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ABSTRACT- Thermal spraying is a very effective surfacemodification technology and is widely used to apply corrosion,high temperature stresses, and abrasive wear and erosion protectivecoatings for various kinds of medical and industrial applications .Many different materials can be used to produce thermal sprayedcoatings, thereby providing effective solutions to above mentionedproblems. These materials include metals, ceramics, polymers, andcombination of these. These coatings can also be utilized forproduction of anodes for cathodic protection of steel reinforcementin concrete. Applications of Thermal spraying are virtuallyunlimited in scale ranging from small fasteners to structures.Advantages of thermal sprayed coatings include lack of cutting

requirements, portability, abrasion and erosion resistance andability to seal or topcoat. Many types of thermal spraying processare available such as Wire arc spraying, Flame spraying, Plasmaspraying, High velocity oxy-fuel coating spraying (HVOF),Detonation spraying and Cold spraying. This paper summarizes theresults of previous research done by various authors on differentcoatings done by Thermal Spraying.

I ndex terms: mechanical properties, surface modification,Thermal spray process

I. INTRODUCTION

Thermal spraying techniques are coating processes inwhich melted materials are sprayed onto a surface. It is animportant and cost effective technique for changing thesurface properties of engineering components with a view toenhancing their durability and performance under a varietyof operating conditions [1, 2]. Thermal spraying can providethick coatings (approx. thickness range is 20 micrometers toseveral mm, depending on the process and feedstock), overa large area at high deposition rate as compared to othercoating processes suchas electroplating, physical and chemical vapour deposition.Coating materials available for thermal spraying includemetals, alloys, ceramics, plastics and composites.

A typical thermal spray system consists of the spray torch,feeder, media supply, power supply, control console. Typesof thermal spraying are:

Plasma spraying

Detonation spraying

Wire arc spraying

Flame spraying

High velocity oxy-fuel coating spraying (HVOF)

Warm spraying

Cold spraying

The basic steps involved in any thermal coating process

are: Substrate preparation: This usually involves oil/grease

removal and surface roughening. Surface rougheningis necessary for most of the thermal spray processes toensure adequate bonding of the coating to the work

piece.

Masking and fixturing: Masking the part reduces theamount of overspray that an operator must strip afterdeposition.

Coating application:Coatings can be sprayed from rodor wire stock or from powder material. Operators feedmaterials to a flame that melts it. The molten stockthen is stripped from the end of the wire and atomized

by a high-velocity stream of compressed air or othergases, coating the materials onto the work piece.

Finishing: The final step is finishing the work piece.

Most often it is accomplished by grinding and lapping

the work piece

II. LITERATURE REVIEW

a) Plasma arc sprayingIn plasma arc argon and/or nitrogen, with hydrogen orhelium flows through cylindrical copper anode which formsa constricting nozzle. A direct current arc is maintained

between an axially placed tungsten cathode and the outer orexpanding portion of the anode. Gas plasma (ionized gas) isgenerated with a core temperature of about 50,000F. Thecoating powder, with a particle size ranging up to about 100microns, is fed into the plasma stream. The powder is heatedand accelerated by the plasma stream, usually totemperatures above its melting point, and to velocitiesranging from 400 to almost 2,000 ft/sec. The gases chosenfor plasma do not usually react significantly with the

powder particles; however, reaction with the externalenvironment, normally air, may lead to significant changesin the coating. The most significant reaction with metallicand carbide coatings is oxidation. To reduce degradationduring deposition, coatings may be produced using either aninert gas shield surrounding the effluent or by spraying in a

A review on Investigation of different THERMAL

SPRYAING process.

Mukesh Chauhan Himanshu Kala Mr. Amit Joshi

Research Scholar Research Scholar Asst. Professor

Mechanical Engg. Mechanical Engg. Mechanical Engg.GBPEC, Ghurdauri GBPEC, Ghurdauri GBPEC, Ghurdauri


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vacuum chamber under a low pressure of inert gas. Argon isusually used in both cases as the inert gas.[3]

Plasma spray technique

Grisaffe and Spitzig[4] have studied the interaction ofsprayed particles onto a number of polished substrates.Examination of the deposit structure of tungsten andzirconia sprayed onto glass, stainless steel, tungsten andcopper substrates (Grisaffe and Spitzig 1963)) indicated thatthe substrate thermal conductivity exerted the greatestcontrol on the quench rate of the particles and greatlyinfluenced the particle-to-substrate bond. The zirconia

particles splashed considerably more than tungsten particles.

Ying Chun Zhu[5] studied the nanostructure of WCCocoating deposited by plasma arc spray. The result shows thatthe structure of the plasma sprayed WCCo coating is very

complicated. The main structure of the coating is composedof WC grains with a mean particle size of 35 nm. In someregions, the structure is composed of WC grains with amean particle size of 10 nm embedded in an amorphousmatrix. Moreover, some regions of the coating areconstituted completely of amorphous phase. It was alsofound that WC grains have grown to 100 nm in someregions of the coating. The hardness of nano WC-Cocoating is about 18 GPa, which is apparently improvedcomparing with conventional WC-Co coatings, [46].

N. Hegazy[6] employed the plasma spray process onsubstrate of AISI 304 stainless steel with deposition of

AL2O3 ceramic coatings with and without Ni- 5%AL asbond layer. The porosity of the coating was measured byoptical methods. The effectiveness of the type of coatings,

bond layer on the corrosion behavior of the coatings weredetermined through static immersion test in 5%HCL.It was observed that there is a good adhesion between thecoating and substrate and , the coating, bond layer andsubstrate. The interfaces between top ceramic layer and

bond coat and between bond coat and substrate are firm andalmost totally free of material lacks or cracks.The hardness of the alumina coat is nearly 4.5 times higherthan that of the substrate and high corrosion resistance wasobtained with minimum porosity.

Ozkan[7] investigated the effect of coating parameters(spraying distance, substrate temperature, coating thicknessand surface roughness of substrate) on Al2O3 coatings onAISI 304 L stainless steel substrate. The results indicated

that the parameters such as the spraying distance, substratetemperature, coating thickness and substrate roughness werefairly effected the hardness, porosity and surface roughnessof Al2O3 coatings. The lowest surface roughness and thelowest porosity and the highest hardness values of Al2O3coating were obtained for the spraying distance of 12 cm

and the surface roughness of 3.28 Am and the substratetemperature of 500 C. It also found that the increases ofcoating thickness were lowered the hardness and enhancedthe porosity and the coating roughness.

b) High velocity oxy-fuel coating spraying (HVOF)The process utilizes a combination of oxygen with variousfuel gases including hydrogen, propane, propylene,hydrogen and even kerosene. In the combustion chamber,

burning by-products are expanded and expelled outwardthrough an orifice where at very high velocities, often timesthey produce "shock diamonds" exiting the spray gun asshown in Fig. 5 below. Powders to be sprayed via HVOF

are injected axially into the expanding hot gases where theyare propelled forward, heated and accelerated onto a surfaceto form a coating. Gas velocities exceeding Mach 1 have

been reported with temperatures approaching 2,300C.

HOVF[8]

W. Fang[9] investigated WC- based cermets hard coatingsby HVOF, for obtaining the coatings of high hardness, wearresistance, thermal stability and corrosion resistance. Thesurface properties, such as microstructure, hardness and

porosity of WC-CrC-Ni coatings prepared by optimalcoating process (OCP) were investigated. In particular, thefriction and wear behaviours were analyzed for the WC-CrC-Ni coatings, EHC (electrolytic hard chrome) and thesubstrate Inconel 718 (IN 718) both at 25 and 450 C. They

found that the HVOF WCCrCNi coating is veryprotective for alloy surface.

L. Fedrizzi[10] studied the substitution of hard chromiumcoatings with new HVOF cermet coating and they foundthat this process involves very high benefits for theenvironment, as the proposed HVOF technique allows tosubstitute some highly polluting surface treatment

technologies, such as chromium-plating, with a perfectlyclean process from an environmental point of view.


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c) Flame sprayingFlame spray uses combustible gas as a heat source to meltthe coating material. Flame

spray guns are available to spray materials in either rod,wire, or powder form. Most flame spray guns can beadapted to use several combinations of gasesto balance operating cost and coating properties. Acetylene,

propane, methyl-acetylene-propadiene (MAPP) gas, andhydrogen, along with oxygen, are commonly used flamespray gases.[11]

Nuchjira Dejang[12] prepared Al2O3-40wt%TiO2 ceramiccoating using flame spray technique. Microstructure of thecoating was observed using a scanning electron microscopeand phase analysis using x-ray diffractrometry. Densecoating was received , however, defects such as cracks andincompletely melted particles should be minimized byadjusting spraying parameters. Phase transformation ofAl2O3 new Al2TiO5 phase occurred during spraying.

d) Detonation- gun sprayingIn d-gun process consumable powder is fed into the gununder a small gas pressure. Valves are opened to allowoxygen and acetylene to enter the combustion chamber of

the gun. The mixture is then detonated by the sparks fromspark plugs and an explosion occurs immediately. The

temperature of the detonation fuel is about 38000

C, and it isa sufficiently high temperature to melt most of the materials.Immediately after the detonation, hot particles (undergoingmelting) rush toward the target at a very high velocity. Thisfactor is very important for having a well-bonded, densecoating. Detonation cycles are repeated four to eight times

per second and nitrogen gas is used to flush out thecombustion products after each cycle.[13,14,15]

D-GUN setup

Chang-Jiu Li[16] examined the structural features of adetonation gun sprayed Al2O3 coating. It was revealed thatthe detonation sprayed Al2O3 coating has a typical layerstructure similar to that of coating deposited using otherthermal spraying processes. Lamellar bonding at theinterfaces between flattened particles in detonation gunsprayed Al2O3 coating is very poor. The mean bondingration of bonded interface area t apparent bonding surface is

about 10% which is less than one third the value for plasmasprayed Al2O3. However interlocking between flattened

particles is good.

Jia [17] conducted a research on detonation gun coatingwith Fe-SiC composite powders. The FeSiC composite

powder prepared by the mechanical activation process hasbeen used for coating on materials with the detonation gun(D-gun) machine in order to develop a new way for coating.Authors found that the coating layer has fine, homogeneous,dense structure and good wear resistance.The results of SEM and X-ray diffraction (XRD) show that

some reactions happened between Fe and SiC

during the D-gun coating, the FeSi compounds formed andSiC strength the coating layer. It was proved that the

technology combined mechanical alloying with D-gun

coating is a new method for surface modification.

Kamal [18] investigated the microstructure and mechanicalproperties of detonation gun sprayed NiCrAlY + CeO2 alloycoatings deposited on superalloys. The morphologies of thecoatings were characterized by using the techniques such asoptical microscopy, X-ray diffraction and field emissionscanning electron microscopy/energy-dispersive analysis.The coating depicted the formation of dendritic structureand the microstructural refinement in the coating was due to

ceria. Average porosity on three substrates was less than0.58% and surface roughness of the coatings was in the

range of 6.176.94 m. Average bond strength and

microhardness of the coatings were found to be 58 MPa and

697920Hv, respectively

Murthy[19] analyzed the abrasive wear behaviour of WCCoCr and Cr3C220(NiCr) deposited by HVOF anddetonation spray processes. The abrasion tests were doneusing a three body solid particle rubber wheel test rig usingsilica grits as the abrasive medium.

Authors found that the DS coating performs slightly better

than the HVOF coating possibly due to the higher residual

compressive stresses induced by the former process andWC-based coating has higher wear resistance in

comparison to Cr3C2-based coating. Also, the thermally

sprayed carbide-based coatings have excellent wear

resistance with respect to the hard chrome coatings.


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e) Wire arcIn the wire arc spray process, two consumable wireelectrodes connected to a high-current direct-current (dc)

power source are fed into the gun and meet, establishing anarc between them that melts the tips of the wires. Themolten metal is then atomized and propelled toward the

substrate by a stream of air. The process is energy efficientbecause all of the input energy is used to melt the metal.Spray rates are driven primarily by operating current andvary as a function of both melting point and conductivity.

Wire arc coating device.

Yong-xiong CHEN[20] prepared 321 stainlesssteel/aluminium composite coating by arc sprayingtechnique with 321 stainless steel wire as the anode andaluminium wire as the cathode. The traditional 321 stainless

steel coating was also prepared for comparison. Tribologicalproperties of the coatings were evaluated with the ring-block wear tester under different conditions. The structureand worn surface of the coatings were analyzed by scanningelectron microscopy(SEM), X-ray diffractometry(XRD) andenergy dispersion spectroscopy(EDS). The results showthat, except for aluminium phase addition in the 321/Alcoating, no other phases are created compared with the 321coating. However, due to the addition of aluminium, the321/Al coating forms a type of ductile/hard phases inter-deposited structure and performs quite different

tribological behaviour. Under the dry sliding condition, theanti-wear property of 321/Al coating is about 42% lower

than that of 321 coating. But under the oil lubricatedconditions with or without 32 h oil-dipping pre-treatment,the anti-wear property of 321/Al coating is about 9% and5% higher than that of 321 coating, respectively. The anti-wear mechanism of the composite coating is mainly relevantto the decrease of oxide impurities and the strengtheningaction resulted from the ductile/hard phases inter-deposited coating structure.

Shao-Guang Liu[21] prepared cored wires of Three TiAl

series intermetallic compounds, TiAl3, TiAl, and Ti3Al,

which were then used to form coatings on low carbon steel

substrates by arc spraying process. High temperatureerosion (HTE) properties of the coatings were determined in

a laboratory elevated temperature erosion tester. The results

show that the HTE resistance of the coatings prepared using

the cored wires decreased in the order of (from best to

worst) TiAl3, TiAl and Ti3Al. The arc-sprayed coatings

prepared using cored wire containing TiAl3 and TiAl

powders exhibited better or comparable HTE resistance than

that containing a commercial Cr3C2-based composite

powder, although the hardness of the former two coatings

was relatively lower. The laminated structure, which wascharacteristic of the arc spraying coatings, was found on all

the prepared coatings. Oxides resulted from oxidation of

both the cored alloy powders and the mild steel sheaths

were also identified between the laminated layers. Under the

present testing conditions, materials loss of the coatings can

be contributed to brittle breaking, fatigue spalling, cutting

and ploughing mechanisms.

Stefan Lucian Toma[22] conducted a comparative

research of 30 T steel deposits, carried out by two spraying

procedures: the classic wire arc spraying procedure plus a

new one, which represents a combination between the

classic wire arc spraying and the flame spraying procedure.

The use of a new procedure allowed the increase of the

drive jet temperature and the study of its effect created on

30 T deposits properties. The modeling of the arc spraying

and the analysis with finite elements in a coupled field

allowed the determination of the drive jet temperature

variation and the fuel flow, necessary for the temperature to

be maintained over 1000 K at spraying distance of 150 mm.

The investigations carried out on 30 T steel coatings,

obtained by this procedure, demonstrate that the spraying jet

temperature increase determines the average growth of the

coatings adherence over 18% for cylindrical surface and

over 5% for flat surface. Also, these investigations

demonstrate the decrease of the average porosity by over

22% for cylindrical surface and by over 17% for flat

surface.

f) Cold spraying process

COLD SPRAYING is a materials deposition process in

which relatively small particles (ranging in size from

approximately 1 to 50 m in diameter), in the solid state are

accelerated to high velocities(typically 300 to 1200 m/s, or980 to 3940 ft/s), and subsequently develop a coating or

deposit on an appropriate substrate by an impaction process.


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M.K. Decker [23] studies the microstructure and propertiesof cold sprayed coatings of nickel and observed that thetensile properties of cold-sprayed coatings are typicallymuch better than those obtained with other thermal spray

processes. However, the ductility of cold-sprayed coatingsin the as-sprayed condition is typically very low due to the

extensive work hardening that is inherent in the depositionprocess. Heat treating the coatings at high temperatureretrieves sufficient ductility.

W-Y-LI[24] investigated the effect of standoff distance oncoating deposition characteristics in cold spraying. Al, Tiand Cu powders of different sizes were used as feedstocks.It was found that the deposition efficiency was decreasedwith the increase of standoff distance from 10 mm to110 mm for both Al and Ti powders used in this study.However, for Cu powders, the maximum depositionefficiency was obtained at the standoff distance of 30 mm,and then the deposition efficiency decreased with further

increasing the standoff distance to 110 mm. The standoffdistance had a little effect on coating microstructure andmicrohardness for these three powders. Both the stain-hardening effect of the deposited particles and the shot-

peening effect of the rebounded particles take the roles incoating hardness. It was also found that the surface ofsubstrate or previously deposited coating could be exposedto a relatively high gas temperature at a short standoffdistance

Wei HAN[25] studied the elastic modulus of 304stainless steel coating was deposited on the IF steel substrate

by cold gas spraying. The elastic modulus of cold sprayed

304 stainless steel coating was measured using the three-point bend testing and the compound beam theory, and theother mechanic parameters (such as the equivalent flexuralrigidity and the moment of inertia of area) of the coatingswere also calculated using this compound beam theory. It isfound that the calculated results using the above methodsare accurate and reliable. The elastic modulus value of thecold sprayed 304 stainless steel coating is 1. 179 105 MPa,and it is slightly lower than the 304 stainless steel plate(about 2 10

5MPa). It indicates that the elastic modulus of

the cold sprayed coatings was quite different from thecomparable bulk materials. The main reason is that the

pores and other defects are existed in the coatings, and the

elastic modulus of the coatings also depends on variesparameters such as the feed stock particle size, porosity, andprocessing parameters.

Srinivasa R. Bakshi[26] prepared multiwalled carbonnanotube (CNT) reinforced aluminum nanocompositecoatings using cold gas kinetic spraying. Spray drying wasused to obtain a good dispersion of the nanotubes in micron-sized gas atomized AlSi eutectic powders. Spray dried

powders containing 5 wt.% CNT were blended with purealuminum powder to give overall nominal CNTcompositions of 0.5 wt.% and 1 wt.% respectively. Coldspraying resulted in coatings of the order of 500 m in

thickness. Fracture surfaces of deposits show that thenanotubes were uniformly distributed in the matrix.

Nanotubes were shorter in length as they fractured due toimpact and shearing between AlSi particles and the Almatrix during the deposition process. Nanoindentationshows a distribution in the elastic modulus values from 40

229 GPa which is attributed to microstructural heterogeneityof the coatings that comprise the following: pure Al, AlSieutectic, porosity and CNTs.

III. CONCLUSION

The current trends are to design the coating as an integral

part of the component assembly rather than as an add-on to

the substrate, where the property of coating adhesion to the

substrate is of principal interest. Thermal spray coatings

makes it possible to achieve fully effective, maintenance

free protection of steel and concrete structures.

Of all the above discussed processes detonation guncoatings have proven to be successful to both designers andmaintenance engineers as a means of providing dependablewear and corrosion resistant surfaces on machinecomponents operating under difficult service conditions,extending wear life of parts and justifying the expenditurefor coatings on both new and renovated parts equipments.

IV. REFERENCES1. L. Pawlowski, The Science and Engineering of Thermal

Spray Coatings,Wiley, UK, 1995.2. R. Knight, R.W. Smith, Thermal Spray Forming of

Materials, vol.7, ASM Handbook, 19983. [Online]Available:http://www.praxair.com/praxair.nsf/d

63afe71c771b0d785256519006c5ea1/2471692e3b79f13485256ef600676b10/$FILE/Plasma%20Spray%20Process.pdf.

4. Grisaffe S.J., Spitzig W.A., Preliminary investigation ofpgrticle-substrate bonding of plasma-sprayed materials.NASA Tech. Note D-1705 (1963)

5. Y. C. Zhu, C. X. Ding, K. Yukimura, T. D. Xiao, P. R.

Strutt, Ceramics International27 (2001) 669674.6. N. Hegazy, M. Shoeib, Sh. Abdel-Samea , H.Abdel-

Kader, 2009, Effect of Plasma Sprayed AluminaCoating on Corrosion Resistance, ASAT-13-MS-14.

7. Ozkan Sarikaya, 2004, Effect of some parameters onmicrostructure and hardness of alumina coatingsprepared by the air plasma spraying process, Surface &Coatings Technology 190 (2005) 388393.

8. E. Lugscheider, 1992, Technica, v 19, p 19.9. W. Fang, T.Y. Cho, J.H. Yoon, K.O. Song, S.K. Hur,

S.J. Youn, H.G. Chun, Process Tech. (2008),doi:10.1016/j.jmatprotec.2008.08.024.

10. L. Fedrizzi , S. Rossi , R. Cristel , P.L. Bonora ,Electrochimica Acta 49 (2004) 28032814.

11. Robert C. Tucker, Jr., Praxair Surface Technologies,Inc., 1994, Thermal Spray Coatings, ASM Handbook,Volume 5: Surface Engineering.

12. Nuchjira Dejang, Supranee Pitsamai, SittichaiWirojanupatump and Sukanda Jiansirisomboon,microstructure and phase analysis of flame sprayedAl2O3-40wt%TiO2 coating.

13. K. G. Budinski, Surface Engg. For Wear Resistance,N.J., USA, 1988.

14. L. F. Longo, 1985, Thermal Spray Coatings, ASM,USA.

15. V. Meringolo, 1983, Thermal Spray Coating, TappiPress, Atlanta, USA.

16. Chang-ji li, akira ohmori, 1995, surface and coatingtechnology 82(1996) 254-258

17. Jia Chengchang, Li Zhicong, Xie Zizhang; A researchon detonation gun coating with Fe-SiC compositepowders mechanically activated, Materials Science andEngineering A, Volume 263, Number 1,(1999), 96-100.

18. Kamal Subash, Jayaganthan R., Prakash Satya; Mechanical and microstructural characteristics of


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detonation gun sprayed NiCrAlY+ 0.4 wt% CeO2coatings on superalloys, Materials chemistry andphysics, Volume 122, Number 1, (2010), 262-268.

19. Murthy J.K.N., Venkataraman B.; Abrasive wearbehavior of WC-CoCr and Cr3C2-20(NiCr) deposited byHVOF and detonation spray processes, Surface andCoatings Technology, Volume 200, Number 8, (2006),

2642-2652.20. Yong-xiong CHEN,Bin-shi XU,Yan LIU, Xiu-bing

LIANG,Yi XU, Structure and sliding wear behavior of321 stainless steel/Al composite coating deposited byhigh velocity arc spraying technique, Transactions ofNonferrous Metals Society of China Volume 18, Issue 3,June 2008, Pages 603609.

21. Shao-Guang Liu,Jin-Ming Wu,Sheng-Cai Zhang,Shu-JieRong,Zhi-Zhang Li, High temperature erosion propertiesof arc-sprayed coatings using various cored wirescontaining TiAl intermetallics Wear Volume 262,Issues 56, 28 February 2007, Pages 555561.

22. Stefan Lucian Toma, The influence of jet gastemperature on the characteristics of steel coating

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Tran,Microstructure and Properties of Cold SprayNickel, Thermal Spray 2001: New Surfaces for a NewMillennium, C.C.Berndt, K.A. Khor, and E.F.Lugscheider, Ed., ASM International,2001, p 433439

24. W.-Y. Li C. Zhang X.P. Guo G. Zhang H.L. Liao C.-J.Li C. Coddet, Effect of standoff distance on coatingdeposition characteristics in cold spraying, Materials &Design Volume 29, Issue 2, 2008, Pages 297304

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