822 Chiang Mai J. Sci. 2013; 40(5)

Chiang Mai J. Sci. 2013; 40(5) : 822-830http://it.science.cmu.ac.th/ejournal/Contributed Paper

Thermal Spraying and Its Applications in Thai IndustriesPanadda Sheppard*, Kittichai Ninon, Siriwut Phetsantad, Maetee Khailaihong,Chalermchai Sukhonkhet, Hathaipat Koiprasert and Witthawat WongpisarnNational Metal and Materials Technology Center, National Science and Technology Development Agency,Pathumthani, Thailand.*Author for correspondence; e-mail: panaddn@mtec.or.th

Received: 13 February 2013Accepted: 4 March 2013

ABSTRACTThis article provides a brief overview of thermal spraying processes using examples

of their uses in Thai industries. These include solid lubricant coating for polymer andrubber processing, sacrificial anode coating for the corrosion protection of largestructures, ceramic coating for the food industry, hard coating for severe sliding wear incable and wire production and wear resistant coatings for high temperature applications.

Keywords: thermal spraying techniques, thermal spraying applications, hard coating,sacrificial anode coating, high temperature coating

1. INTRODUCTIONIn today’s increasingly demanding

industrial environment driven bytechnological advancement, expectation onmaterial performance is greater than ever.As well as development in structural

materials, surface engineering also plays amajor role in improving the performance,extending the life and enhancing theaesthetic appearance of engineeringcomponents.

Figure 1. Surface engineering techniques.

Surface Engineering Techniques

Removal and coldwork process- Chemical andmechanical cleaninge.g. acid cleaning,pickling, ultrasonicbath, sand blasting

- Abrasive and non-abrasive finishinge.g. milling, EDM,grinding, lapping,electropolishing

- Shot peening

Heat treatment anddiffusion process- Heat treatment e.g.induction hardening,flame hardening,carburising, nitriding

- Conversion coatinge.g. anodizing,chromating,phosphating

- Ion implantation

Overlay coatingprocess- Electro/electrolessplating

- Hot dip galvanizing- Organic painting- Enameling- Physical vapourdeposition

- Chemical vapourdeposition

- Thermal spraying- Hard facing- Lining and cladding

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Figure 1 shows examples of surfaceengineering techniques commonly used inThai industries including cleaning,finishing, heat treatment, conversioncoating and overlay coating processes.Thermal spraying is a relatively recentcoating technology employed to combatwear and corrosion. Thermal sprayedcoating applications include gasket andmechanical seals, grinding gears and threadguides in textile manufacturing, sacrificialanode coatings for large structures, lowfriction coatings for rubber and textileindustries and oxidation resistant coatingsfor high temperature applications. Thetechnique is now routinely used inmaintenance sectors of various industries.In heavy machine-based production, a largefraction of the production cost is definedby the effectiveness and reliability of themaintenance work. In many applications,with some examples to be presented inthis paper, thermal spraying can notonly provide a suitable solution to themaintenance problem, but do so efficientlyand cost effectively.

2. THERMAL SPRAYING TECHNIQUESThe term “thermal spraying” refers to

a process whereby particles of solid, semi-molten or molten material are carried by agas stream to deposit, deform and solidify,producing a solid layer on a substrate. Thethickness of this coating layer variesbetween approximately 50 mm and a fewmillimeters depending on the coatingmaterial and the technique used. Thethermal spraying techniques are usuallydescribed by the nature of the gas stream.The commercial techniques however canbe broadly classified into 2 groups accordingto the energy input to the gas stream; theseare combustion and electric discharge.

2.1 Combustion ProcessesCombustion-type thermal spraying

utilizes the energy released throughcombustion of fuel and oxygen to producea hot gas stream. It can be further classifiedinto 2 types; continuous combustion andrepetitive explosion. The continuouscombustion spraying techniques, namelyflame spraying, HVAF (high velocity airfuel) spraying and HVOF (high velocityoxy fuel) spraying, represent a largefraction of the commercial combustionunits. The process principles are similar toone another whereby fuel gas (e.g. propane,acetylene, hydrogen) is mixed with oxidant(air or oxygen), the mixture is then ignitedin front of the gun or in a small chambercontained within a gun [1]. Coatingmaterial, which can be in a form ofpowder, metallic wire or ceramic rod, isthen fed directly into the combustion zonewhere it starts to melt and the particles arepropelled by the combustion pressure todeposit onto a surface, see Figure 2. Thesetechniques can be employed to producemetallic, ceramic and metal composite(cermet) coatings. Continuous combustionspraying has become a standard coatingtechnique for many applications, mostnotably wear resistant WC-Co HVOF

Figure 2. Schematic diagram of a typicalflame spray gun.

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coatings (see Figure 3) as a hard chromereplacement in an effort to move towardscleaner technology [2,3].

The repetitive explosion process, betterknown as detonation gun spraying orD-gun, employs a similar setup to thecontinuous combustion spraying with thecombustion occurring in cycles rather thancontinuously. One cycle consists of amixing of fuel gas and oxygen in acombustion chamber, injecting coatingpower into the gas mixture, ignition of themixture thus propelling one lot of powderonto the substrate, and finally purging thechamber with nitrogen. The high speeddetonation repeats up to 15 cycles persecond. It has been reported that theD-gun can achieve significantly higherpowder velocity than the HVOF spray gundue to the pulsed explosion and the designof the gun barrel, thus producing a densercoating with superior adhesion strength [4].

2.2 Electric Discharge ProcessesThe electric discharge process converts

electrical energy into thermal energy usedto melt the coating material. It can be

Figure 3. Cross-sectional structure of WC-Co coating produced using HVOFspraying, magnification x1000. Thestructure shows a ceramic-metal compositecoating consisting of continuous Co matrixwith reinforcing WC particles. Someporosity can be observed.

Figure 4. Schematic diagram of a typicalEAS gun.

classed into 2 distinct groups; two-wireelectric arc spraying (EAS) and plasmaspraying.

In the EAS process, the coatingmaterial comes in the form of metallicwires. Two wires, functioning asconsumable electrodes, are separatelyguided into the gun to meet at the tip, seeFigure 4. As current is passed through, thewires arc at the tip. The arc heating issufficient to completely melt the tips. Atthe same time, a compressed air jetpositioned behind the tip atomizes theliquid pool and propels the droplets ontothe substrate where they solidify and buildup a coating, see Figure 5. The spray plumeof EAS is longer than that produced fromthe flame spray gun. It can reach between300-600 mm spraying distances dependingon the pressure of the compressed air jet.The longer reach makes EAS a highlyversatile process. EAS is successfullyutilized in many applications, in particularfor large scale, low cost coatings such asfor boiler tubes and off-shore structureswhere a high throughput is required. Thecoatings produced are mostly metallic e.g.Al, Zn, NiAl, FeAl. Some cored wires areavailable for production of ceramic-metalcomposite coatings.

There are various sub-categories forcommercial plasma spraying groupedaccording to their operational environment,e.g. plasma spraying in air, low-pressure

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Figure 5. Cross-sectional structure of EASAlSi coating, magnification x150. Thecoating is dense with a very small amountof porosity both in the coating and at thecoating interface due to the completemelting of the coating material. Theinterfaces between solidified droplets arealso not visible. Some oxide was formedduring the spraying.

chamber and inert atmosphere. The mostwidely used is undoubtedly air oratmospheric plasma spraying (APS) due toits simpler setup, lower operational cost andhigher versatility. In a typical APS gun, seeFigure 6, a copper anode forms an internalwall of a circular nozzle with a tungstencathode placed inside. As a working gas ispassed through the nozzle, it absorbsenergy from the electric arc discharge andchanges into a high energy state or plasma[5]. The core region of the plasma jet can

Figure 6. Schematic diagram of a typicalplasma gun.

reach a temperature of 10,000 oC or higher,capable of melting most materials. Thecoating powder is fed into the plasma jetwhere it starts to melt and is transportedby the gas pressure onto the substrate. Dueto the extreme temperature of the plasmajet, APS finds many applications in theproduction of ceramic, metal andrefractory metal coatings [6,7]. Thermalbarrier coating (TBC) produced frompartially-stabilised ZrO2 powder with aNiCrAlY bondcoat is one such application,see Figure 7.

Figure 7. Cross-sectional structure of APSZrO2-Y2O3/NiCrAlY coating, magnifica-tion x200. The ceramic ZrO2 coatingcontains a large amount of porosity becausethe high melting point material did notachieve a high degree of melting duringspraying. The adhesion of ZrO2 coating tothe substrate is improved by the applicationof NiCrAlY bondcoat [7].

3. COATING APPLICATIONSThermal spraying enjoys successful

applications in many industries. Thefollowing are some examples of the uses ofthermal spraying in Thai industries.

3.1 Solid Lubricant CoatingIn rubber and polymer processing

machine components, although made from

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hard metals such as flame-hardened steeland hard chrome plated steel, often exhibithigh friction from contact with the rubberand polymer. This is due to the adhesionof polymeric debris onto the surface of themetallic components, resulting in changesin the physical properties of the surface.In components such as moulds, doctorblades, push shoes and conveyor belts, thehigh and inconsistent friction can causean interruption to the line process. Mocoatings produced by EAS or APSprocesses have been successfully employedto reduce the adhesion of the polymericdebris and effectively reduce the surfacefriction of the components. In someapplications where a higher hardness of thecoating is required, a secondary phasesuch as NiBSi and NiCrBSi can be addedto the coating to produce a metal-metalcomposite coating, see Figure 8 [8]. Thecomposite coating possesses higherhardness and wear resistance at the expenseof the low coefficient of friction.

Figure 8. Cross-sectional structure ofAPS Mo-NiCrBSi composite coating,magnification ×800.

Figure 9. Cross-sectional structure of ZnEAS coating on low carbon steel substrateafter 119 hours submersion in sea water at55-60oC (average original thickness is 75 μmand average thickness after test isapproximately 65 μm).

Zn alloy coating (lower than 1%) makesthis coating ideal as a sacrificial anodecoating for corrosion protection ofoffshore and seaside steel structures. Anaccelerated submersion test of Zn-coatedlow carbon steel in warm sea water showedthat the coating remained completely intactwith the substrate throughout the test. Nodegradation was observed at the coatinginterface or on the steel sample. Thinningof the coating occurs as a function of thetesting time, see Figure 9 [9].

3.2 Sacrificial Anode CoatingDense Zn and ZnAl coatings can be

cheaply produced using EAS. The highsusceptibility to corrosion in sea watertogether with the low porosity of the EAS

3.3 Ceramic Coating for ContaminationReduction

In industrialized food processing,metal contamination is a major concern.Metallic particles released from machinecomponents during crushing and pul-verizing processing, mostly due to wear andcorrosion, must be removed before thefood product is packaged. In order toreduce the amount of contamination,manufacturers can opt for machinecomponents with higher wear resistance orfor food-grade components, e.g. stainlesssteel AISI316. APS Al2O3 coating was

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introduced as an alternative wear resistantcoating for such components.

In a test conducted in a pulveriser usingrice husk as a pulverizing load, different typesof blade sample were tested simultaneously.

The results show that by volume, the wear ofAl2O3 coated blades is approximately 40% of aflame-hardened blade and less than 5% of anAISI316 blade [10].

Figure 10. Blade samples after 40 hours of testing in a pulveriser using rice husk as aload.

3.4 Coating for Severe Metal SlidingWear

In the production of metal cables, e.g.low carbon steel, Al and Cu, metal slidingwear or adhesive wear occurs between therelatively soft cable and hard componentssuch as counter roll, cable guide, V-grooveroll, stretch roll and capstan. Some heat-treated components, designed to deformthe cable, are subjected to high stress andsevere sliding wear can occur. The damageshortens the lifetime of the components andreduces the surface quality of the cable.Hard coatings such as WC-Co, Cr2O3 andAl2O3 have been successfully utilized toreduce the sliding wear by reducing themetallic proportion of the surface, thusreducing metal seizure and maintainingsurface friction at a low level. For hardercables, i.e. low carbon steel, HVOF WC-(12-17wt. %) Co is used. For softer cables,i.e. Al and Cu, APS ceramic coating is

Figure 11. Cu tracks on the surfaces ofAl2O3 coated and flame-hardenedcomponents. The flame hardened surfacesuffers more severe wear damage resultingin a larger amount of material transfer tothe Cu counterpart. Cu adhesion is alsoobserved on both surfaces.

preferred due to the better surface finishingof the cable, see Figure 11 and Figure 12.

(a) AISI316 blade. (b) Al2O3 APS blade.

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Figure 12. Al2O3 APS capstan in Cu wireproduction.

3.5 High Temperature Wear ResistantCoating

Components operated at hightemperature such as in burner rigs, hospitalincinerators, boiler tubes and gas reactors,are subjected to oxidation and other formsof high temperature corrosion, see examplein Figure 13. Many such components arealso subjected to wear damage as a resultof erosion, sliding movement duringoperation, vibration, etc, which markedlyshortens their useful lives. NiCr-Cr3C2

HVOF coating has been successfullyemployed in these situations, see Figure 14[11]. The weight proportion of the two

Figure 13. Fuel nozzle for a hightemperature reactor.

Figure 14. NiCr-Cr3C2 HVOF coating,magnification ×1000. The structure showsa ceramic-metal composite coatingconsisting of continuous NiCr matrix withreinforcing Cr3C2 particles. Some porositycan be observed.

constituents varies; generally about 20-25wt. % of NiCr is used. The NiCr acts as aductile matrix providing the coating withsome fracture toughness, good adhesion tothe substrate and at the same time oxidationprotection for the underlying substrate.The reinforcing Cr3C2 particles, on theother hand, provide the coating with highhardness and wear resistance. The coatingcan be used in air up to about 800oC abovewhich excessive oxidation of NiCr degradesthe coating.

4. SUMMARYThermal spraying as a family of

coating processes has gained an increase inpopularity in Thailand in recent years asan alternative surface engineering methodto conventional heat treatments and hardchrome plating. Thermal spraying has alsoallowed for novel coatings to combatagainst wear and corrosion. Detailedstudies into wear and corrosionmechanisms of the coatings and extensivefield tests are still required in order toeffectively utilize this surface technologyto its full potential.

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REFERENCES

1. Pawlowski L., The science andengineering of thermal spray coatings,2nd Edn., Wiley, England, 69-74.

2. Ibrahim A. and Berndt C.C., Fatigueand deformation of HVOF sprayedWC–Co coatings and hard chromeplating, Mater. Sci. Eng. A., 2007; 456:114–119.

3. Flitney B., Alternatives to chrome forhydraulic actuators, SealingTechnology, 2007; 8.

4. Ke P.L., Wu Y.N., Wang Q.M.,Gong J., Sun C. and Wen L.S., Studyon thermal barrier coatings depositedby detonation gun spraying, Surf.Coat. Tech., 2005; 200: 2271– 2276.

5. Heimann R.B., Plasma-spray coating,principles and applications, VCH,Weinheim, Germany, 22-24.

6. Sukhonkhet C., Wongpisarn W. andNiranatlumpong P., Tribologicalproperties of Composite CoatingsContaining Large Al2O3 Particles,Asian Thermal Spray Conference,2009, 22-24 Oct. 2009, Xian, China.

7. Koomparkping T., Damrongrat S.and Niranatlumpong P., PhasePrecipitation in NiCoCrAlYbondcoat at High Temperature,Proceedings of the InternationalThermal Spray Conference, May 2003,Orlando, USA, 1631-1634.

8. Niranatlumpong P. and KoiprasertH., The effect of Mo content inplasma-sprayed Mo-NiCrBSi coatingon the tribological behavior, Surf.Coat. Tech., 2010; 205: 483–489.

9. Niranatlumpong P. and AkeviriyakijC., Corrosion Behaviour of Zn andNiCr Arc-Sprayed Coatings in SaltWater Atmosphere, ST 28, 24-26 Oct.2002, Bangkok, Thailand, 638.

10. Niranatlumpong P., Sukhonket C. andNakngoenthong J., Wear resistantsurface treatment of pulverizer blades,Wear, 2013, in press.

11. Koiprasert H., Dumrongrattana S.and Niranatlumpong P., Thermally-Sprayed Coatings for Protection ofFretting Wear in Land-Based Gas-Turbine Engine, Wear, 2004; 257Pawlowski L., The science andengineering of thermal spray coatings,2nd Edn., Wiley, England, 69-74.

12. Ibrahim A. and Berndt C.C., Fatigueand deformation of HVOF sprayedWC–Co coatings and hard chromeplating, Mater. Sci. Eng. A., 2007; 456:114–119.

13. Flitney B., Alternatives to chrome forhydraulic actuators, Sealing Technology,2007; 8.

14. Ke P.L., Wu Y.N., Wang Q.M.,Gong J., Sun C. and Wen L.S., Studyon thermal barrier coatings depositedby detonation gun spraying, Surf.Coat. Tech., 2005; 200: 2271– 2276.

15. Heimann R.B., Plasma-spray coating,principles and applications, VCH,Weinheim, Germany, 22-24.

16. Sukhonkhet C., Wongpisarn W. andNiranatlumpong P., Tribologicalproperties of Composite CoatingsContaining Large Al2O3 Particles,Asian Thermal Spray Conference,2009, 22-24 Oct. 2009, Xian, China.

17. Koomparkping T., Damrongrat S.and Niranatlumpong P., PhasePrecipitation in NiCoCrAlYbondcoat at High Temperature,Proceedings of the InternationalThermal Spray Conference, May 2003,Orlando, USA, 1631-1634.

18. Niranatlumpong P. and KoiprasertH., The effect of Mo content in

830 Chiang Mai J. Sci. 2013; 40(5)

plasma-sprayed Mo-NiCrBSi coatingon the tribological behavior, Surf.Coat. Tech., 2010; 205: 483–489.

19. Niranatlumpong P. and AkeviriyakijC., Corrosion Behaviour of Zn andNiCr Arc-Sprayed Coatings in SaltWater Atmosphere, ST 28, 24-26 Oct.2002, Bangkok, Thailand, 638.

20. Niranatlumpong P., Sukhonket C. andNakngoenthong J., Wear resistantsurface treatment of pulverizer blades,Wear, 2013, in press.

21. Koiprasert H., Dumrongrattana S.and Niranatlumpong P., Thermally-Sprayed Coatings for Protection ofFretting Wear in Land-Based Gas:1-7.