Application Notes Spray Coatings

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Application

Notes

Metallographicpreparation of thermalspray coatingsThermal spraying was invented in theearly 1900s using zinc for „metallizing”substrates for corrosion protection. Thedevelopment of the plasma spray gun inthe late 50s and 60s made it commerciallyviable to use high temperature materialssuch as ceramics and refractory metalsfor coating materials. In addition to flameand plasma spraying, today thermal spraymethods include high velocity and detona-tion spraying using a multitude of different

spray materials for the most diverse anddemanding applications.

Thermal spray coatings are applied to asubstrate to give a specific surface quali-ty, which it originally does not have. Thusthe bulk strength of a part is given by thesubstrate, and the coating adds superiorsurface qualities such as corrosion, wearor heat resistance.Therefore thermal spray coatings are wide-ly used in the aerospace and power gene-ration industry for new and refurbishedsections and parts for jet engines andgas turbines, compressors and pumps.The properties of some coatings can onlybe fabricated by thermal spraying, usingmainly metals, ceramics, carbides andcomposites as well as mixtures of variousmaterials.

Metallography of thermal spray coatingscan have several purposes:- To define, monitor and control spraying

conditions for quality control- For failure analysis- For developing new products.

The procedure normally involves coatinga test coupon to define and optimize theprocess for the part to be sprayed. Sec-tions of this test coupon are then metal-lographically prepared and examined toassess coating thickness, size and distribu-tion of porosity, oxides and cracks, adhe-sion to base material, interface contamina-tion and presence of unmelted particles.

Cutting: Cracks in the coating due toclamping the sample and using coarsecut-off wheels;Delamination from substrate

Mounting: Insufficient penetration ofmounting resin

Difficulties during metallographic preparation Solution:

- Precision cutting- Vacuum impregnation with epoxy resin- Standardized, reproducible preparation

methods for thermal spray coatings

Electric arc metal spray coating, showing grey oxides

and round, unmelted particles

Crack between a plasma spray coating 500x and the substrate. The crack originates

from cutting

Fig.1: Ceramic spray coating, 200x insufficiently polished

Fig. 2: Same coating as Fig.1, 200x polished correctly

Grinding and polishing: Because of smear-ing of soft materials and pull-outs in brittlematerials, it is difficult to establish andevaluate true porosity



Spray methodsand applicationsof thermal spraycoatings

In the spraying process the coating mate-rial, wire or powder, melts in a high tem-perature heat source in a spray gun and isaccelerated by the flame or plasma jet andprojected towards the substrate. A streamof molten and semi-molten particles im-pinges onto the substrate and forms acoating. When the particles hit the work-piece they mechanically lock onto the sur-

face, deform and cool rapidly. The bondingof single particles is through mechanicalinterlocking, or in some cases metallurgicalbonding or diffusion. High velocity of theparticles leads to better bonding and higherdensity of the coating. For good adhesionto the substrate it is essential that thesurface is roughened by sandblasting andthoroughly degreased and cleaned beforespraying.The various spraying techniques displaydifferent temperatures at the heat sourceand different particle velocities, which,together with the economical aspect, needto be taken into consideration for specificapplications. In the following the mainspraying techniques are briefly describedand some of the most well-known applica-tions of the resulting coatings mentioned:

Flame spraying is the oldest method of ap-plying thermal spray coatings. The coatingmaterial is either wire or powder, which isfed into an oxygen-fuel gas flame. The mol-ten and atomized particles are ejected in a

directed stream through the spraying gunnozzle. Due to the relatively low particlevelocity the oxygen exposure is increased

the workpiece with extremely high kineticenergy. These coatings have an excel-lent density, integrity and adhesion to thesubstrate. Due to the process conditionsthis method is limited to the application ofcarbide coatings, mainly in the aerospaceand aviation industry for wear-resistantcoatings.

In High Velocity Oxy-Fuel Combustionspraying (HVOF) fuel gas and oxygen arefed into a chamber in which combustion

produces a supersonic flame, which isforced down a nozzle increasing its veloc-ity. Powder of coating material is fed intothis stream and the extreme velocity of theparticles when hitting the substrate creates

Fig. 3:

Flame sprayedcoating; Ni5Al

Brass synchronising rings flame-sprayed withmolybdenum for wear resistance

Fig. 4: Electric arc wire-sprayed metal coatingFeCrSiNi and Mn

Flying drops of moltencoating material

Impact on substrate Heat dissipation tosubstrate

Solidification and shrinkingof coating material

and therefore the oxide content in thesecoatings is relatively high (Fig. 3); adhesionand density are moderate (subsequent fus-ing to increase the density is possible).Flame sprayed coatings are used for corro-sion protection and/or wear protection ofstructures and components, surface build-

up and repair of worn shafts, for coatingsmall parts and spots.

Electric Arc spraying uses the heat

of an electric arc between two con-tinuous consumable wire electrodesmade of coating material to melt the wires.The wires intersect in front of a jet ofcompressed air. As the heat from the arcmelts the wires, the compressed air blowsthe molten droplets of the coating materialonto the substrate. The high arc tempera-ture and particle velocity gives this coatinga bond strength and density superior toflame sprayed coatings. However, becauseof the use of compressed air the arcsprayed coatings have a higher percentageof oxides (Fig. 4).The advantage of arc wire spraying is itshigh deposition rate which makes it suit-able for large areas or high volume produc-tion applications: spraying of large struc-tures like bridges and off-shore structureswith corrosion resistant zinc or aluminiumcoatings, reclamation of engineering com-ponents and spraying of electronic compo-nent housing with conductive coatings ofcopper or aluminium.

For Detonation spraying small amounts ofcarbide powder, fuel gas and oxygen areintroduced in a closed tube and exploded.The detonation ejects the powder withmultiple sonic speed and shoots it onto

Unmeltedparticles

Oxides

Voids

Substrate

Principle of layer formation



+ –

Difficulties inthe preparationof thermal spraycoatings

Fig. 5:

HVOF coating of WC/12Co

a very dense, strong coating (Fig. 5). Thevery high kinetic energy of the particleswhen striking the substrate ensures anadequate mechanical bond even withoutthe particles being fully molten. This makesthis spraying method particularly well-sui-ted for spraying of coatings with carbides.Typical applications are tungsten carbidecoatings on air engine turbine componentsand valves, and nickel-chromium coatingsfor oxidation resistance.

Plasma spraying is the most commonmethod for thermal spray coatings, and isapplied as Air Plasma Spraying (APS) orspraying under controlled atmosphere. Anelectric arc is formed between a cathodeand the concentric nozzle of the spray gun.A mixture of gases with a high flow ratealong the electrode is ionised by the arc,and forms plasma. This plasma stream ispushed out of the nozzle, where the pow-der of the coating material is injected intothe plasma jet. The heat and velocity of theplasma jet rapidly melts and acceleratesthe particles so that they are propelled ontothe substrate and form a coating. Plasmasprayed coatings have a denser structurethan flame sprayed coatings (compareFigs. 3 and 6).Plasma spraying has the advantage thatit can spray materials with high meltingpoints such as ceramics or refractory met-als. It is a versatile spraying method for

Cutting: Clamping of spray coated work-pieces for sectioning can introduce cracksin brittle coatings or compress very softcoatings.

Combustion chamber with APS thermal barrier coating,bond coat NiCrAlY, topcoat ZrO ² + Y ² O ³

high quality coatings and used for a widerange of applications, including coatings ontraction surfaces, thermal barrier coatingson turbine combustion chambers, vanesand blades, biocompatible hydroxylapatitecoatings for implants and ceramic coatingson print rolls.

Insulator

Water cooled electrode,cathode

Powder injector (external)

Plasmajet

Coating material

Water cooled nozzle, anode

Plasma gases,primary gases: Ar1, N2secondary gases: H2, He

Current250 - 1000 A

Coolingwater

Plasmagases

Fig. 6: APS coating with NiCr bond coat and titaniumoxide top coating

Fig. 7: Incorrect polish suggests less porosityin the middle of the coating

Cracks introduced through sectioning

Fig. 8: WC/Co spray coating with relief polishshows dark line at resin/coating interface.Can lead to misinterpretation.

Mounting: Cold mounting resins with highshrinkage can cause damage to coatingswith weak adhesion to the substrate; dueto the shrinkage gap the coating is notsupported by resin, which can lead to dela-mination of the coating during grinding andpolishing.

Grinding and polishing: Edge-rounding canlead to uneven polishing and subsequentmisinterpretation of the coating density(Fig. 7). Relief between coating and sub-strate creates a shadow that can be misin-terpreted (Fig. 8).

How to estimate the true porosity in a me-tallographically prepared spray coating isstill a reason for debate, as metallographicgrinding and polishing, if not carried out

Schematic drawing

of plasma spray gun



correctly, can introduce artefacts which arenot part of the coating structure. For exam-ple, in metal or metal/ceramic coatings, thesofter metal is smeared into pores duringgrinding and if not polished properly cancover up the true porosity (see Figs. a-c).In comparison, ceramic coatings are brit-tle and particles break out of the surfaceduring grinding. If not polished thoroughly,these break-outs leave an incorrect impres-sion of a high porosity (see Figs. d-f).

Recommendations forthe preparation of thermalspray coatingsAs there are many different sprayingmaterials with sometimes unusual com-binations, it is important to know thecorrect spraying and substrate material. Itfacilitates to estimate how the materials willbehave under mechanical abrasion. As dif-ferent spraying processes result in different

coating densities and structures it alsohelps to know the spraying method usedon a particular sample in order to estimatethe expected porosity and oxide content.

Cutting: Selection of the cut-off wheel isbased on the substrate material, whichis usually metallic. A wheel with a looserbond (soft) is preferable to a denser bond(hard) as brittle particles of the coating aredragged out by a hard cut-off wheel. This isparticularly importantwhen cutting parts

with ceramic coatings.Even if the coating isceramic, it constitutesonly a small percent-age of the total crosssectional area and does not need to be cutwith a diamond cut-off wheel. Usually sec-tioning is possible with a soft aluminiumoxide wheel. If the ceramic coating is verythick and dense a resin-bonded diamondcut-off wheel can be used as an alternative.

A thin piece of styrofoam between clampsand sample can help to protect brittle andvery soft coatings from being damaged.

d) Ceramic spray coating after fine grinding

a) Metal spray coating after fine grinding

b) Same coating as in a) polished with 3 µm

c) Same coating as in b) after final polish

e) Same coating as in d) polished with 3 µm

f) Same coating as in e) after final polish

When cutting pieces other than test cou-pons, for instance samples for failureanalysis, it is important to ensure that theworkpiece is clamped into the cut-off ma-chine in such a way that the cut-off wheelis cutting into the coating towards thesubstrate, and not from the substrate intothe coating. As the bond of the coating ismainly mechanical, it can delaminate fromthe substrate due to the drag of the cut-offwheel.

Particularly fragile or thin coatings canfirst be vacuum impregnated with coldmounting epoxy resin, and then the microsections are cut and remounted for grind-ing and polishing. This ensures maximumsupport to the coating during sectioning.

The appearance of cracks in a coatingafter final polishing may or may not bethe result of cutting. It is recommended toregrind and polish the sample. If the crackis from cutting it will usually disappear, ifit is inherent in the coating it will reappear,or cracks will surface in other areas of thecoating.

Mounting: Cold mounting with epoxy resin(EpoFix, CaldoFix) is recommended asspray coatings are very easily damagedduring hot compression mounting(Figs. 9 and 10).In general, vacuum impregnation is rec-ommended for all coatings. The depth of

impregnation varieswith the degree of

open porosity andinterconnectionsbetween the pores.Very porous coatingscan be easier impreg-

nated than denser ones, and coatings withless than 10% porosity can not be impreg-nated successfully. As it can be difficult todistinguish voids filled with transparentor translucent mounting resins from thestructural elements of the coating, it helpsto mix a fluorescent dye (Epodye) into the

cold mounting resin. Viewed with a longpass blue filter and a short pass orangefilter in the microscope, the fluorescent dye

Nickel flame spray coatingwith 15% graphite



Step PG FG

Surface SiC-paper 220# MD-Largo

Suspension DiaPro Allegro/Largo*

Force [N] 180 180

Time Until plane 5 min.

Grinding

Lubricant Water

rpm 300 150

Step DP 1 DP 2**

Surface MD-Dac MD-Nap

Suspension DiaPro Dac* DiaPro Nap B*

Force [N] 180 120

Time 5 min. 1 min.

Polishing

rpm 150 150

will colour those voids yellow which havebeen filled with resin by the impregnation(Fig.11 and 12).Unfortunately this method is not alwaysapplicable for ceramic coatings, becauseceramics are translucent and the wholecoating appears fluorescent.

Grinding and polishing: As a general ruleplane grinding should start with the finestpossible silicon carbide paper to avoid cre-ating artificial porosity by fracturing brittleparticles. Exceptions can be very dense orthick ceramic coatings, which are plane

ground more efficiently with diamond(with e.g. MD-Piano 220). For high samplevolumes or large parts, which need to be

Fig.12: Same coating as in Fig.11 in fluorescent light

Fig.11: WC/Co plasma spray coating in bright field

Fig.10: Same coating as in Fig. 9, cold mounted 200x

examined as a whole, plane grinding with astone may be preferred as it is faster.Whichever method is use, one must alwaysbe aware that the first preparation stepshould aim to removeany cracks that arisefrom cutting withoutintroducing newdamage from coarsegrinding.

To retain flatness and assure a good mate-rial removal rate, fine grinding is preferablydone with diamond on a composite finegrinding disc. For ceramic coatings the finegrinding disc MD-Allegro is recommended,and for metal coatings MD-Largo. A tho-rough polishing on a silk cloth (MD-Dur orMD-Dac) will retain the flatness of the sam-ple and guarantee the removal of smearedmetal.

Metal coatings can be fine polished eitherwith 1 µm diamond or a colloidal silica

(OP-U) on a soft cloth. It is not recom-mended to use the colloidal silica suspen-sion OP-S for polishing metal spray coat-ings as it creates too much relief. However,OP-S is suitable for the final polishing ofceramic coatings as it gives a good con-trast to the structure.

In the trial stage for establishing prepara-tion methods both silicon carbide anddiamond grinding can be tried to find outwhich is the more suitable plane grinding

method. The same applies to the final pol-ishing step, for which 1µm diamond mightin some cases be preferable to colloidalsilica.

In general it is recommended that, if pos-sible, a standard procedure is always usedfor all coatings. With automatic preparationequipment it is possible to control prepara-tion parameters, which guarantees consist-ent results and excellent reproducibility. Bykeeping the preparation conditions con-stant, it can then be assumed that sudden

differences in the microstructure in mostcases reflect differences in the sprayingprocess and not in the preparation process.

The preparation method in the tableabove has successfully been used for themost common coatings. The data are for6 mounted samples, 30 mm diameter,clamped into a holder. DiaPro diamondsuspension can be replaced by DP-Sus-pension 9 µm, 3 µm and 1 µm respectively,applied with blue lubricant.

Etching: In general, etchants that are rec-ommended for a specific material can alsobe used for spray coatings of this material.

It can be expected that the more similarthe substrate and coating materials are, themore even the etching attack will be.

Valid for 6 mounted samples, 30 mm diam. clamped

in a holder.

Remarks:

*Alternatively DiaPro diamond suspension can be replacedby DP-Suspension, P, 9 µm, 3 µm and 1 µm respectively,

applied with blue lubricant.

**Alternatively, this diamond polishing step can bereplaced by a polishing step with colloidal silica (OP-U for

metal, OP-S for ceramic coatings) for 30-60 sec.

Standard preparation method forthermal spray coatings

Fig.9: Damage to ceramic spray coating 200x

due to hot compression mounting



Struers A/SPederstrupvej 84DK-2750 Ballerup, DenmarkPhone +45 44 600 800Fax +45 44 600 [email protected]

www.struers.com

05.05 / 62142005. Printed in Denmark by From & Co. - 42

Application NotesMetallographic preparation of thermal spray coatings

Elisabeth Weidmann, Anne Guesnier, Struers A/S,Copenhagen, DenmarkBrigitte Duclos, Struers S.A.S., Champigny, France

AcknowledgementWe wish to thank Sulzer Metco AG, Wohlen, Switzerland,for its cooperation and supplying information material.Special thanks go to J. Hochstrasser and P. Ambühl forsharing their extensive knowledge with us and supplyingthe following images for reproduction: photo of sprayingprocess and large micrograph on page 1; drawing:Principle of particle movement, photo synchronisingrings and micrographs on page 2; drawing, photocombustion chamber and all micrographs on page 3 andmicrograph of nickel flame sprayed coating on page 4.We thank Richard Compton, Zimmer, Inc. USA, for

the photo of the acetabular cup shell and the SEMphotomicrograph on page 6.

BibliographyMetallographic preparation of thermally sprayedorthopaedic devices, Richard C. Compton, Zimmer, Inc.,USA, Structure 28, 1995Summary Report of the Plasma Spray CoatingsSymposium at Struers, Copenhagen, May 25th to 27th,1988Universal metallographic procedure for thermal spraycoatings, S. D. Glancy, Structure 29, 1996Materialographic characterization of modern multilayercoating systems used for hot-gas components in largegas turbines for static power generation, A. Neidel, S.Riesenbeck, T. Ulrich, J. Völker, Chunming Yao, SiemensPower Generation, Berlin, Structure 2/2004

Thermally sprayedacetabular cup shell

SEM photomicrograph of thermally sprayed surfaceof acetabular cup shell

Coatings sprayed in a controlled atmos-phere have few or no oxides and it is dif-ficult to recognize the coating structure.Therefore these types of coatings need tobe contrasted with chemical etching.

Vacuum sprayed coatings on nickel andcobalt based superalloys can be etchedwith the same solutions used for the sub-strate, or electrolytically with 10% aqueousoxalic acid.

The structure of coatings containing mo-lybdenum can be revealed by using thefollowing etchant:

50 ml water50 ml hydrogen peroxide (3%)50 ml ammonia

Caution: Always follow the recommendedsafety precautions when working withchemical reagents.

SummaryThermal spray coatings are widely used to

give or improve a specific surface qualityor function to a workpiece. Different spray-ing methods result in different character-istics of the coatings, and they are mainlyapplied for corrosion, heat and wear resist-ance. Metallographic examination of spraycoatings includes estimation of porosity,oxides and unmelted particles as well asadhesion to the substrate. Because incor-rect grinding and polishing procedures caninfluence the evaluation of the true porosityit is very important that metallographic

preparation is carried out systematicallyand that the results are reproducible. Preci-sion cutting with the correct cut-off wheelis recommended to avoid cracks in thecoating. Mounting should follow with slowcuring epoxy. Coarse grinding introducesthe most damage to the coating and shouldtherefore be carried out with the finest gritpossible. To avoid relief fine grinding withdiamond on a rigid disc is recommended,followed by a thorough diamond polish ona silk cloth.

It is particularly important to be aware thatmetal coatings behave differently to cera-mic coatings under mechanical abrasion

and that the diamond polish needs to belong enough to reveal the true porosity.

The recommended preparation procedureis based on experience and gives excel-lent results for the majority of commonthermal spray coatings. However, it shouldbe noted that for some specific proprietarycoatings the polishing times may need tobe adjusted.

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Application Notes Spray Coatings

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