheless, mixtures of metals andceramics can be successfully sprayed ontometallic surfaces and, conversely, metalpowders can be successfully applied to ce-ramic substrates. Apparently the presenceof a metallic component appears to com-pensate for the lack of ductility of the ce-

The range of materials and the metal-lurgical quality of cold spray depositslargely depend on the range of velocitiesthat can be attained as well as the me-chanical characteristics of the particlesand substrate.

Helium is known to provide the high-est supersonic flow velocities and, there-fore, is capable of successfully depositingthe widest range of materials in an oxy-gen-free environment. However helium isexpensive, which requires the use of spe-cialized recovery/recycling equipment tominimize cost. Nitrogen is cheaper andcapable of providing an oxygen-free coat-

===~~~~

ramic component. In general, the rangeof materials that could be successfullysprayed include pure metals such as AI,Zn, Cu, Fe, Ni, Ti, Nb, and Ta, compos-ites such as AI-AIZO3' AI-B4C, Cu-W, Cu-Pb-Sn, Co-WC, and ceramics such as TiOz(Ref. 11). Table 1 provides a list of knownapplications for cold spray. Figure 11shows the microstructure correspondingto an AI-AIZO3 cold spray deposit on graycast iron using air as the carrier gas.

The Benefits of Cold Spray

One of the greatest benefits of coldspray is its ability to deposit material atlow temperatures. It virtually suppressesany metallurgical transformations or ther-mal reactions in either the deposited orsubstrate materials. Because grain growth,phase changes, recrystallization, thermalstresses, and oxidation are prevented; theprocess is uniquely suitable to deposit awide range of sensitive and advanced ma-terials. Other added benefits of this tech-nology include the following:

tiple applications in many industries.However, much more research is neededto better understand and optimize theprocess so that its applicability can be ex-tended into fields where existing practiceshave limitations. For example, since coldspray does not thermally affect the mi-crostructure of powders or substrates, theprocess may be suitable for the depositionof nanocrystalline materials. Also, theability of cold spray to produce composi-tional gradients could be further exploredto develop ways of producing functionalthree-dimensional structures that maybenefit from gradient structures.

Many new material combinations andapplications are yet to be developed andtherefore, the technology will go throughmultiple iterations in the endless pursuitfor new and better ways of processing en-gineering materials. .

Buzdygar, T. V. 2002. Apparatus for gas-dynamic coating. U.S. Patent 6,402,050.

8. Maev, R. G., Strumban, E.,Leshchynsky, \1:, and Beneteau, M. 2004.Supersonic induced mechanical alloytechnology and coatings for automotiveand aerospace applications. Presentationat TSS Cold Spray 2004, Akron, Ohio.

9. Dyhuizen, R. C., et al. 1999. J. Ther-mal Spray Technology: 8(4) pp. 559-564.

10. Assadi, H., Gartner, F., Stoltenhoff,T., and Kreye, H. 2003. Bonding mecha-nism in cold gas spraying, Acta Materialia51, pp. 4379-4394.

11. Kreye, H. 2004. The cold sprayprocess and its potential for industrial ap-plications. Helmut Schmidt University,presentation at TSS Cold Spray 2004,Akron, Ohio.

12. McCune, R.C. 2003. Potential ap-plications of cold spray technologies in au-tomotive manufacturing. Thermal Spray2003: Advancing the Science and Applyingthe Technology, pp. 63-70. C. Moreau andB. Marple, eds. Materials Park, Ohio.ASM Int'l.

Acknowledgments

The author deeply acknowledges allthe valuable contributions by the team atSupersonic Spray Technologies, a divisionof CenterLine Windsor Ltd.; Prof. RomanMaev, University of Windsor, Canada; Dr.Volf Leshchynsky, University of Windsor,Canada; Prof. Bertrand Jodoin, Univer-sity of Ottawa, Canada; and Prof. GaryRankin, University of Windsor, Canada.

. Extremely high density and com.paction of deposited material

. No thermally induced residualstresses

. Minimal or no surface preparationrequired

. A wide selection of coating materi-als, including dissimilar materials References

. Ability to custom-build the chemistryof multilayer deposits

. No generation of toxic gases, radia.tion, or undesirable chemical reactions

. Waste powder can be recycled

. Operates under normal temperature,pressure, and humidity conditions

. Well-defined localized coatings withnear-net shape characteristics

. The shape of the deposit can be pre.determined by the nozzle geometry.

Future Trends

Cold spray is an emerging technologythat addresses many of the shortcomingsof conventional thermal spray processes.The prospects of cold spray as an alterna-tive process are wide open for novel ap-plications unachievable with current prac-tices. Already, cold spray has found mul-

1. Handbook of Thennal Spray Technol-ogy. 2004. Materials Park, Ohio. ASMInt'l.

2. Alkhimov, A. P., Kosarev, B. F., andPapyrin, A. N. 1990. A method of cold gasdynamic deposition. Dokl. Akad. NaukSSR 315, pp. 1062-1065.

3. Alkhimov, A. P., Papyrin, A. N.,Kosarev, V. F., Nesterovich, N. I., andShushpanov, M. M. 1994. Gas-dynamicspraying method for applying a coating. U.S.Patent 5,302,414.

4. Van Steenkiste, T. R., Smith, J. R.,and Teets, R. E. 2002. Aluminum coatingsvia kinetic spray with relatively large par-ticles. Surface & Coatings Techn., Elsevier,154: pp. 237-252.

5. Ajdelsztajn, L., Zuniga, A., Jodoin,B., and Lavernia, E. J. Cold Gas DynamicSpraying of a High Temperature Al Alloy,Surf Coat. Technology, in press.

6. Ajdelsztajn, L., Jodoin, B., Kim, G.E., and Schoenung, J. Cold Spray Deposi-tion of Nanocrystalline Aluminum Alloys,Met. and Mat. Trans. A, in press.

7. Kashirin, A. I., Klyuev, O. F., and