1998 thermal spray processes

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01.2012 EBSCOhost: Thermal Spray Processes

TECHNICAL UNIVERSITYGHEORGHE ASACHIIASI

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Thermal Spray Processes.Authors: van den 8erge[a], Frank M.J.Source: Advanced Materials & Processes; Dec98, Vol. 154 Issue 6, p31, 4p, 1 Color Photograph, 1

Diagram, 1 ChartDocument Article

Type:Subject Terms: *METAL spraying

*COATINGSNAlCS/lndustry 325510 Paint and Coating Manufacturing

Codes:Abstract: Presents an overview of thermal spray processes. Plasma spray; Wire arc spray; Flame

spray; Detonation gun; Coating characteristics; Highvelocity oxygen fuel thermal spray.Full Text Word 2073

Count:ISSN: 08827958

Accession 1363845Number:

Database: Academic Search Complete

THERMAL SPRAY PROCESSESAn OverviewThermal spray comprises a group of processes in which a heat source converts metallic or nonmetallicmaterials into a spray of molten or semimolten particles that are deposited onto a substrate. Any material thatdoes not sublimate or decompose at temperatures close to its melting point can be applied by thermal spray,as long as it is available in wire or powder form.

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Thermal spray coatings offer practical and economical solutions to a variety of industrial problems. They aremost commonly applied to resist wear, heat, oxidation, and corrosion; provide electrical conductivity orresistance; and restore worn or undersized dimensions. Although the coating techniques have been around forsome time, ongoing improvements are leading to lower application costs and a better understanding of howthese coatings work. When properly selected and applied, thermally sprayed coatings can reduce downtime,lower production costs, and improve production yields.Thermal spray is somewhat related to the welding process. In welding, the added material is actually fused tothe base metal, forming a metallurgical bond; whereas a thermally sprayed coating generally adheres to thesubstrate through a mechanical bond. Nonetheless, some thermal spray processes are capable of achievingmechanical bond a Member of ASM International strengths that exceed 70 MPa (10 ksi). Figure 1 shows the wirearc thermal spray process at work. This article provides a review of basic thermal spray technologies,including plasma spray, wire arc spray, flame spray, detonation gun, and HVOF.Plasma sprayThe plasma spray process requires a plasma gun or torch to generate an arc, which creates fire plasma byionizing a continuous flow of argon gas that is injected into the arc. The arc is struck between a water-cooledcopper anode and a tungsten cathode. This type of process is also referred to as non-transferred arc spraying,because the arc is confined to the plasma gun. It is generally operated at energies in the neighborhood of 40 to100 kW.The plasma is a conductive gas with an extremely high internal working temperature (around 10,000 degrees Cor 18,000 degrees F). However, little heat is transferred by the plasma, so the part being sprayed remainsrelatively cool. For example, the process temperature of an 8 kg (20 Ib) part will stay around 100 degrees C (210degrees F). Because of the high internal operating temperature, this process is ideally suited for spraying high-melting-point materials such as ceramics and refractory metals.The plasma's high heat causes a large increase in the volume of inert gas introduced, and this produces a high-speed gas jet that accelerates the molten particles and propels them toward the substrate at high velocities.High particle velocities result in dense coatings with high bond strengths.The plasma transferred arc (PTA) process is somewhat of a hybrid between plasma spraying and welding. Inthis process, an arc is struck between the nonconsumable electrode of the plasma torch and the workpieceitself. The feedstock, in the form of wire or powder, is introduced into the resulting external plasma. The materialis melted and puddled onto the substrate, producing a metallurgical bond similar to welding, but with a lot lessdilution. This process is capable of producing dense and smooth coatings, but it is not capable of applyingceramics.

Wire arc sprayThe wire arc spray process, like plasma spraying, requires an electrical heat source to melt materials. In thiscase, the feedstock consists of two conductive metal wires. These two wires act as electrodes that arecontinuously consumed as the tips are melted by heat from the electrical arc that is struck between them. Anatomizing gas shears off the molten droplets and propels them toward the substrate.The atomizing gas is usually compressed air, but it can also be an inert gas such as nitrogen or argon.Compressed air causes oxidation of metal particles, resulting in a large amount of metal oxide in the coating.Because of this, the coating is harder and more difficult to machine than the coating's source material. This canbe a disadvantage because some coatings have to be ground. However, the increased hardness can also



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enhance wear resistance.EBSCOhost: Thermal Spray Processes

In addition, the temperature of the arc far exceeds the melting point of the sprayed material, resulting in theformation of superheated particles. Consequently, localized metallurgical interactions or diffusion zones develop,which enable achievement of good cohesive and adhesive strengths.The wire-arc process also operates at higher spray rates than the other thermal spray processes. The sprayrate, which is dependent on the applied current, makes this process relatively economical.Flame sprayIn the flame spray process, powder or wire materials are melted through the release of chemical energytriggered by a combustion process. A fuel gas (or liquid) is burned in the presence of oxygen or compressed air.Acetylene fuel gas is most frequently selected, due to its high combustion temperature (3100 degrees Cor 5625degrees F) and low cost. Propane, hydrogen, MAPP, and natural gas are also common choices. The flamemelts the feedstock, and also accelerates and propels the molten particles. Compressed shop-air is also usedto assist and boost the particle velocities. However, a compressed inert gas such as argon or nitrogen ispreferred if oxidation is a concern.The setup of a flame spray system is relatively inexpensive and mobile. A basic setup requires only a flamespray torch, a supply of oxygen, and a fuel gas. To increase safety, the setup might have to be augmented withan enclosed spray booth and exhaust.Because its particle velocities are lower than those of the other thermal spray processes, flame spray coatingsare usually of lower quality; they have higher porosity and lower cohesive and adhesive strengths. However,coating quality can be improved by a "spray-and-fuse" process. After the coating is applied by flame spray, thecombustion process is repeated to raise the substrate temperature to the point at which the previously appliedcoating starts to melt. Fusing temperatures exceed 1040 degrees C (1900 degrees F). The final coating isextremely dense and well-bonded by a metallurgical bond. A disadvantage of this technique is the high substratetemperature required and the possibility for deformation of the part.Detonation gunThe detonation gun (D-gun) process involves an intermittent series of explosions, which melt and propel theparticles onto the substrate. Specifically, a spark plug ignites a mixture of powder and oxygen-acetylene gas in abarrel. Mer ignition, a detonation wave accelerates and heats the entrained powder particles. After eachdetonation, the barrel is purged with nitrogen gas, and the process is repeated several times per second.Coatings produced by the detonation gun process are of excellent quality. The particle velocities are high, so thecoatings are dense and exhibit high bond strengths. The drawback is that the process is relatively expensive tooperate. It also produces noise levels that can exceed 140 decibels, and requires special sound and explosion-proof chambers.HVOF sprayThe high velocity oxygen fuel (HVOF) thermal spray process is closely related to the flame spray process,except that combustion takes place in a small chamber rather than in ambient air. The HVOF combustionprocess generates a large volume of gas caused by the formation and thermal expansion of such exhaustgases as carbon dioxide and water vapor.These gases must exit the chamber through a narrow barrel several inches long. Because of the extremely highpressure created in the combustion chamber, the gases exit the barrel at supersonic velocities, thereby



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01.2012 EBSCOhost: Thermal Spray Processesaccelerating the molten particles. Although the particles do not reach the speed at which the gases are traveling,they do reach very high velocities. Particle velocities of over 750 m/s (2500 fVs) have been measured. Thesehigh particle speeds, and subsequent high kinetic energy, translate into dense coatings with some of the highestbond strengths possible.Coating characteristicsThe goal of all these thermal spray processes is to provide a functional coating that meets all of the necessaryrequirements. The quality of a coating depends on the final function of the coating, and can be determined byevaluating a number of coating characteristics.Characteristics that can be evaluated to determine coating quality include:microstructure (porosity, unmelts, oxidation level)macrohardness (Rockwell B or C) and microhardness (Vickers or Knoop)bond strength (adhesive and cohesive)corrosion resistancewear resistancethermal shock resistancedielectric strength.One of the best ways to gage the overall quality of a thermally sprayed coating is to examine a polished crosssection that reveals the microstructure of the coating, as shown in Fig. 3. A critical aspect of the microstructureis the amount of porosity in the coating, which can be as high as 20% or as low as 1%. It is very difficult to spraycoatings with a porosity of below 1% and this is rarely achieved. In most cases, the coating should be as denseas possible, because low porosity usually results in better wear and corrosion resistance. However, thermalbarrier coatings need higher porosity to achieve the necessary low heat conductivity.Also revealed in the coating microstructure are the so-called "unmelts." These are particles which, due to someprocessing conditions, did not melt completely and are incorporated into the coating. In most cases, unmeltsdisrupt the overall microstructure and lower the cohesive strength of the coating.Oxidized particles are also revealed in the microstructure. When metals or alloys are sprayed, part of themetal/alloy will be oxidized in flight. The amount of oxidation strongly depends on the process and its conditions.For example, wire-arc spraying can result in a large amount of oxides in the coating, especially whencompressed air is the atomizing medium.However, it should be noted that a higher oxide content in the coating is not always bad, and in some cases it isactually encouraged. The higher level of oxides results in a harder coating with somewhat improved wearcharacteristics. A cross section reveals the oxides as dark lines that surround the individual particles (the skin,so to speak).Coating selectionMetal forming, paper and pulp, paper converting, printing (including offset and flexographic), chemical,petrochemical, textile, infrastructure, food processing, automotive, medical, power generation, and aerospace alltake advantage of thermally sprayed coatings. For each application, the coating is selected to perform one ormore functions. The five most encountered functions are wear resistance, heat and/or oxidation resistance,corrosion resistance, electrical conductivity or resistance, and the restoration of worn or undersized dimensions.



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01.2012 EBSCOhost: Thermal Spray ProcessesBasically, coatings fall into three categories: metals/alloys, ceramics, and cermets. Almost every metal and alloyavailable can be sprayed in some form. Frequent choices include copper, tungsten, molybdenum, tin, aluminum,and zinc. Frequently sprayed alloys include steels (carbon and stainless), nickel/chromium, cobalt-base alloys,nickel-base alloys, bronzes, brass, and Babbitts. Ceramic materials are usually metal-oxide ceramics such aschromium oxide, aluminum oxide (also called alumina), alumina-titania composites, and stabilized zirconias.Cermets are coatings that combine a ceramic and a metal or alloy. Two examples include tungsten carbide (theceramic constituent) in a cobalt matrix, and chromium carbide in a nickel-chromium matrix.Overall, thousands of different products and components are coated with great success. In addition, newapplications are developed daily. While it is not possible to provide examples of every application and whatcoating is best for the specific function, the table illustrates the broad range of applications and industries thatare served.The functions and applications of thermal spray coatingsFunction

ApplicationCoating

Wear resistanceAdhesive wear

Bearings, piston rlngs, hydraulic press sleevesChrome oxide, Babbitt, carbon steel

Abrasive wearGuide bars, pump seals, concrete mixer screwsTungsten carbide, alumina / titania, Steel

Surface fatigue wearDead centers, cam followers, fan blades (jet engines), wearrings (land based turbines)Tungsten carbide, copper / nickel/indium alloy, chromiumcarbide

ErosionSlurry pumps, exhaust fans, dust collectorsTungsten carbide, Stellite (Deloro-Stellite Co.)



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Heat resistanceBurner cans/baskets (gas turbines), exhaust ductsPartially stabilized . .ZlrCOnla

Oxidation resistanceExhaust mufflers, heat treating fixtures, exhaust valve

stemsAluminum, nickel/chromium alloy, Hastelloy (HaynesInternational Co.)

Corrosion resistance

Pump parts, storage tanks, food handling equipmentStainless steel (316), aluminum, Inconel (Inco AlloysInternational), Hastelloy

Electrical conductivityElectrical contacts, ground connectorsCopper

Electrical resistanceInsulation for heater tubes, soldering tipsAlumina

Restoration of dimensionsPrinting rolls, undersize bearingsCarbon steel, stainless steel



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Fig. 1--Wire arc thermal spray process at vortc.

O>:;.:!Ii~d1 ' ", < I ;d . .

Fig. 2 -- Types of thermal spray processes. Fig. 3 -- Schematic of a cross-section of thermally sprayed metal.

By Frank M.J. van den Berge[a], Stork Cellramic Inc. Milwaukee, Wis.

* rv1emberof ASM International



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1998 thermal spray processes

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