Thermal Spray Processes and Application Examples
This article provides a brief description of commercially important thermal spray processes and gives examples of applications and application requirements. The processes covered are flame, wire arc, plasma, high-velocity oxyfuel processes, detonation gun, and cold spray methods. Examples are provided of the applications in aerospace, automotive, and medical device industries as well as the use of thermal spray as an additive manufacturing technique.
Thermal spray is a versatile, adaptable, potentially cost-effective technology in which a wide range of metals, ceramics, polymers, and composites can be applied to almost any metal to protect against wear, corrosion, abrasion, high temperature, chemicals, and erosion. It can also rebuild worn parts of almost any metal, and can be applied by hand or by robots, in the field or in the factory. Because of these capabilities, the technology has increased part life, improved efficiency, and reduced repair costs in the aircraft, automotive, mining, and power generation industries, as well as other industries and many consumer products.
The thermal spray process requires a material in wire or powder form that is transferred to the surface by heat and/or kinetic energy, forming a protective layer. Thermal spray equipment basically consists of a spray gun, materials that are sprayed, a carrier gas, and simple to highly sophisticated controls. Equipment, materials, processes, and controls may be designed for specific applications. This versatility enables engineers in almost every industry throughout the world to improve the function of their equipment and structures.
This article provides a brief description of commercially important thermal spray processes and gives examples of application areas. More detailed coverage of thermal spray processes and applications are provided in ASM Handbook, Volume 5A, Thermal Spray Technology.
In the flame spray process, a combustible gas serves as the heat source that melts the coating material, which may be in rod, wire, or powder form. Commonly selected gases include acetylene, propane, methyl-acetylene-propadiene (MAPP) gas, and hydrogen, along with oxygen. In general, changing the nozzle and/or air cap is all that is required to adapt the gun to different alloys, wire sizes, or gases. For all practical purposes, the rod and wire guns are similar.
The flame spray process is characterized by low capital investment, high deposition rates and efficiencies, low cost of equipment maintenance, and relative ease of operation. In general, as-deposited (or cold spray) flame-sprayed coatings exhibit lower bond strengths, higher porosity, a narrower working temperature range, and higher heat transmittal to the substrate than most other thermal spray processes.
The flame spray process is widely used for the reclamation of worn or out-of-tolerance parts, frequently with nickel-base alloys. Bronze alloys may be sprayed for some bearings and seal areas. Blends of tungsten carbide and nickel-base alloys are applied for wear resistance. Zinc is commonly coated on bridges and other structures for corrosion resistance.
Figure 1 illustrates typical flame spray guns.
The electric-arc (wire-arc) spray process differs from the other thermal spray processes in that no external source such as gas flame or electrically induced plasma provides heat. Instead, two electrically opposed charged wires, comprising the spray material, are fed together in such a manner that a controlled arc strikes at the intersection. The molten metal on the wire tips is atomized and propelled onto a prepared substrate by a stream of compressed air or other gas.
Figure 2 is a schematic of a typical electric-arc spray device.
In general, electric-arc spray exhibits higher bond strengths than flame spray, in excess of 69 MPa (10,000 psi) for some materials. Deposition rates of up to 55 kg/h (120 lb/h) have been achieved for some nickel-base alloys. Substrate heating is lower than in flame spray processes, due primarily to the absence of a flame touching the substrate. The electric-arc process is generally less expensive to operate than the other processes. Electrical power requirements are low, and with few exceptions, no expensive gas such as argon is necessary.
However, the process most commonly entails relatively ductile, electrically conductive wire about 1.5 mm (0.060 in.) in diameter. Therefore, electric-arc spray coatings of carbides, nitrides, and oxides are not currently practical; however, the recent development of cored wires permits the deposition of some composite coatings containing carbides or oxides. By using dissimilar wires, it is possible to deposit pseudo-alloys. A less expensive wear surface can be deposited when one wire, or 50% of the coating matrix, is an inexpensive filler material.
Electric-arc coatings are widely chosen for high-volume, low-cost applications such as zinc corrosion-resistant coatings. In a more unusual application, metal-face molds can be made with a fine spray attachment available from some manufacturers. Molds made in this way can duplicate extremely fine detail, such as the relief lettering on a printed page.
Plasma spray is based on a superheated gas known as a plasma. A gas, usually argon but occasionally including nitrogen, hydrogen, or helium, is allowed to flow between a tungsten cathode and a water-cooled copper anode. An electric arc is initiated between the two electrodes by a high-frequency discharge and then sustained with dc power. The arc ionizes the gas, creating a high-pressure gas plasma. The resulting increase in gas temperature, which may exceed 30,000 °C, expands the volume of the gas and increases its pressure and velocity as it exits the nozzle. (Gas velocities, which may be supersonic, should not be confused with particle velocities.)
Powder is usually introduced into the gas stream either just outside the torch or in the diverging exit region of the nozzle (anode). It is both heated and accelerated by the high-temperature, high-velocity plasma gas stream (Fig. 3). Torch design and operating parameters are critical in determining the temperature and velocity achieved by the powder particles.
The powder velocities usually achieved in plasma spray deposition range from about 300 to 550 m/s. Temperatures are usually at or slightly above the melting point. Generally, higher particle velocities and temperatures above the melting point, but without excessive superheating, yield coatings with the highest densities and bond strengths.
The density of plasma spray coatings is usually much higher than that of flame spray coatings and is typically in the range of 80 to 95% of theoretical. Coating thickness usually ranges from about 0.05 to 0.50 mm (0.002 to 0.020 in.) but may be much thicker for some applications (e.g., dimensional restoration or thermal barriers). Bond strengths vary from less than 34 MPa (5000 psi) to greater than 69 MPa (10,000 psi).
Plasma spray done in an inert atmosphere and/or low-pressure chamber has become a widely accepted practice, particularly in the aircraft engine industry. Inert-atmosphere, low-pressure plasma spray systems have proven to be an effective means for applying complex, hot corrosion-resistant coatings of the Ni-Co-Cr-Al-Y type to high-temperature aircraft engine components without oxidation of the highly reactive constituents. In fact, plasma spray can be used to produce coatings of virtually any metallic, cermet, or ceramic material.
High-Velocity Oxyfuel (HVOF)
In the high-velocity oxyfuel (HVOF) process, fuel such as propane, propylene, or hydrogen is mixed with oxygen and burned in a chamber. (In some cases, liquid kerosene may be used as a fuel and air as the oxidizer.) The products of combustion expand through a nozzle where the gas velocities may become supersonic. Powder is introduced into the nozzle and is heated and accelerated, achieving velocities of up to about 550 m/s.
Figure 4 is a schematic of a high-velocity oxyfuel system.
With appropriate equipment, operating parameters, and choice of powder, coatings with high density and with bond strengths frequently exceeding 69 MPa (10,000 psi) can be achieved. Coating thicknesses are usually in the range of 0.05 to 0.50 mm (0.002 to 0.020 in.), but substantially thicker coatings can occasionally be used, when necessary, with some materials.
HVOF processes can produce coatings of virtually any metallic or cermet material and, for some HVOF processes, most ceramics. Those few HVOF systems that burn acetylene as a fuel are necessary to apply the highest-melting-point ceramics such as zirconia or some carbides. HVOF coatings have primarily been used for wear resistance to date, but their field of applications is expanding.
In the detonation gun process, a mixture of oxygen and acetylene, along with a pulse of powder, is introduced into a barrel and detonated by a spark. The high-temperature, high-pressure detonation wave moving down the barrel heats the powder particles to their melting points or above and accelerates them to a velocity of about 750 m/s.
By changing the fuel gas and some other parameters, the detonation gun process achieves velocities of about 1000 m/s. This is a cyclic process, and after each detonation the barrel is purged with nitrogen and the process is repeated at up to about 10 times per second. Instead of a continuous swath of coating as in the other thermal spray processes, a circle of coating about 25 mm (1 in.) in diameter and a few micrometers thick is deposited with each detonation. A uniform coating thickness on the part is achieved by precisely overlapping the circles of coating in many layers. Typical coating thicknesses are in the range of 0.05 to 0.50 mm (0.002 to 0.02 in.), but thinner and much thicker coatings can be used.
Figure 5 is a schematic of a detonation gun system.
Detonation gun coatings have some of the highest bond strengths and lowest porosities of the thermal spray coatings. In fact, they have been the benchmark against which the other coatings have been measured for years.
In the detonation gun process, the extremely high velocities and consequent kinetic energy of the particles cause most of the coatings to be deposited with residual compressive stress, rather than the tensile stress typical of most thermal spray coatings. This is particularly important relative to coating thickness limitations and the effect of the coating on the fatigue properties of the substrate.
Virtually all metallic, ceramic, and cermet materials can be deposited by detonation gun processes. Detonation gun coatings are used extensively for wear and corrosion resistance as well as for many other types of protection. They are frequently specified for the most demanding applications, yet often can be the most economical choice because they provide such long life.
The distinguishing feature of the cold spray process compared with conventional thermal spray processes is its carrier gas preheat temperatures in the range of 0 to 700 °C (32 to 1290 °F), a range that is generally lower than the melting temperature of the coating particle materials. The nozzle exit temperature is substantially lower than the gas preheat temperature, further lowering the temperature excursions of the feedstock particles. Consequently, deleterious effects of high-temperature oxidation, evaporation, recrystallization, residual stresses, and other concerns are minimized or eliminated.
Figure 6 is a schematic of a cold spray system.
Materials applied by the cold-spray process include pure metals, ferrous and nonferrous alloys, composites, and cermets. Current and potential applications for cold spray coatings include electrical/electronic applications (copper and Fe-NdFeB), aerospace (MCrAlY), automotive (Al and Zn), chemical (Ti), free-standing structures for rapid prototyping, multilayered sleeves (Zn-Cu-Al), and conductive polymers.
Copper is perhaps the most studied feedstock material for the cold-spray process, as these coatings combine very low porosity and low oxygen content, which contribute to excellent electrical properties. Cold-sprayed copper deposits also display excellent tensile properties. Direct-write electrical metallization of copper was one of the earliest envisioned applications for cold-spray technology.
Thermal Spray Applications
Thermal spray technology improves the quality of an astonishing array of products that we see or use every day (Fig. 7). In products ranging from jet engines to automotive components to biomedical implants, thermal spray technology reduces cost and extends useful life. Industrial machinery, chemical process equipment, power generation equipment, and other products that we don’t see every day last longer and are more productive because of reduced corrosion and wear. This article provides just a few examples of significant application areas. A much greater range of thermal spray applications is addressed in ASM Handbook, Volume 5A.
Some of the first applications for thermal spray were in the aerospace industry, specifically for the hot sections of jet engines. To make these engines smaller and more efficient, they had to run hotter, but this meant that materials had to withstand higher temperatures, more corrosion attack, and greater fatigue stress. To enable materials to survive in this environment, a plasma spray technology called low-pressure plasma spray (LPPS) was developed to coat the hot sections of jet engines with MCrAlY alloys, where M is a metal element such as nickel or molybdenum, or an alloy such as nickel-cobalt. The LPPS technology produces the lowest oxide levels and the highest density of all the thermal spray processes, as well as excellent bond strength. Typical thickness of this coating is only 0.125 mm (0.005 in.).
Jet engines are now major consumers of thermal spray, with hundreds of components protected. For example, refractory metals such as molybdenum, tungsten, and rhenium are applied in both the cold and hot sections of engines to reduce friction. Such coatings offer a secondary benefit by facilitating disassembly of the component group at rebuilding time. They are typically applied by air plasma spray (APS) or high-velocity oxy fuel (HVOF) processes.
Thermal barrier coatings consist of a low thermal-conductivity ceramic layer deposited over an MCrAlY bond coat. The ceramic coating is usually zirconia (ZrO2), but pure zirconia exhibits a phase change as the temperature approaches 425 °C (800 °F), resulting in a substantial volume change that can subsequently generate internal stresses and lead to premature coating failure. Oxide stabilizers are therefore added to the zirconia. Yttria (Y2O3) is the most widely used stabilizer, and the material is commonly known as yttria-stabilized zirconia, or YSZ.
Thermal spray protects dozens of parts in automobiles, including piston rings, cylinder bores, exhaust components, alternator covers, brake disks, and many others. For example, exhaust headers are metallized with aluminum via the wire-combustion process. As a result of the coating, stock engines show a 6 to 10% increase in power, and more than a 5% increase in gas mileage.
To protect cast aluminum-silicon cylinder bores, a plasma coating process called Rotoplasma has been developed. The plasma coating typically consists of iron/molybdenum composites or iron/iron-oxide composites. The coefficient of friction against the piston ring is reduced by as much as 30%, and fuel consumption is reduced by 2 to 4%.
Medical implants can now be sprayed with porous coatings that encourage the ingrowth of bone, thus facilitating implant fixation and reducing the need for bone cements and screws. Titanium is the most widely used coating because it has essentially no reaction with bone or soft tissue. Both commercially pure titanium and the workhorse Ti-6Al-4V alloy are deposited with about 30% porosity and typical pore size ranging from 0.05 mm. The technology allows for tight control of particle size, porosity, and thickness. Coatings up to 1.25 mm (0.05 in.) are deposited on hip implants, but dental implants are much thinner. These materials are typically sprayed by low-pressure plasma spray (LPPS) because the process eliminates the need for heat treatment.
In some implants, biomedical coatings with compositions similar to that of bone are needed to actively encourage bone ingrowth. In these cases, hydroxyapatite ceramic, a calcium phosphate similar to bone, may be sprayed. Hydroxyapatite is usually applied by LPPS and may be sprayed directly onto the implant or over another coating.
Additive manufacturing represents a family of technologies in which layers of powder are built up to form a net-shape or near-net-shape part. Two growing areas of this technology for thermal spray are direct fabrication of parts and tool repair.
Thermal spray has been used in combination with stereolithography (SLA) patterns made of polymers. The polymer pattern can be sprayed with zinc or aluminum to make tools for plastic parts. For such applications, tooling costs can be half to only 5% of conventional tooling.
Air plasma spray (APS) processing has been used to repair tooling such as casting molds. For example, APS coatings consisting of 70% yttria-stabilized zirconia and 30% CoNiCrAlY have significantly improved service life of centrifugal casting molds, in one case extending life from 20 to 200 runs.
The cold spray process has produced aluminum metal matrix composites (MMCs) to net shape. Cold spray can produce almost any MMC as a coating or in bulk form by simply blending the matrix and dispersant powders.
New Applications Development
In an effort to advance the technology and increase applications, industries are collaborating with thermal spray providers, universities, and government research organizations. One such collaboration is the Consortium for Thermal Spray Technology, headquartered at the Center for Thermal Spray Research at Stony Brook University in New York. Original equipment manufacturers such as Alcoa, Caterpillar, General Electric, and Boeing are collaborating with thermal spray equipment makers such as Sulzer Metco, carrier gas providers such as Praxair, powder makers such as Saint-Gobain, and coating applicators such as Cincinnati Thermal Spray.
The consortium also includes government entities such as the National Institute of Standards and Technology and Oak Ridge National Laboratory, as well as universities such as the University of California at Santa Barbara and the University of Modena Italy.
Portions of this article are adapted from
James H. Clare and Daryl E. Crawmer, Thermal Spray Coatings, Cleaning, Finishing, and Coating, Vol 5, 9th ed., Metals Handbook, p 1982
Robert C. Tucker, Jr., ed., Thermal Spray Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 497–509, https://doi.org/10.31399/asm.hb.v05.a0001282
ASM Thermal Spray Society, Thermal Spray Processes and Application Examples, Thermal Spray Technology: Accepted Practices, By ASM Thermal Spray Society, ASM International, 2022, p 10–19, https://doi.org/10.31399/asm.tb.tstap.t56040010
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