05348G Sample

Introduction to CoatingDesign and Processing

SEVERAL KEY PROCESSING STEPS arerequired to produce optimized thermal spraycoatings. For example, to ensure adequate bond-ing of a coating, it is critical that the substratebe properly prepared. The substrate surface mustbe clean and usually must be roughened aftercleaning by grit blasting or some other means.Masking and preheating of the substrate are mostcommonly carried out prior to the applicationof thermal spray coatings; for more detailedinformation, see the section Processing Priorto Coating in this article as well as the articlePrecoating Operations in the Pre- and Post-coating Processing Division of this Volume.Coating quality also depends on spray processvariables and parameters such as part tempera-ture control, gun and substrate motion, spray pat-tern, and deposition efficiency and deposit rate.Postcoating operations further enhance the

quality of thermal spray coatings. As discussedin the section Postcoating Processing in thisarticle and in greater detail in the article Post-coating Operations of the Pre- and Postcoat-ing Processing Division of this Volume, theseoperations include finishing treatments (suchas grinding and polishing), densification treat-ments (fusion, hot isostatic pressing, and heattreating), sealing, and testing and inspection.In addition to these processing steps and

parameters, it is important to understand howto design coatings for optimal performance.Thermal spray coatings are fabricated by com-bining equipment (spray guns, process controls,manipulators, etc.), feedstock materials (pow-ders, wires, or rods), and engineering and pro-cess know-how (experience). Only when allcomponents of the spray process are used cor-rectly does a well-designed coating result.This introductory article reviews the advan-

tages and disadvantages associated with thermalspray processing. Good processing practiceallows the advantages of thermal spray coatingsto be realized, while minimizing some of thedisadvantages.

Advantages and Disadvantages ofThermal Spraying

Thermal spray coatings exhibit a uniquemicro-structural architecture. Coatings are formed as

melted and partially melted particles of differentsizes impact substrates at a rateof perhapsonemil-lion particles per second and build up one upon theother, as shown in Fig. 1. Figure 2 shows the typi-cal lamellar (layered) thermal spray coatingmicrostructure that results from particulate depo-sition. This unique microstructure imparts severaladvantages and disadvantages.

Advantages

Many of the defects identified in Fig. 2 arecontrollable through appropriate equipment

and feedstock selection, but by far the greatestinfluence on coating structure comes from theactual processing step. Properties of a sprayedcoating stemming from the lamellar shape ofthe splats and residual porosity can be over-come by postdeposition treatments. Porositycan sometimes be a benefit, as in the cases foroil retention on bearing surfaces, for chemicallyactive structures such as batteries, for boneattachment in orthopedic implants, and so on.Rapid particle cooling rates and the lamellarsplat shapes are two features that distinguishthermal spray coatings from other coatings.Brittleness, hardness, anisotropic properties,and residual stresses are the result of very rap-idly cooled and flattened particles. Properlyapplied thermal spray coatings have many usesand offer several advantages.

Fig. 1 Schematic of the thermal spray process

Fig. 2 Thermal spray coating. Buildup of a thermalspray coating is a chaotic process. Molten

particles spread out and deform (splat) as they strike thesubstrate, at first keying onto asperities on the substratesurface, then interlocking to one another. Voids canoccur if the growing deposit traps air. Particlesoverheated in the spray jet can become oxidized.Unmelted particles may simply be embedded in theaccumulating deposit.

ASM Handbook, Volume 5A, Thermal Spray TechnologyR.C. Tucker, Jr., editor

Copyright # 2013 ASM InternationalW

All rights reserved

www.asminternational.org

A wide range of materials can be depos-ited as coatings, including metals, metalalloys, oxide and nonoxide ceramics, plastics,cermets, and composite structures comprisedof metals, ceramics, and plastics, can be appliedusing thermal spray. Competitive coating pro-cesses do not afford this versatility.Low Processing Costs. Rapid deposition

rates on the order of 1 to 45 kg/h (2 to 100 lb/h)or more can be achieved. Typically, 2 to 7 kg/h(5 to 15 lb/h) is normal practice. Rapid spray ratesand high deposit efficiencies result in relativelylow processing costs.Wide Range of Coating Thicknesses. Coat-

ing thicknesses from 25 mm to 6.5 mm (0.001 to0.250 in.) are used today (2012). Thicker coat-ings are possible using electric arc spray, coldspray, and vacuum plasma spray (VPS). Coat-ing capabilities on the basis of thickness are:

Wide application range. Thermal spray coat-ings function effectively for a broad range ofsurface modifications.

Wear resistanceabrasive, adhesive, gall-ing, antifretting, cavitation, and erosion

Multilayered thermal barrier coatings com-prising metallic bond coats and oxideceramic topcoats

Abradable and abrasive coatings for gas-turbine engines

Atmospheric and aqueous corrosion control High-temperature oxidation resistance and

corrosion control Electrical resistance and conductivity Net and near-net shape component manu-

facturing Metal- and ceramic-matrix composite

structures

Equipment Costs and Portability. Basicthermal spray equipment is relatively low incost compared to competitive coating processesand can, to some extent, be made portable.Exceptions to this are VPS and fully integratedthermal spray systems.Minimal Thermal Degradation of the Sub-

strate. With proper control, there is little risk ofthermally degrading the substrate during spraying.In terms of substrate interaction, thermal sprayingis a relatively cold process, and substrates areusually kept below 65 C (150 F). Higher tem-peratures are often used to produce enhancedbonding or unique coating characteristics.

Disadvantages

As with all coating technologies, thermalspray has some limitations. Understandingthese limitations is important so that engineerscan design effective solutions to the challengesof surface modification. Failures have occurredbecause coating and substrate limitations werenot considered or were poorly understood.Porosity. Thermal spray coatings generally

exhibit some porosity, allowing the passage ofgases and/or liquids through to the coating/sub-strate interface. Porosity is controllable down to

levels below approximately 0.1% with mostprocesses. Postdeposition treatments such asfusion, sintering, or hot isostatic pressing candecrease porosity and close up splat boundaries.Surface and interconnected porosity can beclosed off using a variety of sealing methods,including anaerobic sealers, epoxies, phenolicsealers, and so on. Connected porosity is mini-mized by using high-velocity oxyfuel, VPS,and cold spray processes.Anisotropic Properties. Thermal spray coat-

ings are inherently anisotropic in the as-sprayedcondition. That issue is discussed elsewhere inthis Volume.Line-of-Sight Process. Thermal spray depo-

sition of coatings on simple geometricalconfigurations is relatively straightforward.Complex geometries, such as gas-turbine air-foils, require automated, robotically-controlledmanipulation of the gun and/or substrateto properly address the coating surface. Devia-tion from spraying normal to the surface cancompromise coating properties (Fig. 3a). As arule of thumb, low-velocity spray processesneed to stay within 15 of normal (Fig. 3b).Higher-velocity processes can tolerate off-axisspraying up to 45 (Fig. 3b). Porosity tendsto increase and coating cohesion tends todecrease, along with deposit efficiency, beyondthese limits.

Processing Prior to Coating

Processing of thermal spray coatings mayinvolve some or all of the steps outlined in thisarticle. Each step represents an essential part ofthe planning process that must be consideredbefore and during the coating process. Forinstance, a first-time repair application willrequire some attention to all of the steps shown,whereas some of the steps can be omitted for anoriginal-equipment-manufactured component.More information on processing prior to coat-ing can be found in the article PrecoatingOperations in the Pre- and Postcoating Pro-cessing Division of this Volume.Thermal spray begins with proper surface

preparation, including cleaning, roughening,and deburring, chamfering, or radiusing edges,and so on. Repairing worn parts often requiresundercutting to remove damaged surface.After cleaning, masking protects selected

component areas, adjacent to the area beingcoated, from unwanted abrasive and/or sprayparticle impact. Common masking methodsinclude metal shadow masks, high-temperaturetapes, and paint-on compounds. Some tapesoffer protection from both abrasive grit andsprayed particles without damage.Preheating, the next step before spraying, is

used to drive off moisture and present a warm,dry surface to the first impacting particles. Alu-minum, copper, and titanium substrates nor-mally are not preheated. Excessive preheatingin either time or temperature should be avoided,because it may oxidize the surface.

Substrate Metallurgy andThermal History

Substrate material type, hardness, and priorheat treatment must be evaluated before pro-ceeding. Understanding metal alloy type andheat treatment helps determine the correct sur-face preparation and preheat temperatures touse during spraying. Thermal spray coatingscan be applied to virtually any substrate,including metals, metal alloys, wood, plastic,glass, paper, and so on.Substrates with hardnesses less than 45 HRC

can be cleaned, roughened, and sprayed usingmost standard techniques. When hardnessexceeds 40 to 45 HRC, surface rougheningwith common abrasive grits and techniques isdifficult to achieve. Procedures for handlinghardened substrates are covered later. Therewill be little change to heat treated substratesif preheat temperatures are kept below 65 C(150 F).Care should be taken when preheating mate-

rials that oxidize easily, such as copper, tita-nium, aluminum, zirconium, uranium, and soon, to minimize oxidation prior to coatingdeposition. When spraying reactive materials,limit preheat temperatures to 65 C (150 F)and spray immediately.

Fig. 3 Thermal spray stream positions. (a) Good. (b)Acceptable. (c) Minimum acceptable. HVOF,

high-velocity oxyfuel

2 / Thermal Spray Processes and Coatings

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Castings or powder metallurgical (P/M) com-ponents may require special cleaning, becauseof inherent porosity. This often includes bakingout trapped moisture and processing fluidlikecutting oils.

Undercutting for Repair andDimensional Restoration

When parts come in for repair, the surface isusually damaged due to wear and/or corrosion.Repair procedures call for removal of the dam-aged surface material by undercutting into thesubstrate. Undercutting removes some goodmaterial as well as the damaged surface inorder to provide a uniform flat surface to acceptthe new coating. It is important to rememberthat undercutting reduces the cross-sectionalarea and correspondingly reduces the tensileand ultimate strength of the part being restored.Sprayed coatings will not restore the originaltensile strength, even though the cross-sectionalarea will have been restored. The undercut sur-face area can be prepared by grit blasting,macroroughening, or bond coating, dependingon the depth of undercut and the coating mate-rial chosen for spraying. It is important to reit-erate that macroroughening can reduce fatiguelife as well as the strength of the part.

Substrate Preparation, PartConfiguration, andCoating Location

In addition to the following discussion ofsubstrate preparation and part configurationand coating location considerations, more infor-mation can be found in the Pre- and Postcoat-ing Processing Division of this Volume.The substrate should be inspected for surface

defects such as cracks, scratches, dents, poros-ity, organic contamination, and so on. Superfi-cial flaws will be replicated in the sprayedcoating, more so with thinner coatings. Conven-tional coatings do not add strength to the sub-strate and cannot be used to repair damagethat may affect structural integrity.Engineering drawings and job routers should be

examined for coating location, coating thickness,and other coating attributes prior to cleaning, sur-face preparation, and coating. Understanding thepart geometry is important to fixturing and toolingin order to maintain the proper spray angle.The following acceptable practices and designconsiderations should be considered for compli-ance with coating requirements (Fig. 4):

When the substrate target area is undercut toaccept a coating (Fig. 4a), always chamfer orradius the corners before spraying. One ruleof thumb is that the radius should equal orexceed the sprayed coating thickness.

Likewise, the substrate edges as shownin Fig. 4(b) should be chamfered or radiused.Again, a 45 chamfer or radius equalto the coating thickness is suggested or

recommended. Edge effects common to allcoating techniques cause unacceptable stres-ses at the edges, which in turn will lead tocracking and coating separation.

Figure 4(c) shows the acceptable practicesfor depositing a coating onto a substrateand against a shoulder or boss. Using

procedures similar to undercutting, cornersmust be radiused or chamfered. Sharp cor-ners against a shoulder are likely to resultin porosity and voids.

Substrate surface preparation prior to thermalspraying is absolutely essential. Steps must be

Fig. 4 Design of substrate geometry for thermal spray coating processes

Introduction to Coating Design and Processing / 3

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undertaken correctly for the coating to performto design expectations. For a one-of-a-kind partto be coated or repaired, surface preparation,masking, and setup may account for 75% ofthe total time required to complete the job.Considering labor and overhead expenses, thetask should be done correctly the first time.Cleaning. The first step in preparing a sub-

strate surface for spraying is to removeorganic and inorganic contaminants such asoil, grease, paint, rust, scale, and moisture.Contaminants remaining on the substrate willaffect adhesion.Vapor degreasing is commonly used to

remove oils, greases, and other solvent-solubleorganic compounds. It is fast, economical, andefficient. Hot vapor degreasing is a variationthat is even faster and more efficient. Porousmaterials such as castings, P/M parts, or castirons should be soaked for 15 to 30 min to dis-solve hidden contaminants that may have pene-trated the substrate. Baking may be required toremove contaminants from these porous struc-tures. Large parts can be steam cleaned orsubmerged in solvents. Manual cleaning withsolvents is often all that is necessary to removesurface greases and oils from machined compo-nents. Common solvents include:

Alcohol Acetone Aqueous washer solutions containing acetic

acid

To avoid the possibility of corrosive crack-ing, chlorinated solvents must be avoided whencleaning titanium and titanium alloys.Baking. When solvent soaking does not work

for porous materials such as castings or P/Mparts, baking at 232 C (450 F), actual parttemperature, for 1 h will vaporize and removemost deeply penetrated oils. If not removed,residual oils will bleed out of the substrateonto the surface during preheating, coating,and/or after spraying.Ultrasonic cleaning can also be used to dis-

lodge some contaminants in confined or hiddenareas. Commercial equipment is available,together with the manufacturers recommendedcleaning solutions. Check to be sure that thesolution chosen is compatible with the substratematerial. Ultrasonic cleaning can also be usedto remove residual entrapped grit after gritblasting. The substrate must be oriented, how-ever, so that dislodged grit can fall free of thesubstrate.Dry abrasive blasting rigorously removes

surface contaminants. This process is used toremove old sprayed coatings, mill scale, paint,burrs, corrosion products, and oxides. Neveruse the same abrasive equipment for bothcleaning and roughening, because contaminantsfrom cleaning could contaminate the surface.Roughening. Second to cleaning, roughen-

ing is the most important step in preparing thesurface to accept the sprayed coating. Commonmethods used to roughen surfaces include:

Grit blasting Machining or macroroughening Application of a bond coat, where grit blast-

ing is not possible

Combinations of thesemethods are often used, forexample, bond coats applied over grit-blasted sur-faces to provide the highest bond strength, or gritblasting over macroroughened surfaces. Properlycleaned and roughened surfaces provide the criti-cal interface upon which the first layer of coatingparticles impact. A properly prepared surface hasthe following attributes:

Cleanliness provides white metal-to-metalcontact for interatomic and metallurgicalinteractions between the substrate andsprayed particles.

Roughening provides significantly increasedsurface area for particle-to-substrate contactand increased atomic and metallurgicalinteraction.

Dry abrasive grit blasting is the most com-monly used surface-roughening technique. Dryabrasive particles are propelled toward the sub-strate at relatively high speeds. On impact withthe substrate, the sharp, angular particles act likechisels, cutting small irregularities into the sur-face. Note that masking of the part may be neces-sary to confine the grit blasting to the area to becoated. Masking is discussed subsequently.The amount of plastic deformation of the sur-

face is a function of the angularity, size, den-sity, and hardness of the particles, in additionto the speed and angle at which the particlesare directed toward the substrate. The result ofgrit blasting places the outer layer of the sub-strate in residual compressive stress. Withmaterials prone to fatigue, such as titanium,substrates are sometimes shot peened beforedry abrasive grit blasting to induce a deep com-pressive stress.Grit-Blasting Variables. Variables that

determine the size and geometry of the surfaceirregularities include the size, shape, hardness,specific gravity, and chemistry of the abrasiveparticles, as well as their speed and angle ofimpingement. Hardness of the base materialand uniform coverage of the substrate are otherimportant variables.Abrasive Grain Size. Because the mass of an

abrasive grain varies as the cube of its diameter,the force of the impact of a 20-mesh (840 mm)abrasive particle will, for example, be eighttimes that of a 40-mesh (420 mm) particle(assuming the particles are equiaxed). Conse-quently, the larger particle will cut much deeperinto the surface, all else being equal. FollowingANSI B74.12, Tables 2 and 3, common gritsizes for thermal spray are 16, 24, 36, 60,80, 120, and 240. Grit selection is based onroughness requirements and substrate toleranceto the grit.Surface roughness values are generally

expressed in microinch (min.) or micrometer(mm) units. The American National Standards

Institutes standard for surface roughness mea-surements calls for Ra, which the arithmeticaverage deviation from the mean (ANSIB46.1-1978, Surface Texture, AmericanSociety of Mechanical Engineers). Other defini-tions of roughness, such as root mean square,are falling into disuse. There are many othersignificant attributes of roughness that areequally, if not more, important in definingroughness. Those parameters help to define thepopulation density of irregularities and surfacearea. See Fig. 5 for a guide to surface roughnessexpected from alumina of various grit sizes ontypical substrates.Particle Shape (or Morphology). The angu-

larity of the abrasive particles profoundlyaffects the depth and shape of the pits cut intothe substrate surface. Rounded shot will makesmooth, rounded indents in the surface, produc-ing roughness with little added surface area.Angular grit crushed from chilled iron shot pro-vides a greater roughness and surface area. Ingeneral, abrasive particulate with sharp, angularcutting edges is required.Particle Hardness. Blasting particles should

be hard enough to cut efficiently but not so brit-tle as to shatter and embed excessively. A cer-tain amount of toughness (ductility) isdesirable to avoid rapid breakdown.Specific Gravity (Density) of Particles.

Angular chilled iron grit has nearly twice thedensity of the heaviest nonmetallic abrasiveand so will have nearly twice the impact energyat the same velocity, for equal-sized particles.This will result in deeper pitting.Grit Velocity. With any type of abrasive, the

force of the impact varies directly with theimpact speed. Hence, higher pressures resultin deeper pitting. With nonmetallic abrasives,

Fig. 5 Typical surface finish obtained using aluminumoxide abrasive. Curves illustrate the range of

results obtainable by pressure and abrasive sizevariations. The exact results will be influenced bymaterial differences, nozzle selection, and othervariables. RMS, root mean square


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high pressures result in rapid shattering of gritparticles, and for this reason, blast pressuresare reduced to save on grit. As a rule, blastmachines of the suction-feed type provide onlyone-third to one-half the speed possible withthe pressure-pot type of blast machines. It hasbeen demonstrated and reported that increasinggrit size is more effective in increasing rough-ness than increasing air pressure.Angle of Impact. It is generally considered

that the angle of impingement of the blasting besimilar to that of the spraying, maintained asclose to 90 to the surface as possible for mostmaterials, but may be varied to lower dependingon the substrate materials and grit type.Substrate Material Hardness. It is difficult to

obtain sufficient roughness for a strong bond tohard metals or ceramics through grit blasting.This is largely true because it is difficult tocut the hardened surface to the required depthto create roughness. Hard materials lack ductil-ity, and the surface cannot be displaced in amanner that provides rough asperities. On hard-ened materials, the use of larger grit sizes withharder grit will effectively fracture, creatingroughness. This approach has substrate limita-tions. However, when coupled with effectivepreheating, it can produce good bonding.Uniformity of the Grit Blasting. Grit blasting

should be accomplished in one uniform traverse ofthe surface being blasted. This is best accom-plished with automation but can be done by expe-rienced operators. The intent is to maximizeroughness while minimizing embedment of thegrit and avoiding work hardening of the substrate.Abrasive Grit-Blasting Equipment. There

are three basic types of grit-blasting equipment:

Pressure machines or pressure pots Suction-type nozzles Centrifugal or airless blasting machines

Pressure machines provide higher abrasivespeeds and use air more efficiently thansuction-feed types (Fig. 6a). For work on largepieces or for continuous production blasting,suction-type systems are used. Properly engi-neered systems provide a means for removingdust and ultrafine particles from the abrasiveto maintain proper grit sizing.Grit velocity and blasting efficiency, with

pressure-type machines, depend primarily onnozzle diameter and air pressure. Table 1 showsthe consumption of air, sand, and chilled irongrit for various nozzle sizes and air pressures,and the horsepower needed to drive them.Suction-type nozzles (Fig. 6b) do not provide

as high an abrasive speed as pressure blasters,and they are less efficient with respect to useof air; however, they are very convenient forsmall work, on-site/field work, and continuousblasting operations. The abrasive is recycledso that periodic reloading is not required.Centrifugal blasting machines are well suited

for production blasting, particularly for largeparts with thick cross sections. With this equip-ment, grit is thrown in a fan-shaped pattern

against the substrate by a rapidly revolving pad-dle wheel. In some cases, the work is also rotatedto expose all surfaces to the blast. Othermachinesmay use two or more wheels that can be posi-tioned to suit the substrate geometry. This kindof equipment ismuchmore efficient than air-typeblast generators in terms of power used.Blasting abrasives may include the follow-

ing types.Angular chilled iron grit is practical for use

with centrifugal blasting equipment in preparingsurfaces for flame spraying. Chilled iron grit hashardnesses up to 60 to 62 HRC and can be usedon substrate materials as hard as 40 to 45 HRC.Chilled iron is often called steel grit, but steel grithas a hardness of only 43 to 48 HRC and is toosoft for most blasting. Steel grit becomes dulland does not produce the sharply defined surfacedetails needed for proper bonding of sprayedcoatings. Chilled iron grit stays sharp longer,and some fracturing occurs during use to producenew, sharp cutting edges.Maintaining the right grit-size distribution

requires that blasting equipment is capable ofscreening and dust collection during blasting.Grit size should be the smallest that will pro-duce the desired surface characteristics.Alumina (Al2O3) grit works well in blasting

cabinets of both designs. The specific gravityof alumina is just over half that of chilled iron,so that this type of abrasive is readily picked upby the suction feed and is effectively acceler-ated through the blasting nozzle. The rate of

breakdown is not excessive when used at mod-erate air pressures.Brown, fused alumina is the more commonly

used grit medium and is extremely hard (>9 onthe Mohs scale); when properly crushed, it hassharp cutting edges. Alumina grains of 240-and 150-mesh size are shown in Fig. 7.Alumina grit is also used for hard surfaces

(>45 HRC) and on nonferrous materials whereembedded steel grit could result in corrosionproblems.Crushed flint for blasting is available in all

mesh sizes. Some users feel that crushed flint issuperior to ordinary silica sand. It is very clean,and the particles are extremely angular and sharp.Flint is, of course, a formof silica, and proper pre-cautions against silicosis must be taken.Crushed garnet is available in several areas.

The composition of garnet varies widely, andsome grades may contain free silica and thusmandate respiratory protection. As may beexpected, the breakdown rate also varies withgarnets from different sources. Some gradesare almost dust-free when blasting, while othersproduce nearly as much dust as silica sand.Silicon carbide (SiC) is also available

in many mesh sizes. However, for coatingapplications where high temperatures areencountered, it is not recommended becauseof its reactivity with various substrates andcoating materials.Crushed slag is a black, glassy grit made from

the slag produced by certain types of furnaces.

Fig. 6 (a) Pressure-type and (b) suction-type grit-blasting machines. Adapted from Ref 1


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Ordinarily, it contains no free silica, but thisshould be verifiedwith the supplier. Crushed slagis usually angular and well graded.Machining and Macroroughening. Macro-

roughening is usually accomplished by machin-ing grooves or threads into the surface to besprayed (Fig. 8). Typically, rough-machinedsurfaces are also grit blasted prior to spraying.Surfaces roughened to this magnitude are oftenused for thick coatings to restrict shrinkagestresses and to disrupt the lamellar pattern ofparticle deposition in order to break up theshear stresses parallel to the substrate surface(Fig. 9). Caution should be used with this tech-nique. Threading reduces cross section and canaffect load capability and increase fatigue.When to Start Spraying. It is always good

practice to begin thermal spraying of a compo-nent as soon as possible after completion of sur-face preparation. Freshly exposed clean metal ismetallurgically active and readily susceptible tocontamination and oxidation. Fingerprints orcontact with foreign materials may severelyaffect coating adhesion. If a freshly preparedsurface must be stored before spraying, itshould be kept covered and stored in a clean,dry area with the temperature maintained abovethe dewpoint, but preferably within two hoursdepending on substrate materials.Application of a Bond Coat. Using a bond

coat is a common method of obtaining a

roughened, textured surface and to mitigatecoefficient of thermal expansion (CTE)-induceddifferential expansion between the coating andsubstrate.The following guidelines can be applied to

determine when to use bond coats:

A minimum of surface preparation is possi-ble. This is particularly useful on thin sub-strates where blasting may cause warpageor distortion.

Increased reliability when combining gritblasting with bond coating

Use for hard substrates that cannot be effec-tively roughened by grit blasting

Use for corrosion protection of the substrate Use to help accommodate differences in

CTE between topcoats and substrates When the topcoat has a thickness limitation,

bond coatings may be applied to build upextra thickness for repair work.

Bond coat thicknesses are typically between 75and 125 mm (0.003 and 0.005 in.).Bond Coat Materials. Bond coats can be

applied by wire- and powder-flame spray,plasma arc spray, high-velocity oxyfuel(HVOF), and electric arc spray. Most materialsare available in the form of powders or wires.Some typical materials, their chemistry, andservice temperatures are listed in Table 2.

Masking

Various masking techniques have beendeveloped to protect components next to thecoating area from impact by overspray parti-cles. These methods include:

Shadow masks High-temperature tapes, most commonly

two-sided, silicone tape to resist blastingand spray, 3M stencil tape for blasting andspray resistance, and glass-filled tape forboth blasting and spraying

Paint-on masking compounds

Metal shadow masks are placed approxi-mately two to three times the total coating thick-ness away from the part to be sprayed and infront of the spray torch, shielding selected areasfrom the direct spray stream (Fig. 10). The tar-geted spray area may be stationary or rotating.The mask is typically stationary and securelyaffixed. As the spray stream is traversed acrossthe target area and onto the mask, the unwantedspray collects on the mask and is prevented fromreaching the substrate. The coating will end in anarrow feathered band rather than as a sharplydefined line (Fig. 10b). In contrast to the featherededge, note that a contactmask (Fig. 10a), similar tothe tape that is discussed in this section, leaves asharp edge, which may act as a stress raiser that

Table 1 Consumption of air, horsepower, sand, and grit for various nozzle sizes and air pressures

Air pressure, psig

Consumable 50 60 70 80 90 100

0.19 in. (4.75 mm) nozzle size

Air, cfm 26 30 33 38 41 45Horsepower 3.6 4.6 5.5 6.9 7.9 9.3Sand/h 188 214 245 270 298 330Grit/h 470 532 612 674 748 824





0.38 in (9.50 mm) nozzle size


0.44 in. (11 mm) nozzle size




cfm, cubic feet per minute


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often leads to disbonding of the coating. Metalmasks can be reused, subject to the amount ofcoating buildup and/or distortion due to heat fromthe spray process.Sheet metal masks are preferred for HVOF

spraying, because tapes do not stand upto high-velocity particle impact and the highthermal output of the gun.Tape masks are applied by hand wrapping

areas that do not require a coating. After spray-ing, the tape is easily removed, along withaccumulated overspray, leaving the protectedareas clean. Many different tapes are availablecommercially, some for resistance to grit blast-ing and others for thermal spraying. The besttapes for the job have sufficient toughness toresist both grit blasting and heat resistance towithstand hot process gases and particle impactduring spraying. The coating line when the tapeis removed after the application of thin coatingsleaves a neat and clean edge, but if the coatingis thick, chipping is likely to result where bridg-ing over the tape has occurred.Masking Holes and Complex Geometries.

Further masking techniques are shown in

Fig. 11. Radiused corners are protected usingcurved masks to keep unmelted particles andother debris from contaminating the coating.Dowel and screw holes can be masked usingsimple tapered silicone rubber plugs whosetop surfaces are above the surface being coatedand a slightly larger diameter than the hole, sotapered, low-stress edges will result.

Masking compounds are useful forproviding short-term protection from overspraybut are not useful for protection from gritblasting. Commercially available compoundscomprise refractory particles, such as graphite,carried in water- or solvent-soluble binders.These can be applied by brushing or spraying.When dry, the film prevents overspray fromsticking, while the refractory particles provideshort-term heat protection. Excessive exposureto the spray stream, however, will cause thesecompounds to break down, after which sprayparticles may stick. Residual compound is eas-ily removed by rinsing with water or solvents,depending on the carrier solubility. Maskingcompounds are easy and convenient to use inspite of their shortcomings. Epoxy/puttylikecompounds are also suitable for plasma andHVOF masking needs.

Preheating

Preheating of substrates prior to thermalspraying is a normal and acceptable practice.A no-powder preheat pass using the flame orplasma is often used to drive off adsorbed mois-ture from the surface of the part. Higher-temperature preheats can be used to offset theCTE mismatches between the coating and sub-strate. Coating at an elevated substrate temper-ature can place the coating in compressionafter cooldown. This approach is not practicalfor all applications but can be useful for moder-ate operating temperatures.Preheating Temperatures. As a general

rule, a single preheat pass is sufficient to driveoff adsorbed moisture. Larger parts may requiremore preheat passes or supplemental heating.Preheating temperatures are part, coating, andapplication dependent and typically range from100 to 150 C (210 to 300 F). As soon as thepreheat temperature is reached, spraying mustbegin.Although preheating is an accepted and com-

mon practice, preheating is not recommendedfor some materials. Carbon- and glass-fiber-reinforced composites have matrices that aretemperature-sensitive. To remove moisturefrom sensitive substrates, an alcohol rinse priorto spraying is used.Excessive preheating will cause surface oxi-

dation. Materials such as copper, aluminum,titanium, uranium, beryllium, and zirconiumoxidize readily in air, producing a layer that

Fig. 7 Aluminum oxide (Al2O3) particles. Originalmagnification: 130. (a) 240mesh. (b) 150mesh

Fig. 8 Examples of surface preparation by threading orgrooving. Adapted from Ref 2

Fig. 9 Comparison of thermal spray coatings depositedon macroroughened and smooth surfaces. (a)

Sprayed metal over grooves; shrinkage constrained bygrooves. (b) Sprayed metal on smooth surface; effect ofshear stress on bond due to shrinkage. Adapted from Ref 3

Table 2 Common bond coat materials

Coating Wire Powder

Service temperature

C F

Molybdenum Yes Yes 315 60080Ni-20Al . . . Yes 845 155090Ni-5Mo-5Al . . . Yes . . . . . .95Ni-5Al Yes Yes 845 155095NiCr-5Al Yes Yes 980 180095FeNi-5Al Yes Yes 1200 165090FeNi-5Mo-5Al . . . Yes 650 1200


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inhibits good bonding. Preheat and coating tem-peratures should be kept lower, below 100 C(212 F), for high-expansion alloys such as the300-series stainless steels, superalloys, andnickel-base alloys. Lower-expansion coatingssuch as carbides and cermets will fail due toCTE mismatches when applied to these alloysat higher temperatures. However, careful selec-tion of coating material relative to the substratecomposition and preheating temperatures canresult in the formation of compounds at theinterface that produce extremely strong bonds.Atmospheres. Spraying done under inert

cover, either by inert gas shrouding or byVPS, can take advantage of very high preheat-ing because the deleterious effects of oxygenare minimized. For example, gas turbine bladesthat are to be plasma sprayed with MCrAlYcoatings in low-pressure inert gas chambersare often preheated to 815 C (1500 F).

Coating Deposition

Spray Stream

The spray stream is fundamental to under-standing thermal spray coatings. Spray pro-cesses involve the acceleration of particulatematter in a cold to molten or semimolten statetoward a substrate to form a coating. Particletrajectory, temperature, and velocity at anymoment in time in the gas stream constitutethe spray stream or pattern.To understand what is happening in a hot jet

stream, refer to Fig. 12, which shows particletrajectories in a plasma jet. While the figureshows particles entering the gas stream radially,they could just as well be introduced along thecenterline of the jet or generated by atomiza-tion, as in wire flame spray or electric arc spray.Some particles travel along the jet centerline,others take an intermediate position, and stillothers ride along the periphery of the jet. Fineparticles riding the periphery may, in fact,never enter the main jet, and coarse particlesmay pass through the jet and take an outsideposition on the opposite side to their injectionpoint. On impact with the substrate, particlesimpacting virtually simultaneously make upthe spray pattern.Figure 13 depicts the spray pattern in cross

section. In a planar view, parallel to the sub-strate, the spray pattern is circular or oval inshape. The deposit at an instant in time is essen-tially Gaussian in cross section, where, at thecenter, the highest degree of melting hasoccurred. Moving radially out from the center,temperatures and velocities are lower, resultingin slightly different microstructures. Each parti-cle will have a unique time/temperature/velocity profile as it passes through the plumeor jet. By maintaining tight control of the feed-stock and parameters, the stream can produce astatistically significant number of properlymelted and accelerated particles.The spray-pattern phenomena described in

this section and shown in Fig. 12 and 13 apply

Fig. 10 Coatings resulting from (a) contact masks versus (b) shadow masks

Fig. 11 Examples of how to use masking techniques for special configurations


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to all thermal spray processes. Variables com-mon to each process, such as total gas flow,energy levels, particle size distribution, specificgravity, and carrier gas flow, combine to affecttime, temperature, and mass. These in turn affectporosity, unmelted particles, and oxides quantita-tively. The spray pattern varies between pro-cesses more as a function of gas stream velocity.

Coating Buildup

As the spray stream is manipulated overthe part, overlapping occurs. After severalpasses, a coating such as that shown in Fig. 14results. A typical coating may include someporosity, unmelted or resolidified particles,and oxides. Depending on hood and dust collec-tor design, masking and tooling, and airflowaround the part, fumes from vaporized feedstockmay be trapped in the layers between passes.Effect of Part Geometry on Coating

Buildup. Figure 15 illustrates two basic con-cepts related to part geometry. First is thatinclusion of unmelts and other debris is reducedwhen spraying small target areas. This effect isillustrated in Fig. 15(a). The shaft diameter issmaller than the spray pattern diameter; conse-quently, one-half of the spray stream actuallymisses the substrate. The larger target areashown in Fig. 15(b), however, captures theentire spray pattern on the substrate when tra-versed across and/or up and down. To statethe obvious, process efficiency is poor whencoating small parts, because of the low targetefficiency.The second concept is that small, round parts

do not behave as flat surfaces, due to the angleof impingement. As a rule of thumb, curvedsurfaces begin to approximate flat surfaces atradii above 40 mm (1.5 in.).Substrate Cooling Devices. Supplemental

air cooling has two distinct advantages: to con-trol part temperature and to remove overspray

and fume debris from the coating surface.Clean, dry air is a must. Condensation and com-pressor oils are detrimental to coatings. Tospray large target areas, an ancillary air-coolingdevice can be attached to the spray gun or themanipulator following the gun, such that astream of compressed air blows dust and debrisfrom the substrate surface in advance of, andfollowing, the traversing spray stream. Fig-ure 16 shows an air device positioned for spray-ing a large shaft. Here, debris is blown off thesurface before it can be embedded into the coat-ing. When spraying flat surfaces, as shown inFig. 15(b), air coolers should be positioned onboth sides of the spray stream to help removedust and debris. Air coolers are usually adjustedto precede and follow the spray torch along thetraverse direction. However, air cooling can beplaced anywhere on the part as long as it doesnot interfere with the spray stream. The typeof air jet, placement, and air volume are deter-mined by the temperature required and thepotential to create and embed debris.Cooling with air may be the most common

type or source of cooling; however, other typesmay be used, including liquid CO2, which isvery efficient, or Ar or N2 gases in specializedapplications.Relationship between Particle Melting

and Coating Structure. In addition to the effectsof particles in the periphery of the spray stream,resulting from poorly classified materials or non-optimized process parameters, considerationshould be given to particles in the spray streamand how variations in the degree of melting andvelocity affect coating structure. Particles in thecenter of the spray pattern may include one or allof the following states on impact:

Fully molten, or just above the materialmelting temperature

Superheated, well above the melting temper-ature and perhaps close to the vaporizationpoint

Semimolten, with the outside liquid but thecore still solid

Molten then resolidified while in flightbefore impact

Under- and overaccelerated particles

Fully melted particles at or just above thematerial melting point arrive at the substrate,impact, flow, and flatten (Fig. 17). Particulatematerial spreads and cools rapidly as heatis conducted into the substrate. The classiclamellar splat morphology is thus achieved(Fig. 17b). Lamellar particle thickness dependson particle velocity, particle temperature, sub-strate temperature, and other minor factors.Superheated particles may splash on impact,sending out fine droplets radially that canend up in the coating as debris (Fig. 17c).Debris formed from splatter differs from debrisproduced in the jet, in that splatter lodges inthe coating, building up at the first ridge thatstops its radial travel. Air blasting does notremove splatter debris. Superheating is a condi-tion that generally should be avoided, in thatit produces fumes, oxidation, and lowereddeposition efficiency. Particles that resolidifyin flight after having been melted may eithernot deposit or they may appear as embeddedunmelted particles, usually with clearly discern-ible oxide layers on the particle surfaces(Fig. 17a).During development and as a quality-control

measure, sprayed coatings should be examinedmetallographically in cross section to see theseeffects. Additional information on spray proces-sing and microstructures can be found in thearticles Coating Structures, Properties, andMaterials and Metallography and ImageAnalysis in this Volume.

Process Variations

The controllable parameters for each of themajor thermal process types are different,but all share some common fundamentals.Equipment and theory are covered extensivelyin the article Thermal Spray Processes in

Fig. 12 Particle trajectories in a typical plasma spray stream

Fig. 13 The spray pattern, illustrating particledeposition and the effect of size and debris

on thickness and porosity in cross section


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this Volume. It is important that the reader isfamiliar with various equipment theory andparameter variations in order to understand thecause-and-effect relationship between inputparameters and sprayed coatings, as discussedin this section.Converting feedstock material (whether pow-

ders, wires, or rods) into a coating requires boththermal energy for melting and gas flow foratomization and/or particle acceleration to pro-pel the particles toward the substrate. The com-bination of available energy and jet velocityacting on the mass of the feedstock directlyaffects particle temperature, velocity, and tra-jectory. Table 3 shows a simple overview com-paring thermal energy and particle velocity foreach process. In general, powder- and wire-flame spray and electric-arc-sprayed coatingsare generated from high heat input and low par-ticle velocities. These coatings tend to exhibithigher oxide contents and higher porosity. Elec-tric arc has a stronger potential to create fumes,where powder flame has a stronger tendency toproduce unmelted particles in the coating. As aresult of the higher particle velocity, particlespresent in plasma-sprayed coatings are flattenedmore than in flame-sprayed coatings. Theresulting coatings are also denser, with finerporosity. High-kinetic-energy processes suchas detonation gun, HVOF, and cold spray pro-duce very dense coatings with less embeddeddebris. In addition to the input spray parametersthat control the spray pattern, particle heatingand velocity, substrate temperature control,and torch-to-substrate relative motion are alsocritical for producing desired coatings.

Process Temperature Control

During spraying, it is necessary to control thetemperature of both the sprayed coating and thesubstrate to avoid substrate degradation, oxida-tion, and coating failure due to differentialexpansion.

Substrate Degradation. As with preheating,mentioned previously, there are limitations tosubstrate temperature during spraying. Lower-temperature-substrate materials or matrix mate-rials may be damaged by allowing the part tooverheat during the spray process.Oxidation. Coating layers deposited and

allowed to reach high temperatures duringspraying will oxidize and appear to darken. Sur-face oxides so formed leave a plane of weak-ness in the coating, between passes, and couldlead to delamination under applied stress duringservice. The critical oxidizing temperature foreach material varies.Shrinkage/Expansion. Controlling the CTE

mismatch between the coating and substratefollows the same guidelines as mentioned underthe preheating section. Along with the sub-strate, sprayed coatings shrink as they cool.The CTE differences can exert considerableshear stress along the interface and lead to pos-sible separation or distortion (Fig. 18). Sub-strate materials having a high CTE that areallowed to expand because of high temperaturesduring the spray process will experience cata-strophic disbonding at the substrate/coatinginterface.

Manipulation

Thermal spraying is a line-of-sight process.As mentioned previously, the substrate coatingarea should preferably be normal (at 90) tothe spray stream. The fundamental objectiveof torch and substrate motion during sprayingis to present the target area to the spray streamin a steady, consistent, and repeatable manner,always maintaining the same torch-to-substratespray angle, standoff (spray distance), and pitchor increment. Manipulation is commonly doneby moving the gun with an X-Y manipulatoror robot. The part is often stationary, as withflat surfaces, or is rotated on a turntable orlathe, as in the case of cylinders. More complex

components may require the use of coordinatedmotion between the robot and external axis ofmotion.Manipulation Variables. Controlled relative

movement of the gun with respect to the part iscritical for continuous and even deposition ofthe coating. Critical parameters are standoffdistance, surface speed, angle of impingement,and pitch or increment.Standoff distance is the distance between

the face of the gun and the part or workpiece.This distance is critical because the optimalparticle temperature and velocity occur inone plane along the length of the spray stream.

Fig. 14 Typical coating microstructure, containing unmelted particles, oxides, porosity, and debris. (a) Unmeltedparticles. (b) Oxides. (c) Debris. (d) Fine particles. (e) Porosity

Fig. 15 How part geometry affects coating deposition.(a) A small target has peripheral debris

missing the substrate, while (b) a larger target collects allthe peripheral debris and oxides.


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Tolerance for standoff is a function of particlevelocity. Higher-velocity processes are gener-ally more tolerant of variation than the lower-velocity processes.Angle of impingement has been mentioned

several times in this article. Again, higher-

velocity processes such as HVOF are more tol-erant of off-axis spraying than low-velocityprocesses such as conventional flame spraying.Pitch or increment, that is, the overlap

between subsequent strokes of the gun/spraystream, is important to coating uniformity.

Pitch is the term used for rotating elementssuch as rolls. As the roll turns, the gun musttravel across the surface such that the pitch(thread) does not create a barber pole effect.The same holds true for planar surfaces, wherethe increment must be set and controlled to pre-vent striping. As a rule, lower-velocity pro-cesses that have wider spray patterns alsohave wider pitch/increment values. Conversely,high-velocity processes have narrower pitch/increment values. Using rules-of-thumb, pitch/increment values for HVOF and high-velocityplasma may range from 3 to 6 mm (0.13 to0.25 in.), while flame and electric arc may be6 to 12 mm (0.25 to 0.50 in.).One final consideration would be the deposi-

tion rate in terms of unit thickness per passbuildup, that is, mm/pass or mils/pass. Thisparameter is controlled by the speed of thespray stream across the surface of the part, thatis, surface speed. Generally, the material feedrate is held constant and manipulation is spedup or slowed down to produce the requiredmm/pass or mils/pass. Another rule-of-thumbis that for higher-density coatings, thinner mm/pass or mils/pass layers are better; for example,plasma-sprayed alumina may be 13 mm/passor 0.5 mil/pass. The common baselinesurface speed seems to be approximately500 mm/s (20 in./s) but varies widely about thatpoint. Consider the spray pattern shown inFig. 13 and then refer to Fig. 19.

Process Efficiency

Process efficiency (PE) is the product of tar-get efficiency and deposit efficiency. Targetefficiency (TE) is the ratio of the spray patternenvelope to the target area on the substrate.Deposition efficiency (DE) is a ratio of thespray material that impacts the substrate to thespray material that actually adheres to the sub-strate. TE DE = PE. For example, if the spraystream impinges the prepared surface of thepart half of the time, the TE would be 50%. Ifonly half of the sprayed material that impingesthe prepared area adheres to the prepared sur-face, then the DE is also 50%. The PE is there-fore 0.50 0.50 = 0.25, or 25%.Measurement of TE is a geometric calcula-

tion, based on the physical dimensions of theprepared coating area and the total workingenvelope of the robotic/manipulator spray pat-tern. Deposition efficiency, on the other hand,is a measurement made for a specific gun/para-meters set, with a fixed feedstock material, agiven set of manipulation variables, and con-trolled, known substrate temperature.Deposit efficiency can be measured easily.

One simple way to measure DE is to use aflat plate with a known width, for example,a 150 mm (6 in.) square that is 3 mm(0.013 in.) thick. After properly preparing thesurface, set the manipulation for the appropriatestandoff, increment, surface speed, and normalangle of impingent. Stay within the vertical

Fig. 16 Positioning of an air-cooling device to blow debris off the surface before it rotates into the center sprayregion and is incorporated into the coating

Fig. 17 Effect of the degree of particle melting at or just before impact on shape and coating structure. (a) Particle isheat softened or beginning to resolidify. At impact, it does not flow out and begins to lift at the edges. (b)

Properly melted particle impacts and flows to form a well-bonded classical lamellar splat shape. (c) Superheatedparticle impacts and splashes, creating debris, satellites, and dusting.


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limits of the plate, but program the manipula-tion to extend beyond the horizontal limits ofthe plate by 100 mm (4 in.). Use air coolingto keep the plate temperature below 65 C(150 F).With a known surface speed, width of the

plate, weight of the plate, number of strokesacross the plate, and material feed rate, onecan calculate the mass buildup on the plateper unit time impinging the plate, against thefeed rate.To carry the example a step further:

150 mm plate width 500 mm/s traverse rate 100 strokes across and off the plate 60 g/min feed rate 500 g initial plate weight 520 g final plate weight

150mm 100 strokes=500mm=s 0:5min on part

0:5min 60 g=min 30 g sprayed on part

520 g final500 g initial=30 g 0:667; or 67%DE

Note that the substrate temperature is critical.The DE is directly related to temperature.Allowing the temperature to rise will producefalse-high DEs.

Statistical Design Techniques

As noted earlier in this article, many vari-ables/parameters require study before a coatingcan be optimized. Statistical design techniquesreduce the number of experimental runs orparameter iterations needed for coating

optimization. The results obtained are oftenunambiguous and at a minimum cost. Statisticaldesign methods encourage the systematic studyof the many variables that confront coatingsengineers. Furthermore, these studies providedata on how variables interact and how theyinfluence coating properties, as well as a plan

Fig. 18 Effect of coating shrinkage on interfacial shearstresses. The sprayed metal cools, creating a

tensile stress in the coating and a compressive stress inthe underlying substrate material. These stresses maydeform the substrate or weaken the bond between thesprayed coating and the substrate, leading to debondingof the coating. The bottom illustration showsdeformation of a thin-section substrate and debonding ofthe coating due to stresses generated by cooling of thesprayed metal. Adapted from Ref 2

Table 3 Heat energy input and particle velocity for common thermal spray processes

Input heat energy to particle Output particle velocity(a)

Process High Low Highest High Medium high Medium Low

Flame wire X XFlame powder X XAPS X XHigh-velocity plasma X XVPS X XElectric wire arc X XHigh-velocity oxyfuel X XDetonation gun X XCold spray X X

(a) Particle speed ranges from a high of approximately 1000 m/s (3000 ft/s) to a low of 25 m/s (80 ft/s). Further variations within each process depend on the particle size, material type, and gas velocity.

Fig. 19 How to obtain an even coating layer by indexing the torch after each pass. See text for details.


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for constructively changing input parameters toaffect the most desirable properties.Designs of experiments (Ref 3) and Taguchi

factorial experiments (Ref 4) are two com-monly used statistical techniques. The funda-mental approach for each is the same:A matrix of selected variables versus a seriesof experimental runs is established. Table 4 isa Taguchi L-8 fractional factorial matrix takenfrom Ref 5. Nerz et al. (Ref 5) chose six vari-ables represented by high and low values. Thenumber of variables chosen determines thematrix size and number of runs. Typically, afractional factorial matrix is constructed withvariables deemed most likely to influence coat-ing results, rather than a complete factorial ofall variables.The matrix shown in Table 4 includes a maxi-

mum of six variables and eight experimentalruns. Selected coating properties are measuredfor each run. To understand themagnitude of var-iable influence on coating properties, an analysisof variance (ANOVA) is performed.AnANOVAis a Taguchi analysis of results showing how eachvariable, both high and low, influences results.Another method for evaluating factorial experi-ments is to use analysis of means, developed byOtt using the Null hypothesis (Ref 6). Thismethod is a statistical approach employing gra-phics and statistical theory.

Postcoating Processing

Most thermal spray coatings require addi-tional processing after the coating is deposited.These include sealing, heat treatment, and fin-ishing. Postcoating processing is described in

detail in the article Postcoating Operationsin the Pre- and Postcoating Processing Divi-sion of this Volume.

REFERENCES

1. C.C. Berndt, Ed., Thermal Spray: Interna-tional Advances in Coatings Technology,Proc. International Thermal Spray Conf.,May 29June 5, 1992 (Orlando, FL), ASMInternational, 1992

2. T.F. Bernecki, Ed., Thermal Spray Researchand Applications, Proc. Third NationalThermal Spray Conf., May 2025, 1990(Long Beach, CA), ASM International, 1991

3. T.F. Bernecki, Ed., Thermal Spray Coatings:Properties, Processes and Applications,Proc. Fourth National Thermal Spray Conf.,May 410, 1991 (Pittsburgh, PA), ASMInternational, 1992

4. P. Fauchais, State of the Art for the Under-standing of the Physical PhenomenaInvolved in Plasma Spraying at AtmosphericPressure, Proc. National Thermal SprayConf., Sept 1417, 1987 (Orlando, FL),ASM International

5. D.L. Houck, Ed., Thermal Spray: Advancesin Coatings Technology, Proc. FirstNational Thermal Spray Conf., Sept 1417,1987 (Orlando, FL), ASM International,1988

6. D.L. Houck, Ed., Thermal Spray Technol-ogy: New Ideas and Processes, Proc. SecondNational Thermal Spray Conf., Oct 2427,1988 (Cincinnati, OH), ASM International,1989

SELECTED REFERENCES

R.A. Fisher, The Design of Experiments,Oliver and Boyd, 1935

D.A. Gerdeman and N.L. Hecht, Arc PlasmaTechnology in Materials Science, SpringerVerlag, 1972

J. Hancock, Ultrasonic Cleaning, SurfaceEngineering, Vol 5, ASM Handbook, ASMInternational, 1994, p 4447

H.S. Ingham and A.P. Shepard, Ed.,Metallizing Handbook, 7th ed., Vol 13,Metallizing Engineering Co., Inc. (Metco),1959

H.S. Ingham and A.P. Shepard, Ed., Metal-lizing Handbook, 7th ed., Metallizing Engi-neering Co., Inc. (Metco), 1959, p A-10,28, 65, 136, 137

T. Kostilnik, Mechanical Cleaning Sys-tems, SurfaceEngineering,Vol 5,ASMHand-book, ASM International, 1994, p 5566

E.J. Kubel, Thermal Spraying Technology:From Art to Science, Adv. Mater. Proc.,Vol 132 (No. 6), Dec 1987, p 6980

F.N. Longo, Ed., Thermal Spray Coatings:New Materials, Processes and Applications,Proc. National Thermal Spray Conf., Oct31Nov 2, 1984 (Long Beach, CA), Ameri-can Society for Metals, 1985

J.E. Nerz et al., Taguchi Analysis of ThickBarrier Coatings, Proc. NTSC, May 1990(Long Beach, CA), p 670

E. Pfender, Fundamental Studies Asso-ciated with the Plasma Spray Process,Proc. National Thermal Spray Conf.,Sept 1417, 1987 (Orlando, FL), ASMInternational

V.L. Rupp and K. Surprenant, Solvent ColdCleaning and Vapor Degreasing, SurfaceEngineering, Vol 5, ASM Handbook, ASMInternational, 1994, p 2132

T.E. Shahnazarian, Statistical Methods forQuality Control, Part 2, Technical Semi-nars, New Canaan, CT

T.J. Steeper, D.J. Varacalle, Jr., G.C. Wil-son, and V.T. Berta, Use of Thermal SprayProcesses to Fabricate Heater Tubes forUse in Thermal-Hydraulic Experiments,Thermal Spray Coating: Properties, Pro-cesses, and Applications, Proc. NationalThermal Spray Conf., May 410, 1991(Pittsburgh, PA), ASM International, 1992

G. Taguchi and S. Konishi, TaguchiMethods: Orthogonal Arrays and LinearGraphics, ASI Press, 1987

Thermal Spraying: Practice, Theory andApplication, American Welding Society,1985

Table 4 Taguchi matrix variables and constants

Experiment Spray distance, mm Spray rate, g/min Carrier flow, scfh Application temperature, C Particle size, mm Power, kW

1 76 30 12 177 100 402 76 30 15 343 53 503 76 76 12 177 53 504 76 76 15 343 100 405 152 30 12 343 100 506 152 30 15 177 53 407 152 76 12 343 53 408 152 76 15 177 100 50

Levels

Constants Variables Low(1) High(2)

Gun Metco 3 APG Spray distance, mm 76 152Nozzle Metco 315 Spray rate, g/min 30 76Powder port No. 2 Carrier flow, scfh 12 15Plasma gas Argon Application temperature, C 177 343Pressure 75 psi Particle size, mm 100 53Flows 60 scfh Power level, kW 40 50

scfh, surface cubic feet per hour


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